Biological and clinical considerations in making maxillo mandibular

relation records:

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Introduction

Jaw relations are defined as any one of the many relations of the mandible to the maxillae (Boucher -3)

Maxillomandibular relationship is defined as any spatial relationship of the maxillae to the mandible; any one of the infinite relationships of the mandible to the maxilla. (Glossary of prosthodontic terms, 1999-1)

These relations may be of orientation, vertical and horizontal relations. They are grouped as such because the relationship of the mandible to the maxillae is in the three dimensions of space i.e., sagittal, vertical and horizontal planes. (Gunnar E Carlson-2)

The occlusal surfaces of the teeth determine the relation of the mandible to the maxillae when the natural teeth are present, thereby aiding in mastication, phonetics and the general appearance of the patient. With the turn of events the natural teeth are lost due to trauma or disease, thus oral rehabilitation is at a standstill and has to be achieved by the process of jaw relations by restoring the lost orofacial balance and comfort of the patient.

When the mandible goes through functional and parafunctional movements, the relationship it assumes defy description because of their complexity. when the mandible is at rest, definite relationship to the cranium or the maxilla can be established. Thus one needs to study certain static relationships to understand the motions made by the mandible in function.

Biologic consideration:

A good prosthodontic treatment bears a direct relation to the structures of the temporomandibular articulation, since occlusion is one of prime concern to the prosthodontist during the treatment of patients with complete denture prosthesis prosthesis. The temporomandibular joints affect the complete denture prosthesis prosthesis prosthesis and likewise the complete denture prosthesis prosthesis prosthesis affect the health and function of the joints. Therefore a knowledge of the interrelationship of the bony structures, tissue resislency, muscle function, movements of the lips, facial muscles, muscles of mastication, occlusions of the teeth, temporomandibular joints and overriding mental attitudes seem indispensable for treatment of edentulous patients.

Review of literature John D. Rugh, carl J. Drago in 1981 suggested that in an upright position, certain jaw muscle must be in slight contraction to maintain the jaw in clinical rest position ,what has been reffered to as “Clinical Rest Position” may be more approximately called an upright posturel position.

Manns, Miralles & Guerraro in 1981 suggested that there is a decrease of electrical activity in the three muscles as VD increases. This may be explained by the passive elastic force of muscles carrying larger part of load on muscle as it s length increases. Further more, the action of opening the mouth implies a mechanism of reciprocal innervation with nervous impulses that excite the motor neurons of mandible depressor muscles & inhibit those of elevator muscles.

. Ito et al suggested that a wide range of condylar loading could occur during unilateral biting & chewing at second molar. If the ratio of force of masseter muscle on working side to force of masseter muscle on the balancing side is large, condylar loading on the working side condyle will be greater than on the balancing side condyle. If this ratio is low, condylar loading will be greater on the balancing side.

Franco Mongini in 1986– suggested that

a. Extensive remodeling of TMJ takes place

thoroughout adult life, leading the marked typical

changes in shape.

b. The degree of remodeling & a new shape imposed on the condyles are closely related to changes in the dentition. The influence of the latter is both direct, as in the close relations between the edentulismand remodeling indeces and between the index of abrasion & condyle shape, & indirect as the cause of defective occlusal contacts. Similar changes in shape may in fact, be observed in patients with complete dentitions & varying degrees of edentulism.

C . Characteristic alterations in the shape of the condyles may be brought about as the result of condylar displacement in centric occlusion. Symmentric posterior displacement appears to occur more frequently in older subjects with fewer teeth. Other forms of displacement are caused by the loss of one or a few teeth, malocclusion of various kinds & eruption of wisdom teeth

d. The accepted definition of “Centric Relation” does not appear applicable to posterior displacement of one or both condyles in centric occlusion.

e. Remodeling of condyles can, to a certain extent, be considered as a functional adaptation of the joint to a new occlusion situation and may be a distance prescursor of symptoms of a pain-dysfunction syndrome in some subjects. It may reasonably be supposed that in other subjects satisfactory readjustment is achieved, and no disterbances appear.

Temporomandibular Joint(TMJ) (Gray’s Anatomy-11)

(okeson-7)

The area where craniomandibular articulation occurs is called temporomandibular joint. The TMJ is by far the most complex joint in the body. It provides for hinging movement in one plane and therefore can be considered as ginglymoid joint. At the same time it also provides for a gliding movements, which classifies it as arthroidal joint. Thus it has been technically considered a ginglymoarthroidal joint.

the TMJ is formed by the mandibular condyle fitting into the mandibular fossa of the temporal bone. Separating these two bones from direct articulation is the articular disc. The TMJ is classified as compound joint.

By definition a compound joint requires the presence of atleast three bones, yet the TMJ is made up only two bones. Functionally the articular disc serves as a nonossified bone that permits the complex movement of the joint. Since the articular disc functions as a third bone the cranionmandibular articulation is considered as a compound joint.

The articular disc is composed of dense fibrous connective tissue devoid of any blood vessels or nerve fibres. In saggital plane it can be divided into three regions according to thickness. The central area is thinnest and is called the intermediate zone. Both anterior and posterior to the intermediate zone the disc becomes considerably thicker.

In the normal joint the articular surface of the condyle is located on the intermediate zone of the disc. The precise shape of the disc is determined by the morphology of the condyle and mandibular fossa

The articular disc is attached posteriorly to an area of loose connective tissue that is highly visualized and innervated. This is known as retrodiscal tissue. Superiorly it is bordered by a lamina of connective tissue that contains many elastic fibres, the superior retrodiscal lamina. Since this region consists of two areas it has been referred to as Bilaminary Zone.

The superior retrodiscal lamina attaches the articular disc posteriorly to the tympanic plate. At the lower border of the retrodiscal tissues is the inferior retrodiscal lamina, which attaches the inferior border of the posterior edge of the disc to the posterior margin of the articular surface of the condyle. The inferior retrodiscal lamina is composed chiefly of collagenous fibres. The remaining body of the retrodiscal tissue is attached posteriorly to a large ligament that surrounds the entire joint, the Capsular Ligament. The superior and inferior attachments of the anterior region of the disc are also by the capsular ligament.

(Sahler L.G, Morris T.W – 69)

Like the articular disc, the articular surfaces of the mandibular fossa and condyle are lined with dense fibrous connective tissue rather than hyaline cartilage as in most other joints. The fibrous connective tissue in the joints affords several advantages over hyaline cartilage. Its is generally less susceptible than hyaline cartilage to the effects of aging and therefore less likely to break down over time. Also is has a much greater ability to repair than does hyaline cartilage.

The articular disc is attached to the capsular ligament, not only anteriorly and posteriorly but also medially and laterally. This divides the joint into two distinct cavities.

The upper or superior cavity is bordered by the mandibular fossa and the superior surface of the disc. The lower or inferior cavity is bordered by the mandibular condyle and the inferior surface of the disc. The internal surface of the cavities are surrounded by specialized endothelial cells that form a synovial lining. This lining produces synovial fluid which fills both joint cavities. Thus the TMJ is referred to as synovial joint. The synovial fluid serves two purposes.

1. Since the articular surfaces of the joint are non vascular, the synovial fluid acts as a medium for providing metabolic requirements to these tissues.

2. It lubricates the articular surfaces by two mechanisms; boundary lubrication and weeping lubrication. (Shengyi. T, Yinghuax – 70)

Ligaments (Jeffrey P. Okeson)

As in any joint system, ligaments play an important role in protecting the structures. The ligaments of the joint are made up of collagenous connective tissues, which do not stretch. They do not enter actively in joint function bit instead act as passive restraining devices to limit and restrict joint movement. There are three functional ligaments that support the TMJ: (1) the collateral ligaments, (2) the capsular ligament and (3) the temporomandibular ligament. There are also two accessory ligaments: (4) the sphenomandibular and (5) the stylomandibular.

Collateral (discal) ligaments

The collateral ligaments attach the medial and lateral borders of the articular disc to the poles of the condyle. They are commonly called the discal ligaments, and there are two. The medial discal ligament attachees the medial edge of the disc to the medial pole of the condyle. The lateral discal ligament attaches the lateral edge of the disc to the lateral pole of the condyle. They cause the disc to move passively witht eh condyle as it glides anteriorly and posteriorly. These ligaments are responsible for hinging movement of the TMJ, which occurs between the condyle and the articular disc.

Capsular ligament

The entire TMJ is surrounded and encompassed by the capsular ligament. The fibers of the capsular ligament are attached superiorly to the temporal bone along the borders of the articular surfaces of the mandibular fossa and articular eminence. Inferiorly the fibers of the capsular ligament attached to the neck of the condyle. It acts to resist any medial, lateral or inferior forces that tend to separate or dislocate the aricular surfaces. A significant function of the ligament is to encompass the joint, thus retaining the synovial fluid. It is well innervated and provides proprioceptive feedback regarding the positional movement of the joint.

Temporomandibular ligament

The lateral aspect of the capsular ligament is reinforced by strong tight fibers that make up the lateral ligament or temporomandibular ligament. The TM ligament is composed of two parts. An outer oblique portion and an inner horizontal portion. The outer portion extends from the outer surface of the articular tubercle and zygomatic process posteroinferiorly to the outer surface of the condylar neck. The inner horizontal portion extends from the outer surface of the articular tubercle and zygomatic process posteriorly and horizontally to the lateral pole of the condyle and posterior part of the articular disc. The oblique portion of the TM ligament resists excessive dropping of the condyle and therefore acts to limit the extent of mouth opening. The inner horizontal portion of TM ligament limits posterior movement of the condyle and disc.

Sphenomandibular ligament

It is one of the accessory ligaments of the TMJ. It arises from the spine of sphenoid bone and extends downward and laterally to a small bony prominence on the medial surface of the ramus of the mandible called the lingual. It does not have any significant effects on the mandibular movements.

Stylomandibular ligament

It arises from the styloid process and extends downwards and forwards to the angle of the posterior border of the ramus of the mandible. It limits excessive protrusive movements of the mandible.

In understanding the function of this structure it is important to recognize that the mandibular fossa does not normally participate in joint activities except for its anterior wall, which forms the posterior slope of the articular eminence. The functional bony element of this joint, should be perceived as two convex structures, namely the condyle and articular eminence. The superior and posterior areas of the fossa do not participate in bearing functional loads. Such loads are normally borne by the posterior slope of the articular eminence, where the fibrous connective tissue is thickest on the posterior slope and crest of the articular eminence. It has been hypothesized that the natural dentition carries most of the compressive load so that the joint is not ordinarily required to withstand such forces.

The loss of natural dentition may therefore place additional compressive forces on the temporomandibular joint, which is then required to adapt to these new functional demands. Continued stress beyond the adaptive capabilities of the articular tissues may lead to degenerative joint diseases. The collagen fibres become “unmasked” under the compressive loads and uncontrolled and aberrant remodeling ensues and portions of the articular tissues may break down leading to a subluxation of the mandible.

Thus recording of the centric relation position becomes difficult. The edentulous patients are more susceptible to degenerative joint diseases, particularly those individuals whose tissues cannot adapt adequately to the functional changes. Although there is no evidence to suggest that properly constructed complete denture prosthesis prosthesis can reverse the course of this disease, there is an empirical possibility that its progression may be prevented or slowed by reestablishment of more normal types of functional relationships and activities.

The articular disc or meniscus plays a prominent part in the movement of the mandible. Though it has very little movement in the first opening movements when the condyle merely rotates, it undergoes extensive movements when the mandible makes wider opening movements or protrusive movements. The disk can move forward and back over the condyle but cannot move from side to side.

Unhealthy temporomandibular joints complicate the registration of jaw relation records and sometimes even preclude them completely. Centric relation depends on both structural and functional harmony of osseous structures, the intraarticular tissue and the capsular ligaments if it is to be a function position. If these specifications cannot be fulfilled, the patient will not have a centric relation or for that matter provide the prosthodontist with a recordable one.

The auricolotemporal, the posterior deep temporal nerves and the mesenteric nerves innervate the temporomandibular joints. (Gray’s Anatomy -11)

Muscles of Mastication (Gray’s Anatomy-11)

The energy that moves the manndible and allows function of masticatory system is provided by muscles. There are four pairs of muscles making up a group called “muscles of mastication”

1. Masster

2. Temporalis

3. Medial Pterygoid

4. Lateral Pterygoid. - Depressor of Mandible

} Elevators of mandible

The accessory muscles of mastication are:

1. Suprahyoid muscles (Myelohyoid, Digastric, Stylohyoid, Geniohyoid, Hyoglossus)

2. Infra Hyoid Muscles (Sternothyroid, Sternohyoid, Thyrohyoid, Omohyoid)

3. Facial Muscles (Buccinator, Orbicularis oris, Zygomaticus major, Zygomaticus minor, Mentalis, Levator anguli oris)

4. Muscles of back of neck (Scalenus anterior, Scalenus medius, Scalenus posterior, Splenius capitus, Levator scapulae, suboccipital muscles)

5. Muscles of side of neck (Splenius capitus, Semispinalis capitus)

There are three groups of muscles that act to depress the mandible. (Guyton A. C)

1. The suprahyoid muscles (Digastrics, mylohyoid, geniohyoid and stylohyoid) and platysma act as a group and are primarily responsible for opening the mandible.

2. The infrahyoid muscles (Sternothyroid, Sternohyoid, Thyrohyoid, Omohyoid) act to stabilize the hyoid bone so that the suprahyoid muscles can act.

3. The lateral pterygoid muscles pull the condyles forward or medially as the other group of muscles act.

The Masseter

The masster is a rectangular muscle which originates from the zygomatic arch and extends downwards to the lateral aspect of the lower border of the ramus of the mandible. Its insertion on the mandible extends from the region of second molar at the inferior border posteriorlt to include the angle.

The muscle is made up of two portions or heads: superficial portion and deep portion. As fibres of the masseter contract mandible is elevated and the the teeth are brought into contact. Masseter is a powerful muscle which provides necessary force to chew effeciently. Its superfical portion also aids in protruding the mandible. When the mandible is protruded and biting force is applied the fibres of the deep portion stabilizes the condyle against the articular eminence.

Masseter

TEMPORALIS MUSCLE

The temporal muscle(temporalis) is a large, fan-shaped muscle that originates from the temporal fossa and the lateral surface of the skull. Its fibres come together as they extend downward between the zygomatic arch and the lateral surface of the skull to to form a tendon that inserts on the coronoid process and anterior border of the ascending ramus.

Fibres of temporalis are classified into three types according to their direction and their distinct function.

Anterior vertical fibres

Middle oblique fibres

Posterior horizontal fibres.

When the entire temporalis contracts, it elevates

the mandible and the teeth are brought into contact. If

only anterior portions contract the mandible is

elevated. Contraction of the middle portion will

elevate and retruded the mandible. Function of the

posterior portion is controversial. Although it would

appear that contraction of this portion retrudes the

mandible, DuBrul suggest that the fibers below the

root of the zygomatic process are the only significant

ones and therefore contraction causes elevation and

only slight retrusion.

Temporalis

MEDIAL PTERYGOID

The medial (internal) pterygoid muscle originates from the pterygoid fossa and extends downward, backward and outward to insert along the medial surface of the mandibular angle.

Along with masseter forms a muscle that supports the mandible at the mandibular angle. When its fibres contract, the mandible is elevated and the teeth are brought into contact.

Unilateral contraction along with lateral pterygoid will bring about a mediotrusive movement of the mandible.

LATERAL PTERYGOID

Lateral pterygoid is described as two distinct portions.

1 Inferior portion (or) belly

2.Superior portion(or) belly

The inferior lateral pterygoid muscle originates at the outer surface of the lateral pterygoid plate and extends backward, upward and outward to its insertion primarily on the neck of the condyle. When the right and left inferior lateral pterygoids contact simultaneously, the condyles are pulled down the articular eminences and the mandible is protruded. Unilateral contraction creates a mediotrusive movement of the condyle and causes a lateral movement of the mandible to the opposite side.

The superior lateral pterygoid muscle is considerably

smaller than the inferior and originates at the infratemporal

surface of the greater sphenoid wing, extending almost

horizontally, backward and outward to insert on the articular

capsule the disc and the neck of the condyle.

The functions of these two portions are different and

nearly opposite . and hence described as inferior lateral

pterygoid and superior lateral pterygoid. Superior lateral

peterygoid is considerably smaller than the inferior. This is

responsible for keeping the disc properly aligned with the

condyle during function.

Lateral and medial pterygoids

Functional neuroanatomy and physiology of the masticatory system.(Jeffery P. Okeson-7)

1. Muscle

Motor Unit: The basic component of the neuromuscular system is the motor unit, which consists of a number muscle fibers that are innervated by one motor neuron. Each neuron joins with a muscle fiber at a motor end plate. When the neuron is activated, the motor end plate is stimulated to release small amounts of acetylcholine, which initiates the depolarization of muscle fibers. Depolarization causes the muscle fibers to shorten or contract. Fewer the muscle fibers per motor neuron, more precise is the movement. Hundreds to thousands of motor units along with blood vessels and nerves are bundled together by connective tissue and fascia to make up the muscle.

2. Neurologic structures

The neurons: Each skeletal muscle has both sensory and motor innervations. The sensory or afferent neurons carry information from the muscle to the central nervous system at both the spinal cord and higher center levels. The type of information carried by the afferent nerve fibers most often depends on the sensory nerve endings. Some nerve endings relay sensation of discomfort and pain, as when the muscle is fatigued or damaged. Others provide information regarding the state of contraction of relaxation of the muscle. Still others provide information regarding joint and bone positions (proprioception)

Once the sensory information has been received and processed by the central nervous system, regulatory information is returned to the muscles by way of the motor or efferent nerve fibers.

The information from the tissues outside the CNS needs to be transferred into the CNS and onto the higher centers in the brainstem and the cortex for interpretation and evaluation. Once this information is evaluated, appropriate action must be taken. The higher centers then send information down the spinal cord and back out to the periphery to an efferent organ for the desired action. The primary afferent neuron (first order neuron) receives stimulus from the sensory receptor. This impulse is carried by the primary afferent neuron into the CNS by way of dorsal root to synapse in the dorsal horn of spinal cord with a secondary neuron (second order neuron). The impulse is then carried by the second order neuron across the spinal cord to the anterolateral spinothalamic pathway that ascends to the higher centers. Multiple interneurons (third and fourth order, etc) are involved with the transfer of this impulse to the thalamus and cortex.

3. Brainstem and Brain (Guyton A.C-10 and okeson-7)

Once the impulse has been passed to the second order neurons, these neurons carry them to the higher centers for interpretation and evaluation. Numerous centers in the brainstem and brain help to give meaning to the impulses. The Prosthodontist should remember that numerous interneurons may be involved in transmitting the impulses onto higher centers. The important areas that will be reviewed are spinal tract nucleus, the hypothalamus, the limbic structures and the cortex. They are discussed in the order by which neural impulses pass on to the higher centers. (Okeson J.P – Bell’s orofacial pain)

a. Spinal tract nucleus

Throughout the body, primary afferent neurons synapse with the second order neurons in the dorsal horn of the spinal column. Afferent input from the face and oral structures, does not enter the spinal cord by way of spinal nerves. Instead, sensory input from the face and mouth are carried by way of fifth cranial nerve (Trigeminal nerve). The cell bodies of the trigeminal afferent neurons are located in the large gasserian ganglion. Impulses carried by trigeminal nerve enter directly into the brainstem in the region of Pons to synapse in the trigeminal spinal nucleus. The brainstem-trigeminal nucleus complex consists of two main parts.

i) Main sensory trigeminal nucleus (receives periodontal and some pulpal afferents)

ii) The spinal tract of trigeminal nucleus (Delaat A)

• Subnucleus oralis

• Subnucleus interpolaris

• Subnucleus caudalis

The subnucleus caudalis has been implicated in trigeminal nociceptive mechanisms based on electrophyiological observations of nociceptive neurons. (Sessle B.J, Dostrovsky J.O)

The subnucleus oralis appears to be a significant area of this trigeminal-brainstem complex for oral pain mechanisms. (Lund J.P, Donga R., Widmer C.G, Stohler C.H)

b. Reticular formation (Guyton A C-10)

After the primary afferent neurons synapse in the spinal tract nucleus, the interneurons transmit the impulses up to the higher centers. The interneurons ascend by way of several tracts passing through an area of the brainstem called the reticular formation. Within the reticular formation are concentrations of cells or nuclei that represent centers for various functions. The reticular formation plays an extremely important role in monitoring impulses that enter the brainstem. The reticular formation controls the overall activity of the brain by either enhancing the impulses on to the brain or by inhibiting the impulses. This portion of the brainstem has an extremely important influence on pain and other sensory input.

c. Thalamus (Jeffery p okeson-7)

The thalamus is located in the very centre of the brain, with the cerebrum surrounding it from the top and sides and the mid-brain below. It is made up of numerous nuclei that function together to interrupt impulses. Almost all impulses from the lower regions of the brain, as well as from the spinal cord, are relayed through synapses in the thalamus before proceeding to the cerebral cortex. The thalamus acts as a relay station for most of the communication between the brainstem, cerebellum, and cerebrum. While impulse arise to the thalamus, the thalamus makes assessments and directs the impulses to appropriate regions in the higher centers for interpretation and response.

d. HypothalamusThe hypothalamus is a small structure in the middle of the

base of the brain. Although it is small, its function is great, the hypothalamus is the major center of the brain for controlling internal body functions, such as body temperature, hunger, and thirst. Stimulation of the hypothalamus excites the sympathetic nervous system throughout the body, increasing the overall level of activity of many internal parts of the body, especially increasing heart rate and causing blood vessel construction. An increased level of emotional stress can stimulate the hypothgalamus to up regulate the sympathetic nervous system and greatly influence nonciceptive impulses entering the brain. This simple statement should have extreme meaning to the clinician managing pain10 .

e. Limbic structuresThe word limbic means border. The limbic system

comprises the border structures of the cerebrum and the diencephalons. The limbic structures function to control our emotional and behavioral activities. Within the limbic structures are centers, or nuclei, that are responsible for specific behaviors, such as anger, rage etc. The limbic structures also control emotions, such as depression, anxiety, fear or paranoia.

Impulses from the limbic system leading into the hypothalamus can modify any or all of the many internal bodily functions controlled by the hypothalamus. Impulses from the limbic system feeding into the midbrainm and medulla can control such behavior as wakefulness, sleep, excitement and attentiveness.

f. Cortex (okeson-7)

This cerebral cortex represents the outer region of the cerebrum and is made up predominantly of gray matter. The cerebral cortex is the portion of the brain most frequently associated with the thinking process, even though it cannot provide thinking without simultaneous action of deep structures of the brain. The cerebral cortex is the portion of the brain in which essentially all of our memories are stored, and it is also the area most responsible for our ability to acquire our many muscle skills. The basic psychologic mechanisms by which the cerebral cortex stores either memories or knowledge of muscle skills are not known.

In most areas the cerebral cortex is about 6mm thick and contains an estimated 50 to 80 billion nerve cell bodies. Perhaps 1 billion nerve fibers lead away from the cortex, and comparable numbers lead into it. These nerve fibers pass to other areas of the cortex, to and from deeper structures of the brain; some travel all the way to the spinal cord.

Different regions of the cerebral cortex have been identified to have different functions. A motor area is primarily involved with coordinating motor function; (precentral gyrus) a sensory area receives somatosensory (post central gyrus) input for evaluation. Areas for specials senses, such as visual and auditory areas, also are found. (Guyton-10)

THE SENSORY RECEPTORS (William F. Ganong-71)

Sensory receptors are neurologic structures or organs located in the tissues that provide information to the central nervous system regarding the status of these tissues. As in other areas of the body, various types of sensory receptors are located throughout the tissues that make up the masticatory system. There are specialized sensory receptors that provide specific information to the afferent neurons and thus back to the central nervous system. Some receptors are specific for discomfort and pain. Others provide information regarding the position and movement of the mandible and associated oral structures. These movement and positioning receptors are called proprioceptors.

The masticatory system utilizes four major types of sensory receptors to monitor the status of its structures: (1) the muscle spindles, which are specialized receptor organs found in the muscle tissue; (2) the Golgi tendon organs, located in the tendons; (3) the pacinian corpuscles, located in tendons, joints, periosteum, fascia and subcutaneous tissues, and (4) the nociceptors, found generally throughout all the tissues of the masticatory system

a. Muscle spindles (Jeffery P Okeson-7)

Skeletal muscles consist of two types of muscle fiber: the first is the extrafusal fibers, which are contractible and make up the bulk of the muscle, the other is the intrafusal fibers, which are only minutely contractile. A bundle of intrafusal muscle fibers bound by a connective tissue sheath is called a muscle spindle. The muscle spindles are interspersed throughout the skeletal muscles and aligned parallel to the extrafusal fibers. Within each muscle spindle the nuclei of the intrafusal fibers are arranged in two distinct fashions. Chainlike (nuclear chain type) or clumped (nuclear bag type)

There are two types of afferent nerves that supply the intrafusal fibers. They are classified according to their diameters. The larger fibers conduct impulses at a higher speed and have lower thresholds. Those that end in the central region of the intrafusal fibers are the larger group (la) and are said to be the primary endings (so-called annulospiral endings.) Those that end in the poles of the spindle (away from the central region) are the smaller group (II) and are the secondary endings (so-called flower spray endings)

The afferent neurons originating in the muscle spindles of the muscles of mastication have their cell bodies in the trigeminal mesencephalic nucleus.

The intrafusal fibers receive efferent innervation by way of fusimotor nerve fibers, alpha nerve fibers, which supply the extrafusal. There are two manners in which the afferent fibers of the muscle spindles can be stimulated: generalized stretching or elongation of the entire muscle (extrafusal fibers) and contraction of the intrafusal fibers by way of the gamma efferents. The muscle spindles can only register the stretch and cannot differentiate between these two activities. Therefore the activities are recorded as the same activity by the central nervous system.

The extrafusal muscle fibers receive innervation by way of the alpha efferent motor neurons. Most of these have their cell bodies in the trigeminal motor nucleus.

From a functional standpoint the muscle spindle acts as a length monitoring system. It constantly feeds back information to the central nervous system regarding the state of elongation or contraction of the muscle.

AFFERENT FIBERS II

AFFERENT FIBERS IA

EFFERENT

FIBERS (γ ) EFFERENT

FIBERS (α )

EXTRAFUSAL FIBERS

CAPSULE OF MUSCLE FIBER

NUCLEAR CHAIN INTRAFUSAL FIBER

NUCLEAR BAG INTRAFUSAL FIBER

INTRAFUSAL FIBER

b. Golgi tendon organsThe golgi tendon organs are located in the muscle tendon

between the muscle fibers and their attachment to the bone. They occur in series with the extrafusal muscle fibers and not in parallel as with the muscle spindles. Each of these sensory organs consists of tendinous fibers surrounded by lymph spaces enclosed within a fibrous capsule. Afferent fibers enter near the middle of the organ and spread out over the extent of the fibers. Tension on the tendon stimulates the receptors in the Golgi tendon organ. Therefore contraction of the muscle also stimulates the organ. Likewise, an overall stretching of the muscle creates tension in the tendon and stimulates the organ.

At one time it was thought that the Golgi tendon organs had a much higher threshold than the muscle spindles and therefore functioned solely to protect the muscle from excessive or damaging tension. It now appears that they are more sensitive and are active in reflex regulation during normal function. The Golgi tendon organs primarily monitor tension whereas the muscle spindles primarily monitor muscle length.

c. Pacinian CorpusclesThe pacinian corpuscles are large oval organs made up of

concentric lamellae of connective tissue. At the center of each corpuscle is a core containing the termination of a nerve fibre. These corpuscles are found the tendons, joints, periosteum, tendinous insertions, fascia, and subcutaneous tissue. There is a wide distribution of these organs, and because of their frequent location in the joint structure they are considered to serve prinicp0ally for the perception of movmement and firm pressure (not light touch).

d. NociceptorsGenerally nociceptors are sensory receptors that are

stimulated by injury and transmit this information to the central nervous system by way of the afferent nerve fibres. Nocieceptors are located throughout most of the tissue s in masticatory system. There are several general types; some respond exclusively to noxious mechanical and thermal stimuli; other respond to a wide range of stimuli, from tactile sensation to noxious injury; still others are low threshold receptors specific for light touch, pressure, or facial hair movement. The last type is some times called mechanoreceptors.

NEUROMUSCULAR FUNCTIONFunction of the sensory receptors:

The dynamic balance of the head and neck muscles previously described is possible through feedback provided by the various sensory receptors. When a muscle is passively stretched, the spindles infor the central nervous system of this activity. Active muscle contraction is montitored by both the Golgitendon organs and the muscle spindles. Movement of the joints and tendons stimulates the pacinian corpuscles, which relay this information to the central nervous system. Pain as well as fine movement and tactile sensations are monitored through the nociceptors. All these sensory organs provide constant feedback to the central nervous system. This input is continually monitored and evaluated both day and night, during both activity and relaced periods. The central nervous system evaluates and organizes the sensory input and initiates appropriate efferent input to create a desired motor function. Most of the efferent pathways running from the higher centrers to the muscles of mastication pass through the trigeminal motor nucleus.

Neuromuscular regulation of mandibular motion: (Boucher-3)

The muscles that move, hold, or stabilize the mandible do so because they receive impulses from the central nervous system. The impulses that regulate mandibular motion may arise at the conscious level and result in voluntary mandibular activity. They also may arise from subconscious levels as a result of the stimulation of oral or muscle receptors or of activity in other parts of the central nervous system. When a closing movement occurs, the neurons to the closing muscles are being excited and those to the opening muscles are being inhibited. Impulses from the subconscious level, including the reticular activating system, also regulate muscle tone, which plays a primary role in the physiological rest position of the mandible.

Certain receptors in mucous membranes of the oral cavity can be stimulated by touch, thermal changes, pain or pressure. Other receptors located principally in the periodontal ligaments, mandibular muscles, and mandibular ligaments provide information as to the location of the mandible in space and are called proprioceptors. The impulses generated by stimulation of these oral receptors travel to the sensory nuclei of the trigeminal nerve or, in the case of proprioceptors, to the mesencephalic nuclei.

From there they are transmitted –

(1) By way of the thalamus to the sensorimotor cortex (conscious level) to produce a voluntary change in the position of the mandible;

(2) By way of the reflex arc to the motor nuclei of the trigeminal nerve and directly back to the mandibular muscles to cause an involuntary movement of the mandible or

(3) By a combination of these two under the influence of subcortical areas such as the hypothalamus, basal ganglia, or reticular formation.

REFLEX ACTIONA reflex action is the response resulting from a stimulus

that passes as an impulse along an afferent neuron to posterior nerve root or its cranial equivalent, where it is transmitted to the efferent neuron leading back to the skeletal muscle. Although the information is sent to the higher center influence. A reflex action may be monosynaptic or polysynaptic. A monosynaptic reflex occurs when the afferent fiber directly stimulates the efferent fiber in the central nervous system. A polysynaptic reflex is present when the afferent neuron stimulates one or more interneurons in the central nervous system, which in turn stimulate the efferent nerve fibers.

Two general reflex actions are important in masticatory system

1. The myotatic reflex2. The nociceptive reflex.

Myotatic (stretch) reflex: (Dale. R.A-22) the myotatic or stretch reflex is the only monosynaptic jaw reflex is the only monosynaptic jaw reflex. When skeletal muscle is quickly stretched, this protective reflex is elicited and brings about a contraction of the stretched muscle.

The myotatic reflex can be demonstrated by observing the masseter as a sudden downward force is applied with a small rubber hammer. As the muscle spindles within the masseter suddenly stretch, afferent nerve activity is generated from the spindles. These afferent impulses pass into the brainstem is the trigeminal motor nucleus by way of the trigeminal mesencephalic nucleus, where the primary afferent cell bodies are located. These same afferent fibers synapse with the alpha efferent motor neurons leading directly back to the extrafusal fibers of the masseter.

Clinically this reflex can be demonstrated by relaxing the jaw muscles, allowing the teeth separate slightly. A sudden downward tap of the chin will cause the jaw to be reflexly elevated. The masseter contracts, resulting in tooth contract.

The myotatic reflex occurs without specific response from the brain and is very important in determining the resting postion of the jaw. If there were complete relatxation of the muscles that support the jaw, the forces of gravity would act to lower the jaw and separate and articular surfaces of the TMJ. To prevent this dislocation, the elevator muscles (and other muscles) are maintained in a mild state of contraction (called muscle tonus). The myotatic reflex is a principal determinant of mucle tonus in the elevator muscles. As gravity pulls down on the mandible, the elevator muscles are passively stretched, which also creates stretching of the muscle spindles. Thus passive stretching causes a reactive contraction that relieves the stretch on the muscle spindle.(Hellsing and klineberg -23)

Nociceptive (flexor) reflex: (Stohler C S –20) The nociceptive or flexor reflex is a polysynaptic reflex to noxious stimuli and therefore is considered to be protective. In masticatory stystem this reflex becomes active when a hard object is suddently encountered during masticatory.

As the tooth is forced down on the hard object, a noxious stimulus is received by the tooth and surrounding periodontal structures. The associated sensory receptors trigger afferent nerve fibers, which carry the information to the interneurons in the trigeminal motor nucleus. Not only must the elevatory muscles be inhibited to prevent jruther jaw closure on the hard object, but the jaw opening muscles must be activated to bring the teeth away from potential damage. As the afferent information from the sensory receptors reaches the interneurons, two distinct actions are taken excitatory interneurons leading to the efferent fibers of the jaw opening muscles are stimulated. This action causes therse muscles to contract. At the same time the afferent fibers stimulate inhibitory interneurons, which have their effect on the jaw elevating muscles and cause them to relax. The overall lresult is that the jaw quickly drops and the teeth are pulled away from the object causing the noxious stimulus.

The myotatic reflex protects the masticatory system from sudden stretching of a muscle the nocieceptive reflex protects the teeth and supportive structures from dameage created by sudden and unusually heavy functional forces.

Influence of opposing tooth contacts: (Gunner E Carlsson - 2)

An important aspect of many jaw movements includes the contacts of opposing teeth. The manner in which the teeth occlude is related not only to the occlusal surfaces of the teeth themselves but also to the muscles, TMJs, and neurophysiological components including the patient’s mental well being. When patients wearing complete denture prosthesis prosthesis bring their teeth together in centric or eccentric positions within the functional range of mandibular movements, the occlusal surfaces of the teeth should meet evenly on both sides. In this manner, the mandible is not deflected from its normal path of closure, nor are the dentures displaced from the residual ridges. In addition, when mandibular movements are made with the opposing teeth of complete denture prosthesis prosthesis in contact, the inclined planes of the teeth should pass over one another smoothly and not disrupt the influences of the condylar guidance posteriorly and the incisal guidance anteriorly.  

Mandibular movements . (Jeffery .P Okeson -7)

Mandibular movements occur as a complex series of interrelated three-dimensional rotational and translational activities. It is determined by the combined and simultaneous activities of both temporomandibular joints.

Types of movements

Two types of movement occur in the temporomandibular joint;

1. Rotation or hinge movement

2. Translatory movement

Rotational Movement (Lindaver. S J –72)

Rotational movement occurs as movement within the inferior cavity of the joint. It is thus movement between the superior surface of the condyle and the inferior surface of the articular disc. Rotational movement of the mandible can occur in all three reference planes; horizontal, frontal (vertical), and sagittal. In each plane it occurs around a point, called the axis.

Translational Movement

Translation can be defined as a movement in which every point of the moving object has simultaneously the same velocity and direction. In the masticatory system it occurs when the mandible moves forward, as in protrusion. The teeth, condyles, and rami all move in the same direction and to the same degree. Translation occurs within the superior cavity of the joint between the superior surface of the articular disc and the inferior surface of the articular fossa.

Sagittal Plane Border and Functional movements

Mandibular motion viewed in the sagittal plane can be seen to have four distinct movement components

1. Posterior opening border

2. Anterior opening border

3. Superior contact border

4. Functional

Posselt’s Curve

Horizontal Plane Border And Functional Movements:

When mandibular movements are viewed in the horizontal plane, a rhomboid shaped pattern can be seen that has four distinct movement components plus a functional component:

1. Left lateral border

2. Continued left lateral border with protrusion

3. Right lateral border

4. Continued right lateral border with protrusion.

CR

CO

1

24

3

Frontal (Vertical) Border and Functional Movements:

When mandibular motion is viewed in the frontal plane, a shield-shaped pattern can be seen that has four distinct movement components along with the functional component:

1. Left lateral superior border

2. Left lateral opening border

3. Right lateral superior border

4. right lateral opening border

The biologic factors which include the anatomy and physiology of the temporomandibular joints, the axes around which the mandible rotates, the actions of muscles and ligaments, contacts of opposing teeth and the neuromuscular integration must be well understood by the prosthodontist during the treatment of edentulous patients.

A. Roy MacGregor described the following procedure of adjusting the upper and lower record

blocks during jaw relation.

TRIMMING THE UPPER RECORD BLOCK

When trimming the rim there are four main considerations and they must be taken in the order given.

• Labial fullness: The lip is normally supported by the alveolar process and teeth which, at this stage, are represented by the base and rim of the record block. Therefore, the labial surface must be cut back or added to until a natural and pleasing position of the upper lip is obtained.

2. The height of occlusal rim: It should be trimmed vertically until it represents the amount of anterior teeth intended to show below the lip at rest. The average adult shows approximately 3mm of upper central incisors when the lips are just parted, but there are many variations from this amount which should be accepted as a guide rather than a rule

A greater length of tooth than normal may be shown if the patient has:

a. A short upper lipb. Superior protrusionc. An Angle’s Class II malocclusion of natural teeth

And less will be shown:

a. With a long upper lipb. In most old people, owing to attrition of natural teeth and some loss of tone of the orbicularis oris muscle

3. Anterior plane: Generally the plane to which the anterior teeth should be set, and to which the rim must be trimmed, is parallel to an imaginary line joining the pupils of the eyes or a line at right angles to the midsagittal plane of the face.

4. The anteroposterior plane: This plane indicates the position of occlusal surfaces of the posterior teeth and is obtained in conjunction with the anterior plane. The rim is trimmed parallel to Ala-tragus line (an imaginary line running from the external auditary meatus or tragus of the ear to the lower border of ala of the nose). It has been found from the study of many cases that the occlusal plane of natural teeth is usually parallel to this line

Thus when the rim has been trimmed to these planes it indicates the place of orientation for setting the artificial teeth.

GUIDELINES

1. The centre line or midlineIn the normal natural dentition, the upper central

incisors have their mesial surfaces in contact with an imaginary vertical line which bisects the face and, for esthetic reasons, it is desirable that the artificial substitutes should occupy the same position. Few human faces are symmetrical. Therefore there can be no hard and fast rule for determining the centre line, which thus depends on the artistic judgement of the prosthodontist.

The following aids are suggested as a help in deciding where to mark a vertical line on the labial surface of the upper rim

• Where it is crossed by an imaginary line from the centre of the brows to the centre of the chin.• Immediately below the centre of the philtrum• Immediately below the centre of the labial tubercle• At the bisection of the line from one corner to the other corner of the mouth, when the lips are relaxed.• Where it is crossed by a line at right angles to the interpupillary line from a point midway between the pupils when the patient is looking directly forwards.• Midway between the angles of the mouth when the patient is smiling.

2. High lip line This is a line just in contact with the lower border of the

upper lip when it is raised as high as possible unaided, as in smiling or laughing. It is marked on the labial surface of the rim and indicates the amount of denture which may be seen under normal conditions, and thus assists in determining the length of tooth needed. 3. Canine lines

These mark the corners of the mouth when the lips are relaxed and are supposed to coincide with the tips of the upper canine teeth but are only accurate to within 3 or 4 mm. These lines give some indication of the width to be taken up by the six anterior teeth from tip to tip of the canines.

TRIMMING THE LOWER RECORD BLOCK

Having trimmed and marked the upper block, all that now requires to be done is to trim the lower block so that when it occludes evenly with the upper, the mandible will be separated from the maxilla by the same distance that it was when the natural teeth were in occlusion. The location of the occlusal plane posteriorly will ultimately be determined by the height of the mandibular anterior teeth and anterior 2/3 rd of retromolar pads. After recording the tentative occlusal vertical relation and the centric relation position, the maxillary occlusion rims are oriented to the opening axis of the jaws with the help of the face bow.

Orientation Relations

Orientation relations are those that orient the mandible to the cranium in such a way that when the mandible is kept in its most posterior unstrained position, the mandible can rotate in the sagittal plane around an imaginary transverse axis passing through or near the condyles

Transverse horizontal axis or Hinge Axis is defined as an imaginary line around which the mandible may rotate within the saggital plane.

The ‘Terminal Hinge position or retruded contact position, is defined as the guided occlusal relationship occurring at the most retruded position of the condyles in the joint cavities. A position that may be more retruded that the centric relation position.

The Face bow

1. The face bow is an instrument used to record the spatial relationship of the maxillae to some anatomic reference and transfer this relationship to an articulator. Customarily this reference is a plane established by a transverse horizontal axis and a selected anterior point.

- Glossary of prosthodontic terms, 1987

2. A caliper like instrument used to record the spatial relationship of the maxillary arch to some anatomic reference point or points and then transfer this relationship to an articulator; it orients the dental cast in the same relationship to the opening axis of the articulator. Customarily, the anatomic references are the mandibular condyles transverse horizontal axis and one other selected anterior point; called also as hingebow

- (Glossary of prosthodontic terms, January 1999 –1)

The face bow is a caliper like device that is used to record the relationship of the jaws to the temporomandibular joints or the opening axis of the jaws and to orient the casts in this relationship to the opening axis of the articulator. (Boucher. 10th ed)

A face bow is used to record the three dimensional

relation of the maxillae to the cranium. The face bow record is used to orient the maxillary cast to the articulator this procedure is called the face bow transfer. Mandibular opening and closing movement are reproduced when the transverse horizontal axis is coincident with the articulator hinge axis. In order to create precise occlusion, the casts would be oriented correctly which depends on an accurate face bow transfer. (Lucia 1960)

Types of Face bow:

   There are two types of face bows.

1. Kinematic face bow

2. Arbitrary face bow – Facial type

- Earpiece type

Review Literature:

The study of hinge axis opening of the mandible and the need to accurately locate it has occupied many distinguished workers over the years.

Locating the transverse hinge axis was first discussed by Campion (1902), who felt that the axis of the articulator should coincide with that of the patients.

Gysi (1910), in his treatise stated “the mandible in opening and closing rotates around another center, which, however has no influence in the setting up of teeth for articulators, and therefore need not be considered in construction of an articulation”

Other important workers in this field were Bennet (1908, 1924), Needles (1923, 1927), and Wardsworth.

Stansberry (1928), was dubious about the value of face bows and adjustable articulators. He thought that since an opening movement about the hinge axis took the teeth out of contact, the use of these instruments was ineffective except for the arrangement of the teeth in centric occlusion. In his opinion, the plain line hinge type of articulator was just as effective.

Mclean (1937) stated; “the hinge functions of the lower portion of the temporomandibular joints are still disputed and little understood”. The hinge portion of the jaw has two function of great importance to Prosthodontists

First, the hinge portion of the joint is the great equalizer for disharmonies between the gnathodynamic factors of occlusion when occlusions are synthesized on articulator without accurate hinge axis orientation, there may be minor cuspal conflicts, which must be removed by selective spot grinding.

The second function of the hinge portion of the joint is inherent in the fact that in it takes place all changes of the level of biting closure, commonly called opening or closing the bite.”

Regarding the satisfactory construction of full dentures, he said that opening or closing the bite on a articulator with an incorrect hinge axis location would result in unsatisfactory occlusion of a dentures when they were placed in the mouth. When the hinge axis on the articulator was too far forward compared with its location on a patient, closing the interocclusal distance would result in the dentures meeting prematurely posteriorly. If the axis was too far posteriorly, premature contact would occur anteriorly. If the axis was too low, the lower denture would be forward of centric relation. If too high, the lower denture would be posterior to centric occlusion. The conclusion was that any alteration in the interocclussal distance must be made in the mouth or by the use of a hinge articulator. If the latter were to be use, then the hinge axis must be determined as a stationary point (i.e. rotatory but not translatory) over the head of the condyle during hinge axis movements and not by palpation or anatomical location.

 

McCollum (1939), was one of the leading advocated of the hinge axis theory and published a very important series of articles concerning restorative remedies. He stated: “In 1921 I became convinced that the opening and closing center of the mandible was a most important factor in dental articulation and that its determination was preliminary to the transferring to an articulating instrument a record of jaw relations..

       In his articles he lauded Snow for his discovery of the face bow and its use and at the same time he critici1zed Gysi on his views of the hinge axis and for saying that changing vertical dimension is a chair side operation. McCollum also described how be came to demonstrate conclusively the existence of the definite opening and closing axis by using a face-bow rigidly attached to the lower teeth with an orthodontic appliance. He found wide variation in anatomic location of the points and between sides of the same individual. He said that the hinge axis point remained constant throughout life.

Other important workers in this field were Higley (1940), Stuart (1947), Logan (1941), McLean (1944), and Branstad (1950).

      Robert. G. Schallhorn (1947), studying the arbitrary center and kinematic center of the mandibular condyle for face bow mountings concluded that using the arbitrary axis for face bow mountings on a semi adjustable articulator is justified. He says that since, in over 95% of there subjects, the kinematic center lies within a radius of 5 mm. from the arbitrary center.

Craddock and Symmons (1952), considered that the accurate determination of the hinge axis was only of academic interest since it would never be found to be move that a few millimeters distant from the assumed center in condyle itself.

Posselt (1952), conducted extensive studies on the hinge axis. He found that the extent of hinge opening between the upper and lower incisor teeth was 19.2 mm. 1.9mm.

Page (1952), described the ‘hinge bow’ developed by Mc Collum in 1936 as one of the most important contributions made to dental science.

Lucia (1953) stated “the practical importance of the hinge axis and hinge axis transfer to an articulator is of tremendous importance. “ without a hinge axis transfer he thought it impossible to diagnose an occlusal problem.

Bandrup – Wognesen (1953), discussed the theory and history of face bows. He quoted the work of Beyron who had demonstrated that the axis of movement of the mandible did not always pass through the centers of the condyle. They concluded that complicated forms of registration were rarely necessary for practical work.

Other very important workers in this field were Laurizten (1951), Clapp (1952), Sloane (1951), Granger (1952), Lucia (1953), Sicher (1954), Thompson (1954), page (1955), Collet (1955), Kornfield, (1955), Trapozzano (1955), and Beck and Morrision (1956)

       Teteruck and Lundeen (1966), evaluated the accuracy of the ear face bow and concluded that only 33% of the conventional axis locations were within 6 mm of true hinge axis as compared to 56.4% located by ear face – bow. They also recommended the use of ear bow for its accuracy, speed of handling, and simplicity of orienting the maxillary cast.

Thorp, Smith, & Nicholos ( 1978), evaluated the use of face bow in complete denture prosthesis occlusion. Their study revealed very small differences between a hinge axis face bow Hanau 132-SM face bow, and Whipmix ear-bow.

Neol D. Wilkie 1979, analyzed and discussed five commonly used anterior points of reference for a facebow transfer.

He said that not utilizing a third point of reference may result in additional and unnecessary record making, an unnatural appearance in the final prosthesis and even damage to the supporting tissues. He suggest the use of the axis-orbital plane because of the ease of marking and locating orbital and therefore the concept is easy to teach and understand.

Bailey J.O.J.R.. and Nowlin T.P in 1981 in their study concluded that face-bow transfer utilization orbital as the third point of reference does not accurately establish the relationship of the Frankfurt horizontal to the occlusal plane on the articulator.

Elwood. H. Staele et al 1982, evaluated esthetic considerations in the use of face-bow.

Goska and Christensen (1988), investigated cast positions using different face-bows. They concluded that it was not possible to establish clinical superiority between one type of face bow and another because the casts are mounted in relation to anatomic land marks that vary from subject to subject.

Parts of a Face Bow (Winkler –5, Whipmix manufacturers manuel –25)

It consists of a “U” shaped frame or assembly that is large enough to extend from the region of the temporomandibular joints to a position 2-3 inches in front of the face and wide enough to avoid contact with the sides of the face. The facia type of face bow has condyle rods that contact the skin over the temporomandibular joints. Whereas in the ear piece type it is known as a condylar compensator since their location on the articulator approximately compensates for the distances the external auditory meatuses are posterior to the transverse opening axis of the mandible. The part that attaches to the occlusion rims is the fork. The fork is attached to the face bow by means of a locking device, which also serves to support the face bow, the occlusion rims and the cast while they are being attached to the articulator.

Kinematic Face bow (Rosensteil –26)

The Kinematic face bow is initially used to accurately locate the hinge axis. It is attached to a clutch, which in turn attaches to the mandibular teeth. As the mandible makes opening and closing movements the condylar styli move in an arc. Their position is adjusted until they exhibit pure rotation and not translation, when the mandible is opened and closed. The points of rotation are marked on the skin and this determines the true hinge axis. The mandibular clutch is removed and the face bow is attached to the maxillary arch. The true rotation points are again used to orient the tips of the condylar styli .

Kinematic location of the hinge axis works well when natural mandibular teeth remain to stabilize the clutch mechanism. However, they are generally not used for complete denture prosthesis prosthesis fabrication because the resiliency of the soft tissues and the resultant instability of the mandibular record base make precision location of the rotational centers almost impossible.

Arbitrary face bow: (Rosensteil –26)

The arbitrary type of face bow is so called because it uses arbitrarily located marks on the skin at the condyle points as the hinge axis position.

1. Facia type: (winkler-5, McCollum -28) In the facia type the condyle rods are positioned on a line extending from the outer canthus of the eye to the superior inferior center of the tragus and approximately 13mm. anterior to the distal edge of the tragus of the ear.

This locates the condyle rods within 5mm. of the true center of the opening axis of the jaws. The presence of an assistant is required to hold the bow while the prosthodontist without clamping the condyle rods centers the device so that equal readings are obtained on both sides. The wing nut of the clamp is tightened to hold the face bow in place on the occlusal fork attached to the maxillary occlusion rim.

2. Ear piece type: the earpiece face bow is designed to fit into the external auditory meatuses. Here also the fork is attached to the maxillary occlusion rim. The whip mix, Hanau earpiece and Denar slide matic face bow are equipped with plastic earpieces at the condylar ends of the bow. When an earpiece face bow is removed, it is attached to the articulator by orienting “centering holes” in the earpieces on the side of the condylar housings of the articulator. With the denar slidematic face bow, the anterior portion of the apparatus is removed from the bow proper and supported in the articulator by a special jig, which replaces the incisal guide table.

All articulators require either an arbitrary or specific third point of reference for articulating the maxillary cast. This is done with an orbitate pointer or a nasion relator .(Neol D Wilkie –32)

It is important to remember that the critical relationship being transferred is between the maxillae and the hinge axis, to raising or lowering the anterior part of the face bow does not alter this relationship. Varying the position of the anterior part of the face bow will create a change in the absolute values for the condylar guidance settings. However, as long as eccentric records are used to determine condylar guidance’s after the casts are mounted the values for condylar guidance will be equivalent relative to the mounting of the casts.(Ulf Posselt –30, Cristensen R L-31)

Whip Mix Model 9600

Face bow

# Description

1 Screw

2 T- Screw

3 T- Screw

4 Horizontal clamp

5 Toggle clamp

6 Lock washer

7 Toggle clamp

8 Retaining ring

9 Bite fork

10 Cross bar assembly

11 HEX nut

12 Face bow (Right)

13 Center locking nob

14 Face bow (Left)

15 Upright post

16 Nose piece shaft

17 Face bow nob

18 Nose piece

19 Washer

The Plane of orientation

The maxillary cast in the articulator is the baseline from which all occlusal relationships start and it should be positioned in space by identifying three points, which cannot be on the same line. The plane is formed by two points located posterior to the maxillae and one point located anterior to it. The posterior points are referred to as the posterior points of reference and the anterior one is known as the anterior point of reference.

Posterior points of reference: (Neol D. Wilkie –32)

The position of the terminal hinge axis on either side of the face is generally taken as the posterior reference points.

Terminal Hinge position is the most retruded hinge position. The limits of opening at this position have been determined to be around 12 to 15 degrees or 19 to 20mm at incisal edges.

Location of the Posterior References Points:

Prior to aligning the face bow on the face, the posterior reference points must be located and marked.

The posterior points are located by

• Arbitrary method

• Kinematic method.

The Anterior points of reference (Neol D Wilkie-32,Baily JoJr-33)

It was important to ascertain at what level in the articulator the occlusal plane should be placed. The selection of the anterior point of the triangular spatial plane determines which plane in the head will become the plane of reference when the prosthesis is being fabricated. The prosthodontist can ignore but cannot avoid the selection of the anterior point. The act of affixing a maxillary cast to an articulator relates the cast to the articulaaror’s hinge axis, to the vertical axes, to the condylar determinants to the anterior guidance, and to the mean plane of the articulator.

Reasons for selecting an anterior point of reference ( Neol D Wilkie –32)

1. When three points are used the position can be repeated, so that different maxillary casts of the same patient can be positioned in the articulator in the same relative position to the end controlling guidance’s. For this reason it is important to identify the mark permanently or be able to repetitively measure an anterior point of reference as well as the posterior points of reference.

2. A planned choice of an anterior reference point will allow the prosthodontist and the auxiliaries to visualize the anterior teeth and the occlusion in the articulator in same frame of reference that would be used when looking at the patient. For example, when using the Frankfort horizontal plane as the plane of reference, the teeth will be viewed as though the patient were standing in a normal postural position with the eyes looking straight ahead.

3. An occlusal plane not parallel to the horizontal in the beginning steps of denture fabrication may be unknowingly located incorrectly because of a tendency for the eye to subconsciously make planes and line parallel. Therefore the prosthodontist may wish to initially establish the restored occlusal plane parallel to the horizontal in order to better control the occlusal plane in its final position.

4. The prosthodontist may wish to establish a baseline for comparison between patients, or for the same patient at different periods of time.

Selection of an anterior reference point (Neol D Wilkie-32)

The various anterior points that may be used are as follows.

1. Orbitale: In the skull, orbitale is the lowest point of the infraorbital rim. On a patient it can be palpated through the overlying tissue and the skin. One orbitale and the two posterior points that determine the horizontal axis of rotation will define the axis orbital plane. The orbitale is transferred from the patient to the articulator with the help of an orbital pointer on the face bow or by raising the face bow itself to the level of the orbitale.

Advantage:

A) It is easy to locate and mark the orbitale.

B) The concept is easy to teach and understand.

Disadvantage:

Relating the maxillae to the axis orbital plane will slightly lower the maxillary cast anteriorly from the position that would be established if the Frankfort horizontal plane were used.

2.     Nasion  minus  23mm:    According  to  Sicher another  skull  landmark,  the  nasion,  can  be approximately located in the head as the deepest part of  the midline  depression  just  below  the  level  of  the eyebrows.  The nasion guide, or positioner, of the face bow, which is designed to be used with the whip- mix Articulator,  fits  into  this depression.   This guide  can be moved  in and out, but not up and down,  from  its attachment to the face bow crossbar.  The crossbar is located  23mm.  below  the  midpoint  of  the  nasion positioner.    When  the  nasion  guide  of  face  bow  is positioned  anteriorly  on  the  nasion  the  crossbar will be  in  the  approximate  region  of  orbitale.    The  face-bow  crossbar  and  not  the  nasion  guide  is  the  actual anterior reference point locator.

    3.     Obitale minus 7 mm

          4.        Alae  of  the  nose:    this  method  uses  the Campers line as the plane of orientation – the right or left  ala  is  marked  on  the  patient  and  the  anterior reference pointer of the face-bow is set.  This relation is then transferred to the articulator 

 

A. Whip Mix face bow

B. Hanau articulator with arbitrary face bow

C. Dentatus articulator with arbitrary face bow

D. Modified Whip Mix face bow

Face bow transfer (Sloane R B –34)

An  arbitrary mounting  of  the maxillary  cast  without  a face-bow  transfer  can  introduce  errors  in  the  occlusion of  the  finished denture.   A  faulty or  careless mounting, with or without a face bow, will obviously lead to errors in cast  inclination that can seriously affect  the condylar inclination.   A  face bow  transfer  is  essential when  cusp teeth  are  used  allows  minor  changes  in  the  occlusal vertical dimension without having  to make new maxillo mandibular  records,  and  is  also  most  helpful  in supporting  the maxillary cast while  it  is being mounted on the articulator.

Arbitrary Axis for various Face bows  (winkler-5,Thorp-35)

When using a Hanau face-bow, a Rechey condylar marker is used to scribe an arc about 13mm. anterior to the external auditory meatus. Using a ruler, held so that it runs from the corner of the eye to the top of the tragus of the ear, place a mark where this line intersects the arc made by the condyle marker.  This locates the arbitrary axis for the Hanau face bow condyle rods, which is within 2 mm of the true center of the opening axis of the jaws.  If desired, a plane of orientation can be determined by utilizing the infraorbital notch as a third point of reference with the infraorbital pointer of the Hanau face-bow, Whereas for whip mix face bow locating an arbitary axis is not necessary when using the Whip Mix articulator, since it was designed and constructed after much research with a built in locator.  The inserting of plastic earpieces into the external auditory meatus automatically locates the face bow in the proper 

Arbitrary axis for denar slidematic face bow:

      The  Slidematic  face  bow  uses  the  external  auditory meatus for determining the arbitrary hinge axis location.  A built  in reference pointer aligns  the  face bow with  the horizontal reference plane.   The anterior reference point is  marked  on  the  patient’s  right  side  using  the  Denar reference  plane  locator.    The  point  is  43 mm.  above  the incisal  edge  of  the  right  central  or  lateral  incisor  for  a dentulous patient.  For an edentulous patient this distance is  measured  up  from  the  lower  border  of  the  upper  lip when the lips are relaxed.

Face  Bow  Transfer  -  Whip-Mix  Face  Bow (Winkler –7)

Attach  the maxillary  stabilized  base  to  the  bite  fork.   Insert in the mouth and have the patient hold it in place with  both  thumbs  using  light  pressure,  or  place  the lower base in the mouth and close against the bite fork.  The  face bow  is  carried  to patient’s  face,  and  the  face bow fork toggle assembly is slipped onto the stem of the bow  fork;  the  plastic  earpieces  are  inserted  into  the external auditory meatus and brought slightly forward.  The nasion relator assembly is attached to the face bow; the plastic nosepiece should rest on the nasion, and the face bow is tightened.  The face bow is locked to the bite fork.  The positioning of the face bow and locking of the bite fork to the face bow must be done carefully or the purpose of the face bow transfer is defeated.  The entire assembly is then carried to the articulator 

The upper cast is attached to the articulator.  The proper  use  of  the  face  bow  prevents  errors  of occlusion in the finished dentures during eccentric movement  of  the  lower  jaw within  the  functional range.

 

Indications for Face Bow Use.  ( Heartwell  –4, bandrup-morgsen-36)

When the disharmonies in occlusion resulting from failure to use the face bow are analyzed, it can be concluded that the face bow should be used when.

1.  Cusp form teeth are used

2.  Balanced occlusion in the centric positions is desired

3   A definite cusp fossa or cusp tip to cusp incline relation is desired.

4.  When  interocclusal check records are used  for verification of jaw positions.

5   When the occlusal vertical dimension is subject of change, and alterations  of  tooth  occlusal  surfaces  are  necessary  to accommodate the change

6    To diagnose existing occlusion in-patient’s mouth.

Vertical Jaw relations

Introduction:

     A vertical  jaw relation  is defined as  the distance between two selected points, one on the maxillae and one on  the mandible.   That  is,  they are  established by  the  amount  of  separation  of  the  two  jaws  in  a vertical direction under specified conditions.

      The physiologic  rest  position  of  the mandible  as related  to  the  maxillae  and  the  relations  of  the mandible  to  the  maxillae  when  the  teeth  are  in occlusion are  the  two dimensions of  jaw separation of  primary  concern  in  complete  denture  prosthesis prosthesis constructions.

Thus vertical jaw relations are classified as (Boucher –3)

1.      The vertical relation of rest position

2.      The vertical relation of occlusion

3.      The differences between the vertical relation of rest and  the  occluding  vertical  relation,  also  known  as “freeway space.”

The  “rest  vertical  relation”  is  the  distance  measured when the mandible is in the rest position.

In infants and in edentulous adults the vertical relation of rest position is established by muscles and gravity and is assumed only when  the muscles  that  open  and  close  the jaws are in a state of minimal contraction to maintain the posture of the mandible.

 REVIEW OF LITERATURE

   Wallisch (1906) was the first to define physiologic rest position.

    In  the  late  1920’s,  Sicher  and  Jandler  described  the role of the musculature in controlling the posture of the mandible  and  stated  that  the  rest  position  of  the articulation is that  in which the mandible is at a slight distance  from  the  maxilla  and  in  this  position  the mandible  is  kept  against  gravity  by  the  forces  of  the closing muscles.

     Niswonger  (1934)  was  perhaps  the  first investigator to study extensively the rest position of the mandible.   He established that the interocclusal distance measured 4/32  inch  i.e. 3 mm.  in majority of  the  cases  and  that  the  patients  whose  vertical dimension of occlusion was excessive complained of soreness  of  the  residual  ridges  due  to  mastication, and  once  this  space  was  developed  after  tissue changes,  the  patient  was  able  to  masticate  with satisfaction and comfort.

      Many  observers  pointed  out  the  role  of  muscles physiology  in  limiting  the  extent  to  which  vertical dimension of occlusion could be increased.

Mershon  (1938  )  contended  that  muscles  cannot lenghthen  to  accommodate  an  increase  in  bony  size,  but rather bone adapts itself to the length of the muscles.

Tench ( 1939 ) felt that the functional  length of the muscles  could not be  increased after observing  failures of restorations constructed at an excessive vertical dimension of occlusion.

Gillis ( 1941 ) stated the mandibular rest position is not  artificially  but  naturally  established  and  that  the interocclusal  distance  did  not  vary  greatly  between different individuals, with average of 3mm. 

        Schlosser  (1941)  conducted  a  series  of  phonetic experiments  indicating  that  the  movements  of  the mandible  during  speech  were  subject  to  habitual fixation.   He  concluded  that  edentulous  patients were repeatedly able to bring the mandible to and identical rest position by sounding the letter ‘m’

.       Thompson  (1946)  reported  on  the  cephalometric analysis  of  the  rest  position  in  edentulous  and  semi edentulous adults and concluded that if the mandible is carried to a greater than normal rest position by dental restorations.    The  mandible  will  return  to  its preordained  position  at  the  expense  of  the  alveolar process or by the intrusion of occluding teeth.

     Sicher (1954) felt that the mandibular rest position was completely  dependent  on  the  tonicity  of  the musculature and that only in disturbed muscle forms as in disease, over work  and  nervous  tension  could  the  rest  position  vary from normal.   He  also  pointed  out  that  since  the muscle tonus  is  fairly  constant  for  each  individual,  the mandibular rest position is fairly position

Another school felt that rest position was variable.

In  the  1930’s  when  the  ground  work  of  the concept of constancy was being developed, Harris and Hight  (1936  )  reasoned  that  the vertical dimension of occlusion was  dependent  on  the    occlusal  contacts  in the closing movements of  the mandible. They felt that reduction  of  the  vertical  dimension  occlusion  was caused  by  abrasion  of  teeth,  loss  of  posterior  teeth, resorption of ridges under dentures and faulty dental work.  Hence,  the  correct  vertical  opening  in edentulous patients was debatable. 

      Leof  (1950)  stressed  that  muscles  tone  rather  than muscle  length  controls  the  rest  position  and  that  muscle tone  can  and  does  vary  by  exercise  or  excessive  rest.   Hypertonicity  of  mandibular  muscles  through  grinding habits may  interfere  with  the maintenance  of  a  constant rest  position  and  result  in  a  reduction  of  the  normal interocclusal distance.

        Atwood  (1956)  conducted  a  longitudinal radiographic  analysis  of  face  height  using  a combination  of  swallowing  and  phonetics  before  and after extraction and demonstrated variability within a sitting;  between  sittings  and  between  readings,  with and without dentures.  He concluded that rest position is  a  dynamic  rather  than  a  static  concept  and  that  it varied  from  person  to  person  and  within  the  same person.    A  cine  fluoroscopy  technique  coupled  with electronics  was  suggested  to  provide  a  better  insight into the variability of rest position.

Tallgren ( 1957 ) studied the changes in adult face height by means of  cephalometrics  and  her  findings were  similar  to  that  of Olsen and  Atwood  in  showing  a  certain  instability  of  the  rest  position after removal of teeth.

Swerdlow ( 1964 ) studied a group of  immediate denture patients over a period of 6 months. He recorded cephalometrically, changes in  occlusal  vertical  dimension,  rest  vertical  dimension  and interocclusal  distance during  the  transition  from natural  teeth  to immediate dentures. He concluded that (a) the phonetic method of recording  rest  position  gave  consistent  values  for  interocclusal distance than did the swallowing method. (b) The occlusal vertical dimension and rest vertical  dimension increased initially and then decreased markedly  in the 6 months of wearing dentures. (c) The interocclusal  distance  adjusted  itself  to  accommodate  to  the variations  in  facial  vertical  dimension.  (d)  and  a  change  in mandibular  load  appeared  to  influence  the  rest  position  of  the mandible.

“Physiologic  rest  position”  is  the  postural  position  of  the mandible  when  an  individual  is  resting  comfortably  in  an upright  position  and  the  associated muscles  are  in  a  state  of minimal contractual activity.

This  position  is  controlled  by  the  muscles  that  open,  close, protrude  and  retrude  the mandible  and  further  is  controlled by  the  position  of  the  head,  which  modifies  the  effect  of gravity.    The  force  applied  by  the  Jaw  opening  muscles  is added to the force of gravity, when the head  is upright.    In a reclining patient, gravity does not pull the mandible down and so one may find the distance between the jaws to be less than it is when the head is upright.  When observations of physiologic rest  position  are  being  made,  the  patients’  head  should  be upright and unsupported.

  The second thing that establishes the vertical relation of the mandible  to  the  maxillae  is  the  occlusal  stop  provided  by teeth or occlusion rims.  The “occlusal vertical relation” is the distance  measured  when  the  occluding  members  are  in contact.

    In the course of a lifetime, many things happen to natural teeth.  Some are lost, some are abraded so that they lose their clinical crown length, dental caries attacks some of them, and restorations fail to maintain their full clinical crown length.   Even dentulous patients may have a reduced occlusal vertical relation.    The  pre-extraction  occlusal  vertical  relation  may not  be  a  reliable  indication  of  the  vertical  relation  to  be incorporated  in complete denture prosthesis prosthesis.   But any information available about the occlusal vertical relation with  natural  teeth  should  not  be  ignored  and modifications from it should be made as indicated.

The  health  of  the  periodontal membranes  that  support the  natural  teeth  and  the  health  of  the  mucosa  of  the basal  seat  for  dentures  depend  on  rest  from  occlusal forces  to  maintain  their  health.    For  this  reason,  an interocclusal  rest  space  between  the  maxillary  and mandibular teeth is essential for the opening and closing muscles  and  gravity  to  be  in  balance when  the muscles are  in  a  state  of  minimum  tonic  contraction.    The interocclusal rest space is the difference between the rest vertical  relation  and  the  occlusal  vertical  relation  and amounts to 2-4 mm. in a vertical direction if observed at the position of the first pre molars.

      Once  the  vertical  relation  of  rest  position  has  been determined  it  is  easy  to  adjust  the  vertical  relation  of  the occlusion  rims  sufficiently  to  provide  for  the  necessary interocclusal distance

 

   Other vertical relation such as the vertical relations of the two jaws when the mouth is half open or wide-open are of no significance in the construction of dentures.

 Methods   ( Boucher-3)

   Many methods have been proposed for determination of the correct  vertical  relation  of  the  mandible  to  the  maxillae.   Some  of  them  have  been  offered  as  ‘Scientific”,  but  as  yet none is accurate.  Others have been offered as helpful aids to good  clinical  judgment.    All  those  currently  in  use  will  be discussed

Other classifications:

The methods for determining the vertical maxillomandibular relations can be grouped roughly into two categories.

1.      The mechanical methods

2.      The Physiologic methods

The use of esthetics as a guide combines both the mechanical and physiologic approaches to the problem.

 

Mechanical Method:

1.      Ridge relations

a)     Distance of incisive papilla from mandibular incisors: The incisive papilla is used to measure the patients’ vertical relation  since  it  is a  stable  landmark and  is changed  little by resorption of  the residual alveolar ridge.   The distance of  the  incisive  papilla  from  the  incisal  edge  of  the mandibular  incisors  is  about  4  mm.  in  the  natural dentition.  The incisal edge of the maxillary central incisor is  an  average  of  6mm.  below  the  incisive  papilla.    So  the average vertical  overlap  of  the  opposing  central  incisor  is about 2 mm. the disadvantage of this method is the absence of  lower  teeth  and  so  is  only  useful  in  the  treatment  of single dentures.

b)  Parallelism of the ridges:  Paralleling of the ridges, plus a 5 degree opening in the posterior region as suggested by sears,  often  gives  a  clue  to  the  correct  amount  of  jaw separation.    This  theory  if  used  alone,  is  not  reliable; because  many  patients  present  such  marked  resorption that the use of this rule would generally close the vertical relation.   But when considered with other observations,  it may be of value.   However,  in most patients the teeth are lost  at  irregular  intervals  and  the  residual  ridges  are  no longer parallel

 

2. Measurement of former dentures:(Majid Bissasu-39)

Measurements are made between the borders of the maxillary and  mandibular  dentures  by  means  of  a  boley  gauge  and corresponding alterations can be made in the new denture to compensate the occlusal wear.

 

3. Pre-extraction records  (Ricketts –40, Crabtree 41 ) -:  It is frequently possible to see the patient before he or she becomes edentulous.  In such cases one can usually establish the occlusal position, record it in some manner and transfer this record to the edentulous situation.  This is a relatively easy procedure and can be accomplished in several ways 

    a) Profile radiograph:   the exposure of a full lateral  radiograph  is  made  with  the  teeth  in occlusion,  and  after  extraction  trial  plates  are made  to  an  apparently  correct  vertical  relation.   They are  inserted,  the patient closes on them and another  radiograph  is  taken.    The  two  films  are compared  and  necessary  adjustment  is  made  to bring  the  mandible  in  correct  position  as  in  the initial film.  The image should have approximately 1:1  ratio  to  the  patient.      Disadvantages  include inaccuracy due  to  enlargement of  the  image,  it  is time consuming and it may result  in too frequent exposure to radiation.

  b)  Profile Photographs (Alexander Morton –42):  Profile photographs are made and enlarged  to  life  size.   The photographs  should  be  made  with  the  teeth  in maximum  occlusion.    Measurements  of  anatomic landmarks  on  the  photograph  are  compared  with measurements of the face, using the same landmarks.  These  measurements  can  be  compared  when  the records  are made  and  again when  the  artificial  teeth are  tried  in.   Disadvantage of  this method  is  that  the angulation  of  the  photograph  might  differ  with  the patients. Posture.

      c) Lead wire adaptation (Crabtree  Ballard):   Lead wires  may  be  adapted  carefully  to  pre  extraction profiles,  and  this  contour  is  transferred  to  a cardboard.  The resultant cutout is stored until after extraction.    When  the  prosthodontist  estimates  the vertical relation using the trial plates, the cardboard cutout  is  placed  against  the  profile  in  order  to  see whether  the  facial  contour  has  been  maintained  or reestablished.  It is not in common use today. 

      d)    Swenson’s method (Swenson-70):    Swenson suggested  that acrylic  resin  face marks made before  the extraction,  and  later  when  the  patients  is  rendered edentulous,  it  is  fitted  on  the  face  to  see  whether  the vertical relation has been restored properly.  Drawbacks of this method is that, it is time consuming requires lot of skill  and  experience  with  the  use  of  facial  impressions and casts for the fabrication of artificial facial parts and lastly the face assumes a different topography in the erect posture  from  that  in  the  recumbent  or  semirecumbant position.

e)     The Dakometer (Fenn, Lidlow, Gimson-6):  This instrument records  both  the  vertical  relation  and  position  of  upper central  incisors.    In most  cases  recording  can  be  obtained with an error range of 1mm.

 f)    Willis gauge (Willis 1935-43), Toolson and Smith 1982-44): This instrument  is  used  for  recording  vertical  height  before extraction.  The arm is placed in contact with the base of the nose, and the arm is moved along the slide till it lightly but firmly  touches  the  lower border of  the  chin.  It  is  locked  in position by the screw. The distance on the scale is recorded on the patients’ chart. It is not an accurate method as there may be verifications is applying pressure.

g)    Articulated casts (Howard J. Merkely 1953-45): These are of practical  value  in  the  assessment  of  the  vertical  relation.     Measurements  can  be  made  of  the  casts  in  occlusion  and relatively stable points.

4) POST EXTRACTION METHODS: 

a) Niswonger’s methods (Niswonger 1934) suggested a method for determining the vertical dimension that is commonly used today.  The patient is seated so that the Ala-Tragal line is parallel with the floor. Two markings are made, one on the upper lip below the nasal septum other in the most prominent part of the chin. The patient is told to swallow and relax. The distance between the marks is recorded.  Subsequently, the occlusion rims are constructed so that when they occlude, the measured distance is 1/8 inch less than the original measurement. 

This 1/8-inch average freeway space falls within 2-4 mm. This method has  the disadvantage that  the marks move with the skin and sometimes it is difficult to obtain two constant measurements of the rest position. However, when combined with other observations  this  technique  is reasonably reliable.

b) Willis method: Willis believed that the distance from the pupils  of  the  eye  to  the  rima oris  should be  equal  to the  distance  from  the  base  of  the  nose  to  the  inferior border of the chin, when the occlusion rims are in contact.

c) Concepts of equal thirds: Some observers suggested that the face can be divided into equal thirds, the forehead, the nose and lips and chin. This concept is of little practical value since the points of measurements are vague.

d) Silverman’s closest speaking space (Mayer M. Silverman-47) : Which measures the vertical relation in phonetic method, must not  be  confused  with  freeway  space.  The  freeway  space establishes  vertical  relation when  the muscles  involved  are  at minimal  tonic  contraction  and  the  mandible  is  in  its  rest position.  The  closest  speaking  space  measures  the  vertical relation  when  the  mandible  and  the  muscles  involved  are  in physiologic function of speech. The occlusion rims are placed in the mouth and the height is adjusted until a minimum of space exists when the patient pronounces the letter “S”. It may vary from 1 to 10 mm, but the 2mm average will generally prevent an increase in vertical relation. 

           e)  Boos method (Power Point):  Boos (1940) found that  there  is  a point  of maximum biting power.   He  stated that the patient registers the greatest amount of pressure on a spring dynometer at a point considerably more open than the denture occlusion.

The  bimeter  is  attached  to  an  accurately  adapted mandibular  record  base.    A metal  plate  is  attached  to  the vault  of  an  accurately  adapted  maxillary  record  base  to provide  a  central  bearing  point.    The  vertical  relation  is adjusted  by  turning  the  cap.    The  gauge  indicating  the pounds  of  pressure  generated  during  closure  at  different degrees of jaw separation.  When the maximum power point has been determined the set is locked.  Plaster registrations are made and the cast is transferred to an articulator 

Investigators  agree  that  such  a  device  offers  no  more accuracy than Niswonger’s or Silverman’s method.

7. Lytle’s method (Neuromuscular perception) (Lytle 1964-48):   A central bearing device attached  to accurately adapted record bases permits the patient to experience through neuromuscular perception at the different vertical relation.

The  central  bearing  plates  are  mounted  directly  on  the adapted  record  bases,  and  adapting  the  plates  to  the  patient’s interarch distance.   The bearing pin  is  adjusted until  the mouth  is opened beyond the physiologic rest position.  The pin is lowered by half  turn at a  time, having  the patient make  two  sharp  contacts at each time, until the patient signifies that he has closed too far.  The procedure is repeated starting from an over closed position.  At one stage  the  patient  signifies  that  he  has  reached  excessive  opening.   The  procedure  is  repeated  by  moving  the  jackscrew  arrangement vertically  until  the  patient  adjudges  the  vertical  relation  proper.   This  position  is  then  examined  by  the  prosthodontist  to  determine that it is reasonable.

a)      Electromyography  (Mannsa  A.,  Miralles  1981-49)  :    Rest position  of  the  mandible  can  be  determined  by  means  of electromyography, which would record the minimal activity of  the muscles.   All muscles  show  greater  activity  in  other positions  than  in  rest  position.  Electromyography  is  not  a practical  method  of  determining  vertical  relations  in  the dental office because

The equipment is too expensive

The  operator  should  have  considerable  knowledge  and experience  in  the  field  of muscle  physiology  before  he  can attempt to interpret the tracings

Lastly,  it  is  indicated  that  a  five  electrode  reference arrangement is needed in order to minimize the error

Physiologic methods:

1. Physiologic rest position (Niswonger 1934, Shanahan 1955 - 50):  Registration  of  the  jaws  in  physiologic  rest  position  gives an  indication  to  a  relatively  correct  vertical  relation when used  with  other  methods.    After  the  insertion  of  the occlusion rims into the patients’ mouth, the patient is asked to  swallow  and  let  the  jaws  to  relax.    Then  the  lips  are carefully parted to see how much space is present between the  occlusion  rims.    The  patients  must  allow  the prosthodontist to separate the lips without moving the jaws or lips. 

      This interocclusal rest space should be between 2-4 mm.  when  viewed  in  the  premolar  region.    The interarch space and rest position can be measured by indelible  dots  or  adhesive  tapes  on  the  face.    If  the difference  is  greater  than 4 mm.  the occlusal  vertical relation would be considered too small.  If the occlusal vertical relation is less than 2 mm. the occlusal vertical relation would be assumed to be great.

2)   Phonetics (Silverman  1955  –  51,  Edward  J. Mehringer  1963  -  52):   speech is used as an aid in determining the vertical relation.  The patient is asked to repeat the letter ‘M’ until he is aware of the contacting of the lips.  The patient is asked to stop all jaw movements when the lips touch and the distance between the two points of reference are measured. The production of the  h,  s  and  j  sounds  bring  the  anterior  teeth  very  close together when correctly placed the lower incisors are moved forward to a position nearly directly under the upper central incisor  and  almost  touching  them.    If  the  distance  is  too large,  a  vertical  relation  of  occlusion  that  is  too  small may have been established.  If the anterior teeth touch when these sounds  are  made,  the  vertical  relation  of  occlusion  is probably too great. 

3).   Facial expression (Alexander  L.  Marton-42):    the experienced  prosthodontist  learns  the  advantage  of recognizing  the  relaxed  facial  expression when  the  jaws are at rest.  In normally related jaws the lips will be even anteroposteriorly  and  in  slight  contact.    The  lips  of  a patient  with  a  retruded  mandible  will  not  be  even;  the lower lip will be distal to the upper and not in contact.  In the case of protruded mandible, the lips will not be evenly related  anteroposteriorly  and  the  lower  lip  will  be anterior  to  the  upper  lip  and  not  in  contact.    The  skin around  the  eyes  and  over  the  chin  will  be  relaxed.   Relaxation  around  the  nares  reflects  unobstructed breathing.

4)  Swallowing threshold (Brente  L. Ward-53,  Shanahan-50,  Ismail  and George-54):    the  position  of  the  mandible  at  the  beginning  of  the swallowing  act has been used as  a  guide  to  the  vertical  relation.   The theory behind the method is that when a person swallows, the teeth come together with very light contact at the beginning of the swallowing  cycle.    If  denture  occlusion  is  continuously  missing during  swallowing  the  occlusal  vertical  relation  may  be insufficient. The technique involves building a cone of soft wax on the lower denture base so that it contacts the upper occlusion rim with the jaws too wide open.  The flow of saliva is stimulated and the repeated action of swallowing will gradually reduce the height of  wax  cone  to  allow  the mandible  to  reach  the  level  of  occlusal vertical relation.  The length of time this action is carried out and relative  softness  of  the  wax  cone  will  affect  the  results.    It  is difficult  to  find consistency  in  the  final vertical positioning of  the mandible by this method.

   5)     Tactile sense (Heartwell-4):  The patient’s tactile sense is used as a guide  to  the determination of  the occlusal vertical relation.    The  occlusion  rims  are  inserted  into  the  patients’ mouth and he  is  instructed  to  open and  close until  the  rims contact.  The patient is asked if the rims appear to touch too soon,  whether  the  jaws  seem  to  close  too  far  before  they touch, or if the height feels just right.  This method is not very effective  with  senile  patients  or  those  who  have  impaired neuromuscular coordination’s.

Effects of Increased vertical Relation:(Peter E. Dawson-55)

1)    Discomfort to the patient

2)    Trauma:  by the jamming effect of the teeth coming into contact sooner than expected may cause not only discomfort but also pain owing to the brusing of the mucous membrane by these sudden and frequent blows.

3)    Loss of freeway space:  which may be lead to 

   a)   Muscular  fatigue  of  any  one  or  group  of  muscles  of mastication

              b)    Trauma  caused by  the  constant  pressure  on  the mucous membrane and 

                  c)     Annoyance  from  the  inability  to  find  a comfortable resting position.

        4)   Clicking teeth:    the  tongue  which  has  become accustomed to the presence of teeth in certain fixed positions and during speech helps to produce sounds without the teeth coming into contact.  When there is increase in vertical height opposing  cusps  frequently  meet  each  other,  producing  an embracing  clicking  or  clattering  sound.    This  effect  is  also produced during eating.

          5)    Appearance:    the  face has  an  elongated appearance since at rest the lips are parted and closing them together will produce an expression of strain

          6)     Bone  residual  alveolar  ridge  undergoes  rapid resorption

 

Effects  of  decreased vertical  relation:(Peter  E.  Dawson-55, Boucher-3)

1)      Inefficiency – which is due to the fact that the pressure with  which  it  is  possible  to  exert  with  the  teeth  in  contact decreases considerably with over closure because the muscles of  mastication  act  from  attachments,  which  have  been brought close together.

 

2)      Cheek biting:    In  some  cases  where  there  is  a  loss  of muscular tone, as well as reduced vertical height, the flabby cheek tends to become trapped between the teeth and bitten during mastication.

   3)     Appearance:  the general effect of over closure on facial expression is of increased age.  There is close approximation of nose to chin, the soft tissues sag and fall in and the lines on the face  are  deepened.    The  lips  loose  their  fullness  and  the vermilion borders are reduced to approximate a line.

    4)    Angular cheilitis:  Reduced vertical relation results in a crease  at  the  corners  of  the  mouth  beyond  the  vermilion border  and  the  deep  fold  thus  formed  becomes  bathed  in saliva, thus leading to infection and soreness.

 5)    Pain in the TMJ’s:    Trauma  in  the  region  of  the temporomandibular  fossa  may  be  attributed  to  a  reduced vertical relation with symptoms like obscure pains, discomfort, clicking sounds, headaches and neuralgia.

6)    Costen’s syndrome:  is stated to be the result of prolonged over closure

 

       If it is suspected that these various pathologic conditions are due to a reduced interarch distance, the dentures should be considered as treatment dentures.  The vertical relation of occlusion  should  be  built  up  gradually  in  successive  sets  of dentures.    Complete  restoration  of  the  original  occlusal vertical relation in one set of dentures would likely result in a failure  because  patients  will  not  be  able  to  accommodate themselves to this great changes in so short a period of time.

 

HORIZONTAL JAW RELATIONS

 

Introduction

Horizontal  jaw  relations  are  those  that  are  established anteroposteriorly  and mediolaterally,  and  so  are  classified as,

1.      Centric relations

2.      Eccentric relations: which include the 

(a)  Protrusive and

(b)  Left and right lateral movements.

The principles  of  good  occlusion  apply  to  the  dentulous  as well  as  the  edentulous  patients.    However,  different requirements  are  necessary  in  the  occlusion  for  complete denture  prosthesis  prosthesiss  because  artificial  teeth  are not attached to  the bone  in same manner as natural  teeth.   Thus  an  occlusion  that  is  physiologically  acceptable  or desirable  may  not  be  applicable  for  complete  denture prosthesis  prosthesiss.    Stability  of  the  complete  denture prosthesis prosthesis can be maintained if the opposing teeth meet evenly on both sides of the arch when the teeth contact anywhere within the normal functional range of mandibular movement.   An occlusion  that provides  these  even contacts can  only  be  developed  with  centric  occlusion  in  harmony with centric relation.

 

 

Definition: Centric relation

Centric relation is defined as a maxillomandibular relationship in  which  the  condyles  articulate  with  the  thinnest  avascular portion  of  their  respective  disks  with  the  complex  in  the anterior  superior  position  against  the  shapes  of  articular eminences.  This position is independent of tooth contact.  This position  is  discernible  when  mandible  is  directed  superiorly and  anteriorly  and  restricted  to  a  purely  rotatary movement about a transverse horizontal axis.

(Glossary of prosthodontic terms 1999 – 1)

      It  is  a  bone  to  bone  relation  and  is  classed  as  horizontal relation because variations from it occur in horizontal plane.

  An  eccentric  maxillomandibular  is  any  relationship  of  the mandible to the maxilla other than centric position.

 (Glossary of prosthodontic terms 1999 – 1)

Review of literature

 Direct check bite interocclusal recording :

It  is  one  of  the  oldest  type  of  Centric  record  .  In  1796, PHELLEP PFAFF, the prosthodontist of  Fredrick, Germany was  the  first  to  describe  this  technique  of  ‘taking  a  Bite” .Until  end    of  Ninteeth  century,  it  was  the  most  commonly used  method.  The  direct  interocclusal  record  during  that period, was a non precession joint record obtained by placing a  thermoplastic  material  ,  Usually  wax  or  a  compound between  the  edentulous  ridges  and  having  the  patient  close into the material . This was known as the  “mush”  “ biscuit” or “squash” bite.

Significance of centric relation: (Boucher-3)

1.       This  position  is  more  definite  than  the  vertical relation and  is  independent of  the presence or absence of teeth.

2.       It  is  recordable  and  reproducible  over  a  period  of time

3.       Centric  relation  serves  as  a  reference  relation  for establishing an occlusion

4.       When  centric  relation  and  centric  occlusion  of natural  teeth  do  not  coincide  the  periodontal  structures around the natural teeth are endangered.

      5)      When  centric  relation  and  centric  occlusion  of artificial  teeth  do  not  coincide  there  is  instability  of dentures  and  the  patient  will  be  subjected  to  pain  and discomfort

   6)    Errors in mounting the casts on the articulator can be  detected,  when  the  centric  relation  is  used  as  the horizontal reference position.

      7)     An  accurate  centric  relation  record  properly orients  the  lower  cast  to  the  opening  axis  of  the articulator and the mandible

      8)      Accurately  recorded  centric  relation  when transferred to the articulator permits proper adjustments of  the  condylar  guidance  for  the  control  of  eccentric movements of the instrument.

Methods and Materials

There  are  two  different  concepts  in  the making  of  centric relation records, each with its own objectives.

In  one  concept  the  records  should  be made  with minimal closing pressure so that the tissues supporting the bases will not  be  displaced  while  the  record  is  being  made.    The objective of  this  concept  is  for  the opposing  teeth  to  touch uniformly  and  simultaneously  at  their  first  contact  of  the teeth.   This uniform contact will not stimulate the patients to clench and relax the closing muscles  in periods between mastication. (Boucher-3)

The  second  concept  is  that  the  records  should  be  made under heavy closing pressure  so  that  the  tissues under  the recording bases will be displaced while  the record  is being taken.  The objective is to produce the same displacement of the soft tissues as would exist when heavy closing pressures are applied on  the dentures.   Thus  the occlusal  forces will be  evenly  distributed  over  the  supporting  residual  ridges when  the  dentures  are  under  heavy  occlusal  loads.    If  the distribution  of  the  soft  tissues  is  uneven,  the  teeth  would contact  unevenly  when  they  first  touch.    This  uneven contact  tends  to  stimulate  nervous  patients  to  clench  and relax  the  closing  muscles  of  the  jaws,  which  may  cause soreness  under  the  denture  bases  and  changes  in  the residual ridges. (Boos-57)

The use of a technique based on minimal closing pressure seems to produce the best results for most patients.

The patient should be assisted and trained to retrude the mandible  to centric  relation.   Certain difficulties will be encountered when trying to retrude the mandible.  Some of  these  difficulties  may  be  biologic,  some  psychologic and some mechanical

(Kingrey R.H –56)

1.         Biologic difficulties: arise form the lack of coordination of opposing muscles when the patient is requested to close the jaws in the retruded position. This lack of synchronization between the protruding and retruding muscles may be caused by habitual eccentric jaw positions adopted by patients to accommodate to malocclusions.

2. Psychologic difficulties involve both the prosthodontist and the patient. The more the prosthodontist becomes irritated over the apparent lack of ability of the patient to retrude the mandible, the more confused the patient becomes and less likely the patient is to respond to the directions provided. The prosthodontist must be prepared to calmly spend adequate time in securing the centric relation records.

3)      Mechanical difficulties arise because of poorly fitting bases. It is essential that the bases on which the records are made fit perfectly and do not interfere with each other. The amount of pressure the patient exerts at the time of registering centric relation is difficult to control. Minimal pressure seems to produce the best results.

Methods for assisting the patient to retrude the mandible:

1. The patient is instructed to let his or her jaw relax pull it back and to close slowly and easily on the back teeth.

2. The patient is instructed to get the feeling of pushing his upper jaw out and then to close the back teeth together

3. Instruct the patient to protrude and retrude the mandible repeatedly while the patient holds the finger lightly against the chin.

4. Boo’s series of stretch relax exercises: the patient is instructed to open wide and relax, to move the jaws to the left and relax, to move the jaw to the right and relax and to move the jaw forward and relax in series of movements. The results to be expected are for the patient to be able to follow the prosthodontist’s directions in moving the jaw to centric relation and to the desired eccentric relations.

      5.       The patient can be instructed to turn the tongue backward towards the posterior border of the upper denture and to close the rims together until they meet. The disadvantage with this method is the likelihood of displacing the mandibular record base by the action of the tongue.

6. The patient is asked to swallow and conclude the act with the blocks in contact. However a person can swallow when the mandible is not completely retruded. This method must be verified by other techniques.

7. Tapping the occlusion rim on back teeth together rapidly and repeatedly is used to help the patient retrude the mandible. But it is difficult to record these positions and a patient can easily tap in a slightly protrusive or lateral position.

8.      Tilting the head backward at the neck will place tension on the inframandibular muscles and tend to pull the mandible to a retruded position. However, it is difficult to obtain registrations with the head in this position because of the awkwardness of insertion and removal of the occlusion rims from the mouth when the head is so tilted

9. Massaging or palpation of the temporal and masseter muscles can be done to relax them. Contraction of the temporal muscle can be felt when the mandible is in or near its retrusive position and the patient is asked to open and close.

Methods of recording centric relation:(Boucher-3)

The three primary requirements for making a centric relation record are.

1. To record the correct horizontal relation of the mandible to the maxillae.

2. To exert equalized vertical pressure.

3. To retain the record in an undistorted form until the casts have been accurately mounted on the articulator

The various methods used for recording centric relation may be classified as

1. Functional methods

a) Needle – Hose method

b) Patterson’s method

c) Meyer’s method

2. Excursive methods (graphic method)

a) Intra oral tracing

b) Extra oral tracing

3.   Tactile or interocclusal check records

4. Terminal hinge axis method

Functional methods

Review of literature

Functional or “Chew in” records were another method described to record C.R. These were first discussed in the dental literature around 1910. All functional method of recording C.R. require a stable record base of this record base is dislodged the record will be in accruable patient must have good new muscular coordination to participate in such according procedure.

NEEDLES (1923) mounted studies on maxillary occlusion rims, and these studies engraved arrow tracings into compound rims on the mandibular arch. After the rims were removed from the oral cavity, they were reassembled with the functional grooves in place

Patterson (1923) was also known for promoting the use of functional records Patterson prepared a trough in the maxillary and mandibular occlusion rims and these thoughts were filled with a plaster mixture again, the patient was asked to move his mandible and continue the motion until appropriate curvature has been formed on the rims. This was said to equalize pressure and provides uniform contact in all exclusive movement.

Meyer (1934) also developed a functional technique in which soft wax occlusion rims were used and wax paths were formed in these rims during functional movements. Then a plaster index was made of this wax palter, and teeth were set opposing this generated plaster index.

House (1930’s) described a technique of recording mandibular movements and registering the CR position using the House technique, shellac trial denture bases with attached wax occlusion rims were made. A preliminary jaw relation records was obtained and the maxillary cast and occlusion rim was obtained and the maxillary cast and occlusion rim was mounted on the House articulator. Wax was then placed in the premolar and molar regions bilaterally on the mandibular occlusion rim. It was then replaced by a block of impression compound. These compound blocks had a occlusal surface simulating an average curve of spee and curve of Wilson. Four triangular shaped studs with cutting edges were then placed in the maxillary wax occlusion rims opposite then blocks.

The new occlusion rims were then inserted intra orally and the patients carried out mandibular movement during these movements, the studies engraved 4 separate needle point gothic arch recording into the blocks of compound. This was called a “CHEW IN” recordings. One of the casts was then remounted. According to the new gothic arch recording the condylar elements of the articulator were adjusted

Late Swanson gave the result of his studies and suggested that House technique was desirable but it needed some improvements such as

1.     An accurate recording of terminal things axis with a face bowl

2. In Jaw relations records made on intimately adapted denture bases or on the final impression

3. Using the central bearing screw when performing the chew in recording so that the vertical dimension of occlusion could be maintained

4. The recording made with a soft material rather than a hard material.

5. The recording used with on articulator capable of stimulating jaw movement and with fossae that could be molded

 

Swanson then described a procedure where he improved on the house technique using TMJ technique. The TMJ Kinematic face bow was used to locate the hinge axis, the recording platter were attached to impressions rather than stabilized trial bases, and a central bearing screw was incorporated. A recording was made and the molded fossae with TMJ articulator were their formed from the generated Gothic arch tracing

One of the most famous promoters of functional records was BOOS (1959). He developed the GNATHODYNAMOMETER. Using this instrument, he determined the vertical and horizontal position where maximum biting force could be generated. This instrument was mounted on the mandibular occlusion rim, and it had a central bearing point that occluded agents a plate on the maxillary rim plaster registrations were also used intra orally with the gnathodynamometer in place and with the patient exerting biting pressure.

BOOS claimed that OPTIMUM OCCLUSAL POSITION and the position of maximum biting force are coincident. He believed that all registrations be made under this biting force with displacement of the soft tissue. He thought that this displacement would also occur in function.

Functional methods:

1. Needles House method (Needles 1923 – 58, House 1930): They used compound occlusion rims with four metal styli placed in the maxillary rim. When the mandible moves with the styli contacting the mandibular rim, they cut four diamond shaped tracings. The tracings incorporate the movements in 3 planes and records are placed on a suitable articulator to receive and duplicate the record.

2. Pattersons method (Patterson 1923-59): this method uses wax occlusion rims. A trench is made in the mandibular rim and a mixture of half plaster and half carborundum paste is placed in the trench. The mandibular movements generate compensating curves in the plaster and carborundum. When the paste is reduced to the predetermined vertical height of occlusion, the patient is instructed to retrude the mandible and the occlusion rims are joined together with metal staples.

3. Meyers (Meyers 1934 – 60) used soft wax on the occlusion rims to establish a generated path. Tinfoil was placed over the wax and lubricated. The patient performed the functional movement to produce a wax path. A plaster index was made of the wax path and the teeth were set to the plaster index

Excursive methods

Review of literature

The earliest graphic recordings were based on studies of mandibular movements by Balckwill in 1866. The intersection of the aracs produced by the right and left condyles formed the apex of what is known as the Gothic arch tracing. The first known “needle point tracing” was by hesse in 1897 and the technique was improved and popularized by Gysi around 1910.

GYSIS tracer was an intraoral incisal tracer in which the plate was attached to the mandibular rim and a spring loaded pin was mounted on the maxillary rim. The rims were made of modelling compound to maintain the vertical dimension of occlusion. When a good tracing was recorded the patient held the rims on the open of the tracing while notches were scored in the rims for orientation.

CLAPP described the use of a Gysi tracer which was attached directly to the impression trays.

SEARS used lubricated rims for easier movement . He placed the needle point tracer on the mandibular rim and the plate on the maxillary rim. He believed this made the angle of the tracing more acute and more easily discernable. They would then cement the rim together for removal.

PHILLIPS (1927) recognized that any lateral movement of the jaw would cause interference of the rims resulting in a distorted record. He developed a plate for the upper rim and a tripoded ball bearing mounted on a jack screw for the lower rim . This innovation , was named the “CENTRAL BEARING POINT” which was supposed to produce civilization of pressure on the edentulous ridges.

STANSBERY (1929) introduced a technique which incorporated a curved plate corresponding to Monsons curve mounted on the upper rim and a Central Bearing Screw was attached to the lower plate corresponding to the reserve Monsoons curve. After the tracing was made a biconcave centric registration was obtained using plaster.

SILVERMAN Later gothic recording methods used the central bearing point to produce gothic are tracing various tracing devices were designed by FLIGHT, PHILLIPS, TERRELL, SEARS, HOUSE, MESSERMAN and others.

The Graphic recording and check bite records received much praise & Criticism Critics of gothic much tracing stated that equalization of pressure did not occur prognathic and retrognathic patients could not be used and flabby tissues and large tongue could cause shifting of the bases & finally  too much of patient cooperatives was needed.

SILVERMAN used on infra oral gothic are tracing to locate the biting point of a patient. The patient was told to bite hard on the tracing plate. This developed the functional resultant of the closing muscle, which would retrude the mandible. The indentation made by the patient would be used for the centric record whether or not it corresponded to the Gothic arch open.

Excursive methods :(Balkwell 1866-62, Hesse 1897, Gysi 1910-63, Philips 1927-61)

The most common form of excursive recordings is the Gothic arch tracing. This may be employed intra orally or extraorally. Intra oral tracers:

(Coble, Ballard, Messiermann)

The Intraoral arrow point tracer combines a central bearing and tracing device. It has pointed screw in bearing and a tracing device mounted on the maxillary rim and a plate mounted on the mandibular rim. The plate is covered with a marking substance. The central bearing pin (Hardy and Pleasure) is connected to the proper vertical relation and when the rims are in place, the patient is instructed to perform lateral and protrusive movements. As these movements are performed, the Gothic arch form is traced on the plate.

Advantages:

The advantage of Intraoral tracers is that the bearing-tracing device is strong enough to resist biting pressures and can be held in position by means of a locking disk Disadvantages:

1. Relative difficulty in visualizing the tracing2. Since the Intraoral tracings are small it will be difficult to find the true apex.3. The tracer must be seated in a hole at the point of the apex to assure accuracy when injecting plaster between the occlusion rims. If the patient moves the rims before they are secured, the records shift on their basal seat which destroys the accuracy

Extra Oral tracing :(Sears-64 and Gysi-63)

The extraoral tracer is always combined with an intra oral bearing device to ensure equalization of pressure on the bases. A needlepoint tracing made on a tracing table coated with carbon or wax is used to indicate the relative position of the upper and lower jaws in the horizontal plane. These tracings are shaped like a Gothic arch and so are sometimes referred to as Gothic arch tracings. They are also known as arrow point tracing.The tracing is not acceptable unless appointed apex is developed, a blunted apex usually indicates an acquired functional relationship and a sharp apex usually indicates the position of centric relation. Double tracings indicate lack of coordinated movements at a different vertical dimension. A graphic tracing to determine centric relation is made at the predetermined vertical relation of occlusion, which harmonizes centric relation with centric occlusion and the antero posterior bone-to-bone relation.

After tentative centric relation record has been made with soft wax at the predetermined vertical relation of occlusion, the rims are mounted on the articulator with the help of the face bow transfer. The lower rim is reduced by 2mm. while maintaining the occlusal plane. Central bearing devices are placed on and fastened to the occlusion rims taking care to center them laterally and anteroposteriorly. The tracing devices are then attached securely to the rims. The stylus is attached to the maxillary rim and the recording plate to the mandibular rim. This arrangement develops and arrow point tracing the apex anteriorly. The patient is seated with his head upright and in a comfortable position. The record bases are inserted and inspected for stability. After confirming that there is no interferences between the occlusion rims when the mandible is moved in any direction, the stylus is lowered to the recording plate to determine that it contacts the recording plate during mandibular movements.

The stylus is retracted and training exercises are conducted with the patient. The tips of the index and middle fingers are placed under the mandible in the bicuspid area and the thumb, under the mandible near the chin to gently apply guiding pressure. There is a possibility of dislodging the mandibular rim by exerting excessive pressure. When the patient executes the mandibular movements proficiently, a thin coating of precipitated chalk in denatured alcohol is applied evenly with a brush on the tracing plate. An acceptable tracing is developed by dropping the stylus to the record plate. When a definite arrow point tracing with a sharp apex is made, the patient is asked to retrude the mandible to centric relation, at which time the point of the stylus must be at the point of the apex. Quick setting plaster is injected between the rims and allowed to harden. The assemblies are removed and the mandibular cast is mounted with the new record.

Advantages of extra oral tracers.1. The tracing point is usually much larger than its

intra oral counterpart because they are made further from the centers of rotation and the apex is more discernible

2. The extra oral tracings are visible while the tracings are being made. Therefore, the patient can be guided and directed more intelligently during the mandibular movements.

3. The stylus can be observed in the apex of the tracing during the process of injecting plaster between the occlusion rims and no hole is required.

Graphic methods are the most accurate visual means of making a centric relation record with mechanical instruments, but all graphic tracings are not necessarily accurate.

Direct check bite interocclusal recordings

Review of literature

It is one of the oldest type of Centric record . In 1796, PHELLEP PEAFF, the dentist of Fredrick, Germany was the first to describe this technique of ‘taking a Bite” .Until end of Ninteenth century, it was the most commonly used method. The direct interocclusal record during that period, was a non precession joint record obtained by placing a thermoplastic material , Usually wax or a compound between the edentulous ridges and having the patient close into the material . This was known as the “mush” “ biscuit” or “squash” bite.

In 1905 CHRISTENSEN was the one of early authors to use impression wan for bite records.

Lates in 1910 GREENE described a mush bite made from modeling compound in which be used a plaster wash to achieve a more accurate record. Occlusion rims were later added to the technique to provide a more stable base.

Gradually these procedures evolved into interocclusal records as they are usually done today. Small amount of wax,compound ,plaster and zinc oxide euginol paste were used as the materials for the registration of the records.

Hanau (1929) was one of the first individuals to be concerned about equalization of pressure when recording the bite. He coined the word “Realeff” which is formed by the beginning letters of the words “resilient and like effect” this became a major factor in check bite techniques

SCHUYLER (1932) observed that if the recording medium was not of uniform density and Viscosity uneven pressures would be transmitted to the record bases which would cause a disharmony of occlusion. He said that modelling compound was preferable to the wan for occlusal records because it can be softened more evently cools slower and doesn’t distort as much as wax..

Wright (1939) described the four factors he believed affected the accuracy of records as resiliency of lessves ,saliva film fit of the bases and pressure applied and concluded that the best technique was to record the occlusal record at zero pressure.

Trapozzano (!955) stated that wax check bite method was the technique of preference.

Payne (1955) & Hickey (1964) stated a preference for plaster.

Payne (!959) Wrote it was important to avoid torsion when recording centric relation and felt that plaster or zinc oxide euginol paste was more accurate.

Hanau Block & others agreed the zero pressure philosophy.

Schyler, Payne and Trapozzano among others advocated the use of light pressure.

The problem of pressure in any record was recognized by Boucher (1960). Who wrote , In additions to technical errors are the errors which occur as a result of failure to control jaw activities and pressure to the time of registration.

The importance of verifying the interocclusal records has been stressed by Greene (1910), Schyler (!932) Trapozzano (1955) and Beck (!960).

Interocclusal check records: (Phellep Pfaff 1796)

It is particularly indicated in situation of 1. Abnormally related jaws2. Supporting tissues that are excessively displaced3. Large awkward tongues4. Uncontrollable or abnormal mandibular movements5. To check the occlusion of teeth in trial dentures.

The record is made by placing a nick and notch on the

maxillary rim and a trough on the mandibular rim in the region of the posterior teeth. The patient is made to retrude the mandible by applying any one of the methods mentioned earlier and recording materials used are waxes, impression compound, dental plaster and zinc oxide eugenol paste

Terminal hinge axis (McCollum –28, Stuart 1921):

Terminal hinge axis is an imaginary line between the temporomandibular joint, around which the mandible may rotate without translatory movements.

As in the determination of the physiologic transverse hinge axis, the mandible is in its most retruded relation to the maxilla when a centric relation record is made therefore if the upper cast is correctly oriented to the hinge axis of the articulator by an accurate face bow transfer, the lower cast will also be correctly oriented to the opening axis of the instrument when it is mounted with an accurate centric relation record. This is true because the mandible is in the most retruded unstrained position to the maxilla both for locating transverse hinge axis and for recording centric relation

Eccentric relation records (Rahn, Gysi 1929 - 63, Craddock):The eccentric maxillomandibular relations are those that

are anterior or lateral to centric relation and those anterior to it are known as protrusive relations. The purpose in making an eccentric relation record is to adjust the horizontal and lateral condylar inclinations. So that the articulator jaw members perform eccentric movements equivalent, but not identical to the relative movements of the mandible to the maxillae. These adjustments permit the condylar elements to travel to and from the centric to eccentric positions and make it possible to arrange the teeth for complete denture prosthesis prosthesiss in balanced occlusion.

The eccentric positions to be recorded are protrusive and right and left lateral. Protrusive and lateral maxillomandibular records are made by functional, graphic and direct check records.

The graphic method of making eccentric maxillomandibular relation records is performed at the same sitting and with the same equipment used for centric relation record. A protrusive interocclusal record can register the influence of the condylar paths over the movements of the mandible. It enables the condylar guidance of the articulator to be set to an approximately of the paths of condylar movements in the patient. Mandibular protrusive movements depend upon and most follows the contour of the glenoid fossa, which understandably does not resemble the straight path of articulator. After the mandibular cast has been mounted in centric relation, the recording devices are replaced in the patient’s mouth.

A distance of 5-6 mm is measured from the apex of the arrow point tracing on the protrusive tracing and is marked. The patient is instructed to protrude until the point of the stylus rests in the marked point. Quick setting dental plaster is injected to seal the rims. The horizontal condylar arrangements on the articulator are released by turning the lock nuts. The incisal guide pin is raised to about one half inch from the top of the guide table. The record bases are seated on the casts and the locknuts are manipulated on one side, then the other. The lock nuts are secured with positive finger pressure and the calibrations are recorded on the plaster mounting.

Lateral Movements (Winkler-5)

Lateral movements are complex activities in most humans. They are of paramount importance to the practitioner, for they influence the intercuspation of teeth in working mastication. Bennett movement is the bodily side shift of the whole mandible that occurs in lateral movements varying in degree from very little to considerable. The right and left lateral mouth records are used to set the articulator, in an attempt to reproduce the lateral movements of the mandible.

Lateral interocclusal records (Whip-Mix) : Before makings these records, set the side shift guide at 45 degree. Remove the plastic piece from its protrusive position, but do not destroy the tracing. If the lateral marks are destroyed, make a new tracing. Six millimeters from CR along the LeftLateral line make a mark; center the hole of the plastic piece over it, and sticky wax in place. Guide the patient’s mandible until the tracer penetrates the hole. Make right and left indices. Return bases with attached plaster to the articulator, and it will be noted that the right condyle ball is in protrusion. The side shift guide should be brought into contact with the ball from its 45-degree position. Also, the condylar inclination may need some minor adjustment. The same procedure is followed for the opposite side, a right lateral record to adjust the left side of the articulator.

Lateral relation records:Lateral positions can be recorded and used to establish lateral guidance on certain adjustable articulators. Graphic methods are used in the same manner as protrusive relation record except that the records required are one of the right lateral and one of the left lateral.Wax check bites are taken to lateral positions and it is desirable to have more than on record at each position.

Hanau recommended a formula to arrive at an acceptable lateral inclination.

L=H/8=12Where L=Lateral condylar inclination in degrees and

H=Horizontal condylar inclination in degrees as established by the protrusive relation record.

The value of this formula has not been proved or disapproved.

Conclusion

The biologic considerations that have been enumerated should be essentially considered by the prosthodontist during Jaw relation procedures to relate its useful clinical application in the healthy service of the edentulous patients. A blind orientation of the maxillary cast on an articulator may result in an error so slight that a face bow transfer appears to the unnecessary. But since the procedure is not complicated nor time consuming the chances of incorporating an error should not be taken.

Since there is no precise scientific method of determining the correct vertical relations, the registration of vertical relation depends upon the clinical experience and judgment of the prosthodontist – an art rather than a science.

It is obvious that the skill of the prosthodontist and the cooperation of the patient are probably the most important factors in securing an accurate centric relation records. The use of various methods enables the prosthodontist to make just preliminary and tentative determinations of the various jaw relation records. The final determination cannot be made by any method until the teeth are set in position in the wax trial dentures and verified in the mouth.

I would conclude with this statement by Weinberg (1959) which puts the controversial subjects of hinge axis theories, occlusion and mandibular movements into perspective;

“The true value of our individual work can be measured only by the degree of finesse with which we practice the art of prosthodontistry rather than by the particular school of thought to which we adhere”

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