5 2 Australian Dental Journal, February, 1965

Temporomandibular joint dysfunction at the genera1 practitioner level. Part 11. Functional anatomy*

W. 0. Read, B.D.S. (Syd.)

Functional anatomy Before discussing the various symptoms and

signs mentioned in the first part of this dis- cussion, it is felt that an outline of the anatomy of the parts involved is necessary. However, as it is intended also to discuss function, the anatomical part of this section will be in the nature of a brief review.

Sicher'l) has drawn attention to three things which distinguish the temporomandibular articulation from most other joints:

1. The articular surfaces are not covered by hyaline cartilage but by an avascular flbrous tissue, which may contain a few cartilage cells.

2. The two bone structures carry teeth, which have a n effect on movement.

3. The two joints function as a single unit, as any movement in one joint is reflected in the other.

I t is further complicated by the articular disc, which divides the joint into two com- partments, and gives us two joints in one- between condyle and disc on the one hand, and between disc and temporal bone on the other. It is a hinge joint with a movable socket (Fig. 1).

A. The bony structures The bony structures consist of: 1 . Condyle. The condyle has a transverse

ridge at the summit, terminating medially and laterally just below the articular surface

* Presented at the 17th Australian Dental

Sicher, H.-Oral anatomy, St. Louis, The C. V. Congress, Perth, May, 1964.

Mosby Co., 3rd ed., 1960 (p. 162).

in two distinct tubercles. The articular sur- face, which faces upwards and forwards, ie. covered by the fibrous tissue layer mentioned: above.

2. The receptacle for the condyle is the articular fossa, which is part of the squamous. temporal, and which is bounded medially, laterally and posteriorly by bony ridges which limit gross displacement of the condyles in these directions. The roof of the fossa. which1

Fig. 1.-Diagrammatic representation of the tem- poromandibular joint. (Rees, L. A., Brit. D. J.).

separates it from the middle cranial fossa, is very thin-even translucent in some skulls, which indicates that this is not a stress- bearing area. This is further borne out by the fact that the fibrous tissue covering is here little thicker than a periosteum. Anteriorly, however, we find the articular eminence, where the fibrous tissue is at i ts thickest on the posterior slope and prominence. This is the main stress-bearing area.

Australian Dental Journal, February, I965

B. Articular disc This is an oval, dense sheet of very firm

fibrous tissue, in which Rees”) has described

53

four 1.

2.

3.

4.

distinct zones, (Mg. 2): Anterior band - moderately thick but narrow. Intermediate band - much thinner, for flexibility. Posterior band - much the thickest and widest. Bilaminar zone-in two layers. (a) Upper stratum, composed mainly of fibro- elastic tissue. (a) Lower stratum - composed mainly of fibrous tissue, with but little elastic tissue. These layers contain nerves and blood vessels.

bYg. 2.--Diagrainmatic regresent:ition of the disc and its attnrhmrnta. (Kees, L. A,, Brit. 1). .J.) .

The attachments of the disc and fibrous capsule are closely related:

The disc is firmly attached to the condylar tubercles below the medial and lateral poles of the condyle. This firm attachment forms the axis for the lower half of the joint, allow- ing a visor-like movement of the disc relative to the condyle, and ensures that disc and rondyle move antero-posteriorly together.

Around the disc’s circumference is fused the fibrous capsule, so completely that the two are barely distinguishable. It supplies a compara- tively loose attachment to the margin of the temporal articular surface to accommodate the movement in the upper half of the joint.

Anteriorly attachment is above to the anterior margin of the articular eminence, below to the front of the condyle’s articular

. (9) Rees, L. A.-The structure and function of the mandibular joint. Brit. D. J., 9 6 : 6 , 125-133 (March 1 6 ) 1 9 5 4 .

surface. Here also is found the tendinous insertion of the external pterygoid muscle.

Posteriorly the capsule blends with the disc’s bilaminar zone whose upper stratum attaches to the posterior wall of the articular fossa, the lower stratum attaching to the posterior sur- face of the condyle.

Generally the fibrous capsule is rather thin, except for the lateral border, which is thickened to form the temporomandibular ligament, extending from the root of the zygoma down- wards and backwards to the neck of the condyle.

Lining the fibrous capsule is the synovial capsule for the elaboration of the synovial fluid.

C. Relevant muscles 1. Masseter. The most superficial, and power-

ful. Superficial and deep parts are distinguish- able, the deeper fibres running slightly posteriorly.

2. Temporal, perhaps the most important postural muscle of the mandible. The anterior fibres run practically vertically, the posterior fibres horizontally.

3. Internal pterygoid, the counterpart of the masseter, and works synergistically with it.

These three muscles are basically elevators, with the fibres which run more posteriorly and horizontally providing retraction.

4. External pterygoid, presenting a larger inferior head and a smaller superior head.

The uppermost and most medial fibres insert into the antero-medial surface of the capsule, and into the disc itself. Most of the flbbres, i.e., most of the superior head, and the entire inferior head, insert into the anterior surface of the neck of the condyle.

I t is noteworthy that the direction of the upper head from its origin is downwards, backwards and outwards, and so, by its partial insertion into the disc and capsule, tends to maintain intimate contact of the disc against the articular eminence during contraction. These muscles provide protmct$on of the mandible, when contracting bilaterally, lateral movement, when one acts singly.

5. Suprahyoids, the relevant muscles of this group being digastric, genio-hyoid, and mylo-hyoid, which depress the mandible.

This does not complete the list of muscles involved, for if these three supra-hyoid muscles

(8 ) Sicher, H.-Op. n’t. p. 136.

5 4 Auatralian Dental Journal, February, I965

Through reflex activity, they are responsible, amongst other things, for maintenance of muscle tone, and assistance in muscle coordination.“’

They also occur in the gingival and palatal tissues and most importantly in the lower third of the periodontal mernbrane.t6) Here they assist the proprioceptors in the mandi- bular musculature to direct the movements of the mandible, and in this they afford a more delicate sensitivity than those in the muscles. We all know how difficult i t can be to take a “bite” registration in the edentulous case, where the conditioned reflex made possible by these receptors has been lost through loss of teeth. I t follows that these proprioceptors are responsible for alteration of mandibular closure to avoid premature contacts.

Before considering the functional application of the foregoing, it is first necessary to de- scribe three fundamental positions of the mandible which have great clinical significance.

1. The rest o r postural position is estab- lished a t about three months, and remains relatively constant throughout life.@’ It depends primarily on an equilibrium of muscle tone between the jaw opening muscles and the jaw closing muscles, but is affected by many things such as body and head position and psychic factors, such as pain, sleep. I t is the position from which most movement starts, being the normal position or posture of the mandible.

Sicher points out that clinically the rest position is of great importance, as i t sets the limit for bite raising procedures. If the bite is opened to or beyond the rest position, the elevator muscles are stretched beyond their normal resting length, stimulating the pro- prioceptors which cause reflex contractions perhaps to the extent of painful spasm. This may lead to damage to the teeth and their supporting structures and/or the joints, or in the edentulous person may lead to painful damage to the soft tissues and eventual loss of alveolar bone.“’

2. The occlusal or intercuspal position is the intermaxillary relationship when inter- cuspation has occurred after closure, i.e.,

are to depress the mandible, it is necessary for the hyoid bone, which is suspended be- tween the supra-hyoids and the infra-hyoids, to be fixed. This requires the infra-hyoids as a group to stabilize the hyoid bone, which they achieve by isometric contraction.

Furthermore, the digastric and genio-hyoid are comparatively weak muscles, and in order to achieve depression of the mandible, auto- matic relaxation of the powerful elevator muscles must coincide.

Taking this a step further, the muscles of mastication all take their origin from various parts of the cranium. Unless the cranium itself is stabilized, it is obvious that elevation of the mandible will be interfered with-you would nod your head every time you closed your mouth. This brings into the picture a formidable array of muscles affecting the posi- tion of the cranium itself.

The importance of this will be apparent when one considers the incredibly intricate coordination of muscles necessary for what would appear, on the surface, to be a simple action like opening and closing one’s mouth. Apart from isotonic contraction of the muscles actually performing the action, there must be relaxation of the antagonists, and isometric contraction, or stabilizing of the muscles con- cerned with the posture or attitude of the two movable platforms, the cranium and hyoid bone.

Of perhaps even greater significance clini- cally is the diverse nerve supply of all these muscles and adjacent structures which per- haps explains the widespread and varied loca- tions of pain due to dysfunction of this intrirate mechanism.

D. Taeth n n d supporting tissues Finally, we must consider the teeth and

supporting tissues. The teeth, by variations in shape and position, have a definite effect on mandibular movements. In the supporting tissues we are concerned at the moment with the sensory mechanism they provide. This is partly protective, as mentioned earlier ( the rapid reflex opening on biting, say, a chop bone), and partly proprioceptive.

Proprioceptors are a type of receptor organ which subconsciously tell us the position of various parts of the body-i.e., without the necessity for looking-and are mainly con- cerned with muscles, joints and tendons.

Samson \Vright-Applied ~ihysiology. London, Oxford liniversity Press. 10th ed.. 1961 to. 2 6 4 ) .

1:) Bernick. S.-Innervation of the teeth and perio- dontium. D. Clin. N. America, Philadelphia, W. B. Saunders Co.. 1959 ( D . 5 0 3 ) .

(Q1 Posselt. I..-Physiology of occlusion and rehabi- litation. Oxford, Blackwell Scientific Publica- tions, 1 9 6 2 (p. 3 0 ) .

(a Sirher, H.-Op. cit . (p. 1 7 3 ) .

Australian Dental Journal, February, I965

where the teeth of both jaws are meshing tightly.(*) It is separated from the rest posi- tion by a variable amount of 2-4 mm. which we know as the freeway space.

The occlusal position, then, determines the relationship of the mandible to the cranium when the jaws are closed, and thus is funda- mental in determining the condyle-fossa relationship. Ideally, this offends neither joints nor muscles.

3. Thc centric position is achieved where teeth, joints and musculature are in perfect balance.(#) Ideally this coincides with the occlusal position as mentioned before. I t i s not, however, the most retruded position that the musculature can achieve, being up to 1 mm. anterior to it.

However, several factors can separate the occlusal and centric positions. Malpositioned teeth, poor restorations and prostheses can provide premature contacts which are not appreciated by the periodontal proprioceptors.

During function they maintain a continuous discharge of impulses, which result in the muscles attempting to guide the mandible into a position of less interference. Finally a posi- tion is found where the protective stimuli are at the lowest level and, by the conditioned reflex formed, this position becomes the acclusal position a t the expense of normal muscle function. Lateral displacement may occur because the muscles will strive for bilateral contact (even a t the expense of posi- iional symmetry), resulting in condylar dis- placement.

Further, overartivity of the muscles, through fatigue and accumulation of metabolic waste products, may result in pain and spasm.

Clinically these must first be relieved if accurate functional analysis records are to be obtained, as will be described later.

Realization that the closure pattern is a conditioned reflex requiring constant reinforce- ment is the rationale of the temporary use of bite plates as an aid to diagnosis. By keeping the teeth apart, the conditioned reflex cannot be reinforced and becomes in- operative. Muscle action eventually returns to normal and accurate records can be obtained.

Perhaps i t would be as well to explain what happens in muscle spasm, as frequent mention is made in the Iiterature. Spasm is the result

5 5

( h ) Posselt, 1'.-Op. cit. (p. 3 5 ) . '") Sicher. 1q.--Op. C i t . (p. 175).

of continuous bombardment of impulses over the afferent pathways, due to muscular over- activity such as has been described. Holmes('o) defines i t as "sudden involuntary contractions of a muscle or group of muscles. They range in magnitude from a single inconspicuous

VIP. :%.--Stages in the forward movement of the condyle when the mouth i s opened. (From Schwartz, L.-Disorders of the temporo- manrlihular joint, Philadelphia, W. B. Saunders

Cornimny, 1959 . )

twitch of a small number of fibres in a muscle, to strong painful contractions or cramps, involving most or all of the fibres in one or more muscles. These spasms may be inter- mittent and repetitive, or sustained, and may vary widely in frequency".

110) Holmes, T. I€. in Schwaria, L.-Disorders of the temporomandibular joint. Philadelphia, U'. B. Snunders Co.. 19.79 (p. 3 6 ) .

56 Australian Dental Journal, February, I965

Functional movements Let us now consider what happens within

the joint itself.(11) In the occlusal position, disc and condyle

lie within the articular fossa as shown (Fig. 3) with the posterior band in the deepest part of the fossa, overlying the transverse ridge of the condyle.

Fig. 4.-Relati@nship of the condyle to the posterior band of the disc when clicking occurs

in the early stages of movement.

Remembering that the disc is firmly attached to the two tubercles mentioned earlier, the first movement of opening resembles a hinge movement about this axis, which brings the transverse ridge on to the intermediate band. As opening continues, disc and condyle move forwards together on to the articular eminence, the condyle still rotating about the bicondylar axis which brings the transverse ridge further forward on the intermediate band.

This forward movement, of course, is due to the external pterygoid acting on both disc and condyle.

Finally, stretching of the tissues of the bilaminar zone slows the disc’s progress, but the condyle continues to rotate about its axis, until the transverse ridge passes under the anterior band and may even finish up just anterior to the articular eminence in extreme opening.

(11) Iiees, L. A.---Op. c i t . (p. 182).

Closure follows closely the reverse of this procedure, the external pterygoid holding the disc in position during a n intial rotation back- wards, then maintaining the relationship of transverse ridge to intermediate band until the posterior band returns to the articular fossa, and the transverse ridge passes under its inferior surface.

Note that there is no muscle to retract the disc-whose backward movement is due partly to recoil of the fibro-elastic tissue of the bilaminar zone, and partly through passively following the condyle, due to the disc’s firm attachment to the condylar tubercles.

The lateral pterygoid, however, is respon- sible for stabilizing the disc and maintaining’ its relationship to the condyle.

Disturbance of this relationship is probably the most common reason for the clicking joint. During opening and closing, the transverse ridge is related to the thin intermediate band of the disc. If. however, the condylar ridge

Fig. 5.-Relationship of the condyle to the anterior band of the disc when clicking occurs

a t extreme opening of the mouth.

encounters either the posterior or anterior band at the wrong time or place, a click will occur. For example, the click on early open- ing is frequently due to a postero-superior displacement of the condyle, i.e., above and behind the posterior band in the occlusal position (Fig. 4) . On initial rotation the

Australian Dental Journal, February, I965 5 7

ridge has to snap under the posterior band. and maintaining the proper disc-condyle The common click a t extreme opening, or at relationship as we saw before. the beginning of closure, is due to the ridge passing under the anterior band (Fig. 6 ) . Uncoordinated muscle action or muscle spasm, There is no doubt that ‘uspa’ interference acting on disc, or condyle (or both) can can exist without causing symptoms; however, readily upset the correct relationship at any generally clinical signs can and should be stage, and is generally considered the cause observed. When cuspal interference exists,

excessive function at this point will obviously of clicking. result in abnormal wear, compared with the What happens in the joint during other overall degree of wear; there may also be mandibular movements follows on this basic areas of abnormal lack of wear. mechanism, as every movement from the

Alternatively, the brunt may be carried by occlusal position involves a degree of opening the supporting tissues, resulting i n periodontal to rlear the incisal or buccal overbite. How- disturbance. The tooth may become loose or ever, in lateral movement and protraction, even move. which involves action by one or both external

Finally, the muscles themselves, through pterygoids, coordination of disc and condyle over-activity caused by continually searching is essential for normal movement. In passing, for the most effective and compatible contact lateral movement implies rotation about the position, may hypertrophy or exhibit fatigue, working side condyle; however, this condyle which can lead to muscle spasm and pain. does not remain stationary, but moves siightly

laterally and perhaps downwards-this bodily Whether Or not symptoms Occur is due to shift being known as the Bennett Movement. the faculty of adaptation, which varies from

person to person and from time to time.(’)

Adaptation

Mastication It is not intended to describe the intricacies

of mastication, but the following points are considered relevant.

Masticatory movements are automatic, con- sisting of a number of conditioned reflexes. This reflex pattern varies from person to person but is remarkably stable for a n individual.

Sicher(’*) discusses both the cutting and grinding movements of mastication in three stages :

1. Free movement of lower jaw-opening and protraction and/or translation.

2. Closing - until teeth contact, and leading to-

3. An articulatory movement - returning to the occlusal position under tooth guidance.

He points out that force is applied in the second and third stages, and that muscle balance is very important during the second stage, where great force is applied with the mandible not yet stabilized by tooth contact. Again the external pterygoid’s stabilizing action comes to the rescue, preventing dis- placement whilst the condyle is balanced on the posterior slope of the articular eminence,

(12) Sicher. H.-OD. c i t . (D. 1 x 2 ) (13) Possect, u.--(ip. cit. ‘ip. -71 j . (14) Berry, D. C.-The temporomandibular syn-

drome. J. Pros. Den., 1 3 : 6, 1122-1129 (Nov.- Dec.) 1963 .

In most cases, adaptive changes occur which enable the system to function, although it cannot be regarded as normal function. When the upper limits of tolerance and adaptation are reached, symptoms occur. This can be caused by a general effect, such as nervous tension (i.e., a n “anxiety state”) or a local effect such as loss of a tooth, or any mild pathological change, or even a change of diet. Pain limits movement and provides rest, which allows tissue recovery. If the system is overloaded again, dysfunction and pain recur, usually at a lower threshold; recovery is slower and attacks become more frequent. Bruxism creates a difficulty because the normal protective reflexes are dulled, and adaptation is more difficult. Here we have a combination of both general and local factors, namely, mental stress and cuspal interference. Furthermore, the movements can greatly exceed the normal functional range. Alteration to the occlusion in any way-be it a restora- tion or a n extraction-can trigger off bruxism in the susceptible patient. It has been observed that any mutilation of the masticatory system may be tolerated in the absence of mental stress or nervous tension.(’4) However, if this supervenes, pain and dysfunction may occur, however perfect the mechanics of the system may be.

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