ORIENTATION JAW RELATION SUBMITTED BY :- AMISHA JAIN

DEFINITION The jaw relation when the mandible is kept in its most posterior position, it can rotate in the sagittal pane around an imaginary transverse axis passing through or near the condyles.

- It can be recorded with a face-bow.

FACE BOW Definition :- A caliper like device which is used to

record the relationship of the maxillae and/or the mandible to the temporomandibular joints.

Parts of a Face – bow The component parts of a face-bow are :- a. U- shaped frame – It is a U-shaped metallic bar

that forms the main frame of the face-bow. All other components are attached to the frame with the help of clamps. This records the plane of the cranium.

b. Condylar rods – These are two small metallic rods on either side of the free end of the U-shaped frame that contacts the skin over the TMJ. They help to locate the hinge axis or the opening axis of the temporomandibular joint.

Face- bows that have a condylar rod, to record the true hinge axis, are called Kinematic face-bows. Earpiece face-bows do not record the true hinge axis and hence they are called Arbitary face-bows.

c. Bite Fork – This is a U- shaped plate, which is attached to the occlusal rims while recording the orientation relation. It is attached to the frame with the help of a rod called the stem.

d. Locking device – This part of the face-bow helps to attach the bite fork to the U- shaped frame. This also supports the face-bow, occlusal rims and the casts during articulation.

It consists of a transfer rod and a transverse rod. The ‘U’- shaped frame is attached to the vertical transfer rod.

The horizontal transverse rod connects the transfer rod with the stem of the bite fork. The assembly where the transverse rod gets automatically positioned when tapped is called an auto-adjusting or self-centering assembly.

e. Orbital pointer:- it is designed to mark the anterior reference point and can be locked in position with a clamp. It is present only in the arbitary face-bow.

TYPES OF FACE-BOWS Face-bows can be classified as follows:-i. Arbitary face-bow- Facia type- Earpiece type- Hanau face-bow (Spring bow)- Slidematic (Denar)- Twirl bow- Whipmixii. Kinematic or hinge bow

1. Arbitary face-bow: Most commonly used face-bow in

complete denture construction. The hinge axis is approximately located. The condylar rods are positioned approximately 13mm anterior to the auditory meatus on a line running from the outer canthus of the eye to the top of the tragus also called the cantho-tragal line. This is done using a Richey condylar marker.

2. Facia type :- The hinge axis or the posterior

reference point is 13mm anterior to the external auditory meatus and the anterior reference point is the orbitale.

3. Earpiece type:- The posterior reference point

is the external auditory meatus and the anterior reference point is the orbitale.

4. Hanau face-bow:- Most commonly used face-bow.

5. Slidematic type (Denar):- This face-bow has an electronic

device, which gives the reading that can be seen in the anterior region. This reading denotes one-half of the patient’s inter-condylar distance. The posterior reference point for this instrument is the external auditory meatus and the anterior reference point is 43 mm superior to the incisal edge of the upper central incisor for dentulous patients.

In an edentulous patient the anterior reference point is 43mm superior to the lower border of the upper lip in a relaxed state.

6. Twirl bow:- It is an arbitrary type of face-bow that

does not require any physical attachment to the articulator. It is not commonly used for CD construction. It relates the maxillary arch to the frankfort’s horizontal plane.

7. Whipmix face-bow:- These face-bows have a built-in

hinge axis locator. It automatically locates the hinge axis when the earpieces are placed in the external auditory meatus.

8. Kinematic face-bow:- This face-bow is generally used for

the fabrication of fixed partial denture and full-mouth rehabilitation.

RECORDING THE ORIENTATION JAW RELATION USING AN ARBITARY FACE-BOW

The patient is seated in a comfortable position with his head upright and supported by the headrest.

A point is marked 13mm in front of the auditory meatus on a line running from the outer cantus to the superior border of the tragus.

Guidelines for anterior teeth placement are marked on the maxillary occlusal rim.

A notch index about 2 mm deep is made in the first molar region. This helps to position the fac-bow.

The mandibular occlusal rim is placed in the patient’s mouth and reduced such that sufficient space is available between the two rims to accommodate the bite fork.

The U-shaped frame of the face-bow along with the condylar rods is positioned on the patient. The condylar rods are moved and placed on the posterior reference points marked 13mm in front of the auditory meatus on the canthotragus line.

Aluwax is softened and shaped to the form of a horseshoe. The thickness of the bite fork and the wax together should not be more than 6mm.

A thin layer of petroleum jelly is applied on both the occlusal rims to facilitate easy separation.

The maxillary and mandibular occlusal rims are inserted into the patient’s mouth.

The bite fork with the wax is inserted into the patient’s mouth. The midline of the bite fork should coincide with the midline of the maxillary occlusal rim. The stem of the bite fork should be parallel to the sagittal plane.

The patient is asked to close his mouth till both the occlusal rims get embedded into the bite fork.

The stem of the bite fork is locked to the transverse rod of the face-bow.

Any alterations in the position of the condylar rods should be checked.

The infraorbital notch is palpated and marked. The orbital pointer is made to touch the infraorbital notch and locked in position to the ‘U’ frame with an orbital clamp.

The entire face-bow assembly along with the occlusal rim is removed from the mouth and positioned (transferred) in the articulator.

RECORDING THE ORIENTATION JAW RELATION USING A KINEMATIC FACE-BOW

The kinematic face-bow is attached to the mandible occlusal rim and the hinge axis is located based upon the opening movements of the mandible.

FABRICATION OF A CLUTCH: The first step involves the fabrication of a

clutch. The clutch is a device, which relates the face-bow to the mandibular residual alveolar ridge. It is an occlusal rim made of impression compound with a bite fork attached to it.

For dentulous patients, the clutch resembles a cap splint with a bite fork attached to it. It is usually fabricated and cast in aluminium.

A record base is fabricated over the mandibular cast.

An occlusal rim made of impression compound is built upon the record base.

The bite fork is attached to this occlusal rim. Extending outside from the bite fork is its

horizontal stem, which lies parallel to the sagittal plane.

CLINICAL PROCEDURE The clutch along the stem is placed in the

patient’s mouth. The hinge bow is attached to the stem of the

bite fork and the condylar rods are located at a point 13mm in front of the auditory meatus on the canthotragus line.

The patient should be placed in a semi-supine position and his mandible should be guided to produce opening and closing movements, which are purely rotational. The patient should be asked to make eccentric movements.

The condylar rods will move during the mandibular movements. The point at which the condylar rods show pure rotation indicates the terminal hinge axis. If the patient opens his mouth widely then the condylar rods will move anteriorly.

The condylar rods are locked in this position and the face-bow is removed.

A tatoo representing the terminal hinge axis should be marked on the patient’s face for further reference.