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Jaw relation in complete denture

Dr.shanai M.

Jaw Relation (Maxillo-mandibular relationship)

Any longitudinal relation of the maxilla to the mandible
Jaw relation record:It is a registration of any positional relationship of the mandible relative to the maxilla. These records may be made at any vertical, horizontal, or lateral orientation; it is also known as: Maxillo-mandibular record, maxilla-mandibular registration.

Jaw Relation types

Classification of the jaw relation

ORIENTATION JAW RELATION

This relation gives the angulation of the maxilla in relation to the base of the skull. It is necessary to do orientation jaw relation before carrying out other jaw relation.
The position of the maxilla or mandible in the skull is recorded using an instrument called the face bow.
Face bow is mainly used when the vertical dimension of occlusion is expected to be altered.
Use of face bow minimizes occlusal errors in the restoration as the casts will be oriented as close to as they are in the patient.

Face bow

It is a caliper-like device used to record the relationship of the maxillary arch to the temporomandibular joints or the opening axis of the jaws, and then transfer this relationship to the opening axis of the articulator.

The face-bow basically contains

1- U-frame: it represents the plane of the cranium. It is the main frame of the face-bow; all other components are attached to the frame.
It should be large and wide enough to avoid contact with the sides of the face.





2- Bite fork: it represents the plane of the maxilla. It is a U-shaped plate, which is attached to the occlusal rims (3 mm below) while recording the orientation relation

3- Locking device: it locks the first two sections without altering their plane.

4-Condyle rods: These are two small rods on either side of the free end of the frame that contacts the skin over the TMJ, over the hinge axis and transverse the hinge axis of TMJ by attaching to the condylar shaft in the articulator.



5-Orbital pointer with clamp: It is designed to mark the anterior reference point (infraorbital notch) and can be locked in position with a clamp. It is present only in the arbitrary face- bow.

Types of face-bow

Arbitrary face-bow: It is the most commonly used face-bow in complete denture construction. The hinge axis is approximately located, it orients the maxilla to an arbitrary hinge axis and transverse it to the articulator and it does not exactly match the articulator axis to the actual hinge axis. It is simplistic, less accurate, requires less complicated equipment and time. It is used with semi-adjustable articulators.

The Hinge Axis Can Be Designed

Either arbitrarily the condylar rods are positioned approximately 13 mm anterior to the posterior margin of the tragus of ear along a line drawn from the outer canthus of the eye to the center of the tragus (cantho-tragus line) this point called (Beyron's point).

Or by the position of the earpieces in the external auditory meatus; The difference in the position of the earpiece position is accommodated by the design of the articulator and its earpiece receiver points (auditory pin).

2- Kinematic face-bow: A face-bow attached to the mandible with caliper ends (condyle rods) that can be adjusted to permit the accurate location of the true axis of rotation of the mandible. It is generally used for the fabrication of fixed partial denture and full mouth rehabilitation. It is generally not used for complete denture fabrication because it requires a long and complex procedure to record the orientation jaw relation. It orients the maxilla to the actual hinge axis and transverse it to the articulator, it is most sophisticated, most accurate, required more elaborate equipment and time. It is used with fully-adjustable articulators.

Importance of the face -bow

1- The mounting of maxillary cast without face-bow transfer can produce errors in the occlusion of the finished denture.


2- A face-bow transfer allows minor changes in the occlusal vertical dimension on the articulator without having to make new maxilla-mandibular records.

3- It is helpful in supporting maxillary cast while it is being mounted on the articulator.