Orientation relation

It is defined as the jaw relation when the mandible is kept in its most posterior position, it can rotate in the sagittal plane around an imaginary transverse axis passing through or near the condyles. This record 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 casts on the articulator must relate to the hinge axis of the instrument in as nearly as possible the same way as the jaws relate to the patient s arc of closure. This relation can be recorded by mean of the face-bow.

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.
PARTS OF FACE-BOW
The face-bow basically contains three sections:
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; see figure (6-2).
3- Locking device: it locks the first two sections without altering their plane.
Also the face-bow contains
? 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.
? 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
1- 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 semiadjustable articulators.

The hinge axis can be designated:
• 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; figure (6-6). 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.

The axis can be designated by:
• The first step involves the fabrication of clutch (it is occlusal rim made of impression compound with a bite fork tightly attached to it). Once the clutch has been attached to the mandible and hinge bow attached to it, guide the patient in making only hinge opening and closing movement. Left and right styli are attached via a face-bow to a clutch, remain stationary (draw point) if aligned with the actual axis of rotation, if the stylus is positioned forward or backward, above or below the actual axis, it will travel one of the arcs indicated by the arrows when the mandible makes a rotational movement. Thus, the arc indicates in what direction an adjustment should be made to the stylus position.

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