Dental Anatomy: Terminal Hinge Axis and Facebow

Study Notes

The terminal hinge axis is the imaginary line around which the mandible rotates during opening.
This rotation is considered pure and occurs in centric relation.
The rotation averages about 12° or ranges between 10-25mm of incisal opening.

A caliper-like device used to record the jaw's relationship to the hinge axis.
It allows the casts to be oriented in the same relationship to the hinge axis of the articulator.

Maxillary (arbitrary) facebow:
- U-shaped bow with a bite fork, condylar rods, and an anterior reference point indicator.
- The condylar rods are placed on the assumed condylar position.
- The anterior reference point can be the infra-orbital rim or the nasion.
- Calibrated condylar rods, spring facebows, and slidematic facebows exist to ensure the maxilla is centrally positioned.
Mandibular or kinematic face bow (Hinge axis face bow):
- U-shaped bow attached to the mandible by a clamp that fixes the lower occlusion block.
- Two pointed condylar rods are attached to the bow.
- Uses:
  - Locating the exact terminal hinge axis before using the maxillary face bow.
  - Recording centric relation.

Arbitrary:
- Placing the hinge axis 11-13mm anterior to the tragus of the ear on a line extending from the outer canthus of the eye to the upper margin of the external auditory meatus.
Kinematically:
- Attach the kinematic facebow to the mandible.
- Ask the patient to open and close their mouth slightly while in the most retruded position at the proper vertical dimension of occlusion.
- The condylar rods will draw arcs on paper attached to the skin.
- Adjust the rod positions towards the center of the arcs.
- Repeat the process until the rods rotate in a point, indicating the terminal hinge axis.

Accurately locate the condylar axis either with the mandibular facebow or arbitrarily.
Attach the bite fork to the maxillary occlusion rim parallel to the occlusal plane and insert it into the patient's mouth.
Support the maxillary block with the mandibular occlusion block.
Connect the stem of the bite fork to the universal joint.
Adjust the condylar rods to touch the points representing the condylar axis and ensure both rods have equal calibrations with tension.
Tighten the clamps and then the bite fork clamp.
Place the third reference point (infra-orbital notch or nasion).
Remove the entire assembly with careful attention to the bite fork's position.

Place the articulator and face bow on a flat surface.
Adjust the condylar rods of the face bow to match the calibrations on the articulator's extended condylar rods.
Bring the occlusion rim level with the incisal pin's notch on the articulator.
Use a cast support to prevent sagging of the maxillary cast during mounting.
Ensure the top of the incisal pin is level with the surface of the articulator's upper member.