Dental Anatomy: Terminal Hinge Axis and Facebows

Terminal Hinge Axis

• An imaginary line around which the mandible rotates during opening and closing
• Occurs in centric relation, a position where the teeth are in maximum intercuspation
• The movement of the condyle during early opening is assumed to be pure rotation around this axis

Facebows

• A device used to record the relationship of the jaws to the hinge axis
• Helps orient casts in the same relationship to the hinge axis of the articulator

Types of Facebows

• Maxillary (arbitrary) Facebow: A caliper-like device used to record the position of the maxillary arch relative to the hinge axis.
  • Consists of a U-shaped bow, a bite fork, and posterior and anterior reference point indicators.
  • Posterior reference point indicators:
    • Facia facebow: placed on the patient's skin over the assumed hinge axis position
    • Earpiece facebow: inserted into the external auditory meatus
  • Anterior reference point indicators:
    • Infra orbital pointer: placed on the lowest margin of the infra-orbital rim
    • Nasion pointer: placed on the nasion
  • Mechanism of Intercondylar Distance Equilibration:
    • Calibrated condylar rods: graduated rods adjusted bilaterally for equal readings
    • Spring facebow: made of spring steel, the bow widens and narrows until condylar rods contact posterior reference points
    • Slidematic facebow: anterior horizontal arm with sliding arms to adjust for medio-lateral movements
• Mandibular or Kinematic Facebow (Hinge axis facebow): a U-shaped bow attached to the mandible by a clamp. This helps locate the exact terminal hinge axis.

Determine Terminal Hinge Axis

• Arbitrary: located 11-13 mm 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 face bow to the mandible
  • Patient opens and closes their mouth slightly in the most retruded position at the proper vertical dimension of occlusion
  • Condylar rods draw arcs on paper attached to the skin
  • Adjust rods towards the center of these arcs
  • Repeat until the rods rotate in a point, representing the terminal hinge axis (the "Still point")

Registration of a Maxillary Face Bow Record

• Locate the condylar axis using the mandibular face bow or arbitrary by drawing a line on the patient's face.
• Attach the bite fork to the maxillary occlusion rim parallel to the occlusal plane.
• Insert bite fork and mandibular occlusion block in the patient's mouth.
• Slip the stem of the bite fork into the universal joint.
• Adjust condylar rods to equal calibrations with slight tension.
• Tighten clamps.
• Place the third reference point (infra-orbital notch or nasion).
• Release condylar rods and remove assembly carefully.

Mounting the Maxillary Cast on the Articulator

• Position the articulator and facebow on a flat surface.
• Adjust graduated condylar rods until they show equal calibrations.
• Adjust the anterior part of the bow until the lower edge of the occlusion rim is level with the incisal pin of the articulator.
• Use a cast support to prevent sagging of the maxillary cast while mounting.
• The top of the incisal pin should be level with the surface of the upper member of the articulator.