Dental Anatomy: Terminal Hinge Axis and Facebows

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Questions and Answers

What is the purpose of drawing a line from the upper margin of the external auditory meatus to the outer canthus of the eye?

To ensure proper insertion of the mandibular occlusion block.

To align the bite fork parallel to the occlusal plane.

To locate the condylar axis accurately. (correct)

To measure the distance of the maxilla from the mandible.

Why should both condylar rods show equal calibrations when adjusting them?

To achieve balanced occlusion in the articulator. (correct)

To prevent the assembly from becoming misaligned.

To ensure that the face bow is symmetrical.

To maintain slight tension in the rods for accurate readings.

At what point should the clamp of the bite fork be tightened?

Once the infra-orbital notch or nasion reference point is placed.

Before attaching the bite fork to the maxillary occlusion rim.

Immediately after inserting the assembly into the patient's mouth.

After adjusting the condylar rods to the condylar axis. (correct)

What role does the cast support (Jig) play during the mounting of the maxillary cast?

It prevents the maxillary cast from sagging. Signup and view all the answers

What should be the position of the lower edge of the occlusion rim during the adjustment of the anterior part of the face bow?

In line with the notch present on the incisal pin. Signup and view all the answers

What is the terminal hinge axis also known as?

Transverse axis Signup and view all the answers

What is the purpose of the facebow in dental practices?

To record the relationship of the jaws to the hinge axis Signup and view all the answers

Which of the following components is NOT part of the maximum (arbitrary) face bow?

Crown holder Signup and view all the answers

How is the hinge axis determined when using the facia facebow?

Over the patient's skin at the hinge axis position Signup and view all the answers

What is the range of the incisal opening during rotation around the terminal hinge axis?

10-25 mm Signup and view all the answers

Which type of reference point is associated with the infraorbital pointer in the maxillary facebow?

Anterior reference point Signup and view all the answers

What type of facebow is also referred to as the hinge axis face bow?

Kinematic face bow Signup and view all the answers

What is the role of the graduated condylar rod in the maxillary facebow?

To indicate the assumed condylar position Signup and view all the answers

What is the primary purpose of the calibrated condylar rods in an articulator?

To ensure equal readings are achieved bilaterally. Signup and view all the answers

Which facebow is recognized for its ability to allow medio-lateral movements?

Slidematic facebow Signup and view all the answers

How is the terminal hinge axis located using the arbitrary method?

By placing it 11-13 mm anterior the tragus on a specific line. Signup and view all the answers

What is the significance of the 'Still point' in kinematic face-bow procedures?

It represents the center of the arcs drawn by the condylar rods. Signup and view all the answers

What component of the articulator is responsible for contacting the posterior reference point?

Spring facebow Signup and view all the answers

Which mechanism is primarily used to centralize the maxilla within the facebow?

Calibrated condylar rods adjustment. Signup and view all the answers

Why is the mandibular or kinematic face bow crucial before using a maxillary face bow?

To determine the exact terminal hinge axis. Signup and view all the answers

What material is commonly used to make a spring facebow?

Spring steel Signup and view all the answers

Study Notes

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.

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Description

Explore the concepts of terminal hinge axis and facebows in dental anatomy. Understand how these tools are used to record jaw relationships and aid in dental procedures. Delve into the specifics of different types of facebows and their applications.