Biomechanics of the Temporomandibular Joint (TMJ)

Introduction

• Discussion on the biomechanics of the TMJ.
• Focus on the process of mouth opening.
• Two phases of mouth opening:
  • Early Phase
  • Late Phase

Mouth Opening

Major Osteokinematic Movement

• Mandibular Depression: The mandible moves downward.
• Not a simple downward translation like scapular depression.
• Combination of two movements:
  • Bilateral Downward Condylar Rotation
  • Bilateral Anterior Condylar Translation
• Rotation occurs in the early phase.
• Translation occurs in the late phase.

Role of Muscles

• Lateral Pterygoid Muscle: The only mastication muscle involved in mandibular depression.
• Assisted by gravity.

Symmetric Movement

• For mandibular depression, elevation, protrusion, and retrusion, identical actions occur at both TMJs.
• Both joints do the same thing simultaneously.

Early Phase of Mouth Opening

• Greater rotation than translation.
• Anterior Roll of Mandibular Condyle:
  • Occurs about the articular disc within the mandibular fossa.
  • Produces net downward rotation of the mandible.
• Occurs bilaterally (in both TMJs).
• Some anterior translation, but outweighed by downward rotation.

Late Phase of Mouth Opening

• Greater anterior translation than downward rotation.
• Anterior translation also includes some inferior movement.
• Anterior Slide of Mandibular Condyles:
  • Appears as if condyles are dislocating from the mandibular fossa.
  • Articulates within the inferior joint cavity.
• Increased tension on the articular disc.
• At full mouth opening:
  • Intermediate region of the disc resides between the mandibular condyle and the articular tubercle.
  • Maximum stretch on the superior retrodiscal tissue.

Mouth Closing

• Major osteokinematic movement is Mandibular Elevation.
• Closing is a reversal of opening:
  • Early phase involves posterior condylar translation.
  • Late phase involves upward mandibular rotation.

Conclusion

• Early phase of opening: Downward mandibular rotation.
• Late phase of opening: Anterior mandibular translation (with some inferior translation).
• Closing reverses these movements.