TMJ Biomechanics and Function

TMJ Biomechanics: Function

• TMJ performs bilateral actions, including mastication, talking, and swallowing
• The joint works with the tongue and teeth during the mastication process

TMJ Biomechanics: Anatomy

• The TMJ is a bicondylar synovial joint between the condyle in the lower jaw and the mandibular fossa of the temporal bone
• The joint has a wide range of rotation and translation, with an articular disk cushioning the forces related to mastication
• The disk separates the joint into two synovial joint cavities: the inferior joint cavity and the larger superior joint cavity

Mandibular Condyle

• The condyle has a convex shape with short medial and lateral poles
• The medial pole is more prominent than the lateral pole
• The condyle is covered by a thin layer of fibrocartilage

Mandibular Fossa

• The fossa is part of the temporal bone and forms the joint with the condyle

Articular Disk of the TMJ

• The disk is made of dense fibrocartilage
• It lacks blood supply and sensory innervation
• The disk's main function is to generate greater congruency and reduce contact pressure

Stability of the TMJ

• Passive stability is provided by the fibrous capsule, articular disk, lateral temporomandibular ligament, and retrodiscal tissue
• Active stability is provided by muscles

Osteokinematics of the TMJ

• The TMJ has four movements: protrusion, retrusion, lateral excursion, and depression-elevation
• Protrusion and retrusion involve anterior and posterior movements without significant rotation
• Lateral excursion involves side-to-side movement of the mandible, guided by the mandibular fossa and articular disk
• Depression and elevation involve opening and closing the mouth

TMJ Arthrokinematics

• The TMJ typically involves bilateral action
• During rotational movement, the mandibular condyle rolls relative to the inferior surface of the disk
• During translational movement, the mandibular condyle and disk slide together
• The disk usually moves in the direction of the translating condyle

Protrusion and Retrusion

• Protrusion involves the mandibular condyle and disk translating anteriorly and slightly downwards
• Retrusion involves the mandibular condyle and disk translating posteriorly and slightly upwards

Lateral Excursion

• Primary movement involves side-to-side translation of the condyle and disk within the fossa
• Secondary movement involves slight rotational components

Depression and Elevation

• The early phase of opening the mouth involves primary rotation of the mandible relative to the temporal
• The late phase involves a gradual transition from primary rotation to primary translation
• At the end of opening, the axis of rotation shifts inferiorly

Kinetics of the TMJ

• The masseter muscle elevates the mandible and brings teeth into contact
• The temporalis muscle elevates the mandible and slightly retrudes it
• The medial pterygoid muscle elevates the mandible and protrudes it
• The lateral pterygoid muscle depresses the mandible and protrudes it
• The superior head of the lateral pterygoid adjusts disc position