14. TMJ.pdf

TOPIC 13: TMJ BIOMECHANICS Function Bilateral action Mastication, talking, swallowing Mastication process: together with tongue and teeth. Bicondylar synovial joint between condyle in lower jaw and the mandibular fossa of the temporal bone. Arthrology of the TMJ Is a synovial jo...

TOPIC 13: TMJ BIOMECHANICS Function Bilateral action Mastication, talking, swallowing Mastication process: together with tongue and teeth. Bicondylar synovial joint between condyle in lower jaw and the mandibular fossa of the temporal bone. Arthrology of the TMJ Is a synovial joint that permits a wide range of rotation as well as translation, an articular disk cushions the potentially large and receptive forces related to mastication. The disc separates the joint into two synovial joint cavities. -The inferior joint cavity is between the inferior aspect of the disc and Mandibular condyle. -The larger superior joint cavity is between the superior surface of the disc and the part of bone formed by the mandibular fossa. Anatomy of the TMJ Mandibular condyle Convex shape. It possesses short projections known as medial and lateral poles. The medial pole is more prominent than the lateral. Covered by a thin layer of fibrocartilage. Mandibular fossa Articular disk of the TMJ Dense fibrocartilage Lacks blood supply and sensory intervention Main function: generate greater congruency. The disc position protects the condyle as it slides forward during the later phase of opening the mouth widely. The articular disc maximizes the congruency within the TMJ to reduce contact pressure. It adds stability to the joint and helps guide the condyle of the mandible during the movement. Stability of the TMJ Passive stability Fibrous capsule provides more stability with lateral movements. Articular disc Lateral temporomandibular ligament Retrodiscal tissue (contains collagen and elastic tissue) Active stability Muscles Osteokinematics of the TMJ PROTRUSION-RETRUSION LATERAL EXCURSION DEPRESSION-ELEVATION Protrusion and Retrusion Anterior movement without significant rotation Essential component for maximum opening of the mouth Retrusion: opposite movement, important for closing the widely opened and protruded mouth. Lateral Excursion Side to side movement of the mandible. usually combines with rotations and other type of translations. Line of movement guided by mandibular fossa and articular disc. Depression and Elevation Depression causes the mouth to open. -Maximum opening occurs during yawning or singing. Elevation of the mandible closes the mouth - action used to grind food during mastication. TMJ Arthrokinematics Movement of the mandible typically involves bilateral action of the TMJs. Abnormal function in one joint naturally interferes with the function of the other. During rotational movement the mandibular condyle rolls relative to the inferior surface of the disc, and during the translational movement the mandibular condyle and disc slide essentially together. The disc usually moves in the direction of the translating condyle. Protrusion and Retrusion PROTRUSION: Mandibular condyle and disc translate anteriorly and slightly downwards, relative to the fossa. RETRUSION: Mandibular condyle and disc translate posteriorly and slightly upwards, relative to the fossa. Lateral excursion Primary: side to side translation of the condyle and disc within the fossa. Secondary: Slight rotational components. During left lateral excursion, the left condyle form a pivot point within the fossa as the right condyle rotates slightly anterior and medially. Depression and Elevation The arthrokinematics of opening the mouth are depicted for an early and a late phase. The early phase: -first 35% to 50% of the ROM. -Primalry rotation of the mandible relative to the temporal. -the condyle rolls posteriorly within the concave inferior surface of the disc. -the rolling motion swings the body of the mandible inferiorly and posteriorly.  The late phase: -consists of the final 50% to 65% of the total ROM. -this phase is marked by a gradual transition from primary rotation to primary translation. -during the translation the condyle and disc slide together in a forward and inferior direction. -At the end of opening, the axis of rotation shifts inferiorly. Kinetics of the TMJ Masseter -Bilateral contraction: Elevates the mandible to bring teeth into contact. -Unilateral contraction - slight ipsilateral excursion. Temporalis -Bilateral contraction elevate the mandible, and slightly retrude it. -Unilateral contraction of the temporalis, therefore, as when chewing in a side to side manner, causes ispilateral excursion of the mandible Medial pterygoid -Bilaterally: mandible elevation and slight protrusion. -Unilateral contraction: contralateral lateral excursion. Lateral pterygoid -Unilateral contraction produces: depression -Bilateral contraction: strong protrusion of the mandible * Special role of the superior head of the lateral pterygoid in adjusting disc position. Secondary muscles of mastication: suprahyoid and infrahyoid muscle. Main function: depression of the mandible. B D B A  A 2 B D B