TMJ Biomechanics: Understanding the Temporomandibular Joint

TMJ Biomechanics

• The Temporomandibular Joint (TMJ) is a bilateral action joint that is one of the most continuously used joints in the body.
• It is responsible for mastication, talking, and swallowing, and requires synchrony and synergy for correct jaw function.

Anatomy of the TMJ

• The TMJ is a bicondylar synovial joint between the condyle of the lower jaw (mandible) and the mandibular fossa of the temporal bone.
• The joint has a wide range of rotation and translation, and is cushioned by an articular disc that separates the joint into two synovial joint cavities.
• The inferior joint cavity is between the inferior aspect of the disc and the mandibular condyle.
• The larger superior joint cavity is between the superior surface of the disc and the segment of bone formed by the mandibular fossa.

Mandibular Condyle

• The mandibular condyle has a convex shape and possesses short projections known as 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 mandibular fossa is a segment of bone formed by the temporal bone.

Articular Disc

• The articular disc is a dense fibrocartilage that lacks blood supply and sensory innervation (except the periphery).
• The disc's main function is to generate greater congruence and reduce contact pressure.
• The disc's thickness varies between its anterior and posterior regions.
• The disc position protects the condyle as it slides forward across the articular eminence during the later phase of opening the mouth.
• The disc also adds stability to the joint and helps guide the condyle of the mandible during movement.

Stability of the TMJ

• Passive stability is provided by the fibrous capsule, articular disc, lateral temporomandibular ligament, and retrodiscal tissue.
• Active stability is provided by the musculature.

Osteokinematics of the TMJ

• The TMJ has three main movements: protrusion-retrusion, lateral excursion, and depression-elevation.
• Protrusion and retrusion involve anterior and posterior translation of the mandible without significant rotation.
• Lateral excursion involves side-to-side translation of the mandible, with an average of 11mm translation considered normal.
• Depression and elevation involve opening and closing of the mouth, with a 50mm average opening from the frontal area.

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 translational movement, the mandibular condyle and disc slide essentially together.
• The disc usually moves in the direction of the translating condyle.

TMJ Arthrokinematics-Protrusion and Retrusion

• Protrusion involves anterior translation of the mandibular condyle and disc.
• Retrusion involves posterior translation of the mandibular condyle and disc.

TMJ Arthrokinematics-Lateral Excursion/Deviation

• Lateral excursion involves side-to-side translation of the condyle and disc within the fossa.
• Secondary rotational components occur during lateral excursion.

TMJ Arthrokinematics: Depression and Elevation

• Depression and elevation involve a combination of rotation and translation among the mandibular condyle, articular disc, and fossa.
• The early phase of opening involves primary rotation of the mandible relative to the temporal.
• The late phase of opening involves a gradual transition from primary rotation to primary translation.

Kinetics of the TMJ

• Masseeterus muscle has deep and superficial heads with similar functions.
• Bilateral contraction of the masseterus elevates the mandible and brings teeth into contact, and also protrudes the mandible slightly.
• Unilateral contraction of the masseterus causes slight ipsilateral excursion.

Kinetics of the TMJ (continued)

• Temporalis muscle has bilateral contraction that elevates the mandible and slightly retrudes it.
• Unilateral contraction of the temporalis causes slight ipsilateral excursion of the mandible.
• Medial pterygoid muscle has two heads with similar functions.
• Bilateral contraction of the medial pterygoid elevates the mandible and slightly protrudes it.
• Unilateral contraction of the medial pterygoid causes slight ipsilateral excursion of the mandible.