Temporomandibular joint

Definition

The temporomandibular joint (TMJ) is the bilateral synovial joint that articulates the mandible (lower jaw) to the temporal bone of the skull. It is unique in the human body as a ginglymoarthrodial joint, meaning it allows for both hinging (rotation) and gliding (translation) movements.

The two joints must function in unison to facilitate mastication, speech, and yawning.

Anatomy and Structure

1. Articular Surfaces

Mandibular Condyle: The superior-posterior part of the mandibular ramus.
Mandibular Fossa (Glenoid Fossa): The depression in the temporal bone.
Articular Eminence: The raised bony projection anterior to the fossa that guides the condyle during translation.

2. The Articular Disc (Meniscus)

The TMJ is divided into upper and lower compartments by a biconcave articular disc made of dense fibrocartilage (unlike most synovial joints which use hyaline cartilage).

The disc is avascular and non-innervated in its central load-bearing zone.
It is attached posteriorly to the retrodiscal tissue (bilaminar zone), which is highly vascular and innervated.

3. Ligaments

The joint is stabilized by several key ligaments:

Temporomandibular (Lateral) ligament: Prevents excessive posterior and lateral displacement.
Sphenomandibular ligament: An accessory ligament.
Stylomandibular ligament: Limits excessive protrusion of the mandible.

Biomechanics and Movement

TMJ movement occurs in two distinct phases:

Rotation (Hinging): Occurs in the lower compartment (between the condyle and the disc) during the first 20–25 mm of mouth opening.
Translation (Gliding): Occurs in the upper compartment (between the disc and the temporal bone) as the condyle moves forward down the articular eminence for wide opening.

Clinical Significance

Temporomandibular Disorders (TMD)

TMD is a broad term encompassing pain and dysfunction of the TMJ and the masticatory muscles.

Internal Derangement: Most commonly Disc Displacement with Reduction, characterized by a "click" or "pop" as the disc snaps back into position during opening.
Disc Displacement without Reduction: Often leads to "closed lock," where the patient has limited opening because the disc acts as an obstruction.

Dislocation (Subluxation)

Occurs when the condyle moves anteriorly past the articular eminence and becomes trapped, preventing the patient from closing their mouth.

Bruxism

Chronic grinding or clenching can lead to flattening of the condyle, perforation of the disc, or hypertrophy of the associated muscles (e.g., masseter muscle).