Temporomandibular joint (TMJ)
The temporomandibular joint (TMJ), also known as the gynglymoarthrodial joint (diarthrosis - freely movable joint), is a paired joint between the squamous part of the temporal bone and ramus of the mandible. Therefore, it is a connection between the lower jaw and skull.
This joint is located on the lateral part of the face - anterior to the auricle and tragus of the ear. As the TMJ provides a wide range of movements of the lower jaw, it also participates in the digestion process and is associated with the teeth.
Articulating structures of TMJ
Several structures articulate in the joint, and they include the following:
- Mandibular condyle and head of the mandible
- Mandibular (glenoid) fossa of the temporal bone
- Articular eminence (tubercle) of the temporal bone
- Articular disc
This joint is unique as the articulating surfaces of both bones actually do not articulate because they are separated by an articular disc.
The articular disc is a biconcave plate located within the joint cavity between the articular surfaces of the mandibular condyle and temporal bone. Its outer border is fused with the joint capsule.
The articular disc divides the joint cavity into two smaller joint cavities (superior and inferior) known as the floors or compartments. Therefore, the TMJ is a compound joint.
The superior compartment of the TMJ is limited by the temporal bone and articular disc (disco-temporal), while the inferior compartment is limited by the articular disc and mandible (disco-mandibular). The upper floor is a plain-gliding joint responsible for translational movements, while the lower floor is a hinge joint for rotational movements discussed in the next slides.
The inferior compartment is slightly smaller than the superior floor. Both contain synovial fluid. The articular surfaces are covered by fibrocartilage, not hyaline cartilage, like it is in other joints.
The articular disc is made of dense fibrous connective tissue. It contains three discrete zones - anterior, posterior and intermediate zones. The center of the disc is avascular, and it also does not contain any nerve tissue. The articular disc receives neurovascular supply from the periphery - from the area known as the retrodiscal pad.
The retrodiscal pad (or bilaminar zone) contains loose connective tissue rich in blood vessels and nerves. This area binds the articular disc to the condyle of the mandible and temporal bone by fibers known as the superior and inferior lamina. The retrodiscal pad helps to prevent posterior dislocation of the joint.