The TMJ is a synovial joint. All synovial joints have a capsule composed of dense, irregular connective tissue on the outer surface and what is known as a synovial membrane on the inner surface. The outer capsule stabilizes the joint by limiting movement and as the fibers are interwoven forms an impervious shell around the joint. The synovial membrane has cells that make synovial fluid which is about the consistency of an egg white and acts as a lubricant for the joint. To further reduce friction the mandibular condyle and the mandibular fossa are both covered in smooth hyaline cartilage.
The articular capsule is not the only connective tissue that limits movement of the joint. The stylomandibular ligament extends from the styloid process to the inferior-posterior surface of the mandible. This ligament angles posteriorly from the mandible it limits forward movement of the joint. A second ligament, the sphenomandibular ligament runs from the spine of the sphenoid bone vertically to the lingula which is located, as mentioned previously, on the inner surface of the ramus where the mandibular foramen is found. This ligament runs vertically so limits the downward movement of the mandible. The last important ligament of the joint is the temporomandibular ligament which runs from the zygomatic process of the temporal bone back to the posterior portion of the condylar process. Given that it is angled posteriorly as it descends to the mandible it limits movement of the joint in a posterior as well as inferior direction.
There are also bone limits to the movement as the condyle sits in a hollow, the articular fossa. The posterior wall of the fossa is known as the post-glenoid process as the other name for the mandibular fossa is the glenoid fossa. Anterior to the fossa is the articular eminence which is not so much a stop as a hinderance. As we will soon see the mandible slides down this process during opening. However, if the limiting structures do not work properly the condyle can get across the eminence and the person will not be able to close as the eminence now acts to limit posterior movement. To return the condyle to the fossa one must push the mandible both inferiorly and posteriorly simultaneously.
One last piece of the puzzle is there is articular cartilage found between the two bones. Known as the meniscus, this is a tough piece of fibrocartilage that is draped over the condyle with extensions on the medial and lateral to prevent it dislodging in those directions. Posteriorly it is connected by connective tissue to the joint capsule which is firmly attached the temporal bone and anteriorly it is connected to the superior head of the lateral pterygoid muscle. As the mandible is moved anteriorly the cartilage is pulled by the muscle and the posterior connective tissue which was like a loose rope gets tauter. This limits the movement of the cartilage anteriorly. Keeping the cartilage centered on the condyle takes some coordination between the structures pulling on and limiting movement of both structures and often goes awry.
To tie this together one must realize that the lower compartment of the joint behaves like a normal condyloid joint and can both rotate in a horizontal axis around the joint but also vertically thus allowing one to protrude one side by pulling it down the articular eminence (upper compartment movement) while the other side stays in the fossa but rotates (lower compartment movement). Opening generally combines both the lower compartment rotation and the upper compartment translation. To fully understand the TMJ you must understand the muscles that move it also but that is outside the scope of this section but will be covered in the section on muscles.Diagram Reference Guide