Internal derangements represent a group of functional disorders that arise from abnormalities in the anatomy and/or positional relationships of the TM joint structures. A review of TMJ anatomy demonstrates that the disc is attached to the poles of the condyle by the medial and lateral collateral ligaments. Many internal derangement disorders arise from alterations of the integrity or lengths of these ligamentous attachments. Once these ligaments become elongated the disc is allowed more freedom to move within the joint. The disc will often begin to assume an anteromedial position in relationship with the condyle (Fig. 2.4). When the disc is in this more forward and medial position, function of the joint can be somewhat altered. As the mouth opens and the condyle moves forward, a short distance of translatory movement can occur between the condyle and the disc until the condyle once again assumes its normal position on the thinnest area of the disc (intermediate zone). Once it has translated over the posterior surface of the disc to the intermediate zone, inter-articular pressure due to joint loading maintains this relationship and the disc is again carried forward with the condyle through the remaining portion of the translatory movement. Upon closing, the disc reassumes its abnormal position on the condyle in the closed joint position. Once in the closed joint position, the disc is again free to move according to the demands of its functional attachments. In this condition, the disc will assume the most anteromedial position allowed by the discal attachments and its own morphology.

As the disc is more chronically repositioned forward and medially by action of the superior lateral pterygoid muscle, the discal ligaments are further elongated. With continuous thinning of the posterior border of the disc and further elongation of the disc ligaments, the disc can move through the discal space and be trapped anterior to the condyle. (Fig. 2.4). When this occurs, the condyle can now function or load the retrodiscal tissues which may be associated with pain.

The important feature of this functional relationship is that the condyle translates across the disc to some degree when movement begins. This type of movement does not occur in the normal joint. During such movement the increased interarticular pressure may prevent the articular surfaces from sliding across each other smoothly. The disc can stick or be bunched slightly, causing an abrupt movement of the condyle over it into the normal condyle-disc relationship. A clicking sound often accompanies this abrupt movement. Once the joint has clicked, the normal relationship of the disc and condyle is reestablished and this relationship is maintained during the rest of the opening movement. A second click can occur as the disc is re-displaced during the later stages of closing the mouth. This is called “reciprocal clicking” [50]. As the disc displacement progresses, the condyle actually begins to function behind the disc with loading occurring on the retrodiscal tissues (Fig. 2.4).

As the disc is more chronically repositioned forward and medially, the discal ligaments are further elongated. With continuous thinning of the posterior border of the disc and further elongation of the disc ligaments, the disc can move further forward and be trapped anterior to the condyle. This often is accompanied by folding of the disc into a ball-like shape. When this occurs it will initially lead to a decrease in how far the condyle can move forward. Therefore the patient will have the sensation that he or she cannot open the mouth completely. This has been called a “closed lock” (Fig. 2.5) [50] since the patient feels he or she is locked near the closed mouth position. Patients may report pain when the mandible is moved to the point of limitation, but pain does not always accompany this condition [51–54].

If a closed lock continues, the condyle will be constantly positioned on the retrodiscal tissues. These tissues are not anatomically structured to accept forces, but they often remodel to form a functional pseudodisc. However, as force is applied, some likelihood arises that these tissues may break down in some patients [55–57]. With this breakdown comes tissue inflammation and pain (retrodiscitis).

It is important to appreciate that pain is not always a factor with these conditions. Pain is not generated by the disc, since it is aneural. The structures that are able to produce pain are the connective tissues such as the ligaments and the very highly innervated retrodiscal tissues. If these structures are loaded quickly due to a sudden disc shifting, pain is likely. However, if the changes occur slowly over time, these tissues are often able to adapt and pain may not be associated.

When internal derangements of the TMJ occur, the structures affected by these changes often respond. The most common tissues affected are the retrodiscal tissues and the articular surfaces of the condyle and articular eminence. These changes can result in adaptation or destruction, depending upon many factors. Some of these factors are the acuteness of the changes as well as the intensity and duration of the loading. There are also important biologic and genetic factors that may regulate the patient’s ability to repair tissues.

If the articular surfaces of the condyle become affected, the subarticular bone receives additional loading and changes can occur. Similar changes also can occur in the absence of internal derangements. These changes represent a group of disorders that are considered joint arthritides. The most common type of TMJ arthritis is osteoarthritis (sometimes called degenerative joint disease). Osteoarthritis represents a destructive process by which the bony articular surfaces of the condyle and fossa become altered. It is generally considered to be the body’s response to increased loading of a joint [95]. As loading forces continue and the articular surface becomes softened (chondromalacia), the subarticular bone begins to resorb. Progressive degeneration eventually results in loss of the subchondral cortical layer, bone erosion, and subsequent radiographic evidence of osteoarthritis [96]. It is important to note that radiographic changes are only seen in later stages of osteoarthritis and may not reflect the clinical symptoms accurately.

Osteoarthritis is often painful, and jaw movement accentuates the symptoms. Crepitation (multiple grating joint sounds) is a common finding with this disorder. Osteoarthritis can occur any time the joint is overloaded, but is most commonly associated with disc displacements [97, 98] or perforation [99]. Once the disc is displaced and the retrodiscal tissues break down, the condyle begins to articulate directly with the fossa accelerating the destructive process. In time, the dense fibrous articular surfaces are destroyed and bony changes occur. Radiographically, the surfaces seem to be eroded and flattened. Any movement of these surfaces creates pain, so jaw function usually becomes very restricted. Although osteoarthritis is in the category of inflammatory disorders, it is not a true inflammatory condition. With appropriate treatment and reduction of joint loading, the arthritic condition can become adaptive. The adaptive stage has been referred to as osteoarthrosis [95, 100].

Other types of arthritides can certainly affect the temporomandibular joint. The most common after osteoarthritis is rheumatoid arthritis. This is thought to be an autoimmune disorder and therefore has its origin in systemic factors. The juvenile form of this problem can produce significant joint changes as well as serious occlusal problems. Other common causes of TMJ arthritis are traumatic arthritis, infectious arthritis, psoriatic arthritis, and hyperuricemia (gout). These conditions are not reviewed in this chapter. Other texts can be reviewed for a more thorough review of TMJ arthritides [2, p. 317–61].