Anatomy of the Temporomandibular Joint

The TMJ is the articulation between the condyle of the mandible and the glenoid fossa of the temporal bone (Figure 10-1). It is a highly specialized joint that differs from other joints because of the fibrocartilage that covers the bony articulating surfaces, its ginglymoarthrodial (rotational and translational) movement, the fact that its function and overall health are dictated by jaw movement, and its dependence on the contralateral joint. An articular disk is interposed in the space between the temporal bone and the mandible. This disk divides the space into an upper compartment (superior joint space) and a lower compartment (inferior joint space) (Figure 10-1).

Translational movements occur in the upper compartment, whereas the lower compartment functions primarily as the hinge or rotational component. The superior and inferior spaces contain synovial fluid, which is produced by the synovial membrane that lines the joint. The synovial fluid provides nourishment and lubrication of the avascular structures. The articular disk is attached to the lateral and medial aspects of the condyle, to the superior belly of the lateral pterygoid muscle, and to the joint capsule (Figure 10-1). The articular disk and the bony surfaces are avascular (i.e., they do not contain blood vessels) and devoid of nerve fibers. The joint is further surrounded and protected by the fibrous connective tissue joint capsule. The primary innervation of the TMJ capsule and disk attachments is from the auriculotemporal nerve with secondary innervation from the deep temporal and masseteric nerves. The function of the disk is to separate the forces resulting from rotation and translation, absorb shock, improve the fit between bony surfaces, protect the edges of the articulating surfaces, distribute weight over a larger area, and spread the lubricating synovial fluid.

Understanding the location and action of the muscles of mastication is important in the evaluation of disorders affecting the TMJ. Palpation of these muscles during a clinical evaluation is done to determine whether muscle spasm or dysfunctional muscle activity is occurring. The muscles of mastication comprise major muscles about the facial region that govern the movement of the mandible. They include the masseter, temporalis, medial pterygoid, lateral pterygoid, anterior digastric, and mylohyoid (suprahyoids) (Figures 10-2 to 10-4). The function of these muscles is to create the mandibular envelope of motion. Three of these muscles, the masseter, medial pterygoid, and temporalis, are elevator muscles that, when activated, close the mandible. The opening, or depressor, function is accomplished mainly by the lateral pterygoid muscle with some help from the anterior digastric muscle. Studies have shown that the two components of the lateral pterygoid muscle are active at different times in the functioning of the mandible (Figure 10-4). The superior portion of the muscle seats the articular disk on the eminence of the articulating surface. The inferior belly is attached to the mandibular condyle and functions during mouth opening.

Normal Joint Function

The harmonious function of the TMJ depends on various factors. The anatomic relationship of the condyle-disk complex governs the smooth functioning of the mandible. This articulation, along with the muscles of mastication, provides the movement of the mandible. The muscles of mastication are the machinery that powers mandibular movement; the anatomic joint structures such as the condyle, articular eminence, and disk act as the gears or bearings of the jaw.

In normal joint function the jaw begins at a rest position of maximal occlusal contact. In this position the mandibular condyle rests within the glenoid fossa, with the articular disk situated between the condyle, roof of the glenoid fossa, and articular eminence (see Figure 10-1, A and B). The first phase of opening is characterized by rotational (hinge) movement of the condyle, followed by anterior translation (sliding movement) to approximately the anterior peak of the articular eminence. During translation the disk assumes a more posterior position in relation to the condyle. The inferior and superior joint spaces assume different configurations during each of these movements.

Temporomandibular disorders (TMDs) are caused by abnormalities in the functioning of the TMJ or associated structures. TMDs, which have been a clinical and diagnostic challenge in dentistry for many years, are reported to affect up to 36 million adults in the United States. Hippocrates documented jaw dysfunction problems as early as the fifth century BCE. However, it was not until 1934 that James Costen, an otolaryngologist, described a group of symptoms centered around the jaw and ear. Costen syndrome included such symptoms as impaired hearing, ear pain, tinnitus, dizziness, burning in the throat and tongue, headache, and trismus. Although most if not all of Costen’s explanations have been refuted, the dental profession’s interest was stimulated by his view that malocclusion plays a pivotal role in this condition. The 1980s were marked by a growing interest in TMDs facilitated by advances in pain neurophysiology, multicenter research, the American Academy of Craniomandibular Disorders (later to become the American Academy of Orofacial Pain), the American Dental Association, and pivotal work published in 1986 by Sanders for the treatment of closed lock by arthroscopy.

Although up to 75% of the adult population has at least one sign, with 33% having at least one symptom, most studies suggest that clinically significant TMD-related jaw pain, dysfunction, or both affects 10% to 16% of the adult population. The majority of patients with TMDs are female aged between 20 and 40 years, and some researchers suggest that female sex hormones may have a role in the pathogenesis. Furthermore, TMDs are often associated with comorbid conditions such as depression, rheumatoid arthritis, chronic fatigue syndrome, chronic headache, fibromyalgia, sleep disturbances, and irritable bowel syndrome.