Temporomandibular Disorders. Robin J. M. Gray

2 Clinical Aspects of Anatomy, Function, Pathology, and Classification

      The joint anatomy, histology, structure, capsule, synovial membrane, and fluid, ligaments

The articulatory system comprises the temporomandibular joints (TMJs) and, intra‐articular discs, mandibular/jaw muscles and occlusion.

      In the simplest terms, the temporomandibular joint is the articulation between the upper and lower jaws. The teeth form the contacts between the upper and lower jaws, and the muscles are the motors that move the mandible. This system is unique in that the TMJs are paired; any stimulus that affects one joint or any other single part of the articulatory system can have a ‘knock‐on effect’ in the rest of the system.

      It is important to have an understanding of anatomy not only to be able to differentiate between what is physiological and what is pathological but also to understand the objectives of some treatment options.

The TMJ (Figure 2.1) is a synovial diarthrodial joint, which means that the joint is lubricated by synovial fluid, and the joint space is divided into two separate compartments by means of an intra‐articular disc. The movements that take place in the compartments are predominantly a sliding movement in the upper joint space between the upper surface of the disc and the inferior surface of the glenoid fossa, and a rotational movement in the lower joint space between the head of the condyle and the undersurface of the intra‐articular disc. Unlike the articular surfaces of other synovial joints, where the surfaces are typically lined by hyaline cartilage, the articular surface of the TMJ is covered by a layer of fibrocartilagenous tissue. It was thought that this arrangement reflected a non‐load bearing functional role for the TMJ; however, a more likely explanation is that, because the covering layer of the condyle is derived from intramembranous ossification, rather than endochondrol ossification, it therefore lacks the endochondrol template from which hyaline articular cartilage is derived.

Figure 2.1 The temporomandibular joint

      (M. Ziad Al‐Ani, Robin J.M. Gray.)

Histology

There are four distinct layers or zones described in the articular surface of the condyle and mandibular fossa. These layers are the articular zone, proliferative zone, cartilagenous zone, and calcified zone (Figure 2.2):

      1 The articular zone is dense fibrous connective tissue and forms the outer functional surface of the condyle head. As a result of this fibrous connective tissue layer, it is suggested that it is less susceptible to the effect of ageing and breakdown over time. In addition, despite a poor blood supply, it has a better ability to repair, good adaptation to sliding movement, and the ability to act as a shock absorber when compared with hyaline cartilage.

      2 The proliferative zone is mainly cellular and is the area in which undifferentiated germinative mesenchyme cells are found. This layer is responsible for the proliferation of the articular cartilage and the proliferative zone is capable of regenerative activity and differentiation throughout life.

      3 The cartilagenous zone contains collagen fibres arranged in a criss‐cross pattern of bundles. This offers considerable resistance against compressive and lateral forces but becomes thinner with age.

      4 The calcified zone is the deepest zone and is made up of chondrocytes, chondroblasts, and osteoblasts. This is an active site for remodelling activity as bone growth proceeds.

Figure 2.2 The four distinct zones described in the articular surface of the condyle and mandibular fossa

      (M. Ziad Al‐Ani, Robin J.M. Gray.)

The joint capsule

The joint capsule (Figure 2.3) envelops the articular disc and is attached superiorly to the rim of the glenoid fossa and articular eminence and inferiorly to the neck of the condyle. Posteriorly it is attached to the bilaminar zone and anteriorly becomes continuous with the pterygoid muscle attachment. Although it is thin both anteriorly and posteriorly, it is strengthened laterally by the lateral temporomandibular ligament which is not a discrete ligament but a thickened part of the capsule.

Figure 2.3 The joint capsule

      (M. Ziad Al‐Ani, Robin J.M. Gray.)

Synovial membrane

The glistening inner surface of the capsule comprises the synovial membrane. At birth, this membrane covers all internal joint surfaces but is lost from articular surfaces as function commences. The flexibility of the inner surface of the capsule is increased by finger‐like projections (villi) of the synovial membrane which disperse the synovial fluid.

      The function of the synovial membrane is considered to be

       regulatory because it controls electrolyte balance and nutrients

       secretory via the interstitial cells

       phagocytic.

      Synovial fluid is a clear, pale‐yellow, viscous solution secreted by the synovial tissues and consists mainly of an ultrafiltrate of plasma enriched with a proteoglycan‐containing hyaluronic acid synthesised by synovial cells. The high viscosity of this fluid is a result of the presence of sodium hyaluronate which provides lubrication. The synovial fluid also allows removal of degradation products from the joint space, lubrication of the joint surfaces, and nutrition of the vascular parts of the joint.

Ligaments

       The temporomandibular ligament

The temporomandibular ligament is a strong band of fibrous tissue originating as a thickening of the lateral aspect of the joint capsule.

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