TMJ is a compound craniomandibular joint in which the articular disc serves as a third non-ossified bone hat permits the complex movements of the joint. The articular disc is composed of dense fibrous connective tissue which is non-innervated and non-vascularized for the most of it.

Posteriorly – The disc is attached to the highly vascularized and innervated connective tissue called as retrodiscal tissues.

Posterosuperiorly: The retrodiscal lamina rich in elastic fibers attachs the disc to the tympanic plate.

Posteroinferiorly: The inferior retrodiscal lamina rich in collagenous fibers attaches the disc to the posterior margin of the articular disc of the condyle.

Anteriorly- The disc is attached to the capsular ligaments.

Antero-superiorly – Capsular ligaments attaches the disc to the anterior margin of the articular surface of the temporal bone.

Anteroinferiorly – Capsular ligament attaches the disc to the anterior margin of the articular surface of the condyle.

In between the capsular ligaments anteriorly the disc is also attached by the tendinous fibers to the superior lateral plerygoid muscle. The attachments of the capsular ligaments anteriorly, posteriorly and medially, laterally divides the joint into upper and lower cavities. The internal surfaces of the cavities are bound by the specialized endothelial cells that form synovial lining. These cells along with the synovial fringe located at the anterior border of the retrodiscal tissues, forms synovial fluid that fills the joint cavities.

Synovival fluid acts as

1. Medium for providing metabolic requirement to the non-vascular tissues of the articular surface.

2. Lubricant in bringing about smooth frictionless movements between the articular surfaces of the disc, condyle and fossa.

It gives the disc tensile strength and stiffness and maintain the shape. They can resist tension parallel to their orientation. The collagen fibers mainly run anteroposteriorly in the inter mediate zone. Thus presenting greater tensile modulus and strength anteroposteriorly than mediolateraly in the intermediate zone (6-8). In the anterior and posterior bands the collagen fibres run mainly mediolaterally thus presenting larger tensile modulus and strength in the mediolateral direction. Collagen fibers exhibit waviness. In tension there is first straightening of the waves without the lengthening of collagen fibers (9). On application of further load, the collagen fibers begin to extend and become load bearing. Also, the collagen network present slight permeability to interstitial fluid (10,11). On loading, the disc initially bears it by the pressurization of the incompressible fluid without much deformation of the collage network (12). However, on further load application, the permeable collagen network transmits the fluid. Thus, transferring the load the collagen fibers. These collagen fibers rearranges once the water is squeezed out (13,14).This squeezing out of water and deformation to collagen network is reversible if the disc is not deformed beyond the physicologic strain range (15). This pumping action and diffusion of synovial fluid through the disc into the joint cavity helps in nourishing the joint cartilage (weeping lubrication) (16). However, it has been reported that after unloading some of the energy used to deform the disc is not released immediately. According to Beek et al the return of tissue fluid from outside the disc and recovery of the disc to its original shape is relatively slow (17). This residual strain could be an important factor in the permanent deformation of the disc (Clenching).

These consists of a core of protein attached to glycosaminoglycans (GAG) sulphate side chains. These are hydrophillic, stiff viscoelastic material with a large molecular size that are intertwined through out the collagen network. As these are hydrophilic in nature, they tend to bind water, leading to expansion of the matrix. The tension in the collagen network counteracts the pressure caused by the expansion of the matrix, thus supporting the joint loading. Further loading leads to squeezing out of the fluid. New equilibrium is therefore achieved which is reversed on the release of load. Thus proteoglycans indirectly modulate the stiffness of the collagen network.

There are a group of

a) Small proteoglycans – Core proteins to which one/two side chains are attached (18,19).

b) Large proteoglycans – Aggrecan contains both chondroitin sulphate and keratan sulphate (20).

The large proleoglycans are present in greater concentration in the central part of the intermediate zone and the anterior, posterior bands of the disc that encounters heavy compressive joint loading, thus maintaining the resilience of the disc (21,22,23). During compression these larger molecules interfere with the flow of the fluid out of the disc resulting in increased compressive modulus compared to the medial and lateral regions of the intermediate zone, that contain greater concentration of smaller proteoglycan. When loading occurs in the jaw-closed position, the deformations in the disc are spread through out the entire intermediate zone, while translation of the condyle in the forward direction to obtain a protrusive or open jaw position leads to a concentration of the deformation in the lateral part of the disc.

Proteoglycans are largely responsible for the compressive modulus of the disc and collagen fibers for its tensile modulus. Compressive modulus is generally smaller (14-17 Mpa) than the tensile modulus (22-26 Mpa). Also, it has been seen that the static loading reduces proteoglycan synthesis and dynamic loading increase it. This is considered as an important factor in maintaining the homeostasis of the joint cartilage (Burger et al, 1992, Kukoskio et al, 1997; Quim et al 1998) (24-26).

Shear stresses may also occur in the disc in the range of (1.0Mpa -1.75Mpa) as the articular surfaces compressing the disc are not parallel. The shear stresses in the disc are sensitive to the frequency and direction of joint loading (Mowet at 1992).

It was found that Ca²⁺ and phosphorus content of the articular disc increased progressively with aging where as the sulphus contents decreased lightly with aging.

Calcification of the disc of the TMJ were studied by Masato Jibiki et al in 1999 and stated that the calcifications were recognized more frequently posteriorly than anteriorly, and were related to disc perforations. They suggested that the disc degeneration which may occur as a result of aging or mechanical stress, causes calcifications.

The glycosaminoglycan content relative to the tissue fluid markedly increases with age. This increase in the content of sulphated glycosaminoglycan may elevate the osmotic pressure, thus increasing the compressive stiffness of the disc (Nakano & Scott 1996).

The collagen content of the disc increases with age relative to the water content which remains constant. This leads to increase in elasticity of the disc as the aging occurs (23).