54 Histology of the bilaminar zone
The superior stratum (1), genu vasculosum (2), and inferior stratum (3) can be clearly distinguished from one another. Sensory and sympathetic nerve fibers provide pain perception and regulation of blood-vessel tonus. Here the neuropeptides A and Y effect vasoconstriction (Lundberg et al. 1990, Grundemar and Hakanson 1993) while vasodilation is brought about by the vasoactive intestinal peptide, the peptide histidine-isoleucine amide and acetylcholine (Widdicombel991).
55 Progressive adaptation (fibrosis)
Chronic overloading brings about fibrosis (arrows) and reduction of the number of blood vessels. Such fibrosis can be seen in 64-90% of patients, depending on the position of the disk. Posterior and posterosuperior condylar displacement without adaptation of the bilaminar zone is a common cause of joint pains. Therefore, previous adaptation of the bilaminar zone can be considered a favorable factor for treatment.
56 Function and structural adaptation of the bilaminar zone
In addition to supplying nutrients and proprioception, the inferior stratum is of special importance in stabilizing the disk in the sagittal plane. Increased functional loading can lead to its fibrosing. Our own studies indicate that in spite of mechanical loading, fibrosis does not occur in 10-36% of joints. Chronic nonphysiological overloading usually results in perforation, overextenion, or inflammation.
Joint Capsule
The bony parts of the temporomandibular joint are enclosed in a thin fibrous capsule. In addition to lateral, medial, and posterior capsule walls, there is an anterior wall that can be divided into upper and lower portions. The medial and lateral walls are reinforced by the similarly named medial and lateral ligaments (Schmolke 1994, Loughner et al. 1997). Attachment of the disk to the lateral and medial poles of the condyle is independent of the capsular structure (Fig. 60). The boundaries of the superior attachment of the capsule to the temporal bone are shown in Figure 30.
Because of its loose connective-tissue structure the anterior capsule wall cannot withstand as much loading as the other parts of the capsule (Koritzer et al. 1992, Johannson and Isberg 1991). The insertion of the capsule on the condyle is superficial and it lies at different levels on different sides of the condyle (Figs. 58, 61). Anterior disk displacements are accompanied not only by overextension of the inferior stratum, but also by stretching of the lower anterior capsule wall (Scapino 1983). The amount of extension is directly related to the amount of anterior disk displacement (Katzberg et al. 1980).
57 Joint capsule in the sagittal plane
By applying artificial traction on the specimen, the anterior portions of the upper and lower joint capsules (arrows) have been made more clearly visible. Posteriorly the joint spaces are bounded by the superior stratum (1) and inferior stratum (2). The posterior capsule wall lies behind the genu vasculosum. The type-III receptors of the capsule are only activated by heavy tensile loads on the lateral ligament and serve then to stimulate the elevator muscles (Kraus 1994).
58 Attachment of the capsule to the condyle
Schematic representation of the attachment of the joint capsule in the sagittal plane. The band-like insertion is significantly broader posteriorly than anteriorly. Because of the insertion of the lateral pterygoid muscle on the anterior surface of the condyle, the anterior part of the joint capsule attaches much higher anteriorly than posteriorly. The values given are based on measurements made on 39 human temporomandibular joints (Brauckmann 1995).
59 Overdistended capsule
Anterior disk displacement requires not only a stretching of the inferior stratum (1), but also a distention of the lower anterior wall of the joint capsule (arrows). However, because the connective tissue of the anterior capsule wall is much looser, disk displacement depends almost exclusively on posterior loading vectors and the adaptability of the inferior stratum. A downward movement of the condyle as shown here without downward movement of the disk is possible only with extensive stretching of the inferior stratum.
The interior surface of the capsule is covered by synovial membrane (Dijkgraaf et al. 1996a, b). The synovial cells form synovial fluid which serves to bring nutrients to the avascular cartilage of the joint surfaces and to reduce friction. Lubrication of the joint surfaces is accomplished through two mechanisms (Okeson 1998). One is the displacement of synovial fluid from one area to another by jaw movements. The other is the ability of the cartilage to store a limited amount of synovial fluid. Under functional pressure the fluid is again released to ensure minimal friction within the joint, in spite of static and dynamic toads (Shengyi and Xu 1991).
A second important function of the joint capsule is proprioception. Receptors are divided into four types (Wyke 1972, Clark and Wyke 1974, Zimny 1988). Type I have a low threshold, adapt slowly, provide postural information, and have a reflexive inhibiting effect on the antagonistic muscles. Likewise, type I have a low threshold but adapt quickly and provide information about movements. Type III have a high threshold and are slow to adapt. Type IV receptors stand ready for sensory pain perception and do not “fire” under normal conditions.
60 Disk and capule attachments in the frontal plane
Macroscopic anatomical preparation of a temporomandibular joint in the Frontal plane. Although the insertion of the disk on the condyle at the condylar poles has been described by some as an attachment through the joint capsule in the form of a “diskocapsular system” (Dauber 1987), other studies (Solberg et al, 1985, Bermejo et al. 1992) identify two separate connective-tissue structures for attachment to the condyle, one for the disk (1) and the other for the capsule (2).
61 Attachment of the joint capsule to the condyle
Schematic representation of the capsule attachment in the frontal plane. The collagen fibers of the disk and capsule insert somewhat lower on the lateral than on the medial surface of the condyle. It is not known to what extent the band of insertion is shifted superiorly when there is contracture of the capsule. However, shortening of the capsule walls does change the activity of the mechanoreceptors and thereby the activity of the muscles of mastication (Kraus 1994).
62 Function and structural adaptation of the joint capsule
The primary functions of the capsule are proprioception and nourishment of the fibrocartilaginous joint surfaces. Increased functional loading of the joint can result in either stretching or contraction