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 of the capsule. A chronic loading that exceeds the physiological limits activates the type-IV receptors through inflammation or rupture, resulting in pain.
Ligaments of the Masticatory System
The ligaments of the masticatory system, as in all other freely movable joints, have three main functions: stabilization, guidance of movement, and limitation of movement. From a functional view, limitation of movement is the most important function (Mankin and Radin 1979, Osborn 1995). There are different interpretations concerning the number and nomenclature of the ligaments found within the masticatory system (Sato et al. 1995). Five or six ligaments have been described: lateral ligament, stylomandibular ligament, sphenomandibular ligament, discomalleolar (Pinto’s) ligament and Tanaka’s ligament. Sometimes the collateral attachment fibers between disk and condyle are included in the list as the lateral and medial collateral ligaments of the disk (Yung et al. 1990, Kaplan and Assael 1991, Okeson 1998), although from a functional viewpoint, this is not accurate.
The lateral ligament or temporomandibular ligament is made up of two parts: a deep, more horizontal part and a superficial, more vertically oriented part (Arstad 1954, Sicher and DuBrul 1975, Kurokawa 1986). The horizontal part limits retrusion (Hylander 1992) as well as laterotrusion (DuBrul 1980) and thereby protects the sensitive bilaminar zone from injury. The vertical part of the lateral ligament, on the other hand, limits jaw opening (Osborn 1989, Hesse and Hansson 1988). The superficial portions of the lateral ligament contain Golgi tendon organs (Thilander 1961). These nerve endings are very important for the neuromuscular monitoring of mandibular movements (Hannam and Sessle 1994, Sato et al. 1995). For this reason, anesthetizing the lateral portion of the joint permits a 10-15% increase in jaw opening (Posselt and Thilander 1961).
Lateral ligament
63 Situation with jaws closed
Unlike in formalin-fixed preparations, the lateral ligament (arrows) is usually clearly distinguishable in fresh preparations. The initial rotation during an opening movement is limited by the superficial part of the lateral ligament (von Hayek 1937, Burch and Lundeen 1971). Further opening of the jaws can occur only after protrusion has relieved tension on the ligament, following which the ligament is again stressed by renewed rotation (Osborn 1989).
64 Situation with jaws open
jaw opening is restricted by the length of the lateral ligament from its origin to its insertion. However, if the condyle can slip past the apex of the tubercle (eminence), the ligament (arrows) will no longer have this limiting effect. In addition, the lateral ligament will now impede retrusive and laterotrusive movements of the condyle (Posselt 1958. Brown 1975, Osborn 1989).
65 Function and structural adaptation of the ligaments
The chief function of the ligaments is to limit movement and thereby protect sensitive structures. In addition, they stabilize the joint and take over guidance functions (Rocabado and Iglarsh 1991). Depending upon the proportions of the types of collagen within the ligament and the direction of the functional overload on the joint, ligaments may become either stretched or shortened.
The stylomandibular ligament is a part of the deep fascia of the neck and runs from the styloid process to the posterior edge of the angle of the mandible. While part of the ligament inserts onto the mandible, its largest part radiates into the fascia of the medial pterygoid muscle (Sicher and DuBrul 1975). Although the stylomandibular ligament is relaxed during jaw opening, it restricts protrusive and mediotrusive movements (Burch 1970, Hesse and Hansson 1988).
