stratum is made up of tight collagen fibers (Rees 1954, Wilkes 1978, Luder and Bobst 1991). In the bilaminar zone the collagen fibers are more loosely organized and run more or less in the sagittal plane (Mills et al. 1994 b). The fibers of both strata stream into the pars posterior of the disk and there intertwine with the transverse fibers of the pars posterior and the sagittal fibers of the pars intermedia (Scapino 1983). The elastic fibers in the bilaminar zone have larger diameters than those of the disk and are concentrated predominantly in the superior stratum (Rees 1954, Scapino 1983, Mills et al. 1994a).
51 Macroscopic anatomical preparation
Left: With the jaws closed the bilaminar zone (1) fills the space posterior to both the pars posterior (2) I and the condyle (3). The inferior stratum stabilizes the disk on the condyle in the sagittal plane. An overextension of the bilaminar zone through posterosuperior displacement of the condyle is an essential precondition for an anterior disk displacement to occur.
Right: With the mouth open the genu vasculosum (1) fills with blood. The superior stratum (2) and inferior stratum (3) can be easily identified.
52 Variants of the postero-superior attachment
Left: Type A insertion. The superior stratum and the posterior joint capsule run separately to their insertions in the fissures. This type of insertion occurs most often in the medial portion of the joint.
Right: Type B insertion. Here the superior stratum and the posterior joint capsule merge before reaching the fissures and continue posterosuperiorly as one uniform, undifferentiated structure. This variant is the second most common in the medial portion of the joint.
53 Variants of the postero-superior attachment
Left: Type C insertion. The superior stratum inserts on the glenoid process because the fissures are completely filled by the posterior portion of the joint capsule. This type of insertion is found most frequently in the lateral part of the joint.
Right: Type D insertion. In this rare variant no posterior capsule structure can be demonstrated histologically. The posterior boundary is formed by the parotid fascia.
The superior stratum is attached posteriorly to the bony auditory meatus, the cartilaginous part of the auditory meatus, and the fascia of the parotid gland (Scapino 1983). Four insertion variations can be distinguished (Bumann et al. 1999).
The inferior stratum inserts on the posterior side of the condyle below the fibrocartilaginous articulating surface and is responsible for stabilizing the disk on the condyle. Anterior disk displacement is possible only when the predominantly collagenous inferior stratum becomes overstretched. The superior stratum, on the other hand, is responsible for retracting the articular disk, especially during the initial phase of closure, but is of lesser importance in the occurrence of anterior disk displacement (Eriksson et al. 1992). These facts are very important to consider in the diagnosis and treatment of disk displacements. Continuous posterior or posterosuperior loading of the bilaminar zone eventually leads to fibrosis and sometimes to the formation of a pseudodisk (Hall et al. 1984, Isberg et al. 1986, Kurita et al. 1989, Westesson and Paesani 1993, Bjornland and Refsum 1994).
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
