as the result of regressive or progressive adaptation. Therefore, the loads borne by the lateral and medial portions of the joint during function are also influenced by the morphology of the articular protuberance (Öberg et al. 1971, Hylander 1979. Hinton 1981).
44 Relationships in the medial part of the joint
Schematic drawing (modified from Christiansen et al. 1987) of the positional relationships in the medial portion of a left temporomandibular joint. This finding also emphasizes the fundamental principles of physiological joint movements. As with all other joints, the temporomandibular joint has a passive “play” space in all directions and is thus not confined to any border position. Average values: 1 = 3.4 mm; 2 = 4.4 mm
Articular Disk
The articular disk can be divided into three regions based upon their function: the partes anterior, intermedia, and posterior. The primary functions of the disk are to reduce sliding friction and to dampen load spikes (McDonald 1989, Scapino et al. 1996). The extracellular matrix of the Fibrocartilaginous disk consists primarily of type I and type II collagen (Mills et al. 1994b). The orientation of the collagen fibers in the disk displays a typical pattern (Knox 1967, Scapino 1983). In the pars intermedia dense bundles of collagen fibers run approximately in a sagittal direction. These intertwine with the exclusively transverse fibers of the pars anterior and pars posterior (Takisawa et al. 1982). Elastic fibers are found in all parts of the disk (Nagy and Daniel 1991) but are more numerous in the pars anterior and in the medial portion of the joint (Luder and Babst 1991). A reduction in the thickness of the disk results in an exponential increase in the load it experiences (Nickel and McLachlan 1994). The more rapidly a load is applied, the “stiffer” the disk reacts (Chin et al. 1996). The inferior stratum and the convexity of the pars posterior help stabilize the disk on the condyle.
45 Alignment of fibers within the disk and their attachment to the condyle
Macroscopic anatomical preparation of the disk-condyle complex of a right temporomandibular joint The collagen fibers of the pars posterior (1) and the pars anterior (2) run from the medial to the lateral pole of the condyle (Moffet 1984), making possible a wide range of movement of the disk relative to the condyle in the sagittal plane. The fibers of the pars intermedia (outlined area), on the other hand, run in a more sagittal direction. The medial pterygoid muscle (3) makes its insertion at the anteromedial region.
46 Cranial view
A view from above of the disk in Figure 45 after removal of the condyle, the fibers in the pars posterior (1) and pars anterior (2) can be seen more clearly. Histologically the disk is composed of dense collagenous connective tissue with a few embedded chondrocytes (Rees 1954). In the pars anterior and pars posterior the chondrocytes are found in clusters, but in the pars intermedia (outlined) they are arranged uniformly. Part of the bilaminar zone (3) can be seen attached at the distal border of the pars posterior.
47 Inferior view of the same disk
In this view the insertion of a portion of the superior head of the lateral pterygoid muscle (1) can be clearly seen. The remaining fibers of the superior head insert on the condyle. This preparation also demonstrates the insertion of the lateral (2), anterior (3), and medial (4) borders of the joint capsule. In the posterior part of the joint the capsule is connected to the posterior surface of the condyle by the stratum inferium (5) of the bilaminar zone (see p. 47).
Anatomical Disk Position
In a physiological temporomandibular joint, the pars posterior of the disk lies on the superior portion of the condyle. In the “centric condylar position” the thinnest part of the disk, the pars intermedia, is located between the anterosuperior convexity of the condyle and the articular protuberance (van Blarcom 1994). This finding is also supported by studies using measurements and mathematical models (Bumann et al. 1997, Kubein-Meesenburg 1985). The pars anterior lies in front of the condyle (Steinhardt 1934, Wright and Moffet 1974, Scapino 1983). The disk is attached to the medial and lateral poles of the condyle by means of the transversely aligned collagen fibers of the pars anterior and pars posterior. Viewed by itself, this anatomical arrangement with the condyle allows a great degree of movement during active mandibular movements (see p. 46). The disk exhibits viscoelastic properties under compressive loads. Its resistance is strengthened by the arrangement of the collagen fibers (Shengyi and Xu 1991). The elastic fibers within the disk serve primarily to restore the shape of the disk after a load has been removed (Christensen 1975).
48 Anterosuperior aspect of the disk-condyle complex
Macroscopic anatomical preparation of a left temporomandibular joint showing the relationship between disk and condyle. The lateral half of the disk has been removed for a clearer view. The dorsal border of the pars posterior is near the region of the apex of the condyle. From a functional point of view, this broad description is not very helpful for diagnostic purposes because the physiological position of the pars posterior depends to a large extent upon the inclination of the protuberance.
49 Anterolateral aspect of the disk-condyle complex
The same preparation in half profile. Here the pars posterior (1), pars intermedia (3), and pars anterior (2) can be clearly distinguished. Although the posterior border of the pars posterior lies over the apex of the condyle, the pars intermedia is in front of the anterosuperior convexity (arrows) of the condyle. The pars anterior is 2.0 mm thick, the pars intermedia 1.0 mm thick, and the pars posterior 2.7 mm thick (Gaa 1988).
50 Function and structural adaptation of the disk
Functionally, the disk serves as a “moveable fossa” for the condyle. Because of its unique tissue structure it can cushion and dampen peaks of force. Progressive adaptation differs from regressive in that the former is reversible. Strictly speaking, there is no “positive” tissue reaction in the disk because functional loads as well as continuous nonphysiological loads result in deformation.
Bilaminar Zone
The posterior portion of the temporomandibular joint has been variously referred to as the bilaminar zone (Rees 1954), retroarticular plastic pad (Zenker 1956), retroarticular pad (DuBrul 1988), retrodiskal fat pad (Murikami and Hoshino 1982), or trilaminar zone (Smeele 1988). It consists of an upper layer (superior stratum) and a lower layer (inferior stratum) (Rees 1954. Griffin and Sharpe 1962). Between these two layers lies the genu vasculosum with its numerous vessels, nerves, and fat cells (Griffin and Sharpe 1962). The superior stratum is composed of a loose network of elastic and collagen fibers, fat, and blood vessels (Zenker 1956). By contrast, the
