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 inferior 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