view of a temporal bone with partially ossified fissures. The lateral half of the squamotympanic fissure is completely ossified (arrows). The superior stratum of the bilaminar zone can now insert only into the periosteum in this region. It has been shown that these fissures are ossified in more than 95% of patients with disk displacement, whereas in joints without disk displacement normal fissure formation prevails (Bumann et al. 1991).
However, the maturation process of these cells is delayed by functional demands (Kantomaa and Hall 1988). Loading reduces the intracellular concentration of cyclic adenosine monophosphate (cAMP). This increases the rate of mitosis and suppresses the ossification process relative to the proliferation of cartilage (Copray et al. 1985). Furthermore, the proteoglycane content of cartilage correlates with its ability to withstand compressive loads (Mow et al. 1992).
The hypothesis that structures of the temporomandibular joint are subjected to compressive loads during function has been around for many decades and is supported by a number of experimental studies (Hylander 1975, Hinton 1981, Taylor 1986, Faulkner et al. 1987, Boyd et al. 1990, Mills et al. 1994a). Studies using finite element analysis (FEA) also verify that during function, temporomandibular joint structures are subject to variable loads depending upon the individual static and dynamic occlusion (Korioth et al. 1994a, b). Different types of loads also bring about different responses in bone. When erosive changes are found in the condyle, the trabecular bone volume (TBV) of the temporal portion of the joint is significantly higher (25%) than when the condyle is unchanged (16%; Flygare et al. 1997).
30 Inferior view of the temporal cartilaginous joint surface and capsule attachment
Caudal view of the left temporomandibular joint of a newborn. The bony portions have been separated from the periosteum up to the circular bilaminar zone. Part of the zygomatic arch (1) can be seen near the right border of the photograph. The fibrocartilaginous articular surfaces over the articular protuberance are thickened medially and laterally (arrows). When covered with synovial fluid they allow movements with virtually no friction (Smith 1982).
31 Sagittal histological section showing buildup of the temporal joint components
The temporal portion of the joint can be divided into four functional components: 1 postglenoidal process, 2 glenoid fossa, 3 articular protuberance, and 4 apex of the eminence. As a rule, no cartilage can be identified within the fossa. The average thickness of the fibrous cartilage over the protuberance and the eminence is between 0.07 and 0.5 mm (Hansson et al. 1977). As this photograph shows, there can be considerable variation in thickness within the same individual.
32 Function and structural adaptation of the articular eminence
A summary of the basic anatomical changes in the temporal joint tissues. Increased functional loading will cause hypertrophy through secondary cartilage formation and bone deposition (progressive adaptation). Persistent nonphysiological loading (massive influences) leads to deforming or degenerative changes. This regressive adaptation is accompanied by more or less noticeable rubbing sounds, some times in combination with pain.
Mandibular Condyle
Human condyles differ greatly in their shapes and dimensions (Solberg et al. 1985, Scapino 1997). From the time of birth to adulthood the medial-lateral dimension of the condyle increases by a factor of 2 to 2.5, while the dimension in the sagittal plane increases only slightly (Nickel et al. 1997). The condyle is markedly more convex in the sagittal plane than in the frontal plane.
The articulating surfaces of the joint are covered by a dense connective tissue that contains varying amounts of chondrocytes, proteoglycans, elastic fibers and oxytalan fibers (Hansson et al. 1977, Helmy et al. 1984, Dijkgraaf et al. 1995). The composition and geometric arrangement of the extracellular matrix proteins within the fibrous cartilage determine its properties (Mills et al. 1994 a, b). Cartilage that can absorb and distribute compressive loads is characterized by a matrix with high water content and high molecular weight chondroitin sulfate in a network of type II collagen (Maroudas 1972, Mow et al. 1992). A low level of functional demand upon the joint leads to an increase of type I collagen and a reduction of type II (Pirttiniemi et al. 1996). Interleukin la inhibits the matrix synthesis of chondrocytes, while the transforming growth factor TGF-b promotes it (Blumenfeld et al. 1997). The collagen fibers of the fibrocartilaginous joint surfaces are oriented mainly in a sagittal plane (Steinhardt 1934).
33 Condyle dimensions
Left: Width of condyle in the frontal plane (Solberg et al. 1985). The average condylar width is significantly greater in men (21.8 mm) than in women (18.7 mm).
Center: Anteroposterior dimension of the central portion shown in the sagittal plane (Öberg et al. 1971; minimum and maximum in parentheses).
Right: Anteroposterior dimension of the condyle in the horizontal plane. There is no significant difference between men (10.1 mm) and women (9.8 mm).
34 Functional joint surface
Histological preparation showing a physiological fibrocartilaginous joint surface (thin arrows) of the condyle of a 58-year-old individual. In spite of the intact joint surface on the condyle, the pars posterior (1) of the disk is flattened and the functional fibrocartilaginous temporal surface of the joint on the articular protuberance shows degenerative changes (outlined arrows). The subchondral cartilage has not yet been affected and would appear intact on a radiograph.
35 Buildup of the condylar cartilage
Histologically, the secondary cartilage of the condyle is made up of four layers:
1 Fibrous connective-tissue zone
2 Proliferation zone with undifferentiated connective-tissue cells
3 Fibrous cartilage zone
4 Enchondral ossification zone
Other structures shown are:
5 Eminence
6 Disk
7 Condyle
Contributed by R. Ewers
Joint surface cartilage must permit frictionless sliding of the articulating structures while at the same time it must be able to transmit compressive forces uniformly to the subchondral bone (Radin and Paul 1971). Hypomobility of the mandible results in a more concentrated loading of the joint surfaces. Even if the forces in the masticatory system remain the same, the load per unit of area on the cartilage will be increased when there
