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 is hypomobility. The amount of structural change depends upon the amplitude, frequency, duration, and direction of the loads (Karaharju-Suvanto et al. 1996).
In joints that have undergone erosive changes, the percentage of trabecular bone volume (21%) and the total bone volume (54%) are significantly higher than the corresponding 15% and 40% found in condyles without these changes (Flygare et al. 1997). Degenerative changes therefore are closely associated with nonphysiological loading of the joint surfaces.
36 Intercondylar distance
Left: Sex-specific data on the distances between pairs of medial poles and lateral poles of the condyle (after Christiansen et al. 1987). The numbers given are average values. A difference of 5-10 mm in the Intercondylar distance will have a corresponding effect on the tracings of condylar movements and the accuracy of simulated movements in the articulator (see pp. 216 and 243).
Right: Schematic drawing illustrating the intercondylar angle.
37 Condylar shapes in the frontal plane
According to Yale et al. (1963) 97.1% of all condyles fall into one of four groups based upon their frontal profile. These are described as either flat (A), convex (B), angled (C), or round (D). The relative frequencies of occurrence are taken from the works of Yale et al. (1963), Solberg et al. (1985), and Christiansen et al. (1987). The condyle form affects the radiographic image of this part of the joint in the Schüller projection (Bumann et al. 1999) and the loading of the joint surfaces (Nickel and McLachlan 1994).
38 Function and structural adaptation of the condyle
Summary of the basic anatomical and functional changes in the condylar portion of the joint. Increased functional loading will stimulate cartilaginous hypertrophy (= progressive adaptation) that is not noticeable clinically. Continuous nonphysiological loading of the condyle can lead to degeneration, deformation, and even ankylosis (Dibbets 1977, Stegenga 1991). These changes may be accompanied by pain or, with sufficient adaptation, they may progress painlessly.
Positional Relationships of the Bony Structures
The position of the condyle