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 relative to the articular protuberance has been a subject of controversy in dentistry for many years (Lindblom 1936, Pullinger et at. 1985). A well-defined condylar position oriented to the maximal occlusion is especially relevant to extensive dental treatment (Spear 1997). In the past, to transfer the jaw relations to an articulator the condyles were always placed in their most posterosuperioi position because this relationship could be most easily reproduced (Celenza and Nasedkin 1979). Under purely static conditions the condylar position is dependent upon the shape of the fossa, the inclination of the protuberance, and the shape of the condyle. In the 1970s this led to the assignment of a geometric centric position of the condyle in the fossa (Gerber 1971). However, the dimensions of the joint space are quite variable in both the sagittal plane (anterior, posterior, and superior) and the transverse plane (medial, central, and lateral) (Pullinger et al. 1985, Hatcher et al. 1986, Christiansen et al. 1987, Bumann et al. 1997). For this reason the concept of an anatomical orientation is untenable, and the radiographic techniques (p. 148) are unsuitable for determining a therapeutic condylar position (Pullinger and Hollender 1985). Therefore the current definitions of centric relation are geared more toward the functional conditions (van Blarcom 1994, Dawson 1995, Lotzmann 1999). It has been demonstrated experimentally that the surfaces of the temporomandibular joint are subjected to loads of 5-20 N (Hylanderl979, Brehnan et al. 1981, Christensen et al. 1986). In a patient’s habitual occlusion this force is partially intercepted by the occluding premolars and molars. Tooth loss can lead to higher joint loading and regressive adaptation (van den Hemel 1983, Christensen et al. 1986, Seligman and Pullinger 1991). However, if the joint’s capacity for adaptation is sufficiently great, degenerative changes may be avoided (Helkimo 1976. Kirveskari and Alanen 1985. Roberts et al. 1987). The direction of functional loading is anterosuperior against the articular protuberance (Dauber 1987). Clear evidence for this is the presence of the load-induced secondary cartilage on the joint surfaces in this region.
39 Sagittal relationships
Macroscopic anatomical preparation showing the relation of the fossa, disk, and condyle to one another in the sagittal plane. Because the shapes of fossae and condyles vary so greatly, it is not possible to determine a universally applicable measurement of the condylar position. Although the physiological (i.e. centric) condylar position is defined as the most anterosuperior position with no lateral displacements (arrows), this position depends upon the basic neuromuscular tonus.
40 Frontal relationships
Macroscopic anatomical preparation showing the relation of the fossa, disk, and condyle to one another in the frontal plane. In this plane, too, there is no standard geometric arrangement of condyle and fossa because of the variability of the hard and soft tissues (Yung et al. 1990). In this preparation the disk (arrows) is displaced laterally. Structures of the bilaminar zone (1) can be identified in the medial portion of the joint. The close proximity of the joint to the middle (2) and inner ear (3) can also be observed.
41 Horizontal relationships
A right temporomandibular joint viewed from above showing the relation of the fossa, disk, and condyle to one another in the horizontal plane. The lateral portion of the joint is near the left border of the picture. Near the upper border a section through the external auditory meatus can be seen (1). The roof of the fossa has been removed. Near the center of the picture lies the transition from the pars posterior (2) to the bilaminar zone (3). The central perforation was created during sectioning, and through it can be seen the upper surface of the condyle (arrow).
Positioning of the condyles on the protuberances is accomplished exclusively through the antagonistic activity of the neuromuscular system and from a functional standpoint requires no border position.
42 Relationships in the frontal plane
Schematic depiction of the joint space relationships in the frontal plane. A number of studies have reported that the dimensions found in the lateral, central, and medial parts may vary greatly (Christiansen et al. 1987, Vargas 1997). Although the lateral portion is affected more frequently by degenerative changes, the width of the joint space is usually least at its center (blue line).
43 Contours on the temporal surface of the joint
Schematic drawing (modified from Hasso et al. 1989) of the contours in the lateral (green), central (blue), and medial (red) regions of the joint. The entire protrusive functional path is represented as a convex bulge
