upper and lower joint spaces arise through the formation of multiple small splits in the dense mesenchyme from which the condyle, disk, and fossa arose previously.
The lower joint space appears first at about the tenth week (50-65 mm CRL), but later the upper joint space overtakes it in its development (Burdi 1992). At first the space is extensively compartmentalized, and it is only later that the individual cavities merge (Bontschev 1996). The lower joint space lies close to the embryonic condyle.
The upper joint space appears after about the twelfth week (60-70 mm CRL) and spreads posteriorly and medially over Meckel’s cartilage with its contour corresponding to that of the future fossa. After week 13 the lower joint space is already well formed as the upper joint space continues to take shape. From its beginning, the upper joint space has fewer individual islands of space and grows more rapidly than the lower joint space. After week 14 both joint spaces are completely formed. During weeks 16-22 the lumens of the chambers become adapted to the contours of the surrounding rounding bone. The fibrocartilaginous articular disk develops from the concentrated mesenchyme between the two joint spaces. The articular disk is not visible until the CRL is 70 mm. Even before formation of the joint spaces the disk is already thinner at its center than at the periphery and this leads to its final biconcave form (Bontschew 1996). The peripheral portions are not sharply demarcated from the surrounding loose mesenchyme. In fetuses with a CRL of 240 mm, the mesenchymal tissue changes into dense fibrous connective tissue. At this stage the peripheral region has a greater blood supply than the central region. According to Moffet (1957), compression of the disk between the temporal bone and the condyle results in an avascular central zone. At the beginning of its development the disk lies closer to the condylar process than to the future fossa. At this stage there is still a layer of loose mesenchyme between the temporal bone and the upper joint space. It is only after a CRL of 95 mm has been reached that the condylar process and the fossa become closer and the mesenchymal layer disappears.
Joint development
23 Twenty-sixth week
Completely formed human temporomandibular joint with physiolgical lower and upper joint spaces. Trabecula-like structures can be identified in both joint spaces where the disk has not yet separated completely from the temporal and condylar portions. At present it has not been conclusively determined whether or not this type of incomplete separation could be one cause of disk adhesions.
24 Development of the joint spaces
Above: Three-dimensional reconstruction from a series of histological sections of the developing joint space (yellow) of a right temporomandibular joint. In the center of the picture is the condyle (1); to the right of it lies the coronoid process (2). To the left behind the condyle is Meckel’s cartilage (3). The upper joint space arises approximately 2 weeks after the lower. Below: Three-dimensional reconstruction of the lower joint space (green) of the S3me joint. Initially the mesenchyme in the condylar region (1) is still uniformly structured, but in weeks 10-12 it begins to tear in several places mesial and distal to the condyle. The resulting clefts run together to form the lower joint space. A region of concentrated mesenchyme remains between the two joint spaces, from which the fibrocartilaginous articular disk is later formed. Contributed by R. J. Radianski (Figs. 23-25)
25 Development of the lateral pterygoid muscle
Three-dimensional representation of the insertion of the lateral pterygoid muscle (1) onto a left temporomandibular joint. As the muscle develops from the eleventh week, its upper belly attaches to the condyle, capsule, and disk while its lower belly attaches only to the condyle (2). At no time during development do the fibers of the lateral pterygoid muscle make direct contact with Meckel’s cartilage (Ögütcen-Toller and juniper 1994).
26 Development of the human temporomandibular joint
Graphic representation (modified from van der Linden et al. 1987) of prenatal development of the human temporomandibular joint showing its relationship to the CRL and age. First to form are the bony structures and the disk. Development of the joint capsule is accompanied by development of the upper and lower joint spaces. It is most interesting that prenatal mandibular movements can be observed as early as weeks 7-8 (Hooker 1954, Humphrey 1968), even though most of the joint structures and even the muscle insertions do not develop until a few weeks later. It is assumed that the movements are made possible by the primary jaw joint between Meckel’s cartilage and malleus-incus (Burdi 1992).
Glenoid Fossa and Articular Protuberance
The temporal portion of the joint can be divided into four functional parts from posterior to anterior: postglenoidal process, glenoid fossa, articular protuberance, and apex of the eminence. The inclination of the protuberance to the occlusal plane varies with age and function (Kazanjian 1940), but is 90% determined at the age of 10 years (Nickel et al. 1988). Three fissures can be found at the transition to the tympanic plate of the temporal bone: the squamotympanic, petrotympanic, and petrosquamous fissures (Fig. 28). In patients with disk displacement, these fissures are frequently ossified (Bumann et al. 1991). Under physiological conditions the only parts of the temporal portion of the joint that are covered with secondary cartilage are the protuberance and the eminence (Fig. 31). Secondary cartilage is formed only when there is functional loading. Before the fourth postnatal year stimulation of the cells of the perioseum leads to the formation of secondary cartilage (Hall 1979, Thorogood 1979, Nickel et al. 1997J. With no persisting functional load the chondrocytes of the condyle would differentiate into osteoblasts (Kantomaa and Hall 1991).
27 Inclination of the articular protuberance to the occlusal plane
This graph (adapted from that of Nickel et al. 1988) indicates the inclination of the posterior slope of the eminence (articular protuberance) in relation to the occlusal plane. Accordingly, at the age of 3 years the eminence has reached 50% of its final shape (Nickel et al. 1997). Between the tenth and twentieth year there is a difference of only 5°. The study material originates from the osteological collection of Hamman-Todd and Johns Hopkins. Cleveland Museum of Natural History.
28 Joint region of the temporal bone
Inferior view of the temporal portion of a defleshed temporomandibular joint. Near the upper border of the picture is the articular eminence (1) and at the far left is the external auditory meatus (2). In the posterior portion of the fossa the squamotympanic fissure (3) is found laterally, and the petrosquamous (4) and petrotympanic (5) fissures are found medially. Both the superior stratum of the bilaminar zone and the posterior portion of the joint capsule, and sometimes also the fascia of the parotid gland can insert into these fissures.
29 Ossification of the fissures and disk displacement
Inferior
