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 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.