Abstract
One of the possible causative factors of the increasing trend of temporomandibular joint problems in recent years, is the evolutionary reduction and decreased growth of the human masticatory system. The purpose of this study was to investigate the secular reductive changes in this joint in Japanese people.
Materials used were 279 X-ray cephalogramsof Japanese crania from the Jomon, Yayoi, Kofun, Kamakura, Muromachi, Yedo and Meiji periods, and 419 X-ray cephalograms of living Japanese obtained from 6 areas. Linear measurements of 7 items, i. e. condylar process height, condylar process basal width, mandibular neck width, condylar width, condylar height, articular fossa width and articular fossa depth were carried out. Secular changes were analyzed for these measurements using the t-test and rank correlation.
Jomon people had prominent condylar processes with wide condylar width. The condylar processes became longer in the Yayoi period. In addition, the width of condyles and mandibular necks were slightly reduced and condylar heads became narrower. The articular fossa, on the other hand, became both wider and deeper. This indicated that the ratio of condylar width to fossa width became smaller, resulting in a morphological disharmony between the condyle andarticular fossa. In the Kofun period, the condylar and mandibular neck widths reached their maximum and the condylar process became thicker and more sturdy. At the same time the articular fossa width decreased slightly, but the size and depth of the fossa remained appropriate for the condylar head. Thus, the size of the condyle and of the articular fossa were proportional. In the Kamakura period and Muromachi period, the condylar processes became shorter whilst the condylar and mandibular neck widths became slightly narrower. Moreover, both the width and depth of the articular fossa began todecrease. From the Yedo period to the present, marked morphological changes have occurred.For example, the condylar process has become thinner and more delicate and the top of the condyle more narrow. In contrast, the articular fossa has tended to increase both in width and depth until the present. As a result of these changes, a morphological disharmony of the temporomandibular joint, (i.e., a narrower condyle than articular fossa), has become apparent.
As a general trend, secular changes of the joint structure have been observed on the condylar process, whilst the changes of the articular fossa have shown no such definite tendency. The condylar process had a tendency to become more slender with time, and the condylar width showed highly significantly diminution. Of considerable interest was the significant decrease in the ratio of condylar width to the fossa width in the Yayoi and modern period.
