It has been well established that the growth rate of the condylar cartilage of the mandible can be altered in response to changes in its biomechanical environment. Yet surprisingly little is known of the factors which regulate this response or of the limits within which these factors are effective. Although most investigations have focussed on the presumed influence of the protrusive jaw musculature, recent studies have suggested that a critical factor may well be the alteration in the pattern or intensity of compressive loading produced by the protrusion. However, very few empirical data are available concerning the response of the condylar cartilage to compressive articular forces, especially with regard to the intermittent forces that are transmitted to the cartilage during mastication and incision. The most well- controlled studies of this question have been conducted in vitro, but the applicability of these findings to the living animal needs to be evaluated. The proposed research will investigate the response of the condylar cartilage to intermittent compressive forces in vivo. Specific loading modalities will be imposed on the cartilage by chronic stimulation of the jaw elevator musculature using fine wire indwelling electrodes, thereby permitting the magnitude, frequency, and pattern of joint loading to be varied for comparative purposes. Information on condylar cartilage response to a range of loading amplitudes will be sought, as well as to the complete removal of articular forces (effected by a molar bite splint). In addition, the possible influence of the frequency and the pattern of loading remains an almost completely unexplored, but potentially significant, area of inquiry to be examined using this methodology. Rate of mitotic activity, matrix synthesis, and mineralization/bone formation in the condylar cartilage will be evaluated using biochemical ((3H)- thymidine, (35S)-sulfate incorporation and (45Ca)-uptake), histochemical, and radioautographic techniques. The data obtained should deepen our understanding of the capability of the condylar cartilage, and perhaps other secondary cartilages, to respond to biophysical stimuli. A more detailed appreciation of the adaptive limits of this tissue, especially with regard to those types of stimuli required to maximize the magnitude or duration of the response, should be of obvious benefit for therapeutic purposes.
