Quantitative estimation method and device of articular cartilage have been needed to evaluate various new treatment methods for joint disease. We proposed the non-contact ultrasonic evaluation method and investigated the accuracy using animal study and in vitro study. The advantage of the ultrasonic method is sensitive to the change of articular cartilage in the early stage of joint disease. The mechanism of the sensitivity, however, was not cleared. The objection of this paper was that the mechanism was investigated using numerical analysis. The relation between the distribution of solid phase and signal intensity of ultrasonic echo was simulated. The result showed that the signal intensity was in proportion to the quantity of solid phase near the cartilage surface. The simulation suggests that the ultrasonic method estimates the degree of overall cartilage damage or repair from the information near cartilage surface.
