The thickness of the subchondral plate and its correlation with the thickness of the uncalcified articular cartilage in the human patella.

The regional thickness distributions of the subchondral plate and the unmineralized part of the articular cartilage were morphometrically determined in normal human patellae, and the correlation coefficient for each specimen calculated from the paired measurements. For this purpose the patellae were embedded in methyl methacrylate and cut as serial sections, which were assessed with a Vidas image-analyzing system (Kontron). The values obtained were used to reconstruct the individual and average thickness distributions and to calculate the correlation coefficients for each subject. Both the thickness of the subchondral plate and that of the cartilage revealed regular distributions which, however, followed different patterns. Central regions with maximum values from which the thickness decreased concentrically towards the periphery were found in both. However, the distribution patterns of the unmineralized cartilage and the subchondral plate could be clearly distinguished, both by the position of the maxima and by the arrangement of the isocrassids (contour lines of equal thickness). The thicknesses of the two tissues showed a correlation between 0.38 and 0.82 (mean 0.6). We attribute this to their different reactions to the type of stress acting upon them. It appears that the thickness of the subchondral plate is principally determined by stresses acting over a longer period of time with low frequency, whereas the thickness of the articular cartilage seems to be a response to intermittent dynamic stresses of a higher frequency.