Fig. 1.

Measurement of mean thickness of cartilage. Convex segment (a): the mean thickness ($\overline{l}$) of the cartilage (dotted area) is the difference of the outer and inner radius of the annulus segment that has an area equal to the area (A) of the cartilage segment of the same α angle. Mean thickness can be calculated with the formula for the area of an annulus segment. The striped area represents subchondral bone. r—outer radius of the constructed annulus segment. Concave segment (b): measuring the thickness of cartilage on a concave segment. Dotted and striped areas represent cartilage and subchondral bone, respectively. A—area of the cartilage segment of α, $\overline{l}$—mean thickness, r—inner radius of the constructed annulus segment. Straight segment (c): measuring the thickness of cartilage on a straight segment. Dotted and stripped areas represent cartilage and subchondral bone, respectively. A—area of the cartilage segment, $\overline{l}$—mean thickness, x—width of the constructed rectangle and the selected segment of cartilage