To assess matrix changes and chondrocyte viability during static and continuous repetitive mechanical loading in mature bovine articular cartilage explants.
Cartilage explants were continuously loaded either statically or cyclically (0.5 Hz) for 1-72 h (max. stress 1 megapascal). Cell death was assessed using fluorescent probes and detection of DNA strand breakage characteristic of apoptosis. Cell morphology and matrix integrity were evaluated using histology and transmission electron microscopy.
Repetitive loading of articular cartilage at physiological levels of stress (1 megapascal) was found to be harmful to only the chondrocytes in the superficial tangential zone (STZ) and depended on the characteristics (static vs cyclic) and duration (1-72 h) of the applied load. The chondrocytes in the middle and deep zone remained viable at all times. Static loads caused cell death at an early time (3 h) as compared with cyclic loads (sinusoidal, 0.5 cycles per s for 6 h). The amount and extent of cell death peaked at 6 h of cyclic loading, and did not change in subsequent experiments run for longer periods of time (up to 72 h). There was no indication of fragmented nuclear DNA but there was evidence of injurious cell death (necrosis) by electron microscopy. Morphological analysis of cartilage repetitively loaded for 24 h showed matrix damage only in the uppermost superficial layer at the articular surface, reminiscent of the early stages of osteoarthritis.
Cell death in mature cartilage explants occurred after 6 hours of continuous repetitive load or 3 h of static load. Cell death was directly related to the mechanical load, as control (free-swelling) explants remained viable at all times. The excessive, repetitive loading conditions imposed are not physiological, and demonstrate the deleterious effects of mechanical overload resulting in morphological and cellular damage similar to that seen in degenerative joint disease.