How severe must repetitive loading be to kill chondrocytes in articular cartilage?

OBJECTIVE: Little is known about the effects of severe repetitive loading on articular cartilage chondrocytes, even though epidemiological studies associate this type of loading with osteoarthritis. We hypothesize that repetitive loading can kill cartilage chondrocytes in a dose-related manner.
DESIGN: Large cartilage-on-bone specimens were cut from the patella groove of bovine knees obtained directly from a slaughterhouse. Cartilage was loaded using a flat impermeable indenter in such a manner that the loaded region was supported naturally by surrounding cartilage and subchondral bone. Specimens received 3600 cycles of compressive loading at 1 Hz, with the peak load lying in the range 1-70% of the force required to damage cartilage in a single loading cycle (35 MPa). Cell viability was assessed in thick sections of loaded and control cartilage using a paravital staining method: fluorescein diacetate stained live cells green, and propidium iodide stained dead cells red. The assay was validated on cartilage which had been subjected to repeated freeze-thaw cycles to kill the chondrocytes.
RESULTS: Paravital staining revealed 100% cell death after one freeze-thaw cycle at -196 degrees C and three cycles at -20 degrees C. Baseline chondrocyte viability was 80% in unloaded cartilage, and viability decreased when applied compressive loading exceeded 6 MPa. Above this threshold, cell viability was inversely proportional to applied stress. When gross damage to the cartilage surface first became evident, above 14 MPa, 40% of cells remained viable. Load-induced chondrocyte death was greatest in the surface zone, and extended beyond the loaded area. Electron micrographs indicated that some cells were dying by apoptosis.
CONCLUSIONS: Some chondrocytes are much more vulnerable to repetitive mechanical loading than others, suggesting that vigorous activity may lead to cell death in articular cartilage.