Objective: Little is known about the effects of severe repetitive loading o
n articular cartilage chondrocytes, even though epidemiological studies ass
ociate this type of loading with osteoarthritis. We hypothesize that repeti
tive 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 load
ed using a flat impermeable indenter in such a manner that the loaded regio
n was supported naturally by surrounding cartilage and subchondral bone. Sp
ecimens 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 secti
ons of loaded and control cartilage using a paravital staining method: fluo
rescein diacetate stained live cells green, and propidium iodide stained de
ad 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 degreesC and three cycles at -20 degreesC. Baseline chondrocy
te viability was 80% in unloaded cartilage, and viability decreased when ap
plied compressive loading exceeded 6 MPa. Above this threshold, cell viabil
ity 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, a
nd extended beyond the loaded area. Electron micrographs indicated that som
e cells were dying by apoptosis.
Conclusions: Some chondrocytes are much more vulnerable to repetitive mecha
nical loading than others, suggesting that vigorous activity may lead to ce
ll death in articular cartilage. (C) 2001 OsteoArthritis Research Society I
nternational