Reduction in tensile strength of cartilage precedes surface damage under repeated compressive loading in vitro

An experimental protocol for the fatiguing and tensile testing of articular cartilage has been established. Samples were taken from the interpatellar groove of bovine femurs collected post-slaughter, split into two test groups and subjected to a cyclically varying compressive load of approximately 65 N for 64,800 cycles or 97,200 cycles. The cartilage was then removed from the underlying bone and two specimens, one from the indented region and one from an unindented region – the control, were taken from it and prepared for subsequent tensile testing using notched specimens. From data collected during tensile testing, a value of maximum tensile stress was calculated for each sample. The underlying bone was examined for evidence of microdamage using the basic fuchsin method. A decrease in values of maximum tensile stress (p<0.05) for the indented sample of each paired group of samples loaded for 97,200 cycles was found. In contrast, those fatigued for only 64,800 cycles showed no such difference. Examination of the underlying bone of these specimens revealed no evidence of trabecular failure and crack formation beneath both the indented and control regions. It is postulated that the fatiguing process used in this experiment induces trauma in the cartilage causing a weakening of the interfibril connections which link collagen fibrils in the matrix, leading to a reduction in tensile strength. This weakening occurs, however, without the appearance of fibrillation on the cartilage surface or any evidence of failure in the bony structure which supports it.