Effects of freezing on the mechanical properties of articular cartilage.

Preventing cartilage injury is important in minimizing the long term debilitating effects of osteoarthritis. Accurate subfracture injury prediction must take into account the possible effects that freeze thaw cycles may have on the mechanical properties of cartilage tissue. This paper addresses this concern with matched pair testing of various low temperature storage techniques against fresh control groups. Ten matched pairs of bovine knees were used for testing, five pairs for a -20 degrees C slow freeze cycle and five pairs for a -80 degres C flash freeze cycle. Controlled mechanical indention tests were performed on the bovine articular cartilage-on-bone specimens to compare stiffness, peak stress, and loading energy of the cartilage. Findings showed that a slow freeze cycle or flash freeze cycle caused cartilage stiffness to decrease by 37% and 31% respectively, which was statistically significant in both cases (p< or =0.01). Compressive stress at this strain was also lowered by 31% with a slow freezing process (p=0.03). A similar trend was observed with compressive stress in the flash freeze specimens, although the 37% decrease was not found to be statistically significant (p=0.08). These results may be indicative of a weakened extracellular matrix structure caused by the freeze-thaw process. It is still unclear whether these changes in mechanical properties will result in a change in injury susceptibility for articular cartilage.