Effect of dynamic loading on the frictional response of bovine articular cartilage

The objective of this study was to test the hypotheses that (1) the steady-state friction coefficient of articular cartilage is significantly smaller under cyclical compressive loading than the equilibrium friction coefficient under static loading, and decreases as a function of loading frequency; (2) the steady-state cartilage interstitial fluid load support remains significantly greater than zero under cyclical compressive loading and increases as a function of loading frequency. Unconfined compression tests with sliding of bovine shoulder cartilage against glass in saline were carried out on fresh cylindrical plugs (n=12), under three sinusoidal loading frequencies (0.05, 0.5 and 1 Hz) and under static loading; the time-dependent friction coefficient μeff was measured. The interstitial fluid load support was also predicted theoretically. Under static loading μeff increased from a minimum value (μmin=0.005±0.003) to an equilibrium value (μeq=0.153±0.032). In cyclical compressive loading tests μeff similarly rose from a minimum value (μmin=0.004±0.002, 0.003±0.001 and 0.003±0.001 at 0.05, 0.5 and 1 Hz) and reached a steady-state response oscillating between a lower-bound (μlb=0.092±0.016, 0.083±0.019 and 0.084±0.020) and upper bound (μub=0.382±0.057, 0.358±0.059, and 0.298±0.061). For all frequencies it was found that μub>μeq and μlb<μeq (p<0.05). Under cyclical compressive loading the interstitial fluid load support was found to oscillate above and below the static loading response, with suction occurring over a portion of the loading cycle at steady-state conditions. All theoretical predictions and most experimental results demonstrated little sensitivity to loading frequency. On the basis of these results, both hypotheses were rejected. Cyclical compressive loading is not found to promote lower frictional coefficients or higher interstitial fluid load support than static loading.