(8b) Testing-Configuration Effects in Assessing Cartilage Lubrication

Recent theoretical and experimental developments suggest that the pressurization of the interstitial fluid is essential in cartilage lubrication [J. Biomech. Eng. 119 (1997) 81 and J. Orthopaedic Res. 22 (2004) 565]. The present work confirms this understanding, and demonstrates that the biphasic lubrication model proposed by Ateshian and coworkers is consistent with experimental measurements performed using a pin-on-disc tribometer in which cartilage samples can be placed on the pin, on the disc, and on both surfaces.

More importantly, our results show that, under constant load and constant sliding velocity, (I) the friction coefficient between a cartilage pin and an alumina disc increases with sliding distance until a plateau of ~0.27 is reached; (II) the friction coefficient between a cartilage pin on a cartilage disc increases only slightly as the sliding distance increases and remains close to ~0.07; and that (III) the friction coefficient between an alumina pin and a cartilage disc decreases as the sliding distance increases until it reaches a plateau of ~0.06. Our results confirm that the interstitial fluid support strongly regulates the frictional response in articular cartilage, and suggest that, because the metal (or cartilage) pin on cartilage disc testing configuration is the one most representative of physiological conditions, it should be implemented to test wear-and-tear characteristics of prosthetic devices as well as the efficacy of medical implants designed to improve lubrication in articular joints.