Assessing the friction properties of synovial joint lubrication by tracking pendulum motion

Osteoarthritis (OA) is the most common degenerative joint disease. In the advanced stages of OA, patients suffer from severe pain, functional limitations in the affected joints, and restriction of mobility. In the animal test to develop therapeutic agents of intra-articular injection for knee OA, it is important to understand the functional properties of synovial joints, and to establish precise measurement techniques. The pendulum test can mechanically evaluate the friction properties of joints. In this study, the frictional properties of the synovial joint were measured using a novel system based on the equation of motion for pendulum joint motion. It was hypothesized that the motion during the pendulum test included rotation motion and translation motion and that the instantaneous center of rotation changes occurred. It was also hypothesized that the coefficient of friction would fluctuate within one cycle in response to changes in the instantaneous center of rotation. To test these hypotheses, the motion of a rabbit knee joint was measured using a pendulum test with three laser displacement sensors, and the instantaneous center of rotation was calculated. It considered the fluctuation of the instantaneous center of rotation and measured the fluctuation of the coefficient of friction in one cycle using a novel calculation method. The trajectory of the instantaneous center of rotation was obtained, and changes in the motion phase were observed. The coefficients of friction were not uniform but were dependent on the phase of the joint motion. The coefficients of friction fluctuated significantly with a cycle of half of the joint motion.