The arthrotripsometer is an apparatus for sensing and recording the instantaneous frictional properties and deformations of articular cartilage in vitro under conditions simulating human locomotion.

The frictional force measurements would have been straightforward were it not that the center of rotation of the twenty-one canine ankles studied varied throughout each oscillation. Folding of the friction-force curves about the end points of oscillation served to determine the instantaneous center of rotation and coefficient of friction. The center of rotation was determined as the mid-point between the two curves in the central portion of travel and extrapolated to the trailing end of each oscillation. At the start of each half cycle of oscillation, there occurred an abrupt transition and then a slower buildup of the coefficient of friction. These end effects are attributed to both shearing and small compressive deformations (plowing) of the articular cartilage. The plowing component of sliding friction decreased with increasing speed. The coefficient of friction decreased with increasing load. The friction was much lower in joints than in metallic or rubber bearings. When immersed in physiological saline solution, the mean sliding coefficient of friction of the ankles was .0124 (fifty-seven tests); whereas, bovine synovial fluid reduced this value by an additional 54 per cent to µ = .0057 (nineteen tests). The recovery of articular cartilage from compressive deformation was much quicker when the joint was oscillated than when it was stationary.