Tribometer for measuring coefficients of friction of uneven surfaces like articular cartilage.

To examine coefficients of friction (COFs) of articular cartilage, it is necessary to use cartilage as a friction partner. Irregularities of surfaces require special tribometers and calculation methods. The aim of this study was to establish a tribometer system for measuring a low COF of cartilage and to develop and validate an algorithm that takes the irregularities into consideration. We used a pin-on-plate tribometer that allows a vertical displacement of the pin to follow the surface of the plate and developed an algorithm that takes these irregularities into account. We were, thus, able to take into consideration a forward and backward movement, an upward and downward movement, and different force ratios. The algorithm was validated using a spherical POM (polyoxymethylene) pin against a stainless steel plate at slope angles up to 24°. First examinations with articular cartilage against articular cartilage samples of a stifle joint of a pig were then performed. The newly developed tribometer worked well when POM against a stainless steel hump was examined. The COF increased for slope angles steeper than ±15°. There was an interaction between the COF and the slope angle, but not for the range within ±15°. Cartilage examinations revealed COFs as published in the literature. The tribometer and the algorithm were suitable for the detection of low COF of irregular surfaces of the plate within a range of ±15°. The COF resulting from the forward and backward movements should be averaged.