Clinical reports of surgical intervention options, such as spacers or hemi-arthroplasties, particularly for treatment of young arthritic patients, have been poor [1]. Knowledge of the tribology of the cartilage-prosthesis interaction of these devices would potentially provide an insight to the reasons for the premature failure of these devices and the development of more appropriate intervention treatment solutions for arthritic patients. Frictional studies of articular cartilage have been reported, using simple pin-on-plate geometric configurations [2], which do not accurately represent the geometric and stress conditions in the natural joint. A more representative model, based on the medial compartment of the knee joint has been developed in the Part 1 of this study [3] for the pre-clinical tribological testing the natural joint and their related arthroplasty devices. Bearing geometry is an important consideration in limiting wear, as shown in congruous meniscal knee replacement, which exhibited lower wear rates than incongruous designs [4,5]. The aim of this study was to use a unicompartmental hemi-arthroplasty model to examine the effect of tibial conformity and stress on the friction and wear of articular cartilage.

Experiments were conducted in an anatomic pendulum friction simulator (SimSol, UK) using the medial femoral condyle of a bovine knee joint articulating against two conforming stainless steel (316L) tibial plates (R=50mm and 100mm). A simplified physiologic knee loading profile was applied represent both low loading and much higher physiological loading conditions, with peak load between 259N – 1.5kN). Tests were conducted in 25% bovine serum and run for 3600 and 300 cycles under the low and high loading conditions respectively. The motion was cycled at 1Hz with amplitude between −10°–13.1°. Cartilage wear was assessed qualitatively from surface roughness measurements using a surface profile using a surface profilometer (Taylor Hobson, UK). The friction and wear of cartilage articulating against the conforming tibial plates were compared to a positive control flat tibial plate model [3]. The conforming plate models were found to produce significantly lower cartilage friction and surface damage (μ=0.022–0.035, Ra=0.136–0.145μm) than the flat plate model (μ=0.078, Ra=2.70μm). No damage on the cartilage surface was observed under low loads, however, under higher, more physiological loading cartilage friction increased (μ=0.08) in the conforming plate model, with a significant surface damage. An anatomic unicompartmental knee joint model has been developed to successfully examine the effect of counterface conformity on cartilage friction and wear for pre-clinical testing of a hemi-arthroplasty device. Counterface conformity was shown to significantly reduce cartilage friction and wear. This was attributed to the increased surface area and reduced stresses experienced in comparison to an incongruent bearing articulation.