An experimental model of tibial counterface polyethylene wear in mobile bearing knees: The influence of design and kinematics

Abstract

Current designs of mobile bearing knees have different kinematics at the tibial counterface articulation; unidirectional represented by linear tracks and rotating platform designs, and multidirectional represented by reduced constraint designs with motion of the tibial surface in A–P and M–L directions simultaneously. One fifth scale experimental models of the tibial counterface articulation have been developed with mean contact stresses of 0.6 MPa. The unidirectional model had a linear reciprocating motion with a 10 mm stroke, the multidirectional model had a reciprocating motion with a 10 mm stroke and simultaneous rotation of $\pm$ 7.5 ${}^\circ$ . Six specimens of GUR415 polyethylene were tested for each model, sliding on polished cobalt chrome counterfaces with Ra $<$ 0.01 $\mu$ m in 25% bovine serum lubricant. The mean $\pm$ STERR wear rates were: unidirectional 0.045 $\pm$ 0.015 mm $^{3}$ /million cycles and multidirectional 0.44 $\pm$ 0.15 mm $^{3}$ /million cycles. Applying the scaling factor of 5, the predicted wear rates in actual knee prostheses were: unidirectional 0.23 mm $^{3}$ /million cycles and multidirectional 2.2 mm $^{3}$ /million cycles. The order of magnitude increase in wear rate was statistically significant ( $p={}$ 0.05).