Tribological evaluation of materials for replacement knee joint by a new simulator

Abstract

The evaluation of wear and friction behavior in vitro requires realistic tribological conditions. These conditions are primarily load, motion and contact geometry. Since a knee joint has unique functions, structure, and tribological conditions, the simulation of knee joint in the pin-on-flat configuration as commonly used is not realistic. In view of this shortcoming, an oscillating link knee joint simulator is proposed. Wear tests on this simulator indicated that it provides a better representation of the wear damage which is very much similar to that observed from retrieved knee joints. The comparison of wear modes from the simulator with those of the reciprocating arrangements indicated that the cylinder-on-flat arrangement was also suitable for evaluative testing while the pin-on-flat arrangement was not. The effect of speed and load on the wear behavior is examined and the wear mechanisms for all of these cases are studied.

The wear and friction of UHMWPE and PEEK, and the composites of these materials was studied in a knee joint simulator. The latter produced a cyclic loading roughly of the sinusoidal form which was intended to simulate the tribological conditions in an actual knee joint. The specific materials tested were UHMWPE and that reinforced with PTFE, short carbon fiber, and continuous carbon fiber. The tests were also done on PEEK and that reinforced with short carbon fiber. Wear tests were run in Ringer solution at a frequency of 1.58 cycle/s and the cyclic load fluctuating between 60 and 140 MPa. It was found that UHMWPE had the highest wear resistance. The addition of PTFE and short carbon fiber to UHMWPE decreased its wear resistance and increased the intensity of fatigue cracking. The coefficient of friction in all of these cases was very low. The wear rate of PEEK was very high compared to that of UHMWPE and the wear mechanism changed to delamination as opposed to fatigue in UHMWPE.