The influence of femoral condylar lift-off on the wear of artificial knee joints.

In vivo fluoroscopic studies of patients with total knee replacements (TKRs) have shown lift-off of the femoral condyles from the tibial insert. This study investigated the influence of femoral condylar lift-off on the ultra-high molecular weight polyethylene (UHMWPE) wear of fixed bearing (FB) and rotating platform mobile bearing (RP MB) total knee replacements, using a physiological knee joint simulator. In the absence of lift-off, the RP MB knees exhibited a lower wear rate of 5.2 +/- 2.2 mm3 per million cycles (mm3/MC) compared with 8.8 +/- 4.8 mm3/MC for the FB knees. The presence of femoral condylar lift-off was found to accelerate the wear of the FB and RP MB knees tested in this study to 16.4 +/- 2.9 and 16.9 +/- 2.9 mm3/MC respectively. For the RP MB knees the increase in wear rate was more marked, resulting in a similar wear rate for both designs of knee under lift-off conditions. In both cases the medial condyle displayed more wear damage. This study has shown that a small amount of abduction/adduction lift-off and medial-lateral shift increases wear and that the increase in wear is design dependent. In this simulator test, lift-off was simulated on every cycle, whereas the amount of wear and effect of lift-off clinically would depend on the frequency of occurrence of lift-off in vivo.