WEAR OF CERAMIC ON POLYETHYLENE HIP REPLACEMENTS UNDER CONDITIONS OF SWING PHASE MICROSEPARATION

Abstract

Following total hip replacement surgery, fluroscopy studies have shown that a mean separation of 2 mm can occur between CoCr femoral heads and UHMWPE acetabular cups during the swing phase of gait [1]. In vivo and in vitro studies [2, 3] of alumina ceramic on ceramic hip replacements have demonstrated that swing phase microseparation followed by the impact of the femoral head on the acetabular insert rim can lead to accelerated wear. However, wear remains low. A similar trend was observed when metal on metal hip replacements were tested under microseparation conditions [4]. The purpose of the current study was to examine the wear of ceramic on polyethylene bearings under standard and microseparation conditions.

A physiological hip simulator was used, loads and motions were applied to approximate in vivo conditions. The alumina ceramic heads and polyethylene cups were 28 mm in diameter and were tested for 5 million cycles in 25% new born calf serum at 1 Hertz. Microseparation was achieved by displacing the femoral head inferiorly during swing phase, where the head contacted the inferior cup rim and was laterally displaced. On heel strike the head contacted the superior cup rim prior to relocation.

The volumetric wear of the polyethylene inserts was approximately four times less under microseparation conditions (5.6 ± 5.3 mm3 per million cycles), in comparison to standard conditions (25.6 ± 5.3 mm3 per million cycles). Deformation of the cup rim was observed, but some of this was attributed to creep. It is postulated that this reduction in wear was due to the separation of the components in swing phase improving the entrapment of lubricant, hence reducing wear via a squeeze film lubrication mechanism. In conclusion, surgical procedures that produce a small and controlled amount of joint laxity and microseparation may lead to a reduction in wear of the polyethylene acetabular cups.