The direction of penetration of the femoral head following total hip replacement has been shown at revision to be superomedial, superior or superolateral. However, it is important to study well functioning components to describe normal patterns of wear. The aim of this study was to characterise the 3D direction of penetration in standard and HXLPE.

A prospective double blind randomised control trial was conducted using Radiostereometric Analysis (RSA). Fifty-four subjects were randomised to receive hip replacements with either UHMWPE liners or HXLPE liners. All subjects received a cemented CPT stem and uncemented Trilogy acetabular component (Zimmer, Warsaw, IN, USA). The 3D penetration of the head into the socket was determined to a minimum of 7 years.

The direction of penetration between one and seven years was in a superior and lateral direction for both groups. In the HXLPE group there was no significant penetration in the coronal or sagittal planes (superiorly 0.009 mm/yr, 95% confidence interval, ±0.045, p1 = 0.68, laterally 0.003mm/yr, CI 0.031, p1 = 0.85). In the UHMWPE group there was significant penetration 0.059 mm/yr superiorly (CI 0.042, p1 = 0.01) and 0.049 mm/yr laterally (CI 0.044, p1 = 0.03). The anterior-posterior steady state penetration was not significant in either group (HXLPE p1 = 0.39, UHMWPE p1 = 0.37).

We have previously demonstrated that the penetration in the first year is creep-dominated and is in the proximal direction. From one year onwards the superolateral direction of penetration is probably due to wear. The steady-state wear direction is the same in both bearings types. It is likely that creep occurs in the direction of the Joint Reaction Force i.e. superomedial, whereas wear is perpendicular to the axis of rotation and therefore superolateral. This work may enable us to develop more accurate models for predicting wear in total hip arthroplasty.

Pseudotumours (soft-tissue masses relating to the hip joint) following metal-on-metal hip resurfacing arthroplasty (MoMHRA) have been associated with elevated serum and hip aspirate metal ion levels, suggesting that pseudotumours occur when there is increased wear. This study aimed to quantify the wear of implants revised for pseudotumours and a control group of implants revised for other reasons of failure.

A total of 30 contemporary MoMHRA implants in two groups were investigated: (1) 8 MoMHRA implants revised due to pseudotumour; (2) 22 MoMHRA implants revised due to other reasons of failure. The linear wear of retrieved implants was measured using a Taylor-Hobson Roundness machine. The average linear wear rate was defined as the maximum linear wear depth divided by the duration of the implant in vivo.

In comparison with the non-pseudotumour implant group, the pseudotumour implant group was associated with: (1) significantly higher median linear wear rate of the femoral component: 8.1 um/year (range 2.75-25.4 um/year) vs. 1.97 um/year (range 0.82-13.00 um/year), p=0.002; and (2) significantly higher median linear wear rate of the acetabular component: 7.36 um/year (range 1.61-24.9 um/year) vs. 1.28 um/year (range 0.18-3.33 um/year), p=0.001. Wear on the acetabular cup components in the pseudotumour group always involved the edge, indicating edge-loading of the bearing.

Significantly greater linear wear rates of the MoMHRA implants revised due to pseudotumour support the in vivo elevated metal ion concentrations in patients with pseudotumours. This study is the first to confirm that pseudotumour occurs when there is increased wear at the MoM articulation. Furthermore, edge-loading may be the dominant wear generation mechanism in patients with pseudotumour.

Introduction

Unidirectional mobile bearing knees (RP) were developed to optimise the tibio-femoral articulation in an effort to enhance function and reduce polyethylene wear. The self-aligning bearing should also benefit the patello-femoral joint further improving outcome. This study was designed to assess whether these potential benefits are realised in the clinical setting.