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.