RANDOMISED PROSPECTIVE STUDY OF FOUR BEARING SURFACE COMBINATIONS IN TOTAL HIP REPLACEMENT. A PRELIMINARY REPORT ON METAL ION LEVELS.

Abstract

The search for the ideal bearing surfaces to be used in Total Hip Replacement continues. The current “best” materials are felt to be various combinations of metal, ceramics and cross linked polyethylene. This study aims to identify the best combination with the lowest side effect profile.

In February 2004 a prospective randomised trial on different bearing surfaces was started. The combinations selected were ceramic on cross linked polyethylene, ceramic on ceramic, metal on metal and ceramic on metal. Institutional ethics clearance was obtained. In all patients uncemented femoral stems are used, and an uncemented porocoated acetabular shell. 28mm Head size was selected. Blood samples have been taken to measure the metal ion concentrations in all patients. These are measured pre operatively, and repeated at intended follow up visits at 3 months, 1, 3, 5 and 10 years post operative using a graphite furnace atomic absorption spectrometer.

Between February 2004 and 2006 seventy hips have undergone total hip replacement. There are 85 patients (11 bilateral). 40% are males and 60% female. The average age at operation is 52 years (17 to 72). 46% Hips are left and 54% right. Follow up includes blood samples and the Harris Hip Score. Complications to date have been surgeon related, with three femoral components needing early revision for technical reasons. This has not affected the bearing surfaces. Ten patients have hetero-topic ossification. Cup inclination averages at 48 degrees (32 degrees to 69 degrees). Post operative blood metal ion levels are compared to the patient’s pre-operative level. To date there is no increase in the metal ion levels for the ceramic/cross linked poly ethylene and ceramic/ceramic articulations. The ceramic metal group is providing intermediate raised metal ion levels, and the highest metal ion levels are in the metal on metal articulation group. In the laboratory the ceramic on metal articulation demonstrates the least wear of all the groups studied, with metal on metal second. The high level of metal ions in the latter groups has always been of concern.

This study demonstrates a lower blood level of metal ions in the ceramic on metal group. If the in vivo wear rate in this group is as good as the laboratory wear, it becomes a very attractive bearing surface in younger active patients.