PRELIMINARY BLOOD METAL ION LEVELS IN A CERAMIC-ON-METAL BEARING AS PART OF A PROSPECTIVE RANDOMISED THA BEARING SURFACE TRIAL.

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. Laboratory studies suggest that ceramic-on-metal articulations may provide distinct advantages. 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, with an uncemented porocoated acetabular shell. A uniform 28mm femoral 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 follow up visits at 3 months and 1 year, with further follow up at 3,5 and 10 years post operatively. Whole blood ion levels are measured using a graphite furnace atomic absorption spectrometer.

Between February 2004 and April 2006 one hundred and ten hips have undergone total hip replacement. There are 105 patients (5 bilateral). 40% are males and 60% female. The average age at operation is 52 years (17 to 72). 49% hips are left and 51% 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. Radiological and clinical assessment shows no difference between the different bearing surface groups.

Post operative whole 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-on-cross linked polyethylene and ceramic-on-ceramic articulations. The ceramic-on-metal group is providing moderately raised metal ion levels, and the highest metal ion levels are in the metal-on-metal articulation group. At one year, the ceramic-on-metal group demonstrates a drop to close to pre-operative levels and these are still significantly lower than the metal-on-metal group. This confirms laboratory studies on the ceramic-on-metal articulation, which demonstrate significantly lower wear than comparable metal-on-metal articulations. 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 continues to replicate the laboratory wear studies, this articulation becomes a very attractive bearing surface in younger active patients, and may well become a bearing surface of choice in the future.