147. A PROSPECTIVE RANDOMISED CLINICAL TRIAL: DOES HEAD SIZE EFFECT METAL ION LEVELS IN METAL-ON-METAL THAS?

Purpose: Metal-on-metal articulations are an increasingly popular choice as an alternate bearing surface in total hip arthroplasty (THA) and Resurfacing implants. One advantage of a metal-on-metal bearing is the use of larger diameter femoral heads with hip simulator data demonstrating reduced wear. We performed a prospective, multicentre, randomized, blinded clinical trial comparing 28mm to 36mm metal-on-metal bearings assessing multiple validated outcome measures and serum, erythrocyte and urine metal ions. Method: Ninety-one patients were randomized to receive a metal-on-polyethylene (34), a 28mm metal-on-metal (25) or a 36mm metal-on-metal (32) insert. All patients received the same acetabular and femoral component. Patients were evaluated pre-operatively, at 6, 12 months and annually thereafter, including an evaluation of serum, erythrocyte and urine cobalt, chromium, and titanium, outcome measures (WOMAC, SF-12, Harris Hip Score) and radiographs. Results: At a minimum two years follow-up there were no differences in WOMAC, SF-12, Harris Hip scores or radiographs. Patients receiving metal liners had significantly (p Conclusion: Both cobalt and chromium ion measurements were significantly elevated in the blood and urine of the patients randomized to receive the metal-on-metal bearings at all time intervals. There were no differences seen between the 28mm and 36mm metal-on-metal bearings, keeping all other variables identical. The larger diameter bearing therefore provides the potential clinical advantages of improved range of motion and stability, while providing a similar metal ion profile. While reduced wear is seen with larger diameter metal-on-metal bearings in-vitro, we could not demonstrate a reduction in blood or urine metal ion levels in-vivo.