18 – METAL ION RELEASE: COMPARISON OF FOUR LARGE DIAMETER METAL-ON-METAL BEARING ARTICULATIONS

Abstract

Purpose: Tribological studies of hip arthroplasty suggest that larger diameter metal-on-metal (MOM) articulations would produce less wear than smaller diameter articulations. Other advantages of these large femoral head implants include better stability with lower dislocation rates and improved range of motion. The aim of the present study was to compare chromium (Cr), cobalt (Co) and titanium (Ti) ion concentrations up to one year after different large diameter MOM total hip arthroplasties (THAs).

Method: One hundred and twelve patients were randomized to receive large (femoral head > 36 mm diameter) metal-on-metal articulation THA (LDH) from one of the following companies: Zimmer, Smith & Nephew, Biomet or Depuy. Samples of whole blood were collected pre-operatively and post-operatively at six months and one year. Cr, Co and Ti concentrations were measured by high-resolution mass spectrometry in an independent laboratory. All LDH implants have a modular Cr-Co tapered sleeve for leg length adjustment, except for Biomet with its sleeve made of Ti. All groups had Ti stems, and Zimmer and Biomet had, in addition, a Ti acetabular porous surface for secondary fixation. We undertook statistical analysis (SPSS 14.0) with p< 0.05 as significant.

Results: The groups were comparable in respect to pre-operative parameters (age, gender ratio, body mass index, etc.) as well as post-operative functional scores at six months and one year. We found that Biomet, Depuy and Smith & Nephew LDH had similar Co ion levels at 12 months post-op with 1.5, 1.4 and 1.6 ug/L, respectively. Durom LDH had the highest Co level with 2.3 ug/L (p< 0.01 versus the three other groups). The highest Ti ion levels were observed in the Zimmer group with 3.2 ug/L (p< 0.01 versus the three other groups) and the Biomet group with 2.0 ug/L (p=0.01 versus Zimmer and NS versus the other 2). Ti levels tripled versus pre-op for BHR and ASR (0.5 versus 1.5 and 0.5 versus 1.4 ug/L).

Conclusion: Different implant factors may influence metal ion levels measured in whole blood: articular surface wear and implant passive corrosion. Zimmer’s Durom LDH presents higher Co levels than the other groups. Since previously-published Durom hip resurfacing (same bearing characteristics as Durom LDH) showed much lower Co ion results, the modular sleeve may be incriminated. The plasma-sprayed acetabular surface of Zimmer’s and Biomet’s components seems to be responsible for the significant difference in Ti versus the other implants. Biomet’s plasma-sprayed Ti appears to be less prone to corrosion than Durom’s plasma spray coating. When evaluating metal ion release from MOM THA, total metal load from the implants should be considered, and newer implant designs should be evaluated scientifically before their widespread clinical use. LDH-THA should be seen as an improvement and should not be blamed as the source of metal ion release when a specific implant produces unsatisfactory results.