Purpose of the study: Tribology studies of total hip arthroplasty (THA) have demonstrated that large diameter head in metal-on-metal bearings produce fewer wear particles than small diameter heads. The other advantages of this option are better stability, less dislocation, and optimal joint range of motion. The purpose of this work was to compare blood levels of chromium, cobalt and titanium six months and one year after implantation of different models of large diameter metal-on-metal THA.

Material and methods: We conducted a retrospective comparative and randomised study including 110 patients who had been implanted with a larger diameter head THA/Zimmer? Smith and Nephez, Biomet or Depuy. The metal ion concentrations (Cr, Co, Ti) were measured in whole blood by an independent laboratory using high-resolution mass spectrometry (HR-ICP-MS). Blood samples were drawn preoperatively and postoperatively at six months and one year.

Results: At six months, the concentrations of metal ions in whole blood expressed as mean (range) for Cr (μg/L) Co (μg/L) and Ti (μg/L) were, respectively: Zimmer 1.3 (0.4/2.8) 1.7 (0.9/6.8) 2.5 (0.6/6.7); Smith and Nephew 2.0 (0.7/4.2) 2.1 (0.5/6.6) 1.1 (0.5/4.1); Biomet 1.2 (0.4/2.2) 0.9 (0.3/3.4) 1.4 (0.8/2.4); Depuy 1.7 (0.5/3.2) 1.9 (0.3/4.2) 1.3 (0.5/3.9). There was a significant difference between groups for Cr (p=0.006), Co (p=0.047) and Ti (p< 0.001). The Biomet implants presented the lowest concentrations for Cr and Co; the Zimmer implants gave the highest levels of Ti.

Discussion and Conclusion: Several implant-related factors affect blood concentrations of metal ions: contact surfaces leading to “active” abrasion but also wear in other parts of the implant giving rise to “passive” corrosion. Bearing wear is related to the diameter of the head, its roughness, its spherical shape, joint clearance, the manufacturing technique (forging, casting) and its carbon content. The Biomet head corresponds to a better compromise for these different factors. Passive corrosion can result from an exposed metal surface or from metal to metal contact. This explains the high level of Ti ions found for all implants tested since titanium is not present in the bearings.

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.