Low-Carbon Metal-on-Metal Articulations for Hip Arthroplasties--Evaluation of Wear and Histology after 11 to 17 Years

Abstract

High-carbon (HC) alloys for hip arthroplasties were preferred to low-carbon (LC) alloys for a long time because of their structurally hard carbide content. We opted for an LC alloy in 1994, because we expected it to be subject to less wear on account of its more homogeneous structure. Prompted by early complications not seen with ceramic-on-polyethylene mating surfaces, LC metal-on-metal articulations were, however, given up by us in early 1999. A series of implants retrieved after 11 to 17 years was now studied to establish the actual amount of wear. Potential tissue reactions associated with hypersensitivity were also evaluated histologically and correlated with the measured wear and the amount of metal particles in capsular tissue. Ten patients with LC metal-on-metal hip implants were the subjects of analyses after a mean follow-up time of 13.9 years. They underwent revision surgery because of osteolysis, cup loosening without dislocation, late infection in 1 patient and pain. The implant positions at the time of retrieval were the same as on the postoperative radiographs. Wear was determined in keeping with ISO 14242-2 and by SEM. In addition, periprosthetic tissue including the joint capsule and interface membranes were obtained for histological analysis. The amount of metal particles and the extent of lymphocyte infiltration were determined with the method described by Willert et al. Tissue alterations were evaluated histologically for signs of ALVAL using the method of Campbell et al. and correlated with the amount of wear and metal particles. The mean maximum linear wear rate was found to be 1.6 (1.0-2.1) µm/year. Our data also showed a mean rate of 0.32 mm³/year (range, 0.22-0.47 mm³/year). This is equivalent to an annual metal release of 2.7 (1.9-3.9) mg/year. No corrosion or corrosion products were present on the ball heads and their taper. All mating surfaces studied by SEM showed signs of abrasion. Sporadically, additional abrasions in the submicrometer range were detected on the ball head surfaces. These were caused by corundum particles detached from the blasted implant surfaces and interpreted as third-body wear below the level of quantification. Signs of impingement were absent. Histologically, metal wear particles of variable amount were detected in all cases. In 4 of them solid corrosion products were present in the tissue. The ALVAL scores were 5 to 10 (moderate to high), but did not correlate with the measured wear and the amount of metal particles. The wear of LC articulations found after years of implant survival was very low. In fact, it was lower than the data reported for HC articulations and would permit implants to function for decades in engineering terms. However, their survival is limited by tissue alterations associated with hypersensitivity. These tissue alterations may contribute to implant loosening as also reported for HC articulations. They are apparently not related to the actual amount of wear and may reflect adaptive immunological processes.