Evaluation of a metal-on-metal total hip arthroplasty

Abstract

Background: After the introduction of the Metasul system by Weber in 1988, total hip arthroplasty (THA) with metal-on-metal (MoM) bearings became increasingly popular in the 1990s. MoM bearings aimed to reduce the problem of aseptic loosening associated with polyethylene wear particles from conventional polyethylene cups. The following introduction of hip resurfacing added positive factors as bone sparing and large caliber heads, reducing the risk of dislocations. After almost two decades of increasing use of MoM prostheses, reports of implant failure and adverse reactions to metal became numerous. Starting in 2009, a dramatic decrease in the use of MoM bearings has been seen.

Patients and methods: Paper I was an experimental in vitro study measuring the amount of metal worn off from a MoM prosthesis with a 28-mm diameter head, using a hip simulator. Papers II-IV were all based on a prospective, randomized, controlled clinical trial including 85 patients allocated into two groups. One group was operated with a MoM prosthesis while the other group received a similar prosthesis with a conventional metal-on-polyethylene (MoP) bearing. All components were cemented and 28-mm heads were used. Clinical outcome and serum metal ion concentrations were evaluated at short-term (three, six, 12 and 24 months, paper II), medium-term (7 years, paper III) and long-term (16 years, paper IV) follow-up. Overall longterm survival of the implants was estimated after a mean of 16 years (paper IV).

Results: A so-called run-in phase with higher initial wear from MoM implants was seen in the joint simulator testing within the equivalent of the first year of use. As expected, patients with MoM bearings showed higher cobalt (Co) and chromium (Cr) ion concentrations compared to patients in the MoP group at all time-points from 3 months to 16 years postoperatively. The percentage of circulating HLA DR+ CD8+ T-cells was higher in the MoM compared to MoP group (10.6 vs. 6.7 %; p = 0.03) and was positively correlated to systemic concentrations of Co and Cr. At the last follow-up 16 years after the index surgery, radiographic result and clinical performance was similar in both groups (Harris hip score: MoM=91, MoP=95, p=0.28) and no difference between groups was seen regarding implant survival (MoM: 93 % [95% CI: 83-100%]; MoP: 95 % [95% CI: 87-100%], p=0.99).

Conclusions: This thesis evaluated a 28-mm head THA, comparing a MoM bearing with a conventional MoP articulation. Both articulations performed equally well in regard to long-term implant survival and clinical outcome. Due to the absence of clinical superiority of the MoM bearing and concerns about biological effects of the MoM articulation, the use of MoP articulations is recommended.