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Comparable outcomes seen with cobalt-chromium and oxidized zirconium knees
Hui C. J Bone Joint Surg.2011; doi:10.2106/JBJS.I.01753.
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Australian investigators of this level 1 study discovered no significant differences regarding subjective, clinical and radiographic results during a 5-year follow-up comparing cobalt-chromium and oxidized zirconium femoral components in patients with both types of implants.
Catherine Hui, MD, FRCS, and colleagues studied 40 patients with osteoarthritis who were treated with bilateral total knee arthroplasty using a cruciate-retaining design. The investigators randomized the patients to receive a cobalt-chromium femoral component in one knee and an oxidized xirconium implant in the other limb. Patients in this double-blinded study underwent radiographic evaluation, and their outcomes were measured using the British Orthopaedic Association patient satisfaction scale, WOMAC, KOOS and Knee Society score.
The investigators found similar results in all measures between the groups at several time points during the 5-year follow-up. The investigators discovered that 38% of patients preferred the cobalt-chromium femoral component and 18% favored the oxidized zirconium implant. Overall, 44% of patients did not prefer one implant over the other at 5 years after surgery. “There were no adverse effects associated with the use of oxidized zirconium femoral implants,” the authors wrote in their conclusion.
The study by Hui and colleagues demonstrates perhaps the best study to date comparing the use of oxidized zirconium with a more conventional cobalt-chromium alloy for the bearing surface of a total knee replacement. The strength of the study is the study design (a prospective, double-blind, randomized clinical trial) that provides the highest level of evidence of the clinical findings — namely, no difference between the bearing surfaces, including the fact that only the bearing surface differed between the two implants, since the shapes of the implant components were the same. The weakness is that at 5 years of follow-up, it may be too early to conclude that a difference might not emerge from longer follow-up, a weakness that the authors understand.
Despite their statement that, “To our knowledge, this is the first report of analysis of polyethylene liners retrieved from oxidized zirconium femoral implants in total knee replacements,” there has indeed been a previous report from our institution, Hospital for Special Surgery, on a study in which matched pairs of retrieved cobalt alloy and oxidized zirconium implants were examined at retrieval (Heyse TJ, Chen DX, Kelly N, et al. Matched-pair total knee arthroplasty retrieval analysis: Oxidized zirconium vs. CoCrMo. Knee. 2010 Sep 22. [Epub ahead of print]). Using a similar approach in grading damage to the polyethylene (PE) bearing surfaces, we found that the amount of damage to the tibial PE inserts was significantly lower with oxidized zirconium components than inserts that had been in patients with cobalt-chromium femoral components. Unlike the findings of Hui et al., this would suggest that there is an advantage to oxidized zirconium, though we have no clinical data to support that apparent advantage.
– Timothy Wright, PhD
Kirby Chair of Orthopedic Biomechanics
Hospital for Special Surgery
New York
- Disclosure: His department receives research funding from Smith & Nephew, the maker of the oxidized zirconium implants, but the funding is for a project that does not deal at all with the topic of the Hui article.