In the field of arthroplasty, the orthopaedist selects an implant system with the expectation of both short and long-term restoration of function and improvement in quality of life. The use of evidence-based outcomes for implant selection adds the dilemma of an inherent time lag, given that standards for implant survival run into a second, or third, decade. That same time lag confounds the clinical vetting of newly developed implant technology, despite encouraging laboratory and short-term results. For widespread adoption to occur, new technology not only must cause no harm but also must meet, and ideally exceed, currently available technology standards for performance and longevity.
An oxidized zirconium (OxZr) bearing surface represents one such technology, purported to offer an advantage regarding longevity for total knee arthroplasty. Simulator studies have shown that OxZr total knee arthroplasty implants, compared with implants of like design with a cobalt-chromium (CoCr) bearing surface, yield improved polyethylene wear properties1,2. Those laboratory findings have been extrapolated to predict greater longevity of OxZr total knee arthroplasty implants because of decreased polyethylene wear, associated osteolysis, and implant loosening. As a potential longevity enhancer, OxZr total knee replacements have been projected to have clinical import for all patients having a total knee arthroplasty, but particularly those who are younger and more active.
The long-term clinical advantage of an OxZr total knee replacement bearing surface has not yet been demonstrated. To evaluate that question, Vertullo et al. reviewed the cases of >17,000 patients in a national registry who underwent total knee arthroplasty employing a specific implant system (cemented, cruciate-retaining Genesis II with non-cross-linked polyethylene; Smith & Nephew). They evaluated outcomes at up to 12 years of follow-up with regard to implant survivorship, comparing differing femoral bearing surfaces, CoCr versus OxZr (Oxinium; Smith & Nephew). Prior literature has shown this total knee arthroplasty design to have strong survival into its second decade of use, albeit without differentiation of CoCr versus OxZr bearing surfaces3,4.
Overall, Vertullo et al. found no significant difference for the 12-year cumulative percent revision (CPR) due to all causes (4.8% for CoCr and 7.7% for OxZr), nonseptic causes, or osteolysis or loosening (0.6% for CoCr and 1.1% for OxZr). The prevalence of OxZr total knee arthroplasty implants was 34% overall, and was greater in younger age-stratified subcohorts (e.g., 64% for those <55 years old and 50% for those 55 to 64 years old). The only age-related difference was found with patients who were ≥75 years old, for whom OxZr total knee arthroplasty had an increased hazard ratio (HR) of revision because of osteolysis or loosening.
Before drawing conclusions from this study, we should heed the authors’ notice about its limitations. Despite stratification on the basis of age, the data may still be exposed to an implant selection bias with OxZr potentially being favored in patients with higher polyethylene wear expectations, a confounding factor that could contribute to a higher revision rate for patients with OxZr total knee arthroplasty implants. Additionally, while not significant, seemingly disparate CPR and HR outcomes may partially reflect the methods of data analysis. We also cannot reach conclusions regarding the comparative survival of CoCr versus OxZr on cross-linked polyethylene, as that was not studied.
Nevertheless, the crux of this study remains intact. Despite follow-up into a second decade, within no age cohort did OxZr total knee arthroplasty implants offer a survival advantage, suggesting that the potential desired benefit of OxZr regarding polyethylene wear reduction might not present during the initial 12-year period. Given the track record of the total knee arthroplasty system employed3, one could conjecture that an advantage with an OxZr total knee arthroplasty design might accrue only after (much) longer follow-up.
A reasonable goal for modern arthroplasty is to reliably provide a successful total knee replacement lasting 25 to 30 years. Even with total knee arthroplasty technology considered older by modern standards, survival of >90% at >25 years of follow-up has been demonstrated5. Prior literature has shown a reasonable durability projection toward reaching that goal with both the CoCr and OxZr versions of this total knee arthroplasty design3,4. However, this study, with follow-up to 12 years, does not support the superiority of OxZr over CoCr and, at present, does not support the premise that an OxZr total knee arthroplasty bearing surface is more suitable for younger patients and patients at increased risk of polyethylene wear.
↵* The authors indicated that no external funding was received for any aspect of this work. The Disclosure of Potential Conflicts of Interest forms are provided with the online version of the article.
- Copyright © 2017 by The Journal of Bone and Joint Surgery, Incorporated