Abstract
Introduction
Cross-linked polyethylene in total hip arthroplasty has demonstrated excellent long-term wear resistance, leading to its acceptance as the standard bearing used in hip replacement. Adoption in knee replacement has been tentative, as the cross-linking process can decrease the polyethylene mechanical properties. The current study's purpose was examining survivorship of a fixed bearing knee replacement system featuring a moderately cross-linked polyethylene (MXLK) bearing, a cobalt chrome (CoCr) tibial tray with a highly polished top surface, and a new polyethylene-to-tray locking mechanism. The MXLK is made of ultra-high molecular weight GUR1020 resin irradiated with 5 Mrad gamma radiation, followed by a free radical quenching remelting annealing process, above the 135 degree melting point, that provides wear and fatigue resistance, and oxidative stability.
Materials & Methods
From November 2005 to June 2008, 539 PFC Sigma primary total knee replacements (TKA's) were prospectively entered into this non-comparative, multicenter, multinational study. Average age at time of surgery was 67 years, 57% were female, average body mass index was 30.4 kg/m2, and the dominant diagnosis was osteoarthritis (97%). Kaplan-Meier (KM) survivorship was the primary endpoint with primary event definition being removal of any component for any reason. The time variable was one of the following: time to revision, time to death, or time to last follow-up. Additional endpoints included: American Knee Society scores (knee and function), Oxford Knee score (range 12 to 60), SF-12 scores; radiographically assessed rates of radiolucent lines (RLL's) and osteolysis. RLL's greater than 2mm were counted. Progressive RLL's were those increasing in width from an earlier follow-up interval. Adjacent RLL's were defined as RLL's in adjacent zones. Complete RLL's were defined as RLL's completely around a component. This report provides 5-year results in this ongoing study with a 10-year final endpoint.
Results
The primary endpoint 5-year KM survivorship estimate (Figure 1) was 96.7% (95% C.I.: 94.7% – 98.0%). There were 15 revisions reported through 5 years, and another 5 thereafter. The average 5-year AKS score was 86.5 points, which was a 46.7 point improvement versus pre-operative (see Figure 3 for all clinical outcomes). RLL's, progressive RLL's, adjacent RLL's, and complete RLL's of the 529 knees with x-rays were as follows, respectively: 3.78% (N=20 knees), 0.57% (N=3), 0.38% (N=2), and 0%.
Discussion
Polyethylene wear after knee replacement surgery is a major concern for both orthopaedic surgeons and prosthetic manufacturing companies. As a result, manufacturing and design changes have been made to increase longevity of knee replacement systems. Lower wear rates were observed for fixed bearing inserts in polished CoCr trays compared with inserts in relatively rough titanium trays in both in vitro wear studies (Billi 2010, J Biomech) and in vivo studies (Berry 2012, CORR). Results of the current study demonstrate excellent 5-year survivorship that are consistent with a recent report (Kindsfater 2015, J Arthroplasty) on the same device, materials, and the class of primary TKA. Additional follow-up to 10 years is underway with these patients.
