Correlation Between In Vivo Oxidation and Mechanical Properties In Contemporary Polyethylene Bearings

Introduction: Ultra-high molecular weight polyethylene oxidation remains an important concern in total joint arthroplasty, since it eventually results in a significant drop in mechanical properties that may compromise implant longevity, especially for TKA [1-3]. In vivo oxidation of polyethylene can occur in the presence of unstabilized macroradicals within the polymer and molecular oxygen uptake from body fluids and tissues [1-3]. The oxidation index (OI) is widely used to quantify oxidation in polyethylene (ASTM F2102), and the ultimate load from the small punch test has been used to quantify its ultimate strength (ASTM 2183). Previously, researchers have suggested that an OI of greater than 3 was associated with "critical" loss in mechanical properties and a propensity for fatigue damage in vivo [3]. The interpretation of OI levels less than 3 remains unclear. The objective of this study was to employ ASTM techniques to examine the correlation between in vivo oxidation levels and mechanical properties at the articular surface of retrieved contemporary acetabular liners and tibial inserts. We hypothesized that an ASTM OI below 1 can be interpreted as "low" oxidation and would not significantly affect the ultimate strength of polyethylene. Finally, the influence of oxidation and mechanical properties on wear performance and surface damage was also explored. Methods: A total of 157 polyethylene components (80 acetabular liners and 77 tibial inserts) were collected after revision surgery. Of the 80 acetabular liners, 63 were annealed highly crosslinked (Crossfire, Stryker) and 17 conventional gamma inert sterilized, whereas all the tibial inserts were gamma inert sterilized. Clinical and polyethylene information, including implantation times, patient characteristics, shelf lives and polyethylene resin type, were also available (Table 1). Femoral head penetration was measured in intact acetabular liners implanted for periods longer than 1 year. In addition, surface damage scores per Hood [4] were obtained for tibial inserts. FTIR analysis following ASTM F2102 was conducted on polyethylene sections microtomed from retrievals. To avoid the interference of absorbed lipids, polyethylene sections underwent heptane extraction (6 hours) prior to the oxidation analysis. Maximum oxidation indexes were calculated from FTIR spectra at the articulating surface of the acetabular liners, and tibial inserts. Finally, mechanical properties were assessed by means of the small punch technique (ASTM F2183). Near surface and subsurface small punch specimens were prepared from superior (worn) and inferior (unworn) regions of the bearing for retrieved liners. Similarly, small punch test specimens were machined from the lateral condyle of tibial inserts near the surface and at the subsurface of anterior, posterior, medial and lateral locations. General linear models were developed to explore correlations and examine differences in oxidative, mechanical and wear performance (p<0.05 level of significance) Results: Patient cohorts corresponding to acetabular liners were statistically comparable with respect to age, weight and activity level (p≥0.4). All the gamma inert sterilized polyethylene retrievals oxidized regardless of polyethylene formulation. Among retrieved acetabular liners, the highest oxidation (1.6) was found in the annealed highly crosslinked group. In contrast, ultimate loads were significantly higher regardless of location for annealed compared to conventional acetabular liners (p≤0.002). Retrieved acetabular liners with oxidation levels less than 1 demonstrated no significant correlation between oxidation and ultimate load (p≥0.09). General linear models confirmed a significant correlation between both oxidation and ultimate loads at the superior subsurface (p=0.02) for conventional liners. Similar correlations were found for oxidation and ultimate loads near the superior surface (p<0.0001) and subsurface (p=0.02) of annealed liners. Ultimate loads dropped up to 40% for oxidation levels near 1 in conventional liners. In contrast, annealed liners registered a 31% drop when oxidation reached levels close to 2 (Figure 1). With regard to tibial inserts, 1900H retrievals had significantly higher oxidation than retrievals produced from GUR 1050 (p=0.048; Student’s t-test) and GUR 1020 (p=0.0002). Again, ultimate loads significantly decreased with oxidation (p<0.0001; linear models), regardless of polyethylene resin (Figure 2). 1900H retrievals had, however, slightly higher ultimate loads at zero oxidation levels with respect to the other resins. Ultimate load and oxidation were not correlated in tibial inserts with oxidation equal or less than 1 (p≥0.2; linear models). With respect to wear performance, there were no significant correlations between femoral penetration rate of acetabular liners or bearing surface damage scores of tibial inserts and oxidation (p=0.2 and p=0.75, respectively) or near surface mechanical properties (p≥0.34 and p≥0.28). Discussion: The current study provides a quantitative basis for interpreting ASTM OI values based on its effect on mechanical properties. Our findings supported the hypothesis that ASTM oxidation levels below 1 do not have a significant effect on the mechanical performance of polyethylene. Above that oxidation level, polyethylene mechanical properties start to decrease, confirming a transition in mechanical behavior when oxidation increases from low (OI<1) to moderate (OI≥1). The lack of correlation of oxidation with clinical performance in the current study is consistent with previous research, suggesting that a critical level of oxidation (OI>3) may be necessary to compromise the mechanical properties of polyethylene. Acknowledgements: Supported by research grants from NIH (NIAMS) R01 AR47904, Stryker Orthopedics, Zimmer, Inc., and Sulzer Settlement. References: [1] Kurtz et al., CORR, 2006; [2] Medel et al., J Biomed Mat Res, 2008; [3] Currier et al. JOA 2007; [4] Hood et al. JBMR 1983.