Can Sequentially-irradiated and Annealed Highly Cross-linked Polyethylene Inserts Thinner than Eight-millimeters Be Utilized in Total Knee Arthroplasty?

The routine use of highly cross-linked ultra-high molecular weight polyethylene (UHMWPE) has remained controversial secondary to the possibility of decreased material properties when compared to conventional UHMWPE. The aim of the present study was to evaluate if thin, sequentially-irradiated, and annealed highly cross-linked UHMWPE tibial inserts would have improved wear properties, while maintaining mechanical integrity, compared to conventional UHMWPE during biomechanical testing under aligned and malaligned conditions. Polyethylene inserts (4.27 and 6.27 mm) manufactured from GUR 1020-UHMWPE were cyclically loaded to analyze for wear. All wear scars were visually examined after loading using scanning electron microscopy (SEM). Volume loss was plotted versus cycle count with linear regression analysis yielding wear rates. There was no statistical difference in wear between both thicknesses for all testing conditions. During aligned condition testing, the volumetric wear rate for sequentially-irradiated and annealed polyethylene thicknesses of 4.27 and 6.27 mm was 4.0 and 4.4 mm3/million cycles; and during malaligned conditions, it was 13.9 and 15.1 mm3/million cycles. For conventional polyethylene during aligned conditions, the volumetric wear rate was 33.0 and 22.8 mm3/million cycles; and during malaligned conditions it was 50.0 and 50.8 mm3/million cycles. By SEM evaluation, condylar wear surfaces for conventional and sequentially-irradiated and annealed polyethylene displayed surface ripples typical of adhesive wear. There were no observed visible differences between the wear scars for conventional compared to sequentially-irradiated and annealed polyethylene with no evidence of fatigue failure. This study demonstrated no differences between polyethylenes with thicknesses of 4.27 and 6.27 mm. This strengthens the conclusion that sequentially-irradiated and annealed highly cross-linked UHMWPE can be utilized in total knee arthroplasty. The successful wear properties of 4.27 mm liners could mean that smaller tibial resections leading to bone stock preservation could be utilized in patients undergoing total knee arthroplasty, although further in-vivo studies are needed.

Effect of acetabular cup abduction angle on wear of ultrahigh-molecular-weight polyethylene in hip simulator testing.

The effect of acetabular component positioning on the wear rates of metal-on-polyethylene articulations has not been extensively studied. Placement of acetabular cups at abduction angles of more than 40° has been noted as a possible reason for early failure caused by increased wear. We conducted a study to evaluate the effects of different acetabular cup abduction angles on polyethylene wear rate, wear area, contact pressure, and contact area. Our in vitro study used a hip joint simulator and finite element analysis to assess the effects of cup orientation at 4 angles (0°, 40°, 50°, 70°) on wear and contact properties. Polyethylene bearings with 28-mm cobalt-chrome femoral heads were cycled in an environment mimicking in vivo joint fluid to determine the volumetric wear rate after 10 million cycles. Contact pressure and contact area for each cup abduction angle were assessed using finite element analysis. Results were correlated with cup abduction angles to determine if there were any differences among the 4 groups. The inverse relationship between volumetric wear rate and acetabular cup inclination angle demonstrated less wear with steeper cup angles. The largest abduction angle (70°) had the lowest contact area, largest contact pressure, and smallest head coverage. Conversely, the smallest abduction angle (0°) had the most wear and most head coverage. Polyethylene wear after total hip arthroplasty is a major cause of osteolysis and aseptic loosening, which may lead to premature implant failure. Several studies have found that high wear rates for cups oriented at steep angles contributed to their failure. Our data demonstrated that larger cup abduction angles were associated with lower, not higher, wear. However, this potentially "protective" effect is likely counteracted by other complications of steep cup angles, including impingement, instability, and edge loading. These factors may be more relevant in explaining why implants fail at a higher rate if cups are oriented at more than 40° of abduction.