Effect of Radiation, Heat, and Aging on In Vitro Wear Resistance of Polyethylene

Abstract

Radiation cross-linking increases the wear resistance of polyethylene used in total hip replacement. Radiation also generates residual free radicals, which are detrimental to long-term properties of polyethylene. Two approaches are used to stabilize the residual free radicals and terminally sterilize the components. One is postirradiation annealing with gas sterilization and the other is postirradiation melting with gamma sterilization in nitrogen. The hypothesis of the current study is that postirradiation annealing followed by gamma sterilization in nitrogen will result in more free radicals in polyethylene than gamma sterilization either in air or in nitrogen alone. To test this hypothesis, concentration of residual free radicals was quantified in polyethylene that was annealed and gamma sterilized in nitrogen and control polyethylenes gamma sterilized in air versus in nitrogen. Three crosslinked polyethylenes that were melted and gas sterilized also were included in the study. The effects of residual free radicals were studied by accelerated aging. Oxidation levels and weight loss in bidirectional pin-on-disk tests were determined before and after aging. Polyethylene that was subjected to postirradiation annealing and gamma sterilization resulted in 58% more residual free radicals than control polyethylenes. Weight loss of the annealed polyethylene increased by 16-fold on accelerated aging and had three times higher oxidation levels than that measured in control polyethylenes after aging. In contrast, polyethylenes that were stabilized with postirradiation melting and terminally gas sterilized showed no detectable residual free radicals. Accelerated aging did not affect the weight loss and oxidation levels of melted polyethylenes.

Registry Outcomes of Unicompartmental Knee Arthroplasty Revisions

“…In our registry, the CRR is higher for primary UKA compared with TKA, indicating a decreased overall survivorship for UKA compared with TKA, a finding consistent with those reported previously [8,9,15,17,20]. In contrast, no difference was found in the secondary CRR between the rev-UKA and rev-TKA groups.…”

“…Numerous studies have cited consistent modes of UKA failure, including progressive adjacent compartment arthritis, aseptic femoral or tibial component loosening, and polyethylene wear [3,4,6,9,16,17,24,25]. We sought to determine how rev-UKAs perform in a community registry population in comparison to revTKAs.…”

“…The proposed advantages of UKA include preservation of femoral and tibial bone stock, a less invasive surgery, improved knee range of motion [2] and kinematics, and shorter postoperative lengths of hospitalization [22]. Although early midterm studies cited a high rate of satisfactory results after UKA [12], long-term survivorship and outcome continue to be a concern [9,13,20]. Although these concerns are not without foundation, they have been addressed with advancements in prosthetic component design, instrumentation and surgical technique, and appropriate patient selection [5,7,27].…”

“…Some reports have cited reliable UKA survivorship of greater than 90% for more than 10 years after implantation [1,11,19,21,26]. When compared with TKA as an index arthroplasty procedure, however, the cumulative revision rate for UKA tends to be higher [8,9,15,17,20].…”

Mobile Versus Fixed Bearing Medial Unicompartmental Knee Arthroplasty: A Series of 375 Patients

“…Component loosening has been proposed to be one of the leading causes of conversion to TKA [13]. In mobile bearing implants, the motion and shear force transmission from the mobile bearing interface should theoretically lead to low rates of component loosening.…”