2006 OTTO AUFRANC AWARD PAPER: Significance of In Vivo Degradation for Polyethylene in Total Hip Arthroplasty

Abstract

Our research group developed an implant retrieval program to study in vivo degradation of polyethylene. We now have evidence to support our hypothesis that degradation of radiation-sterilized polyethylene occurs in the body for not only historical gamma air sterilized liners, but also for conventional gamma inert sterilized (ArCom) and annealed highly crosslinked polyethylene (Crossfire) liners as well. Our research has also led to the discovery that the most severe manifestations of in vivo oxidation typically occur in regions of the liner experiencing minimal wear, such as the rim of the component, where the body fluids (containing oxidizing species) have access to the polyethylene. Our data from historical, ArCom, and Crossfire retrievals all point to a similar scenario in which the femoral head limits the in vivo oxidation of polyethylene at the bearing surface. Consequently, provided rim impingement does not occur, and the polyethylene locking mechanisms remain relatively isolated from oxidizing fluid, in vivo oxidation does not seem to be clinically important in the first 10 years of implantation for conventional gamma sterilized polyethylene. We conclude that in vivo degradation should be included among the list of potential long-term failure modes for modular polyethylene components for total hip arthroplasty.

Do First-generation Highly Crosslinked Polyethylenes Oxidize In Vivo?

“…Our data suggest contemporary gamma inert and annealed liners are associated with the highest levels of rim oxidation and hydroperoxide content and are consistent with previous studies suggesting the femoral head may play a role in mitigating oxidation at the bearing surface [20,21,30]. Moderate oxidation (average OI [ 1) was evident at the rim in annealed and gamma inert liners, but over half of the retrieved annealed liners exhibited severe rim oxidation (average OI [ 3).…”

“…The rims of acetabular components do not necessarily undergo repetitive mechanical loading throughout the duration of implantation. On the other hand, several recent studies [7,21,29] have demonstrated evidence of impingement (34%-56% of retrieved liners) at the rim of both historical and contemporary liner designs. With repeated loading at the rim, it remains unknown whether elevated rim oxidation may play a role in the long-term clinical performance of annealed and gamma inert liners.…”

History and Systematic Review of Wear and Osteolysis Outcomes for First-generation Highly Crosslinked Polyethylene

“…One decade after their clinical introduction, our knowledge of the clinical failure modes and reasons of revision for first-generation HXLPEs remains extremely limited. For annealed polyethylene, which contains residual free radicals, in vivo oxidation has been observed at the rim [78], and rim delamination has been identified in case studies of chronic instability and impingement [35]. The mechanical properties and oxidation of the bearing surface in retrieved annealed bearings have not yet shown evidence of degradation with implantation [78], suggesting the femoral head reduces in vivo oxidation at this location.…”

“…For annealed polyethylene, which contains residual free radicals, in vivo oxidation has been observed at the rim [78], and rim delamination has been identified in case studies of chronic instability and impingement [35]. The mechanical properties and oxidation of the bearing surface in retrieved annealed bearings have not yet shown evidence of degradation with implantation [78], suggesting the femoral head reduces in vivo oxidation at this location. For remelted polyethylenes, rim fracture has been reported in several case studies requiring revision of the bearing [42,57,58,101,131].…”

“…On the one hand, the crosslinking imparted by a single sterilization dose (25-40 kGy) of gamma radiation substantially improved its wear resistance [29,55] and contributed to the long-term clinical success of the metal-on-polyethylene wear couple used in hip and knee arthroplasty to this day. Gamma sterilization in the presence of air also had a drawback in that it rendered the polymer vulnerable to oxidation before implantation [81,113] and during in vivo exposure [30,36,78,82,129]. It would take decades, however, for gamma sterilization in air to be challenged as the industry standard for polyethylene, because the deleterious effects of oxidation in polyethylene take many years to manifest [81].…”