Evaluating the suitability of highly cross‐linked and remelted materials for use in posterior stabilized knees

Abstract

Posterior stabilized (PS) knee designs are a popular choice for cruciate sacrificing knee arthroplasty procedures. The introduction of PS inserts fabricated from highly cross-linked and remelted Ultra High Molecular Weight Polyethylene (UHMWPE) has recently generated concern as these materials have been shown to possess reduced mechanical properties. This study investigated whether highly cross-linked and remelted UHMWPE material (referred to as XRP) can be expected to perform similarly to historical gamma-air polyethylene, which has suffered few reported incidences of tibial post failure. Never-implanted gamma-air PS tibial inserts shelf-aged 14 years were examined and compared to XRP materials. Evaluation of oxidation levels, impact toughness, and fatigue strength demonstrated never-implanted gamma-air PS tibial inserts to possess nonuniform mechanical properties. Despite severe oxidation along the exterior of gamma-air tibial posts, comparatively low oxidation levels at the center of the tibial posts corresponded to sufficiently high mechanical properties. XRP material (75 kGy) showed superior impact toughness over shelf aged gamma-air material; however, tibial post fatigue testing demonstrated XRP material (100 kGy) to be less resistant to fatigue failure than historical gamma-air material. Results from this study indicate that XRP materials (100 kGy) may demonstrate an inferior resistance to tibial post failure than historical polyethylene.

Grafted Vitamin-E UHMWPE may increase the durability of posterior stabilized and constrained condylar total knee replacements

“…Delamination is related to high subsurface contact stresses in TKR . Elevated oxidation levels, a consequence of chain scission and free radical formation during irradiation of CPE, have been noted in the subsurface region and can contribute to delamination through local mechanical property change . Moreover, studies of clinically retrieved HXPE are showing evidence of slow but progressive in vivo oxidation which has been postulated to result from reactions between residual or newly created free radicals with oxygen in vivo , and alternatively by oxidation of in vivo absorbed lipids such as squalene and cholesterols (components of synovial fluids in the joint), which occurs via similar free radical‐based mechanism as polyethylene.…”

“…Remelted and annealed HXPE have shown reduced initial mechanical and fatigue properties compared to conventional UHMWPE (CPE), both of which may raise the incidence of tibial post fractures. In an in vitro post fatigue comparison of shelf‐aged CPE to 100 kGy HXPE, it was found that CPE was stronger . However, it has been shown that with appropriate design considerations, such as increasing the tibial post thickness, HXPE can perform as well as CPE articular surfaces in in vitro impingement loading conditions .…”

Effectiveness of anti‐oxidant polyethylene: What early retrievals can tell us

“…Evaluation of the change of toughness (the ability of the material to resist crack propagation) with oxidation was not possible for these antioxidant retrievals. However, toughness of all the UHMWPE materials represented by the retrievals in this study varied with irradiation dose . As the irradiation dose used to cross‐link these antioxidant materials was increased above a conventional sterilization dose, the toughness of the materials was decreased compared to conventional gamma‐inert sterilized polyethylene retrievals.…”

Clinical trade‐offs in cross‐linked ultrahigh‐molecular‐weight polyethylene used in total joint arthroplasty

“…These fractured acetabular components, while not commonplace, are likely due to the adoption of a more flaw intolerant material in a design containing notches in highly stressed locations of the component. TKR failures have indicated cam post impingement, delamination, as well as fatigue and fracture of posts 56–61. The use of XLPE in TKR warrant caution as the tibial components are subjected to high cyclic contact stresses and can have stress concentration in their cam post designs.…”