Surface cross‐linked UHMWPE can enable the use of larger femoral heads in total joints

Abstract

Limiting cross-linking to the articular surfaces of ultrahigh molecular weight polyethylene (UHMWPE) to increase wear resistance while preventing detrimental effects of cross-linking on mechanical strength has been a desirable goal. A surface cross-linked UHMWPE can be achieved by blending UHMWPE with a free radical scavenger, such as vitamin E, consolidating the blend into an implant shape, extracting the vitamin E from the surface, and radiation cross-linking the surface extracted blend. This process results in high cross-link density in the vitamin E-depleted surface region because vitamin E hinders cross-linking during irradiation. In this study, we described the properties of successful extraction media and the manipulation of the wear and mechanical properties of extracted, irradiated blends. We showed that these formulations could have similar wear and significantly improved mechanical properties compared to currently available highly cross-linked UHMWPEs. We believe that these materials can enable thinner implant forms and more anatomical designs in joint arthroplasty and may provide a feasible alternative to metal-on-metal implants. The catastrophic periprosthetic consequences of metalon-metal (MOM) hip implants 1 have increased demands for longevity with alternative bearings. MOM benefits were decreased risk of dislocation 2 and increased range of motion, both resulting from the larger femoral head sizes (36-60 mm diam.) that could be accommodated. Conversely, alternative bearings comprised of metal heads and ultrahigh molecular weight polyethylene (UHMWPE) acetabular liners are typically limited to smaller sizes (commonly 32-40 mm) for two reasons.First, current hip joint technology is usually modular with a metal acetabular shell and a UHMWPE liner, making a metal-on-polyethylene (MOP) system thicker than a comparable MOM. Thus, the jump distance for the femoral head to dislocate from the acetabulum is larger in MOM implants (average of 28 mm compared to 16 mm for MOP). Second, the recommended UHMWPE thickness is at least 6 mm, albeit based on a study of conventional gamma-in-air sterilized oxidation-prone UHMWPE, 3 which is not the norm today.4 Highly cross-linked UHMWPEs, 5 developed to provide superior wear and oxidation resistance, 6,7 have significantly reduced fatigue strength. 8,9These materials have decreased wear dramatically at 10 years of clinical use 10 and showed superior oxidation resistance in vivo compared to conventional UHMWPE. At the same time, concerns exist about fatigue-related fracture and failure when using thinner implants, especially at the rim and in locking mechanisms, which are susceptible to damage under adverse loading conditions such as impingement. Therefore, a need exists to develop MOP bearing systems with improved fatigue strength to enable use of thinner UHMWPE bearings with larger femoral heads. This may also enable the use of UHMWPE bearing surfaces with the possibility of fewer dislocations and larger range of motion, perhaps equivalent to the larger ...

Natural Polyphenols Enhance Stability of Crosslinked UHMWPE for Joint Implants

“…The decreased mechanical strength and fatigue resistance remain as concerns [14,15]. Alternatively, stabilization of irradiated UHMWPE with vitamin E has improved strength and toughness [35][36][37][38]. Natural polyphenols have been used in food and pharmaceutics as potent antioxidants [3,11].…”

Chromium nitride coating for large diameter metal-on-polyethylene hip bearings under extreme adverse hip simulator conditions

“…A larger diameter, however, might allow for a greater range of motion [5] and reduce the clinical risk of dislocation which is one of the leading causes for revision in metal-on-polyethylene bearings behind osteolysis and loosening [6,7]. The lower wear rates achieved in the latest generation of orthopaedic polyethylene [8] has led many to propose the use of this material in larger diameters hip replacements [9].…”

Surface cross‐linked ultra high molecular weight polyethylene by emulsified diffusion of dicumyl peroxide

“…In the second method, peroxide can diffusion into an already consolidated UHMWPE sample at below the peroxide decomposition temperature followed by heating above that temperature allows cross‐linking if the sample in regions where peroxide is able to penetrate . The main benefit of the latter method is to limit cross‐linking to the surface of the polymer intended for use as a bearing surface, resulting in increased wear resistance on the surface with improved toughness in the bulk compared with a uniformly cross‐linked UHMWPE …”