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 ...
Current standards for testing cemented femoral hip stems involve potting the distal third of the stem and applying a cyclic load, see ISO 7206-8. 1995(E). However, this procedure does not test implants for the most prevalent mode of failure — loosening of the prosthesis from the host bone (Malchau et al., 1993). To quantify the expected longevity of implants in a pre-clinical test, a protocol that establishes the rate of loosening of a prosthesis must be developed. Using Radiostercophotogrammetry (RSA), Karrholm et al. (1994) have correlated migration of more than 1 mm within two years of implantation with early loosening of the implant. Therefore, by quantifying the migration of a prosthesis in a laboratory test for the equivalent of two years post implantation, its risk of loosening in vivo can be assessed. In the past, the motion of cemented implants in one direction has been quantified (Manley et al., 1987); whilst other researchers have performed a more complete analysis of the motion of prostheses, but for a limited number of loading cycles (e.g. Schneider et al., 1989). In most of these studies, prosthesis insertions were performed by surgeons. Not only does this introduce subjectivity into the pre-clinical test, but in the case of cemented replacements, variations in stem orientation and cementing pressures will occur. This paper presents the design and initial validation of procedures which may be used for pre-clinical testing of cemented hip stems.
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