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 ...

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Post-irradiation melting of ultra-high molecular weight polyethylene (UHMWPE) reduced the oxidation potential of UHMWPE in vivo. After mid-term (5-10 years) use in vivo, there is detectable oxidation in irradiated and melted joint implant retrievals. The absorption of the synovial fluid lipid squalene was identified as a possible factor initiating oxidation. We investigated the role of lipids in UHMWPE oxidation by asking: (1) Do other synovial fluid lipids initiate oxidation in irradiated and melted UHMWPE?; (2) What is the effect of the absorption of multiple lipids on UHMWPE oxidation?; (3) How does lipid-initiated oxidation in vitro compare to what is observed in long-term retrievals? We diffused emulsified single and mixed lipids into irradiated and melted UHMWPE and accelerated aged them. We analyzed the oxidation in these samples and in four long-term highly crosslinked, irradiated, and melted Longevity™ UHMWPE liner retrievals (in vivo for up to 190 months) using Fourier Transform Infrared Spectroscopy (FTIR). We showed that lipids other than squalene could initiate oxidation in UHMWPE and that the types of absorbed lipids determined the amount of resultant oxidation. Although mixed lipids doping and accelerated aging reproduced the average and maximum oxidation values and oxidation products observed in vivo, the oxidation depth profile and its effect on cross-link density was different. One reason for this was the variability of oxidation in retrievals, suggesting additional factors contributing to oxidation. The understanding of oxidative processes in vivo and the development of clinically relevant in vitro protocols to evaluate implant materials is crucial for their long-term performance. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1833-1839, 2018.

Background: Atlantoaxial (C1-C2) osteoarthritis (AAOA) causes severe suboccipital pain exacerbated by lateral rotation. The pain is usually progressive and resistant to conservative therapy. Posterior fusion surgery is performed to stabilise the C1-C2 segment. This is the first Australian study reporting the outcome of posterior atlantoaxial fixation including hybrid fixations performed for AAOA. Methods: All patients who underwent posterior atlantoaxial fixation surgery for AAOA from 2005 to 2015 at our institutions were enrolled (N=23). Patient demographics and surgical technique were recorded. These techniques included transarticular screw (TAS) fixation using image guidance with iliac crest bone graft and supplemental posterior Sonntag wiring, or C1-C2 lateral mass fixation (Harms technique). Some patients required a combination of fixation due to anatomical variation. Primary outcome measures including patient satisfaction, pain, disability scores and range of motion were recorded for all patients pre-and postoperatively. Post-operative assessment was supplemented with CT and X-ray imaging. Results: Twenty-three patients (19 women, 4 males, mean age 71.8±6.3 years) underwent surgical fixation. Eight underwent TAS fixation, 8 had Harms fixation, and 7 had a hybrid fixation. All patients reported statistically significant improvement in pain scores [Visual Analogue Scale (VAS) 9.4 pre-op compared to 2.9 post-op, P<0.005]. Disability scores [Neck Disability Index (NDI)] were statistically significantly reduced from 72.2±12.9 pre-operatively to 18.9±11.9 post-operatively, P<0.005. Mean followup was 55.3±36.1 months. Results did not vary according to the construct type. Ninety-five point five percent of patients showed radiographic evidence of fusion. Ninety-one percent of patients said they would undergo the surgery again. Conclusions: Posterior atlantoaxial fixation with TAS and Harms constructs are highly effective for the surgical treatment of intractable neck pain secondary to atlantoaxial lateral mass osteoarthritis (AAOA). Surgery offers a high rate of symptom relief. If anatomical variability exists, both transarticular and pedicle screw fixation could be safely used in the same patient.