Orhun K. Muratoglu

Advances in the processing, sterilization, and crosslinking of ultra-high molecular weight polyethylene for total joint arthroplasty

Despite the recognized success and worldwide acceptance of total joint arthroplasty, wear is a major obstacle limiting the longevity of implanted UHMWPE components. Efforts to solve the wear problem in UHMWPE have spurred numerous detailed studies into the structure, morphology, and mechanical properties of the polymer at every stage of its production from original resin into stock material and final fabricated form. Scientific developments in this field are occurring at an accelerating rate, and periodic review of UHMWPE technology is therefore increasingly necessary. The present article provides a four-part comprehensive review of technological advancements in the processing, manufacture, sterilization, and crosslinking of UHMWPE for total joint replacements. The first part of this article describes the recently updated nomenclature of UHMWPE, including the process of resin production and conversion to stock material. The second part outlines the methods of manufacturing UHMWPE into joint replacement components and provides overviews of alternate forms of UHMWPE, namely carbon-fiber reinforced UHMWPE (Poly II) and UHMWPE recrystallized under high temperature and pressure (Hylamer). The third part summarizes the sterilization and degradation of UHMWPE. Newly developed methods for accelerating the oxidation of UHMWPE after sterilization (for preconditioning of test specimens), as well as methods for quantifying the oxidation of UHMWPE, are also discussed. Finally, the fourth part reviews the development and properties of crosslinked UHMWPE, a promising alternate biomaterial for total joint replacements.

Unified wear model for highly crosslinked ultra-high molecular weight polyethylenes (UHMWPE).

Crosslinking has been shown to improve the wear resistance of ultra-high molecular weight polyethylene in both in vitro and clinical in vivo studies. The molecular mechanisms and material properties that are responsible for this marked improvement in wear resistance are still not well understood. In fact, following crosslinking a number of mechanical properties of UHMWPE are decreased including toughness, modulus, ultimate tensile strength, yield strength, and hardness. In general, these changes would be expected to constitute a precursor for lower wear resistance, presenting a paradox in that wear resistance increases with crosslinking. In order to understand better and to analyze this paradoxical behaviour of crosslinked UHMWPE, we investigated the wear behavior of (i) radiation-crosslinked GUR 1050 resin, (ii) peroxide-crosslinked GUR 1050 resin and (iii) peroxide-crosslinked Himont 1900 resin using a bi-directional pin-on-disk (POD) machine. Wear behavior was analyzed as a function of crystallinity, ultimate tensile strength (UTS), yield strength (YS), and molecular weight between crosslinks (Mc). The crosslink density increased with increasing radiation dose level and initial peroxide content. The UTS, YS, and crystallinity decreased with increasing crosslink density. While these variations followed the same trend, the absolute changes as a function of crosslink density were different for the three types of crosslinked UHMWPE studied. There was no unified correlation for the wear behavior of the three types of crosslinked UHMWPE with the crystallinity, UTS and YS. However, the POD wear rate showed the identical linear dependence on Mc with all three types of crosslinked UHMWPEs studied. Therefore, we have strong evidence to propose that Mc or crosslink density is a fundamental material property that governs the lubricated adhesive and abrasive wear mechanisms of crosslinked UHMWPEs, overriding the possible effects of other material properties such as UTS, YS and crystallinity on the wear behavior.

Toughening mechanism of rubber-modified polyamides

Abstract Rubber-modified polyamides were probed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), small-angle X-ray scattering and various mechanical tests. TEM studies showed that in tough samples the crystalline orientation in the interparticle region is of a distinctly different character. The lamellae are organized perpendicular to the rubber-matrix interface, while the hydrogen-bonded planes of low slip resistance are aligned parallel to these interfaces. Based on this observation a model is proposed to elucidate the deformation and toughening mechanisms of these materials. Further SEM studies in the stress-whitened regions of both the tensile bars and Izod specimens revealed the evolution of a cavitation process in the rubber particles. The shape and the size of the cavities in tough samples is related to the initial morphology of the matrix.

Vitamin E diffused, highly crosslinked UHMWPE: a review

Highly crosslinked UHMWPE has become the bearing surface of choice in total hip arthroplasty. First generation crosslinked UHMWPEs, clinically introduced in the 1990s, show significant improvements compared to gamma sterilised, conventional UHMWPE in decreasing wear and osteolysis. These crosslinked UHMWPEs were thermally treated (annealed or melted) after irradiation to improve their oxidation resistance. While annealing resulted in the retention of some oxidation potential, post-irradiation melted UHMWPEs had reduced fatigue strength due to the crystallinity loss during melting. Thus, the stabilisation of radiation crosslinked UHMWPEs by the diffusion of the antioxidant vitamin E was developed to obtain oxidation resistance with improved fatigue strength by avoiding post-irradiation melting. A two-step process was developed to incorporate vitamin E into irradiated UHMWPE by diffusion to obtain a uniform concentration profile. Against accelerated and real-time aging in vitro, this material showed superior oxidation resistance to UHMWPEs with residual free radicals. The fatigue strength was improved compared to irradiated and melted UHMWPEs crosslinked using the same irradiation dose. Several adverse testing schemes simulating impingement showed satisfactory behaviour. Peri-prosthetic tissue reaction to vitamin E was evaluated in rabbits and any effects of vitamin E on device fixation were evaluated in a canine model, both of which showed no detrimental effects of the inclusion of vitamin E in crosslinked UHMWPE. Irradiated, vitamin E-diffused, and gamma sterilised UHMWPEs have been in clinical use in hips since 2007 and in knees since 2008. The clinical outcome of this material will be apparent from the results of prospective, randomised clinical studies.

Poly(vinyl alcohol)–acrylamide hydrogels as load-bearing cartilage substitute

Poly(vinyl alcohol) (PVA) has been advanced as a biomaterial for the fabrication of medical devices to be used as synthetic articular cartilage because of its viscoelastic nature, high water content, and biocompatibility. Key material requirements for such devices are high creep resistance to prevent mechanical instability in the joint and high water content to maintain a lubricious surface to minimize wear and damage of the cartilage counterface during articulation. The creep resistance of PVA hydrogels can be increased by high temperature annealing; however this process also collapses the pores, reducing the water content and consequently reducing the lubricity of the hydrogel surface [Bodugoz-Senturk H, Choi J, Oral E, Kung JH, Macias CE, Braithwaite G, et al. The effect of polyethylene glycol on the stability of pores in polyvinyl alcohol hydrogels during annealing. Biomaterials 2008;29(2):141-9.]. We hypothesized that polymerizing acrylamide (AAm) in the pores of the PVA hydrogel would minimize the loss of lubricity during annealing by preventing the collapse of the pores and loss of water content. Increasing AAm content increased porosity and equilibrium water content and decreased the coefficient of friction, tear strength, crystallinity, and creep resistance in annealed PVA hydrogels.

The effects of high dose irradiation on the cross-linking of vitamin E-blended ultrahigh molecular weight polyethylene

Vitamin E-stabilized, highly cross-linked ultrahigh molecular weight polyethylene (UHMWPE) is a promising oxidation and wear resistant UHMWPE with improved mechanical strength in comparison with the first generation, irradiated and melted UHMWPE. One approach of incorporating vitamin E in UHMWPE is through blending of vitamin E in UHMWPE powder followed by consolidation and radiation cross-linking. However, radiation cross-linking efficiency of UHMWPE decreases in the presence of vitamin E. Therefore an optimum vitamin E concentration and radiation dose level need to be determined to achieve a cross-link density comparable to 100-kGy irradiated and melted UHMWPE, which has shown excellent wear properties in vivo. We investigated the cross-link density and mechanical properties of vitamin E-blended UHMWPEs as a function of vitamin E concentration in the blend and gamma irradiation doses up to 200kGy. We found that 0.3wt% vitamin E-blended UHMWPE could not be cross-linked above a cross-link density achieved at a radiation dose of 65kGy for virgin UHMWPE and 1.0wt% vitamin E-blended UHMWPE could not be cross-linked above a cross-link density achieved at a radiation dose of 25kGy for virgin UHMWPE even when the these UHMWPEs were irradiated to a radiation dose of 200kGy. In addition, higher plasticity at vitamin E concentrations at and above 0.3wt% indicated that increased chain scissioning may be prevalent. Since the wear resistance of this irradiated UHMWPE would be expected to be low, vitamin E concentrations equal to or above 0.3wt% are not recommended for subsequent irradiation to achieve a wear resistant cross-linked UHMWPE. The long-term oxidative stability of irradiated blends with low vitamin E concentrations has yet to be studied to determine an optimum between cross-link density and long-term oxidative stability.

Third-body wear of highly cross-linked polyethylene in a hip simulator.

The wear performance of a radiation cross-linked melted ultrahigh-molecular-weight polyethylene (UHMWPE) articulating against 28-mm cobalt chrome femoral heads in the presence of third-body particulate debris was investigated in a hip simulator and compared with the wear of conventional UHMWPE. Particles of aluminum oxide or bone cement containing barium sulfate were added to the serum. In the presence of aluminum oxide particles, the incremental wear rates of conventional UHMWPE averaged as high as 149 +/- 116 mg/million cycles compared with 37 +/- 38 mg/million cycles for the highly cross-linked components. The difference in the average weight loss was statistically significant at P <.01. With bone cement particles, the conventional UHMWPE components had an average incremental wear rate of 19 +/- 5mg/million cycles, and the wear rate of the highly cross-linked UHMWPE components was 0.5 +/- 0.7 mg/million cycles.

A new mechanism of oxidation in ultrahigh molecular weight polyethylene caused by squalene absorption

Although synovial fluid lipids were found to absorb in ultrahigh molecular weight polyethylene (UHMWPE) total joint implants in vivo, their effect on the oxidation of the polymer was not known. Current understanding of the oxidation and oxidative stability of UHMWPE joint implants is focused on the presence or elimination of radiation-induced free radicals, which are long-lived and can react with oxygen over the long term. Recently, we found unexplained oxidation in irradiated and melted UHMWPE components that were exposed to bodily fluids then stored on the shelf despite being free of detectable free radicals at the time of implantation. Thus, we hypothesized that lipids absorbed from the synovial fluid in vivo could initiate and accelerate oxidation of UHMWPE even in the absence of detectable residual free radicals. We found that squalene, a precursor in cholesterol synthesis and a synovial fluid lipid with unsaturated bonds, accelerated oxidation in irradiated and melted UHMWPE under in vitro accelerated aging conditions. This result represents a paradigm shift in our understanding of oxidative stability of UHMWPE and prompts further investigation of in vivo oxidation mechanisms as well as the development of relevant in vitro aging models.

Ex Vivo Stability Loss of Irradiated and Melted Ultra-High Molecular Weight Polyethylene

BACKGROUND Radiation crosslinking reduces wear of ultra-high molecular weight polyethylene (UHMWPE), and subsequent annealing or melting increases oxidative stability. Little is known about the oxidative stability of polyethylene total joint components after in vivo service and subsequent shelf storage in air. METHODS We analyzed thirty-four surgically retrieved, radiation crosslinked acetabular liners to determine their oxidative stability after in vivo service (range, 0.5 to 84.0 months). Oxidation was determined at the time of explantation. After shelf storage in air (range, 7.0 to 72.0 months), oxidation, crosslink density, and thermal properties were determined. Oxidation of one control liner that was shelf-aged in air (for eighty-four months) was also determined. RESULTS At the time of explantation, all components showed minimal oxidation; however, oxidation levels increased during shelf storage, with a concomitant decrease in crosslink density and increase in crystallinity. Increasing oxidation, increasing crystallinity, and decreasing crosslink density correlated with the duration of ex vivo storage. The shelf-aged control liner showed no detectable oxidation. CONCLUSIONS The oxidation and loss of crosslink density of the irradiated and melted UHMWPE was surprising. Two potential mechanisms that might alter the oxidative stability of UHMWPE in vivo are cyclic loading and absorption of lipids. Both of these mechanisms can generate new free radicals in UHMWPE and can initiate and propagate its oxidation.

Optical analysis of surface changes on early retrievals of highly cross-linked and conventional polyethylene tibial inserts

Retrieved tibial liners of highly cross-linked and conventional polyethylene were examined for articular and backside surface damage. Surfaces were graded for pitting, machine-mark loss, scratching, abrasion, delamination, and embedded debris. Whereas no difference existed in the damage score for the 2 groups, the highly crosslinked group showed significantly less elimination of machine marks. Wear, surface plastic deformation, or a combination, could account for the damage on these components. Only 1 of the highly crosslinked polyethylene inserts was available for destructive testing. That insert was melted to activate the shape memory, and thus differentiate, between wear versus plastic deformation. Nearly all changes on the articular and backside surfaces disappeared upon melting, and original machining marks reappeared, suggesting that the surface changes for that component were primarily the result of plastic deformation and not material removal.