Avram A. Edidin

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.

Plasticity-induced damage layer is a precursor to wear in radiation-cross-linked UHMWPE acetabular components for total hip replacement

The mechanism for the improved wear resistance of cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) remains unclear. This study investigated the effect of cross-linking achieved by gamma irradiation in nitrogen on the tribologic, mechanical, and morphologic properties of UHMWPE. The goal of this study was to relate UHMWPE properties to the wear mechanism in acetabular-bearing inserts. Wear simulation of acetabular liners was followed by detailed characterization of the mechanical behavior and crystalline morphology at the articulating surface. The wear rate was determined to be directly related to the ductility, toughness, and strain-hardening behavior of the UHMWPE. The concept of a plasticity-induced damage layer is introduced to explain the near-surface orientation of the crystalline lamellae observed in the wear-tested acetabular liners. Cross-linking reduces abrasive wear of acetabular components by substantially reducing--but not eliminating--the plasticity-induced damage layer that precedes abrasive wear.

Fixation of Acetabular Cups without Cement in Total Hip Arthroplasty. A Comparison of Three Different Implant Surfaces at a Minimum Duration of Follow-up of Five Years*

We evaluated 377 patients (428 hips) who had been managed, by a total of fourteen surgeons at twelve clinical sites in the United States and Europe, with a porous-coated press-fit acetabular cup, a hydroxyapatite-coated threaded screw-in cup, or one of two similar designs of hydroxyapatite-coated press-fit cups between April 1987 and November 1992. The same type of hydroxyapatite-coated femoral stem was inserted without cement in all patients. After a minimum duration of follow-up of five years (mean, 7.9 years; range, 5.3 to 9.1 years), one (1 per cent) of the 131 hydroxyapatite-coated threaded cups, two (2 per cent) of the 109 porous-coated press-fit cups, and twenty-one (11 per cent) of the 188 hydroxyapatite-coated press-fit cups had been revised because of aseptic loosening. A common radiographic sign of impending failure of the hydroxyapatite-coated press-fit cups was radiolucency at the interface between the implant and the subchondral bone beneath it. This radiolucency usually was seen initially more than two years after implantation. Radiographic evaluation of the 383 acetabular implants that were in situ at the time of the most recent follow-up showed that 123 (99 per cent) of the 124 hydroxyapatite-coated threaded cups, 101 (98 per cent) of the 103 porous-coated cups, and 139 (89 per cent) of the 156 hydroxyapatite-coated press-fit cups were stable with osseous ingrowth (as indicated by the absence of radiolucency at the interface and the absence of migration within the acetabulum). The probability of revision due to aseptic loosening was significantly greater for the hydroxyapatite-coated press-fit cups than it was for the hydroxyapatite-coated threaded cups or the porous-coated press-fit cups (p < 0.001 for both comparisons). Within the group of patients who had a hydroxyapatite-coated press-fit cup, the probability of revision due to aseptic loosening was significantly greater in association with a young age (p = 0.003), female gender (p = 0.02), the use of a femoral head with a diameter of thirty-two millimeters (p = 0.018), and the use of a thin polyethylene liner (p < 0.001). We found that the hydroxyapatite-coated threaded cups and the porous-coated press-fit cups continued to perform well more than five years after the operation. The hydroxyapatite-coated press-fit cups that were revised probably failed because the fixation interface beneath the cup could not sustain the tensile stresses that were imposed between the cup and the bone by the activity of the patient. Our data suggest that, in the specific biomechanical environment of the acetabulum, physical interlocking between the cup and the supporting bone beneath it may be a prerequisite for long-term stability.

Thermomechanical behavior of virgin and highly crosslinked ultra-high molecular weight polyethylene used in total joint replacements

Three series of uniaxial tension and compression tests were conducted on two conventional and two highly crosslinked ultra-high molecular weight polyethylenes (UHMWPEs) all prepared from the same lot of medical grade GUR 1050. The conventional materials were unirradiated (control) and gamma irradiated in nitrogen with a dose of 30 kGy. The highly crosslinked UHMWPEs were gamma irradiated at room temperature with 100 kGy and then thermally processed by either annealing below the melt transition at 100 degrees C or by remelting above the melt transition at 150 degrees C. The true stress-strain behavior of the four UHMWPE materials was characterized as a function of strain rate (between 0.02 and 0.10 s(-1)) and test temperature (20-60 degrees C). Although annealing and remelting of UHMWPE are primarily considered as methods of improving oxidation resistance, thermal processing was found to significantly impact the crystallinity, and hence the mechanical behavior, of the highly crosslinked UHMWPE. The crystallinity and radiation dose were key predictors of the uniaxial yielding, plastic flow, and failure properties of conventional and highly crosslinked UHMWPEs. The thermomechanical behavior of UHMWPE was accurately predicted using an Arrhenius model, and the associated activation energies for thermal softening were related to the crystallinity of the polymers. The conventional and highly crosslinked UHMWPEs exhibited low strain rate dependence in power law relationships, comparable to metals. In light of the unifying trends observed in the true stress-strain curves of the four materials investigated in this study, both crosslinking (governed by the gamma radiation dose) and crystallinity (governed by the thermal processing) were found to be useful predictors of the mechanical behavior of UHMWPE for a wide range of test temperatures and rates. The data collected in this study will be used to develop constitutive models based on the physics of polymer systems for predicting the thermomechanical behavior of conventional and crosslinked UHMWPE used in total joint replacements.

The yielding, plastic flow, and fracture behavior of ultra-high molecular weight polyethylene used in total joint replacements

The yielding, plastic flow, and fracture behavior of UHMWPE plays an important role in wear and failure mechanisms of total joint replacement components. The primary objective of this study was to compare the yielding, plastic flow, and fracture behavior of two implantable grades of UHMWPE (GUR 1120 vs 4150 HP). The first part of this work explored the hypothesis that up to the polymer yield point, the monotonic loading behavior of UHMWPE displays similar true stress strain behavior in tension and compression. Uniaxial tension and compression tests were conducted to compare the equivalent true stress vs strain response of UHMWPE up to 0.12 true strain. During monotonic loading, the equivalent true stress strain behavior was similar in tension and compression up to the yield point. However, investigation of the unloading behavior and permanent plastic deformations showed that classical deviatoric rate independent plasticity theory may dramatically overpredict the permanent strains in UHMWPE. A secondary goal of this study was to determine the ultimate true stress and strain for UHMWPE and to characterize the fracture surfaces after failure. Using a fracture mechanics approach, the critical flaw sizes were used in combination with the true ultimate stresses to predict the fracture toughness of the two resins. A custom video-based strain measurement system was developed and validated to characterize the true stress-strain behavior up to failure and to verify the accuracy of the incompressibility assumption in calculating the true stress-strains up to failure. In a detailed uncertainty analysis, theoretical expressions were derived for the relative uncertainty in digital video-based estimates of nominal strain, true strain, homogeneous stress, and true stress. Although the yielding behavior of the two UHMWPE resins was similar, the hardening and plastic flow behavior clearly discriminated between the GUR 1120 and 4150 HP. A statistically significant difference between the fracture toughness of the two resins was also evident. The long-term goal of this research is to provide detailed true stress strain data for UHMWPE under uniaxial tension and compression for future numerical simulations and comparison with more complex multiaxial loading conditions.

Mortality Risk for Operated and Nonoperated Vertebral Fracture Patients in the Medicare Population

Vertebral compression fractures (VCFs) are associated with increased mortality risk, but the association between surgical treatment and survivorship is unclear. We evaluated the mortality risk for VCF patients undergoing conservative treatment (nonoperated), kyphoplasty, and vertebroplasty. Survival of VCF patients in the 100% U.S. Medicare data set (2005–2008) was estimated by the Kaplan‐Meier method, and the differences in mortality rates at up to 4 years were assessed by Cox regression (adjusted for comorbidities) between operated and nonoperated patients and between kyphoplasty and vertebroplasty patients. An instrumental variables analysis was used to evaluate mortality‐rate difference between kyphoplasty and vertebroplasty patients. A total of 858,978 VCF patients were identified, including 119,253 kyphoplasty patients and 63,693 vertebroplasty patients. At up to 4 years of follow‐up, patients in the operated cohort had a higher adjusted survival rate of 60.8% compared with 50.0% for patients in the nonoperated cohort (p < .001) and were 37% less likely to die [adjusted hazard ratio (HR) = 0.63, p < .001]. The adjusted survival rates for VCF patients following vertebroplasty or kyphoplasty were 57.3% and 62.8%, respectively (p < .001). The relative risk of mortality for kyphoplasty patients was 23% lower than that for vertebroplasty patients (adjusted HR = 0.77, p < .001). Using physician preference as an instrument, the absolute difference in the adjusted survival rate at 3 years was 7.29% higher in patients receiving kyphoplasty than vertebroplasty (p < .001), compared with a crude absolute rate difference of 5.09%. This study established the mortality risk associated with VCFs diagnosed between 2005 and 2008 with respect to different treatment modalities for elderly patients in the entire Medicare population.

Radiation and chemical crosslinking promote strain hardening behavior and molecular alignment in ultra high molecular weight polyethylene during multi-axial loading conditions

The mechanical behavior and evolution of crystalline morphology during large deformation of eight types of virgin and crosslinked ultra high molecular weight polyethylene (UHMWPE) were studied using the small punch test and transmission electron microscopy (TEM). We investigated the hypothesis that both radiation and chemical crosslinking hinder molecular mobility at large deformations, and hence promote strain hardening and molecular alignment during the multiaxial loading of the small punch test. Chemical crosslinking of UHMWPE was performed using 0.25% dicumyl peroxide (GHR 8110, GUR 1020 and 1050), and radiation crosslinking was performed using 150 kGy of electron beam radiation (GUR 1150). Crosslinking increased the ultimate load at failure and decreased the ultimate displacement of the polyethylenes during the small punch test. Crosslinking also increased the near-ultimate hardening behavior of the polyethylenes. Transmission electron microscopy was used to characterize the crystalline morphology of the bulk material, undeformed regions of the small punch test specimens, and deformed regions of the specimens oriented perpendicular and parallel to the punch direction. In contrast with the virgin polyethylenes, which showed only subtle evidence of lamellar alignment, the crosslinked polyethylenes exhibited enhanced crystalline lamellae orientation after the small punch test, predominantly in the direction parallel to the punch direction or deformation axis. Thus, the results of this study support the hypothesis that crosslinking promotes strain hardening during multiaxial loading because of increased resistance to molecular mobility at large deformations effected by molecular alignment. The data also illustrate the sensitivity of large deformation mechanical behavior and crystalline morphology to the method of crosslinking and resin of polyethylene.

In vivo degradation of polyethylene liners after gamma sterilization in air.

BACKGROUND Ultra-high molecular weight polyethylene degrades during storage in air following gamma sterilization, but the extent of in vivo degradation remains unclear. The purpose of this study was to quantify the extent to which the mechanical properties and oxidation of conventional polyethylene acetabular liners treated with gamma sterilization in air change in vivo. METHODS Fourteen modular cementless acetabular liners were revised at an average of 10.3 years (range, 5.9 to 13.5 years) after implantation. All liners, which had been machined from GUR 415 resin, had been gamma-sterilized in air; the average shelf life was 0.3 year (range, 0.0 to 0.8 year). After removal, the components were expeditiously frozen to minimize ex vivo changes to the polyethylene prior to characterization. The average duration between freezing and testing was 0.6 year. Mechanical properties and oxidation were measured with use of the small-punch test and Fourier transform infrared spectroscopy, respectively, in the loaded and unloaded regions of the liners. RESULTS There was substantial regional variation in the mechanical properties and oxidation of the retrieved liners. The ultimate load was observed to vary by >90% near the surface. On the average, the rim and the unloaded bearing showed evidence of severe oxidation near the surface after long-term in vivo aging, but these trends were not typically observed on the loaded bearing surface or near the backside of the liners. CONCLUSIONS The mechanical properties of polyethylene that has been gamma-sterilized in air may decrease substantially in vivo, depending on the location in the liner. The most severe oxidation was observed at the rim, suggesting that the femoral head inhibits access of oxygen-containing body fluids to the bearing surface. This is perhaps why in vivo oxidation has not been associated with clinical performance to date.

The biomechanical effects of kyphoplasty on treated and adjacent nontreated vertebral bodies.

It remains unclear whether adjacent vertebral body fractures are related to the natural progression of osteoporosis or if adjacent fractures are a consequence of augmentation with bone cement. Experimental or computational studies have not completely addressed the biomechanical effects of kyphoplasty on adjacent levels immediately following augmentation. This study presents a validated two–functional spinal unit (FSU) T12–L2 finite element model with a simulated kyphoplasty augmentation in L1 to predict stresses and strains within the bone cement and bone of the treated and adjacent nontreated vertebral bodies. The findings from this multiple-FSU study and a recent retrospective clinical study suggest that changes in stresses and strains in levels adjacent to a kyphoplasty-treated level are minimal. Furthermore, the stress and strain levels found in the treated levels are less than injury tolerance limits of cancellous and cortical bone. Therefore, subsequent adjacent level fractures may be related to the underlying etiology (weakening of the bone) rather than the surgical intervention.

Influence of Mechanical Behavior on the Wear of 4 Clinically Relevant Polymeric Biomaterials in a Hip Simulator

The elastic and large-deformation mechanical behavior of 4 materials with known clinical performance was examined and correlated with the wear behavior in a hip simulator. Acetabular liners of a commercially available design were machined from ultra-high molecular weight polyethylene (UHMWPE), high-density polyethylene (HDPE), polytetrafluoroethylene (PTFE), and polyacetal and wear tested in a multidirectional hip joint simulator. Elastic and large-deformation mechanical behavior was directly measured from the wear-tested liners using the small punch test. The finite element method was used to compute elastic modulus from the measured small punch test initial stiffness, and the contact stress for the liners was calculated using the theory of elasticity solution. Positive, statistically significant correlations were observed between the hip simulator wear rate and the initial peak load, ultimate load, and work to failure from the small punch test. Negative correlations were observed between the wear rate and the elastic modulus and contact stress. The results of this study support the hypothesis that the large-deformation mechanical behavior of a polymer plays a greater role in the wear mechanisms prevalent in total hip replacements than the elastic behavior.