Gamma-irradiated cross-linked polyethylene in total hip replacements--analysis of retrieved sockets after long-term implantation

Comprehensive Review on Biomaterials and Their Inherent Behaviors for Hip Repair Applications

Developing biomaterials for hip prostheses is challenging and requires dedicated attention from researchers. Hip replacement is an inevitable and remarkable orthopedic therapy for enhancing the quality of patient life for those who have arthritis as well as trauma. Generally, five types of hip replacement procedures are successfully performed in the current medical market: total hip replacements, hip resurfacing, hemiarthroplasty, bipolar, and dual mobility systems. The average life span of artificial hip joints is about 15 years, and several studies have been conducted over the last 60 years to improve the performance and thereby increase the lifespan of artificial hip joints. Present-day prosthetic hip joints are linked to the wide availability of biomaterials. Metals, ceramics, and polymers are some of the most promising types of biomaterials; nevertheless, each biomaterial has advantages and disadvantages. Metals and ceramics fail in most applications owing to stress shielding and the emission of wear debris; ongoing research is being carried out to find a remedy to these unfavorable responses. Recent research found that polymers and composites based on polymers are significant alternative materials for artificial joints. With growing research and several biomaterials, recent reviews lag in effectively addressing hip implant materials' individual mechanical, tribological, and physiological behaviors. This Review comprehensively investigates the historical evolution of artificial hip replacement procedures and related biomaterials' mechanical, tribological, and biological characteristics. In addition, the most recent advances are also discussed to stimulate and guide future researchers as they seek more effective methods and synthesis of innovative biomaterials for hip arthroplasty application.

Thermal, Mechanical and Tribological Properties of Gamma-Irradiated Plant-Derived Polyamide 1010

In this study, we investigated the influence of the gamma-irradiation dose and the addition of the cross-linking agent (triallyl isocyanurate (TAIC)) on the thermal, mechanical and tribological properties of plant-derived polyamide 1010 (PA1010). PA1010 and PA1010/TAIC were extruded using a twin screw extruder and injection molded. These specimens were then irradiated with gamma-ray in air with doses of 20 and 50 kGy. After gamma-irradiation, the specimens were heat-treated to remove the free radicals generated in the polymer. The combination of gamma-irradiation and the addition of TAIC significantly changed the crystal structures of PA1010. Glass transition temperature increased with the addition of TAIC and, in particular, with increasing gamma-irradiation dose. Moreover, PA1010/TAIC showed a rubbery plateau originating from cross-links by gamma-irradiation, which was observed in the temperature regions above the melting point in DMA measurements. Mechanical properties such as strength, modulus and hardness, and tribological properties such as frictional coefficient, specific wear rate and limiting pv (pressure p × velocity v) value of PA1010 improved with change in the internal microstructure with the gamma-irradiation and addition of TAIC.

Global diversity in bearings in primary THA

Choice of articulating materials, head size and the design of the articulation will become decisive for the long-term performance of a total hip arthroplasty (THA) and especially in terms of risk for dislocation and wear-related problems. Here we account for common alternatives based on available studies and the evidence that can be derived from them.Metal or ceramic femoral heads articulating against a liner or cup made of highly cross-linked polyethylene and ceramic-on-ceramic articulations have about similar risk for complications leading to revision, whereas the performance of metal-on-metal articulations, especially with use of big heads, is inferior. The clinical significance of problems related to ceramic-on-ceramic articulations such as squeaking remains unclear. With use of current technology ceramic fractures are rare.Large femoral heads have the potential to increase the range of hip movement before impingement occurs and are therefore expected to reduce dislocation rates. On the other hand, issues related to bearing wear, corrosion at the taper-trunnion junction and groin pain may arise with larger heads and jeopardize the longevity of THA. Based on current knowledge, 32-mm heads seem to be optimal for metal-on-polyethylene bearings. Patients with ceramic-on-ceramic bearings may benefit from even larger heads such as 36 or 40 mm, but so far there are no long-term reports that confirm the safety of bearings larger than 36 mm.Assessment of lipped liners is difficult because randomized studies are lacking, but retrospective clinical studies and registry data seem to indicate that this liner modification will reduce the rate of dislocation or revision due to dislocation without clear evidence of clinically obvious problems due to neck-liner impingement.The majority of studies support the view that constrained liners and dual mobility cups (DMC) will reduce the risk of revision due to dislocation both in primary and revision THA, the latter gaining increasing popularity in some countries. Both these devices suffer from implant-specific problems, which seem to be more common for the constrained liner designs. The majority of studies of these implants suffer from various methodological problems, not least selection bias, which calls for randomized studies preferably in a multi-centre setting to obtain sufficient power. In the 2020s, the orthopaedic profession should place more effort on such studies, as has already been achieved within other medical specialties, to improve the level of evidence in the choice of articulation when performing one of the most common in-hospital surgical procedures in Europe. Cite this article: EFORT Open Rev 2020;5:763-775. DOI: 10.1302/2058-5241.5.200002.

Functional Characteristics and Mechanical Performance of PCU Composites for Knee Meniscus Replacement

The potential use of fiber-reinforced based polycarbonate-urethanes (PCUs) as candidate meniscal substitutes was investigated in this study. Mechanical test pieces were designed and fabricated using a compression molding technique. Ultra-High Molecular Weight Polyethylene (UHMWPE) fibers were impregnated into PCU matrices, and their mechanical and microstructural properties evaluated. In particular, the tensile moduli of the PCUs were found unsuitable, since they were comparatively lower than that of the meniscus, and may not be able to replicate the inherent role of the meniscus effectively. However, the inclusion of fibers produced a substantial increment in the tensile modulus, to a value within a close range measured for meniscus tissues. Increments of up to 227% were calculated with a PCU fiber reinforcement composite. The embedded fibers in the PCU composites enhanced the fracture mechanisms by preventing the brittle failure and plastic deformation exhibited in fractured PCUs. The behavior of the composites in compression varied with respect to the PCU matrix materials. The mechanical characteristics demonstrated by the developed PCU composites suggest that fiber reinforcements have a considerable potential to duplicate the distinct and multifaceted biomechanical roles of the meniscus.

Simultaneous measurement of friction and wear in hip simulators

We propose and have evaluated a method to measure hip friction during wear testing on a popular multi-station hip simulator. A 6-degree-of-freedom load cell underneath the specimen sensed forces and torques during implant wear testing of simulated walking. This included internal–external and adduction–abduction rotations which are often neglected during friction testing on pendulum-type machines. Robust mathematical analysis and data processing provided friction estimates in three simultaneous orthogonal rotations, over extended multi-million cycle wear tests. We tested various bearing couples including metal-on-plastic, ceramic-on-plastic, and metal-on-metal material couples. In one test series, new and intentionally scratched CoCrMo 40-mm-diameter femoral heads were tested against conventional ultrahigh-molecular-weight polyethylene, highly cross-linked, and highly cross-linked with vitamin E versions. The scratching significantly increased friction and doubled the wear of all groups. Before scratching, friction levels for the aforementioned plastic groups were 0.056 ± 0.0060, 0.062 ± 0.0080, and 0.070 ± 0.0045, respectively, but after scratching increased to 0.088 ± 0.018, 0.076 ± 0.0066, and 0.082 ± 0.0049, respectively, all statistically significant increases (p = 0.00059, 0.00005, 0.0115, respectively). In another test series of 44-mm femoral head diameter hips, metal-on-plastic hips with conventional ultrahigh-molecular-weight polyethylene showed the lowest friction at 0.045 ± 0.0085, followed by highly cross-linked with 0.046 ± 0.0035 (not significantly different). In a ceramic-on-plastic design with conventional ultrahigh-molecular-weight polyethylene, higher friction 0.079 ± 0.0070 was measured likely due to that ceramic surface being rougher than usual. Metal-on-metal hips were compared without and with a TiN coating, resulting in 0.049 ± 0.014 and 0.097 ± 0.020 friction factors, respectively (statistically significant, p < 0.001), and the coating wore away on all coated hips eventually. Higher friction mostly correlated with higher wear or damage to femoral heads or implant coatings, except for the highly cross-linked wear resistant ultrahigh-molecular-weight polyethylene which had slightly higher friction, confirming the same finding in other independent studies. This type of friction measurements can help screen for clamping and elevated wear of metal-on-metal and resurfacing total hip replacements, surgical malpositioning, and abraded and otherwise damaged surfaces.

Biphasic and boundary lubrication mechanisms in artificial hydrogel cartilage: A review

Various studies on the application of artificial hydrogel cartilage to cartilage substitutes and artificial joints have been conducted. It is expected in clinical application of artificial hydrogel cartilage that not only soft-elastohydrodynamic lubrication but biphasic, hydration, gel-film and boundary lubrication mechanisms will be effective to sustain extremely low friction and minimal wear in daily activities similar to healthy natural synovial joints with adaptive multimode lubrication. In this review article, the effectiveness of biphasic lubrication and boundary lubrication in hydrogels in thin film condition is focused in relation to the structures and properties of hydrogels. As examples, the tribological behaviors in three kinds of poly(vinyl alcohol) hydrogels with high water content are compared, and the importance of lubrication mechanism in biomimetic artificial hydrogel cartilage is discussed to extend the durability of cartilage substitute.

Influences of dehydration and rehydration on the lubrication properties of phospholipid polymer-grafted cross-linked polyethylene

Surface modification by grafting of biocompatible phospholipid polymer onto the surface of artificial joint material has been proposed to reduce the risk of aseptic loosening and improve the durability. Poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC)-grafted cross-linked polyethylene (CLPE) has shown promising results for reducing wear of CLPE. The main lubrication mechanism for the PMPC layer is considered to be the hydration lubrication. In this study, the lubrication properties of PMPC-grafted CLPE were evaluated in reciprocating friction test with rehydration process by unloading in various lubricants. The start-up friction of PMPC-grafted CLPE was reduced, and the damage of PMPC layer was suppressed by rehydration in water or hyaluronic acid solutions. In contrast, the start-up friction of PMPC-grafted CLPE increased in fetal bovine serum solution, and the damage for PMPC layer was quite noticeable. Interestingly, the start-up friction of PMPC-grafted CLPE was reduced in fetal bovine serum solution containing hyaluronic acid, and the damage of the PMPC layer was suppressed. These results indicate that the rehydration by unloading and hyaluronic acid are elemental in maximizing the lubrication effect of hydrated PMPC layer.

Characterization of a highly cross-linked ultrahigh molecular-weight polyethylene in clinical use in total hip arthroplasty

This article reports on a commercially available extensively cross-linked ultrahigh molecular-weight polyethylene (HXPE) produced by subjecting molded GUR 1050 ultrahigh molecular-weight polyethylene (UHMWPE) to 100 +/- 10 kGy of electron beam radiation followed by melt annealing and sterilization by gas plasma. When compared to contemporary conventional molded GUR 1050 UHMWPE sterilized by 37 kGy of gamma radiation, the HXPE material has enhanced wear properties, has no detectable free radicals, and is resistant to oxidation and oxidative-related material property changes. The relative wear improvement of the HXPE is maintained in the presence of bone cement or alumina particles. The HXPE produced greater than 90% fewer wear particles in all size ranges and statistically significantly (P < .0001) smaller average-size particles than did the conventional UHMWPE.

Oxidation and wear of 100-Mrad cross-linked polyethylene shelf-aged for 30 years

Some previous studies suggest that aging influences wear and oxidatively degraded nonsterilized ultra-high-molecular-weight polyethylene (UHMWPE) exhibits decreased wear resistance. We therefore asked whether shelf-aging storage conditions influenced degradation and wear resistance of gamma-irradiated UHMWPE. We examined oxidation and wear of 100-Mrad gamma-irradiated UHMWPE (100-Mrad polyethylene) cups shelf-aged for 30 years without (n=2) or with (n=2) packages. The oxidation index of the unpackaged 100-Mrad polyethylene surface (4) was higher than that of the packaged one (2.7). The packaged 100-Mrad polyethylene cup exhibited a high wear resistance with a steady wear rate of 0.5 mg/10(6) cycles. In contrast, the unpackaged 100-Mrad polyethylene exhibited an extremely high initial wear rate of 187.9 mg/10(6) cycles over the first 0.25 x 10(6) cycles with a subsequently reduced wear rate of 5 mg/10(6) cycles after 5 x 10(6) cycles. Packaging over long periods inhibits surface oxidation and maintains the wear resistance of gamma-irradiated UHMWPE cups.

Radiation cross-linking in ultra-high molecular weight polyethylene for orthopaedic applications

The motivation for radiation cross-linking of ultra-high molecular weight polyethylene (UHMWPE) is to increase its wear resistance to be used as bearing surfaces for total joint arthroplasty. However, radiation also leaves behind long-lived residual free radicals in this polymer, the reactions of which can detrimentally affect mechanical properties. In this review, we focus on the radiation cross-linking and oxidative stability of first and second generation highly cross-linked UHMWPEs developed in our laboratory.

Comparison of hip simulator wear of 2 different highly cross-linked ultra high molecular weight polyethylene acetabular components using both 32- and 38-mm femoral heads

Much evidence suggests that ultra high molecular weight polyethylene (UHMWP) irradiated to 9.5 Mrad has lower wear than UHMWP given 5 Mrad. Curiously, highly cross-linked polyethylenes gain weight during hip simulator testing. We postulated that (a) UHMWP irradiated to 9.5 Mrad would wear less than UHMWP irradiated to 5 Mrad UHMWP, (b) articulation against 38-mm heads would accentuate this difference, and (c) weight gain of highly cross-linked material reflects the inadequacy of load soak controls. We compared 9.5 versus 5 Mrad polyethylene in hip simulator wear tests, with both load soak corrections and with novel "motion soak" corrections. The 9.5-Mrad material wore less than 5-Mrad material for 32- and 38-mm heads. Motion soak corrections were more accurate than load soak corrections.

Two- to 9-year clinical results of alumina ceramic-on-ceramic THA

From June 1997 to June 2003 we performed 194 total hip arthroplasties on 173 patients using alumina ceramic-on-ceramic bearings as part of a prospective United States Food and Drug Administration/Investigational Device Exemption study. The average patient age at surgery was 49.9 years. Minimum followup time was 2 years (mean 4.3 years, range 2-9 years). We evaluated survival rate, implant- and nonimplant-related complications. Clinical outcomes included the Merle d'Aubigné score. We assessed radiographs for signs of osteolysis, component loosening, and implant wear. No patients had osteolysis and there were no hip dislocations. Implant survivorship for all hips with aseptic revision of any component was 96% (CI, 91-100) at 9 year; survivorship for hips without prior surgery was 99.3%, (CI, 98-100). There was a 1.7% incidence of implant-related complications. Our data help confirm two United States FDA/IDE studies on alumina ceramic-on-ceramic total hip arthroplasty that reported low aseptic revision rates and low revision rates for instability. Total hip arthroplasty using alumina ceramic-on-ceramic implants is a safe and reliable procedure in the comparably young and active patient.

In vitro comparison of frictional torque and torsional resistance of aged conventional gamma-in-nitrogen sterilized polyethylene versus aged highly crosslinked polyethylene articulating against head sizes larger than 32 mm

BACKGROUND

The advent of highly crosslinked polyethylene has allowed the re-evaluation of the use of femoral heads larger than 32 mm for metal-on-polyethylene total hip arthroplasties. However, the effect of larger heads on the frictional torque of highly crosslinked polyethylene is unknown.

METHODS

We performed an in vitro examination of the effect of larger chrome cobalt femoral heads (40 mm diameter) on the frictional torque and torsional resistance of hip articulations on aged liners of polyethylene that were sterilized by gamma rays while in nitrogen, and aged highly crosslinked polyethylene. The frictional torque at the femoral head articulation was usually higher for the highly crosslinked polyethylene than for the conventional polyethylene. The aged conventional liners oxidized considerably, which led to gross failure of the polyethylene at the anti-rotation portion of the rim. The aged crosslinked polyethylene showed no such failures despite the higher frictional torque.

INTERPRETATION

Our findings suggest that in terms of torsional resistance to fatigue when studied as a device, rather than as an isolated material, under these conditions, aged highly crosslinked polyethylene is preferable to aged conventional polyethylene.

Contact stress assessment of conventional and highly crosslinked ultra high molecular weight polyethylene acetabular liners with finite element analysis and pressure sensitive film

Stress magnitude and distribution of both conventional polyethylene versus a crosslinked polyethylene in the liner of a total hip replacement (THR) were examined using finite element analysis and pressure sensitive film. Both types of polyethylene were assessed against head sizes of 22 and 28 mm with 5-mm thick polyethylene liners and head sizes of 28, 38, and 46 mm with 3-mm thick polyethylene liners. Liners with 5-mm conventional polyethylene represented successful combinations with long track records. Our hypothesis was that although the combination of the large head and the lower modulus of the highly crosslinked polyethylene would lead to lower stresses, the stresses would be excessive if the liner was extremely thin at 3 mm. Von Mises stresses at the articulating surface of the highly crosslinked liners were lower, when compared to conventional polyethylene, in every THR size examined. Specifically, however, the 38- and 46-mm inner diameter (ID) highly crosslinked polyethylene even at the extreme of only 3-mm thick had lower stresses than the 22-mm ID conventional liner of 5-mm thickness. These data indicate that the use of a large head against highly crosslinked material even at 3-mm thickness results in lower stresses than in an existing conventional 22-mm head and 5-mm thick combination. Obviously, other considerations will influence the minimum thickness to be recommended.

The measurement of creep in ultrahigh molecular weight polyethylene: a comparison of conventional versus highly cross-linked polyethylene

Quantification of creep of highly cross-linked polyethylene would enable separation of creep from wear when evaluating femoral head penetration into polyethylene. We compared creep magnitude of a highly cross-linked versus conventional polyethylene in the laboratory. Twelve acetabular liners of each material were tested, 6 of which had a 32-mm inner diameter (ID) and 6 had 28-mm ID. Creep was measured using coordinate measuring machines during loading at 2 Hz without motion to 4 million cycles. Penetration into 32-mm ID conventional liners reached 97 microm versus 107 microm for highly cross-linked material, not significant. Penetration into 28-mm conventional liners was 132 microm versus 155 microm for highly cross-linked material (P = .017). Ninety percent of the creep had occurred by 2.5 million cycles.

Analysis of a retrieved UHMWPE acetabular cup crosslinked in air with 1000 kGy of gamma radiation

The oxidation and degradation that accompany high dose gamma irradiation in air for crosslinking and sterilization are important because they could affect the clinical performance of polyethylene total joint implants. We report a clinical case of a 1000 kGy gamma-irradiated, highly crosslinked ultra-high molecular weight polyethylene acetabular cup retrieved 24 years postoperatively. Performance evaluations included absorbed radiation dose, total wear penetration, average wear rate, crystallinity, oxidation, and equibiaxial tensile properties. A retrieved acetabular cup of the same grade of polyethylene but gamma-sterilized using a conventional dose was used as control. The highly crosslinked and control cups took in about 1300 and 30 kGy of radiation, respectively, as measured using a trans-vinylene index. Direct dimensional measurements revealed average wear rates of the highly crosslinked and control cups were 0.04 and 0.06 mm/year, respectively. The oxidation index of the highly crosslinked cup was very high (0.679), but equivalent to that of a 1000 kGy irradiated reference cup. The retrieved highly crosslinked cup showed much higher equibiaxial ultimate tensile strength than the retrieved control cup. Based on these observations, the increased wear resistance and equibiaxial tensile properties that resulted from extensive crosslinking in the presence of air were partially offset by the adverse effects caused by immediate oxidation during the process.

Comparison of the wear rates of twenty-eight and thirty-two-millimeter femoral heads on cross-linked polyethylene acetabular cups in a wear simulator

BACKGROUND

The use of larger femoral head sizes in total hip arthroplasty has been shown to reduce the rate of dislocation and to increase the range of motion; however, such components have been associated with unacceptably high polyethylene wear rates. Studies have shown dramatic differences in wear rates between nominally cross-linked polyethylene (i.e., polyethylene that is cross-linked during radiation sterilization) and elevated cross-linked polyethylene (i.e., polyethylene that is cross-linked to a higher degree than that obtained by radiation sterilization alone). The aim of this study was to test the effect of increased cross-linking and of increased head size on polyethylene wear rates.

METHODS

Four groups of acetabular liners obtained from a single manufacturer, including 28-mm-diameter nominally cross-linked, 32-mm-diameter nominally cross-linked, 28-mm-diameter elevated cross-linked, and 32-mm-diameter elevated cross-linked polyethylene liners, were tested. Three implants from each group were tested in a twelve-station hip wear simulator with use of 90% bovine serum as a lubricant. The liners were articulated with the appropriately sized cobalt-chromium femoral head. Additional liners from each design were subjected only to the same load without motion to serve as load-soak controls to account for any weight gain due to fluid absorption. Gravimetric analysis was performed every 500,000 cycles for a total of five million cycles.

RESULTS

Nominally cross-linked liners demonstrated mean wear rates of 14.97 and 16.92 mg per million cycles for the 28-mm and 32-mm head sizes, respectively. Both of the elevated cross-linked liners had significantly lower wear rates than the nominally cross-linked liners, with a mean of 1.51 and 2.57 mg per million cycles for the 28-mm and 32-mm head sizes, respectively (p < 0.001).

CONCLUSION

The dramatic reduction in wear rates with polyethylene cross-linking, even with the larger head size, may increase the potential for use of 32-mm head components in total hip arthroplasty.

Long-term in vivo wear performance of porous-coated acetabular components sterilized with gamma irradiation in air or ethylene oxide

This study evaluated the long-term in vivo wear performance of 2 groups of well-functioning cementless acetabular cups sterilized by different methods. The first group included 31 hips that were implanted with AML TriSpike cups (DePuy, Warsaw, IN) sterilized by gamma-irradiation in air. The second group included 28 hips implanted with Arthropor cups (Joint Medical Products, Stamford, CT) that were sterilized with ethylene oxide. Time-dependent variations in the radiographic wear rates were compared within each group. Changes in the wear rates between 4- and 16-year follow-up times for the TriSpike cups were not significant (P=.09), and there was no evidence to suggest a trend toward substantially increasing wear rates with longer follow-up times. Among the Arthropor cups, the wear rates remained relatively constant between 2 and 14 years of follow-up evaluation. Although clinically apparent late increases in radiographic head penetration rates were not evident, we will continue to monitor all patients for evidence of accelerated wear at late follow-up.

Wear of conventional and cross-linked ultra-high-molecular-weight polyethylene acetabular cups against polished and roughened CoCr femoral heads in a biaxial hip simulator

The wear of acetabular cups made from conventional gamma-sterilized, and electron-beam cross-linked ultra-high-molecular-weight polyethylene was studied with a biaxial hip wear simulator. The femoral heads were either polished or roughened so that they represented the type of roughening and the value of surface roughness (R(a) = 0.14-0.18 micro m) observed in the roughest explanted femoral heads. The lubricant was diluted calf serum, and the test length 3 million cycles. The mean wear rate and standard deviation of conventional polyethylene cups against polished and against roughened heads was 11.6 +/- 0.07 and 64.4 +/- 10.1 mg per 1 million cycles, respectively. The latter value closely corresponds to that measured from explanted Charnley prostheses. Against polished heads, cross-linked polyethylene cups showed net weight gain, whereas against roughened heads, their mean wear rate was 2.4 +/- 0.3 mg per 1 million cycles. The mean equivalent circle diameters of polyethylene wear particles produced in the above four categories were: conventional/roughened 0.32 micro m, cross-linked/roughened 0.29 micro m, conventional/polished 0.28 micro m, cross-linked/polished 0.23 micro m. The size ranges and shapes were close to those seen in particles isolated from tissue samples. In conclusion, the tests indicated that electron-beam irradiation effectively reduces the harmful polyethylene wear particle production in total hip arthroplasty.

Effect of terminal sterilization with gas plasma or gamma radiation on wear of polyethylene liners

BACKGROUND

Although terminal sterilization with gamma radiation in air promotes cross-linking, which improves wear resistance, it also results in free radicals, which can oxidize and degrade the mechanical properties of polyethylene liners used for total hip arthroplasty. For this reason, non-cross-linked polyethylene components have also been sterilized with chemical surface treatments, such as gas plasma. In this study, we tested the hypothesis that conventional polyethylene liners cross-linked by sterilization with gamma radiation in air had better in vivo wear performance than non-cross-linked liners sterilized with gas plasma.

METHODS

We retrospectively reviewed the wear rates in a series of hips treated with a Duraloc 100 cup, a 28-mm femoral head, and an Enduron liner that had been sterilized with either gamma radiation (sixty-one hips followed for a mean of 5.2 years) or gas plasma (sixty-three hips followed for a mean of 3.9 years). The irradiated liners had been stored with access to ambient oxygen for an average of 1.0 year (range, 0.05 to 5.72 years) prior to implantation. Multiple linear regression was used to assess the effect of the sterilization method on the wear rate while accounting for the possible influence of other factors, including liner geometry, femoral head material, patient gender, cup abduction angle, and age at surgery.

RESULTS

The polyethylene liners that had been sterilized with gamma radiation in air had a significantly lower wear rate than did the gas-plasma-sterilized liners (0.097 compared with 0.19 mm/yr, p < 0.001). The sterilization method (p < 0.001) and age at surgery (p = 0.001) were the only factors that we analyzed that correlated with the wear rate.

CONCLUSIONS

The in vivo wear of conventional polyethylene liners that had been sterilized with gamma radiation in air was, on the average, 50% less than that of non-cross-linked liners sterilized with gas plasma.

LEVEL OF EVIDENCE

Therapeutic study, Level III-2 (retrospective cohort study). See Instructions to Authors for a complete description of levels of evidence.

Cross-linking to improve THR wear performance

Polyethylene wear and associated osteolysis can limit the longevity of total hip replacement. In recent years, many improvements have been made in the consolidation, manufacture, and sterilization of polyethylene acetabular components. These improvements provided reduced polyethylene wear and prolonged usefulness of total hip replacement. Recent advances in extensively cross-linking polyethylene offer the possibility to substantially further reduce wear in total hip replacement. Hip simulator wear testing demonstrates an order of magnitude reduction in wear resulting from cross-linking GUR 1050 polyethylene by exposure to 100 kGy of electron beam radiation followed by annealing to encourage cross-linking and to reduce residual free radicals. Clinical investigation will be required to validate the wear advantage of these materials in vivo. (Hip International 2002; 2: 103-7).