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Highly Cross-linked Polyethylene in Total Hip Arthroplasty Abstract Although total hip arthroplasty is a common and highly successful procedure, its long-term durability has been undermined by the cellular response to polyethylene wear debris and the subsequent effects on periprosthetic bone. Research elucidating the effects of sterilization on polyethylene wear has facilitated the development of a more wear-resistant material—highly cross-linked polyethylene. Laboratory testing has demonstrated that highly cross-linked polyethylene has markedly improved wear resistance compared with conventional polyethylene under a variety of conditions. Early clinical data have supported these results. To make informed decisions about this already widely available and frequently used product, the practicing orthopaedic surgeon should have a basic understanding of the production process as well as knowledge of the most current laboratory and clinical data. T otal hip arthroplasty (THA) is one of the most successful sur- gical procedures ever developed. Ce- mented and cementless component fixation provide excellent pain relief, return of function, and intermediate longevity in patients with degenera- tive conditions of the hip. Ultra- high–molecular-weight polyethylene (UHMWPE) articulating with a metal head has been the p redominant bearing surface since the inception of modern THA. Although the success of this bearing couple is well docu- mented, clinical studies and retrieval analyses have shown that polyethylene wear and osteolysis are the ma- jor factors limiting the longevity of THA. 1-5 Extensive research undertaken to elucidate the physical and biologic mechanisms behind polyethylene wear and osteolysis 6,7 has led to the development of highly cross-linked UHMWPE. The term highly cross- linked polyethylene is commonly used to describe this new generation of polymers. We use this term to describe intentionally cross-linked material; however, different manufacturers use proprietary methods to produce various levels of cross- linking in their components. Labora- tory work and early clinical trials have demonstrated that highly cross-linked polyethylene is significantly more wear-resistant than conventional polyethylene. A synthesis of the manufacturing processes, in addition to laboratory and clinical data regarding highly cross-linked UHMWPE, may aid the orthopaedic surgeon in making an informed deci- sion regarding THA. Alexander C. Gordon, MD Darryl D. D’Lima, MD Clifford W. Colwell, Jr, MD Dr. Gordon is Orthopaedic Surgeon, Illinois Bone and Joint Institute, Morton Grove, IL. Dr. D’Lima is Director, Orthopaedic Research Laboratories, Scripps Center for Orthopaedic Research and Education, La Jolla, CA. Dr. Colwell is Director, Musculoskeletal Center; Director, Scripps Center for Orthopaedic Research and Education; and Shiley Chair, Orthopaedic Research, Scripps Center for Orthopaedic Research and Education. None of the following authors or the departments with which they are affiliated has received anything of value from or owns stock in a commercial company or institution related directly or indirectly to the subject of this article: Dr. Gordon, Dr. D’Lima, and Dr. Colwell. Reprint requests: Dr. Gordon, Illinois Bone and Joint Institute, 9000 Waukegan Road, Morton Grove, IL 60053. J Am Acad Orthop Surg 2006;14: 511-523 Copyright 2006 by the American Academy of Orthopaedic Surgeons. Perspectives on Modern Orthopaedics Volume 14, Number 9, September 2006 511 Polyethylene Resins and Manufacturing Polyethylene Resin Polyethylene is simply a repeat- ing chain of ethylene monomer molecules; the modifiers low-density, high-density, and ultra-high–molecular weight refer to the molecular weight, chain length, and arrangement of the polymer chains. The condensed polymers have crystalline and amorphous regions, the percentage and arrangement of which affect the properties of the material (Figure 1). In general, polymers with higher percentages of crystalline regions have higher elastic moduli and demonstrate better resistance to crack propagation, but they may be more susceptible to the effects of oxidation. 8 Ruhrchemie AG, a predecessor company of Ticona, began commercial manufacture of UHMWPE resin in the 1950s. Ticona is currently the leading manufacturer of medical- grade UHMWPE resin, with plants in Bishop, Texas and Oberhausen, Germany . A ll Ticona resins have the designation “GUR,” followed by a numeric modifier. Their medical- grade resins are named GUR 1020, 1120, 1050, and 1150. The first of the four numerals (1) indicates that the polymer is designated for orthopaedic implantation. The second nu- meral notes the presence (1) or absence (0) of calcium stearate. The third digit is an indicator of molecular weight, and the fourth is an inter- nal corporate code. Hercules Powder manufactured another UHMWPE resin known as the 1900 series. Most recently produced by Basell Polyolefins, the 1900 series line (1900 and 1900H) has re- mained the same as when Hercules produced it. In 2002, Basell sold the 1900 resin technology and ceased production of this product. Although new 1900 resin is not being produced, some orthopaedic device manufacturers have stockpiled the material and continue to use it in their implants. The 1900 series resins have a lower mean molecular weight and larger mean particle size than do the GUR 1050 resins, which may affect their clinical performance. Edidin et al 9 andWonetal 10 studied the effects of resin type and manufacturing method on the wear and degradation of the 1900 and GUR resins. Won et al 10 analyzed retrieved tibial bearings from the Miller- Galante (MG) I and II (Zimmer, War- saw, IN) knee arthroplasty designs. The tibial bearings had the same ge- ometry; both were gamma-sterilized in air. T he MG-I bearings were made from direct compression-molded 1900 resin, and the MG-II components were manufactured from extruded GUR 415 stock. The researchers found notably higher rates of delamination and subsurface damage consistent with oxidation in the MG-II components. They concluded that compression-molded 1900 resin was more resistant to oxidation than GUR material. Edidin et al 9 studied differences between 1900 and GUR resins in the laboratory setting using an accelerated aging technique to determine sus- ceptibility to oxidation. They found that after accelerated aging and gamma sterilization in air, compression- molded GUR 1050 and 1900 resins as well as extruded 1050 stock de- graded similarly in mechanical testing. The authors noted more rapid degradation of the 1900 resins than of the GUR resins under similar testing conditions, but only minor differences in the post-aging mechanical properties of the three resins. Implant Fabrication Implant manufacturers obtain UHMWPE as powdered resin or stock material from converting com- panies, such as Poly Hi Solidur (Fort Figure 1 A, Molecular structure of ethylene and of ultra-high–molecular-weight polyethylene (UHMWPE). n = the degree of polymerization. B, Crystalline and amorphous regions of UHMWPE. (Reproduced with permission from Kurtz SM: The UHMWPE Handbook: Ultra-High Molecular Weight Polyethylene in Total Joint Replacement. San Diego, CA: Elsevier Academic Press, 2004, pp 4, 6.) Highly Cross-linked Polyethylene in Total Hip Arthroplasty 512 Journal of the American Academy of Orthopaedic Surgeons Wayne, IN) and Perplas Medical (Bacup, Lancashire, UK). Compo- nents are fabricated from the resin by direct compression molding or machined from converted stock sup- plied by ram-extruded bars or molded sheets. The mechanical properties of the final product are affected by the specific temperature, pres- sure, and cooling rate used in the processes. 8,11 Direct compression- molded components made from 1900 resin have demonstrated excellent clinical performance despite being sterilized by gamma radiation in air. In his study of direct compression-molded components in the hip and knee, Ritter 12 found osteolysis in 2.5% of hips at a mean of 21 years and in 0% of knees at a mean of 8 years. He concluded that the clinical performance of direct compression- molded 1900 resin is superior to that of other polyethylene components. Because there is no clear consensus on the best resin or fabrication method for UHMWPE bearings in THA, orthopaedic implant manufacturers decide which resin and fabrication method best suits their implants. Sterilization and Aging of Conventional Polyethylene Polyethylene THA bearings are sterilized by one of two general methods—surface treatment and irradiation. These methods, as well as numerous other variables in the sterilization process, have specific effects on the in vitro and in vivo performance of UHMWPE acetabular liners. Surface Treatment The two commonly used surface sterilization treatments are ethylene oxide (EtO) gas and gas plasma. Al- though highly toxic, EtO is well- suited for polyethylene because the gas does not chemically react with the component. Safe and effective EtO sterilization requires special en- vironmental conditions during sterilization and appropriate timing to allow the gas to diffuse in and out of the component. Gas plasma treatment is performed at a lower temperature and in a shorter time frame than EtO surface sterilization. In gas plasma treatment, less toxic sub- stances (eg, peracetic acid, hydrogen peroxide gas plasma) are used to eliminate potential contamination. This method is newer than EtO, and data regarding its use are limited. 11 Irradiation Gamma radiation and its effects on the mechanical properties of polyethylene have been well docu- mented, with a resultant large-scale overhaul of polyethylene production for THA. Irradiation of polyethylene causes cleavage of the polymer chains, leading to the production of free radicals. After radiation, the chains may bond at their original scission point or cross-link with one another. When neither occurs, the cleaved end of the polymer chain re- mains a free radical. When sterilization and packaging of the component take place in the presence of oxygen, the free radicals generated by the radiation are able to combine with oxygen molecules during stor- age and after implantation (Figure 2). This leaves the component susceptible to the effects of oxidation, which are now known to adversely affect its mechanical properties. In a retrieval analysis of components from multiple manufacturers, Sutula et al 13 investigated the subsurface white band found in their re- trievals. Infrared spectroscopy demonstrated that this subsurface white band corresponded to an area of high oxidation and was present only in components sterilized by gamma irradiation in air. The appearance of this band was time-dependent, and all components in which the white band was observed had been sterilized more than 3 years before the ob- servation. The authors found that the presence of the subsurface white band corresponded with decreased tensile strength, severe embrittlement of the subsurface zone, and an increased incidence of rim cracking and delamination in retrieved liners (Figure 3). McKellop and coauthors 14,15 ex- amined the effects of sterilization method, calcium stearate addition, and thermal aging on the wear performance of UHMWPE in two hip simulator studies. Before initiating the studies, all irradiated samples received a mean dose of 2.7 Mrad. Despite differences in molecular weight and the presence or absence of calcium stearate, gas plasma–sterilized components demonstrated wear rates comparable with each other. Among components not subjected to accelerated aging, the EtO- sterilized samples had significantly higher wear rates than those sterilized with gamma radiation in air (P = 0.0001) or in a vacuum (P = 0.0001). Additionally, components that were Figure 2 Effects of irradiation in an oxygen environment on UHMWPE. (Reproduced with permission from Greenwald AS, Bauer TW, Ries MD, Committee on Biomedical Engineering, Committee on Hip and Knee Arthritis: New polys for old: Contribution or caveat? J B one Joint Surg Am 2001; 83(suppl 2):27-31.) Alexander C. Gordon, MD, et al Volume 14, Number 9, September 2006 513 gamma radiated in air had significantly higher wear rates than did those irradiated in a vacuum (P = 0.01) (Figure 4). After thermal aging, all gamma- irradiated cups, including those sterilized with methods to decrease oxidation (eg, ion implantation, nitrogen packaging, oxygen scavenger) demonstrated oxidative degradation and a subsequent increase in wear. The unsterilized and gas plasma– treated cups wore at the same rates before and after thermal aging, and both types of cups showed no oxidation. After accelerated aging, cups that were gamma-irradiated in air had the highest wear rates of all test specimens (Figure 5). The investigators concluded that prior to aging, the cross-linking effect of radiation—even in an oxygen environment—provided improvements in wear compared with components that were never sterilized or were EtO-sterilized. After oxidation and Figure 5 Wear rates of six types of polyethylene for two cycle intervals after artificial aging for 14 days at 80°C. All gamma-sterilized cups had a mean radiation dose of 2.7 Mrad. (Reproduced with permission from McKellop H, Shen FW, Lu B, Campbell P, Salovey R: Effect of sterilization method and other modifications on the wear resistance of acetabular cups made of ultra-high molecular weight polyethylene: A hip-simulator study. J Bone Joint Surg Am 2000;82:170 8-1725.) Figure 3 Photographs of Charnley components that were never implanted. The section without the band (top) was never sterilized. The section with the pronounced white band (bottom) was sterilized by gamma radiation in air 14 years earlier. (Reproduced with permission from Sutula LC, Collier JP, Saum KA, et al: The Otto Aufranc Award: Impact of gamma sterilization on clinical performance of polyethylene in the hip. Clin Orthop Relat Res 1995;319:28-40.) Figure 4 Wear rates of six types of polyethylene for two cycle intervals without artificial aging. All gamma-sterilized cups had a mean radiation dose of 2.7 Mrad. (Reproduced with permission from McKellop H, Shen FW, Lu B, Campbell P, Salovey R: Effect of sterilization method and other modifications on the wear resistance of acetabular cups made of ultra-high molecular weight polyethylene: A hip-simulator study. J Bone Joint Surg Am 2000;82:1708-1725.) Highly Cross-linked Polyethylene in Total Hip Arthroplasty 514 Journal of the American Academy of Orthopaedic Surgeons embrittlement of the polymer, however, the advantage of irradiation is lost. Sychterz et al 16 studied sterilization variables in a clinical setting. They reviewed radiographs of patients who had undergone cementless THA fixation whose conventional acetabular liners had been sterilized with (1) gamma radiation in air, (2) gamma radiation in a vacuum and barrier-packaged, or (3) gas plasma. The liners that were gamma-irradiated in a vacuum and those irradiated in air wore at significantly lower rates than did those sterilized by gamma radiation in air or gas plasma (P < 0.01). The authors also concluded that the c ross-linking provided by gamma sterilization, even in air, provided better wear resistance than did gas plasma in conventional polyethylene. Before the introduction of highly cross-linked polyethylene, two products meant to be improvements on conventional polyethylene were mar- keted but subsequently discontin- ued—highly crystalline UHMWPE (Hylamer, DePuy, Warsaw, IN) and carbon fiber–reinforced polyethylene (Poly II, Zimmer). Hylamer has been more extensively studied than Poly II in THA, with reports of Hylamer wearing at rates comparable to those of conventional polyethylene. 15,16 De- spite this finding, some studies 17,18 indicate high wear rates and severe osteolysis in patients implanted with Hylamer liners sterilized by gamma radiation in air. In a retrieval analysis, Collier et al 19 noted that for a given level of oxidation, Hylamer liners that were gamma-sterilized in air sustained more wear and damage than did conventional polyethylene sterilized in the same manner. They suggested that the increased crystal- linity of Hylamer makes it more susceptible to oxidation than conventional polyethylene. Most reports of Poly II are from the knee arthroplasty literature, but one report o f carbon fiber polyethylene in THA discussed two instances of severe tissue reac- tion and prosthetic loosening associated with this material. 20 Highly Cross-linked Polyethylene Manufacturing The highly cross-linked components available for implantation are machined from ram-extruded bar stock of GUR 1050 resin. Although the exact methods are proprietary and differ among manufacturers, the steps to produce cross-linked polyethylene follow the same general sequence: radiation cross-linking, thermal treatment, and terminal sterilization 21 (Figure 6). The first step is a cross-link–induc- ing radiation dose of 2.5 to 10 Mrad provided by cobalt 60 (gamma) or an electron beam source. This is followed by thermal treatment, in which the polyethylene is heated below, at, or above its melting temperature, depending on the manufacturer. This step is meant to quench free radicals, allowing the polyethylene chains to preferentially cross-link, thus diminishing the chances for oxidative degradation. The heating methods, which are proprietary, may be combined with electron beam irradiation because this process mea- surably heats the polymer. The final step is terminal sterilization and barrier packaging. Te rminal sterilization of these components is usually a surface treatment, but some manufacturers use a sterilizing dose of gamma radiation in an inert atmosphere. In a study attempting to determine the effects of these specific steps, Muratoglu et al 22 found higher levels of free radicals and more post-aging oxidation in polymers Figure 6 1050 Extruded rod Machine cup Radiation (1) 5 Mrad (2) 10 Mrad 7.5 Mrad radiation 125 C Warming oven Warming oven 3 Mrad Sterilize, N 2 Heat above melt (>135 C) Heat anneal 9.5 Mrad Electron beam 10 Mrad Electron beam Heat anneal in package Machine cup Machine cup Heat above melt (>135 C) Sterilize (1) Gas plasma (2) 2.5 Mrad St erilize, N 2 /Vacuum Machine cup Machine cup Ethylene oxide sterilize Gas plasma sterilize Process Heat stabilized CISM (cold irradiated subsequent melt) CIAN (cold irradiated adiabatic non-melt) WISM (warm irradiated subsequent melt) Longevity Durasul Zimmer Stryker Howmedica Osteonics Crossfire (1) Marathon (2) XLPE (1) DePuy/ Johnson & Johnson (2) Smith+Nephew Duration Product Company Stryker Howmedica Osteonics ° Heat above melt C) WIAM (warm irradiated adiabatic melting) (>135 Ethylene oxide Zimmer ° ° ° Processing steps for highly cross-linked polyethylene, by manufacturer. (Reproduced with permission from Greenwald AS, Bauer TW, Ries MD, Committee on Biomedical Engineering, Committee on Hip and Knee Arthritis: New polys for old: Contribution or caveat? J Bone Joint Surg Am 2001;83(suppl 2):27-31.) Alexander C. Gordon, MD, et al Volume 14, Number 9, September 2006 515 treated with sub-melt temperature annealing and terminal gamma sterilization (Crossfire; Stryker How- medica Osteonics, Mahwah, NJ) than in those that were melted and gas sterilized after cross-linking radiation (Longevity; Zimmer). In a retrieval analysis of explanted cross- linked liners from different manufacturers, Bhattacharyya et al 23 hy- pothesized that Crossfire would show more in vivo oxidation than melt-stabilized polyethylene, such as Longevity or Durasul (Zimmer). Within 3 years of implantation, the authors found elevated oxidation levels and one component with a subsurface white band among the Crossfire liners; they did not detect any oxidation in the other two types. Bhattacharyya et al 23 concluded that the free radicals formed by sub-melt temperature annealing and gamma sterilization can lead to in vivo oxidation. Laboratory Studies Laboratory studies have demonstrated that higher degrees of cross- linking improve wear resistance and decrease particulate volume in a hip simulator. 24 Although increased femoral head size causes increased volumetric wear rates in hips implanted with conventional polyethylene, 25 wear-simulator studies of highly cross-linked polyethylene have demonstrated greatly diminished wear compared with conventional polyethylene in liners articulating with 22-, 28-, 32-, and 46-mm heads. Hermida et al 26 compared highly cross-linked liners with nominally cross-linked liners articulating with 28- and 32-mm femoral heads. The highly cross-linked liners had been sub-melt temperature annealed and sterilized with gamma radiation; the nominally cross-linked liners were polyethylene that was conventional- ly sterilized by gamma radiation in nitrogen. The 28- and 32-mm highly cross-linked liners had significantly (P < 0.001) less wear than did their conventional counterparts, but the wear of 28- and 32-mm highly cross- linked cups did not differ significantly (Figure 7). The authors concluded that larger femoral head size may not be predisposed to increased wear in highly cross-linked liners. Muratoglu and colleagues 27,28 have extensively studied electron beam cross-linked, melt-annealed, and EtO-sterilized (Durasul) UHMWPE. They studied the mechanical properties, oxidation levels, effect of femoral head size, and wear rates compared with those of conventional polyethylene. The authors found markedly less wear of the highly cross-linked liners compared with gamma-sterilized/inert implants for femoral head sizes ranging from 22 to 46 mm. After weighing the components, they determined that there was no detectable wear from the highly cross-linked specimens and that the head penetration noted was solely the result of plastic deformation. This was corroborated by the presence of machining marks on the cross-linked liners after 20 million cycles; these marks had been worn away on the conventional polyethylene specimens. The mechanical and molecular analysis of Durasul showed no oxidation after accelerated aging and no evidence of free radicals, but it did demonstrate a decrease in ultimate tensile strength (UTS) and yield strength compared with gamma-sterilized/inert polyethylene. Despite the inferior mechanical properties of the highly cross- linked polyethylene, the testing results fell well within American So- ciety for Testing and Materials (ASTM) standards for medical-grade UHMWPE. However, ASTM standards do not imply that a polyethylene component is suitable for clinical use and do not include a specification for fracture toughness. The diminished crack propagation resistance of cross-linked polyethylene may have clinical implications. Other researchers have tested highly cross-linked polyethylene under more adverse conditions, such as wear in the presence of a third body or a rough countersurface. One study comparing gamma/nitrogen–cross- linked, barrier-packaged polyethyl- Figure 7 Cumulative wear rates of highly cross-linked (X) and nominally cross-linked (O) acetabular liners articulating with 28- and 32-mm heads. (Reproduced with permission from Hermida JC, Bergula A, Chen P, Colwell CW Jr, D’Lima DD: Comparison of the wear rates of twenty-eight and thirty-two-millimeter femoral heads on cross-linked polyethylene acetabular cups in a wear simulator. J Bone Joint Surg Am 2003;85:2325-2331.) Highly Cross-linked Polyethylene in Total Hip Arthroplasty 516 Journal of the American Academy of Orthopaedic Surgeons ene articulating with femoral heads of differing surface roughness reported significantly (P = 0.004) less wear of the cross-linked liners. 29 These investigators found that severely roughened balls (surface roughness, 0.9 µm) during and after the initial wear-in period had wear rates higher than that of the conventional polyethylene articulating with a smooth surface, thus negating the effects of cross-linking on wear. Although it is not known wheth- er this degree of roughening occurs in vivo, Minakawa et al 30 attempted to quantify the third-body damage of retrieved femoral heads. They determined that cobalt-chrome heads could suffer varying degrees of damage; cobalt-chrome heads had a mean surface roughness of 0.4 µm on their most damaged areas. The authors found a single component with damage >2.0 µm, which sug- gests that the conditions in the study by McKellop et al 29 could oc- cur in vivo. Bragdon et al 31 compared the wear resistance of gamma/nitrogen and cross-linked polyethylene in an environment of polymethylmethacry- late (PMMA) or alumina third-body particles. As expected, the specimens with the alumina particles wore much more than did those with PMMA or without third-body particles. Although the authors found that the presence of a very hard third body (eg, alumina) affected the wear of cross-linked polyethylene, PMMA particles had a very small effect. The cross-linked liners in this study demonstrated significantly (P < 0 .0001) l ess wear than did conventional polyethylene in all testing conditions. Taylor et al 32 also studied the effects of PMMA particles on wear of cross-linked polyethylene. They found lower wear in the cross-linked specimens than in controls; however, they did note significant surface damage and wear rates that were much higher than those reported by Bragdon et al. 31 Although much research has focused on the wear rates of cross- linked polyethylene, other reports have focused on the characterization of the wear particles generated during these tests. Ingram et al 33 mea- sured the size of wear particles produced by wearing 5- and 10-Mrad cross-linked polyethylene against smooth and rough surfaces, then tested their biologic activity by de- termining the levels of tumor necro- sis factor-α production by macro- phages cultured with the wear debris. They found that increased levels of cross-linking, associated with wear from the rougher surface, led to a higher percentage of debris in the submicron range and increased biologic activity. Wear against a smooth surface resulted in nanometer-sized particles in non- and cross-linked specimens, thus de- creasing their biologic activity. These data suggest that although ab- solute wear is decreased with cross- linking, the particles generated are biologically active and have the potential to induce osteolysis. Collier et al 34 obtained cross- linked acetabular liners from six US orthopaedic implant manufacturers to determine the effects of the differing manufacturing techniques on the mechanical properties, crystal- linity, and pre- and post-aging oxidation levels of the various components (Table 1). Their goal was to determine the properties of clinical- ly available polyethylene liners and relate those properties to the wear rates published by the manufacturers. The authors did not do a head- to-head comparison of wear rates. These cups were subjected to an accelerated aging protocol. Before aging, all test cups showed no to low initial oxidation rates; however, Du- rasul, Crossfire, and ArCom did have higher “as received” oxidation l evels than did a standard reference polyethylene. After accelerated aging, the Longevity, C rossfire, and ArCom liners demonstrated significantly more oxidation than their as- received counterparts (P < 0.01), while the others had no change in oxidation level. The Longevity liners had the lowest initial oxidation level of the six test specimens, and the authors thought that its increase in oxidation after aging was not enough to affect its mechanical properties. Mechanical testing demonstrated a range of UTS (34 to 59 MPa) and a smaller range of tensile strength at yield point (19 to 24 MPa) for the as- received components (Table 2 ). Af- ter aging, ArCom, Durasul, and Crossfire liners demonstrated a decrease in UTS, and ArCom, Reflec- tion, and Crossfire liners showed significant differences in yield point compared with their as-received counterparts (Table 3). All materials tested exceeded the ASTM standard (27 MPa) for UTS and tensile stress at yield point (19 MPa) in non–cross- linked polyethylene. Collier et al 34 found that the UTS of the materials was stratified by radiation dose; the components receiv- ing >5 Mrad had significantly (P < 0.01) lower values than did the reference polyethylene before aging. In contrast, the tensile strength a t y ield point was stratified by the heating method rather than radiation dose. The components that were heated at or above their melting temperature showed significantly (P < 0.01) lower values than did the reference material. The investigators concluded that even intentionally cross-linked polyethylene is not immune to oxidation and free-radical formation; the varying oxidation levels and sus- ceptibility to oxidation after aging were dependent on the processing conditions. The authors also stated that increasing the radiation dosages appears to produce lower wear rates, as reported by the manufacturers, but also results in lower toughness. The appendix of the article 34 in- cludes a paragraph from each manufacturer regarding the rationale for the manufacturing processes used in the production of its components. Subsequent to the 2003 publica- tion of the study by Collier et al, 34 Alexander C. Gordon, MD, et al Volume 14, Number 9, September 2006 517 Smith & Nephew increased the cross-linking radiation dose from 5 to 10 Mrad; Stryker Howmedica Os- teonics introduced a new cross- linked polyethylene (X3) that is se- quentially irradiated and annealed; and Biomet began production of Ar- Com XL, an intentionally cross- linked and heat-stabilized version of its direct compression-molded Ar- Com product. No published clinical or laboratory results are available on these updated products. Concern about the loss of fracture toughness and more brittle nature of highly cross-linked polyethylene has been the topic of numerous reports. Baker and colleagues 36-38 conducted several studies to elucidate the fatigue resistance and fracture toughness of cross-linked polyethylene. Their hypothesis was that the higher degree of cross-linking, leading to a restriction of chain mobility in the amorphous regions of polyethylene, would decrease the plasticity of the polymer, resulting in a material that is less resistant to crack propagation. The true stress at break point and the resistance to crack propagation were inversely related to the cross- linking radiation dose and were attributed to a decrease in plasticity at the fracture tip. The results of these studies suggest that for clinical situ- ations in which stress concentra- tions and surface defects may exist, a lower degree of cross-linking may be safer. Clinical Evaluation and Retrieval Analysis Extensive laboratory data exist on the wear resistance of cross-linked polyethylene, but few clinical studies are available. The published studies (Table 4) are generally consistent with the laboratory data, but long- term follow-up is not yet available. Three randomized, prospective eval- uations of highly cross-linked polyethylene with 2-year clinical follow- up have been published. 39-41 In a study of cemented THA, Di- Table 1 Cross-linked Material Tested by Collier et al 34 Material (Manufacturer) Resin Fabrication Radiation Source Dose to Cross-link Annealing ArCom (Biomet, Warsaw, IN) 1900H Direct compression-molded or machined from molded bar Gamma 2.5 to 4 Mrad None Marathon (DePuy, Warsaw, IN) 1050 Machined from extruded bar Gamma 5 Mrad Above melt temperature (150°C) Reflection XLPE (Smith & Nephew, Memphis, TN) 1050 Machined Gamma 5 Mrad (subsequently changed to 10 Mrad) At melt temperature (136°C) Durasul (Zimmer, Warsaw, IN) 1050 Machined from compression-molded sheet Electron beam 9.5 Mrad Above-room-temperature pre-heat before electron beam; melt anneal; controlled heat and cooling rates; warm irradiation with adiabatic melting Crossfire (Stryker Howmedica Osteonics, Mahwah, NJ) 1050 Machined Gamma 7.5 Mrad (subsequently irradiated with 3 Mrad) Below melt temperature (>120°C) Longevity (Zimmer) 1050 Compression molded and machined Gamma 10 Mrad Above-room-temperature pre-heat before electron beam; process between cold irradiation with subsequent melt and warm irradiation with adiabatic melting Adapted with permission from Collier JP, Currier BH, Kennedy FE, et al: Comparison of cross-linked polyethylene materials for orthopaedic applications. Clin Orthop Relat Res 2003;414:289-304. Highly Cross-linked Polyethylene in Total Hip Arthroplasty 518 Journal of the American Academy of Orthopaedic Surgeons gas et al 39 compared Durasul with conventional polyethylene sterilized by gamma irradiation in nitrogen. Head penetration into the liner was evaluated with radiostereometric analysis (RSA) at 1 and 2 years post- operatively. Head penetration seen on supine radiographs was similar between groups at 1 and 2 years, but was approximately 50% less in the highly cross-linked group in the same time period as seen on stand- ing radiographs. Additionally, n o dif- ference was found between groups with respect to component migra- tion or the appearance of radiolucent lines. Most of the head penetration in the first year was attributed to plastic deformation of the socket. In another study, 40 these same investigators presented their 3-year data with the cemented liners and introduced a new study of bilateral hybrid THAs. The cemented cup study results were similar to those in the previous report, with lower head penetration rates in the highly cross-linked group at 3-year follow- up. The hybrid hip study used an RSA method to compare Longev- ity liners with polyethylene that was compression-molded, gamma- irradiated in nitrogen, and implanted into a cementless cup. During the first year, head penetration rates of the two polyethylenes were not significantly different, but at 2 years, significantly (P < 0.0005) less head penetration was observed in the cross-linked components. The authors concluded that the similar early head penetration rates generally reflect creep and not wear. Using a digital radiographic technique in a randomized, prospective evaluation with 2-year follow-up, Martell et al 41 compared Crossfire with polyethylene irradiated in ni- Table 2 Mechanical Properties of As-Received Acetabular Liners Cross-linked Material Yield Point (MPa) Probability Value* Ultimate Tensile Strength (MPa) Probability Value* Elongation (%) Probability Value* ArCom 24 ± 0.8 P < 0.01 59 ± 4.7 0.3826 240 ± 38 P <0.01 Marathon 21 ± 0.5 P < 0.01 56 ± 7.0 0.1892 300 ± 14 P < 0.01 Reflection XLPE 20 ± 1.3 P < 0.01 56 ± 7.1 0.1895 300 ± 20 P < 0.01 Crossfire 22 ± 1.0 0.8177 53 ± 5.3 P < 0.01 230 ± 17 P < 0.01 Durasul 19 ± 1.6 P < 0.01 34 ± 3.4 P < 0.01 330 ± 19 P < 0.01 Longevity 21 ± 1.1 0.0271 43 ± 5.3 P < 0.01 250 ± 25 P < 0.01 HSS Reference UHMWPE 35 21.7 ± 1.0 — 58 ± 4.7 — 380 ± 10 — *Probability values are for the t-test between the cross-linked materials and the Hospital for Special Surgery (HSS) reference ultra-high–molecular-weight polyethylene (UHMWPE). Adapted with permission from Collier JP, Currier BH, Kennedy FE, et al: Comparison of cross-linked polyethylene materials for orthopaedic applications. Clin Orthop Relat Res 2003;414:289-304. Table 3 Mechanical Properties of Acetabular Liners After 28 Days of Artificial Aging Cross-linked Material Yield Point (MPa) Probability Value* Ultimate Tensile Strength (MPa) Probability Value* Elongation (%) Probability Value* ArCom 25 ± 1.4 P < 0.01 40 ± 8.1 P < 0.01 300 ± 60 P < 0.01 Marathon 21 ± 1.5 0.3877 56 ± 5.7 0.8699 290 ± 14 P < 0.01 Reflection XLPE 21 ± 1.4 0.0393 58 ± 7.3 0.2643 300 ± 37 0.9858 Crossfire 24 ± 1.3 P < 0.01 48 ± 7.2 P < 0.01 280 ± 37 P < 0.01 Durasul 20 ± 0.7 0.0279 30 ± 7.1 P < 0.01 280 ± 74 P < 0.01 Longevity 21 ± 1.0 0.1662 43 ± 9.8 0.8006 240 ± 35 0.0718 *Probability values are for the t-test between the “as received” and aged cross-linked material properties Adapted with permission from Collier JP, Currier BH, Kennedy FE, et al: Comparison of cross-linked polyethylene materials for orthopaedic applications. Clin Orthop Relat Res 2003;414:289-304. Alexander C. Gordon, MD, et al Volume 14, Number 9, September 2006 519 trogen and barrier-packaged. The authors noted a marked (40% to 50%) decrease in the two-dimensional linear, two-dimensional volumetric, and three-dimensional linear wear rates in the highly cross-linked group. Head penetration seen in the first year after implantation was mostly caused by plastic deformation, not by true wear. Heisel et al 42 performed a nonrandomized study comparing Marathon cross-linked polyethylene with conventional polyethylene sterilized by gamma irradiation in air; they found an 81% decrease in volumetric wear in the cross-linked group after 2 years. Using regression analysis to control for the differences between groups, they determined that the type of polyethylene was the only significant variable influencing volumetric wear rates. The study with the longest follow-up to date, published by Dorr et al, 43 compared Durasul with gamma/nitrogen polyethylene after 5 years of clinical use. In a retrospec- tive study of 37 Durasul hips matched to historical controls, direct radiographic measurements were used to calculate the mean annual head penetration rates; the investigators found that a digital measure- ment technique did not provide accu- rate data. The “bedding-in” period for Durasul was approximately 2 years, while that of the conventional polyethylene was 1 year. From 2 to 5 years, the linear wear rate of Durasul was approximately 50% less and the annual head penetration rate was 60% to 75% less than conventional polyethylene during the same period. In a prospective, nonrandomized study using RSA, Rohrl et al 44 compared wear rates of cemented stems articulating with either cemented gamma/air or Crossfire polyethylene. In contrast with other studies, the bedding-in period was only 2 months, and the wear rates were linear for both groups thereafter. From 2 to 24 months, an 85% reduction in wear was noted in the Crossfire group, and cross-linked polyethylene demonstrated significantly (P < 0.001) lower wear rates than did gamma/air polyethylene without increased migra- tion or radiolucencies. In all of the aforementioned studies, wear rates were lower for cross- linked polyethylene than for controls. Larger differences between conventional and cross-linked polyethylene were found when the controls were gamma-sterilized in air versus in an inert environment, again demonstrat- ing the inferior wear characteristics of gamma/air polyethylene. In addition to the clinical studies, in vivo behavior of highly cross- linked polyethylene after a relative- ly short service life has been studied using retrieval analysis. Bradford et al 45 studied 21 cross-linked Durasul liners revised 2 to 24 months after implantation. Pitting, scratches, and surface cracking were common findings, but no liners d emonstrated bur- nishing or severe wear (Figure 8). The authors postulated that the cracking was likely the result of the diminished ductility and fatigue resistance of the polymer and concluded that the in vivo wear patterns of highly cross-linked polyethylene differ from those occurring in a hip simulator. The significance of this finding is that hip simulators did not accurately predict the in vivo performance of a given material. Other researchers attribute a different significance to the surface findings of explanted cross-linked liners, however. Muratoglu et al 46 also studied liners not revised for wear with a service life of 2 weeks to 10 months. They used a melt-recovery technique to test their hypothesis that the surface scratching represented plastic deformation, not true wear. Their most common findings were light and heavy surface scratches; a few specimens had polished areas. The melt- recovery process was used to recover the machining marks, if present. Five of the seven liners treated with this process had complete or near- complete recovery of the original machining marks. The authors con- Table 4 Summary of Highly Cross-linked Polyethylene Clinical Studies Author Cross-linked Polyethylene (Fixation) Conventional Polyethylene Follow-up (years) Wear Reduction of Cross-linked Polyethylene Digas et al 39 Durasul Gamma/Nitrogen 2 50% linear wear Digas et al 40 Durasul (cemented) Gamma/Nitrogen 3 50% linear wear Longevity (hybrid) Gamma/Nitrogen 2 62% linear wear Martell et al 41 Crossfire Gamma/Nitrogen 2 40% to 50% linear wear Heisel et al 42 Marathon Gamma/Air 2 81% volumetric wear Dorr et al 43 Durasul Gamma/Nitrogen 5 50% linear wear; 60% to 75% less head penetration Rohrl et al 44 Crossfire Gamma/Air 2 85% linear wear Highly Cross-linked Polyethylene in Total Hip Arthroplasty 520 Journal of the American Academy of Orthopaedic Surgeons

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