Department of Orthopaedic Surgery, Graduate School of Medical Sciences, Kyushu University, Maidashi, Higashi-ku, Fukuoka , Japan

Transcription

1 Wear Resistant Performance of Highly Cross-Linked and Annealed Ultra-High Molecular Weight Polyethylene against Ceramic Heads in Total Hip Arthroplasty Taishi Sato, Yasuharu Nakashima, Mio Akiyama, Takuaki Yamamoto, Taro Mawatari, Takashi Itokawa, Masanobu Ohishi, Goro Motomura, Masanobu Hirata, Yukihide Iwamoto Department of Orthopaedic Surgery, Graduate School of Medical Sciences, Kyushu University, Maidashi, Higashi-ku, Fukuoka , Japan Received 27 January 2012; accepted 1 May 2012 Published online 29 May 2012 in Wiley Online Library (wileyonlinelibrary.com). DOI /jor ABSTRACT: The purpose of this study was to examine the effects of ceramic femoral head material, size, and implantation periods on the wear of annealed, cross-linked ultra-high molecular weight polyethylene (UHMWPE) (XLPE) in total hip arthroplasty compared to non-cross-linked conventional UHMWPE (CPE). XLPE was fabricated by cross-linking with 60 kgy irradiation and annealing. Femoral heads made from zirconia and alumina ceramics and cobalt chrome (CoCr) of 22 or 26 mm diameter were used. In this retrospective cohort study, the femoral head penetration into the cup was measured digitally on radiographs of 367 hips with XLPE and 64 hips with CPE. The average follow-up periods were 6.3 and 11.9 years, respectively. Both XLPE creep and wear rates were significantly lower than those of CPE (0.19 mm vs mm, mm/year vs mm/year, respectively). Zirconia displayed increased wear rates compared to alumina in CPE; however, there was no difference among head materials in XLPE (0.0008, , and mm/year for zirconia, alumina, and CoCr, respectively). Neither head size or implantation period impacted XLPE wear. In contrast to CPE, XLPE displayed low wear rates surpassing the effects of varying femoral head material, size, implantation period, and patient demographics. Further follow-up is required to determine the long-term clinical performance of the annealed XLPE. ß 2012 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 30: , 2012 Keywords: polyethylene; wear; total hip arthroplasty; cross-linked; annealing Osteolysis is related to particulate ultra-high molecular weight polyethylene (UHMWPE) wear debris and is considered an important cause of aseptic loosening and late implant failure in total hip arthroplasty (THA). 1 3 Thus, UHMWPE wear is one of the major determinants influencing THA survivorship. Highly cross-linked UHMWPE (XLPE) is an alternative bearing surface developed to improve wear resistance and decrease osteolysis. XLPE has been studied extensively in vitro and has substantially decreased wear rates compared to non-cross-linked conventional UHMWPE (CPE) in hip simulator studies. 4,5 Preliminary clinical studies of XLPE in patients undergoing THA have confirmed these promising findings with a 30 80% reduction in wear compared to CPE at 3 5 years. 6,7 XLPE is fabricated using irradiation at kgy for cross-linking and thermal treatment after irradiation to reduce free radicals and increase durability and cross-link density. Methods to prepare XLPE include radiation (gamma ray or electron beam), thermal treatment (remelted or annealed), and sterilization. 8 These methods, which influence mechanical properties, crystallinity, and oxidation states, are not equivalent, and the resulting clinical performances differ. The type of material and the size of femoral head are additional factors that influence wear Cobaltchromium alloy (CoCr) replaced stainless steel and titanium alloy due to its high wear resistance. 13 Correspondence to: Yasuharu Nakashima (T: þ ; F: þ ; yasunaka@ortho.med.kyushu-u.ac.jp) ß 2012 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. Ceramic heads of alumina or zirconia, display a smoother surface finish and improved resistance to scratching compared to metal heads. 14 The wear of CPE against zirconia is 5 10 times lower than stainless steel in vitro. 15 However, no significant difference was found comparing the wear rate of zirconia and CoCr in vivo. 10,16 Another study reported excessive UHMWPE wear due to phase transformation and thermal transduction properties of zirconia ceramic implanted between 1997 and 1999 in longer follow-up periods. 17 Thus, any benefits from ceramics remain controversial. These findings suggest the need to determine the efficacy of annealed XLPE coupled with ceramic and metal femoral heads. At present, studies of XLPE wear are limited by small sample sizes and short follow-up periods. They also lack a comparison to ceramic femoral heads. We therefore examine the effects of ceramic head material, head size, and implantation period on the wear of annealed XLPE at a maximum of 9 years of follow-up, compared with those of CPE. MATERIALS AND METHODS Patients This study was approved by our institutional review board. Between 2000 and 2006, a consecutive series of 367 hips in 335 patients who had primary THAs using XLPE were retrospectively reviewed (XLPE group). The average follow-up period was 6.3 years ranging from 4 to 9 years. The average age at THA was 61.8 years. The cause leading to THA was osteoarthritis in 324 (88.3%) hips, rheumatoid arthritis in 19 hips (5.2%), and osteonecrosis of the femoral head in 24 hips (6.5%). Sixty-four hips in 58 patients who had primary THAs using CPE between 1994 and 1998 were analyzed for 2031

2 2032 SATO ET AL. comparison. The average follow-up period was 11.9 years ranging from 7 to 16 years. The average age at operation was 59.6 years. The initial diagnosis in the CPE group was osteoarthritis in 53 hips (82.8%), rheumatoid arthritis in 5 hips (7.8%), and osteonecrosis of femoral head in 6 hips (9.4%). Patient demographics are summarized in Table 1. No significant difference was identified between UHMWPE groups, head material, or head size groups with gender, age at surgery, BMI and the latest follow-up Merle d Aubigne score (p ¼ preoperatively and 0.14 at last follow-up). Implant All patients received the identical cementless acetabular and femoral component system [AMS & PerFix, Japan Medical Materials Corp. (JMM), Osaka, Japan]. The hemispherical acetabular component and tapered cylindrical femoral component were made from hydroxyapatite-coated titanium alloy substrate. 18 The materials and mechanical properties of the head are shown in Table 2. With alumina and zirconia ceramic heads, each powder was molded into the form of a bar by a cold isostatic press, and heads were machined from the bar. The machined ceramic heads were sintered and subjected to a hot isostatic press, after which they reached over 99% theoretical density. Surfaces were polished to an average roughness R a between 0.01 and 0.02 mm. The polished heads were sterilized by ethylene oxide gas or gamma-ray irradiation. The CoCr femoral heads were manufactured from bar stock according to ASTM F799 standards specification for Co 28Cr 6Mo Alloy. After machining, these heads were polished to an average roughness ranged between 0.01 and 0.04 mm. The polished CoCr femoral heads were sterilized by gamma-ray irradiation. Operation and Follow-Up The operation was performed through a standard posterior approach by two experienced hip surgeons (YN and TY), each having performed >100 THAs/year. Clinical and radiographic follow-ups were performed at 3 and 6 months postoperatively and annually thereafter. The Merle d Aubigne hip score was recorded annually for clinical assessment. 19 UHMWPE Gamma ray irradiated and annealed XLPE liners (Aeonian, JMM) were used in the XLPE group. To manufacture XLPE, a calcium stearate-free GUR 1050 UHMWPE resin was utilized to create a compression-molded sheet stock, which was then machined into the final implant geometry. Prior to machining, the compression-molded sheet was subjected to a gamma-ray irradiation of 35 kgy in air to achieve the desired level of cross-linking and annealed at 1108C for 12 h in nitrogen to reduce free radicals. After machining, the liners were sterilized with 25 kgy of gamma-ray irradiation in nitrogen. The CPE liners, not irradiated or thermally treated, were manufactured from ram-extruded bars made of GUR415 UHMWPE resin containing calcium stearate. After machining, these liners were sterilized by ethylene oxide gas. Wear Measurement The penetration of the heads into the UHMWPE liners was measured on radiographs at 1 week, and 1, 4, 6, and 8 years postoperatively and at the latest follow-up. At each followup, standardized AP radiographs of the pelvis were taken in a supine position with the bilateral hip joints in the neutral position, with the beam center on the pubic tubercle. A 2D linear penetration was measured using a computer-assisted method (PolyWare, Draftware Developers, Vevay, IN) for which the accuracy is mm. 20 Radiographs were digitized into tagged image file format files at a resolution of 300 dots/in. and read by the software. The software calculated head penetration by identifying the centers of the head and the acetabular component with an edge detection method. UHMWPE creep deformity is a recognized phenomenon in the early postoperative periods after THA. 5,10 To reduce the disproportionate effects of creep, a radiograph taken at 1 year postoperatively was used as the initial one for the Table 1. Patient Demographics CPE (n ¼ 58) XLPE (n ¼ 335) p-value Hips No. of men/women 24/40 50/ Age at surgery (year) a Body mass index a Follow-up (year, range) b 12.1 ( ) 6.1 ( ) < Inclination of the cup 42.0 (26 60) 41.6 (31 58) 0.67 Merle d Aubigne hip score Preoperative a Final follow-up a Head materials (no. of the hip) Zirconia Alumina CoCr 0 20 Head size (no. of the hip) 22 mm (all zirconia) 26 mm CPE, conventional UHMWPE; XLPE, highly cross-linked UHMWPE. The continuous variables and nominal variables were analyzed with use of the Mann Whitney s U-test and a Chi-square test, respectively. The values of p < 0.05 were considered significant. a The values are given as the mean SD. b The values are given as the mean with the range in parentheses.

3 XLPE WEAR AGAINST CERAMIC HEAD 2033 Table 2. Chemical Composition and Mechanical Properties of the Femoral Heads Zirconia Alumina CoCr Chemical composition (wt%) ZrO 2 þ HfO 2 þ Y 2 O 3 > 99.0 Al 2 O 3 > 99.5 Co: Bal, Cr: 26 30, Mo: 5 7 Density (g/cm 3 ) Crystal grain size (mm) Fracture toughness (Mpcm 0.5 ) Hardness (Hv) 1,400 1, Elasticity (GPa) Surface roughness (mm) 0/ measurement of steady wear rate. The steady wear rate for each group was calculated as an average of individual patient wear rates. In addition, the mean creep in each group was evaluated. To assess the correlation between cup position and wear rate, the inclination angle of the cup was measured using the radiograph at 1 week postoperatively. The inclination angle was defined as an angle between the teardrop line that joins the right and left distal margin of the teardrop and the line that crosses the lateral and medial margins of the base of the cup. No significant difference was found in the inclination angle between the two groups (Table 1). Two independent observers (TS and MA) performed all measurements. For inter and intra correlation reliability evaluation, 10 randomly selected radiographs were measured twice in a blind manner during the course of two sessions 1 month apart. Intra-observer reliabilities, evaluated with by intraclass correlation coefficients, were excellent (range, for TS and for MA, respectively). The interclass correlation coefficient ranged from 0.97 to Statistical Analysis The sample size was determined on the basis of an overall alpha error of 0.05 and a statistical power of 80%. Steady wear rate, creep, gender, age at THA, BMI, and Merle d Aubigne scores were compared between CPE and XLPE and between head materials in the 22 and 26 mm heads. These factors were all continuous variables, and a normal distribution test was performed with a Shapiro Wilk test. A statistical comparison was carried out with the Levene test for homoscedasticity and subsequently with the Mann Whitney s U-test. Gender and disease were nominal variables, and a statistical comparison was done using a Chi-square test. The values of p < 0.05 were considered significant. RESULTS Comparison of the Creep and Steady Wear Rate between CPE and XLPE UHMWPE creep and subsequent steady phase wear were noted in both groups (Fig. 1A and B). The average creep was significantly lower in the XLPE group (0.44 mm in CPE vs mm in XLPE, p < , Table 3). The steady wear rate was 0.09 mm/year in the CPE group and mm/year in the XLPE group, with significantly less wear in the XLPE group (p < , Table 3). The wear rates through 1 8 years and 8 16 years in the CPE group were and mm/year, respectively, showing no significant difference (p ¼ 0.41). Similarly, the wear rates through 1 4 years and 4 9 years in the XLPE group were and mm/year, respectively, showing no significant difference (p ¼ 0.15). No significant effect was noted among patient demographic data on the wear rate in either group. The inclination angle of the cup had no correlation with steady wear rate in either group (p ¼ 0.62 for CPE and p ¼ 0.51 for XLPE). Effect of Head Materials The creep and wear rate in the three groups are shown in Table 4. The wear rate of zirconia and alumina ceramics in the CPE group was and mm/year, respectively, indicating significantly more penetration with zirconia heads (p < ). In contrast, the wear rates of zirconia, alumina, and CoCr in the XLPE group were , , and mm/year, respectively. No significant difference of the wear rates was found among the three head materials (p ¼ 0.45). The wear rates in head materials did not show significant differences between 1 to 4 years and 4 to 9 years (p ¼ 0.25 for zirconia, p ¼ 0.12 for head and, p ¼ 0.55 for CoCr, respectively; data not shown). Effect of Head Size In the XLPE group with zirconia femoral heads, no significant difference in creep was apparent between the 22 and 26 mm sizes (p ¼ 0.81, Table 5). The wear rate of mm/year in the 26 mm head was not significantly different (p ¼ 0.84) from mm/year measured in the 22 mm head despite varying XLPE thickness (10.1 mm average for 22 mm head and 8.3 mm average for 26 mm head). In both head sizes, wear rates did not show significant differences between 1 to 4 years and 4 to 9 years (p ¼ 0.69 for 22 mm, p ¼ 0.08 for 26 mm; data not shown). Clinical Results Eight patients in the CPE group underwent revision surgeries to exchange the UHMWPE liner and femoral head due to the excessive UHMWPE wear and the acetabular osteolysis in the CPE group. In the XLPE

4 2034 SATO ET AL. Figure 1. (A) Scatter plots of wear in the CPE group. The broken line represents the regression. The penetration in the CPE group increased linearly, whereas that in the XLPE group remained constant. CPE: conventional UHMWPE, XLPE: highly cross-linked UHMWPE. (B) Scatter plots of wear in the XLPE group. The solid line represents the regression. group, one patient underwent revision due to recurrent dislocation and another due to the neck fracture of the stem. No obvious periprosthetic osteolysis was observed in the XLPE group in this study period. No other complications such as surgical site infection, symptomatic deep vein thrombosis, or paresis associated with THA were noted in either group. DISCUSSION We examined the effects of femoral head material, size, and implantation periods on XLPE wear in THA Table 3. Comparison of Creep and Steady Wear Rate between Polyethylene Groups CPE XLPE p-value Creep (mm) < Steady wear < rate (mm/year) CPE, conventional UHMWPE; XLPE, highly cross-linked UHMWPE. The values are given as the mean SD. The continuous variables were analyzed with use of the Mann Whitney s U-test. The values of p < 0.05 were considered significant. compared to those on CPE wear. Both XLPE creep and steady wear rates were significantly lower than those of CPE. Zirconia femoral heads resulted in significantly more wear than alumina in CPE, whereas no difference was found among head materials in XLPE. This is the first study comparing zirconia and alumina heads against XLPE resulting in no difference of wear rate with an average of 6 years follow-up. Head size and implantation period did not show any effects on XLPE wear rates. As reported in the literature, the XLPE wear rate is generally much lower than CPE. 6,21 McCalden et al. showed in their randomized control trial that the wear rate of non-cross-linked UHMWPE with a CoCr head was >10 times higher than cross-linked UHMWPE irradiated by 100 kgy (0.051 mm/year vs mm/ year). Kurtz et al. 8 reported in their systematic review that the usage of XLPE resulted in a 30% wear reduction compared to CPE. Our results of mm/year in XLPE and 0.09 mm/year in CPE revealed a greater advantage of XLPE, compatible with these reports. 8 UHMWPE creep is a well-recognized phenomenon seen in the early periods after implantation. Penetration of the femoral head at 1 year after operation is thought to result from combined creep and wear. 5,10 XLPE creep was significantly less than that of CPE (0.19 mm vs mm, respectively). The material and size of the head did not have significant effects on creep in either XLPE or CPE. Cross-linking is accompanied with hardening of the material possibly contributing to the lesser amounts of creep seen in XLPE. XLPE wear properties largely depend on radiation dose and thermal treatment. Increased radiation with subsequent remelting lead to better wear due to higher cross-link densities and lower levels of free radicals. 22,23 Muratoglu et al. 23 showed that the cross-link density increased with increasing radiation dose and that the wear rate decreased with increasing radiation dose up to 200 kgy. Gencur et al. 22 reported the thermal treatment temperature influenced crystallinity and residual free radicals. In our study, however, XLPE with 60 kgy irradiation and annealing showed wear rates of mm/year up to 9 years after implantation. Röhrl et al. 24 reported that the head penetration after 10 years for the submelt-annealed XLPE with 75 kgy irradiation was 0.02 mm/year without creep. These results suggest that the comparatively low dose radiation and annealing also increased wear resistance. The GUR materials were different between the UHMWPE groups. Meding et al. 25 reported significant differences in wear rate among resins (GUR 415, 1050, and Himont 1900), with the UHMWPE liner made from GUR 1050 resin having the lowest wear rate of 0.05 mm/year. Therefore, a difference in GUR material possibly influenced the wear rate. Conversely, the wear rate was significantly different between zirconia and alumina heads even with the same GUR 415 resin

5 XLPE WEAR AGAINST CERAMIC HEAD 2035 Table 4. Comparison of Creep (A) and Steady Wear Rate (B) among Head Materials Zirconia Alumina Co Cr p-value (A) Creep CPE (mm) XLPE (mm/year) (B) Steady wear rate CPE (mm) < XLPE (mm/year) CPE, conventional UHMWPE; XLPE, highly cross-linked UHMWPE. The values are given as the mean SD. The continuous variables were analyzed with use of the Mann Whitney s U-test. The values of p < 0.05 were considered significant. in the CPE group. These results suggest that numerous factors influence UHMWPE wear. The inclination angle of the cup can be one of those factors. Waewsawangwong and Goodman reported fracture of an XLPE implant and concluded that failure was caused by local stress concentration due to cup malposition. 26 Crowninshield et al. 27 reported in a finite element analysis that excessive inclination caused tensile stress at the superior rim. Georgiades et al. 28 reported that the wear rate was significantly greater when the cup was placed in >458 inclination. In our study, the inclination angle ranged from 278 to 608; however, no patient sustained polyethylene fracture. The inclination angle also did not correlate with the wear rate of XLPE. However, further observation is needed for the long-term survival of XLPE. Effects of head materials on UHMWPE wear remain controversial. Ceramic heads are thought to offer improved lubrication, a smoother surface finish, and improved resistance to scratching, all leading to decreased wear. 14,16 However, zirconia heads reportedly result in increased wear rates and subsequent periprosthetic osteolysis when coupled with CPE. Liang et al. showed that the mean linear wear rate of CPE against zirconia heads was mm/year, significantly greater than alumina wear rates (0.078 mm/year). 17 Similarly, the wear rate of zirconia and alumina in CPE was and mm/year, respectively, indicating significantly more wear with zirconia heads in our study. Review of the literature indicates that the aging of zirconia in the body contributes to these clinical results. 29 Zirconia has the greatest mechanical strength in the tetragonal phase, Table 5. Comparison of Creep and Steady Wear Rate between 22 and 26 mm Head in Zirconia Group 22 mm 26 mm p-value Creep (mm) Steady wear rate (mm/year) The values are given as the mean SD. The continuous variables were analyzed with use of the Mann Whitney s U-test. The values of p < 0.05 were considered significant. although this is the most unstable phase. Transformation from the tetragonal to the monoclinic phase is accompanied by a 3% increase in ceramic head volume. Other reports indicate more than 20% monoclinic transformation with substantial surface roughening of a zirconia head retrieved at 3 years. 30 Nevertheless, the wear rate of XLPE was not influenced by head material in our study despite using the same zirconia femoral head. The wear rate of zirconia, alumina, and CoCr in XLPE was , , and mm/year without significant differences among them. Kawate et al. 31 also reported no difference in wear of XLPE between zirconia and CoCr heads. This is the first study comparing the wear rate of zirconia and alumina heads, showing the effect of using XLPE is far more important than head material. With XLPE, no clear benefit seems to exist with using ceramic heads. A larger diameter femoral head has an advantage in terms of dislocation due to a larger oscillation angle and jumping distance prior to dislocation, whereas the higher risk of excessive wear is suggested by a longer sliding length. With CPE, wear was reported to increase with a larger head; 32 however, this was not the case with XLPE. According to Lachiewicz et al., no association exists between head sizes from 26 to 40 mm and the linear penetration rate in XLPE coupled with a CoCr head. 14 Hammerberg et al. 33 also reported XLPE wear rates after an average 3.6-year follow-up of and mm/year for mm and mm femoral heads, respectively. These agree with our result; no difference in wear rate was noted between 22 and 26 mm. These reports and our results suggested decreased sensitivity of XLPE to the head diameter. Patient demographics such as age and BMI did not show significant effects on the wear rate of XLPE, although these factors were reported to correlate with the CPE wear rate. 34 Implantation duration also did not influence XLPE wear rate, showing cross-linking stability at a maximum of 9 years of follow-up in our study. Several limitations must be noted in this study. First, our cohorts were not randomized; the non-controlled design provided limited data. The difference in the numbers of patients in the CPE and XLPE groups

6 2036 SATO ET AL. was large. In the CPE group, the number of the patients using the same prosthesis except UHMWPE was 64 in this study periods. However, each group was not significantly different in terms of several wear related factors, such as gender, age, BMI, and Merle d Aubigne hip score. Secondly, the edge-detection technique as our primary wear measurement tool potentially has its own limits. This is evidenced by the substantial number of hips that demonstrated negative wear values, as has also been reported by other authors. 5,7 On the other hand, Devane and Horne 20 reported the accuracy of the measurement using the absolute value. No clear consensus exists on how to deal with this potential problem. Future development of the measurement tool with accuracy and precision are expected. Third, the size range of the femoral heads was small. Recently, 32 mm or larger sized heads have been adopted, whereas only 22 and 26 mm heads were used during the period of this study. Finally, the sample number in the CoCr group was smaller compared to the other groups. The CoCr head was adopted occasionally, resulting in the lower statistical power of comparison. In conclusion, our data showed marked improvement in wear resistance with annealed XLPE against ceramic femoral heads. The wear rate of 60 kgy irradiated and annealed XLPE was significantly lower than that of CPE. XLPE showed lower wear rates up to 9 years, surpassing the effects of head material, head diameter, implantation periods, and patient demographics. Longer follow-up is required to illustrate improved clinical performance and longevity of the implants. ACKNOWLEDGMENTS Y.N. and T.Y. received lecture fees from the Japan Medical Materials Corporation for subjects unrelated to this study. REFERENCES 1. Harris WH The problem is osteolysis. Clin Orthop Relat Res 311: Goodman SB, Ma T Cellular chemotaxis induced by wear particles from joint replacements. Biomaterials 31: Sochart DH Relationship of acetabular wear to osteolysis and loosening in total hip arthroplasty. Clin Orthop Relat Res 363: Collier JP, Currier BH, Kennedy FE, et al Comparison of cross-linked polyethylene materials for orthopaedic applications. Clin Orthop Relat Res 414: Manning DW, Chiang PP, Martell JM, et al In vivo comparative wear study of traditional and highly crosslinked polyethylene in total hip arthroplasty. J Arthroplasty 20: Ise K, Kawanabe K, Tamura J, et al Clinical results of the wear performance of cross-linked polyethylene in total hip arthroplasty: prospective randomized trial. J Arthroplasty 24: Engh CA Jr, Stepniewski AS, Ginn SD, et al A randomized prospective evaluation of outcomes after total hip arthroplasty using cross-linked marathon and non-crosslinked enduron polyethylene liners. J Arthroplasty 21: Kurtz SM, Gawel HA, Patel JD History and systematic review of wear and osteolysis outcomes for first-generation highly crosslinked polyethylene. Clin Orthop Relat Res 469: Derbyshire B, Fisher J, Dowson D, et al Comparative study of the wear of UHMWPE with zirconia ceramic and stainless steel femoral heads in artificial hip joints. Med Eng Phys 16: Geller JA, Malchau H, Bragdon C, et al Large diameter femoral heads on highly cross-linked polyethylene: minimum 3-year results. Clin Orthop Relat Res 447: Nakahara I, Nakamura N, Nishii T, et al Minimum five-year follow-up wear measurement of longevity highly cross-linked polyethylene cup against cobalt-chromium or zirconia heads. J Arthroplasty 25: Lachiewicz PF, Heckman DS, Soileau ES, et al Femoral head size and wear of highly cross-linked polyethylene at 5 to 8 years. Clin Orthop Relat Res 467: Agins HJ, Alcock NW, Bansal M, et al Metallic wear in failed titanium-alloy total hip replacements. A histological and quantitative analysis. J Bone Joint Surg Am 70: Saikko VO, Paavolainen PO, Slatis P Wear of the polyethylene acetabular cup. metallic and ceramic heads compared in a hip simulator. Acta Orthop Scand 64: Kumar P, Oka M, Ikeuchi K, et al Low wear rate of UHMWPE against zirconia ceramic (Y-PSZ) in comparison to alumina ceramic and SUS 316L alloy. J Biomed Mater Res 25: Stilling M, Nielsen KA, Soballe K, et al Clinical comparison of polyethylene wear with zirconia or cobaltchromium femoral heads. Clin Orthop Relat Res 467: Liang B, Kawanabe K, Ise K, et al Polyethylene wear against alumina and zirconia heads in cemented total hip arthroplasty. J Arthroplasty 22: Nakashima Y, Hayashi K, Inadome T, et al Hydroxyapatite-coating on titanium arc sprayed titanium implants. J Biomed Mater Res 35: d Aubigne RM, Postel M The classic: functional results of hip arthroplasty with acrylic prosthesis Clin Orthop Relat Res 467: Devane PA, Horne JG Assessment of polyethylene wear in total hip replacement. Clin Orthop Relat Res 369: McCalden RW, MacDonald SJ, Rorabeck CH, et al Wear rate of highly cross-linked polyethylene in total hip arthroplasty. A randomized controlled trial. J Bone Joint Surg Am 91: Gencur SJ, Rimnac CM, Kurtz SM Fatigue crack propagation resistance of virgin and highly crosslinked, thermally treated ultra-high molecular weight polyethylene. Biomaterials 27: Muratoglu OK, Bragdon CR, O Connor DO, et al Unified wear model for highly crosslinked ultra-high molecular weight polyethylenes (UHMWPE). Biomaterials 20: Röhrl MR, Nirbrant B, Nilsson KG No adverse effect of submelt-annealed highly crosslinked polyethylene in cemented cups. Acta Orthop 83: Meding JB, Keating EM, Davis KE Acetabular UHMWPE survival and wear changes with manufacturing techniquies. Clin Orthop Res 469: Waewsawangwong W, Goodman SB Unexpected failure of highly cross-linked polyethylene acetabular liner. J Arthroplasty 27:1 4.

7 XLPE WEAR AGAINST CERAMIC HEAD Crowninshield RD, Maloney WL, Wentz DH Biomechanics of large heads. Clin Orthop Res 429: Georgiades G, Babis GC, Kourlaba G, et al Effect of cementless acetabular component in total hip arthroplasty for congenital hip disease. J Arthroplasty 25: Haraguchi K, Sugano N, Nishii T, et al Phase transformation of a zirconia ceramic head after total hip arthroplasty. J Bone Joint Surg Br 83: Higuchi F, Shiba N, Inoue A, et al Fracture of an alumina ceramic head in total hip arthroplasty. J Arthroplasty 10: Kawate K, Ohmura T, Kawahara I, et al Differences in highly cross-linked polyethylene wear between zirconia and cobalt-chromium femoral heads in Japanese patients: a prospective, randomized study. J Arthroplasty 24: Hirakawa K, Bauer TW, Hashimoto Y, et al Effect of femoral head diameter on tissue concentration of wear debris. J Biomed Mater Res 36: Hammerberg EM, Wan Z, Dastane M, et al Wear and range of motion of different femoral head sizes. J Arthroplasty 25: Mall NA, Nunley RM, Zhu JJ, et al The incidence of acetabular osteolysis in young patients with conventional versus highly crosslinked polyethylene. Clin Orthop Relat Res 469: