Effect of alumina femoral heads on polyethylene wear in cemented total hip arthroplasty

Transcription

1 Effect of alumina femoral heads on polyethylene wear in cemented total hip arthroplasty OLD VERSUS CURRENT ALUMINA K. Tanaka, J. Tamura, K. Kawanabe, M. Shimizu, T. Nakamura From Kyoto University, Japan W e examined the behaviour of alumina ceramic heads in 156 cemented total hip arthroplasties, at a minimum follow-up of eight years. They were divided into three groups according to the size of the femoral head; 22, 26, and 28 mm. We measured polyethylene wear radiologically using a computer-aided technique. The linear wear rate of polyethylene sockets for the 28 mm heads was high (0.156 mm/year), whereas those for the 22 and 26 mm heads were relatively low (0.090 and mm/year, respectively). Moreover, the surface roughness data of retrieved femoral heads clearly showed maintenance of an excellent surface finish of the current alumina. We conclude that the alumina ceramic femoral heads currently used are associated with a reduced rate of polyethylene wear. J Bone Joint Surg [Br] 2003;85-B: Received 14 October 2002; Accepted after revision 18 February 2003 Polyethylene wear has been a major cause of periprosthetic osteolysis, leading to aseptic loosening after total hip arthroplasty (THA). 1-3 In order to reduce such wear, for more than 20 years, ceramic femoral heads have been used with polyethylene cups. 4-9 Alumina ceramics, which previously had been used clinically as a ceramic-on-ceramic combination, gave great promise because of their excellent wear characteristics and high resistance to scratching. 10 In laboratory tests of wear, many investigators have shown that the wear rate of polyethylene against alumina is lower than that against metal Although early clinical results of the alumina-on-polyethylene bearing combination were excellent, K. Tanaka, MD, Orthopaedic Surgeon J. Tamura, MD, PhD, Instructor K. Kawanabe, MD, PhD, Associate Professor M. Shimizu, MD, PhD, Instructor T. Nakamura, MD, PhD, Chairman and Professor of Orthopaedics Department of Orthopaedic Surgery, Faculty of Medicine, Kyoto University, 54 Shogoin, Kawahara-cho, Sakyo-ku, Kyoto , Japan. Correspondence should be sent to Professor T. Nakamura British Editorial Society of Bone and Joint Surgery doi: / x.85b $2.00 mid- and long-term wear of this combination has been controversial and some authors have questioned its use. 6,8,9 There are, however, differences between the old alumina, which was used in the early 1980s, and the newer material, which is currently in use. 16 Newer alumina ceramics seem better in terms of purity of composition, grain size, mechanical properties and surface quality. It is still premature to draw conclusions concerning the effect of alumina femoral heads on reducing polyethylene wear. In this study, therefore, our aim was to evaluate the effect of femoral heads made of new alumina on polyethylene wear. Patients and Methods Between March 1981 and December 1992, we carried out 225 consecutive primary cemented THAs in 181 patients using Kyocera hip prostheses (Bioceram series and KC series: Kyocera, Kyoto, Japan), in which alumina femoral heads articulated with all-polyethylene acetabular components. Of these, we selected only the 171 hips with a minimum follow-up of eight years, in order to assess the rate of polyethylene wear. We excluded 14 hips because of missing or incomplete radiological records. Four hips had been revised within eight years of the primary operation, three for aseptic loosening and these were included, and one for infection and this was excluded. The remaining 156 hips (124 patients) were divided into three groups for measurement of wear based on the size of the femoral head (Table I). The 28 mm heads were used between 1981 and 1988; the 26 mm heads between 1988 and 1991, and the 22 mm heads between 1991 and The preoperative diagnosis included osteoarthritis (primary or secondary) in 149 hips, avascular necrosis in four and rheumatoid arthritis in three. Since 1988, the manufacturer has made improvements in the properties of alumina ceramics, enabling the femoral head size to be reduced from 28 mm diameter (old alumina) to 26 or 22 mm (current alumina). The old alumina contained 7% of yttria, while the current material is high purity alumina containing >99.8% Al 2 O 3. Table II compares the material properties of the old and current alumina. Current alumina has a greater strength and a much smaller grain size as seen on SEM (Fig. 1). With regard to the femoral components, the Bioceram stem was designed for patients with dysplastic hips, which VOL. 85-B, No. 5, JULY

2 656 K. TANAKA, J. TAMURA, K. KAWANABE, M. SHIMIZU, T. NAKAMURA Table I. Details of the 124 patients and the implants used in cemented THA between 1981 and 1992 Alumina head (mm) Type of alumina Containing 7% yttria High purity High purity Implantation period 1981 to to to 1992 Number of hips Gender Male Female Mean age in years (range) 50.1 (37 to 61) 57.8 (29 to 77) 58.3 (42 to 74) Mean follow-up in years (range) 13.6 (6.4 to 17.8) 10.8 (8.5 to 12.5) 9.2 (8.0 to 10.9) Diagnosis (number of hips) Osteoarthritis Rheumatoid arthritis Avascular necrosis Table II. Material properties of old, containing 7% yttria, and current alumina, high purity, used in the implants Old alumina Current alumina Density (g/cm 3 ) Bending strength (MPa) Vickers hardness (HV) Grain size (µm) are frequently seen in Japan, and it has been described in detail elsewhere. 8,9,17 The KC stem (Kyocera) was similar in design to the Charnley prosthesis, but was made of stainless steel or its alloy, while the Charnley prosthesis was made of titanium alloy. The Bioceram stem had a 28 mm modular alumina head, and the KC stem had a 26 or 22 mm modular alumina head. All the acetabular polyethylene components in both series were machined from compression-moulded GUR 412 sheets and were sterilised with ethylene oxide gas. Radiological analysis. We measured polyethylene wear radiologically by determining the migration of the centre of the femoral head relative to the centre of the cup, based on the computer-aided technique of Sychterz et al. 18 For each patient, the initial postoperative and the most recent anteroposterior radiographs of the pelvis were selected. First, the radiographs were scanned using a scanner (GT9500, EPSON, Nagano, Japan) to generate 600 dpi TIFF images. Using the custom software Image-Pro Plus version 4.0 (Media Cybernetics, Maryland), an observer digitised about ten points around the periphery of the femoral head on the radiograph and also around the periphery of the polyethylene cup at the cement-cup interface. This software provided the best-fit circles and their centres for the head and cup from the digitised points, by which the two-dimensional coordinate for each centre was obtained on the radiograph (Fig. 2). By comparing the co-ordinates of the two centres on the initial postoperative and most recent radiographs, the Fig. 1 SEM (a) of old and (b) of current alumina ( 2500). THE JOURNAL OF BONE AND JOINT SURGERY

3 EFFECT OF ALUMINA FEMORAL HEADS ON POLYETHYLENE WEAR IN CEMENTED TOTAL HIP ARTHROPLASTY 657 Fig. 2 Radiological measurement of polyethylene wear. BP1 to 10, digitised points around the periphery of the polyethylene socket; BP11 to 20, digitised points around the periphery of the femoral head; BC1, centre of the socket; BC2, centre of the femoral head; D1, distance between two centres. amount and the direction of penetration of the femoral head into the polyethylene were determined after correction for pelvic tilting and magnification. Although the migration of the femoral head thus determined may consist not only of true wear, but also creep deformation of polyethylene, we defined it as linear wear for the purpose of this study. The direction of wear was also defined relative to a vertical line drawn through the centre of the femoral head and perpendicular to an inter-teardrop line, as described by Livermore, Ilstrup and Morrey. 19 The wear angle was defined as positive if the direction was medial to this line, and negative if lateral to it. In addition, volumetric wear was calculated from the linear wear data using the same equation as Hashimoto et al, 20 Maruyama et al 21 and Sychterz et al. 22 We determined the annual wear rate by dividing the total wear by the duration of time after implantation. In order to eliminate interobserver errors, all measurements were undertaken by the same observer (KT). To validate this technique, we compared the radiological measurements with direct measurements using ten cups retrieved from nine patients at revision surgery. These were chosen because their wear measurements had not been disturbed by excessive osteolysis or significant migration of the cup on the radiographs taken just before revision. The linear wear was measured directly from the retrieved polyethylene cups using a three-dimensional co-ordinate measuring machine (BHN305; Mitutoyo Corporation, Tokyo, Japan). For each patient, we also measured the polyethylene wear radiologically on five occasions using the method described above. Analysis of retrieved femoral heads. We analysed 11 femoral heads retrieved at revision surgery from ten patients with regard to surface roughness; seven were of old and four of current alumina. As controls, eight retrieved stainlesssteel heads from Charnley prostheses were also analysed. A contacting two-dimensional profilometer (S-405; Kosaka Laboratory Ltd, Tokyo, Japan) determined the surface roughness (Ra and Rmax) of the retrieved heads. The adopted cut-off and evaluation lengths were 0.8 mm for both. Ra is defined as the arithmetic mean roughness, while Rmax is the largest peak-to-valley height. Measurements of roughness were made at the apex and at the equator of the femoral head and on two lines perpendicular to each other at each point. Statistical analysis. The data were statistically analysed using a one-way ANOVA, with Scheffe s method as a posthoc test. Differences with p < 0.05 were considered to be statistically significant. VOL. 85-B, No. 5, JULY 2003

4 658 K. TANAKA, J. TAMURA, K. KAWANABE, M. SHIMIZU, T. NAKAMURA Table III. Comparison between the direct and radiological measurement of polyethylene wear in ten implants obtained from ten women at revision THA Wear (mm) Case Age Time in situ (yrs) Type of prosthesis Direct (A) Radiological (B)* Difference (B - A) Charnley ± Bioceram ± Bioceram ± Bioceram ± Bioceram ± Bioceram ± Bioceram ± KC (26) ± KC (22) ± KC (22) ± Mean ± 0.27 *the values are expressed as the mean and the SD of five measurements positive values indicate overestimation and negative values indicate underestimation Table IV. Wear data, at the last follow-up, expressed as mean ±SD, for 156 hips (124 patients) after THA with 28, 26 or 22 mm ceramic heads Alumina head (mm) Number of hips Socket angle (degrees) 39.0 ± ± ± 6.6 Polyethylene thickness (mm) 7.9 ± ± ± 1.1 Linear wear (mm) ± ± ± Linear wear rate (mm/year) ± ± ± Volumetric wear (mm 3 ) ± ± ± Volumetric wear rate (mm 3 /year) 91.1 ± 47.2* 50.1 ± ± 28.4 Wear angle (degrees) 4.7 ± ± ± 16.1 *significantly higher than for 26 and 22 mm heads (p < ) Results Validation testing of the measuring technique revealed a good correlation between the radiological and direct measurements of polyethylene wear (R 2 = 0.95), and that the radiological measurement underestimated the direct measurement by a mean of 0.11 mm (Table III). Moreover, measurements by the same observer were reproducible; the SD of five measurements ranged from 0.04 to 0.13 mm. The mean age at operation for the 28 mm group (50.1 years) was significantly lower than that for the 26 mm and 22 mm (57.8 and 58.3 years, respectively). This is because alumina ceramic heads were selectively used in younger patients and metal heads were used in older patients during the same period as the 28 mm group. There were significant differences in the mean lengths of follow-up among the three groups: 13.6 years for 28 mm, 10.8 for 26 mm and 9.2 for 22 mm. Of the 156 hips, 23 had been revised at the latest followup; 20 in the 28 mm group, one in the 26 mm group, and two in the 22 mm group. The wear data are summarised in Table IV. Although there were no significant differences in the cup abduction angle among the three groups, there were significant differences in the thickness of polyethylene. The mean linear wear rates ranged from to mm/year for the 28 mm group, from to mm/year for the 26 mm group, and from to mm/year for the 22 mm group. The mean volumetric wear rates ranged from 14.7 to mm 3 /year for the 28 mm group, from 2.7 to mm 3 /year for the 26 mm group, and from 2.5 to mm 3 /year for the 22 mm group. The volumetric wear rate for the 28 mm group of patients was significantly higher than that for the other two groups (p < ), whereas there were no significant differences between the 22 mm and 26 mm groups (p = 0.07). Interestingly, the mean wear angle was negative for the 22 mm group of patients and positive for the other two groups (p < 0.05). The surface roughness data for the retrieved femoral heads clearly showed the maintenance of an excellent surface finish of the current alumina (Table V). We did not compare the three groups statistically, because the number for the current alumina group was so small (n = 4) and the follow-up time for these patients was short. Both Ra and Rmax values on the apex for the current alumina heads (0.013 ± µm Ra and ± µm Rmax) were lower than those for both the old alumina (0.028 ± µm Ra and ± µm Rmax) and the stainless-steel heads (0.026 ± µm Ra and ± µm Rmax). It should be noted that the values for the roughness for the stainless steel heads were similar to those for the old alumina heads. THE JOURNAL OF BONE AND JOINT SURGERY

5 EFFECT OF ALUMINA FEMORAL HEADS ON POLYETHYLENE WEAR IN CEMENTED TOTAL HIP ARTHROPLASTY 659 Table V. Surface roughness data of 11 retrieved femoral heads, unused heads and eight stainless-steel controls, expressed as the mean ± SD (range) Surface roughness (µm) Apex Equator Number Time in vivo (years) Ra Rmax Ra Rmax Unused heads 22 mm alumina 0.02 to to to to mm alumina 0.01 to to to to mm alumina 0.02 to to to to 0.15 Retrieved heads Current alumina (22- or 26 mm) ± 1.5 (8.0 to 11.4) ± (0.01 to 0.02) ± (0.09 to 0.13) ± (0.01 to 0.02) ± (0.08 to 0.17) Old alumina (28 mm) ± 1.1 (13.0 to 16.3) ± ± ± ± (0.02 to 0.04) Stainless steel controls (22 mm) ± 1.9 (11.2 to 16.0) ± (0.01 to 0.05) (0.22 to 1.15) ± (0.07 to 0.47) (0.01 to 0.04) ± (0.01 to 0.10) (0.11 to 0.60) ± (0.06 to 0.86) Discussion The measurement of polyethylene wear is very important when predicting the longevity of a hip arthroplasty. The technique for measuring it, as described by Livermore et al, 19 has been used in many studies. 7,8,23-29 In cemented THA using all-polyethylene sockets, however, the measurements are subjective as they depend on the manual determination of the centre of the femoral head and the direction of maximum wear. Since our technique is based on the concept of vector wear analysis as described by Sychterz et al 18 or Martell and Berdia, 30 the amount and direction of wear are objectively determined from the co-ordinates of the centres of the head and the cup on the initial and latest radiographs. Additionally, even if the identification of the cement-polyethylene interface may be difficult when determining the direction of the maximum wear, the choice of well-recognisable points on the rest of the interface should provide the best-fit circle and its centre. We believe that computer-aided methods for measuring the wear of all-polyethylene sockets are preferable as they reduce the subjectivity of measurement. In recent years, several authors have questioned the use of alumina femoral heads in THA. Sychterz et al 18 reported no significant difference in the wear rate of polyethylene between alumina and cobalt-chrome femoral heads at a mean of seven years in cementless THA. Hasegawa et al 8 and Haraguchi et al 9 have recently investigated the longterm results of cemented THA with Bioceram prostheses and reported mean linear wear rates of 0.10 mm/year at 13 years and 0.15 mm/year at 13.8 years of follow-up, respectively. The mean linear wear rate of mm/year for the 28 mm group (Bioceram) in our study was disappointing, compared with the reported wear rates of all-polyethylene cups articulating with 28 mm metal heads which ranged from 0.05 to 0.12 mm/year. 19,23,24,31 By contrast, the wear rate of 0.09 mm/year for the 22 mm group was satisfactory compared with the recently reported values for the wear rate of polyethylene with 22 mm metal heads which ranged from 0.08 to 0.15 mm/year. 19,23,26,28,32-34 Wang and Essner 15 found that ceramic heads were effective in reducing polyethylene wear because of the inability of third-body particles to adhere to their surfaces rather than due to their resistance to scratching. On the other hand, Hasegawa et al 8 reported that the retrieved alumina heads (28 mm Bioceram) showed smooth surfaces without scratches, but there was debris stuck to the gaps on the surfaces. Therefore, we infer from these findings and our data that the increased surface roughness, in particular Rmax values, and the high wear rate for the old alumina heads may be due to the adherence of such third-body particles and the resultant abrasive wear process, while we believe that the better wear achieved in the current alumina groups is mainly attributable to the improved quality of the surface. Our study is limited because it is retrospective and there were significant differences in the age of the patients and the thickness of the polyethylene among the three groups. We are aware that young age and male gender, with higher activity, are risk factors for increased wear. We consider that the differences in the ages of the patients had little influence on the wear results, because Japanese patients after THA tend to be relatively inactive regardless of age and most of our patients are women. Although whether polyethylene thickness is correlated to increased wear remains controversial, we think that our results may be affected, partly, by this factor. 9,25,35 The wear data for the 22 mm group are, however, comparable, but not superior, to the excellent wear rates reported for 22 mm metal heads in some studies, with the preservation of excellent surface finish for the current alumina in vivo. 33,34 Metal heads with improved surface quality may show excellent wear performance. 36,37 These data should not be interpreted to mean that the current alumina heads are disappointing, because the possible advantageous effects of the current alumina heads in this study may have been masked by other factors including implant design, the quality of the polyethylene and the method of sterilisation. 25,27,29,38-41 In particular, since sterilisation by gamma irradiation may affect the cross-linking of polyethylene, VOL. 85-B, No. 5, JULY 2003

6 660 K. TANAKA, J. TAMURA, K. KAWANABE, M. SHIMIZU, T. NAKAMURA even if it is done in air, sterilisation with ethylene oxide gas in our study may have been disadvatageous In summary, our study has shown that alumina heads are not always associated with reduced polyethylene wear if the initial surface quality is poor, but that there may be reduced wear with the current alumina heads which have excellent surface quality. No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. References 1. Amstutz HC, Campbell P, Kossovksy N, Clarke IC. Mechanism and clinical significance of wear debris-induced osteolysis. Clin Orthop 1992;276: Schamlzried TP, Jasty M, Harris WH. Periprosthetic bone loss in total hip arthroplasty: polyethylene wear debris and the concept of the effective joint space. J Bone Joint Surg [Am] 1992;74-A: Jiranek WA, Machado M, Jasty M, et al. Production of cytokines around loosened cemented acetabular components: analysis with immunohistochemical techniques and in situ hybridization. J Bone Joint Surg [Am] 1993;75-A: Schuller HM, Marti RK. Ten-year socket wear in 66 hip arthroplasties: ceramic versus metal heads. Acta Orthop Scand 1990;61: Zichner LP, Willert HG. Comparison of alumina-polyethylene and metal-polyethylene in clinical trials. Clin Orthop 1992;282: Sychterz CJ, Engh CA Jr, Young AM, Hopper RH Jr, Engh CA. Comparison of in vivo wear between polyethylene liners articulating with ceramic and cobalt-chrome femoral heads. J Bone Joint Surg [Br] 2000;82-B: Urban JA, Garvin KL, Boese CK, et al. Ceramic-on-polyethylene bearing surfaces in total hip arthroplasty: seventeen to twenty-one-year results. J Bone Joint Surg [Am] 2001;83-A: Hasegawa M, Ohashi T, Tani T. Poor outcome of 44 cemented total hip arthroplasties with alumina ceramic heads: clinical evaluation and retrieval analysis after years. Acta Orthop Scand 2001;72: Haraguchi K, Sugano N, Nishii T, et al. Analysis of survivorship after total hip arthroplasty using a ceramic head. Clin Orthop 2001;391: Clarke IC, Gustafson A. Clinical and hip simulator comparisons of ceramic-on-polyethylene and metal-on-polyethylene wear. Clin Orthop 2000;379: Semlitsch M, Lehmann M, Weber H, Doerre E, Willert HG. New prospects for a prolonged functional life-span of artificial hip joints by using the material combination polyethylene/aluminium oxide ceramic/ metal. J Biomed Mater Res 1977;11: 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 1991;25: Weightman B, Light D. The effect of the surface finish of alumina and stainless steel on the wear rate of UHMW polyethylene. Biomaterials 1986;7: Saikko VO, Paavolainen PO, Slatis P. Wear of the polyethylene acetabular cup: metallic and ceramic heads compared in a hip simulator. Acta Orthop Scand 1993;64: Wang A, Essner A. Three-body wear of UHMWPE acetabular cups by PMMA particles against CoCr, alumina and zirconia heads in a hip joint simulator. Wear 2001;250: Willmann G. Ceramic femoral head retrieval data. Clin Orthop 2000;379: Iida H, Matsusue Y, Kawanabe K, et al. Cemented total hip arthroplasty with acetabular bone graft for developmental dysplasia: longterm results and survivorship analysis. J Bone Joint Surg [Br] 2000;82- B: Sychterz CJ, Engh CA Jr, Shah N, Engh CA Sr. Radiographic evaluation of penetration by the femoral head into the polyethylene liner over time. J Bone Joint Surg [Am] 1997;79-A: Livermore J, Ilstrup L, Morrey B. Effect of femoral head size on wear in wear of the polyethylene acetabular component. J Bone Joint Surg [Am] 1990;72-A: Hashimoto Y, Bauer TW, Jiang M, Stulberg BN. Polyethylene wear in total hip arthroplasty: volumetric wear measurement of retrieved acetabular components. Trans Orthop Res Soc 1995;20: Maruyama M, Capello WN, D Antonio JA, Jaffe WL, Bierbaum BE. Effect of low friction ion treated femoral heads on polyethylene wear rates. Clin Orthop 2000;370: Sychterz CJ, Moon KH, Hashimoto Y, et al. Wear of polyethylene cups in total hip arthroplasty: a study of specimens retrieved post mortem. J Bone Joint Surg [Am] 1996;78-A: Callaghan JJ, Pedersen DR, Olejniczak JP, Goetz DD, Johnston RC. Radiographic measurement of wear in 5 cohorts of patients observed for 5 to 22 years. Clin Orthop 1995;317: Bankston AB, Cates H, Ritter MA, Keating EM, Faris PM. Polyethylene wear in total hip arthroplasty. Clin Orthop 1995;317: Oonishi H, Wakitani S, Murata N, et al. Clinical experience with ceramics in total hip replacement. Clin Orthop 2000;379: Hirakawa K, Mitsugi N, Koshino T, et al. Effect of acetabular cup position and orientation in cemented total hip arthroplasty. Clin Orthop 2001;388: Sakalkale DP, Sharkey PF, Eng K, Hozack WJ, Rothman RH. Effect of femoral component offset on polyethylene wear in total hip arthroplasty. Clin Orthop 2001;388: Kesteris U, Ilchamnn T, Wingstrand H, Onnerfalt R. Polyethylene wear in Scanhip arthroplasty with a 22 or 32 mm head: 62 matched patients followed for 7-9 years. Acta Orthop Scand 1996;67: Urquhart AG, D Lima DD, Venn-Watson E, Colwell CW Jr, Walker RH. Polyethylene wear after total hip arthroplasty: the effect of a modular femoral head with an extended flange-reinforced neck. J Bone Joint Surg [Am] 1998;80-A: Martell JM, Berdia S. Determination of polyethylene wear in total hip replacements with use of digital radiographs. J Bone Joint Surg [Am] 1997;79-A: Cates HE, Faris PM, Keating EM, Ritter MA. Polyethylene wear in cemented metal-backed acetabular cups. J Bone Joint Surg [Br] 1993;75-B: Eggli S, z Brun S, Gerber C, Ganz R. Comparison of polyethylene wear with femoral heads of 22 mm and 32 mm: a prospective, randomised study. J Bone Joint Surg [Br] 2002;84-B: Kobayashi S, Eftekhar NS, Terayama K, Iorio R. Risk factors affecting radiological failure of the socket in primary Charnley low friction arthroplasty: a 10- to 20-year followup study. Clin Orthop 1994;306: Wroblewski BM, Fleming PA, Siney PD. Charnley low-frictional torque arthroplasty of the hip: 20- to 30-year results. J Bone Joint Surg [Br] 1999;81-B: Nashed RS, Becker DA, Gustilo RB. Are cementless acetabular components the cause of excess wear and osteolysis in total hip arthroplasty? Clin Orthop 1995;317: Lancaster JG, Dowson D, Isaac GH, Fisher J. The wear of ultra-high molecular weight polyethylene sliding on metallic and ceramic counterfaces representative of current femoral surfaces in joint replacement. Proc Inst Mech Eng [H] 1997;211: Kobayashi S, Saito N, Horiuchi H, Iorio R, Takaoka K. Poor bone quality or hip structure as risk factors affecting survival of total hip arthroplasty. Lancet 2000;355: Bankston AB, Keating EM, Ranawat C, Faris PM, Ritter MA. Comparison of polyethylene wear in machined versus molded polyethylene. Clin Orthop 1995;317: Affatato S, Bordini B, Fagnano C, et al. Effects of the sterilisation method on the wear of UHMWPE acetabular cups tested in a hip joint simulator. Biomaterials 2002;23: Karrholm J, Digas G, Nivbrant B, Rohrl S, Thanner J. Increased femoral head penetration using polyethylene sterilised with ethylene oxide. Trans Orthop Res Soc 2001;47: McKellop HA, Shen FW, Campbell P, Ota T. Effect of molecular weight, calcium stearate, and sterilisation methods on the wear of ultra high molecular weight polyethylene acetabular cups in a hip joint simulator. J Orthop Res 1999;17: THE JOURNAL OF BONE AND JOINT SURGERY