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Wear of Highly Cross-linked Polyethylene Acetabular Cup in Japan
The Journal of Arthroplasty Vol. 21 No. 7 2006
Wear of Highly Cross-linked Polyethylene Acetabular Cup in Japan Hironobu Oonishi, MD, PhD,* Sok Chol Kim, MD,* Yasuhiro Takao, MD,* Masayuki Kyomoto, MSc,y Mikio Iwamoto, MSc,y and Masaru Ueno, PhDy
Abstract: The wear characteristics of ultra–high-molecular-weight polyethylene acetabular cups irradiated with 6.0 to 7.5 Gy of c-irradiation were studied in a hip simulator and in vivo in 70 hips for 6 to 39 months. The linear wear was 2.5% to 4.5% of the wear seen in conventional polyethylene cups. A biphasic wear pattern was observed over time with a relatively large wear in the first year and a subsequent leveling of the wear rate curve. The linear wear rate in the second phase (steady state) was 0.006 mm/y. The extent of decrease in clinical wear of 6.0-Mrad polyethylene (Aeonian, Kyocera Corp, Kyoto, Japan) cups is comparable with the decrease of the in vitro wear measured by the hip simulation test. Key words: total hip arthroplasty, highly cross-linked polyethylene, wear, clinical results, hip simulator. n 2006 Elsevier Inc. All rights reserved.
beam or c ray irradiation has, therefore, been extensively investigated. When a high-energy beam, such as a c ray, is irradiated on a polymer, free radicals are formed by a scission of the molecular chains. This is followed by retermination and cross-linkage of the molecules. High-dose c-irradiation on UHMWPE severs the C2C or C2H bonds, which then results in cross-linking and subsequent chemical bonding of C;O and C2C. In 1970, Shikita and Oonishi [4], collaborating with Hisashi Igaki, started development of a crosslinked polyethylene treated with high-dose c irradiation, which was aimed at improved wear properties as well as creep resistance [5]. Based on the results of this and other researches, a limited number of polyethylene cups prepared with cirradiation of almost 100 Gy (100-Gy polyethylene) were first tried in 1971 for hip arthroplasty by Shikita and Oonishi and were used until 1978 when the company ceased marketing the cup. Good clinical results with this 100-Gy polyethylene have been observed for more than 30 years [6,7]. Many researchers have reported the linear wear rate of acetabular cups made with conventional Polyethylene to be between 0.1 and 0.2 mm/y. The wear of 100-Gy polyethylene
As a bearing couple for artificial joint systems, the combination of the ultra–high-molecular-weight polyethylene (UHMWPE) acetabular component and metal (in particular, Co-Cr alloy) for the femoral component still remains the most popular materials for joint arthroplasty since its first application by Charnley in 1962. However, osteolysis caused by wear particles of UHMWPE has emerged as a serious issue [1,2]. To decrease such particles from UHMWPE, different combinations of the bearing couple or improvements of the bearing materials themselves have been attempted. In 1966, Awatani reported the beneficial effects of radiation to the surface layer of polyethylene [3]. Cross-linking of UHMWPE caused by electron From the *H. Oonishi Memorial Joint Replacement Institute, Tominaga Hospital, Osaka-City, Japan; and yResearch and Development Corporate Division, Japan Medical Materials Corporation, OsakaCity, Japan. Submitted October 30, 2004; accepted August 6, 2005. No benefits or funds were received in support of the study. Reprint requests: Hironobu Oonishi, MD, PhD, H. Oonishi Memorial Joint Replacement Institute, Tominaga Hospital, 4-48, 1-Chome, Minato-Machi, Naniwa-Ku, Osaka-City 556-0017, Japan. n 2006 Elsevier Inc. All rights reserved. 0883-5403/06/1906-0004$32.00/0 doi:10.1016/j.arth.2006.03.009
944
Wear of Highly Cross-linked Polyethylene ! Oonishi et al 945
measured using radiographs was reduced to 16%, on average, compared with conventional polyethylene, and retrieved cups showed wear decreased to 3% to 10%. These data evidenced the lasting effect of cross-linking by high-dose c-irradiation. In a 100-Gy polyethylene cup retrieved after clinical use for more than 25 years, most free radicals remained but without progressive oxidation of the polyethylene, although oxidation in retrieved conventional polyethylene cups was found to have progressed. Since 1998 several different highly cross-linked polyethylene cups irradiated with an energy beam in the range of 5.0 to 10.5 Gy have been produced and used extensively [8]. The source and dose of the high-energy beam, including c ray and electron beams, differs among manufacturers. There are many reports of in vitro testing on highly crosslinked polyethylene made by various manufacturing techniques in this exposure range [9,10]. In earlier reports, highly cross-linked polyethylene made using dosages from 5.0 to 10.5 Gy showed an 80% to 90% reduction in wear rate. However, only limited clinical results of this reduced irradiated polyethylene are available to date. In the present study, the wear rate of in vitro simulation and in vivo clinical (radiological) investigation, using highly cross-linked polyethylene (Aeonian, Kyocera Corp, Kyoto, Japan) irradiated with 5.0-Gy to 7.5-Gy c-irradiation (6.0-Gy polyethylene [Aeonian]), was studied. We measured linear wear as well as volumetric wear to determine the ongoing properties of this material. Although the follow-up period was short, we feel this early report will provide an indication of the in vitro and in vivo wear properties of this material.
Hip Simulation Study of 6.0-Mrad polyethylene The in vitro wear test was performed using a hip simulator (Shore Western, Inc, Monrovia, Calif). A mixture of 30% bovine serum, 20 mmol/L of EDTA, and 0.2% sodium azide was used as lubrication. A load simulating a physiologic loading curve with a 2 kN peak load was added with a period 1 Hz and a 408 arc cup rotation. The acetabular component was tested with a 22-mm zirconia ceramic femoral head (Bioceram, Kyocera Corp). Wear was measured by a gravimetric method. The weight of the cup was measured every 200 000 cycles. The volumetric change was then calculated from the weight loss over time. To control for the effect of water absorption during the test, 3 pieces of the same size and material were immersed in the same lubricant for the same period, and the weight gain of these control cups was then measured. The true weight loss was calculated as the difference between the experimental and the control weight. After 3.0 106 cycles, an apatite powder with an average grain size of 1.7 lm was added into the bearing space to evaluate the properties after third-body wear. Testing then continued until a total of 5.4 106 cycles were completed. Clinical Wear Study of 6.0-Mrad polyethylene (Aeonian) An alumina femoral head of 28 mm in diameter was used in all patients (Fig. 1). The 6.0-Mrad polyethylene (Aeonian) acetabular cup was fixed with an Interface Bioactive Bone Cement technique in all cases [11]. The clinical data consisted of 70 joints operated on between January 2000 and
Materials and Methods The 6.0-Mrad polyethylene (Aeonian) acetabular cups, both experimental and clinical, were manufactured using the following process. The compression-molded UHMWPE (GUR1050 resin, Perplas Medical Ltd, UK) bar stock was c-irradiated with 3.5 Gy in air and annealed at 1108C in N2 for 12 hours. The cups were then machined from this bar stock to 22-mm inner diameter and 44-mm outer diameter and then c-sterilized with a dose of 2.5 to 4.0 Gy in N2. Thus, the total dose of c-irradiation was 6.0 to 7.5 Gy. Control cups were prepared using conventional polyethylene, sterilized with ethylene oxide gas (EOG polyethylene). Three units each of the experimental and control cups were prepared.
Fig. 1. A sample radiograph showing no wear in a 6.0Mrad polyethylene (Aeonian)–fitted patient. A, Total hip prosthesis; B, Radiograph after surgery.
946 The Journal of Arthroplasty Vol. 21 No. 7 October 2006 January 2001. The average age of the patients was 61 years (range, 20-89 years) at the time of operation, and the average follow-up period was 28 months (range, 6-39 months). Primary diseases were osteoarthritis of the hip in 63 joints, femoral head necrosis in 4, congenital dislocation of the hip in 2, and femoral neck fracture in 1. Demographics of the cup thickness in the 70 joints was 1 cup of 7 mm, 5 of 8 mm, 31 of 9 mm, 26 of 10 mm, 5 of 11 mm, and 2 of 13 mm. Of 70 cups, however, 1 of 8 mm, 4 of 9 mm, and 2 of 13 mm were excluded from the evaluation because the follow-up term was shorter than 1 year. The in vivo wear of the acetabular cup was measured using frontal view plain radiographs. Wear measurement was performed at 1 month post operation and, afterward, every 6 months and recorded as the change in distance between the cup and the femoral head center. The femoral head center was determined using a computerized algorithm provided by Vector Works 10.5 (A & A Corp, Tokyo, Japan). Data input to the software consisted in the 3 outer points of the femoral head, as measured in the radiograph (Fig. 1). The volumetric wear was calculated from the linear wear and the wear direction after an equation developed by Kabo et al [12].
Results Hip Simulation Wear Study Fig. 2 shows the conventional EOG polyethylene acetabular cup with a linear wear pattern. The wear
Fig. 3. Comparison of 6.0-Mrad polyethylene (Aeonian) wear rate with conventional polyethylene in vivo. -x-, wear trend of 6.0-Mrad polyethylene (Aeonian); --,wear trend of conventional polyethylene.
in 6.0-Mrad polyethylene (Aeonian) was 2.5% to 4.5% (mean, 3.0%) of the wear of the EOG polyethylene. Visual inspection of Fig. 2 shows that the wear of 6.0-Mrad polyethylene (Aeonian) appears to be very low, in comparison with EOG polyethylene. After the addition of apatite powder, the 6.0Mrad polyethylene (Aeonian) showed a decrease in wear of 15% of the wear of EOG polyethylene showing that 6.0-Mrad polyethylene (Aeonian) sustains low wear properties even in the presence of foreign substances. Clinical Wear Study
Fig. 2. Simulation study comparing 6.0-Mrad polyethylene (Aeonian) to EOG polyethylene. 6.0 Mrad, 6.0Mrad polyethylene (Aeonian).
Fig. 1 shows an example of a radiograph at 2 years and 11 months post operation in which no distinguishable change was found, compared with the radiograph immediately after operation. Fig. 3 shows the in vivo 6.0-Mrad polyethylene (Aeonian) cups with a linear wear rate of 0.10 F 0.05 mm/y and a volumetric wear rate of 25.4 F 27.9 mm3/y. For the first year after surgery, a steep increase in the wear rate of about 0.2 mm was seen, followed by leveling with a tendency of gradual increase over the time of the study. We defined the initial phase up to 1 year post operation as the initial linear wear rate and the subsequent phase 1 year
Wear of Highly Cross-linked Polyethylene ! Oonishi et al 947
Fig. 4. The comparison of wall thickness of the acetabular cups as a function of in vivo linear wear rate and volumetric wear rate. Asterisk indicates non significance; 5, linear wear rate; n, volumetric wear rate.
post operation as the steady linear wear rate (Fig. 3). The 6.0-Mrad polyethylene (Aeonian) showed an initial linear wear rate of 0.232 mm/y and a steady linear wear rate of 0.006 mm/y. The EOG polyethylene cups showed an initial linear wear rate of 0.104 mm/y and a steady linear wear rate of 0.098 mm/y, giving almost no difference between initial and steady linear wear rates in the case of the EOG polyethylene cups [13]. In the case of 6.0-Mrad polyethylene (Aeonian), the steady linear wear rate decreased to 6% compared with EOG polyethylene (Fig. 3). A significant difference was not found in either the linear or volumetric wear rates among groups of different thickness (8,9,10, and 11 mm), but a rather high wear tendency was observed in the 8-mm group (Fig. 4).
Discussion In this clinical wear study, the 6.0-Mrad polyethylene (Aeonian) cups showed a steady linear wear rate of 0.006 mm/y, and the EOG polyethylene cups showed a steady linear wear rate of 0.098 mm/y. The clinical wear of polyethylene cups irradiated with less than 10 Mrad decreased to 6% of conventional polyethylene in this shortterm examination. The literature reports linear wear rates of highly cross-linked polyethylene: 0.008 mm/y by Manning et al [14] for Zimmer cups; 0.09 mm/y by Digas et al [15] for the Durasul cups (Zimmer, Inc, Warsaw, Ind); 0.02 mm/y by Heicel et al [16]for the Marathon cups (DePuy, Orthopaedics Inc, Warsaw, Ind); and 0.12 mm/y by Martel et al [17] for the Crossfire cups (Stryker Howmedica Osteonics, Inc, Mahwah, NJ).
All of these highly cross-linked polyethylene cups showed significantly decreased wear rates compared with that of conventional polyethylene cups. The extent of decrease in clinical wear of 6.0Mrad polyethylene (Aeonian) is comparable with the decrease of the in vitro wear measured by hip simulation test. The hip simulation test was conducted using 22-mm femoral heads. In clinical usage, 28-mm femoral heads were used for improved range of motion as well as prevention of luxation. In addition, zirconia femoral heads were used in the hip simulation test, and alumina femoral heads were used clinically. Both alumina and zirconia have shown good in vitro wear properties [18]. Zirconia is a comparatively new material of about 20 years and with additional clinical experience may eventually exceed alumina [19]. However, clinical experience with alumina is more than 30 years, we used alumina for this clinical study because of its high reliability [20]. The 6.0-Mrad polyethylene (Aeonian) cups showed high initial linear wear rate of 0.232 mm/y. After the first year, the steady linear wear rate became low and steady. We propose that creep deformation of highly cross-linked polyethylene caused high initial wear. Muratoglu et al [21] also reported that initial wear of highly cross-linked polyethylene was due to the creep deformation. In addition, it is known that creep resistant properties generally improve in the highly cross-linked polyethylene annealed under the melting point (1108C in this study) [22,23]. Hopper et al [24] reported that initial wear rate (0-2 years of follow-up data) of non–cross-linked Enduron polyethylene (Depuy Orthopaedics Inc, Warsaw, Ind) is higher than initial wear rate of cross-linked Marathon polyethylene. Therefore, we speculate that creep deformation of the EOG polyethylene is higher than creep deformation of 6.0-Mrad polyethylene (Aeonian) because the EOG polyethylene is not highly crosslinked, but we did not measure the initial linear wear of the EOG polyethylene. The follow-up period of this 6.0-Mrad polyethylene (Aeonian) study was short. In past studies, we reported that the long-term clinical wear (by radiograph and retrieval) of 100-Mrad polyethylene cups decreased to 3% to 17% compared with conventional polyethylene [25]. We expect that 6.0-Mrad polyethylene (Aeonian) will show favorably low and steady wear properties in long-term clinical studies, although detailed follow-up is, of course, necessary. In a cup of less than 8 mm in thickness, 100-Mrad polyethylene did not show any difference in wear over time [26]; in addition, 6.0-Mrad
948 The Journal of Arthroplasty Vol. 21 No. 7 October 2006 polyethylene (Aeonian) did not show any difference. We previously reported that thicker cups have advantages in wear behavior and that both linear and volumetric wear decrease as the wall thickness of the cup increases in conventional polyethylene [27]. When size of the femoral head was studied in relation to wall thickness, it was shown that a wall thickness of more than 10 mm showed little correlation with head diameter in volumetric wear [28]. This supports the interpretation that the effect of creep deformation on wear behavior is not large at 10 mm. On the other hand, no significant difference was found either in the linear or the volumetric wear rate among groups of different thicknesses in highly cross-linked polyethylene cups. It is known that creep resistant properties generally improve in the highly crosslinked polyethylene annealed under the melting point (creep deformation: conventional polyethylene N highly cross-linked polyethylene N100-Mrad polyethylene). It therefore appears that lower dose polyethylene is more susceptible to creep deformation [23], compared with 100-Mrad polyethylene. The preliminary observation in this study is still short clinical term. Future observations may be to support that assumption by the increased wear during the first year of in vivo use. Consequently, future observations that take creep deformation into account are important with the highly cross-linked polyethylene cups. In addition, long-term follow-up including wear and creep deformation is still needed to support this preliminary observation.
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8.
9.
10.
11.
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Acknowledgments
14.
The authors thank Takuji Shikita, MD, Hisashi Igaki, PhD, and the research group of Mizuho Medical Instruments, Ltd, for cooperation in the development of 100-Mrad cross-linked polyethylene.
15.
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References 17. 1. Harris WH. The problem is osteolysis. Clin Orthop 1995;311:46. 2. Sochart DH. Relationship of acetabular wear to osteolysis and loosening in total hip arthroplasty. Clin Orthop 1999;363:135. 3. Awatani J, Hashimoto M. Effect of radiation in the surface layer of polyethylene samples. J Soc Mater Sci in Japanese 1966;15:149. 4. Shikita T, Oonishi H. Abnutzung und sehmierung der totalendoprothesen des hqftgelenkes. Der Totals
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Hqftgelenkersatz. Georg Thieme Verlag Stuttgart, Germany; 1973. p. 15. Oonishi H, Takayama Y, Tsuji E. Improvement of polyethylene by irradiation in artificial joints. Radiat Phys Chem 1992;39:495. Oonishi H, Clarke IC, Masuda S, et al. Study of retrieved acetabular sockets made from high-dose, cross-linked polyethylene. J Arthroplasty 2001; 16:129. Iwaki H, Minoda Y, Kobayashi A, et al. In vivo quantitative analysis of high cross-linked polyethylene particles in total hip replacement. Am Acad Orthop Surg 2002;13. Muratoglu OK, Mark A, Vittetoe DA, et al. Polyethylene damage in total knees and use of highly crosslinked polyethylene. J Bone Joint Surg 2003; 85A:7. McKellop H, Shen FW, Lu B, et al. Development of an extremely wear-resistant ultra high molecular weight polyethylene for total hip replacements. J Orthop Res 1999;17:157. Muratoglu OK, Bragdon CR, O’Connor DO, et al. A novel method of crosslinking ultra–high-molecularweight polyethylene to improve wear, reduce oxidation, and retain mechanical properties: recipient of the 1999 HAP Paul Award. J Arthroplasty 2001; 16:149. Oonishi H, Kadoya Y, Iwaki H, et al. Total hip arthroplasty with a modified cementing technique using hydroxyapatite granules. J Arthroplasty 2001; 16:784. Kabo JM, Gebhard JS, Loren G, et al. In vivo wear of polyethylene acetabular components. J Bone Joint Surg 1993;75:254. Oonishi H, Igaki H, Takayama Y. Comparisons of wear of UHMW polyethylene sliding against metal and alumina in total hip prostheses. Bioceramics 1989;1:272. Manning DW, Chiang PP, Martell JM, et al. In vivo wear of traditional vs. highly crosslinked polyethylene. 50th Annual Meeting of the Orthop Res Soc 2004;1478. Digas G, K7rrholm J, Thanner J, et al. Highly crosslinked polyethylene in cemented THA. Clin Orthop 2003;417:126. Heicel C, Silva M, Dela Rosa MA, et al. Short-term in vivo wear of cross-linked polyethylene. J Bone Joint Surg 2004;86A:748. Martell JM, Verner JJ, Incavo SJ. Clinical performance of a highly cross-linked polyethylene at two years in total hip arthroplasty: a randomized prospective trial. J Arthroplasty 2003;18:55. Willmann G. Ceramics for total hip replacement— what a surgeon should know. Orthopedics 1998;21:173. Christel P, Meunier A, Heller M, et al. Mechanical properties and short-term in-vivo evaluation of yttrium-oxide-partially-stabilized zirconia. J Biomed Mater Res 1989;23:45.
Wear of Highly Cross-linked Polyethylene ! Oonishi et al 949 20. Boutin P, Christel P, Dorlot JM, et al. The use of dense alumina-alumina ceramic combination in total hip replacement. J Biomed Mater Res 1988;22:1203. 21. Muratoglu OK, Greenbaum ES, Bragdon CR, et al. Surface analysis of early retrieved acetabular polyethylene liners. A comparison of conventional and highly crosslinked polyethylenes. J Arthroplasty 2004;19:68. 22. Streicher RM. Ionizing irradiation for sterilization and modification of high molecular weight polyethylenes. Plast Rubber Process Appl 1988;10:221. 23. Ikada Y, Nakamura K, Ogata S. Characterization of ultrahigh molecular weight polyethylene irradiation with g-rays and electron beams to high doses. J Polym Sci Polym Chem 1999;37:159. 24. Hopper RH, Young AM, Orishimo KF, et al. Correlation between early and late wear rates in total hip
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arthroplasty with application to the performance of marathon cross-linked polyethylene liners. J Arthroplasty 2003;18:60. Oonishi H, Kadoya Y. Wear of high-dose gammairradiation polyethylene in total hip replacements. J Orthop Sci 2000;5:223. Oonishi H, Tsuji E, Kim YY. Retrieved total hip prostheses. Part I The effects of cup thickness, head sizes and fusion defects on wear. J Mater Sci Mater Med 1998;9:393. Oonishi H, Tsuji E, Kim YY. Retrieved total hip prostheses. Part II Wear behavior and structural changes. J Mater Sci Mater Med 1998;9:575. Oonishi H, Iwaki H, Kin N, et al. The effects of polyethylene cup thickness on wear of total hip prostheses. J Mater Sci Mater Med 1998;9:475.
Wear of Highly Cross-linked Polyethylene Acetabular Cup in Japan Hironobu Oonishi, MD, PhD,* Sok Chol Kim, MD,* Yasuhiro Takao, MD,* Masayuki Kyomoto, MSc,y Mikio Iwamoto, MSc,y and Masaru Ueno, PhDy
Abstract: The wear characteristics of ultra–high-molecular-weight polyethylene acetabular cups irradiated with 6.0 to 7.5 Gy of c-irradiation were studied in a hip simulator and in vivo in 70 hips for 6 to 39 months. The linear wear was 2.5% to 4.5% of the wear seen in conventional polyethylene cups. A biphasic wear pattern was observed over time with a relatively large wear in the first year and a subsequent leveling of the wear rate curve. The linear wear rate in the second phase (steady state) was 0.006 mm/y. The extent of decrease in clinical wear of 6.0-Mrad polyethylene (Aeonian, Kyocera Corp, Kyoto, Japan) cups is comparable with the decrease of the in vitro wear measured by the hip simulation test. Key words: total hip arthroplasty, highly cross-linked polyethylene, wear, clinical results, hip simulator. n 2006 Elsevier Inc. All rights reserved.
beam or c ray irradiation has, therefore, been extensively investigated. When a high-energy beam, such as a c ray, is irradiated on a polymer, free radicals are formed by a scission of the molecular chains. This is followed by retermination and cross-linkage of the molecules. High-dose c-irradiation on UHMWPE severs the C2C or C2H bonds, which then results in cross-linking and subsequent chemical bonding of C;O and C2C. In 1970, Shikita and Oonishi [4], collaborating with Hisashi Igaki, started development of a crosslinked polyethylene treated with high-dose c irradiation, which was aimed at improved wear properties as well as creep resistance [5]. Based on the results of this and other researches, a limited number of polyethylene cups prepared with cirradiation of almost 100 Gy (100-Gy polyethylene) were first tried in 1971 for hip arthroplasty by Shikita and Oonishi and were used until 1978 when the company ceased marketing the cup. Good clinical results with this 100-Gy polyethylene have been observed for more than 30 years [6,7]. Many researchers have reported the linear wear rate of acetabular cups made with conventional Polyethylene to be between 0.1 and 0.2 mm/y. The wear of 100-Gy polyethylene
As a bearing couple for artificial joint systems, the combination of the ultra–high-molecular-weight polyethylene (UHMWPE) acetabular component and metal (in particular, Co-Cr alloy) for the femoral component still remains the most popular materials for joint arthroplasty since its first application by Charnley in 1962. However, osteolysis caused by wear particles of UHMWPE has emerged as a serious issue [1,2]. To decrease such particles from UHMWPE, different combinations of the bearing couple or improvements of the bearing materials themselves have been attempted. In 1966, Awatani reported the beneficial effects of radiation to the surface layer of polyethylene [3]. Cross-linking of UHMWPE caused by electron From the *H. Oonishi Memorial Joint Replacement Institute, Tominaga Hospital, Osaka-City, Japan; and yResearch and Development Corporate Division, Japan Medical Materials Corporation, OsakaCity, Japan. Submitted October 30, 2004; accepted August 6, 2005. No benefits or funds were received in support of the study. Reprint requests: Hironobu Oonishi, MD, PhD, H. Oonishi Memorial Joint Replacement Institute, Tominaga Hospital, 4-48, 1-Chome, Minato-Machi, Naniwa-Ku, Osaka-City 556-0017, Japan. n 2006 Elsevier Inc. All rights reserved. 0883-5403/06/1906-0004$32.00/0 doi:10.1016/j.arth.2006.03.009
944
Wear of Highly Cross-linked Polyethylene ! Oonishi et al 945
measured using radiographs was reduced to 16%, on average, compared with conventional polyethylene, and retrieved cups showed wear decreased to 3% to 10%. These data evidenced the lasting effect of cross-linking by high-dose c-irradiation. In a 100-Gy polyethylene cup retrieved after clinical use for more than 25 years, most free radicals remained but without progressive oxidation of the polyethylene, although oxidation in retrieved conventional polyethylene cups was found to have progressed. Since 1998 several different highly cross-linked polyethylene cups irradiated with an energy beam in the range of 5.0 to 10.5 Gy have been produced and used extensively [8]. The source and dose of the high-energy beam, including c ray and electron beams, differs among manufacturers. There are many reports of in vitro testing on highly crosslinked polyethylene made by various manufacturing techniques in this exposure range [9,10]. In earlier reports, highly cross-linked polyethylene made using dosages from 5.0 to 10.5 Gy showed an 80% to 90% reduction in wear rate. However, only limited clinical results of this reduced irradiated polyethylene are available to date. In the present study, the wear rate of in vitro simulation and in vivo clinical (radiological) investigation, using highly cross-linked polyethylene (Aeonian, Kyocera Corp, Kyoto, Japan) irradiated with 5.0-Gy to 7.5-Gy c-irradiation (6.0-Gy polyethylene [Aeonian]), was studied. We measured linear wear as well as volumetric wear to determine the ongoing properties of this material. Although the follow-up period was short, we feel this early report will provide an indication of the in vitro and in vivo wear properties of this material.
Hip Simulation Study of 6.0-Mrad polyethylene The in vitro wear test was performed using a hip simulator (Shore Western, Inc, Monrovia, Calif). A mixture of 30% bovine serum, 20 mmol/L of EDTA, and 0.2% sodium azide was used as lubrication. A load simulating a physiologic loading curve with a 2 kN peak load was added with a period 1 Hz and a 408 arc cup rotation. The acetabular component was tested with a 22-mm zirconia ceramic femoral head (Bioceram, Kyocera Corp). Wear was measured by a gravimetric method. The weight of the cup was measured every 200 000 cycles. The volumetric change was then calculated from the weight loss over time. To control for the effect of water absorption during the test, 3 pieces of the same size and material were immersed in the same lubricant for the same period, and the weight gain of these control cups was then measured. The true weight loss was calculated as the difference between the experimental and the control weight. After 3.0 106 cycles, an apatite powder with an average grain size of 1.7 lm was added into the bearing space to evaluate the properties after third-body wear. Testing then continued until a total of 5.4 106 cycles were completed. Clinical Wear Study of 6.0-Mrad polyethylene (Aeonian) An alumina femoral head of 28 mm in diameter was used in all patients (Fig. 1). The 6.0-Mrad polyethylene (Aeonian) acetabular cup was fixed with an Interface Bioactive Bone Cement technique in all cases [11]. The clinical data consisted of 70 joints operated on between January 2000 and
Materials and Methods The 6.0-Mrad polyethylene (Aeonian) acetabular cups, both experimental and clinical, were manufactured using the following process. The compression-molded UHMWPE (GUR1050 resin, Perplas Medical Ltd, UK) bar stock was c-irradiated with 3.5 Gy in air and annealed at 1108C in N2 for 12 hours. The cups were then machined from this bar stock to 22-mm inner diameter and 44-mm outer diameter and then c-sterilized with a dose of 2.5 to 4.0 Gy in N2. Thus, the total dose of c-irradiation was 6.0 to 7.5 Gy. Control cups were prepared using conventional polyethylene, sterilized with ethylene oxide gas (EOG polyethylene). Three units each of the experimental and control cups were prepared.
Fig. 1. A sample radiograph showing no wear in a 6.0Mrad polyethylene (Aeonian)–fitted patient. A, Total hip prosthesis; B, Radiograph after surgery.
946 The Journal of Arthroplasty Vol. 21 No. 7 October 2006 January 2001. The average age of the patients was 61 years (range, 20-89 years) at the time of operation, and the average follow-up period was 28 months (range, 6-39 months). Primary diseases were osteoarthritis of the hip in 63 joints, femoral head necrosis in 4, congenital dislocation of the hip in 2, and femoral neck fracture in 1. Demographics of the cup thickness in the 70 joints was 1 cup of 7 mm, 5 of 8 mm, 31 of 9 mm, 26 of 10 mm, 5 of 11 mm, and 2 of 13 mm. Of 70 cups, however, 1 of 8 mm, 4 of 9 mm, and 2 of 13 mm were excluded from the evaluation because the follow-up term was shorter than 1 year. The in vivo wear of the acetabular cup was measured using frontal view plain radiographs. Wear measurement was performed at 1 month post operation and, afterward, every 6 months and recorded as the change in distance between the cup and the femoral head center. The femoral head center was determined using a computerized algorithm provided by Vector Works 10.5 (A & A Corp, Tokyo, Japan). Data input to the software consisted in the 3 outer points of the femoral head, as measured in the radiograph (Fig. 1). The volumetric wear was calculated from the linear wear and the wear direction after an equation developed by Kabo et al [12].
Results Hip Simulation Wear Study Fig. 2 shows the conventional EOG polyethylene acetabular cup with a linear wear pattern. The wear
Fig. 3. Comparison of 6.0-Mrad polyethylene (Aeonian) wear rate with conventional polyethylene in vivo. -x-, wear trend of 6.0-Mrad polyethylene (Aeonian); --,wear trend of conventional polyethylene.
in 6.0-Mrad polyethylene (Aeonian) was 2.5% to 4.5% (mean, 3.0%) of the wear of the EOG polyethylene. Visual inspection of Fig. 2 shows that the wear of 6.0-Mrad polyethylene (Aeonian) appears to be very low, in comparison with EOG polyethylene. After the addition of apatite powder, the 6.0Mrad polyethylene (Aeonian) showed a decrease in wear of 15% of the wear of EOG polyethylene showing that 6.0-Mrad polyethylene (Aeonian) sustains low wear properties even in the presence of foreign substances. Clinical Wear Study
Fig. 2. Simulation study comparing 6.0-Mrad polyethylene (Aeonian) to EOG polyethylene. 6.0 Mrad, 6.0Mrad polyethylene (Aeonian).
Fig. 1 shows an example of a radiograph at 2 years and 11 months post operation in which no distinguishable change was found, compared with the radiograph immediately after operation. Fig. 3 shows the in vivo 6.0-Mrad polyethylene (Aeonian) cups with a linear wear rate of 0.10 F 0.05 mm/y and a volumetric wear rate of 25.4 F 27.9 mm3/y. For the first year after surgery, a steep increase in the wear rate of about 0.2 mm was seen, followed by leveling with a tendency of gradual increase over the time of the study. We defined the initial phase up to 1 year post operation as the initial linear wear rate and the subsequent phase 1 year
Wear of Highly Cross-linked Polyethylene ! Oonishi et al 947
Fig. 4. The comparison of wall thickness of the acetabular cups as a function of in vivo linear wear rate and volumetric wear rate. Asterisk indicates non significance; 5, linear wear rate; n, volumetric wear rate.
post operation as the steady linear wear rate (Fig. 3). The 6.0-Mrad polyethylene (Aeonian) showed an initial linear wear rate of 0.232 mm/y and a steady linear wear rate of 0.006 mm/y. The EOG polyethylene cups showed an initial linear wear rate of 0.104 mm/y and a steady linear wear rate of 0.098 mm/y, giving almost no difference between initial and steady linear wear rates in the case of the EOG polyethylene cups [13]. In the case of 6.0-Mrad polyethylene (Aeonian), the steady linear wear rate decreased to 6% compared with EOG polyethylene (Fig. 3). A significant difference was not found in either the linear or volumetric wear rates among groups of different thickness (8,9,10, and 11 mm), but a rather high wear tendency was observed in the 8-mm group (Fig. 4).
Discussion In this clinical wear study, the 6.0-Mrad polyethylene (Aeonian) cups showed a steady linear wear rate of 0.006 mm/y, and the EOG polyethylene cups showed a steady linear wear rate of 0.098 mm/y. The clinical wear of polyethylene cups irradiated with less than 10 Mrad decreased to 6% of conventional polyethylene in this shortterm examination. The literature reports linear wear rates of highly cross-linked polyethylene: 0.008 mm/y by Manning et al [14] for Zimmer cups; 0.09 mm/y by Digas et al [15] for the Durasul cups (Zimmer, Inc, Warsaw, Ind); 0.02 mm/y by Heicel et al [16]for the Marathon cups (DePuy, Orthopaedics Inc, Warsaw, Ind); and 0.12 mm/y by Martel et al [17] for the Crossfire cups (Stryker Howmedica Osteonics, Inc, Mahwah, NJ).
All of these highly cross-linked polyethylene cups showed significantly decreased wear rates compared with that of conventional polyethylene cups. The extent of decrease in clinical wear of 6.0Mrad polyethylene (Aeonian) is comparable with the decrease of the in vitro wear measured by hip simulation test. The hip simulation test was conducted using 22-mm femoral heads. In clinical usage, 28-mm femoral heads were used for improved range of motion as well as prevention of luxation. In addition, zirconia femoral heads were used in the hip simulation test, and alumina femoral heads were used clinically. Both alumina and zirconia have shown good in vitro wear properties [18]. Zirconia is a comparatively new material of about 20 years and with additional clinical experience may eventually exceed alumina [19]. However, clinical experience with alumina is more than 30 years, we used alumina for this clinical study because of its high reliability [20]. The 6.0-Mrad polyethylene (Aeonian) cups showed high initial linear wear rate of 0.232 mm/y. After the first year, the steady linear wear rate became low and steady. We propose that creep deformation of highly cross-linked polyethylene caused high initial wear. Muratoglu et al [21] also reported that initial wear of highly cross-linked polyethylene was due to the creep deformation. In addition, it is known that creep resistant properties generally improve in the highly cross-linked polyethylene annealed under the melting point (1108C in this study) [22,23]. Hopper et al [24] reported that initial wear rate (0-2 years of follow-up data) of non–cross-linked Enduron polyethylene (Depuy Orthopaedics Inc, Warsaw, Ind) is higher than initial wear rate of cross-linked Marathon polyethylene. Therefore, we speculate that creep deformation of the EOG polyethylene is higher than creep deformation of 6.0-Mrad polyethylene (Aeonian) because the EOG polyethylene is not highly crosslinked, but we did not measure the initial linear wear of the EOG polyethylene. The follow-up period of this 6.0-Mrad polyethylene (Aeonian) study was short. In past studies, we reported that the long-term clinical wear (by radiograph and retrieval) of 100-Mrad polyethylene cups decreased to 3% to 17% compared with conventional polyethylene [25]. We expect that 6.0-Mrad polyethylene (Aeonian) will show favorably low and steady wear properties in long-term clinical studies, although detailed follow-up is, of course, necessary. In a cup of less than 8 mm in thickness, 100-Mrad polyethylene did not show any difference in wear over time [26]; in addition, 6.0-Mrad
948 The Journal of Arthroplasty Vol. 21 No. 7 October 2006 polyethylene (Aeonian) did not show any difference. We previously reported that thicker cups have advantages in wear behavior and that both linear and volumetric wear decrease as the wall thickness of the cup increases in conventional polyethylene [27]. When size of the femoral head was studied in relation to wall thickness, it was shown that a wall thickness of more than 10 mm showed little correlation with head diameter in volumetric wear [28]. This supports the interpretation that the effect of creep deformation on wear behavior is not large at 10 mm. On the other hand, no significant difference was found either in the linear or the volumetric wear rate among groups of different thicknesses in highly cross-linked polyethylene cups. It is known that creep resistant properties generally improve in the highly crosslinked polyethylene annealed under the melting point (creep deformation: conventional polyethylene N highly cross-linked polyethylene N100-Mrad polyethylene). It therefore appears that lower dose polyethylene is more susceptible to creep deformation [23], compared with 100-Mrad polyethylene. The preliminary observation in this study is still short clinical term. Future observations may be to support that assumption by the increased wear during the first year of in vivo use. Consequently, future observations that take creep deformation into account are important with the highly cross-linked polyethylene cups. In addition, long-term follow-up including wear and creep deformation is still needed to support this preliminary observation.
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Acknowledgments
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The authors thank Takuji Shikita, MD, Hisashi Igaki, PhD, and the research group of Mizuho Medical Instruments, Ltd, for cooperation in the development of 100-Mrad cross-linked polyethylene.
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