The similarity of the wear rates with or without metal backing for either type of PE was consistent with the computational model of Maxian et al. , which predicted less than 1% difference in the wear rates of PE liners with or without metal backing.
The observation that the wear rate did not increase as the thickness of the cups decreased was consistent with the results of Kelly et al.  who compared the wear rates of acetabular cups of conventional and highly crosslinked PEs in a type of hip simulator closely comparable to that used in our study but with the cups mounted in greater abduction (“near impingement”). The conventional PE cups were gamma sterilized in nitrogen at 3 Mrad and had a 36-mm inner diameter with a 7.9-mm wall thickness (Stryker Orthopaedics, Mahwah, NJ). The PE for the highly crosslinked cups was crosslinked at 9 Mrad using three gamma doses of 3 Mrad, each followed by annealing (X3®; Stryker). These highly crosslinked cups had a 36-mm inner diameter with a 7.9-mm thickness or a 44-mm inner diameter with a 3.8-mm thickness. The mean wear rates were 25, 1.8, and 1.8 mg per million cycles for the conventional cups, 7.9-mm-thick crosslinked cups, and 3.8-mm-thick crosslinked cups, respectively. These authors concluded “crosslinked PE may allow for liners that are thinner than has been traditionally accepted” . However, they also cautioned their results were for an “idealized cup position” and did not take into account the potential effects of edge loading, subluxation, or artificial aging, which has caused fracture of conventional and highly crosslinked liners.
In the FE modeling, the trend for the contact stress to increase as the thickness of the liner decreased (Fig. 8) was consistent with the results obtained by Bartel et al.  using analytical solutions and FE analysis. Although the general trend also was in agreement with the results of Plank et al.  who evaluated the contact stress of conventional and crosslinked PE using FE analysis and pressure-sensitive film for the comparable situation of a 38-mm ball and a 3-mm-thick PE liner, the contact stress calculated by these authors was 36% lower and von Mises stress was 39% higher than those in our study. This difference might be attributable to a number of factors, such as differences in the material properties, the geometry of the specimens, the clearance between the ball and liner, and the bonding condition assumed between the metal shell and the liners. For example, Bartel et al.  found a bonded interface, as used in our study, resulted in a higher contact stress and lower von Mises stress than a debonded interface, as used by Plank et al.  and Kurtz et al. .
Considering only the increase in the magnitudes of the contact stresses in these studies as the thickness of the liner was reduced, one would predict a corresponding increase in the rate of wear. For example, all other factors equal, Archard’s equation  predicts the volumetric rate of wear is directly proportional to the contact stress. Thus, it was surprising the lowest mean rate of wear in our study occurred with the thinnest (3-mm) liners. This trend might have been due, in part, to the contact area being smallest with the 3-mm liners, as indicated by the FE model. In the wear test, this might have been sufficient to more than offset the effect of the increased contact stress, leading to a net reduction in the volumetric rate of wear. In addition, the higher stresses in the thinner liners might have increased the total viscoelastic deformation of the PE, increasing the conformity between the ball and the liner in the contact zone and thereby the potential for at least partial separation of the bearing surfaces by a layer of lubricant, which would tend to reduce the rate of wear.
Taken together, the results of our study indicate, for a 5-Mrad crosslinked PE liner with the acetabular component in proper orientation, the diameter of the ball may be increased and the thickness of the PE decreased within the dimensional limits investigated to improve the resistance to dislocation, with little or no corresponding increase in the volumetric rate of wear and a modest increase in the stresses within the PE.
We thank Peter Liao and Don McNulty of DePuy Orthopaedics, Inc., for their valuable contributions to the design of the study and the interpretation of the results.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
1. Archard, J. The temperature of rubbing surfaces. Wear.
1958; -59: 438-455.
2. Bartel, DL., Bicknell, VL. and Wright, TM. The effect of conformity, thickness, and material on stresses in ultra-high molecular weight components for total joint replacement. J Bone Joint Surg Am.
1986; 68: 1041-1051.
3. Bitsch, RG., Loidolt, T., Heisel, C., Ball, S. and Schmalzried, TP. Reduction of osteolysis with use of Marathon cross-linked polyethylene: a concise follow-up, at a minimum of five years, of a previous report. J Bone Joint Surg Am.
2008; 90: 1487-1491. 10.2106/JBJS.F.00991
4. Bragdon, CR., Greene, ME., Freiberg, AA., Harris, WH. and Malchau, H. Radiostereometric analysis comparison of wear of highly cross-linked polyethylene against 36- vs 28-mm femoral heads. J Arthroplasty.
2007; 22: 125-129. 10.1016/j.arth.2007.03.009
5. Clarke, IC., Good, V., Anissian, L. and Gustafson, A. Charnley wear model for validation of hip simulators-ball diameter versus polytetrafluoroethylene and polyethylene wear. Proc Inst Mech Eng H.
1997; 211: 25-36. 10.1243/0954411971534656
6. Clarke, IC., Gustafson, A., Jung, H. and Fujisawa, A. Hip-simulator ranking of polyethylene wear: comparisons between ceramic heads of different sizes. Acta Orthop Scand.
1996; 67: 128-132.
7. Collier, JP., Currier, BH., Kennedy, FE., Currier, JH., Timmins, GS., Jackson, SK. and Brewer, RL. Comparison of cross-linked polyethylene materials for orthopaedic applications. Clin Orthop Relat Res.
2003; 414: 289-304. 10.1097/01.blo.0000073343.50837.03
8. D’Antonio, JA., Manley, MT., Capello, WN., Bierbaum, BE., Ramakrishnan, R., Naughton, M. and Sutton, K. Five-year experience with Crossfire®
highly cross-linked polyethylene. Clin Orthop Relat Res.
2005; 441: 143-150. 10.1097/00003086-200512000-00024
9. Devane, PA., Horne, JG., Martin, K., Coldham, G. and Krause, B. Three-dimensional polyethylene wear of a press-fit titanium prosthesis: factors influencing generation of polyethylene debris. J Arthroplasty.
1997; 12: 256-266. 10.1016/S0883-5403(97)90021-8
10. Digas, G., Karrholm, J., Thanner, J., Malchau, H. and Herberts, P. Highly cross-linked polyethylene in total hip arthroplasty: randomized evaluation of penetration rate in cemented and uncemented sockets using radiostereometric analysis. Clin Orthop Relat Res.
2004; 429: 6-16. 10.1097/01.blo.0000150314.70919.e3
11. Dorr, LD., Wan, Z., Shahrdar, C., Sirianni, L., Boutary, M. and Yun, A. Clinical performance of a Durasul highly cross-linked polyethylene acetabular liner for total hip arthroplasty at five years. J Bone Joint Surg Am.
2005; 87: 1816-1821. 10.2106/JBJS.D.01915
12. Duffy, GP., Wannomae, KK., Rowell, SL. and Muratoglu, OK. Fracture of a cross-linked polyethylene liner due to impingement. J Arthroplasty.
2009; 24: 158-19.
13. Elfick, AP., Hall, RM., Pinder, IM. and Unsworth, A. Wear in retrieved acetabular components: effect of femoral head radius and patient parameters. J. Arthroplasty.
1998; 13: 291-295. 10.1016/S0883-5403(98)90174-7
14. Elfick, AP., Hall, RM., Pinder, IM. and Unsworth, A. The effect of socket design, materials and liner thickness on the wear of the porous coated anatomic total hip replacement. Proc Inst Mech Eng H.
2001; 215: 447-457. 10.1243/0954411011536046
15. Garvin, KL., Hartman, CW., Mangla, J., Murdoch, N. and Martell, JM. Wear analysis in THA utilizing oxidized zirconium and crosslinked polyethylene. Clin Orthop Relat Res.
2009; 467: 141-145. 10.1007/s11999-008-0544-5
16. Geerdink, CH., Grimm, B., Ramakrishnan, R., Rondhuis, J., Verburg, AJ. and Tonino, AJ. Crosslinked polyethylene compared to conventional polyethylene in total hip replacement: pre-clinical evaluation, in-vitro testing and prospective clinical follow-up study. Acta Orthop.
2006; 77: 719-725. 10.1080/17453670610012890
17. Geller, JA., Malchau, H., Bragdon, C., Greene, M., Harris, WH. and Freiberg, AA. Large diameter femoral heads on highly cross-linked polyethylene: minimum 3-year results. Clin Orthop Relat Res.
2006; 447: 53-59. 10.1097/01.blo.0000218742.61624.80
18. Glyn-Jones, S., Isaac, S., Hauptfleisch, J., McLardy-Smith, P., Murray, DW. and Gill, HS. Does highly cross-linked polyethylene wear less than conventional polyethylene in total hip arthroplasty? A double-blind, randomized, and controlled trial using roentgen stereophotogrammetric analysis. J Arthroplasty.
2008; 23: 337-343. 10.1016/j.arth.2006.12.117
19. Halley, D., Glassman, A. and Crowninshield, RD. Recurrent dislocation after revision total hip replacement with a large prosthetic femoral head. A case report. J Bone Joint Surg Am.
2004; 86: 827-830.
20. Heck, DA., Partridge, CM., Reuben, JD., Lanzer, WL., Lewis, CG. and Keating, EM. Prosthetic component failures in hip arthroplasty surgery. J Arthroplasty.
1995; 10: 575-580. 10.1016/S0883-5403(05)80199-8
21. Hermida, JC., Bergula, A., Chen, P., Colwell, CW, Jr, and D’Lima, DD. Comparison of the wear rates of twenty-eight and thirty-two-millimeter femoral heads on cross-linked polyethylene acetabular cups in a wear simulator. J Bone Joint Surg Am.
2003; 85: 2325-2331.
22. Hirakawa, K., Bauer, TW., Hashimoto, Y., Stulberg, B. and Wilde, AH. Effect of femoral head diameter on tissue concentration of wear debris. J Biomed Mater Res.
1997; 36: 529-535. 10.1002/(SICI)1097-4636(19970915)36:4<529::AID-JBM11>3.0.CO;2-5
23. Jacobs, CA., Christensen, CP., Greenwald, AS. and McKellop, H. Clinical performance of highly cross-linked polyethylenes in total hip arthroplasty. J Bone Joint Surg Am.
2007; 89: 2779-2786. 10.2106/JBJS.G.00043
24. Kabo, JM., Gebhard, JS., Loren, G. and Amstutz, HC. In vivo wear of polyethylene acetabular components. J Bone Joint Surg Br.
1993; 75: 254-258.
25. Kelly, NH., Rajadhyaksha, AD., Wright, TM., Maher, SA. and Westrich, GH. High stress conditions do not increase wear of thin highly crosslinked UHMWPE. Clin Orthop Relat Res.
2010; 468: 418-423. 10.1007/s11999-009-1154-6
26. Kurtz, SM., Edidin, AA. and Bartel, DL. The role of backside polishing, cup angle, and polyethylene thickness on the contact stresses in metal-backed acetabular components. J Biomech.
1997; 30: 639-642. 10.1016/S0021-9290(96)00181-9
27. Kurtz, SM., Pruitt, L., Jewett, CW., Crawford, RP., Crane, DJ. and Edidin, AA. The yielding, plastic flow, and fracture behavior of ultra-high molecular weight polyethylene used in total joint replacements. Biomaterials.
1998; 19: 1989-2003. 10.1016/S0142-9612(98)00112-4
28. Leung, SB., Egawa, H., Stepniewski, A., Beykirch, S., Engh, CA, Jr, and Engh, CA, Sr, Incidence and volume of pelvic osteolysis at early follow-up with highly cross-linked and noncross-linked polyethylene. J Arthroplasty.
2007; 22: 134-139. 10.1016/j.arth.2007.04.006
29. Livermore, J., Ilstrup, D. and Morrey, B. Effect of femoral head size on wear of the polyethylene acetabular component. J Bone Joint Surg Am.
1990; 72: 518-528.
30. Maxian, TA., Brown, TD., Pedersen, DR., McKellop, HA., Lu, B. and Callaghan, JJ. Finite element analysis of acetabular wear: validation and backing and fixation effects. Clin Orthop Relat Res.
1997; 344: 111-117. 10.1097/00003086-199711000-00012
31. McKellop, H., Clarke, IC., Markolf, K. and Amstutz, H. Wear characteristics of UHMW polyethylene: a method for accurately measuring extremely low wear rates. J. Biomed Mater Res.
1978; 12: 895-927. 10.1002/jbm.820120611
32. McKellop, H., Shen, FW., DiMaio, W. and Lancaster, J. Wear of gamma-crosslinked polyethylene acetabular cups against roughened femoral balls. Clin Orthop Relat Res.
1999; 369: 73-82. 10.1097/00003086-199912000-00008
33. McKellop, H., Shen, FW., Lu, B., Campbell, P. and Salovey, R. Development of an extremely wear-resistant ultra high molecular weight polyethylene for total hip replacements. J Orthop Res.
1999; 17: 157-167. 10.1002/jor.1100170203
34. McKellop, HA. The lexicon of polyethylene wear in artificial joints. Biomaterials.
2007; 28: 5049-5057. 10.1016/j.biomaterials.2007.07.040
35. Min, BW., Song, KS., Kang, CH., Won, YY. and Koo, KH. Polyethylene liner failure in second-generation Harris-Galante acetabular components. J Arthroplasty.
2005; 20: 717-722. 10.1016/j.arth.2004.11.006
36. Moore, KD., Beck, PR., Petersen, DW., Cuckler, JM., Lemons, JE. and Eberhardt, AW. Early failure of a cross-linked polyethylene acetabular liner: a case report. J Bone Joint Surg Am.
2008; 90: 2499-2504. 10.2106/JBJS.G.01304
37. Muratoglu, OK., Bragdon, CR., O’Connor, D., Perinchief, RS., Estok, DM, II, Jasty, M. and Harris, WH. Larger diameter femoral heads used in conjunction with a highly cross-linked ultra-high molecular weight polyethylene: a new concept. J Arthroplasty.
2001; 16: 24-30. 10.1054/arth.2001.28376
38. Muratoglu, OK., Bragdon, CR., O’Connor, DO., Jasty, M. and Harris, WH. A novel method of cross-linking ultra-high-molecular-weight polyethylene to improve wear, reduce oxidation, and retain mechanical properties. Recipient of the 1999 HAP Paul Award. J Arthroplasty
2001; 16: 149-160. 10.1054/arth.2001.20540
39. Oonishi, H., Clarke, IC., Yamamoto, K., Masaoka, T., Fujisawa, A. and Masuda, S. Assessment of wear in extensively irradiated UHMWPE cups in simulator studies. J Biomed Mater Res A.
2004; 68: 52-60. 10.1002/jbm.a.10108
40. Paul JP. Loading on normal hip and knee joints on joint replacements. In: Schaldach M, Hohmann D, eds. Advances in Artificial Hip and Knee Joint Technology
. Berlin, Germany: Springer-Verlag; 1976:53.
41. Plank, GR., Estok, DM, 2nd, Muratoglu, OK., O’Connor, DO., Burroughs, BR. and Harris, WH. Contact stress assessment of conventional and highly crosslinked ultra high molecular weight polyethylene acetabular liners with finite element analysis and pressure sensitive film. J Biomed Mater Res B Appl Biomater.
2007; 80: 1-10.
42. Rohrl, SM., Li, MG., Nilsson, KG. and Nivbrant, B. Very low wear of non-remelted highly cross-linked polyethylene cups: an RSA study lasting up to 6 years. Acta Orthop.
2007; 78: 739-745. 10.1080/17453670710014509
43. Salvati, EA., Wright, TM., Burstein, A. and Jacobs, J. Fracture of polyethylene acetabular cups. J Bone Joint Surg Am.
1979; 61: 1239-1242.
44. Shaju, KA., Hasan, ST., D’Souza, LG., McMahon, B. and Masterson, EL. The 22-mm vs the 32-mm femoral head in cemented primary hip arthroplasty long-term clinical and radiological follow-up study. J Arthroplasty.
2005; 20: 903-908. 10.1016/j.arth.2005.02.005
45. Tower, SS., Currier, JH., Currier, BH., Lyford, KA., Citters, DW. and Mayor, MB. Rim cracking of the cross-linked longevity polyethylene acetabular liner after total hip arthroplasty. J Bone Joint Surg Am.
2007; 89: 2212-2217. 10.2106/JBJS.F.00758