Commentary & Perspective

Commentary & Perspective on
"Wear and Surface Cracking in Early Retrieved Highly Cross-Linked Polyethylene Acetabular Liners"
by Letitia Bradford, MD, et al.

Commentary & Perspective by
Roy Crowninshield, PhD*,
Fort Wayne, Indiana

In this issue of The Journal, Bradford et al. report an analysis of retrieved, highly cross-linked acetabular liners. Their aim is to determine whether the observed early in vivo wear behavior has been accurately predicted by hip-simulator studies of this material. The retrieved specimens were found to have surface cracking, abrasion, pitting, and scratching, and they conclude that these damage modes were not predicted in joint simulator studies of this material.

An alternative view can be found in relevant recently published works not cited by these authors. Rieker et al.¹ described ripples with microcracks on the surface of both clinically retrieved and hip-simulator-tested highly cross-linked polyethylene specimens. This work directly contradicts the assertion of Bradford et al. that this polyethylene damage mode is not predicted in hip-simulator studies. Muratoglu et al.² previously reported scratching of the articular surface on both conventional and highly cross-linked retrieved acetabular components. They also described a method of articular surface examination that helps separate wear from non-wear surface damage.

Although the specimens were retrieved from patients with loose, porous metal acetabular components, Bradford et al. suggest that the highly cross-linked articular components were functioning in vivo without any abrasive third-body particles and thus should be expected to have little articular surface damage. I would think it more likely, however, to expect to find articular surface damage from third-body particles in this setting.

Hip-simulator testing of highly cross-linked polyethylene has included adverse wear conditions with third-body particles^3,4 and roughened femoral heads⁵. These tests under adverse conditions do produce surface damage and higher rates of wear compared with an optimal articulating environment. These studies also predict that highly cross-linked polyethylene components experience less wear under these conditions than do conventional polyethylene components.

Thus, the evaluation of clinical wear^6,7 of highly cross-linked acetabular components is generally consistent with prior joint simulator testing^3,8,9 and the analyses of clinically retrieved implants². Therefore, the conclusion of Bradford et al. that there is a discrepancy between in vivo and in vitro wear surfaces of highly cross-linked acetabular components is not well supported by the data presented.

On the contrary, the articular surface damage reported in this paper appears similar to that reported by others in both clinical retrieval and in vitro hip-simulator testing^1-4,10. Ongoing clinical studies of highly cross-linked acetabular components do indicate that wear performance is consistent with prior joint-simulator studies^6,7.

*The author did not receive grants or outside funding in support of his research or preparation of this manuscript. The author received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity (Zimmer, Inc.). No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the author is affiliated or associated.

References

1. Rieker CB, Konrad R, Schon R, Schneider W, Abt NA. In vivo and in vitro surface changes in a highly cross-linked polyethylene. J Arthroplasty. 2003;18(7 Suppl 1):48-54.
2. Muratoglu OK, Greenbaum ES, Bragdon CR, Jasty M, Freiberg AA, Harris WH. Surface analysis of early retrieved acetabular polyethylene liners: a comparison of conventional and highly crosslinked polyethylenes. J Arthroplasty. 2004;19:68-77.
3. Crowninshield DR, Laurent MP, Yao JQ, Bhambri SK, Gsel RA, Gilbertson LN, Swarts DF. Cross-linking to improve THR wear performance. Hip. 2002;12:103-7.
4. Laurent MP, Yao JQ, Gilbertson LN, Johnson TS, Swarts DF, Blanchard CR, Crowninshield RD. The wear of crosslinked UHMWPE in the presence of abrasive particles: hip and knee simulator studies. In: Kurtz S, Gsell R, Martell J, editors. Crosslinked and thermally treated ultra-high molecular weight polyethylene for joint replacements (ASTM Special Technical Publication, 1445). West Conshohocken, PA: American Society for Testing and Materials International; 2004. p 86-103.
5. McKellop H, Shen FW, DiMaio W, Lancaster JG. Wear of gamma-crosslinked polyethylene acetabular cups against roughened femoral balls. Clin Orthop. 1999;369:73-82.
6. Digas G, Kärrholm J, Thanner, J Malchau H, Herberts P. Highly cross-linked polyethylene in total hip arthroplasty: randomized evaluation of penetration rates in cemented and uncemented sockets using radiostereometric analysis. Read at the Thirty-Second Open Scientific Meeting of the Hip Society and the Tenth Combined Open Meeting of the Hip Society and AAHKS; 2004 Mar 13; San Francisco, CA.
7. Digas G, Herberts P, Kärrholm J,Thanner J, Malchau H. Crosslinked vs. conventional polyethylene in bilateral hybrid THR randomised radiostereometic study. Presented as a poster at the Annual Meeting of the Orthopaedic Research Society; Mar 7-10; San Francisco, CA.
8. McKellop H, Shen FW, Lu B, Campbell P, Salovey R. Development of an extremely wear-resistant ultra high molecular weight polyethylene for total hip replacements. J Orthop Res. 1999;17:157-67.
9. Muratoglu OK, Bragdon CR, O'Connor DO, Jasty M, Harris WH. A novel method of cross-linking ultra-high molecular-weight polyethylene to improve wear, reduce oxidation, and retain mechanical properties. J Arthroplasty. 2001;16:149-60.
10. Dowling JM, Atkinson JR, Dowson D, Charnley J. The characteristics of acetabular cups worn in the human body. J Bone Joint Surg Br. 1978;60:375-82.

Copyright © 2004 by the The Journal of Bone and Joint Surgery, Inc.