J. Rod Davey, M.D., FRCSC
Toronto Western Hospital
Toronto, ON

Osteolysis and implant loosening induced by ultra high molecular weight polyethylene wear debris remains one of the most important factors affecting the success of total hip arthroplasty. Wear is a complex problem that can be affected by patient risk factors, surgical technique and implant design and materials. It is essential to reduce wear debris in order to minimize the problems of osteolysis and loosening. Recently improved component design, alternate bearing surfaces, as well as improving the ultra high molecular weight polyethylene have addressed this.  

Cross-linking of polyethylene has been reported to significantly reduce wear in laboratory testing. Cross-linked polyethylenes can be manufactured by a number of methods. Energy (e.g., gamma radiation) is used to form an increased number of cross-links between the polyethylene molecular chains. Degradative free radicals that form during the cross-linking are removed through thermal treatments of the material. The cross-linking process has been reported to decrease wear ten-fold compared to standard polyethylene. Cross-linking can reduce the ultimate tensile strength, fatigue strength, and elongation to failure of polyethylene - therefore the cross-linking process should provide a balance between these physical properties and the resistance to wear.

McKellop et al¹ compared cross-linked polyethylene to standard polyethylene in hip simulator tests lasting as long as five million cycles. The gamma radiated cross-linked cups exhibited dramatically improved resistance to wear and excellent resistance to oxidation.

Debris in the submicron size range is believed to have greater osteolytic potential than larger particles. Endo et al² studied irradiated cross-linked polyethylene, chemically cross-linked polyethylene and non cross-linked polyethylene. Both cross-linked polyethylenes showed better wear resistance than the non cross-linked polyethylene. However, in the rough and scratched counterface conditions, the gamma irradiated cross-linked material produced higher wear rates than the non cross-linked polyethylene. Clinically, this could result in higher wear rates if the head or cup was damaged or scratched. Analysis of the wear debris showed that the majority of the volume of the chemically cross-linked polyethylene wear debris was in the most biologically active size range (0.1 to 0.5 micron).

Although laboratory testing supports the use of cross-linked polyethylene, how the material behaves in vivo is the ultimate test. Oonishi, Kadoya and Masuda ³ evaluated retrieved, gamma irradiated cross-linked polyethylene sockets at 15 and 16 years after implantation. Decreased surface oxidation was seen in these specimens compared to non cross-linked sockets. The cross-link was stable and was retained for a long period both in vivo and in ambient air.

Recently there has been interest in using larger femoral heads to improve stability of the hip. Muratoglu et al⁴ investigated the wear behavior of electron-beam cross-linked polyethylene with femoral head diameters ranging from 22 to 46 mm. The simulated gait studies showed that wear was independent of head size for the range of femoral heads studied. Even for the 46 mm femoral head, wear was reduced significantly out to 11 million cycles of simulated gait.

Laboratory testing and the limited clinical information on cross-linked polyethylene has been favorable to date. Cross-linked polyethylene looks like a promising alternative to conventional non cross-linked polyethylene and to other alternative bearing surfaces (e.g., metal on metal, ceramic on ceramic). Postmarket surveillance of all these materials is required to determine whether the improvements in wear resistance seen in the laboratory carry over to the long-term clinical situation.

References

1. McKellop H., Shen F.W., Lu B., Campbell P., Salovey R. Development of an extremely wear-resistant ultra high molecular weight polyethylene for total hip repalcements. J Orthop res 1999 Mar;17(2):157-67
   
   
2. Endo M.M., Barbour P.S., Barton D.C., Fisher J., Tipper J.L., Ingham E., Stone M.H. Comparative wear and wear debris under three different counterface conditions of crosslinked and non-crosslinked ultra high molecular weight polyethylene. Biomed Mater Eng 2001;11(1);23-35
   
   
3. Oonishi H., Kadoya Y., Masuda S. Gamma-irradiated cross-linked polyethylene in total hip replacements-analysis of retrieved sockets after long-term implantation. J Biomed Mater Res 2001;58(2):167-71
   
   
4. Muratoglu O.K., Bragdon C.R., O'Connor D., Perinchief R.S., Estok D.M. 2nd, Jasty M., Harris W.H. Larger diameter femoral heads used in conjunction with a highly cross-linked ultra-high molecular weight polyethylene:a new concept. J Arthroplasty 2001 Dec;16(8 Suppl 1):24-30