The dramatic wear results reported by S&N are highly dependent upon the type of polyethylene used.  In addition, the results of this test are not statistically significant.

  • S&N used ethylene oxide (EtO) sterilized conventional polyethylene for this test.  When conventional polyethylene is sterilized using gamma irradiation, crosslinking of the polyethylene occurs, which equates to additional wear benefit.  However, when EtO is used to sterilize conventional polyethylene, no crosslinking occurs; therefore, wear rates are higher.2,4
  • Additionally, Zimmer Prolong Highly Crosslinked Polyethylene has shown an 81% reduction in wear over gamma-sterilized conventional polyethylene. 5  In another S&N test comparing Oxinium femoral heads with CoCr femoral heads articulating with highly Crosslinked polyethylene, the cups showed no significant difference in wear.6  Zimmer believes that when the polyethylene and metallic articulating surfaces are optimized in a metal-on-polyethylene articulation, the hardness of the metal is not the main determinant of wear.
  • In the study claiming 85% wear reduction, S&N used only three components of each type.  The P value for the S&N test was 0.097. 7  Common scientific practice is to infer statistical significance when the P value is less than or equal to 0.050 and the significant level obtained is nearly twice the acceptable standard.

References

  1. Ries MD, Salebi A, Widding K, Hunter G. Polyethylene wear performance of oxidized zirconium and cobalt chrome knee components under abrasive conditions. Scientific Exhibit No. SE16: 69th Annual Meeting of the AmericanAcademy of Orthopaedic Surgeons; February 13-17, 2002; Dallas, Texas.
     
  2. Ries MD, Scott ML, Jani S. Relationship between gravimetric wear and particle generation in hip simulators: conventional compared with cross-linked polyethylene. J Bone Joint Surg. 2001; 83:116-122.
     
  3. McKellop H, Shen F, Lu B, Campbell P, Salovey R. Effect of sterilization method and other modifications on the wear resistance of acetabular cups made of ultra-high molecular weight polyethylene: a hip-simulator study. J Bone Joint Surg. 2000; 82-A(12):1708-1725.
     
  4. McKellop H, Shen F, Campbell P, Ota T. Effect of molecular weight, calcium stearate, and sterilization methods on the wear of ultra high molecular weight polyethylene acetabular cups in a hip joint simulator.  J Orthop Res. 1999;17(3):329-339.
     
  5. Laurent MP, Yao JQ, Bhambri S, et al. High cycle wear of highly crosslinked UHMWPE tibial articular surfaces evaluated in a knee wear simulator. 28th Annual Meeting Transactions, Society for Biomaterials. 2002:712.
     
  6. Good V, Ries MD, Barrack RL, Widding K, Gordon BH, Heuer D. Reduced wear with oxidized zirconium femoral heads. Scientific Exhibit No. SE213: 70th Annual Meeting of the AmericanAcademy of Orthopaedic Surgeons; February 5-9, 2003; New Orleans, Louisiana.
     
  7. Spector M, Ries MD, Bourne RB, Sauer WS, Long M, Hunter G. Wear performance of ultra-high molecular weight polyethylene on oxidized zirconium total knee femoral components. J Bone Joint Surg. 2001;83-A(suppl 2, pt 2):80-86.
     
  8. Haman JD, Bechtol N, Jacobs JJ, Rosenberg AG, Berger RA, Wimmer MA. Tibial post damage in TKA’s is associated with tibial plateau damage . Paper presented at: 49th Annual Meeting of the Orthopaedic Research Society; 2003.
     
  9. Banks SA, Harman MK, Hodge WA. Mechanism of anterior impingement damage in total knee arthroplasty. J Bone Joint Surg. 2002;84-A(2):37-42.
     
  10. Puloski SKT, McCalden RW, MacDonald SJ, Rorabeck CH, Bourne RB. Tibial post wear in posterior-stabilized total knee arthroplasty: an unrecognized source of polyethylene debris. J Bone Joint Surg. 2001;83-A:390-397.
     
  11. Yao JQ, Blanchard CR, Lu X, et al. Improved resistance to wear, delamination and posterior loading fatigue damage of electron beam irradiated, melt-annealed, highly crosslinked UHMWPE knee inserts. Standard Technical Publication 1445. ASTM International. 2004.
     
  12. Yao JQ, Gsell R, Laurent MP, Gilbertson LN , Swarts D, Blanchard CR, Crowninshield RD. I mproved delamination resistance of melt-annealed electron-beam irradiated highly crosslinked UHMWPE knee inserts. 28th Annual Meeting Transactions, Society for Biomaterials. 2002:60.
     
  13. Yao JQ, Lu X, Laurent MP, Gilbertson LN, Crowninshield RD. Pitting/delamination resistance of melt-annealed highly crosslinked UHMWPE . 27th Annual Meeting Transactions, Society for Biomaterials. 2001:482.
     
  14. Maher SA, Furman BD, Wright TM. Reduced fracture toughness of enhanced cross-linked polyethylene is not associated with increased wear damage. 28th Annual Meeting Transactions, Society for Biomaterials. 2002:542.
     
  15. ASTM Metal Handbook: Specifications for Hot-Rolled and Cold-Finished Zirconium and Zirconium Alloy Bars, Rod, and Wire Products. 1990.
     
  16. Niki Y, Matsumoto H, Otani T, Yatabe T, Kondo M, Yoshimine F, Toyama Y. Screening for symptomatic metal sensitivity: a prospective study of 92 patients undergoing total knee arthroplasty. Biomaterials. 2005;26:1019-1026.
     
  17. Hallab N, Merritt K, Jacobs JJ. Metal sensitivity in patients with orthopaedic implants. J Bone Joint Surg. 2001;83-A(3):428-436.