|
Morphology of Wear Debris from Non-crosslinked, Gamma-Air Sterilized (Crosslinked) and Gamma-Crosslinked-Remelted Low-Wear Polyethylenes
McKellop, H., Campbell, P., Shen, F.
Abstract
Radiation crosslinking with remelting markedly improves the resistance of UHMW polyethylene to wear and oxidation, but these advantages could be offset if the morphology of the wear debris was changed in a manner that substantially increased its osteolytic potential. One hip simulator study1 reported that, despite having a substantially lower volumetric wear rate, the wear debris from a polyethylene crosslinked at 5 Mrads had a greater volume of small particles and, therefore, greater osteolytic potential than a non-crosslinked polyethylene. However, the accuracy of the volume calculations from that study is suspect, since the total volume of wear debris for the two polyethylenes was about 2 times greater than the volume loss of the respective cups, which is not physically possible. Furthermore, the appropriate control material for assessing the osteolytic potential of a newly modified polyethylene is gamma-air sterilized polyethylene, as used in the vast majority of prosthetic joints during the past 20 to 30 years and on which the reports relating volumetric wear to the incidence and severity of osteolysis have been based.2,3 In the present study, UHMW wear particles were recovered by digesting and filtering the serum lubricant from hip simulator wear tests. The particles were imaged in a scanning electron microscope, and the volume of each particle was calculated by modeling round particles as spheres and fibrils as cones. The total volume of debris per million cycles was plotted as a function of particle size, knowing the total volume loss of the respective cups (determined by weighing), i.e., 41.4, 26.7 and 5.0 mm3 per million cycles for non-crosslinked polyethylene, gamma air-sterilized-crosslinked (historical control), and 5-Mrad-remelted crosslinked polyethylene, respectively. The debris from the non-crosslinked cups contained more than twice the volume of fibrils than debris from the gamma-air cups, and the fibrils were longer (averaging 1.7mm and 1.1 mm, respectively). There were few fibrils in the debris from the 5 Mrad-remelted polyethylene. Relative to gamma-air polyethylene, the size distribution of the round particles was shifted slightly upward with non-crosslinked polyethylene, whereas the debris from the 5 Mrad-remelted poly covered the same size range as that from the gamma-air sterilized historical controls, but with substantially less volume across the range, corresponding to the 82% reduction in total wear. If the relative wear volumes and particle size distributions measured in the present study also occur with hip prostheses in clinical use, the incidence and severity of osteolysis should be substantially reduced with acetabular cups of 5 Mrad-remelted polyethylene.2,3
1) M. Scott et al., Trans ORS, 1, 2001 ; 2) H. McKellop, Assessment of Wear of Materials for Artificial Joints. Chapter 16, The Adult Hip, J. Calaghan, A. Rosenberg and H. Rubash, Eds., Lippencott-Raven, New York, 231-246, 1998. 3) P.C. Oparaugo et al., Acta Orthop. Scand. 72(1), 22-28, 2001
|
