Effects of Counterface Roughness and Conformity on the Tribological Performance of Crosslinked and Non-crosslinked Medic
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Effects of Counterface Roughness and Conformity on the Tribological Performance of Crosslinked and Non-crosslinked Medical-Grade Ultra-High Molecular Weight Polyethylene A. D. Chawan,1 A. M. Chakravartula,1 J. Zhou,1 L. A. Pruitt,1,3 M. Ries,2 and K. Komvopoulos1 1
Department of Mechanical Engineering, University of California, Berkeley, CA 94720 Department of Orthopaedics, University of California, San Francisco, CA 94131 3 Department of Bioengineering, University of California, Berkeley, CA 94720 2
ABSTRACT The tribological behavior of crosslinked ultra-high molecular weight polyethylene (UHMWPE) was compared to that of non-crosslinked UHMWPE, used as control sample. A reciprocating pin-on-disk tribometer was used to determine the effects of countersurface roughness and conformity on wear mechanisms occurring during the initial stage of sliding. Pin samples of two different radii of curvature were slid against medical-grade Co-Cr alloy disks with surface roughness ranging from 0.005 to 0.04 µm in a lubricant of bovine serum. Normal loads were chosen to provide physiological contact stresses. The focus of this study was on the dependence of early wear mechanisms on surface roughness and conformity. Although a correlation between coefficient of friction data and dominant wear mechanisms was not observed, different wear mechanisms were found between control and crosslinked UHMWPE. The results of this study provide insight into the differences of the initial wear behavior of noncrosslinked and crosslinked UHMWPE used in total joint replacements. INTRODUCTION Ultra-high molecular weight polyethylene (UHMWPE), articulating against a ceramic or metal counterface, is the prime load-bearing material system in most total joint replacements. While the intrinsic mechanical properties of UHMWPE make this polymer well suited for this purpose, methods to further improve the in vivo mechanical behavior of UHMWPE are still under development, the principal objective being to reduce the amount of submicron-sized polymer wear particles generated through joint motion. Accumulation of such fine wear debris elicits a foreign body response leading to osteolysis, and eventually to loosening of the prosthesis necessitating revision surgery. Increasing the wear resistance of the polymer surface is thus a high priority in the area of orthopaedics research, as it can be correlated to an increase of the expected life of total joint replacements. UHMWPE is a semicrystalline polymer. In its non-crosslinked form it is about 50% crystalline. The amorphous phase is well above its glass transition temperature, rendering the polymer susceptible to chain orientation or texture development under contact sliding conditions. Polymer chains in bulk UHMWPE exhibit random orientation. Chain crosslinking through chemical means or radiation results in decreased crystallinity (~20%) and inhibits polymer chain reorientation near the surface in a direction parallel to the direction of sliding. Since such chain re-arrangement is the precursor to wear in non-cro
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