TRIBOLOGICAL EVALUATION OF PMPC-GRAFTED HYDRATED BEARING SURFACE USING MULTIDIRECTIONAL PIN-ON-DISK TESTER

The modification of bearing surfaces with poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) is known to increase the hydration of the surfaces and decrease the wear of the substrates. PMPC grafting to acetabular liner of total hip arthroplasty showed a drastic reduction of cross-linked polyethylene (CLPE) wear in a long-term hip simulator test and achieved a good short-term clinical result. To apply this technique to other joint prostheses, the wear resistance under various conditions needs to be evaluated because every joint has a different wear mode. ASTM F732 gives a method that disk shaped polymer specimen is loaded with hemispherical pin using pin-on-disk tester, which is suitable for hydrated polymer because the lubricant is supplied every loading cycle on the surface. The purpose of this study is to evaluate the performance of PMPC-grafted hydrated CLPE under multidirectional wear condition in anticipation of applying PMPC to various prostheses. The CLPE disks of 3 or 6-mm in thickness were machined from a bar stock. The PMPC was grafted onto the CLPE surfaces using a photoinduced polymerization of MPC in aqueous medium. All disks were irradiated with a total amount of 75-kGy gamma-ray. The wear resistance of the CLPE and PMPC-grafted CLPE disks against Co-Cr-Mo alloy pin was evaluated using Ortho-POD pin-on-disk tester. The disks were fixed to the tester with a Ti-6Al-4V alloy plate that has screw hole in the center. The test conditions were a static load of 213 N, sliding shape of 5 mm × 10 mm rectangular, frequency of 1 Hz and maximum cycles of 1.0 × 10 6 [Fig. 1]. Gravimetric wear was determined by weighing the disks and soak controls were used to compensate for the fluid absorption. After the wear test, volumetric changes of sliding and backside surfaces of disks were evaluated using a noncontact optical three-dimensional profiler. The PMPC-grafted surface showed decrease in the gravimetric wear drastically [Fig. 2]. The thickness of CLPE had no substantial effect on the wear resistance. Three-dimensional profile measurements of sliding surfaces detected a substantial volumetric penetration; the corner of sliding track were deeper than the straight-line portion. Backside extrusion was observed in all disks. The thickness of CLPE affected both volumetric penetration and backside extrusion for both untreated and PMPC-grafted CLPE. The PMPC grafting had no discernible effect on volumetric changes [Fig. 3]. Results of this study revealed: (1) the PMPC-grafted surface decreases wear of CLPE, however, the thickness of disk has no effect, in contrast, (2) thinner thickness of CLPE increases the volumetric changes including penetration in sliding surface and extrusion in back surface but the PMPC-grafted surface has no effect. Gravimetric wear did not correlate with the volumetric penetration in sliding surface because the volumetric penetration might be caused by not only the wear but also the creep deformation. In conclusion, hydrated bearing surface and thickness of bearing substrate are essential for the wear and fatigue resistance properties for an increasing longevity of artificial joint. In addition, PMPC grafting is a promising technique for increasing the longevity of various joint prostheses.