Abstract
Statement of Purpose:
The wear rate of Ultra High Molecular Weight Polyethylene (UHMWPE) in joint replacements has been correlated to both contact area and contact stress in the literature, [1], [2]. In both publications and our experiment, UHMWPE articulated with a polished surface of cobalt-chromium alloy was evaluated using a Pin-On-Disk (POD) apparatus (AMTI) implementing bi-directional movement.
In publication [1], volumetric wear was independent of normal load and dependent upon increasing contact area. The results demonstrated that increasing contact stress decreased wear rates twofold. In publication [2], at maximum cross-shear, wear was proportional to nominal contact area and wear factors normalized to area are more appropriate than load based wear factors. In both studies, the contact surface areas of the POD pins were reduced by decreasing the diameters of the POD Pins.
In our experiment, the contact area was dependent on textured POD Pin 390 (T390) which had low wear [3]. T390 reduced the normal POD contact area from 71 mm2 to 8.26 mm2. Hydroxylapatite (HA) particles were introduced to the serum to simulate third body wear debris. We hypothesized that the normal POD Pins would have greater wear rates than the textured POD Pins. A measurement of 0.14 mg HA particles per 250 mL of serum was used for each test 0.33 million cycles.
Methods:
The GUR 1020 resin XLK POD Pins were gamma irradiated to 50 kGy in a vacuum package and then remelted. Three (3) T390 POD pins and nine (9) untextured XLK POD Pins were used. Three untextured XLK POD Pins were tested against three T390 POD pins. The other six (6) untextured XLK POD Pins were used as soak controls. Each pin articulated against a polished, high carbon wrought CoCr metal alloy counterface (ASTM F1537; diameter = 38.1 mm; thickness = 12.7 mm). Wear rate tests were for 1.98 million cycles. In order to perform the t-test analysis, the wear rates for each pin were given by the slope of the linear regression line through the individual data points (cycle count, cumulative wear), excluding the (0, 0) point.
Results:
The probability for the means between the T390 POD pins and the untextured XLK POD Pins was *p = 0.009. T390 wear rates were statistically significant as compared to the untextured XLK POD Pin wear rates. The T390 POD Pin is illustrated in Figure 1. Figures 2 and 3 summarize the wear rates between T390 POD Pins and the untextured POD Pins with and without HA particles.
Conclusions:
The wear rates between T390 and untextured POD pins did not take into account that the POD pins were not cleaned using a solution to remove potentially embedded HA particles. The follow-on experiment will use a special cleaning method to remove all HA particles after each test cycle.
