Performance analysis of the RandomPOD wear test system

Abstract

The type of relative motion between the bearing surfaces of prosthetic joints is known to strongly influence their wear behaviour. The previously validated 16-station wear simulator of the pin-on-disc type, called RandomPOD, was used to study the wear of a conventional, gamma-sterilized ultra-high molecular weight polyethylene (UHMWPE). The counterface was polished CoCr and the lubricant was diluted calf serum. Two test conditions were compared, random motion/random load and circular translation/static load. With random motion, the accumulated change of the direction of sliding was 2.8 times higher than that with circular translation. The test duration with both test conditions was 880 h. Random motion/random load resulted in a mean wear factor 23 per cent higher than that produced with circular translation/static load. The difference was statistically significant. The wear mechanisms however were similar and in agreement with clinical observations. As earlier studies have shown that the type of load is of secondary importance, the present study confirms the earlier findings that the type of relative motion is tribologically of fundamental importance. In particular, the complex, yet biomechanically realistic non-cyclic motion, represented by the random track, resulted in a wear factor significantly higher than that produced by a fixed slide track shape.

Effect of CoCr Counterface Roughness on the Wear of UHMWPE in the Noncyclic RandomPOD Simulation

“…Although the average sliding velocity in the RandomPOD, 15.5 mm/s, is relatively low, the cumulative change of the direction of sliding is high, 500°/s on the average. The wear factors were significantly higher compared with those obtained with cyclic input, while the wear mechanisms were largely similar [2,3,5]. The multidirectionality of the relative motion is known to be of fundamental importance with respect to the UHMWPE wear [6].…”

“…The computer-controlled, 16-station RandomPOD wear test system, the test procedure and the test specimens for hip and knee wear simulation have been described elsewhere [1][2][3][4].…”

“…They represented fresh material, i.e., they were neither shelf nor artificially aged. Both the hip wear simulations (flat-on-flat, 8 test stations) [1,2], and the knee wear simulations (ball-on-flat, 8 test stations) [3] were performed 6 simultaneously.…”

“…It was shown in earlier RandomPOD (random motion and load pin-on-disk) studies that valid simulations of wear mechanisms of both the prosthetic hip and the prosthetic knee can be performed with the same motion and load input provided that the shapes of the specimens are designed appropriately [1][2][3][4]. The design principle of the RandomPOD was such that certain limits were set for the range of motion, velocity, acceleration and its derivative, and for the rate of change of the load so that instead of producing different activities sequentially, all biomechanically and tribologically relevant activities are produced at once.…”

“…1) were found to be suitable for the simulation of the wear mechanisms of ultra-high molecular weight polyethylene (UHMWPE) as used for the bearing surfaces of the total hip and the total knee prostheses, respectively [1][2][3][4]. The non-cyclic characteristics of the motion input signals that resulted in increased multidirectionality of the biaxial relative motion was shown to influence the wear behavior.…”