In vitro wear simulation on the RandomPOD wear testing system as a screening method for bearing materials intended for total knee arthroplasty

Abstract

The 16-station RandomPOD wear test system, previously validated for prosthetic hip wear, was used in the simulation of knee wear mechanisms with a ball-on-flat test configuration.This consisted of a CoCr pin with a ground and polished spherical bearing surface (radius 28 mm) against a conventional, gamma-sterilized UHMWPE disk in serum lubrication. The biaxial motion, consisting of x and y translations, and the load were non-cyclic. Relative to the disk, the center of contact wandered within a circle of 10 mm diameter, and the average sliding velocity was 15.5 mm/s (range 0 to 31 mm/s). The load varied non-cyclically between 0 and 142 N (average 73 N). In the 60-day test with 16 similar wear couples, moderate adhesive wear, the principal wear mechanism of a well-functioning prosthetic knee, dominated. This showed as a burnished, circular wear mark (diameter 13.2 mm, area 137 mm 2 ). The wear factor was 2.04 ± 0.03 × 10 -6 mm 3 /Nm (mean ± 95 per cent confidence limit). For the first time a truly multidirectional, realistic and uniform, large capacity pin-ondisk simulation of knee wear mechanisms was implemented.

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.…”