Abstract
The temporal change of the direction of sliding relative to the ultrahigh molecular weight polyethylene (UHMWPE) component of prosthetic joints is known to be of crucial importance with respect to wear. One complete revolution of the resultant friction vector is commonly called a wear cycle. It was hypothesized that in order to accelerate the wear test, the cycle frequency may be substantially increased if the circumference of the slide track is reduced in proportion, and still the wear mechanisms remain realistic and no overheating takes place. This requires an additional slow motion mechanism with which the lubrication of the contact is maintained and wear particles are conveyed away from the contact. A three-station, dual motion high frequency circular translation pin-on-disk (HF-CTPOD) device with a relative cycle frequency of 25.3 Hz and an average sliding velocity of 27.4 mm/s was designed. The pins circularly translated at high frequency (1.0 mm per cycle, 24.8 Hz, clockwise), and the disks at low frequency (31.4 mm per cycle, 0.5 Hz, counter-clockwise). In a 22 million cycle (10 day) test, the wear rate of conventional gamma-sterilized UHMWPE pins against polished CoCr disks in diluted serum was 1.8 mg per 24 h, which was six times higher than that in the established 1 Hz CTPOD device. The wear mechanisms were similar. Burnishing of the pin was the predominant feature. No overheating took place. With the dual motion HF-CTPOD method, the wear testing of UHMWPE as a bearing material in total hip arthroplasty can be substantially accelerated without concerns of the validity of the wear simulation.
Discussion
For the first time, a substantially accelerated, multidirectional pin-on-disk study was performed with the most common material combination used in prosthetic joints, conventional gamma-inert-sterilized UHMWPE against polished CoCr, so that no overheating took place and the wear mechanisms were realistic. The increase of the lubricant temperature (4 1C) relative to the environment temperature was moderate, and it was in line with that measured in the 12-station CTPOD device (Saikko, 1998). Burnishing is a phenomenon typical of retrieved UHMWPE prosthetic components (Kurtz, 2009; McKellop et al., 1995). The absence of overheating was attributable to the fact that the average sliding velocity, 27.4 mm/s, was not excessive with respect to actual prosthetic joints. The sliding velocity varied between 9.1 mm/s and 40.5 mm/s which is a range typical of contemporary prosthetic hips (Calonius and Saikko, 2003). The average sliding velocity was actually lower than the constant sliding velocity in the established 1 Hz CTPOD devices, 31.4 mm/s (Saikko, 1998; Saikko, 2005). With the dual CTPOD motion, the lubrication of the contact was maintained, and wear debris was conveyed away from the contact which was not miniaturized but was of real size, 63.6 mm2, and still the test was highly accelerated with respect to the cycle frequency, 25.3 Hz. An additional advantage of the present test protocol was that the effect of temporal degradation of serum was reduced as the lubricant change interval was 24 h only.
The most commonly used cycle frequency in wear testing of orthopaedic biomaterials is 1 Hz (Affatato et al., 2008; Kurtz, 2009). A frequency of 2 Hz, resulting in an average sliding velocity of 60 mm/s, has been used in pin-on-disk testing (Bragdon et al., 2001). With a hip simulator also, 2 Hz has been used (Bragdon et al., 1996). In these two studies, the wear of UHMWPE with 2 Hz was close to that produced with 1 Hz. An exceptional 5 Hz singleaxis, water lubricated hip simulator test has been done (Pappas et al., 1995), but the maximum sliding velocity in the test apparently was close to 200 mm/s, which is excessive. In 1 Hz testing, efficiency can be increased by large capacity, such as 100 test stations (Saikko, 2005). Another way of increasing the efficiency is to increase the cycle frequency, as shown in the present study.
With similar specimens, load and lubricant, the mean wear rate with 1 Hz frequency in the SuperCTPOD test (sliding velocity constant 31.4 mm/s) was 3.40 mg/106 cycles (n¼100), corresponding to 0.29 mg per 24 h, and the mean wear factor was 1.63 106 mm3/Nm (Saikko, 2005). Compared with these results, the wear per 24 h with dual motion in the present study was 6.1 times higher, the wear factor was 7.1 times higher, whereas the wear rate expressed as mg/106 cycles was one fourth. This indicates that the short motion is not as efficient in completely removing the wear particle from the pin wear surface as the large motion is, but still the wear in a unit time and the wear factor were substantially increased. It is possible that some of the particles removed by the short motion remained between the contacting surfaces and were not readily conveyed into the lubricant bath, and thus they reattached and did not reduce the weight of the UHMWPE pin. The importance of the large motion in conveying wear debris away from the contact was proven by the test with short motion only which showed a wear rate of an order of magnitude lower than that with dual motion. The type of wear with short motion only (Fig. 6) indicated local deficiency of lubrication. In addition, occasional, subtle scratches were observed, the diameter of which was close to the designed slide track diameter, 0.32 mm, of the short circular translation, which proved that the drive mechanism worked as intended.
A limitation of the study was that only one material combination was tested. The applicability of the novel HF-CTPOD method to combinations other than conventional UHMWPE against polished CoCr needs to be evaluated with additional studies in the future.