تجزیه و تحلیل عملکرد از سیستم تست سایش POD تصادفی

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% 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.

The type and rate of wear of prosthetic joints strongly depend on the type of relative motion between the articulating surfaces [1]. Laboratory wear tests for orthopaedic biomaterials have shown that the way in which the direction of sliding changes is of fundamental importance [2] and [3]. Circular translation [4] has been shown to produce the highest wear rate together with realistic wear mechanisms [5]. Many different activities take place daily, and the relative motions in a certain activity, say, level walking, do not remain unchanged. Until recently, mainly fixed slide track patterns have been used in wear testing, which undoubtedly differs from the clinical situation [6]. Therefore, there is a growing interest in orthopaedic tribology research to use more diverse motion input in wear simulation studies [7]. A new wear test method and device of a pin-on-disc type, called RandomPOD, was recently described and validated [8]. With a certain type of biomechanically realistic random motion between the pin and the disc, the wear factor of conventional, non-irradiated ultra-high molecular weight polyethylene (UHMWPE) was almost twice that with circular translation. The basic idea in the RandomPOD is that the biaxial random motion includes a wide variety of track features with a view to producing wear that is as realistic as possible. The principle of random motion and random load is unique and it differs fundamentally from all earlier methods used in this field. The main differences between the validation paper [8] and the present study are that this time, the UHMWPE pins were gamma-sterilized, the tests were longer, and the lubricant temperature was lower. Moreover, the sliding velocity of circular translation was halved so that it was equal to the average velocity of random motion. The distribution of the random travel was presented for the first time. The hypothesis was that with the clinically most used bearing material, gamma-sterilized UHMWPE, the mean wear factor with random motion exceeds that obtained with a fixed, multidirectional motion.

With conventional, gamma-sterilized UHMWPE, the complex, yet biomechanically realistic non-cyclic motion and load resulted in a mean wear factor significantly higher than that produced by a fixed slide track and load. The standard deviations of the wear factors (n=16) were low. Under both test conditions, the wear mechanisms were similar to those observed clinically. In the field of orthopaedic tribology, fixed conditions are commonly used in wear testing, but there is a growing interest in using more diverse motion and load input to further improve the wear simulation. The findings of the present study indicate that this trend is well justified.