The development of a new hip simulator for the study of bearing materials used in total hip replacements has led to several findings which add important new information to the understanding of wear process of ultra-high molecular weight polyethylene, the most commonly used bearing material today for total joint replacements. Using this hip simulator which is capable of applying the physiological motion pathways occurring during gait to total hip components which are held in the correct anatomical position under the complex loading conditions of the hip in gait, the authors have shown that physiological motion pathways produce very different wear rates and morphology of the wear surface than unidirectional reciprocating pathways. Scanning electron microscopy studies show striking differences in the morphology of the wear surfaces of the polyethylene depending upon the relative motions of the components. Wear rates, surface morphology and particle debris generation consistent with clinical and retrieved studies are achieved when physiological conditions are simulated.