Evaluation of the superficial characteristics of articular cartilage using evanescent waves in the friction tests with intermittent sliding and loading.

Articular cartilage plays an important role in the lubrication of synovial joints because of its peculiar characteristics. In this work, the frictional and superficial characteristics of articular cartilage were evaluated simultaneously during intermittent sliding and loading. The apparatus used for the analysis of the articular surface was based on the evanescent waves, where a laser light was reflected at the interface between a prism and a specimen of articular cartilage. Friction forces were measured due to the sliding of specimens on the prism. Images of reflected light were analyzed and attenuation of the reflectance was associated with the presence of collagen fibers near the interface, which interacted with the evanescent waves because of the high refractive index of these fibers. Specimens were tested in the intervals of 5.5 min with an interruption of 10 and 30 s in the sliding and loading. Results indicated a decrease in the both friction coefficient and attenuation of reflectance after the unloading. The level of reduction of the friction as well as of the attenuation of reflectance increased as the time of unloading increased. Decrease of friction after unloading was related to the decrease of collagen contents, or increase in water contents, at the articular surface, which was observed through the decrease of the attenuation of reflectance. Results indicated that the increase in the water content at the articular surface and the rehydration ability of articular cartilage after unloading could be responsible for the maintenance of friction in low levels.