biotribology

Fluid load support in the migrating contact area: How much migration is necessary?

It is well-accepted that cartilage maintains interstitial fluid load support under long-term joint loading because contact migration leaves insufficient time for fluid exudation. However, it’s also evident that the benefits of migration dissipate as range of motion first approaches the contact length, a situation typical of moving diarthrodial joints, and then zero—typical of static joints. This study aims to elucidate the transition from full fluid load support to zero fluid load support under restricted ranges of motion. Testing was performed on osteochondral plugs using varied probe sizes, loads and track-lengths at Pe >> 1; fluid load support, contact area, and contact stress were quantified in-situ. Fluid load support depended primarily on the migration length per unit contact length (S*) and maintained maximal magnitude (F*=100%) at S* > 10. At S* < 10, it varied as a sigmoidal function of S*, falling to F* = 50% by S* = 0.1 on average. This transition migration length was independent of probe radius and varied slightly, yet significantly with contact area, load, and contact stress over the ranges tested. When migration length approached the contact length, the fluid load support of cartilage fell below that predicted by the established mechanics of migrating contacts. Based on our results, we propose a simple analytical correction that should be used when S*<10. These results demonstrate that fluid retention and load support are impaired by reduced activity and reduced ranges of motion, especially given the relatively short tracks of most joints at full range of motion.
Listed In: Biomechanical Engineering, Biomechanics, Biotribology