Range-of-motion affects cartilage fluid load support: functional implications for prolonged inactivity.

Joint movements sustain cartilage fluid load support (FLS) through a combination of contact migration and periodic bath exposure. Although there have been suggestions that small involuntary movements may disrupt load-induced exudation during prolonged inactivity, theoretical studies have shown otherwise. This work used well-controlled explant measurements to experimentally test an existing hypothesis that the range-of-motion must exceed the contact length to sustain non-zero FLS. Smooth glass spheres (1.2-3.2 mm radius) were slid at 1.5 mm/s (Péclet number >100) against bovine osteochondral explants under varying normal loads (0.05-0.1 N) and migration lengths (0.05-7 mm) using a custom instrument. In situ deformation measurements were used to quantify FLS. Non-zero FLS was maintained at migration lengths as small as 0.05 mm or <10% the typical contact diameter. FLS peaked when track lengths exceeded 10 times the contact diameter. For migration lengths below this threshold, FLS decreased with increased contact stress. Migration lengths far smaller than the contact diameter can sustain non-zero FLS, which, from a clinical perspective, indicates that fidgeting and drifting can mitigate exudation and loss of FLS during prolonged sitting and standing. Nonetheless, FLS decreased monotonically with decreased migration length when migration lengths were less than 10 times the contact diameter. The results demonstrate: (1) potential biomechanical benefits from small movement (e.g., drifting and fidgeting); (2) the quantitative limits of those benefits; (3) and how loads, movement patterns, and mobility likely impact long term FLS.

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