The Modes and Competing Rates of Cartilage Fluid Loss and Recovery

Abstract

Cartilage loses, recovers, and maintains its thickness, hydration, and biomechanical functions by adjusting relative rates of load-induced fluid loss and competitive fluid recovery under varying joint-use conditions. While the mechanics and implications of fluid loss have been studied extensively, those of fluid recovery have not. This study isolates, quantifies, and compares rates of cartilage recovery from three known modes: (1) passive swelling – fluid recovery within a nominally unloaded contact; (2) free swelling – fluid recovery at an exposed surface; (3) tribological rehydration – fluid recovery within a loaded contact area during sliding. Following static loading of adult bovine articular cartilage to between 100 and 250 μm of compression, passive swelling, free swelling, and tribological rehydration rates were 0.18 ± 0.08 μm/s, 1.7 ± 0.3, and 1.8 ± 0.6 μm/s, respectively; the corresponding exudation rate was 0.06 ± 0.04 μm/s. For this range of compressions, we detected no significant difference between passive swelling and exudation rates. However, free swelling and tribological rehydration rates, those associated with joint articulation, were 10-fold faster than passive swelling and exudation rates. While previous studies show how joint articulation prevents fluid loss indefinitely, this study shows how quickly joint articulation reverses fluid loss following static loading (at 30:1 the exudation rate). These competitive recovery rates suggest that joint space and function may be best maintained throughout an otherwise sedentary day using brief but regular physical activities (e.g. a 5-minute walk).