The modes and competing rates of cartilage fluid loss and recovery.

ABSTRACT

Cartilage loses, recovers, and maintains its thickness, hydration, and biomechanical functions based on competing rates of fluid loss and recovery under varying joint-use conditions. While the mechanics and implications of load-induced 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 static unloaded contact area; (2) free swelling - unrestricted fluid recovery by 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 500 µm of compression, passive swelling, free swelling, and tribological rehydration exhibited average rates of 0.11 ± 0.04, 0.71 ± 0.15, and 0.63 ± 0.22 µm/s, respectively, over the first 100 s of recovery; for comparison, the mean exudation rate just prior to sliding was 0.06 ± 0.04 µm/s. For this range of compressions, we detected no significant difference between free swelling and tribological rehydration rates. However, free swelling and tribological rehydration rates, those associated with joint articulation, were ∼7-fold faster than passive swelling rates. While previous studies show how joint articulation prevents fluid loss indefinitely, this study shows that joint articulation reverses fluid loss following static loading at >10-fold the preceding 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 activity. STATEMENT OF SIGNIFICANCE: Cartilage loses, recovers, and maintains its thickness, hydration, and biomechanical functions based on competing rates of fluid loss and recovery under varying joint-use conditions. While load-induced fluid loss is extremely well studied, this is the first to define the competing modes of fluid recovery and to quantify their rates. The results show that the fluid recovery modes associated with joint articulation are 10-fold faster than exudation during static loading and passive swelling during static unloading. The results suggest that joint space and function are best maintained throughout an otherwise sedentary day using brief but regular physical activities.