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Concentration polarization of hyaluronan on the surface of the synovial lining of infused joints

Hyaluronan (HA) in joints conserves the lubricating synovial fluid by making trans‐synovial fluid escape almost insensitive to pressure elevation (e.g. effusions, joint flexion). This phenomenon, ‘outflow buffering’, was discovered during HA infusion into the rabbit knee joint cavity. It was also found that HA is partially reflected by the joint lining (molecular sieving), and that the reflected fraction R decreases as trans‐synovial filtration rate Q is increased. It was postulated therefore that outflow buffering is mediated by HA reflection. Reflection creates a HA concentration polarization layer, the osmotic pressure of which opposes fluid loss. A steady‐state, cross‐flow ultrafiltration model was previously used to explain the outflow buffering and negative R‐vs.‐Q relation. However, the steady‐state, cross‐perfusion assumptions restricted the model's applicability for an infused, dead‐end cavity or a non‐infused joint during cyclical motion. We therefore developed a new, non‐steady‐state model which describes the time course of dead‐end, partial HA ultrafiltration. The model describes the progressive build‐up of a HA concentration polarization layer at the synovial surface over time. Using experimental parameter values, the model successfully accounts for the observed negative R‐vs.‐Q relation and shows that the HA reflected fraction (R) also depends on HA diffusivity, membrane area expansion and the synovial HA reflection coefficient. The non‐steady‐state model thus explains existing experimental work, and it is a key stage in understanding synovial fluid turnover in intact, moving, human joints or osteoarthritic joints treated by HA injections.