Interaction of intraarticular hyaluronan and albumin in the attenuation offluid drainage from joints

Objective. How is fluid volume regulated in joints? Fluid drainage rate is an important factor, both in normal joints and those with effusions, Hyalur onan and albumin, separately, are known to attenuate drainage, conserving s ynovial fluid volume in the presence of raised joint pressure, Hyaluronan a nd albumin normally coexist, however, in joint fluid. The objective was to determine their interactive effect on drainage. Methods. The fluid escape rate from the joint cavity through synovium was m easured at controlled intraarticular pressures using a rabbit knee model in vivo. One joint contained 4 mg/ml hyaluronan and the other contained 4 mg/ ml hyaluronan plus 20 mg/ml albumin, as in normal synovial fluid. Hyalurona n-albumin interactions were assessed in vitro by viscometry and osmometry, Results. Hyaluronan alone greatly attenuated fluid escape. Drainage rates p lateaued at 4-5 mu l/minute as pressure was raised, because the opposition to drainage increased with pressure. Addition of albumin to hyaluronan shif ted the opposition-versus-pressure relation upward and further attenuated d rainage by 22.5%, despite a small fall in the viscosity of the mixture. Osm ometry showed a small synergistic interaction. Analysis of aspirates showed that less than or equal to 8% of albumin molecules in the draining fluid w ere reflected by the synovial lining (compared with 79% of hyaluronan molec ules). Conclusion. Hyaluronan and albumin act together at normal concentrations to conserve synovial fluid in the presence of raised drainage pressures. Hyal uronan has the greater effect, acting osmotically by way of a concentration polarization boundary layer. Attenuation of this effect in arthritic effus ions with low hyaluronan concentrations is one of several factors limiting fluid accumulation and, hence, the size of the effusion.