Measurement of synovial fluid volume: a new dilution method adapted to fluid permeation from the synovial cavity

A new dilution method was developed to measure the synovial fluid (SF) volume in a single knee joint of a rabbit. In this dilution method, we used 2 different kinds of dextrans, one having a mean molecular weight (M(r)) of 487,000 and labelled with fluorescein isothiocyanate (FITC) as a marker, and the other M(r) of 300,000 to give hypertonicity to the solution. By employing these high M(r) dextrans in their optimal concentrations, we were able to minimize the inevitable transsynovial shift of the fluid and marker after intraarticular injection. The original SF volume was then calculated using the fluorescence intensities of the injected and retrieved fluids. By determining the accurate volume of SF, it became possible to measure per joint the protein and hyaluronic acid contents in SF as well as the total and differential cell counts. The results obtained by this new method were reasonable and consistent, except for volume, with the reported results based on pooled samples from multiple joints. We believe that the method can contribute to the analysis of SF changes under various conditions in small laboratory animals.