Estimation of joint fluid volume in the knee joint of rabbits by measuring the endogenous calcium concentration

Objectives The purpose of this study was to develop a new simple, and more efficient method for estimating joint fluid volume in the knee of experimental animals by measuring endogenous calcium concentration, which is maintained rigidly at a constant level in the body fluid. Methods Calcium concentrations in the plasma and joint cavity lavage were colorimetrically measured in normal and papain-induced arthritic rabbits. The joint fluid volume was estimated by dividing the total calcium concentration in the joint cavity lavage (μg/joint) by calcium concentration in the plasma (μg/ml). To confirm the relevancy of this method, radioactivity in the plasma and joint cavity lavage was determined after intravenously injecting 3 H 2 O. The correlation between the joint fluid volumes obtained from the two different methods was examined to evaluate the validity of the method involving measurement of the calcium concentration. Results The joint fluid volumes of normal rabbits were estimated at 401 μl/joint and that in the arthritic rabbits at 680 μl/joint by measuring calcium concentration, respectively. These values were not significantly different from those estimated by radioactivity in the plasma and joint cavity lavage (normal: 425 μl/joint, arthritis: 761 μl/joint). A statistically significant correlation was observed between the values obtained from the two methods (r = 0.985). Conclusion This method is considered useful for the evaluation of therapeutic medicine for arthritis or for hydrarthrosis research using experimental animal models.