Synovial fluid--its mass, macromolecular content and pressure in major limb joints of the rabbit

Synovial fluid from rabbit elbow, shoulder, hip and knee was analysed to establish the normal levels of parameters relevant to fluid exchange viz. fluid mass per joint, hydraulic pressure, temperature, colloid osmotic pressure (COP), protein, albumin and glycosaminoglycan (GAG) concentrations. Fluid mass was greatest in the least congruous joint, the shoulder (43 +/- 4 mg), cf. 6 +/- 2 mg in the highly congruous hip. In the knee (24 +/- 4 mg) the mean thickness of the fluid layer was calculated to be 30 microns. Fluid pressure was subatmospheric in all joints (mean -2.8 +/- 0.4 cmH2O, elbow, to -5.7 +/- 0.3 cmH2O, knee), as in many other interstitial spaces. Colloid osmotic pressure was substantial (mean 11.4 +/- 0.9 cmH2O, shoulder, to 13.1 +/- 1.0 cmH2O, elbow), being 40-46% of serum level. Comparison of synovial fluid results with COP versus concentration curves in vitro indicated that the fluid's COP was primarily generated by its protein content (22-30 g l-1, 64% albumin) rather than GAG (4.0-5.8 g l-1). The GAG was 95% hyaluronate and 5% sulphated GAG. Algebraic summation of the hydraulic and colloid osmotic pressures of synovial fluid and serum indicated a net filtration gradient from blood to joint cavity. When serum COP was reduced by intravenous saline infusion, synovial fluid mass increased, in accordance with the ultrafiltration hypothesis of synovial fluid formation. The fluid's colloid concentration declined as volume increased. The relation was not a simple dilution curve, but indicated that the newly formed synovial fluid contained greater than or equal to 8 g protein l-1 (14.5% serum concentration); and that hyaluronate was entering the synovial cavity at a rate of greater than or equal to 6.5 micrograms h-1 per joint.