Current and Future Diagnostic Means to Better Characterize Osteoarthritis in the Horse—Routine Synovial Fluid Analysis and Synovial Fluid and Serum Markers

Examination of the synovial fluid should be a routine procedure in the evaluation of arthritic conditions, as it can provide valuable information in addition to that gained by clinical and radiologic examination. Conventional analysis will not provide a specific diagnosis; however, it does give an indication of the degree of synovitis and metabolic derangement within the joint. It is more specific in the diagnosis of infective arthritis as this condition causes the parameters of protein and white cell counts to go beyond a level encountered with other inflammatory conditions. Some basic information on synovial fluid is necessary to appreciate the changes that occur in joint disease. Synovial fluid is a unique tissue fluid. The distribution of electrolytes and most nonelectrolytes between plasma and joint fluid is in accord with the Gibb-Donnan equilibrium concept and indicates that it is mainly a dialysate of plasma with hyaluronan added to it. The intercellular space between the synoviocytes in the synovial membrane acts as an important permeability barrier in this filtration process. The source of hyaluronan is the synovial membrane. Studies of the molecular structure of hyaluronan indicate that it is arranged in a random coil with a moderate degree of stiffness. Hyaluronan provides synovial fluid with a number of unique properties. It imparts a high viscosity to the fluid. It acts as a boundary lubricant for the synovial membrane. There is evidence that the hyaluronan in the synovial fluid also influences the further composition of the fluid. Data suggest that steric hindrance by hyaluronan may obstruct solute passage through the water surrounding the molecules. In this concept of excluded volume, the size and shape of the molecules presented play an important role. Small molecules are allowed through, whereas large ones, such as fibrinogen, are excluded. With this concept, the quantity and physical state of hyaluronan produced under pathologic conditions may well be the primary determinant of