Joint fluid hydrostatic pressures that empty synovial capillaries of red cells have a wide range and correlate with pressures emptying arterioles.

Abstract

To study if, as predicted by calculations based on 3 Starling pressures, the fourth one, capillary pressure, may increase enough to contribute to joint swelling. At magnifying arthroscopy, red cells mostly flow separated from each other in capillaries and precapillary vessels (arterioles). These are emptied of erythrocytes at intraarticular injection of saline to "tamponade" pressures. Patients sent for arthroscopy of the knee joint for mechanical or other lesions, or "internal derangement" (n = 30) or osteoarthritis (n = 26) were studied. The range of capillary (including postcapillary venular) tamponade pressure was 9-70 mm Hg. It correlated (r = 0.536, p < 0.001) with arteriolar tamponade pressure (range 20-100 mm Hg). If intravascular hydrostatic pressure is the main determinant of tamponade pressure, then increase of capillary pressure, possibly due to arteriolar dilatation induced precapillary pressure increase, may be a key mechanism in enhanced formation of synovial fluid.