Physiological compartmentation of fluid within the synovial cavity of the rabbit knee.

1. The relationship between pressure in the suprapatellar region (Psp) of the synovial cavity of the rabbit knee, and volume of fluid (oil or saline) infused into that region displayed a pressure plateau (15 . 5 cm H2O) between 1 . 2 and 2 . 0 ml. 2. No pressure plateau occurred when fluid was infused simultaneously into the suprapatellar and posteromedial regions of the synovial space. 3. Pressure in the posteromedial region (Ppm) did not respond to suprapatellar fluid infusions until onset of the suprapatellar pressure plateau. During the plateau phase, Ppm rose steadily towards Psp. At the end of the plateau phase the two pressures were almost equal and rose in parallel. 4. The plateau phenomen also occurred during aspiration of volume-expanded joints; and was present at all joint angles. 5. It was concluded that the joint space, although anatomically continuous, is divided into two hydraulically separate compartments at physiological pressures. 6. The sites of communication between the two compartments at pathological pressures were explored by casts of the synovial cavity.