An investigation into the validity of subatmospheric pressure recordings from synovial fluid and their dependence on joint angle.

1. Synovial fluid hydrostatic pressures were measured in normal knee (stifle) joints of anaesthetized rabbits using perforated cannulae. Pressures were subatmospheric in seventy out of seventy-two joints, with a mean value of -4.6 cm H2O (range 0 to -12 cm H2O) at joint angles of 120--150 degrees. 2. Similar values were obtained by a wick-in-needle technique (mean -4.0 cm H2O), which along with several other tests indicated that the subatmospheric values were not artifactual. 3. A slow rise in pressure of 1--2 cm H2O per hour in the motionless joint was attributed to a net filtration of fluid into the joint space. 4. Pressure increased as a curvilinear function of joint angle when the joint was flexed passively, the slope of the relationship depending on synovial fluid volume. Pressure also increased on active flexion of the joint, or on passively increasing the tension of soft peri-articular tissues. 5. Positive (above atmospheric) pressures in acutely flexed joints declined with time. Pressures declined less rapidly when synovial fluid was replaced by nonabsorbable paraffin oil. It was concluded that joint fluid can be absorbed by the synovium during acute flexion. 6. Processes which might generate subatmospheric pressures are discussed. The hypothesis is advanced that the flexion-dependent 'trans-synovial pump', possibly in series with a lymphatic pump, may account for the maintenance of a small synovial fluid volume and subatmospheric pressure in the face of a net filtration of fluid from synovial capillaries into the joint space.