The effect of the volume of effusion, joint position and continuous passive motion on intraarticular pressure in the rabbit knee.

The effect of joint position and continuous passive motion (CPM) on intraarticular pressure and joint compliance was studied in 40 knee joints of 20 rabbits. Increments of 0.5 ml normal saline were injected into the knee joint and the intraarticular pressures were recorded with the knees flexed from 40 degrees to 160 degrees and during CPM. Increasing degrees of flexion of the knee produced significantly higher intraarticular pressures. A sinusoidal oscillation in pressure was observed during CPM. The optimum volume of effusion required to simulate the pressures associated with a hemarthrosis was found to be 2 ml. Viscoelastic stretching of the capsule could be demonstrated prior to synovial rupture which occurred with a mean of 4 ml of injected saline.