Time-dependence of the pressure-volume relationship in the synovial cavity of the rabbit knee.

The pressure-volume curve of the synovial cavity of the rabbit knee has been determined by the infusion of a non-absorbable oil into the joint space. Pressure (greater than 3 cm H2O) at a given volume decayed as a curvilinear function of time. In consequence, a plot of volume against pressure immediately after a volume increment (the immediate compliance curve) was steeper than a plot of volume against pressure after 5-20 min (the delayed compliance curve) above atmospheric pressure. Similarly, the compliance curve for a rapidly expanded joint was steeper than the compliance curve for the opposite, slowly expanded joint. Observations 2 and 3 indicated that the joint investment was visco-elastic. Pressure at a given volume during aspiration of the expanded joint was always less than pressure at the same volume during infusion. This hysteresis was substantial, the mean energy dissipation being about 37%. Pressure at a given volume increased as a function of time during aspiration, but the pressure recovery was less pronounced than pressure decay during infusion. When the cycle of infusion and aspiration was repeated, the pressure-volume hysteresis loop was displaced progressively towards the volume axis. Observations 5 and 6 were attributed to plasticity of the joint investment. It is concluded that intra-articular pressure at a given volume depends on the rate of change of volume, the direction of change, and the previous history of the joint. The relevance of these properties to fluid dynamics in other interstitial compartments, in arthritic human joints, and to the neurophysiology of mechanoreceptors in the joint investment is discussed.