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

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Abstract
The pressure-volume curve of the synovial cavity of the rabbit knee was determined by the infusion of a nonabsorbable oil into the joint space. Pressure (> 3 cm H20) at a given volume decayed as a curvilinear function of time. Thus, a plot volume against pressure immediately after a volume inerement (the immediate compliance curve) was steeper than a plot of volume against pressure after 5-20 min (the delayed compliance curve) above atmospheric pressure. The compliance curve for a rapidly expanded joint was steeper than the compliance curve for the opposite, slowly expanded joint. 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 .apprx. 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, apparently because of the plasticity of the joint investment. Apparently, intraarticular 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 and in arthritic human joints as well as to the neurophysiology of mechanoreceptors in the joint investment is discussed.