Pressure‐volume relationships above and below atmospheric pressure in the synovial cavity of the rabbit knee.

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Abstract
The pressure-volume [PV] relationship in the synovial cavity of the rabbit knee was investigated by the infusion of a non-absorbable oil into the cavity. The PV curve was sigmoid. Its slope, dP/dV (elastance), was steep at subatmospheric, physiological pressures, least steep at 5 cm H2O and steepened progressively at higher, pathological pressures. dP/dV was a good approximation to linear functions of P below atmospheric pressure and above 5 cm H2O. Each part of the PV relationship could be described by an exponential expression. Flexion of the joint increased pressure at a given volume. This was mainly due to steepening of the PV curve, and also partly due to a shift towards the pressure axis. PV curves determined by the infusion of normal saline or Krebs solution were grossly distorted, because these fluids were absorbed across the permeable synovium. The physiological significance of the synovial PV curve is discussed and mechanical explanations of the curve are suggested.