Pressure-volume relationships in equine midcarpal joint.

The effects of joint angle, fluid infusion, history-dependence, and time dependence on the pressure-volume (PV) relationships of normal equine midcarpal joints were determined. Horses (n = 24 and 48 midcarpal joints) were anesthetized and placed in dorsal recumbency, and the four midcarpal joint pouches were cannulated for intra-articular pressure (IAP) measurements and recording. Fluid (synovial fluid or saline) was infused or withdrawn through the dorsal joint capsule. The PV curves were sigmoid and best described by IAP = A x e(B x volume) - C, where B is the fractional change in pressure per unit change of volume, and A and C are constants. Compartmentation was not observed. Elastance was greater at sub- than supra-atmospheric pressures, at 90 degrees than 135 degrees angles, and with saline than synovial fluid. Hysteresis was greater at 90 degrees than 135 degrees angle, and with synovial fluid than saline. Elastance progressively increased with sequential distention at high IAPs. IAP relaxation was a positive logarithmic relationship of IAP. These findings suggest an important role of synovial fluid in articular PV relationships and emphasize the role of joint angle, prior distention cycles, and decay of IAP with time in future studies investigating these phenomena.