PRESSURE-VOLUME RELATIONSHIPS IN EQUINE MIDCARPAL JOINT

The effects of joint angle, fluid infusion, history-dependence, and ti me dependence on the pressure-volume (PV) relationships of normal equi ne midcarpal joints were determined. Horses (n = 24 and 48 midcarpal j oints) 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 Fluid (synovial fluid or saline) was infused or withdrawn through the dorsal joint capsule.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, wher e B is the fractional change in pressure per unit change of volume, an d A and C are constants. Compartmentation was not observed. Elastance was greater at sub- than supra-atmospheric pressures, at 90 degrees th an 135 degrees angles, and with saline than synovial fluid. Hysteresis was greater at 90 degrees than 135 degrees angle, and with synovial f luid than saline. Elastance progressively increased with sequential di stention at high IAPs. IAP relaxation was a positive logarithmic relat ionship of IAP. These findings suggest an important role of synovial f luid in articular PV relationships and emphasize the role of joint ang le, prior distention cycles, and decay of IAP with time in future stud ies investigating these phenomena.