VASCULAR AND TRANSSYNOVIAL FORCES OF THE ISOLATED STATIONARY EQUINE JOINT

Objective-To provide quantitative assessment of forces affecting filtr ation of synovial fluid in response to incremental changes in arterial and venous hemodynamics. Animals-7 clinically normal adult horses. Pr ocedure-Using a stationary, isolated metacarpophalangeal joint prepara tion, blood flow (Qa(cir)), tissue perfusion, arterial pressure (Pa-ci r), venous pressure (Pv(cir)), transsynovial fluid flow, total vascula r resistance, vascular compliance, and tissue compliance were evaluate d before and after arterial and venous pressure manipulations. At isog ravimetric conditions, pre- and postcapillary resistance and ratios, o smotic reflection coefficient (sigma(d)), capillary pressure, net filt ration pressure, and transitional microvascular pressure were determin ed. Results-Synovial tissue blood flow was similar before, immediately after, and 3.5 hours after joint isolation. The sigma(d) for the join t was low, owing to the high oncotic pressure of synovial fluid at fil tration-independent states. Transsynovial flow occurred in preference to lymph flow because of the high permeability of synovial tissue (low sigma(d)). Synovial fluid production and transfluid flow (synovium we ight gain) increased at Pa-cir > 200 mm of Hg, indicating a threshold phenomenon for synovial filtration. Net filtration pressure > 6 mm of Hg is needed to effect an increase in synovial fluid flow, and pressur e of approximately 11 mm of Hg is necessary to increase lymphatic flow . Vascular compliance in the joint was low, but increased markedly wit h Pv(cir). Vascular and tissue compliance increased with increased Pa- cir. Vascular compliance changes caused by increased arterial pressure were minimal, compared with those caused by increased venous pressure owing to the greater elastance of arteries and the larger muscular ar terial wall. Conclusion-This isolated joint preparation permitted eval uation of codependent hemodynamic, microvascular, and transsynovial fl ow responses to hemodynamic manipulations. Synovial tissue permeabilit y was markedly affected by increased vascular forces altering filtrati on pressures toward synovial fluid production.