Synovial capillary distribution in relation to altered pressure and permeability in knees of anaesthetized rabbits

1. The hydraulic conductance of the synovial lining of the rabbit knee increases greatly at intra-articular pressures (IAP) above 9 cmH2O. A structural cause was sought by fixing synovium in situ at less than or equal to 5 cmH2O IAP (ten animals) or 25 cmH2O IAP (five animals) and examining histological sections morphometrically. 2. The synovial lining was found to be a highly deformable sheet of very vascular connective tissue, with 47 x 10(3)-73 X 10(3) capillaries per cm2 section. 150-260 cm2 endothelial surface per cm3 tissue and a vascular volume of 2.4-5.7%. 3. The thickness of the lining averaged 14-19 microns at low IAP and was reduced at high IAP; in suprapatellar synovium, where changes were most marked, thickness fell by 24-47%. The loose subsynovial space expanded. 4. The average distance separating capillary near-edges from the joint cavity approximately halved from 3.75 and 7.47 microns at low IAP (harmonic and arithmetical means respectively) to 1.82 and 3.35 microns at high IAP. Capillaries remained patent and their number density did not change significantly at high IAP. 5. It is concluded that a reduction in the extravascular path length for fluid exchange contributes to the increase synovial conductance at high IAP, but the path length changes were not sufficient to account fully for the conductance changes.