Pressure induced deformation of the interstitial route across synovium and its relation to hydraulic conductance.

The ease with which fluid passes across the synovial lining (i.e., the lining's hydraulic conductance) is enhanced when intraarticular fluid pressure (IAP) is raised acutely to pathological levels in rabbit knees. A structural basis for this pathophysiological change was sought by morphometry of synovial sections from rabbit knees fixed in situ at less than or equal to 5 cm H2O and 25 cm H2O IAP. Light and electron microscopy showed that the main structural changes induced in areolar synovium by raising IAP to 25 cm H2O were (1) a reduction in synovial thickness to 56% control value; (2) an increase in the area of interstitium exposed at the synovial surface (3) an increased proximity of the synovial capillaries to the joint lumen, the mean distance of capillaries from the surface falling from 8.9 +/- 0.5 microns (n = 391) to 3.3 +/- 0.3 microns (n = 92: p less than 0.001). The capillary profiles showed slight compression under 25 cm H2O IAP, but no collapse. The ratio of interstitial area to thickness is the geometric factor governing hydraulic conductance. The maximum change in interstitial area/thickness was 6.8 times for the blood-joint barrier. A change of this magnitude accounts partly (but not fully) for the experimentally observed conductance changes; and it highlights the importance of capillary depth as a factor governing exchange in joints.