Synovial joints are lined by a thin sheet of specialized require the transport of water, electrolytes, nutrients and mesenchymal tissue called synovium, synovial intima or plasma proteins into the joint cavity from capillaries just synovial lining. This cellular, well-vascularized sheet is only below the synovial surface, and the drainage of fluid, 10-20 #sm thick in the rabbit. Its primary roles are the metabolic end-products, proteins, lubricants and cartilage production of the lubricating synovial fluid and the delivery degradation products from the joint cavity into the subof nutrients to the avascular articular cartilage. These roles synovium, which is a less cellular zone of connective tissue t To whom correspondence should be addressed.

1. When intra-articular pressure is raised to pathological values (> 9 cmH2O) by saline, the hydraulic conductance of the synovial lining increases manyfold. The increase at 25 cmH2O is only partially accounted for by stretching of the tissue and has been ascribed to washout and/or dilution of interstitial matrix biopolymers. This suggestion was tested in this study by sampling synovium from control joints (rabbit knees) and from joints perfused with saline to 25 cmH20, and analysing them quantitatively for collagen, chondroitin sulphate, heparan sulphate and hyaluronan. 2. Pressure and trans-synovial flow measurements showed that in samples taken at 25 cmH2O the conductance of the synovial lining had increased by a factor of 5-23 + 1P5 (mean + S.E.M.) over the conductance at low pressures (just above atmospheric pressure). 3. The tissue concentrations of collagen and the sulphated glycosaminoglycans (GAGs) were reduced by similar amounts after perfusion to 25 cmH2O, namely to 628 -70-4% of control. The hyaluronan concentration by contrast was not significantly reduced (106 % of control). 4. The reduction in collagen concentration (fixed material) indicated increased interstitial hydration. The closely similar reduction in sulphated GAGs indicated that dilution rather than washout of these components was occurring. The hyaluronan results could be explained by synthesis in vivo at a rate of > 91 jug h-1 (ml synovium)-1 (possibly a non-basal rate under the conditions of the experiment, i.e. raised pressure and a stretched hydrated membrane). 5. Because interstitial hydraulic drag is related to biopolymer concentration by a power function, the overall matrix dilution observed here was more than sufficient to explain the rise in synovial lining hydraulic conductance at 25 cmH2O when taken in conjunction with stretching of the synovial lining (increased area, reduced thickness).The hydraulic conductance of the synovial lining couples trans-synovial fluid movement to intra-articular pressure and is thus an important factor in synovial fluid dynamics. Measurements of trans-synovial flow in the rabbit knee, both in vivo and post mortem, show that the conductance of the synovial lining increases when intra-articular pressure is raised to pathological levels (above -9 cmH2O), such as occurs in joint effusions (Edlund, 1949). This causes a marked steepening of the pressure-flow relation and faster trans-synovial flows ('yield' phenomenon). Above -9 cmH2O each successive, applied step in pressure causes an increment in conductance to a new level, i.e. the pressure-conductance relation is graded above -9 cmH2O (Levick, 1980). A monotonic relation between conductance and intra-articular pressure above -9 cmH2O has been confirmed by measurements of blood-to-joint cavity conductance, i.e.

The synovial intercellular space is the path by which water, nutrients, cytokines, and macromolecules enter and leave the joint cavity. In this study two structural factors influencing synovial permeability were quantified by morphometry (Delesse's principle) of synovial electronmicrographs (rabbit knee), namely interstitial volume fraction Vv.1 and the fraction of the interstitium obstructed by collagen fibrils. Mean Vv.1 across the full thickness was 0.66 +/- 0.03 SEM (n = 11); but Vv.1 actually varied systematically with depth normal to the surface, increasing nonlinearly from 0.40 +/- 0.04 (n = 5 joints) near the free surface to 0.92 +/- 0.02 near the subsynovial interface. Tending to offset this increase in transport space, however, the space "blocked" by collagen fibrils also increased nonlinearly with depth. Bundles of collagen fibrils occupied 13.6 +/- 2.4% of interstitial volume close to the free surface but 49 +/- 4.8% near the subsynovial surface (full-thickness average, 40.5 +/- 3.5%), with fibrils accounting for 48.6-57.1% of the bundle space. Because of the two counteracting compositional gradients, the space available for fibril-excluded transport (hydraulic flow and macromolecular diffusion) was relatively constant > 4 microns below the surface but constricted at the synovium-cavity interface. The space available to extracellular polymers was only 51-53% of tissue volume, raising their effective concentration and hence the lining's resistance to flow and ability to confine the synovial fluid.