Interstitial pressure gradients around joints; location of chief resistance to fluid drainage from the rabbit knee

Abstract

The hypothesis has been advanced that synovium offers the main resistance to fluid escape from joints, even though it is under 20 pm thick. To test this, fluid was infused into the knee joint cavity of anaesthetised rabbits to set up a pressure gradient, then the profde of periarticular interstitial fluid pressure (Pif) was measured by advancing a micropipette, connected to a servo-null pressure recorder, in steps through a periarticular tissue 'window' until the joint cavity was entered. With intra-articular pressure (Pi) raised to 15 cmHzO (the pressure of an acute joint effusion) the pressure gradient d P & x (where x is distance) across the synovial lining was 0.47 f 0.04 cmH20 pm-' (n = 10 joints). This was 23.5-fold greater than the gradient in the subsynovinm (0.02 & 0.01 cmHzO ,urn-'; P < 0.0001, Student's t test), indicating that the hydraulic resistivity of the subsynovium is 14% of that of the synovium. The pressure profde was not altered by circulatory arrest. To test the hypothesis further, the effect of a stab perforation of the synovial lining on fluid drainage rate a) was studied. Perforation raised both & and the conductance term d&dPj more than 10-fold (n = 6 joints; P < O.OOO1, ANOVA). The results thus support the view that, despite its thinness, the synovial lining offers the main hydraulic resistance to fluid drainage from a synovial joint. Experimental Physiology (2001) 86.6,739-747.

Non‐linear dependence of interstitial fluid pressure on joint cavity pressure and implications for interstitial resistance in rabbit knee

“…2001) was applied over the P j range used to evaluate synovial resistance, namely 2–8 cmH 2 O. Scott et al. (2001) reported that the change in pressure with distance, d P if /dΔ x , was 23.5‐fold greater in the synovium than in the subsynovium, which strongly supports the view that the synovium has a much greater resistivity than the subsynovium.…”

“…In the last three groups P j was raised and held constant by a continuous infusion of saline into the joint cavity at a rate that matched fluid drainage across the lining down the measured pressure gradient. Gradients at 15 cmH 2 O are a subset of results reported by Scott et al. (2001).…”

“…The remaining sheet of tissue separating the intra‐articular fluid from the atmosphere was ∼200 μ m thick and comprised the synovial lining of thickness 12–21 μ m backed by the subsynovium. (Scott et al. 2001).…”

“…In other cases P j was raised to the required level and held there for 20 min before commencing the P if measurements. Previous work established that 20 min is sufficient to establish a steady‐state gradient of tissue fluid pressure (Scott et al. 2001).…”

“…The principal object of the present study was to test the assumption that is constant by measuring the profile of interstitial fluid pressure ( P if ) around a joint cavity over the relevant range of P j . A recently developed method for measuring the peri‐articular P if profile using glass micropipettes (Scott et al. 2001) was applied over the P j range used to evaluate synovial resistance, namely 2–8 cmH 2 O.…”