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

Aims: Synovium retains lubricating fluid in the joint cavity. Synovial outflow resistance estimated as dP j /dQ s (P j , joint fluid pressure and Q s trans-synovial flow) is greater, however, than expected from interstitial glycosaminoglycan concentration. This study investigates whether subsynovial fluid pressure increases with intra-articular pressure, as this would reduce the estimated resistance estimate. Methods: Interstitial fluid pressure (P if ) was measured as a function of distance from the joint cavity in knees of anaesthetized rabbits, using servonull pressure-measuring micropipettes and using an external 'window'. Joint fluid pressure P i was either endogenous (-2.4 ± 0.4 cmH 2 O, mean ± SEM) or held at ∼4, 8 or 15.0 cmH 2 O by a continuous intra-articular saline infusion that matched the trans-synovial interstitial drainage rate. Results: At endogenous P j the peri-articular P if was subatmospheric (-1.9 ± 0.3 cmH 2 O, n = 19). At raised P j the P if values became positive. Gradient dP if /dx was ∼20 times steeper across synovium than subsynovium. P if close to the synovium-subsynovium border (P* if ) increased as a non-linear function of P j to 1.4 ± 0.2 cmH 2 O (n = 23) at P j = 4.3 ± 0.1 cmH 2 O: 2.3 ± 0.2 cmH 2 O (n = 17) at P j = 7.6 ± 0.2 cmH 2 O: and 3.0 ± 0.4 cmH 2 O (n = 26) at P j = 15 ± 0.2 cmH 2 O (P = 0.03, ANOVA). Conclusions: Synovial resistivity is ∼20× subsynovial resistivity. The increase in P* if with P j means that true synovial resistance d(P j - P* if )/dQ s is overestimated 1.5× by dP j /dQ s . This narrows but does not eliminate the gap between analysed glycosaminoglycan concentration, 4 mg ml -1 , and the net interstitial biopolymer concentration of 11.5 mg ml -1 needed to generate the resistance.