title: Non-linear dependence of interstitial fluid pressure on joint cavity pressure and implications for interstitial resistance in rabbit knee.
abstract: OBJECTIVE
  Synovium retains lubricating fluid in the joint cavity. Synovial outflow resistance estimated as dPj/dQs (Pj, joint fluid pressure and Qs 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 (Pif) was measured as a function of distance from the joint cavity in knees of anaesthetized rabbits, using servo-null pressure-measuring micropipettes and using an external 'window'. Joint fluid pressure Pj was either endogenous (-2.4 +/- 0.4 cmH2O, mean +/- SEM) or held at approximately 4, 8 or 15.0 cmH2O by a continuous intra-articular saline infusion that matched the trans-synovial interstitial drainage rate.
  
  
  RESULTS
  At endogenous Pj the peri-articular Pif was subatmospheric (-1.9 +/- 0.3 cmH2O, n = 19). At raised Pj the Pif values became positive. Gradient dPif /dx was approximately 20 times steeper across synovium than subsynovium. Pif close to the synovium-subsynovium border (Pif*) increased as a non-linear function of Pj to 1.4 +/- 0.2 cmH2O (n = 23) at Pj = 4.3 +/- 0.1 cmH2O : 2.3 +/- 0.2 cmH2O (n = 17) at Pj = 7.6 +/- 0.2 cmH2O: and 3.0 +/- 0.4 cmH2O (n = 26) at Pj = 15 +/- 0.2 cmH2O (P = 0.03, anova).
  
  
  CONCLUSIONS
  Synovial resistivity is approximately 20x subsynovial resistivity. The increase in Pif*with Pj means that true synovial resistance d(Pj-Pif*)/dQs is overestimated 1.5x by dPj/dQs. 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.
authors.affiliation: Department of Experimental, Environmental Medicine and Biotechnology, Faculty of Medicine and Surgery, Monza, Italy.
authors.forename: D
authors.initials: D
authors.lastname: Scott
authors.forename: J R
authors.initials: JR
authors.lastname: Levick
authors.forename: G
authors.initials: G
authors.lastname: Miserocchi
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D045604:Extracellular Fluid / Q000502:physiology*; D006025:Glycosaminoglycans / Q000032:analysis; D006614:Hindlimb / Q000502:physiology; D006874:Hydrostatic Pressure; D007596:Joints / Q000502:physiology*; D010539:Permeability; D011817:Rabbits; D013582:Synovial Fluid / Q000737:chemistry / Q000502:physiology*
chemical_list: D006025:Glycosaminoglycans