Fluid movement across synovium in healthy joints: role of synovial fluid macromolecules.
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Albumins↗
Biomedical subjects
J R Levick
Publications and source records attributed to J R Levick.
Microvascular architecture and exchange in synovial joints.
The microcirculation of the synovial lining of joints displays many fascinating adaptations to function. One primary function is to supply nutrients to the avascular cartilage, whose chondrocytes are metabolically active but are relatively vast distances from the nearest capillary (> 1 cm in the center of a human knee). Exchange is facilitated by a high density of fenestrated capillaries situated very close to the synovial surface (an arrangement disrupted in rheumatoid synovium) with fenestrations preferentially oriented toward the joint cavity. Even so, diffusion alone is too slow to supply central chondrocytes with glucose. The problem is solved by the synovial microcirculation generating intra-articular fluid (synovial fluid) that transports glucose by convection during joint movement. Synovial fluid is a plasma ultrafiltrate into which hyaluronan has been secreted, and it also serves to lubricate the joint. The joint cavity offers unusually easy access to the interstitial side of the microcirculation because the synovial cell layer is discontinuous, with an interstitial matrix between capillary and joint cavity. This allows the experimental study of the effect of the extravascular Starling "forces" (hydraulic and colloid osmotic pressure) on fluid exchange. One unexpected outcome has been the finding that a substantial part of the hydraulic resistance to fluid transport between blood and joint cavity resides in the series interstitial layer rather than in the capillary wall. Another is that, under appropriate boundary conditions (joint angle, etc.) fluid can simultaneously filter into the cavity in some regions (synovium directly overlying capillaries) and out of the cavity into subsynovium in other regions (synovium in the mesh space between capillaries) to produce a turnover of synovial fluid.
Animals↗
Assessment of truncal edema following breast cancer treatment using modified Harpenden skinfold calipers.
After initial treatment for breast cancer, lymphedema often affects the trunk as well as the arm. Evaluation of truncal swelling by the clinical "pinch test" of the posterior axillary fold is unreliable. Our aim was to develop an objective measurement, using modified Harpenden skinfold calipers. Standard Harpenden skinfold calipers exert a pressure of 12.6 g.mm-2, which rapidly squeezes edema fluid out of the skinfold. Springs were substituted to exert a lighter but relatively constant load (3.7 g.mm-2). Repeated skinfold thickness measurements on the same, normal subject then gave a relative standard deviation (r.s.d.) or coefficient of variation of 5%. The posterior axillary folds of 14 patients (age 56 +/- 13 (s.d.) years) with an average 30% arm swelling were measured using the same procedure. Readings were taken at 10 s, and again after 60 s of sustained application to assess the rate of creep, or deformation with time, attributed to displacement of pressurized interstitial fluid. Two patients had clinically observable axillary fold swelling. Eight patients, including the above two, showed axillary fold swelling by caliper measurement, defined as a 10% increase over the contralateral side (2 r.s.d.'s). Creep was greater on the affected side in all 14 patients. Thus, modified calipers can detect axillary fold edema, and thereby provide an objective method for assessing changes in swelling after lymphedema treatment.
Adult↗
Starling pressures in the human arm and their alteration in postmastectomy oedema.
1. Surgery and radiotherapy to axillary lymph nodes during breast cancer treatment is often followed, commonly years later, by chronic postmastectomy oedema (PMO). PMO is considered a 'high protein' oedema due to reduced axillary lymph drainage. Since oedema formation also depends on fluid input (capillary filtration), we studied the Starling pressures in the affected and contralateral arm. Colloid osmotic pressure was measured in patient serum (pi p) and interstitial fluid (pi i). Subcutis fluid was collected from PMO arms by both wick and aspiration methods, and from the control arm by the wick method only. Interstitial hydraulic pressure (P(i)) was measured by the wick-in-needle method. 2. Oedema pi i was 19.2 +/- 4.1 cmH2O (n = 13, wick) to 16.3 +/- 4.4 cmH2O (n = 41, aspirate; difference not significant; mean +/- S.D. throughout). This was significantly lower than pi i in the control arm (21.4 +/- 3.8 cmH2O, n = 14, P < 0.01, analysis of variance). Also, there was a negative correlation between oedema pi i and the percentage increase in arm volume (correlation coefficient r = -0.35, P < 0.05) in contrast to conventional expectation. 3. Oedema P(i) (1.9 +/- 2.0 cmH2O, n = 28) exceeded the subatmospheric control P(i) (-2.8 +/- 3.0 cmH2O; P < 0.01). Venous and arterial pressures were normal but pi p was subnormal (31.1 +/- 2.7 cmH2O, n = 47). 4. Net pressure opposing capillary blood pressure, P(o), was calculated as P(i) + sigma (pi p-pi i) for a reflection coefficient, sigma, of 0.90-0.99.(ABSTRACT TRUNCATED AT 250 WORDS)
Adult↗
Viscous and osmotically mediated changes in fluid movement across synovium in response to intraarticular albumin.
Fluid exchange in synovial joints involves both interstitial and transcapillary flow. Here the influence of intraarticular albumin, a normal component of intraarticular fluid, on net transsynovial flow was explored by infusing albumin solutions into the synovial cavity of rabbit knees and recording net flow across the synovial lining. The latter is a discontinuous cell layer possessing matrix-filled intercellular spaces and fenestrated capillaries. Albumin solutions (10-250 g.liter-1) of known oncotic pressure and viscosity were infused at a constant pressure between 3 and 18 cm H2O, either in vivo or post mortem. The rate of absorption (outflow) of Krebs solution from the cavity at the same pressure was measured as the control. Albumin reduced the net rate of absorption of fluid from the joint cavity (Qs). The reduction in Qs in vivo, expressed as a fraction of control, was greater than the fractional reduction in the fluidity of the albumin solution (i.e., 1/relative viscosity). At high albumin concentrations in vivo, the net flow reversed direction and became a net filtration into the joint cavity. The change in Qs per unit change in intraarticular oncotic pressure was then 29.7 x 10(-3) microliters.min-1.cm H2O-1. Even when there was continuous net outflow across the lining, however, as at lower albumin concentrations or higher joint pressures, the intraarticular albumin became diluted compared with the infusate. When synovial blood flow was stopped by killing the animal, albumin had less effect on net transsynovial outflow than that in vivo. Outflow was still reduced by albumin but the fractional reduction was now less than the fractional reduction in bulk fluidity. Also, intraarticular dilution was less marked postmortem than in vivo. The observations were compatible with the view that the net transsynovial flow in vivo comprised two opposing flows, namely, capillary (fenestral) filtration into the joint cavity and interstitial flow from cavity to subsynovium. These two flows were affected oppositely by albumin: filtration was enhanced by the oncotic pressure of albumin in the pericapillary interstitium, whereas interstitial flow was reduced by the effect of albumin on fluidity. The results also raised the issues of whether fenestral filtration reduces nearby interstitial albumin concentration and how interstitial and bulk-phase fluidities are related.
Albumins↗
An analysis of the interaction between interstitial plasma protein, interstitial flow, and fenestral filtration and its application to synovium.
An analysis of the interstitial flow of a protein solution is presented, based on physical interactions with extracellular matrix biopolymers. Previous models were extended by incorporation of three features--(a) interstitial osmotic reflection coefficients and attendant partial molecular sieving, (b) the effect of solute exclusion on apparent intramatrix fluid viscosity, and (c) the effect of discrete capillary porosity on near-pore oncotic and hydraulic gradients. The principles were applied to a distributed, finite difference model of the synovial lining and its fenestrated capillaries. Numerical solutions were used to assess the significance of some of the factors involved and predictions were compared with experimental measurements of the transsynovial flow of albumin solutions in the steady state. The following physiologically important points emerged. (i) Steric exclusion of protein reduces the effective interstitial fluid viscosity and creates nonzero interstitial reflection coefficients, which impose a slight nonuniformity of interstitial plasma protein distribution. Together these two effects produce nonintuitive deviation of interstitial flow from Darcy's law when interstitial protein concentration is increased experimentally. (ii) Flow across fenestrae with high reflection coefficients is buffered by steep local gradients in interstitial oncotic and hydraulic pressures immediately outside the fenestrae. Buffering is underestimated by the mean interstitial Starling forces and by treatment of the capillary as if it were uniformly permeable to water. (iii) A bidirectional flow pattern can develop across the synovial lining and can contribute to fluid turnover within the joint cavity.
Animals↗
Analysis of microgram quantities of glycosaminoglycan in milligram samples of microdissected synovial intima.
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Animals↗
Hyaluronan reduces fluid escape rate from rabbit knee joints disparately from its effect on fluidity.
The effect of hyaluronan, the major viscous component of synovial fluid, on trans-synovial flow was measured in rabbit knees. Hyaluronan reduced fluid escape rate by up to 45%. This was much less than the reduction in intra-articular fluidity (1/viscosity). Electron micrographs supported the view that hyaluronan might be partially reflected by synovium, creating a molecular filtercake at the surface.
Animals↗
Quantification of rate and depth of pitting in human edema using an electronic tonometer.
An instrument (tonometer) was developed to measure objectively the rate as well as depth of pitting of edematous limbs under a sudden local load. Displacement versus time curves were obtained in vivo in postmastectomy edema arms and also in vitro (compression of sponges) and were analyzed in terms of spring and dashpot constants. There was no significant difference between the quasi-instantaneous indentation of tissue in the edematous and normal arms (median 2.9mm), and the two correlated strongly (r = 0.91, p < 0.0001). An exponentially slowing indentation followed. The mean difference between initial and final deformation (X infinity-X0) was greater in the swollen arms (5.7mm) than in the normal arms (1.3mm, p < 0.01). The time constant of indentation (tau) was significantly greater in the swollen arms (227s) than in the normal arms (71s). There was no correlation between the duration of the edema and any of the pitting characteristics. There was a significant negative correlation between glycosaminoglycan concentration of interstitial fluid and rate constant 1/tau (r = 0.9, p < 0.01). The tonometer thus provides an objective way of quantifying the rate and depth of pitting edema.
Adult↗
Change in macromolecular composition of interstitial fluid from swollen arms after breast cancer treatment, and its implications.
1. The pathophysiology of chronic arm oedema after treatment of breast cancer was investigated by collecting serum and subcutaneous interstitial fluid from the affected and contralateral arms by the wick method (both arms) and by aspiration (oedematous arm). The fluids were analysed for total protein, albumin, glycosaminoglycan and viscosity, and arm volume was measured. 2. Total protein concentration in the aspirated oedema fluid was 32.4 +/- 7.5 g/l (mean +/- SD throughout; n = 39). Protein concentration in wick fluid from the oedematous arm (35.8 +/- 7.3 g/l, n = 14) was not significantly different from that in aspirated fluid. The oedema protein concentrations were significantly lower than in wick fluid from the non-swollen arm (41.4 +/- 6.7 cmH2O, n = 13, P < 0.01, analysis of variance). This was surprising in view of the common assumption that, the condition being of lymphatic origin, the oedema protein concentration should be raised. 3. The ratio of aspirate protein concentration to serum protein concentration showed a weak but highly significant negative correlation with the percentage increase in arm volume (r = -0.47, n = 35, P < 0.005), again in contrast to conventional expectation. The demonstration of a reduced protein concentration in the swollen arm did not therefore depend solely on a comparison with the wick control results. The volume increased by on average 33% and the ratio of aspirate protein concentration to serum protein concentration averaged 0.52 +/- 0.11 on the swollen side and 0.64 +/- 0.13 on the unaffected side. 4. Serum protein concentration in the patients with arm swelling (61.2 +/- 4.9 g/l) was significantly lower than that in postmastectomy patients without this complication (65.0 +/- 6.2 g/l). Most of the decrease occurred in the albumin fraction (oedema patients, 38.3 +/- 5.1 g/l; control patients, 42.0 +/- 2.1 g/l). In oedema patients receiving the anti-oestrogen tamoxifen serum albumin concentration was on average 2.3 g/l lower than in oedema patients not under medication (P < 0.05, t-test). 5. Glycosaminoglycan concentration in oedema fluid was 0.8 +/- 0.14 g/l (n = 21) and 75% was sulphated. Along with the plasma protein this raised the relative viscosity of the fluid to 1.34 +/- 0.16 (n = 11). 6. The reduction in interstitial protein concentration in the swollen arm, contrary to expectation in lymphoedema, could be explained in several ways. One possible hypothesis in light of reported haemodynamic abnormalities in such arms is that capillary pressure rises, increasing capillary filtration rate.(ABSTRACT TRUNCATED AT 400 WORDS)
Aged↗
Effect of extravascular plasma protein on pressure-flow relations across synovium in anaesthetized rabbits.
1. The effect of extravascular plasma protein on fluid flux through interstitial matrix was investigated in vivo by studying the pressure-flow relation across synovium during intra-articular infusions of protein solutions (usually bovine serum albumin). Synovium is a sheet of non-epithelial cells separated by interstitium-filled gaps, beneath which are fenestrated capillaries: synovium regulates synovial fluid volume and composition. 2. Albumin solutions (10-150 g l-1) of measured oncotic pressure and viscosity were infused at known pressure into the synovial cavity of knees of anaesthetized rabbits. Flow across the synovial lining in the steady state (absorption rate Qs) was recorded at a series of joint pressures (Pj) to define the pressure-flow relation. Krebs solution was infused into the opposite knee as a control (26 animals). 3. Infusion of a low albumin concentration (10 g l-1, bovine or rabbit) or diluted rabbit serum revealed no specific effect of plasma protein on interstitial matrix permeability (cf. specific protein effect on capillary glycocalyx permeability). Physiological (22.5 g l-1) and higher concentrations reduced trans-synovial absorption rate. The slope of the pressure-flow relation was reduced and the pressure intercept displaced to the right (i.e. Pj at zero flow was raised). 4. Slope dQs/dPj correlated negatively with intra-articular viscosity (P = 0.001-0.04), in keeping with viscous interstitial flow. The reduction in normalized slope, however, did not equal the reduction in fluidity (1/viscosity) quantitatively. It is proposed that apparent fluidity within the interstitial matrix is higher than in the bulk phase due to steric exclusion of albumin (radius 3.55 nm) by the interstitial glycosaminoglycans. The latter form spaces of estimated mean hydraulic radius 14-18 nm in synovium. 5. The joint-pressure intercept at zero net trans-synovial flow was displaced 0.015 cmH2O per cmH2O intra-articular oncotic pressure (pi j; S.E.M. +/- 0.006). Thus large trans-synovial osmotic gradients were not maintained at physiological flow velocities. The 1.5% displacement of the Pj intercept by pi j was attributed principally to interstitial albumin exerting pericapillary oncotic pressure and enhancing net Starling filtration pressure. Indeed, net trans-synovial flow at low joint pressure sometimes reversed from absorption to filtration into the joint cavity at high intra-articular oncotic pressures. 6. The displacement of the trans-synovial flow intercept per unit change in intra-articular oncotic pressure, (dQs/d pi j)Pj = 0, was 18 +/- 3 nl min-1 cmH2O-1.(ABSTRACT TRUNCATED AT 400 WORDS)
Anesthesia↗
Evidence for simultaneous bidirectional fluid flux across synovial lining in knee joints of anaesthetized rabbits.
A recent model predicted a local synovial circulation of extravascular fluid in which capillary filtrate entered a joint cavity and fluid simultaneously drained out of the cavity via intercellular pathways distant from capillaries. Here, solutions of albumin (10-250 g l-1) were infused continuously into the knee joint cavity in fifteen anaesthetized rabbits at 3-18 cmH2O pressure and steady-state net trans-synovial flow was recorded. The intra-articular fluid collected after 15-30 min was substantially diluted despite a sustained net efflux from the cavity. This supports the prediction that net efflux comprises an influx of low-protein plasma ultrafiltrate and a larger efflux of joint fluid.
Albumins↗
Subcutaneous interstitial fluid pressure and arm volume in lymphoedema.
Interstitial fluid pressures were measured by the wick in needle method in the swollen and normal arms of 38 patients with lymphoedema resulting from treatment for breast cancer. The mean increase in arm volume, calculated from sequential circumferential measurements, was 33% (range 0.25 to 85.9). Subcutis interstitial fluid pressure in the swollen arm (2.0 cmH2O range -4.5 cmH2O to 6.8 cmH2O) was significantly greater (p < 0.001) than in the contralateral, non-swollen arm (-2.6 cmH2O range -11 cmH2O to 0 cmH2O). Interstitial fluid pressure in the oedematous arm did not correlate with the duration of the swelling (1-324 months), but did correlate with the increase in volume relative to the normal arm (r = 0.38, p < 0.05), the slope ('apparent compliance') being 28 ml.dl-1.cmH2O-1. The pressure-volume curve was less steep than the classic curve for acute oedema of dog limbs (Guyton, 1965). Vascular pressures were normal. The interstitial fluid pressures were not as high as those reported for lymphoedema of the lower extremity (mean 17.9 mmHg, Christensen et al., 1985). Nevertheless, the rise in interstitial fluid pressure by an average of 4.6 cmH2O constitutes a force opposing further microvascular fluid filtration and perhaps promoting fluid drainage out of the arm.
Adult↗
A comparative study by morphometry of the microvasculature in normal and rheumatoid synovium.
Vascularity is said to be increased (subjectively) in rheumatoid arthritis, yet synovial fluid is hypoxic and acidotic. Morphometry by image analysis was used to quantify vascularity in normal and rheumatoid synovium from the knee joint. Capillaries were distributed more deeply in the rheumatoid synovium (93.3 microns, compared with 32.5 microns in normal synovium) and were significantly less densely arranged (80.2/mm2 rheumatoid, 241.5/mm2 normal). The blood volume fraction fell from 2.9% in normal knee synovium to 1.2% in rheumatoid synovium. These results imply that there is impairment of O2 transfer, which contributes to the intraarticular hypoxia, in rheumatoid arthritis.
Adult↗
Asymmetrical effects of albumin on transsynovial fluid movement.
The effect of intraarticular infusions of albumin solution on transsynovial flow was studied in healthy rabbit knee joints and compared with the effect of albumin solution perfused through the synovial microcirculation. Increasing intravascular albumin levels enhanced fluid absorption from the joint cavity, whereas increasing intraarticular albumin levels reduced the absorption rate. The slope of intraarticular pressure-versus-absorption rate plots was reduced by albumin in proportion to the reduction in fluidity (1/viscosity). When joint pressure was held constant, the transsynovial absorption rate was reduced by albumin in excess of the fluidity reduction and even reversed to filtration into the joint cavity. Thus intraarticular albumin acts by a dual mechanism, namely by increasing synovial interstitial fluid viscosity and by exerting a peri-capillary oncotic pressure. However, the latter effect was much less than that of intravascular albumin. Reasons for this are discussed.
Absorption↗
Capillary filtration-absorption balance reconsidered in light of dynamic extravascular factors.
The evidence for the functional importance of extravascular Starling pressures now seems overwhelming, and when these terms are taken into account it is difficult to uphold the traditional conception that upstream microvascular filtration is largely matched by a sustained downstream reabsorption. Transient absorption can occur, however, during spontaneous vasomotion cycles, during sympathetic-induced vasoconstriction and during hypovolaemic hypotension. Sustained absorption is possible in specialized tissues where the interstitium is 'flushed' by an independent stream (intestinal mucosa, renal cortex, lymph nodes). Both theory and experiment show, however, that absorption cannot be maintained across most low-pressure exchange segments due to the finite permeability of microvessels to plasma protein, which leads to a rise in pericapillary interstitial oncotic pressure with time around absorbing microvascular segments. Extravascular hydraulic resistance may be a further determinant of net fluid transfer rate in situations where capillary wall resistance is low.
Absorption↗
A two-dimensional morphometry-based model of interstitial and transcapillary flow in rabbit synovium.
The synovial lining of a joint is a layer of specialized connective tissue, containing fenestrated capillaries, that regulates the volume and composition of fluid in the joint cavity. The hydraulic conductance of the synovial lining and of the plasma-to-cavity pathway is increased by high intra-articular pressures (IAP greater than 9 cmH2O) and this is accompanied by an increase in the interstitial pathway's area/pathlength ratio. In order to assess the contribution of the altered pathway geometry to the conductance changes, and also to evaluate the conductivity of the synovial interstitial matrix (Ki) and local flow patterns, a two-dimensional model of trans-synovial flow was developed from morphometric data at low IAP (5 cmH2O) and high IAP (25 cmH2O). Darcy's law was applied to finite elements within a 'unit cell' of synovium to compute a steady-state pressure field. Elements adjacent to limited discrete porous regions of the capillary wall were additionally subjected to the Starling principle. Appropriate values of Ki and capillary wall conductance were obtained by iteration to match the experimentally measured net conductances. The value of Ki that matched the data at low IAP was 1.4-2.1 x 10(-12) cm4 s-1 dyn-1 (1.4-2.1 x 10(-15) m4 s-1 N-1). This is in the range reported for some other fluid-confining tissues, namely scleral stroma and aortic wall. Comparison between a capillary model with localized porous regions and one with a uniformly distributed permeability showed that the uniformity assumption (commonly used in mathematical modelling) leads to severe underestimation of the local pericapillary pressure gradients. At high IAP, synovial deformation was found to account for 24-50% of the increased hydraulic conductances. To explain the remainder, it was necessary to postulate a rise in Ki, especially in the zone between the joint cavity and capillary wall. Possible mechanisms involving tissue hydration and/or glycosaminoglycan wash-out at high IAP are discussed. The model highlights a need for quantitative biochemical analysis.
Animals↗
An immunohistochemical study of the collagens of rabbit synovial interstitium.
The interstitial pathway from joint cavity to synovial transport vessels contains a complex extracellular matrix. Rabbit knee synovial intima contains 3 fibrous elements--banded collagen fibrils, microfibrils and broad banded aggregates. The distribution of 4 types of collagen in rabbit knee synovium was investigated immunohistochemically. Types I and III collagens are both present, although the binding of anti-type I antibody was weak. Type V collagen, which forms thin fibrils, or is copolymerized with type I collagen, and type VI collagen, which forms broad banded aggregates and microfibrils, are widely distributed throughout the intimal and subintimal matrices.
Animals↗