Fluid Load Support Research Articles

In-situ studies of cartilage microtribology: roles of speed and contact area.

The progression of local cartilage surface damage toward early stage osteoarthritis (OA) likely depends on the severity of the damage and its impact on the local lubrication and stress distribution in the surrounding tissue. It is difficult to study the local responses using traditional methods; in-situ microtribological methods are being pursued here as a means to elucidate the mechanical aspects of OA progression. While decades of research have been dedicated to the macrotribological properties of articular cartilage, the microscale response is unclear. An experimental study of healthy cartilage microtribology was undertaken to assess the physiological relevance of a microscale friction probe. Normal forces were on the orderof50 mN. Sliding speed varied from 0 to 5 mm/s, and two probes radii, 0.8 mm and 3.2 mm, were used in the study. In-situ measurements of the indentation depth into the cartilage enabled calculations of contact area, effective elastic modulus, elastic and fluid normal force contributions, and the interfacial friction coefficient. This work resulted in the following findings: 1) at high sliding speed (V=1-5 mm/s), the friction coefficient was low (μ = 0.025) and insensitive to probe radius (0.8 mm 3.2 mm) despite the 4-folddifference in the resulting contact areas; 2) The contact area was a strong function of the probe radius and sliding speed; 3) the friction coefficient was proportional to contact area when sliding speed varied from 0.05mm/s-5mm/s; 4) the fluid load support was greater than 85% for all sliding conditions (0% fluid support when V=0) and was insensitive to both probe radius and sliding speed. The findings were consistent with the adhesive theory of friction; as speed increased, increased effective hardness reduced the area of solid-solid contact which subsequently reduced the friction force. Where the severity of the sliding conditions dominates the wear and degradation of typical engineering tribomaterials, the results suggest that joint motion is actually beneficial for maintaining low matrix stresses, low contact areas, and effective lubrication for the fluid-saturated porous cartilage tissue. Further, the results demonstrated effective pressurization and lubrication beneath single asperity microscale contacts. With carefully designed experimental conditions, local friction probes can facilitate more fundamental studies of cartilage lubrication, friction and wear, and potentially add important insights into the mechanical mechanisms of OA.

Chondroitin sulphate: an effective joint lubricant?

The effect of chondroitin sulphate (CS) treatment on the friction and deformation characteristics of native and glycosaminoglycan (GAG) deficient articular cartilage was investigated. Friction tests were conducted at 0.4 MPa load, in Static and Dynamic models, to determine the startup coefficient of friction (COF) and dynamic COF, respectively. Native cartilage: For each cartilage pin and plate couple, the COF was determined under three consecutive tests - (1) baseline COF in PBS (2) COF in CS lubricant and (3) COF again in PBS, after 24h CS treatment. GAG deficient cartilage: For each cartilage pin and plate couple, the baseline COF was determined in PBS initially and again following enzymatic treatment to deplete GAGs. The specimens were then soaked in CS solution for 24h and the COF determined again in PBS. In a similar manner, friction tests were replaced with indentation tests to study the deformation of the tissue. CS at 50mg/ml significantly lowered the startup COF of native cartilage both as a lubricant and a treatment solution. In the dynamic model, where the fluid load support is sustained at a high level, CS failed to have any effect on the COF of native cartilage. GAG depletion raised the friction and deformation levels of cartilage, and subsequent CS treatment failed to lower them to their native levels. CS proved to be an effective lubricant for cartilage under mixed-mode lubrication conditions. However, supplemental CS that diffused into the specimens had no influence on the fluid load support of cartilage.

Effect of load variation on the friction properties of articular cartilage

Synovial joints are known for their remarkable load bearing capacity while maintaining very low friction and wear over their lifetime. Although other joint components play a role in the lubrication mechanisms, articular cartilage with its unique biphasic characteristics is believed to be largely responsible for the observed low friction properties in these joints. In the current study, a pin-on-plate machine was used in a cartilage loaded against cartilage configuration to study the effect of load variation on cartilage friction properties. Both static and dynamic loading conditions were tested under three different contact stress levels - 0.2, 0.3, and 0. 4 MPa with phosphate buffered saline as the lubricant. Indentation experiments were performed and modelled through finite-element (FE) method to determine the biphasic material properties of cartilage. A second FE model adopting the derived biphasic properties of cartilage was then used to simulate the static friction tests and determine the interstitial fluid load support in the cartilage tissue under different loads. An increase in contact stress was found to reduce the friction levels significantly ( p < 0.05) between the articulating cartilage surfaces in both dynamic and static models, while the percentage of fluid load support within the cartilage was almost identical. It was shown that while interstitial fluid pressurization of cartilage plays the major role, another extraneous factor or mechanism that influences the friction between cartilage surfaces is needed to completely explain the results observed.

Investigation of the friction and surface degradation of innovative chondroplasty materials against articular cartilage

Understanding the wear of the biomaterial-cartilage interface is vital for the development of innovative chondroplasty. The aim of this study was to investigate a number of biphasic materials as potential chondroplasty biomaterials. Simple geometry friction and wear studies were conducted using bovine articular cartilage pins loaded against a range of single-phase and biphasic materials. The frictions of each biomaterial was compared within simple and protein-containing lubricants. Longer-term continuous sliding tests within a protein containing lubricant were also conducted at various loading conditions to evaluate the friction and degradation for each surface. All single-phase materials showed a steady rise in friction, which was dependent on the loss of interstitial fluid load support from the opposing cartilage pin. All biphasic materials demonstrated a marked reduction in friction when compared with the single-phase materials. It is postulated that the biphasic nature of each material allowed an element of fluid load support to be maintained by fluid rehydration and expulsion. In the longer-term study, significant differences in the articular cartilage pin (surface damage) between the positive control (stainless steel) and the negative control (articular cartilage) was found. The potential biphasic chondroplasty materials produced a reduction in articular cartilage pin damage when compared with the single-phase materials. The changes in surface topography of the cartilage pin were associated with increased levels of friction achieved during the continuous wear test. The study illustrated the importance of the biphasic properties of potential chondroplasty materials, and future work will focus on the optimization of biphasic properties as well as long-term durability, such that materials will more closely mimic the biotribology of natural articular cartilage.

Modelling of fluid support inside articular cartilage during sliding

Considerable attention has been drawn to the lubricatingmechanism of synovial joints. Biphasic lubrication was proposed by Mowand Lai [1] based on the load partitioning mechanism between fluid and solid phases of articular cartilage. Forster et al. [12] carried out experimental studies to investigate the same, whereas Ateshian et al. [13, 17] further investigated it theoretically using interstitial fluid pressurization. Forster and Fisher [15] showed that start-up frictional coefficient was higher than that under a wide range of sliding conditions and that biphasic nature of the cartilage does play a significant role in joint lubrication. The experimental and theoretical studies of frictional behaviour of the cartilage have been carried out by several researchers; however, finite-element modelling is far less in comparison, and mainly focused on deformation rather than sliding conditions. In the present study, two finite-element modelswere developed to study the effect of sliding on fluid load support in the cartilage and its implications to frictional and lubricating characteristics. The results predicted by the static modelwere same as those given by a metallic plate sliding over the cartilage surface, whereas the model of a metallic cylindrical indenter sliding over the cartilage surface with the migrating contact area showed remarkably higher fluid load support for relatively long-time durations. The results agreed with the experimental results presented by Forster and Fisher [15] and Bell et al. [24] and, in general, with those of other researchers with respect to weeping, boosted, and self-generating lubrication mechanism. Further extensive studies need to be carried out to compare the finite-element prediction with the experimental studies under other conditions.

Wall slip and hydrodynamics of two-dimensional journal bearing

In the present paper, based on the limiting shear stress model, a multi-linearity finite element algorithm and quadratic programming technique are used to study the influence of wall slip on the hydrodynamic lubrication performance of a two-dimensional journal bearing (finite length journal bearing). It is found that if the lubricated surfaces are designed as homogeneous slip surfaces, the hydrodynamic force will be decreased. If the shaft surface (rotation) is a slippery surface with very low limiting shear stress, almost no fluid load support can be generated. If the sleeve surface is designed as the homogeneous slip surface, a low fluid load support together with a small friction drag can be obtained. However, if the sleeve surface is designed as an optimized slip surface with a slip zone in the inlet region, a high load support and low friction coefficient can be obtained. Optimization of the shape and the size of the slip zone can give the journal bearing many advanced properties.

Low Friction and High Load Support Capacity of Slider Bearing With a Mixed Slip Surface

The classical Reynolds theory reveals that a converging gap is the first necessary condition to generate a hydrodynamic pressure in a viscous fluid film confined between two solid surfaces with a relative sliding/rolling motion. For hundreds of years, the classical lubrication mechanics has been based on the frame of the Reynolds theory with no slip assumption. Recent studies show that a large boundary slip occurs on an ultrahydrophobic surface, which results in a very small friction drag. Unfortunately, such a slip surface also produces a small hydrodynamic pressure in a fluid film between two solid surfaces. This paper studies the lubrication behavior of infinite width slider bearings involving a mixed slip surface (MSS). The results of the study indicate that any geometrical wedges (gaps), i.e., a convergent wedge, a parallel gap, and even a divergent wedge, can generate hydrodynamic pressure in an infinite slider bearing with a mixed slip surface. It is found that with an MSS, the maximum fluid load support capacity occurs at a slightly divergent wedge (roughly parallel sliding gap) for an infinite width slider bearing, but not at a converging gap as what the classical Reynolds theory predicts. Surface optimization of a parallel sliding gap with a slip surface can double the hydrodynamic load support and reduce the friction drag by half of what the Reynolds theory predicts for an optimal wedge of a traditional slider bearing.

Squeeze fluid film of spherical hydrophobic surfaces with wall slip

Isothermal squeeze film flow of Newtonian fluid between spherical hydrophobic surfaces with wall slip is investigated using a limiting shear stress model and complementary algorithm. Wall slip velocity is controlled by the liquid–solid interface limiting shear stress. It is found that the wall slip dramatically decreases the hydrodynamic support force of the squeeze fluid film. In the case of large wall slip the hydrodynamic support force increases only slightly with the decrease in the film thickness. We find that wall slip decreases with increasing film thickness and limiting shear stress, but increases with increasing fluid viscosity and approaching velocity. An empirical equation is given for prediction of the fluid load support capacity. The possible effect of pressure on wall slip is also discussed. It is found that fluid pressure suppresses wall slip after the proportionality coefficient of limiting shear stress reaches a critical threshold. However, almost no effect is found when it is below this critical threshold. Good agreements exist between the present theoretical predictions and some existing experimental observations.

Biphasic surface amorphous layer lubrication of articular cartilage

The biphasic nature of articular cartilage has been acknowledged for some time and is known to play an important role in many of the biomechanical functions performed by this unique tissue. From the lubrication point of view however, a simple biphasic model is unable to account for the extremely low friction coefficients that have been recorded experimentally, particularly during start-up. In addition, research over the last decade has indicated the presence of a surface amorphous layer on top of articular cartilage. Here, we present results from a finite element model of articular cartilage that includes a thin, soft, biphasic surface amorphous layer (BSAL). The results of this study show that a thin BSAL, with lower elastic modulus, dramatically altered the load sharing between the solid and liquid phases of articular cartilage, particularly in the near-surface regions of the underlying bulk cartilage and within the surface amorphous layer itself where the fluid load support exceeded 85%. By transferring the load from the solid phase to the fluid phase, the biphasic surface layer improves lubrication and reduces friction, whilst also protecting the underlying cartilage surface by ‘shielding’ the solid phase from elevated stresses. The increase in lubrication effectiveness is shown to be greatest during short duration loading scenarios, such as shock loads.

Cartilage Interstitial Fluid Load Support in Unconfined Compression Following Enzymatic Digestion

Interstitial fluid pressurization plays an important role in cartilage biomechanics and is believed to be a primary mechanism of load support in synovial joints. The objective of this study was to investigate the effects of enzymatic degradation on the interstitial fluid load support mechanism of articular cartilage in unconfined compression. Thirty-seven immature bovine cartilage plugs were tested in unconfined compression before and after enzymatic digestion. The peak fluid load support decreased significantly (p < 0.0001) from 84 +/- 10% to 53 +/- 19% and from 80 +/- 10% to 46 +/- 21% after 18-hours digestion with 1.0 u/mg-wet-weight and 0.7 u/mg-wet-weight of collagenase, respectively. Treatment with 0.1 u/ml of chondroitinase ABC for 24 hours also significantly reduced the peak fluid load support from 83 +/- 12% to 48 +/- 16% (p < 0.0001). The drop in interstitial fluid load support following enzymatic treatment is believed to result from a decrease in the ratio of tensile to compressive moduli of the solid matrix.

Effect of dynamic loading on the frictional response of bovine articular cartilage

The objective of this study was to test the hypotheses that (1) the steady-state friction coefficient of articular cartilage is significantly smaller under cyclical compressive loading than the equilibrium friction coefficient under static loading, and decreases as a function of loading frequency; (2) the steady-state cartilage interstitial fluid load support remains significantly greater than zero under cyclical compressive loading and increases as a function of loading frequency. Unconfined compression tests with sliding of bovine shoulder cartilage against glass in saline were carried out on fresh cylindrical plugs ( n=12), under three sinusoidal loading frequencies (0.05, 0.5 and 1 Hz) and under static loading; the time-dependent friction coefficient μ eff was measured. The interstitial fluid load support was also predicted theoretically. Under static loading μ eff increased from a minimum value ( μ min = 0.005 ± 0.003 ) to an equilibrium value ( μ eq = 0.153 ± 0.032 ). In cyclical compressive loading tests μ eff similarly rose from a minimum value ( μ min = 0.004 ± 0.002 , 0.003±0.001 and 0.003±0.001 at 0.05, 0.5 and 1 Hz) and reached a steady-state response oscillating between a lower-bound ( μ lb = 0.092 ± 0.016 , 0.083±0.019 and 0.084±0.020) and upper bound ( μ ub = 0.382 ± 0.057 , 0.358±0.059, and 0.298±0.061). For all frequencies it was found that μ ub > μ eq and μ lb < μ eq ( p < 0.05 ) . Under cyclical compressive loading the interstitial fluid load support was found to oscillate above and below the static loading response, with suction occurring over a portion of the loading cycle at steady-state conditions. All theoretical predictions and most experimental results demonstrated little sensitivity to loading frequency. On the basis of these results, both hypotheses were rejected. Cyclical compressive loading is not found to promote lower frictional coefficients or higher interstitial fluid load support than static loading.

Effects of enzymatic degradation on the frictional response of articular cartilage in stress relaxation

It was recently shown experimentally that the friction coefficient of articular cartilage correlates with the interstitial fluid pressurization, supporting the hypothesis that interstitial water pressurization plays a fundamental role in the frictional response by supporting most of the load during the early time response. A recent study showed that enzymatic treatment with chondroitinase ABC causes a decrease in the maximum fluid load support of bovine articular cartilage in unconfined compression. The hypothesis of this study is that treatment with chondroitinase ABC will increase the friction coefficient of articular cartilage in stress relaxation. Articular cartilage samples (n=34) harvested from the femoral condyles of five bovine knee joints (1–3 months old) were tested in unconfined compression with simultaneous continuous sliding (±1.5mm at 1mm/s) under stress relaxation. Results showed a significantly higher minimum friction coefficient in specimens treated with 0.1μ/ml of chondroitinase ABC for 24h (μmin=0.082±0.024) compared to control specimens (μmin=0.047±0.014). Treated samples also exhibited higher equilibrium friction coefficient (μeq=0.232±0.049) than control samples (μeq=0.184±0.036), which suggest that the frictional response is greatly influenced by the degree of tissue degradation. The fluid load support was predicted from theory, and the maximum value (as a percentage of the total applied load) was lower in treated specimens (77±12%) than in control specimens (85±6%). Based on earlier findings, the increase in the ratio μmin/μeq may be attributed to the decrease in fluid load support.

Response to Comment by Charles W. McCutchen

Contributed by the Bioengineering Division for publication in the JOURNAL OF BIOMECHANICAL ENGINEERING. Manuscript received by the Bioengineering Division January 9, 2004; revision received January 16, 2004. We thank Dr. McCutchen for his comment. We are keenly aware of his hypotheses for cartilage lubrication 12 and agree with one of the fundamental premises he first proposed, namely that hydrostatic pressurization of the interstitial fluid of cartilage supports most of the contact load transmitted across articular surfaces, thereby reducing the interfacial frictional force exerted on the solid matrix of the opposing surfaces. We have previously formulated a cartilage friction model 34, within the frameworks of the biphasic 5 and triphasic 6 theories of cartilage, to quantify this mechanism, and we have recently found excellent agreement between the model predictions and experimental data 7, thus supporting Dr. McCutchen’s original hypothesis. In our model, the direction of interstitial fluid flow at the contact interface is irrelevant to the mechanism of friction reduction by interstitial fluid load support. The only relevant issue is the magnitude and duration of interstitial fluid pressurization. Our recent experimental studies confirm that this pressurization can last for hundreds of seconds or longer 78. From our perspective, it is thus inconsequential whether the interstitial fluid “weeps” out of the tissue or is “boosted” into it. It is only because the direction of flow had been debated historically in the literature that we referred to it in our discussion. We respectfully disagree with Dr. McCutchen’s statement that “Were [weeping flow] not present cartilage would be, in effect, an impervious bearing…” Cartilage is a highly porous tissue, particularly near the articular surface where the porosity can approach 90% 910. As demonstrated in our model 3, it is not necessary for a layer of fluid to separate the articular surfaces in order for its lubrication to be effective. When two cartilage layers come into contact, each with a surface porosity of 90% (or solid fractional content φs=0.1), a fraction of 1−φs2, or 99%, of the apparent contact area consists of fluid-against-fluid or fluid-against-solid contact. Since the fluid is typically highly pressurized under loading, due to the relatively low modulus and permeability of cartilage, most of the contact load at the interface will be supported by fluid, producing negligible friction. Only 1% of the contact area will consist of solid-against-solid contact, where friction occurs. Trapped lubricant pools would further decrease solid-against-solid contact, and thus frictional forces. Thus, even in the absence of weeping flow, our model predicts that the porous nature of cartilage will maintain effective lubrication.

Experimental verification of the role of interstitial fluid pressurization in cartilage lubrication

The objective of the current study was to measure the friction coefficient simultaneously with the interstitial fluid load support in bovine articular cartilage, while sliding against glass under a constant load. Ten visually normal 6-mm-diameter cartilage plugs harvested from the humeral head of four bovine shoulder joints (ages 2–4 months) were tested in a custom friction device under reciprocating linear motion (range of translation ±2 mm; sliding velocity 1 mm/s), subjected to a 4.5 N constant load. The frictional coefficient was found to increase with time from a minimum value of μ min=0.010±0.007 (mean ± SD) to a maximum value of 0.243 ± 0.044 over a duration ranging from 920 to 19,870 s (median: 4,560 s). The corresponding interstitial fluid load support decreased from a maximum of 88.8 ± 3.8% to 8.7 ± 8.6%. A linear correlation was observed between the frictional coefficient and interstitial fluid load support ( r 2=0.96±0.03). These results support the hypothesis that the temporal variation of the frictional coefficient correlates negatively with the interstitial fluid load support and that consequently interstitial fluid load support is a primary mechanism regulating the frictional response in articular cartilage. Fitting the experimental data to a previously proposed biphasic boundary lubrication model for cartilage yielded an equilibrium friction coefficient of μ eq=0.284±0.044. The fraction of the apparent contact area over which the solid cartilage matrix was in contact with the glass slide was predicted at ϕ=1.7±6.3%, significantly smaller than the solid volume fraction of the tissue, ϕ s=13.8±1.8%. The model predictions suggest that mixed lubrication prevailed at the contact interface under the loading conditions employed in this study.

Cartilage interstitial fluid load support in unconfined compression

Under physiological conditions of loading, articular cartilage is subjected to both compressive strains, normal to the articular surface, and tensile strains, tangential to the articular surface. Previous studies have shown that articular cartilage exhibits a much higher modulus in tension than in compression, and theoretical analyses have suggested that this tension–compression nonlinearity enhances the magnitude of interstitial fluid pressurization during loading in unconfined compression, above a theoretical threshold of 33% of the average applied stress. The first hypothesis of this experimental study is that the peak fluid load support in unconfined compression is significantly greater than the 33% theoretical limit predicted for porous permeable tissues modeled with equal moduli in tension and compression. The second hypothesis is that the peak fluid load support is higher at the articular surface side of the tissue samples than near the deep zone, because the disparity between the tensile and compressive moduli is greater at the surface zone. Ten human cartilage samples from six patellofemoral joints, and 10 bovine cartilage specimens from three calf patellofemoral joints were tested in unconfined compression. The peak fluid load support was measured at 79±11% and 69±15% at the articular surface and deep zone of human cartilage, respectively, and at 94±4% and 71±8% at the articular surface and deep zone of bovine calf cartilage, respectively. Statistical analyses confirmed both hypotheses of this study. These experimental results suggest that the tension–compression nonlinearity of cartilage is an essential functional property of the tissue which makes interstitial fluid pressurization the dominant mechanism of load support in articular cartilage.

Experimental verification and theoretical prediction of cartilage interstitial fluid pressurization at an impermeable contact interface in confined compression

Interstitial fluid pressurization has long been hypothesized to play a fundamental role in the load support mechanism and frictional response of articular cartilage. However, to date, few experimental studies have been performed to verify this hypothesis from direct measurements. The first objective of this study was to investigate experimentally the hypothesis that cartilage interstitial fluid pressurization does support the great majority of the applied load, in the testing configurations of confined compression creep and stress relaxation. The second objective was to investigate the hypothesis that the experimentally observed interstitial fluid pressurization could also be predicted using the linear biphasic theory of Mow et al. (J. Biomech. Engng ASME, 102, 73–84, 1980). Fourteen bovine cartilage samples were tested in a confined compression chamber fitted with a microchip piezoresistive transducer to measure interstitial fluid pressure, while simultaneously measuring (during stress relaxation) or prescribing (during creep) the total stress. It was found that interstitial fluid pressure supported more than 90% of the total stress for durations as long as 725±248 s during stress relaxation (mean±S.D., n=7), and 404±229 s during creep ( n=7). When comparing experimental measurements of the time varying interstitial fluid pressure against predictions from the linear biphasic theory, nonlinear coefficients of determination r 2=0.871±0.086 (stress relaxation) and r 2=0.941±0.061 (creep) were found. The results of this study provide some of the most direct evidence to date that interstitial fluid pressurization plays a fundamental role in cartilage mechanics; they also indicate that the mechanism of fluid load support in cartilage can be properly predicted from theory.