The density and strength of proteoglycan-proteoglycan interaction sites in concentrated solutions.

Rheological flow properties of link-stable and link-free proteoglycan (PG) aggregates in concentrated solutions were measured using a cone-on-plate viscometer. A second-order constitutive model, based upon the statistical-network theories of Lodge, [Rheol. Acta 7, 379-392 (1968)] and De Kee and Carreau [J. Non-Newtonian Fluid Mech. 6, 127-143 (1979)], was developed to describe the measured steady and transient flow responses exhibited by the PG solutions. Our measurements confirmed previous experimental findings that the complex shear modulus of PG solutions depends on the frequency of the imposed small-amplitude oscillatory shear, and the apparent viscosity and primary normal-stress difference depend nonlinearly on the shear rate under steady-shear flow conditions [Mow et al., J. Biomechanics 17, 325-338 (1984b); Hardingham et al., J. orthop. Res. 5, 36-46 (1987)]. In the present study, we found that PG solutions exhibit pronounced stress overshoot responses and large hysteresis loop effects. These transient responses were shown to be sensitive to acceleration strain (i.e. the second rate of strain) as well as PG structure (i.e. link-protein stabilization). The model parameters were determined by curvefitting of the second-order constitutive model and experimental data from steady, oscillatory and transient shear flow measurements. Using this network model, we calculated the density of the idealized interaction sites existing in the PG network, and the average strength of these interaction sites. The results indicate that link-protein stabilization of PG aggregates does not change the density of interaction sites formed in the PG network, rather, it increases the average strength of these interaction sites.

On the conditional equivalence of chemical loading and mechanical loading on articular cartilage.

Osmotic pressure loading of articular cartilage has been customarily invoked to be equivalent to mechanical loading. In the literature, this equivalence is defined by the amount of water squeezed from the tissue, i.e. if the amount of water content lost by these two modes of loading are the same, it has been generally regarded that the two loadings are equivalent. This assumption has never been proven. Using the water content lost concept, in this paper, we derived the exact conditions under which an osmotic pressure loading of cartilage can be considered to be equivalent to a mechanical loading. However, the mechanical loading condition satisfying this equivalency criterion, i.e. an isotropic loading delivered via a porous permeable rigid platen uniformly applied all around the specimen, is not practically achievable. Moreover, even if this were achieved experimentally, the interstitial fluid pressure caused by the two loading conditions are not the same. This result has important ramifications for interpretation of experimental data from mechanical stimulations of cartilage explant studies.

A stereophotogrammetric method for determining in situ contact areas in diarthrodial joints, and a comparison with other methods.

Determination of contact areas in diarthrodial joints is necessary for understanding the state of stress within the articular cartilage layers and the supporting bony structures. The present study describes the use of a stereophotogrammetry (SPG) system [Huiskes et al., J. Biomechanics 18, 559-570 (1985) and Ateshian et al., J. Biomechanics 24, 761-776 (1991)] for determining contact areas in diarthrodial joints, using a surface proximity concept similar to the one used by Scherrer et al. [ASME J. biomech. Engng 101, 271-278 (1979)]. This method consists of evaluating the proximity of the articular surfaces to determine joint contact areas using precise geometric models of the joint surfaces obtained from the SPG system, and precise kinematic data, also obtained from SPG. In this study, the SPG method for determining contact areas is compared to other commonly used methods such as dye staining, silicone rubber casting and Fuji film contact measurement techniques which have been often used and reported by other investigators. The bovine glenohumeral joint and the bovine lateral tibiofemoral articulation (without the meniscus) were used to represent congruent and incongruent joints, respectively. While all the methods yielded consistent contact patterns for the incongruent tibiofemoral articulations, the results for the congruent bovine glenohumeral joints showed that the SPG and Fuji film methods were in better agreement than those obtained from the dye staining and silicone rubber casting methods. The advantages of the new SPG method are that it can be used for intact joints, and used repeatedly and quickly thus making contact-area movement analyses possible [Soslowsky et al., J. orthop. Res. 10, 524-534 (1992)]. The results of this comparison study show that the SPG technique is a reliable and versatile method for determining contact areas in diarthrodial joints.

Determination of collagen-proteoglycan interactions in vitro.

The objective of this study was to characterize the physical interactions of the molecular networks formed by mixtures of collagen and proteoglycan in vitro. Pure proteoglycan aggrecan solutions, collagen (type II) suspensions and mixtures of these molecules in varying proportions and concentrations were subjected to viscometric flow measurements using a cone-on-plate viscometer. Linear viscoelastic and non-Newtonian flow properties of these solutions and suspensions were described using a second-order statistical network theory for polymeric fluids (Zhu et al., 1991, J. Biomechanics 24, 1007-1018). This theory provides a set of material coefficients which relate the macroscopic flow behavior of the fluid to an idealized molecular network structure. The results indicated distinct differences between the flow properties of pure collagen suspensions and those of pure proteoglycan solutions. The collagen network showed much greater shear stiffness and more effective energy storage capability than the proteoglycan network. The relative proportion of collagen to proteoglycan is the dominant factor in determining the flow behavior of the mixtures. Analysis of the statistical network theory indicated that the collagen in a collagen-proteoglycan mixture enhances molecular interactions by increasing the amount of entanglement interactions and/or the strength of interaction, while aggrecan acts to reduce the number and/or strength of molecular interactions. These results characterize the physical interactions between type II collagen and aggrecan and provide some insight into their potential roles in giving articular cartilage its mechanical behavior.

Constituents and pH changes in protein rich hyaluronan solution affect the biotribological properties of artificial articular joints.

The relationship between the coefficient of friction and pH value or protein constituents of lubricating fluid, together with viscosity, were studied within a bearing surface model for artificial joint, ultra-high molecular weight polyethylene (UHMWPE) against stainless steel (SUS), using a mechanical spectrometer. Four lubricants were tested in this study: sodium hyaluronate (HA), HA with albumin, HA with gamma-globulin, and HA with (L)alpha-dipalmitoyl phosphatidylcholine ((L)alpha-DPPC). The coefficient of friction between UHMWPE and SUS in HA with albumin or HA with gamma-globulin varied from 0.035 to 0.070 depending on angular velocity and pH. The coefficient of friction in HA or HA with (L)alpha-DPPC varied from 0.023 to 0.045 depending on angular velocity and pH. The variation in pH for HA with albumin had a large effect on the coefficient of friction at low range of angular velocity with viscosity independence. The variation in pH for HA with gamma-globulin had a large effect on the coefficient of friction with viscosity dependence at high angular velocity. The addition of (L)alpha-DPPC showed a small effect on the coefficient of friction at low angular velocity. This study confirms that the presence of albumin in the lubricant promotes pH dependence and viscosity independence of the tribological properties at low speed while the presence of globulin promotes pH and viscosity independence at low speed and promotes pH and viscosity dependence at high speed in the lubrication of UHMWPE against SUS. This study supports the clinical hypothesis that the effect of constituents and pH changes in periprosthetic fluid for the lubrication is a clue toward resolving many complications after total joint replacement.

Proteoglycans and mechanical behavior of condylar cartilage.

Mandibular condylar cartilage functions as the load-bearing, shock-absorbing, lubricating material in temporomandibular joints. Little is known about the precise nature of the biomechanical characteristics of this fibro-cartilaginous tissue. We hypothesized that the fixed charge density associated with proteoglycans that introduces an osmotic pressure inside condylar cartilage will significantly increase the tissue's apparent stiffness. Micro-indentation creep tests were performed on porcine TMJ condylar cartilage at 5 different regions-anterior, posterior, medial, lateral, and central-in physiologic and hypertonic solutions. The intrinsic and apparent mechanical properties, including aggregate modulus, shear modulus, and permeability, were calculated by indentation test data and the biphasic theory. The apparent properties (with osmotic effect) were statistically higher than those of the intrinsic solid matrix (without osmotic effect). Regional variations in fixed charge density, permeability, and mechanical modulus were also calculated for condylar surface. The present results provide important quantitative data on the biomechanical properties of TMJ condylar cartilage.