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Rotator cuff disorders are one of the most common causes of shoulder pain and disability in the aging population but, unfortunately, the etiology is still unknown. One factor thought to contribute to the progression of disease is the external compression of the rotator cuff tendons, which can be significantly increased by age-related changes such as muscle weakness and poor posture. The objective of this study was to investigate the baseline compressive response of tendon and determine how this response is altered during maturation and aging. We did this by characterizing the compressive mechanical, viscoelastic, and poroelastic properties of young, mature, and aged mouse supraspinatus tendons using macroscale indentation testing and nanoscale high-frequency AFM-based rheology testing. Using these multiscale techniques, we found that aged tendons were stiffer than their mature counterparts and that both young and aged tendons exhibited increased hydraulic permeability and energy dissipation. We hypothesize that regional and age-related variations in collagen morphology and organization are likely responsible for changes in the multiscale compressive response as these structural parameters may affect fluid flow. Importantly, these results suggest a role for age-related changes in the progression of tendon degeneration, and we hypothesize that decreased ability to resist compressive loading via fluid pressurization may result in damage to the extracellular matrix (ECM) and ultimately tendon degeneration. These studies provide insight into the regional multiscale compressive response of tendons and indicate that altered compressive properties in aging tendons may be a major contributor to overall tendon degeneration.

The goal of this study was to expand understanding of strain-generated potential (SGP) in ligamentous or tendinous tissues. Most SGP studies in the past have focused on cartilage or bone. Herein, rabbit patellar tendon (PT) was used as a model. Each patellar tendon had two Ag/AgCl electrodes inserted at axial positions of 1/4 and 1/2 from patellar to tibial insertions. Each specimen was electrically isolated, gripped in a servohydraulic test system, and then subjected to a short session of uniaxial haversine tension (2.5 percent maximum strain) at a frequency of 0.5, 1.0, 2.0, or 5.0 Hz. A cyclic (sinusoidal) electrical potential superimposed upon a larger transient (exponentially asymptotic) potential was consistently observed. Upon termination of loading, the cyclic SGP ended, and the shifted baseline of the SGP exponentially decayed and asymptotically returned to a residual potential which over all specimens was not different than the original potential. The transient and cyclic SGPs were frequency dependent (P < 0.001, P = 0.06, respectively). To our knowledge, this transient portion of the SGP, although theoretically predicted by Suh (1996, Biorheology, 33, pp. 289-304) and Chen (1996, Ph.D. thesis, University of Wisconsin-Madison) has not been observed in other experiments using different protocols. Additional PTs were dehydrated and the rehydrated in solution at different pH levels. The magnitude of SGPs increased in basic solution (pH 9.5) but diminished in pH 4.7 buffer. This pH dependency suggests that electrokinetics is the dominant mechanism for the transient and cyclic responses of the SGPs, although this study does not provide direct evidence.

Measured values of the swelling pressure of charged proteoglycans (PG) in solution (Williams RPW, and Comper WD; Biophysical Chemistry 36:223, 1990) and the ionic strength dependence of the equilibrium modulus of PG-rich articular cartilage (Eisenberg SR, and Grodzinsky AJ; J Orthop Res 3: 148, 1985) are compared to the predictions of two models. Each model is a representation of electrostatic forces arising from charge present on spatially fixed macromolecules and spatially mobile micro-ions. The first is a macroscopic continuum model based on Donnan equilibrium that includes no molecular-level structure and assumes that the electrical potential is spatially invariant within the polyelectrolyte medium (i.e. zero electric field). The second model is based on a microstructural, molecular-level solution of the Poisson-Boltzmann (PB) equation within a unit cell containing a charged glycosaminoglycan (GAG) molecule and its surrounding atmosphere of mobile ions. This latter approach accounts for the space-varying electrical potential and electrical field between the GAG constituents of the PG. In computations involving no adjustable parameters, the PB-cell model agrees with the measured pressure of PG solutions to within experimental error (10%), whereas the ideal Donnan model overestimates the pressure by up to 3-fold. In computations involving one adjustable parameter for each model, the PB-cell model predicts the ionic strength dependence of the equilibrium modulus of articular cartilage. Near physiological ionic strength, the Donnan model overpredicts the modulus data by 2-fold, but the two models coincide for low ionic strengths (C0 < 0.025M) where the spatially invariant Donnan potential is a closer approximation to the PB potential distribution. The PB-cell model result indicates that electrostatic forces between adjacent GAGs predominate in determining the swelling pressure of PG in the concentration range found in articular cartilage (20-80 mg/ml). The PB-cell model is also consistent with data (Eisenberg and Grodzinsky, 1985, Lai WM, Hou JS, and Mow VC; J Biomech Eng 113: 245, 1991) showing that these electrostatic forces account for approximately 1/2 (290kPa) the equilibrium modulus of cartilage at physiological ionic strength while absolute swelling pressures may be as low as approximately 25-100kPa. This important property of electrostatic repulsion between GAGs that are highly charged but spaced a few Debye lengths apart allows cartilage to resist compression (high modulus) without generating excessive intratissue swelling pressures.

We have constructed an electrokinetic surface probe capable of applying small sinusoidal currents to the surface of articular cartilage and measuring the resulting current-generated stress with a piezoelectric sensor. Using the probe, we have characterized the electromechanical response of excised discs of normal and chemically modified adult bovine femoropatellar groove cartilage. The measured stress amplitude was proportional to the applied current density and inversely proportional to the excitation frequency, consistent with a poroelastic model. As a function of bath pH, the stress amplitude exhibited a minimum in the range pH 2.4-2.8 and the phase underwent an abrupt 180 degrees transition in the same range, consistent with an electrokinetic mechanism as the origin of the current-generated mechanical stress. Digestion of the tissue with trypsin resulted in a progressive loss of highly charged proteoglycan molecules from the tissue, with a concomitant decrease in the measured stress amplitude. These results support the feasibility of surface measurements as a means of assessing electromechanical transduction in cartilage and of detecting subtle molecular-level degradative changes in the extracellular matrix. This technique of surface spectroscopy provides a new means of nondestructively measuring the material properties of cartilage on intact joints and detecting degradative changes such as those seen in the earliest stages of osteoarthritis.

Studies were conducted of some of the nonequilibrium, electrolyte-activated, electromechanical and osmotic processes that can affect the tensile properties of articular cartilage. We measured changes in tensile force that were induced by altering the ionic environment of strips of cartilage held at fixed length. We compared the kinetics of changes in these macroscopically measured isometric tensile forces to theoretical estimates of the time constants that characterize the underlying physical and chemical mechanisms occurring within the cartilage specimens during the experiment. Changes in the tensile force induced by changing the bath neutral salt concentration surrounding the specimen appear to be rate-limited by the diffusion of the salt into the specimen. That is, the mechanical stress relaxation process resulting from changes in salt concentration seems to be occurring at least as rapidly as the diffusion of salt into the matrix. When the bath concentration of CaCl2 or HCl is varied, the rate of change in the resulting isometric stresses indicates that Ca++ and H+ ions are binding to the cartilage matrix macromolecules.

The compressive stiffness of articular cartilage was examined in oscillatory confined compression over a wide frequency range including high frequencies relevant to impact loading. Nonlinear behavior was found when the imposed sinusoidal compression amplitude exceeded a threshold value that depended on frequency. Linear behavior was attained only by suitable control of the compression amplitude. This was enabled by real time Fourier analysis of data which provided an accurate assessment of the extent of nonlinearity. For linear viscoelastic behavior, a stiffness could be defined in the usual sense. The dependence of the stiffness on ionic strength and proteoglycan content showed that electrostatic forces between matrix charge groups contribute significantly to cartilage's compressive stiffness over the 0.001 to 20 Hz frequency range. Sinusoidal streaming potentials were also generated by oscillatory compression. A theory relating the streaming potential field to the fluid velocity field is derived and used to interpret the data. The observed magnitude of the streaming potential suggests that interstitial fluid flow is significant to cartilage behavior over the entire frequency range. The use of simultaneous streaming potential and stiffness data with an appropriate theory appears to be an important tool for assessing the relative contribution of fluid flow, intrinsic matrix viscoelasticity, or other molecular mechanisms to energy dissipation in cartilage. This method is applicable in general to hydrated, charged polymers.

An electromechanical model for charged, hydrated tissues is developed to predict the kinetics of changes in swelling and isometric compressive stress induced by changes in bath salt concentration. The model focuses on ionic transport as the rate limiting step in chemically modulating electrical interactions between the charged macromolecules of the extracellular matrix. The swelling response to such changes in local interaction forces is determined by the relative rates of chemical diffusion and fluid redistribution in the tissue sample. We have tested the model by comparing the experimentally observed salt-induced stress relaxation response in bovine articular cartilage and corneal stroma to the response predicted by the model using constitutive relations for the concentration dependent material properties of the tissues reported in a related study. The qualitatively good agreement between our experimental measurements and the predictions of the model supports the physical basis of the model and demonstrates the model's ability to discriminate between the two soft connective tissues that were examined.