The importance of fluid-flow-induced shear stress and matrix-induced cell deformation in transmitting the global tendon load into a cellular mechanotransduction response is yet to be determined. A multiscale computational tendon model composed of both matrix and fluid phases was created to examine how global tendon loading may affect fluid-flow-induced shear stresses and membrane strains at the cellular level. The model was then used to develop a quantitative experiment to help understand the roles of membrane strains and fluid-induced shear stresses on the biological response of individual cells. The model was able to predict the global response of tendon to applied strain (stress, fluid exudation), as well as the associated cellular response of increased fluid-flow-induced shear stress with strain rate and matrix-induced cell deformation with strain amplitude. The model analysis, combined with the experimental results, demonstrated that both strain rate and strain amplitude are able to independently alter rat interstitial collagenase gene expression through increases in fluid-flow-induced shear stress and matrix-induced cell deformation, respectively.

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Mechanical loading of articular cartilage can influence chondrocyte metabolism and lead to alterations in cartilage matrix composition. Most previous studies have focused on the effect of cyclic loading on cartilage mechanical properties and proteoglycan synthesis. However, the role of proteoglycans synthesized from cyclically loaded cartilage in response to an acute overload has not been elucidated. Therefore, we conducted studies where low intensity, intermittent cyclic loading was applied to chondral explants prior to an acute unconfined compression on the tissue. The chondral explants were randomly assigned to three groups: 7, 14, and 21 days of 10 cycles of 0.2 Hz sinusoidal loading at 0.5 MPa followed by an unloaded interval of 3,600 s. All explants were then taken to 25 MPa of unconfined compression. Biochemical assays were conducted to determine the tissue proteoglycan and hydroxyproline contents. The results showed cyclic preloading increased the proteoglycan content and mechanically stiffened the explants, making them more resistant to matrix damage and cell death under 25 MPa of unconfined compression up to 14 days. After 21 days of cyclic loading, however, the explants lost compressive stiffness and suffered more extensive damage in the unconfined compression test. This study investigated the role of cyclic loading in response of chondral explants to a potentially damaging, acute overload. In the long term, these types of studies may help understand the role of preconditioning of articular cartilage for in vitro or even in vivo studies of blunt force trauma to a joint. ß

Effective Poisson’s ratio (EPR) of articular cartilage in compression is an important parameter, which is inversely correlated with stiffness of the collagen fibers [1]; and thus, if known, could provide valuable information about integrity of the collagen network in the tissue. Unfortunately, direct determination of the EPR by measuring lateral expansion during unconfined compression tests [2], while being effective, due to it’s destructive nature many times is not desired and/or hard to apply in practice. Optically-determined values of equilibrium EPR for bovine humeral articular cartilage using this method are reported to be in range 0.185±0.0065.

The rabbit model is often used for the study of the mechanical properties of articular cartilage. In numerous cases the authors are investigating the initiation and progression of osteoarthritis. The studies have shown that the mechanical properties of articular cartilage vary across the medial and lateral compartments of the tibial plateau. A review of these data indicates numerous inconsistencies in the location dependent mechanical properties and correlations with the macroscopic and microscopic characteristics of the tissue. For example, Hoch et al. [1] and Rasanen et al. [2] document a higher equilibrium tissue modulus in the medial than the lateral compartments of the tibial plateau, using a linear elastic model of the cartilage due to Hayes et al. [3]. In contrast, a recent study by Roemhildt el al. [4] documents a lower aggregate modulus in the medial versus the lateral compartments of the plateau, based on a linear biphasic model analysis. The study also documents a lower Poisson’s ratio in the medial than lateral facets, while more surface fissuring is noted in the medial compartment. While this surface morphology explains a higher permeability of the tissue in the medial than lateral compartments, the data are inconsistent with a notion set forth by Kiviranta et al. [5]. Using a fibril reinforced biphasic model analysis, this study would suggest a less structurally intact collagen network in the cartilage would yield a higher Poisson’s ratio in the medial than lateral compartments. These various inconsistencies in mechanical properties across the tibial plateau may be, in part, due to limitations of each computational model. The hypothesis of the current study is that a more structurally-based fibril-reinforced, biphasic model analysis of the rabbit cartilage will correlate better with the macroscopic and microscopic aspects of the tissue across the tibial plateau.