Considerations on Joint and Articular Cartilage Mechanics

Abstract

When studying joint degeneration leading to osteoarthritis (OA), it seems imperative that local joint tissue loading is known during normal everyday movement and that the adaptive/degenerative effects of this loading are quantified systematically. Philosophically, we believe the best way to approach this problem is by studying joint degeneration and osteoarthritis in long-term experimental models and by representing diarthrodial joints and the associated tissues with accurate, geometric and structural, theoretical models. Here, we present selected examples of our work representing this approach. Experimentally, we demonstrate that the local loading of joints changes continuously in experimental models of OA, not only because of the changing external and internal loading, but also because of the continuous alterations in joint contact geometry and tissue mechanical properties. Furthermore, we show that single bouts of joint loading affect gene expression, and that gene expression, as well as subsequent joint degeneration is site-specific. In fact, opposing articular surfaces that are exposed to the same loading may degenerate at completely different rates. Finally, we propose a series of theoretical models of articular cartilage and contact mechanics, demonstrating that many of the anisotropic and inhomogeneous properties can be explained by structural elements and their orientation and volumetric concentration across the tissue.

The Mechanical Behaviour of Chondrocytes Predicted with a Micro-structural Model of Articular Cartilage

“…Therefore, the depth-dependent arrangement of structural elements and inclusions (collagen fibres and chondrocytes) greatly determines the material properties of articular cartilage. The depth-dependent stiffness predicted by the TI model agreed well with experimental results of Schinagl et al (1997) Herzog and Federico 2005;Fig. 4).…”

On the anisotropy and inhomogeneity of permeability in articular cartilage

“…We developed a model of a composite material with inclusion phases (such as collagen fibres) that have statistical orientation (Federico et al 2004). Using this model, we addressed issues of anisotropy and inhomogeneity for elasticity, and derived a transversely isotropic, transversely homogeneous model of articular cartilage (TITH model, Federico et al 2005;Herzog and Federico 2006).…”

Experimental measurement and quantification of frictional contact between biological surfaces experiencing large deformation and slip

“…This distance is obtained by taking the dot product of g M1 and the unit normal to the master surface, defined by the cross product of the two tangent vectors. (15) (16) (17) Contact is typically defined as a gap distance of zero (Fernandez and Hunter, 2005), (Pietrzak and Curnier, 1999), and negative magnitudes, representing overlapping surfaces, are prohibited. However, for experimental data where the outermost tracked points lie slightly below the surface, it is practical to define contact as a gap distance below a prescribed threshold, D contact ; negative gap distances would still be impossible, and distances lower than the contact threshold would be possible and represent compression of the surface-most tissue layers.…”

“…In recent years, advancements in the field of contact mechanics have been driven by the formulation of robust finite element approaches and increased computing power. Models have been developed for a variety of physiological (Herzog and Federico, 2006), and even patient-specific (Fernandez and Hunter, 2005), geometries, and more detailed macro-scale boundary conditions have been prescribed by realtime tracking of movement in human subjects (Fernandez and Hunter, 2005). In biological tissues, the complex nature of the materials (e.g.…”