Osmotic Swelling Behavior of Bovine Cartilage

Introduction Cartilage is a complex tissue whose extracellular matrix mainly consists of charged proteoglycan (PG) assemblies imbedded in a fibrous collagen network. PGs attract water and repel each other due to their negative charges generating a high swelling pressure. Collagens are proteins that form the fibrillar meshwork providing cartilage with its tensile strength. Cartilage extracellular matrix is composed of 70 to 80% water, 14 to 18% collagen (mainly collagen type II), 7 to 10% PGs, and about 1 to 2% other collagens (types IX, X, and XI). Between the large collagen fibers are a meshwork of smaller collagen fibers and the PG assemblies. The PG aggregates provide the cartilage with its swelling ability and compressive resistance under external load, while the rigid collagen is necessary for shape retention. Mechanically, cartilage exhibits nonlinear and anisotropic characteristics. In particular, its elastic properties are influenced by the anisotropic arrangement of the collagen fibers, which are parallel to the surface in the superficial zone, randomly oriented in the middle zone, and orthogonal to the cartilage-bone interface in the deep zone. The elastic modulus of articular cartilage is in the range 0.5–1.5 MPa. However, much higher values (up to 15-20 MPa) have been reported in joints during dynamic loads. The high dynamic stiffness of cartilage can be attributed to its relatively low permeability. Water cannot be easily squeezed out of the matrix but is pressurized thus supporting the high stresses. Experimental results also demonstrate anisotropy of diffusion of solute molecules. It was observed that the diffusion coefficient of macromolecules varied throughout the thickness of the tissue due to anisotropic diffusion in different zones. Various biomechanical models have been developed to interpret the anisotropic and non-linear behavior of cartilage matrix. However, an understanding of the relationship between the structure and the functional properties of the tissue remains incomplete at this time. The osmotic compression modulus that defines the load-bearing ability of the tissue can be obtained from the concentration dependence of the osmotic pressure. The aim of this study is to determine the osmotic properties of cartilage in the different zones as a function of the distance from the articular surface. Experimental Materials. A large number of cartilage samples from different sites of the bovine femoral head (20 to 26 months old animals) were studied. The cartilage covering the femoral head of the rear limbs was sampled. 5 mm wide and 0.5 mm thick sections were prepared representing the superficial, middle and deep zones (Figure 1).