Swelling Pressure of Cartilage: Roles Played by Proteoglycans and Collagen

Abstract

Cartilaginous tissues such as articular cartilage and the intervertebral disc are called upon to function under very high pressures which they can do, thanks to the very special properties of their two major components, viz., the proteoglycans (PG) and collagen. The PG, a flexible polyelectrolyte of high fixed charge density has a high osmotic pressure and therefore a tendency to imbibe water and maintain tissue turgor while the collagen mesh, with its good tensile properties, resists undue swelling, thus enabling the proteoglycan-water mixture to exist as a concentrated solution (e.g. Fessler, 1960; Maroudas, 1973). The combination of the two components enables a cartilaginous tissue to exhibit flexibility and to withstand tensile stresses as well as high compressive loads (e.g. Maroudas, 1973; Maroudas, 1979; Weightman and Kempson, 1979). In Part I of the present chapter, we shall describe the organization of cartilaginous tissues and the factors which determine the levels of hydration under different conditions. In particular, we will consider in detail the role played by collagen and proteoglycans in determining the swelling pressure of cartilage. In Part II we shall describe how cartilage deforms when it is subjected to unconfined compression, with special reference to the factors resisting the change in shape and volume. We shall develop a mechanical model explicitly incorporating the osmotic pressure of the PG and the notion of two compartments.