ABSTRACT
Intervertebral discs (IVDs) are the structures lying between the vertebral bodies, and which function is to transmit the mechanical loads exerted on the spine during motion to assure stable mobility. The stress redistribution is achieved by the nucleus pulposus (NP), a water-rich, compressible core, which is surrounded by the annulus fibrosus (AF), a firm, fibrous outer layer. Both AF and NP interface with two end plates that are thin, horizontal layers of hyaline cartilage located at the top and bottom portions of the adjacent vertebral bodies. The IVD structure is presented in Figure 4.1a. The AF consists of concentric lamellae with alternating orientation, creating an angle-ply structure depicted in Figure 4.1b, and is composed of highly organized collagen fiber bundles, rich in type I collagen. These specific composition and structure provide to the AF high resistance to circumferential, longitudinal, and torsional stresses that are created by intradiscal pressure and by spine motion. The NP is a gelatinous matrix made of proteoglycans immobilized in a fibrous collagen network, comprised mainly of aggrecan and type II collagen as indicated in Figure 4.1b. This peculiar extracellular matrix creates a highly osmotic environment that attracts and retains water within the tissue, enabling the NP to sustain high compressive loading. The IVD is also the largest avascular structure in the body. Supply of nutrients is solely insured by diffusion from blood vessels at the disc’s margins, mainly through the end plates (Ogata and Whiteside 1981; Shirazi-Adl, Taheri, and Urban 2010).
