The Intervertebral Disc : Anatomy, Pathophysiology, and Aging

Intervertebral Disc

Starting from the space between the C2 and C3 vertebrae and ending at the L5‒S1 intervertebral space, there exist 23 IVDs that comprise nearly 25% of the total height of the spinal column, which is thickest in the lumbar spine and thinnest in the thoracic spine. The IVDs form a complex with the paired dorsal zygapophyseal joints responsible for the range of motion seen at each spinal level. The function of the three components of this three-joint complex, the IVD disc, and dorsal zygapophyseal joints, are intimately related, as are their effects on each other during degenerative changes.

There are three main components of the IVD: the central gel-like nucleus pulposus (NP), the denser annulus fibrosus that circumscribes the NP, and the bony and cartilaginous end plates (CEPs) that cap the IVD rostrally and caudally ( Fig. 6.1 ).

Nucleus Pulposus

The NP is the central core of the IVD and is the only part of the IVD derived from a remnant of the notochord originating from the endoderm, whereas the other components are derived from mesoderm. The NP is a soft and deformable gelatinous structure composed mostly of water, peptidoglycans, and collagen. Water forms the bulk of the NP’s weight, at 70% to 90%, and peptidoglycans represent roughly half of the NP’s dry weight. The most abundant type of collagen in the NP is type II. Although other collagen types, such as types VI and XI, are found in the NP, it is nearly devoid of type I collagen, unlike the outer annulus fibrosus (AF).

Aggrecans, a type of leucine-rich proteoglycan, are responsible for much of the NP’s loadbearing function and reversibly deformable properties. They are bound centrally to side chains of chondroitin and keratin sulfate and form aggregates by adhering to hyaluronan and collagen fibrils, which results in extensive cross linking that support the structure of the NP. Negatively charged side chains on aggrecans create a hydrophilic environment that promotes water absorption and retention, which is, in part, responsible for the NP’s reversibly deformable properties that are central to its loadbearing functions.

Elastin fibers, found throughout the NP, also contribute to the NP’s ability to retain its shape following loadbearing. Their orientation, observed in a radial distribution from the center to the periphery as well as vertically, which anchors the NP to the end plates, is also crucial to the NP’s ability to maintain its structure within the AF following deformation and transmit loads to the surrounding AF.

Annulus Fibrosus

Circumscribing the NP and making up most of the IVD’s volume is the AF. The AF is composed primarily of bundles of collagen fibers arranged in concentric lamellae, or layers, and can be separated into inner and outer segments. Each annulus is composed of 15 to 25 of these concentrically organized lamellae, and fibrils in adjacent lamellae are oriented in opposite directions giving the AF greater tensile strength when stretched. The fibrils in each lamella are generally oriented 30 degrees from the horizontal axis but can vary from 20 to 55 degrees. Collagen fibers also interact with extracellular matrix proteins to create much of the tensile strength and stiff structural integrity of the annulus.

The inner and outer annuluses have different compositions that have implications for their properties and functions. The inner AF, with a higher ratio of type II to type I collagen, is softer, less dense, and more flexible than the more rigid outer AF, which is composed of roughly 70% type I fibers by dry weight. Cells of the inner AF are more widely spaced compared with the more densely fibroblast-like packed ellipsoidal cells of the outer annulus. There is also a difference in water content between the inner and outer AF, as type II collagen has a 50% to 100% greater water content than type I collagen. The inner annulus also has a higher concentration of proteoglycan, at 30%, that decreases to 10% in the outer annulus. At the periphery of the outer annulus, fibrillar bundles known as Sharpey fibers extend superiorly and inferiorly to anchor the disc into the periosteal fibrils of the adjacent vertebral bodies.

Their aforementioned physiology determines the functions of the inner and outer AF. The inner annulus serves as a transitional zone between the dense fibrous region at the periphery of the disc and the gelatinous inner nucleus. The outer annulus connects the vertebrae above and below the disc and supports the NP’s structure by preventing it from herniating out of the disc space.