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The intervertebral key (IVD) is crucial in the typical functioning of the spine. That is a cushion that fibrocartilage and the major joint between two vertebrae in the spinal column. There room 23 discs in the human being spine: 6 in the cervical region (neck), 12 in the thoracic region (middle back), and also 5 in the lumbar an ar (lower back).

IVDs enable the spine to be versatile without sacrifice a good deal that strength. They also provide a shock-absorbing result within the spine and prevent the vertebrae from grinding together. Lock consist of three significant components: the inner, nucleus pulposus (NP), the outer, annulus (AF) and the cartilaginous endplates that anchor the discs to adjacent vertebrae.

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Clinically relevant Anatomy

The IVD is composed of three distinct materials (Figure 2):

A central nucleus pulposus (NP);A peripheral annulus fibrosus (AF);Two vertebral endplates (VEPs).

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Figure 2: thorough structure the the IVD (adapted from Bogduk 2005)

Nucleus Pulposus

A gel-like framework that sits at the center of the intervertebral disc and also accounts for much of the strength and also flexibility the the spine. That is made of 66% come 86% water v the remainder consist of of primarily type II collagen (it may also contain kind VI, IX, and XI) and also proteoglycans. The proteoglycans encompass the bigger aggrecan and versican that tie to hyaluronic acid, and also several little leucine-rich proteoglycans. Aggrecan is largely responsible because that retaining water in ~ the NP. This structure additionally contains a low density of cells. When sparse, this cells produce the extracellular procession (ECM) commodities (aggrecan, form II collagen, etc.) and also maintain the integrity of the NP.<1>

Annulus Fibrosus

Consists of “lamellae” or concentric layers of collagen fibres <2>. The fibre orientation of every layer that lamellae alternative and therefore permit effective resistance that multidirectional movements. The AF includes an inner and also an outer portion. They differ generally in their collagen composition. While both are mainly collagen, the outer annulus contains mostly kind I collagen, when the inner has actually predominantly kind II. The inner annulus additionally contains much more proteoglycans 보다 the inner.<1> NB collagen form I: skin, tendon, vasculature, organs, bone (the key component of the organic part of bone) type II: cartilage (the key collagenous ingredient of cartilage and is much more flexible)<3>

Vertebral Endplate

An upper and a reduced cartilaginous endplate (each around 0.6– 1 mm thick) sheathe the superior and inferior aspects of the disc. The endplate patent diffusion and also provides the main source of nutrition because that the disc. The hyaline endplate is likewise the last part of the disc come wear through during severe key degeneration.

Plates that cartilage that bind the disc come their particular vertebral bodies.Each endplate covers almost the whole surface of the adjacent vertebral body; only a small rim of bone, referred to as the ring apophysis, about the perimeter of the vertebral body is left uncovered by cartilage.The part of the vertebral body to which the cartilaginous endplate is used is referred to as the vertebral endplate.The endplate covers the cell core pulposus in its entirety; peripherally it stops working to sheathe the entire extent that the annulus fibrosus.

Innervation

The disc is innervated in the outer few millimetres that the annulus fibrosus <5>.

Only the outer third of the AF is vascular and innervated in a non-pathologic state. In ageing and also states the inflammation, both nerve growth and granulation tissue development are stimulated. Additionally, the granulation organization secretes inflammatory cytokines, which additional increases sensitivity come pain sensations<1>.

Vascular Supply and also Nutrition

The IVD is largely avascular, through no major arterial branches to the bowl <6>. The external annular great are offered by tiny branches native metaphysical arteries. Just the external annulus is vascularized. Blood vessels close to the disc-bone junction that the vertebral body as well as those in the external annulus it is provided the NP and inner annulus. Glucose, oxygen and other nutrient reach the avascular areas by diffusion. The same process removes metabolites.<1>

Vital functions

Restricted IV joint motion.Contribution come stability.Resistance to axial, rotational, bending load.Preservation of anatomic relationship.It offers cushioning because that the vertebrae and reduce the stress brought about by impact.They plot shock absorber for the spine.

Biomechanics

Weight bearing: The disc is subjected to various loads, consisting of compressive, tensile and shear stress <8> <9>. Throughout compressive loading, hydrostatic pressure develops within the NP, which thereby disperses the pressures towards the endplates and the AF <10> <11> <10>. This mechanism slows the rate applied loads room transmitted to the nearby vertebra, offering the bowl its shock-absorbing abilities <12>.

Movement: The bowl is additionally involved in permitting movements between vertebral bodies, i beg your pardon include:

Axial compression / distraction;Flexion / extension;Axial rotation;Lateral flexion.

Nuclear migration: Asymmetric compressive loading bowl can cause the NP to move in a direction opposite come the compression <13> <12> <14> <15>. Because that example, throughout forward bending (or flexion) the the lumbar spine, the NP migrates posteriorly or backwards (Figure 4). Conversely, throughout backwards bending (or extension), the cell nucleus is squeezed anteriorly or forwards. This principle is recognized as the dynamic disc version <16>. Return NP migration has been displayed to law predictably in asymptomatic discs, a variable sample of migration occurs in people with symptomatic and/or degenerative IVDs <16>.

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Figure 4: Direction of atom migration in ~ the IVD throughout spinal movements (adapted native McKenzie 1981)

Physiologic Variants

Disc thickness generally increases native rostral come caudal. The thickness the the discs relative to the dimension of the vertebral body is greatest in the cervical and also lumbar regions. This mirrors the increased range of motion discovered in those regions.

In the cervical and lumbar regions, the intervertebral discs room thicker anteriorly. This create the an additional curvature of the spine – the cervical and also lumbar lordoses.<1>

Pathology

There are several terms to explain disc pathologies

Disc bulge ie the one of disc extends past the vertebral bodies.Disc herniation entails the NP. Disc herniation is far-reaching in the it may compress an surrounding spinal nerve. A herniated disc impinges upon the nerve connected with the inferior vertebrae (e.g., L4/L5 herniation affects the L5 nerve root). The most typical site of bowl herniation is in ~ L5-S1, which might be due to the thinning that the posterior longitudinal ligament towards its caudal end. There are three subtypes that herniations:Disc protrusion is characterized by the width of the base of the protrusion is more comprehensive than the diameter of the disc material that is herniated.In disc extrusion, the AF is damaged, enabling the NP come herniate past the typical bounds the the disc. In this case, the herniated product produces a mushroom-like dome that is more comprehensive than the neck connecting it come the body of the NP. The herniation may expand superior or inferiorly relative to the bowl level.In bowl sequestration, the herniated product breaks turn off from the body of the NP.Disc desiccation is typical in aging. The is brought about by the fatality of the cells the produce and also maintain the ECM, consisting of proteoglycans, such as aggrecan. The NP shrinks as the gelatinous type is replaced with fibrotic tissue, to reduce its functionality, and leaves the AF supporting additional weight. This enhanced stress leads the AF come compensate by increasing in size. The resulting flattened disc to reduce mobility and also may impinge top top spinal nerves causing pain and weakness. The is believed to be as result of proteoglycan breakdown, which reduces the water-retaining properties of the NP.

NB: far-reaching research has been placed into method of replacing/re-growing the intervertebral discs. The various methods encompass the replacement of discs with artificial materials, stem cell therapy, and gene therapy.

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Additional Points

There is no intervertebral disc between C1 and also C2, i m sorry is distinct in the spine.