Facts about Intervertebral Disc
The intervertebral disc (IVD) is a cushion of fibrocartilage between the two vertebral bodies in the spine. Vertebral column consists of 33 vertebrae (bones) connected by small joints and intervertebral discs.
There are 23 discs in the human spine: 6 in cervical region (neck), 12 in the thoracic region (middle back), and 5 in the lumbar region (lower back). Disc can be thought of as an organ composed of connective tissue.
They consist of three major components: the inner gelatinous nucleus pulposus (NP), the outer fibrous annulus (AF) and the cartilaginous endplates that anchor the discs to adjacent vertebrae.
The purpose of the disc is to provide flexibility to the spine along with being an effective shock absorber. Thus we can imagine the spine to be a structure comprising of multiple small bones with disc in between connecting the adjacent bones.
Clinically Relevant Anatomy
The IVD consists of three distinct components.
Nucleus Pulposus
A gelatinous structure that sits at the centre of the intervertebral disc. It is made of 66% to 86% water with the remainder consisting of primarily type II collagen and proteoglycans. The water content of the nucleus pulposus decrease with age.
Annulus Fibrosus
Consists of “lamellae” or concentric layers of collagen fibres. The fibre orientation of each layer of lamellae alternate and therefore allow effective resistance of multidirectional movements. The AF contains an inner and an outer portion. They differ primarily in their collagen composition. While both are primarily collagen, theouter annulus contains mostly type I collagen, while the inner has predominantly type II. The organic matrix of the disk increases from the periphery to the centre making the peripheral part weakest and most susceptible for disc prolapse.
Vertebral Endplate
An upper and a lower cartilaginous endplate cover the superior and inferior aspects of the disc. The endplate permits diffusion and provides the main source of nutrition for the disc.
Innervation
The disc is innervated in the outer few millimetres of the annulus fibrosus.
Only the outer third of the AF is vascular and innervated in a non-pathologic state.
Vascular Supply and Nutrition
The intervertebral disc is mostly avascular, with no major arterial branches to the disc. The outer annular layers are supplied by small branches from metaphysical arteries. Only the outer annulus is vascularized. Glucose, oxygen and other nutrients reach the avascular regions by diffusion. The same process removes metabolites.
Biomechanics
Weight bearing: The disc is subjected to various loads, including compressive, tensile and shear stresses. During compressive loading, hydrostatic pressure develops within the Nucleus pulposus, which disperses the forces towards the endplates as well as the annulus fibrosus. This mechanism slows the rate at which applied loads are transmitted to the adjacent vertebra, giving the disc its shock-absorbing abilities.
Movement: The disc is also involved in permitting movements between vertebral bodies, which include:
Nuclear migration: Asymmetric compressive loading disc can cause the Nucleus pulposus to migrate in a direction opposite to the compression.
Physiologic Variants
Disc thickness generally increases from top to bottom. The thickness of the discs relative to the size of the vertebral bodies is highest in the cervical and lumbar regions. This reflects the increased range of motion found in those regions.
In the cervical and lumbar regions, the intervertebral discs are thicker anteriorly. This creates the secondary curvature of the spine.
Pathology
There are several terms to describe disc pathologies
- Disc bulge ie the circumference of disc extends beyond the vertebral bodies.
- Disc herniation: involves the nucleus pulposus. Disc herniation is significant in that it may compress an adjacent spinal nerve. There are three subtypes of herniations:
- Disc protrusion: disc bulge wider than the vertebral bodies causing compression of the exiting roots on one side or both sides.
- Disc extrusion: Annulus fibrosus is damaged, allowing the Nucleus pulposus to herniate beyond the normal bounds of the disc.
- Disc sequestration: the herniated material breaks off from the body of the Nucleus pulposus and may migrate superiorly or inferiorly.
- Disc desication is common in ageing. The water content of the nucleus pulpous reduces with age from 90% in infancy to 74% in eighth decade. The NP shrinks as the gelatinous form is replaced with fibrotic tissue, reducing its functionality, and leaves the AF supporting additional weight. This increased stress leads the AF to compensate by increasing in size. The resulting flattened disc reduces mobility and may impinge on spinal nerves leading to pain and weakness.
Intra discal pressure
The pressure in the disc varies in different positions. There is influx and eflux of water in the disc in various activities and positions thereby giving it the flexibility it has.
Disc gains water in lying position and loses water in positions which increases the pressure inside the disc.
The whole aim of rehabilitation and treatment of prolapsed intervertebral disc is to reduce the intradiscal pressure and muscle strength building.
How Can We Help?
We at Indian Neuro Care are a dedicated team of Neurosurgeon, Orthopedic Surgeon and Physiotherapists. We have been providing a comprehensive care for the patients of chronic back pain that includes conservative treatment in form of medicines, physiotherapy and suggestions for lifestyle changes. When indicated, surgery is undertaken with most advanced technology assuring minimal hospital stay and risks. Our goal is to make the patient return to normal life style as early as possible.
