When the spinal disc causes pain, it is possible to replace the symptomatic disc with an artificial disc. An artificial disc is able to alleviate pain from degenerative disc disease by returning normal function to the spinal segment.
Understanding how a spinal disc causes pain is a good first step in evaluating the potential benefits and drawbacks of artificial disc surgery.
Spinal Disc Anatomy and Function
Artificial disc devices currently approved by the FDA replace all parts of a natural disc. A spinal disc consists of three parts:
- Nucleus pulposus, the center portion of the disc containing a mix of water, sugars, and proteins. Healthy disc nuclei consist of about 77% water.3 The nucleus balances weight placed on the disc, transferring pressure outward.
- Annulus fibrosis, the exterior of the disc is made of overlapping ligaments. The annulus seals the nucleus and maintains flexibility, allowing for bending and twisting without rupture.
- Cartilaginous endplates, two pieces of cartilage that attach the upper and lower surfaces of the disc to the round, flat area of each adjacent vertebrae. The disc endplates play an important role in keeping the disc healthy as they act as gatekeepers for nutrients and water to enter the disc space.
See Spinal Discs
The intervertebral disc has several important functions, serving as a:
- Spacer. Spinal discs sit between each of the bony building blocks of the spine, called vertebrae, and make up nearly a third of the spine’s total height. The disc height provides the joints, muscles, and nerve roots with ample room to function properly. Each pair of vertebrae, with the disc in between, is called a spinal segment, and acts together as a motion joint.
- Shock absorber. Throughout the spine and at each spinal segment, the discs act to absorb loading pressure placed on the spine.
- Motion unit. The disc’s flexibility allows for forward, backward, rotational, and side-to-side movements while helping to stabilize the spine and keep it upright.
Artificial disc replacement surgery seeks to maintain disc function as much as possible, with the primary concern being to maintain movement at the spinal segment.
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How a Spinal Disc Causes Pain
Spinal discs typically have little innervation, meaning pain rarely comes from within the disc itself. Pain fibers are located in the annulus and outer portions of the disc.
Disc pain may be caused by other factors, such as:
- Inflammation. The proteins in the disc interior cause inflammation if they come into contact with nearby muscles, joints, and nerve roots. Inflammation can cause pain, increased muscle tension, muscle spasms, and tenderness around the affected disc, as well as sharp, shooting pains in the back of the leg if a nerve root is affected (sciatica).
- Disc space collapse. A disc’s height decreases as it degenerates and loses water content, placing undue strain on the joints and muscles that support the vertebrae. In some cases, a collapsed disc space can cause a nerve root to become pinched or irritated, leading to sciatica pain.
- Excessive micro-motions. With less cushioning and support from the disc, the spinal segment becomes gradually unstable, leading to excess motion between two vertebrae. This excess motion typically causes muscle tension and strain, may irritate the joints and nerve roots, and can result in formation of bone spurs.
More than one of these processes may occur at the same time as the disc degenerates. Pain may range from mild to severe, and may become worse with prolonged standing or sitting or with certain movements, such as twisting or bending the spine.
- Roberts S, Urban JP. Intervertebral Discs. In: Riihimäki H, Viikari-Juntura E, editors. Musculoskeletal System. Encyclopedia of Occupational Health and Safety. 6. Geneva: International Labor Organization; 2011.