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Your Discs
DISCS: YOUR SPINE’S SHOCK ABSORBERS
Did you know that a child is taller in the morning and shorter at the end of the day? Just measure your child’s height first thing in the morning, then measure it again before bed. You’ll be amazed to see the difference!
Our spine is a mechanical structure that supports our body when standing and sitting and allows us to bend, stoop, squat, twist and turn. With the exception of the first two vertebrae in our neck, each vertebra in our spine is cushioned by a piece of cartilage called a disc, which is held in place by ligaments connected from vertebrae to vertebrae.
OUR DISCS:
A disc is circular and made up of two types of cartilage: a strong balloon filled with a thick gel at the center, called the nucleus pulposus, and a tire-like material, called the annulus fibrosus, which surrounds the nucleus. The gelatin substance found in the nucleus consists of collagen, proteoglycans (PG) and water. The water binds itself with the PGs, and creates a cushion that acts like a shock absorber for our vertebrae during jarring movements such as walking, jumping and running.
As we age, the restraining fibers become calcified, meaning harden, break and lose their ability to keep the nucleus at the center of the disc. When the nucleus (balloon of gel) breaks through the border of the annulus (tire-like material) it is known as a herniated disc often inaccurately called a slipped disc. In reality, discs do not slip from side to side; they become deformed and herniated, also known as nuclear disc protrusions.
DISC DEGENERATION:
As we age, the amount of mucopolysaccharides and water in our discs diminishes from 85% water to only 65%. Young, healthy discs contain a clear border between the nucleus and the annulus. But as we age, our discs begin to degenerate or breakdown. Their consistency becomes more uniform, meaning the nucleus and annulus join together, and the substance becomes drier and flakier. The clear border found in a young, healthy disc is no longer present, making disc protrusions rare after the age of seventy.
As the fluid content of the disc decreases, biomechanical alignment changes. The annulus is no longer under constant tension in relation to the nucleus and is submitted to deformations and strains, which can tear the annular fibers of the weakest parts of the disc. Ruptured or cracked fibers (fissures) allow for further movement of disc material in the direction with the least resistance, usually backwards. When bending forward, the pressure sustained by the discs pushes the nucleus away and moves it towards the back; therefore greater stress will fall on the posterior (back) annular fibers, which are already stressed. Unfortunately, the posterior annular fibers are thinner and its ligaments are weaker than the anterior ones. This process usually results in intradiscal displacement, bulging behind the vertebral borders and disc prolapse and is responsible for backaches and sciatica.
KEEPING OUR DISCS HEALTHY:
Adequate fluid content within a disc is not a natural component, but depends on changes in pressure on the discs; increased load squeezes fluid out of the disc, whereas released pressure allows PGs in the disc to suck in fluid from surrounding tissue. Standing straight and tall or lying down, both low pressure positions, hydrates the disc, whereas sitting; bending or lifting squeezes, high pressure positions, expresses fluid out of the disc.
Since maintaining a disc’s nutrition depends largely on fluid flow, continuous change in the pressure it sustains is of utmost importance. Loading and unloading acts as a pump, and transports waste and metabolites to and from the discs. Therefore, it is important to keep intradiscal pressure as low as possible during activity in order to protect discs against early degeneration. Low-pressure levels can be achieved by adopting a slight lordosis, meaning an increased degree of forward curvature, at the lumbar spine and by regularly changing positions.
When pressure is applied in a symmetric (equal) fashion, the nucleus can distribute the force to all sides and therefore can evenly stretch the annular fibers. In this position, the disc is very strong and during high compressive loads, outward herniation of the nucleus is not seen. However, asymmetrical (unequal) loading creates tension, compression and stress at different locations in the disc encouraging disc displacements, herniation, bulging and protrusion.
CAN DISCS GIVE YOU PAIN?
Disc tissue itself is virtually insensitive, so that displacements are painless, except when they pinch or stretch other and more sensitive tissue, such as a nerve roots, adjoining muscles or tendons.
Because CT scans and MRI’s can only detect the size and the location of a disc displacement and not always show the effect on surrounding tissue, many more disc displacements are detected during imaging than actually cause symptoms. More than 50% of asymptomatic patients, meaning patients without symptoms, over 40 years old have apparent disc displacements. This means that more than 50% of disc displacements are painless.
However, what about the other 50%? Our bodies’ biomechanics often act like a chain reaction. In this case, when disc herniation is present, the adjoining bones, tendons and muscles shift to compensate for the change in alignment and are most likely the cause of your pain: local tendons can become inflamed (tendinitis), muscles irritated (myofascial pain) and nerves pinched (neuropathy).
As you can see, pain can be caused by different conditions and a single finding, such as a herniated disc can lead you down the wrong path. I hope this article has given you an insight on the biomechanics of your discs, has answered the question about children being taller in the morning and has made you understand the importance of being able to adequately describe the location and type of pain to your physician.
By Dr. Yong H. Tsai
Published in The Daytona Beach News-Journal
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