THEM BONES AND JOINTS: Breaking down the vertebra

Tom Neviaser

If you’ve seen a skeleton, you know that the spine looks like a long chain of similar-looking bones. Each individual bone is called a vertebra(VER-ta-bra); the plural is vertebrae (VER-ta-bray). A vertebra has three parts: the large, round, canister-shaped part, which bears most of the weight, is known as the centrum; behind the centrum is the vertebral (ver-TEE-bral) arch or dorsal arch, which protects the spinal cord and the nerves within the circle created. It also has two (right and left) oblong outcroppings or processes covered by a white glistening surface which make up one half of the joints (facet joints) that connect each vertebra to another; finally, the pointed midline outgrowth of this arch is the spinous process, or spine of the vertebra .

If you run your finger down the middle of someone’s neck or back, you can usually feel the tips of these spinous processes, and if   the person is quite thin, you can actually see the tips as well. Each vertebra is separated from the one above and below by

an intervertebral (inter-ver-TEE-bral) disc and circumferentia ligaments holding them all together.

A normal disc has a semi-gelatinous center, the nucleus pulposus(NEW-klee-us pul-PO-sus), surrounded by a dense ring of fibrous cartilage material called the annulus fibrosis (AN-ew-lus fie-BRO-sis)annulus meaning “ring” in Latin and fibrosis meaning many fibers.

This intervertebral disc is formed in such a way that forces placed upon it are dissipated throughout the disc space by the central nucleus pulposus, allowing flexibility of the spine and equal distribution of forces. A simplified way to think of it is by comparing the disc to a golf ball—a really old one, not the kind made today.

As a child, any time I got hold of one with a crack in it, I would remove the white cover and unravel what seemed a mile of rubber-band material to find a little rubber sphere filled with a thick jellylike substance. This made a terrific super-ball because this little sphere could be bounced high in the air and seemingly would keep bouncing almost forever. It certainly kept my attention for hours. I guess you can say this was my X-Box of the day.

The combination of rubber bands and super-ball core carried any force from outside the golf ball to its center where it is reversed sending the force back from whence it came. When a golfer teed off for a drive, the force of his striking the ball traveled to the bands and eventually to the jelly core, making the ball spring off the club head and fly forward. I believe the semi-gelatinous intervertebral disc, with its surrounding fibrous material, reacts to outside forces in a very similar way—a marvelous mechanical wonder. You could say, by compressing this type of golf ball down to a pancake shape, the white covering of a golf ball represents ligaments around the disc, the rubber bands represent the annulus fibrosis, and the super-ball is the nucleus pulposus.

But what would happen to the golf ball if the super-ball core dried up? The force of the club hitting the golf ball would not be distributed equally, right? The ball wouldn’t go far, and hitting it time and again would cause the rubber bands to break under the stress.

These old golf balls did become ineffective over time because the center would dry up and the rubber bands would eventually fragment and break. I believe something very much like this happens in our bodies, probably as early as age 20. For some reason, the “super-ball core” of our intervertebral discs dries up; water is absorbed from the gelatinous center. No one knows why, but this type of phenomenon occurs in many other areas of the body as you will see.

When it happens, the surrounding fibers, the annulus fibrosis,bear the brunt of forces placed on the spine in the course of a day. Doing a job they weren’t designed for, these fibrous rings fragment over time, breaking into small pieces like chunks of crab meat. These fragments can’t distribute the forces from outside, and those forces eventually get the upper hand, pushing the fragments around. In turn, the fragments press repeatedly against ligaments, and the ligaments give and stretch, bulging outward at the weakest point, usually on the right or the left of the back side or posterior aspect of the disc space.

Like any bodily tissue that’s stretched—such as gas-filled intestines or a fluid full urinary bladder—a stretched ligament causes pain that may be referred to nearby muscles. In the case of a bulging cervical disc, it’s the trapezius muscles we discussed above.

How a disc herniates will be covered next month!