Degenerative Disk Disease? There Will be Pain…Maybe
Why Do Spines Degenerate? Hi, I am Dr. John Shim, and this is a common question asked by many of my patients. I hope you had a chance to already view my basic spine anatomy video, as that foundation will make it much easier to understand this Spine Degeneration discussion. To simplify how the spine degenerates, we must make some basic assumptions. The degeneration is a result of aging, not a traumatic event, or known medical condition. As you know, the spine is composed of the functional spine unit. The so called vertebral body sandwich with the intervertebral disc being the center of the sandwich. For this discussion, we will focus on the intervertebral disc. The disc is composed of two main components. The center is the nucleus pulposus. The outer section is the annulus fibrosus. Current-day research indicates the water-filled nucleus changes as we age. There are specific proteins called proteoglycans that have a water capturing property. When we are young, the proteoglycan strongly pulls in water, keeping a nucleus full of water, and maintaining its height and strength. If you MRI most healthy teenagers, the MRIs will show the discs are full of water, and as a result of this water pressure the disc walls, or the annulus are straight. The disc height is full, and tall. There is plenty of space in the foramen, and the nerve has lots of room to safely exit the spine, without any concerns for pinching. As we age, the body starts to produce a different form of proteoglycan. To simplify the discussion, each time the small cells in our body divide to produce another, it loses a bit of a marker called the teleomere. As we lose more and more of the teleomere, our resultant cells start to change. The resultant cells, in the spine, and in the cartilage will produce different proteogylcans. The proteoglycan for aging spines will not have the same water retention properties as younger cells. As a result, the disc is less full of water. The disc height reduces. The water pressure on the annulus lessens, and the annular walls start to sag, or bulge. Yes, this is the disc bulge process. As the disc height reduces, the forces on the facet joints of the spine also change. There is more excursion of the facet joint, and the facet joints may start to slide further against each other. This leads to wear of the cartilage of the facet joints. Because of the wear on the facet joints can be a painful process, the body reacts to decrease the joint forces experience on the individual facet joints surfaces by an enlarging the facet joint surface area. The facet joint enlarge or hypertrophies. This process is how bone spurs form that pinch the nerves as they exit out the foramen. Not only is the height of the formen hole decreased by the shorter disc height, but the facet joints, that are located in the back of the foramen enlarges, and also contributes to the narrowing of the space for the nerves before exiting out the spinal canal. As the discs lose its water content, the disc bulge also contributes to the narrowing of the central canal space of the thecal sac or nerve tube, causing another area of pinching of the nerves. The narrowing of the space of the nerve is the spinal stenosis process. In addition to the narrowing of the space caused by bulging of the disc, and hypertrophy the facet joints, the narrowing of the foramen by the shrinking of the disc space, there is also an enlargement of the ligaments between the vertebral bodies. The ligament that most enlarges is the ligamentum flavum. To understand how this happens, imagine stretching a rubber band. When it's stretched, it thins. When the rubber band is no longer stretched, it thickens. The ligamentum flavum is a ligament that is stretched by a tall, well hydrated disc. When the disc shrinks because it is loosing water content (proteoglycan no longer holds water), it no longer stretches the ligamentum flavum. As a result, it thickens. That thickening can contribute to the narrowing or spinal stenosis condition. While I am oversimplifying the process, I hope you can now understand how the aging process of the spine begins with disc dehydration by change of the proteoglycans. It causes disc height shrinkage, leading to facet hypertrophy, disc bulging and ligament thickening. That all leads to spinal stenosis, and facet arthritis and hypertrophy. This process is called a degenerative cascade. Thank you for staying with this video, as this degenerative cascade process will help explain many of the pains experienced by many of my patients. Thank You!
Have you been told you have Degenerative Disk Disease? If so, welcome to the club. If you live long enough, you will get it. If you think about it, degenerative disk disease really should not be called a disease, as it is the normal process of disk degradation. It is like saying aging is a disease, which we all know it not true.
So, how does the degenerative disk disease process begin? It really starts almost from childhood. As you develop and grow, the intervertebal disk, does get subject to increasing forces. As we have discussed before, the disk is made of two components, the annulus, which is a tough woven outer structure that contains the softer, initially more jelly-like, full of water, shock absorbing center. With a full of water center, there is enough water pressure to maintain the side walls of the disk. The walls are nice and straight. The disk is also firm. With the height of the disk, and strength of the well hydrated disk, it acts as a spacer between the vertebrae. Secondary to the space, nerves can easily exit out of the spinal canal in the holes called the foramen. In the normal disk, there is plenty of room for the nerve to exit out the center of the spinal canal but, with a degenerated disk, the space narrows, potentially pinching a nerve in the process.
As we stated above, when the disk is full of water, the pressure in the disk is such that the disk is of a normal height. But, the side walls are also flat. The often used analogy is that of the car tire. When properly inflated, the tire side walls are flat, and the tire is firm, giving a better, more controlled ride. When the disk starts to lose its water content, which is a necessary component of the degenerative disk disease process, the side walls begin to bulge, and the strength and firmness of the disk reduces. In the side wall bulging process, it can also cause further narrowing of the space for the nerve sacs. By losing some of its firmness, it can allow extra motion of the vertebral bones upon one another.
This extra motion causes the facet joints, located in the back of the spine to rub excessively on one another causing development of facet joint arthritis. Also, the extra motion of the vertebra on one another can lead to instability or slippage of the vertebra, potentially causing further irritation of the nerves. This process can lead to what is called degenerative spondylolithesis.
Luckily, the process is usually very gradual. In fact, there are plenty of people who live their whole life without any significant episodes of back, neck or extremity pain, even though their backs and necks may look horrible to a spine surgeon like me.
In general, by age 20 about 15-20% of the population will have some evidence of degenerative disk disease. By age 40, it will be a majority that will have these changes.
And, unfortunately for most, degenerative disk disease can cause pain. The source of the pain can be secondary to the pinching of the nerves, or the arthritis of the facet joints, or the related pain associated with the muscles attaching to these joints. Also, secondary to the potential development of the spinal instabilities, the muscles often will need to work extra hard to keep the bones properly aligned and not rubbing against each other, or pinching the nerves. This extra work may explain the increasing muscle aches to the neck and back.
Also, with further collapse of these disks, and reduced heights, the overall normal body curvatures, Lordosis, and kyphosis, can be reduced. And, because of the reduction of curvature, and the arthritis to the facet joints, we experience increased stiffness as we age. For the more mature reading audience, I think all these points do sound familiar.
Fortunately, we usually learn to accomodate these changes, and we have medical treatments such as anti-inflammatory medications, muscle relaxers and narcotics to treat the severe episodes. If necessary, we can also use corticosteroid injections to accelerate the reduction of the inflammation. In certain relatively rare situations, surgery may be necessary to help with the pinched nerves or instability.
Help yourself by doing the proper things. Stay disciplined by not smoking, not gaining weight, and eating properly. Get exercise, and stretch.
But, in the end, we need to accept some of this is secondary to the aging process. As I discussed with my patients, it is better than the alternative.
- Cho IY, Park SY, Park JH, Suh SW, Lee SH. MRI findings of lumbar spine instability in degenerative spondylolisthesis. J Orthop Surg (Hong Kong). 2017 May-Aug;25(2):2309499017718907. PubMed PMID: 28685666