Time changes everything. Just as hair turns gray and skin wrinkles, the spine also tends to follow a series of changes. The changes that happen don’t always mean you’re heading for back pain or back problems. They’re merely a reality of aging. Medical scientists describe these predictable changes using a model—the degenerative model of back pain.
In this lesson, you'll need to rely on your knowledge of anatomy. It will help you begin to understand this series of natural changes that occur in the spine with the passage of time. You'll start to see why back pain is common and perhaps why it has affected you. The pain that occurs from these changes is usually mechanical pain. In this session, you'll learn the difference between mechanical pain and a more concerning type of pain called neurogenic pain.
During an office visit for low back pain, healthcare providers may describe how changes in the spine can sometimes cause back pain. The terms degeneration or degenerative disc disease may be used. Although the parts of the spine do change with time and in some sense degenerate, this does not mean your spine is deteriorating and that you are headed for future pain and problems. We're simply using these terms as a starting point for describing what occurs in the spine over time and how the changes may explain the symptoms you may be feeling.
Spine degeneration tends to follow a predictable pattern, or model. This pattern of changes is called the degenerative model of back pain. The model describes a sort of chain reaction of changes in the spine that can lead to back pain.
Recall from Lesson 2 that the discs between the vertebrae have two parts. The inner, spongy part is the nucleus. The ligament that surrounds the nucleus is called the annulus.
The first stages of the degenerative process affect these two structures. At first, the annulus around the nucleus weakens and begins to develop small cracks and tears. The body tries to heal the cracks with scar tissue. But scar tissue is not as strong as the tissue it replaces.
The torn annulus can be a source of pain for two reasons. First, there are pain sensors in the outer rim of the annulus. They create a painful response when the tear reaches the outer edge of the annulus. Second, like injuries to other tissues in the body, a tear in the annulus can cause pain due to inflammation.
The disc continues to change over time. At first, the disc is spongy and firm. The nucleus in the center normally contains a great deal of water, which gives the disc its ability to absorb shock and protect the spine from heavy and repeated forces. But with time, the nucleus begins to lose its water content and becomes dehydrated. The nucleus becomes thick and fibrous, so that it looks much the same as the annulus. As a result, the nucleus isn't able to absorb shock as well. Routine stress and strain begin to take a toll on the structures of the spine.
The disc loses water, causing it to lose some of its fullness and height. As a result, the vertebrae begin to move closer together, and the space between the vertebrae shrinks. This compresses the facet joints along the back of the spinal column. As these joints are forced together, extra pressure builds on the articular cartilage on the surface of the facet joints. This extra pressure can damage the facet joints. Over time, arthritis in the facet joints can develop.
These degenerative changes in the disc, facet joints, and ligaments cause the spinal segment to become loose and unstable. The extra movement causes even more wear and tear on the spine. As a result, more and larger tears often occur in the annulus.
The nucleus may push through the torn annulus and into the spinal canal. This is called a herniated or ruptured disc. The disc material that squeezes out can press against the spinal nerves. The disc also emits enzymes and chemicals that produce inflammation. Pain is caused by the combination of pressure on the nerves and inflammation caused by the chemicals released from the disc.
As the degeneration continues, bone spurs develop around the facet joints and the disc. No one knows exactly why these bone spurs develop. Most medical scientists think that bone spurs are the body's attempt to stop the extra motion between the spinal segments.
Bone spurs can cause problems by pressing on the nerves of the spine where they pass through the openings between the vertebrae (the neural foramina). Pressure around the irritated spinal nerves, called foraminal stenosis, can cause pain, numbness, and weakness in the low back, buttocks, and lower limbs and feet.
A collapsed spinal segment eventually becomes stiff and immobile. Thickened ligaments and facet joints, scarred and dried disc tissue, and protruding bone spurs prevent normal movement. Typically, a stiff joint doesn't cause as much pain as one that slides around too much. So this stage of degeneration may actually lead to pain relief for some people.
The spine changes described here occur with age, much like our hair turns gray. Conditions such as a major back injury or fracture can affect how the spine works, making the changes happen even faster. Daily wear and tear and certain types of vibration can also speed up degeneration in the spine. In addition, strong evidence suggests that smoking speeds up the degeneration. Scientists have also found links among family members, showing that genetics plays a role in how fast these changes occur.
To help you understand the cause of your pain, spine specialists sometimes divide low back pain into two categories:
Mechanical pain is caused by wear and tear in the parts of the lumbar spine. Think of it like a machine that is wearing out. Mechanical pain usually starts from degenerative changes in the disc. As the disc begins to collapse and the space between the vertebrae narrows, the facet joints may become inflamed.
Mechanical pain tends to worsen with activity and ease with rest. It is usually felt in the low back but may spread into the buttocks, hips, and thighs. The pain rarely goes down past the knee. It usually doesn't cause weakness or numbness in the leg or foot because the problem is not from pressure on the spinal nerves.
Neurogenic pain means "pain from nerve injury." Neurogenic pain occurs when spinal nerves are inflamed, squeezed, or pinched. A herniated disc or pinched nerve where it leaves the spine can cause this type of pain. Also, when a disc ruptures, chemicals are released that can inflame the nerves, even if there is no pressure directly on them. Neurogenic pain is more concerning to healthcare providers than mechanical pain because it can damage the nerves and lead to weakness and numbness in the legs.
Pressure, inflammation, and irritation can affect the spinal nerve and cause symptoms in the areas where the nerve travels, rather than in the low back. The problem nerve affects structures away from the spine, such as the skin, joints, or muscles controlled by the nerve. As a result, your back may not hurt, yet you could feel pain, numbness, or weakness in your leg or foot. This indicates there's a problem in the body's electrical wiring. Muscles weaken. Reflexes slow. Sensations of pins, needles, and numbness may be felt where the nerve travels.