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Low Back pain is the second leading cause of disability and work- related injury, affecting 50 million Americans annually. The spine including the disc space unit, is composed of joints. The disc has to adjust to the changing loads. In a 150 lbs. middle- aged man with a healthy back, the stressed axial load on the lower lumbar disc may be up to 2,200 pounds. This force if slowly applied to the vertebral end plates would cause them to fracture far sooner than it would cause a disc to herniate. The load on a disc may be as little as 55 lbs. in the supine position to 550 lbs. in the seated flexed position. Risks of low back pain include micro trauma, disuse, immobilization, sitting, postural stress, vibration, chemicals, driving, and heavy lifting. Of course your occupation can cause problems such as heavy work for greater than 3 years if done before 20 years of age. Exercise is beneficial but can also cause problems if only performed for 1 year or less or more than 15 years. Your back stays healthier if there is a moderate physical occupation. It is no wonder that the spine deteriorates. When it does deteriorate any one component or all components can deteriorate.
COMPOSITION
The back is composed of a bony frame- work that adds partial stability but not all. Bones can deteriorate with use, injury or time which leads to bony spurs called osteophytes, or fractures or compression injuries such as from osteoporosis. Not only can the nerves that innervate the bones be compressed from the fractures or bony spurs, they also can be irritated from edema or inflammation.
Helping to support and move the bones are the muscles, tendons and ligaments. These can become painful not only with over use but also with under use. These structures can become stretched with weight gain, irritated from vascular diseases caused by smoking and other abusive behavior, crushed or torn from injury or atrophied from sitting on the couch watching TV and snacking. The muscles, tendons and ligaments cause pain not only because the nerves that go from them carry their misery, they also cause pain because once they tighten they can irritate the nerves directly, leading to pain. If the muscles are too weak for forces on the spine, then the joints will deteriorate faster.
The joints take most of the back abuse since they are meant to be mobile yet stable. They have to adjust to forces in all planes. The main joints are the facets and the disc. The facets are true synovial joints that allow for flexion and extension and tend to resist axial loads, side- bending and rotation. When the loads exceed the capacity of the joints, the body will try to compensate. In the case of the facets they will thicken over time. The body, to take up the additional strain, will add bony spurs. This leads to narrowing of the central canal, narrowing of the foramen and erosion of the synovial surface causing back pain with movement, especially extension. When the synovium is eroded the nerves become irritated once again. The body will also assist the facets by thickening the ligament, further adding to the narrowed canal. This may then compress the nerves leading to decreased blood and nutrient supply causing a form of ischemia, which is felt as neurogenic claudication or radiculopathy.
The other joint was thought to be the best known - the disc. Surgery and knowledge of the lumbar herniated disc continues to progress since Vesalius first described the intervertebral disc in 1555. Its association to post axial pain was (Continued on page 14)
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