The Pain Generator

The Source of the Pain—“Pain Generator”

It is an almost universally held concept among surgeons and patients that a specific structural lesion is usually the source of pain. If that lesion can be identified and repaired, the pain will resolve. This seems plausible. It seems likely in light of the intensity of back pain that a diagnostic test ought to be able to identify the source of intense pain and point to a solution.

During my first five years of practice it was my assumption that if a patient had experienced low back pain for six months then it was my role to simply find the anatomic source of pain and surgically solve it. I was diligent in this regard. The test I relied on most heavily was a discogram. It is a test where dye is injected into several discs in your lower back. If the patient’s usual pain was produced at a low injection pressure it was considered a positive response. The only patients I did not fuse were those who did not have a positive response or had more than two levels that were positive. I performed dozens of low back fusions and felt frustrated when I could not find a way to surgically solve my patients’ low back pain.

I have a physiatrist friend, Jim Robinson, who is a strong supporter and contributor to the DOCC project. From 1986 to 1992 we both served on the Washington State Worker’s Compensation clinical advisory board in regards to setting standards for various orthopedic and neurosurgical procedures. Our discussions were based on this assumption that there always was an identifiable “pain generator.” It was just a matter of figuring out what test was the best one to delineate it. We did not think in terms of structural versus non-structural sources of pain. We knew about the role of psychosocial stress but did not fully appreciate how large a role it played.

I am defining a structural lesion as one that is distinctly identifiable on a test and the symptoms match the lesion. An example would be a ruptured disc pinching a nerve that causes pain down the leg in the distribution of that nerve. A ruptured disc between the fourth and fifth lumbar vertebrae will cause pain down the side of the leg. This is the pathway of the fifth lumbar nerve root. A ruptured disc between the fifth lumbar and first sacral vertebra will cause pain down the back of the leg, which is the pattern for the first sacral nerve too. If in either of these two examples the pain was going down the front of the leg, it would not be considered the cause of the pain because that is the path of the fourth lumbar nerve root and it does not match.

A non-structural lesion is one where the source of the pain is not distinctly identifiable on a diagnostic test and/or the symptoms do not match.

Degenerated discs are the most debatable example. The “pathology” identified has a significant chance of being normal for that person’s age. Another type of non-structural pathology is that which is identified by a subjective test such an injection into a disc or facet joint. There is a lot of variability in the technical performance of the testing and the patient’s response.
This logic of feeling that all pain has an identifiable source overlooks several key points: First, the soft tissue injury can occur at a level that is below the sensitivity of any diagnostic test. In another scenario, tissues can be irritated without being torn—another undetectable injury. The irritation occurs through inflammation, which is a chemical, not mechanical, irritation.
Patients may become frustrated when pain from an “undetectable” injury doesn’t let up. They begin to feel that no one believes them. How can a problem that cannot be detected continue to cause so much prolonged misery?

However, if you think this through, this type of injury is far more likely to persist than, say, a broken bone. In the spine, once the soft tissues are irritated, they may stay irritated through normal daily activities, sometimes almost indefinitely. If you severely sprained your ankle and kept re-spraining it on a daily basis, how long would it remain painful? On the other hand, broken bones heal in three to four months. The prognosis for a fractured spine has been shown to be better than the prognosis for a muscle sprain.
One of my own “soft tissue” afflictions is tennis elbow. I may set it off when I lift too heavy of a weight at the gym or when I practice my terrible golf swing. I will then suffer for the next six to 18 months with severe pain in either one or both my elbows. It hurts to shake hands, reach up and adjust the lights during surgery, use the surgical instruments, and countless other routine activities. The pain is as severe as any pain I have experienced, and it is persistent. Two years after my last episode, I can still push on the spot on my elbow and slightly feel the irritated area. Yet if I were to have an X-ray, MRI, CT scan, or bone scan of the area, the results would be completely negative. If I were to have a biopsy, there would probably be some inflammatory cells in the tendon area. However, since a biopsy would not change treatment, there would never be a need to do one.

Patients often wonder how soft tissue can be so painful. We know that soft tissues are loaded with pain fibers. They are arranged in a spider web type pattern that are very small and numerous. These irritated soft tissues give rise to some of the most painful conditions such as plantar fasciitis, tennis elbow, muscular tension headaches, chondromalacia of the kneecap, and countless more. Even a heart attack is fundamentally a muscular pain: the heart muscle lacks oxygen and the soft tissue pain fibers around the heart muscle are stimulated.

Although we often cannot identify the exact structural source of the pain, we do know that pain fibers are being stimulated and are sending messages to the brain. The intensity of the pain may increase if more pain fibers are stimulated in your back or if the sensitivity of the brain increases. The final perception of the pain will depend on how many pain areas that are stimulated in the brain. We clearly acknowledge that it is not “imaginary” pain, which is being experienced. It does not matter why the pain fibers in the brain are firing. They are firing and causing real pain.

The term commonly used for low back pain is “axial” pain. This means that pain is located in your “axial skeleton” or the center of your body. There is very little pain in your leg. This discussion is not relevant if your pain is primarily in your leg, a symptom commonly called sciatica. A pinched nerve either from a bone spur or ruptured disc usually causes this pain. The location of sciatic pain will follow the pathway of that specific irritated nerve. Often there is very little if any low back pain. The treatment options for sciatica are specific and much different than those for patients with axial low back pain.

Patients with low back pain may experience vague leg pain, which is typically much less severe than the back pain, doesn’t travel below the knee, and is fairly diffuse. It is called “referred” pain. Nerves that reach the back muscles also indirectly travel to leg muscles and tendons. One way to conceptualize this phenomenon is to compare it to a rock being thrown into a quiet pond. The pain can end up in the leg from a “ripple” effect.

Generally, when low back pain is associated with minimal leg pain, we do not know the exact cause. Potential causes include the disc, the muscles around the spine, the ligaments that hold the spine together, the facet joints that keep the spine aligned, or a combination of all of the above. Physicians can make an exact diagnosis only about 15 percent of the time. (1)Your body is designed to hurt whenever a tissue is injured, so when you feel low back pain, it indicates that the stress threshold for a specific part of the spine has been exceeded. At a minimum, treatment tries to decrease stress to the whole lower back to so any injured structure can heal.