Often the most difficult to diagnose and certainly the most controversial, the basic concept behind spinal instability is that there is an excess of motion between two spinal segments that results in pain or compromised neurologic function. The controversy in diagnosis arises, because in the majority of cases, the motion is not demonstrated, but only inferred solely by the presence of pain. We know that pain can be the result of excess motion. Since there is pain, we infer motion. The problem is that pain is often due to other factors and so that inferring that the pain is due to undetectable motion is a stretch. Surgery that is based on such an inference has a poorer success rate than surgery done in cases of demonstrable instability. In my opinion, this circular logic and the subsequent surgical treatments that follow is the main reason that spine surgery has developed such a poor reputation. What do I mean by this? Instability is often obvious on an X-ray. I had a patient that had had terrible back pain for a number of years. The pain would come on with activity and was relieved when he rested in bed. The patient had tried physical therapy many times and found that the therapy just made the pain worse. When I saw the patient in the office, the exam was unremarkable, as was the MRI. Because the history of the pain was so “mechanical” in nature, I obtained standing flexion and extension X-rays, which showed a clear motion between the L4 and L5 vertebral bodies (L4 moved forward relative to L5 when the patient flexed forward). Remember, the MRI was done with the patient lying on his back and would not have revealed the laxity between the two vertebrae because the pictures were “static” and done with the patient lying down.
This patient was treated by fusing the L4 body to the L5 body, which resulted in much less pain.Most of the time, such dynamic instability is not evident and the diagnosis is made on inference. For example, the facet joint may display excess water content suggesting stress from instability, or the marrow of the bone may show edema suggesting excess stress on the bone from an instability. Spondylolisthesis is a condition where the alignment of the vertebral bodies is altered, also suggesting instability (see below).
How does instability cause pain? Presumably, the supporting structures are stressed by the movement. This results in pain. Instability improves when motion is diminished. This diminishment can happen over time. Ligaments may shorten or become scarred and stiffen. Muscle strengthening may reduce the motion across the abnormal segment by increasing tone. Finally, surgery may eliminate the motion if the two bones are fused together.
This is an anatomically defined condition (a diagnosis made by an imaging study) that refers to one vertebra slipping relative to an adjacent vertebra. The term is derived from the Greek roots spondylo for vertebra and listhesis for slippage. There are many causes for this condition, but the most common causes are labeled as degenerative and isthmic (related to a fracture of a narrow structure connecting two larger structures). The former is thought to be the result of a weakening and ultimate incompetence of the facet joints. It is more common in women over sixty and is thought to be a consequence of the shape of the facet joint, the shape of the spine, and the weakened musculature. It occurs most commonly at the L4-5 segment of the spine.
The isthmic variety is more common in younger men and occurs most commonly at the L5-S1 segment. It is most commonly the result of a stress fracture of a part of the L5 bone called the pars interarticularis—an isthmus or narrowed part of the bone that connects the two larger joints above and below it. This stress fracture is surprisingly common. It likely is the result of an individual’s spine shape, the size of the isthmus, activities performed and growth of the patient. It occurs during the teenage years and may not ever be symptomatic. If it does become symptomatic, it is usually later in life. It is often seen in gymnasts or cricket bowlers and is pervasive among NFL linemen, all of whom require repetitive back extension. The symptoms associated with this condition are typically back pain and/or leg pain. When the symptoms are severe enough to warrant treatment, it is appropriately considered a form of spinal instability.