The pars interarticularis is a small bone that connects one vertebrae to another in the spine. Sometimes this thin piece of bone can sustain a fracture, known as a spondylolysis. When this occurs, the affected vertebrae may become unstable and one may slip forward on the other. This condition is known as isthmic spondylolisthesis.
Isthmic spondylolisthesis occurs most frequently in children and adolescents. It most commonly happens in the lower back, in the lowest lumbar vertebrae. The initial spondylolysis fracture is typically caused by repeated stress to the pars interarticularis, which is not a particularly strong bone.
There are four grades of isthmic spondylolisthesis based on how far the top vertebrae slips over the other. Grade one is relatively mild, 25% or less, and grade two is 25% to 50%; most patients fall into one of these two categories. More severe grade three (50% to 75% slippage) and grade four (more than 75%) cases are less common and will cause very obvious physical symptoms, including a severe swayback, an abdomen that appears swollen outward, and possibly difficulty walking due to tightened hamstring muscles. In the most extreme cases, one vertebrae may completely slip off of the other; the condition is then referred to as spondyloptosis, and is also sometimes known as grade five isthmic spondylolisthesis.
When isthmic spondylolisthesis occurs, particularly the lower grades, it is often painless and causes no symptoms. Many people will never experience problems due to the condition, or symptoms may be very mild. Those who do develop back or leg pain often do so in adulthood, as the discs between the vertebrae start to break down from the force of the unsupported bones pressing on them. Higher grade slips, especially at levels three and four, may be more problematic due to the affect they have on posture and movement.
Treatment for isthmic spondylolisthesis, if necessary at all, is typically approached conservatively at first. Rest, oral steroids, and pain medication such as NSAIDs are often used, and in cases where pain is severe, steroid injections at the site may be helpful. Physical therapy and stretching exercises may also be prescribed, to strengthen the back and relax the hamstrings. Patients who do not find relief from these less invasive techniques may want to consider surgery. The main form of surgical correction for isthmic spondylolisthesis is lumbar spinal fusion; it is normally very effective for correcting the problem, but should be considered a last option as it is a major procedure.