Spondylolisthesis usually affects the lumbar spine, most often at the L4-5 level. In spondylolisthesis, a defect in the supporting column of bone allows the vertebra to shift forward or slip over the vertebra below it.
Isthmic spondylolisthesis refers to a slip that occurs as a result of a defect of the pars interarticularis. The spondylolytic defect may be present at birth but is usually acquired between the ages of 6 and 16 years. The child or teen may not even know there is a problem until an X-ray is taken for some other reason and the condition is seen.
Once the slip has occurred, it rarely continues to progress. This defect often develops over time as a result of a hyperextension injury. Athletes involved in gymnastics, ballet, and football seem to be affected most often.
As the name suggests, degenerative spondylolisthesis is age-related and can affect more than one vertebra. Women ages 50 and older seem to develop this problem. The same thing happens: the vertebral body and forward slip of the bone over the segment below it. But with degenerative spondylolisthesis, the shift narrows the spinal canal where the spinal cord is located. The result is pressure on the spinal cord, discs, and spinal joints with pain and sometimes even more serious symptoms.
Isthmic spondylolisthesis doesn’t progress to become the degenerative type. The greater concern is whether a low-grade isthmic spondylolisthesis (less than 50 per cent slippage) will progress to become a high-grade slip. It’s the high-grade slips that tend to cause symptoms, spinal instability, and even scoliosis (spinal curvature).