Occasionally, one of the 33 bones that comprise the spine “slips” out of position and moves forward onto the vertebra that lies next to it, resulting in a condition known as spondylolisthesis. Back and leg pain often ensues from this condition, which is believed to be caused most often from undue stress and strain being placed on the spine or from a deterioration of a part of the spine itself. And once spondylolisthesis is present, the lower spine can become abnormally shaped and the vertebral canal can become significantly narrowed.

Physicians classify spondylolisthesis into five categories. The dysplatic form—which is congenital or present at birth—is a result of a defect in the facet joint of a vertebra that allows it to slip forward. With isthmic spondylolisthesis there is malformation in the arched portion of the vertebra connecting the facet joints called the pars interarticularis, and the condition usually results from repeated injuries (half of all cases fall into this category) to the affected area such as those experienced by athletes. Additionally, isthmic spondylolisthesis can be classified further into three sub-categories: Type II A results from numerous and repeated fractures (or “stress fractures”) of the pars interarticularii; Type II B is similar to Type II A except that the pars interarticularii lengthen as the fractures are filled in with new bone growth; and Type II C, which, unlike the preceding types, is the result of a sudden and acute break of the pars interarticularii.

When arthritis changes the cartilage in the vertebral joints, degenerative spondylolisthesis—typically appearing in older people—occurs. And if an injury damages the vertebrae severely, resulting in a fracture to the lamina (the bone that forms the wall of the vertibra), pedicle (a segment of the vertebral body) or a facet joint, traumatic spondylolisthesis will see the vertebra slip out of place and forward. Lastly, pathologic spondylolisthesis occurs when a tumor or other disease affects the bone and makes it weak and therefore susceptible to slipping.

When someone is suffering from spondylolisthesis, pain in the lower back is the most common symptom experienced. Often, that pain worsens when the sufferer bends forward, and conversely the pain eases when bending backwards. Occasionally leg pain is also present, a result of the narrowing of bottom of the spinal canal and a corresponding crowding of the nerves located there.

Mobility and flexibility are not usually affected by spondylolisthesis, and indeed the abnormalities of the slip disc may be hard to even detect. Sufferers do report a tightening of the hamstring muscles as well as irregular bouts of back pain and, sometimes, a weakness or numbness in the legs or feet.

Depending on the overall health of the patient, the severity of the symptoms they’re experiencing and what type of slip occurred, treatment for spondylolisthesis may range from medication and physical therapy to the recommendation of some sort of bracing device. And a cessation or curtailing of certain activities will certainly be advised until the slip stabilizes. However, if nerves are being severely impacted and the symptoms that accompany it are severe, spinal decompression surgery—the removal of bone to allow for a relief of nerve compression, performed in conjunction with spinal fusion—may be an option.