The pars interarticularis is a portion of the lumbar spine that joins the upper and lower joints together. The pars is normal in the vast majority of people, however some people are more susceptible to injuring this specific area.
There are three stages of injury to the pars interarticularis:
- stress reaction
- fracture (spondylolysis)
- slippage (spondylolisthesis)
After approximately 8 years of age, certain patients begin to experience abnormal growth and development of this particular region in the bone. Stress reaction or injury may occur when the bone experiences excessive wear and tear from activities of daily living, sports, or a fall. The symptoms may include lumbar pain, stiffness, and hamstring muscle tightness. X-rays may not reveal any abnormality. A bone scan will demonstrate the inflammation in the pars. Treatment consists of relieving the pain and restoring spinal flexibility. After several months, the majority of patients resume most activities.
If the pars "cracks" or fractures, the condition is called Spondylolysis. An X-ray or CT scan confirm the bony abnormality. Treatment is customized based on the severity of symptoms. Anti-inflammatory drugs, physical therapy, and activity modifications will be considered. Prior to a release to activities after the pain resolves, a course of truncal core muscle strengthening (pilates or yoga) may be prescribed to condition the muscles and minimize reinjury.
If the fracture gap at the pars widens, then the condition is called Spondylolisthesis. Widening of the gap leads to the fifth lumbar vertebra shifting forward on the part of the pelvic bone called the sacrum. Normally, the pars interarticularis stabilizes a bony hook that keeps the L5 vertebra from sliding downhill on S1. If the fracture removes this stability, the bone may move forward to varying degrees. Standing lateral spine X-rays are measured to determine the amount of forward slippage. Symptoms may include low back pain or pain in the buttocks or legs related to irritation of the nerve roots. Spondylolisthesis is called “Isthmic” when the chronic fracture leads to the slippage. Another common type of spondylolisthesis in adults is called “degenerative spondylolisthesis”.
Treatment is customized based on the severity of symptoms. Treatment is prescribed to decrease any acute spasm and restore spinal flexibility. Anti-inflammatory drugs, physical therapy, pars injections, and activity modifications will be considered. One of the mainstays of physical therapy treatment is to perform truncal core strengthening exercises. The therapist will caution the patient on avoiding hyperextension maneuvers and excessive abdominal "crunches". Prognosis is affected by the amount of slippage. In general, most patients with less than 50% slippage fend to fare well through adolescence. With slippage of 50% or greater, the potential for additional slippage with growth and aging is greater. The small numbers of patients who do not respond to conservative medical management are evaluated for a spinal fusion.
If the pain, spasm, or slippage increases despite conservative management, then the surgeon will discuss potential spinal fusion.
For a majority of children and adults, fusing the 5th lumbar vertebra to the sacrum is the first choice. The fusion involves removing the loose bony fragments and placing bone graft that will lead to the successful "gluing together" of the two vertebra. Often a cage full of bone is placed in the disc space to increase the likelihood of fusion.
Depending on the degree of the slippage, the bones may be realigned to various degrees. The most important steps are restoring stability and making sure the nerves have no pressure on them.