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There are three stages of injury of the pars interarticularis:

• Stress reaction - this comes from excessive wear and tear due to either daily living, lifestyle, sports, or trauma (e.g., falling). Nothing unusual appears on x-ray images but a bone scan may show increased activity in the region of the pars. Symptoms associated with this stage include:

  • low back pain
  • low back stiffness
  • tightness of the hamstring muscle - a group of large, powerful muscles that passes along the back of the thigh, from the lower pelvis to the back of the shin bone.

• Fracture - fracture or crack in the pars that can be seen on x-ray. It may occur due to repetitive extension or rotation of the lower back where the pars cannot absorb the constant shock and, consequently, develops a stress fracture. Once the pars is injured and the defect created, healing of the fracture is often prevented by anatomic and/or biomechanical forces. This leads to instability at the level of the fracture that paves the way for the upper vertebra to slide forward. This situation creates the progression to spondylolisthesis.

• Slippage - the gap caused by the fracture widens and as a result the 5th lumbar vertebra shifts forward onto the section of the pelvic bone called the sacrum. If the fracture is bilateral and the posterior articulations no long provide stability, anterior slippage of L5 over the sacrum takes place. At this level, the condition is called spondylolisthesis. Because the vertebral motion is abnormal and is chronic, the pars cannot heal properly. As new bone forms around the fracture, it may compress nearby nerves.