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Isthmic spondylolisthesis is caused by a defect or fracture of the pars interarticularis (spondylolysis) In order to understand the progression of spondylolysis to isthmic spondylolisthesis, it is important to understand the conditions leading up to instability of the spine due to injury of the pars interarticularis.

There are three stages of injury of the pars (spondylolysis) that may result in slippage (spondylolisthesis):

• Stress reaction - this occurs from excessive wear and tear due to either activities of 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. Stress on the pars is the initiating factor in the development of spondylolysis. 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 resulting in spondylolysis. Once the pars is injured and the defect is created, healing of the fracture is often prevented by anatomic and/or biomechanical forces and leads to instability at the level of the fracture. This paves the way for the upper vertebra to slide forward (spondylolisthesis).

• Slippage - the gap caused by the fracture widens and as a result the L5 vertebra shifts forward over the sacrum. Slippage typically results from bilateral fracture of the pars which prevents the posterior articulating facets from providing adequate stability. At this stage, 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.

Spondylolysis

Spondylolysis was first described in the medical literature in the mid-1800s and is comprised of the words 'spondylo' (vertebra) and 'lysis' (destruction). As noted above, it involves the degeneration, defective development, or fracture of the pars interarticularis resulting in the potential for slippage of one vertebra over another (typically L5 over S1). It may begin as a stress fracture, but sometimes the cause is unknown. If the bone does not heal normally as it should due to forces exerted on the lower back, spondylosis develops. Normally, posterior facets, ligaments, and intervertebral disks provide resistance to forward displacement of the vertebrae. When there is a defect, fracture, or elongation of the pars, the posterior elements may no longer be strong enough to hold the vertebra in its correct anatomical position. This also increases the stress on the intervertebral disk and may cause it to stretch. If the posterior elements also slide forward, stenosis (narrowing of the spinal canal) and compression on nearby nerves occurs. If the defect in the pars is unilateral, there may be no slippage and no progression to spondylolisthesis.