What is Isthmic Spondylolisthesis?

Spondylolisthesis is when one vertebra (the bones that make up the spine) slips forward over the vertebra below it. Isthmic spondylolisthesis, the most common form of this condition, is caused by a bony defect or fracture in the pars interarticularis, an area located on the roof (lamina) of the vertebrae.

Isthmic spondylolisthesis most commonly affects L4-L5 (the fourth and fifth vertebrae of the lumbar spine—the low back) and S1 (the first vertebra in your sacrum—the region of your spine below the lumbar spine).

Like other types of spondylolisthesis, isthmic spondylolisthesis is given a grade based on how far forward the vertebra has slipped. This grade is important in determining treatment options.

• Grade I is a less than 25% slip
• Grade II is a 25-49% slip
• Grade III is a 50-74% slip
• Grade IV is a 75-99% slip
• Grade V is for a vertebra that has fallen off the vertebra below it (This extreme case of spondylolisthesis is called spondyloptosis.)

High grade spondylolisthesis is Grade III or above, and it can cause spinal stenosis, nerve compression, pain, and nerve injury.