Spondylolisthesis is the translational displacement or non-anatomic alignment of one vertebra relative to the adjacent vertebra. Doctors commonly describe spondylolisthesis as either low grade or high grade, depending upon the amount of slippage. A high-grade slip occurs when more than 50 percent of the width of the fractured vertebra slips forward on the vertebra below it. Patients with high-grade slips are more likely to experience significant pain and nerve injury and to need surgery to relieve their symptoms.