A pars defect at L5 with 4 to 5 millimeters anterolisthesis on S1 means the L5 vertebra has slipped onto the S1 vertebra. A pars defect is an injury to a vertebra that connects the facet joints of the spine, explains the University of Southern California Center for Spinal Surgery.
Anterolisthesis occurs when one vertebra slides forward on the one below, per Spine-Health. Defects to the pars are more formally referred to as spondylolysis, notes Jurch Performance Education. This condition is caused by a genetic weakness to the pars or by repeated microtrauma due to postural habits and repetitive activities. Symptoms associated with a pars defect include pain, loss of mobility, muscle spasm, postural changes and problems with walking.
Anterolisthesis has four graded levels, with level one being the least severe and level four indicating complete slippage off the lower vertebra, according to the Cedars-Sinai Foundation. Symptoms of this condition vary depending on the amount of slippage present, but include problems moving, changes in sensation, loss of bowel and bladder control, pain, and postural changes.