This forward slippage can be the result of many causes and is classified based on the reason for the slip. The majority of spondylolisthesis occurs in the low lumbar spine.
Congenital spondylolisthesis is caused by an anomaly of the bones of the spine.
Because of the abnormal orientation of the bones, the normal ability of the spine to resist slippage is lost and the vertebral body of L5 slips forward on S1. Congenital forms of spondylolisthesis were once thought to be very rare, but as our understanding of this process has improved and imaging with MRI s and CT scans has improved, this type of spondylolisthesis is being identified more frequently. The pars bone connects the upper joint of one vertebra to the lower joint. Spondylolysis simply refers to the defect without slippage. The defect can be painful in and of itself without slippage.