Anterior slippage of a vertebra with pars defect.
Pars defect without anterior slippage.
The Meyerding grading system is used to depict the severity of the spondylolisthesis. The sacral base is subdivided into 4 equal segments. A line drawn parallel to the posterior aspect of the L5 vertebral body will bissect a subsegment of the sacral base to indicate the Meyerding's grade.