Zones of lateral canal as described by Lee. Entrance zone (1) is composed of cephalad and medial aspects of lateral recess, which begins at lateral aspect of thecal sac and runs obliquely down and laterally toward intervertebral foramen. Midzone (2) is located beneath pars interarticularis and just inferior to pedicle and is bound anteriorly by posterior aspect of vertebral body and posteriorly by pars; medial boundary is open to central spinal canal. Exit zone (3) is formed by intervertebral foramen.