A 29 year-old man developed low back pain radiating down the right leg associated with numbness of the lateral foot.  Examination showed an absent right ankle reflex, weakness of right ankle plantar flexion and knee flexion, and patchy loss of sensation over the right lateral foot and posterior lateral leg.

T2-weighted MRI scans: (Left) sagittal; (Right) axial. Note the large disk herniation between the L5 and S1 vertebral bodies.  On the axial scans, the herniation posterior lateral, compressing the exiting right S1 nerve root, resulting in the clinical symptoms and signs of a S1 radiculopathy.  Most disk herniations are posterior lateral, not directly posterior, due to the presence of the tough posterior longitudinal ligament.