Every patient will MR images of the lumbar spine before the surgery. The severity of stenosis is classified according to the cross sectional area of the dura sac at the axial plane on T1-weighted MR images—severe stenosis for less than 76 mm2, moderate stenosis for between 76 and 100 mm2, and mild stenosis for more than 100 mm2. When MRI is contraindicated, CT-myelography will be used instead.