What Causes Nerve Pain In Spine Radiating Down The Thigh?

Thank you. I have nerve pain in the L3, L4, L 5 area that radiates thru the SI area and down the thigh. I have had an epidural and 2 sets of 2 to 4 injections. the last injections lasted well for 6 weeks and moderately for another two. Now the Dr. recommends burning the nerve. What can I expect in the way of length of recovery and the time the procedure will last successfully?