The aim of this study is to assess the efficacy and safety of ozone therapy (vs. placebo) applied into the epidural space through epiduroscopy in Patients with Failed Back Surgery Syndrome. It´s a Prospective, double blind, randomized study. The investigators will study 30 patients of both sexes between 18 and 70 years with chronic back pain after lumbar spine surgery for more than six months. The patients will receive the ozone gas or placebo (oxygen) in the lumbar epidural space, using a spinal endoscope inserted through the sacral hiatus by local anesthesia or sedation. The patients will be evaluated before the procedure, with 30 days, with 3, 6 and 12 months after intervention. This will be run by a researcher who will have no knowledge of the gas (oxygen or ozone) to be injected into the lumbar epidural space with the aid of an epiduroscopic. Will be applied before and after the procedure, pain scales (VAS, McGill, Neuropathic Pain 4, Neuropathic Pain Symptom Inventory), quality of life scale (WHOQOL), functional scales of pain disability (Roland Morris and Oswestry Disabilities Scales). Only one researcher will keep the data confidential until the end of the study.