The purpose of this study is to explore the changes in objective walking parameters induced by medullary stimulation in patients with Fail Back Surgery Syndrome, after chronic stimulation, in comparison with preoperative condition.

After the informed consent signature, a first assessment will be carried out, including: quantified analysis of walking (AQM), felt effort questionnaire, pain questionnaires, functional abilities, limitations, goals and quality of life.

The implantation of the spinal cord stimulation system will be performed in two stages: the first stage, the electrode alone will be implanted and connected to an external test stimulator, allowing a one-week stimulation test.

If the test is positive (EVA pain improvement > or = 50%), the second step will be to connect this electrode to a Spectra WaveWriter™ internal stimulator. This stimulator delivers three different stimulation modalities: tonic, microburst and high frequency. The activated initial stimulation mode will be the tonic one.

After 3 months of "tonic" stimulation, the patient will be treated successively and in random way by 3 other different stimulation modalities during a week: microburst, high frequency, stopped stimulation (Off). The AQM and the different questionnaires will be done again, which will determine which stimulation modality is most beneficial to the patient.

Inclusion Criteria:

• Age > 18 year old and < or = 70 year old
• Patients with chronic neuropathic lumbo-sciatalgia (DN4 score > or = 4/10) as part of a moderate to severe spinal surgery failure syndrome (EVA >50/100), in who a collegial and multidisciplinary indication of medullary stimulation treatment was chosen.
• Patients awith Social Security

Exclusion Criteria:

• Vulnerable persons
• Patients who are invalid and unable to perform a quantified walk analysis
• Contraindication to surgery or anesthesia
• Patient not able to complete questionnaires
• Woman of childbearing age without effective contraception