Inclusion Criteria:  Age > 18 years old BMI > 40 or BMI > 35 with comorbidities due to obesity ( criteria for bariatric surgery) Follow up in Pompidou's Hospital Nutrition service Bariatric surgery intervention ( Sleeve gastrectomy or By pass) Follow up possible for one year or more Being covered by French social insurance Consent to participate in the study  Exclusion Criteria:  History of spine surgery Bariatric surgery postoperative complication Non mastering of french language MRI contraindication Patient under legal protection ( e.g. guardianship)