Spondylolisthesis Treated With an iO-Flex® System Enabled Decompression

Inclusion Criteria: 1. Adult (\> 18 years of age) 2. Leg/buttock pain, with or without back pain 3. Grade 1 degenerative spondylolisthesis (≤25% slippage) 4. NRS pain score for leg pain of 4/10 or greater 5. ODI score of 30/100 or greater 6. Failed non-operative medical management for a period of at least 6 months 7. Confirmed clinical diagnosis of lumbar spinal stenosis 8. Confirmation of central and/or lateral recess stenosis with or without foraminal stenosis, at one level or at two adjacent levels from L2-S1, with concordant symptoms. Subjects with central and/or lateral recess stenosis and spondylolisthesis at one level, may have symptomatic central, lateral recess or foraminal stenosis treated at one adjacent level. A "below-the pedicle" pass to decompress a third segment can be completed provided laminotomies are performed at no more than 2 levels. Subjects that, following an intraoperative decision, have a 3-level laminotomy and decompression will not be considered enrolled. Confirmation of spinal stenosis is shown on MRI or CT scan with evidence of nerve root impingement (displacement or compression) by either osseous or non-osseous elements. 9. Ability to give voluntary, written informed consent to participate in this clinical investigation and ability to participate in follow up examinations and complete patient questionnaires Exclusion Criteria: 1. Back pain only 2. Diagnosis of moderate to severe peripheral neuropathy such as diabetic or idiopathic peripheral neuropathy, by EMG/NCS or a Neurologist. 3. History of pathologic fractures of the vertebrae 4. Primary disc pathology; planned or incidental discectomies will NOT be excluded as long as surgeon believes the disc pathology is not the primary reason for the surgical intervention. 5. Subjects for whom removal of the midline structures is planned, likely or is the result of an intraoperative decision; every attempt should be made to spare the supraspinous, interspinous and facet capsular ligaments. 6. Significant instability of the lumbar spine as defined by ≥ 4mm translational motion between standing lateral view flexion and extension radiographs 7. Prior surgery of the lumbar spine 8. Spondylolisthesis greater than grade 1 (on a scale of 1 to 4) 9. Spondylolysis (pars fracture) at any level in the lumbar spine 10. Degenerative lumbar scoliosis with a Cobb angle of ≥ 25° 11. Vascular claudication in the lower extremities 12. Cauda equina syndrome 13. Evidence of active (systemic or local) infection at time of surgery 14. Paget's disease at involved segment or metastasis to the vertebra, osteomalacia, or other metabolic bone disease 15. Tumor in the spine or a malignant tumor except for basal cell carcinoma. 16. Prisoner or transient 17. Any significant psychological disturbance past or present, psychotic or neurotic that could impair the consent process or ability to complete subject self-report questionnaires 18. Involved in pending litigation of the spine or worker's compensation related to the back 19. Inability to communicate clearly in the English language 20. Morbid obesity (BMI \> 40) 21. Pregnant, nursing, or planning on becoming pregnant. 22. History of narcotic abuse 23. Current involvement in another drug or device clinical trial 24. Uncontrolled diabetes 25. Irreversible coagulopathy or bleeding disorder (patients on anticoagulant therapy may participate. Investigators should follow routine practices for perioperative discontinuation and reinitiation of anticoagulants.) 26. Plans to relocate in the next 2 years 27. Subject unwilling to undergo a blood transfusion, if necessary