Study to Assess the Performance of MagnEtOs Flex Matrix Compared to Cellular Allograft

Inclusion Criteria: 1. Patient is able to read/be read, understand, and provide written informed consent and has signed the IRB approved informed consent. 2. Male or female patient ≥ 18 years old. 3. Patients with leg pain, and/or back pain requiring up to four-level instrumented posterolateral lumbar/thoraco-lumbar fusion (T11 - S1). 4. Failed conservative treatment (physical therapy, bed rest, medications, spinal injections, manipulations, or transcutaneous electrical nerve stimulation) for a minimum period of 3 months prior to study enrollment. Exclusion Criteria: 1. Requires greater than four-level fusion or expected to need secondary intervention within one year following surgery. 2. Had prior PLF fusion or attempted PLF fusion at the involved levels. 3. Had previous decompression at the involved levels. 4. Women who are or intend to become pregnant within the next 12 months. 5. To treat conditions in which general bone grafting is not advisable. 6. In conditions where the surgical site may be subjected to excessive impact or stresses, including those beyond the load strength of fixation hardware (e.g., defect site stabilization is not possible). 7. In case of significant vascular impairment proximal to the graft site. 8. In case of severe metabolic or systemic bone disorders (e.g., osteogenesis imperfecta or Paget's Disease) that affect bone or wound healing. 9. In case of acute and chronic infections in the operated area (soft tissue infections; inflammation, bacterial bone diseases; osteomyelitis). 10. When intraoperative soft tissue coverage is not planned or possible. 11. Undergoing any procedure that allows MagnetOs to come in direct contact with the articular space. 12. Receiving treatment with medication interfering with calcium metabolism. 13. Has leg pain, and/or back pain related to benign or malignant tumor. 14. Has history or presence of active malignancy. 15. Has known substance abuse, psychiatric disorder, or a condition which, in the opinion of the investigator, may influence the healing or ability to comply with protocol requirements. 16. Is involved in active litigation relating to his/her spinal condition. 17. Has participated in an investigational study within 30 days prior to surgery for study devices