MagnetOs™ Granules vs. Autograft in Instrumented Posterolateral Spinal Fusion

Inclusion Criteria: * To be treated with instrumented posterolateral thoracolumbar spinal fusion with the use of iliac crest bone, with or without additional posteriorly inserted interbody devices (PLIF, TLIF), because of (1) deformity, (2) structural instability and/or (3) expected instability as a result of decompression for spinal stenosis; 1. Deformity is defined as a scoliosis in the coronal plane of \>20° and/or a sagittal balance disturbance according the SRS/Schwab classification on standardized standing full spine radiographs; 2. Preoperative instability is defined as a progressive angular deformity or spondylolisthesis in standing radiographs; 3. Decompression for spinal stenosis is done in the occurrence of radiological evidence of stenosis on MRI and clinical leg and/or back pain with one or more of the following phenomena: radiculopathy, sensory deficit, motor weakness, reflex pathology or neurogenic claudication. * Non-responsive to at least 6 months of non-operative treatment prior to study enrollment; * Fusion indicated for one to six levels in the T10 to S2 region. In case of vertebral osteotomies (PSO or VCR) the osteotomized segment will not be included in the assessment of the fusion rate; * Willing and able to understand and sign the study specific Patient Informed Consent; * Skeletally mature between 18 and 80 years of age. Exclusion Criteria: * Any previous surgical attempt(s) for spinal fusion (revision surgery) of the intended segment(s); * Previous treatments that compromise fusion surgery like irradiation; * Previous autologous bone grafting procedures that compromise the quality and amount of iliac crest bone grafting; * Indication for spinal fusion because of an acute traumatic reason, like a spinal fracture; * Active spinal and/or systemic infection; * Spinal metastasis in the area intended for fusion; * Systemic disease or condition, which would affect the subjects ability to participate in the study requirements or the ability to evaluate the efficacy of the graft (e.g. active malignancy, neuropathy, pregnancy); * At risk to be non-compliant e.g.: (recently treated for) substance abuse, detainee, likely to immigrate * Participation in clinical trials evaluating investigational devices, pharmaceuticals or biologics within 3 months of enrollment in this study; * Female patients who intend to be pregnant within 1.5 year of enrollment in the study; * Body mass index (BMI) larger than 36 (morbidly obese); * Being expected to require additional surgery to the same spinal region within the next 6 months; * Current or recent (\<1yr) corticosteroid use equivalent to prednisone ≥5mg/day, prescribed for more than 6 weeks.