Post-market Surveillance Study of FLXfit™ TLIF Interbody Fusion Device

Inclusion Criteria: * Adult (18-70 y/o) * Male or Female * With the following conditions of the lumbar spine as confirmed by advanced imaging (CT or MRI), who is a candidate for primary spinal fusion procedure according to acceptable criteria for such medical conditions: * Degenerative disc disease with up to Grade I spondylolisthesis * Spondylolisthesis * Failure of at least 6-months conservative treatment * BMI \< 40 * Patient to approve no pregnancy during the 24 months of study and no participation in other studies in parallel to this one * Ability to read, understand, and sign informed consent Exclusion Criteria: * Infection, local to the operative site * Signs of local inflammation * Fever or leukocytosis * Pregnancy * Significant mental disorder or condition that could compromise the patient's ability to remember and comply with preoperative and postoperative instructions (e.g. current treatment for a psychiatric/psychosocial disorder, senile dementia, Alzheimer's disease, traumatic head injury) * Prior surgical procedure (with the exception of decompression only procedure) at the index level(s) using the desired operative approach * Prior fusion procedure at an adjacent level * Any other condition which would preclude the potential benefit of spinal implant surgery, such as the presence of tumors or congenital abnormalities, fracture local to the operating site, elevation of segmentation rate unexplained by other diseases, elevation of white blood count (WBC), or a marked left shift in the WBC differential count * Neuromuscular disorder that would engender unacceptable risk of instability, implant fixation failure, or complications in postoperative care * Active local infection in or near the operative region * Active systemic infection and/or disease * Severe osteoporosis or insufficient bone density, which in the medical opinion of the physician precludes surgery or contraindicates instrumentation * Endocrine or metabolic disorders known to affect osteogenesis (e.g. Paget's disease, renal osteodystrophy, hypothyroidism) * Systemic disease that requires the chronic administration of nonsteroidal anti-inflammatory or steroidal drugs * Suspected or documented allergy or intolerance to implant's materials * Symptomatic cardiac disease * Patient unwilling to cooperate with postoperative instructions. * Any case where the implant components selected for use would be too large or too small to achieve a successful result. * Patient having inadequate tissue coverage over the operative site or inadequate bone stock or quality. * Any patient in which implant utilization would interfere with anatomical structures or expected physiological performance. * Prior fusion at the level to be treated. * Back VAS \< 4/10