Aspen Spinous Process Fixation System for Use in PLF in Patients With Spondylolisthesis

Inclusion Criteria: * Age between 50 and older * Diagnosis of mild segmental instability on standing radiography lateral film defined as degenerative spondylolisthesis ≤ 4mm in dynamic imaging (i.e., flexion/extension) * Scheduled for an elective single-level posterior decompression and posterolateral fusion using AspenTM spinous process fixation system * Oswestry Disability Index (ODI) v 2.1 score \>30% * Failed at least 3 months of conservative care (non-surgical) OR has clinical signs of neurological deterioration * Signed Informed Consent Form Exclusion Criteria: Pre-operative * Marked degenerative spondylolisthesis (\>4mm) * Lesions requiring surgery involving two levels * Prior surgery at index level or adjacent level * Prior spinal fusion surgery * Isthmic or lytic spondylolisthesis * Incompetent or missing posterior arch at the affected level (e.g. complete laminectomy, pars defect) * Requires complete laminectomy at level of surgery * Facet joints at implant level are absent or fractured * Vertebral body compromise or acute fracture at implant level * Body mass Index (BMI) ≥ 35 * Known allergy to titanium * Osteoporosis: SCORE (Simple Calculated Osteoporosis Risk Estimation) will be used to screen subjects requiring DEXA. Subjects with T-score \< -2.5 will be excluded * Paget's disease, osteomalacia, or any other metabolic bone disease * Use of medications or any drug known to potentially interfere with bone/soft tissue healing (e.g. chronic systemic steroids) * Planned use of additional segmental fixation (eg. facet screws, anterior/lateral plating, anchored cages) * Use of Bone Morphogenetic Protein (BMP)\* * Unlikely to comply with the follow-up evaluation schedule * In the opinion of the Investigator, Subject has history of chemical substance dependency or significant psychosocial disturbance that may impact the outcome or study participation * Active participation in a clinical trial of another drug or device * Active systemic infection or any other health condition that would preclude surgery * History of invasive malignancy, except if the subject has received treatment and displayed no clinical signs and symptoms for at least five years * Pregnant or planning to become pregnant during study participation * Involvement in active litigation related to back problems at the time of screening * Direct involvement in the execution of this protocol * Pre-existing conditions that could interfere with the evaluation of outcome measures (e.g. musculoskeletal, neuromuscular, etc.) Intra-operative Exclusion * Intra-operative visualization of a \> 4mm spondylolisthesis not previously noted radiographically * Any change in the surgical procedure that violates the inclusion/exclusion criteria. (eg. switching to double level procedure; placing pedicle screws as posterior fixation).