The purpose of this clinical investigation is to evaluate the Aspen™ device compared to pedicle screw instrumentation for posterior fixation in the treatment of patients with degenerative disc disease and/or spondylolisthesis. The Aspen™ device will be compared to pedicle screw instrumentation in Oswestry Disability Index (ODI) improvement and success of fusion.

This is a multi-center, prospective randomized clinical study to evaluate the clinical outcome of subjects with degenerative disc disease (DDD) (defined as back pain of discogenic origin with degeneration of the disc confirmed by history and radiographic studies) and/or spondylolisthesis utilizing either Aspen Spinous Process Fixation System or Pedicle Screws for supplemental posterior instrumentation as an adjunct to an anterior or lateral approach to lumbar interbody fusion.

The primary hypothesis is that change in Oswestry Disability Index (ODI) of subjects in the study group (Aspen) will be non-inferior to the control group (pedicle screw instrumentation). The non-inferiority margin is pre-specified to be 10 ODI score points. If the non-inferiority is established, superiority of Aspen to the control will be tested.

The study will involve up to 25 investigational sites. Enrollment is expected to take approximately 24 months. Subjects will be followed for 2 years, with the anticipated duration of the study being approximately 4 years from start to finish.

Inclusion Criteria:

• Age between 18 and 75 years
• Scheduled for an elective single-level circumferential lumbar fusion by means of supplemental posterior fusion with fixation (allowing for the placement of ASPEN or pedicle screws) for an anterior or lateral interbody fusion
• Diagnosis of primary symptomatic Degenerative Disc Disease (DDD) and/or spondylolisthesis confirmed with appropriate imaging studies and/or positive lumbar discography
• Oswestry Disability Index (ODI) v2.1 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:

• Previous fusion at the operative level
• Spondylolisthesis Grade 3 or more
• 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: Simple Calculated Osteoporosis Risk Estimation (S.C.O.R.E.) > 6 AND Dual-energy X-ray absorptiometry (DEXA) T-score < -2.5
• 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)
• Planned use of Bone Morphogenetic Protein (BMP) for posterolateral fusion*
• 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
• Subject is a prisoner
• Pregnant or planning to become pregnant during the length of 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.)