Will answer study question through other research means.

The purpose of this clinical investigation is to evaluate the clinical outcomes of subject with spondylolisthesis undergoing posterolateral fusion (PLF) surgery utilizing the Aspen™ Spinous Process Fixation System. The Aspen™ device will be evaluated using Oswestry Disability Index (ODI) improvement and success of fusion.

Evaluate the clinical outcomes of subject with spondylolisthesis undergoing posterolateral fusion (PLF) surgery utilizing the Aspen™ Spinous Process Fixation System. The Aspen™ device will be evaluated using Oswestry Disability Index (ODI) improvement and success of fusion.

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).

Patients 50 years and older with spondylolisthesis requiring a single level posterolateral decompression and posterolateral fusion