LLIF Plus Posterior Decompression for Severe Lumbar Spinal Stenosis

Inclusion Criteria: * Male and female patients who are 50 - 80 years of age. * Patients who present with symptomatic Grade I or II degenerative spondylolisthesis at one or two contiguous lumbar levels between L1 and L5 and who are surgical candidates for interbody lumbar fusion surgery; symptoms should include radiculopathy and/or neurogenic claudication with or without back pain. * Patients who have been unresponsive to at least 6 months of conservative treatments or who exhibit progressive neurological symptoms in the face of conservative treatment. * Patients who understand the conditions of enrollment and are willing to sign an informed consent to participate in the study. Exclusion Criteria: * Patients with lumbar pathologies requiring treatment at more than two levels. * Patients who have had previous lumbar fusion surgery. * Patients with congenital lumbar stenosis. * Patients with radiographic confirmation of Grade IV facet joint disease or degeneration. * Patients with noncontained or extruded herniated nucleus pulposus. * Patients with active local or systemic infection. * Patients with rheumatoid arthritis or other autoimmune disease. * Patients who cannot undergo magnetic resonance imaging (MRI). * Patients who are mentally incompetent. * Patients with BMI over 30kg/m2 or less than 18 kg/m2.