XLIF® vs. MAS®TLIF for the Treatment of Symptomatic Lumbar Degenerative Spondylolisthesis With or Without Central Stenosis.

Inclusion Criteria: \- Male and female patients who are at least 18 years of age; \- Patients who present with symptomatic Grade I or II degenerative spondylolisthesis at 1 or 2 contiguous lumbar levels between L1 and L5 and 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 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 2 levels; \- Patients who have had previous lumbar fusion surgery; \- Patients with lytic spondylolisthesis or a defect of the pars interarticularis; \- Patients with radiographic confirmation of Grade IV facet joint disease or degeneration; \- Patients with non-contained or extruded herniated nucleus pulposus