The purpose of this study is to determine differences in the XLIF and TLIF procedures with respect to perioperative variables in adult patients with low-grade symptomatic spondylolisthesis.

The study is a prospective multi-center randomized evaluation of clinical and radiographic outcomes of two commonly used lumbar interbody fusion procedures (XLIF and TLIF) in adult patients with low-grade symptomatic spondylolisthesis. The study seeks to identify differences between the procedures with respect to perioperative variables.

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

Study population will consist of patients from participating site locations.