Lateral lumbar interbody fusion (LLIF), as a minimally invasive technique, is an indirect decompression technique, and its decompression effect is not as thorough as traditional posterior decompression surgery. For certain patients with severe lumbar stenosis, additional posterior decompression is required. However, whether additional posterior decompression is necessary for these patients is unknown. Radiographic predictors of failed indirect decompression via LLIF is unknown. In current randomized, controlled trial , the investigators compare the clinical outcomes of patients with severe lumbar stenosis who received LLIF plus posterior decompression and those without posterior decompression.

The current randomized controlled trial consists of two groups. Patients included in the study are with Schizas's stenosis grades C. Patients in one group received transpsoas lateral lumbar interbody fusion. Patients in the other group received transpsoas lateral lumbar interbody fusion plus posterior decompression. Patients will be followed for one year. Clinical outcomes of the two groups will be compared. And radiographic predictors of failed indirect decompression via LLIF will be analyzed.

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.