Different procedure of percutaneous endoscopic lumbar discectomy (PELD) was with ventral decompression of dural sac on the lumbar spinal stenosis remains unkonwn.The traditional transforaminal endoscopic spine system (TESSYS) of PELD has been used in clnical for many years, but cannot achieve dorsal decompression. A newly developed modified TESSYS procedure, "U" route PELD combining ventral and dorsal decompression was introduced. Nevertheless, the superior between TESSYS and "U" route PELD procedures on treating LSS remains unknown. This study is desinged to recruit degenerative lumbar spinal stenosis patients who underwent TESSYS or "U" rout PELD, recruited from January 2014 to December 2017. These patients will be followed up for 2 years, and assessed the minimum dura sac cross sectional area (mDCSA) by MRI, and visual analogue scale (VAS) and Oswestry Disability Index (ODI) at pre- and post-operation. The global clinical outcomes were evaluated using modified MacNab criteria postoperatively. Thus, comprehensively evaluate the safety and therapeutic effects of the two PELD procedures on LSS treatments.

Inclusion Criteria:

• (1) patients with the diagnosis of degenerative lumbar spinal stenosis (LSS) (central stenosis with or without lateral recess stenosis) on mono or double segments, with the imaging evidence of magnetic resonance images (MRI) and computed tomography (CT); (2) patients presented low back pain, limp and sciatica, and have accepted conservative treatment more than 3 months but failed in symptoms relief; (3) patients agreed to accept TESSYS or the "U" route PELD procedure over other spinal surgeries; (4) patients had lumbar MRI imaging examines in our hospital at pre- and post-operation.

Exclusion Criteria:

• (1) patients with spinal instability, including dynamic instability or more than Grade II spondylolisthesis; (2) patients had spinal surgical history; (3) patients with peripheral nerve disease, systematic infection, bleeding diathesis or high risk of bleeding that cannot tolerate the surgery; (4) patients with mental illness and were uncooperative; (5) patients lost to the follow up.

The degenerative lumbar spinal stenosis patients who underwent TESSYS or "U" rout PELD, recruited from January 2014 to December 2017.