Extraforaminal entrapment of the fifth lumbar spinal nerve by osteophytes of the lumbosacral spine: anatomic study and a report of four cases.

STUDY DESIGN: An anatomic study of the associations between the fifth lumbar spinal nerve (L5 spinal nerve) and a lumbosacral tunnel, consisting of the fifth lumbar vertebral body (L5 vertebral body), the lumbosacral ligament, and sacral ala, and clinical case reports of four patients with lumbar radiculopathy secondary to entrapment of the L5 spinal nerve in the lumbosacral tunnel.

OBJECTIVES: To delineate the anatomic, clinical, and radiologic features and surgical outcome of patients with entrapment of the L5 spinal nerve in the lumbosacral tunnel.

SUMMARY OF BACKGROUND DATA: Although several cadaveric studies on a lumbosacral tunnel as a possible cause of L5 radiculopathy have been reported, few studies had focused on osteophytes of the L5-S1 vertebral bodies as the major component of this compressive lesion, and clinical reports on patients with this disease have been rare.

METHODS: Lumbosacral spines from 29 geriatric cadavers were examined with special attention to the associations between osteophytes of the L5-S1 vertebral bodies and the L5 spinal nerve. Four patients with a diagnosis of the entrapment of the L5 spinal nerve by osteophytes at the lumbosacral tunnel were treated surgically, and their clinical manifestations and surgical results were reviewed retrospectively.

RESULTS: The anatomic study demonstrated osteophytes of the L5-S1 vertebral bodies in seven of the 29 cadavers. Entrapment of the L5 spinal nerve in the lumbosacral tunnel was observed in six of the seven cadavers with L5-S1 osteophytes but in only one of the 22 cadavers without such osteophytes (P < 0.05, chi2 test). All four patients had neurologic deficits in the L5 nerve root distribution. MRI and myelography showed no abnormal findings in the spinal canal, but CAT scans demonstrated prominent osteophytes on the lateral margins of L5-S1 vertebral bodies in all four. Selective L5 nerve block completely relieved all patients of pain but only temporarily. Three patients were treated via a posterior approach by resecting the sacral ala along the L5 spinal nerve, and the other patient was treated by laparoscopic anterior resection of the osteophytes. Pain relief was obtained in the four patients immediately after surgery, but one patient experienced recurrence of pain 1 year after the first surgery and was successfully treated by additional posterior decompression and fusion.

CONCLUSIONS: Extraforaminal entrapment of L5 spinal nerve in the lumbosacral tunnel can cause L5 radiculopathy, and osteophytes of L5-S1 vertebral bodies are a major cause of the entrapment.