[A case of a lumbar spinal synovial cyst located on the midline].

We report a case of lumbar spinal synovial cyst located on the midline. A 72-year-old man was admitted to our hospital with the chief complaint of low back pain radiating to the left buttock and posterior thigh. An MR image revealed an extradural cystic lesion adjacent to the dorsal side of the dural sac at the L4-5 level. The cyst was remote from the facet joints and existed on the midline just in front of the L4 lamina. A CT scan showed a concave deformity of the ventral aspect of the L4 lamina because of compression by the cyst. The patient underwent L4 laminectomy and total removal of the cyst. The cyst was in contact with the anterior surface of the ligamentum flavum and it had no connection with the facet joint. In the histological examination, the cyst was multilobular and lined with synovial epithelium. Therefore the cyst was diagnosed as a synovial cyst. After the operation, the pain radiating to the buttock and thigh completely disappeared. Intraspinal synovial cysts are usually located in the lower lumbar spine and most of them are adjacent to the facet joint. The cysts that are located on the midline are very rare. We review previous reports and discuss clinical and pathological features of spinal synovial cysts.