Sacral insufficiency fracture, usually overlooked cause of lumbosacral pain.

Sacral insufficiency fractures are usually known to develop in elderly patients with osteoporosis without definite trauma history. It is difficult to diagnose the sacral insufficiency fracture at an early stage because lower lumbar diseases, concurrently or not, may also be presented with similar symptoms and signs. We report a rare case of sacral insufficiency fracture who was not diagnosed initially but, instead, showed progressively worsening of clinical symptoms and radiological findings after decompression surgery for upper level lumbar stenosis.