Sacral insufficiency fracture.

Insufficiency fractures of the sacrum are not uncommon and usually occur in osteoporotic bone with minimal or unremembered trauma. However, they appear to be relatively under-diagnosed and this pictorial review aims to highlight the condition, discuss the expected imaging features and some of the potential imaging pitfalls. Owing to its relationship with osteoporosis, the majority occur in elderly females and are frequently bilateral, often presenting as low back pain. Plain radiographs are generally normal and both clinician and radiologist need to consider the possibility of sacral insufficiency fracture to allow prompt accurate diagnosis and correct treatment. Lumbar spine MRI is among the first investigations performed and can enable the correct diagnosis to be made. Occasionally the MR appearances can mimic tumour or osteomyelitis. The "H" sign on an isotope bone scan is considered diagnostic in the right clinical setting, but this sign is often not present. CT is useful to confirm the diagnosis and exclude tumour or infection.