Sacral insufficiency fracture

Four day history of severe lower back pain, exacerbated by movement. There was no history of trauma.

The patient, with a history of rheumatoid arthritis, was admitted with a four day history of worsening severe low back pain, exacerbated by walking. There was no history of trauma or previous back problems. She had previously been taking low dose corticosteroids. There was some tenderness at the lower back and sacrum, examination was otherwise unremarkable. There was no radiation of pain and no radicular symptoms or signs.
Plain radiographs (Fig. 1) were normal. MR imaging demonstrated low T1/high T2 signal intensity in both sacral alae, with enhancement after IV gadolinium chelate (Fig. 2 and 3). On bone scintigraphy there was increased isotope uptake throughout the sacrum, especially in the lateral masses, more marked on the right, with an ‘H’ shape (Fig. 4). CT scanning revealed bilateral sacral alae fractures parallel to the sacro-iliac joints (Fig. 5).
Based upon the clinical features and the imaging findings a diagnosis of bilateral sacral insufficiency fracture was made. Conservative treatment resulted in slow improvement and the patient has returned to normal activities. Follow-up CT scan showed sclerosis from fracture healing with no other abnormality.

Elderly patients often present with low back pain and related symptoms. The features are often not specific and there are many possible differential diagnoses including malignancy, osteomyelitis and referred pain due to abdominopelvic diseases. Insufficiency fractures involving the sacrum are becoming acknowledged as an important and treatable cause. They need not be associated with trauma, are often difficult to identify and may mimic other conditions such as osteomyelitis, or malignancy, even on imaging. The potential to misinterpret is important as there is often a history of prior malignancy in the at-risk patient.
There are several associated risk factors, the commonest being osteoporosis. Patients with rheumatoid arthritis or on corticosteroid therapy are at risk, possibly because of osteoporosis. Other risk factors include pelvic irradiation and conditions that weaken bone such as osteomalacia, Paget’s disease, osteogenesis imperfecta, osteopetrosis and fibrous dysplasia, or according to one group, large Tarlov cysts.
These fractures are often bilateral and occur mostly in the sacral alae (lateral masses), paralleling the sacro-iliac joints. The sacral bodies may be involved. Their imaging features vary depending on the duration and the degree of healing.
Plain radiographs are generally non-contributory. MRI demonstrates low signal intensity on T1-weighted images and high signal intensity on T2-weighted images due to oedema within the bone marrow of the sacral ala. T2-weighted short tau inversion recovery (STIR) images are particularly sensitive. The fracture line is sometimes seen, but this is not usual. MR is sensitive, but usually non-specific and there may be enhancement after IV gadolinium chelate and the fracture may involve one or more sacral bodies. There is a potential to misinterpret as metastasis, especially if there is a previous history of malignancy.
Bone Scintigraphy is sensitive for suspected insufficiency fractures of the sacrum and the sacrum is best imaged on posterior views. Because the lateral masses are usually the site of these fractures and the sacral bodies are relatively spared, the isotope uptake classically produces a butterfly or ‘H’ pattern, which is considered diagnostic in the right setting. However, the fracture may be unilateral, with or without a horizontal bar to the ‘H’ and there may be extension into the iliac crest and other pelvic fractures and the classic ‘H-sign’ may only be seen in as few as 40% of cases, limiting specificity. The H-sign may be asymmetric as in the case presented.
CT is accurate, efficient and specific for diagnosing these fractures. It is helpful in confirming the diagnosis and excluding other pathology, after equivocal scintigraphy or MR. CT typically demonstrates unilateral or bilateral fracture lines in the sacral alae parallel to the SI joints. With healing the fracture lines may become sclerotic. CT can exclude destructive lesions such as malignancy and osteomyelitis, by demonstrating the trabeculae to be intact, apart from disruption at the fracture site. If the diagnosis is not certain, or if there is concern regarding healing, follow-up CT scanning after several months is useful.