Findings: Image #1 is a T1 weighted axial MR image through the sacrum. This image demonstrates a large region of decreased signal intensity involving the right sacral ala with apparent cortical disruption.

Image #2 is an axial CT image through the sacrum which demonstrates a vague focal region of sclerosis within the right sacral ala adjacent to the sacroiliac joint. In addition, there is minimal anterior cortical disruption with periosteal new bone formation.

Diagnosis: Sacral insufficiency fracture.

Discussion: Insufficiency fractures of the pelvis are a major cause of low back pain or hip pain in elderly women with osteoporosis. Insufficiency fractures are defined as fractures secondary to normal physiologic stress on abnormal underlying bone. Although there have been many reported etiologies of insufficiency fractures, osteoporosis is generally accepted as being the most common underlying disease. Other potential risk factors include rheumatoid arthritis, corticosteroid therapy, fibrous dysplasia, Paget's disease, and pelvic irradiation. Patients with pelvic insufficiency fractures usually present with severe low back pain, buttock pain, or groin pain. In addition, the patients may present with hip pain. The most common sites of pelvic insufficiency fractures include the sacral ala as well as the parasymphyseal region of the os pubis and the pubic rami. Insufficiency fractures of the sacral ala are notoriously difficult to diagnose by plain film where the findings are often subtle.

Bone scintigraphy is the most sensitive imaging modality for the detection of occult insufficiency fractures, although the findings are generally nonspecific. The typical pattern is that of an H-shaped region of increased uptake including both the sacral ala interconnected by a horizontal component. Although the H-shaped pattern is considered diagnostic for bilateral sacral ala insufficiency fractures, the findings of increased uptake within a unilateral sacral ala are somewhat more nonspecific. Computed tomography demonstrates sacral insufficiency fractures to be sclerotic bands, linear bands, or a combination of both through the sacral ala. These findings are often demonstrated adjacent to the sacroiliac joint with associated disruption of the anterior cortex of the sacral ala. Magnetic resonance imaging demonstrates insufficiency fractures to be regions of decreased signal intensity on T1 weighted images consistent with edema. Unless there is discrete demonstration of a fracture line, the demonstration of decreased signal on T1 weighted images is a nonspecific finding. The MR appearance may be quite deceiving with lack of linear abnormality and a rounded tumorous appearance. Metastatic disease is often considered first. One needs to remember to consider this diagnosis when dealing with sacral lesions. Most reported cases of sacral insufficiency fractures improve with prolonged periods of bed rest with conservative therapy directed at pain relief.

In summary, a high degree of clinical suspicion is required to diagnose pelvic insufficiency fractures because plain radiographs are often unhelpful.

Reference:
Peh W. Imaging of Pelvic Insufficiency Fracture. Radiographics 1996;16:335-348.

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