Answers

Short answers

1 This clinical presentation of sudden onset and incapacitating sacral pain is typical of sacral insufficiency fracture and can be confirmed by magnetic resonance imaging (MRI) of the sacrum.

2 The most common underlying pathology in sacral insufficiency fracture is osteoporosis, and most such fractures occur in elderly women.

3 Although MRI and isotope bone scan have high diagnostic sensitivity for sacral insufficiency fracture, computed tomography more reliably excludes malignant disease and osteomyelitis.

Long answer
Sacral insufficiency fracture
Spontaneous osteoporotic fracture of the sacrum was originally described in 1982 in three elderly patients who presented with incapacitating back and leg pain.¹ Further case series substantiate that sacral insufficiency fracture is a syndrome occurring predominantly in elderly women with primary osteoporosis.^{2 3} Other identified risk factors include taking corticosteroids, pelvic irradiation, rheumatoid arthritis, vitamin D deficiency, hyperparathyroidism, and malignancy.⁴

The clinical presentation is usually sudden onset pain in the lower back or pelvis and impaired mobility. Pain is exacerbated by weight bearing, and patients often prefer to be supine. Two thirds of patients have no history of trauma.⁵ On physical examination there may be sacral tenderness, and sacroiliac joint tests are often positive. Most fractures occur lateral to the sacral foramina and so rarely cause neurological impairment. However, fractures affecting the sacral foramina or the vertical and transverse central canal of the sacrum may cause lumbosacral radiculopathy or cauda equina syndrome respectively.⁴ Sacral insufficiency fractures are often associated with other fractures of the pelvic rim such as of the pubic rami.⁶

The incidence of sacral insufficiency fracture is unknown, and the diagnosis may often be overlooked. In a two year prospective study of women over the age of 55 years who presented to hospital with low back pain, the prevalence of sacral insufficiency fracture was 1.8%.² Patients may be misdiagnosed with degenerative disease of the lumbar spine, spondylolisthesis, spondylosis, or spinal stenosis or may undergo rigorous investigation for suspected malignancy.

Investigations
Plain radiographs are usually inadequate for diagnosing sacral insufficiency fracture, although they may show other pelvic rim stress fractures. Serum alkaline phosphatase is often mildly elevated. Magnetic resonance imaging of the lumbar and sacral spine is the most sensitive first line screening investigation.⁴ MRI is effective at showing bone oedema and fracture configuration, although their appearances occasionally can mimic those of metastatic bone disease or osteomyelitis.⁷

Isotope bone scan can also be used in the diagnosis of sacral insufficiency fracture.⁴ Sacral insufficiency fractures are frequently bilateral with the fracture line running vertically parallel to the sacroiliac joints. When combined with a horizontal fracture of the sacral body this may give the appearance of an "H" on isotope bone scan,⁸ which is considered diagnostic.⁹ However, the appearances on bone scan may be non-specific, making further imaging necessary.

Although less sensitive than MRI or bone scan in the diagnosis of sacral insufficiency fracture, computed tomography is better at showing the architecture of bone trabeculation and is therefore more effective at excluding tumour or infection in those cases where the MRI or bone scan appearances are non-specific.⁷

Treatment
Treatment of sacral insufficiency fracture is conservative, with analgesia and initial bed rest followed by early mobilisation and rehabilitation. Secondary prevention of osteoporotic fragility fractures with calcium and vitamin D supplementation and bisphosphonates should be considered. Randomised controlled trials have shown an analgesic effect for both calcitonin¹⁰ and intravenous pamidronate¹¹ in acute osteoporotic vertebral fractures, and so these may be of benefit. Some specialist centres have developed the technique of vertebroplasty for the sacrum (sacroplasty),¹² although this technique remains under study and is not widely available. One advantage of doing a sacroplasty is that a biopsy can be obtained.

Conclusion
Although pubic rami fractures are often encountered in hospital practice, sacral insufficiency fracture remains a poorly recognised diagnosis and should be considered in patients with symptoms disproportionate to the radiographic findings.

Cite this as: BMJ 2008;337:a1804