Sacral Insufficiency Fracture (SIF)

Diagnosis Definition

• Sacral insufficiency fractures (SIFs) result from normal stress applied to abnormal bone; antecedent trauma is not identified in two-thirds of patients
• Patients most commonly present with diffuse low back pain, which may radiate to the buttock, hip, or groin
• There is often a delay in diagnosis because clinical symptoms are frequently vague and nonspecific and can mimic a variety of pathologic processes, including radiculopathy and metastatic disease
• SIFs commonly affect elderly women with osteoporosis; other risk factors include pelvic radiation, steroid-induced osteopenia, rheumatoid arthritis, multiple myeloma, Paget disease, renal osteodystrophy, and hyperparathyroidism
• Almost all patients with SIFs are older than 55, with a mean age between 70 and 75 years
• SIFs are unilateral or bilateral with equal frequency

Imaging Findings

• The sacrum is divided into Zone 1 (sacral ala and bone between the sacroiliac joints and the neural foramina; the most common site of SIFs), Zone 2 (neural foramina), and Zone 3 (sacral bodies)
• A characteristic “H” sign is seen on bone scintigraphy
• CT scans show sclerosis in the sacral ala and lateral sacrum paralleling the SI joints, vertical fracture lines on axial images, and full fracture lines on coronal reconstructions
• MR imaging can detect early changes of sacral insufficiency and, similar to bone scintigraphy, has a reported sensitivity at or near 100%
• MR T2 STIR sequences are exquisitely sensitive for the detection of early marrow edema related to SIFs, which can be seen as early as 18 days after symptoms develop
• MRI shows marrow edema (hyperintensity on T2-weighted and STIR images and low signal on T1) and a hypointense fracture line in the area of edema

KEY IMAGES

Pearls

• There is a high incidence of concomitant pelvic insufficiency fractures, especially of the pubic rami and parasymphyseal region
• Neurologic symptoms related to SIFs are unusual

References

1. Lyders EM, Whitlow CT, Baker MD, Morris PP. Imaging and Treatment of Sacral Insufficiency Fractures. American Journal of Neuroradiology 2010; 31(2):201-210
2. Denis F, Davis S, Comfort T. Sacral fractures: an important problem. Clin Orthop Relat Res 1988; 227:67-81