Sacral Insufficiency Fracture

Figure 1: Axial CT image shows diffuse osteopenia, bilateral sacral alar fractures (arrowheads) and patchy scleroses in the sacral ala in an elderly women presenting with low back and pelvic pain without trauma.
Figure 2: Axial T2 MR image with fat suppression shows, in addition to CT, high T2 signal intensity in the sacrum (red arrows). Areas of sclerosis (yellow arrows) are seen as low T2 signal intensity.

Facts: Sacral Insufficiency Fracture

• Fractures of the sacral ala related to axial stress on the underlying deficient elastic resistant sacrum. Frequently bilateral, involving sacral ala parallel to the sacroiliac joints
• Exact incidence unknown but one prospective study shows a frequency of 1.8% in women aged over 55 years old with low back pain. It is more common than generally thought, and remains overlooked as a cause of pain and disability in the elderly
• Risk factors: elderly women, osteoporosis, steroid-induced osteopenia, radiation therapy
• High index of clinical suspicion is key. Suspect SIF in patients with sudden onset of low back and pelvic pain without trauma who are at risk of osteopenia.
• Serum alkaline phosphatase (ALP) are often slightly elevated.

Imaging

• Best imaging tool = MRI.
• MRI can show low T1 and high T2 signal in the sacral ala corresponding to bone marrow edema, and also fracture lines.
• Alternatively, bone scintigraphy requires a minimum time lapse of 48-72 hours from symptom onset to show SIF. Classic "H" pattern require bilateral involvement
• Radiographs are usually inadequate to demonstrate SIF, especially in acute setting and before the development of healing. In healing phase, sclerotic lines may be seen in the sacral alae
• CT findings can be subtle. It can show sclerotic healing lines, and/or acute fracture lines - but can be quite subtle.
• Major differentials on imaging = malignancy and osteomyelitis

Reference:

Tsiridis E, Upadhyay N, Giannoudis PV. Sacral insufficiency fractures: current concepts of management. Osteoporos Int 2006;17:1716-1725.