From the Guidelines
The order should specify an MRI of the sacrum and iliac region without IV contrast as the next imaging study after negative radiographs, due to its high sensitivity and specificity in diagnosing sacral or iliac insufficiency fractures. This recommendation is based on the American College of Radiology (ACR) Appropriateness Criteria for stress (fatigue/insufficiency) fractures, including sacrum, excluding other vertebrae 1. The ACR criteria rate MRI area of interest without IV contrast as a 9 out of 9 in terms of appropriateness for diagnosing sacral or iliac insufficiency fractures after negative radiographs.
When ordering an MRI for a sacral or iliac insufficiency fracture, the following sequences should be included:
- T1-weighted images to show hypointense fracture lines
- STIR sequences to reveal bone marrow edema
- In-phase and out-of-phase MRI sequences to differentiate benign stress fractures from pathologic fractures 1
The use of CT scans or bone scintigraphy may be considered in certain cases, such as when MRI is not available or contraindicated, or when detailed fracture characterization and surgical planning are necessary 1. However, MRI remains the preferred imaging modality due to its high sensitivity and specificity in diagnosing sacral or iliac insufficiency fractures.
Key considerations when diagnosing sacral or iliac insufficiency fractures include:
- Clinical presentation, including patient history and physical examination
- Imaging findings, including radiographs, MRI, and CT scans
- Patient factors, such as osteoporosis or prior radiation therapy
- The potential for misdiagnosis or delayed diagnosis, which can lead to significant pain and disability if left untreated 1
From the Research
Imaging Protocol for Diagnosing Sacral or Iliac Insufficiency Fracture
When ordering an MRI for a sacral or iliac insufficiency fracture, the following specifications should be considered:
- The MRI protocol should include a coronal fat-suppressed T2-weighted image to improve early detection of sacral insufficiency fractures 2
- Diffusion-weighted MRI sequences, such as steady-state free precession (SSFP), can be useful in differentiating sacral insufficiency fractures from metastases of the sacrum 3
- The MRI should cover the lumbar spine and sacrum to detect concomitant pathologies and adequate treatment for patients 2
Key Imaging Findings
The following imaging findings are characteristic of sacral or iliac insufficiency fractures:
- Hypointense lines or fissures on T1- and T2-weighted and STIR MR images 4
- Low signal intensity on T1-weighted MRI and low or high signal intensity on T2-weighted MRI 3
- Strong enhancement on contrast-enhanced T1-weighted MRI 3
- Low signal intensity on diffusion-weighted MRI for sacral insufficiency fractures, whereas metastases typically show high signal intensity 3
Additional Considerations
When diagnosing sacral or iliac insufficiency fractures, it is essential to consider the following:
- The presence of additional pelvic fractures in characteristic locations can help confirm the diagnosis 5
- The clinical and imaging features of sacral insufficiency fractures can be similar to those of lumbar spine pathology, making it essential to be familiar with the features of sacral insufficiency fractures on lumbar spine MRI 6
- MRI can determine the clinical activity of the disease and monitor the response to treatment of active-type insufficiency fractures 4