Is ultrasound an appropriate imaging modality to evaluate sacral fractures, tumors, infection, or sacroiliac joint disease in an adult patient?

Ultrasound is Not Appropriate for Sacral Imaging in Adults

Ultrasound is not routinely obtained and has no supporting evidence for evaluating sacral fractures, tumors, infection, or sacroiliac joint disease in adults. 1

Evidence-Based Imaging Recommendations

For Sacroiliac Joint Disease (Axial Spondyloarthritis)

The American College of Radiology explicitly states that ultrasound is not routinely obtained as the initial imaging modality in the evaluation of suspected axial spondyloarthritis, and there is no relevant literature supporting its use in this setting. 1

Appropriate imaging modalities include:

• Pelvic radiographs as the first-line study to demonstrate chronic structural changes including erosions, sclerosis, and ankylosis 1
• Dedicated sacroiliac joint MRI (not lumbar spine MRI) to identify both inflammatory changes and structural lesions, which may appear 3-7 years before radiographic abnormalities 2

For Sacral Fractures (Insufficiency/Stress Fractures)

Radiographs should be the initial imaging modality for suspected sacral fractures, though sensitivity is low due to overlying bowel gas, fecal material, vascular calcifications, sacral curvature, and copious soft tissue 1

When radiographs are negative or equivocal:

• MRI or CT are the appropriate cross-sectional imaging modalities 1
• There is consensus in the literature that cross-sectional imaging (MRI/CT) should supersede bone scintigraphy as the imaging of choice when radiographs are negative 1
• MRI demonstrates edema patterns within hours and is extremely sensitive for stress fractures 1
• CT is particularly well suited for sacrum and pelvis evaluation, showing periosteal reaction, sclerosis, or fracture lines if the patient has been symptomatic for several weeks 1

For Sacral Infection

While ultrasound has been studied for superficial bone defects in controlled laboratory settings 3, there is no clinical evidence supporting its use for diagnosing sacral osteomyelitis or sacroiliitis in adults. The diagnosis of pyogenic sacroiliitis requires bone scintigraphy or CT of the sacroiliac joint, with fluoroscopic-guided fine-needle aspiration for organism identification 4

For Sacral Tumors

MRI is sensitive but not specific for detecting sacral lesions, as metastatic disease can mimic insufficiency fractures 5. The MRI pattern can be nonspecific and confusing when differentiating sacral insufficiency fractures from metastases 1. Ultrasound has no established role in this evaluation.

Critical Pitfalls to Avoid

• Do not order lumbar spine MRI to evaluate the sacroiliac joints - standard lumbar protocols do not reliably include the SI joints within the field of view 2
• A negative lumbar spine MRI does not exclude sacroiliac joint pathology - you must explicitly order dedicated SI joint imaging 2
• Do not rely on ultrasound for any sacral or sacroiliac pathology - it lacks both evidence and technical capability for deep pelvic bone assessment 1

Practical Algorithm

1. Start with radiographs (AP pelvis for SI joints; AP/lateral for sacral fractures) 1
2. If negative or equivocal, proceed to:
   • Dedicated SI joint MRI for inflammatory disease or early detection 2
   • MRI or CT for suspected fractures 1
   • CT or bone scan with fluoroscopic-guided aspiration for suspected infection 4