Imaging for Sacroiliac Joint Pain
For sacroiliac joint pain, order a radiograph of the pelvis (not a lumbar spine X-ray or hip X-ray), as this is the first-line imaging modality that visualizes the SI joints directly while also capturing the hips. 1
Recommended Imaging Approach
Initial Imaging: Pelvic Radiograph
The 2021 ACR Appropriateness Criteria clearly establish that radiographs of the pelvis are the first-line imaging modality for evaluating suspected SI joint pathology 1. This recommendation is based on:
- The pelvis radiograph captures both SI joints AND the hips simultaneously, which is important since hip pathology can also be involved in inflammatory conditions affecting the SI joints 1
- A standard anteroposterior (AP) view is sufficient—oblique views provide no additional diagnostic benefit 1
- Ferguson views show general agreement with AP views with no superiority of one over the other 1
Why Not Lumbar Spine X-rays?
Lumbar spine radiographs are inadequate for SI joint evaluation. While the SI joints may be partially visible on lumbar spine films, research demonstrates that:
- In 92% of cases where lumbar spine or pelvic radiographs were obtained together, the lumbar spine view provided incomplete visualization of the SI joints 2
- When SI joints appear normal on a lumbar spine radiograph, there is no diagnostic benefit from additional SI joint-specific imaging 2
- A single lateral lumbar view is inadequate for assessing SI joint disease 3
Why Not Hip X-rays Alone?
Hip radiographs focus on the hip joint itself and do not adequately visualize the SI joints. The pelvic radiograph is superior because it captures both anatomical regions in a single study.
Important Caveats and Clinical Considerations
Limitations of Plain Radiography
Be aware that plain radiographs have significant limitations for SI joint evaluation:
- Sensitivity ranges from only 19-72% and specificity from 47-84.5% 1
- Radiographs miss more than half of patients with structural changes when compared to CT as the reference standard 1
- Interobserver agreement is only fair to moderate 1
- Radiographs cannot demonstrate active inflammation—they only show chronic changes (erosions, sclerosis, ankylosis) 1
When to Consider MRI Instead
For patients with short duration of symptoms (early disease), consider MRI of the SI joints as the initial imaging modality 1. This is because:
- Inflammatory changes on MRI can precede radiographic findings by 3-7 years 1
- MRI can detect active inflammation (bone marrow edema, synovitis, enthesitis) that radiographs cannot 1
- This is particularly relevant if you suspect axial spondyloarthritis in its early stages
Additional Spine Imaging
If symptoms are referable to the spine in addition to SI joint pain, add lumbar spine (and cervical spine) radiographs to the pelvic radiograph 1. This allows assessment for:
- Syndesmophytes
- Erosions
- Shiny corners
- Vertebral body squaring
- Ankylosis
Thoracic spine radiographs are not useful for diagnosis due to overlying structures unless there are specific thoracic symptoms 1.
Practical Algorithm
- Order a pelvic radiograph (AP view) as your initial imaging study for SI joint pain
- If symptoms are present <3 months or you strongly suspect early inflammatory disease, consider MRI of the SI joints instead of or in addition to plain films
- If spine symptoms are also present, add lumbar and cervical spine radiographs to the pelvic film
- Do not order dedicated hip X-rays or lumbar spine X-rays alone for SI joint evaluation—they will not adequately visualize the SI joints
This approach minimizes radiation exposure while maximizing diagnostic yield for SI joint pathology.