Lower back pain and hip pain are some of the most common reasons people see a doctor. Many patients go through scans, treatments, and months of discomfort without finding the real source of their pain.
Sometimes the cause is a joint most people have never heard of: the sacroiliac (SI) joint.
This guide covers what the SI joint does, why it can cause so much pain, and which tests actually help diagnose it.
What Are the Sacroiliac (SI) Joints?
The sacroiliac joints sit just behind the hip joints. They connect your lower back to your pelvis, with one on each side.
They may be small, but they do an important job. Force from your lower back travels through the SI joints, then down into your hips and legs. That makes the SI joints a key link between your upper body and lower body.
Why SI Joint Problems Cause So Much Pain
When something goes wrong in the SI joint, the pain doesn’t always stay in one place. SI joint problems can cause:
- Lower back pain
- Hip pain
- Leg pain
The SI joint is a surprisingly common contributor in people with lower back and hip pain. Because the symptoms overlap with spine and hip problems, it’s often overlooked.
Can a Hip MRI Show the SI Joint?
Patients often ask this. The short answer: a hip MRI will show the SI joint, but not in much detail.
Hip MRI: A Zoomed-Out View
A hip MRI captures the SI joint from a distance. It may show major problems, such as:
- Obvious arthritic changes
- Significant inflammation
It does not give the close, detailed look needed to properly evaluate the joint.
Lumbar Spine MRI: Only Part of the Picture
An MRI of the lower back (lumbosacral spine) may capture the top portion of the SI joint, but it won’t show the whole joint.
Pelvis MRI: The Best Way to See the Whole SI Joint
To properly evaluate the SI joint, an MRI of the pelvis is the best choice. It’s focused and zoomed in, and it shows the entire joint. It’s especially useful for spotting inflammation.
Do X-Rays or CT Scans Help?
Yes, depending on what your doctor is looking for.
X-rays and CT scans can show bone changes in the SI joint, such as wear-and-tear arthritis. They’re generally less sensitive than MRI, especially for inflammation.
A simple way to think about it:
- Looking for inflammation? An MRI of the SI joints is most helpful.
- Looking for wear-and-tear (osteoarthritis)? An X-ray or CT scan can be helpful.
Why Imaging Isn’t the Whole Story
Imaging is useful, but it’s only one part of a proper diagnosis. A careful evaluation also includes:
Your Medical History
Your history matters a great deal, especially when a doctor suspects inflammatory back pain, which can be linked to autoimmune conditions.
A Hands-On Physical Exam
A good examination that stresses the SI joints helps a rheumatology specialist pinpoint whether the SI joint is the source of your pain.
SI Joint Dysfunction: When Scans Look Normal
One of the most important points is that SI joint dysfunction often won’t show up on imaging.
SI joint dysfunction usually means instability in the joint. It’s often caused by laxity (looseness) of the ligaments that support it. These ligaments aren’t always visible as a clear problem on an MRI or X-ray.
In these cases, imaging alone won’t give you a diagnosis. An accurate diagnosis comes from:
- Understanding how ligament laxity and instability cause pain
- Identifying exactly where the pain is
- A thorough, hands-on examination
Many patients are told their scans look “normal” even though they’re still in pain. For them, finding a physician who understands SI joint problems can make all the difference.
The Bottom Line: See a Specialist Who Knows the SI Joint
If you have ongoing lower back, hip, or leg pain, the SI joint deserves a closer look. Keep these points in mind:
- A hip MRI shows the SI joint, but not in detail.
- A dedicated pelvis MRI is the best imaging test for the whole SI joint.
- X-rays and CT scans help detect wear-and-tear arthritis.
- SI joint instability often doesn’t appear on any scan.
- A skilled physician’s history and exam are often the most important tools.
The right test matters, and so does the right doctor. A Chicago rheumatologist experienced in SI joint pathology can tell the difference between inflammation, arthritis, and ligament instability. That leads to a more accurate diagnosis and better joint pain relief.
Get Answers for Your Back and Hip Pain
At Chicago Arthritis and Regenerative Medicine, we specialize in finding the real source of chronic pain. Our care covers inflammatory and autoimmune disease treatment, osteoarthritis treatment, and regenerative options such as PRP therapy, all aimed at helping you move comfortably again.
Learn more or schedule your evaluation at ChicagoArthritis.com. We offer arthritis treatment in Chicago and expert regenerative medicine in Chicago.
How Ankylosing Spondylitis Differs in Men and Women
Medications for Ankylosing Spondylitis: What You Need to Know (Best Treatments & FAQs Answered!)
The Best Exercises for Ankylosing Spondylitis: Stay Active and Reduce Pain