The Sacroiliac Joint and Sacrum: Plain-Language Anatomy for Patients
The sacroiliac joints are the strong joints on each side of the sacrum where the spine connects to the pelvis, and they can be a source of low back or buttock pain — but imaging findings alone do not prove that the SI joint is the cause of pain.
In my practice, many patients are surprised to learn that the SI joint is not in the middle of the low back. It sits off to each side, where the sacrum locks into the pelvis.
If your MRI, CT, or X-ray report mentions “degenerative change,” “sclerosis,” “sacroiliitis,” or “bone marrow edema,” the wording can sound serious. Sometimes it is important. Sometimes it is an age-related or stress-related finding. The key is whether the report matches your symptoms, exam, and the actual images.
Suggested diagram: Posterior pelvis anatomy illustration showing the lumbar spine, L5-S1 level, sacrum, right and left SI joints, iliac bones, coccyx/tailbone, and common pain zones.
Caption: The sacroiliac joints connect the sacrum to the pelvis. Pain in this region can overlap with lumbar spine, hip, and tailbone problems, so imaging findings must be matched to the patient’s symptoms and exam.
What Is the Sacrum?
The sacrum is the triangular bone at the base of your spine.
It sits below L5, which means the fifth lumbar vertebra, or the lowest bone in the low back. It sits above the coccyx, also called the tailbone.
The sacrum forms the back part of the pelvis. It connects the spine to the pelvic ring. The pelvic ring is the strong bony circle that supports your body when you stand, walk, and sit.
The sacrum is not a disc or a single movable spinal level like L4-L5. It is a solid, wedge-shaped bone that transfers weight from the spine into the pelvis and legs.
The sacrum also has small openings called sacral foramina. A foramen is an opening in bone. Sacral nerves pass through these openings on their way to the pelvis and legs.
Tailbone pain can feel different from sacral pain. The tailbone is lower than the sacrum. You can read more here: The Coccyx and Tailbone — Coccydynia.
What Are the Sacroiliac Joints?
The sacroiliac joints, often shortened to SI joints, are the joints where the sacrum meets the pelvis.
You have two SI joints:
- One on the right
- One on the left
Each SI joint connects the sacrum to the ilium. The ilium is the large wing-shaped bone on each side of your pelvis.
The SI joints are strong, uneven, tightly supported joints. Their main job is not large motion. Their main job is stability.
They help transfer force from your spine into your pelvis and legs. They also help absorb stress as you walk, climb stairs, bend, and carry weight.
Strong bands of tissue called ligaments hold the SI joints in place. Ligaments connect bone to bone. Around the SI joint, these ligaments are major stabilizers.
Why the SI Joint Moves Less Than Most People Think
The SI joint does move. But it moves only a small amount compared with your hip, knee, or low back.
That small motion is normal.
A small-motion joint can still hurt. Pain can come from inflammation, which means irritation and swelling in tissue. It can also come from stress, ligament irritation, arthritis, which means joint wear or inflammation, or changed mechanics in the pelvis and spine.
Where SI Joint Pain Is Usually Felt
SI joint pain is often felt near the back of the pelvis.
Common locations include:
- Low back pain off to one side
- Buttock pain
- Pain near the back of the pelvis
- Sometimes groin pain
- Sometimes upper thigh pain
SI joint pain can mimic other problems, such as:
- Lumbar disc herniation, which means a low back disc has pushed out and may irritate a nerve
- Sciatica, which means leg pain from irritation of a nerve, often in the low back
- Hip arthritis or hip labral pain. The labrum is a rim of cartilage around the hip socket.
- Lumbar facet pain, which means pain from the small joints in the back of the spine
- Muscular pain
Pain location alone does not prove SI joint dysfunction. Dysfunction means the joint is not moving or loading normally. The same pain zone can come from the SI joint, the lumbar spine, the hip, or nearby muscles.
Why SI Joint Problems Are Often Confused With Lumbar Spine Problems
The SI joint sits close to the lower lumbar spine, especially the L5-S1 segment. L5-S1 is the level between the lowest lumbar vertebra and the sacrum.
That area is also near important lower lumbar nerves. Because of this, buttock and leg-area pain can come from several structures.
A lumbar MRI may show disc degeneration or disc bulges. Disc degeneration means age-related or wear-related changes in the cushions between the spinal bones. A disc bulge means the disc edge extends outward. These findings may or may not explain your pain.
At the same time, SI joint findings may be only partly seen on a lumbar MRI. A lumbar MRI is designed mainly to look at the lumbar spine, not the full pelvis.
In my practice, the hardest part is often separating SI-region pain from pain referred from L5-S1 or a lumbar nerve root. A nerve root is the part of a spinal nerve as it exits the spine.
A careful exam and symptom pattern matter.
Learn more about the lower spine here: The L5-S1 Lumbosacral Segment.
If your main concern is nerve-type leg pain, read: Sciatica: Causes, Diagnosis, and the Treatment Path.
What Imaging May Show in the Sacrum or SI Joints
Imaging can help, but it does not diagnose the pain source by itself.
An X-ray is a picture that shows bones well. X-rays may show:
- Joint narrowing
- Sclerosis
- Bone spurs
- Degenerative change
- Partial or complete fusion
A CT scan, or computed tomography scan, uses X-rays to make detailed cross-sectional pictures. CT is often better for bony detail. It can show:
- Arthritis changes
- Erosions
- Fusion
- Fractures
- Sclerosis
An MRI, or magnetic resonance imaging scan, uses magnets to show bones, discs, nerves, muscles, and inflammation patterns. MRI can show:
- Inflammation
- Bone marrow edema
- Infection
- Tumor
- Stress fracture
- Inflammatory sacroiliitis, depending on the MRI protocol
A lumbar MRI may not fully evaluate the SI joints. A pelvis MRI or dedicated SI joint MRI may answer different questions.
What I look for on MRI is not just whether the report mentions the SI joint. I look at whether the imaging pattern matches the patient’s exact pain location and exam.
Common Imaging Terms Patients May See
Sclerosis means increased bone density. On X-ray or CT, it can look like whitening or thickening of bone. It is often seen with chronic stress, arthritis, prior inflammation, or other causes.
Degenerative change means wear-and-tear arthritis-type change. It is common with age and mechanical stress. It does not automatically mean the SI joint is causing pain.
Sacroiliitis means inflammation of the SI joint. It may be mechanical, inflammatory, infectious, or related to another cause. It does not always mean autoimmune arthritis.
Bone marrow edema means a signal change inside the bone on MRI. Bone marrow is the inner part of bone. Edema means extra fluid-like signal. This can be seen with inflammation, stress reaction, fracture, infection, or other causes, depending on the pattern.
Ankylosis means partial or complete fusion of a joint. Fusion means the joint has become joined together by bone.
Erosion means loss of bone at the joint surface. It can be seen with inflammatory disease, infection, or other joint processes.
None of these words is diagnostic by itself. Each term must be read with your symptoms, physical exam, lab tests if relevant, and the full imaging pattern.
When SI Joint Findings Matter Most
SI joint findings matter more when several clues point in the same direction.
They matter more when:
- Pain is located near the back of the pelvis or SI region
- Symptoms are reproduced by SI joint provocative maneuvers on exam
- Other common causes, such as lumbar nerve compression or hip disease, do not fully explain symptoms
- A diagnostic SI joint injection gives meaningful temporary relief
- Imaging shows active inflammation, fracture, infection concern, or another specific problem
Provocative maneuvers are exam tests that place stress across the SI joint to see if they reproduce your familiar pain. No single test is perfect. A group of tests is usually more useful than one test alone.
A diagnostic SI joint injection means numbing medicine is placed into the SI joint using imaging guidance. If it gives meaningful short-term relief in a selected patient, it can support the SI joint as a pain source.
The finding matters most when the imaging, exam, and symptom pattern all point in the same direction.
Causes of SI Joint and Sacral Pain
Pain near the SI joint can come from many causes. Some are common. Some are uncommon but important.
Possible causes include:
- Mechanical SI joint pain or dysfunction: The joint or nearby ligaments may be stressed by movement, posture, load, or altered mechanics.
- Degenerative arthritis: Wear-related joint change may irritate the SI joint in some people.
- Pregnancy or postpartum ligament changes: Pregnancy and the months after delivery can change load and ligament tension around the pelvis.
- Prior lumbar fusion: Lumbar fusion means surgery that joins two or more spine bones. In some patients, this can change force transfer across the SI joints.
- Trauma or falls: A fall or accident can injure the sacrum, SI joint, or pelvic ring.
- Sacral stress fracture or insufficiency fracture: A stress fracture is a small crack from repeated load. An insufficiency fracture is a stress-type fracture in weakened bone, often from osteoporosis. Osteoporosis means low bone strength.
- Inflammatory arthritis or spondyloarthritis: Spondyloarthritis is a group of inflammatory conditions that can affect the spine and SI joints.
- Infection: This is uncommon, but important when there is fever, severe illness, immune suppression, or concerning imaging.
- Tumor or metastatic disease: Metastatic disease means cancer that has spread from another area. This is uncommon, but important in the right clinical context.
- Referred pain from the lumbar spine or hip: Referred pain means pain felt in one area but coming from another structure.
For a deeper look at this condition, read: Sacroiliac Joint Dysfunction: Why It’s Often Missed.
If osteoporosis or fracture is part of the concern, read: Vertebral Compression Fractures: Osteoporosis, Imaging, and Treatment Options.
How Doctors Decide Whether the SI Joint Is the Pain Source
Imaging is only one part of the diagnosis.
A doctor usually looks at several pieces:
- History: Where is the pain? What triggers it? Was there trauma? Was there pregnancy or recent delivery? Are there inflammatory symptoms, such as morning stiffness that improves with movement?
- Physical exam: This may include watching your walking pattern, checking hip motion, testing strength and reflexes, and performing SI joint stress tests.
- Neurologic exam: This checks nerve function, including strength, sensation, and reflexes.
- Imaging: X-ray, CT, MRI, or other studies may be used to look for arthritis, inflammation, fracture, infection, tumor, or other structural causes.
- Diagnostic injection: In selected cases, an image-guided numbing injection may help confirm whether the SI joint is a main pain source.
Doctors also work to separate SI pain from lumbar radiculopathy. Radiculopathy means pain, numbness, tingling, or weakness caused by irritation or compression of a spinal nerve root. Sciatica is a common form of this pattern.
The SI joint may be one possible pain source. But it should not be diagnosed from one MRI phrase alone.
You can learn more here: Sacroiliac Joint Dysfunction: Why It’s Often Missed.
Treatment Overview: From Conservative Care to Procedures
This is an anatomy page, so this section is only a brief overview.
Many SI-related problems are treated without surgery first.
Common options may include:
- Activity modification
- Physical therapy focused on pelvic, core, and hip mechanics
- Anti-inflammatory medicines when appropriate and safe
- SI belt in selected patients
- Image-guided SI joint injection
- Radiofrequency ablation in selected cases
Radiofrequency ablation means using heat from a special needle to reduce pain signals from targeted nerves.
SI joint fusion is different. SI joint fusion means surgery to join the SI joint so it no longer moves. It is reserved for carefully selected patients after the diagnosis is clear and non-surgical treatment has not helped.
Fusion is not based on an MRI phrase like “degenerative change” or “sclerosis” alone.
For more detail, read: Sacroiliac Joint Dysfunction: Why It’s Often Missed.
How to Read an MRI Report That Mentions the Sacrum or SI Joint
First, check what study was done.
Was it:
- A lumbar MRI?
- A pelvis MRI?
- A dedicated SI joint MRI?
This matters because a lumbar MRI may only partly show the SI joints and sacrum.
Next, look at how the finding is described. Reports may use words like:
- Mild
- Moderate
- Severe
- Acute, meaning recent or active
- Chronic, meaning longer-standing
- Active
- Incidental, meaning found on imaging but not clearly related to symptoms
Then look at the side.
Is the finding on the right, left, or both sides? Does that match where you feel pain?
Also look for words that need careful attention:
- Fracture
- Mass
- Infection
- Edema
- Erosions
- Ankylosis
- Sacroiliitis
- Sclerosis
When I review a report that mentions sacroiliitis, sclerosis, or sacral edema, I want to know the patient’s age, symptoms, trauma history, inflammatory symptoms, and what study was actually performed.
A spine specialist may need the actual images, not just the report. The report is a summary. The images show the pattern.
If your MRI report mentions the sacrum or SI joints and you are not sure whether it explains your pain, SpineClarity can help. You can upload your symptoms, MRI report, and relevant records for a written review by a board-certified spine surgeon. You’ll receive a plain-language interpretation and a suggested next-step category. This is not emergency care and does not replace an in-person physician relationship.
When to Seek Urgent Medical Care
Seek urgent medical care now if you have:
- New loss of bowel or bladder control
- Numbness in the groin, genitals, or “saddle” area
- New or worsening leg weakness
- Fever, chills, or feeling severely ill with back, pelvic, or sacral pain
- Severe pain after a fall, accident, or trauma
- Known cancer history with new unexplained back, pelvic, or sacral pain
- Unexplained weight loss
- Inability to walk or stand because of pain or weakness
These symptoms can point to a serious spine, nerve, infection, fracture, or cancer-related problem.
SpineClarity’s written review service is not for emergencies.
If you are worried about bowel or bladder changes, saddle numbness, or leg weakness, read: Cauda Equina Syndrome: The Spine Emergency Patients Need to Recognize.
FAQ
Where is the SI joint located?
The SI joint is located on both sides of the sacrum where it meets the pelvis. More specifically, it is where the sacrum meets the ilium, the large pelvic bone.
Is the sacrum part of the spine or the pelvis?
Functionally, it is both. The sacrum is the base of the spine and the back part of the pelvic ring.
Can SI joint pain feel like sciatica?
Yes. SI joint pain can mimic some sciatica-like buttock or thigh pain.
True sciatica usually involves irritation of a lumbar nerve root. The diagnosis depends on the symptoms, exam, imaging pattern, and sometimes targeted testing.
Does “degenerative change” in the SI joint mean I need surgery?
Usually, no.
Degenerative findings are common. Treatment depends on symptoms, exam findings, function, and response to non-surgical care. Surgery is not based on that phrase alone.
Can a lumbar MRI see the SI joints?
Sometimes partially.
A lumbar MRI is designed mainly to evaluate the lumbar spine. It is not always designed to fully evaluate the SI joints or sacrum.
What is sacroiliitis?
Sacroiliitis means inflammation of the SI joint.
It can have different causes, including mechanical irritation, inflammatory arthritis, infection, trauma, or other conditions. Context matters.
How do doctors confirm SI joint pain?
Doctors use history, physical exam, imaging when appropriate, and sometimes an image-guided diagnostic injection.
No single test proves SI pain by itself. The diagnosis is stronger when several findings point to the same source.
Can SI joint problems happen after lumbar fusion?
Yes, in some patients.
After lumbar fusion, mechanics and load transfer may change. The SI joints can become one possible pain source when symptoms and exam findings fit.
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