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 matters; sometimes it is just an age- or stress-related finding. What counts is whether the report matches your symptoms, exam, and the actual images.
What Is the Sacrum?
The sacrum is the triangular bone at the base of your spine. It sits below L5 (the fifth lumbar vertebra, the lowest bone in the low back) and above the coccyx, or tailbone.
It forms the back of the pelvis, connecting the spine to the pelvic ring — the strong bony circle that supports you when you stand, walk, and sit. Unlike a movable level such as L4-L5, the sacrum is not a disc; 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 them on their way to the pelvis and legs.
Tailbone pain can feel different from sacral pain, since the tailbone sits lower. Read more: The Coccyx and Tailbone — Coccydynia.
What Are the Sacroiliac Joints?
The sacroiliac (SI) joints are where the sacrum meets the pelvis. You have two — one on the right, one on the left. Each connects the sacrum to the ilium, the large wing-shaped bone on each side of your pelvis.
These are strong, uneven, tightly supported joints. Their job is stability, not large motion: they transfer force from your spine into your pelvis and legs and absorb stress as you walk, climb stairs, bend, and carry weight. Strong bands of tissue called ligaments — which connect bone to bone — hold them in place and act as their major stabilizers.
Why the SI Joint Barely Moves
The SI joint does move, but only a small amount compared with your hip, knee, or low back — and that small motion is normal. A low-motion joint can still hurt. Pain can come from inflammation (irritation and swelling in tissue), stress, ligament irritation, arthritis (joint wear or inflammation), or changed mechanics in the pelvis and spine.
Where SI Joint Pain Shows Up
SI joint pain is often felt near the back of the pelvis. Common locations:
- Low back pain off to one side
- Buttock pain
- Pain near the back of the pelvis
- Sometimes groin or upper thigh pain
It can also mimic other problems:
- Lumbar disc herniation — a low back disc has pushed out and may irritate a nerve
- Sciatica — 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 — pain from the small joints at the back of the spine
- Muscular pain
Pain location alone does not prove SI joint dysfunction (the joint 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 Pain Gets Mistaken for Lumbar Spine Pain
The SI joint sits close to the lower lumbar spine, especially the L5-S1 segment (the level between the lowest lumbar vertebra and the sacrum), and near important lower lumbar nerves. So buttock and leg-area pain can come from several structures.
A lumbar MRI may show disc degeneration (age- or wear-related changes in the cushions between the spinal bones) or a disc bulge (the disc edge extending outward). These findings may or may not explain your pain. And because a lumbar MRI is designed mainly to image the lumbar spine, not the full pelvis, SI joint findings may be only partly seen.
In my practice, the hardest part is often separating SI-region pain from pain referred from L5-S1 or a lumbar nerve root (the part of a spinal nerve as it exits the spine). A careful exam and symptom pattern matter.
Learn more about the lower spine: 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
Imaging helps, but it does not diagnose the pain source by itself.
An X-ray shows bones well and may reveal joint narrowing, sclerosis, bone spurs, degenerative change, or partial-to-complete fusion.
A CT scan (computed tomography) uses X-rays to make detailed cross-sectional pictures and is often better for bony detail — arthritis changes, erosions, fusion, fractures, and sclerosis.
An MRI (magnetic resonance imaging) uses magnets to show bone, discs, nerves, muscle, and inflammation. It can show inflammation, bone marrow edema, infection, tumor, stress fracture, and inflammatory sacroiliitis, depending on the protocol.
A lumbar MRI may not fully evaluate the SI joints; a pelvis MRI or dedicated SI joint MRI answers different questions. What matters to me is whether the imaging pattern matches the patient’s exact pain location and exam.
Imaging Terms You May See
Sclerosis — increased bone density; on X-ray or CT it looks like whitening or thickening of bone. Often seen with chronic stress, arthritis, or prior inflammation.
Degenerative change — wear-and-tear, arthritis-type change, common with age and mechanical stress. It does not automatically mean the SI joint is causing pain.
Sacroiliitis — inflammation of the SI joint. It may be mechanical, inflammatory, infectious, or from another cause, and does not always mean autoimmune arthritis.
Bone marrow edema — a signal change inside the bone on MRI (bone marrow is the inner part of bone; edema is extra fluid-like signal). It can reflect inflammation, stress reaction, fracture, infection, or other causes, depending on the pattern.
Ankylosis — partial or complete fusion of a joint, meaning the joint has become joined by bone.
Erosion — loss of bone at the joint surface, seen with inflammatory disease, infection, or other joint processes.
None of these words is diagnostic by itself. Each must be read alongside your symptoms, exam, lab tests if relevant, and the full imaging pattern.
When SI Joint Findings Matter Most
SI joint findings carry more weight when several clues point the same way:
- Pain is near the back of the pelvis or SI region
- SI joint provocative maneuvers on exam reproduce the pain
- Other common causes, such as lumbar nerve compression or hip disease, don’t 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 stress the SI joint to see if they reproduce your familiar pain; no single test is perfect, so a group of tests is usually more useful than one alone. A diagnostic SI joint injection places numbing medicine into the joint under imaging guidance — meaningful short-term relief in a selected patient can support the SI joint as a pain source.
Causes of SI Joint and Sacral Pain
Pain near the SI joint has many causes — some common, some uncommon but important:
- Mechanical SI joint pain or dysfunction: the joint or nearby ligaments stressed by movement, posture, load, or altered mechanics
- Degenerative arthritis: wear-related joint change that irritates 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: surgery that joins two or more spine bones 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 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 (low bone strength)
- Inflammatory arthritis or spondyloarthritis: a group of inflammatory conditions that can affect the spine and SI joints
- Infection: uncommon, but important with fever, severe illness, immune suppression, or concerning imaging
- Tumor or metastatic disease: cancer that has spread from another area — uncommon, but important in the right context
- Referred pain from the lumbar spine or hip: pain felt in one area but coming from another structure
For a deeper look, read: Sacroiliac Joint Dysfunction: Why It’s Often Missed. If osteoporosis or fracture is a concern, read: Vertebral Compression Fractures: Osteoporosis, Imaging, and Treatment Options.
How Doctors Decide If the SI Joint Is the Source
A doctor usually weighs several pieces:
- History: Where is the pain? What triggers it? Any trauma, pregnancy, or recent delivery? Inflammatory symptoms, such as morning stiffness that improves with movement?
- Physical exam: watching your walking pattern, checking hip motion, testing strength and reflexes, and SI joint stress tests
- Neurologic exam: checking nerve function — strength, sensation, and reflexes
- Imaging: X-ray, CT, or MRI to look for arthritis, inflammation, fracture, infection, tumor, or other structural causes
- Diagnostic injection: in selected cases, an image-guided numbing injection can help confirm whether the SI joint is a main pain source
Doctors also work to separate SI pain from lumbar radiculopathy — pain, numbness, tingling, or weakness from irritation or compression of a spinal nerve root (sciatica is a common form). The SI joint may be one pain source, but it should not be diagnosed from a single MRI phrase.
Learn more: Sacroiliac Joint Dysfunction: Why It’s Often Missed.
Treatment, in Brief
This is an anatomy page, so this is only a brief overview. Most SI-related problems are treated without surgery first. Options may include:
- Activity modification
- Physical therapy focused on pelvic, core, and hip mechanics
- Anti-inflammatory medicines when appropriate and safe
- An SI belt in selected patients
- Image-guided SI joint injection
- Radiofrequency ablation in selected cases
Radiofrequency ablation uses heat from a special needle to reduce pain signals from targeted nerves. SI joint fusion — surgery to join the SI joint so it no longer moves — is different: 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 of the Sacrum or SI Joint
First, check what study was done — a lumbar MRI, a pelvis MRI, or a dedicated SI joint MRI — because a lumbar MRI may only partly show the SI joints and sacrum.
Next, look at how the finding is described. Reports use words like mild, moderate, or severe; acute (recent or active) or chronic (longer-standing); active; or incidental (found on imaging but not clearly related to symptoms).
Then check the side. Is the finding on the right, left, or both — and does that match where you feel pain?
Also watch for words that need careful attention: fracture, mass, infection, edema, erosions, ankylosis, sacroiliitis, and sclerosis.
When I review a report mentioning 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 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
Can SI joint pain feel like sciatica?
Yes. SI joint pain can mimic sciatica-like buttock or thigh pain. True sciatica usually involves irritation of a lumbar nerve root, and telling them apart depends on symptoms, exam, imaging, and sometimes targeted testing.
Can a lumbar MRI see the SI joints?
Only partially. A lumbar MRI is designed mainly to evaluate the lumbar spine, not to fully assess the SI joints or sacrum. A pelvis or dedicated SI joint MRI may be needed.
What is sacroiliitis?
Inflammation of the SI joint. It can be mechanical, inflammatory, infectious, traumatic, or from another cause — context matters.
Does “degenerative change” in the SI joint mean I need surgery?
Usually, no. Degenerative findings are common. Treatment depends on symptoms, exam, function, and response to non-surgical care, not on that phrase alone.
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