Sacroiliitis is inflammation of the sacroiliac joint, one of the largest joints in the body. This joint sits at the base of your spine where the sacrum (the triangular bone at the bottom of your spinal column) connects to the ilium (the broad upper portion of your pelvis) on each side. Its main job is distributing your body weight from your upper body into your pelvis and legs. When it becomes inflamed, the result is pain in the lower back, buttocks, or both, and it can sometimes radiate down one or both legs.
Where the Pain Shows Up
The hallmark of sacroiliitis is pain concentrated in the buttocks, typically in the area between the gluteal folds and the bony ridges you can feel at the top of your pelvis in back. Many people point to a spot just below and to the side of the dimples on their lower back. The pain often feels deep and aching, and it can be easy to confuse with a hip problem or a disc issue in the lower spine.
About half of people with sacroiliac joint problems also experience pain that radiates into the leg. The most common referral site is the back of the thigh, but pain can extend to the knee, groin, or even the bottom of the foot. When it travels down the leg, it can feel a lot like sciatica, complete with numbness, tingling, or a burning sensation. One distinguishing clue: sacroiliac pain generally stays within the nerve distribution of the lower lumbar and upper sacral spine and won’t produce symptoms above the lower back.
Certain movements reliably make things worse. Climbing stairs, sitting cross-legged, and staying in one position (either sitting or standing) for a long time are common triggers. Bending and twisting motions also tend to flare the pain.
Common Causes
Sacroiliitis has several distinct triggers, which broadly fall into inflammatory, mechanical, and infectious categories.
- Inflammatory arthritis. Conditions like ankylosing spondylitis and other forms of spondyloarthritis are among the most well-known causes. In these diseases, the immune system attacks the joint lining, leading to chronic inflammation that can eventually cause the joint to stiffen or fuse. Sacroiliitis is often the earliest sign of ankylosing spondylitis.
- Mechanical stress. Pregnancy is a frequent trigger because hormonal changes loosen the ligaments around the sacroiliac joint while the growing uterus shifts weight distribution. Repetitive stress from activities like running or heavy lifting can also irritate the joint over time.
- Trauma. A fall, car accident, or other direct impact to the pelvis can damage the sacroiliac joint and set off inflammation.
- Infection. Rarely, bacteria can settle in the sacroiliac joint and cause septic sacroiliitis, which typically comes on quickly with severe pain and fever.
How It Differs From Disc Problems
Because sacroiliitis can mimic sciatica and lumbar disc herniation, telling them apart matters for getting the right treatment. Pain from a herniated disc usually originates in the central or slightly off-center lower back and follows a specific nerve path down the leg, often worsening with coughing, sneezing, or bearing down. Sacroiliac pain, by contrast, tends to center right over the joint itself, below the level of the lower spine, and is more likely to be provoked by position changes, twisting, or direct pressure on the pelvis.
That said, the overlap is significant. Referred pain maps from the sacroiliac joint land in the same nerve territories as disc-related pain, which is why physical exam maneuvers and imaging play such an important role in sorting out the source.
How It’s Diagnosed
Diagnosis starts with a set of hands-on provocative tests designed to stress the sacroiliac joint and reproduce your pain. Five standard maneuvers are commonly used: the FABER test (also called Patrick’s test), compression, distraction, thigh thrust, and Gaenslen’s test. If at least three of the five are positive, and one of those is either the thigh thrust or the compression test, sacroiliac dysfunction is the likely culprit. The thigh thrust is considered the most sensitive of the group, while the distraction test is the most specific.
During the FABER test, for example, you lie on your back and the examiner positions your leg so that your ankle rests on the opposite knee, forming a figure-four shape. Pressing the bent knee toward the table while stabilizing the opposite hip bone stresses the sacroiliac joint. If that reproduces your familiar pain, it counts as a positive result.
Imaging
Plain X-rays can show advanced changes like joint erosion or fusion, but they miss early inflammation. In one study comparing the two approaches, standard X-rays caught sacroiliitis only 22% of the time, while MRI detected it in 71% of cases. MRI is now the preferred tool for early diagnosis because it can reveal bone marrow swelling and soft tissue inflammation before any structural damage appears on an X-ray. Radiographic grading uses a 0-to-4 scale, where 0 is normal, 2 represents minimal changes, 3 is clear-cut abnormality, and 4 indicates severe damage or complete fusion of the joint.
When the diagnosis is still uncertain after physical exams and imaging, a guided injection of numbing medication directly into the sacroiliac joint can serve as a diagnostic test. If the injection eliminates your pain, the joint is confirmed as the source.
The Link to Ankylosing Spondylitis
Sacroiliitis that shows up on both sides of the pelvis is a hallmark of ankylosing spondylitis and related spondyloarthropathies. Under the modified New York criteria, the standard classification system, a diagnosis requires at least a grade 3 change on one side or grade 2 changes on both sides, combined with clinical symptoms like chronic back pain and limited spinal mobility.
Not everyone with sacroiliitis progresses to full ankylosing spondylitis. In a study following over 300 patients with early spondyloarthritis who did not meet the New York criteria at the start, only about 5% developed enough radiographic change to meet the criteria after two years. Interestingly, a similar proportion of patients who initially met the criteria actually improved enough over the same period that they no longer qualified. This suggests the disease course is more variable than many people assume.
Treatment Options
First-line treatment is straightforward: over-the-counter anti-inflammatory medications like ibuprofen or naproxen to reduce pain and swelling, combined with physical therapy. A physical therapist can guide you through stretching and range-of-motion exercises for the hips and lower back, along with strengthening work that helps stabilize the joint and improve posture. For many people, this combination is enough to manage symptoms.
When conservative measures fall short, corticosteroid injections delivered directly into the sacroiliac joint under imaging guidance are the next step. These injections are effective for most people in the short term. In clinical trials, roughly 82 to 85% of patients experienced meaningful pain relief at two weeks. By eight weeks, that number dropped to about 64 to 67%, which means the injections provide real but time-limited benefit. Repeat injections may be needed, and some people get longer-lasting relief than others.
For sacroiliitis caused by inflammatory conditions like ankylosing spondylitis, treatment also targets the underlying disease process. Biologic medications that suppress specific immune pathways can reduce joint inflammation and slow structural damage over time.
When Surgery Becomes an Option
Surgery is reserved for people who have tried physical therapy, medication, and injections without adequate improvement. Minimally invasive sacroiliac joint fusion is the most common surgical approach. It involves placing small implants across the joint to promote bone growth and eliminate painful motion.
The procedure is considerably quicker and less demanding than traditional open fusion. Operating time averages around 27 minutes for the minimally invasive version compared to roughly 128 minutes for open surgery. Hospital stays are correspondingly shorter: typically one to two days versus three to five days for open procedures. In studies tracking return to work, patients who were employed before surgery were back at their jobs within three months.
Candidates for fusion generally need at least three of five positive provocative exam maneuvers, pain that matches sacroiliac joint patterns, and a positive response to diagnostic injections before surgery is considered.