SI joint pain is pain originating from the sacroiliac joints, two large joints that sit where your lower spine meets your pelvis on each side. It’s one of the most commonly overlooked sources of lower back pain, estimated to account for 10% to 25% of all chronic low back pain cases. The pain typically centers in the low back and buttock area, and it can range from a dull ache to sharp, stabbing discomfort that makes sitting, standing, or climbing stairs difficult.
Where the SI Joint Is and What It Does
The sacroiliac joints (often just called SI joints) are the largest joints along the spine’s axis. Each one sits between the sacrum, the triangular bone at the base of your spine, and the ilium, the large wing-shaped bone of your pelvis. You have one on each side. Their primary job is transferring the weight of your upper body down through your pelvis and into your legs. Every time you walk, bend, or lift something, these joints absorb and redirect large compressive forces.
Unlike your knee or shoulder, SI joints aren’t designed for a wide range of motion. They move only a few degrees, held tightly in place by some of the strongest ligaments in the body. That limited movement is the point: stability matters more here than flexibility. Problems arise when the joint becomes either too stiff or too loose, or when the surfaces inside the joint become inflamed.
What SI Joint Pain Feels Like
The hallmark sensation is pain right over the joint itself, which you’d feel as a deep ache on one side of your lower back, just below the waistline and slightly off-center. Many people can point to the spot with one finger. But SI joint pain doesn’t always stay put. It commonly spreads into the buttock, and it can travel into the groin or down the upper leg. One useful distinguishing feature: SI joint pain rarely radiates below the knee. If your pain shoots all the way down to your foot, the cause is more likely a pinched nerve in your spine than the SI joint.
The pain can start suddenly after a fall, car accident, or awkward twist, or it can develop gradually over weeks or months without a clear trigger. Prolonged sitting, standing from a chair, climbing stairs, and rolling over in bed are common aggravators. Some people notice it most on one side, though both joints can be affected.
Common Causes and Risk Factors
The most common cause is simply idiopathic, meaning there’s no single identifiable event or condition behind it. Repetitive stress on the joint over time is believed to play a central role in many cases. Beyond that, the causes fall into a few recognizable categories.
Too much movement (hypermobility): When the ligaments holding the SI joint together become lax, the joint moves more than it should. This creates irritation and inflammation. Pregnancy is the most common trigger for this type. During the third trimester, the body produces higher levels of hormones like relaxin, which loosen the muscles and ligaments around the pelvis, back, and abdomen. This loosening is necessary for delivery but can leave the SI joint unstable and painful. Relaxin levels drop after birth but can take up to 12 months to return to pre-pregnancy levels, which is why SI joint pain sometimes lingers well into the postpartum period.
Too little movement (hypomobility): The joint can also become overly stiff, often from arthritis or age-related degeneration. Over time, the cartilage wears down, the joint space narrows, and bone spurs may form. This type is more common in older adults and tends to develop gradually.
Other contributing factors: Imbalances in the muscles and connective tissue around the pelvis, legs with uneven lengths, prior lumbar spine surgery (which shifts extra stress onto the SI joints), and inflammatory conditions like ankylosing spondylitis can all play a role.
How SI Joint Pain Is Diagnosed
Diagnosing SI joint pain is tricky because imaging alone often can’t confirm it. X-rays and MRIs may show degenerative changes, but plenty of people have those changes without any pain. The diagnosis relies heavily on physical examination and, in uncertain cases, a diagnostic injection.
During a physical exam, a clinician will typically perform a cluster of hands-on provocation tests. These involve applying pressure to the pelvis in specific ways to stress the SI joint and see if it reproduces your familiar pain. The standard cluster includes five maneuvers: distraction (pulling the front of the pelvis apart), compression (squeezing the pelvis together), thigh thrust, Gaenslen’s test, and sacral thrust. Having three or more of these tests reproduce your pain is a strong clinical indicator that the SI joint is the source.
When the exam findings are inconclusive or a procedure is being considered, the gold standard is a diagnostic injection. A clinician numbs the SI joint with local anesthetic under imaging guidance, and if your pain drops by at least 75% for a few hours, the joint is confirmed as the pain source. A single diagnostic injection has about an 87% overall accuracy rate, though false-positive results occur roughly 20% of the time. For that reason, some clinicians use two separate injections on different days to increase confidence in the diagnosis.
Treatment Options
Physical Therapy and Stabilization
For most people, treatment starts with physical therapy focused on stabilizing the pelvis. The core muscles that matter most here are the deep abdominal layers, particularly the transverse abdominis and internal oblique. These muscles act like a natural corset around the pelvis, and strengthening them takes stress off the SI joint. A typical program includes exercises in multiple positions (lying on your back, standing, on hands and knees, and side-bridge holds), often with an emphasis on drawing in the lower abdomen and holding for several seconds per repetition.
A pelvic compression belt can also help, especially in the early weeks. Worn low around the hips just above the bony prominence on the outside of each thigh, the belt mimics what your stabilizing muscles do by holding the SI joint snug. Clinical protocols often recommend wearing it during all waking hours for the first four weeks alongside an exercise program. The belt provides external support while you rebuild internal stability.
Injections
If physical therapy alone isn’t enough, steroid injections into the SI joint can reduce inflammation and provide weeks to months of relief. These are typically done under fluoroscopic or ultrasound guidance. They work best as a bridge, buying you time to continue physical therapy and address underlying causes.
Radiofrequency Ablation
For pain that keeps returning, radiofrequency ablation is an option. This procedure uses heat to disrupt the nerves that carry pain signals from the SI joint. A retrospective study of 128 patients found that about 67% achieved a clinically meaningful reduction in pain at three months, and 54% experienced at least a 50% drop in pain scores. Half also reported meaningful improvements in daily function and quality of life. The nerves do regrow over time, so the relief isn’t permanent, but it can last several months to over a year in many cases.
Surgery
SI joint fusion is reserved for cases that don’t respond to less invasive treatments. Minimally invasive techniques use small implants placed through a short incision to lock the sacrum and ilium together, eliminating the painful movement. Recovery typically involves several weeks of limited weight-bearing, and it takes a few months for the bone to fully fuse around the implants. It’s a last-resort option, but for the right patient, it can provide lasting relief.
Why It’s Often Misdiagnosed
SI joint pain is frequently mistaken for a herniated disc, hip bursitis, or piriformis syndrome because the pain patterns overlap. The lower back and buttock are a busy intersection of many potential pain generators, and standard lumbar MRIs won’t show SI joint problems clearly. Studies suggest that 10% to 25% of people being treated for generic “low back pain” actually have the SI joint as their primary source. If you’ve been treated for lower back pain without improvement, particularly if your pain is concentrated on one side and doesn’t travel below the knee, the SI joint is worth investigating.