When people search for the “S1 joint,” they’re almost always referring to one of two structures in the lower back and pelvis: the sacroiliac (SI) joint or the L5-S1 lumbosacral junction. Both involve the sacrum, the triangular bone at the base of your spine, and both are common sources of lower back pain. The SI joint is far more frequently discussed in this context, so that’s where we’ll start.
The Sacroiliac Joint: Where Spine Meets Pelvis
The sacroiliac joint is the largest axial joint in the body. It sits on each side of your lower back, connecting the sacrum to the ilium (the large, wing-shaped bone of your pelvis). You have two of them, one on the left and one on the right. Their primary job is transferring the weight of your upper body from your spine down through your pelvis and into your legs. Every time you walk, run, bend, or simply stand upright, your SI joints are absorbing and distributing load.
Unlike your knee or shoulder, the SI joint doesn’t move much. It allows only a few degrees of rotation and a small amount of gliding motion. That limited movement is by design: the joint is reinforced by some of the strongest ligaments in the body, keeping it stable under heavy compressive and shearing forces. Despite that stability, the joint can become a significant source of pain when something goes wrong.
The L5-S1 Junction: A Different Structure
The L5-S1 joint is where the lowest lumbar vertebra (L5) meets the top of the sacrum (S1). This is a spinal motion segment, not a pelvic joint, and it behaves quite differently from the SI joint. It includes an intervertebral disc, a pair of facet joints, and the surrounding ligaments that allow your lower spine to flex, extend, bend sideways, and rotate.
Because the lumbar spine curves forward (lordosis), the L5-S1 junction bears accentuated forward shearing stress compared to the segments above it. That makes it especially vulnerable to disc herniations, degenerative disc disease, and a condition called spondylolisthesis, where one vertebra slips forward over the one below it. When the disc or bone at L5-S1 compresses the S1 nerve root, it produces a specific pattern of symptoms: pain radiating down the back of the leg, numbness along the outside or bottom of the foot, and weakness when pushing the foot downward (the motion you use to press a gas pedal). A diminished Achilles tendon reflex is another hallmark sign.
How SI Joint Pain Feels
SI joint dysfunction is estimated to be the primary source of pain in 10 to 25 percent of people with chronic lower back pain. That’s a substantial share, yet it’s frequently misdiagnosed as a disc problem or generalized lumbar strain because the pain patterns overlap.
The hallmark of SI joint pain is tenderness right around or just below and toward the midline of the posterior superior iliac spine, the bony bump you can feel at the top of your buttock on either side. Clinicians use something called the Fortin finger test: they ask you to point with one finger to where it hurts. If your finger lands within about one centimeter of that bony landmark, SI joint dysfunction moves up the list of suspects. The pain can also radiate into the buttock, the lateral hip, the groin, and sometimes down into the leg, which is why it gets confused with sciatica.
Certain activities tend to make it worse: prolonged sitting, standing on one leg, climbing stairs, rolling over in bed, or transitioning from sitting to standing. The pain is often one-sided, though both joints can be affected.
How SI Joint Problems Are Diagnosed
There’s no single imaging test that reliably confirms SI joint dysfunction. X-rays and MRIs can show structural changes like arthritis or inflammation, but many people with abnormal-looking joints on imaging have no pain, and many people with significant pain have normal-looking scans. Diagnosis relies heavily on a clinical exam.
Clinicians typically use a cluster of five hands-on provocation tests designed to stress the SI joint and see if they reproduce your pain. These include a distraction test (pressing outward on the front of your pelvis while you lie on your back), a compression test (pressing down on your pelvis while you lie on your side), Gaenslen’s test (flexing one hip while extending the other), a thigh thrust, and a sacral thrust. When three or more of these tests reproduce your familiar pain, the likelihood of SI joint involvement is high.
For a definitive answer, a diagnostic injection can be performed. A small amount of local anesthetic is injected directly into the SI joint under imaging guidance. If you experience at least 75 percent sustained pain relief for the duration of the anesthetic, the joint is confirmed as the pain source.
Treatment for SI Joint Dysfunction
Most people with SI joint pain improve with conservative treatment. Physical therapy is the cornerstone, focusing on strengthening the muscles around the pelvis and core that help stabilize the joint. An SI belt, a supportive band worn around the hips, can provide additional stability during flare-ups. Anti-inflammatory medications help manage pain in the short term.
When conservative measures aren’t enough, the next step is typically a therapeutic injection of corticosteroid and anesthetic directly into the joint. This can provide weeks to months of relief. For people who respond well to injections but find the relief temporary, radiofrequency ablation is an option. This procedure uses heat to disrupt the small nerves that carry pain signals from the joint, and the relief can last six months to a year or longer before the nerves regenerate.
If all of these approaches fail, SI joint fusion surgery becomes a consideration. The procedure involves removing damaged cartilage and bone from the joint, then inserting small titanium implants through the ilium into the sacrum to lock the joint in place permanently. Minimally invasive versions use incisions only a few inches long, and recovery is generally faster than with open surgery. Fusion eliminates motion at the joint entirely, which eliminates the joint as a pain source. Your surgeon will review which nonsurgical treatments you’ve already tried before recommending this step.
SI Joint vs. L5-S1: Telling Them Apart
Because these two structures sit so close together, their pain patterns can look similar. A few distinguishing features help sort them out. SI joint pain tends to center on that specific bony point at the top of the buttock, worsens with position changes, and doesn’t typically cause neurological symptoms like numbness or weakness in the foot. L5-S1 nerve root compression, by contrast, produces shooting pain down the back of the leg with clear sensory or motor deficits: numbness on the outer foot, difficulty pushing off while walking, or a diminished ankle reflex.
The provocation tests described above are specific to the SI joint and won’t reproduce pain from a disc problem. Conversely, a straight leg raise test (lying on your back while someone lifts your extended leg) tends to provoke nerve root pain from a disc herniation but not SI joint pain. In many cases, both structures contribute, and a thorough exam can identify which one is the primary driver.