The sacroiliac joint connects your spine to your pelvis and serves as the primary bridge for transferring the weight of your upper body down into your legs. You have two of them, one on each side, sitting where the triangular bone at the base of your spine (the sacrum) wedges tightly between the two large hip bones. Despite being easy to overlook, these joints play a central role in nearly every movement that involves your trunk and lower body.
How It Transfers Weight
Every time you stand, walk, or lift something, the force of your upper body has to travel from your spine into your pelvis and then into your legs. The sacroiliac joint is where that handoff happens. The sacrum sits wedged between the hip bones like a keystone in an arch, and this shape allows it to resist the shearing forces that come with bearing load. Without this joint functioning properly, the simple act of standing upright would place enormous stress on surrounding structures.
This weight-transfer role is why the sacroiliac joint is implicated in so many cases of lower back pain. Clinical estimates suggest that 15% to 30% of chronic low back pain originates from sacroiliac joint dysfunction, and some studies put the number even higher depending on the population studied.
How Much It Actually Moves
The sacroiliac joint is not built for large, sweeping movements. It allows only a few degrees of rotation and a few millimeters of translation. The two primary motions are called nutation and counternutation. Nutation is a subtle forward tilt of the top of the sacrum relative to the hip bones, and counternutation is the reverse. These tiny movements happen naturally during walking, bending, and shifting your weight from one leg to the other.
This minimal motion is intentional. The joint is designed for stability first, mobility second. Its small range of movement acts as a buffer, absorbing and distributing forces so that no single point in your pelvis or lower spine takes the full impact of activities like running, jumping, or even sitting down hard in a chair.
What Holds It Together
The sacroiliac joint relies on one of the strongest ligament networks in the body. The interosseous sacroiliac ligament, buried deep between the sacrum and the hip bone, forms the primary structural connection. Behind the joint, the posterior sacroiliac ligament anchors the hip bone to the lower segments of the sacrum. Two additional ligaments, one running from the sacrum to a bony point you can feel at the bottom of your pelvis and another connecting to a nearby prominence, work together to prevent the sacrum from tilting too far forward during weight-bearing and walking.
On the front side, the anterior sacroiliac ligament acts as a thickened capsule that resists forward tilting and rotation. There’s also a ligament that connects the lowest lumbar vertebra directly to the top of the hip bone, which limits side-bending. This layered system of restraints is what makes the sacroiliac joint so stable. No single ligament does the job alone; they work as a team, each one checking a different direction of movement.
Differences Between Male and Female Joints
Male and female sacroiliac joints are shaped differently. In men, the joint surface on the hip bone is significantly larger overall, and the angle between the upper and lower portions of that surface is wider. In women, one dimension of the surface is proportionally larger while the overall surface area is smaller. These structural differences likely reflect the competing demands of stability (more important for load-bearing in men) and flexibility (more important for childbirth in women).
This anatomical difference has real consequences. The female sacroiliac joint is inherently more mobile, which contributes to the higher rates of sacroiliac joint pain and dysfunction in women compared to men.
How Pregnancy Changes the Joint
During pregnancy, the body releases a hormone called relaxin, produced by the ovaries and placenta starting in early pregnancy. Relaxin levels climb significantly during the first trimester and stay elevated until late pregnancy, becoming undetectable within days after delivery. Its job is to remodel collagen, the structural protein in ligaments, making the pelvic ligaments more elastic so the pelvis can widen during delivery.
This loosening begins around the tenth to twelfth week of pregnancy. While it serves an essential purpose, it also reduces the stability of the sacroiliac joint. The posterior sacroiliac ligament, for example, can stretch noticeably when the normal curve of the lower back flattens, which commonly happens as pregnancy progresses. This combination of hormonal loosening and postural change is a major reason why pelvic girdle pain is so common during and after pregnancy.
How the Joint Changes With Age
The sacroiliac joint does not stay the same throughout your life. In middle-aged and older adults, degenerative changes are common: the space inside the joint narrows, the bone underneath the cartilage becomes denser (a sign of wear), and bony spurs can develop around the edges. On imaging, these spurs sometimes look like the joint has fused completely, but true internal fusion of the sacroiliac joint is actually rare in normal aging. The bony bridges tend to form along the front surface of the joint rather than through the joint cavity itself.
These changes generally reduce the already-small range of motion, making the joint stiffer over time. For many people this causes no symptoms at all. For others, particularly those with prior injuries or inflammatory conditions, the degenerative process can become a source of chronic pain.
How Sacroiliac Problems Are Identified
Pinpointing the sacroiliac joint as the source of pain is notoriously tricky because the pain it produces often mimics other conditions, particularly lumbar disc problems and hip issues. No single physical exam maneuver is reliable enough on its own. The most useful approach is to combine several provocation tests, which involve positioning and pressing on the pelvis in specific ways to stress the joint, and then checking how many of them reproduce your pain.
Five tests have consistently shown both sensitivity and specificity above 60%: the distraction test, compression test, thigh thrust, sacral thrust, and resisted hip abduction. When three or more of these tests are positive, the likelihood of sacroiliac joint involvement is high, with a negative predictive value of 96%, meaning that if fewer than three tests are positive, there’s a very strong chance your pain is coming from somewhere else. This composite approach is far more accurate than relying on any single test or on patient history alone.