The sacroiliac (SI) joint sits deep beneath thick muscle and ligament, making it one of the harder joints to palpate directly. You can’t feel the joint line itself the way you might feel a kneecap or an ankle bone. Instead, palpation relies on identifying the bony landmarks that frame the joint, feeling the ligaments that cover it, and using movement-based tests to assess how the joint behaves under load.
Finding the Key Bony Landmarks
The SI joint connects the sacrum (the triangular bone at the base of the spine) to the ilium (the large wing-shaped pelvic bone) on each side. The joint surface itself is buried several centimeters deep, but three landmarks on the surface give you a reliable map to its location.
The posterior superior iliac spine, or PSIS, is your primary reference point. These are the two bony bumps you can see as dimples on either side of the low back, roughly at the level of the S2 vertebra. To find them, place your hands on the top of the pelvis (the iliac crests) and slide your thumbs downward and inward until they drop into a firm, rounded bony prominence. The SI joint line runs just medial and slightly inferior to the PSIS on each side.
The sacral tubercles form a vertical ridge running down the midline of the sacrum. You can feel them as a series of small bumps between the two PSIS landmarks. The second sacral tubercle, roughly level with the PSIS, marks the approximate center of the SI joint from top to bottom. The iliac tuberosity, a roughened area of bone just behind the joint surface, sits directly dorsal to the auricular (ear-shaped) part of the joint and can sometimes be felt as a broad, less distinct prominence lateral to the sacral midline.
Palpating the Long Dorsal Sacroiliac Ligament
While the joint surface itself is too deep to touch, the long dorsal sacroiliac ligament (sometimes called the long posterior sacroiliac ligament) is one of the few SI joint structures you can palpate directly. It attaches from the PSIS and adjacent iliac bone above to the third and fourth sacral segments below, running vertically just caudal (below) the PSIS. When you press into this area, a healthy ligament feels like a firm, bone-hard band beneath your fingertip.
Tenderness here is clinically meaningful. Research published in Acta Obstetricia et Gynecologica Scandinavica found that this ligament frequently shows tenderness on palpation in patients with peripartum pelvic pain, and the authors suggested it may be an underrecognized source of symptoms often attributed to the joint itself. When you’re assessing someone with low back or buttock pain, pressing systematically along this ligament, from just below the PSIS downward toward the lower sacrum, can help identify whether extra-articular (ligamentous) structures are contributing. This distinction matters because treatment for ligament-related pain differs from treatment for problems inside the joint capsule.
Patient Positioning and Hand Placement
The most common position for SI joint palpation is prone (face down) on a firm surface. This relaxes the gluteal muscles and paraspinal muscles that otherwise obscure the landmarks. Ask the person to let their arms rest at their sides or above their head, whichever is more comfortable.
Start by placing both thumbs on the iliac crests and walking them toward the midline until you locate each PSIS. From there, slide one thumb medially onto the sacrum to feel the sacral tubercles, while keeping the other thumb on the PSIS. The gap between these two thumbs approximates the width of the SI joint underneath. Apply slow, steady pressure rather than poking, since the overlying tissue is often tender in people with low back pain regardless of the source.
For comparison, palpate both sides sequentially. Asymmetry in landmark height or notable differences in tenderness between left and right can point toward one-sided dysfunction, though these findings need to be interpreted alongside other tests.
The Gillet Test: Palpation During Movement
Static palpation tells you where structures are and whether they’re tender, but the Gillet test (also called the stork test or marching test) uses palpation to assess how the SI joint moves. The person stands while you sit or kneel behind them. Place one thumb on the PSIS and the other on the sacral base at roughly the second sacral tubercle. Then ask the person to lift one knee toward their chest, flexing the hip on the side you’re testing.
The expected finding is that the PSIS thumb drops downward relative to the sacral thumb as the ilium rotates posteriorly during hip flexion. If the PSIS thumb stays level with or rises above the sacral thumb, the test is considered positive, suggesting restricted SI joint movement on that side.
A word of caution on interpretation: a 2018 study in the Journal of Chiropractic Medicine questioned whether this test truly measures SI joint motion or simply reflects asymmetric balance strategies during one-legged stance. The test was positive in all patients included in the study regardless of whether their pain originated from the SI joint, giving it 100% sensitivity but 0% specificity. In practical terms, a negative Gillet test may help rule out SI joint restriction, but a positive result alone doesn’t confirm it.
How Reliable Is SI Joint Palpation?
Palpation-based assessment of the SI joint has well-documented limitations. When researchers compared how consistently different examiners agreed on palpation findings, the results were poor. One review found that agreement for palpation-based motion tests produced a kappa value of negative 0.06, essentially no better than chance. By contrast, pain provocation tests (where you apply specific forces to stress the joint and ask whether it reproduces symptoms) showed kappa values between 0.43 and 0.84, meaning moderate to good agreement between examiners.
This doesn’t mean palpation is useless. It means palpation works best as one piece of a larger assessment. A study on distinguishing SI joint pain from other causes of chronic low back pain found that diagnostic accuracy was 85 to 86% with medical history alone, 87% when imaging was added, and 96% when physical examination testing was included. The physical exam component included both palpation findings and provocation maneuvers. Current multispecialty consensus guidelines note that physical examination and provocative tests have better negative predictive value than positive predictive value, meaning they’re more useful for ruling out the SI joint as a pain source than for confirming it. When confirmation is needed, a diagnostic injection into the joint remains the reference standard.
Distinguishing SI Joint Pain From Lumbar Pain
One of the main reasons people palpate the SI joint is to figure out whether pain is coming from the joint rather than from the lumbar spine or hip. A few palpation findings help with this distinction. SI joint tenderness is typically localized over or just medial to the PSIS, in an area roughly the size of a thumbprint. It often extends into the long dorsal ligament below the PSIS. Lumbar facet joint pain, by comparison, tends to produce tenderness closer to the midline, over the spinous processes or just lateral to them, and higher up.
The Fortin finger test is a simple screen: ask the person to point with one finger to where it hurts most. If they consistently point to a spot within about 2 centimeters of the PSIS, SI joint involvement is more likely. If they sweep their hand across a broad area of the low back, the source is less likely to be isolated to the SI joint.
Combining palpation findings with at least three provocation tests (such as the compression test, thigh thrust, and sacral distraction) gives you a much more reliable clinical picture than any single finding on its own. When three or more provocation tests reproduce the person’s familiar pain pattern and palpation confirms localized tenderness near the PSIS and dorsal ligament, the probability of SI joint involvement is high enough to guide treatment decisions or justify further diagnostic workup.