Palpating the sacroiliac (SI) joint starts with finding the posterior superior iliac spine (PSIS), the bony bump you can feel at the base of your lower back on each side. The SI joint itself sits roughly 4.4 centimeters (about 1.75 inches) toward the midline from the PSIS, buried under thick ligament and muscle. You cannot directly feel the joint space through the skin, but you can reliably locate its position, assess tenderness over it, and use motion-based palpation to evaluate how it moves.
Finding the PSIS
The PSIS is the single most important landmark for SI joint palpation. It’s the pointed crest of the ilium (hip bone) that sits at approximately the level of the second sacral segment. On most people, two visible dimples in the lower back mark the PSIS on each side. If the dimples aren’t obvious, place your hands on the iliac crests (the top of the hip bones) and slide your thumbs downward and toward the spine. You’ll feel a firm, rounded bony prominence on each side. That’s the PSIS.
Once you’ve found the PSIS, you have a reliable reference point. The SI joint line begins just slightly above the PSIS (about 7 to 8 millimeters) and extends roughly 38 millimeters below it. In practical terms, the joint runs from just above the PSIS down to about 1.5 inches below it. The joint is deeper than many people expect: a radiographic study in Pain Physician found the horizontal distance from the PSIS to the back of the SI joint averages about 44 millimeters, or just under two inches. This depth is why you’re palpating over the joint rather than touching it directly.
Static Palpation of the SI Joint
With your subject lying face down (prone), locate both PSIS landmarks. Press firmly with your thumb just medial and slightly inferior to the PSIS. This area overlies the upper portion of the SI joint. Work your pressure in a slow arc from just above the PSIS down to about 1.5 inches below it, staying roughly one to two finger-widths toward the midline. You’re pressing through the long dorsal sacroiliac ligament, the most superficial ligament covering the joint. It runs directly from the PSIS over the back of the joint, so tenderness here often correlates with SI joint irritation.
Compare side to side. Note whether one side reproduces the patient’s familiar pain while the other does not. Also palpate the sacral sulcus, the groove between the sacrum and the ilium. Run your thumb down this groove from the level of the PSIS toward the tailbone. Increased tissue tension, bogginess, or sharp tenderness along this line can point to SI joint involvement.
Below and lateral to the SI joint, you can also palpate the sacrotuberous ligament, which connects the sacrum to the sit bone (ischial tuberosity). Find the ischial tuberosity by pressing into the lower buttock where it meets the upper thigh, then trace upward and medially toward the sacrum. Tenderness here sometimes accompanies SI joint dysfunction because the sacrotuberous ligament blends with fibers of the long dorsal ligament.
The Fortin Finger Test
Before you start hands-on palpation, a quick screening step can help confirm you’re in the right area. Ask the person to point with one finger to the spot that hurts most. Then ask them to do it again. If they point to the same spot both times, within about one centimeter, and that spot is just below and toward the midline from the PSIS, the test is considered positive for SI joint pain. This consistent localization pattern is a useful clue that the SI joint is the pain source rather than the lumbar spine or hip.
Motion Palpation With the Gillet Test
Static palpation tells you about tenderness, but motion palpation assesses whether the SI joint is moving normally. The most widely used motion palpation technique is the Gillet test, sometimes called the stork test.
Have the person stand facing a wall, about a foot away, with fingertips lightly touching the wall for balance. Their feet should be directly under their hips so they’re standing upright, not leaning. Stand or kneel behind them. Place one thumb on the PSIS and the other thumb on the sacrum at approximately the second sacral tubercle (the bony bump you can feel on the midline of the sacrum, roughly level with the PSIS). Position your thumbs so they’re pointing toward each other on a horizontal line.
Ask the person to flex the hip on the side where your thumb is on the PSIS, bringing their knee up toward their chest. As the hip flexes, the ilium should rotate posteriorly. You should feel the PSIS thumb drop downward relative to the sacral thumb. If the PSIS thumb stays level with or rises above the sacral thumb, the test suggests restricted SI joint motion on that side. Repeat on the other side and compare.
Because this is an excursion test, the exact starting position of your thumbs matters less than the relative movement between them. What you’re tracking is whether the PSIS moves inferiorly in relation to the sacrum during hip flexion.
Sex-Based Differences in Anatomy
The SI joint sits in a slightly different position in men and women. In men, the horizontal distance from the PSIS to the joint averages about 46 millimeters, and the joint extends roughly 42 millimeters below the PSIS. In women, the distance is shorter at about 42 millimeters, and the joint extends only about 34 millimeters below the PSIS. Women’s SI joints are also generally smaller and more mobile, which means the area you need to cover during palpation is slightly more compact. Adjusting your palpation field based on these differences can improve your accuracy.
How Reliable Is SI Joint Palpation?
Palpation of the SI joint has a reputation for being inconsistent between examiners, and the research partly supports that concern. When individual palpation or motion tests are studied, inter-examiner reliability ranges from fair to substantial, with adjusted kappa values between 0.52 and 0.84. That’s decent but not strong enough to rely on a single test.
Reliability improves significantly when you combine tests. Clusters of motion palpation and pain provocation tests together reach kappa values of 0.52 to 0.92, which is considered substantial to excellent. The practical takeaway: never base your assessment on one palpation finding alone. Combine your static palpation and Gillet test results with provocation tests (like the FABER, compression, distraction, or thigh thrust tests) to build a more reliable clinical picture. Three or more positive provocation tests in combination are generally considered meaningful.
Distinguishing SI Joint From Lumbar Spine
The SI joint and the L5-S1 spinal segment sit close together, and their pain patterns overlap. A few palpation strategies help you separate them. SI joint tenderness is typically located lateral to the midline, in and around the sacral sulcus and just inferomedial to the PSIS. Lumbar facet joint tenderness sits closer to the midline, directly over the spinous processes or just lateral to them at the L4-L5 or L5-S1 levels.
Press along the spinous processes of L4, L5, and S1 first, noting any tenderness. Then move laterally to the SI joint region. If the person’s pain is reproduced only when you palpate over the sacral sulcus and not over the lumbar spine, that tilts the picture toward the SI joint. Combining this with the Fortin finger test adds another layer: people with true SI joint pain almost always point to a spot near the PSIS, while those with lumbar disc or facet problems tend to point higher or more centrally along the spine.