Posterior Superior Iliac Spine: Anatomy and Function

The posterior superior iliac spine, usually called the PSIS, is a bony projection at the back of each side of the pelvis. It sits where the top rear edge of the iliac bone juts backward, roughly at the level of the second sacral vertebra. You can often spot it from the outside as a small skin dimple on either side of the lower back, and clinicians use it constantly as a reference point for everything from spinal injections to posture assessment. Despite its small size, the PSIS anchors major ligaments and fascia, marks the entry point for bone grafts and bone marrow biopsies, and plays a surprisingly large role in diagnosing low back pain.

The Dimples on Your Lower Back

Many people have a pair of visible indentations just above the buttocks, sometimes called the “dimples of Venus.” These dimples sit directly over the PSIS, where the skin is tethered to the underlying bone by short ligaments, creating a shallow depression. The dimples are not just cosmetic curiosities. Research has found that people who have visible dimples of Venus tend to have a higher pelvic incidence angle, a measure of how steeply the pelvis is tilted relative to the spine.¹ A separate study confirmed that patients with these dimples had a significantly higher pelvic incidence, though the dimple’s presence did not reliably predict whether a surgical corridor through the sacrum would be adequate.²

The association with a more sagittally oriented facet joint is worth noting too. A steeper facet angle can affect how forces are distributed through the lumbar spine, which may be relevant if you are dealing with recurrent low back pain. Still, a visible dimple of Venus by itself does not diagnose anything. It just tells a clinician that the pelvis underneath may be shaped in a particular way.

How Reliable Is Palpation?

Doctors, physical therapists, and chiropractors routinely press their thumbs into your lower back to locate the PSIS. This is one of the most common hands-on assessments in musculoskeletal care, used to estimate spinal levels, check for pelvic asymmetry, and guide needle placement. The problem is that different examiners often disagree about exactly where the PSIS sits.

A systematic review looking at interexaminer agreement for PSIS palpation found only fair reliability across eight studies, with agreement levels too low to support confident clinical use.³ A separate study comparing PSIS palpation with iliac crest palpation did find that the PSIS was more reliably located, but the estimated spinal levels still ranged widely, from the L5-S1 interspace all the way down to the S2 spinous process.⁴ That is a spread of several vertebral segments, which matters if a procedure depends on hitting the right level.

This inconsistency does not mean palpation is useless. It means clinicians should treat it as a rough guide and confirm with imaging when precision matters, particularly for injections or surgical planning. Body composition makes things harder: more subcutaneous tissue means the bony prominence is buried deeper, and the shape of the PSIS itself varies from person to person.

A Landmark for Sacroiliac and Low Back Pain

If you have ever pointed to the spot where your lower back hurts and placed your finger right next to your spine at belt level, you may have been pointing at or near your PSIS. Pain localized to that area is one of the hallmarks of sacroiliac joint dysfunction. The Fortin finger test formalizes this observation: a clinician asks you to point with one finger to where the pain is worst, and if that finger lands within about a centimeter of the PSIS, the sacroiliac joint becomes a prime suspect. In one study, all 16 patients identified by this test were subsequently confirmed to have sacroiliac joint problems through provocation injections.⁵ Pain at or around the PSIS is recognized broadly as characteristic of sacroiliac joint-related pain.⁶

But not all pain near the PSIS comes from the sacroiliac joint itself. Nerves passing close to this bony ridge can get pinched. The middle cluneal nerve, made up of sensory branches from the lower sacral nerve roots, passes between the PSIS and the posterior inferior iliac spine, sandwiched against the sacral ligament. When this nerve gets trapped in that tight space, it can cause severe low back pain that sometimes radiates into the leg, mimicking sciatica.⁷ The superior cluneal nerve is also vulnerable: an anatomical study found that over half of cadaver specimens had at least one branch of this nerve running through a bony-fibrous tunnel near the iliac crest, creating a potential squeeze point.⁸

For clinicians, distinguishing sacroiliac joint pain from cluneal nerve entrapment near the PSIS is important because the treatments differ. Sacroiliac dysfunction may respond to joint injections or stabilization exercises, while nerve entrapment sometimes requires targeted nerve blocks or, in stubborn cases, surgical release. The PSIS is the landmark that ties these diagnoses together geographically, even when the underlying cause is different.

Where Fascia and Force Meet

The PSIS is not just a spot to press on. It is a structural anchor where several layers of connective tissue converge. The thoracolumbar fascia, a broad sheet of dense tissue that spans the entire lower back, fuses into a thick composite right at the PSIS and the sacrotuberous ligament.⁹ This composite acts as a force-transfer hub. When your gluteal muscles, hamstrings, and spinal extensors contract during activities like walking, lifting, or bending, forces from the lower limb and trunk converge at the PSIS region. The fused fascial layers help maintain the integrity of both the lower lumbar spine and the sacroiliac joint.

This matters practically because dysfunction in the fascial system around the PSIS can contribute to instability. If the thoracolumbar fascia is not transmitting force efficiently, perhaps because of injury, deconditioning, or chronic inflammation, the sacroiliac joint may bear loads it is not designed to handle alone. This is one reason that rehabilitation for low back pain often focuses on strengthening the muscles that pull on the thoracolumbar fascia, including the gluteus maximus and the deep spinal extensors. The PSIS sits at the center of that force web.

Bone Grafts and Bone Marrow Biopsies

The posterior iliac crest near the PSIS is one of the most common sites in the body for harvesting bone graft material, particularly for spinal fusion surgery. Surgeons prefer this area because the ilium provides a generous supply of both cortical and cancellous bone, and the anatomy is relatively predictable. That said, the procedure is not without drawbacks. A prospective study tracking patients after posterior iliac crest bone graft harvesting found that the main complication was lingering donor site pain. About 45% of patients still reported some pain at 12 months, though the levels dropped significantly compared to three and six months after surgery. About one in ten patients experienced local sensory loss.¹⁰

Rare but serious vascular complications can also occur. The superior gluteal artery runs close to this region, and damage during harvesting has been reported to cause arteriovenous fistulas requiring embolization to repair.¹¹ Understanding the bony safety zones around the PSIS helps surgeons avoid the sciatic notch and the sacroiliac joint. Anatomical measurements show that the shortest distance from the PSIS to the sciatic notch averages roughly 40 to 44 millimeters, and the distance to the sacroiliac joint averages about 17 to 22 millimeters, with men having slightly larger safety margins.¹²

The PSIS also serves as a key landmark for bone marrow aspiration and biopsy, commonly performed in hematology to diagnose blood cancers and other marrow disorders. A cadaver study demonstrated that approaching the posterior iliac crest from a lateral angle, rather than straight on, caused less damage to nearby nerves and blood vessels and avoided the sacroiliac joint.¹³ More recent work on a posterolateral approach using palpation-based guidance, sometimes supplemented with ultrasound, confirmed correct needle positioning at the PSIS level in all 63 procedures studied.¹⁴ The lateral position also appears more comfortable for patients and more ergonomic for the operator.

Sex Differences in PSIS Shape and Size

The PSIS does not look identical in everyone, and one of the most consistent differences is between males and females. Studies using CT-based 3D reconstruction in Chinese populations have found that female specimens generally have larger linear and thickness measurements at the PSIS compared to males.¹⁵ An earlier morphological study confirmed this pattern, reporting that several key distances and the local bone thickness were significantly greater in women.¹⁶

These differences are dimensional rather than categorical. The overall morphological types, the basic shapes the PSIS can take, are distributed similarly between men and women. The broader female pelvis, an adaptation for childbirth, shifts the position and proportions of the PSIS without creating an entirely different structure. Practically, these size differences influence how easy it is to palpate the PSIS and how much bone is available for graft harvesting or screw placement. Meanwhile, the bony safety zones described earlier tend to be slightly smaller in women, which is clinically relevant for surgeons navigating this area.¹⁷

Measuring Pelvic Tilt and Alignment

Physical therapists and orthopedic specialists frequently measure pelvic tilt by looking at the angle between the PSIS and the anterior superior iliac spine (ASIS) on the same side. If the ASIS sits much lower than the PSIS, the pelvis is considered anteriorly tilted; if they are nearly level, pelvic tilt is minimal. This measurement guides decisions about corrective exercises, shoe lifts for leg length differences, and surgical planning.

The trouble is that pelvic bone shape varies enormously. One study found that the ASIS-to-PSIS angle ranged from 0 to 23 degrees across healthy individuals, with a mean of about 13 degrees. Side-to-side differences in the same person reached up to 11 degrees in tilt and 16 millimeters in bone height. That kind of variability means a clinician could diagnose “pelvic asymmetry” in someone whose pelvis is perfectly functional but simply has uneven bony landmarks. Conversely, true pelvic tilt could be masked when the anatomy creates a misleading angle. This is why imaging, rather than palpation alone, becomes important when making clinical decisions based on pelvic alignment, particularly when leg length discrepancy is suspected.

Tracking Pelvic Motion in Gait Labs

In biomechanics research and clinical gait analysis, reflective markers are placed on bony landmarks to track how the pelvis moves in three dimensions. The standard pelvic model uses the two ASIS points and one or both PSIS points. But in real-world gait labs, markers on the front of the pelvis can get blocked by swinging arms, assistive devices, or body tissue, and markers on the back can be obscured by wheelchair backs or trunk braces.

A study comparing alternative marker placements found that a model using PSIS markers combined with iliac crest markers produced very strong agreement with the standard model across all three planes of motion, with angular errors consistently below about 1.4 degrees.¹⁸ This means that when the front pelvic landmarks are not visible, the PSIS and iliac crest can serve as a reliable backup without sacrificing much accuracy. For patients with obesity or those using walkers, this flexibility can make the difference between a usable gait study and one that has to be thrown out.

The Evolutionary Backstory

The PSIS ended up where it is partly because of how our ancestors began walking upright. When early hominins transitioned to bipedal locomotion, the pelvis reshaped dramatically. The iliac blades flared outward and the overall pelvis became shorter and wider compared to the tall, narrow pelvis of our quadrupedal primate relatives. This flaring pushed the posterior superior iliac spines into a more rearward position, which gave the spinal extensor muscles a better mechanical advantage for holding the trunk upright over two legs.¹⁹ In a four-legged animal, those muscles do not need the same leverage because the spine hangs horizontally like a bridge. In humans, the spine is a vertical column balanced on the sacrum, and every forward lean of the trunk must be countered by the extensors pulling from behind. The posterior placement of the PSIS improves the moment arm for that pull.

This evolutionary compromise is also why the PSIS region is so crowded with ligaments, fascial attachments, and nerves. Bipedalism concentrated enormous mechanical demands at the lumbopelvic junction, and the body packed stabilizing structures into a small space to meet those demands. The tradeoff is that the same crowding creates vulnerability to entrapment, impingement, and pain, problems that are essentially a cost of walking upright.

Avulsion Fractures at the PSIS

The PSIS is rarely fractured in isolation, but it can happen. Avulsion fractures occur when a sudden muscular contraction or traumatic force pulls a fragment of bone away from the main structure. These injuries are more commonly discussed in adolescents, whose growth plates have not yet fused, but they occur in adults too. One reported case involved a 34-year-old woman who arrived at the emergency department with severe back pain after a traumatic injury. Standard pelvic X-rays looked normal, but a CT scan revealed minimally displaced bilateral avulsion fractures at both PSIS sites. She was treated conservatively with pain management and gradual return to activity, and by six months she had returned to normal function.²⁰

The fact that plain X-rays missed the fracture is worth remembering. The overlapping shadows of the sacrum and ilium on a standard pelvic film can hide subtle fractures in this region. If pain is localized precisely over the PSIS after trauma and conventional radiographs look clean, a CT scan may reveal what the X-ray could not. This applies to athletes experiencing sudden onset pain during explosive movements as well, where the erector spinae or gluteal attachments at the PSIS can pull bone fragments free under high load.

Ultrasound-Guided Sacroiliac Joint Injections

When sacroiliac joint pain does not respond to conservative measures, image-guided injections are a common next step. Historically these were done under fluoroscopy or CT guidance, both of which involve radiation. Ultrasound offers a radiation-free alternative, and the PSIS serves as the primary surface landmark for orienting the ultrasound probe and guiding the needle. In a study testing ultrasound-guided injections at two levels of the sacroiliac joint, correct intraarticular needle placement was confirmed by CT in 80% of cadaver joints on the first attempt, and in the four cases that initially missed, injection at the other level succeeded. In living patients, all ten ultrasound-guided injections hit their target, and pain relief averaged well above baseline at three months of follow-up.²¹

The PSIS makes ultrasound guidance feasible because it creates a recognizable hyperechoic (bright) line on the screen that orients the operator to the joint line just medial to it. Without that reliable bony reference point on ultrasound, the sacroiliac joint, which is deep and irregularly shaped, would be much harder to target without radiation-based imaging. For patients who need repeated injections over months or years, avoiding cumulative radiation exposure is a real practical benefit.