The iliac spine is not a single landmark but a set of four bony projections on each side of the pelvis, serving as anchor points for muscles, ligaments, and tendons that keep you upright, moving, and stable. The most commonly referenced is the anterior superior iliac spine (ASIS), the bony point you can feel at the front of each hip, but the anterior inferior (AIIS), posterior superior (PSIS), and posterior inferior (PIIS) iliac spines each play distinct roles in how the hip and trunk function. These small prominences matter far beyond anatomy class: they guide clinical measurements, get injured during sports, shape surgical planning, and even help forensic scientists determine age and sex from skeletal remains.
The Four Iliac Spines and Where They Sit
Your pelvis has a large, wing-shaped bone on each side called the ilium. Along the rim of that wing, four projections jut out at strategic locations. The anterior superior iliac spine (ASIS) sits at the very front of the iliac crest, right where many people rest their hands on their hips. Just below it, the anterior inferior iliac spine (AIIS) projects forward near the top of the hip socket. On the back side, the posterior superior iliac spine (PSIS) marks the upper rear corner of the ilium, often visible as a small dimple in the lower back. Below it, the posterior inferior iliac spine (PIIS) sits closer to the sacroiliac joint.
Each spine anchors specific soft tissues. The ASIS is where the sartorius muscle and the inguinal ligament attach. The AIIS gives origin to the rectus femoris, the powerful quadriceps muscle responsible for kicking and sprinting. The PSIS anchors portions of the sacroiliac ligaments, while the PIIS serves as an attachment for the sacrotuberous ligament, which stabilizes the sacroiliac joint during loading. These aren’t decorative bumps; they’re the mechanical connection points that let your muscles generate force against the skeleton.
How the Iliac Spines Shaped Human Walking
The position and shape of the iliac spines are part of what separates human bipedal walking from the locomotion of other primates. When early hominins began walking upright, the entire pelvis reorganized. The ilium became shorter and broader, and the iliac spines shifted to new positions that favored upright balance. The posterior superior iliac spines, in particular, moved further back as the ilium flared outward. This rearrangement gave the spinal extensor muscles a longer lever arm, making it mechanically easier to hold the trunk erect over two legs instead of four.1Journal of Hip Preservation Surgery. Evolution of the human hip. Part 1: the osseous framework
The anterior spines also changed. The AIIS became more prominent to support the rectus femoris during the swing phase of walking, when the thigh swings forward to take the next step. In quadrupedal primates, the equivalent structures are far less developed because the biomechanical demands are different. The iliac spines, in other words, are part of the anatomical package that makes human gait possible.
Avulsion Fractures in Adolescent Athletes
One of the most clinically relevant injuries involving the iliac spines is the avulsion fracture, where a sudden, forceful muscle contraction pulls a chunk of bone away from the spine it’s attached to. This happens almost exclusively in adolescents, because in growing teenagers the apophysis (the growth plate where muscle tendons attach to bone) hasn’t yet fused to the rest of the pelvis. An explosive sprint, a hard kick, or even an aggressive stretch can tear the bone fragment loose at the weakest link.
A large review of 719 cases found that the AIIS was the most common avulsion site, accounting for about a third of injuries, followed closely by the ASIS at roughly 30%. The ischial tuberosity made up about a fifth of cases. Running and kicking were the leading mechanisms, together responsible for over half the fractures.2Journal of Pediatric Orthopaedics. Acute Pelvic and Hip Apophyseal Avulsion Fractures in Adolescents: A Summary of 719 Cases ASIS avulsions are typically caused by a sudden pull of the sartorius during sprinting, while AIIS avulsions result from forceful contraction of the rectus femoris during kicking.3PubMed Central. Avulsion Fracture of the Anterosuperior Iliac Spine: A Case Report
Most avulsion fractures heal with rest, but the diagnosis can be tricky. Standard X-rays sometimes miss small avulsions in adolescents because the apophysis is still partly cartilage and doesn’t show up well on plain film. Ultrasound has emerged as a practical alternative, especially when families want to avoid the radiation of a CT scan. In one reported case, point-of-care ultrasound confirmed an AIIS avulsion fracture in a teenager after the family declined CT imaging.4PubMed Central. Point-of-care ultrasound detection of anterior inferior iliac spine avulsion fracture in an adolescent: A case report A separate case report of an adolescent soccer player with an AIIS avulsion described a 50% reduction in inflammation within a week of combined shockwave and magnetotransduction therapy, with return to play at six weeks.5PubMed. Pelvic apophysitis and avulsion fracture treated with a regenerative combination of extracorporeal shockwave therapy and extracorporeal magnetotransduction therapy in an adolescent soccer player
Subspine Impingement at the AIIS
The anterior inferior iliac spine has gained attention in orthopedics for a different reason: it can physically collide with the femur during deep hip flexion. When the AIIS is unusually large or projects too far forward, it may contact the front of the thighbone during activities that require bringing the knee up high, such as squatting, climbing stairs, or kicking a ball. This condition is called subspine impingement, and it’s been recognized as a source of extra-articular hip impingement distinct from the more familiar femoroacetabular impingement (FAI) that involves the rim of the hip socket itself.6PubMed Central. Anterior Inferior Iliac Spine (AIIS) and Subspine Hip Impingement
The relationship between AIIS shape and symptoms is less straightforward than it might seem. One imaging study found no significant difference in AIIS morphology between people with symptomatic FAI and those without symptoms. Age, sex, and other FAI features were not risk factors for developing an impingement-prone AIIS shape.7PubMed. Anterior Inferior Iliac Spine Morphology: Comparison of Symptomatic Hips With Femoroacetabular Impingement and Asymptomatic Hips Meanwhile, dynamic CT imaging has revealed that subspine impingement can occur even when the AIIS looks relatively normal on a static scan, particularly when the femur has decreased anteversion (a twist in the thighbone that changes how it meets the pelvis during motion).8PubMed. Subspine Hip Abnormalities: Exploring the Difference Between “Morphology” and “Impingement” The takeaway for anyone with persistent anterior hip pain is that a normal-looking AIIS on a still image doesn’t rule out mechanical impingement during movement.
Meralgia Paresthetica and the ASIS
A nerve called the lateral femoral cutaneous nerve runs from the lumbar spine down into the front and outer thigh, and it passes disturbingly close to the ASIS on its way. In a cadaveric study, the average distance from the ASIS to this nerve was just 8.8 millimeters. About 90% of the time, the nerve lay within two centimeters of the ASIS, and in over three-quarters of specimens it was less than one centimeter away.9PubMed. Anatomy of the lateral femoral cutaneous nerve relevant to clinical findings in meralgia paresthetica
This proximity matters because the nerve is vulnerable to compression right at that spot. When it gets pinched, you develop meralgia paresthetica: a burning, tingling, or numb patch on the outer thigh. Common culprits include tight belts, body armor, tool belts worn at the hip, prolonged standing, weight gain, and even tight-fitting jeans. The condition is usually annoying rather than dangerous, but it can become chronic if the compression isn’t relieved. Surgeons who operate near the ASIS have to be especially careful to avoid damaging this nerve, since its position varies from person to person and there’s no reliable external landmark to predict exactly where it will be.
The ASIS and PSIS in Pelvic Tilt Assessment
Physical therapists and sports medicine professionals routinely use the iliac spines as reference points to assess pelvic alignment. The standard method involves drawing an imaginary line between the ASIS and PSIS on the same side and measuring its angle relative to horizontal. A forward-tilted pelvis (anterior pelvic tilt) shows the ASIS lower than the PSIS; a backward tilt (posterior tilt) shows the opposite. In a study of 120 healthy young adults, about 85% of males and 75% of females had some degree of anterior pelvic tilt, while only about 6 to 7% showed a posterior tilt.10PubMed. Assessment of the degree of pelvic tilt within a normal asymptomatic population
The problem is that the bony landmarks themselves vary considerably between individuals. One study measured pelvic tilt using the ASIS-to-PSIS angle and found values ranging from 0 to 23 degrees, with a mean around 13 degrees. More troublingly, side-to-side asymmetry in the landmarks themselves produced differences of up to 11 degrees in tilt angle and 16 millimeters in innominate height between the left and right sides of the same pelvis.11PubMed Central. Variation in pelvic morphology may prevent the identification of anterior pelvic tilt In plain terms, the bones themselves aren’t symmetrical, so if a clinician palpates both sides and finds a difference, it may reflect how that person’s pelvis is shaped rather than a genuine misalignment.
How Reliable Is Palpation of These Landmarks?
Given how central the iliac spines are to clinical assessment, you might assume that trained professionals can locate them with precision. The evidence is mixed, and in some cases discouraging. One study found that a digitization technique for measuring innominate bone position produced excellent reliability scores when combined with palpation.12PubMed. Inter-tester reliability of non-invasive technique for measurement of innominate motion However, a separate study on trained versus untrained osteopathic examiners found that agreement between examiners was poor across the board for palpation of the ASIS, PSIS, and related landmarks. Even after training sessions, inter-examiner reliability for the ASIS reached only a kappa of 0.24, and for the PSIS just 0.08, both well below what’s considered acceptable for a clinical test.13International Journal of Osteopathic Medicine. The effect of training on the inter-examiner and intra-examiner reliability of the seated flexion test and assessment of pelvic anatomical landmarks with palpation
The authors of that study went so far as to suggest the osteopathic profession should reconsider using these palpation-based tests to diagnose sacroiliac dysfunction, given their poor reliability. For the average person, the practical implication is this: if one practitioner tells you your pelvis is “out of alignment” based on feeling your ASIS and PSIS, a different practitioner examining you the same day might reach a completely different conclusion. The limitation isn’t necessarily with the concept of pelvic asymmetry but with the human hand’s ability to precisely locate these landmarks through skin and soft tissue.
Leg-Length Measurement
One of the most common clinical uses of the ASIS is measuring apparent leg-length discrepancy. The standard bedside technique involves running a tape measure from the ASIS down to the ankle bone (medial or lateral malleolus) on each side and comparing the two values.14Journal of Pediatric Orthopaedics. Measurement Variance in Limb Length Discrepancy: Clinical and Radiographic Assessment of Interobserver and Intraobserver Variability This technique has the advantage of requiring no equipment beyond a tape measure, but it inherits the same limitation described above: if the ASIS itself is hard to locate precisely, or if the two sides of the pelvis are naturally asymmetric, the measurement carries built-in error. Radiographic measurement (standing X-ray with a ruler calibration) remains the gold standard when precision matters, but the tape-measure method persists in daily practice because it’s fast and free.
Surgical Relevance of the Iliac Spines
Surgeons interact with the iliac spines in two main ways: as reference landmarks for placing hardware, and as a source of bone graft material.
In pelvic fracture surgery, the iliac spines serve as starting and ending points for screw trajectories through the ilium. For example, the path from the PSIS to the AIIS produces the longest bony tunnel available for a transiliac screw, measuring about 141 millimeters in males and 129 millimeters in females.15PubMed. Anatomic and radiographic considerations for placement of transiliac screws in lumbopelvic fixations These measurements matter because a longer screw path means more cortical bone contact and stronger fixation. Gender differences in pelvic anatomy influence which landmarks are most reliable for guiding screw placement. A cadaveric study found that the PSIS and PIIS were most reliable for anterior column screws, while the ASIS and ischial tuberosity showed the lowest variation between sexes for posterior column screws.16PubMed Central. Screw placement in percutaneous acetabular surgery: gender differences of anatomical landmarks in a cadaveric study
The iliac crest, just above and between the iliac spines, is a traditional donor site for bone grafts used in spinal fusion and other reconstructive procedures. Harvesting bone from this area provides high-quality graft material, but it comes at a cost. A study of patients who underwent anterior iliac crest bone graft harvesting for cervical spine fusion found that a large percentage reported chronic pain at the donor site, and long-term functional impairment was significant enough that the authors recommended patients be counseled about the risks and alternative graft sources be considered.17PubMed. Donor site morbidity after anterior iliac crest bone harvest for single-level anterior cervical discectomy and fusion This donor site morbidity has driven a shift toward synthetic bone substitutes and cadaveric allograft in many surgical settings, though autograft from the iliac crest is still considered the biological gold standard for fusion.
Forensic Age and Sex Estimation
The iliac spines and surrounding structures are valuable to forensic scientists because the pelvis is one of the most sexually dimorphic bones in the human body. A study of 400 CT scans from a Western Australian population found that pelvic measurements could determine sex with accuracy rates ranging from about 81% using a single measurement (ischial length) to 100% when multiple variables from the complete pelvis were combined. The transverse pelvic outlet and the subpubic angle contributed most significantly to sex discrimination.18PubMed Central. Morphometric analysis of pelvic sexual dimorphism in a contemporary Western Australian population
Age estimation draws on a different feature: the fusion of the iliac crest apophysis. In adolescents and young adults, the iliac crest has a separate growth center that gradually fuses to the main ilium in a predictable sequence. CT-based assessment of this fusion has shown that early stages appear around age 12, with progressive stages at 14, and near-complete fusion beginning around age 17.19PubMed. Computed tomography evaluation of the iliac crest apophysis: age estimation in living individuals These thresholds are useful in legal contexts where determining whether an individual has crossed a legally significant age boundary (such as 14 or 18) matters for criminal proceedings or immigration cases.
Enthesopathy and Inflammatory Disease
The iliac spines are entheses, meaning they’re the spots where tendons and ligaments insert into bone. In inflammatory conditions like axial spondyloarthritis (the family of diseases that includes ankylosing spondylitis), these insertion points can become chronically inflamed, a process called enthesopathy. A CT-based study found that enthesopathy at the hip and pelvis was present in about 40% of patients with axial spondyloarthritis compared to roughly 22% of controls. Smoking, higher spinal disease scores, and use of TNF-alpha blockers were independently associated with enthesopathy in the patient group. Interestingly, radiographic progression of these entheseal lesions over approximately two years was very rare, suggesting that once the bony changes form, they tend to stabilize rather than worsen rapidly.20SpringerLink / Clinical Rheumatology. Prevalence and progression of radiographic enthesopathy at hip and pelvis in patients with axial spondyloarthritis based on CT assessment
Obstetric Assessment and the PSIS
During pregnancy and childbirth, the posterior iliac spines take on a particular significance. The diamond-shaped area of skin over the lower back known as the Michaelis sacral rhombus is defined by the PSIS on each side, and its dimensions are thought to reflect the underlying mobility of the sacroiliac joints. Obstetricians and osteopaths have traditionally assessed this area by watching how its shape changes when a pregnant woman squats, on the premise that greater expansion correlates with a more mobile pelvis. A study of 100 pregnant women in their late trimester found that measuring the sacral area from skin marks in different positions didn’t produce statistically significant differences, while measurements taken through direct finger contact on the bony landmarks (the PSIS) appeared more adequate for estimating pelvic mobility.21PubMed Central. Estimating the Mobility of the Michaelis Sacral Rhombus in Pregnant Women This area remains one of the few practical ways to get a hands-on sense of pelvic mobility without imaging, though its clinical utility for predicting labor outcomes is still debated.