AIIS Hip Anatomy: Impingement, Fractures, and Surgery

The anterior inferior iliac spine, commonly abbreviated AIIS, is a small bony bump on the front of the pelvis that sits just above the hip socket. It serves as an anchor point for the rectus femoris muscle, one of the main muscles that flexes the hip and straightens the knee. When the AIIS is unusually large or positioned too low, or when it has been altered by an old injury, it can physically collide with the front of the thighbone during hip flexion, a condition known as subspine impingement. The AIIS is also a common site of avulsion fractures in young athletes, where a sudden forceful contraction pulls a piece of bone away from the pelvis. Understanding what can go wrong at this small landmark helps explain a surprisingly wide range of hip and groin complaints.

Where Exactly Is the AIIS and What Does It Do

The AIIS sits on the front surface of the ilium, the large wing-shaped bone of the pelvis, just above the upper rim of the acetabulum (the hip socket). It is one of two bony projections on the front of the pelvis; the other, the anterior superior iliac spine (ASIS), is the more prominent bump you can feel at the front of your hip bone when you press on it. The AIIS sits lower and deeper, making it harder to palpate through skin and muscle.

The main tendon that attaches here is the direct head of the rectus femoris. A cadaveric study measured the footprint of that attachment at roughly 13 mm wide and 26 mm long, meaning it covers a surprisingly broad area of the AIIS surface. A second tendon, the reflected head, attaches nearby along the acetabular rim, spanning an even larger area of about 48 mm by 17 mm. Together, these two tendon origins anchor the rectus femoris firmly to the pelvis so it can generate the force needed for kicking, sprinting, and jumping.¹ Critical structures including the femoral nerve, the psoas tendon, and the lateral circumflex femoral artery all pass within a couple of centimeters of the AIIS, which matters greatly during surgery.

Anatomical variation is common at the AIIS, and not just in overall size. A cadaveric study found that the indirect tendon of the rectus femoris displays several different patterns of aponeurotic expansion, with roughly 40% of specimens showing what researchers call a “standard” arrangement and the remaining 60% showing various superior, inferior, or combined expansions.² This variability helps explain why two people with a similarly shaped AIIS can respond quite differently to the same injury or surgical procedure.

The Three AIIS Types and Why They Matter

A widely used classification system sorts AIIS morphology into three types based on how far the bony prominence extends relative to the acetabular rim, using three-dimensional CT reconstructions of the pelvis. In Type I, the ilium wall between the AIIS and the rim is smooth and flat, with no overhang. In Type II, the AIIS extends down to the level of the rim. In Type III, it projects below the rim.

The clinical significance is straightforward: the further down the AIIS extends, the more it restricts hip motion. A study of 53 hips with impingement found that average hip flexion was about 120° in Type I, 107° in Type II, and only 93° in Type III hips. Internal rotation showed an even steeper drop, averaging 21° in Type I versus just 8° in Type III.³ For context, losing nearly 30° of flexion is the difference between comfortably tying your shoes and struggling to reach your foot, and losing that much internal rotation affects pivoting and cutting movements in virtually every field sport.

A Type I AIIS is generally considered normal and rarely causes impingement on its own. Types II and III, however, create a physical block: the protruding bone crashes into the femoral neck when the hip is flexed or internally rotated past a certain point. This collision can damage the labrum (the ring of cartilage lining the hip socket) and cause chronic groin pain. Subspine impingement can exist as a standalone problem or coexist with the more familiar cam and pincer forms of femoroacetabular impingement, and distinguishing between them matters for treatment planning.⁴

AIIS Avulsion Fractures in Young Athletes

In adolescents, the AIIS has not yet fused to the rest of the pelvis. It remains connected through a growth plate, or apophysis, which is the weakest link in the chain of bone-tendon-muscle. A sudden explosive contraction of the rectus femoris, as in a sprint start or a powerful kick, can rip the AIIS clean off the pelvis. This is called an avulsion fracture, and it is one of the most common pelvic injuries in young athletes.

A large review of 719 pelvic avulsion fractures in adolescents found that the AIIS was the single most frequently fractured site, accounting for about a third of all cases. The average patient age was around 15, and nearly four out of five fractures occurred in males. Soccer was by far the most common sport at the time of injury, followed by football and baseball. Running and kicking were the two leading injury mechanisms.⁵ A separate series of 242 cases confirmed these patterns and noted that males were more likely than females to sustain AIIS avulsions specifically, while females were more prone to iliac crest avulsions.⁶

Most AIIS avulsion fractures heal without surgery. The standard approach is rest, activity modification, and a gradual return to sport over several weeks. The catch is what happens afterward: the fracture fragment sometimes heals with extra bone formation, enlarging the AIIS and converting what was previously a normal Type I spine into a Type II or III configuration. One case report described exactly this scenario, where six months of conservative management failed to resolve symptoms after an avulsion fracture, and the patient eventually required surgical excision of the enlarged AIIS and the heterotopic bone that had formed around it.⁷ So while the fracture itself is usually a short-term problem, the remodeled bone can create a long-term impingement issue that only becomes apparent months or years later.

When the AIIS Causes Subspine Impingement

Subspine impingement (SSI) is a form of extra-articular hip impingement, meaning it involves structures outside the hip joint itself. The enlarged or low-hanging AIIS bumps against the femoral neck during hip flexion, and the repeated contact irritates the surrounding soft tissues and can damage the labrum from the outside. Symptoms tend to include anterior groin pain that worsens with activities requiring deep hip flexion: squatting, sitting for long periods, climbing stairs, or kicking a ball. The pain can be frustratingly nonspecific, which is one reason SSI is often overlooked or mistaken for more common intra-articular hip pathology.⁸

SSI does not always travel alone. A propensity-matched study comparing 50 patients with subspine impingement to 150 patients with standard femoroacetabular impingement found that the SSI group was more likely to need labral reconstruction rather than simple repair, suggesting that the extra-articular impingement had caused more severe labral damage. Interestingly, the actual size and location of labral tears and cartilage defects were otherwise comparable between the two groups.⁹ This underscores that SSI may fly under the radar during initial workup but leave a deeper mark on the labrum than expected.

How AIIS Problems Are Diagnosed

Getting the diagnosis right is trickier than you might expect. A standard AP pelvic X-ray can hint at a prominent AIIS, but its accuracy is limited. One study comparing plain radiographs with 3D-CT reconstructions found that when the AIIS appeared to project below the acetabular sourcil on X-ray, the sensitivity for detecting a truly hypertrophic AIIS (Type II or III on CT) was only moderate, around 76%, with specificity at 64%. The one reliable radiographic sign was when the AIIS clearly extended beyond the anterior rim on the X-ray: in those cases, every single patient had a hypertrophic AIIS confirmed on 3D-CT.¹⁰ In practice, this means that while an obvious finding on plain film is trustworthy, a normal-looking radiograph does not rule out the problem, and CT with 3D reconstruction remains the most reliable way to classify AIIS morphology.

MRI adds value by showing soft-tissue changes like labral tears, tendon injuries, and bone marrow edema around the AIIS. Ultrasound has also gained a role, particularly for evaluating the rectus femoris tendon origins and guiding diagnostic injections. A recently described lateral ultrasound approach now allows clinicians to assess both the direct and indirect heads of the rectus femoris tendon at the AIIS without relying solely on MRI.

The Role of Diagnostic Injections

One of the more interesting developments in diagnosing SSI is the use of ultrasound-guided injections directly at the AIIS. The idea is simple: inject a local anesthetic around the AIIS and see if the pain goes away. If groin pain drops by more than half, the AIIS is likely the culprit. A study comparing this injection-based approach with radiographic methods found that imaging alone (CT, MRI) and even intraoperative findings did not consistently agree with each other or with the injection results. The researchers concluded that no single imaging method was sufficiently reliable on its own and recommended combining an ultrasound-guided AIIS injection with radiographic assessment for a more accurate diagnosis.¹¹

A more recent propensity-matched study took this a step further. Patients who had a positive diagnostic AIIS injection before arthroscopic subspine decompression achieved higher rates of clinically meaningful improvement than patients selected for surgery based on CT morphology alone. Both groups improved after surgery, but the injection-positive group did better, suggesting that confirming the pain source before operating leads to sharper patient selection.¹²

Conservative Treatment

Most cases of AIIS-related hip pain begin with nonoperative management. The standard approach includes activity modification (reducing or temporarily stopping movements that provoke impingement), physiotherapy to improve hip mechanics, and anti-inflammatory medication for pain control.¹³ Physiotherapy typically focuses on strengthening the muscles around the hip, improving core stability, and restoring normal movement patterns. For many patients, especially those with mild SSI and no structural labral damage, this is enough.

The threshold for considering surgery is persistent pain after a reasonable trial of conservative care, usually three to six months. There is no hard cutoff, and the decision factors in the severity of symptoms, the degree of bony prominence on imaging, the patient’s activity demands, and whether the diagnostic injection was positive.

Arthroscopic AIIS Decompression

When conservative management fails, the surgical treatment of choice is arthroscopic osteoplasty, meaning the surgeon shaves down the protruding AIIS using a motorized burr inserted through small incisions around the hip. The goal is to reshape the AIIS into a Type I configuration, eliminating the overhang that was colliding with the femoral neck. One described surgical technique uses a 5.5 mm burr and aims for roughly 11 mm of clearance above the acetabular rim.¹⁴

How much bone can safely be removed is an important question. The rectus femoris tendon attaches broadly to the AIIS, and overly aggressive resection risks weakening or detaching it. A cadaveric study found that removing more than 10 mm of bone from the AIIS significantly compromised the rectus femoris origin and should be avoided. On the positive side, even moderate decompression improved the tracking of the iliopsoas tendon through its normal range of motion, an incidental benefit for patients who also have iliopsoas snapping.¹⁵ The broad origin of the direct head on the AIIS, along with a consistently bare area on the anteromedial side of the spine, provides surgeons with a safe zone to work within during resection.¹⁶

Surgical Outcomes and Return to Sport

The results of arthroscopic AIIS decompression are encouraging. A systematic review and meta-analysis pooling data from multiple studies found substantial improvement across all measured outcomes. Average modified Harris Hip Scores jumped from about 61 before surgery to nearly 89 afterward, and the Hip Outcome Score-Sports Specific Subscale climbed from roughly 37 to 83, reflecting a dramatic improvement in sport-related function.¹⁷

For competitive athletes specifically, a propensity-matched study followed patients for at least two years after arthroscopic subspine decompression. About 89% of athletes returned to sport, and 80% achieved a clinically meaningful improvement in their sports-specific hip score. The revision surgery rate was low at under 7%, and outcomes were comparable to those of a matched group of athletes who underwent hip arthroscopy for standard femoroacetabular impingement without SSI.¹⁸ A multicenter observational study also noted that rehabilitation protocols after combined FAI and AIIS osteoplasty were no different from those used for FAI correction alone, and return-to-sport programs were handled identically.¹⁹ In other words, adding the AIIS decompression does not appear to slow recovery.

Iatrogenic AIIS Impingement After Other Hip Surgeries

Subspine impingement does not always develop naturally. It can also be created as a side effect of another procedure, particularly periacetabular osteotomy (PAO), a major open surgery used to treat hip dysplasia. During PAO, the surgeon reorients the acetabulum to increase its coverage of the femoral head. This rotation can inadvertently push the AIIS into a more prominent position, effectively converting a Type I spine into a Type II or III. Researchers have documented high rates of abnormal AIIS subtypes in patients undergoing PAO for hip dysplasia, and some centers now perform a prophylactic AIIS osteoplasty at the time of PAO to preserve range of motion and avoid creating a new impingement problem while fixing an old one.²⁰

When the AIIS Growth Plate Has Not Yet Closed

The AIIS has its own secondary ossification center, a small island of bone that appears during puberty and eventually fuses to the main pelvic bone. A radiographic study tracking the order in which pelvic growth centers appear and close found that the AIIS ossification center appears and closes earlier in females than in males, with girls typically running one to two years ahead. In boys, the AIIS growth center appears after the lesser trochanter and closes after the femoral head physes; in girls, it appears after the ischial tuberosity and closes before the femoral head.²¹

This developmental timeline has practical consequences. While the growth plate is open, the AIIS is vulnerable to avulsion fractures because the cartilaginous junction is weaker than the surrounding bone and tendon. Once the growth plate closes, the risk of avulsion drops dramatically but the risk of chronic impingement from bony morphology takes over. The overlap period, roughly from age 13 to 17, is when adolescent athletes are most at risk: the muscles and tendons are strong enough to generate enormous forces, but the bony anchor has not yet fully solidified. This is why AIIS avulsions cluster so heavily in the mid-teenage years, and why parents and coaches of young athletes playing kicking and sprinting sports should be aware of the injury pattern.

Effect on Walking and Everyday Movement

Patients with femoroacetabular impingement often worry that their gait is affected. Research on this question suggests the impact is surprisingly subtle. A gait analysis study comparing people with symptomatic FAI to healthy controls found that the FAI group walked with slightly less range of motion in the sagittal plane (the forward-and-back direction), but showed no significant differences in side-to-side or rotational hip motion during walking. Joint forces were also comparable between the groups.²² This makes intuitive sense: normal walking does not require the deep flexion or aggressive internal rotation that triggers impingement. The problems tend to surface during athletic activities, prolonged sitting, or tasks that push the hip toward its end range. For patients whose main concern is simply being able to walk comfortably, this is reassuring, though it also means that a normal-looking gait in the clinic does not rule out significant impingement during sport.