The anterior superior iliac spine (ASIS) is a highly recognizable bony projection on the pelvis. Located at the front of the hip, it marks the anterior termination of the iliac crest, the large, curving upper border of the ilium. The ASIS is a prominent landmark in surface anatomy because it is easily felt beneath the skin. This bony prominence exists bilaterally, with one on each side of the lower abdomen, providing an anchor for soft tissues and a baseline for clinical measurements. Understanding the ASIS is foundational for analyzing movement, posture, and orthopedic conditions.
How to Palpate and Locate the ASIS
To locate the ASIS, start with the iliac crest, the long, curved top edge of your hip bone. Place your hands flat on your waist and slide them down until you feel the firm, uppermost edge of the pelvic bone. Trace this ridge forward until it terminates in a distinct, rounded bony point—this is the ASIS.
For accurate location, stand relaxed or lie on your back with your knees bent to relax the overlying muscles. The ASIS feels like a slight corner or notch where the hip bone ends before the thigh begins. Palpating both sides simultaneously allows for comparison, which is often done in clinical settings to assess symmetry.
Key Muscle and Ligament Attachments
The ASIS is a significant origin point for powerful muscles and a strong ligament. The longest muscle in the human body, the Sartorius, originates directly from the ASIS. This strap-like muscle runs across the thigh to the knee, functioning to flex the hip and knee and rotate the thigh outward. Adjacent to the Sartorius, the Tensor Fasciae Latae (TFL) also arises from the ASIS and surrounding iliac crest, contributing to hip flexion, abduction, and stabilization of the hip and knee.
The ASIS also provides the lateral attachment for the inguinal ligament, a dense band of connective tissue. This ligament spans from the ASIS medially to the pubic tubercle, forming the lower border of the anterior abdominal wall. It creates a boundary between the abdomen and the thigh, supporting lower abdominal structures and forming the floor of the inguinal canal.
Importance in Clinical Assessment and Biomechanics
The easily identifiable location of the ASIS makes it an indispensable reference point in orthopedic and biomechanical assessments. Clinicians use it to accurately measure true leg length discrepancy by measuring the distance from the ASIS down to the medial malleolus (the bony prominence on the inside of the ankle). A difference in this measurement between the two sides can indicate a structural imbalance affecting gait and posture.
The ASIS is routinely used to evaluate pelvic alignment and posture, such as when assessing for an anterior pelvic tilt. When the pelvis tilts forward, the ASIS moves lower than the pubic symphysis, altering the length-tension relationship of attached muscles. Its position is also a key factor in calculating the Q-angle, which is the angle formed by a line from the ASIS through the kneecap center to the tibial tuberosity.
The ASIS also acts as a surface landmark for locating deeper anatomical structures, such as McBurney’s point, a common clinical indicator for appendicitis. Because the ASIS is the origin for the strong Sartorius and TFL muscles, it is susceptible to avulsion fractures, particularly in young athletes whose growth plates are still developing.