The Anterior Superior Iliac Spine (ASIS) is a prominent bony landmark located on the front of the pelvis. This projection forms the anterior end of the iliac crest, the curved upper edge of the hip bone. The ASIS is easily felt just beneath the skin, making it a reliable surface reference point for anatomical study and clinical assessment. It acts as a central anchor for several muscles and a major ligament, playing a role in maintaining posture and facilitating lower body movement.
Locating the ASIS
The ASIS is situated at the most forward-projecting point of the pelvis, where the top edge of the hip bone curves forward. To locate it, a person can place their hands on their hip bones and follow the curved ridge (the iliac crest) toward the front of the body. The distinct bony prominence felt at the end of this ridge is the ASIS. This easily identifiable position makes it a crucial reference point in physical examinations, helping practitioners orient themselves when examining the hip region and surrounding soft tissues.
Anchoring Point for Muscles and Ligaments
The ASIS serves as the proximal attachment point for several structures that contribute to hip movement and core stability. The Sartorius muscle, the longest muscle in the human body, originates directly from the ASIS. This muscle crosses the thigh diagonally and is responsible for flexing the hip and knee, movements involved in sitting cross-legged. The Tensor Fasciae Latae (TFL) also attaches here, stabilizing the hip and knee during activities like walking and running.
The ASIS also provides the lateral attachment point for the inguinal ligament. This strong, fibrous band stretches from the ASIS to the pubic tubercle on the pubic bone. The inguinal ligament forms the lower border of the anterior abdominal wall and creates a tunnel for structures like the femoral nerve and artery to pass into the thigh. The combined pull of the attached muscles and the inguinal ligament highlights the ASIS as a nexus governing lower body motion and pelvic integrity.
ASIS in Clinical Assessment
Medical professionals frequently use the ASIS as a fixed point for various diagnostic and measurement techniques. Clinicians rely on the ASIS to assess a patient’s leg length, a measurement known as true leg length discrepancy (LLD). This direct measurement involves using a tape measure to determine the distance from the ASIS down to the medial malleolus (the bony prominence on the inside of the ankle). Comparing this distance between the two legs helps identify structural differences in bone length.
The ASIS is also used in conjunction with the Posterior Superior Iliac Spine (PSIS) to evaluate pelvic tilt. Pelvic tilt refers to the angle of the pelvis in the sagittal plane, balancing the forward (anterior) and backward (posterior) rotation. A clinician may use an inclinometer or palpate both the ASIS and PSIS to determine if the front landmark is lower or higher than the back one. This assessment is an important component of postural evaluation, as excessive anterior or posterior pelvic tilt can be associated with various musculoskeletal conditions.
Common Sources of Pain and Injury
Pain localized to the ASIS often results from excessive traction forces exerted by the attached muscles, especially in adolescent athletes whose skeletons are still maturing. One common issue is apophysitis, which is the inflammation and irritation of the growth plate (apophysis) at the ASIS. This condition occurs due to repetitive microtrauma from athletic activity and is more likely in young individuals whose apophysis has not yet fully fused to the main pelvic bone. The growth plate at the ASIS typically begins to harden into bone between the ages of 13 and 15 and is not fully fused until around ages 21 to 25.
A more acute injury is an avulsion fracture, where a sudden, forceful muscle contraction pulls a small fragment of bone away from the ASIS. This is most often seen in sports involving explosive movements, like sprinting or hurdling, due to the rapid, strong pull of the Sartorius muscle. The mechanism usually involves a sudden, eccentric contraction with the hip extended and the knee flexed. Avulsion fractures can also involve the Tensor Fasciae Latae attachment and are generally treated non-surgically unless the bone fragment is displaced by more than two centimeters.