The Anterior Superior Iliac Spine (ASIS) is a bony landmark located on the front of the hip bone, an important part of the pelvis. This prominent projection marks the anterior extremity of the iliac crest, the uppermost border of the hip bone. The ASIS is a readily palpable structure, serving as a significant anatomical reference point.
Locating the ASIS
To locate the ASIS, feel for a distinct bony prominence at the front and top of the hip, just below the waistline. It is situated on both sides of the pelvis, making it an easily accessible superficial landmark. Its position at the anterior part of the ilium allows it to be felt directly beneath the skin, making it a valuable reference point for medical professionals.
Key Muscle Connections
The ASIS serves as an attachment point for several important muscles, playing a role in lower body movement and stability. Two primary muscles originating here are the Sartorius and the Tensor Fasciae Latae (TFL). The Sartorius, the longest muscle in the human body, extends diagonally across the thigh. This muscle assists in hip flexion, abduction, external rotation, and knee flexion.
The Tensor Fasciae Latae (TFL) also originates at the ASIS. This muscle works with other hip muscles to facilitate hip flexion, abduction, and internal rotation. The TFL contributes to hip and knee stability, particularly during activities like walking and running, through its connection to the iliotibial band.
Important Ligamentous Attachments
The ASIS provides a crucial attachment site for the Inguinal Ligament, a strong band of connective tissue. This ligament originates at the ASIS and extends medially to the pubic tubercle. The Inguinal Ligament forms the lower border of the inguinal canal, an important passageway in the groin. It also helps contain soft tissues, such as nerves and blood vessels, as they pass from the trunk into the lower extremity.
Fascial Connections
Beyond muscles and ligaments, the ASIS is involved with important fascial structures, particularly the fascia lata. The fascia lata is the deep fascia that encases the thigh musculature like a stocking. The ASIS serves as a superior attachment point for this extensive fascial layer.
The iliotibial band (IT band), a thickened lateral portion of the fascia lata, connects to the ASIS through the Tensor Fasciae Latae muscle. The TFL inserts into the iliotibial band, linking the ASIS to this fibrous tract that runs down the side of the thigh to the tibia.
Clinical Relevance of the ASIS
The ASIS is a valuable landmark in various medical contexts. In physical examinations, it is used to assess pelvic alignment and posture. It serves as a starting point for accurately measuring true leg length, a procedure performed from the ASIS to the medial malleolus of the ankle to detect discrepancies. The ASIS also helps pinpoint the location of other anatomical structures, such as the femoral artery and the base of the appendix.
In surgical settings, the ASIS acts as a key anatomical reference for incisions and orientation during orthopedic and general procedures. The ASIS is also relevant in understanding certain injuries, notably avulsion fractures. These fractures occur when a strong, sudden contraction of muscles, like the Sartorius or TFL, pulls a piece of bone away from the ASIS, often seen in young athletes. Recovery typically requires rest and activity modification.