The iliofemoral ligament is a band of connective tissue found at the hip joint that links the ilium, or hip bone, to the femur, the large bone of the thigh. A large, thick, and fibrous structure, it is actually made up of two bands of tissue that run diagonally between the hip and the top of the femur. It is also the strongest ligament in the human body, necessary to maintaining the structural integrity of the hip joint as well as absorbing impact forces and transferring forces from the muscles of the hip to the legs.
With its origin on the hip bone, the iliofemoral ligament arises from two locations: the anterior inferior iliac spine (AIIS) and the acetabulum. The AIIS is a bony ridge on the bottom front part of the lateral protrusion or “wing” of the hip bone, while the acetabulum is the “cup” or socket into which the femur inserts to form the hip joint. The ligament emanates from the rim of the acetabulum. Its fibers run obliquely between the two bones on the anterior side of the joint capsule.
One of three extracapsular ligaments of the hip joint, meaning that it is located outside of the joint capsule, the iliofemoral is accompanied by the ischiofemoral and pubofemoral ligaments. Just as the iliofemoral ligament connects the ilium to the femur, the ischiofemoral ligament connects the ischium bone in the hip to the femur, and the pubofemoral ligament connects the pubis bone in the hip to the femur. The iliofemoral is by far the largest and is situated anterior to the joint, while the ischiofemoral is found on the posterior side of the joint, and the pubofemoral is located on the inferior or underside of the joint.
Both bands of the iliofemoral ligament traverse the front side of the hip joint capsule and attach to the intertrochanteric line, a ridge on the anterior side of the head of the femur. The upper band is the transverse part, which runs perpendicular to the femur bone, and is the stronger of the two. Conversely, the lower band, the descending part, twists around and under the transverse part to run parallel to the femur bone. Together, they resemble an inverted “Y.”
These two bands make up a ligament that is integral to supporting standing posture at the hip joint, particularly when the pelvis is tilted posteriorly, as in tucking the tailbone under. When the hip is tilted as such, the iliofemoral ligament tightens up and holds the ball of the femur firmly against the acetabulum of the hip. Like a column supporting a building, the joint can best withstand downward forces, such as pressing heavy weights overhead, when the pelvis is tilted in this position and the spine, hip, and thigh bones are aligned vertically.
When the pelvis tilts anteriorly and the leg bends forward at the hip, on the other hand, the iliofemoral ligament relaxes and allows for rotation at the joint. This action makes pushing the hips backward to squat or sit down possible. Additionally, the flexibility of this ligament is largely what determines the degree of external rotation permitted at the hip, as when a dancer turns her toes out.