Ligamentous Structures Contributing To Hip Stabilization

The hip capsule is comprised of a series of ligaments, which can be subdivided into functional and anatomic components. The five primary ligaments discussed in the hip are the iliofemoral (lateral and medial arms), pubofemoral, ischiofemoral, the ligamentum teres femoris, and the ligamentum obicularis. The collagen structure of the hip as demonstrated by electron microscopy is similar to that of the shoulder and the elbow [15].

The iliofemoral ligament (also referred to as the Y-ligament of Bigelow) is the largest of the ligaments and reinforces the capsule anteriorly. Originating at the anterior superior iliac spine (ASIS) and the acetabular rim, it inserts at the intertrochanteric line and the front of the greater trochanter. The ischiofemoral ligament supports the capsule posteriorly, fastening the ischial portion of the acetabular rim to the neck of the femur, medial to the base of the greater trochanter. The pubofemoral ligament reinforces the capsule inferiorly, extending from the superior pubic ramus and acetabular rim to the lower femoral neck. These ligaments are connected to each other by the circular ligamentum obicu-laris, which circumvents the femoral neck. The ligamentum teres femoris originates at the acetabular notch from the transverse acetabular ligament, and inserts in the fovea of the femoral head.

The function of these ligaments has been well described in terms of limiting ranges of motion. There is debate in the literature over which ligament might limit what motion. Most authors agree that the iliofemoral ligament limits extension [16], the pubofemoral ligament limits abduction, and the ischiofemoral ligament limits internal rotation. It is thought that with an elongated or surgically resected iliofemoral ligament, the ligamentum teres has a limiting effect on external rotation. There is debate regarding the ligament limitation in other motions and debate as to what role is played by the functional subdivisions of each ligament (such as the lateral and medial iliofemoral ligament) [17]. The ligamentum orbicularis appears to be overlooked as a major key in stability of the hip joint. Traditionally, the ligamentum orbicularis was thought to be relevant only to extension by tightening the posterior capsule [18]. It now appears to play a vital role in stability, particularly in the area where the lateral arm of the iliofemoral ligament and the orbicularis merge together and continue over the anterosuperior portion of the capsule.

Although studies have described independent motions limited by the ligaments, it is believed that they do not function independently. The ligament complex surrounding the hip acts to stabilize the hip in all ranges of motion. Fuss and Bacher [17] discussed three varieties of interconnections between the ligaments as they form the capsule: parallel fibers either join and become one ligament, join and intermingle though separate ligaments, or join by fusing at the borders (pilema, confluens and conjunction fibrarum, respectively). Fuss and Bacher performed a kinematic study on 10 intact pelves secured to a table mount. The ligaments of the hip were removed except for the iliofemoral ligament (medial and lateral arm). The hip was taken through extension, abduction, adduction and internal/external rotation movements (as guided by a grid) and the motion of the ligament was recorded. In many hips, the iliofemoral ligament appeared to lock when the hip was in pure terminal extension without rotation. The ligament moved to the lateral aspect of the femoral head in abduction or external rotation unlocking the major anterior structure. The pubofemoral ligament contribution to the capsular structures is thought to play a role in controlling this motion.

Certain in vivo studies have illustrated the importance of the ligamentous structures in providing stability to the hip joint [19-22]. While standing, the body's center of gravity lies just posterior to the axis of the hip in the sagittal plane, which causes the pelvis to tilt posteriorly on the femoral head [19]. This tilt is opposed by the tensile forces from the stretching of the anterior capsule, implying that the energy required to stand stationary should be compensated by the ligaments without muscular contribution [19]. Gait involves ranges of motion in all three planes. The force for motion is derived from the musculature of the lower limbs, although stability could not be maintained without the liga-mentous capsule. Abnormal functioning of the iliofemoral ligament has been identified as a cause for coxa sultans [20]. Owing to the relatively large tensile forces of the ligaments of the capsule, dislocation of the hip requires high impact forces, except in children, due to their relatively shallow acetabulum [21,22].

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