Richard N Villar and Nicola Santori

Unlock Your Hip Flexors

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The human hip is a ball-and-socket joint contained within a capsule that runs along the axis of the joint from the rim of acetabulum to the neck of femur. The bony architecture of this joint provides the inherent stability required for erect posture but at the same time impairs access to the joint itself.

The gross anatomy of the hip has been well known since ancient times. Nevertheless, the average orthopedic surgeon is often not confident in this area, especially in the knowledge of the intraarticular structures, because hips operated on in an orthopedic department are mostly degenerating joints. The surgeon may not believe it is important to distinguish anatomic structures that he or she is planning to replace.

Arthroscopy has permitted the identification of new pathologic entities in the hip, allowing a detailed analysis of normal and pathologic anatomy. The chances for successful hip arthroscopy depend primarily on the ability of the surgeon to distinguish what is a normal finding from what is abnormal and causing hip symptoms.

development of the normal hip

To understand the biomechanics and the functions of the intraarticular structures, it is important to review some aspects of the development of the hip. Both femur and pelvis are preformed in cartilage. Of the eight ossification centers of the hemipelvis, three form the acetabulum. These are the iliac, ischial, and pubic ossification centers, meeting at the triradiate cartilage. The iliac center appears prenatally at the ninth week of intrauterine life. The ischial and pubic centers appear at the fourth and fifth months, respectively. At birth, the acetabulum is still a cartilaginous cup. Between the ages of 8 and 9 years, ossification of the ac-etabulum begins. Fusion of the ilium, ischium, and pubis within the acetabulum occurs between the ages of 16 and 18 years.1

In the early fetal period, the acetabulum is a deeply set cavity that almost totally encloses the femoral head. As growth proceeds, the shape progressively changes so that at birth it becomes shallower and covers just one-third of a complete sphere. After birth, this process reverses and the cavity steadily deepens once more. The progressive shallowing of the acetab-ulum allows an increased range of movement at the time of birth.

On the femoral side, the center of ossification for the femoral head appears during the first year of life and is located laterally within the head. Similar to the acetabulum, the head also undergoes a change of shape during development. Its anteroposterior diameter is slightly greater than the transverse up to the age of 3 years. At that point, the head becomes almost a perfect sphere. Then the transverse diameter becomes larger, making the head ovoid. Because of this pattern of development of both sides of the hip, the femoral head is poorly contained at the time of birth.

Soft tissue structures of the hip joint arise from the same undifferentiated mesenchyme and similarly undergo a progressive development. By the 11th week of gestation, the capsule, internally lined with synovial cells, the labrum glenoidale, and the ligamentum teres, is well differentiated. The capsule, initially very thin, becomes progressively pluristratified, gaining thickness and strength. It is generally considered the most important structure contributing to hip stability. Joint, and consequent capsular, laxity may occur in either sex, but is more frequent in female subjects.2-4 The labrum increases the depth of the acetabular cavity and progressively takes on a fibrocartilaginous structure during development.

Horii and coauthors5 have evaluated the degrees of coverage of the femoral head, comparing three age groups: A, 6 to 8 years old; B, 9 to 11 years old; and C, 12 to 13 years old and using 10 healthy adults as controls. They found that coverage of the femoral head by the acetabulum in young children was less than in adults at all positions. However, interestingly, the total coverage including the labrum was greater than in adults.

In the lower part of the joint the labrum is normally continuous with the transverse ligament. The ligamentum teres differentiates from the interzone of the embryonic mesenchyme and runs from the femoral head to the medial border of the acetabular fossa behind the transverse ligament. It provides stability and restraint during the early phases of hip development. Furthermore, it gives mechanical support to the artery of the ligamentum teres or foveolar artery. This vessel is an important nutritional source during certain periods of growth. The anatomy of the circulation to the head is not constant during development, but differs significantly at different ages. During fetal life, three groups of vessels supply the hip joint: the lateral epiphyseal, the anterior metaphyseal, and the foveolar artery. From 4 months of infancy onward, the vessels of the ligamentum teres regress. At this stage they play no part in the nutrition of the ossification center; the main supply until the age of 7 years is the lateral epiphyseal vessels. The contribution from the artery of the ligamentum teres increases again in early adolescence (7 to 11 years). It penetrates the ossification center and enlarges toward the rest of the head. For the first time, connection develops between the terminal branches of the lateral epiphyseal artery and the foveolar artery. These are the only two sources of blood supply of the epiphyseal ossification center. The adolescent pattern of blood supply is maintained until approximately 17 years of age when the epiphysis becomes linked with the metaphysis by interconnecting vessels.


The head forms approximately two-thirds of a sphere and is covered throughout with articular cartilage, except at the fovea. Anteriorly, the articular surface extents to the neck. It faces anterosuperomedially and geometrically resembles part of the surface of an ovoid. Kurrat and Oberlander found maximal thick-

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Essentials of Human Physiology

Essentials of Human Physiology

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