Hip joint Anatomy Overview

The Hip joint Anatomy is a ball / socket joint with a synovial type, it consists of the acetabulum and the head / neck of the proximal femur.

Acetabulum of Hip joint Anatomy

The ilium, ischium, and pubis fuse together within the acetabulum and form a deep-seated depression in the lateral pelvis, which allows for the proximal transmission of weight from the axial skeleton to the lower extremity.

The surface of the acetabulum faces laterally, inferiorly, and anteriorly. The superior and posterior margins of the acetabulum are reinforced with a compact cortical bone, which extends the peripheral brim of the fossa, enhancing the stability of the joint during weight-bearing from both flexed and extended positions.

While the majority of acetabular development is determined by the age of eight,10–12 the depth of the acetabulum increases additionally at puberty, because of the development of three secondary centers of ossification.

Around the periphery of the acetabulum is a thickened collar of fibrocartilage known as the acetabular rim, or acetabular labrum that further deepens the concavity and grasps the head of the femur.

The transverse acetabular ligament is a fibrous tissue link spanning the inferior acetabular notch that connects the anteroinferior and posteroinferior horns of the semilunar surface of the acetabulum. The posterior aspect of the ligament attaches to the bone beneath the lunate surface, and the anterior aspect attaches to the labrum. The transverse acetabular ligament contains no cartilage cells. The function of this ligament in the hip is currently unknown.

The articular surface of the acetabulum is limited to an inverted horseshoe-shaped area covering the anterior, superior, and posterior margins. This relatively small contact area, may contribute to the prevalence of hip degenerative joint disease in humans. The articular surface is covered by a thickened layer of hyaline cartilage, which thins near the center of the joint, and is absent over the acetabular notch, the area occupied by the ligamentum teres and obturator artery.

The diameter of the acetabulum is slightly less than that of the femoral head and results in an incongruous fit of the joint surfaces. This incongruity unloads the joint during partial weight-bearing
(PWB), by allowing the femoral head to sublux laterally out of the cup, while in full weight-bearing, the femoral head is forced into the acetabulum.

In addition, elastic deformation of the acetabulum increases joint congruency of the two-joint surfaces in weight-bearing.

The position of maximum articular congruence corresponds to a quadriped position: 90 degrees flexed, slightly abducted, and externally rotated.

Proximal Femur Anatomy

The femur is the strongest and the longest bone in the body. The Proximal Femur Anatomy consists of a head, a neck, and a greater and lesser trochanter.

Approximately two-thirds of the femoral head is covered with a smooth layer of cartilage except for a depression, the fovea capitis, which serves as the attachment of the ligamentum teres.

The femoral head is composed of a trabecular bone core encased in a thin cortical bone shell. Both the trabecular framework of the head and its pliable hyaline cartilage covering contribute toward the shaping of the femoral head within the acetabulum during full weight-bearing. The trabecular bone in the femoral neck and head is specially designed to withstand high loads because of the incorporation of both primary and secondary compressive and tensile patterns.

However, within this trabecular system, there is a point of weakness called the Ward triangle, which is a common site of osteoporotic fracture.

The neck of the femur is located between the shaft of the femur and its head. On the anterior surface of the femoral neck is the rough intertrochanteric line. The femoral neck serves to extend the weight-bearing forces lateral and inferior to the joint fulcrum. The intertrochanteric crest marks the posterior junction between the neck and the shaft of the femur.

The greater trochanter serves as the insertion site for several muscles that act on the hip joint . The lesser trochanter, located on the posteromedial junction of the neck and the shaft of the femur, is created from the pull of the iliopsoas muscle.

The head of the femur is angled anteriorly, superiorly, and medially. The femoral neck is externally rotated with respect to the shaft. The angle that the femoral neck makes with the acetabulum is called the angle of anteversion/declination

Hip Joint Capsule Anatomy

The joint capsule of the hip, a substantial fibrous sleeve, attaches proximally to the pelvis just lateral to the acetabular labrum and extends laterally over the femoral head and neck to attach to the intertrochanteric line anteriorly.

Posteriorly, the capsule attaches to the lateral one-third of the femoral neck allowing for the attachment of the obturator externus tendon in the posterior intertrochanteric fossa. Both the articular cartilage and the joint capsule are thicker anterosuperiorly, where maximal stress and weight-bearing occurs, and thinnest posteroinferiorly.

The joint capsule has three thickened regions that constitute the capsular ligaments.

These anterior capsular thickenings include the iliofemoral ligament and the pubofemoral ligament:

The iliofemoral ligament

The iliofemoral ligament (Y ligament of Bigelow) consists of two parts: an inferior (medial) portion and a superior (lateral) portion. The iliofemoral ligament is the strongest ligament in the body. The ligament is oriented superior laterally and blends with the iliopsoas muscle. By limiting the range of hip extension, this ligament, with the assistance of the pubofemoral ligament, allows maintenance of the upright posture and reduces the need for contraction of the hip extensors in a balanced stance. Hip adduction tightens the superior portion of the iliofemoral ligament.

The pubofemoral ligament

The pubofemoral ligament blends with the inferior band of the iliofemoral, and with the pectineus muscle. The orientation of the pubofemoral ligament is more inferior-medial. Its fibers tighten in extension and abduction and reinforce the joint capsule along the medial surface.

The ischiofemoral ligament

The posterior thickening of the capsule accounts for the ischiofemoral ligament, which winds posteriorly around the femur and attaches anteriorly, strengthening the capsule. The ischiofemoral ligament ligament, which tightens with internal rotation of the hip, is more commonly injured than the other hip ligaments.

All these capsular thickenings/ligaments are taut in hip extension, especially the inferior portion of the iliofemoral ligament. Conversely, all the ligaments are relaxed in hip flexion. In external rotation of the hip, the superior portion of the iliofemoral ligament and the pubofemoral ligament are both taut. Because of their inherent strength, the hip ligaments are only usually compromised with severe macrotrauma involving a fracture/dislocation of the hip.

The ligamentum teres

The ligamentum teres, or capitis femoris ligament, is a structure seen only in the hip joint. This ligament spans the hip joint running from the acetabular notch to the fovis capitis of the femur, attaching the femoral head to the inferior acetabular rim. The ligament is entirely enclosed by synovial membrane, which forms a sheath around the ligament.

Within this sheath, vessels and nerves pass to the femoral head, providing an important source of arterial blood from a tiny posterior branch of the internal iliac artery. This arterial supply is a significant source of blood to the femoral head in infants and children but becomes less significant in adulthood, because of collateral circulation from the circumflex arteries surrounding the femoral neck, although chronic interruption the blood supply of the femoral head has been associated with avascular necrosis and degenerative arthritis.

The ligamentum teres tightens during adduction, flexion, and external rotation and, although it provides very little to stability, it can contribute to symptoms when injured.

Acetabular Labrum Anatomy

The acetabular labrum is a ring consisting of fibrocartilage and dense connective tissue that encases the femoral head and is attached to the acetabular margin. The acetabular labrum of the hip, to a large extent, is analogous to the meniscus of the knee and the labrum of the glenohumeral joint in that it enhances joint stability, decreases the forces transmitted to the articular cartilage, and provides proprioceptive feedback.

The majority of the acetabular labrum is thought to be avascular. However, cadaver studies have shown blood vessels entering primarily the peripheral part of the labrum, with penetration only into the outer one-third of the substance of the labrum. Nerve-endings and sensory end organs in the superficial layers of the labrum participate in nociceptive and proprioceptive mechanisms.

The osseous acetabulum in the hip provides substantial static stability to the hip joint. Deepening of the socket that is provided by the labrum would, therefore, appear to be less important at the hip. The labrum may enhance stability by providing negative intra-articular pressure in the hip joint.

Another proposed function of the labrum is to improve the mobility of the hip by providing an elastic alternative to the bony rim. The labrum, which varies greatly in form and thickness, has a triangular cross section:

1. an internal articular surface,
2. an external surface contacting the joint capsule,
3. a basal surface attached to the acetabular bone and transverse ligaments.

Most of the labrum is composed of thick, type I collagen fiber bundles principally arranged parallel to the acetabular rim, with some fibers scattered throughout this layer running obliquely to the predominant fiber orientation.

The normal microvasculature of the acetabular labrum consists of a group of small vessels located in the substance of the labrum traveling circumferentially around the labrum at its attachment site on the outer surface of the bony acetabular extension. In addition, the labrum is surrounded by highly vascularized synovium that is present in the capsular recess.

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