Hip joint structure and function

STRUCTURE AND FUNCTION OF THE HIP

The pelvic girdle links the lower extremity to the trunk and plays a significant role in the function of the hip as well as the spinal joints. The bones of the hip joint consist of the proximal femur and the pelvis. The unique characteristics of the pelvis and femur that affect hip function are reviewed in this section.

ANATOMICAL CHARACTERISTICS OF THE HIP REGION

Bony Structures

The structure of the pelvis

hip joint structure

and femur are designed for weight bearing and transmitting forces through the hip joint.


The Pelvis

Each innominate bone of the pelvis is formed by the union of the ilium, ischium and pubis bones and therefore is a structural unit. The right and left innominate bones articulate anteriorly with each other at the pubic symphysis and posteriorly with the sacrum at the sacroiliac joints.69 Slight motion occurs at these three joints to attenuate forces as they are transmitted through the pelvic region, but the pelvis basically functions as a unit in a closed chain.

The Femur

The shape of the femur is designed to bear body weight and to transmit ground reaction forces through the long bone, neck, and head to the acetabulum of the pelvis. In the frontal plane there is an angle of inclination (normally 125_) between the axis of the femoral neck and the shaft of the femur. The angle of torsion formed by the transverse axis of the femoral condyles and the axis of the neck of the femur ranges from 8_ to 25_, with an average angle of 12_. There is also slight bowing of the shaft in the sagittal plane.

ANATOMICAL CHARACTERISTICS OF THE HIP REGION

Bony Structures

The structure of the pelvis and femur are designed for weight bearing and transmitting forces through the hip joint.

The Pelvis

Each innominate bone of the pelvis is formed by the union of the ilium, ischium and pubis bones and therefore is a structural unit. The right and left innominate bones articulate anteriorly with each other at the pubic symphysis and posteriorly with the sacrum at the sacroiliac joints.69 Slight motion occurs at these three joints to attenuate forces as they are transmitted through the pelvic region, but the pelvis basically functions as a unit in a closed chain.

The Femur

The shape of the femur is designed to bear body weight and to transmit ground reaction forces through the long bone, neck, and head to the acetabulum of the pelvis. In the frontal plane there is an angle of inclination (normally 125_) between the axis of the femoral neck and the shaft of the femur. The angle of torsion formed by the transverse axis of the femoral condyles and the axis of the neck of the femur ranges from 8_ to 25_, with an average angle of 12_. There is also slight bowing of the shaft in the sagittal plane.69

Hip Joint Characteristics and Arthrokinematics

Characteristics

The hip is a ball-and-socket (spheroidal) triaxial joint made up of the head of the femur and acetabulum of the pelvis. It is supported by a strong articular capsule that is reinforced by the iliofemoral, pubofemoral, and ischiofemoral ligaments. The two hip joints are linked to each other through the bony pelvis and to the vertebral column through the sacroiliac and lumbosacral joints.69,123

Articular Surfaces

The concave bony partner of the hip joint, the acetabulum, is located in the lateral aspect of the pelvis and faces laterally, anteriorly, and inferiorly (see Fig. 20.1). The acetabulum is deepened by a ring of fibrocartilage, the acetabular labrum. The articular cartilage is horseshoeshaped, being thicker in the lateral region where the major weight-bearing forces are transmitted. The central portion of the acetabular surface is nonarticular. The convex bony partner is the spherical head of the femur, which is attached to the femoral neck. It projects anteriorly, medially, and superiorly. The shapes of the articulating surfaces of the hip joint and the reinforcing properties of the capsule and ligaments, as well as the hip musculature, lend mobility coupled with stability for functional tasks that require wide ranges of combined movements, such as squatting, tying shoes while seated, standing up from a chair or walking.

Ligaments

Of the three ligaments that reinforce the joint capsule, the iliofemoral and pubofemoral ligaments are situated anteriorly, whereas the ischiofemoral ligament is located posteriorly. There is general agreement in the literature that thesethree capsular ligaments limit excessive extension of the hip and that the iliofemoral ligament, also known as the Y ligament of Bigelow, is the strongest of the hip ligaments However, there is some dispute as to the functions of each of these ligaments on an individual

basis. The iliofemoral ligament, which reinforces the anterior portion of the capsule, also is thought to limit external rotation of the hip and the pubofemoral ligament, lending support to the inferior as well as anterior portion of the capsule, is believed to limit abduction. Lastly, the ischiofemoral ligament, although reinforcing the posterior aspect of the capsule, may also limit internal rotation and, when the hip is flexed, adduction.

Arthrokinematics of the Hip Joint

During many activities, such as squatting, walking, or doing leg-press exercises, both the pelvis and femur are moving. Therefore, joint mechanics can be described by the movement of the femur in the acetabulum or as the pelvis moving on the femur.

Motions of the femur

 The convex femoral head slides in the direction opposite the physiological motion of the femur. Thus, with hip flexion and internal rotation the articulating surface slides posteriorly; with extension and external rotation it slides anteriorly; with abduction it slides inferiorly; and with adduction it slides superiorly.

Motions of the pelvis

 When the lower extremity is stabilized (fixated) distally, as when standing or during the stance phase of gait, the concave acetabulum moves on the convex femoral head, so the acetabulum slides in the same direction as the pelvis. The pelvis is a link in a closed chain; therefore, when the pelvis moves, there is motion at both hip joints as well as the lumbar spine.

Influence of the Hip Joint on Balance and Posture Control

The joint capsule is richly supplied with mechanoreceptors that respond to variations in position, stress, and movement for control of posture, balance, and movement. Reflex muscle contractions of the entire kinematic chain, known as balance strategies, occur in a predictable sequence when standing balance is disturbed and regained. Joint pathologies, restricted motion, or muscle weakness can impair balance and postural control.

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