BIO-MECHANICS OF HIP JOINT Prepared by: Dr. Hazrat Bilal, PT DPT (IPMR, KMU), MSPT* (IPM&R, KMU)

Hip joint

Introduction The hip joint, or coxofemoral joint, is the articulation of the acetabulum of the pelvis and the head of the femur These two segments form a ball-and-socket joint with three degrees of freedom: flexion/extension in the sagittal plane, abduction/adduction in the frontal plane, and Medial/lateral rotation in the transverse plane.

The primary function of the hip joint is to support the weight of the head, arms, and trunk (HAT) both in static erect posture and in dynamic postures such as ambulation, running, and stair climbing. The hip joint, like the other joints of the lower extremity is structured primarily to serve its weight-bearing functions.

Structure of Hip Joint Proximal Articular Surfaces The cuplike concave socket of the hip joint is called the acetabulum and is located on the lateral aspect of the pelvic bone (innominate or os coxa). Three bones form the pelvis: the ilium, the ischium, and the pubis. Each of the three bones contributes to the structure of the acetabulum

The pubis forms one fifth of the acetabulum, the ischium forms two fifths, and the ilium forms the remainder. Until full ossification of the pelvis occurs between 20 and 25 years of age, the separate segments of the acetabulum may remain visible on radiograph

The acetabulum appears to be a hemisphere, but only its upper margin has a true circular contour, and the roundness of the acetabulum as a whole decreases with age. In actuality, only a horseshoe-shaped portion of the periphery of the acetabulum (the lunate surface) is covered with hyaline cartilage and articulates with the head of the femur

The inferior aspect of the lunate surface (the base of the horseshoe) is interrupted by a deep notch called the acetabular notch. The acetabular notch is spanned by a fibrous band, the transverse acetabular ligament, that connects the two ends of the horseshoe. The transverse acetabular ligament also spans the acetabular notch to create a fibro-osseous tunnel, called the acetabular fossa, beneath the ligament, through which blood vessels may pass into the central or deepest portion of the acetabulum.

The acetabulum is deepened by the fibrocartilaginous acetabular labrum, which surrounds the periphery. The acetabular fossa is nonarticular; the femoral head does not contact this surface The acetabular fossa contains fibroelastic fat covered with synovial membrane.

Center Edge Angle of the Acetabulum Each acetabulum, in addition to its obvious lateral orientation, is oriented on each innominate bone some-what inferiorly and anteriorly. The magnitude of inferior orientation is assessed on radiograph by using a line connecting the lateral rim of the acetabulum and the center of the femoral head. This line forms an angle with the vertical known as the center edge (CE) angle or the angle of Wiberg and is the amount of inferior tilt of the acetabulum.

Using computed tomography (CT), Adna and associates found CE angles in adults to average 38° in men and 35° in women (with ranges in both sexes to be about 22° to 42°).

Acetabular Anteversion The acetabulum faces not only somewhat inferiorly but also anteriorly. The magnitude of anterior orientation of the acetabulum may be referred to as the angle of acetabular anteversion. Adna and associates found the average value to be 18.5° for men and 21.5° for women, although Kapandji cited larger values of 30° to 40°. Pathologic increases in the angle of acetabular anteversion are associated with decreased joint stability and increased tendency for anterior dislocation of the head of the femur.

Acetabular Labrum The entire periphery of the acetabulum is rimmed by a ring of wedge-shaped fibrocartilage called the acetabular labrum The labrum is attached to the periphery of the acetabulum by a zone of calcified cartilage with a well-defined tide-mark. The acetabular labrum deepens the socket and increases the concavity of the acetabulum and grasps the head of the femur to maintain contact with the acetabulum.

Although the labrum appears to broaden the articular surface of the acetabulum, experimental evidence suggests that load distribution in the acetabulum is not affected by removal of the labrum. Histological examination demonstrated free nerve endings and sensory receptors in the superficial layer of the labrum, as well as vascularization from the adjacent joint capsule only in the superficial third of the labrum.

The transverse acetabular ligament is considered to be part of the acetabular labrum, although, unlike the labrum, it contains no cartilage cells. Although it is positioned to protect the blood vessels traveling beneath it to reach the head of the femur, experimental data do not support the role of the transverse acetabular ligament as a load-bearing structure.

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