The head of the femur and the acetabulum are anatomically and functionally related in such a way that changes in either component induce a response in the other. This holds true both for the immature and for the mature skeleton.
The acetabulum is cartilaginous at birth, with a Y-shaped physeal plate (the triradiate cartilage) located between the ilium, ischium, and pubis. The fully developed acetabulum is a hemispheric cavity made deeper by a bony rim lining its contour, to which is attached a fibrocartilaginous labrum. The smooth, hemispheric head of the femur articulates with the acetabular cavity, to which it is firmly attached by the ligament of the femoral head. The bony surfaces of the femoral head and acetabulum are covered with a layer of articular cartilage. These articular cartilages, together with the intra-articular space, form the hip joint space as seen on frontal radiographs. The joint space can be analyzed in adult by dividing it into three portions, superior, axial, and medial (Arm-buster et al. 1978). The axial and superior joint space should be approximately half the medial joint space.
Changes in width of individual portions of the joint space are associated with specific hip diseases. For example, superior joint space loss is frequent in degenerative joint disease, while axial joint space loss is observed in disuse cartilage atrophy following paralysis, primary protrusio acetabuli, rheumatoid arthritis, and ankylosing spondylitis (Wroblewski and Charnley 1982).Axial and superior joint space can be increased in acromegaly (Armbuster et al. 1978).
Several measurements of normal osseous structures of the hip have been determined, such as the acetabular and iliac angle, the angle of anteversion of the femoral neck, and the center-edge angle of Wiberg (Gulan et al. 2000; Engelhardt 1988). Some of these measurements are described in the sections below. Similarly, a number of bony landmarks, including the acetabular rim and line, the ilioischial and iliopubic line, and the teardrop configuration, have been identified as important radiographic signs in the diagnosis of specific disorders involving the hip.
In the following pages, two structural defects of the acetabulum - i.e., small acetabular angle and protrusio acetabuli - are described. Several other acetab-ular configurations, including trident-like, shallow, and steeply oblique acetabuli, are not discussed in detail but are mentioned throughout the chapter as appropriate.
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