Conventional Measurement Methods

Radiographic imaging is the established technique for quantifying the slippage and planning the treatment, which includes correctional surgery. Correctional osteotomy aims toward improving alignment of the acetabulum and proximal femur in order to prevent early arthritic degeneration due to pathologic hip function [3,7,14]. Typically the crucial angles are determined on an anterio-posterior and a lateral radiographic view of the hip. Clinically relevant angles are shaft-neck angle, shaft-physis angle, torsion of the femoral neck, and physeal torsion (Figs 2 and 3), described in more detail later. These angles reflect the deformity of the proximal femur and thereby help the surgeon to indicate and plan a correctional osteotomy [7,14]. However, since radiographs are projection

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FIGURE 1 Anterior (upper image) and superior aspect (lower image) of a 3D display of a slipped capital femoral epiphysis on the left femur. In the superior aspect a cross-section of knee condyles is seen. See also Plate 38.

images, angular measurements derived from them may have distortions due to the projection [8]. If the positioning of the patient is accurate and reproducible, this problem can be corrected. Yet, appropriate positioning, especially in cases of SCFE, is difficult because these patients usually experience hip pain (note the asymmetrical positioning in Fig. 3). The extent to which angular measurements can be off under clinical circumstances is indicated in Section 3.

Since the clinical introduction of CT imaging, this method has also been used to gather information on femoral geometry [17]. In general, cross-sections through the hip and through the knee condyles are taken. This makes it possible to determine the torsion of the femoral neck, physeal torsion (Fig. 4) and acetabular anteversion (Fig. 14, right image). This technique does not suffer from projectional distorsion because of its cross-sectional imaging. Nevertheless, there are also sources for errors in torsional measurements, especially if the femoral shaft or the pelvis are not positioned perpendicularly to the imaging plane. In comparison to the errors made on plain radiographs, these errors are relatively small and in most cases clinically tolerable. On the other hand, CT images of the hip region in general provide only axial cross-sections. Without further processing of these cross-sections this methodology does not allow definition of shaft-neck angle, shaft-physis angle, or acetabular inclination. Because SCFE is a disease with a major three-dimensional problem and because correctional osteotomy is a complex three-dimensional procedure, axial cross-sections alone are not sufficient. We will not address the

FIGURE 2 Anterioposterior plain radiograph of both hips with a SCFE on the left hip. Sketched on the image is also the way to determine the shaft-neck (red, right hip) and shaft-physis angle (blue, left hip). See also Plate 39.

comparison between measurements on 3D computer models and axial CT images because the latter do not address the entire three-dimensional problem. A complete solution may be obtained with 3D computer models and related angular measurements.

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