The cross-sectional shape of the femoral neck was flat and oval (Fig 3).5 The radiographic projection that best showed the smallest offset was obtained when the largest diameter of the flattened femoral neck was parallel to the xray plate. We measured the rotational orientation of the greatest femoral neck ([ρ]) diameter around the long axis of the femoral neck relative to the sagittal plane of the femur (Fig 3) using a caliper. Anteversion of the femoral neck was measured in each femur relative to the posterior aspect of the femoral condyles.5
All radiographic and anatomic measurements were done by two examiners (DM, MB) blinded to the groups. The two examiners made measurements by consensus, not independently. All measurements were repeated (after greater than 6 months) by one of the two examiners (DM) and independently by a third examiner (ML) to determine the intraobserver and interobserver correlations of the measurements. Differences between the two groups in each of the six projections were calculated with unpaired two-tailed t tests. Differences between the six different projections in the same group were calculated using paired two-tailed t tests. Anteversion of the femoral head was measured and correlated with the angle alpha using Pearson's correlation. According to the Bonferroni correction, values of p < 0.01 are considered statistically significant.
For all radiographic projections, the offset angle alpha was larger in the aspherical group compared with the spherical group (Dunn, p < 0.0005; Dunn/45° flexion, p < 0.0005; cross-table/15° IR, p < 0.005; cross-table/NR, p < 0.005; and cross-table/15° ER, p < 0.01). Because of substantial standard deviation (± 16°), we found no difference in the AP projection (Fig 4). The alpha angles for the aspherical and spherical groups were the same.
The Dunn view with 45° hip flexion was the most sensitive projection for detecting a large angle alpha. The alpha angle in the externally rotated cross-table view was smaller (p < 0.05) in all projections except the AP view (Fig 4).
Orientation of the greatest diameter of the femoral neck rho around the long axis of the femoral neck was similar in the spherical group (21° ± 9°) and the aspherical group (25° ± 8°). Antetorsion of the femoral neck did not correlate with the measured offset angle alpha.
The intraobserver and interobserver correlations for all measurements combined were R = 0.95 and R = 0.88, respectively. The best correlation was with the cross-table view in internal rotation (R = 0.97 for intraobserver and interobserver correlations).
A key for prevention of orthopaedic diseases is early appreciation and eventual treatment of predisposing morphologic features. For hip dysplasia, where the insufficient acetabular coverage is a radiographically well-appreciated predisposing factor, ascertaining and correcting this under-coverage can lead to substantial improvement of the course of the disease.11 Predisposing morphologic alterations have not been as well defined for primary (idiopathic) OA. With the new concept of femoroacetabular impingment, acetabular and femoral alterations such as asphericity of the head/neck junction are thought to damage cartilage with subsequent OA.3,5,12,21 To prevent such early changes, timely and reliable diagnosis of routine radiographs is desirable. Our study focused on the radiographic determination of femoral head/neck asphericity as a predisposing factor in femoral-induced femoroacetabular impingement (cam type). Based on the assumption that anterosuperior femoral head asphericity may be hidden in some radiographic projections, we evaluated the optimal radiographic exposure of the femur to identify this disorder.
A limitation of this study is that a small number of desiccated specimens was analyzed. However, differences between the groups were obvious in macroscopic inspection. On conventional radiographs, only the bony structure is evaluated to determine the shape of the head/neck junction, which was preserved in these desiccated specimens. However, in patients, the shape of the bone may be less clearly visible than in the desiccated specimens used in this study; small and hidden irregularities even more likely may be missed. This emphasizes the need for an optimal exposure of the hip on routine radiographs.
There is a continuum from spherical to aspherical femoral head/neck junctions. However, we were interested in identifying differences between radiographic projections rather than the absolute values of the angle alpha of the specimens; the specimens we used seemed adequate for this purpose. When we compared our data for the angle alpha with that of Notzli et al,15 we found similar values (71° versus 50°) as with magnetic resonance imaging (MRI) (74° versus 42°) for abnormal and normal hips. Notzli et al15 found a 12% decrease in the asymptomatic population of 30-year-old volunteers. Although the cartilage layer thickness at the femoral head/neck junction might differ from other parts of the femoral head, this does not seem to alter the data they obtained. We focused on femoral alterations detectable on radiographs, but clinically, the femur must be evaluated in association with the acetabulum (entire hip), as both parts determine the degree of femoroacetabular impingement.
We evaluated six radiographic projections to observe femoral head asphericity. In the same femur, the measured angles of the head/neck offset alpha varied by greater than 30° depending on the radiographic projection. This is partly because of the flat, oval-shaped femoral neck. In cross-section, the oval shape of the femoral neck is rotated anteriorly by a mean rho angle of 23°. This directs the largest diameter of the femoral neck anterosuperiorly, making this the region with the least femoral head/neck offset. In the cross-table view, the anterosuperior ridge of the neck is hidden behind the abnormal part of the femoral neck with external rotation of the femur. With internal rotation, the anterosuperior ridge of the neck appears on the projection (Fig. 1D,E). In the AP view, the critical zone may be hidden behind the normal parts of the femoral neck. The best of the six exposures tested was obtained with the Dunn view where the femur is flexed 45° and the flat geometric shape of the femoral neck is nearly parallel to the xray plate.
Based on the bony structure of the femoral neck, the ideal radiographic position to identify the critical zone most likely would be obtained with the hip flexed 25°, neutrally rotated, and abducted 20°. This position ensures maximum parallel orientation of the flat femoral neck with the xray plate. However, this would require establishing a new standard view. We think this is not indicated as the Dunn view in 45° flexion, which is similar to the Schneider femoral head contour projection,17 provides sufficient information. The superiority of the Dunn view in 45° flexion supports the abnormal femoral head/neck contour being located anterosuperiorly.
We considered it reasonable to use a combination of an AP view of the pelvis and a cross-table view in internal rotation. These projections do not need additional leg holders for the examined extremity. Femoral head/neck asphericity was best detected with the Dunn view in 45° or 90° hip flexion, neutral rotation, and 20° abduction. The cross-table lateral view had a comparable sensitivity, but should be obtained with the leg in approximately 15° internal rotation. These radiographs will minimize false-negative results in patients with suspected femoroacetabular impingement.
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