The exact mechanism for idiopathic osteoarthritis (OA) of the hip is not known. There is evidence that motion-induced femoroacetabular impingement might initiate a progressive degenerative process leading to early OA of the hip.8,9 Acetabular and femoral abnormalities leading to contact between the proximal femur and the acetabular rim can contribute to the development of femoroacetabular impingement.3 Reduced joint clearance is a well-appreciated cause of repetitive contact between the prosthetic femoral neck and the edge of the acetabular component in a malpositioned total hip arthroplasty.6 The native hip is under even tighter constraint, and any contact or shearing forces arising from femoroacetabular impingment can lead to labral, and even more harmful, chondral lesions as the hip cannot escape these forces.3 In femoral-induced femoroacetabular impingment (cam type),5 an aspherical femoral head/neck contour caused by widening of the femoral neck or a reduction in the head/neck offset reduces joint clearance. This causes repetitive impingement between the proximal femur and the acetabular rim.6 The shape and orientation of the acetabulum and the degree of excursion of the femoral neck during hip flexion and internal rotation determines whether femoroacetabular contact occurs at the acetabular rim.
The concept that femoral head/neck deviations lead to OA of the hip is not new. Stulberg et al reported abnormal head/neck configurations of the proximal femur on antero-posterior (AP) radiographs (pistol grip deformity) in 40% of patients with idiopathic OA.20 An abnormal anatomic relationship between the femoral head and neck also is a possible cause of OA.14,16,19,22 Subclinical displacement of the femoral epiphysis has been reported as a risk factor for OA, and the terms head-tilt or post-slip have been used to describe the deformity resulting from a mild slipped capital femoral epiphysis (SCFE).4,5,14,20 Some small series8,9 provide evidence to support femoral-induced (cam type) femoroacetabular impingment in SCFE.16 Similar deformities may occur from malunited femoral neck fractures,1 from morphologic deviations including residual childhood diseases such as Legg-Calvé-Perthes disease, and from surgical interventions such as femoral osteotomies leading to reduction of joint clearance.2,18 The majority of patients with femoroacetabular impingement lack a history of detected predisposing factors.3
Anteroposterior radiographs of the pelvis are the most widely used for initial radiographic assessment of unexplained hip pain These radiographs often appear normal during the early stages of OA according to classic radiologic criteria.10 However, closer examination may show morphologic abnormalities. The most common abnormality is a bony prominence at the anterosuperior head/neck junction that frequently is best seen on the lateral projection.1 Because the aspherical portion of the femoral head/neck contour often is located anterosuperiorly, it might be missed when using AP and lateral views.5 To date, there is no conclusive information on the ideal position for detecting femoral head/neck sphericity/asphericity in suspected femoroacetabular impingement.
We questioned which of six standard radiographic projections would be best to evaluate femoral head/neck asphericity in normal and pathologic femurs. We also questioned whether the cross-sectional shape of the femoral neck could explain possible variations in exposure of the bony prominence on the femoral head/neck junction in different radiographic projections.
MATERIALS AND METHODS
Twenty-one intact femurs were included from a collection of 30 desiccated skeletons with no signs of OA (osteophytes and cysts) or sequelae of childhood disease (Legg-Calvé-Perthes disease, proximal femoral focal deficiency, SCFE). Femurs were provided from the skeletal collection of the Institute of Anatomy, University of Berne, Berne, Switzerland. Before making radiographic measurements, the desiccated femurs were categorized into two groups (spherical and aspherical femoral head/neck junctions) by a spherical template that matched the size of femoral heads.7 A spherical femoral head/neck junction was present when there was normal transition of the head into the neck and normal waist of the femoral neck, not leading to displacement of the spherical template when approaching the head/neck junction of the desiccated specimens. An aspherical femoral head/neck junction showed lift-off of the spherical template7 when approaching the femoral head/neck junction, indicating anterosuperior asphericity from a locally decreased waist of the femoral neck. There were 11 pathologic (aspherical) and 10 normal (spherical) femoral head/neck junctions.
Radiographs were taken in six different standard radiographic projections7: (1) plain AP view in neutral rotation of the femur (AP); (2) cross-table lateral view in 15° internal rotation of the femur (Lat IR); (3) cross-table lateral view in neutral (0°) rotation of the femur (Lat NR); (4) cross-table lateral view in 15° external rotation of the femur (Lat ER); (5) plain AP Dunn view in 90° hip flexion, neutral rotation, 20° abduction (Dunn); and (6) Dunn view in 45° hip flexion, neutral rotation, 20° abduction (Dunn/45°). For the lateral cross-table view, the central xray beam entered the pelvis of the patient, who was positioned supine, horizontally at 45° from the contralateral side from distally, with the examined leg extended and rotated. The contralateral leg was held in flexion to avoid interference with the beam.
Rotation of the femur was determined by the angle between a line parallel to the posterior aspect of the femoral condyles and the radiograph table (table-top method).13 Flexion was determined by the angle between the mechanical long axis of the bone and the radiograph table.
The anterior offset angle alpha11 was measured on each radiograph to identify anterosuperior asphericity at the femoral head/neck junction. Alpha represented the angle formed by a line between the center of the femoral head and the center of the femoral neck, and the line between the center of the femoral head and Point A. Point A represented the point at the femoral head/neck contour where the radius of the femoral head diverged from the femoral neck (Fig 1).15 Such loss of femoral head sphericity usually is found on the anterosuperior aspect of the head/neck junction5 (Fig 2).
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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