Asphericity of the femoral head-neck junction is one cause of FAI that can eventually lead to osteoarthrosis of the hip. The asphericity of the femoral head-neck junction is often underestimated [8
]. To date, there are no studies that compare the accuracy of the visualization of the asphericity on conventional radiographs to the anatomic representation on MRA with radial slices. The aim of this study was to evaluate whether asphericity of the femoral head-neck junction can be missed or underestimated if standard radiographs are normal and whether MRA with radial slices is superior to regular radiographs to visualize osseous abnormalities.
A limitation of this study is the high dropout rate due to imaging that was not suitable for the study. It can be questioned if those additional 41 hips would have altered the results. Theoretically, the distribution of the four groups could be different, however, we believe the influence on the values of alpha angle and height of asphericity is less likely, given the reasonable number of hips in Groups I, II and IV. The interpretation of the results of Group III is limited by the small number that precludes statistical analysis with the calculation of mean values. Whether those hips really have an additional inferior decrease of the femoral head neck offset has to be examined by a larger group of such hips. Another limitation may be the possible bias caused by the referral system. Often patients referred from outside already have had their MRA and while the quality of these examinations did not meet the standards for this study they were sufficient for diagnosis and treatment. This was the case if the deformity causing FAI was large and visible on the standard radiographs. In these cases MRA was only used to assess the state of labral and acetabular cartilage damage. Because hips with subtle deformities rather had a repeat MRA with good quality this may have led to an overrepresentation of such hips in this study. Finally, the precision to match the axis of the radial slices perfectly to the axis of the femoral neck is within 1 to 2 mm. This will lead to an underestimation of the values of the alpha angle and height of asphericity because the measurements are not taken at the largest diameter but just next to it. If there is an error, then towards an underestimation of the true value.
Not surprisingly, the hips in Group IV with radiographic signs in both the anteroposterior and crosstable views had the highest alpha angle and also the most prominent asphericity. In all four groups, an asphericity of the head-neck junction was present on the anterosuperior aspect of the femoral head-neck junction. In group III high alpha angles were also measured inferiorly.
The results in Group I without radiographic signs of asphericity on standard radiographs are interesting. Even in this group, the alpha angle and the asphericity was increased in the anterosuperior region. All these patients would have been missed if diagnosis would have relied on standard radiographs only. For this group that accounts for 34.6% of the study population, it was important to have an additional examination not to overlook the asphericity and the resulting FAI.
Of the two pathomechanisms of FAI, cam impingement with the aspheric head-neck junction is the more important, because it leads early to extensive acetabular cartilage damage [2
]. To assess the shape of the proximal femur, there is a need for an accurate tool to visualize asphericity. Conventional radiographs are not accurate enough to visualize the circumference of the femoral head-neck area of the proximal femur, especially in subtle deformities [16
]. Different radiographic views and imaging techniques have been described to assess osseous deformities of the femoral head-neck junction, referring to standard views [4
]. However, a limitation of conventional radiographs always is that only the outline of the bones are clearly visualized. With MRA with radial slices this problem can be solved. Alternately, CT scans with 3-D reconstructions will also show the deformity accurately, but measurement of the alpha angle is more difficult to obtain [1
]. In addition, concomitant lesions of the labrum and cartilage cannot be visualized as well [14
]. It is an advantage to assess the complete joint with only one examination to assess the deformities and secondary joint damages.
Of great importance is to understand that an aspheric head-neck junction does not necessarily indicate FAI. Motion of the hip depends on the shape of the head-neck junction and the shape of the acetabulum. In a hip with a dysplastic acetabulum asphericity will not cause FAI because the acetabulum is too small and maloriented to permit an abutment between these two joint components. On the other hand, in a deep or a retroverted acetabulum, overcoverage may be such that even in the presence of a normal head-neck junction FAI occurs. This study shows that normal radiographs do not allow the conclusion that the femoral head-neck junction is spherical. Therefore, in patients with clinical symptoms of FAI additional investigations, best with MRA with radial slices, are necessary to rule out FAI.
It is important that the all radial slice planes include the axis of the femoral neck. Often radiologists offer radial slices through the femoral head along a longitudinal axis not coincident with that of the femoral neck. These radial slices are useless for mapping the outline of the femoral neck and cannot contribute to diagnosis and understanding of the pathomorphology. For diagnosing labral and joint cartilage abnormalities, radial slices are helpful but not necessary [28
In the past, previous MR studies have focused on MRA to investigate soft tissue structures like labral and chondral impairments [9
]. However, only the radial slices can show the complete circumference of the bony structure of the proximal femur [10
]. Using radial slices in 50 patients with MRA, Pfirrman et al. reported the largest alpha angles in cam FAI and pincer FAI appear in the anterior-superior position and they recommend radial slices [19
]. The number of radial slices can be varied to focus on the most interesting parts of the joint [16
MRA with radial slices in the axis of the femoral head neck is necessary to visualize the asphericity of the head-neck junction and to establish the diagnosis of cam FAI in modest deformities, which cannot be visualized with conventional radiographic techniques. Without correct imaging, asphericity of the head-neck junction would be underestimated in a high number of patients, 34.6% in this series.