With respect to cam-type FAI, although the deformity was long recognized as a tilt or pistol grip deformity [18, 23, 24], there was no real way to quantify the deformity as one would do using the center edge angle for dysplasia. The introduction of the alpha angle by Nötzli et al.  measured on MRI has provided clinicians and researchers a way to quantify the contour deformity of the femoral head-neck junction and permit the development of treatment guidelines [1, 2, 25]. Having said that, because of the three-dimensional nature of the cam deformity, it is unclear how the alpha angle measured on plain radiographs correlates with MRI measurements. This is critical as numerous authors have solely used alpha angle measurements on plain radiographs to determine the natural history of the cam deformity [2, 14] and evaluate its severity . However, the quantification of the cam deformity using the alpha angle on plain radiographs has yet to be validated. Therefore, we determined (1) the sensitivity and specificity of the plain radiograph alpha angle measurements as compared to MRI and (2) the degree of agreement between the alpha angle measurements obtained from the Dunn view and MRA.
We acknowledge limitations of our study. First, data on what alpha angle values represent a normal femoral head-neck junction are still lacking. We used the current standard of 50.5° as the cutoff value for an alpha angle to diagnose cam impingement [5, 25]. However, some patients may not have had true cam impingement and others may have had impingement but not diagnosed as such. Consequently, the diagnosis of cam FAI cannot be based solely on the alpha angle value but rather on the overall clinical presentation. Further research is required to better define what represents a normal femoral head-neck contour. Second, although we know that the measurement and subsequent interpretation of the alpha angle value on MRI is a function of the imaging plane, we cannot definitely state the exact location on the femoral head-neck junction that is represented by the Dunn view. However, because of the strong correlation (0.702) between alpha angle values on the Dunn view and the axial oblique MRI, we can state with some certainty that the Dunn view is highly representative of the MRI in terms of alpha angle measurements.
We found plain radiographic evaluation with measurement of the alpha angle had a relatively high positive predictive value (> 0.8) with both the crosstable and Dunn views using the axial oblique MRI alpha angle value as the gold standard . In contrast, the use of a single AP radiograph has a sensitivity of only 60% compared to 74% and 90% for the crosstable and Dunn views, respectively. The accurate assessment of the cam deformity is especially critical as Nouh et al.  recently showed a simple visual estimation of the presence or absence of a cam deformity was a poor predictor of an abnormal measured alpha angle. This also corroborates the study of Clohisy et al. , which reported a poor inter- and intraobserver reliability in diagnosing cam-type FAI on plain radiographs when using a simple qualification of presence or absence of sphericity. The greater sensitivity and specificity of the Dunn view over the crosstable lateral and AP radiograph projections in quantifying the alpha angle is consistent with the findings of Meyer et al. . More specifically, our study confirms the findings of Dudda et al.  that the appearance of a spherical femoral head on the AP radiograph cannot exclude the presence of a cam deformity, as 19 of 38 (50%) patients with a spherical head had a cam deformity detected by MRI. The limitations of plain radiography in accurately quantifying the maximal alpha angle is consistent with the MRI studies of Pfirrmann et al.  and Rakhra et al. , which showed the epicenter or peak of the cam deformity can vary between the 1:30 and 3:00 positions. These findings are consistent with the six false-positive cases on the Dunn view where the alpha angle was normal on the axial oblique MR image but abnormal on the 1:30 radial image. It follows from this that the radial series images may be required in borderline cases and may be more sensitive than the oblique axial MRI plane view in detecting a cam deformity.
However, the strong correlation between the alpha values measured on MRI and the Dunn view as shown on the Bland-Altman plot suggests radiographs alone may be adequate for evaluation of the femoral head-neck junction in the investigation and management of cam FAI. More importantly, this emphasizes the importance of a proper and detailed clinical evaluation in determining the underlying pathology of patients complaining of hip pain where diagnostic imaging is complementary to the overall patient evaluation but not the sole tool for diagnosing cam FAI . In addition, the cam deformity is not necessarily focal and can be diffuse along a portion of the femoral head-neck junction [6, 21, 22]. Thus, imaging with MRI and/or CT should be the gold standard in the identification of a cam deformity due to their multiplanar capability.
Traditionally, plain radiographs have been, and still are, the standard for evaluating the severity of bony abnormality and deficiency. In specific relation to the hip, plain radiographs are the mainstay in the evaluation of hip dysplasia and arthritis. This can be said also of FAI, and although previous studies have demonstrated the important role of proper lateral radiographs in the evaluation of the femoral head-neck contour for a cam deformity, this study demonstrates and confirms the accuracy of the alpha angle measurement on plain radiographs in quantifying the cam deformity when compared to the gold standard, multiplanar MRI. However, given the secondary derangement a cam deformity can cause to the labrum and cartilage, MRI will remain a powerful tool in the investigation of FAI as it will give further information to the clinician for prognostication and patient counseling.
We thank Steve Doucette, MSc from the Ottawa Hospital Methods Center for his assistance with data analysis.
1. Allen, DJ., Beaule, PE., Ramadan, O. and Doucette, S. Prevalence of associated deformities and hip pain in patients with cam type femoroacetabular impingement. J Bone Joint Surg Br.
2009; 91: 589-594. 10.1302/0301-620X.91B5.22028
2. Bardakos, NV. and Villar, RN. Predictors of progression of osteoarthritis in femoroacetabular impingement. A radiological study with a minimum of ten years follow-up. J Bone Joint Surg Br.
2009; 91: 162-169. 10.1302/0301-620X.91B2.21137
3. Beaule, PE., Allen, DJ., Clohisy, JC., Schoenecker, PE. and Leunig, M. The young adult with hip impingement: deciding on the optimal intervention. J Bone Joint Surg Am.
2009; 91: 210-221. 10.2106/JBJS.H.00802
4. Beaule, PE., Harvey, N., Zaragoza, EJ., LeDuff, M. and Dorey, FJ. The femoral head/neck offset and hip resurfacing. J Bone Joint Surg Br.
2007; 89: 9-15. 10.2106/JBJS.F.00681
5. Beaule, PE., LeDuff, M. and Zaragoza, EJ. Quality of life following femoral head-neck osteochondroplasty for femoroacetabular impingement. J Bone Joint Surg Am.
2007; 89: 773-779. 10.2106/JBJS.F.00681
6. Beaule, PE., Zaragoza, EJ., Motamedic, K., Copelan, N. and Dorey, J. Three-dimensional computed tomography of the hip in the assessment of femoro-acetabular impingement. J Orthop Res.
2005; 23: 1286-1292.
7. Bland, JM. and Altman, DG. Statistical methods for assessing agreement between two methods of of clinical measurement. Lancet.
1986; 1: 307-310.
8. Clohisy, JC., Carlisle, JC., Beaule, PE., Kim, Y.-J., Trousdale, RT., Sierra, RJ., Leunig, M., Schoenecker, PE. and Millis, M. A systematic approach to the plain radiographic evaluation of the young adult hip. J Bone Joint Surg Am.
2008; 90: 47-66. 10.2106/JBJS.H.00756
9. Clohisy, JC., Carlisle, JC., Trousdale, RT., Kim, Y.-J., Beaule, PE., Morgan, P., Steger-May, K., Schoenecker, PE. and Millis, M. Radiographic evaluation of the hip has limited reliability. Clin Orthop Relat Res.
2009; 467: 666-675. 10.1007/s11999-008-0626-4
10. Dudda, M., Albers, C., Mamisch, T., Werlen, S. and Beck, M. Do normal radiographs exclude asphericity of the femoral head-neck junction? Clin Orthop Relat Res.
2009; 467: 651-659. 10.1007/s11999-008-0617-5
11. Eijer, H., Leunig, M., Mahomed, N. and Ganz, R. Cross-table lateral radiographs for screening of anterior femoral head-neck offset in patients with femoro-acetabular impingement. Hip Int.
2001; 11: 37-41.
12. Ganz, R., Parvizi, J., Leunig, M. and Siebenrock, KA. Femoroacetabular Impingement: a cause for osteoarthritis of the hip. Clin Orthop Relat Res.
2003; 417: 112-120.
13. Gosvig, KK., Jacobsen, S., Palm, H., Sonne-Holm, S. and Magnusson, E. A new radiological index for assessing asphericity of the femoral head in cam impingement. J Bone Joint Surg Br.
2007; 89: 1309-1316. 10.1302/0301-620X.89B10.19405
14. Gosvig, KK., Jacobsen, S., Sonne-Holm, S. and Gebuhr, P. The prevalence of cam-type deformity of the hip joint: a survey of 4151 subjects of the Copenhagen Osteoarthritis Study. Acta Radiol.
2008; 49: 436-441. 10.1080/02841850801935567
15. Harris, WH. Etiology of osteoarthritis of the hip. Clin Orthop Relat Res.
1986; 213: 20-33.
16. Ito, K., Minka-II, MA., Leunig, S., Werlen, S. and Ganz, R. Femoroacetabular impingement and the cam-effect. J Bone Joint Surg Br.
2001; 83: 171-176. 10.1302/0301-620X.83B2.11092
17. Meyer, DC., Beck, M., Ellis, T., Ganz, R. and Leunig, M. Comparision of six radiographic projections to assess femoral head/asphericity. Clin Orthop Relat Res.
2006; 445: 181-185.
18. Murray, RO. The aetiology of primary osteoarthritis of the hip. Br J Radiol.
1965; 38: 810-824. 10.1259/0007-1285-38-455-810
19. Notzli, HP., Wyss, TF., Stoecklin, CH., Schmid, MR., Treiber, K. and Hodler, J. The contour fo the femoral head-neck junction as a predictor for the risk of anterior impingement. J Bone Joint Surg Br.
2002; 84: 556-560. 10.1302/0301-620X.84B4.12014
20. Nouh, MR., Schweitzer, ME., Rybak, L. and Cohen, J. Femoroacetabular impingement: can the alpha angle be estimated? AJR Am J Roentgenol.
2008; 190: 1260-1262. 10.2214/AJR.07.3258
21. Pfirrmann, CW., Mengiardi, B., Dora, C., Kalverer, F., Zanetti, M. and Hodler, J. Cam and pincer femoroacetabular impingement: characterstic MR arthrographic findings in 50 patients. Radiology.
2006; 240: 778-785. 10.1148/radiol.2403050767
22. Rakhra, K., Sheikh, AM., Allen, DJ. and Beaule, PE. Comparison of MRI alpha angle measurement planes in femoroacetabular impingement. Clin Orthop Relat Res.
2009; 467: 660-665. 10.1007/s11999-008-0627-3
23. Solomon, L. Patterns of osteoarthritis of the hip. J Bone Joint Surg Br.
1976; 58: 176-183.
24. Stulberg SD, Cordell LD, Harris WH, Ramsey PL, MacEwen GD. Unrecognized childhood hip disease: a major cause of idiopathic osteoarthritis of the hip. In: Amstutz HC, ed. The Hip, Proceedings of the Third Open Scientific Meeting of the Hip Society.
St. Louis, MO: C.V. Mosby; 1975:212-228.
25. Tannast, M., Siebenrock, KA. and Anderson, S. Femoroacetabular impingement: radiographic diagnosis - what the radiologist should know. AJR Am J Roentgenol.
2007; 188: 1540-1552. 10.2214/AJR.06.0921
26. Tanzer, M. and Noiseux, N. Osseous abnormalities and early osteoarthritis: the role of hip impingement. Clin Orthop Relat Res.
2004; 429: 170-177. 10.1097/01.blo.0000150119.49983.ef