- Email: [email protected]
Assessment of Acetabular Morphology Using the Acetabular Anterior Center-Edge Angle on Modified False-Profile Radiographs
Assessment of Acetabular Morphology Using the Acetabular Anterior Center-Edge Angle on Modified False-Profile Radiographs Michael M. Murphy, M.D., Penny R. Atkins, B.S., Evangeline F. Kobayashi, B.S., Andrew E. Anderson, Ph.D., Travis G. Maak, M.D., Anatoliy V. Nechyporenko, M.D., and Stephen K. Aoki, M.D.
Purpose: To compare radiographic parameters of acetabular morphology between standard and modified false-profile (FP) radiographs. Methods: Standard and modified FP radiographs were obtained in 225 hips in 200 consecutive patients evaluated for hip pain and suspected femoroacetabular impingement. Radiographs were retrospectively reviewed by 2 readers to determine the anterior center-edge angle (ACEA), as assessed to the sourcil and to the bone edge. Inter-rater reliability of radiographic measurements was assessed using the intraclass correlation coefficient. Measurements were evaluated for normality with the Shapiro-Wilk test, averaged between the 2 readers, and compared between views using the paired Wilcoxon test. Results: The intraclass correlation coefficient values for standard and modified FP views were 0.923 and 0.932, respectively, measuring to the sourcil and 0.867 and 0.896, respectively, measuring to the lateral bone edge. The median difference in ACEA measurements to the sourcil was 1 between the standard and modified FP view (45 vs 44 , P < .001). The median difference in ACEA measurements to the bone edge was 2 (34 vs 32 , P < .001). Conclusions: Thirty-five degrees of femoral internal rotation for a modified FP hip radiographic view provides similar clinical information regarding acetabular morphology to that of the standard FP view. Given that the modified FP view also provides better visualization of the anterosuperior head-neck junction cam lesion, the modified FP view may be preferred over the standard FP view in evaluation of hip pain in the young patient. Level of Evidence: Level III, retrospective comparative study.
See commentary on page 3067 adiographic evaluation is a critical component in the initial clinical assessment of the young adult with hip pain. In addition to the anteroposterior (AP) radiograph, which provides an assessment of lateral acetabular coverage, the standard false-profile (FP) radiograph has traditionally been used to assess acetabular morphology.1 Specifically, the anterior center-edge angle (ACEA), or angle of Lequesne, is measured on the FP view and provides an assessment of
R
anterior coverage of the femoral head.2,3 Inadequate acetabular coverage may indicate structural hip dysplasia and instability, whereas overcoverage of the femoral head may indicate pincer-type femoroacetabular impingement (FAI). Recently, a modification of the FP view was proposed that offers potential advantages in the clinical evaluation of patients with hip pain. After the observation that the standard FP view may be useful in
From the Department of Orthopaedics (M.M.M., P.R.A., E.F.K., A.E.A., T.G.M., S.K.A.), Department of Bioengineering (P.R.A., A.E.A.), and Department of Physical Therapy (A.E.A.), University of Utah, Salt Lake City, Utah; Scientific Computing and Imaging Institute (A.E.A.), Salt Lake City, Utah; and Department of Orthopedics and Rehabilitation, University of Wisconsin School of Medicine and Public Health (A.V.N.), Madison, Wisconsin, U.S.A. The authors report the following potential conflicts of interest or sources of funding: T.G.M. is a paid speaker for Arthrex. S.K.A. is a paid education speaker for Stryker Medical. Full ICMJE author disclosure forms are available for this article online, as supplementary material.
Abstract presented at the Annual Meeting of the International Society for Hip Arthroscopy, Melbourne, Australia, October 4-6, 2018. Received November 12, 2018; accepted May 23, 2019. Address correspondence to Stephen K. Aoki, M.D., University of Utah Orthopaedics, 590 Wakara Way, Salt Lake City, UT 84108, U.S.A. E-mail: [email protected] Ó 2019 by the Arthroscopy Association of North America 0749-8063/181337/$36.00 https://doi.org/10.1016/j.arthro.2019.05.048
3060
Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 35, No 11 (November), 2019: pp 3060-3066
3061
ACEA ON MODIFIED FALSE-PROFILE RADIOGRAPH
Fig 1. Patient positioning during radiographic acquisition of standard and modified false-profile views. (A) The standard falseprofile radiograph is made with the patient in the standing position with the pelvis rotated 65 in relation to the cassette and with the foot of the affected hip (right foot in this image) parallel to the cassette. (B) The modified false-profile radiograph is made with the patient in the standing position with the pelvis rotated 65 in relation to the cassette and with the foot of the affected hip (right foot in this image) internally rotated 35 in relation to the cassette. Radiographic beam is depicted by the dashed arrows.
characterizing cam deformity in patients with FAI,4 Atkins et al.5 used computed tomography (CT) surface data from control patients and patients with FAI to quantify the degree of internal femoral rotation that is required to visualize the 12- to 3-o’clock position of the femoral head-neck junction as seen on FP radiographs. It was determined that 35 of internal femoral rotation allowed optimal visualization of the 1- to 2o’clock region of the femoral head-neck junction, which is the most common location of the bony prominence typically observed in patients with a diagnosis of cam FAI.6-8 Although the modified view may provide improved visualization of femoral cam pathoanatomy, it is unclear whether acetabular morphology, in particular assessments of femoral head coverage, is different between the standard and modified FP views. Of note, measurements of acetabular morphology were not reported in the initial description of the modified FP radiograph.5 The authors of the study describing the modified FP view noted that the acetabulum and pelvis appeared to be unaltered compared with the standard FP view. However, other studies have suggested that it can be difficult to establish the rotation of the pelvis during the acquisition of the FP film, which may lead to subtle differences in the definition of the bony landmarks used to define measurements of acetabular morphology.9,10 Because the AP pelvis and FP views are commonly used to assess pelvic morphology,1 it is important to examine whether the modified FP view, as described earlier, affects acetabular morphology measurements prior to widespread clinical adoption. The purpose of this study was to compare radiographic parameters of acetabular morphology between
standard and modified FP radiographs. The hypothesis was that measurements of the ACEA to the sourcil and bone edge made from the modified FP radiograph would not be significantly different from those obtained from the standard FP radiograph.
Methods Subjects and Images Approval for the study protocol was obtained from the institutional review board of our institution (No. 71733). Beginning in May 2016 and continuing until February 2017, standard and modified FP radiographs were included in the initial radiographic series in new patients presenting for evaluation of hip pain, yielding 225 hips in 200 consecutive adult patients. Patients were included regardless of age, diagnosis, and treatment as long as standard and modified FP radiographs were obtained within the same radiographic series in all patients undergoing an evaluation of hip pain. No patients were excluded because all patients in the series met the aforementioned inclusion criteria. Standard FP radiographs were acquired as previously described.1,3 In brief, with the patient in a standing position, the affected hip was positioned against the cassette and the pelvis was angled 65 from the cassette with the affected foot positioned parallel to the cassette (Fig 1A). Modified FP radiographs were acquired similarly but with the affected foot angled 35 from the cassette, as described by Atkins et al.5 (Fig 1B). To ensure standardization of radiographic technique, 35 and 65 angle markings relative to the cassette were installed on the floor using tape in each radiographic suite used during the study period and technicians were
3062
M. M. MURPHY ET AL.
Fig 2. Representative radiographic images of standard (A) and modified (B) false-profile views of a right hip. The 2 radiographs provide similar projections of acetabular morphology.
trained in the acquisition technique. The center of the beam was positioned on the presumed location of the femoral head for the affected limb, and digital radiographs were acquired for the standard FP view (Fig 2A) and modified FP view (Fig 2B). Radiographs from consecutive patients were retrospectively reviewed by 2 trained independent blinded readers (E.F.K. [medical student] and M.M.M. [orthopaedic fellow]) to determine the ACEA to the sourcil and to the bone edge, as previously described by Hanson et al.9 To obtain the ACEA, the center of the femoral head was estimated from a circle that
best fit the contour of the femoral head. The longitudinal axis was defined by a vertical line on the radiograph. The ACEA to the sourcil was measured between a line from the center of the femoral head parallel to the longitudinal axis and a line from the center of the femoral head to the most lateral aspect of the sourcil, defined as the sclerotic line along the acetabular roof (Fig 3A). The ACEA to the bone edge was measured between a line from the center of the femoral head parallel to the longitudinal axis and a line from the center of the femoral head to the most lateral aspect of the acetabular bone (Fig 3B).
Fig 3. Anterior center-edge angle (ACEA) to the sourcil (A) and bone edge (B) on a representative modified false-profile radiograph of a right hip. To obtain the ACEA, the center of the femoral head is estimated from a circle that best fits the contour of the femoral head. The longitudinal axis is defined by a vertical line on the radiograph. The ACEA to the sourcil is measured between a line from the center of the femoral head parallel to the longitudinal axis and a line from the center of the femoral head to the most lateral aspect of the sourcil, defined as the sclerotic line along the acetabular roof. The ACEA to the bone edge is measured between a line from the center of the femoral head parallel to the longitudinal axis and a line from the center of the femoral head to the most lateral aspect of the acetabular bone.
3063
ACEA ON MODIFIED FALSE-PROFILE RADIOGRAPH Table 1. Radiographic Measurement of ACEA on Standard and Modified FP Radiographs Measurement Sourcil Bone edge
Radiograph Standard FP Modified FP Standard FP Modified FP
ACEA, 34 [28, 37] 32 [26, 36] 45 [40, 49] 44 [39, 48]
ICC 0.923 0.932 0.867 0.896
Reader Difference, 2 [0, 3] 2 [0, 4] 1 [ 1, 3] 1 [ 1, 3]
View Difference, R1: 1 [0, 3] R2: 1 [ 1, 3] R1: 1 [0, 3] R2: 2 [ 1, 5]
P Value <.001 <.001
NOTE. Data are presented as median [interquartile range] because data were not normally distributed. The reader difference was calculated as the difference between R1 and R2. The view difference was calculated as the difference between the measurement on the standard and modified FP radiographs for R1 and R2. P values represent the statistical significance of this difference; P < .05 was considered significant. ACEA, anterior center-edge angle; FP, false profile; ICC, intraclass correlation coefficient; R1, reader 1; R2, reader 2.
Statistical Analysis Measurements of the ACEA from both views were evaluated for inter-rater reliability using the intraclass correlation coefficient (ICC). ICC values less than 0.5, between 0.5 and 0.75, between 0.76 and 0.9, and greater than 0.9 are indicative of poor, moderate, good, and excellent reliability, respectively. Measurements were evaluated for normality with the Shapiro-Wilk test, averaged between the 2 readers, and compared between views using the paired Wilcoxon test. All statistical analyses were completed in the R program (R Foundation for Statistical Computing, Vienna, Austria; https://www.R-project.org/).
Results The ICC values indicated good to excellent reliability for the ACEA measurements in the 2 radiographic views (Table 1). The ICC values for the ACEA to the sourcil and bone edge on standard FP radiographs were 0.923 and 0.867, respectively. The ICC values for the ACEA to the sourcil and bone edge on modified FP radiographs were 0.932 and 0.896, respectively. The median difference in ACEA measurements between views and between readers was 2 or less for all measurements (Table 1). The median ACEA to the sourcil on the standard and modified FP radiographs was 34 and 32 , respectively (P < .001). The median ACEA to the bone edge on the standard and modified FP radiographs was 45 and 44 , respectively (P < .001). Post hoc power analyses indicated a power of 0.36 for the sourcil-based measurements of the ACEA with an effect size of 0.15 and indicated a power of 0.76 for the bone measurements with an effect size of 0.25. Given the sample size in this study, effects larger than 0.26 would have a power greater than 0.80.
Discussion In this study, the differences in radiographic measurements of acetabular morphology between standard and modified FP radiographs were small. The median value for ACEA measurements to the sourcil on the modified FP view was 2 less than that on the standard FP view. Similarly, the median value for ACEA
measurements to the bone edge on the modified FP view was 1 less than that on the standard FP view. The ICC values showed good to excellent interobserver reliability for all measurements and were comparable to previously reported values for measurements of the lateral center-edge angle (LCEA) and ACEA.10-12 Thus, we can conclude that the modified FP view can be used to assess acetabular morphology with similar results to those obtained using the standard FP view. Although small differences in the ACEA were found between the standard and modified FP views, it is unlikely that these differences are of clinical significance. Normal values for the ACEA are suggested to be between 20 and 40 , with values less than 20 considered dysplastic.1,11 Thus, we believe it is unlikely that a difference of 2 or less in measurements between views, as found in our study, would affect the radiographic diagnosis, except in cases in which the ACEA was borderline (e.g., 18 -22 ). In a previous study, while not specifically measuring the ACEA, Mast et al.13 showed a similar inter-rater reliability ICC value of 0.73 and a standard error of measurement of 3.0 mm when measuring the acetabular LCEA on standard pelvic radiographs. In cases in which the ACEA yields borderline results, further CT volumetric imaging may be helpful to clarify whether anterior coverage is deficient.10,14-16 When radiographic techniques or views are being modified, it is important for clinicians to consider whether similar information can be drawn from existing, and accepted, views. Although the projection of pelvic morphology would not be expected to be altered with isolated internal rotation of the affected leg during study acquisition, it would be careless to assume that such a modification would not affect acetabular positioning and radiographic measurements. Our study suggests that essentially the same information regarding acetabular morphology can be obtained using standard and modified FP radiographs. This finding is important given that the clinical interpretation of these radiographs and the measurements made from these images, such as the ACEA, play a critical role in clinical decision making and selection of an appropriate surgical or nonoperative treatment intervention.4,5
3064
M. M. MURPHY ET AL.
Similarly to the ACEA, the LCEA is used to measure femoral head coverage in the medial-lateral plane. Although the LCEA and ACEA are often obtained when evaluating patients with hip pain, discrepancies exist regarding the radiographic landmarks used to define both measurements.4,17 Specifically, the lateral edge of the acetabular roof, initially described by Wiberg,18 has been interpreted as the lateral border of the sourcil or the lateral edge of the acetabular bone. Specifically, some studies in which the LCEA was measured used the sourcil1,11,15,19,20 whereas others used the lateral bone edge.14,21-24 In further studies, the landmarks used to define the LCEA were not clearly specified.25-28 Similarly, in studies of the ACEA, both sourcil1,11,29,30 and lateral bone edge14,31 landmarks have been used. By their very nature, lateral bone edge measurements have larger values than measurements to the sourcil, which can have clinically significant implications, such as classifying a hip as being within the normal range when it is actually deficient in coverage.5 Thus, we encourage investigators to clearly define which landmarks they are using when reporting the ACEA and LCEA. In our study, we found similar values between the standard and modified FP views when using the sourcil, as well as the lateral bone edge, providing further evidence that the modified FP view is appropriate independently of what landmark is chosen. The modified FP view has several advantages compared with the standard FP view. As previously shown, the modified FP view provides improved visualization of the anterosuperior femoral head-neck junction, the area often pathologically involved in FAI.5 On the basis of the results of our study, we believe the modified FP radiograph is a valuable view for evaluating young adults with hip pain because it affords an assessment of both femoral and pelvic morphology on a single radiograph and may obviate independent evaluation of singular radiographic parameters via separate radiographic views, thus limiting patients’ radiation exposure. Other radiographic projections of the hip, such as the 90 Dunn or crosstable lateral view, provide visualization of the femoral head-neck junction,6,8,32 but information regarding pelvic and acetabular morphology has not yet been established in these views. The AP pelvic radiograph provides an assessment of lateral acetabular coverage and pelvic version and visualizes the 12o’clock radial location of the superior femoral headneck junction.32 This provides a further rationale to acquire a modified FP view as part of the diagnostic workup. This finding is particularly important considering the cost and radiation exposure associated with magnetic resonance imaging and CT imaging, which are often required when plain radiographs do not provide sufficient information to direct clinical decision making.6,33,34
Limitations Our study used rotation of the foot relative to the cassette during image acquisition. The use of foot rotation to alter femoral rotation assumes that little or no rotation through the knee or ankle occurs during acquisition. It is unknown whether there is a direct correlation between internal foot rotation and internal femoral rotation. Accordingly, the technique does not accommodate for patients with excessive femoral and/ or tibial torsion. In addition, although our protocol for the modified FP view called for patients to internally rotate the affected foot during image acquisition, we did not directly observe every patient during acquisition of the film. Thus, there may be some variability in the degree of pelvic and foot rotation across patients. Still, this variability would likely be encountered when acquiring the standard FP view. Therefore, we believe our results are generalizable. It is important to note that the modified FP view is not difficult to acquire, and thus, other practitioners who use radiographic films to assess hip morphology should be able to integrate this modified FP protocol into their clinic without major difficulty. Given our study’s sample size, post hoc power analyses indicated a power of 0.76 for the bone measurements with an effect size of 0.25. Given the sample size in this study, effects larger than 0.26 would have a power greater than 0.80. Thus, with the current sample size, it is possible that the statistically significant differences identified in ACEA measurements between views came from a false rejection of the null hypothesis, that is, a difference was found when one did not exist. However, differences in ACEA measurements between views were very small, and thus, it is unlikely that eliminating errors in measurement or errors due to inconsistencies in patient positioning would change the conclusions of our study.
Conclusions Thirty-five degrees of femoral internal rotation for a modified FP hip radiographic view provides similar clinical information regarding acetabular morphology to that of the standard FP view. Given that the modified FP view also provides better visualization of the anterosuperior head-neck junction cam lesion, the modified FP view may be preferred over the standard FP view in evaluation of hip pain in the young patient.
References 1. Clohisy JC, Carlisle JC, Beaule PE, et al. A systematic approach to the plain radiographic evaluation of the young adult hip. J Bone Joint Surg Am 2008;90:47-66 (suppl 4). 2. Ganz R, Parvizi J, Beck M, Leunig M, Notzli H, Siebenrock KA. Femoroacetabular impingement: A cause
ACEA ON MODIFIED FALSE-PROFILE RADIOGRAPH
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
for osteoarthritis of the hip. Clin Orthop Relat Res 2003;417: 112-120. Lequesne M, de Sèze S. False profile of the pelvis. A new radiographic incidence for the study of the hip. Its use in dysplasias and different coxopathies. Rev Rhum Mal Osteoartic 1961;28:643-652 [in French]. Hellman MD, Mascarenhas R, Gupta A, et al. The falseprofile view may be used to identify cam morphology. Arthroscopy 2015;31:1728-1732. Atkins PR, Kobayashi EF, Anderson AE, Aoki SK. Modified false-profile radiograph of the hip provides better visualization of the anterosuperior femoral head-neck junction. Arthroscopy 2018;34:1236-1243. Domayer SE, Ziebarth K, Chan J, Bixby S, Mamisch TC, Kim YJ. Femoroacetabular cam-type impingement: Diagnostic sensitivity and specificity of radiographic views compared to radial MRI. Eur J Radiol 2011;80:805-810. Rakhra KS, Sheikh AM, Allen D, Beaule PE. Comparison of MRI alpha angle measurement planes in femoroacetabular impingement. Clin Orthop Relat Res 2009;467: 660-665. Siebenrock KA, Wahab KH, Werlen S, Kalhor M, Leunig M, Ganz R. Abnormal extension of the femoral head epiphysis as a cause of cam impingement. Clin Orthop Relat Res 2004;418:54-60. Hanson JA, Kapron AL, Swenson KM, Maak TG, Peters CL, Aoki SK. Discrepancies in measuring acetabular coverage: Revisiting the anterior and lateral center edge angles. J Hip Preserv Surg 2015;2:280-286. Mittal A, Bomar JD, Jeffords ME, Huang MT, Wenger DR, Upasani VV. Defining the lateral edge of the femoroacetabular articulation: Correlation analysis between radiographs and computed tomography. J Child Orthop 2016;10:365-370. Anderson LA, Gililland J, Pelt C, Linford S, Stoddard GJ, Peters CL. Center edge angle measurement for hip preservation surgery: Technique and caveats. Orthopedics 2011;34:86. Bouttier R, Morvan J, Mazieres B, et al. Reproducibility of radiographic hip measurements in adults. Joint Bone Spine 2013;80:52-56. Mast NH, Impellizzeri F, Keller S, Leunig M. Reliability and agreement of measures used in radiographic evaluation of the adult hip. Clin Orthop Relat Res 2011;469: 188-199. Dolan MM, Heyworth BE, Bedi A, Duke G, Kelly BT. CT reveals a high incidence of osseous abnormalities in hips with labral tears. Clin Orthop Relat Res 2011;469: 831-838. Hansen BJ, Harris MD, Anderson LA, Peters CL, Weiss JA, Anderson AE. Correlation between radiographic measures of acetabular morphology with 3D femoral head coverage in patients with acetabular retroversion. Acta Orthop 2012;83:233-239. Sakai T, Nishii T, Sugamoto K, Yoshikawa H, Sugano N. Is vertical-center-anterior angle equivalent to anterior coverage of the hip? Clin Orthop Relat Res 2009;467: 2865-2871.
3065
17. Ogata S, Moriya H, Tsuchiya K, Akita T, Kamegaya M, Someya M. Acetabular cover in congenital dislocation of the hip. J Bone Joint Surg Br 1990;72:190-196. 18. Wiberg G. Studies on dysplastic acetabulum and congenital subluxation of the hip joint with special reference to the complications of osteoarthritis. Acta Chir Scand 1939;83(suppl 58). 19. Jessel RH, Zurakowski D, Zilkens C, Burstein D, Gray ML, Kim YJ. Radiographic and patient factors associated with pre-radiographic osteoarthritis in hip dysplasia. J Bone Joint Surg Am 2009;91:1120-1129. 20. Sankar WN, Brighton BK, Kim YJ, Millis MB. Acetabular morphology in slipped capital femoral epiphysis. J Pediatr Orthop 2011;31:254-258. 21. Al-Ghamdi A, Rendon JS, Al-Faya F, Saran N, Benaroch T, Hamdy RC. Dega osteotomy for the correction of acetabular dysplasia of the hip: A radiographic review of 21 cases. J Pediatr Orthop 2012;32:113-120. 22. Hosalkar H, Munhoz da Cunha AL, Baldwin K, Ziebarth K, Wenger DR. Triple innominate osteotomy for Legg-Calve-Perthes disease in children: Does the lateral coverage change with time? Clin Orthop Relat Res 2012;470:2402-2410. 23. Philippon MJ, Wolff AB, Briggs KK, Zehms CT, Kuppersmith DA. Acetabular rim reduction for the treatment of femoroacetabular impingement correlates with preoperative and postoperative center-edge angle. Arthroscopy 2010;26:757-761. 24. Werner CM, Copeland CE, Ruckstuhl T, Stromberg J, Turen CH, Bouaicha S. Relationship between Wiberg’s lateral center edge angle, Lequesne’s acetabular index, and medial acetabular bone stock. Skeletal Radiol 2011;40: 1435-1439. 25. Kappe T, Kocak T, Bieger R, Reichel H, Fraitzl CR. Radiographic risk factors for labral lesions in femoroacetabular impingement. Clin Orthop Relat Res 2011;469: 3241-3247. 26. Nepple JJ, Brophy RH, Matava MJ, Wright RW, Clohisy JC. Radiographic findings of femoroacetabular impingement in National Football League Combine athletes undergoing radiographs for previous hip or groin pain. Arthroscopy 2012;28:1396-1403. 27. Polkowski GG, Novais EN, Kim YJ, Millis MB, Schoenecker PL, Clohisy JC. Does previous reconstructive surgery influence functional improvement and deformity correction after periacetabular osteotomy? Clin Orthop Relat Res 2012;470:516-524. 28. Ross JR, Zaltz I, Nepple JJ, Schoenecker PL, Clohisy JC. Arthroscopic disease classification and interventions as an adjunct in the treatment of acetabular dysplasia. Am J Sports Med 2011;39:72S-78S (suppl). 29. Chosa E, Tajima N. Anterior acetabular head index of the hip on false-profile views. New index of anterior acetabular cover. J Bone Joint Surg Br 2003;85:826-829. 30. Zingg PO, Werner CM, Sukthankar A, Zanetti M, Seifert B, Dora C. The anterior center edge angle in Lequesne’s false profile view: Interrater correlation, dependence on pelvic tilt and correlation to anterior
3066
M. M. MURPHY ET AL.
acetabular coverage in the sagital plane. A cadaver study. Arch Orthop Trauma Surg 2009;129:787-791. 31. Crockarell JR Jr, Trousdale RT, Guyton JL. The anterior centre-edge angle. A cadaver study. J Bone Joint Surg Br 2000;82:532-534. 32. Nepple JJ, Martel JM, Kim YJ, Zaltz I, Clohisy JC, Group AS. Do plain radiographs correlate with CT for imaging of cam-type femoroacetabular impingement? Clin Orthop Relat Res 2012;470:3313-3320.
33. Dudda M, Albers C, Mamisch TC, Werlen S, Beck M. Do normal radiographs exclude asphericity of the femoral head-neck junction? Clin Orthop Relat Res 2009;467: 651-659. 34. Saito M, Tsukada S, Yoshida K, Okada Y, Tasaki A. Correlation of alpha angle between various radiographic projections and radial magnetic resonance imaging for cam deformity in femoral head-neck junction. Knee Surg Sports Traumatol Arthrosc 2017;25:77-83.
Purpose: To compare radiographic parameters of acetabular morphology between standard and modified false-profile (FP) radiographs. Methods: Standard and modified FP radiographs were obtained in 225 hips in 200 consecutive patients evaluated for hip pain and suspected femoroacetabular impingement. Radiographs were retrospectively reviewed by 2 readers to determine the anterior center-edge angle (ACEA), as assessed to the sourcil and to the bone edge. Inter-rater reliability of radiographic measurements was assessed using the intraclass correlation coefficient. Measurements were evaluated for normality with the Shapiro-Wilk test, averaged between the 2 readers, and compared between views using the paired Wilcoxon test. Results: The intraclass correlation coefficient values for standard and modified FP views were 0.923 and 0.932, respectively, measuring to the sourcil and 0.867 and 0.896, respectively, measuring to the lateral bone edge. The median difference in ACEA measurements to the sourcil was 1 between the standard and modified FP view (45 vs 44 , P < .001). The median difference in ACEA measurements to the bone edge was 2 (34 vs 32 , P < .001). Conclusions: Thirty-five degrees of femoral internal rotation for a modified FP hip radiographic view provides similar clinical information regarding acetabular morphology to that of the standard FP view. Given that the modified FP view also provides better visualization of the anterosuperior head-neck junction cam lesion, the modified FP view may be preferred over the standard FP view in evaluation of hip pain in the young patient. Level of Evidence: Level III, retrospective comparative study.
See commentary on page 3067 adiographic evaluation is a critical component in the initial clinical assessment of the young adult with hip pain. In addition to the anteroposterior (AP) radiograph, which provides an assessment of lateral acetabular coverage, the standard false-profile (FP) radiograph has traditionally been used to assess acetabular morphology.1 Specifically, the anterior center-edge angle (ACEA), or angle of Lequesne, is measured on the FP view and provides an assessment of
R
anterior coverage of the femoral head.2,3 Inadequate acetabular coverage may indicate structural hip dysplasia and instability, whereas overcoverage of the femoral head may indicate pincer-type femoroacetabular impingement (FAI). Recently, a modification of the FP view was proposed that offers potential advantages in the clinical evaluation of patients with hip pain. After the observation that the standard FP view may be useful in
From the Department of Orthopaedics (M.M.M., P.R.A., E.F.K., A.E.A., T.G.M., S.K.A.), Department of Bioengineering (P.R.A., A.E.A.), and Department of Physical Therapy (A.E.A.), University of Utah, Salt Lake City, Utah; Scientific Computing and Imaging Institute (A.E.A.), Salt Lake City, Utah; and Department of Orthopedics and Rehabilitation, University of Wisconsin School of Medicine and Public Health (A.V.N.), Madison, Wisconsin, U.S.A. The authors report the following potential conflicts of interest or sources of funding: T.G.M. is a paid speaker for Arthrex. S.K.A. is a paid education speaker for Stryker Medical. Full ICMJE author disclosure forms are available for this article online, as supplementary material.
Abstract presented at the Annual Meeting of the International Society for Hip Arthroscopy, Melbourne, Australia, October 4-6, 2018. Received November 12, 2018; accepted May 23, 2019. Address correspondence to Stephen K. Aoki, M.D., University of Utah Orthopaedics, 590 Wakara Way, Salt Lake City, UT 84108, U.S.A. E-mail: [email protected] Ó 2019 by the Arthroscopy Association of North America 0749-8063/181337/$36.00 https://doi.org/10.1016/j.arthro.2019.05.048
3060
Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 35, No 11 (November), 2019: pp 3060-3066
3061
ACEA ON MODIFIED FALSE-PROFILE RADIOGRAPH
Fig 1. Patient positioning during radiographic acquisition of standard and modified false-profile views. (A) The standard falseprofile radiograph is made with the patient in the standing position with the pelvis rotated 65 in relation to the cassette and with the foot of the affected hip (right foot in this image) parallel to the cassette. (B) The modified false-profile radiograph is made with the patient in the standing position with the pelvis rotated 65 in relation to the cassette and with the foot of the affected hip (right foot in this image) internally rotated 35 in relation to the cassette. Radiographic beam is depicted by the dashed arrows.
characterizing cam deformity in patients with FAI,4 Atkins et al.5 used computed tomography (CT) surface data from control patients and patients with FAI to quantify the degree of internal femoral rotation that is required to visualize the 12- to 3-o’clock position of the femoral head-neck junction as seen on FP radiographs. It was determined that 35 of internal femoral rotation allowed optimal visualization of the 1- to 2o’clock region of the femoral head-neck junction, which is the most common location of the bony prominence typically observed in patients with a diagnosis of cam FAI.6-8 Although the modified view may provide improved visualization of femoral cam pathoanatomy, it is unclear whether acetabular morphology, in particular assessments of femoral head coverage, is different between the standard and modified FP views. Of note, measurements of acetabular morphology were not reported in the initial description of the modified FP radiograph.5 The authors of the study describing the modified FP view noted that the acetabulum and pelvis appeared to be unaltered compared with the standard FP view. However, other studies have suggested that it can be difficult to establish the rotation of the pelvis during the acquisition of the FP film, which may lead to subtle differences in the definition of the bony landmarks used to define measurements of acetabular morphology.9,10 Because the AP pelvis and FP views are commonly used to assess pelvic morphology,1 it is important to examine whether the modified FP view, as described earlier, affects acetabular morphology measurements prior to widespread clinical adoption. The purpose of this study was to compare radiographic parameters of acetabular morphology between
standard and modified FP radiographs. The hypothesis was that measurements of the ACEA to the sourcil and bone edge made from the modified FP radiograph would not be significantly different from those obtained from the standard FP radiograph.
Methods Subjects and Images Approval for the study protocol was obtained from the institutional review board of our institution (No. 71733). Beginning in May 2016 and continuing until February 2017, standard and modified FP radiographs were included in the initial radiographic series in new patients presenting for evaluation of hip pain, yielding 225 hips in 200 consecutive adult patients. Patients were included regardless of age, diagnosis, and treatment as long as standard and modified FP radiographs were obtained within the same radiographic series in all patients undergoing an evaluation of hip pain. No patients were excluded because all patients in the series met the aforementioned inclusion criteria. Standard FP radiographs were acquired as previously described.1,3 In brief, with the patient in a standing position, the affected hip was positioned against the cassette and the pelvis was angled 65 from the cassette with the affected foot positioned parallel to the cassette (Fig 1A). Modified FP radiographs were acquired similarly but with the affected foot angled 35 from the cassette, as described by Atkins et al.5 (Fig 1B). To ensure standardization of radiographic technique, 35 and 65 angle markings relative to the cassette were installed on the floor using tape in each radiographic suite used during the study period and technicians were
3062
M. M. MURPHY ET AL.
Fig 2. Representative radiographic images of standard (A) and modified (B) false-profile views of a right hip. The 2 radiographs provide similar projections of acetabular morphology.
trained in the acquisition technique. The center of the beam was positioned on the presumed location of the femoral head for the affected limb, and digital radiographs were acquired for the standard FP view (Fig 2A) and modified FP view (Fig 2B). Radiographs from consecutive patients were retrospectively reviewed by 2 trained independent blinded readers (E.F.K. [medical student] and M.M.M. [orthopaedic fellow]) to determine the ACEA to the sourcil and to the bone edge, as previously described by Hanson et al.9 To obtain the ACEA, the center of the femoral head was estimated from a circle that
best fit the contour of the femoral head. The longitudinal axis was defined by a vertical line on the radiograph. The ACEA to the sourcil was measured between a line from the center of the femoral head parallel to the longitudinal axis and a line from the center of the femoral head to the most lateral aspect of the sourcil, defined as the sclerotic line along the acetabular roof (Fig 3A). The ACEA to the bone edge was measured between a line from the center of the femoral head parallel to the longitudinal axis and a line from the center of the femoral head to the most lateral aspect of the acetabular bone (Fig 3B).
Fig 3. Anterior center-edge angle (ACEA) to the sourcil (A) and bone edge (B) on a representative modified false-profile radiograph of a right hip. To obtain the ACEA, the center of the femoral head is estimated from a circle that best fits the contour of the femoral head. The longitudinal axis is defined by a vertical line on the radiograph. The ACEA to the sourcil is measured between a line from the center of the femoral head parallel to the longitudinal axis and a line from the center of the femoral head to the most lateral aspect of the sourcil, defined as the sclerotic line along the acetabular roof. The ACEA to the bone edge is measured between a line from the center of the femoral head parallel to the longitudinal axis and a line from the center of the femoral head to the most lateral aspect of the acetabular bone.
3063
ACEA ON MODIFIED FALSE-PROFILE RADIOGRAPH Table 1. Radiographic Measurement of ACEA on Standard and Modified FP Radiographs Measurement Sourcil Bone edge
Radiograph Standard FP Modified FP Standard FP Modified FP
ACEA, 34 [28, 37] 32 [26, 36] 45 [40, 49] 44 [39, 48]
ICC 0.923 0.932 0.867 0.896
Reader Difference, 2 [0, 3] 2 [0, 4] 1 [ 1, 3] 1 [ 1, 3]
View Difference, R1: 1 [0, 3] R2: 1 [ 1, 3] R1: 1 [0, 3] R2: 2 [ 1, 5]
P Value <.001 <.001
NOTE. Data are presented as median [interquartile range] because data were not normally distributed. The reader difference was calculated as the difference between R1 and R2. The view difference was calculated as the difference between the measurement on the standard and modified FP radiographs for R1 and R2. P values represent the statistical significance of this difference; P < .05 was considered significant. ACEA, anterior center-edge angle; FP, false profile; ICC, intraclass correlation coefficient; R1, reader 1; R2, reader 2.
Statistical Analysis Measurements of the ACEA from both views were evaluated for inter-rater reliability using the intraclass correlation coefficient (ICC). ICC values less than 0.5, between 0.5 and 0.75, between 0.76 and 0.9, and greater than 0.9 are indicative of poor, moderate, good, and excellent reliability, respectively. Measurements were evaluated for normality with the Shapiro-Wilk test, averaged between the 2 readers, and compared between views using the paired Wilcoxon test. All statistical analyses were completed in the R program (R Foundation for Statistical Computing, Vienna, Austria; https://www.R-project.org/).
Results The ICC values indicated good to excellent reliability for the ACEA measurements in the 2 radiographic views (Table 1). The ICC values for the ACEA to the sourcil and bone edge on standard FP radiographs were 0.923 and 0.867, respectively. The ICC values for the ACEA to the sourcil and bone edge on modified FP radiographs were 0.932 and 0.896, respectively. The median difference in ACEA measurements between views and between readers was 2 or less for all measurements (Table 1). The median ACEA to the sourcil on the standard and modified FP radiographs was 34 and 32 , respectively (P < .001). The median ACEA to the bone edge on the standard and modified FP radiographs was 45 and 44 , respectively (P < .001). Post hoc power analyses indicated a power of 0.36 for the sourcil-based measurements of the ACEA with an effect size of 0.15 and indicated a power of 0.76 for the bone measurements with an effect size of 0.25. Given the sample size in this study, effects larger than 0.26 would have a power greater than 0.80.
Discussion In this study, the differences in radiographic measurements of acetabular morphology between standard and modified FP radiographs were small. The median value for ACEA measurements to the sourcil on the modified FP view was 2 less than that on the standard FP view. Similarly, the median value for ACEA
measurements to the bone edge on the modified FP view was 1 less than that on the standard FP view. The ICC values showed good to excellent interobserver reliability for all measurements and were comparable to previously reported values for measurements of the lateral center-edge angle (LCEA) and ACEA.10-12 Thus, we can conclude that the modified FP view can be used to assess acetabular morphology with similar results to those obtained using the standard FP view. Although small differences in the ACEA were found between the standard and modified FP views, it is unlikely that these differences are of clinical significance. Normal values for the ACEA are suggested to be between 20 and 40 , with values less than 20 considered dysplastic.1,11 Thus, we believe it is unlikely that a difference of 2 or less in measurements between views, as found in our study, would affect the radiographic diagnosis, except in cases in which the ACEA was borderline (e.g., 18 -22 ). In a previous study, while not specifically measuring the ACEA, Mast et al.13 showed a similar inter-rater reliability ICC value of 0.73 and a standard error of measurement of 3.0 mm when measuring the acetabular LCEA on standard pelvic radiographs. In cases in which the ACEA yields borderline results, further CT volumetric imaging may be helpful to clarify whether anterior coverage is deficient.10,14-16 When radiographic techniques or views are being modified, it is important for clinicians to consider whether similar information can be drawn from existing, and accepted, views. Although the projection of pelvic morphology would not be expected to be altered with isolated internal rotation of the affected leg during study acquisition, it would be careless to assume that such a modification would not affect acetabular positioning and radiographic measurements. Our study suggests that essentially the same information regarding acetabular morphology can be obtained using standard and modified FP radiographs. This finding is important given that the clinical interpretation of these radiographs and the measurements made from these images, such as the ACEA, play a critical role in clinical decision making and selection of an appropriate surgical or nonoperative treatment intervention.4,5
3064
M. M. MURPHY ET AL.
Similarly to the ACEA, the LCEA is used to measure femoral head coverage in the medial-lateral plane. Although the LCEA and ACEA are often obtained when evaluating patients with hip pain, discrepancies exist regarding the radiographic landmarks used to define both measurements.4,17 Specifically, the lateral edge of the acetabular roof, initially described by Wiberg,18 has been interpreted as the lateral border of the sourcil or the lateral edge of the acetabular bone. Specifically, some studies in which the LCEA was measured used the sourcil1,11,15,19,20 whereas others used the lateral bone edge.14,21-24 In further studies, the landmarks used to define the LCEA were not clearly specified.25-28 Similarly, in studies of the ACEA, both sourcil1,11,29,30 and lateral bone edge14,31 landmarks have been used. By their very nature, lateral bone edge measurements have larger values than measurements to the sourcil, which can have clinically significant implications, such as classifying a hip as being within the normal range when it is actually deficient in coverage.5 Thus, we encourage investigators to clearly define which landmarks they are using when reporting the ACEA and LCEA. In our study, we found similar values between the standard and modified FP views when using the sourcil, as well as the lateral bone edge, providing further evidence that the modified FP view is appropriate independently of what landmark is chosen. The modified FP view has several advantages compared with the standard FP view. As previously shown, the modified FP view provides improved visualization of the anterosuperior femoral head-neck junction, the area often pathologically involved in FAI.5 On the basis of the results of our study, we believe the modified FP radiograph is a valuable view for evaluating young adults with hip pain because it affords an assessment of both femoral and pelvic morphology on a single radiograph and may obviate independent evaluation of singular radiographic parameters via separate radiographic views, thus limiting patients’ radiation exposure. Other radiographic projections of the hip, such as the 90 Dunn or crosstable lateral view, provide visualization of the femoral head-neck junction,6,8,32 but information regarding pelvic and acetabular morphology has not yet been established in these views. The AP pelvic radiograph provides an assessment of lateral acetabular coverage and pelvic version and visualizes the 12o’clock radial location of the superior femoral headneck junction.32 This provides a further rationale to acquire a modified FP view as part of the diagnostic workup. This finding is particularly important considering the cost and radiation exposure associated with magnetic resonance imaging and CT imaging, which are often required when plain radiographs do not provide sufficient information to direct clinical decision making.6,33,34
Limitations Our study used rotation of the foot relative to the cassette during image acquisition. The use of foot rotation to alter femoral rotation assumes that little or no rotation through the knee or ankle occurs during acquisition. It is unknown whether there is a direct correlation between internal foot rotation and internal femoral rotation. Accordingly, the technique does not accommodate for patients with excessive femoral and/ or tibial torsion. In addition, although our protocol for the modified FP view called for patients to internally rotate the affected foot during image acquisition, we did not directly observe every patient during acquisition of the film. Thus, there may be some variability in the degree of pelvic and foot rotation across patients. Still, this variability would likely be encountered when acquiring the standard FP view. Therefore, we believe our results are generalizable. It is important to note that the modified FP view is not difficult to acquire, and thus, other practitioners who use radiographic films to assess hip morphology should be able to integrate this modified FP protocol into their clinic without major difficulty. Given our study’s sample size, post hoc power analyses indicated a power of 0.76 for the bone measurements with an effect size of 0.25. Given the sample size in this study, effects larger than 0.26 would have a power greater than 0.80. Thus, with the current sample size, it is possible that the statistically significant differences identified in ACEA measurements between views came from a false rejection of the null hypothesis, that is, a difference was found when one did not exist. However, differences in ACEA measurements between views were very small, and thus, it is unlikely that eliminating errors in measurement or errors due to inconsistencies in patient positioning would change the conclusions of our study.
Conclusions Thirty-five degrees of femoral internal rotation for a modified FP hip radiographic view provides similar clinical information regarding acetabular morphology to that of the standard FP view. Given that the modified FP view also provides better visualization of the anterosuperior head-neck junction cam lesion, the modified FP view may be preferred over the standard FP view in evaluation of hip pain in the young patient.
References 1. Clohisy JC, Carlisle JC, Beaule PE, et al. A systematic approach to the plain radiographic evaluation of the young adult hip. J Bone Joint Surg Am 2008;90:47-66 (suppl 4). 2. Ganz R, Parvizi J, Beck M, Leunig M, Notzli H, Siebenrock KA. Femoroacetabular impingement: A cause
ACEA ON MODIFIED FALSE-PROFILE RADIOGRAPH
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
for osteoarthritis of the hip. Clin Orthop Relat Res 2003;417: 112-120. Lequesne M, de Sèze S. False profile of the pelvis. A new radiographic incidence for the study of the hip. Its use in dysplasias and different coxopathies. Rev Rhum Mal Osteoartic 1961;28:643-652 [in French]. Hellman MD, Mascarenhas R, Gupta A, et al. The falseprofile view may be used to identify cam morphology. Arthroscopy 2015;31:1728-1732. Atkins PR, Kobayashi EF, Anderson AE, Aoki SK. Modified false-profile radiograph of the hip provides better visualization of the anterosuperior femoral head-neck junction. Arthroscopy 2018;34:1236-1243. Domayer SE, Ziebarth K, Chan J, Bixby S, Mamisch TC, Kim YJ. Femoroacetabular cam-type impingement: Diagnostic sensitivity and specificity of radiographic views compared to radial MRI. Eur J Radiol 2011;80:805-810. Rakhra KS, Sheikh AM, Allen D, Beaule PE. Comparison of MRI alpha angle measurement planes in femoroacetabular impingement. Clin Orthop Relat Res 2009;467: 660-665. Siebenrock KA, Wahab KH, Werlen S, Kalhor M, Leunig M, Ganz R. Abnormal extension of the femoral head epiphysis as a cause of cam impingement. Clin Orthop Relat Res 2004;418:54-60. Hanson JA, Kapron AL, Swenson KM, Maak TG, Peters CL, Aoki SK. Discrepancies in measuring acetabular coverage: Revisiting the anterior and lateral center edge angles. J Hip Preserv Surg 2015;2:280-286. Mittal A, Bomar JD, Jeffords ME, Huang MT, Wenger DR, Upasani VV. Defining the lateral edge of the femoroacetabular articulation: Correlation analysis between radiographs and computed tomography. J Child Orthop 2016;10:365-370. Anderson LA, Gililland J, Pelt C, Linford S, Stoddard GJ, Peters CL. Center edge angle measurement for hip preservation surgery: Technique and caveats. Orthopedics 2011;34:86. Bouttier R, Morvan J, Mazieres B, et al. Reproducibility of radiographic hip measurements in adults. Joint Bone Spine 2013;80:52-56. Mast NH, Impellizzeri F, Keller S, Leunig M. Reliability and agreement of measures used in radiographic evaluation of the adult hip. Clin Orthop Relat Res 2011;469: 188-199. Dolan MM, Heyworth BE, Bedi A, Duke G, Kelly BT. CT reveals a high incidence of osseous abnormalities in hips with labral tears. Clin Orthop Relat Res 2011;469: 831-838. Hansen BJ, Harris MD, Anderson LA, Peters CL, Weiss JA, Anderson AE. Correlation between radiographic measures of acetabular morphology with 3D femoral head coverage in patients with acetabular retroversion. Acta Orthop 2012;83:233-239. Sakai T, Nishii T, Sugamoto K, Yoshikawa H, Sugano N. Is vertical-center-anterior angle equivalent to anterior coverage of the hip? Clin Orthop Relat Res 2009;467: 2865-2871.
3065
17. Ogata S, Moriya H, Tsuchiya K, Akita T, Kamegaya M, Someya M. Acetabular cover in congenital dislocation of the hip. J Bone Joint Surg Br 1990;72:190-196. 18. Wiberg G. Studies on dysplastic acetabulum and congenital subluxation of the hip joint with special reference to the complications of osteoarthritis. Acta Chir Scand 1939;83(suppl 58). 19. Jessel RH, Zurakowski D, Zilkens C, Burstein D, Gray ML, Kim YJ. Radiographic and patient factors associated with pre-radiographic osteoarthritis in hip dysplasia. J Bone Joint Surg Am 2009;91:1120-1129. 20. Sankar WN, Brighton BK, Kim YJ, Millis MB. Acetabular morphology in slipped capital femoral epiphysis. J Pediatr Orthop 2011;31:254-258. 21. Al-Ghamdi A, Rendon JS, Al-Faya F, Saran N, Benaroch T, Hamdy RC. Dega osteotomy for the correction of acetabular dysplasia of the hip: A radiographic review of 21 cases. J Pediatr Orthop 2012;32:113-120. 22. Hosalkar H, Munhoz da Cunha AL, Baldwin K, Ziebarth K, Wenger DR. Triple innominate osteotomy for Legg-Calve-Perthes disease in children: Does the lateral coverage change with time? Clin Orthop Relat Res 2012;470:2402-2410. 23. Philippon MJ, Wolff AB, Briggs KK, Zehms CT, Kuppersmith DA. Acetabular rim reduction for the treatment of femoroacetabular impingement correlates with preoperative and postoperative center-edge angle. Arthroscopy 2010;26:757-761. 24. Werner CM, Copeland CE, Ruckstuhl T, Stromberg J, Turen CH, Bouaicha S. Relationship between Wiberg’s lateral center edge angle, Lequesne’s acetabular index, and medial acetabular bone stock. Skeletal Radiol 2011;40: 1435-1439. 25. Kappe T, Kocak T, Bieger R, Reichel H, Fraitzl CR. Radiographic risk factors for labral lesions in femoroacetabular impingement. Clin Orthop Relat Res 2011;469: 3241-3247. 26. Nepple JJ, Brophy RH, Matava MJ, Wright RW, Clohisy JC. Radiographic findings of femoroacetabular impingement in National Football League Combine athletes undergoing radiographs for previous hip or groin pain. Arthroscopy 2012;28:1396-1403. 27. Polkowski GG, Novais EN, Kim YJ, Millis MB, Schoenecker PL, Clohisy JC. Does previous reconstructive surgery influence functional improvement and deformity correction after periacetabular osteotomy? Clin Orthop Relat Res 2012;470:516-524. 28. Ross JR, Zaltz I, Nepple JJ, Schoenecker PL, Clohisy JC. Arthroscopic disease classification and interventions as an adjunct in the treatment of acetabular dysplasia. Am J Sports Med 2011;39:72S-78S (suppl). 29. Chosa E, Tajima N. Anterior acetabular head index of the hip on false-profile views. New index of anterior acetabular cover. J Bone Joint Surg Br 2003;85:826-829. 30. Zingg PO, Werner CM, Sukthankar A, Zanetti M, Seifert B, Dora C. The anterior center edge angle in Lequesne’s false profile view: Interrater correlation, dependence on pelvic tilt and correlation to anterior
3066
M. M. MURPHY ET AL.
acetabular coverage in the sagital plane. A cadaver study. Arch Orthop Trauma Surg 2009;129:787-791. 31. Crockarell JR Jr, Trousdale RT, Guyton JL. The anterior centre-edge angle. A cadaver study. J Bone Joint Surg Br 2000;82:532-534. 32. Nepple JJ, Martel JM, Kim YJ, Zaltz I, Clohisy JC, Group AS. Do plain radiographs correlate with CT for imaging of cam-type femoroacetabular impingement? Clin Orthop Relat Res 2012;470:3313-3320.
33. Dudda M, Albers C, Mamisch TC, Werlen S, Beck M. Do normal radiographs exclude asphericity of the femoral head-neck junction? Clin Orthop Relat Res 2009;467: 651-659. 34. Saito M, Tsukada S, Yoshida K, Okada Y, Tasaki A. Correlation of alpha angle between various radiographic projections and radial magnetic resonance imaging for cam deformity in femoral head-neck junction. Knee Surg Sports Traumatol Arthrosc 2017;25:77-83.