title: Do plain radiographs correlate with CT for imaging of cam-type femoroacetabular impingement?
abstract: BACKGROUND
  Three-dimensional imaging (CT and MRI) is the gold standard for detecting femoral head-neck junction malformations in femoroacetabular impingement, yet plain radiographs are used for initial diagnostic evaluation. It is unclear, however, whether the plain radiographs accurately reflect the findings on three-dimensional imaging.
  
  
  OBJECTIVE
  We therefore: (1) investigated the correlation of alpha angle measurements on plain radiographs and radial reformats of CT scans; (2) determined which radiographic views are most sensitive and specific in detecting head-neck deformities present on CT scans; and (3) determined if specific radiographic views correlated with specific locations on the radial oblique CT scan.
  
  
  METHODS
  We retrospectively reviewed 41 surgical patients with preoperative CT scans (radial oblique reformats) and plain radiographs (AP pelvis, 45° Dunn, frog lateral, and crosstable lateral). Alpha angles were measured on plain radiographs and CT reformats.
  
  
  RESULTS
  The complete radiographic series was 86% to 90% sensitive in detecting abnormal alpha angles on CT. The maximum alpha angle on plain radiographs was greater than that of CT reformats in 61% of cases. Exclusion of the crosstable lateral did not affect the sensitivity (86%-88%). The Dunn view was most sensitive (71%-80%). The frog lateral showed the best specificity (91%-100%). Substantial correlations (intraclass correlation coefficients, 0.64-0.75) between radiograph and radial oblique CT position were observed, including AP/12:00 (superior), Dunn/1:00 (anterolateral), frog/3:00 (anterior), and crosstable/3:00 (anterior).
  
  
  CONCLUSIONS
  For diagnostic and treatment purposes, a three-view radiographic hip series (AP pelvis, 45° Dunn, and frog lateral) effectively characterizes femoral head-neck junction malformations.
  
  
  METHODS
  Level II, diagnostic study. See Guidelines for Authors for a complete description of levels of evidence.
authors.affiliation: Department of Orthopaedic Surgery, Washington University School of Medicine, 660 S Euclid, 11300 West Pavilion, Campus Box 8233, St Louis, MO, USA.
authors.forename: Jeffrey J
authors.initials: JJ
authors.lastname: Nepple
authors.forename: John M
authors.initials: JM
authors.lastname: Martel
authors.forename: Young-Jo
authors.initials: YJ
authors.lastname: Kim
authors.forename: Ira
authors.initials: I
authors.lastname: Zaltz
authors.forename: John C
authors.initials: JC
authors.lastname: Clohisy
mesh_terms: D000293:Adolescent; D000328:Adult; D005260:Female; D057925:Femoracetabular Impingement / Q000000981:diagnostic imaging*; D005270:Femur Head / Q000000981:diagnostic imaging*; D005272:Femur Neck / Q000000981:diagnostic imaging*; D006801:Humans; D008297:Male; D008875:Middle Aged; D011237:Predictive Value of Tests; D012189:Retrospective Studies; D012680:Sensitivity and Specificity; D014057:Tomography, X-Ray Computed; D055815:Young Adult