title: Ultrasound is as reliable as plain radiographs in the diagnosis of cam-type femoroacetabular impingement.
abstract: BACKGROUND
  In the diagnosis of femoroacetabular impingement (FAI), plain radiographs are accepted as the initial imaging method. However, there is no consensus regarding the optimal lateral view, and radiographs can underestimate the asphericity of the head-neck junction. Our research question was if ultrasound has at least the same reliability as X-ray and can be used as an alternative or additional method in the initial imaging of FAI.
  
  
  METHODS
  Forty patients with a median age of 39 years were consecutively included after diagnosis of cam-type FAI on magnetic resonance imaging (MRI). All patients underwent radiography involving a plain anteroposterior-view, frog-leg lateral view, and ultrasound of the hip joint in the ventral longitudinal section at 20° internal rotation. Parameters measured by MRI, radiographs, and ultrasound were the alpha angle, anterior offset, offset ratio, and anterior femoral distance.
  
  
  RESULTS
  No significant difference between the alpha angle on MRI (64.8°), the frog-leg view (66.3°), or ultrasound (65.6°) could be detected. Comparable correlation was found between the alpha angle on MRI and the frog-leg lateral view (r = 0.73; p < 0.0001) and between the alpha angle on MRI and sonograms (r = 0.77; p < 0.0001). The intra-class correlation coefficient for measurements using ultrasound was 0.81-0.98, and using radiographs was 0.83-0.99, with the exception of measurements involving the anterior offset on the frog-leg lateral view (0.61 and 0.64).
  
  
  CONCLUSIONS
  Ultrasound is as reliable as plain radiographs in the diagnosis of cam-type FAI and can serve as an alternative or additional method in initial imaging.
authors.affiliation: Clinic for Orthopedics/Traumatology/Sports Medicine, Klinikum Agnes Karll Laatzen/Klinikum Region Hannover, Hildesheimer Str. 158, 30880, Laatzen, Germany. solveig.lerch@krh.eu.
authors.forename: Solveig
authors.initials: S
authors.lastname: Lerch
authors.affiliation: Clinic for Radiology and Nuclear Medicine, Hildesheimer Str. 158, 30880, Laatzen, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Kasperczyk
authors.affiliation: Clinic for Orthopedics/Traumatology/Sports Medicine, Klinikum Agnes Karll Laatzen/Klinikum Region Hannover, Hildesheimer Str. 158, 30880, Laatzen, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Berndt
authors.affiliation: Clinic for Orthopedics/Traumatology/Sports Medicine, Klinikum Agnes Karll Laatzen/Klinikum Region Hannover, Hildesheimer Str. 158, 30880, Laatzen, Germany.
authors.forename: Oliver
authors.initials: O
authors.lastname: Rühmann
mesh_terms: D000328:Adult; D005260:Female; D057925:Femoracetabular Impingement / Q000000981:diagnostic imaging*; D006801:Humans; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D011859:Radiography; D015203:Reproducibility of Results; D014463:Ultrasonography
keywords: Alpha angle; Femoroacetabular impingement; Frog leg; Hip; MRI; Ultrasound