title: Does radiographic beam angle affect the radiocapitellar ratio measurement of subluxation in the elbow?
abstract: BACKGROUND
  Radial head alignment is the key to determine elbow reduction after treatment of subluxations or Monteggia fractures. The radiocapitellar ratio (RCR) quantifies the degree of subluxation, by evaluating radial head alignment with the capitellum of the humerus; this ratio is reproducible when measured on true lateral radiographs of nonsubluxated elbows. However, the impact of beam angulation on RCR measurement is unknown.
  
  
  OBJECTIVE
  Our hypotheses were that the RCR of the nonsubluxated elbow would remain in the normal range as the beam angle changed and that the RCR variability would increase for the subluxated elbow with small deviations in the beam angle.
  
  
  METHODS
  Radiographs were taken of six healthy cadaveric extremities using beam angles ranging from -20° to 20° along the inferosuperior axis and from -20° to 20° along the dorsoventral axis. The same views then were taken of the six arms with anterior radiocapitellum subluxation followed by posterior radiocapitellum subluxation. RCRs were measured by one observer. As a reference value, the RCR was measured in the 0° to 0° position and the difference between each RCR in a nonreference position was subtracted from each RCR reference to obtain the delta-RCR. An ANOVA was performed to assess the main and interactive effects on the RCR measured in each C-arm position compared with the RCR measured on a true lateral radiograph.
  
  
  RESULTS
  The RCR remained in the normal range even as the beam angle of the C-arm varied between -20° and 20°. The position of the beam did not affect the RCR in anteriorly subluxated elbows (p = 0.777), whereas RCR variation increased especially in the presence of posterior radial head subluxation when the C-arm position was 10° or more out of plane (p = 0.006). The inferosuperior malposition of the C-arm had a greater impact on quantification of radial head alignment measurement. Despite that, the RCR measurement is reliable in reduced and subluxated elbows on lateral radiographs with a C-arm position deviation of as much as 20°.
  
  
  CONCLUSIONS
  Identification of a subluxated elbow could be made on any lateral radiograph with a beam angulation deviation of as much as 20°. This suggests that the RCR is a useful diagnostic tool for clinical and research purposes, although for subluxated elbows, it is important to pay careful attention to the inferosuperior position of the C-arm.
authors: Université de Montréal, Montréal, QC, Canada.
authors: Fannie
authors: F
authors: McCann
authors: Fanny
authors: F
authors: Canet
authors: Emilie
authors: E
authors: Sandman
authors: Yvan
authors: Y
authors: Petit
authors: Dominique M
authors: DM
authors: Rouleau
mesh_terms: D000328:Adult; D000368:Aged; D000704:Analysis of Variance; D001696:Biomechanical Phenomena; D002102:Cadaver; D016022:Case-Control Studies; D004551:Elbow Joint / Q000000981:diagnostic imaging* / Q000503:physiopathology; D005260:Female; D006801:Humans; D006811:Humerus / Q000000981:diagnostic imaging* / Q000503:physiopathology; D004204:Joint Dislocations / Q000000981:diagnostic imaging* / Q000503:physiopathology; D008297:Male; D008875:Middle Aged; D011237:Predictive Value of Tests; D011859:Radiography; D011884:Radius / Q000000981:diagnostic imaging* / Q000503:physiopathology; D016059:Range of Motion, Articular; D015203:Reproducibility of Results; D055815:Young Adult
references: J Shoulder Elbow Surg. 2008 Mar-Apr;17(2):281-6
references: 18069007
references: Orthopedics. 2012 Feb;35(2):138-44
references: 22300997
references: Psychol Bull. 1979 Mar;86(2):420-8
references: 18839484
references: Can Assoc Radiol J. 1994 Aug;45(4):287-91
references: 8062119
references: J Shoulder Elbow Surg. 2003 Jul-Aug;12(4):391-6
references: 12934037
references: J Bone Joint Surg Am. 1981 Apr;63(4):529-35
references: 7217119
references: J Bone Joint Surg Am. 1998 Dec;80(12):1733-44
references: 9875931
references: J Shoulder Elbow Surg. 2012 May;21(5):574-9
references: 21724422
references: Eur Spine J. 2008 Jan;17(1):30-5
references: 17846802
references: J Bone Joint Surg Am. 2009 Apr;91(4):950-4
references: 19339581
references: J Emerg Med. 1990 May-Jun;8(3):265-9
references: 2373834
references: Injury. 1989 Nov;20(6):365-7
references: 2628337
references: Hand Clin. 2004 Nov;20(4):389-404
references: 15539095
references: J Hand Surg Am. 2010 Feb;35(2):228-32
references: 20061094
references: Clin Orthop Relat Res. 1967 Jan-Feb;50:7-30
references: 6029028
references: Skeletal Radiol. 2008 Jun;37(6):535-9
references: 18343920