, Volume 471, Issue 8, pp 2556-2562

Does Radiographic Beam Angle Affect the Radiocapitellar Ratio Measurement of Subluxation in the Elbow?

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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.


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.


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.


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°.


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.

Each author certifies that he or she, or a member of his or her immediate family, has no funding or commercial associations (eg, consultancies, stock ownership, equity interest, patent/licensing arrangements, etc) that might pose a conflict of interest in connection with the submitted article.
All ICMJE Conflict of Interest Forms for authors and Clinical Orthopaedics and Related Research editors and board members are on file with the publication and can be viewed on request.
Each author certifies that his or her institution approved the human protocol for this investigation, that all investigations were conducted in conformity with ethical principles of research, and that informed consent for participation in the study was obtained.
The work was performed at the Hôpital du Sacré-Cœur de Montréal, in the Laboratoire d’Imagerie et d’Orthopédie affiliated with École de Technologie Supérieure.