Baumann angle and radial-ulnar overlap: a radiographic study to control for the angle of the x-ray beam

Baumann's angle (BA) is the most common radiographic measurement used to assess coronal plane alignment of the distal humerus. However, it can vary greatly based on the x-ray beam angle, which can lead to multiple radiographs causing excessive radiation exposure, cost, and discomfort for the acutely injured child. The lateral capitellohumeral angle is a measure of sagittal plane alignment, and its variability has been reported. In this study, we sought to determine whether there were surrogate measures that could act as internal controls for the angle of the x-ray beam to give an accurate BA.

Elbow radiographs from uninjured children stratified by age into 6 groups were reviewed. BA and lateral capitellohumeral angle as well as several predetermined measurements that could be potential surrogate measures for the angle of the x-ray beam were performed. Statistical significance was found between BA and radial-ulnar overlap (RUO), which was analyzed further and plotted in linear graph fashion.

The Pearson correlation coefficient (0.58) between BA and RUO was significant at P=0.001. The average BA was 71±7.2 degrees (1 SD) and the average RUO was 0.34±0.26. The following linear graph equation was obtained to define the relationship between RUO and BA: BA=12.36 (RUO)+67. Further derivation gives us an equation to "correct" BA for a given RUO: corrected BA=measured BA-12.36 (RUO-0.34). Application of this formula lessened the SD of BA from 7.2 to 5.9 degrees and decreased the percentage of BA measurements outside of 1 SD from 30% to 14%.

RUO is a reliable surrogate measure to control for the angle of the x-ray beam and improve the reliability when measuring BA that can be easily applied in the clinical setting.

The ability to accurately correct the BA based on a given RUO gives the physician the confidence to adequately interpret imperfect radiographs in the emergency room setting and minimize repeat radiographs.