Determining Measurement Error for Bohler's Angle and the Effect of X-Ray Obliquity on Accuracy.

BACKGROUND: Bohler's angle (BA) is the most commonly utilized radiographic measurement in the study of calcaneus fractures and has been shown to be prognostic in nature. Therefore, it is critical that the measurement of BA be accurate as both therapeutic and prognostic information relies on it. Oblique lateral radiographs can be a cause of error in BA measurements. However, measurement error and the effects of X-ray beam obliquity on BA have not been established in the literature. The purpose of this study was to determine measurement error and understand the effects of X-ray beam's obliquity on the measurement of BA.

METHODS: A cadaver specimen was imaged using a C-arm to obtain a perfect lateral radiograph of the ankle and slightly oblique lateral views in the anterior, posterior, cephalad, and caudad directions in 5° increments (21 images). Metallic beads were then placed on the anterior calcaneal process, posterior facet, and the superior aspect of the posterior tuberosity, and the same 21 images were then obtained. The metallic beads placed on the reference radiographs allowed the authors to accurately measure BA for each image and served as reference for the corresponding test radiographs. Thirty-four orthopaedic staff members participated in the study and used DICOM measurement tool to measure BA on each of the 21 test radiographs. The measurements were then compared to the measurements of BA from the reference radiographs to determine error in measurement.

RESULTS: A total of 714 different measurements were obtained. Average measurement error was 6° (95% confidence interval = -4° to 15°). The difference between the observed BA measurements compared to the true BA measurements increased with increasing X-ray obliquity.

CONCLUSIONS: Measurement error for BA is ±6° and increases most with cephalad oblique radiographs. Orthopaedic surgeons' ability to accurately measure BA significantly decreases with increasing obliquity of the lateral radiograph.

LEVELS OF EVIDENCE: Level V: Cadaver bench study.