Applications and Error Ratios of Calibration Techniques in EOS, Orthoroentgenogram, and Teleoroentgenogram for Length Measurement: A Comparative Study

Background: Accurate length measurements of extremity bones are essential in treating limb deformities and length discrepancies in children. Objective: This study aimed to determine errors in common techniques used to measure lower limb lengths in children. Methods: Precision and instrument errors in length measurements were studied utilizing electro-optical system (EOS), orthoroentgenogram, and teleoroentgenogram The goal was to measure a 70-cm metallic rod phantom (average length of the lower extremity of a 10-year-old boy in the 50th percentile) in 3 phases. In Phase 1, the length measurements were performed in an EOS unit with internal calibrations, a magball/magstrip in various scan positions, and measurement with TraumaCAD software. In Phase 2, the measurements were repeated utilizing a single radiation "shot" teleoroentgenogram. In Phase 3, an orthoroentgenogram was utilized with a radiopaque ruler reference. The reliability and validity of measurements were calibrated by 4 physicians (a radiologist, senior orthopaedic attending, and 2 orthopaedic fellows). Results: EOS measurements utilizing internal references had excellent accuracy (for a 700-mm real length, magnification error (ME)] of 0.09%. Teleoroentgenogram with a magball reference and measurements performed with automatic calibration by TraumaCAD program results in ME of 1.83% with insignificant intraobserver/interobserver difference. Teleoroentgenogram with a magball or magstrip reference measured manually showed that the magball has higher intraobserver/interobserver variance than magstrip, with a 6.60 and 0.33-mm SD, respectively. The length by manual measurement utilizing the magstrip has ME of 2.21%. Orthoroentgenogram is accurate with ME of 0.26%, but does not allow anatomical analysis and is also radiation-costly. Conclusion: EOS and orthoroentgenogram are very accurate for length measurements. Teleoroentgenogram is less accurate in measuring length; however, addition of an external reference (magball, magstrip) placed lateral to the target improves accuracy. Automatic calibration with computer-based analysis of the external reference improves the accuracy more than manual calibration. If manual calibration is utilized, the length measurement is less accurate with the magball than the magstrip.