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.

LEVEL OF EVIDENCE

Level II-comparative in vitro study.