Angular accuracy of plain radiographic measurements in leg alignment: Teleoroentgenogram versus orthoroentgenogram

The purpose of the present study was to evaluate the accuracy of the measurements associated with leg alignment on a teleoroentgenogram and an orthoroentgenogram.

The models being irradiated were manufactured and represented 10° varus and 15° varus leg alignment, in which the true values of hip-knee-ankle angle (HKAA), mechanical axis (MA) length, and percentage of MA (%MA) were already known. HKAA, MA length, and %MA were measured in various radiographic conditions of the teleoroentgenogram and orthoroentgenogram. Then, the differences between the radiographic measurement values and the corresponding true values were analyzed.

Regarding HKAA, the teleoroentgenogram provided accurate angular measurements with minimal differences between the measurement and true value in both the 10° and 15° varus models, irrespective of the radiographic condition. In the orthoroentgenogram, the modified method measured accurate HKAA; however, the standard method showed significant angular measurement errors with a 0.6° and 1.0° difference from the true value in the 10° and 15° varus models, respectively. This angular measurement error of HKAA in the standard orthoroentgenogram became significantly larger with object-to-image distance. Regarding MA length, the orthoroentgenogram exhibited accurate length measurements. In contrast, the teleoroentgenogram showed a significant length measurement error for the MA length. Regarding %MA, significant differences from the true values of 2.0% and 2.4% were observed in the modified orthoroentgenograms of the 10° and 15° varus models, respectively. The teleoroentgenogram and standard orthoroentgenogram reproduced the accurate measurement value of %MA in the 10° and 15° varus models.

A teleoroentgenogram is a reliable modality for accurate angular measurements such as HKAA and %MA. An orthoroentgenogram has the potential to measure both HKAA and length accurately if the radiographic condition was modified; however, measurement error in %MA may occur.