Virtual 3D femur model to assess femoral version: comparison to the 2D axial slice approach.

BACKGROUND: Quantifying femoral version is crucial in diagnosing femoral version abnormalities and for accurate pre-surgical planning. There are numerous methods for measuring femoral version, however, reliability studies for most of these methods excluded children with hip deformities.

OBJECTIVE: To propose a method of measuring femoral version based on a virtual 3D femur model, and systematically compare its reliability to the widely used Murphy's 2D axial slice technique.

MATERIALS AND METHODS: We searched our imaging database to identify hip/femur CTs performed on children (<18 years old) with a clinical indication of femoral version measurement (September 2021-August 2022). Exclusion criteria were prior hip surgery, and inadequate image quality or field-of-view. Two blinded radiologists independently measured femoral version using the virtual 3D femur model and Murphy's 2D axial slice method. To assess intrareader variability, we randomly selected 20% of the study sample for re-measurements by the two radiologists >2 weeks later. We analyzed the reliability and correlation of these techniques via intraclass correlation coefficient (ICC), Bland-Altman analysis, and deformity subgroup analysis.

RESULTS: Our study sample consisted of 142 femurs from 71 patients (10.6±4.4 years, male=31). Intra- and inter-reader correlations for both techniques were excellent (ICC≥0.91). However, Bland-Altman analysis revealed that the standard deviation (SD) of the absolute difference between the two radiologists for the Murphy method (mean 13.7°) was larger than that of the 3D femur model technique (mean 4.8°), indicating higher reader variability. In femurs with hip flexion deformity, the SD of the absolute difference for the Murphy technique was 17°, compared to 6.5° for the 3D femur model technique. In femurs with apparent coxa valga deformity, the SD of the absolute difference for the Murphy technique was 10.4°, compared to 5.2° for the 3D femur model technique.

CONCLUSION: The 3D femur model technique is more reliable than the Murphy's 2D axial slice technique in measuring femoral version, especially in children with hip flexion and apparent coxa valga deformities.