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

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Abstract

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.

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Data availability

The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

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Funding

No funding was received to assist with the preparation of this manuscript.

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Authors and Affiliations

  1. Department of Radiology, Boston Children’s Hospital, 300 Longwood Ave, Boston, MA, 02115, USA

    Jade Iwasaka-Neder, Sarah D Bixby, M Alejandra Bedoya, Delma Y Jarrett, Donna Agahigian & Andy Tsai

  2. Department of Pediatrics, Boston Children’s Hospital, 300 Longwood Ave, Boston, MA, 02115, USA

    Enju Liu

  3. Department of Radiology, New York Presbyterian Hospital-Weill Cornell, 525 East 68th Street, New York, NY, 10065, USA

    Delma Y Jarrett

Authors
  1. Jade Iwasaka-Neder
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  2. Sarah D Bixby
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  3. M Alejandra Bedoya
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  6. Donna Agahigian
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Contributions

A.T. conceived, supervised and supported the study. J.I.N. collated the data, performed the statistical analysis and drafted the initial manuscript and multimedia. S.D.B. and M.A.B. interpreted the images. D.Y.J. and D.A. conceived and implemented the 3D model. E.L. helped perform the statistical analysis. All authors reviewed and approved the final manuscript.

Corresponding author

Correspondence to Jade Iwasaka-Neder.

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Supplementary Information

The virtual 3D femur model method applied to two different patients. A femur from a patient without coxa valga or hip flexion deformity (first femur), and a femur from a patient with both coxa valga and hip flexion deformity (second femur) are illustrated. The virtual 3D femur model method consists of rotating the entire model in the sagittal direction to allow an axial point-of-view of both femoral condyles and the femoral neck. The user has the option of viewing the femur from a caudal-to-cranial point-of-view (where the angle was assessed as 20º and 23º on the first and second femurs, respectively), or from a cranial-to-caudal point-of-view (where the angle was assessed as 19º and 22º on the first and second femurs, respectively). The center of rotation is the mid femoral shaft, and a single angle is drawn between the femoral neck axis and the posterior aspect of the femoral condyles. (MP4 1797 kb)

Murphy’s 2D axial slice methodology applied to a femur from a patient without coxa valga or hip flexion deformity. The center of the femoral head is determined by triangulation with coronal images. The cursor is placed at the center of femoral head on the axial plane while the CT images are advanced caudally to the base of the femoral neck, which is defined as at the level of the lesser trochanter. The proximal angle is formed by the line connecting these two points and the horizontal plane (8º on this example). The axial CT images demonstrating the posterior aspects of the femoral condyles are identified. The distal angle is drawn between a line tangential to the posterior margin of the femoral condyles and the horizontal plane (9º on example). In this example, there was internal rotation of the femoral condyles, and therefore, the distal angle was added to the proximal angle to determine the overall angle of torsion, resulting in 17º of femoral version. (MP4 3486 kb)

The effect of hip flexion on the lesser trochanter and femoral head positioning. In the example, the same femur is measured three times – in a position simulating no hip flexion, and in other two positions simulating increasing degrees of hip flexion. The movie clip aims to illustrate the femur of a patient laying in the CT scanner. To understand this example, the reader needs to imagine how the axial CT slices would look like in the three different positions. In the first position, the patient lays flat on the table, and the femoral condyles and femoral neck are aligned to the horizontal plane. The femoral head is anterior to the femoral neck, and the femoral version is measured as 16º. Both the 3D femur model method and the Murphy method would arrive to a similar measurement. In the following two positions, the patient lays down with the hips slightly flexed and substantially flexed, respectively. An increase in the hip flexion angle causes the femoral head to shift in relation to the base of the femoral neck (specifically, at the level of the lesser trochanter). The hip flexion positions the femoral head posteriorly to the femoral neck. If the second position were to be measured on CT slices via the Murphy method, the posteriorly positioned femoral head would result in an apparent retroversion, as depicted by the proximal angle being measured as -18º and the distal angle measured as 4º in external rotation, resulting in a femoral version of -22º. The third position further exaggerates this artifact by representing a patient laying with the hips substantially flexed. The increased flexion exaggerates the shift in position of the femoral head in regards to the femoral neck. The proximal angle becomes -33º and the distal angle becomes 8º in external rotation, resulting in a femoral version of -41º. The same femur had three different femoral version measurement based on different degrees of hip flexion, ranging from positive when the flexion was not present, to negative when it was. This explains the negative values observed in the flexed subgroup of our analysis. Since the entire length of the femur is not visible on the 2D axial slices, it becomes challenging to visualize and account for this false impression of retroversion. On the 3D femur model, the entire femur is seen and its position aligned to the horizontal plane, thus avoiding projectional misinterpretations and allowing accurate measurements of femoral version. (MP4 1137 kb)

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Iwasaka-Neder, J., Bixby, S.D., Bedoya, M.A. et al. Virtual 3D femur model to assess femoral version: comparison to the 2D axial slice approach. Pediatr Radiol 53, 2411–2423 (2023). https://doi.org/10.1007/s00247-023-05758-8

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