Neck-shaft angle measurement in children: accuracy of the conventional radiography-based (2D) methods compared to 3D reconstructions

Abstract

Aim of this study was to examine the accuracy of widely used conventional radiography-based (2D) neck-shaft angle measurements compared to 3D reconstruction. In our retrospective study, EOS 2D/3D images of 156 patients (312 limbs) were selected from our database (4-16 years old: 6 girls and 6 boys/year), where no pathology was revealed. Using the 2D modality of the EOS method neck-shaft angle was measured using the "biggest diameter" and "circle fitting" techniques to define the femoral neck axis and 1/3, 1/2 and full femur to determine the femoral shaft axis. EOS 3D reconstructions of same images were also performed and a comparison of 2D and 3D results was made. We did not find any significant difference between accuracy of the four examined 2D methods, although the deviation between 2 and 3D results was considerable (average difference: 5.11-5.58°, p < 0,001). In 31% of the cases, difference was more than 10°. Only femoral torsion showed significant influence on the difference (correlation coefficient: 0.380, p < 0.001). We did not find a clinically significant difference between the examined 2D methods, although their accuracy was highly questionable compared to 3D results. We suggest using any 3D imaging method for surgical planning and in uncertain cases.

Figure 1

The EOS 2D/3D Imaging System. (a) The EOS 2D/3D Imaging System and the typical positioning during examination (source: www.eos-imaging.com). (b) The basic operation of the imaging system: upright biplanar slot-scanning X-ray imaging (source: www.eos-imaging.com). (c) 3D reconstruction opportunities of the lower limb and spine (source: www.eos-imaging.com). (d) Example of a typical EOS 2D/3D lower limb image-pair used in this study.

Figure 2

Neck-shaft angle measurement methods (OM, 11 years old, girl). (a) Biggest diameter-1/3 femur: 139.7°. (b) Circle fitting-1/3 femur: 138.1°. (c) Circle fitting-1/2 femur: 138.8°, Circle fitting-full femur: 137.1°. (d) SterEOS 3D reconstruction of the femur-true NSA: 123.6°.

Figure 3

The average difference of the 3D reconstruction and the plain radiography-based NSA measurement results, in addition the average femoral torsion.

Figure 4

Pile chart of the difference between the 3D reconstructions and each 2D measurement result. Blue—the absolute difference between the 2D and 3D results is less than 3° (7.6–9.6%). Red—the absolute difference between the 2D and 3D results is between 3–5° (13.6–16.7%). Green—the absolute difference between the 2D and 3D results is between 5–10° (42.4–44.6%). Yellow—the absolute difference between the 2D and 3D results is more than 10° (31.0–34.3%).