CT scan-based measurement technique to assess axial torsion of forearm bones

Rotational malunion of forearm fractures may severely impair pronosupination. Accurate preoperative assessment of torsional deformities is therefore critical, yet no standardized technique exists to quantify axial rotation of the radius and ulna, and the validity of using the contralateral limb as a reference remains uncertain. This study aimed to develop a simple, reproducible CT-based method for measuring forearm torsion and to determine side-to-side anatomical symmetry in normal individuals.

A manual measurement technique based on cross-sectional two-dimensional CT images was applied to 30 normal right forearms by two independent observers. Torsion angles were calculated between defined proximal and distal axes of the radius and ulna. Inter- and intraobserver reliability was assessed using Pearson correlation. To analyse bilateral symmetry, automated measurements were obtained from 490 paired radii and 451 paired ulnae from the three-dimensional CT scans obtained from the Stryker Orthopaedic Modeling and Analytics imaging database of healthy volunteers. Measurements were performed using consistent bony landmarks.

The two-dimensional CT scan method demonstrated excellent reproducibility for radial torsion (interobserver r = 0.89; intraobserver r = 0.95) as well as for ulnar torsion, using the radial notch as a proximal landmark (Pearson co-efficient r = 0.81 and r = 0.93). In three-dimensional CT scan analysis, right-left torsion differences were <15° in 98% of radii and 91% of ulnae, with correlation coefficients of 0.6-0.8 across measurements.

The measurements based on standard axial slices of two-dimensional CT scans provide a reliable and clinically feasible tool to quantify forearm torsion. In over 90% of individuals, the contralateral limb may serve as a valid reference, supporting its routine use in preoperative planning of corrective derotational osteotomies for forearm malunion.

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