Correlation of Radial and Ulnar Shaft Bowing Using the 3D Central Line Method.

Forearm rotation depends on the multiplanar bowing of the radius and ulna. Despite the importance of the fundamental 3D anatomic understanding of the forearm for fracture fixation or implant design, all morphological studies have applied only two-dimensional methods rather than analyzing the true full 3D space. No study investigated the intra-pair relation of ulna/radius either. Therefore, we processed CT scans of 30 healthy forearms into 3D models. Each bone was divided into 10 segments along the rotation axis, creating 11 cross-sections with central points. Connecting these central points yielded the Central-Line. Bowing was quantified as the Bowing-Index (area under the Central-Line). Bowing was analyzed in ap, sagittal and the newly defined Main-Deformity-Axis (MDA) plane. Maximal radial deviation averaged 6.4 ± 2.3 mm in frontal, 1.3 ± 3.3 mm sagittal, and 7.2 ± 1.8 mm in MDA plane over all forearms. Maximal ulnar deviation was greater with 8.2 ± 2.9 mm frontal, 9.5 ± 1.7 mm sagittal, and 12.3 ± 2.1 mm in MDA. MDA orientation angle (angular tilt in the dorso-radial direction in the axial plane) was dorso-radial, with significantly larger out-of-plane bowing in the ulna (60.4° vs. 15.0°; p < 0.01), yielding an average intra-bone-pair-difference of 45.5° ± 33.0°. A negative correlation was found between radius and ulna MDA angles (r = -0.40, p = 0.030). The Bowing Index was higher in the ulna across all planes (p ≤ 0.011), with a significant negative correlation between Ulna and Radius (MDA plane, r = -0.38, p = 0.036). Distal ulna analysis revealed consistent dorsal and slight ulnar tilt. The Central Line Method enables 3D characterization of forearm bowing, identifying deformity planes beyond frontal/sagittal axes. For the first time, significant negative correlations between radial and ulnar bowing were demonstrated. These insights hold clinical relevance for fracture fixation, osteotomy planning, and implant design.