Purpose: Surgical procedures for impaired forearm rotation such as for chronic radial head dislocation remain controversial. We hypothesized that the morphological axis of the proximal radius is important for stable forearm rotation, and we aimed to clarify the relationship between the morphological axis and the kinematic axis of the proximal radius using four-dimensional computed tomography (4DCT). Methods: Ten healthy volunteers were enrolled. Four-dimensional CT of the dominant forearm during supination and pronation was obtained. The rotation axis of forearm rotation was calculated from all frames during supination and pronation. The principle axis of inertia, which represents the most stable rotation axis of a rigid body, was calculated for the proximal radius by extending its surface data incrementally by 1% from the proximal end. The angle between the kinematic rotation axis and the morphological rotation axis of each length was calculated. Results: The rotation axis of the forearm was positioned on the radial head 0.0 mm radial and 0.4 mm posterior to the center of the radial head proximally and 2.0 mm radial and 1.2 mm volar to the fovea of the ulnar head distally. The principle axis at 15.9% of the length of the proximal radius coincided with the forearm rotation axis (kinematic axis). Individual differences were very small (SD 1.4%). Conclusion: Forearm rotation was based on the axis at 16% of the length of the proximal radius. This portion should be aligned in cases of severe morphological deformity of the radial head that cause “rattling motion” of the radial head after reduction procedures.
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