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The aim of this study was to clarify the mechanisms of rotational restriction in malunited forearm diaphyseal fractures.We retrospectively analyzed the cases of 18 patients with malunited forearm diaphyseal fractures and rotational restriction. All patients underwent bilateral computed tomography (CT) of the forearm in maximum supination, pronation, and neutral positions. From these images, we created 3-dimensional (3-D) bone surface models. We quantified the 3-D deformities, identified instances of osseous impingement between the radius and the ulna during forearm rotation, calculated the path length of the central band (CB) of the interosseous membrane, and measured forearm range of motion.Sixteen patients had extension deformity of the radius (the RE group) and 2 had flexion deformity (the RF group). In the RE group, extension deformity of the radius and valgus deformity of the ulna had significant negative correlation with pronation range of motion (R = −0.50, p = 0.046) and supination range of motion (R = −0.63, p = 0.027), respectively. Osseous impingement was mainly observed during pronation (15 of 16 patients). The CB path with the largest changes in length originated from the distal CB attachment area of the radius and ran toward the proximal area of the ulna (the transverse CB). The transverse CB significantly increased in length in supination compared with that in pronation (p < 0.001). Therefore, tightness of the transverse CB appeared to cause supination restriction in the RE group. In the RF group, osseous impingement caused supination restriction. The greatest increases in the transverse CB length were observed in pronation in the RF group, which appeared to cause pronation restriction.In the RE group, pronation restriction was associated with osseous impingement that was due to extension deformity of the radius, and supination restriction was associated with CB tightness that was due to valgus deformity of the ulna. In the RF group, our results suggested that pronation restriction was caused by CB tightness and that supination restriction was caused by osseous impingement.Three-dimensional corrective osteotomy for extension deformity of the radius in malunited forearm diaphyseal fractures would improve rotational restriction by relieving osseous impingement during pronation and CB tightness during supination.