Is the Fovea Ulnaris Truly Isometric during Forearm Rotation?—An In Vivo Retrospective Analysis Using Superimpositions of Three-Dimensional Reconstructions

The fovea ulnaris is considered to be the center of rotation on the ulnar head during forearm rotation. The purpose of this study was to investigate whether the fovea ulnaris is truly isometric during forearm rotation in vivo. The three-dimensional reconstruction models of 21 wrist computed tomography images taken in supination and pronation were investigated. The models were superimposed so that the two ulnar heads were in the same position. Numerous points were set on the surface of the ulnar head with a mean distance of 0.2 mm between the nearest two points. Then, the models were superimposed with respect to the radius, and the distance between the same points on the ulnar head in pronation and supination (DFR) was measured. The rotation center was defined as the point with the shortest DFR. The isometric point was defined as a rotation center with a DFR of less than 0.2 mm. An isometric point was found in three cases and not in 18 cases. The distance the rotation center moved during forearm rotation (DFR of the rotation center) ranged from 0.1 mm to 2.4 mm. The position of the rotation center in the radioulnar direction was significantly correlated with the translation of the ulnar head and the amount of forearm rotation. The rotation center was located more ulnarly when the translation of the ulnar head or the amount of forearm rotation was greater. The isometricity of the foveal insertion of the TFCC during forearm rotation may not be consistent in vivo. The center of rotation on the ulnar head during forearm rotation appears to shift ulnarly with increasing translation of the ulnar head or forearm rotation.