In vivo motion analysis of forearm rotation utilizing magnetic resonance imaging.

OBJECTIVE: To analyse in vivo normal forearm rotation utilizing the image processing method on axial magnetic resonance imaging. DESIGN: Rotatory motion and translation of the radius and rotation of the ulna were analysed in vivo, utilizing features of the bone image; axial centre of gravity and axis of the bone. METHODS: Axial magnetic resonance images at the proximal, middle and distal portion of the right forearms of 20 healthy volunteers were examined in five rotational positions, maximum pronation, 45 degrees pronation, neutral, 45 degrees supination and maximum supination. The axis of forearm rotation, the rotation angle of the radius, the rotation of the radius around its axial centre of gravity, and the rotation of the ulna around its axial centre of gravity were calculated on these axial MR images. RESULTS: The radius rotates in a near-circular ellipse around its rotational centre located on a line through the radial head and the ulnar head from maximum supination to 45 degrees pronation. There was discrepancy between the calculated maximum rotational angle of the radius and the pronation-supination angle showed on the goniometer. This discrepancy was due to motion at the radiocarpal, midcarpal and carpometacarpal joints. The rotation of the ulna ranges within 6 degrees. At maximum pronation, there was palmar translation of the radius. CONCLUSIONS: The forearm rotation is not a simple rotatory motion but a complex motion with rotation and translation. The palmar translation of the radius at maximum pronation may be caused by incongruity of the distal radioulnar joint and contraction of the pronator quadratus muscle. RELEVANCE: To date there exist few methods for the accurate and non-invasive assessment for motion analysis of the forearm rotation. Such a technique utilizing magnetic resonance image characteristics is, however, helpful to reveal accurate self- and relative-rotatory motion of both radius and ulna as well as the translation movement on forearm rotation in normal forearm.