Contribution of the Interosseous Membrane to Distal Radioulnar Joint Constraint Hiroshi Watanabe, MD, Richard A. Berger, MD, PhD, Lawrence J. Berglund, BS, Mark E. Zobitz, MS, Kai-Nan An, PhD, Rochester, MN Purpose: Although forearm injuries are accompanied frequently by rupture to the interosseous mem- brane (IOM) diagnosis of the extent of IOM injury is difficult. In this study we evaluated distal radioulnar joint (DRUJ) laxity caused by both partial and complete IOM disruption and compared these quanti- tative measurements with the common clinical manual evaluation of DRUJ laxity and dislocatability. Methods: Human cadaveric forearms (n = 8) were used in this study. Skin, muscles, and tendons were removed. The specimens were mounted on an experimental apparatus that allowed the radius to move freely about the fixed ulna. Tests were performed in neutral rotation, 60° pronation, and 60° supination. Under various conditions of IOM sectioning testing was performed by volary and dorsally translating the radius relative to the ulna in the coronal plane of the radius. Testing was performed both qualitatively as would be performed in the clinic and quantitatively with an instrumented probe. Results: Our results show that dorsal dislocation of the radius relative to the ulna strongly suggests distal IOM rupture. Disengagement of the radius from the DRUJ indicated injury to the distal and middle IOM. The distal IOM constrained volar and dorsal laxity of the radius at the DRUJ in all forearm rotation positions. The midportion of the IOM constrained laxity except in the volar direction of the pronated forearm. The proximal IOM did not constrain the proximal radius except dorsally for the pronated forearm position. Conclusions: The IOM, in particular the distal IOM, plays an important role in constraining dorsal dislocation of the radius at the DRUJ. (J Hand Surg 2005;30A:1164 –1171. Copyright © 2005 by the American Society for Surgery of the Hand.) Key words: Distal radioulnar joint, instability, interosseous membrane. It generally is assumed that the pronator quadratus, 1 ulnocarpal ligament, extensor carpi ulnaris sub- sheath, 2 dorsal radioulnar ligament, volar radioulnar ligament, 3–7 interosseous membrane (IOM), 3,5,6,8,9 bony anatomy (sigmoid notch), 6,10 and the distal radioulnar joint (DRUJ) capsule contribute to DRUJ stability. 8,9 Among these structures the triangular fibrocartilage complex (TFCC) is the major stabilizer of the DRUJ and the IOM is a secondary stabilizer of the DRUJ. 3,5,6,9,10 Prior studies have focused on the IOM functioning as a longitudinal stabilizer of the forearm rather than as a dorsal–volar stabilizer of the DRUJ. 11–15 From the Biomechanics Laboratory, Division of Orthopedic Research, Mayo Clinic College of Medicine, Rochester, MN. Received for publication February 9, 2005; accepted in revised form June 22, 2005. No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. Funding for this study was received from the Mayo Foundation. Corresponding author: Kai-Nan An, PhD, Biomechanics Laboratory, Division of Orthopedic Research, Mayo Clinic College of Medicine, Rochester, MN 55905; e-mail: an.kainan@mayo.edu. Copyright © 2005 by the American Society for Surgery of the Hand 0363-5023/05/30A06-0010$30.00/0 doi:10.1016/j.jhsa.2005.06.013 1164 The Journal of Hand Surgery