The “Boutonniere Effect” of the Fixed Ulnar Head during Pronation

Abstract The translational/rotational stability of the whole ulna during forearm rotation is unclear and controversial. Our hypothesis was that there is no translational/rotational motion of the whole ulna during forearm rotation and that it was not influenced by the flexed or extended position of the elbow. In this cadaveric study of 21 fresh specimens, we analyzed the stability of the whole ulna during forearm rotation in a custom-made frame with the elbow either in 90° flexion or maximal extension. Pinning of the ulnar diaphysis was used to assess and video record the behavior of the ulna during forearm rotation. Our results suggest that the whole ulna remains fixed during forearm rotation. In addition, we found that the rotational motion of the radius-hand unit around a fixed ulna produces a progressive “boutonniere effect” of the ulnar head protruding from supination to pronation between the extensor digiti minimi and extensor carpi ulnaris tendons under the dorsal extensor retinaculum. During the wrist clinical testing of distal radioulnar joint (DRUJ) instability in neutral, supination and pronation, moving the radius-hand unit with respect to the fixed ulnar head is preferable than moving the ulna with respect to the radius. When analyzing computed tomography scan or magnetic resonance imaging axial cuts, reorienting the DRUJ images to a conventional orientation of the fixed ulnar head may help not only to more accurately assess the position of forearm rotation in which the axial cut was done but also to better analyze DRUJ disorders. Lastly the concept of a radius-hand unit rotating around a fixed ulna during forearm rotation helps differentiating the well-known anatomical forearm rotation axis from the myriads of functional forearm rotation axes that may be observed when combining forearm rotation with prehension of a tool with the fingers.