[Factors in the stabilization of the ulnar head]

The authors studied the stability of the ulnar head by dissections of the radio-ulnar joint and experimental ligamentous removals in 150 specimens. They emphasize the importance of the tendon of the extensor carpi ulnaris and medial collateral ligament tightened by Kuhlmann's sling which firmly apply the ulnar head against the anterior edge of the sigmoid potch of the radius. Transsection of the separate fibro-osseous tunnel of the extensor carpi ulnaris with forward creeping of the tendon out of its groove enlarges the size of the gap between extensor carpi ulnaris and extensor digiti quinti; if in addition both fibro-cartilaginous disk and medial collateral ligament are torn, the ulnar head dislocates dorsally. Such a displacement can be obtained by full supination following forced pronation. The volar dislocation of the ulnar head occurs more rarely; it might be caused by insufficient extensor carpi ulnaris and ruptured fibro-cartilaginous disk and yielded by shallow sigmoid notch and blunt anterior border. So muscular structures avoid dorsal displacement of the ulnar head actively whereas only fibro-osseous structures passively prevent from anterior displacement.