Purpose of the study: Investigate the longitudinal stability and instability of the forearm.

Material and methods: The interosseous membranes of 30 formol-treated forearms were dissected under 4-fold magnification and translumination. The radial and ulnar heads wee resected to eliminate the two radioulnar articulations before performing sequential cuts to identify the different networks of the interosseous membrane. The ulna was maintained in a fixed position allowing proximal and distal displacement of the radius. We studied the medial border of 100 radii, noting the bony eminences and their relations with the configuration of the membrane.

Cases of neglected fractures of the radial cup without injury to the distal radioulnar joint and cases of polyarthritis treated by double resection radioulnar and Sauvé-Kapandji) presenting good longitudinal stability were studied.

The possiblity of using the extensor indicis for primary reinforcement of the interosseous membrane was studied on cadaver specimens. Applied to an acute case of Essex-Lopresti syndrome, this original technique provided good intraoperative stability.

Results: The fibers of the interosseous membrane design two planes, an anterior and a posterior plane. The anterior fibrrs descend distally and medially from the radius. They can be divided into proximal (horizontal) descending fibers, intermediary (short oblique) descending fibers, and distal (long oblique) descending fibers. The posterior fibers rise proximally and medially from the radius to reach the ulna. They form two planes: proximal ascending (short oblique) fibers and distal (long oblique) ascending fibers which are inconstant. These planes are in relation with the origin of the extrinsic wrist extensors.

The main fibers are: the intermediary descending fibers and the proximal ascending fibers. They insert on the interosseous tubercle of the radius, a constant eminence situated on the medial border of the radius 8.4 cm from the elbow.

The thre groups of descending fibers limit proximal translation of the radius. The proximal fibers can limit excessive distal translation. The proximal and distal ascending fibers limit distal translation of the radius. A full thicknes tear of the anterior plane is necessary to achieve proximal displacement. Longitudinal stability is maintained in neglected fractures of the distal without rupture of the interosseous membrane and in operated polyarthritis with resection of the two radioulnar joints.

Conclusion: The fibers of the interosseous membrane describe two planes where the fibers run in opposing directions. Each plane limits radial displacement in a different direction. The ideal reconstruction would restore the two planes, but it is essential to reconstruct at least the intermediary descending fibers and the proximal ascending fibers. However, in routine practice, translation of the radius is generally proximal, so reconstruction of the middle segment appears to be sufficient if it associated with a reinforcement transfer of the extensor indicis to the proximal radius.