[The radio-ulnar unit. Its functional importance in fractures of both bones of the forearm].

The forearm has been composed of two bones since our very old ancestor Ichthyostega, which settled on dry land 300 million years ago. The fish fins, transformed into legs, already comprised two bones, which only became useful much later. This "frame", composed of 2 bones rotating around a longitudinal axis, is the most "practical" mechanical solution to allow longitudinal rotation of the distal extremity, but this rectangle must be pliable in a diagonal plane by means of the two conjoint radioulnar joints and a flexible hinge, the interosseous membrane. This system can only function when certain conditions are respected: strict alignment of the axes and simultaneous functioning of the two joints, maintenance of the "coordinated" shape of the two bones, an intact system of union of the two bones and coaptation of the joints. These anatomical and functional devices may be compromised by many pathological conditions, which is why the pronation-supination function of the forearm is so important and so vulnerable.