This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
When the displacement is posterior the lower end of the upper fragment is tilted backward by the contraction of the triceps muscle. This causes a marked projection on the back of the forearm below the elbow (Fig. 335).
In treating this injury the forearm should be placed in at least partial extension (complete extension is usually not necessary) so as to relax the triceps muscle.
When a person receives a blow in the region of the junction of the upper and middle thirds of the ulna on its posterior surface the fragments are pushed forward and an angular deformity is produced, the apex of the angle pointing toward the anterior surface. The force of the blow is not expended entirely on the ulna but, having broken it, continues and pushes or dislocates the radius forward (Fig. 336).
In these injuries the fracture of the ulna is readily recognized, but the dislocation of the head of the radius is often overlooked. If the dislocation is not reduced subsequent flexion of the elbow will not be possible much if any beyond a right angle. The contraction of the biceps not only favors this luxation by pulling the radius forward but tends to cause it to recur after replacement.
Reduction is to be attempted by supinating and flexing the forearm to relax the biceps and making direct pressure anteroposteriorly on the radius to force the head back into place. The radius may be kept in place by dressing the arm with the elbow in a position of complete flexion.
Fig. 336. - Fracture of the upper third of the ulna, with anterior angular displacement of the fragments and anterior dislocation of the head of the radius.