Both bones are most frequently fractured, generally as a result of direct violence, both bones breaking about the same level, in the middle or lower third. Displacement depends chiefly on the fracturing force, and may be in almost any direction. The upper fragments tend to be flexed by the biceps and brachialis anticus, and to be drawn together by the pronator radii teres, while the lower are drawn together by the pronator quadratus, and pulled up by the long flexor and extensor muscles. Both pronators tend generally to slightly pronate the radius. If the tendency to drawing together of the fragments be not corrected, cross union may result, and pronation and supination movements be lost. The use of interosseous pads for correction, however, is dangerous, and is apt to cause pain from pressure on the median nerve, and swelling, or even gangrene, from pressure on the vessels. Fracture of the radius alone is generally caused by indirect violence, as from falls on the hand ; that of the ulna alone by direct violence, as when the arm is raised to defend a blow. When the radius is broken aoove the insertion of the pronator radii teres, the upper fragment is fully supinated by the supinator brevis, and flexed by the biceps, which further assists the supination. The lower fragment is drawn toward the ulna, and pronated by the two pronators. As the upper fragment is small, the limb is generally best placed in flexion and full supination in treating this fracture. When the fracture occurs below the insertion of the pronator radii teres, the upper fragment takes up a position midway between pronation and supination, the pronators and supinators counteracting one another ; is flexed by the biceps, and drawn toward the ulna by the pronator radii teres. The lower fragment is drawn toward the ulna, and pronated by the pronator quadratus, while the supinator longus, attached to the styloid of the ulna, further tilts the upper end of the lower fragment toward the radius. Fracture of the ulna alone generally occurs in the lower segment of the bone, and displacement is generally slight.

The lower end of the radius is a favourite seat of myeloid sarcoma, which is sometimes treated, owing to its comparatively slight malignancy, by removal of the lower portion of the bone. This is perhaps best done through an incision along the radial sulcus. The lower end of the radius is the most massive part of the bone, is quadrilateral, and curved forwards. The carpal articular surface is triangular in outline, and is inclined down and forwards. Its outer portion articulates with the scaphoid, and the inner with the semilunar.

Colles' fracture occurs within inch of the lower articular surface, where the narrow compact shaft meets the wide and cancellous extremity. It is caused by indirect violence, generally from falls on the outstretched palm, and the displacement is caused by the fracturing force. Probably at the moment of impaction the forearm forms with the ground an angle of less than 6o degrees, and hence the force is borne by the lower end of the radius alone, which is broken off and driven backwards. Not merely is the lower fragment (a) displaced backwards ; it is also (b) rotated backwards, so that the articular surface looks backwards as well as downwards. This is due to the prominence of the posterior edge of the bone receiving an undue share of the shock, (c) Further, the lower fragment is rotated, so that it looks down and outwards, instead of down and inwards, owing to the prominence of the radial margin of the bone receiving a great portion of the shock from the thumb, while the triangular fibro-cartilage holds the ulnar border of the fragment in position. Thus the styloid of the radius comes to be on a level with, or even higher than, that of the ulna, and the hand is thrown markedly to the radial side. Partial impaction generally occurs, the upper fragment being driven into the lower, which may be splintered, and the internal lateral ligament of the wrist is frequently torn, while in some cases the ulnar styloid may be broken off. When the angle of impact is over 6o degrees, the force generally travels up the bones of the forearm, and either a sprain of the wrist or dislocation of the elbow occurs. In one or two cases, where the patient fell on the back of the hand, the position of the fragments has been reversed. Some hold that the displacement is due to muscular action- supinator longus, extensors of thumb and radial extensors- while some French authorities state that it is due to tearing of the bone by strain on the wrist ligaments. The epiphysis of the lower end of the radius, which includes the ulnar facet and insertion of the supinator longus, joins the shaft about the twentieth year, and is occasionally separated.

Amputation through the forearm should be performed as near the wrist as possible. In the lower part a circular, and in the upper part a modified circular, amputation, with equal antero-posterior skin-flaps, is best. After retracting the skin-flaps, the muscles, vessels, etc., are divided circularly, the interosseous membrane cut, and the bones sawn across at the same level after reflection of the periosteum. The periosteal flaps are then drawn over the cut ends, the radial, ulnar, and anterior interosseous vessels ligatured, and the wound stitched.