THE FOREARM AND WRIST 77 



distal to the tip of the styloid process of the ulna between the 

 extensor and flexor carpi ulnaris tendons. 



Colles' Fracture occurs from a fall on the outstretched 

 hand. According to Chiene, the forearm is at an angle of less 

 than 60 to the horizontal when the palm of the hand strikes the 

 ground. The line of force cuts through the distal end of the 

 radius instead of passing along the bone, as it does if the forearm 

 is at an angle of more than 60 to the ground when the injury 

 occurs. Impaction of the proximal fragment into the distal 

 is not uncommon, as the fracture is transverse and occurs 

 through the spongy tissue of the distal end of the bone. The 

 line of force drives the distal fragment backwards and tends 

 to thrust it proximally, but the medial edge of the distal end 

 of the radius is attached to the ulna by the triangular articular 

 disc (p. 75), which slips backwards over the head of the latter 

 bone. The styloid process of the radius swings round the arc 

 of a circle whose radius is the length of the triangular articular 

 disc, together with the width of the lower end of the radius. 

 In this way the hand is deviated to the radial side, and the 

 displacement becomes three-fold. When the styloid processes 

 are compared (p. 66), they are found to be on the same level or 

 the radial is the more proximal. 



If the ulnar styloid also is fractured, then the displacement 

 is backwards and upwards, but the upward displacement is 

 proportionately greater than in the typical Colles' fracture. 

 Smith's Fracture (reversed Colles') occurs from a fall on the 

 back of the hand, the wrist being flexed. The line of force cuts 

 through the anterior surface of the distal end of the radius and 

 the distal fragment is thrust forwards and proximally. The 

 hand is deviated to the radial side as in Colles' fracture and 

 for the same reason. 



Ossification of the Distal Ends of the Radius and Ulna. 

 Secondary centres for the distal ends of the Radius and Ulna 

 appear about the third and sixth years, respectively. The 

 distal epiphyses of both bones include the styloid processes 

 and the articular areas. Growth continues longer in the wrist 

 region than it does at the elbow, and the distal epiphyses do 

 not unite till between twenty and twenty-four. 



Separation of the distal epiphyses of the radius and ulna 

 may occur between the ages of seven and twenty. Owing to 

 the line of attachment of the capsule of the wrist (p. 76), this 

 injury does not open into the joint. 



