254 SURGICAL APPLIED ANATOMY. [Chap. xv. 



the arm, and the usual result is either a severe sprain 

 of the wrist, or a dislocation of the bones of the fore- 

 arm backwards at the elbow joint, (b) The carpal 

 surface of the radius slopes forwards, and therefore 

 the posterior edge of the bone receives the greater 

 part of the shock ; there is, as a result, rotation of the 

 lower fragment backwards on the transverse diameter 

 of the fore-arm, (c) The carpal surface of the radius 

 slopes downwards and outwards to the radial edge of 

 the arm ; therefore the radial edge of the bone receives 

 the principal part of the shock through the ball of the 

 thumb. As a result, this edge of the lower fragment 

 is displaced upwards to a greater extent than the 

 ulnar edge of the fragment, which remains firmly 

 attached to the ulna by the triangular ligament." 

 This rotation also depends upon, the integrity of the 

 inferior radio-ulnar ligament in a typical Colles' frac- 

 ture. These ligaments hold on to the ulnar part of 

 the lower fragment, and prevent its being displaced 

 to so great an extent as is the radial part of the frag- 

 ment. By means of this rotatory displacement, the 

 tips of the two styloid processes come to occupy the 

 same level, or the radial process may even mount above 

 the ulnar. In nearly every case there is some pene- 

 tration of the fragments, the compact tissue on the 

 dorsal aspect of the upper fragment being driven (by 

 a continuance of the force that broke the bone), into 

 the cancellous tissue on the palmar aspect of the lower 

 fragment. It is only in very rare instances that the 

 fragments are so separated as to ride the one over the 

 other. In such cases the radio-ulnar ligaments are 

 probably ruptured, and the wrist ceases to present the 

 typical deformity of a Colles' fracture. That the 

 deformity in Colles' fracture is due to the nature and 

 direction of the force is indirectly proved by an 

 isolated case or so, where the patient fell upon the 

 back of the hand, with the result that the radius was 



