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.
The joint between the lower ends of the radius and ulna embraces not only the portion between these two bones but also that between the lower end of the ulna and the upper surface of the triangular fibrocartilage. This latter is attached by its apex to a depression on the outer side of the root of the styloid process of the ulna, and by its base to the rough line on the radius separating the radio-ulnar from the radiocarpal articulation (Fig. 341).
Fig. 341. - The wrist-joint and inferior radioulnar articulation.
This serves as the main bond of union between the lower ends of the radius and ulna. It is strong and blends with the internal lateral ligament. Thus the hand has an attachment to the inner side of the radius by means of the internal lateral ligament and triangular cartilage.
The capsular ligament serves to retain the synovial fluid in the joint. It is thin and filmy and possesses no strength, and therefore is useless in limiting movements.
These ligaments are simply a few bands which pass across from the radius to the ulna. They are not strong enough to be efficient in limiting movements of the bones.
As has already been pointed out (page 304) the movements of pronation and supination have as their axis a line drawn through the middle of the head of the radius, the styloid process of the ulna, and the ring finger. They embrace in ordinary use a range of about 140 degrees which can be increased by forced effort to 160 degrees (Fig. 342).
These movements are limited by various factors, the most prominent being in pronation the contact of the soft parts and bones, as the radius obliquely overlies the ulna, and in supination by the biceps (the most powerful of the supinators) having reached the dead centre.
There is no communication between the radio-ulnar joint above and the radiocarpal joint below, except when, as occasionally happens, the triangular cartilage has a perforation.
During pronation and supination the lower end of the radius moves with the hand, but the lower end of the ulna remains at rest: hence it is that the styloid process of the radius always retains the same position in relation to the hand. When it is desired to identify the styloid process of the radius, one needs only to follow the metacarpal bone of the thumb up to the snuff-box at the upper edge of which the styloid process can always be felt. Also, to identify the styloid process of the ulna, one must not use the hand as a guide because the hand changes its position in relation to the ulna; but, as the ulna remains quiet, its styloid process can be found by following the posterior surface down to its extremity.
As the interarticular triangular cartilage is fastened by its base to the ulnar edge of the radius and by its apex to the base of the styloid process of the ulna, it travels with the hand in the movements of pronation and supination.
Fig.342. - Axis of rotation.