The pronator quadratus in motions and in stabilization of the radius and ulna at the distal radioulnar joint.

Based upon the findings of dissections of 15 forearms and clinical observations in patients with disturbed architecture of the distal radioulnar joint, the pronator quadratus was found to be composed of two heads. A superficial head originated from the ulna and passed transversely to its insertion into the radius. It averaged 5.1 cm. in length, 4.5 cm. in width, 0.2 cm. in thickness, with a cross-sectional area of 0.95 cm.2 and a contractile volume of 2.6 cm.3. The deep head ran obliquely from a more proximal origin on the ulna to a distal insertion on the radius, with an average length of 4.0 cm., average width of 3.2 cm., and a thickness of 0.4 cm. Its average cross-sectional area was 1.64 cm.2 and its contractile volume 2.5 cm.3. A group of fibers occasionally was noted deep to both heads, running at right angles to them and paralleling the direction of the fibers of the interosseous membrane. The superficial head initiates pronation while the deep head coapts the joint surfaces and stabilizes the joint.