1. The pathology of rupture of the ulnar collateral ligament of the thumb (gamekeeper's thumb) has been presented. 2. A new procedure which provides dynamic stabilization of the metacarpophalangeal joint of the thumb in the function of pinch by advancement of the insertion of the adductor pollicis has been described. 3. It has been emphasized that arthrodesis rather than adductor advancement is indicated in the presence of arthrosis.