1. The normal thumb metacarpophalangeal joint varies greatly in range of motion and stability as evidenced by a clinical study of one thousand hands. Arthrodesis of the joint should be avoided if possible; but if necessary, the absence of motion does not limit thumb function greatly.

2. One hundred and forty-five cases of soft-tissue injury of the thumb metacarpophalangeal joint have been reviewed. These included twenty-six dorsal dislocations, twenty-seven radial or ulnar injuries with complete medial or lateral instability, and ninety-two sprains with slight instability.

3. The pathological findings in the totally unstable joints were variable. Plaster immobilization when maintained for four weeks produced satisfactory results in many of these injuries. Immediate or early surgery is not indicated until the results of a trial with immobilization in a plaster cast have been carefully evaluated. Surgical treatment should then be considered if there is pain and functional impairment.

4. Dorsal dislocation of the thumb metacarpophalangeal joint seldom results in instability. This observation suggests that the collateral ligaments are relaxed in extension and are stretched or partially torn and not avulsed in most instances.

5. Stability of the thumb metacarpophalangeal joint can be determined by clinical and roentgenographic stress testing when both thumbs are compared.