joints.) Innervation All interosseous muscles of the hand are innervated by the deep branch of the ulnar nerve. Function The dorsal interossei abduct the index, middle, and ring fingers. The first dorsal interosseous is also able to rotate the index finger slightly at the metacarpophalangeal joint and assist adductor pollicis in thumb adduction. Metacarpophalangeal joint flexion All interossei pass dorsal to the transverse metacarpal ligament but slightly volar to the flexion-extension axes of rotation of the MP joints. In effect, their ability to flex at the MP joints will depend on the position of the MP joints: When the MP joints are extended, all interossei pass through the flexion-extension axes of the MP joints and their contribution there is therefore negligible, though they still play important roles as joint stabilizers (i.e. preventing MP hyperextension). At the same time, the interossei (and abductor digiti minimi) lie relatively far from the anterior-posterior axis of the MP joints and are consequently effective abductors and adductors during MP extension. Because the dorsal interossei are predominantly in the proximal group they are more effective at the MP joints than the palmar interossei and, therefore, abduction is stronger than adduction at the MP joints. When the MP joints are being flexed, the position of the interossei moves away volarly from the flexion-extension axes of the MP joints until they are nearly perpendicular to the proximal phalanx. At the same time, the increasingly taut collateral ligaments of the MP joints cancel out the abduction-adduction component to increase the force of flexion (resulting in a strong grip). At full flexion, the transverse metacarpal ligament restricts the interossei. Interphalangeal joint extension When the MP joints are extended, effective IP joint extension can be achieved by all interossei in the distal group (i.e. all except the two outermost muscles, the first interosseus and abductor digiti minimi) because they are attached directly to the extension mechanism. This IP extension is therefore stronger than MP abduction/adduction which is produced by continued action except for the index and little fingers. When the MP joints are flexed, the transverse metacarpal ligament enhances the function of the distal interossei by acting as a pulley and preventing them from becoming slack, further increasing the effectiveness of IP extension. Clinical relevance First dorsal interosseous compartment syndrome Compartment syndrome rarely occurs in the first dorsal interosseous compartment of hand. This condition is caused by excessive use of hand,