OVERVIEW
Other Name: Extensor indicis.
Muscle Test Group: Extensor digitorum communis, extensor indicis proprius, and extensor digiti minimi.
Key Points:
• Extensor indicis proprius is tested in a muscle group with extensor digitorum communis and extensor digiti minimi.
• Extends the index finger.
• Extrinsic muscle.
• In the fourth extensor compartment of the wrist; it has the most distal muscle belly of the all the tendons within the fourth extensor compartment.
• On the dorsum of the index metacarpal phalangeal joint, it likes ulnar to the extensor digitorum communis tendon to the index finger.
Tendon Transfers: This nerve is useful for tendon transfer procedures because it provides redundant function.
EXAMINATION
Extensor indicis proprius (EIP) is examined and tested with extensor digitorum communis (EDC) and extensor digiti minimi. EIP runs parallel with the tendon of the EDC to the index finger on the ulnar/lateral side and inserts into the extensor expansion of the index finger with EDC. Refer to extensor digitorum communis for examination.
Muscle Function: Extends the metacarpo-phalangeal joint. In conjunction with the lumbricalis and interossei, extends the interphalangeal joint of the index finger. May assist in adduction of the index finger.
Relevant Anatomy:
• In the fourth extensor compartment of the wrist; it has the most distal muscle belly of the all the tendons within the fourth extensor compartment.
• On the dorsum of the index metacarpal phalangeal joint, it likes ulnar to the extensor digitorum communis tendon to the index finger.
Innervation:
• Roots: C6, C7, C8.
• Nerve: Radial nerve, posterior interosseous nerve.
• Innervation Route: C6, C7, C8 → radial nerve → posterior interosseous nerve → extensor indicis proprius.
Origin: Posterior surface of the body of the ulna distal to the origin of the extensor pollicis longus and interosseous membrane.
Insertion: Into the extensor expansion of the index finger with the extensor digitorum communis tendon.