Nerve Root Innervation of Extensor Indicis Proprius
The extensor indicis proprius muscle is innervated by the posterior interosseous nerve (a terminal branch of the radial nerve), which derives from the C7 and C8 nerve roots.
Anatomical Pathway
The innervation pathway follows a specific anatomical course:
The posterior interosseous nerve is the terminal motor branch of the radial nerve that provides motor innervation to the extensor carpi ulnaris and the extensors of the thumb and fingers, including the extensor indicis proprius 1.
The nerve supply to the extensor indicis proprius arises from the deep branch of the radial nerve (posterior interosseous nerve) after it emerges from the supinator muscle and sends branches to the superficial forearm extensors 2.
The extensor indicis proprius branch of the posterior interosseous nerve contains predominantly motor fibers and has been studied for its potential use in nerve transfer procedures 3.
Clinical Significance
Understanding the nerve root origin is essential for several clinical scenarios:
Posterior interosseous neuropathy presents with characteristic "finger drop" and partial "wrist drop" due to involvement of the extensor muscles, including the extensor indicis proprius 1.
The extensor indicis proprius muscle originates from the posterior surface of the ulna and adjacent interosseous membrane, inserting on the index finger, making it vulnerable to compression at specific anatomical sites like the arcade of Frohse and leash of Henry 4, 1.
Anatomical variations of the extensor indicis proprius occur in approximately 16% of cases, which can have implications for reconstructive hand surgery and tendon transfer procedures 4.
Common Pitfalls
Distinguishing posterior interosseous neuropathy from more proximal radial neuropathy requires careful clinical examination and electrodiagnostic studies, as the clinical presentations can overlap 1.
The extensor indicis proprius branch can be used for nerve transfer procedures to reconstruct the deep branch of the ulnar nerve or thenar branch of the median nerve, given its similar diameter and fascicular composition 3.