The adductor pollicis is the deepest thenar muscle and the primary provider of key pinch force, generating the powerful adduction grip between the thumb pad and the lateral index finger. It is the largest thenar muscle and the only thenar muscle innervated by the ulnar nerve rather than the median nerve. Its weakness produces the classic Froment sign, where the thumb IP joint hyperflexes using the FPL (median nerve) to substitute for lost adductor pollicis key pinch.
| Origin | Base of the second and third metacarpals, capitate, and adjacent carpal bones; Palmar surface of the third metacarpal shaft |
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| Insertion | Ulnar side of the base of the thumb proximal phalanx and the ulnar sesamoid |
| Nerve Supply | Deep branch of the ulnar nerve (C8, T1) |
| Blood Supply | Deep palmar arch |
| Actions | Adduction of the thumb toward the palm; Assists in thumb MCP flexion; Provides the primary force for key pinch between thumb and index finger lateral surface |
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Key pinch between thumb and index lateral surface, used for turning keys, holding cards, and any lateral grip task, is primarily generated by the adductor pollicis. Its ulnar nerve innervation means it is the thenar muscle spared in pure median nerve palsy but lost in ulnar palsy.
The Froment sign tests adductor pollicis function by asking the patient to hold a piece of paper between thumb and index finger while the examiner pulls it away. In ulnar nerve palsy the patient cannot generate key pinch force and compensates by hyperflexing the thumb IP using the intact FPL (median nerve), producing the characteristic bent thumb posture. This is one of the most specific clinical signs in peripheral nerve examination.
The adductor pollicis is palpable in the first web space deep to the first dorsal interosseous, becoming firm during forceful thumb adduction toward the palm.
Ulnar nerve palsy producing inability to generate key pinch force without thumb IP compensation (Froment sign), resulting in significantly reduced grip strength for lateral pinch tasks.
Ulnar collateral ligament injury at the thumb MCP joint from valgus stress, producing instability that mimics adductor pollicis weakness because the adductor aponeurosis may be interposed between the torn ligament ends (Stener lesion).
