Gantzer’s Muscle (Accessory Head of Flexor Pollicis Longus)
Forearm & Wrist
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Gantzer’s muscle is like a ‘bonus thumb-flexor helper.’ It is clinically important because it may compress the anterior interosseous nerve, weakening the thumb and index-finger flexion needed for a strong ‘OK sign.’ A meta-analysis reported the accessory head of flexor pollicis longus in about 48% of upper limbs.
Function
Assists flexor pollicis longus in flexing the thumb, especially at the interphalangeal joint. Because it blends with FPL, its action supports thumb flexion and grip/pinch strength.
Origin
Variable. Common origins include the deep surface of flexor digitorum superficialis, the medial epicondyle of the humerus, the coronoid process of the ulna, or nearby proximal-forearm flexor structures.
Insertion
Usually blends into the flexor pollicis longus muscle belly or tendon, commonly along the medial/ulnar side of FPL.
Innervation
Most commonly the anterior interosseous nerve (a branch of the median nerve); some variants receive direct median-nerve supply.