The abductor digiti minimi is the most medial hypothenar muscle, forming the visible ulnar border of the hand and abducting the little finger away from the ring finger. It is the first hypothenar muscle encountered in ulnar nerve assessment and its weakness is an early sign of deep ulnar nerve palsy. In pianists and guitarists who repeatedly strike or press the little finger, ADM hypertrophy can cause visible hypothenar asymmetry.
| Origin | Pisiform bone and pisohamate ligament |
|---|---|
| Insertion | Ulnar side of the base of the proximal phalanx of the little finger |
| Nerve Supply | Deep branch of the ulnar nerve (C8, T1) |
| Blood Supply | Ulnar artery |
| Actions | Abduction of the little finger; Assists in flexion of the little finger MCP joint; Assists in opposition of the little finger |
|---|
Spreading the little finger away from the ring finger during grasping wide objects and framing the medial hand border, it also contributes to the little finger opposition movement used in power grip.
ADM weakness and hypothenar wasting are among the first signs of ulnar nerve palsy at the wrist level. A positive Froment sign (thumb IP flexion substitution during key pinch from adductor pollicis weakness) combined with ADM wasting and interosseous muscle wasting confirms severe ulnar nerve compromise requiring surgical assessment. The ADM is also tested to localise an ulnar nerve lesion between the elbow and wrist.
The ADM is palpable as the distinct rounded muscle bulk forming the ulnar border of the hand, becoming firm during little finger abduction against resistance.
ADM and hypothenar muscle atrophy from ulnar nerve palsy producing visible medial hand flattening, little finger weakness, and interosseous muscle wasting indicating advanced ulnar nerve compression requiring surgical decompression or repair.
