Topic
The abductor digiti minimi (ADM) is a superficial muscle located along the ulnar border of the palm in the hypothenar eminence of the hand. It is the most medial of the hypothenar muscles and lies superficial to the flexor digiti minimi brevis and opponens digiti minimi. The ADM plays a key role in abducting the little finger (fifth digit) away from the ring finger and contributes to grip and hand stability. It also assists in flexion of the metacarpophalangeal joint and extension of the interphalangeal joints through its connection with the extensor expansion.
Synonyms
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Abductor of the little finger
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Abductor minimi digiti
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Abductor digiti quinti
Origin, Course, and Insertion
Origin: From the pisiform bone, the pisohamate ligament, and the tendon of the flexor carpi ulnaris.
Course: Muscle fibers run distally along the ulnar side of the hand, forming a flat tendon near the base of the little finger.
Insertion: Ulnar side of the base of the proximal phalanx of the fifth digit and the extensor expansion of the same finger.
Tendon Attachments
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The tendon passes along the medial side of the fifth metacarpal and inserts into both the base of the proximal phalanx and dorsal digital expansion.
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It may send a slip to the extensor digiti minimi tendon, facilitating combined extension and abduction.
Relations
Superficial: Palmar fascia and skin of the hypothenar eminence
Deep: Flexor digiti minimi brevis and opponens digiti minimi
Medially: Ulnar border of the hand
Laterally: Flexor tendons of the little finger
Proximally: Pisiform and ulnar artery and nerve branches
Distally: Proximal phalanx and extensor expansion of the little finger
Nerve Supply
Deep branch of the ulnar nerve (C8, T1)
Arterial Supply
Ulnar artery, via its deep palmar branch and ulnar digital artery to the little finger
Function
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Abduction: Moves the little finger away from the ring finger at the metacarpophalangeal joint.
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Flexion: Assists in flexing the proximal phalanx of the fifth digit.
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Extension: Through the extensor expansion, aids in extending the distal phalanges.
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Grip assistance: Contributes to hypothenar support and cupping of the hand during grasp.
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Stabilization: Maintains the ulnar border of the palm during object manipulation.
Clinical Significance
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Ulnar nerve injury: Paralysis or weakness of ADM leads to loss of little finger abduction and atrophy of the hypothenar eminence.
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Muscle hypertrophy or fibrotic bands: May compress the ulnar nerve in Guyon’s canal.
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Tendon tears or strain: Rare but can occur in repetitive hand use or trauma.
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Surgical relevance: Important landmark in ulnar nerve decompression and hypothenar flap surgeries.
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Imaging importance: MRI evaluates ADM for denervation changes, trauma, and space-occupying lesions in the hypothenar region.
MRI Appearance
T1-weighted images:
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Normal muscle: intermediate signal intensity with distinct fascicular pattern.
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Tendon: low signal (dark) extending to proximal phalanx of the little finger.
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Fatty tissue of hypothenar region: bright, providing contrast with muscle.
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Chronic denervation: increased intramuscular fat causing hyperintense signal.
T2-weighted images:
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Normal muscle: intermediate-to-dark signal, slightly darker than on T1.
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Tendon: uniformly dark.
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Pathology: edema or inflammation produces bright hyperintense signal in acute injury or myositis.
STIR:
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Normal muscle: intermediate-to-dark signal.
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Pathology: bright hyperintense signal indicating edema, strain, or nerve-related denervation changes.
Proton Density Fat-Saturated (PD FS):
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Normal ADM: intermediate-to-dark signal with smooth margins.
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Muscle strain or tendinitis: bright focal hyperintensity within or around tendon.
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Excellent for identifying subtle peritendinous edema or fascial inflammation.
T1 Fat-Sat Post-Contrast:
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Normal muscle: mild homogeneous enhancement.
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Active inflammation or tear: focal enhancement at musculotendinous junction.
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Denervation or chronic fibrosis: little to no enhancement with volume loss and fatty infiltration.
CT Appearance
Non-Contrast CT:
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Muscle: soft-tissue density, well defined in the hypothenar region.
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Tendon: linear low-density structure extending to the base of the little finger.
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Calcification or chronic scarring may appear as localized high-density foci.
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Useful for assessing bony attachment sites at the pisiform and proximal phalanx.
Post-Contrast CT (standard):
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Normal muscle: homogeneous mild enhancement.
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Inflamed or injured muscle: increased enhancement and surrounding soft-tissue edema.
MRI image
MRI image
MRI image
CT image
