Topic Overview

✋ Intrinsic Muscles of the Hand

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Overview

• The intrinsic muscles are those confined entirely within the hand, acting on digits for fine motor control.

• They allow precision grip, opposition, and delicate movements of the thumb and fingers.

• Classified into five groups:

  1. Thenar muscles (thumb)

  2. Hypothenar muscles (little finger)

  3. Lumbricals

  4. Interossei (palmar and dorsal)

  5. Adductor pollicis

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1. Thenar Muscles

👉 Mnemonic: “All Oppose, Flex, and Abduct the Thumb”

A. Abductor Pollicis Brevis

• Origin: Flexor retinaculum, scaphoid, and trapezium.

• Insertion: Base of proximal phalanx of thumb.

• Nerve Supply: Recurrent branch of median nerve (C8, T1).

• Action: Abducts thumb at CMC and MCP joints.

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B. Flexor Pollicis Brevis

• Origin: Flexor retinaculum and trapezium.

• Insertion: Base of proximal phalanx of thumb.

• Nerve Supply: Superficial head → Median nerve; Deep head → Ulnar nerve.

• Action: Flexes thumb at MCP joint.

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C. Opponens Pollicis

• Origin: Flexor retinaculum and trapezium.

• Insertion: Lateral border and shaft of 1st metacarpal.

• Nerve Supply: Median nerve.

• Action: Opposition of thumb – brings tip across to meet fingers.

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D. Adductor Pollicis

• Origin:

  • Oblique head → Bases of 2nd & 3rd metacarpals, capitate.

  • Transverse head → Shaft of 3rd metacarpal.

• Insertion: Medial base of proximal phalanx of thumb.

• Nerve Supply: Deep branch of ulnar nerve.

• Action: Adducts thumb toward palm.

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2. Hypothenar Muscles

👉 Mnemonic: “All Little Fingers Act Opposite to Thumb”

A. Abductor Digiti Minimi

• Origin: Pisiform bone.

• Insertion: Base of proximal phalanx of little finger.

• Nerve Supply: Deep branch of ulnar nerve.

• Action: Abducts little finger.

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B. Flexor Digiti Minimi Brevis

• Origin: Hook of hamate and flexor retinaculum.

• Insertion: Base of proximal phalanx of little finger.

• Nerve Supply: Deep branch of ulnar nerve.

• Action: Flexes little finger at MCP joint.

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C. Opponens Digiti Minimi

• Origin: Hook of hamate and flexor retinaculum.

• Insertion: Medial border of 5th metacarpal.

• Nerve Supply: Deep branch of ulnar nerve.

• Action: Draws 5th metacarpal anteriorly → opposes little finger toward thumb.

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3. Lumbricals

Origin

• From tendons of flexor digitorum profundus.

  • Lateral two (1st and 2nd): From lateral two tendons (unipennate).

  • Medial two (3rd and 4th): From medial two tendons (bipennate).

Insertion

• Lateral side of extensor expansion of corresponding fingers.

Nerve Supply

• 1st and 2nd → Median nerve.

• 3rd and 4th → Deep branch of ulnar nerve.

Action

• Flex MCP joints and extend IP joints of fingers (writing position).

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4. Interossei

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A. Palmar Interossei (3 muscles)

👉 Mnemonic: “PAD – Palmar ADduct”

• Origin: From shafts of 2nd, 4th, and 5th metacarpals.

• Insertion: Bases of proximal phalanges and extensor expansions of same digits.

• Nerve Supply: Deep branch of ulnar nerve.

• Action: Adduct fingers toward midline (3rd finger).

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B. Dorsal Interossei (4 muscles)

👉 Mnemonic: “DAB – Dorsal ABduct”

• Origin: From adjacent sides of metacarpals.

• Insertion: Bases of proximal phalanges and extensor expansions of 2nd–4th digits.

• Nerve Supply: Deep branch of ulnar nerve.

• Action: Abduct fingers from midline (3rd finger).

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Testing of Some Intrinsic Muscles

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1. Abductor Pollicis Brevis

• Ask patient to lift thumb perpendicular to palm (as in hitchhiking).

• Weakness → median nerve lesion (carpal tunnel).

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2. Adductor Pollicis

• Place a paper between thumb and index; patient asked to hold it while examiner pulls.

• If thumb flexes at IP joint → Froment’s sign positive → ulnar nerve palsy.

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3. Lumbricals

• Ask patient to flex MCP joints while keeping IP joints extended (“writing position”).

• Inability → ulnar or median nerve lesion.

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4. Interossei

• Palmar interossei test: Ask to hold a paper between fingers → slipping indicates paralysis.

• Dorsal interossei test: Ask to spread fingers apart and resist pressure.

• Weakness → ulnar nerve injury.

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5. Opponens Pollicis

• Ask patient to touch thumb to little finger tip.

• Inability → median nerve injury (ape-hand deformity).

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Dissection of Intrinsic Muscles of Hand

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Steps

1. Reflect the palmar aponeurosis after exposing flexor tendons.

2. Identify thenar muscles on radial side:

   • Abductor pollicis brevis (superficial)

   • Flexor pollicis brevis

   • Opponens pollicis (deep)

   • Adductor pollicis (deep, near 1st dorsal interosseous)

3. On ulnar side, identify hypothenar muscles:

   • Abductor digiti minimi (superficial)

   • Flexor digiti minimi brevis

   • Opponens digiti minimi (deep)

4. Expose lumbricals arising from FDP tendons and entering extensor expansions.

5. Deep to these, reveal palmar and dorsal interossei between metacarpals.

6. Preserve ulnar and median nerves, and superficial/deep palmar arches.