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Abductor Pollicis Brevis (APB)

OVERVIEW
Key Points:
• Abducts the thumb.
• Thenar eminence muscle (an intrinsic hand muscle).

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EXAMINATION
Muscle Function:

Primary:

• Palmar abduction of the thumb at the carpal-metacarpal and metacarpal-phalangeal joints.

Secondary:

• Opposition and interphalangeal joint extension of the thumb.
• May assist in flexion and medial rotation of the metacarpal-phalangeal joint.

Palpation: Palpate thenar musculature, the abductor pollicis brevis is the most superficial muscle and can be felt superficial and slightly more ulnar to the underlying opponens pollicis muscle.

Strength Testing: Position – the forearm in supination. Stabilize – the hand. Resist – thumb abduction at the proximal phalanx (in the direction of thumb adduction).

Or place the patient’s thumb in abduction and tell the patient not to move as the examiner tries to push the thumb down towards the palm.

Possible substitutions:  Opponens pollicis, abductor pollicis longus, flexor pollicis brevis.

Relevant Anatomy:
Innervation:

• Roots: C8, T1. 
• Nerve: Median nerve; thenar branch.
• Innervation Route:  C8, T1 → median nerve → thenar branch → abductor pollicis brevis branch.

Origin: Flexor retinaculum, tubercle of trapezium and tubercle of scaphoid bone.

Insertion: Splits into two tendon slips going to lateral base of proximal phalanx of thumb and extensor apparatus of thumb.