Abductor Pollicis Brevis

The abductor pollicis brevis (APB) is a superficial intrinsic muscle of the hand and a key component of the thenar eminence. It is primarily responsible for abducting the thumb at the carpometacarpal and metacarpophalangeal joints, playing a crucial role in thumb opposition, grip, and fine motor function.

Location

The APB is located on the lateral aspect of the palm, forming the most superficial and lateral part of the thenar eminence. It lies superficial to the flexor pollicis brevis and lateral to the opponens pollicis.

Structure

This short, flat, triangular muscle spans from the flexor retinaculum and carpal bones to the proximal phalanx of the thumb.

• Origin: Flexor retinaculum, scaphoid, and trapezium bones.
• Insertion: Lateral side of the base of the proximal phalanx of the thumb and the extensor expansion.
• Fiber Orientation: Vertical, running from proximal lateral palm to thumb base.

Function

The abductor pollicis brevis serves multiple functional roles:

• Thumb Abduction: Moves the thumb away from the palm in a perpendicular (sagittal) plane at the carpometacarpal and metacarpophalangeal joints.
• Assists in Opposition: Plays a supporting role in bringing the thumb toward the fingertips.
• Thumb Stabilization: Helps stabilize the thumb during gripping and pinching motions.

Innervation

The muscle is innervated by the recurrent branch of the median nerve (C8–T1).

Blood Supply

• Superficial palmar branch of the radial artery
• Branches from the superficial palmar arch

Relations

The APB lies superficial and lateral within the thenar eminence. It is bordered medially by the flexor pollicis brevis and deeper structures like the opponens pollicis. The palmar cutaneous branch of the median nerve lies superficial to it.

Surface Anatomy

The abductor pollicis brevis forms the most prominent surface bulge of the thenar eminence and becomes especially visible during active thumb abduction. It can be palpated just lateral to the base of the thumb.

Development

The APB originates from mesenchymal condensations of the thenar muscle mass during embryonic hand development. Its innervation by the median nerve reflects its anterior (preaxial) compartment origin.

Anatomical Variations

• May have variable insertion into the extensor hood of the thumb.
• May receive additional slips from adjacent thenar muscles or fascia.
• In rare cases, the muscle may be underdeveloped or absent, especially in congenital hand anomalies.

Clinical Significance

• Median Nerve Injury: Paralysis of the APB results in weakness of thumb abduction and impaired opposition (ape hand deformity).
• Tinel’s Sign: Tapping over the carpal tunnel may cause tingling over the APB region in carpal tunnel syndrome.
• Electromyography (EMG): Commonly used to assess APB function in nerve conduction studies for diagnosing carpal tunnel syndrome.

Comparative Anatomy

In non-human primates, the abductor pollicis brevis is less developed or absent, reflecting limited thumb mobility. In humans, its specialization allows for fine thumb movements that enable tool use, writing, and intricate manual tasks.