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Flexor Pollicis LongusEdit

The flexor pollicis longus (FPL) is a key extrinsic muscle of the forearm that enables precise control of the thumb. It functions mainly to flex the distal phalanx of the thumb, a motion essential for tip-to-tip pinch and fine manipulation, such as gripping small objects or threading a needle. The muscle is part of the deep flexor compartment of the forearm and the tendon courses into the hand through the carpal tunnel to reach its distal insertion on the thumb. Its proper function complements the intrinsic thenar muscles and the flexor pollicis brevis in producing coordinated thumb movements.

Anatomy

Origin and insertion

The FPL originates from the anterior surface of the radius and the adjacent interosseous membrane, with occasional minor contributions from neighboring structures in the forearm. The tendon travels distally, passes through the carpal tunnel, and inserts on the base of the distal phalanx of the pollex (thumb). This arrangement places the tendon in close relationship with other long finger flexors within the first flexor compartment of the wrist. For reference, the thumb is also referred to as the pollex in anatomical terminology pollex.

Innervation

The muscle is typically innervated by the anterior interosseous nerve (AIN), a branch of the median nerve. The AIN supplies the FPL as well as several other deep forearm flexors, reflecting a shared motor supply pattern for this muscle group. Lesions affecting the AIN can impair FPL function and manifest in characteristic motor signs.

Blood supply

The FPL receives blood through the anterior interosseous artery, which accompanies the nerve through the forearm. This vascular supply supports the muscle during high-demand or repetitive thumb flexion tasks.

Biomechanics and motion

As the primary muscle responsible for flexing the distal phalanx of the thumb, the FPL plays a central role in precision grip. It works in concert with intrinsic thumb muscles such as the opponens pollicis and the abductor pollicis brevis to position the thumb for grasping and manipulating small items. The FPL’s action also contributes to flexion at the metacarpophalangeal joint indirectly, by enabling stable contact with objects during pinching and opposition.

Function and clinical relevance

Normal function

In healthy individuals, the FPL enables tip-to-tip pinch and fine thumb movements. Its action is especially important when the thumb must oppose the index finger or participate in delicate manual tasks, such as writing or manipulating small hardware.

Clinical significance

  • Anterior interosseous syndrome (AINS) is a classic example of a nerve-related impairment affecting the FPL. Compression or injury to the anterior interosseous nerve can weaken thumb flexion at the distal phalanx, leading to difficulty with precision pinch and an “OK” sign test that fails to bring the tips of the thumb and index finger together crisply.
  • Tendinous and pulley pathologies in the first flexor compartment, including stenosing tenosynovitis at the first annular pulley (A1 pulley), can involve the FPL tendon and contribute to thumb pain and reduced motion. This is often discussed alongside related conditions such as trigger thumb, where tendon thickening or pulley constriction causes catching or locking of the thumb.
  • Traumatic injuries to the forearm, elbow, or proximal radius can, in some cases, affect FPL function indirectly by injuring the nerve supply or the tendon’s trajectory, underscoring the integrated nature of forearm anatomy in hand movement.
  • Imaging and diagnosis: Ultrasound and MRI are commonly used to assess FPL integrity and tendon pathology, especially when symptoms suggest a deep forearm or wrist origin. These modalities help differentiate tendonitis, trigger phenomena, and nerve-related weakness.

Variants and evolutionary context

Anatomical variation in the FPL is relatively uncommon but well documented. In some individuals, the FPL may show variant insertions or accessory slips that alter its moment arm or interactions with adjacent tendons. Rarely, the nerve supply may diverge from the typical anterior interosseous pattern, though such variants are not the norm.

From an evolutionary perspective, the forearm flexors, including the FPL, illustrate the specialization that supports dexterous hand function in humans. The coordination between extrinsic forearm muscles and intrinsic hand muscles has contributed to the fine motor abilities that distinguish human manual dexterity.

See also