Palmaris Longus

The palmaris longus is a superficial flexor muscle of the forearm, notable for its anatomical variability. It plays a minor role in wrist flexion and tensing the palmar aponeurosis. Interestingly, it is absent in about 10–20% of individuals, either unilaterally or bilaterally, without significant functional deficit.

Location

The palmaris longus is located in the anterior (flexor) compartment of the forearm. It lies superficial to the flexor digitorum superficialis and medial to the flexor carpi radialis.

Structure

The muscle has a relatively short belly and a long, slender tendon that extends distally to the palm.

• Origin: Medial epicondyle of the humerus via the common flexor tendon.
• Insertion: Palmar aponeurosis and flexor retinaculum of the hand.
• Shape: Fusiform muscle with a long tendon, often visible on resisted wrist flexion in individuals who possess it.
• Fiber Direction: Longitudinally oriented from origin to tendon.

Function

Although functionally minor, the palmaris longus contributes to:

• Wrist Flexion: Assists in flexing the wrist, though weak compared to other wrist flexors.
• Palmar Fascia Tension: Tenses the palmar aponeurosis, aiding grip strength and stabilization of the palm during object handling.

Innervation

The palmaris longus is innervated by the median nerve (C7–C8), which passes deep to the muscle as it enters the hand.

Blood Supply

• Branches of the ulnar artery and radial artery provide vascular supply to the muscle.

Anatomical Variations

The palmaris longus is one of the most variable muscles in the human body. Variations include:

• Absence: Most commonly absent muscle in the forearm.
• Duplicated muscle: Rarely, there may be two palmaris longus muscles.
• Reversed muscle: Muscle belly distally located with a proximal tendon.
• Accessory slips: May insert into nearby tendons, fascia, or even the thenar muscles.

Clinical Significance

• Tendon Grafting: Commonly harvested for tendon reconstruction surgeries (e.g., Tommy John surgery, ligament repair) due to its expendable nature and long tendon.
• Carpal Tunnel Syndrome: Although the palmaris longus does not pass through the carpal tunnel, its tendon may sometimes contribute to compression in cases of anatomical variation.
• Cosmetic Use: Prominence of the tendon is often used to demonstrate wrist anatomy or in medical training for locating the median nerve.

Surface Anatomy

The tendon of the palmaris longus can be seen or palpated by touching the tips of the thumb and little finger together and flexing the wrist. It is typically the most superficial and central tendon in the anterior wrist region.

Embryological Development

The palmaris longus arises from mesodermal tissue during limb bud development. Its high rate of agenesis is thought to reflect evolutionary regression, as it is more functional in non-human primates.

Associated Tests

Several clinical tests can check for the presence of the palmaris longus:

• Schaeffer's Test: Opposition of the thumb and little finger with wrist flexion makes the tendon prominent if present.
• Mishra's Tests I & II: Detect partial agenesis or assess insertion sites.