Palmaris Longus Muscle: Anatomy, Function, Imaging

Topic

Palmaris longus muscle

The palmaris longus is a slender, superficial flexor muscle of the forearm. It is highly variable, absent unilaterally or bilaterally in up to 15–25% of individuals. When present, it forms a long, thin tendon that becomes prominent in wrist flexion. Functionally, it tenses the palmar aponeurosis and assists in wrist flexion.

This muscle serves as an important surgical landmark and a popular tendon graft donor for reconstructive procedures such as tendon repair, ligament reconstruction, and ptosis correction. Its tendon lies superficially between the flexor carpi radialis and flexor carpi ulnaris, making it easy to identify clinically.

Synonyms

Long palmar muscle
Superficial wrist flexor of forearm

Origin, Course, and Insertion

Origin: Medial epicondyle of the humerus via the common flexor tendon.
Course: Descends superficially in the anterior compartment of the forearm, lying anterior to the flexor digitorum superficialis.
Insertion: Terminates as a long, slender tendon that passes anterior to the flexor retinaculum and inserts into the palmar aponeurosis. Occasionally, fibers blend with the thenar fascia.

Tendon Attachments

The tendon passes superficial to the flexor retinaculum (unlike the median nerve, which runs deep).
In some individuals, a bifid tendon or variant insertion into the flexor retinaculum occurs.
The tendon is often used in reconstructive tendon grafting due to its length and expendability.

Relations

Superficial: Skin and antebrachial fascia
Deep: Median nerve and flexor digitorum superficialis
Medially: Flexor carpi ulnaris tendon
Laterally: Flexor carpi radialis tendon
Distally: Palmar aponeurosis and flexor retinaculum

Nerve Supply

Median nerve (C7–C8)

Arterial Supply

Ulnar artery (via muscular branches)
Radial artery (via small perforating branches)
Anterior ulnar recurrent artery may contribute minor twigs

Venous Drainage

Superficial venous network of the forearm draining into the median cubital and basilic veins
Deep venous drainage via accompanying veins of the ulnar and radial arteries

Function

Flexion of the wrist: Assists flexor carpi radialis and ulnaris.
Tensing of the palmar aponeurosis: Increases grip and palmar tension.
Minimal role in finger motion but stabilizes wrist during forceful flexion.
Clinical landmark: Median nerve lies deep and slightly lateral to the tendon at the wrist.

Clinical Significance

Anatomical variation: May be absent, duplicated, reversed, or digastric.
Surgical donor: Commonly used in tendon grafting, ligament reconstruction (e.g., Tommy John surgery), and ptosis repair.
Entrapment site: Hypertrophy or anomalous insertion may compress the median nerve at the wrist.
Rupture or strain: Rare, but may occur in athletes or manual laborers.
Imaging role: Important to identify its presence, course, and relationship with the median nerve before surgical planning.

MRI Appearance

T1-weighted images:

Muscle belly: intermediate signal intensity, well delineated from surrounding fat.
Tendon: low signal (dark) linear band extending distally to the palmar aponeurosis.
Surrounding fat: bright, outlining the tendon clearly.
Absent muscle: replaced by fat or fibrous strand.

T2-weighted images:

Normal muscle: intermediate-to-low signal, slightly darker than on T1.
Tendon: very low signal (black line).
Pathology: partial tear or tendinopathy shows bright hyperintense focus within the tendon.
Edema or strain: focal or diffuse bright signal within muscle belly.

STIR:

Normal muscle: intermediate-to-dark signal.
Pathology: bright hyperintense regions indicating inflammation or strain.
Clear differentiation from peritendinous fat and fascia.

Proton Density Fat-Saturated (PD FS):

Normal tendon: dark continuous band.
Pathologic tendon: bright intratendinous signal in partial tear or tendinitis.
Muscle: intermediate-to-dark, with bright foci in areas of edema or trauma.

T1 Fat-Sat Post-Contrast:

Normal muscle: mild homogeneous enhancement.
Tendinitis or tenosynovitis: focal or circumferential enhancement around tendon sheath.
Post-surgical scar: peripheral enhancement with central low-signal fibrosis.

CT Appearance

Non-Contrast CT:

Muscle: soft-tissue density, homogenous attenuation.
Tendon: linear dense structure anterior to flexor retinaculum.
Chronic calcification or ossification in old tendinopathy may appear as small dense foci.

Post-Contrast CT (standard):

Muscle enhances homogeneously.
Inflamed peritendinous tissues may show enhancement.
Helpful for detecting postoperative changes, foreign body, or calcific tendinitis.

MRI images