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Palmaris Longus Muscle Anatomy

The palmaris longus, a small fusiform muscle, is absent on one or both sides (usually the left) in approximately 14% of people, but its actions are not missed. It has a short belly and a long, cord-like tendon that passes superficial to the flexor retinaculum and attaches to it and the apex of the palmar aponeurosis.

The palmaris longus tendon is a useful guide to the median nerve at the wrist. The tendon lies deep and slightly medial to this nerve before it passes deep to the flexor retinaculum.

The palmaris longus muscle is one of the superficial layer of the anterior compartment of the forearm that contains:

  1. Pronator teres,
  2. Palmaris longus,
  3. Flexor carpi ulnaris.
  4. Flexor Carpi Radialis

Palmaris Longus Muscle Anatomy

Origin:

The palmaris longus muscle originates from the medial epicondyle of the humerus, a common flexor tendon origin.

Insertion:

The muscle has a short belly and a long, cord-like tendon that traverses superficially to the flexor retinaculum, attaching to the distal half of the flexor retinaculum and the apex of the palmar aponeurosis.

See Also: Forearm Muscles Anatomy & Function

Actions

The primary actions of the PL muscle include:

  • Flexing the Hand: The muscle assists in flexing the hand at the wrist joint.
  • Tightening the Palmar Aponeurosis: It contributes to tightening the palmar aponeurosis, which helps in gripping objects.

Innervation

The palmaris longus is innervated by the median nerve, specifically by the C7 and C8 nerve roots. This innervation plays a crucial role in the muscle’s functionality and its clinical testing.

Arterial Supply

The ulnar artery supplies blood to the palmaris longus, ensuring its metabolic needs are met during its activity.

OriginMedial epicondyle of humerus
InsertionDistal half of flexor retinaculum and palmar aponeurosis
InnervationMedian nerve (C7 and C8) (C7, C8)
Blood SupplyUlnar artery
ActionFlexes hand (at wrist) and tightens palmar aponeurosis
palmaris longus muscle

Clinical Significance

Absence of the Palmaris Longus

In about 14% of individuals, the palmaris longus muscle is absent, most commonly on the left side. Despite its absence, the functionality of the hand remains unaffected due to compensatory mechanisms by other muscles in the forearm.

Palmaris Longus as a Surgical Guide

The tendon of the palmaris longus is a valuable landmark for identifying the median nerve at the wrist. The tendon lies deep and slightly medial to the median nerve before the nerve passes deep to the flexor retinaculum. This relationship is particularly useful during surgical procedures involving the wrist.

Testing the Palmaris Longus

To test the functionality and presence of the palmaris longus, flex the wrist and pinch the pads of the little finger and thumb together tightly. If the tendon is present and functioning normally, it can be seen and palpated easily.

Anatomical variations of the palmaris longus

The palmaris longus muscle was present in 92.5% of specimens. Three types of palmaris longus muscle were identified based on the morphology of its insertion (types I-III) and these were further subdivided into three subgroups (A-C) according to the ratio of the length of the muscle belly and its tendon. The most frequent was type I (78.8%), where the tendon attached to the palmar aponeurosis, and subtype B, where the tendon-to-belly ratio was 1–1.5 (41.1%). 

TypeInclusion criteria for the typeSubtypeInclusion criteria for the subtype
Type I• Origin of the muscular part of the medial epicondyle of the humerus
• The tendon was inserted to the palmar aponeurosis
ATendon to muscle ratio < 1
BTendon to muscle ratio 1–1,5
CTendon to muscle ratio > 1,5
Type II• Origin of the muscular part of the medial epicondyle of the humerus
• The tendon was bifurcated (lateral and medial division); the lateral division was inserted to the palmar aponeurosis, and medial division was inserted to the flexor retinaculuum of the wrist
ATendon to muscle ratio < 1
BTendon to muscle ratio 1–1,5
CTendon to muscle ratio > 1,5
Type III“Rare variants” non-conforming definition Types I and II

References & More

  1. Clinical Anatomy by Regions, Richard S. Snell
  2. Gray’s Anatomy: The Anatomical Basis of Clinical Practice, Susan Standring
  3. Cooper DW, Burns B. Anatomy, Shoulder and Upper Limb, Hand Palmaris Tendon. [Updated 2023 Aug 28]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: Pubmed
  4. Olewnik Ł, Wysiadecki G, Polguj M, Podgórski M, Jezierski H, Topol M. Anatomical variations of the palmaris longus muscle including its relation to the median nerve – a proposal for a new classification. BMC Musculoskelet Disord. 2017 Dec 19;18(1):539. doi: 10.1186/s12891-017-1901-x. PMID: 29258498; PMCID: PMC5738140. Pubmed
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