Palmaris longus

Introduction

The palmaris longus is a slender, superficial muscle of the forearm that is absent in a variable percentage of the population. Although it is often considered functionally redundant, it holds significant clinical importance in reconstructive surgery and tendon grafting. Understanding its anatomy, variations, and functional role is essential for clinicians and surgeons.

Anatomy of the Palmaris Longus

Origin

The palmaris longus originates from the medial epicondyle of the humerus through the common flexor tendon. This proximal attachment allows it to contribute to wrist flexion and tension of the palmar fascia.

Insertion

Distally, the muscle continues as a long, slender tendon that inserts primarily into the palmar aponeurosis. In some individuals, fibers may also attach to the flexor retinaculum of the wrist.

Muscle Belly and Tendon

The muscle belly is short and fusiform, giving rise to a long, cord-like tendon that extends over the anterior wrist. The tendon is superficial, making it easily visible and palpable when the wrist is flexed and fingers are opposed.

Blood Supply

The palmaris longus receives its blood supply from branches of the ulnar artery, which provide nourishment to both the muscle belly and the tendon.

Innervation

The median nerve, typically from the C7-C8 nerve roots, innervates the palmaris longus. This innervation allows coordinated contraction with other flexor muscles of the forearm.

Function

• Flexion of the wrist
• Tension of the palmar aponeurosis to stabilize the hand during grip
• Assists in hand mechanics, although its role is often considered minor

Anatomical Variations

• Complete absence of the muscle in one or both forearms, observed in up to 15% of the population
• Accessory slips or bifid tendons may be present
• Variations are clinically important for tendon grafting and surgical planning

Clinical Significance

Palmaris Longus in Tendon Grafting

The palmaris longus tendon is commonly harvested for reconstructive surgery due to its superficial location and expendable function. It is frequently used in tendon grafts for the hand, wrist, and fingers, as well as in ligament reconstruction procedures.

• Provides a long, strong tendon suitable for grafting
• Minimal functional deficit when harvested
• Readily accessible in the anterior forearm

Physical Examination

Several clinical tests can confirm the presence of the palmaris longus muscle:

• Schaeffer’s Test: The patient opposes the thumb and little finger while flexing the wrist to make the tendon visible.
• Thompson’s Test: Fist formation with wrist flexion to accentuate the tendon.
• These tests are important before tendon harvest or surgical procedures.

Injury and Pathology

• Tendon rupture or laceration is rare due to its superficial and non-critical role.
• Overuse injuries may occur in athletes or individuals performing repetitive wrist motions.
• Occasionally implicated in compression syndromes or tenosynovitis.

Surgical Considerations

• Harvesting techniques must preserve surrounding structures such as the median nerve.
• Used in flexor tendon reconstruction, ligament repairs, and tendon transfers.
• Complications are uncommon but may include tendon adhesion or localized weakness.

Embryology and Development

Developmental Origin

The palmaris longus develops from the embryonic flexor mass of the forearm muscles. It arises from mesenchymal cells that differentiate into muscle fibers during early fetal development.

Tendon Formation

The tendon forms as the muscle elongates distally, establishing its insertion into the palmar aponeurosis. Variations in tendon length or absence result from differences in embryologic development.

Genetic and Evolutionary Considerations

• The presence or absence of the palmaris longus may be influenced by genetic factors.
• It is considered a vestigial muscle in humans, reflecting evolutionary changes from primate ancestors.
• Its variability highlights the evolutionary adaptation of hand function and grip mechanics.

Comparative Anatomy and Evolution

Presence in Other Primates and Mammals

The palmaris longus is present in most primates and some other mammals, where it plays a more significant role in grasping and locomotion. In species such as chimpanzees, gorillas, and orangutans, the muscle contributes to powerful flexion of the wrist and digits, aiding in climbing and arboreal activities.

Evolutionary Significance

• In humans, the muscle is considered vestigial due to its minimal contribution to modern hand function.
• Anatomical variability, including agenesis, reflects evolutionary adaptation to decreased reliance on wrist flexion for locomotion.
• Studies of evolutionary anatomy provide insight into the functional changes in the human forearm and hand over time.

References

1. Standring S, editor. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. London: Elsevier; 2020.
2. Canale ST, Beaty JH. Campbell’s Operative Orthopaedics. 14th ed. Philadelphia: Elsevier; 2021.
3. Loukas M, Hullett J, Louis RG, Tubbs RS. Palmaris longus muscle: A review of anatomy, variability, and clinical significance. Folia Morphol (Warsz). 2012;71(1):1–6.
4. Thompson NW, Mockford BJ. Clinical anatomy of the palmaris longus tendon. J Hand Surg Br. 2001;26(2):140–3.
5. Stecco C, et al. Functional anatomy of the palmaris longus: A review. Clin Anat. 2014;27(3):394–400.
6. Roohi MA, et al. Prevalence and clinical significance of the palmaris longus tendon in humans. Anat Sci Int. 2010;85(4):195–200.
7. Chopra P, et al. Evolutionary and surgical perspectives on the palmaris longus. Clin Anat. 2015;28(6):738–45.
8. Loukas M, et al. The palmaris longus tendon in reconstructive surgery. Surg Radiol Anat. 2008;30(1):39–45.