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Palmaris longus muscle
Palmaris longus muscle
Palmaris longus muscle
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Palmaris longus muscle
Palmaris longus tendon shown during wrist flexion with a clenched fist
Details
OriginMedial epicondyle of humerus (common flexor tendon)
InsertionPalmar aponeurosis and flexor retinaculum of the hand
ArteryUlnar artery
NerveMedian nerve
ActionsWrist flexor
AntagonistExtensor carpi radialis brevis, extensor carpi radialis longus, extensor carpi ulnaris
Identifiers
Latinmusculus palmaris longus
Greekμακρός παλαμικός μυς
TA98A04.6.02.029
TA22482
FMA38462
Anatomical terms of muscle

The palmaris longus is a muscle visible as a small tendon located between the flexor carpi radialis and the flexor carpi ulnaris, although it is not always present. Reviews report rates of absence in the general population ranging from 10–20%; however, the rate varies in different ethnic groups.[1][2][3] Absence of the palmaris longus does not have an effect on grip strength.[4] The lack of palmaris longus muscle does result in decreased pinch strength in fourth and fifth fingers. The absence of palmaris longus muscle is more prevalent in females than males.[5]

The palmaris longus muscle can be observed by touching the pads of the fourth finger and thumb and flexing the wrist. The tendon, if present, will be visible in the midline of the anterior wrist.

Structure

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Palmaris longus is a slender, elongated, spindle shaped muscle, lying on the medial side of the flexor carpi radialis. It is widest in the middle, and narrowest at the proximal and distal attachments.[6]

It arises mainly from the medial epicondyle of the humerus via the common flexor tendon. It also takes origin from the adjacent intermuscular septa and from the antebrachial fascia.[6]

It ends in a slender, flattened tendon, which passes over the upper part of the flexor retinaculum and inserts onto the central part of the flexor retinaculum and lower part of the palmar aponeurosis. Frequently, it sends a tendinous slip to the short muscles of the thumb.[6]

Nerve supply

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The palmaris longus is innervated by the median nerve.[7]

Variation

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Palmaris Longus: misplaced or absent
The fleshy belly occasionally migrates distally and comes to lie close above the wrist, as here.
Palmaris Longus was absent in 98 of 716 dissected limbs (i.e., in 13.7% of 358 paired limbs, 26 times in both limbs, 26 in the right only, and 20 in the left only. (R. K. George).[8]

The palmaris longus muscle is a variable muscle. The most common variation is its absence. Several in vivo and in vitro studies have documented the prevalence or absence of the PL tendon in different ethnic groups. Between 5.5 and 24% of Caucasian populations (European and North American) and 4.6 to 26.6% of Asian populations (Chinese, Japanese, Indian, Turkish, Malaysian) have been reported to lack the PL tendon.[9]

There are also variations related to its form. It may be tendinous above and muscular below; or it may be muscular in the center with a tendon above and below; or it may present two muscular bundles with a central tendon; or finally it may consist solely of a tendinous band. The muscle may be double, or missing entirely. Slips of origin from the coronoid process or from the radius have been seen. Partial or complete insertion into the fascia of the forearm, into the tendon of the flexor carpi ulnaris and pisiform bone, into the scaphoid, and into the muscles of the little finger have been observed.

Clinical significance

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Use in tendon grafts

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Cross-section through the middle of the forearm

The palmaris longus muscle is the most popular for use in tendon grafts for the wrist due to the length and diameter of the palmaris longus tendon, and the fact that it can be used without producing any functional deformities. When a tendon becomes ruptured in the wrist, the palmaris longus tendon may be removed from the flexor retinaculum and grafted to take the place of the ruptured tendon. The tendons most commonly replaced or supplemented by the palmaris longus tendon when ruptured are the long flexors of the fingers and the flexor pollicis longus tendon.[10]

The palmaris longus muscle itself is a weak flexor, and provides no substantial flexing force that would inhibit movement in the wrist if its tendon were cut and moved elsewhere. The palmaris longus may contribute and assist in thumb abduction movements; an action necessary to open the hand.[11][12] If the palmaris longus muscle is not available for harvesting in an individual, the anatomically homologous plantaris muscle in the leg may be taken instead.[13] Using the patient’s own tendon is advantageous, as it does not introduce foreign material into the body.

Carpal tunnel syndrome and palmaris longus variants

[edit]

Of the known anatomical variants of the palmaris longus, the reverse belly of the palmaris longus may be localized within the carpal tunnel producing symptoms of carpal tunnel syndrome. Knowledge of this variation is important to prevent unnecessary carpal tunnel release surgery, in which the median nerve compression may remain unresolved due to the presence of this palmaris longus variant.[14]

Other animals

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Evolution

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The evolutionary interpretation of the muscle's absence is that humans inherited the muscle through common descent, and numerous animals that humans share a common ancestor with (such as the orangutan) still actively employ the muscle.[15] Close primate relatives (such as the chimpanzee and gorilla) also do not actively employ the muscle, and hence they also demonstrate the same variability.[16] The common descent principle suggests that at some stage our ancestors employed the muscle actively. The thumb apparatus (and particularly the thenar muscle group) then started developing in the hominin branch, and consequently the Palmaris longus became vestigial. As there is no apparent evolutionary pressure (positive or negative) concerning the muscle, it has remained largely unaffected by evolutionary processes.[17]

See also

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This article uses anatomical terminology.

References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Palmaris longus muscle

View on Grokipedia
from Grokipedia
The Palmaris longus muscle is a small, fusiform-shaped muscle situated in the superficial anterior compartment of the , serving as one of the extrinsic . It originates from the via the and the adjacent intermuscular septa, forming a short muscular belly that quickly transitions into a long, slender tendon. This tendon passes anterior to the flexor retinaculum and inserts primarily into the , with occasional attachments to the flexor retinaculum or the . Innervated by branches of the (root values C6-T1), the muscle receives its blood supply from the anterior ulnar recurrent artery, a branch of the . The primary function of the palmaris longus is to weakly flex the and tension the , contributing minimally to overall hand or fine motor activities. Its absence, which occurs in approximately 14-26% of individuals globally (with rates varying by from as low as 3% in some Asian populations to over 50% in certain Turkish groups), results in no significant functional deficit, making it phylogenetically vestigial in humans. Anatomical variations are common, including (complete absence), reversed bellies (where the tendon is proximal and muscle distal), duplication, or accessory slips inserting into nearby structures like the flexor carpi ulnaris or thenar muscles, with an overall variation incidence of about 9%. Clinically, the palmaris longus holds value as an expendable graft for reconstructive procedures, such as repairing lacerated flexors in the hand, reconstructing the lip or nasal alar, or augmenting surgeries, due to its superficial location and lack of critical function. It can be easily assessed for presence via clinical tests like the Schaeffer or Thompson tests, where resisted flexion or opposition makes the prominent. Aberrant variations may rarely contribute to compression in or serve as a surgical landmark for identifying the during procedures.

Anatomy

Origin and insertion

The palmaris longus muscle originates proximally from the via the , as well as from the adjacent antebrachial . This attachment places it within the superficial layer of the anterior flexors, sharing the common origin with other flexor muscles such as the flexor carpi radialis and flexor carpi ulnaris. Distally, the muscle inserts into the and the flexor retinaculum (also known as the transverse carpal ligament). The may occasionally send a slip to the abductor pollicis brevis, but the primary insertion fans out to reinforce the central portion of the . The palmaris longus follows a superficial course in the anterior , with a short, spindle-shaped muscle belly located in the proximal to middle third, transitioning into a long, slender that passes over the flexor retinaculum without entering the . It lies medial to the flexor carpi radialis and lateral to the flexor carpi ulnaris throughout its path, remaining superficial to the flexor digitorum superficialis and the , with the nerve passing deep to the at the .

Structure and function

The palmaris longus is a small, fusiform-shaped muscle located in the superficial anterior compartment of the , characterized by a spindle-shaped muscular belly in the proximal or middle third that transitions distally into a long, thin . This is flat and narrow, typically measuring 108–125 mm in length in adults, and broadens as it interweaves with the fibers of the just distal to the flexor retinaculum. The overall morphology is slender and elongated, spanning much of the length with high variability, though the combined muscle belly and typically range from approximately 17–38 cm in total extent. The primary function of the palmaris longus is as a weak flexor of the , providing only minimal contribution to overall flexion force due to its small size and superficial position. It also tenses the during hand closure, which enhances grip stability and protects underlying neurovascular structures in the palm by improving the tension and adherence of the overlying . This tensing action aids in maintaining palmar integrity during forceful grasping activities. In secondary roles, the palmaris longus helps stabilize the palmar fascia during thumb opposition by providing synergistic support to the thenar muscles, though it offers no significant contribution to finger flexion. Biomechanically, it acts in synergy with the flexor carpi radialis and flexor carpi ulnaris to facilitate balanced flexion, but its absence—observed in up to 26% of individuals—results in no measurable impairment to function or strength due to compensatory from these primary flexors.

Innervation and vascular supply

The palmaris longus muscle is innervated primarily by the , with motor fibers originating from spinal roots C5 through T1. These fibers arise as a muscular branch of the in the , entering the proximal muscle belly to provide exclusive motor innervation without a significant sensory component, consistent with its role as a purely motor . The vascular supply to the palmaris longus derives mainly from branches of the , particularly the anterior ulnar recurrent artery, which provides arterial to the muscle in the proximal . Additional contributions may come from the and, in some cases, the median artery, ensuring adequate oxygenation for the superficially located muscle belly. Venous drainage occurs via accompanying venae comitantes that join the superficial venous system, ultimately feeding into the .

Anatomical variations

The palmaris longus muscle displays considerable anatomical variability, with complete absence being the most common deviation, affecting approximately 20% of individuals worldwide based on a and of clinical studies. This rate varies significantly by population, ranging from as low as 4.5% in East Asians to 26.2% in South and Southeast Asians and 26.3% in Caucasians. Absence is more frequently reported in females (20-25% in several populations) than males (8-15%), though overall differences are not statistically significant across meta-analyses. Bilateral absence occurs in about 3.9% of cases. Other notable variations include the reversed form, in which the tendon originates proximally from the medial epicondyle while the muscle belly lies distally; duplicated or accessory bellies, sometimes bilateral; fusion with the adjacent ; and aberrant tendons that insert into muscles rather than the typical . These morphological deviations occur in roughly 9% of individuals with a present muscle and are more common on the right side. Detection of variations relies on clinical tests such as Schaeffer's, where the subject opposes the to the and flexes the to elicit tendon prominence, or Thompson's test, involving resisted wrist flexion with extended fingers. provides a reliable, non-invasive confirmation with over 90% accuracy compared to clinical examination, while MRI offers detailed visualization for complex cases. Population-specific differences in absence prevalence are attributed to genetic factors, with inheritance patterns suggesting a dominant trait modified by polygenic influences. Despite these variations, the absence or altered form of the palmaris longus does not impair hand or overall upper extremity function.

Clinical relevance

Surgical applications

The palmaris longus serves as a primary autologous graft in due to its superficial location in the distal , which facilitates easy access and harvest with minimal disruption to surrounding structures. Its non-essential nature ensures no significant functional loss upon removal, as absence of the muscle does not impair grip or pinch strength. Typically yielding a length of approximately 12 cm and width of 4 mm, it provides sufficient material for various grafts while exhibiting low donor site morbidity. In hand surgery, the is commonly employed for reconstructing flexor tendon injuries, where it acts as an interpositional graft to restore digit motion following trauma or rupture. For instance, in cases of flexor pollicis longus rupture, palmaris longus grafting has demonstrated favorable outcomes, including improved flexion and minimal extension lag. It is also utilized in extensor tendon reconstruction for patients with multiple ruptures, achieving mean total active motion of 239° and fingertip-to-palm distance of 5 mm at five-year follow-up. Additionally, the tendon supports ligamentoplasty in basal joint through arthroscopic-assisted suspensionplasty, stabilizing the and alleviating pain while preserving . Beyond orthopedic applications, the palmaris longus tendon finds use in cosmetic and oculoplastic procedures. In upper , it functions as an autologous filler, increasing vertical lip height by 204% and lateral projection by 180% with sustained mobility at one-year follow-up and low relapse rates of 7-9%. For ptosis correction in children with poor levator function, it serves as a frontalis sling material, achieving successful elevation without recurrence or complications like or exposure over 30-month follow-up. Harvesting involves a transverse incision at the to isolate the and a longitudinal incision in the mid-forearm for retrieval, avoiding injury to adjacent wrist flexors through careful or use of a stripper. Complications are rare, with reported incidents such as misidentification or flexion occurring infrequently due to the tendon's distinct .00029-7/fulltext) Compared to alternatives like plantaris or synthetic grafts, the palmaris longus offers advantages as an expendable donor with ready availability in about 85% of individuals and no need for additional incisions beyond the harvest site, reducing operative time and scarring. Its autologous composition minimizes rejection risks, making it preferable for procedures requiring durable, biocompatible tissue.

Pathological associations

Anomalous variants of the palmaris longus muscle, such as reversed or accessory tendons, can compress the within the , contributing to the development of (CTS). For instance, a reversed palmaris longus with its muscle belly distal to the tendon has been documented to cause compression at the , leading to CTS-like symptoms including paresthesias. Similarly, accessory palmaris longus tendons passing abnormally through the have been identified as a cause of persistent entrapment, necessitating surgical revision in some cases. Studies indicate a higher of an intact palmaris longus (93.6%) among CTS patients compared to the general (approximately 85%), suggesting its presence may play a role in CTS by increasing carpal tunnel pressure during tendon loading. Rare cases of median nerve entrapment arise from anomalous muscle bellies of the palmaris longus, particularly in the distal forearm or wrist, where the hypertrophic or misplaced belly directly impinges on the nerve. This entrapment can mimic CTS but often requires imaging for differentiation, as the anomalous muscle may not be apparent on standard clinical tests. In the context of forearm trauma, anomalous palmaris longus variants, such as a reversed muscle, can contribute to compartment syndrome by increasing intracompartmental pressure during exertion or injury, leading to ischemic complications in the volar forearm. For example, surgical resection of a reversed palmaris longus has resolved chronic exertional compartment syndrome in affected individuals following repetitive trauma. The absence of the palmaris longus does not predispose individuals to any or impair hand function, as confirmed by multiple anatomical studies showing no associated disorders. Conversely, variants of the muscle are diagnosed in cases of persistent hand pain through (EMG) to detect associated nerve compression or (MRI) to visualize anomalous structures, aiding in targeted management. The palmaris longus has no direct involvement in conditions such as De Quervain's tenosynovitis or trigger finger.

Comparative anatomy and evolution

Occurrence in other species

The palmaris longus muscle is present across a wide range of mammalian , particularly those that utilize their s for and locomotion, where it functions primarily as a flexor. In domestic mammals such as dogs, cats, and horses, the muscle is consistently observed in the superficial flexor compartment of the , contributing to carpal flexion and stabilization during quadrupedal movement. For instance, in felids like the , , and , the palmaris longus arises from the medial and inserts via multiple tendons into the and carpal structures, aiding in precise control. Similarly, in such as rats, the muscle is reliably present and supports function in . However, its development varies, with more robust forms in relying on forelimb ambulation and reduced or absent in aquatic mammals where demands are minimal. Among primates, the palmaris longus exhibits notable variation in presence and morphology, reflecting locomotor adaptations from arboreal to terrestrial lifestyles. It is invariably present in prosimians, such as lemurs (), where it features short tendons, muscle fibers for greater force generation, and a high total length-to-tendon ratio (e.g., 5.16 ± 0.49 in Propithecus), enabling strong flexion during climbing. In (e.g., Sapajus), the muscle remains fully developed with intermediate tendon lengths (ratio 3.81 ± 1.07) and fibers, supporting arboreal grasping. monkeys (e.g., Macaca) show fibers and shorter bellies (ratio 2.37 ± 0.12), indicating reduced force but sustained presence. In great apes, it is always present in orangutans (Pongo; ratio 1.89 ± 0.15) and (), where longer s facilitate brachiation and suspensory locomotion, but variable or degenerate in chimpanzees (Pan; 32.1% absence in a sample of 28 specimens) and gorillas (15–63% absence or rudimentary). Overall, length increases and muscle belly shortens from prosimians to humans, correlating with a shift toward and diminished reliance on . The palmaris longus is absent in non-mammalian vertebrates, including birds, most reptiles, and amphibians, though homologous or analogous flexor structures exist for limb or movement. In birds (e.g., chickens, Gallus), no palmaris longus is identified; instead, the flexor digitorum longus handles digit flexion in the wing. Amphibians such as frogs (Rhinella) and salamanders (Ambystoma) lack the muscle, relying on a unified flexor digitorum communis for basic limb flexion. Among reptiles, it is generally absent in crocodilians (e.g., ) and most (e.g., Timon), but present in some lepidosaurs like and semiaquatic (e.g., Trachemys), where it assists in paddling. In , no direct homolog exists, but analogous radial and ulnar fin ray flexors in species like teleosts enable similar undulatory propulsion. These patterns underscore the muscle's emergence tied to mammalian terrestrial adaptations. Functionally, the palmaris longus is more prominent and forceful in quadrupedal mammals, where it contributes to weight-bearing and stability during forelimb extension, as seen in the pennate architecture of prosimians and for enhanced grip and propulsion. In contrast, its role diminishes in bipeds like humans, appearing vestigial with minimal contribution to or function beyond minor tensioning of the . This reduction aligns with decreased forelimb loading in upright postures, leading to greater variability and frequent absence.

Evolutionary significance

The palmaris longus muscle traces its phylogenetic origins to the flexor compartment in early tetrapods, where it likely represented a degenerate flexor derived from more primitive radial carpal flexors, as evidenced by comparative analyses across amphibians, reptiles, and mammals. In synapsids, the lineage leading to mammals, it evolved as part of an expanded flexor system adapted for enhanced mobility, becoming variably present in therian mammals while absent in monotremes, indicating a retention and modification during mammalian radiation. This muscle's development is regulated by , which govern limb patterning and contribute to its morphological variability across vertebrates. Within , the palmaris longus exhibits a of loss correlated with shifts away from toward terrestrial adaptations, including and precision grip associated with tool use in hominins. It is consistently present in arboreal orangutans (0% absence), aiding wrist flexion during suspensory behaviors, but shows increasing agenesis in more terrestrial African apes: approximately 32% in chimpanzees and up to 63% in . In humans, absence rates average around 14% globally (ranging 5–65% across populations), higher than in chimpanzees and reflecting relaxed selective pressure as other forearm flexors compensated for its functions. As a vestigial structure in modern humans, the palmaris longus likely persisted from arboreal ancestors to enhance grip on branches but became redundant with the dominance of precision-oriented hand use and bipedal posture, imposing no fitness disadvantage upon its absence. This neutral trait underscores non-adaptive, evolution in , where genetic and developmental factors linked to HOX expression allow for its variable retention without impacting overall locomotor or manipulative capabilities.

References

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