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The palmaris longus muscle is a weak flexor located in the anterior forearm. It is a variable muscle, with the most common variation being its absence. The evolutionary interpretation of its 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. The muscle plays an important function in the anatomy of the grip and thumb abduction movements.
| Characteristics | Values |
|---|---|
| Name | Palmaris longus muscle |
| Location | Centrally in the anterior forearm |
| Function | Wrist flexion, thumb abduction, tensioning the palmar aponeurosis, stabilising the elbow joint |
| Tendon Graft | Widely used for the upper limb |
| Variability | May be tendinous above and muscular below, or muscular in the centre with a tendon above and below, or may present two muscular bundles with a central tendon, or may consist solely of a tendinous band |
| Prevalence | 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) lack the PL tendon |
| Evolutionary Significance | Provides proof of evolution, as it is a vestigial muscle that humans inherited through common descent but no longer actively employ |
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What You'll Learn
Palmaris longus muscle function
The palmaris longus is a muscle in the wrist that may not be present in all individuals. It is a slender, elongated, and spindle-shaped muscle, lying on the medial side of the flexor carpi radialis. The palmaris longus tendon is located between the flexor carpi radialis and the flexor carpi ulnaris muscles. The median nerve passes between the tendons of these two muscles, and the nerve provides innervation to the palmaris longus.
The palmaris longus is part of the superficial forearm flexor group, and it contributes to wrist flexion and tensioning the palmar aponeurosis. It aids in flexing the hand on the wrist, working together with muscles like the flexor carpi ulnaris and flexor carpi radialis. The specific function of the palmaris longus is to provide weak wrist flexion, and it may also assist in thumb abduction movements, which are necessary for opening the hand.
The absence of the palmaris longus muscle does not cause any functional deficits or decrease grip strength. However, it does result in decreased pinch strength in the fourth and fifth fingers. The presence or absence of this muscle can be assessed through clinical tests such as the “Bunched Finger” test and the “Hooked Finger” test.
The palmaris longus is commonly used in tendon grafts for the wrist due to the length and diameter of its tendon. It can be harvested and grafted to replace ruptured tendons in the wrist without causing any functional deformities. The muscle is also associated with certain clinical conditions, such as carpal tunnel syndrome, where its reverse belly may localize within the carpal tunnel and produce related symptoms.
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Palmaris longus absence
The palmaris longus muscle is a weak flexor muscle in the anterior of the forearm. It is a variable muscle, with the most common variation being its absence. The prevalence of its absence across the world ranges between 1.5% and 63.9%, with ethnic variations being well-known. For example, the prevalence of absence in the south of Iran is 30.7%, while in the north, it is 13.2%. In a study of medical students in Tehran, the capital of Iran, the prevalence was reported as 22.8%.
The muscle's absence is thought to be due to evolution. Humans inherited the muscle through common descent, and while numerous animals that humans share a common ancestor with, such as the orangutan, still actively employ the muscle, close primate relatives like chimpanzees and gorillas do not. The evolutionary principle suggests that at some stage, our ancestors actively used the muscle. As the thumb apparatus, particularly the thenar muscle group, started developing in the hominin branch, the palmaris longus became vestigial. As there is no apparent evolutionary pressure concerning the muscle, it has remained largely unaffected by evolutionary processes.
The palmaris longus muscle aids in wrist flexion and tensioning the palmar aponeurosis. It helps to stabilize the elbow joint when fully extended and contributes to maintaining the hand's grip while holding certain objects. The muscle is also important in surgery, particularly tendon grafting, as it can be used without causing major functional impairment. It can be used as a replacement for ruptured tendons in the hand and fingers.
The presence of the palmaris longus muscle can be determined through various clinical tests that involve positioning the wrist and hand in certain ways to put the muscle in a state of tension so that its tendon is visible and palpable. Schaeffer's test is a commonly used method, where the patient lays their thumb against their little finger and flexes their wrist. If the tendon is not observable or touchable, other tests such as Pushpakumar's test, Thomson's test, and Mishra's test can be used for confirmation.
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Palmaris longus in surgery
The palmaris longus is a long, slender muscle, usually found in the superficial volar compartment of the forearm. However, it is absent in a significant percentage of the population, with some studies reporting that almost 30% of people may lack this muscle in one or both forearms. The absence of the palmaris longus is more prevalent in females than in males.
The muscle is located centrally in the anterior forearm, aiding in wrist flexion and tensioning the palmar aponeurosis. It assists in thumb abduction movements, which are necessary for opening the hand. Additionally, it helps stabilize the elbow joint when fully extended. Due to its location and function, the palmaris longus plays an important role in the anatomy of the grip.
The tendon of the palmaris longus is of particular interest to surgeons, as it is frequently used in tendon grafts for the wrist. Its length, diameter, and ability to be utilized without causing functional deformities make it ideal for this purpose. When a tendon in the wrist is ruptured, the palmaris longus tendon can be grafted in its place, most commonly replacing or supplementing the long flexors of the fingers and the flexor pollicis longus tendon.
The palmaris longus is also used in surgical procedures for correcting claw-finger deformities, opponensplasty, and radial palsy. Its attributes, such as its length, parallel tendon fibres, and mechanical advantage, make it well-suited for these procedures. Furthermore, the absence of the palmaris longus does not result in functional loss in grip and pinch strength, making it a favourable option for surgical interventions.
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Palmaris longus clinical tests
The Palmaris longus (PL) tendon can be assessed with clinical tests of varying reliability. The tests aim to make the tendon prominent by eliciting its flexor action and then identifying it through inspection and palpation. Here are some of the common clinical tests for detecting the PL tendon:
Schaeffer's Test
This test involves maintaining the forearm at 90 degrees and then moving the thumb in opposition towards the little finger with the wrist partially flexed.
Thompson's Test
Details of this test are scarce, but it has been performed on participants.
Mishra's Test I
This test includes passive hyperextension of the metacarpophalangeal joints along with active flexion of the wrist.
Mishra's Test II
The thumb is abducted against resistance with the wrist partially flexed.
Pushpakumar's "Two-Finger Sign" Method
The index and middle fingers are extended, while the wrist and other fingers are fully flexed with the thumb opposed and flexed.
AIIMS Test
Subjects are asked to touch the ulnar aspect of the base of the little finger with the tip of the thumb with the wrist in flexion.
Bunched Finger Test
Participants are asked to place their hand flat on a table with the palm facing upwards. They then bunch their fingers (as if preparing to smite their forehead) and exert firm pressure at the opposing fingertips. Next, they flex the wrist, making the tendon stand out prominently. If the forearm is then pronated by 5°-10°, the tendon is seen even more prominently.
Hooked Finger Test
Participants stretch out their arm and hook their fingers. The examiner then hooks their fingers into the patient's and locks them together. The patient then attempts wrist flexion while the examiner gives resistance, making the PL tendon stand out prominently.
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Palmaris longus evolutionary significance
The palmaris longus muscle is a weak flexor that aids in wrist flexion and thumb abduction movements. It is located centrally in the anterior forearm and helps to stabilise the elbow joint when fully extended. The evolutionary interpretation of the muscle's absence suggests that humans inherited the muscle through common descent, and while numerous animals that humans share a common ancestor with (such as the orangutan) still actively employ the muscle, close primate relatives (such as the chimpanzee and gorilla) do not.
The common descent principle indicates that at some stage, our ancestors actively employed the muscle. As the thumb apparatus (specifically the thenar muscle group) began to develop in the hominin branch, the palmaris longus became vestigial. The muscle has remained largely unaffected by evolutionary processes as there is no apparent evolutionary pressure (positive or negative) concerning it.
The palmaris longus is the most popular tendon for use in wrist grafts due to its length and diameter, and the fact that it can be used without causing functional deformities. The tendon is also of great importance to surgeons, as it is widely used as a tendon graft for the upper limb. The muscle's absence or presence can be detected through clinical tests that manipulate the wrist and hand into certain positions, thus putting the muscle in a state of tension so that its tendon is both visible and palpable.
The palmaris longus is also associated with carpal tunnel syndrome, as its loading increases carpal tunnel hydrostatic pressure more than any other tendon passing through. However, it is not a proven independent risk factor for the development of the condition.
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Frequently asked questions
The palmaris longus is a weak flexor muscle that is located centrally in the anterior forearm. It is a variable muscle and the most common variation is its absence. It is believed that humans inherited the muscle through common descent and it has remained largely unaffected by evolutionary processes.
The palmaris longus muscle aids the flexor carpi ulnaris and flexor carpi radialis muscles in performing a balanced flexion of the hand on the wrist. It also helps in wrist flexion, thumb abduction movements, and stabilising the elbow joint when fully extended.
There are several clinical tests that can be performed to detect the palmaris longus muscle. These tests involve certain positions of the wrist and hand to put the muscle in a state of tension so that its tendon is visible and palpable. One such test involves maintaining the forearm at 90 degrees and then moving the thumb towards the little finger with the wrist partially flexed.
The absence of the palmaris longus muscle is common and does not cause any significant issues. If the muscle is not available for surgical procedures, other options such as the anatomically homologous plantaris muscle in the leg can be considered.
