The palmaris longus is a muscle visible as a small tendon between the flexor carpi radialis and the flexor carpi ulnaris, although it is not always present. It is absent in about 14 percent of the population; however, this number varies greatly in African, Asian, and Native American populations. Absence of the palmaris longus does not have an effect on grip strength. The palmaris longus muscle can be seen 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.
The palmaris longus is supplied by the median nerve.
Variation
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 and 4.6 to 26.6% of Asian populations have been reported to lack the PL tendon. 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
Use in tendon grafts
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. 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. 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. Using the patient’s own tendon is advantageous, as it does not introduce foreign material into the body.
Other animals
Evolution
The evolutionary interpretation of the muscle's absence is fairly simple. Humans inherited the muscle through common descent, and numerous animals that we share a common ancestor with still actively employ the muscle. Close primate relatives also do not actively employ the muscle, and hence they also demonstrate the same variability. The common descent principle suggests that at some stage our ancestor employed the muscle actively. The primate branch then began to evolve the thumbapparatus and consequently the Palmaris longus became vestigial. As there is no apparent pressure concerning the muscle, evolution has largely left it alone. Its persistence might possibly be due to the fact that the necessary changes to the gene involved in losing the muscle may have other consequences, but this is only speculation.
