Morphological variability of the palmaris longus muscle in human fetuses

The PL muscle, like the plantaris muscle, is one of the most variable muscles in human body [1, 3‐7, 9, 21‐23, 25‐27, 29, 32‐34]. This is due to the rudimental function of this structures. More specifically, the PL muscle is well developed in species with a significantly higher ratio of upper limb weight to body weight. As this ratio is quite low in humans, i.e., the PL is less developed and its role in the functioning of the hand is reduced, it may present morphological variation [19].

With the development of the forelimb as a prehensile organ, the long flexor muscles of the forearm start to partially atrophy in a caudocranial direction. These muscles develop from the flexor mass, which is further divided into superficial and the deep layers. From the deep layer arise the flexor digitorum superficialis, flexor digitorum profundus and flexor pollicis longus, while the superficial layer gives rise to the pronator teres, flexor carpi radialis and ulnaris, and the PL [19]. Among vertebrates, the palmaris longus is restricted to mammals and is most well developed in species with a weight-bearing gait. The PL is always present in Orangutans, but is variably absent in chimpanzees and gorillas. Described as one of the muscles with the most anatomical variability, it is classified as a muscle in phylogenetic regression. Morphogenetically, its tendon and muscles are developed and regulated by the HOX gene [2, 13].

Anson et al. [3] and Reinman et al. [29] presented the first major study on the anatomical variations of the PL in adults. In 1600 upper extremities, the incidence of any kind of anomaly (except for the agenesis) was 45 in 540 consecutive arms, half of which concerned variations in position and form. In the remaining half, variations included additional slips and substitute structures (15 cases), duplication of the PL (four cases) and aberrancies of attachment (three times) [3, 29].

Although many original studies and case reports describe morphological variants of the PL muscle in adults, very few concerned fetuses [1, 17]. The present study distinguishes nine types of PL. Type I was the most common and was primary reported in both fetal studies [1, 17]. Type II, with a bifurcated distal tendon, was the second most common type, occurring in 22% of cases. Albay et al. [1] also report a similar type, but while the ‘second division’ inserted to the flexor retinaculum in our study, it passed deep to it. Kocabiyik et al. [17] did not report this type. Surprisingly, neither authors [1, 17] reported variants that gave additional slips to the abductor pollicis brevis (our third most common type—type III) or that were characterized by tendon–belly–tendon morphology (our types IV and V).

On the other hand, our type VI (reversed tendon–belly morphology), identified in 2% of cases, was also described by Albay et al. [1] and Kocabiyik et al. [17]. This variant is clinically significant because in adults it may predispose to median nerve compression by the muscular part of the PL [20, 22, 25]. A similar type to our Type VII (1%), characterized by partial doubling of the belly (muscular part), was described by Albay et al. [1], although in their case the muscular part was single and only the tendon was double. Type VIII (trifurcation of the tendon; 3% of cases) and Type IX (which was connected with flexor carpi ulnaris in distal part; 1% of cases) were not reported by Albay et al. [1] or Kocabiyik et al. [17]. Comparing the present results with those of previous studies, it can be concluded that many variations of the PL observed on the fetuses correspond to variants occurring in adults; however, Type V, VIII and IX did not [3, 4, 12, 14, 25, 29, 33].

In adults, the PL muscle may be completely absent in between 1.5 and 63.9% of examined subjects, depending on the population.

In the present study, the PL was found to be absent in 60 limbs (37.5%). We observed bilateral absence of the PL in 26.25% of fetuses, which is comparable with results of Albay et al. [1] (32.75%) and Kocabiyik et al. [17] (29.2%). More variation has been observed regarding unilateral absence, with it being reported in 22.5% of cases in the present study, 10.34% by Albay et al. [1] and 20.8% by Kacobiyik et al. [17]. For fetuses as well as for adults, the PL is more likely to be absent in female subjects and on the left side [1, 16, 29]. One study based on an Iranian population found the PL to be absent more often in men than in women, but this difference was not significant [23]. In the present study, male and female subjects presented the same rate of unilateral absence, although it was more common for the PL to be absent on the right side in males, but on the left in females.

Due to the fact that the PL is very often used for transplants, attention should be paid to its relationship to the median nerve. One potential grafting complication is a tendon tear. The place where it tears is most likely to occur at the tendon split, this being the location where the MN crosses the tendon. Our previous study on adult identified a crossing of the MN with PL tendon in most types, except for the single case, where a fusion was observed between the palmaris longus muscle and the flexor carpi ulnaris muscle [25]. At the cross point, it was observed that the MN was wider and thicker than the PL tendon [25]; a similar observation was made in the present study for all types apart from Type VII, and no crossing was observed for Type IV. In addition, five examples of Type I and one case of Type V did not cross the MN. In our previous studies conducted on adult humans, we observed a crossing point between the PL and MN in all types except the type that was a fusion with flexor carpi ulnaris [25]. The present study proves that in Type IV there was no place at the crossing point between the PL and MN, while in Types I and V there were cases of non-crossing. These are the first reports on the relationship between the PL and MN. Knowledge about the variability of these structures is essential for proper planning of surgical procedures.

This study has some limitations. Due to the great variability of the PL, the proposed classification is heterogeneous and it depends on several morphological details, such as the presence of accessory division, type of origin or insertion. Nevertheless, as it was created for research purposes, it has to be detailed rather than clinically simple and immediately usable in every day practice. Secondly, we are aware the presented types and their relation to the MN may change with gestation age and after birth. However, the variability of the PL muscle in the fetus is the best starting point for analyzing its variability in adult humans.