The current report describes an uncommon three-headed PM with two accessory heads and a bipartite insertion. In the current literature, only two case reports of a three-headed PM have been described, while the identified insertions were not similar to the present case.
From a developmental point of view, the three muscles of the superficial compartment of the posterior leg are derivatives of a common pre-muscular mass. The PM’s anlage is located anteriorly to the SM and is partially covered by the GM. In a 12-mm embryo, the PM mass is not differentiated only from GM. Lastly, in a 17-mm embryo, the PM is differentiated from the GM [2], and differentiation alterations could lead to variant muscle formation.
The PM muscular origin has been systematically investigated by Olewnik et al. [10], and classified into the following six different origin patterns:
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PM of type I corresponded to a typical origin from the lateral femoral condyle (LFC), the GM lateral head, and the knee joint capsule, in 48.4%.
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PM of type II corresponded to an origin from the knee joint capsule and the GM lateral head, identified in 25%.
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PM of type III is characterized by an origin from the LFC, and the knee joint capsule (10.15%).
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PM of type IV includes an origin from the LFC, the knee joint capsule, and the iliotibial band (6.25%).
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PM of type V includes an origin from the LFC (8.6%).
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PM of type VI (2 cases only, 1.6%) were rare PM variants. One variant (0.8%) corresponded to a double PM (two muscular parts and two distinct tendons), and the other variant (0.8%) was characterized as “bifurcated PM” (two muscular parts that fused into a single tendon).
Hence, the three-headed PM identified in the current report was not observed in the PM systematic morphological study by Olewnik et al. [10]. Based on this classification system, Wasniewska et al. [15] studied the PM in human fetuses and concluded the lack of a bipartite PM, i.e., a duplicated PM (distinct double origins) or bifurcated PM (distinct double insertions). Hence, the duplicated or bifurcated PM corresponds to a rare variant, while the three-headed variant is even rarer. The imaging study by Herzog et al. [7] is a unique study referring to the accessory PM, based on a review of 1000 magnetic resonance imaging (MRI). The authors identified the PM accessory form in 6.3% (63 cases), reporting a prevalence higher than the cadaveric studies (1.6%). However, it is unclear if the PM muscular part, as well as PM variant forms can be easily visualized via imaging.
The existence of PM accessory heads, such as the one described in the present case, has been published before. Olewnik et al. [10] identified a “double PM” and a “bifurcated PM” in 0.8% of their sample, per each form. During a forensic autopsy, Smedra et al. [14] observed a PM with two muscular heads that fused to a common tendon, which corresponds to Olewnik et al. [10] “bifurcated PM”. Futa et al. [3] observed another morphological variant similar to Smedra et al. [14]. Contrariwise, Kurtys, and co-authors [8] and Heo et al. [6] identified during dissection an accessory PM that corresponded to the Olewnik et al. [10] “double PM”. The two rare cases observed by Olewnik et al. [10] have also been described as case reports by the aforementioned studies.
In the published data literature, only two reports described the occurrence of a three-headed PM [9, 12]. Olewnik et al. [12] identified this rare variant in a female cadaver during dissection. The first head originated from the posterior femoral surface, the 2nd head from the LFC, and the GM lateral head, and the 3rd head originated exclusively from the GM lateral head. In this case, all head tendons created a common tendon [12]. Maslanka et al. [9] identified a slightly different three-headed PM that fused into a common tendon, with a 1st head originating from the LFC and fused with Kaplan fibers (another variant consisting of connections between the iliotibial band and the distal femur). The other two heads originated from the knee joint capsule and the LFC [9]. The three-headed PM is a very uncommon variant (only two reports), while the identified bipartite insertion has not been reported. Olewnik et al. [12] and Maslanka et al. [9] reports concerning the three-headed PM concluded a common tendon of the variant muscle, while in the current case, the two accessory heads had a bipartite insertion (lateral and medial attachments) via a short and distinct tendon into the calcaneal tendon and via a musculoaponeurotic expansion into the MFC.
Zielinska et al. [16] first reported the occurrence of a four-headed PM, a variant with four independent muscular heads that fuse into a common tendon.
Interestingly, in the current literature, there have been theories for a possible relationship between the PM and palmaris longus muscle (PLM). Some studies supported that the two muscles were homologous and equivalent [1]; however, other studies cited in Gonera et al. [5] review, pointed out many differences between them. Diogo and Molnar's [1] theory could be disputed from the embryological background of the muscles. The upper limb muscle development precedes the lower limb muscles; specifically, the PM is fully developed and differentiated in a 14mm embryo length, while the PM is not fully differentiated until a 17mm embryo length [2]. Therefore, Diogo et al. [2] highlighted that these two structures appear in different ontogenetic order and are derived from different primordia. Further studies investigating the embryology, comparative anatomy, function, and possible relationship with the PLM are adequate to extend our knowledge of this enigmatic muscle.
The PM clinical significance mainly corresponds to its tendon, and it has been proven that be involved in the mid-portion Achilles tendinopathy [5]. Different morphological types of this tendon can exist, and Gonera et al. [5] have proposed a classification system with 10 types, while they highlighted that some types are more prevalent to cause Achilles tendinopathy [5]. Except for pathologic conditions, the PM tendon is a very good candidate for potential donor graft due to its unique morphology (long and thin tendon). The following clinical situations concerning this tendon could potentially help surgeons, such as the replacement of the lateral ankle ligaments, hand tendon reconstruction, and reinforcement of ruptured Achilles tendon [5].