Via Research Recognition Day Program VCOM-Carolinas 2025

Case Reports

Unilateral Bicipital Plantaris Muscle: A Cadaveric Case Report Maren Timms, OMS ll 1 , Jeremy Bose, DO 1 , David Jaynes, PhD 1 . 1. Edward Via College of Osteopathic Medicine, Spartanburg, SC

Introduction The plantaris muscle (PM) is a leg muscle, found in the posterior superficial compartment, the tendon of which passes between the soleus and gastrocnemius. It is often considered vestigial but plays an essential role in proprioception by providing sensory feedback about the position of the gastrocnemius and soleus muscles. This study investigates the occurrence of a bicipital plantaris muscle belly in a 58-year-old male cadaver. The research aims to highlight the anatomical and clinical implications of this variant, particularly its relevance in surgical applications such as tendon grafting. Additionally, it examines its potential effects on posterior leg pathology, including Achilles tendinopathy and tennis leg syndrome. A 58-year-old male cadaver was observed to have a bicipital PM in the left leg during a routine dissection by one of the authors at The Edward Via College of Osteopathic Medicine. The PM was classified based on morphological features and tendon insertions (Figure 1). Both PMs emerged from the oblique popliteal ligament (OPL). The muscle bellies were well demarcated, with the medial muscle belly (PM 1) wider at origin than the lateral muscle belly (PM-2) (Figure 2). The tendons of both PMs (PT-1 and PT-2) coursed distally to insert on the calcaneus, both in proximity to the Achilles (Not shown). PT-1 inserted medial to the Achilles, while PT-2 coursed between the soleus and gastrocnemius muscles to join the Achilles tendon (Figure 3). This is a variant of normal PT insertion, thus designated as the ectopic plantaris, PT-2. According to Waśniewska - Włodarczyk et al. [1], PT-1 fits into Type 1 of the eight-fold classification system of the PM in fetuses since PT-1 is more medially implanted than PT-2. It falls within Type A of the classification scheme proposed by Olewnik et al. [2] as shown in figure 1. It is assumed that PM-1 is normotopic and PM-2 is ectopic based on muscle belly breadth and insertion sites. Case Description

The PM tendon has been used as grafts due to its accessible bony insertion and high degree of tensile strength. Variants similar to PT-2 with insertion into the Achilles tendon are likely to have the needed tensile strength for grafts. Previous cadaveric studies have shown that the PM tendon possesses sufficient bony stock insertion for use in flexor tendon reconstruction of the digits, hand, and forearm [5], while also having a higher median elastic modulus than ankle ligaments, allowing it to effectively bear loads [6]. When there are absolute contraindications to the Brostrom-Gould approach for persistent lateral ankle instability, usage of grafts is an alternative [7]. Santos et al. reports two cases of PM tendon use in flexor pollicis longus reconstruction with variable outcomes [8]. Future research is needed to assess the potential of the PM tendon in hand injuries and to investigate loss of proprioceptive input within the posterior superficial compartment of the leg from PM harvest. Conclusions Variations of the PM previously cited have noted variable origins, insertions, and additional muscle bellies. Awareness of these variations is crucial for accurate palpation of the popliteal fossa and Achilles tendon. Variants similar to PT-2 that insert into the Achilles tendon may result in additional clinical manifestations of Achilles tendon rupture since the normotopic PT inserts medial to the tendon. Furthermore, non isolated rupture of the PM can contribute to clinical manifestations of tennis leg due to its significant role in balance and stability. This proprioceptive loss highlights the importance of considering the role of the PM in both diagnosis and treatment. Nevertheless, PM tendon usage as a graft remains clinically significant. Surgeons should be aware of bicipital PM variants, particularly if they are involved in flexor tendon reconstruction procedures of the hand.

Figure 3. Two separate tendons (PT-1 and PT-2) of the plantaris muscle on the left lower limb

Figure 2. Proximal part of PM-1 and PM-2 inserting into the oblique popliteal ligament (OPL)

Discussion The PM was first studied in the 19 th century, and variable morphologies have been documented since 1875. The PM is most notable for proprioception due to its high density of muscle spindle fibers that communicate with the dorsal-column medial lemniscus pathway via type 2 afferent fibers. Non-isolated PM rupture can contribute to "tennis leg", a condition caused by muscle tears from excessive eccentric loading on the ankle with the knee extended [3]. While gastrocnemius muscle injury is the most common etiology of tennis leg, PM tears can occur alongside gastrocnemius tears, as shown in MRI studies [4}. The involvement of a PM tear could impact the clinical manifestations due to proprioceptive loss, manifesting as impaired balance and stability. Variants like PM-2, with an additional muscle belly inserting near the gastrocnemius, may increase PM involvement. Ectopic PM could also contribute to this risk and should be considered clinically. Future studies may provide insight into the role of an ectopic plantaris in this condition.

References

The authors would like to thank the donor whose contribution made this research possible. We extend our sincere appreciation to his family for honoring their loved one’s decision to donate. Their decision has not only been appreciated but also has been profoundly impactful. Acknowledgements

Figure 1. Olewnik et al. (2017) major types of the PM tendon insertion.

2025 Research Recognition Day

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