Bilateral Accessory Plantaris Muscles With Variant Origins and Insertions: A Case Report

The plantaris muscle typically originates at the lower portion of the lateral supracondylar line of the femur and oblique popliteal ligament of the knee joint and inserts medial to the Achilles tendon at the calcaneal tuberosity. Functionally, this muscle is unclear, as it could be for proprioception of the leg or flexion of the knee and ankle. Additionally, it is absent in some individuals. If present, the tendon is often mistaken for the tibial nerve, which is why it is known as the "fool's nerve." This case study involves the discovery of a bilateral plantaris muscle duplication in an 88-year-old Japanese woman during a routine cadaveric dissection. This deviates from the more common unilateral presence of an accessory plantaris muscle that is predominantly reported in previous studies. Additionally, both the accessory origin and insertion differ between the two. The right accessory muscle belly originated lateral to the primary, and the tendon terminated in the tendon of the lateral head of the gastrocnemius. Conversely, the left accessory muscle belly originated superomedial to the primary muscle, where the corresponding tendon terminated in the tendon of the medial head of the gastrocnemius. Anatomical variations, including duplication, of the plantaris muscle remain an area of ongoing research due to its clinical relevance with Achilles tendinopathy, tendon grafts, and surgeries. This case reports bilateral asymmetrical accessory plantaris muscles and tendons, which can alert physicians to avoid anesthetic complications when performing popliteal or tibial nerve blocks.