Complex accessory tendon anatomy of the pes anserinus: a unique combination of accessory tendons in a right knee

BACKGROUND: The pes anserinus (PA) is a tendinous complex located on the anteromedial surface of the proximal tibia, formed by the conjoined insertions of the sartorius, gracilis, and semitendinosus muscles. It contributes to internal rotation and flexion of the tibia, providing stability to the medial knee compartment. CASE REPORT: During the routine dissection of a 75-year-old female cadaver, an unusual PA variant was found. On the right limb, four accessory tendons were identified: one accessory sartorius tendon (aSART), one accessory gracilis tendon (aGRT), and two accessory semitendinosus tendons (aSTT1 and aSTT2). Distally, the aSTT1 fused with the aGRT and subsequently joined the aSART, forming a single fascial band that inserted slightly inferior to the typical PA footprint. No corresponding variations were present on the contralateral side. The coexistence of accessory slips from all three PA muscles appears exceedingly rare. CONCLUSIONS: This case report presents a unique variant of PA, emphasizing the extent of its anatomic variability. Awareness and recognizing such variants of PA is crucial for surgeons, as this knowledge may help prevent the misidentification of tendons during medial knee procedures and tendon graft harvesting.