Background:

The posterior compartment of the leg typically contains three muscles in the superficial flexor group: the gastrocnemius, plantaris, and soleus. The gastrocnemius muscle forms the major bulk of the posterior compartment of the leg and is instrumental to walking and posture. The gastrocnemius has medial and lateral heads that originate from the medial and lateral condyles of the femur, respectively. The motor nerve supplying the two gastrocnemius heads is the tibial nerve, receiving fibers from the L5, S1, and S2 nerve roots. The posterior leg is cutaneously supplied by the L4-S2 nerve roots. This biarticular muscle distally inserts into the posterior surface of the broad membranous Achilles tendon. However, a third head of the gastrocnemius, is an extremely rare accessory muscle bundle of the gastrocnemius muscle that covers the surface of the popliteal fossa. There has been scarce representation in the literature of bilateral gastrocnemius tertius muscles.

Case Presentation:

In this poster, we present a case report of bilateral anomalous gastrocnemius tertius muscles discovered in a 67-year-old male cadaveric who died of complications related to vascular dementia. Upon routine educational dissection of the cadaver, an extra muscle belly was discovered bilaterally, medial to each gastrocnemius lateral heads, which was determined to be tertius heads of the gastrocnemius muscle. The tertius heads were separated from the gastrocnemius medial and lateral heads by a fascial sheath. Muscle fibers were arranged in a unipennate pattern. The tertius heads attached proximally to the lateral condyle of the femur – at the same attachment point of the lateral head – and distally contributed to the Achilles tendon. The belly length was 24 cm, and width was 2.5 cm. The tertius head muscles were both innervated by a unique branch of the tibial nerve that entered the muscle belly. Vasculature to the tertius heads originated from additional branches of the popliteal artery and veins. The proximal tendon and belly of this muscle were in proximity to the tibial nerve and artery.

Discussion:

The gastrocnemius is a powerful plantar flexor and knee flexor. Therefore, this muscle provides significant propulsive force regarding gait mechanics. Anomalies of this muscle bundle is important to recognize as they can have significant implications on future patient outcomes. To our knowledge, bilateral gastrocnemius tertius muscles have only been reported in literature once before. Although believed to be a benign anatomical variant, this can potentially lead to pathologies such as Popliteal Artery Entrapment Syndrome, sural nerve entrapment, and deep vein thrombosis. Compression of these structures by the tertius head may cause numbness, tingling, swelling, and signs of muscle hypoxia in the posterior leg. Therefore, clinicians should be aware of this variant and its clinical relevance. Tertius head-related posterior leg vessel compression should be considered on differential when treating patients with signs and symptoms of popliteal fossa swelling and pain. Finally, it could give rise to future reports with similar presentations that lead to better understanding of the physiology of this anomaly.