An Incidental Discovery of the Tibiocalcaneus Internus Muscle of Testut: An Exceedingly Rare Variation of the Soleus Muscle

This report describes an incredibly rare variant of the soleus, the tibiocalcaneus internus muscle of Testut, observed during routine cadaveric dissection. In our cadaveric dissection, we observed this tibiocalcaneus internus muscle of Testut as an accessory muscle in the posterior compartment of the leg, which originates at the soleal line of the tibia and travels deep to the flexor retinaculum, ending in a tendinous insertion on the medial surface of the calcaneus, medial to the tendon of Achilles. Due to the few existing reports of this anomalous finding, any occurrence warrants reporting to expand the knowledge in clinical practice. Although it remains asymptomatic during the lifetime, this variant muscle may have clinical relevance, particularly in diagnostic imaging and in cases of unexplained medial ankle pain. By understanding this rare anatomical variation, clinicians, surgeons, and radiologists could avoid misdiagnosis and unnecessary interventions.