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

## TL;DR

This paper reports a rare anatomical variation of the soleus muscle, which could impact clinical diagnosis and treatment of ankle issues.

## Contribution

The study documents a rare occurrence of the tibiocalcaneus internus muscle of Testut, contributing to anatomical and clinical knowledge.

## Key findings

- The tibiocalcaneus internus muscle of Testut was found as an accessory muscle in the posterior leg compartment.
- This muscle originates at the tibia's soleal line and inserts on the calcaneus, medial to the Achilles tendon.
- Its presence may explain unexplained medial ankle pain and affect diagnostic imaging interpretation.

## Abstract

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.