Tibiocalcaneus internus muscle

Last revised by Dr Joachim Feger on 10 Sep 2021

The tibiocalcaneus internus muscle is a rare accessory muscle of the ankle and an anatomical variant with an unknown prevalence.

  • origin: medial crest of the lower third of the tibia
  • insertion: medial surface of the calcaneus approximately 1-2 cm anterior to the Achilles tendon

The tibiocalcaneus internus muscle travels from the posterior medial tibial surface of the lower leg to the medial surface of the calcaneus near the calcaneal tuberosity deep in the flexor retinaculum within the tarsal tunnel 1-3. It runs posterior to the neurovascular bundle and posteromedially to the flexor hallucis longus muscle and tendon and posterolateral to the flexor digitorum longus muscle 2,3.   

The tibiocalcaneus internus muscle looks like ordinary muscle and can be identified on MRI of the ankle by its course and location posterior to the neurovascular bundle flexor hallucis longus and flexor digitorum longus musculotendinous units within the flexor retinaculum and its insertion site at the posterior medial calcaneal surface 2,3.

The tibiocalcaneus internus muscle can be mistaken for an accessory soleus muscle or a flexor digitorum accessorius longus muscle, it might be also mistaken for pathology 1-3.

The tibiocalcaneus internus muscle can potentially cause tarsal tunnel syndrome 1-3.

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