Peroneocalcaneus internus muscle

Last revised by Henry Knipe on 25 Nov 2022

The peroneocalcaneus internus muscle, also known as fibulocalcaneus internus muscle of MacAlister, is a rare accessory muscle of the ankle and an anatomical variant with an estimated prevalence of about 1%. It is often bilateral if present.

The peroneocalcaneus internus muscle takes its course beneath and parallel to the flexor hallucis longus muscle and medial to the peroneus brevis muscle from the lower third of the fibula to the medial surface of the calcaneus inferior to the sustentaculum tali deep to the flexor retinaculum 1-4. There might be some muscle fiber interdigitation with the flexor hallucis longus muscle belly fibers in the proximal portion 2.

Above the tibiotalar joint, the peroneocalcaneus internus muscle can be seen just posterolateral to the flexor hallucis longus musculotendinous unit. At the level of the posterior talar groove, it can be seen lateral and below the sustentaculum tali, it can be seen inferolateral to the flexor hallucis longus tendon before it inserts into a small tubercle on the undersurface of the sustentaculum 2,3.

The peroneocalcaneus internus muscle was originally discovered by the German anatomist Johann Friedrich Meckel in 1815 and then further described by the Irish anatomist Alexander MacAlister (1844–1919) in 1872 4-7.

The peroneocalcaneus internus muscle displaces the flexor hallucis longus muscle anteromedially and might have the following clinical implications 2,3:

  • might be mistaken for a pathological condition
  • a potential altered approach during ankle arthroscopy with the danger of neurovascular damage

The following clinical conditions might have a relation to a peroneocalcaneus internus muscle 1,2:

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