Presentation
Clinical symptoms of bilateral tarsal tunnel syndrome.
Patient Data
An accessory posterior ankle muscle belly is observed, located deep to the flexor retinaculum and lateral to the flexor hallucis longus muscle. The corresponding tendon attaches below the sustentaculum tali and the tendon sheath contains a small amount of fluid. The posterior tibial neurovascular bundle and both the flexor hallucis longus and accessory muscle appear normal.
In addition, there is a marked thickening of the central band enthesis of the plantar fascia with a partial-thickness tear.
No other abnormalities.
Green = accessory peroneocalcaneus internus muscle and tendon
Blue = flexor hallucis longus muscle and tendon
Yellow = tibial nerve
Red = posterior tibial artery and veins
Both ankles have an identical accessory muscle deep to the flexor retinaculum and lateral to the flexor hallucis longus.
Case Discussion
The posterior ankle accessory muscle belly deep to the flexor retinaculum and lateral to the flexor hallucis longus is consistent with a peroneocalcaneus internus muscle.
This rare anatomical variant (prevalence: 1%) is usually asymptomatic and frequently bilateral, as in this case. Especially during joint movement, the bulk of this accessory muscle can cause medial displacement of the flexor hallucis longus impinging the adjacent posterior tibial neurovascular bundle. Therefore, these accessory muscles can be a potential cause of the tarsal tunnel syndrome symptoms as bilaterally experienced by this patient. Solely based on imaging, it is however difficult to be certain of the relationship between the presence of a peroneocalcaneus internus muscle and the symptoms.
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