The plantaris muscle gives rise to a thin, ribbon-like tendon running medially between the soleus and gastrocnemius to insert on the medial calcaneus adjacent to (but separate from) the Achilles tendon. The plantaris tendon runs along the medial border of the Achilles tendon in the lower leg and may be mistaken for a longitudinal split of the Achilles on ultrasound. The two tendons are separated by a thin layer of paratenon but are in close proximity at the calcaneal insertion zone.
The plantaris tendon is harvested for tendon graft in hand and foot surgery (including EPL reconstruction and staged flexor tendon grafting), providing 30-40 cm of slender tendon through a transverse ankle incision. On MRI and ultrasound, distinguishing a normal plantaris tendon from a longitudinal Achilles split tear is a common diagnostic challenge: the plantaris lies posteromedial to the Achilles and is separate, while Achilles longitudinal splits are within the tendon substance. Plantaris tendon involvement in midportion Achilles tendinopathy has been proposed as a pain generator requiring specific plantaris release or excision.
The plantaris tendon running medially alongside the Achilles can be misidentified as a longitudinal Achilles split tear on ultrasound, leading to inappropriate surgical planning; identifying the plantaris as a separate thin structure posteromedial to the main Achilles tendon on both transverse and longitudinal ultrasound views prevents this diagnostic error.
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