Accessory Plantaris Muscle

To be experienced in msk anatomy means to know also the anatomical variants; today I show you the pictures of a young football player with a “bad feeling” on the postero-lateral compartment of the knee, where a palpable nodularity is noted during the physical exam. With dynamic ultrasound investigation a well defined muscular structure adjacent to the lateral gastrocnemius insertion is noticed; mri scans confirmed the presence of the accessory muscle belly.

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Coronal T2 weighted (left) – Sagittal T1 weighted (right) Mri scans (0.3 Tesla) of the same patient.

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Comparison between Mri and ultrasound imaging modalities.

The accessory plantaris muscle is most frequently located with the origin of the normal plantaris muscle, but in this case merged with the origin of the lateral head of the gastrocnemius. These anatomical variants are frequently asymptomatic, but their relationship and effect on adjacent structures can mimic different kind of pathologies.

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The Forgotten Tendon

Hello, let me introduce myself. My name is plantaris tendon and I don’t know why nobody consider me; maybe because I’m not present in about 10-12% of the population? I’m the longest tendon in the human body. My muscle belly arise from the inferior part of the lateral supracondylar ridge of the femur, at a position slightly superior to the origin of the lateral head of gastrocnemius. I pass posterior to the knee joint in an inferomedial direction where I was born as a tendon, side by side with medial gastrocnemius and soleus; my insertion is into the Achilles tendon but sometimes you can find me also in medial side of the calcaneus. I act to weakly plantar flex the ankle joint and flex the knee joint. The only one that remember me is the orthopaedic surgeon, because in some situations he uses my body to repair the Achilles tendon. Everytime a sonographer finds a fluid collection along my medial course, says that medial head of gastrocnemius is injured. Ok, my isolated tears are less frequent than medial gastrocnemius strains and clinically similar, but the prognosis is different. Please, don’t forget about me. Please.

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Axial T2w spair mri scan of a 2nd degree strain of the plantaris tendon  of a professional football player. Medial head of gastrocnemius is normal.

Sagittal ultrasound exam of the same patient.

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Ultrasound sagittal scan with Power-Doppler evaluation.