Third head of gastrocnemius, also known as accessory head of gastrocnemius, is a normal anatomical variation where there is an accessory muscle bundle of the gastrocnemius muscle in addition to the medial and lateral heads.
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Epidemiology
It is the most common variation of the gastrocnemius. Its occurrence was estimated to be between 2.0% and 5.5% of the normal population 1,4.
Clinical picture
Most of the cases are asymptomatic. It can cause compression upon popliteal vessels resulting in clinical symptoms such as popliteal artery entrapment syndrome resulting in claudication in young patients and popliteal vein entrapment 2. Rarely, it can cause sural nerve entrapment 5.
Radiographic features
It is usually seen in cross-sectional imaging modalities and best identified with MRI. It arises near midline of posterior distal femur and joins the medial or lateral head of gastrocnemius muscle, coursing lateral to the popliteal vessels. The size varied from a thin threadlike muscle to a rather bulky muscle.
History and etymology
It was first described in 1813 by Kelch 3 and was further detailed by Frey in 1919 4.
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