Like the gastrocnemius muscle, the plantaris muscle spans both the knee and the ankle joints. It has a proximal attachment at the lateral supracondylar line of the femur just superior and medial to the lateral head of the gastrocnemius muscle. The distal attachment is via a long tendon that courses between the medial head of the gastrocnemius and the soleus muscle as it inserts onto the medial aspect of the calcaneus adjacent to the Achilles tendon. The planta-ris tendon ranges from 7 to 13 cm in length with the myotendinous junction occurring at the level of the origin of the soleus muscle at the proximal tibia. The tendon may be absent in 7% to 20% of the population and is often harvested for reconstruction of tendons or ligaments [107,114].
Injury to the plantaris tendon can mimic injury to the medial head of the gastrocnemius and distinguishing the two can be clinically difficult. Patients will feel a sudden pop with pain and swelling in the posterior calf. Often, injury to the plantaris muscle belly coexists with injury to the ACL and posterolateral structures in the knee while tendon injuries tend to be isolated and related to tennis leg. A partially or completely torn muscle at the level of the myotendinous junction can result in an intermuscular hematoma between the medial head ofthe gastrocnemius and soleus muscles . More distally, ruptures of the plantaris tendon are depicted on MRI as a fluid collection between the medial head of the gastrocnemius muscle and soleus muscle with retraction of the muscle appearing as a mass. Some authors believe that the presence of a hematoma in this location favors the diagnosis of injury to the medial head of the gastrocnemius muscle given the avascularity of the plantaris tendon [107,114]. Treatment is similar to injuries of the gastrocnemius muscle (Fig. 26).
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