Additional Tendinous Origin and Entrapment of the Plantaris Muscle

Abstract

The human plantaris muscle (PM) is a small, spindle-shaped muscle that originates from the femur just above the lateral condyle and the adjacent posterior ligament of the articular capsule, in close association with the lateral head of the gastrocnemius. Its proximal region varies from a thin fibrous structure to a muscle belly, equal in size to the lateral head of the gastrocnemius. Ordinarily it terminates in a flat, slender tendon that inclines medially while descending between the gastrocnemius and the soleus on its way to the inner border of the calcaneal tendon. Extending along the medial edge of this tendon, it usually terminates on the dorsal surface of the calcaneum and in the neighboring fibrous tissues.1 Le Double (1897), Henle (1871), and Daseler and Anson (1943) described that the PM may originate in the following areas: (i) the lower part of the linea aspera; (ii) the posterior ligament of the knee at the intercondylar space; (iii) the fascial covering of the popliteus; (iv) the fibula, between the flexor hallucis longus and the peroneus longus; (v) the oblique line of the tibia, under cover of the soleus; or (vi) the lateral condyle of the femur above the origin of the lateral head of the gastrocnemius. The insertion of the plantaris tendon (PT) is likewise subject to considerable variations, with its attachment to neighboring structures taking place at almost any point along the tendon’s course. PT insertion has been reported in the following areas: (i) the soft tissues between the muscle bellies of the gastrocnemius and the soleus, (ii) the inner border of the calcaneal tendon, (iii) the dorsomedial surface of the calcaneal tendon at the latter’s insertion, (iv) the bursa between the calcaneal tendon and the calcaneum, (v) the fibrous and fatty tissues situated immediately in front of the calcaneal tendon, and (vi) the plantar aponeurosis.1–3 According to Moore and Dalley (2006), the PM is often found to be absent.4 Daseler and Anson (1943) found that the muscle was absent in 6.67% of 750 lower extremities that they examined.1 The presence of a double PM has also been reported in the medical literature.5