Neuromuscular partitioning of the gastrocnemius based on intramuscular nerve distribution patterns: implications for injections

Spasticity of the gastrocnemius is commonly treated with botulinum toxin injections; however, the optimal injection sites within each head have not been evaluated in relation to neuromuscular partitions. The purpose of the present study was to (1) document the intramuscular innervation patterns of the medial and lateral heads of gastrocnemius using 3 dimensional modeling; (2) determine if the medial and lateral heads of gastrocnemius are neuromuscularly partitioned; and (3) propose botulinum toxin injection strategies based on these findings. In this cadaveric study (n=24) the extramuscular and intramuscular innervation was serially dissected followed by digitization and 3D reconstruction and/or photography of the innervation pattern throughout the muscle volume. Intramuscular innervation patterns were defined to determine if the heads of gastrocnemius were neuromuscularly partitioned and based on these findings approaches for botulinum toxin injections were proposed. In all specimens except one, both heads of the gastrocnemius received independent innervation from three discrete nerve branches. Therefore, each head had three neuromuscular partitions defined by location as superior, inferomedial and inferolateral. In one specimen, the lateral head also received nerve branches via the soleus that innervated the inferolateral partition distally. Functionally, independent activation of the neuromuscular partitions of the gastrocnemius may result in differential contribution of the partitions to knee flexion and ankle plantarflexion. To capture all partitions, four injection sites into each belly were proposed. Future clinical studies are needed to determine if there is improved spasticity reduction by targeting neuromuscular partitions.