Ultrasonography of needle insertion site for the flexor digitorum longus related to the saphenous nerve

Research Objectives: To investigate the flexor digitorum longus (FDL) anatomy associated with the saphenous nerve using ultrasonography and to determine the appropriate needle insertion point for needle electromyography of the FDL.

Design: Cross-sectional study.

Setting: A tertiary clinic center.

Participants: Healthy volunteers between 21 and 68 years of age (N=25; 13 men, 12 women; 50 legs).

Interventions: High-resolution ultrasonography of the FDL and saphenous nerve in the leg.

Main Outcome Measures: The cross sectional area (CSA) of the FDL and the distance from the saphenous nerve to the medial margin of the tibia were measured in three levels of the leg. Level_1 is the junction of the middle and distal third of the leg, level_3 is the midpoint of leg length measured from the medial tibial plateau to the medial malleolus, and level_2 is the midpoint in between the level_1 and level_3.

Results: The CSA of the FDL in level_1, level_2, and level_3 was 107.4±38.7, 147.9 ± 46.1, and 125.5 ± 34.6 mm2, respectively. The saphenous nerve was observed at a mean distance of 7.5±2.7 mm in level_1, 10.7±3.6 mm in level_2, and 13.5 ± 4.0 mm in level_3 from the tibia. The FDL was covered with the soleus in 5, 14, and 33 legs in level_1, level_2, and level_3, respectively.

Conclusions: The appropriate needle insertion point to the FDL is the midpoint between level_1 (the junction of the middle and distal third of the leg) and level_3 (midpoint of the leg) within approximately 5 mm from the medial margin of the tibia.

Author(s) Disclosures: No conflicts of interest or financial support.