To establish the safest approach to needle electrode insertion into the flexor pollicis longus (FPL) regarding possible needle injury to the superficial radial nerve (SRN) or radial artery by ultrasonography.

We evaluated 54 forearms of 27 healthy subjects. Three levels were defined in the forearm. Level 1 is the junction of the middle and distal third of the forearm, level 3 is the midpoint of forearm length, and level 2 is the midpoint between two levels. At each level, the distance between the most prominent point of the radius and the SRN (region A), the distance between the SRN and the radial artery (region B), and the depth from the skin surface to the FPL were measured.

The distance of region A was 1.20±0.41 cm in level 1, 1.62±0.45 cm in level 2, and 1.95±0.49 cm in level 3. The distance of region B was 1.02±0.29 cm in level 1, 0.61±0.24 cm in level 2, and 0.37±0.19 cm in level 3. The depth from the skin surface to the FPL was 0.92±0.20 cm in level 1, 1.14±0.26 cm in level 2, and 1.45±0.29 cm in level 3.

The safest needle insertion point to the FPL is the middle of the forearm within approximately 0.8 cm from the most prominent point of the radius. We recommend that the needle is inserted at the above point perpendicular to the skin surface until the needle meets the FPL at a depth of approximately 1.45 cm from the skin surface.