Classification of the External Branch of the SLN [54] .

Type 1: External branch of the SLN crosses the superior thyroid vessels 1 cm or more above a horizontal plane passing through the upper border of superior thyroid pole.

Type 2a: The nerve crosses the vessels less than 1 cm above the plane.

Type 2b: The nerve crosses the vessels below the plane.

・ Cernea type 2b is the most common position.

・ Cernea type 2b nerves are also the most vulnerable to injury during thyroidectomy [55] .