Classification of the External Branch of the SLN  .
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  .