Image A identifies the surface landmarks for performing an interscalene block. In image B, the patient is flexing the neck to tense the sternocleidomastoid muscle for easier identification of the posterior border of the clavicular head and the interscalene groove.
CC: cricoid cartilage; SCM-S: sternal head of the sternocleidomastoid muscle; SCM-C: clavicular head of the sternocleidomastoid muscle.