Anatomic landmarks for localization of the spinal accessory nerve.

This anatomical study examines the anatomic topography and landmarks for localization of the spinal accessory nerve (SAN) during surgical dissections in 40 fresh human cadavers (2 females and 38 males; ages from 22 to 89 years with a mean of 60 years). In the submandibular region, the SAN was found anteriorly to the transverse process of the atlas in 77.5% of the dissections. When the SAN crossed the posterior belly of the digastric muscle, the mean distance from the point of crossing to the tendon of the muscle was 1.75 +/- 0.54 cm. Distally, the SAN crossed between the two heads of the SCM muscle in 45% of the dissections and deep to the muscle in 55%. The SAN exited the posterior border of the sternocleidomastoid muscle in a point superior to the nerve point with a mean distance between these two anatomic parameters of 0.97 +/- 0.46 cm. The mean overall extracranial length of the SAN was 12.02 +/- 2.32 cm, whereas the mean length of the SAN in the posterior triangle was 5.27 +/- 1.52 cm. There were 2-10 lymph nodes in the SAN chain. In conclusion, the nerve point is one of the most reliable anatomic landmarks for localization of the SAN in surgical neck dissections. Although other anatomic parameters including the transverse process of the atlas and the digastric muscle can also be used to localize the SAN, the surgeon should be aware of the possibility of anatomic variations of those parameters. Similar to previous investigations, our results suggest that the number of lymph nodes of the SAN chain greatly varies.