Background: The course of the spinal accessory nerve in the neck is long and
superficial rendering it at high risk of injury during procedures performed in the
posterior triangle. The majority of spinal accessory nerve injuries are iatrogenic in
nature. This is associated with significant morbidity including reduction in shoulder
movements, drooping of the shoulder, winging of the scapula and neuropathic
pain. Knowledge of the nerve anatomy reduces the risk of intra-operative nerve
injury. Traditional teaching describes the point of entry into the posterior triangle
as the intersection between the upper and middle third of the posterior border
of sternocleidomastoid. The aim of this study was to determine whether this is
in fact the case and if so, whether this landmark can reliably be used to identify
the spinal accessory nerve in order to improve patient outcomes.
Materials and methods: The spinal accessory nerve was identified unilaterally
in 26 cadavers. The total length of sternocleidomastoid was measured as well
as the length along the posterior border from the inferior aspect of the mastoid
process to the point at which the accessory nerve enters the posterior triangle
of the neck. These measurements were used to calculate the ratio of the entry
point of the nerve into the posterior triangle along the length of the posterior
border of sternocleidomastoid from its superior insertion point. The mean ratio
was 0.35 with 95% confidence intervals of 0.33 to 0.36.
Results and Conclusions: Our findings confirm the traditional description of the
entry point of the spinal accessory nerve into the posterior triangle of the neck. We
describe a so-called ‘safe zone’ inferior to the midpoint of the posterior border of
sternocleidomastoid within which the spinal accessory nerve is unlikely to be found,
thereby reducing the risk of iatrogenic injury