The Inferior Nuchal Line as a Simple Landmark for Identifying the Vertebral Artery During the Retrosigmoid Approach

Abstract BACKGROUND The V 3 segment of the vertebral artery (V 3 -VA) is at risk during various approaches to the craniovertebral junction. Several landmarks have been defined to identify V 3 -VA, but these landmarks are not routinely exposed during a retrosigmoid (RS) approach, where musculocutaneous dissection inferiorly towards the foramen magnum can threaten this arterial segment. OBJECTIVE To find a landmark that will identify the V 3 -VA during the RS approach, and analyze the inferior nuchal line (INL) as this novel landmark. METHODS The anatomic relationships between the INL and the V 3 -VA were assessed in 7 cadaveric heads through RS exposure in the lateral position. RESULTS The INL is an L-shaped bony ridge with horizontal (medial) and vertical (lateral) arms, with the vertical arm being more conspicuous in all specimens (INL V ). The mean depths of the V 3 -VA relative to the medial and lateral ends of the INL V were (mean ± standard deviation) 24.9 ± 7.1 mm, and 8.3 ± 3.2 mm, respectively. In all specimens, the V 3 -VA was located inferior and anterior to the INL V . CONCLUSION The INL provides an important landmark during RS approach that can protect the V 3 -VA from inadvertent injury or identify it for use in an interpositional bypass. The INL V identifies the region of the suboccipital triangle where the V 3 -VA is embedded. INL V is routinely seen during the RS approach, making it more relevant than other classic landmarks such as the transverse process of C1, C1 posterior arch, and the atlantomastoid line that are not exposed during the RS approach.