The Inferior Nuchal Line: A Simple Landmark to Avoid Vertebral Artery Injury during the Retrosigmoid Approach

Introduction: The V3 segment of the vertebral artery (V3–VA) is at risk during various approaches to the craniovertebral junction. Several landmarks have been proposed to protect and/or identify the V3–VA. However, none of these landmarks are routinely exposed during a retrosigmoid approach where the V3–VA may be at risk while performing its musculocutaneous stage, especially when the exposure is needed to be extended inferiorly toward the foramen magnum. The aim of this study is to assess the anatomical features of the inferior nuchal line (INL) as a landmark to protect and/or identify the V3–VA during the retrosigmoid approach.

Methods: Seven cadaveric heads underwent retrosigmoid approach in the lateral position. The V3–VA depth was measured relative to three points on the INL: the lateral and medial INL ends as well as its mid-point. The vertical distance between the V3–VA and the suboccipital bone was measured and the occipital condyle, the distance between its medial and lateral ends were measured from the posteromedial and posterolateral points on the occipital condyle, respectively.

Results: The INL was an L-shaped bony ridge with horizontal and vertical arms. The (lateral) vertical arm was more conspicuous in all specimens (INLV) and its mean length was 25.4 ± 3.9 mm. The depths of the V3–VA relative to the medial, middle, and lateral ends of the INLV were 24.9 ± 7.1 mm, 15.7 ± 5.5 mm, and 8.3 ± 3.2 mm, respectively. The medial depth was shown to be deeper than the middle depth (p < 0.01) and the middle depth was deeper than the lateral depth (p < 0.01). In all specimens, the V3–VA was located inferior, and anterior to the INLV. The distance from V3–VA to suboccipital bone was 7.8 ± 1.7 mm.

Conclusion: The V3–VA is consistently found deep (i.e., anterior and inferior) to the INLV during the retrosigmoid approach. The distance between V3–VA and INL was largest at the lateral end and smallest at the medial end. Using subperiosteal dissection and avoiding electrocautery inferior to the INLV are simple methods to avoid injury to the V3–VA during a retrosigmoid approach.

No conflict of interest has been declared by the author(s).