The Asterion to Transverse Process of the Atlas Line as a Surgical Landmark: Anatomical and Radiological Study

Introduction: Several landmarks have been proposed to predict the anatomical location and trajectory of the sigmoid sinus, with variable degrees of reliability. Even with the advent of neuro-navigation technology, such landmarks continue to be crucial in planning and performing complex approaches to the posterolateral skull base. By combining two major dependable structures, asterion (A) and transverse process of the atlas (TPC1), we studied the utility of the A-TPC1 line in relation to the sigmoid sinus and in partitioning surgical approaches to the region.

Methods: Six cadaveric heads (12 sides) were dissected to expose the posterolateral skull base, including the mastoid and suboccipital bone, TPC1 and suboccipital triangle, distal jugular vein and internal carotid artery and lower cranial nerves in the distal cervical region. We inspected the line between the asterion and TPC1, before and after drilling the mastoid and occipital bones exposing the sigmoid sinus. We studied the relationship of the sigmoid sinus trajectory and major muscular elements related to the line. We also retrospectively reviewed 31 CT angiograms of the head and neck (total 61 sides) from our PACS system, excluding posterior fossa or cervical pathologies. Using the Fujitsu Synapse 3D segmentation software, bone and vessels were reconstructed in three-dimensions. We measured the distance between the A-TPC1 line and sigmoid sinus at different levels: digastric point (DP), and maximal and minimal distances above and below the digastric notch. The Rhoton collection and clinical cases were reviewed to further illustrate the utility of the A-TPC1 line.

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Results: The A-TPC1 line averaged 65 mm in length, and was found to be consistently posterior to the sigmoid sinus in all cadaver specimens, coming closest to it at the level of the digastric notch. Using the transverse-asterion line as a rostro-caudal division and the skull base as a horizontal plane, we divided the major surgical approaches to the posterolateral skull base into four quadrants: distal cervical/extreme lateral and jugular foramen (anteroinferior), presigmoid/petrosal (anterosuperior), retrosigmoid/suboccipital (posterosuperior) and far lateral/foramen magnum regions (posteroinferior). Case illustrations were found to illustrate its utility in planning the surgical approach. From a radiographic perspective, the A-TPC1 line was also posterior to the sigmoid sinus in all sides. It came closest to the sinus at the level of digastric point (DP) (average 7 mm posterior, range 0–18.7 mm). The maximal distance above the DP averaged 10.1 mm (3.6–19.5 mm), and that below the DP 8.8 mm (−2 to 20 mm).

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Conclusion: The A-TPC1 line is a helpful landmark that is reliably found posterior to the sigmoid sinus in cadaveric specimens and radiographic CT scans. It can corroborate the accuracy of neuronavigation, assist in minimizing the risk of sigmoid sinus injury, and is a useful tool in planning surgical approaches to the posterolateral skull base, both preoperatively and intraoperatively.