Microsurgical Anatomy of the Triangles of the Posterior Fossa: Neurosurgical Relevance

Introduction: Tumors of the posterior fossa, which often distort regional anatomy, can be located either dorsal/lateral or ventral/medial to the cranial nerves. Further identification of anatomical landmarks and assigning a relative relationship to neurovascular elements be an important adjunct during surgical decision-making.

Objectives:

1. To describe a system of three anatomically-based triangles of the posterior fossa according to the relative position of neurovascular complexes.

2. To describe these triangles with clinical cases.

Method: Ten fully embalmed cadaveric specimens injected with colored silicon were studied bilaterally (20 sides). Retrosigmoid, extended retrosigmoid and far lateral approaches were studied and triangles were projected from key osseous landmarks (asterion, superior nuchal line (SNL), mastoid emissary vein foramen (MEVF) and condylar emissary vein foramen (CEVF). Neurovascular complexes, as previously described by Rhoton, forming these triangles consisted of the following components: cranial nerves, superior petrosal vein, tentorium, petrous bone, and brainstem. Clinical cases were described ([Fig. 1]).

Result: Topographic anatomy was divided in superior, middle, and inferior regions.

Superior: Formed by the asterion and SNL and corresponds with the upper neurovascular complex. The asterion was found superior to the SNL in seven sides, and at the same level to the SNL in three sides. In addition, it was found in the same position of the transversal sinus in 8 sides and above to the transversal sinus in two sides. The distance between the asterion and the (MEVF) was on average 19.75 mm. The petrous–tentorial triangle, formed by the superior petrosal venous complex, tentorium, petrosal and tentorial surfaces of the cerebellum. The petrous–acousticofacial triangle (PAFT), formed by the superior petrosal venous complex, acousticofacial complex, petrous bone, and pons.

Middle: formed by the MEVF and corresponds with the middle neurovascular complex. The MEVF was present in eight sides and absent in two sides; distance to the SNL was on average 13.25 mm. The MEVF was found laterally to the sigmoid sinus in all sides (average 10 mm). Acousticofacial–trigeminal triangle (AFTT), formed by the acousticofacial complex trigeminal nerve–cerebellar petrosal surface. The Acousticofacial–Glossopharyngeal triangle (AFGT), formed by the acousticofacial complex, CN IX, petrous bone, and brainstem ([Figs. 2]–[4]).

Inferior: formed by the CEVF and the occipital condyle and corresponds with the lower neurovascular complex. It was present in all sides and the distance between the MEVF and CEVF tracing a perpendicular line was on average 32.25 mm. The distance between SNL and CEVF was on average 47.10 mm and the distance between the SNL and occipital condyle was on average 50.70 mm. The vagoaccesory triangle formed by the cisternal segment of the vagus-spinal root of the accessory nerve and the medulla. The suprahypoglossal triangle (SHT) formed by the vagus, spinal, and the hypoglossal cranial nerves. The infrahypoglossal triangle (IHT) formed by the spinal, hypoglossal, and the medulla.

Conclusion: The development of anatomically based triangles in navigation the posterior fossa may facilitate in organization and categorization of regional anatomy. Explicitly, these triangles may serve as a reliable in deciding the appropriate surgical route in addressing pathologies of the cerebellopontine angle.

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