Microsurgical Anatomy of the Safe Entry Zones to the Anterolateral Pons in an Anterior Transpetrosal Approach

Introduction: The anterior transpetrosal approach provides excellent exposure of the middle petroclival region and anterolateral pons. Intraoperative nerve monitoring and tractography have significantly expanded the role of surgical intervention in the management of brainstem lesions, along with a better understanding of the anatomy of safe entry zones and their relationships to critical white matter tracts and nuclei of the brainstem. We evaluate the accessibility of these safe entry zones to the anterolateral pons through an extended middle fossa anterior transpetrosal approach.

Methods: Extended middle fossa anterior petrosectomies were performed on 5 cadaveric heads (10 sides) injected with colored latex. The surgical corridor was extended anteriorly to expose the abducens nerve, posteriorly to the superior semicircular canal, laterally until the geniculate ganglion, and medially until the petrous ridge was completely removed. The porus trigemini were opened and drilling of the posterior third of the floor of Meckel’s cave was performed allowing mobilization of the trigeminal nerve inferiorly. A qualitative evaluation of the accessibility of the main safe entry zones to the anterolateral pons, including the peritrigeminal, supratrigeminal, and lateral pontine zones was performed,

Results: The peritrigeminal zone was completely exposed allowing for visualization of both the trigeminal and facial nerve exit zones. The supratrigeminal zone was visualized and partially accessible after gentle inferior mobilization of the trigeminal nerve. The working angle in the lateral pontine zone was found to be inhibited by the cochlea; however, it improved following posterior extension of bone removal in the postmeatal triangle.

Conclusion: The anterior petrosectomy provides excellent exposure of the peritrigeminal zone. Mobilization of the trigeminal nerve allows for adequate exposure of the supratrigeminal zone. Access to and surgical freedom around the lateral pontine zone was improved by additional bone removal in the postmeatal triangle.

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