Access to the Petrous Apex: An Anatomical Study through an Endoscopic Endonasal Approach and a Transorbital Endoscopic Approach

Introduction: The endoscopic endonasal approach (EEA) and the transorbital endoscopic approach (TOEA) provide distinct exposures and use different trajectories to access the petrous apex (PA). The EEA primarily reveals the inferior and medial aspect of the PA, while the TOEA provides an exposure similar to the anterior transpetrosal approach, that it is the superior surface of the PA.

Objective: This study aims to describe the surgical anatomy of the petrous apex and provide a step-by-step guide for performing both the EEA and TOEA to the PA.

Materials and Methods: The study involved the dissection of four sides from two cadaveric head specimens using a 4-mm 18-cm rigid endoscope (Karl Storz) with 0-, 30-, and 45-degree lenses.

Results: To perform an EEA to the PA the key steps are: a wide sphenoidotomy, uncinectomy, maxillary antrostomy, etmoidectomy and perform a transmaxillary transpterigoid approach accessing to the lateral recess of the sphenoid sinus. Once the foramen lacerum is identified, proceed with the skeletonization of the paraclival ICA, drilling of the sphenoid sinus floor and middle clivus. Then removal of the mandibular strut and lingual process of the sphenoid bone will enable to lateralize the paraclival ICA and expose the medial PA ([Fig. 1]). To expose the inferior PA, perform a sublacerum approach ([Fig. 2]), by transecting the connection between the foramen lacerum and pterygosphenoidal fissure, proceed with mobilization of the eustachian tube. Remove the petrous process of the sphenoid bone, and then identify the petroclival fissure and VI nerve. The EEA provides a ventral perpesctive of the inferomedial PA, namely the petroclival fissure, and by using a medial to lateral trajectory, exposes the prepontine cistern and anterior surface of the brainstem, while lateral exposure is limited by the ICA.

For a TOEA, the procedure involved a superior eyelid incision, subperiosteal dissection, medial mobilization of the periorbit, identification of the superior and inferior orbital fissures, removal of the lateral orbital rim to enhance maneuverability, drilling of the greater sphenoid wing to expose the temporal dura, transection of the meningo-orbital band, and peeling of the middle fossa to identify cranial nerves III, V1, IV, V2, and V3 ([Fig. 1]). Exposure and drilling of the PA are delimited by the greater superficial petrosal nerve, petrous ridge, V3, and the projection of the internal acoustic canal. [Fig. 4] shows the exposure once the drilling is completed. This approach affords a straight view of the superior PA, and uses a lateral to medial trajectory, allowing exposure of the cavernous sinus, middle fossa and extent to the posterior fossa, accessing the anterolateral brainstem, V root and the VII–VIII complex.

Conclusion: The EEA and TOEA provide excellent exposure of the superior and inferomedial portions of the petrous bone, respectively. These approaches are viable options for operating on tumors located in the PA. However, it is crucial to have a thorough understanding of the complex anatomy involved in this region to tailor the approach.

Publication History

Article published online:
07 February 2025