Four embalmed and injected cadaveric heads were used for this study. All heads were naïve from prior cranial operations. KARL STORZ VITOM® HOPKINS® Telescope 0˚; KARL STORZ HOPKINS ® Straight Forward Telescope ˚0; and KARL STORZ HOPKINS ® Forward-Oblique Telescope 30˚; endonasal skull base instruments (KARL STORZ United States, El Segundo, CA) were used for the dissections.
A total of 4 cadaveric heads were dissected in the following sequence: All 4 specimens underwent eTONES approach on one side. Three of the eTONES approaches were aimed at generating maximal exposure of the petrous ridge to allow an extensive petrosectomy. The final specimen underwent a slightly varied superior eTONES procedure, as described next, to provide surgical access to the anterior petrous apex and petroclival region. All four specimens then underwent traditional Kawase subtemporal anterior petrosectomy on the contralateral side to the eTONES. Of note, on a single specimen, an attempt was made to expose the petrous ridge via TONES with only lateral orbital rim removal. This approach provided an insufficient degree of freedom for surgical instruments and was therefore followed by eTONES according to the describe protocol. All specimens underwent thin sliced head CT scans before and after the bilateral petrosectomy. Using Brainlab iPlan Cranial 3.0 the anatomic limitations and size of petrosectomy were measured on all 8 sides and compared between eTONES and the traditional Kawase approach.
eTONES dissection Protocol:
A surgical microscope or exoscope is used for the initial exposure.
1. A superior eyelid skin incision is marked from the supraorbital notch medially to 2 cm lateral to the lateral canthus.
2. Orbicularis oculi muscle is dissected in a superior-lateral direction to expose the superior and lateral orbital rims.
3. The superior orbital rim is exposed starting medially at the supraorbital foramen, and the lateral orbital rim (fronto-zygomatic arch) exposed inferiorly to the zygoma.
4. The anterio-superior insertion of the temporalis muscle is dissected and reflected posteriorly to fully expose the fronto-zygomatic arch.
5. The globe is dissected in the periorbital plane and retracted medially.
6. A burr hole is made along the lateral edge of the superior orbital rim and frontal dura is exposed.
7. Epidural dissection of the frontal tip is performed by advancing a dissector in the epidural plane from the burr hole to the supraorbital foramen.
8. The medial vertical bony cut is marked on the superior orbital rim, lateral and adjacent to the supraorbital foramen.
9. Starting at the burr hole, while protecting the frontal dura with the epidural dissector, using bone scalpel or oscillating saw, a horizontal bony cut is fashioned along the superior orbital rim to the medial mark adjacent to the supraorbital foramen.
10. A vertical cut is performed at the medial mark and connected with the initial horizontal cut. This second vertical cut is continued posteriorly along the orbital roof for 2 cm, while protecting the globe and frontal dura.
11. A bony cut of the lateral orbital rim is performed next, beginning at the burr hole superiorly and ending at the root of the zygoma inferiorly.
12. While retracting the globe, an inferior horizontal cut of the lateral orbital rim is fashioned, just superior to the root of the zygoma.
13. While retracting the globe inferio-medially, using a chisel and hammer the final bony cut of the supero-lateral orbit is completed by linking the previous cuts.
14. The single piece superior and lateral orbital rims are removed.
15. The superficial lateral orbital wall and greater sphenoid wing are drilled next while the globe is retracted medially.
16. The recurrent meningeal artery transversing the lateral orbital wall is identified, coagulated and incised.
17. The Superior Orbital Fissure (SOF) and Inferior Orbital Fissure (IOF) are identified deep to the greater sphenoid wing.
18. Following SOF and IOF identification, the residual lateral orbital wall is drilled and the temporal dura is exposed.
An neuroendoscope is used from this point for increased magnification and illumination.
19. SOF and IOF mark the superior, medial, and inferior borders of the approach. The remaining lateral orbital wall is drilled once these landmarks are clearly identified.
20. The SOF and IOF lateral bone edge is removed to expose a continuous dural plane extending from the periorbita anteriorly to the temporal dura.
Standard eTONES – extended middle petrosectomy and exposure of Cerebello-Pontine Angle (CPA) and Internal Acoustic Canal (IAC):
21. The IOF bone edge is drilled flush with the middle fossa floor.
22. The middle fossa floor is then dissected [posterior-medially, exposing the epidural plane, and the foramen ovale with the third trigeminal division (V3). This marks the medial interdural dissection plane.
23. The interdural plane is dissected posteriorly between the medial temporal dura and the cavernous sinus lateral wall marked by the trigeminal divisions.
24. Interdural dissection is continued posteriorly towards the Gasserian Ganglion (GG).
25. Once the GG is identified, the epidural plane is dissected posterior-laterally to expose the anterior petrous ridge.
26. The superior petrosal sinus (SPS) marking the petrous ridge, and the Greater Speno-Palatine Nerve (GSPN), overlying the petrous ICA are identified as superior and inferior drilling limits, respectively.
27. The petrous ridge is drilled lateral to the Gasserian Ganglion, inferior to the SPS, and superior to GSPN to expose Internal Acoustic Canal (IAC) and the posterior fossa dura.
28. Dura is opened to expose the Cerebello-Pontine Angle (CPA) and Internal Acoustic Canal (IAC) contents.
Superior eTONES – petroclival and petrous apex exposure:
24. The lesser sphenoid wing is drilled just superior-lateral to the Meningo-Orbital Band (MOB) and inferior frontal dura is exposed.
25. The MOB is incised to detach the middle fossa dura from the periorbita.
26. The interdural plane is dissected between the middle fossa dura laterally and the cavernous sinus lateral dura.
27. The superior division of the trigeminal nerve (V1) and the Occulomotor nerve (CNIII) are identified medial to the dissection plane.
28. Interdural dissection is continued posterior-medially along V1 until the petrous apex is identified, at the petroclival junction superior to the GG.
29. The anterio-medial petrous apex is drilled to expose posterior fossa dura.
30. Dura is opened to expose the superior-medial CPA and ventral midbrain.
Key features of the described approach are shown in Figure 1