Anterior Petrosectomy

Mohammad Abolfotoh and Khaled El-Bahy

33.1 Introduction

Anterior petrosectomy, which means drilling the apex, is the surgical approach that is defined by the removal of the petrous part of temporal bone to facilitate the exposure of the posterior cranial fossa primarily through any of the middle cranial fossa approaches. It may be done after removal of the zygomatic arch, following either a pterional (fronto-temporal), zygomatic, cranio-orbito-zygomatic approach, or through temporal and subtemporal approaches (see Chapters 15, 17, 19, 20, 21 respectively); however, these approaches require significant temporal lobe retraction and give limited access to lower brainstem.

33.2 Patient Positioning (Fig. 33.1)

• Position: The patient is positioned supine.

• Body: The trunk is elevated 20° from horizontal.

• Head: The head is extended and rotated 30° to the contralateral side, then fixed to three point Mayfield fixator. The head should not be over-extended or over-turned to avoid stretching the neck and losing the corridor to the petrous apex.

Fig. 33.1 Supine position for the anterior petrosectomy approach. The chest is elevated, the head is rotated to the other side and fixed by 3 pins head clamp.
Abbreviations: IL = incision line.

• The ipsilateral shoulder should be elevated on a roll to avoid stretch of neck veins.

• The aim is to keep the zygoma at the highest point in the patient head and almost horizontal, and middle fossa floor should be almost vertical.

33.3 Skin Incision (Figs. 33.1, 33.2)

Skin incision depends on the craniotomy.

• Curvilinear skin incision for zygomatic, cranio-orbitozygomatic, and pterional approaches.

◦ Starting point: Incision starts 5 mm just in front of the tragus. Care should be taken to avoid injuring facial nerve branches and the tragus. The way to extend this point down (not routinely needed) is to extend it very superficially in the skin in front of the ear lobule then up to turn around the mandibular angle then down again in the transverse crease of the neck.

◦ Course: Incision runs upward just behind the hairline.

◦ Ending point: Incision line ends on the midline behind the hairline; it could be extended beyond the midline to facilitate retraction of the skin flap to the anterior cranial base (if needed).

• Variants: Other skin incisions such a straight temporal skin incision for temporal craniotomy or inverted U-shaped incision around ear pinna for subtemporal approach.

33.3.1 Critical Structures

• Superficial temporal artery should be preserved to avoid wound bleeding during surgery, also for better blood supply to the flap (and may be the muscle), and for external-internal carotid (EC-IC) bypass (see Chapter 48).

33.4 Soft Tissues Dissection

Soft tissue dissection techniques are already described in Chapters 15 and 17.

33.5 Craniotomy/Middle Fossa (Zygomatic) Approach (Fig. 33.2)

• Zygomatic osteotomy

◦ Zygomatic arch is freed from the deep fascia at its upper edge and kept attached to the masseter muscle at its lower edge. Alternatively, it can be completely freed from muscle insertions and removed.

◦ Two V-shaped osteotomies are done on the zygoma. The anterior cut should be placed just behind the malar eminence, while the posterior cut is made just in front of the posterior root of the zygoma. The osteotomy has to be shaped as a “V,” in order facilitate further reconstruction.

Fig. 33.2 Zygomatic arch (black dots), temporal craniotomy (blue line), the main burr hole (blue dot), at the posterior temporal bone, above the posterior root of the zygomatic arch.
Abbreviations: BH = burr hole; IL = incision line; Z = zygoma.

◦ At this point the temporal muscle together with the zygomatic arch can be retracted downward through the zygomatic arch defect.

◦ Zygomatic osteotomy gives excellent exposure flush to the middle cranial base and avoids the need for temporal lobe retraction.

• Temporal craniotomy

◦ Single burr hole:

– The main burr hole in this approach is a posterior basal temporal burr hole.

– It is placed just above the root of the zygoma.

– A second burr hole might be placed at the keyhole if frontal or orbital extension is needed (cranio-orbitozygomatic extension).

◦ Cut: The temporal craniotomy might be performed by a craniotome (cranio-zygomatic approach). It is tailored according to the pathology.

◦ As mentioned above, the zygoma might be removed separately, or kept attached to the masseter muscle or the cranial flap.

33.6 Middle Fossa Dissection

33.6.1 Identification of the Greater Superficial Petrosal Nerve (GSPN)

• The middle meningeal artery has to be identified on the dural surface at foramen spinosum. Once identified, the artery has to be obliterated and coagulated (Fig. 33.3).

◦ TIP: Leaving a few millimeters stump of the artery at the foramen spinosum enables the surgeon to use it as a landmark for surgical orientation.

• The mandibular nerve (V3) might be identified anteriorly and medially to the foramen spinosum, at its entrance into the foramen ovale.

• Close to V3 the greater superficial petrosal nerve (GSPN) might be found. It has to be highlighted that GSPN runs in a shallow bony groove, and that it is accompanied by the lesser petrosal nerve and a blood vessel (the petrosal branch of the middle meningeal artery). Many surgeons find this very tiny blood vessel helpful as a marker of the GSPN (Fig. 33.4).

◦ TIP: In this region of the middle fossa floor fibrous bundles of dura are usually encountered, which might be easily mistaken as the GSPN (Fig. 33.5).

• The GSPN must be followed backward toward the facial hiatus. In 16% of cases the genu of the facial nerve is protruding from the facial hiatus.

◦ TIP: The separation of the dura at the middle fossa floor in this area should start from posterior to anterior to avoid damaging the nerve.

• Traction on GSPN should be avoided as it might cause facial nerve palsy.

• GSPN division is not recommended as in most cases there is no need for that; nerve division might cause eye dryness, because of the damage on preganglionic parasympathetic fibers to the pterygopalatine ganglion. Moreover, nerve preservation is im portant, given the GSPN is considered a landmark for the horizontal part of the internal carotid artery (ICA) and the petrous apex.

33.6.2 Identification of the ICA

• One of the most important reasons to identify and preserve the GSPN is to identify the petrous carotid artery. The horizontal part of the petrous ICA runs in the bony petrous canal, which is located roughly parallel and just below the nerve in the foramen lacerum (Fig. 33.6).