Orbitozygomatic Craniotomy

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Orbitozygomatic Craniotomy


Gregory P. Lekovic, L. Fernando Gonzalez,
Felipe Santos, and Marc S. Schwartz


 


The guiding principle of skull base surgery is the avoidance of brain retraction by the removal of additional bone. The orbitozygomatic craniotomy may augment exposure under the frontal lobe, through the sylvian fissure, along the tip of the temporal lobe, and in other locations. The initially described approach includes removal of the superior and lateral orbital rim as well as the zygomatic arch. This approach may be tailored to specific pathology as well. This chapter describes the standard orbitozygomatic craniotomy as well as several variations that may be used to either reduce or extend dissection as indicated for different situations.


The main advantages of the orbitozygomatic approach are (1) the supraorbital rim is removed from the surgeon’s line of sight, facilitating an upward and oblique view to the interpeduncular fossa and third ventricle; and (2) the temporalis muscle is retracted laterally and inferiorly, instead of anteriorly, by removing the arch of the zygoma, thus giving the surgeon a much wider corridor within which to approach deep-seated lesions for which the alternative would be a traditional subtemporal approach. Finally, the extent of bone removal minimizes brain retraction and makes the orbitozygomatic approach ideal for extradural approaches to the cavernous sinus and anterior petrous apex.


image Surgical Anatomy


The frontal branch of the facial nerve leaves the pes anserinus in the parotid, lies deep to the temporoparietal fascia, and may be injured during scalp dissection. The temporoparietal fascia is in continuity with the galea superiorly and the superficial musculoaponeurotic system (SMAS) inferiorly. There is an avascular loose areolar tissue layer between this and the underlying deep temporal fascia that is in continuity with the subgaleal plane. Superiorly, the deep temporalis fascia is a single sheet that splits into two layers (superficial layer and deep layer of the deep temporal fascia) enveloping the superficial temporal fat pad. In addition, care should be taken to preserve, whenever possible, the superficial temporal artery. Though in our experience vascular compromise of the flap has not been an issue when this artery is sacrificed during exposure, its preservation may be critical in those rare instances where a low-flow extracranial to intracranial bypass is needed.


The medial extent of the orbitozygomatic craniotomy is usually demarcated by the supraorbital notch, through which travel the supraorbital nerve and an associated vascular bundle. When encased completely in bone or more laterally situated than usual, it can be easily mobilized with an osteotome if needed. The inferior extent of the orbitozygomatic exposure is limited by the zygomaticofacial foramen. The posterior border of the bony exposure runs along a line connecting the supraorbital to infraorbital fissures. The key landmark for the successful completion of the orbitozygomatic osteotomies is this latter completely extracranial fissure in the inferolateral wall of the orbit. It can be palpated from within the orbit with a Penfield No. 4 dissector, and is the starting point for the reciprocating saw cut through the malar eminence. Similarly, it is the point to which the reciprocating cuts through the sphenoid bone from the superior orbital fissure connect. Hence, at the completion of the “full” orbitozygomatic (OZ) osteotomy, the surgeon removes the roof and lateral wall of the orbit en bloc. This facilitates orbital reconstruction and reduces the risk of enophthalmos postoperatively.


image Preoperative Workup


Gadolinium-enhanced magnetic resonance imaging is essential for planning for all tumors of the skull base; for intraorbital tumors and for those in the petrous apex and or clivus, fat suppression should be obtained as well. For paraclinoid aneurysms of the internal carotid artery, computed tomography angiography (CTA) is very helpful in determining the relationship of the aneurysm origin to the optic strut; this in turn helps differentiate cavernous sinus aneurysms from those that risk rupture into the subarachnoid space. In fact, digital subtraction angiography can usually be avoided for most aneurysms; high-resolution preoperative CTA or magnetic resonance angiography (MRA) is sufficient for surgical planning.


image Surgical Technique


Monitoring


We routinely monitor somatosensory evoked potentials intraoperatively; depending on the nature of the pathology, motor evoked potentials or electrocorticography may be indicated as well. In our experience motor evoked potentials require total intravenous anesthesia, as inhalational anaesthetics cause degradation of the motor response over time.


Anesthetic Considerations


Serial compression devices are placed on all patients, and all patients receive perioperative antibiotics. We prefer cefuroxime 1.5 g IV. Intravenous mannitol up to 1 g/kg may be given to facilitate brain relaxation. As mentioned above, muscle relaxants are avoided or eliminated to optimize electrophysiologic monitoring. For vascular lesions, anesthesiology should be prepared to administer barbiturates for burst suppression, and to induce hypotension if needed.


Variations


Full Orbitozygomatic Craniotomy

The patient is rigidly fixated in a Mayfield head holder and positioned slightly vertex down with the head rotated to the contralateral side, so that the ipsilateral malar eminence is at the top of the operative field. A fingerbreadth of hair along the planned incision is clipped and the skin infiltrated with local anesthetic. The scalp incision extends from the root of the zygoma, just anterior to the tragus, to past midline, at the level of the contralateral midpupillary line, just behind the hairline (Fig. 1.1). The pericranium should be elevated separately in case the frontal sinus is violated during the orbital osteotomies.




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Fig. 1.1 Frontotemporal scalp incision just behind the hairline. The anterior limb of the incision is typically brought across midline.


Injury to the frontal branch of the facial nerve can be avoided either by performing a subfascial dissection of the temporalis fascia, or through an interfascial dissection. This latter approach is more time-consuming. Practically speaking, the nerve travels in the fat pad between the superficial and deep layers of the temporalis fascia within a fingerbreadth of the supraorbital rim; hence, injury to the nerve can reliably be avoided by performing a subfascial dissection from this point forward (approximately 3 cm behind the supraorbital rim at the temporal line).


Next, the supraorbital rim, frontozygomatic process, and malar eminence are exposed by elevating the frontal pericranium and temporalis fascia with a sharp-edged periosteal elevator. The root of the zygoma can usually be palpated, and the superficial investment of the temporalis fascia on the temporal zygomatic arch dissected free (Fig. 1.2).


Next, the periorbita is elevated from the roof of the orbit with a Penfield No. 1 elevator. Thin cottonoids or Telfa strips may be used both to facilitate the dissection and protect the periorbita from tearing (Fig. 1.3). It is often impossible to elevate the periorbita without tearing it. This may result in the periorbital fat herniating through the opening and obscuring the surgeon’s line of sight, in which case the fat can be shrunk with bipolar coagulation and small rents in the periorbita tacked closed with 4–0 Nurolon suture.


The temporalis muscle is then split and its deep periosteal attachment elevated from inferior to superior, staying in the direction of the fibers of the muscle. We prefer to split the muscle sharply and obtain hemostasis with bipolar electrocautery, rather than incising the muscle with the Bovie, to minimize temporalis atrophy (Fig. 1.4). The temporalis muscle is then reflected anteriorly and secured with fishhooks. The pterion is identified, and bur holes placed on either side of the greater wing of the sphenoid. A standard, kidneybean–shaped pterional flap is then elevated. Although for the “two-piece” OZ craniotomy it is not critical for a bur hole to be placed at the true McCarty “keyhole,” positioning this flap as close to the floor of the anterior fossa (i.e., as close to the orbital roof) as possible and incorporating as much of the greater wing of the sphenoid as possible greatly facilitates visualization of the subsequent orbitozygomatic osteotomies (Fig. 1.5).



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Fig. 1.2 Reflected scalp showing exposure of the orbital rim and zygoma. Care is taken to expose medial to the supraorbital bundle. The root of the zygoma is exposed circumferentially.



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Fig. 1.3 Elevation of periorbita using a sharp-edged instrument such as a Penfield No. 1 elevator. Cottonoid or Telfa strips may be useful in protecting orbital contents, which tend to herniated through the very thin and typically adherent periorbita.

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Apr 14, 2018 | Posted by in NEUROSURGERY | Comments Off on Orbitozygomatic Craniotomy
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