Anterior clinoidectomy is an important tool for neurovascular and skull base surgery. This technique is cardinal in expanding access to the sella, proximal carotid artery, and central skull base. The goal of anterior clinoidectomy is to the reveal the more proximal ophthalmic and clinoidal segments of the internal carotid artery (ICA) while unroofing the optic canal and skeletonizing the proximal optic nerve. This maneuver expands the optico-carotid and carotid-oculomotor access windows and therefore the operative corridor to the interpeduncular cisterns. Both the carotid artery and optic nerve are partially untethered and can be more safely and gently mobilized.

Controversy surrounds the value of clinoidectomy for “appropriately selected” cases. Moreover, there is no consensus on the indications and benefits of extradural versus intradural clinoidectomy. Many reports of modified variations of clinoidectomy have complicated the technical complexity of this osteotomy and have further dampened some of my colleagues’ enthusiasm for adopting this technique. This chapter aims to clarify and simplify the appropriate indications for clinoidectomy.

The indications and advantages of extradural clinoidectomy are:

1. The extradural space allows a more “aggressive” osteotomy of the medial sphenoid wing and clinoid while protecting the intradural neurovascular structures during drilling.
2. This maneuver is less selective and more inclusive for various pathologies.
3. It is more beneficial for resection of medial sphenoid wing meningiomas as it allows early devascularization of the tumor and decompression of optic nerve. The tumor-infiltrated clinoid can be removed. Upon early decompression of the nerve, further tumor manipulation may not place the nerve at risk of traction injury.
4. This maneuver is also used as part of the extradural middle fossa approach to the lesions of the cavernous sinus.

The indications and advantages of intradural clinoidectomy are:

1. The intradural space allows limited osteotomy of the clinoid while the intradural structures are potentially at risk during drilling and are exposed to bone dust.
2. This maneuver is more selective and the osteotomy can be tailored to the underlying pathology.
3. It is desirable for clip ligation of ophthalmic and paraclinoid artery aneurysms since bony removal can be tailored and clinoidectomy can be executed under careful monitoring of the aneurysm to prevent manipulations that could potentially place the aneurysm at risk of intraoperative rupture. Partial clinoidectomy may also be necessary for proximal posterior communicating (P-Comm) artery aneurysms.

An intradural clinoidectomy may be associated with a smaller risk of cerebrospinal fluid (CSF) rhinorrhea for two reasons. First, by performing an early intradural inspection of the pathoanatomy, a clinoidectomy may be deemed unnecessary. Second, a partial clinoidectomy tailored to the pathology at hand may obviate the need for a complete clinoidectomy. These two factors may lead to a decreased need for osteotomy and subsequent violation of the sphenoid and even the ethmoid sinuses. The risk of oculomotor nerve injury may also be slightly decreased with extradural clinoid resection. This difference may be due to medial bone removal early in surgery during an intradural versus extradural clinoidectomy.

Adherence of the aneurysm dome to the clinoidal dura is a contraindication for extradural clinoidectomy. In addition, erosion of the clinoid by the aneurysm dome and the presence of carotidoclinoid foramen demand an intradural technique.

Ultimately, surgeon’s preference and comfort level dictates “which side of the dura” is entered to execute anterior clinoidectomy. I prefer the extradural approach due to my experience level with this procedure. I also prefer to avoid drilling within the intradural space if at all possible to minimize the risk of injury to the neighboring structures.

Note that intradural clinoidectomy can be tailored and not all the steps mentioned below are mandatory. Initial intradural inspection of the lesion dictates the need for the extent of clinoid osteotomy.

The extent of clinoid aeration (pneumatized clinoid) may be evaluated on preoperative CT scan and predicts the need for more robust plugging of the bony defect after clinoidectomy to avoid a CSF fistula. Thin-slice CT also determines the ossification of the clinoidal ligaments, specially the carticoclinoid and interclinoid ligaments. For further details on the anatomical variations of the anterior clinoid process, please refer to the Extradural Clinoidectomy chapter.

In the case of proximal paraclinoid aneurysms, control over the ICA must be established via neck dissection and the neck area is prepared, especially in the case of ruptured aneurysms.

Extension of the aneurysm neck proximal to the distal dural ring should be assessed on a preoperative CT angiogram. Although following clinoidectomy, opening of the outer dural ring provides access to additional few millimeters of aneurysm neck, significant extension of certain paraclinoid aneurysm necks into the cavernous sinus precludes the use of an open microsurgical route for clip ligation.

The frontotemporal dura defines the superior and lateral borders to the clinoid while the periorbita and optic strut limit the anterior and inferior borders. The clinoid segment of the internal carotid artery (ICA) provides the posterior limit inferomedially. Inferolaterally, the neurovascular structures of the cavernous sinus lie under a layer of periosteal dura emerging anteriorly into the superior orbital fissure.

Given the large number of neurovascular structures in this region, the importance of any additional created space or mobility of any nerves or key vessels is immediately apparent clinically. A major impediment to learning this technique is a lack of three-dimensional understanding of the clinoid and the surrounding vital structures. I will attempt to simplify the germane operative anatomy.

The following images emphasize three osteotomies required for disconnection of the clinoid

1. Osteotomy along the medial lesser sphenoid wing to disconnect the lateral connection
2. Osteotomy along the orbital roof to disconnect the anterior connection
3. Drilling within the clinoid to create an “egg shell” and disconnect the anteroinferior connection to the optic strut

After the completion of the above three steps, the shell of clinoid can be mobilized and dissected away from the clinoidal ligaments.

Most aneurysms and tumors arising from this region can be accessed via a standard pterional or orbitozygomatic craniotomy tailored to their specific configuration. The lesser sphenoid wing is removed and the superior orbital fissure may be skeletonized through the extradural approach as part of the standard or extended pterional craniotomy.

If there is concern about a pneumatized clinoid, a small piece of temporalis muscle may be placed within the defect and reinforced with a small amount of fibrin glue. The rest of the dura is closed in a standard fashion.

The patient is monitored for CSF rhinorrhea, changes in visual acuity, and diplopia. Postoperative rhinorrhea is first managed through temporary lumbar drainage. This maneuver is often effective for cessation of the leak if the fistula is suspected at the level of clinoid resection. If not effective, the patient will be returned to the operating room for repacking of the clinoidectomy defect.

• Extradural clinoidectomy is my preferred method. For paraclinoid aneurysms adherent to the dura, tumors associated with highly hyperostotic bone, or patients with ossification of clinoid ligaments, an intradural clinoidectomy or combined approach is more reasonable.

• "Egg-shelling” the medial clinoid is a safe method of performing a tailored clinoidectomy.
• Regardless of the technique used for clinoidectomy (drilling or ultrasonic bone curettes), copious irrigation is paramount to avoid thermal injury to the optic nerve.
• Once the medial clinoidectomy is performed, bony removal can be extended to the optic strut, lesser sphenoid wing and roof of the optic nerve. These maneuvers can be carried out based on the location and features of the pathology at hand.

Contributor: Marcus A. Acioly, MD, PhD

DOI: https://doi.org/10.18791/nsatlas.v2.ch09.2