clinoidectomia

ANATOMY Surgical Anatomy and Technique

Sebastien C. Froelich, M.D. Department of Neurosurgery, University of Cincinnati College of Medicine, Cincinnati, Ohio, and Department of Neurosurgery, University of Strasbourg, Strasbourg, France

Khaled M. Abdel Aziz, M.D., Ph.D. Department of Neurosurgery, University of Cincinnati College of Medicine, Cincinnati, Ohio

Nicholas B. Levine, M.D. Department of Neurosurgery, University of Cincinnati College of Medicine, Cincinnati, Ohio

Philip V. Theodosopoulos, M.D. The Neuroscience Institute, Department of Neurosurgery, University of Cincinnati College of Medicine, and Mayfield Clinic, Cincinnati, Ohio

Harry R. van Loveren, M.D. Department of Neurosurgery, University of South Florida, Tampa, Florida

REFINEMENT OF THE EXTRADURAL ANTERIOR CLINOIDECTOMY: SURGICAL ANATOMY OF THE ORBITOTEMPORAL PERIOSTEAL FOLD OBJECTIVE: Extradural removal of the anterior clinoid process is technically challenging because of the limited exposure. In our study of the extradural anterior clinoidectomy, we describe anatomic details and landmarks to facilitate sectioning of the orbitotemporal periosteal fold and elevation of the temporal fossa dura from the superior orbital fissure. We assess the morbidity associated with these procedures as well as compare the indications, advantages, and disadvantages of intra- versus extradural clinoidectomy. METHODS: Of five formalin-fixed cadaveric heads, four were used for cadaveric dissections and one was used for histological examination. RESULTS: Sectioning of the orbitotemporal periosteal fold revealed a cleavage plane between the temporal fossa dura and a thin layer of connective tissue that covers the superior orbital fissure. The lacrimal nerve coursed immediately medial to this surgically created cleavage plane. The superior orbital vein crossed laterally under the cranial nerves, which pass through the superior orbital fissure. This vein is particularly vulnerable as it is composed only of endothelium and a basal membrane. CONCLUSION: Both intra- and extradural techniques for anterior clinoidectomy are important parts of the neurosurgical armamentarium. Sharp incision of the orbitotemporal periosteal fold to increase the extradural exposure of the anterior clinoid process should be made at the level of the sphenoid ridge and restricted to the periosteal bridge. Subsequent blunt elevation of the temporal fossa dura should be performed; however, peeling of the temporal fossa dura should be limited to avoid cranial nerve morbidity. KEY WORDS: Anterior clinoidectomy, Extradural clinoidectomy, Orbitotemporal periosteal fold, Superior orbital fissure, Surgical technique Neurosurgery 61[ONS Suppl 2]:ONS179–ONS186, 2007

DOI: 10.1227/01.NEU.0000280094.37231.D5

Jeffrey T. Keller, Ph.D. The Neuroscience Institute, Department of Neurosurgery, University of Cincinnati College of Medicine, and Mayfield Clinic, Cincinnati, Ohio Reprint requests: Jeffrey T. Keller, Ph.D., c/o Editorial Office, Department of Neurosurgery, University of Cincinnati College of Medicine, 231 Albert Sabin Way, ML 0515, Cincinnati, OH 45267-0515. Email: [email protected] Received, June 14, 2006. Accepted, May 14, 2007.

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nterior clinoidectomy is a technically challenging procedure because of the deep location of the anterior clinoid process (ACP) and the presence of eloquent surrounding structures. An intradural technique is preferred by most neurosurgeons because it provides wide exposure of the ACP and direct observation of the internal carotid artery (ICA) and optic nerve. Unroofing the optic canal and drilling the ACP extradurally are more difficult tasks that require precise knowledge of the threedimensional anatomy of the ACP and surrounding neurovascular structures. To increase the extradural exposure of the ACP, Dolenc (12, 13) and Dolenc and Roger

(14) proposed division of the periosteal fold between the periorbita and the dura of temporal fossa. He emphasized the risk of injuring the cranial nerves that course through the superior orbital fissure (SOF). Recent reports propose more extensive elevation of the temporal fossa dura from the SOF to increase the exposure of the ACP before drilling (4, 28). In so doing, Cranial Nerves (CNs) III and IV and branch V1, which course through the SOF, are exposed, and may be at risk of injury. In our study, we provide anatomic details and useful landmarks for safely sectioning the orbitotemporal periosteal fold, elevating the temporal fossa dura from the SOF, and performing anterior clinoidectomy. We assess the

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morbidity associated with these procedures and compare the indications, advantages, and disadvantages of intra- versus extradural clinoidectomy.

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MATERIALS AND METHODS Five formalin-fixed cadaveric heads were injected with colored silicone in accordance with our previously described technique (34). Four specimens were used for cadaveric dissections, and one was used for histological examination of the cavernous-orbital region.

Cadaveric Dissection Each head was fixed in a Mayfield headholder and positioned for a standard pterional approach. An extradural anterior clinoidectomy was performed using an operative microscope (Zeiss Contraves; Zeiss, Inc., Montpelier, MD). The clinoid space, anterior cavernous sinus, and SOF were explored for additional anatomic study.

Histological Examination The cavernous-orbital region including the ACP and optic canal was removed en bloc and placed in 10% neutral-buffered formalin solution for 2 weeks. Each specimen was decalcified in Decalcifier I and II (Surgipath Medical Industries, Richmond, IL) for 4 weeks and then dehydrated, processed in a Citadel 2000 processor (Shandon, Pittsburgh, PA), and embedded in paraffin. Coronal sections of 6-μm thickness were obtained using a Reichert-Jung 2055 microtome (Cambridge Instruments, Heidelberg, Germany). Two of 15 sections were additionally processed by heating at 65⬚C for 98 hours, and were stained with hematoxylin-eosin and Masson’s trichrome, respectively.

FIGURE 1. A superior view of the clinoid space. A, dura has been incised over the superior aspect of the ACP. The ACP and orbital roof have been removed. B, the transparent layer of periosteum covering the inferior aspect of the ACP has been incised. CN III is overcrossed anteriorly by CN IV and the frontal branch of CN VI. OTPF, orbitotemporal periosteal fold. Courtesy of Mayfield Clinic, Cincinnati, OH.

At the level of the SOF, the outer and inner layers of the lateral wall of the cavernous sinus separate from one another (21). The inner layer, which is composed of the cranial nerve perineurium and surrounding connective tissue, extends with the

nerves into the posterior orbit. The outer layer is formed by the temporal fossa dura (Fig. 2B). Along the lateral margin of the SOF, the periosteal layer of the dura mater is contiguous with the periosteal layer of the periorbita. This bridge of periosteum at the lateral edge of the SOF appears in the surgical field as a periosteal fold stretched between the periorbita and temporal fossa dura (Fig. 3A). The width of the periosteal fold varies according to the shape of the lateral end of the narrow part of the SOF, which ranges from round and large to thin and sharp. The length of the orbitotemporal periosteal fold corresponds to the distance between the temporal fossa dura and periorbita. This length varies according to the bone thickness of the greater sphenoid wing that laterally closes the lateral narrow part of the SOF. A complex network of highly variable small dural veins, and eventually the orbitomeningeal artery, course into the orbitotemporal fold. In one specimen, the orbitomeningeal artery coursed through a distinct bony canal (Hyrtl’s canal) that is located lateral to the superolateral edge of the SOF. Sectioning the orbitotemporal periosteal fold along the edges of the SOF reveals a cleavage plane between the temporal fossa dura and the inner layer of the lateral wall of the cavernous sinus (Fig. 3B) (7, 21). The lacrimal nerve runs tangentially to the lateral narrow part of the SOF below the lesser wing of the sphenoid and immediately superior or superolateral to the superior orbital vein (SOV). The lacrimal nerve, which was hardly dissectible from the SOV and thick surrounding connective tissue, always lies immediately medial to this surgically created cleavage plane (Fig. 2C).The distance between this plane and the lacrimal nerves at the level of the orbitotemporal periosteal fold varied from 1.2 to 4 mm. The SOV exits the muscular cone between the lateral and superior rectus. It then laterally crosses under the annulus of

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RESULTS The clinoid space is the surgically created space obtained after the subperiosteal resection of the ACP (8, 19, 22, 35). Precise knowledge of its limits and underlying eloquent structures is mandatory for performance of anterior clinoidectomy.

Clinoid Space The clinoid space is limited superiorly and laterally by the dura that covers the superior and lateral aspects of the ACP. Its medial aspect is formed by the optic nerve sheath (Figs. 1 and 2). Just below the optic nerve sheath is the inferior root of the lesser sphenoid wing (optic strut), which extends from the base of the ACP to the body of the sphenoid bone (Fig. 2B). The size and shape of the optic strut varied in our specimens with diameter ranging from 3 to 7 mm and section shapes extending from round to oval. The clinoid segment of the ICA runs along the posterior margin of the optic strut and is covered by a thin periosteal membrane (Figs. 1A and 2A). The inferior surface of the clinoid space is formed by a thin layer of periosteum that covers the oculomotor nerve. The trochlear and frontal nerves cross over the oculomotor nerve anteriorly (Fig. 1B).

SOF and Orbitotemporal Periosteal Fold

EXTRADURAL ANTERIOR CLINOIDECTOMY

FIGURE 2. Coronal histological sections of the right cavernous orbital region. Sections are at the levels of clinoidal segment C5 of the ICA (A), the optic strut (B), and the lateral narrow part of the SOF (C). OA, ophthalmic artery; FN, frontal nerve; LN, lacrimal nerve, NcN, nasocilliary nerve; LSW,

lesser sphenoid wing; LRM, lateral rectus muscle; MRM, medial rectus muscle; IRM, inferior rectus muscle; GSW, greater sphenoid wing; SRM, superior rectus muscle; SS, sphenoid sinus. Courtesy of Mayfield Clinic, Cincinnati, OH.

ally toward the ACP, and the sphenoid ridge is drilled using a high-speed diamond drill. After the superolateral bony edge of the SOF is opened, the orbitotemporal periosteal fold is visualized and incised using a No. 15 blade. The incision should be made at the level of the sphenoid ridge to avoid injury of the CNs coursing through the SOF, especially the lacrimal nerve (Fig. 2B). The FIGURE 3. A frontotemporal approach depicts exposure of the ACP before (A) and after (B) release of the orbitotemlevel of the incision determines poral periosteal fold (OTPF) lateral to the lacrimal nerve. Courtesy of Mayfield Clinic, Cincinnati, OH. the position of the cleavage plane into this multilayer connective tissue dural wall that covers the SOF. The more medial Zinn and the CNs that pass through the SOF. At this point, the the incision of the orbitotemporal periosteal fold, the thinner SOV wall is particularly fragile, because it is composed only of will be the remaining dura that covers the CNs passing through endothelium and a basal membrane covered by the temporal the SOF. Incision of this fold also reveals the orbitomeningeal fossa dura (Fig. 2C). Posteriorly, only a thin layer of connective artery that is divided. Coagulation of this artery should be mintissue, which is essentially composed of perineurium, sepaimized because of its close proximity to the lacrimal nerve. rates this surgical plane from CNs III and IV and branch V1 The cleavage plane created between the temporal fossa dura (Fig. 2, A and B). and the connective tissue covering the SOF is then extended Extradural Extended Anterior Clinoidectomy: posteriorly to increase the exposure of the lateral and superior Surgical Technique and Landmarks aspects of ACP (Fig. 4). Elevation of the dura should stop when exposure of the ACP is considered sufficient for drilling. After the standard frontotemporal craniotomy is performed, Depending on the size of the ACP, in most cases it is unnecesthe dura covering the middle and anterior cranial fossa is elesary to extend the cleavage plane more posteriorly and uncover vated from the greater sphenoid wing and the orbital roof. The CNs III and IV and branch V1 that pass through the SOF. dura is progressively elevated from the sphenoid ridge medi-

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FIGURE 4. Extradural anterior clinoidectomy technique and section of the orbitotemporal periosteal fold (OTPF) using the right frontotemporal approach. A, OTPF incision should be made at the level of the sphenoid ridge to avoid injury to the CN coursing through the SOF. B, temporal

The superior bony edge of the optic canal is then identified (Fig. 4B), and the dorsal and lateral walls of the optic canal are unroofed using a diamond drill under continuous irrigation (Fig. 4C). The last remnants of cortical bone can be removed with a microdissector. Once the optic canal is unroofed, the central cancellous bone of the ACP is removed, and the cortical bone is thinned. Care must be taken when the drilling is directed anterolaterally to avoid damage of the underlying CN III. The optic strut is drilled anteromedially, inferior to the optic nerve sheath. With additional drilling in the posterior direction, the clinoid segment of the ICA comes into view (Fig. 4D). After the ACP is freed from the optic strut, the clinoid tip becomes mobile. The clinoid tip is finally dissected from the petroclinoid and interclinoid ligament and is carefully twisted and pulled out with a microronguer.

fossa dura is elevated from the contents of the SOF. C, optic canal is identified and unroofed. D, ACP is freed from the optic strut and the clinoid tip is removed. LSW, lesser sphenoid wing. Courtesy of Mayfield Clinic, Cincinnati, OH.

potential complications, and the advantages and disadvantages of extra- versus intradural clinoidectomy. Recent reports describe sectioning of the orbitotemporal periosteal fold to increase ACP exposure and thus facilitate extradural anterior clinoidectomy (4, 28). In our cadaveric study, we restricted incision of this fold to the periosteal bridge. Made at the level of the sphenoid ridge, the incision was safely performed lateral to the lacrimal nerve. Thereafter, only blunt dissection was necessary to create a cleavage plane between the temporal fossa dura and the connective tissue that covers the SOF. We discuss the potential morbidity associated with this surgical step that exposes the contents of the SOF, including CNs III and IV and branch V1.

Indication of Anterior Clinoidectomy: Extradural versus Intradural Technique

Anterior clinoidectomy is a technically challenging procedure that requires a clear understanding of its indications,

Anterior clinoidectomy has been described as part of the approach to ophthalmic and clinoidal segment aneurysms to improve visualization of the aneurysm neck and ophthalmic artery and to establish proximal control of the ICA (5, 9, 12, 15,



DISCUSSION


18, 20, 23, 29–31, 39, 40). Partial anterior clinoidectomy may also be necessary for posterior communicating aneurysms to additionally expose the aneurysm neck and the posterior communicating artery, which may be obstructed by the ACP (30, 40). The intradural technique is usually preferred as an intraoperative option for exposing the aneurysm neck. Using this technique, before drilling, the surgeon identifies CN II, the ICA, and the aneurysm, which may erode the ACP. The intradural technique is also recommended when a caroticoclinoid foramen is identified from computed tomographic scan before surgery. Anterior clinoidectomy and unroofing of the optic canal with mobilization of the optic nerve and ICA have also been used to increase the size of the opticocarotid triangle and carotid oculomotor window for access to the interpeduncular cistern lesions and basilar bifurcation aneurysms (10, 16, 26, 37, 41, 42). Both extra- and intradural techniques have been used, in accordance with the surgeon’s preference. Anterior clinoidectomy has been described as part of the extradural cranial base approach to cavernous sinus lesions (12, 20, 35, 38). In such cases, the extradural technique is preferred, because the neurovascular structures remain protected. However, indications for intracavernous surgery have been tempered during the last decade in response to advances in radiosurgery, which allow for durable control of meningiomas with low morbidity rates and endovascular techniques that can be used to treat intracavernous vascular lesions. Finally, for patients with meningiomas that have a dural base over the ACP, anterior clinoidectomy is indicated to remove hyperostotic bone that may give rise to recurrence (1, 6, 32, 33). The extradural technique is advocated because it allows for early control of the dural base and feeding vessels before tumor resection. For patients with large sphenocavernous or clinoidal meningiomas, the extradural technique also allows for early identification of the ICA and the optic nerve proximal to the tumor.

Complications Performing anterior clinoidectomy remains a challenging task with potential complications owing to its deep location between the ICA and CNs II and III. Injuries to CN II and III can be caused by unexpected motion of the drill or by heat, in the case of insufficient irrigation (24). Even without intradural manipulation of CN III, Nutik (29) reported that oculomotor paresis and/or palsy occurred postoperatively in 3 of 30 patients. Nutik also conveyed that the superior division of the CN III, which lies close to the ACP, was more likely to be injured. Yonekawa et al. (41) reported that decreased visual acuity and/or inferior visual field defects occurred in 3 of 40 patients who underwent unroofing of the optic canal; similarly, Nutik (29) reports this complication occurred in 1 of 30 patients. According to our study, CN IV and the frontal branch of CN V, which lies below the base of ACP, may also be at risk during clinoidectomy. Although reported rarely, the C5 segment of the ICA or a paraclinoidal aneurysm, especially when associated with bony erosion of the ACP, can be injured during clinoidectomy (25,

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27). According to Seoane et al. (36), the presence of an ophthalmic artery coursing through the optic strut should not be overlooked when the optic strut is being drilled. Lower morbidity rates have been recently reported in association with the use of an ultrasonic bone curette. This tool does not spin, and it therefore offers better stability; additionally, it incorporates a cooling irrigation system (3, 17). Finally, opening of the sphenoid sinus or the ethmoid air cells during anterior clinoidectomy can lead to postoperative cerebrospinal fluid leak, meningitis, or pneumoencephalos. When the optic canal is unroofed, drilling medially to it should be avoided because of the risk of entering the sphenoid sinus or ethmoid air cells (2, 11). Pneumatization of the ACP can occur occasionally through the optic strut and less often through the superior wall of the optic canal (2, 11). When an air cell is encountered, great care should be taken to keep the mucosa intact and to close the opening with muscle, bone wax, and/or fibrin glue.

Sectioning of the Orbitotemporal Periosteal Fold One drawback of the extradural anterior clinoidectomy technique is the limited extent of elevation of the frontotemporal dura over the ACP. The trajectory of CN II and the exact position of the ICA may be difficult to ascertain before drilling, and the space available for this procedure is particularly limited. The tip of the ACP, which firmly anchors to the inter- and petroclinoid ligaments, is usually dissected blindly. Bleeding from the diploic veins and/or from the venous plexus surrounding the C5 segment of the ICA can also make the last steps of the anterior clinoidectomy difficult. Several authors have recently described the incision of the orbitotemporal periosteal fold at the level of the lateral edge of the SOF as a key step to increase the exposure of the ACP, thus facilitating the extradural clinoidectomy (4, 28). Dolenc (12, 13) and Dolenc and Roger (14) first described this technical nuance as part of the surgical approach to cavernous sinus lesions. With division of the orbitotemporal periosteal fold, the dura mater can be further elevated from the superior and lateral aspects of the ACP. Cranial nerve III (below the inferolateral aspect of the ACP) and the intracranial entrance of the optic canal are easily identified, thereby allowing for enhanced appreciation of the trajectory to CNs II and III before one begins to drill. The clinoid tip can be removed under direct vision rather than being dissected and removed blindly. Dolenc (12, 13) and Dolenc and Roger (14) emphasized that the orbitotemporal periosteal fold should be cut using curved scissors with the tips pointing posteriorly, and that its incision should not exceed 4 mm because of the risk of injury to the lacrimal branch of CN VI. Using our technique, incision of this fold is restricted to the periosteal bridge. When made at the level of the sphenoid ridge, the incision can be safely performed lateral to the lacrimal nerve. Thereafter, only blunt dissection is necessary to create a cleavage plane between the temporal fossa dura and the connective tissue that covers the SOF. In our cadaveric specimens, the lacrimal nerve coursed tangentially to the lat-


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eral part of the SOF, just medial to this surgically created cleavage plane. To reduce the risk of injury to the SOF contents, Coscarella et al. (4) proposed that incision of the periosteal fold medial to the foramen rotundum should be the first step in elevating the temporal fossa dura covering the lateral aspect of the SOF. After they visualized the nerves passing through the SOF, Coscarella et al. divided the orbitotemporal periosteal fold. Noguchi et al. (28) also advocated peeling both the temporal fossa dura from the SOF and the anterior aspect of the cavernous sinus to increase the exposure of the ACP. In this scenario, elevation of the temporal fossa began at the lateral edge of the SOF (as we also described). Neither of these groups reported any morbidity associated with the elevation of the temporal fossa dura from the SOF. In our opinion, exposure of the SOF contents is unnecessary when an extradural anterior clinoidectomy is performed, unless peeling and removal of the outer layer of the cavernous sinus is planned for other purposes. We found that one should be careful when elevating the temporal fossa dura from the SOF and the cavernous sinus, because it is sometimes associated with postoperative transient diplopia and facial numbness. Several factors may account for such morbidity, including traction of CNs III and IV and branch V1 when peeling the temporal fossa dura; ischemia caused by disruption of the blood supply to these nerves; sectioning of the more superficial fascicules of CN VI during elevation of the temporal fossa dura from the SOF; and finally, the superior orbital vein that drains into the anterior cavernous sinus is particularly vulnerable when the dura is peeled from the SOF. Use of coagulation to control the bleeding from this vein can also result in CN damage. Therefore, when performing an extradural anterior clinoidectomy, the surgeon should stop the elevation of the dura when adequate exposure of the ACP is achieved. Depending on the shape and volume of the ACP, visualization of CNs III and IV and branch V1 is unnecessary in most cases.

REFERENCES

Anterior clinoidectomies, whether performed extra- or intradurally, are difficult procedures with limited indications, and each has its particular advantages and disadvantages. These techniques should be part of a surgeon’s armamentarium, especially for treating cranial base tumor and vascular lesions. When one is performing an extradural clinoidectomy, division of the orbitotemporal periosteal fold is a useful step to increase exposure of the ACP. The sphenoid ridge at the level of the superolateral edge of the SOF is a useful landmark for incision of this fold. The incision should be made at the level of the sphenoid ridge and parallel to the direction of the sphenoid ridge, thereby entering a safe cleavage plane between the temporal fossa dura and the SOF contents. However, elevation of the temporal fossa dura over CNs III and IV and branch V1 that pass through the SOF should be minimized because of potential morbidity to these nerves.

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CONCLUSIONS


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COMMENTS

F

roelich et al. nicely describe relevant technical aspects of anterior clinoidectomy. The review of technical issues and indication is thorough, and the references were carefully selected. We do agree with the authors that the opening of the orbitotemporal dural fold is a key part of the procedure. This can be complemented with a “mini peeling” of the anterior part of the lateral wall of the cavernous sinus, as recently described, to improve exposure and help the surgeon with respect to spatial orientation; this is especially the case for less-experienced neurosurgeons. This procedure, which was first reported by Dolenc and has been subsequently reviewed by many authors, opens a major chapter of modern neurosurgery. The resection of the clinoid allows the surgeon to safely enter the cavernous sinus. As the authors clearly point out, resection of meningiomas or secreting adenomas with invasion to the cavernous sinus is now less frequent as a result of the refinement of radiosurgery. Nevertheless, there are still some lesions with aggressive evolution despite the use of radiosurgery (such as chordomas and some aggressive prolactinomas that are refractory to dopaminergic

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agonists), and in these selected cases, the neurosurgeon must offer a safe resection of the lesion. The advantages of clinoid resection are multiple and include unroofing of the anteromedial triangle of the cavernous sinus; liberation of the optic nerve after removal of the optic canal roof, opening of the falciform ligament and the optic nerve sheath; and liberation of the carotid artery (after complete release of the distal dural ring allows safe mobilization of the carotid artery) with less risk of laceration or thrombosis. In the case of large pituitary adenomas with severe elongation of the optic pathway, we remove the anterior clinoid invariably to improve visual outcome and achieve a radical resection of the lesion, especially the portion medial to the ipsilateral carotid artery and beneath the optic nerve, which are usually blind spots during the conventional pterional transsylvian approach. This is a nice description of anterior clinoidectomy that clearly discusses most of the major surgical concepts to consider when dealing with lesions of the paraclinoid region. David Rojas-Zalazar Jorge Mura Evandro P. de Oliveira São Paulo, Brazil

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n this article, the authors have studied the “orbitotemporal periosteal fold” and the extent to which it may be safely sectioned in performing extradural anterior clinoidectomy. The findings are quite useful. However, in performing clinoidectomy for intracranial aneurysms that are located close to the clinoid process (e.g., clinoidal aneurysms, posterior paraclinoid aneurysms, giant paraclinoid aneurysms), I prefer to use an intradural technique. In such a situation, the orbitotemporal periosteal fold and the dura mater over the clinoid may be maintained as a layer and retracted medially with sutures, while the aneurysm is concurrently under view intradurally. The clinoidectomy is still performed extradurally. A temporary occlusion of the cervical internal carotid artery may sometimes be necessary while manipulating the bone pieces near the aneurysm. Laligam N. Sekhar Seattle, Washington

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roelich et al. describe the anatomy and histology of the orbitotemporal periosteal fold and a technique for an extradural anterior clinoidectomy. The technique involves incising the orbitotemporal periosteal fold at the level of the sphenoid ridge and using careful blunt dissection to expose the anterior clinoid process. The contribution of Professor Dolenc to this field has been profound. At our institution, we most frequently use an anterior clinoidectomy for ophthalmic aneurysms. Recognizing that an extradural approach is reasonable, we prefer an intradural approach for several reasons. First, the degree of anterior clinoidectomy required is best known at the time of aneurysm exposure, which enables us to avoid extra drilling and save time. Furthermore, aneurysm adhesions are best visualized from an intradural approach. For this application, we have found the ultrasonic claw dissector (Synergetics, St. Charles, MN) to be a safe alternative to the air drill. Andrew Little Robert F. Spetzler Phoenix, Arizona

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roelich et al. present resection of the anterior clinoid process (ACP), which is systematically and anatomically very correctly described. It is true that for a long time, many neurosurgeons dealing with the pathologies in this region were reluctant to use the extradural approach and to perform clinoidectomy because there was too little space. This


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is why many of them preferred to perform partial clinoidectomy from the intradural side. With all due respect to these neurosurgeons, the intradural clinoidectomy, in particular for vascular lesions, should be avoided because it is too dangerous. We now have at hand a description of the extended extradural approach with which the ACP is nicely visualized from the inferolateral side, so that the initial steps of drilling the ACP start on the opposite side from the possible aneurysm and the usual position of the optic nerve and the internal carotid artery. This approach has been repeatedly described and published (1–3, 5). If this fact is not convincing, then the latest publication (4) does summarize more than 2000 patients with pathologies at the central cranial base for whom the anterior clinoidectomy was routinely performed. In all of this clinical work, not a single patient suffered any morbidity as a result of ACP resection. The experience collected through experimental, neuroanatomical, and in particular, clinical work, gives us sufficient experience to objectively defend the extradural ACP resection as opposed to the intradural one. Each individual patient requires detailed discussion and relevant consideration of the underlying anatomical circumstances. Most important of these are pneumatization of the ACP, and/or pneumatization of the walls of the optic canal, as well as erosion of the ACP as a result of pathologies (trigeminal schwannomas, aneurysms, parasellar hemangiomas) in contrast with hypertrophy of the ACP caused by ACP meningiomas (in particular, en plaque meningiomas). There is no doubt that in aneurysm pathology, the origin of the aneurysm as well as the circumference of the ICA, from which the pathology originates, are important factors. Clinoidectomy should be performed from the opposite side to where the pathology is located, and this is only possible when the ACP is sufficiently exposed by cutting the duplicature of the dura and connecting the temporal lobe and the periosteum of the orbital cavity, thereby providing safe passage to the neural and vascular structures from the parasellar to the orbital space.

One can agree that ACP resection offers an important additional corridor to reach different pathologies in the central cranial base in addition to the parasellar space and around the ACP. This is an important reason why all of the approaches to the pathologies at the dorsum sellae as well as in the retrosellar and clival regions and/or the region of the oculomotor trigone and even more posteriorly require ACP resection. Personally, I’m convinced that without practical knowledge of performing this procedure, every neurosurgeon will have problems dealing with the pathologies in the central cranial base regions. Again, personally, I wouldn’t describe the ACP resection as very dangerous. I would rather describe it as a difficult procedure that requires a lot of training via microanatomical dissections in the laboratory to achieve the mandatory skill required to dare to perform it. We should not put fear into the bones of young neurosurgeons; we have to convince them that they must acquire skill in the laboratory to accept the truth that everything is experience-dependent. I would like to add that, despite the long list of references, the most important (the Pioneer’s) is still missing.



Vinko V. Dolenc Ljubljana, Slovenia

1. Dolenc VV: Anatomy and Surgery of the Cavernous Sinus. Wien, Springer-Verlag, 1989. 2. Dolenc VV: Frontotemporal epidural approach to trigeminal neurinomas. Acta Neurochir (Wien) 130: 55–65, 1994. 3. Dolenc VV: Intracavernous carotid artery aneurysms. in: Spetzler RF, Carter LP (eds): Neurovascular Surgery. New York: McGraw-Hill, 1995, pp 659–673. 4. Dolenc VV: Transcranial epidural approach to pituitary tumors extending beyond the sella. Neurosurgery 41: 542–552, 1997. 5. Dolenc VV, Rogers L: Microsurgical Anatomy and Surgery of the Central Skull Base. Wien, Springer-Verlag, 2003.

clinoidectomia

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