Extradural anterior clinoidectomy

Technical note

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✓ The anterior clinoid process (ACP), located on the skull base, is a relatively small structure, although its removal provides enormous gain in facilitating the management of lesions—either tumors or aneurysms—in the paraclinoid region and upper basilar artery. The extensive surgical field gained contributes to safer exposure of the neurovascular elements in the vicinity while avoiding excessive and hazardous retraction of the brain. In this report the authors present a technically simpler avenue for performing an extradural anterior clinoidectomy after reviewing the anatomy of the ACP and its anatomical variations. Additionally, the original Dolenc procedure and its subsequent derivatives are compared and contrasted to the authors' simpler and less laborious technique. Different clinical situations in which to use the procedure are described based on the authors' experience from 60 cases (40 aneurysm cases and 20 tumor cases) during a 4-year period.

Abstract

✓ The anterior clinoid process (ACP), located on the skull base, is a relatively small structure, although its removal provides enormous gain in facilitating the management of lesions—either tumors or aneurysms—in the paraclinoid region and upper basilar artery. The extensive surgical field gained contributes to safer exposure of the neurovascular elements in the vicinity while avoiding excessive and hazardous retraction of the brain. In this report the authors present a technically simpler avenue for performing an extradural anterior clinoidectomy after reviewing the anatomy of the ACP and its anatomical variations. Additionally, the original Dolenc procedure and its subsequent derivatives are compared and contrasted to the authors' simpler and less laborious technique. Different clinical situations in which to use the procedure are described based on the authors' experience from 60 cases (40 aneurysm cases and 20 tumor cases) during a 4-year period.

Anterior clinoidectomy is a technique of paramount importance in the management of aneurysms involving the paraclinoid region3,4,11,19,22,23 and the upper basilar artery.2,6 In addition, removal of the ACP has facilitated the removal of certain tumors in this vicinity to provide better access to the anterior portion of the cavernous sinus or the posterior portion of the orbit.2,5,17,23 The benefits of an anterior clinoidectomy include the following: 1) enhanced visualization of the ophthalmic artery and the proximal neck of its aneurysm; 2) early decompression and mobilization of the optic nerve; and 3) improved surgical exposure as evidenced by Evans and colleagues7 in a cadaveric study whose results revealed a twofold increase in the exposed length of the optic nerve and the opticocarotid triangle as well as a three- to fourfold increase in the maximum width of the opticocarotid triangle. The extradural anterior clinoidectomy procedure coupled with the opening of the optic sheath was introduced into the neurosurgical armamentarium in 1985 by Dolenc.3 The procedure allows for the complete removal of the ACP with minimal brain retraction, while the dura mater acts as a natural barrier to protect neurovascular structures and does not expose the subarachnoid space to bone debris.

We report on a technically simpler means of performing an extradural anterior clinoidectomy after a review of the anatomy of the ACP and its anatomical variations. The key element in the procedure involves cutting the MOB and its named vessels once the SOF is unroofed, which facilitates dural elevation as well as exposure of the ACP and the intracranial aperture of the optic canal. The ACP is safely disengaged from its three supporting structures: 1) the lesser wing of the sphenoid bone; 2) the roof of the optic canal; and 3) the optic strut. Finally, the ACP is centrally hollowed prior to its extirpation to avoid damage to the oculomotor nerve, which is very close to the lateral underside of the ACP. In this report, the original Dolenc procedure and its subsequent derivatives are compared and contrasted to our simpler and less laborious technique. We report on different clinical situations from our experience in 60 cases (40 aneurysm cases and 20 tumor cases) over a 4-year period.

Anatomy of the ACP

The ACP is a spine of bone that projects from the posteromedial border of the lesser wing of the sphenoid. This bone projection bridges the roof of the cavernous sinus and superiorly has the form of a triangular mass (Fig. 1A) that overhangs the proximal portion of the intradural carotid artery. Anteriorly, the base of the ACP has one lateral and two medial components: 1) laterally, it is the continuation of the lesser wing of the sphenoid bone overlying the SOF (Fig. 1B); and 2) medially, the base connects to the sphenoid bone via the anterior and posterior roots. The anterior root, located superiorly, is the continuation of the flat roof of the optic canal. The posterior root, located inferomedially, is the optic strut20 and connects the ACP to the basisphenoid bone. Parkinson20 describes the optic strut as a small pillar of bone with a rounded anterior aspect and a posterior sharp edge corresponding respectively to the leading and trailing edge of an aerofoil. The optic strut forms the lateral and ventral walls of the optic canal.13,18,20 It is also completely surrounded by dura mater from the middle and anterior fossae.

Fig. 1.
Fig. 1.

Photographs representing the left ACP. A: The ACP forms a triangular spine of bone. It is a posteromedial bone continuation of the lesser wing of the sphenoid bone, and its base merges with the roof of the optic canal. Hatched areas represent the base of the ACP, which is removed using a diamond drill in the direction indicated by the arrow, which in turn facilitates detaching of the ACP from two of its three supporting structures, and the body of the ACP, which is centrally hollowed prior to its extirpation. B: The ACP is located between the optic canal medially and the SOF laterally. It is also connected to the sphenoid bone by its posterior root, the optic strut, which forms the lateral and ventral walls of the optic canal. Hatched area represents the portion of the optic strut that is drilled off as needed after drilling of the base of the ACP, as indicated in panel A. C: Dural elevation is performed and directed medially toward the orbital roof overlying the optic canal. Asterisk represents the dural fold entering the apex of the SOF. FL = frontal lobe dura; LWS = lesser wing of the sphenoid bone; TL = temporal lobe dura. D: The SOF is unroofed, which involves complete removal of the lesser wing of the sphenoid (LWS) overlying the superolateral margin of the SOF and the partial removal of the greater wing of the sphenoid (GWS) forming the inferolateral wall of the SOF. The apex of the SOF is exposed to reveal the MOB. E: The MOB has been coagulated and cut to leave a stump. The dura propia of the temporal tip has been peeled away from the inner cavernous membrane (ICM), and the mandibular division of the trigeminal nerve (V2) entering the foramen rotundum is exposed, thus facilitating further dural elevation in a posteromedial direction to expose the ACP without difficulty. F: The anterior root of the ACP is removed using a diamond burr and constant irrigation in a lateral to medial direction. The anterior root of the ACP is defined as the lateral continuation of the flat roof of the optic canal to the base of the ACP. G: A circumferential dissection plane (arrowheads) has been established between the centrally hollowed ACP and its surrounding dural folds to facilitate extirpation without damage to the neural structures. Of particular importance is the oculomotor nerve, which is the closest to the body and base of the ACP and runs in the medial wall of the SOF, just below the lateroinferior side of the ACP. H: A cadaveric intracranial view following an extradural anterior clinoidectomy in which the optic nerve (ON) is deflected medially by an instrument. The third, fourth, and sixth cranial nerves; proximal dural ring; ophthalmic artery; and posterior communicating artery perforators are indicated.

The ACP is generally composed circumferentially of dense cortical bone with an interior of cancellous bone that may contain small venous channels, which in turn communicate with the cavernous sinus, and the diploic veins of the orbital roof. On occasion, the ACP is pneumatized and contains air cells that communicate with the sphenoid sinus via the optic strut, which resembles a bone conduit.8,12,20 Although pneumatization is rare, its potential existence should be verified preoperatively on CT scans to take steps to avoid CSF leaks. The ACP is attached to a number of dural folds: 1) the falciform ligament extends from the planum sphenoidale over the roof of the optic canal and attaches to the base of the ACP; 2) the anteromedial aspect of the tentorium; 3) anterior and posterior petroclinoid ligament; and 4) interclinoid dural folds.

The course of the clinoidal segment of the ICA is intimately related to the inferior aspect of the ACP. The junction of the optic strut and the inferomedial aspect of the ACP are grooved by the clinoidal segment of the ICA. The posterior medial projection of the ACP, located posterior and superior to the optic strut, frequently contains a rounded impression, which accommodates the lateral surface of the clinoidal segment of the ICA. The oculomotor, trochlear, and ophthalmic divisions of the trigeminal and abducens nerves run together in a dural fold from the lateral wall of the cavernous sinus located just below the lateral side of the ACP to the medial wall of the SOF. Of these nerves, the oculomotor one is the closest to the body and base of the ACP. Nutik19 reports that the oculomotor nerve often grooves the inferolateral surface of the ACP. Note that the dural layer inferior to the ACP, which adjoins the lower dural ring of the clinoidal segment of the ICA to the dural covering of the oculomotor nerve, is referred to as the “carotico-oculomotor membrane.”10 This dural membrane can be thin and friable, although data from two different cadaveric studies indicate that the membrane is incomplete 16% of the time.12,21

The posterior tip of the ACP often projects medially behind the lateral portion of the clinoidal segment of the ICA and may be involved in an osseous bridge forming a bone foramen for the ICA. In a study involving a total of 70 ACPs, Kim and colleagues15 reported the occasional presence of an interosseous bridge from the ACP to either the middle (13%) or the posterior clinoid process (6%). The presence of an interosseous bridge from the ACP would prevent an extradural resection and necessitate a combined intra- and extradural procedure.

The optic canal is located at the orbital apex. It is bounded by the body of the sphenoid bone medially, the lesser wing of the sphenoid bone superiorly, and the optic strut laterally and inferiorly (Fig. 1B). The optic canal assumes a vertically oval shape at its orbital end, where it measures approximately 5 to 6 mm in diameter. In its central portion it is round in cross-section, and on the cranial end it is oval in the horizontal plane. The canal attains its adult size by the age of 3 years.9 In approximately 4% of normal individuals the ophthalmic artery will notch the canal floor, forming a keyhole deformity.14 The optic canal is 8 to 12 mm in length.

The ACP has a mean length of 7.7 mm (range 3–8 mm)15 from the roof of the optic canal to its tip (Fig. 1A). Its mean width is 5.6 mm (range 4–7.8 mm) and mean height is 5.3 mm (range 4–6.8 mm) at the widest portion.12 In summary, the ACP has three supporting structures: 1) the lesser wing of the sphenoid bone; 2) the roof of the optic canal; and 3) the optic strut. Extradural removal of the ACP involves disengaging the ACP from these three structures.

Extradural Removal of the ACP

The extradural removal of the ACP is performed using microscopic magnification and illumination after the sphenoid ridge is flattened following a modified orbitozygomatic craniotomy.1 First, the dura is elevated and directed medially toward the orbital roof overlying the optic canal to delineate the intracranial roof of the optic canal (Fig. 1A and C). The SOF is then identified and unroofed (Fig. 1B and D), which involves removing two bone structures with either rongeurs or a diamond burr: 1) complete removal of the lesser wing of the sphenoid bone overlying the superolateral margin of the SOF—the first of three supporting structures for the ACP; and 2) partial removal of the greater wing of the sphenoid bone, which forms the inferolateral wall of the SOF. This step provides mobility for the contents of the SOF and exposes the dural fold at the apex of the SOF (Fig. 1D) where the MOB is located with its named artery and vein. There is no need to unroof the foramen rotundum unless required by the pathology at hand. The MOB together with its vessels can be coagulated and divided with impunity (Fig. 1E) for a distance of 3 to 5 mm, and a cleavage plane by sharp dissection is established between the dura propia of the temporal tip and the inner cavernous membrane. The temporal tip dura is then peeled away from the anterior aspect of the cavernous sinus and the orbital apex. This maneuver is key because it facilitates further dural elevation in a posteromedial direction along the lateral aspect of the ACP. It also facilitates dural elevation medially to identify the intracranial aperture of the optic canal. This cleavage plane can also be extended, if required, along the second and third divisions of the trigeminal nerve on the middle fossa floor and medially to the tentorial edge.

The optic canal—the second of three supporting structures for the ACP—is unroofed from a lateral to medial direction (Fig. 1A, hatched area with embedded arrow) by using a 2-mm diamond burr (Fig. 1F) and constant-cooling irrigation to prevent thermal damage of surrounding neural structures from the elevation in bone temperature. Care is taken not to open either the ethmoid or sphenoid sinus at the medial edge of the optic canal. If the sinuses are entered, we do not violate the mucous membrane and close the bone opening with wax. The ACP is now connected only at the floor of the optic canal by the optic strut (Fig. 1B)—the third of three supporting structures for the ACP—which is drilled off as needed. A final step involves centrally hollowing out the dense cortical bone in the center of the ACP with the aid of a diamond burr and constant-cooling irrigation. During this procedure, the surgeon must be ever cognizant about the relative positions of the optic nerve, the carotid artery, and the oculomotor nerve with reference to the ACP. A circumferential dissection plane between the surrounding dural folds and the centrally shelled ACP is established (Fig. 1G). Caution is exercised at the site of the carotico-oculomotor membrane, which is friable and occasionally incomplete,12,21 prior to extirpating the ACP by using fine rongeurs. As a direct extension of this procedure, the optic sheath and the distal dural ring are excised following dural opening, which allows early decompression and mobilization of the optic nerve with minimal brain retraction. This technique was successfully used during a 4-year period for the removal of the ACP in aneurysm pathology (40 cases) and selective tumorous lesions (20 cases).

Discussion

In this paper we compare and contrast the original Dolenc procedure and its derivatives3,23 with a simpler and less laborious technique that can be easily implemented (Table 1). First, the required dural elevation commences from the SOF following the flattening of the sphenoid ridge and is directed medially toward the optic canal (Fig. 1C). This step is less cumbersome compared with performing dural separation from the orbital roof to the SOF.3 The key maneuver in this extradural technique is the excision of the MOB after unroofing the SOF (Fig. 1D), which can be performed without risk to the third or fourth cranial nerve. This procedure provides for an easier lateral extradural exposure of the ACP when the dura propia is peeled off the inner cavernous membrane through sharp dissection (Fig. 1E). If bleeding occurs from the inner membrane of the cavernous sinus, it is easily halted with oxycellulose and gentle compression. Subsequent to exposure of the ACP, the dura overlying the posterior orbital roof can be easily elevated in a medial direction to identify the intracranial aperture of the optic canal without ambiguity. These advantages are not easily available in the other extradural procedures in the literature,2,3,23 because the dura remains tethered by the MOB and the dura propia of the temporal tip.

TABLE 1

Extradural removal of the ACP*

Craniotomy Procedure
VariablePterional (Dolenc, 1985)Orbitozygomatic (Day, et al., 1994)Pterional (Yonekawa, et al., 1997)Modified Orbitozygomatic (Current Study)
procedural step 1flattening of sphenoid ridgesphenoid ridge shaved flat along w/ regularitiesflattening of sphenoid ridgeflattening of sphenoid ridge
 of orbital roof & ant middle fossa
procedural step 2dura separated medially fromdura elevated from a lat to medial direction fromdura separated medially from ACPdural elevation medially from
 orbital roof to SOF laterally FR to anterosuperior ethmoidal artery up to orbital roof overlying optic the SOF to orbital roof overlying
 sheath OC
procedural step 3roof of orbit removed fromskeletonize lat dural wall of the SOF &sphenoid bone overlying lat marginSOF is opened; removal of lesser
 orbital edge along w/ sphenoid unroof FR of dural insertion into SOF is wing overlying the superior
 wing backward to ACP drilled medially in a perpendicular lat margin of SOF &
 & to entrance of OC direction to the ON & ∼1 cm ant to partial removal of greater
 the falciform ligament wing to expose inferior lat
 margin of SOF
procedural step 4partial removal of lat orbitalunroof OCOC is unroofed from its medialMOB (w/ named vessels) is
 wall btwn SOF & FR margin laterally to include lat divided at apex of SOF
 border of OC & optic strut
procedural step 5ACP drilled off medially &central portion of ACP drilled, leaving thin shelldissection plane created btwn duradura propia of temporal tip elevated
 separated from lat wall of OC of bone & ACP from inner cavernous
 membrane from SOF to FR
 or FO if required
procedural step 6OC unroofed from lat to medialACP dissected from medial wall of SOF & thinACP is removed en bloc; if en blocdura elevated from inner cavernous
 direction w/o opening the shell of bone removed removal impossible, remnants membrane in a pst &
 ethmoid sinus are removed in piecemeal fashion or medial direction to expose
 drilled. Small piece of optic strut ACP & intracranial aperture
 usually remains laterocaudally to of OC
 ON & can be drilled if required
procedural step 7central portion of ACP drilled,ACP tip extirpatedunroof OC & drill off optic strut
 leaving thin shell of bone
procedural step 8blunt dissection of shelled ACPdura propia of temporal tip elevated from innercentral portion of ACP drilled,
 from dura cavernous membrane leaving thin shell of bone
procedural step 9ACP extirpatedMOB w/ vessels are divided at apex of SOFcircumferential dissection plane
 created btwn the dura & centrally
 shelled ACP
procedural step 10cleavage plane is developed at junction of temporalACP extirpated
 dura & periorbital fascia, elevating dura
 of inner cavernous membrane from SOF to FO
differences
 unroofing FRnot performedperformednot performedperformed only when required
 by pathology
 excising MOBnot performedperformed after ACP extirpationnot performedperformed after opening SOF

Ant = anterior; FO = foramen ovale; FR = foramen rotundum; OC = optic canal; ON = optic nerve; pst = posterior.

Second, the ACP can be disengaged from its base by drilling from a lateral to medial direction toward the midpoint of the intracranial opening of the optic canal under direct vision (Fig. 1A, hatched area with embedded arrow) to expose the optic nerve in the optic canal. The trough created is progressively deepened and the optic strut is drilled off as required (Fig. 1B, hatched area). Only for large and difficult paraclinoid aneurysms should the optic canal be completely opened to facilitate subsequent untethering of the optic nerve intradurally with less than gentle manipulation to avoid visual deficits. In addition, one should avoid violating either the ethmoid or sphenoid sinuses to avert CSF rhinorrhea when completely opening the optic canal. Our technique is simpler compared with others in the literature, which are lengthy because of an inability to judge the location of the intracranial aperture of the optic canal. This characteristic is demonstrated in the reports of Yonekawa, et al.,23 who proposed that drilling of the ACP should commence at the sphenoid bone overlying the lateral margin of the dural insertion into the SOF and continue medially in a perpendicular direction to the optic nerve at a point 1 cm anterior to the falciform ligament, and Dolenc,4 who, in a recent report on carotid artery—ophthalmic artery aneurysms, advocated drilling of the ACP starting on its inferolateral side and proceeding in a posteromedial direction toward the intracranial aperture of the optic canal.

Last, the ACP should be centrally shelled or hollowed out using a diamond drill to the point that the walls can be easily fractured and circumferentially dissected free of the surrounding dural folds (Fig. 1G). In this respect, the extradural approach facilitates removal of the ACP, because of the dural protection provided to the neurovascular elements in the vicinity, and provides a tidier option, because it does not expose the subarachnoid space to bone debris—advantages unavailable in the intradural debulking procedure. En bloc removal of the ACP is not recommended,23 given that it would likely require additional extradural manipulation. The risk imposed by this action involves damage to the oculomotor nerve, which runs close to the lateral undersurface of the ACP.12,19 These patients can exhibit temporary postoperative dysfunction of the oculomotor nerve, which can range from a palsy in which the superior division of the oculomotor nerve is involved to complete ptosis. Functional recovery of the damaged nerve usually returns in 3 months.

When there is concern for intradural pathology or anomaly, the extradural anterior clinoidectomy can always be performed with the aid of intradural surveillance or a combined intradural procedure. In the event of dorsally pointing paraclinoid aneurysms, the ACP can be removed extradurally through intradural surveillance. When surgery is performed following subarachnoid hemorrhage of an aneurysm in close anatomical association with the ACP, however, removal of the extradurally hollowed ACP should be performed intradurally to avoid inadvertent rupture from extradural manipulation. Furthermore, particularly with paraclinoid aneurysms, it is important to establish preoperatively through CT studies whether the ACP is eroded, which could result in fatal intraoperative rupture during drilling of the ACP by using either an extradural22 or an intradural16 technique. In the event of an eroded ACP, Korosue and Heros16 have suggested drilling the base of the ACP while leaving a thin collar of bone around the aneurysm dome. On occasion, the ACP forms an interosseous bridge with either the middle or posterior clinoid process to form the caroticoclinoid foramen,15 a feature that can be identified on preoperative imaging. In such cases, the ACP should be disengaged from its three supporting structures and hollowed extradurally, although its tip should be removed intradurally.

A pneumatized ACP8,12,20 is usually connected to the sphenoid sinus by the bone conduit of an optic strut. Although an extradural procedure reduces the risk of a CSF leak after the removal of a pneumatized ACP, we recommend that the optic strut be plugged tightly with muscle or wax and sealed with fibrin glue and a fascial graft. This cautionary step is performed to prevent tension pneumocephalus as well as CSF rhinorrhea. On the other hand, a pneumatized ACP does not usually require central hollowing because the walls can be easily fractured. Nevertheless, a pneumatized ACP should be clearly distinguished from an eroded one, which is identified by a round “punched out” defect16 on CT scans.

Conclusions

This modified extradural anterior clinoidectomy offers the following advantages: 1) technical simplicity; 2) decreased incidence of neurovascular damage; 3) obviates postoperative CSF leaks and tension pneumocephalus; and 4) the option of either intradural surveillance or a combined intradural procedure when required. It is key initially to unroof the SOF and excise the MOB—steps that facilitate dural elevation and exposure of the ACP and the intracranial aperture of the optic canal without ambiguity. One can safely disengage the ACP from its three supporting structures: 1) the lesser wing of the sphenoid bone; 2) the roof of the optic canal; and 3) the optic strut. Finally, the ACP is centrally hollowed prior to its extirpation to avoid damage to the oculomotor nerve, which is very close to the lateral underside of the ACP. The technique presented here represents further refinements to the original Dolenc procedure and its derivatives.

Acknowledgments

The first and second authors (A.N. and V.B.) contributed equally to this work. We thank the medical photographers at Oregon Health & Science University, specifically Richard York, for photographic assistance and Shirley McCartney, Ph.D., for assistance in figure preparation and editing.

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Article Information

Address reprint requests to: Johnny B. Delashaw Jr., M.D., Department of Neurological Surgery, Oregon Health & Science University, 3181 Sam Jackson Park Road, Portland, Oregon 97239. email: delashaw@ohsu.edu.

© AANS, except where prohibited by US copyright law.

Headings

Figures

  • View in gallery

    Photographs representing the left ACP. A: The ACP forms a triangular spine of bone. It is a posteromedial bone continuation of the lesser wing of the sphenoid bone, and its base merges with the roof of the optic canal. Hatched areas represent the base of the ACP, which is removed using a diamond drill in the direction indicated by the arrow, which in turn facilitates detaching of the ACP from two of its three supporting structures, and the body of the ACP, which is centrally hollowed prior to its extirpation. B: The ACP is located between the optic canal medially and the SOF laterally. It is also connected to the sphenoid bone by its posterior root, the optic strut, which forms the lateral and ventral walls of the optic canal. Hatched area represents the portion of the optic strut that is drilled off as needed after drilling of the base of the ACP, as indicated in panel A. C: Dural elevation is performed and directed medially toward the orbital roof overlying the optic canal. Asterisk represents the dural fold entering the apex of the SOF. FL = frontal lobe dura; LWS = lesser wing of the sphenoid bone; TL = temporal lobe dura. D: The SOF is unroofed, which involves complete removal of the lesser wing of the sphenoid (LWS) overlying the superolateral margin of the SOF and the partial removal of the greater wing of the sphenoid (GWS) forming the inferolateral wall of the SOF. The apex of the SOF is exposed to reveal the MOB. E: The MOB has been coagulated and cut to leave a stump. The dura propia of the temporal tip has been peeled away from the inner cavernous membrane (ICM), and the mandibular division of the trigeminal nerve (V2) entering the foramen rotundum is exposed, thus facilitating further dural elevation in a posteromedial direction to expose the ACP without difficulty. F: The anterior root of the ACP is removed using a diamond burr and constant irrigation in a lateral to medial direction. The anterior root of the ACP is defined as the lateral continuation of the flat roof of the optic canal to the base of the ACP. G: A circumferential dissection plane (arrowheads) has been established between the centrally hollowed ACP and its surrounding dural folds to facilitate extirpation without damage to the neural structures. Of particular importance is the oculomotor nerve, which is the closest to the body and base of the ACP and runs in the medial wall of the SOF, just below the lateroinferior side of the ACP. H: A cadaveric intracranial view following an extradural anterior clinoidectomy in which the optic nerve (ON) is deflected medially by an instrument. The third, fourth, and sixth cranial nerves; proximal dural ring; ophthalmic artery; and posterior communicating artery perforators are indicated.

References

1.

Balasingam VNoguchi AMcMenomey SODelashaw JB Jr: Modified osteoplastic orbitozygomatic craniotomy. Technical review. J Neurosurg 102:9409442005J Neurosurg 102:

2.

Day JDGiannotta SLFukushima T: Extradural temporopolar approach to lesions of the upper basilar artery and infrachiasmatic region. J Neurosurg 81:2302351994J Neurosurg 81:

3.

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