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Surgical anatomy of the anterior clinoid process
Journal of Clinical Neuroscience (2004) 11(3), 283–287 0967-5868/$ - see front matter ª 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jocn.2003.08.005
Surgical anatomy
Surgical anatomy of the anterior clinoid process Phuong Huynh-Le
MD,
Yoshihiro Natori
MD PHD,
Tomio Sasaki MD PHD
Department of Neurosurgery, Neurological Institute, Graduate School of Medical Sciences, Kyushu University, Fukuoka, Japan
Summary We studied the surgical anatomy of the anterior clinoid process (ACP) and its adjacent structures in cadaver heads. We paid special attention to the anatomical relationships between the ACP and adjacent structures to determine the surgical landmarks for safe anterior clinoidectomy. Thirty-five cadaver heads were dissected and the ACP regions were examined in 55 skull sides. We observed that in eight sides the ACP had been pneumatized from the sphenoid sinus. The caroticoclinoid foramen was revealed in only eight sides. The extra-ocular nerves ran forward to the superior orbital fissure at the inferolateral aspect of the ACP, with the oculomotor nerve being closest. The posterolateral area of the carotico-oculomotor membrane was thin and incomplete in nine sides. The study clarified the anatomical relationship between the ACP and its surrounding structures, and identified the major variations experienced. We used these to identify anatomical landmarks to assist the surgeon in the planning of a safe and effective anterior clinoidectomy. ª 2003 Elsevier Ltd. All rights reserved. Keywords: anterior clinoid process, caroticoclinoid foramen, clinoid space, carotico-oculomotor membrane, surgical anatomy
INTRODUCTION The anterior clinoid process (ACP) is a bony projection on the medial extremity of the posterior border of the lesser sphenoid wing.1 The need for removal of this structure has been emphasized in reports on surgical treatment of aneurysms of the carotidophthalmic artery, the internal carotid artery (ICA), the intracavernous ICA, the upper basilar artery, and for tumours of the clinoid region.2–10 Anterior clinoidectomy provides improved exposure of structures in and around the optic nerve, the ICA, and the optic canal (OC). It also increases mobilization of the intracranial ICA and optic nerve with less brain retraction.11–15 Many reports have described anterior clinoidectomy as part of a more extensive approach.2–4;6;9;11;16–19 Others have described a few of the anatomical features of the ACP.20–24 However, data supporting surgical applications are rare, and a detailed anatomical description of the ACP is therefore needed. In this study, we examined the surgical anatomy of the ACP and its relationship to surrounding structures in cadavers, with the aim to demonstrate a proper surgical strategy for anterior clinoidectomy and to avoid surgical complications during its removal. MATERIAL AND METHOD The heads of 25 cadavers fixed in formalin and of 10 fresh autopsy subjects (19 male and 16 female) were used. We excluded those sides of skulls that were used for other studies. In all we examined 28 left and 27 right sides. The age of the individuals ranged from 18 to 91 years at death, with an average of 71.8 years. We removed the ACP using a stepwise extradural dissection. Care was taken to avoid damage to adjacent neural and vascular structures. Anatomical landmarks were identified and the same
Received 10 July 2003 Accepted 25 August 2003 Correspondence to: Yoshihiro Natori MD PhD, Department of Neurosurgery, Neurological Institute, Graduate school of Medical Sciences, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan. Tel.: +81-92-642-5523; fax: +81-92-642-5526; E-mail: [email protected]
investigator dissected all specimens. To compare the inferolateral distances between the ACP and related neural structures we chose three crossing coronal planes for sectioning. The first plane was at the level of the tip of the ACP (A). The second crossed through the junction between the medial edge of the ACP and the posterior edge of the roof of the OC (B). The other plane was at the centre of the posterior edge of the roof of the OC (C) (Fig. 3). Data were measured directly using a micrometer to the nearest 0.1 mm through a surgical microscope. We grouped the surrounding anatomical relationships into two sets. The first included measurements of the superior aspect of the ACP and the relationship of the tip of the ACP to surrounding structures. The second group included measurements of the inferior aspect of the ACP and its relationship to nearby neural structures. RESULTS Superior surface of the ACP From a superior view, the ACP appears as a triangular mass with its tip projecting medioposteriorly. Its base has an indefinite limit and has three sites continuous with the adjacent parts of the lesser sphenoid wing. Anteriorly, the base of the ACP continues with the medial end of the sphenoid ridge. Medially, the base connects to the body of the sphenoid bone by two elements of the lesser sphenoid wing: the posterior and anterior roots. The anterior root is flat and forms the roof of the OC by extending medially from the base of the ACP. The posterior root, called the optic strut, extends from the lower margin of the base of the ACP to the body of the sphenoid bone. The posterior root forms the anterior part of the lateral and inferior walls of the OC.21;25;26 The ACP comprises a single bony cortex enclosing a narrow bone. However, if pneumatization of the sphenoid bone is widespread via the optic strut, it can form a hollow process with a thin bony wall. We observed two sides in which the ACP had been completely pneumatized. In these two skulls, the optic strut resembled a thin bony duct. Partial pneumatization was also found in six other sides with more than half of the ACP being pneumatized. The ACP was sometimes united with either the middle or posterior clinoid process by a fibrous or osseous bridge. A complete 283
284 Huynh-Le et al.
Table 1 Measurements of the anatomical relationships of the ACP to surrounding structures Anatomical structures measured
Measurements (mm) Mean
A–B A–C A–D A–E A–F A–G A–H A–I A–J A–K B–E D–B D–C Height of ACP Width of ACP Length of the optic strut Width of the optic strut Width of the clinoid space
5.2 9.2 20.6 10.3 5.8 7.5 7.8 6.7 5.0 10.3 7.6 17.7 18.1 5.3 5.6 6.0 3.1 3.5
Range 4.0–7.1 6.2–12.0 16.3–24.6 8.7–15.5 2.3–8.0 4.7–11.5 4.2–10.1 3.0–9.4 2.7–7.2 5.5–14.5 6.0–8.8 14.8–21.2 14.2–21.7 4.0–6.8 4.0–7.8 2.8–8.9 2.0–4.7 2.0–6.0
Notes: (A) The tip of the anterior clinoid process. (B) The junction of the medial edge of the ACP and the posterolateral edge of the roof of the optic canal. (C) The centre point of the posterior edge of the roof of the optic canal. (D) The lateral end of the superior orbital fissure. (E) The anterior edge of the optic strut base. (F) The posterior edge of the optic strut base. (G) The anterior point of the upper dural ring. (H) The origin of the ophthalmic artery. (I) The anterior point of the dural entry of the oculomotor nerve. (J) The lateral point of the optic nerve at a right angle from the apex of the ACP. (K) The posterior clinoid process.
bony connection between the ACP and the middle clinoid process forms a caroticoclinoid foramen (CCF). This was found in eight of the sides studied (Fig. 2), and in four of these, the ACP united with both the middle and posterior clinoid processes. The anatomical relationships of the ACP to surrounding structures from superior aspect are shown in Table 1 and Fig. 1(A), and are indicated schematically in Fig. 1(B). Inferior surface of the ACP Inferolateral surface From this aspect, the ACP is contiguous with the extra-ocular nerves. The oculomotor and trochlear nerves, three major branches of the ophthalmic nerve and the abducens nerve, form a neural bundle running under the lower margin of the ACP. Table 2 shows the distances from the ACP to the neural bundle at three different levels, and Fig. 3 demonstrates the relationship schematically. Inferomedial surface Removal of the ACP reveals a region located inferomedially to the ACP and termed the clinoid space. On average, this is 3.5 mm long (range 2–6 mm) (Fig. 4(A)). The dural layer separating this space from the ACP is the deep (inner) layer, formed from that of the roof of the cavernous sinus and covering the inferior surface of the ACP. This layer also extends medially to surround the ICA as the lower dural ring, and turns upward continuously along the clinoid segment of the ICA to fuse with the upper dural ring (Fig. 4(B)). The part between the oculomotor nerve and the lateral part of the lower dural ring, extending continuously to the posterior clinoid process, is also part of the deep dural layer termed the carotico-oculomotor membrane. This membrane is thin and sometimes incomplete. There is a small triangular region of this membrane, located posterolaterally to the clinoid segment of the Journal of Clinical Neuroscience (2004) 11(3), 283–287
ICA where the clinoid space tapers. This area might be the thinnest and most fragile region of the roof of the cavernous sinus (asterisk in Fig. 4(B)). It was thin and incomplete in nine sides of our specimens. The anatomical relationships of the ACP to surrounding structures from inferomedial aspect are shown in Table 1 and are indicated in Fig. 4.
DISCUSSION Superior surface of the ACP Removal of the ACP has been reported in many studies.3–8;10;12;20;22;24 Several authors have also described techniques for the removal of the ACP, including extradural and intradural approaches.4;11;13;19;24;27 As Nutik has pointed out,24 the direction of the optic canal and the junction of the medial edge of the anterior clinoid process with the posterolateral edge of the roof of the optic canal should be noted (point B in Fig. 1(B)). In surgery, however, point B is difficult to observe even using an intradural technique because of the overhang of the ACP. Using the extradural technique, Yonekawa 27 advised that drilling of the ACP should begin at the portion of the sphenoid bone that overlies the lateral margin of the dural insertion into the superior orbital fissure. Drilling should be directed medially and make a right angle to the course of the optic canal. Dolenc,11 in his report on carotidophthalmic aneurysms, recommended that drilling of the ACP should start on its inferolateral side and proceed in a posteromedial direction. From our study, we observed that the lateral margin of the superior orbital fissure (point D in Fig. 1(B)) could be determined easily. Moreover, the centre point of the posterior bony edge of the roof of the OC (point C in Fig. 1(B)) was also easy to identify. This is a good anatomical landmark, easily identified by the surgeon using the common anterolateral approach (Fig. 1(A)). In addition, it is also constant compared with other anatomical landmarks at the medial aspect. Under any approach from anterior-frontal or anterior-lateral views, the surgeon can easily dissect the outer dural layer covering the superior surface of ACP medially up to the orbital roof, posteromedially to the posterior edge of the roof of the OC and falciform ligament. Here, we identified a triangle bounded by points A, C and D in Fig. 1(B) (Table 1). This should be determined clearly before starting to drill the ACP. Point C of the triangle gives the medial limit, and visualizing line D–C gives an anterior limit to the drilling region. Drilling should begin at point D, the portion of the sphenoid bone that overlies the lateral margin of the dural insertion into the superior orbital fissure. The drill should be directed along line D–C on the superior surface of the ACP (Fig. 1(B)). Next, drilling should shift to be parallel with line D–C and posterolaterally across the ACP to its opposite edge. However, before drilling is complete, it should be carried out anterolaterally between the optic canal and the lesser sphenoid wing, and deeper into the posterior portion of the optic strut. A completed transaction at the base of the ACP should be carried out at this stage to free it. Moreover, a plane of dissection between the ACP and the dura should be detached carefully using a microdissector. The final bony attachments of the ACP should be grasped using a small rongeur. From the above triangular landmark, we can extend removal to the medial border of the optic canal or simply remove the ACP combined with opening the optic canal on its lateral and superior aspects.6;11;28;29 If the anterior and middle clinoid process have fused to form the CCF (Fig. 2), removing the ACP can become difficult, especially during detachment of the tip of the ACP. Moreover, it is impossible to retract or mobilize the ICA even after circumferª 2003 Elsevier Ltd. All rights reserved.
Surgical anatomy of the anterior clinoid process 285
Fig. 1 (A) Anterolateral view of the left anterior clinoid process region of a cadaveric specimen. This view simulates the surgical view. The photograph shows the relationship between the anterior clinoid process and surrounding structures. Key: ACP, anterior clinoid process; ICA, internal carotid artery; ON, optic nerve and III, oculomotor nerve. (B) Schematic drawing of the superior view of the left anterior clinoid process (ACP), showing the relationship between the tip of the ACP and surrounding structures. Key: A, tip of the anterior clinoid process; B, junction of the medial edge of the ACP and the posterior edge of the roof of the optic canal; C, centre point on the posterior bony edge of the roof of the optic canal; D, point of the lateral margin dura entering into the superior orbital fissure; E, anterior edge of the optic strut base; F, posterior edge of the optic strut base; G, most anterior point of the upper dural ring; H, origin of the ophthalmic artery; I, anterior point of dural entry of the oculomotor nerve and K, posterior clinoid process.
Fig. 2 Photograph of the left parasellar region of a cadaveric specimen from a superior view. The dura covering the anterior clinoid process has been removed. The dura and the falciform ligament of the optic canal are also partly removed. The photograph shows the tip of the anterior clinoid process fused and connecting with the middle clinoid process, forming the caroticoclinoid foramen. Key: ACP, anterior clinoid process; MCP, middle clinoid process; ICA, internal carotid artery; ON, optic nerve; Falciform Lig., Falciform ligament and Caroticoclinoid F., Caroticoclinoid Foramen.
Table 2
Fig. 3 Schematic drawing of the lateral view of the left anterior clinoid process (ACP), showing the relationship between the ACP and cranial nerves from an inferolateral aspect. Three imaging coronal planes sectioning at three differential levels of the ACP are shown, with three small schematic drawings demonstrating the relationships between the ACP and cranial nerves III, IV, and V at three different levels. Key: (A) First coronal plane at the level of the tip of ACP. (B) Second coronal plane at the junction point of the medial edge of ACP and the posterior edge of the roof of the optic canal. (C) Another coronal plane at level of the centre point on the posterior bony edge of the roof of the optic canal. ACP, anterior clinoid process; ICA, internal carotid artery; ON, optic nerve; III, oculomotor nerve; IV, Trochlear nerve and V, trigeminal nerve.
Anatomical relationships between the ACP and cranial nerves From the inferior aspect of ACP to III
Coronal plane at the tip of the ACP Coronal plane at point B Coronal plane at point C
1.5 mm (0.6–2.9) 0.7 mm (0.2–1.2) 0.5 mm (0.2–1.2)
From the inferior aspect of ACP to IV
From the inferior aspect of ACP to V
4.1 mm (2.0–5.6) 2.8 mm (0.6–5.0) 0.9 mm (0.4–3.5)
5.7 mm (2.5–7.2) 4.2 mm (1.8–6.2) 2.2 mm (0.5–5.0)
Notes: (ACP) The anterior clinoid process. (III) Oculomotor nerve. (IV) Trochlear nerve. (V) Trigeminal nerve. (B) The junction of the medial edge of the ACP and the posterolateral edge of the roof of the optic canal. (C) The centre of the posterior edge of the roof of the optic canal.
entially dissecting the dural rings of the ICA.23 Our specimens revealed the presence of a CCF in eight of the 55 sides (14%). Therefore, preoperative identification of the CCF by imaging evaluation is important and requires careful attention. ª 2003 Elsevier Ltd. All rights reserved.
Removal of the ACP should be performed using a diamond drill in short bursts.11 After each drilling period, the wall of the hollowed ACP should be checked carefully. In two of our sides, the ACP had been pneumatized completely via the optic strut. In Journal of Clinical Neuroscience (2004) 11(3), 283–287
286 Huynh-Le et al.
Fig. 4 (A) Schematic drawing of the superoposterior view of the left anterior clinoid process (ACP), showing a coronal section of the ACP at the clinoid space. Key: ACP, anterior clinoid process; ICA, internal carotid artery; ON, optic nerve; X, width of ACP; Y, height of ACP; Z, width of the clinoid space. (B) Superolateral view of the left clinoid space of a cadaveric specimen. The anterior clinoid process (ACP) and dura covering on the superior surface of the ACP have been removed. The dural cuff surrounding the oculomotor nerve has been partly removed. The photograph shows the relationship between the ACP and surrounding structures from an inferomedial aspect. The dura that line the lower margin of the ACP and extends medially above the oculomotor nerve to surround the internal carotid artery and form the lower dural ring is referred to as the carotico-oculomotor membrane. Key: ACP, anterior clinoid process; ICA, internal carotid artery; ON, optic nerve; III, oculomotor nerve; OS, optic strut. (?) Posterolateral region of the carotidoculomotor membrane.
addition, in six other sides, partial pneumatization had occurred. Inadvertent opening of the sphenoid sinus thus needs to be avoided during surgery. Inferior surface of the ACP Direct trauma to the cranial nerves while removing the ACP is also a possibility. Table 2 shows that the oculomotor nerve is very close to the body and base of the ACP. Thus, care needs to be taken when dissecting the plane between the ACP and the dura inferolaterally to avoid injury to this nerve.25 Moreover, drilling of the ACP should be performed with continuous irrigation. On reaching the bony cortex of the ACP, its thin bony attachment should be broken with a small curette or a small rongeur after gently dissecting it free from the dura using a microdissector. The deep dural layer separates the lower margin of the ACP from the neural structures inferolaterally. This layer is very thin and friable. It separates the cavernous sinus from the clinoid space.21 In our material, the carotico-oculomotor membrane was very thin in nine (16%) sides at the posterolateral region of the clinoid space. Umansky29 reported that the membrane was incomplete in five of 30 specimens. Consequently, the surgeon needs to take care when dissecting the inferomedial region of the ACP posterior to the clinoid space. Bleeding from the anterior part of the roof of the cavernous sinus may occur, but it is easy to control using oxycellulose with cotton sponge compression. However, it is easy to damage the oculomotor nerve by tight packing in this area. CONCLUSION We examined the surgical anatomy of the ACP as well as its anatomical relationships to surrounding structures. We suggest some significant surgical landmarks to assist in performing proper and safe anterior clinoidectomy. REFERENCES 1. Warwick R, Williams PL et al. Gray’s Anatomy. 35th edn, Longman Group Ltd., Edinburgh; 1973: 275. 2. Al-Mefty O. Clinoid meningiomas. J Neurosurg 1990; 73: 840–849. 3. Batjer HH, Kopitnik TA, Giller CA, Samson DS. Surgery for paraclinoidal carotid artery aneurysms. J Neurosurg 1994; 80: 650–658.
Journal of Clinical Neuroscience (2004) 11(3), 283–287
4. Day JD, Giannotta SL, Fukushima T. Extradural temporopolar approach to lesions of the upper basilar artery and infrachiasmatic region. J Neurosurg 1994; 81: 230–235. 5. Dolenc VV. A combine epi- and subdural direct approach to carotid-ophthalmic aneurysms. J Neurosurg 1985; 62: 667–672. 6. Heros RC, Nelson PB, Ojemann RG, Crowell RM, DeBrun G. Large and giant paraclinoid aneurysms: Surgical techniques, complications, and results. Neurosurgery 1983; 12: 153–163. 7. Kobayashi S, Kyoshima K, Gibo H, Hegde S, Takemae T, Sugita K. Carotid cave aneurysms of the internal carotid artery. J Neurosurg 1989; 70: 216–221. 8. Korosue K, Heros R. “Subclinoid” carotid aneurysm with erosion of the anterior clinoid process and fatal intraoperative rupture. Neurosurgery 1992; 31: 356–360. 9. Matsuoka Y, Hakuba A, Kishi H, Nishimura S. Direct surgical treatment of intracavernous internal carotid artery aneurysms: Report of four cases. Surg Neurol 1986; 26: 360–364. 10. Perneczky A, Knosp E, Vorkapic P, Czech Th. Direct surgical approach to infraclinoidal aneurysms. Acta Neurochir (Wien) 1985; 76: 36–44. 11. Dolenc VV. A combine transorbital–transclinoid and transsylvian approach to carotid-ophthalmic aneurysms without retraction of the brain. Acta Neurochir Suppl (Wien) 1999; 72: 89–97. 12. Matsuyama T, Shimomura T, Okumura Y, Sasaki T. Mobilization of the internal carotid artery for basilar artery aneurysm surgery. J Neurosurg 1997; 86: 294–296. 13. Morgan F. Removal of anterior clinoid process in the surgery of carotid aneurysm. Schweiz Arch Neurol Neurochir Psychiat 1972; 111: 363–368. 14. Sato S, Sato M, Oizumi T et al. Removal of anterior clinoid process for basilar tip aneurysm: Clinical and cadaveric analysis. Neurol Res 2001; 23: 298–303. 15. Seoane E, Rhoton Jr AL, de Oliveira E. Microsurgical anatomy of the dural collar and rings around the clinoid segment of the internal carotid artery. Neurosurgery 1998; 42: 869–884. 16. Drake CG, Vanderlinden RG, Amacher AL. Carotid-ophthalmic aneurysms. J Neurosurg 1968; 29: 24–31. 17. Knops E, Muller G, Perneczky A. The paraclinoid carotid artery: Anatomical aspects of a microneurosurgical approach. Neurosurgery 1988; 22: 896–901. 18. Ochiai O, Wakai S, Nagai M. Preoperative angiographical prediction of the necessity to removal of the anterior clinoid process in internal carotid-posterior communicating artery aneurysm surgery. Acta Neurochir (Wien) 1989; 99: 117–121. 19. Yasargil MG, Gasser JC, Hodosh RM, Rankin TV. Carotid-ophthalmic aneurysms: Direct microsurgical approach. Surg Neurol 1977; 8: 155–165. 20. Evans JJ, Hwang YS, Lee JH. Pre- versus post-anterior clinoidectomy measurements of the optic nerve, internal carotid artery, and opticocarotid triangle: A cadaveric morphometric study. Neurosurgery 2000; 46: 1018–1021. 21. Inoue T, Rhoton AL, Theele D, Barry ME. Surgical approaches to the cavernous sinus: A microsurgical study. Neurosurgery 1990; 26: 903–932. 22. Kim JM, Romano A, Sanan A, van Loveren HR, Keller J. Microsurgical anatomical features and nomenclature of the paraclinoid region. Neurosurgery 2000; 46: 670–680. 23. Lee HY, Chung IH, Choi BY, Lee KS. Anterior clinoid process and optic strut in Koreans. Yonsei Med J 1997; 38: 151–154.
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24. Nutik SL. Removal of the anterior clinoid process for exposure of the proximal intracranial carotid artery. J Neurosurg 1988; 69: 529–534. 25. Natori Y, Rhoton AL. Microsurgical anatomy of the superior orbital fissure. Neurosurgery 1995; 36: 762–775. 26. Parkinson D. Optic strut: Posterior root of Sphenoid. Clin Anat 1989; 2: 87–92.
ª 2003 Elsevier Ltd. All rights reserved.
27. Yonekawa Y, Ogata N, Imhof HG. Selective extradural anterior clinoidectomy for supra- and parasellar processes. J Neurosurg 1997; 87: 636–642. 28. Guidetti B, Torre EL. Management of carotid-ophthalmic aneurysms. J Neurosurg 1975; 42: 438–442. 29. Umansky F, Alberto Valarezo MD, Josef Elidan MD. The superior wall of the cavernous sinus: A micro anatomical study. J Neurosurg 1994; 81: 914–920.
Journal of Clinical Neuroscience (2004) 11(3), 283–287
Surgical anatomy
Surgical anatomy of the anterior clinoid process Phuong Huynh-Le
MD,
Yoshihiro Natori
MD PHD,
Tomio Sasaki MD PHD
Department of Neurosurgery, Neurological Institute, Graduate School of Medical Sciences, Kyushu University, Fukuoka, Japan
Summary We studied the surgical anatomy of the anterior clinoid process (ACP) and its adjacent structures in cadaver heads. We paid special attention to the anatomical relationships between the ACP and adjacent structures to determine the surgical landmarks for safe anterior clinoidectomy. Thirty-five cadaver heads were dissected and the ACP regions were examined in 55 skull sides. We observed that in eight sides the ACP had been pneumatized from the sphenoid sinus. The caroticoclinoid foramen was revealed in only eight sides. The extra-ocular nerves ran forward to the superior orbital fissure at the inferolateral aspect of the ACP, with the oculomotor nerve being closest. The posterolateral area of the carotico-oculomotor membrane was thin and incomplete in nine sides. The study clarified the anatomical relationship between the ACP and its surrounding structures, and identified the major variations experienced. We used these to identify anatomical landmarks to assist the surgeon in the planning of a safe and effective anterior clinoidectomy. ª 2003 Elsevier Ltd. All rights reserved. Keywords: anterior clinoid process, caroticoclinoid foramen, clinoid space, carotico-oculomotor membrane, surgical anatomy
INTRODUCTION The anterior clinoid process (ACP) is a bony projection on the medial extremity of the posterior border of the lesser sphenoid wing.1 The need for removal of this structure has been emphasized in reports on surgical treatment of aneurysms of the carotidophthalmic artery, the internal carotid artery (ICA), the intracavernous ICA, the upper basilar artery, and for tumours of the clinoid region.2–10 Anterior clinoidectomy provides improved exposure of structures in and around the optic nerve, the ICA, and the optic canal (OC). It also increases mobilization of the intracranial ICA and optic nerve with less brain retraction.11–15 Many reports have described anterior clinoidectomy as part of a more extensive approach.2–4;6;9;11;16–19 Others have described a few of the anatomical features of the ACP.20–24 However, data supporting surgical applications are rare, and a detailed anatomical description of the ACP is therefore needed. In this study, we examined the surgical anatomy of the ACP and its relationship to surrounding structures in cadavers, with the aim to demonstrate a proper surgical strategy for anterior clinoidectomy and to avoid surgical complications during its removal. MATERIAL AND METHOD The heads of 25 cadavers fixed in formalin and of 10 fresh autopsy subjects (19 male and 16 female) were used. We excluded those sides of skulls that were used for other studies. In all we examined 28 left and 27 right sides. The age of the individuals ranged from 18 to 91 years at death, with an average of 71.8 years. We removed the ACP using a stepwise extradural dissection. Care was taken to avoid damage to adjacent neural and vascular structures. Anatomical landmarks were identified and the same
Received 10 July 2003 Accepted 25 August 2003 Correspondence to: Yoshihiro Natori MD PhD, Department of Neurosurgery, Neurological Institute, Graduate school of Medical Sciences, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan. Tel.: +81-92-642-5523; fax: +81-92-642-5526; E-mail: [email protected]
investigator dissected all specimens. To compare the inferolateral distances between the ACP and related neural structures we chose three crossing coronal planes for sectioning. The first plane was at the level of the tip of the ACP (A). The second crossed through the junction between the medial edge of the ACP and the posterior edge of the roof of the OC (B). The other plane was at the centre of the posterior edge of the roof of the OC (C) (Fig. 3). Data were measured directly using a micrometer to the nearest 0.1 mm through a surgical microscope. We grouped the surrounding anatomical relationships into two sets. The first included measurements of the superior aspect of the ACP and the relationship of the tip of the ACP to surrounding structures. The second group included measurements of the inferior aspect of the ACP and its relationship to nearby neural structures. RESULTS Superior surface of the ACP From a superior view, the ACP appears as a triangular mass with its tip projecting medioposteriorly. Its base has an indefinite limit and has three sites continuous with the adjacent parts of the lesser sphenoid wing. Anteriorly, the base of the ACP continues with the medial end of the sphenoid ridge. Medially, the base connects to the body of the sphenoid bone by two elements of the lesser sphenoid wing: the posterior and anterior roots. The anterior root is flat and forms the roof of the OC by extending medially from the base of the ACP. The posterior root, called the optic strut, extends from the lower margin of the base of the ACP to the body of the sphenoid bone. The posterior root forms the anterior part of the lateral and inferior walls of the OC.21;25;26 The ACP comprises a single bony cortex enclosing a narrow bone. However, if pneumatization of the sphenoid bone is widespread via the optic strut, it can form a hollow process with a thin bony wall. We observed two sides in which the ACP had been completely pneumatized. In these two skulls, the optic strut resembled a thin bony duct. Partial pneumatization was also found in six other sides with more than half of the ACP being pneumatized. The ACP was sometimes united with either the middle or posterior clinoid process by a fibrous or osseous bridge. A complete 283
284 Huynh-Le et al.
Table 1 Measurements of the anatomical relationships of the ACP to surrounding structures Anatomical structures measured
Measurements (mm) Mean
A–B A–C A–D A–E A–F A–G A–H A–I A–J A–K B–E D–B D–C Height of ACP Width of ACP Length of the optic strut Width of the optic strut Width of the clinoid space
5.2 9.2 20.6 10.3 5.8 7.5 7.8 6.7 5.0 10.3 7.6 17.7 18.1 5.3 5.6 6.0 3.1 3.5
Range 4.0–7.1 6.2–12.0 16.3–24.6 8.7–15.5 2.3–8.0 4.7–11.5 4.2–10.1 3.0–9.4 2.7–7.2 5.5–14.5 6.0–8.8 14.8–21.2 14.2–21.7 4.0–6.8 4.0–7.8 2.8–8.9 2.0–4.7 2.0–6.0
Notes: (A) The tip of the anterior clinoid process. (B) The junction of the medial edge of the ACP and the posterolateral edge of the roof of the optic canal. (C) The centre point of the posterior edge of the roof of the optic canal. (D) The lateral end of the superior orbital fissure. (E) The anterior edge of the optic strut base. (F) The posterior edge of the optic strut base. (G) The anterior point of the upper dural ring. (H) The origin of the ophthalmic artery. (I) The anterior point of the dural entry of the oculomotor nerve. (J) The lateral point of the optic nerve at a right angle from the apex of the ACP. (K) The posterior clinoid process.
bony connection between the ACP and the middle clinoid process forms a caroticoclinoid foramen (CCF). This was found in eight of the sides studied (Fig. 2), and in four of these, the ACP united with both the middle and posterior clinoid processes. The anatomical relationships of the ACP to surrounding structures from superior aspect are shown in Table 1 and Fig. 1(A), and are indicated schematically in Fig. 1(B). Inferior surface of the ACP Inferolateral surface From this aspect, the ACP is contiguous with the extra-ocular nerves. The oculomotor and trochlear nerves, three major branches of the ophthalmic nerve and the abducens nerve, form a neural bundle running under the lower margin of the ACP. Table 2 shows the distances from the ACP to the neural bundle at three different levels, and Fig. 3 demonstrates the relationship schematically. Inferomedial surface Removal of the ACP reveals a region located inferomedially to the ACP and termed the clinoid space. On average, this is 3.5 mm long (range 2–6 mm) (Fig. 4(A)). The dural layer separating this space from the ACP is the deep (inner) layer, formed from that of the roof of the cavernous sinus and covering the inferior surface of the ACP. This layer also extends medially to surround the ICA as the lower dural ring, and turns upward continuously along the clinoid segment of the ICA to fuse with the upper dural ring (Fig. 4(B)). The part between the oculomotor nerve and the lateral part of the lower dural ring, extending continuously to the posterior clinoid process, is also part of the deep dural layer termed the carotico-oculomotor membrane. This membrane is thin and sometimes incomplete. There is a small triangular region of this membrane, located posterolaterally to the clinoid segment of the Journal of Clinical Neuroscience (2004) 11(3), 283–287
ICA where the clinoid space tapers. This area might be the thinnest and most fragile region of the roof of the cavernous sinus (asterisk in Fig. 4(B)). It was thin and incomplete in nine sides of our specimens. The anatomical relationships of the ACP to surrounding structures from inferomedial aspect are shown in Table 1 and are indicated in Fig. 4.
DISCUSSION Superior surface of the ACP Removal of the ACP has been reported in many studies.3–8;10;12;20;22;24 Several authors have also described techniques for the removal of the ACP, including extradural and intradural approaches.4;11;13;19;24;27 As Nutik has pointed out,24 the direction of the optic canal and the junction of the medial edge of the anterior clinoid process with the posterolateral edge of the roof of the optic canal should be noted (point B in Fig. 1(B)). In surgery, however, point B is difficult to observe even using an intradural technique because of the overhang of the ACP. Using the extradural technique, Yonekawa 27 advised that drilling of the ACP should begin at the portion of the sphenoid bone that overlies the lateral margin of the dural insertion into the superior orbital fissure. Drilling should be directed medially and make a right angle to the course of the optic canal. Dolenc,11 in his report on carotidophthalmic aneurysms, recommended that drilling of the ACP should start on its inferolateral side and proceed in a posteromedial direction. From our study, we observed that the lateral margin of the superior orbital fissure (point D in Fig. 1(B)) could be determined easily. Moreover, the centre point of the posterior bony edge of the roof of the OC (point C in Fig. 1(B)) was also easy to identify. This is a good anatomical landmark, easily identified by the surgeon using the common anterolateral approach (Fig. 1(A)). In addition, it is also constant compared with other anatomical landmarks at the medial aspect. Under any approach from anterior-frontal or anterior-lateral views, the surgeon can easily dissect the outer dural layer covering the superior surface of ACP medially up to the orbital roof, posteromedially to the posterior edge of the roof of the OC and falciform ligament. Here, we identified a triangle bounded by points A, C and D in Fig. 1(B) (Table 1). This should be determined clearly before starting to drill the ACP. Point C of the triangle gives the medial limit, and visualizing line D–C gives an anterior limit to the drilling region. Drilling should begin at point D, the portion of the sphenoid bone that overlies the lateral margin of the dural insertion into the superior orbital fissure. The drill should be directed along line D–C on the superior surface of the ACP (Fig. 1(B)). Next, drilling should shift to be parallel with line D–C and posterolaterally across the ACP to its opposite edge. However, before drilling is complete, it should be carried out anterolaterally between the optic canal and the lesser sphenoid wing, and deeper into the posterior portion of the optic strut. A completed transaction at the base of the ACP should be carried out at this stage to free it. Moreover, a plane of dissection between the ACP and the dura should be detached carefully using a microdissector. The final bony attachments of the ACP should be grasped using a small rongeur. From the above triangular landmark, we can extend removal to the medial border of the optic canal or simply remove the ACP combined with opening the optic canal on its lateral and superior aspects.6;11;28;29 If the anterior and middle clinoid process have fused to form the CCF (Fig. 2), removing the ACP can become difficult, especially during detachment of the tip of the ACP. Moreover, it is impossible to retract or mobilize the ICA even after circumferª 2003 Elsevier Ltd. All rights reserved.
Surgical anatomy of the anterior clinoid process 285
Fig. 1 (A) Anterolateral view of the left anterior clinoid process region of a cadaveric specimen. This view simulates the surgical view. The photograph shows the relationship between the anterior clinoid process and surrounding structures. Key: ACP, anterior clinoid process; ICA, internal carotid artery; ON, optic nerve and III, oculomotor nerve. (B) Schematic drawing of the superior view of the left anterior clinoid process (ACP), showing the relationship between the tip of the ACP and surrounding structures. Key: A, tip of the anterior clinoid process; B, junction of the medial edge of the ACP and the posterior edge of the roof of the optic canal; C, centre point on the posterior bony edge of the roof of the optic canal; D, point of the lateral margin dura entering into the superior orbital fissure; E, anterior edge of the optic strut base; F, posterior edge of the optic strut base; G, most anterior point of the upper dural ring; H, origin of the ophthalmic artery; I, anterior point of dural entry of the oculomotor nerve and K, posterior clinoid process.
Fig. 2 Photograph of the left parasellar region of a cadaveric specimen from a superior view. The dura covering the anterior clinoid process has been removed. The dura and the falciform ligament of the optic canal are also partly removed. The photograph shows the tip of the anterior clinoid process fused and connecting with the middle clinoid process, forming the caroticoclinoid foramen. Key: ACP, anterior clinoid process; MCP, middle clinoid process; ICA, internal carotid artery; ON, optic nerve; Falciform Lig., Falciform ligament and Caroticoclinoid F., Caroticoclinoid Foramen.
Table 2
Fig. 3 Schematic drawing of the lateral view of the left anterior clinoid process (ACP), showing the relationship between the ACP and cranial nerves from an inferolateral aspect. Three imaging coronal planes sectioning at three differential levels of the ACP are shown, with three small schematic drawings demonstrating the relationships between the ACP and cranial nerves III, IV, and V at three different levels. Key: (A) First coronal plane at the level of the tip of ACP. (B) Second coronal plane at the junction point of the medial edge of ACP and the posterior edge of the roof of the optic canal. (C) Another coronal plane at level of the centre point on the posterior bony edge of the roof of the optic canal. ACP, anterior clinoid process; ICA, internal carotid artery; ON, optic nerve; III, oculomotor nerve; IV, Trochlear nerve and V, trigeminal nerve.
Anatomical relationships between the ACP and cranial nerves From the inferior aspect of ACP to III
Coronal plane at the tip of the ACP Coronal plane at point B Coronal plane at point C
1.5 mm (0.6–2.9) 0.7 mm (0.2–1.2) 0.5 mm (0.2–1.2)
From the inferior aspect of ACP to IV
From the inferior aspect of ACP to V
4.1 mm (2.0–5.6) 2.8 mm (0.6–5.0) 0.9 mm (0.4–3.5)
5.7 mm (2.5–7.2) 4.2 mm (1.8–6.2) 2.2 mm (0.5–5.0)
Notes: (ACP) The anterior clinoid process. (III) Oculomotor nerve. (IV) Trochlear nerve. (V) Trigeminal nerve. (B) The junction of the medial edge of the ACP and the posterolateral edge of the roof of the optic canal. (C) The centre of the posterior edge of the roof of the optic canal.
entially dissecting the dural rings of the ICA.23 Our specimens revealed the presence of a CCF in eight of the 55 sides (14%). Therefore, preoperative identification of the CCF by imaging evaluation is important and requires careful attention. ª 2003 Elsevier Ltd. All rights reserved.
Removal of the ACP should be performed using a diamond drill in short bursts.11 After each drilling period, the wall of the hollowed ACP should be checked carefully. In two of our sides, the ACP had been pneumatized completely via the optic strut. In Journal of Clinical Neuroscience (2004) 11(3), 283–287
286 Huynh-Le et al.
Fig. 4 (A) Schematic drawing of the superoposterior view of the left anterior clinoid process (ACP), showing a coronal section of the ACP at the clinoid space. Key: ACP, anterior clinoid process; ICA, internal carotid artery; ON, optic nerve; X, width of ACP; Y, height of ACP; Z, width of the clinoid space. (B) Superolateral view of the left clinoid space of a cadaveric specimen. The anterior clinoid process (ACP) and dura covering on the superior surface of the ACP have been removed. The dural cuff surrounding the oculomotor nerve has been partly removed. The photograph shows the relationship between the ACP and surrounding structures from an inferomedial aspect. The dura that line the lower margin of the ACP and extends medially above the oculomotor nerve to surround the internal carotid artery and form the lower dural ring is referred to as the carotico-oculomotor membrane. Key: ACP, anterior clinoid process; ICA, internal carotid artery; ON, optic nerve; III, oculomotor nerve; OS, optic strut. (?) Posterolateral region of the carotidoculomotor membrane.
addition, in six other sides, partial pneumatization had occurred. Inadvertent opening of the sphenoid sinus thus needs to be avoided during surgery. Inferior surface of the ACP Direct trauma to the cranial nerves while removing the ACP is also a possibility. Table 2 shows that the oculomotor nerve is very close to the body and base of the ACP. Thus, care needs to be taken when dissecting the plane between the ACP and the dura inferolaterally to avoid injury to this nerve.25 Moreover, drilling of the ACP should be performed with continuous irrigation. On reaching the bony cortex of the ACP, its thin bony attachment should be broken with a small curette or a small rongeur after gently dissecting it free from the dura using a microdissector. The deep dural layer separates the lower margin of the ACP from the neural structures inferolaterally. This layer is very thin and friable. It separates the cavernous sinus from the clinoid space.21 In our material, the carotico-oculomotor membrane was very thin in nine (16%) sides at the posterolateral region of the clinoid space. Umansky29 reported that the membrane was incomplete in five of 30 specimens. Consequently, the surgeon needs to take care when dissecting the inferomedial region of the ACP posterior to the clinoid space. Bleeding from the anterior part of the roof of the cavernous sinus may occur, but it is easy to control using oxycellulose with cotton sponge compression. However, it is easy to damage the oculomotor nerve by tight packing in this area. CONCLUSION We examined the surgical anatomy of the ACP as well as its anatomical relationships to surrounding structures. We suggest some significant surgical landmarks to assist in performing proper and safe anterior clinoidectomy. REFERENCES 1. Warwick R, Williams PL et al. Gray’s Anatomy. 35th edn, Longman Group Ltd., Edinburgh; 1973: 275. 2. Al-Mefty O. Clinoid meningiomas. J Neurosurg 1990; 73: 840–849. 3. Batjer HH, Kopitnik TA, Giller CA, Samson DS. Surgery for paraclinoidal carotid artery aneurysms. J Neurosurg 1994; 80: 650–658.
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