Introduction

   The incidence of internal carotid-posterior communicating (ICPCom) artery aneurysm is 17-22% of all intracranial aneurysms.¹⁴⁾ In the generally accepted method, it is possible to successfully clip an aneurysmal neck without any difficulty. However, if the aneurysmal neck is wide, the aneurysmal sac is giant, the aneurysmal sac is hidden by the anterior clinoid process(ACP) or its dome is located in the ventral portion or low-lying ICPCom aneurysm, it is difficult to open the proximal aneurysmal neck and we encounter a barrier in controlling bleeding in case of premature rupture of aneurysm. This results in disastrous outcomes.
   It has been stressed that ACP needs to be resected in sur-gery for internal carotid-opthalmic and giant internal carotid artery aneurysms.^9)17)25) Occasionally, however, this procedure is also needed in surgery for ICPCom artery aneurysm for exposing the proximal aneurysmal neck. Although the technique itself is not difficult, it is of great importance for neurosurgeons to predict prior to surgery whether the procedure is necessary.
   The purpose of this study is to determine angiographic standards for predicting the necessity of this procedure prior to surgery for an ICPCom artery aneurysm by conducting a study of preoperative angiography that was successfully clipped after resection of the ACP.

Material and Methods

   One hundred and forty-nine patients with aneurysm were treated between 1999 and 2003(50 anterior communicating, 38 MCA bifurcation, 40 ICPCom, 5 basilar tip, 1 ICA bifurcation, and 15 miscellaneous). Sixteen patients with ICPCom aneurysm were treated between 1999 and 2003 with the resection of the ACP prior to the application of the clip on the neck of the aneurysm. In the most of them, we measured preoperative angiographic findings with parameters of Ochiai et al. or Kim et al.¹⁰⁾²⁰⁾ Then, we considered to resect the ACP or not. We decided to resect to the ACP for better operative view and proximal controlling duration operation in others. We retrospectively analyzed preoperative cerebral angiographies and clinical and operative findings. For the evaluation of the indication of the anterior clinoidectomy, preoperative cerebral angiographies were analyzed for measurement of various radiometric parameters such as angle A(the angle between the midline of the skull and the axis of the C1⁶⁾ segment on A-P view), angle B(the angle between the axis of the C1 and C2 segments on A-P view), and distance C (the distance between the tip of the ACP and the most proximal portion of the aneursymal neck on the lateral view)(Fig. 1). The clinical findings in the 16 patients are summarized in Table 1. In addition, we checked the duration required for temporary clipping by an intraoperative Doppler device (Fig. 2).
   The extradural approach was used to resect the ACP in all the cases. Extradural clinoidectomy was accomplished as described below. The position of the patient and fixation of the head are the same as those in the standard pterional approach.²³⁾ Following craniotomy and flattening of the sphenoid ridge by the usual drilling procedure, the dura is separated from the ACP up to the orbital roof over the optic sheath medially. With the aid of an operating microscope, the ACP is subsequently drilled away extradurally using a diamond burr. The ACP removal extends to the medial border of the optic nerve, to the lateral border of the dural entry into the superior orbital fissure, and approximately 1 cm anterior to the falciform fold of the optic nerve. Drilling is begun at the portion of the sphenoid bone that overlies the lateral margin of dural insertion into the superior orbital fissure. The drill is pointed medially at a right angle to the course of the optic canal. Further, the optic canal is unroofed from its medial margin laterally. Precautions should be taken in order to avoid injury of the underlying optic sheath or to carry the unroofing beyond the medial border of the optic canal and thereby open the ethmoid sinus. The lateral border of the roof of the optic canal, including the optic strut, must be completely transsected at this stage in order to render the ACP suitable for resection. A plane of dissection between the dura and the ACP is created using a microdissector, and the ACP is finally removed en bloc using a small rongeur.²⁶⁾

Results

   Among these 16 patients with ICPCom aneurysm, 5 were males and 13 were females and the mean age was 50.5 years. With the exception of one case, all the other cases exhibited a ruptured aneurysm. There were seven cases of Rt. ICPCom aneurysm and nine of Lt. ICPCom aneurysm. The aneurysmal sac size ranged from 4 to 17 mm(mean 6.5 mm). We classified the direction of ICPCom aneurysm according to Kodama's classification(Fig. 3).¹²⁾
   The direction of aneurysm was posteroinferior in nine patients, posterior in two, posterosuperior direction in two, inferior direction in three. The mean value of the radiographic measurement in the cerebral angiography in 16 patients is as follows：angle A ranged from 15 to 80 degrees(mean±SD, 42±5 degrees), angle B ranged from 70 to 150 degrees(mean±SD, 110±15 degrees), distance C ranged from 2 to 9 mm (mean±SD, 4.5±1 mm)(Table 3)(Fig. 4). The posterior communicating artery of the fetal type in one of the 16 patients. In all the cases no difficulties were encountered during clip application. Sufficient space existed between the aneurysmal neck and the ACP.

Discussion

   The ACP is a posterior and medial continuation of the lesser sphenoid wing, and it is connected to the body of the sphenoid bone by two roots- a superior root and an inferior root. The superior root is flat, forms the roof of the optic canal, and continues as the planum sphenoidale. The inferior (optic strut) root forms the lateral and ventral walls of the optic canal and connects the lesser wing with the basisphenoid bone. The ACP was is normally composed of a thin shell of cortical bone and an inner trabecular(cancellous) bone.¹¹⁾ The length of the ACP, from the level of the optic canal roof to the tip of the process, was measured in dry crania. In Koreans, the basal width and length of the anterior clinoid process were found to be 9.63±1.49 mm and 9.18±1.55 mm, respectively. The difference between the width and the length was larger on the right side than on the left side. The thickness of the process at the root portion was variable and its mean value was 5.32±1.07 mm.¹⁵⁾ The length and basal width of the anterior clinoid process in Koreans were smaller and thicker than those of the Americans.⁸⁾¹⁵⁾
   The surgical removal of the ACP, i.e., anterior clinoidectomy, exposes the clinoid(C5) segment of the ICA. Following the publication of the original clinical report by Dolec,⁴⁾ anterior clinoidectomy has become an essential approach for entering the anterior cavernous sinus, to expose aneurysm of the C4-C6 segments of the ICA^2)5)19)25) and to resect the dural attachment and hyperostotic bone of clinoid, medial sphenoid wing, and cavernous sinus meningiomas.²⁶⁾ Recently, anterior clinoidectomy has been advocated as a part of the transcavernous approach to produce wider exposure of basilar artery aneurysm of the upper clival region.³⁾¹⁶⁾
   Although the intradural technique has the advantage of enabling a direct view of important surrounding intradural structures, it is not always safe since it also carries the risk that the drilling burr will make direct contact with these structures, including the optic nerve, ICA, and intracranial aneurysms.¹³⁾²¹⁾ Extradural clinoidectomy has several advantages over intradural clinoidectomy. First, anatomical orientation is easily attained by identifying the dural extension into the superior orbital fissure and the optic canal, and, therefore, an extensive resection is possible. In contrast, when the intradural technique is used, both the extent of bone removal as well as the exposure gained tend to be limited. Second, the dura protects the intradural structures. Third, the procedure can be performed considerably faster than the intradural technique, because of the greater delicacy involved in performing the latter safely.²⁶⁾
   Several neurosurgeons have stressed the need to resect the ACP in certain patients with ICPCom aneurysms. Yasagil, who performed this procedure in 16 of his personal series of 173 ICPCom aneurysm operations, stated that the ACP might cover the proximal portion of the aneurysmal neck when the origin of the PComA was quite proximal or the ACP was long.²³⁾ Fox reported a case from his series: the ACP had to be resected because of its remarkable overgrowth.⁷⁾ Sano also mentioned his experience on removal on the ACP during surgery for an ICPcom aneurysm.²²⁾ In our series, removal of the ACP was necessary in 16 of the 40 patients with ICPCom aneurysm. The aneurysm was located immediately lateral to the abrupt curvature formed by the C2 and C1 segments of the ICA, where the origin of the posterior communicating artery and the proximal aneurysmal neck were hidden under the ACP due to an unusual oblique course of the C2 segment. Our results explained that this condition can be predicted before clipping by analyzing a cerebral angiogram because the oblique course of the ICA was well reflected in the size of angles A and B. If angle A is greater than 47 degrees and angle B is less than 95 degrees, the distance C is less than 4.5 mm, it needed to resect the ACP. In the Ochiai series, if angle A was greater than 59.3 degrees, angle B was less than 85.0 degrees, or distance C was less than 9.6 mm, it was necessary to resect the ACP. Further, if the posterior communicating artery was of the fetal type, resection of the ACP was necessary.²⁰⁾ According to Chrzanowski, the mean±SD of angle A in 100 normal subjects was 31.5±11.3 degrees.¹⁾ In the Kim series, resection of the ACP was necessary in 4(12%) of the 33 patients with ICPCom aneurysm. The ACP should be resected in clipping of the low-lying ICPCom aneurysm and in the posterior-inferior direction if the distance between the tip of the ACP and the most proximal portion of the aneurysmal neck is shorter than 5.6 mm or the height of the aneurysm in relation to the imaginary perpendicular line to the interclinoid line on the lateral view is less than -0.5 mm.¹⁰⁾ In the Nagasawa series, ACP or anterior petroclinoid fold was resected in 7 of the 83 patients with ICPCom aneurysm. If the distance of proximal side of the neck from ACP is less than 4 mm or its height from the baseline between the ACP and posterior clinoid process is less than 1 mm, it was necessary to resect the ACP or the anterior petroclinoid fold.¹⁸⁾
   In aneurysmal surgery, we should always be aware of the degree of difficulty involved in proximal control of the parent artery. This is particularly true in ICPCom aneurysm surgery because premature rupture of an aneurysm may have a disastrous outcome. The present study showed that the necessity of resection of the ACP in ICPCom aneurysm surgery can be predicted from the preoperative angiograms. One of the advantages of our angiographic criteria is their simplicity in clinical use. Since bony structures are not involved. the criteria are also applicable even to digital subtraction angiography.

Conclusion

   We have removed of the ACP in 16 of the 40 ICPCom aneurysms. The mean values of angle A, angle B, and distance C are 42±5 degrees, 110±15 degrees and 4.5±1 mm, respectively. We did not encounter any difficulty in clipping in any of the cases in which there was no premature rupture of the aneurysm. Most of cases had a good outcome.
   The authors suggest that it is necessary to analyze angiographic findings, clinical and operative findings of between the group of ICPCom aneurysms with resection the ACP and the group of ICPCom aneurysms without resection the ACP in successful clipping.

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