Microneurosurgical management of aneurysms at A4 and A5 segments and distal cortical branches of...

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Surgical Neurology 70 (2008) 232–246www.surgicalneurology-online.com

Aneurysm-Rainbow Team/Helsinki

Microneurosurgical management of aneurysms at the A2 segmentof anterior cerebral artery (proximal pericallosal artery)

and its frontobasal branchesMartin Lehecka, MDa, Reza Dashti, MDa, Juha Hernesniemi, MD, PhDa,⁎,

Mika Niemelä, MD, PhDa, Timo Koivisto, MD, PhDb, Antti Ronkainen, MD, PhDb,Jaakko Rinne, MD, PhDb, Juha Jääskeläinen, MD, PhDb

aDepartment of Neurosurgery, Helsinki University Central Hospital, 00260 Helsinki, FinlandbDepartment of Neurosurgery, Kuopio University Hospital, 70211 Kuopio, Finland

Received 6 December 2007; accepted 1 March 2008

Abstract Background: Aneurysms originating from the A2 segment of ACA and its frontobasal branches are

Abbreviations: 3Dthe A3 segment; ACACMA, Callosomarginangiography; FPA, FrIntracerebral pressureaneurysm; MCA, Midartery of Heubner; SA

⁎ CorrespondingE-mail address: ju

0090-3019/$ – see frodoi:10.1016/j.surneu.2

rare, forming less than 1% of all IAs. There are only few reports on management of A2As. In thisarticle, we review the practical anatomy, preoperative planning, and avoidance of complications inthe microsurgical dissection and clipping of A2As.Methods: This review, and the whole series on IAs, is mainly based on the personalmicroneurosurgical experience of the senior author (JH) in two Finnish centers (Helsinki andKuopio), which serve, without patient selection, the catchment area in Southern and Eastern Finland.Results: These two centers have treated more than 10000 patients with IAs since 1951. In the KuopioCerebral Aneurysm Database of 3005 patients and 4253 IAs, there were 35 patients carrying 35 A2As,forming 1% of all patients with IAs, 0.8% of all IAs, and 3% of all ACA aneurysms. Twenty-one (60%)patients presented with ruptured A2As with ICH in 11 (52%) and IVH in 7 (33%). Nineteen patients(54%) had multiple aneurysms.Conclusions: A2As are often small, even when ruptured, with relatively wide base, and they arefrequently associated with ICHs of IVHs. Our data suggest that A2As rupture at smaller size thanIAs in general. The challenge is to select appropriate approach, locate the aneurysm deep inside theinterhemispheric fissure, and to clip the neck adequately without obstructing branching arteries at thebase. Unruptured A2As also need microneurosurgical clipping even when they are small.© 2008 Elsevier Inc. All rights reserved.

Keywords: Aneurysm; Anterior cerebral artery; Clipping; Distal; Frontopolar artery; Microanatomy; Microsurgical technique; Pericallosal artery;Subarachnoid hemorrhage

, 3-dimensional; A1A, Aneurysm of the A1 segment; A2A, Aneurysm of the A2 segment and its frontobasal branches; A3A, Aneurysm of, Anterior cerebral artery; ACoA, Anterior communicating artery; ACoAA, ACoA aneurysm; AdistA, distal pericallosal artery aneurysm;al artery; CSF, Cerebrospinal fluid; CTA, Computed tomographic angiography; DACA, Distal ACA; DSA, Digital subtractionontopolar artery; IA, Intracranial aneurysm; ICA, Internal carotid artery; ICG, Indocyanine green; ICH, Intracerebral hematoma; ICP,; IVH, Intraventricular hemorrhage; LSO, Lateral supraorbital approach; M1A, Proximal MCA aneurysm; MbifA, MCA bifurcationdle cerebral artery; MdistA, Distal MCA aneurysm; MLA, Medial lenticulostriate arteries; OFA, Orbitofrontal artery; RAH, RecurrentH, Subarachnoid hemorrhage; STA, superficial temporal artery.author. Tel.: +358 50 427 0220; fax: +358 9 471 [email protected] (J. Hernesniemi).

nt matter © 2008 Elsevier Inc. All rights reserved.008.03.008

mailto:[email protected]

http://dx.doi.org/10.1016/j.surneu.2008.03.008

Fig. 1. Illustration demonstrating segments and branches of ACA andlocation of A2As.

233M. Lehecka et al. / Surgical Neurology 70 (2008) 232–246

1. Introduction

Aneurysms of the ACA can be classified into 5 differentgroups: A1As or proximal ACA aneurysms, ACoAAs,A2As or proximal pericallosal aneurysms, A3As or classicalpericallosal aneurysms, aneurysms of the A4 and A5segments and distal cortical branches, or AdistAs (Table 1,Fig. 1) [7]. The last 3 groups, also called DACA aneurysms,are further divided into 7 subgroups according to micro-neurosurgical criteria (Fig. 2).

1.1. Proximal pericallosal artery aneurysms (A2As)

The A2As are located either directly on the pericallosalartery, between ACoA and genu of corpus callosum (A2segment), or on one of its frontobasal branches. Proximalpericallosal artery aneurysms (A2As) are rare, forming lessthan 1% of all IAs, with few reports on their management[11,25,26,39,45,49,58,70,75,90,94,95]. They are small insize and often associated with ICH when ruptured[25,31,43,59,70,72,75]. They are difficult to reach as theylie deep in the interhemispheric fissure closely attached tothe surrounding brain tissue. Their involvement withperforating arteries arising both from ACoA and A2 segmentas well as frequent arterial anomalies of this region makestheir identifications and microneurosurgical treatment chal-lenging [5,54,87].

1.2. Purpose of review

This review, and the whole series on IAs, is intended forneurosurgeons who are subspecializing in neurovascularsurgery. The purpose is to review the practical anatomy,preoperative planning, and avoidance of complications in themicrosurgical dissection and clipping of A2As.

1.3. Authors

The microneurosurgical technique in this review ismainly based on the personal experience of the senior author(JH) in two Finnish centers (Helsinki and Kuopio), whichserve, without selection, the catchment area in Southern andEastern Finland. These two centers have treated more than10000 aneurysm patients since 1951.

The data presented in our series of articles represents3005 consecutive patients harboring 4253 IAs from theKuopio cerebral aneurysm database (1977–2005). The aimis to present a consecutive, nonselected, population-based

Table 1Five categories of ACA aneurysms (see Fig. 1)

Category Location

A1A A1 segment, between ICA bifurcation and ACoAACoAA ACoAA2A A2 segment and its frontobasal branches, between ACoA and

genu of corpus callosumA3A A3 segment, curving around genu of corpus callosumAdistA A4 and A5 segments or distal branches such as CMA

series of IAs. This database is not reflective of the personalseries of the senior author (JH) alone.

2. Occurrence of A2As

The reported incidence of A2As has been 0.2% to 1% ofall IAs or about 5% to 22% of all DACA aneurysms[11,25,26,39,45,49,58,70,75,90,94,95]. Tables 2-5 present theclinical data on the 35 A2A patients in the consecutive and

Fig. 2. Microsurgical division of DACA aneurysms with emphasis on A2As.

able 4tracerebral hematoma, IVH, and acute hydrocephalus associated withneurysm rupture on different ACA segments

A1As ACoAAs A2As A3As AdistAs

uptured aneurysms 12 715 21 97 10H only 3 (25%) 33 (5%) 6 (29%) 13 (13%) 2 (20%)H withIVH component

0 (0%) 74 (10%) 5 (24%) 14 (14%) 2 (20%)

H only 2 (17%) 137 (19%) 2 (10%) 12 (12%) 0 (0%)reoperativehydrocephalus

5 (42%) 317 (44%) 7 (33%) 32 (33%) 1 (10%)

Table 2Patients with ACA aneurysms in a consecutive and population based seriesof 3005 patients with 4253 IAs from 1977 to 2005 in the Kuopio CerebralAneurysm Data Base

No. of patients No. of aneurysms

Whole series 3005 4253Patients with primary SAH 2365 3325Patients without primary SAH 640 928ACA aneurysms 1145 1179A1As 23 (2%) 23 (2%)ACoAAs 898 (78%) 921 (78%)A2As 35 (3%) 35 (3%)A3As 163 (14%) 174 (15%)AdistAs 26 (2%) 26 (2%)Ruptured ACA aneurysms 855 855A1As 12 (1%) 12 (1%)ACoAAs 715 (84%) 715 (84%)A2As 21 (2%) 21 (2%)A3As 97 (11%) 97 (11%)AdistAs 10 (1%) 10 (1%)Fusiform ACA aneurysms 6 6Fusiform A1A 2 2Fusiform ACoAA 3 3Fusiform A2A 1 1Fusiform A3A 0 0Fusiform AdistA 0 0

234 M. Lehecka et al. / Surgical Neurology 70 (2008) 232–246

population-based series of 3005 patients with 4253 IAs from1977 to 2005 in the Kuopio Cerebral Aneurysm Database. Ofthe 3005 patients, 1145 (38%) had 1179 ACA aneurysms(Table 2). Therewere 35 patients with 35A2As, 0.8%of all the4253 IAs, 3% of all 1179 ACA aneurysms, and 15% of all the235 DACA aneurysms.Most of the A2Aswere at the origin ofFPA. There was no difference between sides with 18 (51%)A2As on the left and 17 (49%) A2As on the right. Only oneA2A, a ruptured one, was fusiform. Giant A2As are extremelyrare [13,14,16,22,25,42,48,51,56,57,71,75,79,82], only one inour series (Table 3).

Table 3Characteristics of 35 A2As

Ruptured Unruptured Total

No. of aneurysms 21 14 35Median aneurysm

size (mm)7 (range, 2–25) 3 (range, 2–6) 6 (range, 2–25)

Aneurysm sizeSmall (b7 mm) 14 (67%) 9 (64%) 23 (66%)Medium (7-14 mm) 5 (24%) 5 (36%) 10 (29%)Large (15-24 mm) 1 (5%) 0 (0%) 1 (3%)Giant (≥25 mm) 1 (5%) 0 (0%) 1 (3%)Aneurysm sideRight 10 (48%) 7 (50%) 17 (49%)Left 11 (52%) 7 (50%) 18 (51%)

ICH 11 (52%) – –Temporal 0 – –Frontal 10 – –Parietal 1 – –IVH 7 (33%) – –Preoperative

hydrocephalus7 (33%) – –

Data are given as number of aneurysms.

TIna

RICIC

IVP

2.1. Ruptured and unruptured A2As

In our series, 855 (73%) of the 1179 ACA aneurysmspresented with SAH, of which 21 (2%) were A2As (Table 2).Of our 35 A2As, 21 (60%) were ruptured and 14 (40%)unruptured (Table 3). Their size distribution is presented inTable 3. Of the 21 ruptured A2As, 14 (67%) were smallerthan 7 mm, suggesting that even small unruptured A2Aswould require occlusive therapy.

2.2. Intracerebral hematoma andintraventricular hematoma

Ruptured DACA aneurysms bleed frequently into theadjacent brain [23,43,72,75]. Of the 21 patients withruptured A2A, ICH was present in 11 (52%), and IVH, in7 (33%) patients (Table 3), the highest frequency among theACA aneurysms (Table 4). If A2A bleeds in the frontal lobe,there is often intraventricular extension (Table 4).

2.3. Associated aneurysms

Distal ACA aneurysms are often associated with otheraneurysms [11,25,49,59,92,95]. In our series, 16 (46%) ofthe 35 patients had at least 1 associated aneurysm (Table 5),most frequently on the MCA. None had multiple A2As.

3. Microsurgical anatomical considerations of A2As

3.1. Anterior cerebral artery

The microneurosurgical anatomy of ACA and its brancheshas been well described [2,5,20,27,33,44,54,55,83,92].The ACA, the smaller of the 2 terminal branches of the

able 5atients with an A2A and possible associated aneurysms

Ruptured A2A Unruptured A2A Total A2A

atients with A2A 21 14 35atients with asingle aneurysm

16 (76%) 3 (21%) 19 (54%)

atients withmultiple aneurysms

5 (24%) 11 (79%) 16 (46%)

Associated A2A 0 0 0Associated aneurysms atother sites

5 11 16

TP

PP

P

Data are given as numbers of patients.


ICA, arises at the medial end of the Sylvian fissure lateral tothe optic chiasm. It traverses the optic chiasm or the opticnerve and ascends in front of the lamina terminalis in thelamina terminalis cistern. Before entering the interhemi-spheric fissure, it is connected to the opposite ACA via theACoA (Fig. 1). In the interhemispheric fissure, the left andright ACA trunks run parallel along the corpus callosum, inthe pericallosal cistern. Both trunks give origin to severalcortical, subcortical, and callosal branches, some of whichmay cross over to the opposite side. Cortical branches of theACA supply the anterior two thirds of the medial aspects ofthe cerebral hemisphere as well as the superior portions of thesuperior frontal, precentral, and postcentral gyri [54].

The ACA is divided into 5 segments, A1 to A5, accordingto Fischer (Table 1, Fig. 1) [17]. The A1 segment is locatedbetween the ICA bifurcation and the ACoA. The A2 segmentextends from the ACoA to the region between the rostrumand the genu of the corpus callosum. The A3 segment curvesaround the genu of corpus callosum and ends at the rostralpart of the body of the corpus callosum. The A4 and A5segments follow the superior surface of the corpus callosumwith a virtual plane of division at the level of the coronarysuture. Traditionally, the ACA has been divided into aproximal part (A1) and a distal part (A2-A5), the latter alsocalled the pericallosal artery [20,27,33,54,84,94].

3.2. A1 segment of ACA

The A1 arises from the ICA in the carotid cistern, andwith a medial and somewhat anterior course, it enters thecistern of the lamina terminalis. At this point, the A1segment is encased by thick arachnoid bands that extendfrom the olfactory triangle to the lateral side of the opticnerve. When mobilizing the A1 segment, these bands requirecareful dissection, especially when toughened by previousSAHs. The course of the A1 segment varies highly in lengthand dominance, and it may loop under the frontal lobe[65,91]. Importantly, 1 A1 is dominant to the other in mostcases [91]. The A1s join the ACoA complex mostly abovethe chiasm (70%), less frequently above the optic nerves(30%) [64].

3.3. Anterior communicating artery complex

The ACoA is the fundamental anastomotic part of theanterior circle of Willis. Detailed microneurosurgical anatomyof the ACoA complex and its anatomical variations is reportedby Yaşargil [91] and others [2,15,20,29,44,55,64,67,77,81].The location and the orientation of the ACoA complex varieshighly with respect to the optic chiasm and the anterior skullbase, depending on the diameters, lengths, and courses ofthe A1s [55,64,91]. In 30 brains studied by Serizawa et al [67],2 ACoAs (6%) were high above the optic chiasm. The length,shape, and diameter of the ACoA also varies [55,64,91]. Theaverage diameters of ACoAs and A1s were 1.6 and 2.6 mm,respectively, in 50 adult brains, and the length of ACoA wasbetween 2 and 3 mm (0.3-7.0 mm) [55]. In cross sections,

ACoAs can be round, triangular, or even flat. The diameter ofthe A1-A2 junctions was equal in 74%, larger in the right sidein 14%, and larger in the left side in 12% [55].

3.4. Perforating branches of ACoA

It is difficult to overestimate the importance of theperforators of the ACoA complex region. The complexmicroneurosurgical anatomy of the perforating branchesarising from the ACoA complex has been described in detail[2,3,15,38,44,55,64,67,77,83,86,91]. Their supply areas,laterality, pattern of origin, and number vary highly[15,44,55,67,83,86,91]. Most of them originate, such as theACoAAs, from the side of the dominant A1 in case ofunequal A1s and from the medial part of ACoA with equalA1s [91]. Importantly, the perforators usually arise from thesuperior or posterior surface of the ACoA, rarely from itsanterior or inferior surface [55]. Perforators may arise frommultiple, fenestrated, or even hypoplastic and anomalousbranches of the ACoAs [3,44,67,86,91]. The ACoAperforators supply infundibulum of the pituitary, optic chiasmand superior part of optic nerve, anterior hypothalamus andlamina terminalis, anterior perforating substance, rostrumand genu of corpus callosum, anterior commisure, anteriorcingulate gyrus, parolfactory gyrus, paraterminal gyrus,septum pellucidum, and some parts of the limbic systemincluding column of fornix [2,3,15,44,55,64,67,83,86,91].Anastomosis between some of these perforators, in particular,hypothalamic branches, are frequent [44,67].

Aneurysms of the very proximal A2 trunk may involveperforating branches of the ACoA. They may be endangeredduring dissection, coagulation, application of standard ortunnel clips, or trapping procedures, especially with wide-based A2As and associated arterial anomalies [44]. Impair-ment of these perforators during surgery may cause a widerange of neurological symptoms including memory deficits,changes of personality, and electrolyte imbalances [67,91].

3.5. A2 segment and interhemispheric fissure

The A2 segment originates at the junction of the A1and the ACoA (Fig. 1). It ascends in the lamina terminaliscistern, in front of the lamina terminalis, and enters theinterhemispheric fissure and the callosal cistern with acourse toward the genu of the corpus callosum. The A2terminates at the junction of the rostrum and the genu ofthe corpus callosum where the A3 segment starts [54,91].Inside the interhemispheric fissure, A2s were side by sidein 18%, the left A2 anteriorly in 48%, and the right A2anteriorly in 34% [55]. The free margin of the anterior falxis well above the corpus callosum. The A2 segment isentirely below the free margin, which allows free shift andcrossover of its branches across the midline [54]. Thismeans that both A2 segments can be reached from aunilateral approach [25,92]. The depth of falx varies. Thecingulate gyri may be so adherent to each other that duringthe surgical exposure, they may be mistaken for the corpus

Fig. 3. A: Azygos ACA with A2 trunk aneurysm as seen on CTA. B:Intraoperative picture of the same patient during anterior intehemisphericapproach showing the azygos ACA (PerA) with the bifurcation (arrow), thefalx (F) and the corpus callosum (CC) (see also video A2A-3).


callosum [92]. The interhemispheric fissure is so narrowthat only limited amount of CSF can be removed.

3.6. Arterial branches of A2 segment

The A2 segment gives rise to three major branches: theRAH, the OFA, and the FPA (Fig. 1). These arteries can bedistinguished by their final destination but not by theirdiameters. High variation in the origin and size of thesebranches makes it impossible to define a standard vascularpattern for the A2s [33,54,84].

3.6.1. Recurrent artery of HeubnerThe origin of the RAH and its course in relation to the

A1 trunk is highly variable. In most cases, it originates fromthe first few mm of the A2 segment or the A1-A2 junctionand in only about 10% from the distal A1 segment[4,15,19,29,55,81,91,93]. Rare origin from the orbitofrontalbranch is known [19]. The RAH supplies the anteriorinferior striatum, anterior limb of the internal capsule,olfactory region, anterior hypothalamus (overlapping withMLAs from A1), frontobasal cortex, and subcortical whitematter [15,19,52,55,77,91]. In 60 hemispheres, RAH wasmissing in 3% and duplicated in 12% [19]. The diametervaries highly (0.2-2.9 mm), and in rare cases, the RAH is asthick as the A1 [55]. Independent of its point of origin, theRAH directs immediately backward towards the A1segment, and then it follows the superior wall, the anteriorwall, or the posterior wall of the A1 trunk. In case of thesuperior course (61%), the first few millimeters of the RAHare often attached to the A1 trunk by arachnoid bands, andtheir differentiation is of great importance before temporaryclipping of A1 [19]. The distal part of the RAH travelsfreely in the subarachnoid space before penetrating into thebrain at the base of the lateral and medial olfactory stria,with a highly variable branching pattern (1-12 branches)[15,19,55,91]. These perforators are often of the samecaliber as the lenticulostriate arteries [2]. Consequently, it isvery important to preserve the RAH to avoid ischemicinjury to the anterior perforating substance.

3.6.2. Orbitofrontal arteryThe OFA arises more distally on the A2 than the RAH

(Fig. 1). It generally courses downward and forward towardthe floor of the anterior cranial fossa to reach the level of theplanum sphenoidale. It supplies the gyrus rectus, olfactorybulb and tract, and the medial part of the orbital surface of thefrontal lobe [33,54,84].

3.6.3. Frontopolar arteryThe FPA arises after the OFA and courses anteriorly

toward the medial subfrontal surface and the frontal pole. Itcrosses the subfrontal sulcus and supplies portions of themedial and lateral surfaces of the frontal pole [64]. It isdirected more anteriorly than laterally, which helps todistinguish it from the RAH. Both the OFA and the FPAcan be identified in over 95% of hemispheres [84]. They donot always arise from the A2 trunk but may arise also fromthe A1 trunk or even the CMA [27,33,64,84].

3.6.4. Basal perforating branchesA2 gives also rise to small basal perforating branches

which overlap with the ACoA perforators. These A2perforators are usually 4 in number, and they supply theanterior hypothalamus, septum pellucidum, medial portionof the anterior commissure, pillars of the fornix, andanteroinferior part of the striatum [54,55]. They passposteriorly to enter the optic chiasm, lamina terminalis,and anterior forebrain below the corpus callosum [54].

Fig. 4. A2A (black arrow) at the origin of FPA (white arrow), preoperative(A) and postoperative (B) image.


Medial portions of the rostrum and the genu of the corpuscallosum are mainly supplied by the subcallosal artery,which arises from the ACoA [83].

3.7. Anatomical anomalies of A2 segment

The important anomalies involving the A2 segment are theazygos ACA, bihemispheric ACA, triplication of ACA, andcrossover branches of the ACA [5,20,38,41,54,91]. Theazygos ACA is a single trunk distal to the A1 segments, so itis the supplier of the both hemispheres (Fig. 3). In thebihemispheric ACA, one A2 is hypoplastic and the larger A2gives origin to most cortical branches. In case of thetriplication of ACA, the middle A2, also called thehemispheric type of the medial callosal artery [83], isprominent and supplies the corpus callosum. Crossover

Fig. 5. CTA images of small, unruptured A2 trunk aneurysm (arrow) as se

branches, found in 26% to 64% of hemispheres, are sent fromDACA to the contralateral hemisphere where they supply asmall medial area [54,73]. Severing of crossover branchesmay cause injury in the contralateral side to the approach.

3.8. Venous structures

The veins draining the lateral convexity of the frontallobe are either ascending to the superior sagittal sinus ordescending to the superior sylvian vein. The ascending veinsare frontopolar, anterior frontal, middle frontal, posteriorfrontal, precentral, and central veins [50,63]. The ascendingveins of the medial frontal surface join the ascendingconvexity veins along the superior rim of the frontal lobe toform subdural bridging veins that empty to the superiorsagittal sinus [1,33,50,53,63,69]. These bridging veins pose

en on sagittal view (A), axial view (B), and 3D reconstruction (C).

Fig. 6. Ruptured A2Awith expansive frontal ICH with IVH component (A). On CTA (B) and DSA (C), we see fusiform aneurysm (arrow) located at the distalpart of the A2 segment.


an obstacle for the surgical approaches into the interhemi-spheric fissure. The venous pattern varies highly. Allbridging veins should be left intact if possible, and sufficientworking space should be searched in between them. Thelarger the damaged or sacrificed vein, the higher the risk ofvenous infarction. Aplastic superior sylvian vein requiresspecial attention because of high risk of venous infarctiondue to reduced venous collateral flow [53].

3.9. Relation to corpus callosum

The pericallosal artery (A2-A5 segments) travels alongthe corpus callosum for most its course (Fig. 2). It makes awide arch around the genu of corpus callosum (A3 segment)and then goes posteriorly along the superior aspect of thecorpus callosum. Türe at al [83] reported irregular course inalmost 40% so that at least one of the A3 to A5 segmentscoursed in the cingulate sulcus instead of the callosal sulcus.The genu of the corpus callosum is semicircular or oval insagittal section, with a mean anteroposterior diameter of11.2 mm [33]. The interhemispheric approach should beselected so that the genu does not obstruct the view to theA2 segment.

3.10. Location and orientation of A2As

The A2As may originate from the A2 trunk itself or fromits frontobasal branches (OFA and FPA) (Fig. 2). The mostfrequent location is on the A2 trunk at the origin of the FPA(Fig. 4). The direction of the dome depends on the point oforigin of the aneurysm. Most A2 trunk aneurysms aredirected forward; a few, upward. A2As on the OFA or FPAusually point forward, sometimes upward (Figs. 3 and 5).A2As are usually small with a relatively broad base andfragile wall, and arterial branches are often originating from

their base. A2As are often attached to the pial surface ofeither frontal lobe.

4. Imaging of A2As

Digital subtraction angiography is still the present goldstandard in many centers. Multislice helical CTA is theprimary modality in our center as noninvasive, safe, andquick imaging method with a comparable sensitivity andspecificity as DSA in aneurysms larger than 2 mm[21,28,30,68,78,85,88,89]. In addition, it allows the dis-closure of calcifications in the arterial walls and quickreconstruction of 3D images [32].

For intraoperative navigation toward A2As, 3D CTA orDSA reconstructions should be evaluated for the bilateralangioarchitecture of the A1-A2 region and its variations andanomalies; the number of pericallosal arteries, the correctparent artery (right or left) and its diameter, distance of theaneurysm from the A1–A2 junction, orientation of the domeand branching arteries at the neck, location of the A2A withrespect to the genu of the corpus callosum, and distance ofthe A2 segment and the A2A from both the cortical surfaceand the anterior fossa (Figs. 3-5, 6 and 8). In the workstation,3D CTA images can be rotated accordingly to show thesurgeon's view and to design a suitable bony exposure. ICHmay dislocate the interhemispheric fissure.

5. Microsurgical strategy with A2As

A2As are rare, and it is difficult to gain experience intheir microneurosurgical treatment [6,11,25,26,39,43,45,47,49,59,70,75,90,92,95]. Nevertheless, clipping is shown tobe an efficient and long-lasting treatment for DACA

Table 6Videos on microsurgical management of IAs in this and previouspublications of the series

Aneurysm and publication Video

M1As [10] 1-Craniotomy and exposure (LSO)2-Clipping of temporally oriented M1A3-Clipping of frontally oriented M1A4-Clipping of giant M1A

MbifAs [9] 1-Clipping of intertruncal-type MbifA2-Clipping of inferior-type MbifA3-Clipping of lateral-type MbifA4-Clipping of insular-type MbifA5-Contralateral approach to bilateral MbifAs

MdistAs [8] 1-Clipping of MdistA2-Clipping of MdistA

A1As [7] 1-Craniotomy and clipping of A1A2-Craniotomy and clipping of fusiform A1A3-Contralateral approach to A1A

ACoAAs,previous study

1-Clipping of downward ACoAA2-Clipping of forward ACoAA 13-Clipping of forward ACoAA 24-Clipping of intertruncal ACoAA5-Clipping of backward ACoAA6-Clipping of large complex ACoAA7-Clipping of double ACoAA8-Interhemispheric approach for high ACoAA9-Clipping of fusiform ACoAA

A2As,present study

1-Craniotomy for interhemispheric approach2-Clipping of proximal A2 trunk A2A3-Clipping of distal A2 trunk A2A4-Clipping of distal FPA A2A


aneurysms [40]. A2As pose certain unique problems andoften require a different surgical trajectory than other anteriorcirculation aneurysms. A2As are difficult to reach. Theoptimal trajectory requires detailed knowledge of themicrosurgical anatomy of the A1-A2 region and carefulstudy of images. A2As are usually situated high above theoptic chiasm, so the LSO would require significant elevationof the frontal lobe and resection of the gyrus rectus. Theanterior interhemispheric approach requires tedious dissec-tion deep in the narrow space, with poor control of theparent A2.

5.1. Neuroanesthesiological principles

A general review of our neuroanesthesiological principleshas been published previously [62].

5.2. Intracerebral hematoma

Ruptured A2As are frequently associated with ICH, 52%in the Kuopio series (Table 3), and the narrow CSF space andadhesions may add to this. The ICH is usually frontal and canbe located on either side (Fig. 6A). We remove massive ICHswhen feasible. Frontal ICHs are likely to cause late cognitivedeficits [74].

5.3. Acute hydrocephalus

In case of acute hydrocephalus, 33% in the Kuopio series(Table 3), ventricular drainage can be applied to reduce ICPand to lower the risk of brain damage, in most cases, afteracute securing the ruptured aneurysm. Before or afterclipping, additional CSF can be released, depending on theapproach, through fenestration of the lamina terminalis orsmall callosotomy into the lateral ventricle.

5.4. Approach and craniotomy

A2As on the proximal A2 segment can be reached eitherthrough the LSO or the anterior interhemispheric approach[23]. The approach is chosen according to the size andorientation of the A2A, its vertical distance from the anteriorskull base, possible vascular anomalies, previous surgeries(arachnoid adhesions), rupture status of the A2A, projectionof the dome, and surgeon's preferences. In acute SAH, weprefer the LSO because of (a) the possibility of fenestratingthe lamina terminalis, (b) proximal control of the ACoAcomplex, and (c) easier orientation. The interhemisphericapproach with no usual anatomical landmarks becomes evenmore difficult in acute SAH with swollen brain and bloodclots obstructing the view. A catheter inserted through thefenestration in the lamina terminalis into the third ventriclecan be left for ICP monitoring and further CSF removalpostoperatively. The main difficulty is to localize the A2A,regardless of the approach. Intraoperative DSA may improvenavigation during the interhemispheric approach, but in theLSO, the oblique head position is a limiting factor.Neuronavigator may help early navigation to A2As beforeCSF removal and frontal lobe mobilization.

5.4.1. Lateral supraorbital approachThe LSO is a more subfrontal and less invasive

modification of the pterional approach for the anteriorcirculation aneurysms [23]. It provides a good view towardsthe ACoA complex and, with a slightly extended gyrus rectusresection in the midline, also to the proximal part of the A2segment. Lateral supraorbital approach can also be used toreach aneurysms of the frontobasal branches of the A2, butthe vertical distance of 1.5 cm from the anterior skull base isthe limit. The distance from the A1-A2 junction to the base ofthe aneurysm is measured from images and used intraopera-tively to determine extent of the gyrus rectus resection.

The LSO craniotomy, described in detail elsewhere [23], isdemonstrated on video in our article on M1As of the MCA inSurgical Neurology (Table 6) [10]. Briefly, the head fixed tothe head frame is (a) elevated clearly above the cardiac level,(b) rotated 20° to 30° toward the opposite side, (c) tiltedlaterally for optimal visualization of the ACoA complex andthe proximal parts of the A2s, and (d) neck is kept in neutralposition (no extension). It is our practice to adjust the positionof the fixed head and body during the operation as needed[24]. Following an oblique frontotemporal skin incision, a 1-layer skin-muscle flap is retracted frontally with springhooks, and the superior orbital rim and the anterior zygomaticarch are exposed. The upper part of the temporal muscle it issplit and retracted towards the zygomatic arch. The extent ofcraniotomy depends on the surgeon's experience and


preferences. It may be tailored according to the localizationand projection of the A2A, its relation to the anterior skullbase, and the presence of ICH. Usually, a small LSOcraniotomy is all that is necessary (the keyhole principle). Asingle burr hole is placed just under the temporal line in thebone, that is, the superior insertion of the temporal muscle. Abone flap of 3 × 3 cm is detached mostly by side-cutting drill,and the basal part can be drilled before lifting. Dura is incisedcurvilinearly with the base sphenoidally. Dural edges areelevated by multiple stitches, extended over craniotomydressings. From this point on, all surgery is performed underthe operating microscope, including the skin closure.

5.4.2. Anterior interhemispheric approachThe anterior interhemispheric approach is used for more

distally located A2As. The gap between the cingulate gyri isnarrow and deep, with possible strong adhesions. Blood clotsand brain swelling in acute SAH make this approach moredemanding. The exposure of the pericallosal artery and theaneurysm depends on the course of the vessel, its location inrelation to the corpus callosum, projection of the dome, andassociated ICH. For a right-handed neurosurgeon, the right-sided approach is more convenient as both pericallosalarteries can be reached under the inferior margin of falx fromthe same side. Left-sided ICH or left-sided associatedanterior circulation aneurysms may require left-sidedapproach. Some authors have used the bifrontal basalanterior interhemispheric approach [6,76]. We feel that theunilateral approach is less invasive and faster and providesequal exposure to the aneurysms located deep in theinterhemispheric fissure.

For the anterior interhemispheric approach, the patient isin supine position with the head fixed in a head frame andelevated about 20° above the heart level. The head should bein neutral position with the nose pointing exactly upwards.Tilting the head to either side increases the chance that thebone flap is placed too laterally from the midline. This wouldmake the entrance into the interhemispheric fissure andnavigation there more difficult. The head is slightly extendedaccording to how proximal or distal the A2A lies. Inthe optimal position the trajectory is almost vertical. Ifintraoperative DSA is considered, the frame pins should beplaced accordingly.

After minimal shaving, an oblique skin incision with itsbase frontally is made just behind the hairline, over themidline, extending more to the side of the planned bone flap(video A2A-1). Location and extent of skin incision dependson the hair line, dimensions of the frontal sinuses, and theorientation of the A2A. A 1-layer skin flap is reflectedfrontally with spring hooks. Bicoronal skin incision isunnecessary because strong retraction with hooks oftenallows anterior enough exposure of the frontal bone.Paramedian frontal craniotomy is performed as anterior aspossible without opening the frontal sinuses. If this happens,the sinuses should be packed and isolated with fat or musclegrafts and covered with pericranium. The bone flap is placed

slightly over the midline to allow better retraction of the falxmedially. The size of the bone flap depends both on thesurgeon's experience and on the presence of ICH. Weusually use a 3- to 4-cm diameter flap. Too small a flap maynot provide sufficient room for working between thebridging veins. In most patients, only 1 burr hole in themidline over the superior sagittal sinus at the posterior borderof the bone flap is needed. Through this hole, bone can bedetached from the underlying dura. One has to be carefulwith the underlying sagittal sinus, particularly in the elderlywith very adherent dura. The bone flap is removed using aside cutting drill. High-speed drill can be used to smooth theedges or to enlarge the opening if necessary.

The dura is opened under a surgical microscope as aC-shaped flap with its base at the midline. The incision isfirst made in the lateral region and then extended towards themidline in the anterior and the posterior direction to preventopening of the superior sagittal sinus. Bridging veins may beattached to the dura for several centimeters along themidline. Careful dissection and mobilization of these veins isnecessary. It is usually during the opening of the dura thatunwelcome damage to the bridging veins takes place. Duraledges are elevated with multiple stitches extended over thecraniotomy dressings.

5.5. Removal of ICH

In case of large ICH and lack of space (Fig. 6A), a smallcortical incision is made accordingly, and hematoma ispartially evacuated to gain space. This may risk the reruptureof A2A, which would be difficult to control through the ICHcavity. In removing the ICH clot, before or after clipping,only minor force should be applied not to sever perforatingarteries. Intracerebral hematoma in the immediate vicinity ofthe aneurysm should be left in place until the proximal andthe distal control have been obtained.

5.6. CSF drainage

In case of LSO, we release CSF from the supracellarcistern. In acute SAH, the dissection is then continued fromthe supracellar cistern subfrontally towards the optic chiasmand the lamina terminalis, which is opened for additionalremoval of CSF.

The interhemispheric approach provides less space as thecallosal cistern is shallow and not much CSF can beremoved. In unruptured A2As, this space will usually do. Inacute SAH, CSF can be released by a puncture to the lateralventricle at the lateral border of craniotomy. The alternativeis puncturing the corpus callosum by closed bipolar forcepsmedial to the pericallosal artery followed by opening of theforceps to create a small channel for CSF release. Partialremoval of ICH may also provide enough room.

5.7. Dissection toward A2A

We use a handheld syringe to expose and to expandplanes for further dissection, i.e., the water dissectiontechnique of Toth [46]. Arachnoid membranes and strands


are cut sharply by microscissors, which can be also used as adissector when closed. Use of retractors is kept at minimum,and they are not routinely used at the beginning of theapproach. Instead, bipolar forceps in the right hand andsuction in the left, with cottonoids of different sizes asexpanders, are used as microretractors [24]. When theinterhemispheric fissure is widely opened and the frontallobe mobilized, the retractor may be used to retain somespace for clipping, but otherwise avoided.

5.7.1. LSO approachFor the LSO approach, it is important to evaluate the

distance between the ACoA complex and the optic chiasm,the courses and lengths of the A1 segments, the number andthe orientation of the A2 segments, the distance of the A2Afrom the A1-A2 junction, and branches in the vicinity of theaneurysm. Computed tomographic angiography should becarefully reviewed for possible calcifications in the arteriesand the aneurysm wall.

Dissection is directed along the orbital roof towards theipsilateral optic nerve, which is identified (video A2A-2).Arachnoidal adhesions are cleared, and the base of thefrontal lobe is mobilized. Cerebrospinal fluid is releasedfrom the supracellar cistern, and arachnoid membranesaround the optic nerve are cut. The ICA is identified on thelateral side of the optic nerve and followed towards thebifurcation to identify the beginning of the A1 segment.Dissection is then oriented toward the chiasm, and the medialpart of the frontal lobe is mobilized. It is often unnecessary toopen the Sylvian fissure for better visualization of the ACoA.In patients with the ACoA high above the optic chiasm,opening of the proximal Sylvian fissure gives a better viewon the ACoA complex and the proximal A2 segments. If theunilateral A1 segment is rather straight, it can be followeddirectly to the ACoA complex. If A1 is very tortuous, it maybe easier to identify the interhemispheric fissure first and toperform a small subpial resection of the gyrus rectus. Theextent of gyrus rectus resection depends on the distancebetween the ACoA and the optic chiasm, which can bemeasured from the images. The RAH and the perforatorsentering the optic chiasm should not be damaged. The goal isto identify the both A1s, the ACoA, and the origins of theboth A2s. Once the proximal control of the A1s is obtained,dissection is continued distally along the appropriate A2,toward the aneurysm.

5.7.2. Interhemispheric approachFor the interhemispheric approach, the images should be

evaluated for several microsurgical aspects: depth of thefalx; depth of the corpus callosum; depth and course of thepericallosal and the callosomarginal arteries; size, orienta-tion, and origin of the dome; and possible dislocations dueto ICH.

Upon entering the interhemispheric fissure, bridging veinsmay obstruct the view, preventing even the slightestretraction of the frontal lobe. The veins are likely to restrictthe working space and one may have to work between them.

It may help to dissect some of them for a few centimeters fromthe brain surface which is very difficult. One may have tosacrifice a smaller vein, at the risk of venous infarctionthough. Extensive and long-lasting use of retractors,preventing venous flow, may have the same result as severingof bridging veins.

Inside the interhemispheric fissure, after clearing thearachnoidal adhesions, dissection is directed along the falxtowards the anterior margin of the genu of the corpuscallosum (video A2A-3). At the inferior border of the falx,the dissection plane is identified between the cingulate gyriattached to each other (Fig. 7A). The pericallosal artery maybe found already in the cingulate sulcus, but in most cases,the dissection must be continued deeper toward the corpuscallosum, identified by its white color and transverse fibers.Taking the attached cingulate gyri as the corpus callosum orother paired arteries as the pericallosal arteries leads toserious problems of navigation.

Once inside the callosal cistern, both pericallosal arteriesare visualized, realizing that they can be on either side of themidline. The artery that leads to the A2A is identified andfollowed to the proximal direction towards the aneurysm. Atedious and careful dissection is performed in the deep andnarrow proximal interhemispheric fissure (Fig. 7B). The aimis to identify the distal part of the parent A2 trunk.Landmarks of help are the origin of the CMA (Fig. 7A),the genu of the corpus callosum, and possible vascularanomalies recognized in the images. After the distal A2 hasbeen identified, the trajectory is directed slightly anteriorly,towards the ACoA complex.

Since the A2A is now approached along the distal parentA2, the dome with its possible rupture site is likely to get inthe way, obstructing the view to the proximal parent A2, thesite for temporary clipping. Premature rupture at this phase,with little room and no proximal control, may cause greatdifficulties. The dome often extends more to one side and isembedded in the pial layer of the cingulate gyrus. This mayallow traversing along the opposite gyrus to get proximalcontrol of the parent A2, often the most difficult part of theinterhemispheric approach. The direction of the A2A maychange due to retraction, and bloodmay obscure the anatomy,making identification of the aneurysm difficult. Strongretraction is likely to cause intraoperative rupture. We donot use partial callosal resection to enlarge exposure in theinfracallosal region [12]. With appropriate head positioningand well-placed bone flap, it is possible to obtain adequatevisualization of the A2As without any callosal resection [35].

6. Dissection and clipping of A2A

6.1. General principles

Small size, thin wall, and a relatively broad baseinvolving branches make the dissection of A2As tedious inthe narrow working space. The proximal and the distalparts of parent arteries as well as all the adjacent branches

Fig. 7. Intraoperative picture demonstrating the narrow interhemispheric space at the A3 segment (A) and A2 segment (B) of the ACA. Pericallosal artery (PerA)is running between the strongly attached cingulate gyri (Cleft and Cright) below the lower margin of the falx (F) with dissection plane marked by arrows.Callosomarginal artery originates at the A3 segment.


should be unhurriedly and painstakingly visualized beforethe final clipping. A small subpial resection is oftennecessary to allow the mobilization and visualization of thewhole A2A dome.

6.2. Dissection under temporary clipping of arteries

Temporary clipping facilitates sharp dissection of theA2A and the adjacent arteries. Dissection and preparation ofthe sites for the temporary clip(s) should be performed withblunt tipped bipolar forceps or with microdissector. Onetemporary clip, usually a small one, curved or straight, isapplied proximal to the aneurysm. Extreme care should betaken not to involve the RAH. Distal A2As are usually wellabove the origin of the RAH. When the main part of the baseis dissected, a short, straight pilot clip is applied and thetemporary clip is removed. When removed, the temporaryclip should be first opened carefully in place to test if anyunwanted bleeding occurs. Removal in rush can be followedby heavy bleeding and great difficulties in placing the clipback. Furthermore, while removing the temporary clip, eventhe slightest resistance should be noted as a possibleinvolvement of a small branch in the clip or its applier. Ifaccess to the proximal A2 trunk cannot be achieved, directpilot clipping is the only choice, usually a small microclipapplied to the aneurysm base. Longer clips may involve orkink side branch(es).

6.3. Clipping of the A2A base

A proper selection of clips with different shapes andlengths of blades, and applicators, suiting the imaginganatomy of the A2A, should be ready for use. A limitedselection of final clips is needed when temporary clipping ofthe arteries and bipolar shaping of the aneurysm dome isused. A2As are generally small with many surrounding smallbranches. To prevent kinking or occlusion of adjacentbranches, the smallest but adequate final clip should beselected. If bipolar reshaping is not considered, the blade of asingle occluding clip should be one and a half times thewidth of the base. We prefer inserting first a pilot clip to theA2A dome, preferring Sugita clips for their wide openingdistance and blunt tips. The pilot clip is later changed for asmaller and lighter final clip, after reshaping of the dome bybipolar coagulation. In the interhemispheric approach, onehas to insert the pilot clip at a rather early phase of domedissection, if the proximal parent artery cannot be visualized,or a premature rupture occurs. Adequate dissection, propersizes of clips, and painstaking checking that the clip bladesare well placed up to their tips are required to preserve theadjacent branches. If the first clip slides, exposing some ofthe neck, another clip may be applied proximal to the firstone for final closure (“double clipping”). Removal of theretractors and cottonoids may lead to kinking of the parent

Fig. 8. Unruptured A2A (arrow) at the distal FPA as seen on preoperative (A) and postoperative (B) CTA (see also video A2A-4).


artery or compression of the perforators by the clip blades orthe clip itself. The flow has to be checked once more, andpapaverine is applied.

6.4. A2A rupture before clipping

A2As may rupture while entering the interhemisphericfissure or dissecting the aneurysm base. The risk is highbecause most A2As are oriented so that their dome isencountered before visualization of the proximal artery.Control should be first attempted via suction and compres-sing the bleeding site with cottonoids. We always keep asecond suction prepared and functional. Sudden and shorthypotension by cardiac arrest, induced by intravenousadenosine, can be used to facilitate quick dissection andapplication of a pilot clip in case of uncontrolled bleeding[62]. A pilot clip may be inserted to a ruptured secondarypouch if visible. Otherwise, temporary clips are insertedproximally on the parent A2 or the both A1s, possibly alsoon A2 branches, to allow dissection of the base and finalclipping. A small- and thin-walled A2A may rupture at itsneck during dissection. In such case, under temporaryclipping, reconstruction of the base involving a part of theparent A2 in the clip should be attempted.

6.5. Intraoperative verification of clipping

We routinely use micro-Doppler to check the patency ofthe proximal and distal arteries and branches, but unexpectedocclusions are sometimes seen in postoperative angiography[37]. We routinely use intraoperative non-invasive ICGinfrared angiography [60,61]. It helps the orientation duringdissection, and it visualizes wall thickness and plaques,perforating arteries, and incomplete neck occlusion. Indo-cyanine green angiography reduces the need for invasiveintraoperative angiography for clipping control, but DSA isstill required in giant and complex aneurysms.

6.6. Resection of A2A dome

When appropriate, not risking the branching arteries orother structures, we resect the aneurysm dome for final checkof closure and for research purposes [18,80]. This policyteaches one to dissect domes more completely and avoidclosure of the perforators.

7. Frontobasal aneurysms

A2As located on the frontobasal branches of the A2segment are very rare (Fig. 8). They are frequently saccularand small. We approach them through the LSO or theinterhemispheric approach, depending on their distancefrom the anterior fossa. When close to the skull base, weuse the LSO with the benefit of familiar anatomicallandmarks. It is very important to measure the distancefrom the A1-A2 junction to the aneurysm in the images anduse it intraoperatively. A small frontobasal resection isusually needed to find the aneurysm. In the interhemi-spheric approach, the angle of the approach and the distanceof the aneurysms from the cranial vault should be evaluatedin the imaging (video A2A-4). A small, straight final clip isusually needed to exclude these aneurysms from thecirculation. Neuronavigator may help finding these aneur-ysms, but brain shift after CSF release and frontal loberetraction may be confounding.

8. Associated aneurysms

A2As are often associated with other aneurysms[11,25,49,59,92,95]. In our series, 46% of our 35 A2Apatients and 24% of those with ruptured A2A had at least oneadditional aneurysm (Table 4). Our strategy is to clip allaneurysms that can be exposed through the same approach.This may not be advisable if clipping of the ruptured


aneurysm is difficult or the brain is swollen due to acuteSAH. This technique of clipping multiple aneurysms atdifferent locations is not recommended at early learningcurve. In acute SAH, if there are problems in clipping theruptured aneurysm, one should not continue with theunruptured one(s). We do not recommend multiple cranio-tomies for ruptured cases.

9. Giant A2As

Giant A2 to A5 (pericallosal) aneurysms of are extremelyrare—less than 20 published cases [13,14,16,22,25,42,48,51,56,57,71,75,79,82]. There was only one giant A2A in ourseries of 4253 aneurysms (Table 3). Giant A2As may mimicsymptoms of frontal tumors. Magnetic resonance imagingand DSA are essential for correct diagnosis. Clipping isconsidered if A2 and its branches are not heavily involved inthe base. Removal of intraluminal thrombus under temporaryclipping may be necessary to decompress the aneurysm basefor the application of the final clip(s). If direct clipping doesnot seem possible, bypass and subsequent proximal occlusionof the parent A2 may be considered [34,36,54].

10. Fusiform A2As

Fusiform A2As are extremely rare—only 1 patient in ourseries (Fig. 6). Wrapping, proximal occlusion, excision,trapping, parent artery occlusion with preoperative bypass,and reconstruction can be considered [66].

11. Bypass operations and arteriotomies

Preoperative bypass, side-to-side A3-A3 bypass orarterial translation (eg, STA-ACA), may be considered ingiant or fusiform A2As [34,36,54]. Acute SAH makesbypasses extremely demanding. A comprehensive neuro-vascular team should be prepared to perform intraoperativearteriotomies, for example, to remove coils or thrombi, andintraoperative bypasses, also in case of emergency.

Acknowledgments

The authors thank Mr Ville Kärpijoki for excellenttechnical assistance.

Appendix A. Supplementary video

Supplementary data associated with this article canbe found, in the online version, at doi:10.1016/j.surneu.2008.03.008.

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Commentary

This is another article in the series of Aneurysm Clippingfrom Hernesniemi et al. The article and the videos areexcellent and contain many good pointers for aneurysmsurgeons. These A2 aneurysms are rare. It is important tonote the authors' two approaches to A2 aneurysms. Oneapproach, the lateral supraorbital approach, has a limit in theheight of the A2 that can be reached from that approach.Aneurysms located further distally on the A2 must bereached by the interhemispheric approach. In addition, alarger paramedian craniotomy higher in the frontal bone isanother way to reach these aneurysms. The frontal interhemi-spheric approach was popularized by Suzuki from Japan. It isa very nice way to see the extent of the A1 and A2 vesselshead on. As the authors describe, splitting the hemispheres ismore difficult because of the interhemispheric adhesions.Suzuki's approach is not widely used now, but it is anexcellent way to get to the anterior cerebral arteries. Frommy point of view, I always want to obtain proximal controlwhether by the interhemispheric approach or by lateralapproach. This principle prevents surprise aneurysm rupture.I also isolate the A2s distal to the aneurysm so that the wholeclipping procedure is more controlled. These are smallcomments to an article on the technique of aneurysm surgeryfor rare lesions of the A2 segment that is well done and verypractical. Read this excellent article and look at the videos.

James Ausman, MD, PhD

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