Introduction

The pterional or frontosphenotemporal craniotomy remains one of the most used and versatile approaches in vascular (and oncological) neurosurgery, being originally described over 100 years ago [85].

After decades of attempting subfrontal approaches to the sellar region, in 1914, Heuer developed the first frontotemporal craniotomy [61]. However, it was not until the 1950s when the term ‘pterional craniotomy’ was first coined by Hamby. Hamby already emphasised the importance of tailoring the craniotomy to the patient’s specific pathology, a frontolateral approach for anterior communicating artery (ACoA) aneurysms and a frontotemporal craniotomy for middle cerebral artery (MCA) aneurysms [34]. In the 1970s, Yasargil’s perfected and popularised this ‘pterional’ approach with incorporation of the microscope and microsurgical techniques, highlighting the true potential of this approach [85]. This approach has become common practice in modern neurosurgery.

Despite its popularity, there are some well-described disadvantages with this classic technique, particularly temporal muscle wasting with functional and aesthetic consequences including facial asymmetry, discomfort with eyewear, temporomandibular joint dysfunction and mastication pain [7, 35]. A large craniotomy with an extensive exposure of brain cortex also increases the risk of iatrogenic injury; hence, customising the pterional craniotomy to a patient’s specific pathology has been a natural evolution for many surgeons. Furthermore, the general principle of the relation between the pterional craniotomy and the Sylvian fissure cannot be overemphasised [5, 86].

Over the last decades, many variants have been described. This review examines the different described techniques, discusses their adaptations and advocates for adoption of standardised nomenclature.

Methods

Literature search

A search strategy was devised according to the 2020 Preferred Reporting Items of Systematic Reviews and Meta-Analyses (PRISMA) statement [63]. An electronic search of the databases Medline, Scopus, Embase, Web of Science and Cochrane library databases was performed from 1st January 1956 to 17th of July 2023. Articles were limited to the English language. To identify articles investigating the variations in the pterional approach, the following search terms were applied: (pterional OR minipterional OR supraorbital) AND (approach OR craniotomy OR technique) with prior checking in the MeSH database to include synonyms.

The database search was further supplemented by a search of the reference lists of included studies as well as checking the related article function provided by each database. Titles and abstracts were screened to identify potentially relevant studies. All potentially relevant articles, or articles where it was unclear based on the abstract, were assessed by reviews of the full-text articles.

Articles were deemed eligible if they: (1) specifically described the surgical technique in detail; (2) state the technique is a unique modification or novel technique and use unique nomenclature that differentiates it from previous iterations; (3) published in a peer-reviewed journal; (4) the approach is specifically designed for vascular pathology managed microscopically or with endoscopic assistance; (5) the technique was indicated for management of aneurysms of the supraclinoid internal carotid artery, proximal middle cerebral artery and proximal anterior cerebral artery/anterior communicating artery complex.

Article were excluded when: (1) the primary goal of the article was not presenting a surgical technique; (2) they present a novel subdural and subarachnoid corridor but use a previously described craniotomy; (3) only demonstrate technique in cadaver specimens; (4) article did not undergo peer review, such as a letter to the editor; (5) the approach has only been used to treat neoplastic pathology; (6) article only presents management of soft tissue dissection without a novel craniotomy; (7) article describes a pure transcranial endoscopic approach; (8) the focus was posterior circulation, distal anterior cerebral artery or distal middle cerebral artery aneurysms.

Data extraction

All data was reviewed independently by two authors (NC and JV) and discrepancies cross checked in a consensus meeting.

The following data was obtained from the included studies: title and year of publication, name of the approach, type of skin incision, described craniotomy location, true anatomical location of the craniotomy, dural incision, approach, target zone exposure or indication for treatment of what type of aneurysms.

Quality assessment

We used a modified quality assessment tool incorporating the Cochrane Collaboration tool to assess the methodological quality of the included articles [37]. The quality assessment tool (Table 1) assessed the following: demographic details, pre-operative variables, post-operative variables, complications and learning curve. The same two authors (NC and JV) then evaluated the risk of bias in the individual articles using a modified version of the Cochrane Collaboration method (Table 2). Discrepancies were resolved after discussion and consensus amongst all authors.

Table 1 Quality assessment tool
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Table 2 Grading of quality assessment
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Results

Study selection

From the literature search, 3546 articles were identified through searching Medline, Embase, Scopus, Web of Science and Cochrane library databases. Six additional articles were also identified by screening reference lists of included articles and searching the recommended article section of applicable databases. In total, 3552 articles were screened with 74 articles being read in full and 27 of these articles being excluded. The common reason for exclusion involved 21 articles [4, 9, 11, 16, 18, 19, 26, 27, 30, 46, 49, 52, 54, 62, 65,66,67,68, 73, 80, 84] and is because the authors cited another article when describing the surgical technique. The other reasons for exclusion included as follows: one article [23] designing an approach to treat distal anterior cerebral artery aneurysms, one article [21] designed an approach that was not for cerebrovascular pathology, two articles [29, 71] only described their technique being used in cadavers and two articles [24, 28] only described a new subdural corridor without a novel craniotomy. Figure 1 contains the flowchart of study selection. The included 47 articles [1,2,3, 6,7,8, 10, 12, 14, 15, 17, 20, 22, 25, 31, 32, 35, 36, 38,39,40,41,42,43, 45, 48, 51, 53, 55,56,57,58,59,60, 64, 70, 72, 74,75,76,77,78,79, 81,82,83, 85] were reviewed in detail.

Fig. 1
figure 1

PRISMA flowchart demonstrating the process of study selection

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Study characteristics

1976

Yasargil et al. [85] describe the fronto-spheno-temporal craniotomy which involves an incision starting 1 cm superior to the anterior aspect of the auricle and extends to the temporal crest in a direction perpendicular to the zygoma ending at the widows peak. Four burrholes are described, being located just superior to the frontal zygomatic suture under the linea temporalis, second burrhole in the frontal bone 3–4 cm above it, third burrhole in the parietal bone along the linea temporalis and fourth burrhole in the squamous temporal bone behind the spheno-temporal line. They term this the ‘pterional’ craniotomy’ and it gives a subfrontal and transsylvian approach to aneurysms of the anterior circulation.

1982

Jane et al. [38] describe the supraorbital approach which involves a bicoronal incision with a fronto-spheno-zygomatic craniotomy requiring a burrhole at the midline level of the orbital ridge and a second burrhole just behind the zygomatic process. This grants a subfrontal approach to orbital tumours, ACoA aneurysms, pituitary tumours, craniopharyngiomas, parasellar and olfactory groove meningiomas.

1987

Al-Mefty [3] described the supraorbital-pterional approach to manage large central skull base lesions, including giant basilar aneurysms. The approach involves a bicoronal incision with a single piece craniotomy involving fronto-spheno-zygomatico-temporal bone, including the orbital rim.

1989

Schlitt et al. [77] describe the osteoplastic pterional craniotomy which is a fronto-spheno-temporal osteoplastic craniotomy that starts with an incision at the zygomatic process 2 cm anterior to the external auditory canal and curves frontally towards the lateral third of the eyebrow. The first burrhole is posterior and inferior to the anterior point of the superior temporal line, second burrhole posterior temporal above the zygomatic root. A rongeur is used craniectomies the remaining bone and fracturing across the gap. They do not define the approach, but describe the approach as granting access to internal carotid artery (ICA), ACoA and MCA aneurysms.

1992

Chehrazi et al. [15] described the temporal transsylvian approach. This started with a skin incision at the zygomatic arch 1 cm anterior to the auricle and extending superiorly and anteriorly to the superior extent of the ‘keyhole’. A single burrhole is placed over the exposed sphenoid wing of the sphenoid with a fronto-spheno-temporal craniotomy being raised. This grants a transsylvian corridor and allows treatment of all aneurysms of the anterior circulation.

1996

Hardland et al. [35] described the modified pterional, which involved a 5–6-cm curvilinear incision immediately behind the hairline from the superior temporal line to the midpoint of the zygomatic arch. A single burrhole is placed over the pterion and this is extended as a craniectomy using rongeurs, removing fronto-spheno-temporal bone. This grants a transsylvian corridor and allows treatment of MCA, ACA, posterior communicating artery (PCoA), ophthalmic and terminal carotid aneurysms.

1998

Van Lindert et al. [83] described the supraorbital keyhole approach. The skin incision starts lateral to the supraorbital nerve and finishes at the lateral edge of the eyebrow in front of the zygomatic process. A single burrhole is placed in the temporal fossa just behind the superior temporal line. The subsequent craniotomy can have three variants depending on the target area to be reached and includes as follows: frontal, fronto-sphenoidal and fronto-spheno-temporal. Through a subfrontal corridor, all aneurysms of the anterior circulation can be reached.

Paladino et al. [64] also described the eyebrow keyhole approach with a skin incision that is poisitoned in the lateral two-thirds of the eyebrow. A single burrhole is made 1 cm above the supraorbital rim and 1 cm lateral to the supraorbital nerve to raise a frontal craniotomy. This grants a subfrontal corridor and allows treatment of ACoA and anterior choroidal aneurysms.

2001

Steiger et al. [79] describe the transorbital keyhole approach with an incision that starts 1 cm in front of the tragus and slightly above the zygomatic arch, extending frontally towards the midline. The first burrhole is placed over the keyhole of the pterional craniotomy, and the second is placed above the upper orbital rim at the medial aspect of the planned craniotomy. This raises a fronto-spheno-zygomatic craniotomy granting a subfrontal corridor to treat ACoA aneurysms.

Dare et al. [20] also described the eyebrow incision-minisupraorbital craniotomy with orbital osteotomy.

2002

Ramos-Zuniga et al. [72] described the trans-supraorbital approach involving a 3-cm incision made through the eyebrow between the pupil median line and the external rim of the zygomatico-orbital joint. They describe an en bloc craniotomy within the supraorbital foramen, zygomatico-orbital joint, orbital arch with a 1-cm extension into the depth of the orbital roof. This raises a frontal craniotomy which grants a subfrontal corridor for treatment of aneurysms on the ICA, ACoA and MCA.

2003

Andaluz et al. [7] describe the one piece orbitopterional approach. Involving a hair sparing incision starting 1 cm below the zygoma and follows the hairline approximately 3 cm beyond the midline. The first burrhole is at the frontosphenoidal suture about 1 cm behind the frontozygomatic junction. Second burrhole is superior to the root of the zygoma, and these are connected to raise an orbitopterional craniotomy involving fronto-spheno-zygomatico-temporal bones. This grants a subfrontal corridor and allows access to the ipsilateral optic nerve, optico-carotid cistern, ACoA and potential suprasellar and infrachiasmatic tumours.

Kang et al. [39] described a pterional craniotomy with keyhole for treatment of supratentorial aneurysms. This variant involved a single burrhole in the posterior temporal fossa.

Reisch et al. [74] describe the supraorbital keyhole craniotomy (Table 3).

Table 3 Study characteristics of the included articles
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2005

Hernesniemi et al. [36] describe the lateral supraorbital (LSO) approach involving a frontotemporal incision behind the hairline and does not go as low as the conventional pterional incision. A single burrhole is placed posteriorly just below the insertion of the temporalis muscle. This allows elevation of a fronto-sphenoidal craniotomy granting a subfrontal and transsylvian corridor allowing access to the whole anterior part of the anterior circle of Willis, sellar, suprasellar region and anterior part of the basilar artery.

Nathal et al. [59] described the sphenoid ridge approach.

Kelleher et al. [40] described the cranio-orbital approach (Table 3).

2006

Figueiredo et al. [31] described the supraorbital minimodified orbitozygomatic craniotomy. Involving an arcuate incision starting at the base of the zygomatic arch 1 cm anterior to the tragus extending to the contralateral midpupillary line. A single burrhole at MacCarty keyhole and then raising a craniotomy involving the fronto-spheno-zygomatic bone. Grants a transsylvian and subfrontal corridor allowing access to the ipsilateral MCA bifurcation, ipsilateral ICA bifurcation, basilar artery bifurcation, contralateral ICA bifurcation, ACoA and contralateral MCA.

Cheng et al. [17] also described the pterion minicraniotomy.

2007

Figueiredo et al. [32] described the mini-pterional craniotomy. This approach involved an arcuate incision starting 1 cm above the base of the zygomatic arch at the anterior border of the hairline and extending superiorly and curving towards the ipsilateral mid-pupillary line. A single burrhole is made superior to the frontozygomatic suture under the linea temporalis. A circular craniotomy is commenced by carrying the craniotome posteriorly and at the stephanion curving inferiorly to include the pterion then curving anteriorly back to connect with the burrhole. This raises a fronto-spheno-temporal craniotomy and allows a transsylvian and subfrontal corridor. The authors found no statistically significant difference between this exposure and that of a conventional pterional.

Mori et al. [56] also described the pterional keyhole craniotomy through an outer canthal incision (Table 3).

2008

Andaluz et al. [6] described the transeyelid, supratarsal, transorbital roof minicraniotomy. Involving an incision of 2.5 to 3.5 cm in the upper eyelid along the eyelid crease, 10 mm superior to the upper lid margin and 6 mm above the lateral canthus at its lateral extent. A single burrhole at the MacCarty keyhole and then raising a fronto-spheno-zygomatic craniotomy. This allows a subfrontal corridor to access the ipsilateral optic nerve and optico-carotid cistern.

Mori et al. [57] described the lateral supraorbital approach via a periorbital skin incision.

Brydon et al. [10] described the supra-orbital minicraniotomy (Table 3).

2011

Kim et al. [41] described the osteomyoplastic monoblock pterional craniotomy which involved a fronto-spheno-parieto-temporal craniotomy with temporalis muscle still attached following a keyhole burrhole, superior osteotomy along the frontal bone limited by temporalis and an inferior osteotomy along the squamosal temporal bone.

Abdel Aziz et al. [1] described the transpalpebral approach which involved an incision through the superior eyelid crease up to 2.5 cm from the lateral canthus and 1 piece fronto-orbital craniotomy.

Mori et al. [55] described the individualised pterional keyhole based on 3D virtual osteotomy. This involved a W shaped made in front of the hairline with a burrhole over the pterion and a further 25 mm craniectomy based on pre-planned location. This created a transsylvian corridor to treat MCA aneurysms.

2013

Mocco et al. [53] described the minimally invasive pterional keyhole approach starting with a hockeystick incision anterior to the tragus and above the posterior root of zygoma extending up towards the temporal line staying behind the hairline. An elliptical craniotomy is raised that grants a transsylvian corridor.

Tang et al. [82] described the modified supraorbital keyhole approach which is started with a skin incision made into the skin crease or the eyebrow and creating a fronto-sphenoidal craniotomy.

2015

Deshaies et al. [22] described the minimally invasive thumb sized pterional craniotomy technique which involves an incision starting at the midline behind the hairline extending towards the tragus in a gentle curve ending 3 mm anterior to the tragus. A 3–4 cm fronto-spheno-temporal craniotomy is raised.

2016

Kim et al. [42] described the modified supraorbital keyhole approach which is started with a conventional pterional skin incision and involves a fronto-spheno-temporal craniotomy.

Lazukova et al. [45] described the modified lateral supraorbital approach which is the same as the lateral supraorbital approach except for the application of an orbitozygomatic stitch.

Dzhindzhikha et al. [25] described the mini-orbitozygomatic craniotomy started with an eyebrow incision from the pupillary within the eyebrow, sometime extending beyond. Raising a single bone flap involving frontal bone, roof of orbit and zygomatic bone.

2017

Mandel et al. [48] described the modified transpalpebral, supratarsal, transorbital roof minicraniotomy involving a skin incision in the upper lid crease from mid pupillary line to the lateral canthal angle. A mini fronto-orbital craniotomy is raised that involves frontal, sphenoid and zygomatic bones.

Ahn et al. [2] described the superficial temporal artery sparing mini pterional approach which involves a curvilinear incision starting above the STA bifurcation.

Andrade-Barazarte et al.[8] described the extended lateral supraorbital craniotomy and extradural anterior clinoidectomy.

Petridis et al. [70] described the modified mini pterional subfrontal supratentorial approach.

Cavalcanti et al. [12] described the minisphenoidal approach.

2018

Takeda et al. [81] described the distal transsylvian keyhole approach.

Kocaman et al. [43] described the modified lateral supraorbital approach.

2020

Chandra et al. [14] described the fronto-orbital variant of the supraorbital keyhole craniotomy (f-SOKHA).

Martinez-Perez et al. [51] described the extradural minipterional approach, which was also separately described by Mura et al. [58] using the same name.

Sattur et al. [76] described the extended lateral orbital approach.

2022

Nerntengian et al. [60] described the mini-spheno-supraorbital craniotomy.

Rychen et al. [75] described the Sylvian keyhole approach.

Secere et al. [78] described the modified osteoplastic pterional craniotomy.

Difference in skin incision

There were two broad types of incisions that are described. The first involves an incision in the eyebrow or eyelid that may extend beyond the orbit into the temporal fossa. There are 14 articles [1, 6, 10, 14, 20, 25, 48, 56, 57, 64, 72, 74, 76, 82, 83] that describe this type of skin incision. The second type of incision is curvilinear in shape and is either behind the hairline of the temporal fossa and forehead, on the hairline, or slightly anterior. There are 28 articles [2, 7, 8, 12, 15, 17, 22, 31, 32, 35, 36, 39,40,41,42,43, 45, 51, 58,59,60, 70, 75, 77,78,79, 81, 85] that describe this type of skin incision. Two articles [3, 38] describe a bicoronal incision, one article [53] describes question mark/hockey stick incision over the temporal fossa and forehead and another article [55] describes a W-shaped incision.

Difference in temporalis dissection

There were two common types of temporalis dissection. The most common method involved incising through all layers of the temporalis fascia and muscle and splitting the temporalis muscle with subperiosteal dissection, described in 18 articles [1, 6, 14, 15, 20, 32, 35, 43, 55,56,57, 64, 72, 74, 76, 79, 82, 83]. The other common method was described in 12 articles [2, 7, 31, 39, 40, 42, 51, 53, 58, 60, 81, 85] and involved an interfascial or subfascial dissection technique with mobilisation of the muscle separately in a different direction. Four articles [12, 17, 22, 36] describe a single myocutaneous flap. Three articles [41, 77, 78] describe an osteoplastic craniotomy. One article [75] describes a T-shaped incision with subperiosteal dissection of each limb, and another article [70] describes incising the muscle along the attachment to the STL and dissecting subperiosteally. Four articles [3, 10, 38, 48] did not describe the method of temporalis dissection.

Difference in anatomical location of craniotomy

A total of seven locations for a craniotomy are described (Fig. 2). Nineteen articles [2, 12, 15, 22, 32, 35, 39, 42, 51, 53, 56, 59, 70, 76,77,78, 81, 83, 85] described a craniotomy that involved removing bone from the fronto-spheno-temporal area (cyan). Eight articles [1, 6, 20, 25, 31, 38, 48, 79] described a craniotomy that involved removing bone from the fronto-spheno-zygomatic area (dark blue). Nine articles [8, 14, 36, 45, 55, 57, 60, 82, 83] described a craniotomy that involved removing bone from the fronto-sphenoidal area (pink). Five articles [10, 43, 64, 72, 74] described a craniotomy involving only frontal bone (yellow). Three articles [3, 7, 40] described a fronto-spheno-temporo-zygomatic craniotomy (orange). One article [17] described a temporo-sphenoidal craniotomy (red), an article [41] described a fronto-spheno-parieto-temporal craniotomy (purple). One article [75] described a fronto-temporal bone removal, which was not able to be demonstrated on Fig. 2.

Fig. 2
figure 2

Schematic illustration demonstrating the anatomical locations of the described craniotomies. Each coloured line represents a different anatomical location for the reviewed craniotomies: Cyan, fronto-spheno-temporal; dark blue, fronto-spheno-zygomatic; pink, fronto-sphenoidal; yellow, frontal; orange, fronto-spheno-temporo-zygomatic; red, temporo-sphenoidal; purple, fronto-spheno-parieto-temporal

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Difference in approach

The majority of articles described a combination of subfrontal and/or transsylvian approaches. Twenty articles [1, 6,7,8, 10, 14, 20, 38, 42, 43, 45, 48, 57, 64, 70, 72, 74, 79, 82, 83] described a pure subfrontal approach. Thirteen articles [2, 12, 15, 17, 35, 51, 53, 55, 56, 58, 59, 75, 81] describe a purely transsylvian approach. Twelve articles [22, 25, 31, 32, 36, 39,40,41, 60, 76, 78, 85] described both subfrontal and transsylvian approaches. One article [3] described a subfrontal, transsylvian and subtemporal approach.

Study quality

Twenty-six of the included articles were deemed of good methodological quality. Sixteen articles were deemed of moderate quality and five articles were deemed poor quality (refer to Table 4 for the full assessment).

Table 4 Quality assessment consensus table
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Discussion

A proposition for standardised nomenclature

We have described the exhaustive number of techniques, variations and modifications described for fronto-spheno-temporal or pterional craniotomy. Ultimately, these 47 different articles can be distilled into two types of incisions (frontotemporal or periorbital), two types of craniotomies (frontal or a craniotomy involving sphenoid such as fronto-spheno-temporal/fronto-spheno-zygomatic/fronto-sphenoidal) and two approaches (subfrontal or transsylvian). There are several modifiers that have been described at each of these stages such as the length of the incision, management of the underlying muscle, size of the craniotomy, an osteotomy including the supraorbital bar [1, 6, 7, 20, 32, 40, 72], osteoplastic craniotomy [41, 77, 78] or use of an extradural anterior clinoidectomy [8, 51, 58].

Minipterional, pterional keyhole, sphenoid ridge approach, Sylvian keyhole, distal transsylvian approach, thumbsize pterional, minisphenoidal and minimal invasive pterional keyhole are just a few of the many names that were identified in the systematic review. More recently, extended minipterional and nanopterional have been described, but did not meet inclusion criteria of this review [47, 50].

These articles create the impression that aneurysm surgery is an outside-in concept. It implies that surgeons must learn a large number of these craniotomies and their named modifications to treat certain target lesions. This concept is misleading as it is really considered from inside-out, despite not being articulated in this way. The majority of the included articles focus on naming the type of craniotomy, or minor variation in the exposure. They all still largely target the same anatomical zone.

In clinical practice, surgeons examine the lesion to be treated and then consider the target zone they want exposed. This will vary by surgical goal (multiple aneurysms, complexity, requirement for deconstructive or reconstructive techniques) and surgeon experience (increasing experience gives confidence to manage with a smaller exposure). The surgeon then considers what approaches grant them the necessary degrees of freedom and visibility, before finally considering the craniotomy and skin incision that will enable that approach. By having so many articles, it is easy to overlook this essential component of designing the neurosurgical procedure for your patient. When the first description appeared of the minipterional in 2007, Bernard George highlighted the importance of considering the general principle of the location of the pterional craniotomy to the Sylvian fissure, rather than to ‘precisely describe a surgical technique and to consider any little change or variation as a new technique’.

The inside-out concept is more evident in articles describing different techniques in skull base oncology surgery. The focus of these articles is to describe the anatomical exposure at the target zone, the degree of freedom offered by the approach and the surgeon’s unique way of exposing these structures. The surgeon then offers case examples of pathology that can be addressed safely using the technique described. It is far less common in the skull base oncology literature to find articles describing all of the different neoplasms addressed through an extremely specific craniotomy or named modification. Moreover, the authors will present an article that is based on their experience treating a specific pathology and the variety of approaches they have had to utilise to achieve their surgical goals. Shifting towards this paradigm for vascular neurosurgery would be beneficial.

We advocate for more simplified and consistent nomenclature. Craniotomies were traditionally described based on the calvarial bones incorporated into the craniotomy. Atlay et al. describe this in detail during a historical perspective on the frontotemporosphenoidal (FTS) approach [5]. Variations were named when major changes occurred compared to the FTS approach, such as Hakuba et al. [33] orbitozygomatic infratemporal approach, Drake [13] half and half approach, and Pellerin et al. [69] orbitofrontomalar approach.

We propose a standardised and intuitive nomenclature to simplify the way these approaches are described, similar to the original description based on the overlying calvarial bones (Fig. 2).

Based on the ‘inside-out’ concept, we advocate to describe all approaches based on the target zone, subdural or subarachnoid corridor being used to approach the aneurysm, the constituent bones included in the craniotomy and the location of the associated incision. A major approach-related consideration is the target zone to be exposed, which is defined by the specific aneurysm and surgical goal. Using this new system of nomenclature applied to an ACoA aneurysm first requires understanding of the anatomic and therapeutic considerations specific to the aneurysm. What direction does the dome of the aneurysm project? If superiorly projecting, do you consider interhemispheric, subfrontal or transsylvian? If inferiorly projecting, do you consider subfrontal or transsylvian? Which side is the dominant A1 and will there be benefit from approaching the aneurysm on that side? Once the target zone and approach corridor are defined the constituent bones involved in the craniotomy become evident and the associated incision to grant this access can be planned based on surgeon experience. If additional areas of bone removal are required, such as an anterior clinoidectomy, these modifiers are defined along with the bones being removed in the craniotomy. This is because the removal of these structures is being done to access the target zone, and therefore is entirely determined by the target zone and corridor required to treat the aneurysm. The additional removal of bone and the shape and type of incision have considerable variation.

The inside-out concept can be considered as a pyramid with the target zone at the apex of the pyramid, as it is essentially the aneurysm morphology to be treated. The corridor, craniotomy and skin incision are then listed in order of increasing degrees of freedom.

For example, an inferiorly projecting ACoA aneurysm would be defined by the following: the target zone (inferiorly projecting ACoA aneurysm), corridor (right/left-sided subfrontal), craniotomy (frontal/fronto-sphenoidal/fronto-spheno-temporal), incision (eyebrow/frontotemporal scalp) (Fig. 3). The long hand description could then be an inferiorly projecting ACoA aneurysm approached subfrontally through an eyebrow incision and right-sided fronto-sphenoidal craniotomy. A short hand description would be to simply describe the bones involved; a right-sided fronto-sphenoidal approach.

Fig. 3
figure 3

Proposal of how to apply the ‘inside-out’ concept for aneurysm surgery and a way to simplify the approach related nomenclature

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Another example describing a right M1/2 aneurysm would be defined by the following: the target zone (right M1/2 aneurysm with a dome projecting frontally and a long M1), corridor (right transsylvian), craniotomy (fronto-sphenoidal/fronto-spheno-temporal/spheno-temporal), incision (frontotemporal). Therefore, the long hand description would be as follows: a right M1/2 aneurysm approached transsylvian through a frontotemporal scalp incision and fronto-spheno-temporal craniotomy. Associated short hand description being a right-sided fronto-spheno-temporal approach.

Ultimately the success of an operation is determined by the surgeon tailoring their incision, craniotomy and approach to the patient and their pathology. The cut-off at which size truly affects patient-outcome is currently unknown, particularly because surgeon experience will likely have a greater affect then the size of the incision or the amount of bone removed. The use of a standardised nomenclature would help clarify this.

It is interesting to note that the International Society on Minimally Invasive Neurosurgery recently published a consensus statement regarding standardising nomenclature in an effort to define the variety of keyhole procedures that are described in the literature [44]. The result of this standardisation will allow for more effective comparisons to be examined.

Limitations

A potential limitation of this article is the narrow scope that is applied to only consider articles relating to vascular neurosurgery. By excluding all the articles focusing on oncological neurosurgery, a large number of important articles will have been excluded. However, the purpose of this review was to highlight the trend in the vascular neurosurgery literature of renaming minor and inconsequential modifications. This trend is not as prevalent in oncological neurosurgery, which is why the review did not consider these articles.

Conclusion

We have demonstrated a systematic review of modifications and variations for the pterional craniotomy that exists in the literature. There is an exhaustive number of minor variations that do not serve to expand the general principles of aneurysm surgery. We advocate for a simplified and standardised nomenclature when considering surgical management of aneurysms that can describe the incision and craniotomy type, but should focus on the approach and target zone exposure.