Since the introduction of the endoscopic transorbital approach (ETOA), the areas in which surgical treatment is possible with this method have been expanded in the course of several studies.1–3 Anterior petrosectomy in the posterior fossa area was possible with ETOA, and a new method to access lesions in the insular region via the ETOA has also been reported.4–6 Additional research has also been conducted to efficiently use the surgical space by modifying the ETOA with insertion of a small port.7,8 In particular, it was observed that even a little widening of the ETOA entry site through lateral orbital rim (LOR) osteotomy has tremendous advantages in endoscopic surgery.9 Due to the nature of endoscopic surgery, even if the entry site is only slightly wider, the angle to the target and surgical freedom of operation are significantly increased.
Surgical freedom and angle of attack of horizontal movement can be obtained via an ETOA with LOR osteotomy, but vertical movement is limited in the conventional ETOA. Due to this vertical movement limitation, some lesions of the anterior cranial fossa are difficult to access via an ETOA or an ETOA with LOR osteotomy (Fig. 1A). We observed that if the vertical range of the ETOA could be increased, it would be possible to access several lesions that are difficult to reach with this approach (Fig. 1B).
Schematic illustration of the ETOA with SLOR (ETOA c SLOR) and ETOA with LOR (ETOA c LOR) osteotomies. A: Resection of the anterior medial part of the tumor was not possible through ETOA with LOR osteotomy. B: Whole tumor resection of anterior clinoid meningioma was possible through ETOA with SLOR osteotomy. C: Compared to ETOA with LOR osteotomy, the entry site of ETOA with SLOR osteotomy was larger and SOR osteotomy was additionally performed. D: Concept illustration of ETOA with SLOR osteotomy. Panel D: Artist: Yurie Lee. Copyright Ju Hyung Moon. Published with permission. Figure is available in color online only.
We devised an ETOA with superior-lateral orbital rim (SLOR) osteotomy to increase the surgical freedom of vertical as well as horizontal movement (Figs. 1 and 2). The purpose of this study was to confirm the feasibility and the clinical efficacy of ETOA with SLOR osteotomy.
Surgical technique of ETOA with SLOR. A: The skin incision can be made using either transeyelid or below-the-eyebrow techniques. B and C: The one-piece bone flap is made by osteotomy of the SOR and the LOR together. D: Like the conventional ETOA method, SLOR osteotomy is performed after drilling the lateral orbital wall, temporal bone, and frontal bone. It is possible that SLOR osteotomy can be performed first, after which frontal and temporal bones can be drilled out. E and F: Anterior clinoidectomy is performed after drilling the orbital roof, lesser sphenoid wing, and optic strut, which are the structures that make up the ACP. Artist: Dong-Su Jang. Copyright Ju Hyung Moon. Published with permission. Figure is available in color online only.
Methods
Surgical Technique of ETOA With SLOR Osteotomy
The skin incision was made using either transeyelid or below-the-eyebrow techniques.1,2,10,11 The one-piece bone flap was created by osteotomy of the superior orbital rim (SOR) and the LOR together (Fig. 2B and C). Similar to the conventional ETOA method, SLOR osteotomy was performed after drilling the lateral orbital wall, temporal bone, and frontal bone. The SLOR osteotomy could be performed first, after which frontal and temporal bones can be drilled out (Fig. 2C). The extent of bone removal was tailored to the surgical goal (Fig. 2D). In the case of anterior clinoidal meningioma, anterior clinoidectomy was performed after drilling the orbital roof, lesser sphenoid wing, and optic strut, which are the structures that make up the anterior clinoid process (ACP) (Fig. 2E).12 The meningo-orbital band (MOB) was cut if necessary. After tumor removal, we used an artificial dura mater substitute for the reconstruction of dural defects and acellular allogenic dermis for reconstruction of the posterior orbital wall. The orbital rim was repositioned and fixed with application of miniplates.
The cadaveric study committee of the Yonsei University College of Medicine approved this study. A total of 10 sides of 5 cadaveric heads were used in this study. Cadaveric heads were prepared by the Thiel embalming and ethanol-glycerin fixation method and then injected with silicone rubber injection compounds (MICROFIL; Flow Tech, Inc.) to fill and opacify vessels. Prior to the dissection, the cadaveric heads were subjected to CT scanning; images were uploaded to a cranial navigation system platform (Stryker navigation system; Stryker Corp.). ETOA with SLOR osteotomy was performed on one side of the head, and ETOA with LOR osteotomy was performed on the other side; ETOA with LOR osteotomy was performed as described previously in the literature.9 One target point was defined: the tip of the ACP. The horizontal and vertical limitations to the target were identified by the neuronavigation system (Stryker Corp.). The angle of attack was measured in the axial and sagittal planes for the target. The maximal horizontal and vertical angles of the target were measured using Mimics software (Materialise Mimics Co., Ltd.) (Fig. 3). Surgical freedom was calculated by incorporating the surgical field volume. To illustrate the concept of ETOA with SLOR and ETOA with LOR osteotomy, we generated a 3D reconstruction model (Figs. 1C and 3). All anatomical dissections were performed at the surgical anatomy laboratory of Yonsei University College of Medicine and Severance Hospital. The anatomical dissections were performed using a rigid endoscope 4 mm in diameter and 18 cm in length, equipped with 0° and 30° optic lenses (Stryker neuroendoscopy; Stryker Corp.).
The angle of attack and schematic illustration of endoscopic accessible range. A: The maximal horizontal and vertical angle of the target (ACP) of ETOA with SLOR and ETOA with LOR osteotomy. The horizontal and vertical angle to the target point in ETOA with SLOR is wider than ETOA with LOR osteotomy. B: The accessible range of endoscopic view and manipulation are wider through ETOA with SLOR than through ETOA with LOR osteotomy. Panel B: Copyright Ju Hyung Moon. Published with permission. Figure is available in color online only.
Statistical Analyses
The comparisons between the data were analyzed using an independent t-test with p < 0.05 considered as statistically significant, using SPSS Statistics for Macintosh version 25.0 (IBM Corp.).
Results
Cadaveric Study
Craniotomy
For ETOA with SLOR osteotomy, the degree of craniotomy of the frontal base bone was increased compared to conventional ETOA and ETOA with LOR osteotomy. Because accessing the anteromedial frontal base lesion is relatively easy, the attack range on the frontal bone was widened and the degree of craniotomy could be increased (Figs. 3B, 4E, and 4F).
Cadaveric study. A–D: Craniotomy. A and B: The entry site and the frontal base craniotomy of ETOA with SLOR are larger than conventional ETOA with LOR osteotomy. C and D: The bone flap of ETOA with LOR and ETOA with SLOR osteotomy. E–J: Extradural phase. E and F: Exposed frontal dura lesion of ETOA with SLOR is larger than conventional ETOA with LOR osteotomy. G and H: The space for the surgical instruments and the endoscope was widened, so that bone drilling could be performed very conveniently. I and J: Because the frontal dura could be retracted easily in ETOA with SLOR osteotomy, the view of the orbital roof and sphenoid ridge is improved over ETOA with LOR osteotomy. K–M: Anterior clinoidectomy. Because the limitation of vertical movement of the device was improved with frontal dura retraction, the orbital roof could be easily accessed and the path of the optic nerve could be easily identified. Anterior clinoidectomy could be performed easily and safely. N–S: Intradural phase. N–Q: It was possible to easily reach the medial side of the ACP dura and frontal base area (planum sphenoidale and tuberculum sellae area) and the medial area of the contralateral optic nerve through ETOA with SLOR osteotomy. Retraction of the frontal lobe was possible by using the brain retractor in ETOA with SLOR osteotomy. R: The accessibility to the anterior frontal base lesion through ETOA with LOR osteotomy was very limited. S: The closed and direct view of the anterior frontal base lesion was possible through ETOA with SLOR osteotomy. It is possible to access the contralateral anterior skull base lesion as well as the ipsilateral one through ETOA with SLOR osteotomy. A-com An = AComA aneurysm; ASB = anterior skull base; C. = contralateral; F = frontal lobe; FD = frontal dura; N = nerve; ON = optic nerve; PO = periorbita; T = temporal lobe; TD = temporal dura. Panels A, B, and E–S: Copyright Ju Hyung Moon. Published with permission. Figure is available in color online only.
Extradural Phase and Anterior Clinoidectomy
After SLOR osteotomy, the space for moving the surgical instruments and the endoscope was widened, so additional bone drilling could be performed conveniently. Given that the frontal dura can be retracted easily in ETOA with SLOR osteotomy, the view of the frontal base and the sphenoid ridge is superior to that for ETOA with LOR osteotomy (Fig. 4G–J). For ETOA with SLOR osteotomy, when the anterior clinoidectomy was performed, the limitation of vertical movement of the device, which is due to the SOR with the conventional ETOA and ETOA with LOR osteotomy (Fig. 4G and I), is improved (Fig. 4H, J, and K). Thus, the orbital roof, which is the structure constituting the ACP, could be easily accessed and the path of the optic nerve could be easily identified (Fig. 4J and K). The orbital roof and optic strut could be removed safely (Fig. 4J–L).
Intradural Phase
It was possible to easily access the medial side of the ACP dura and frontal base area (planum sphenoidale and tuberculum sellae) and the medial area of the contralateral optic nerve, which are difficult to reach via the existing ETOA or ETOA with LOR osteotomy (Fig. 4N–Q). In the case of ETOA with LOR osteotomy, access to the frontal base lesion is greatly restricted due to limitations in vertical movement (Fig. 4P and R). Although this region could be viewed by an endoscope, it was very difficult to acquire sufficient access for surgical instruments (Fig. 4R). Alternatively, in the case of ETOA with SLOR osteotomy, a close and direct view of the endoscope was achievable compared to the ETOA and ETOA with LOR osteotomy, and a surgical space in which a surgical manipulation in the area was possible was acquired (Fig. 4P–S).
Retraction of the frontal and temporal lobes was possible using the brain retractor in ETOA with SLOR osteotomy, allowing us to secure surgical space in addition to that provided by the wider frontal bone removal area. Access to the anterior cerebral artery (ACA) A1 and the anterior communicating artery (AComA) through ETOA with SLOR osteotomy was easy, and manipulation of vascular structures was also possible. In the case of an AComA aneurysm, which was detected in one cadaver head, it was possible to secure sufficient space for vascular clipping (Fig. 4S). Also, because the additional brain retraction force through suction or surgical instruments was reduced, the operator was very comfortable during the procedure. It was also convenient for very delicate procedures, such as arachnoid dissection and vascular dissection with ETOA with SLOR osteotomy.
Attack Range and Surgical Freedom
Horizontal and vertical movement to the target point in ETOA with SLOR osteotomy was statistically increased relative to the ETOA with LOR procedure (Supplemental Table 1). Surgical freedom and accessibility to the surgical target lesion in ETOA with SLOR osteotomy were significantly increased compared to ETOA and ETOA with LOR osteotomy (Supplemental Tables 1 and 2).
Clinical Implications
We performed a total of 6 cases of brain tumor resection via ETOA with SLOR osteotomy (Table 1 and Fig. 5). Four cases were performed with eyebrow incision and 2 cases with eyelid incision. In 4 cases, SLOR osteotomy was performed first, followed by ETOA craniotomy; ETOA craniotomy was performed first and followed by SLOR osteotomy in 2 cases. Total tumor removal was achieved in all cases, and the procedures were comfortable for the operators. In 3 cases of anterior clinoidal meningioma, the origin of the tumor was considered to be ACP dura, so anterior clinoidectomy was performed. Because manipulation of the extended dural margin and origin dura was possible in these cases, Simpson grade I resection in 1 case and Simpson grade II resection in 2 cases were achieved. As shown in the illustrative cases, views and surgical procedures were possible not only for the ipsilateral optic nerve and dura site, but also for the medial anterior skull base (planum sphenoidale and tuberculum sellae) area and the medial site of the contralateral optic nerve (Figs. 6 and 7). Specifically, when dissection was performed near adjacent vascular structures, retraction of surrounding tissues by using additional surgical instruments was possible, enabling the performance of delicate procedures. Even without anterior clinoidectomy, enhanced surgical freedom with SLOR osteotomy could facilitate the surgical manipulation and reduce the crowding of surgical instruments.
Clinical and demographic characteristics of 6 patients who underwent ETOA with SLOR osteotomy
| Case No. | Sex/Age (yrs) | Diagnosis | Tumor Extension | Skin Incision | ACP Removal | EOR | Postop CSF Leak | Preop Visual Function | Visual Outcome | Preop Neuro Sx Other Than Visual Sx (postop improvements, yes/no) | New Postop Complication | Pt Satisfaction w/ Cosmetic Result |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | M/47 | Anterior clinoidal meningioma | Frontal & temporal | Eyebrow | Yes | GTR (SG I) | No | Normal | Stable | None | No | Good |
| 2 | F/63 | Anterior clinoidal meningioma | Frontal & temporal | Eyebrow | Yes | GTR (SG II) | No | Decreased | Improved | None | No | Good |
| 3 | F/45 | Anterior clinoidal meningioma | Frontal | Eyelid | Yes | GTR (SG II) | No | Normal | Stable | None | No | Good |
| 4 | M/34 | Chondrosarcoma | Temporal | Eyelid | No | GTR | No | Normal | Stable | CN III palsy (yes) | No | Good |
| 5 | M/40 | Trigeminal schwannoma | Temporal | Eyebrow | No | GTR | No | Normal | Stable | CN V2 numbness (yes), CN VI palsy (no), CN III palsy (yes) | Transient aggravation of CN VI palsy | Good |
| 6 | M/54 | Trigeminal schwannoma | Temporal & posterior fossa | Eyebrow | No | GTR | No | Normal | Stable | CN V1, 2, 3 numbness (no) | No | Good |
CN = cranial nerve; EOR = extent of resection; neuro = neurological; pt = patient; SG = Simpson grade; Sx = symptoms.
Six clinical cases treated with the ETOA with SLOR osteotomy (1 case per row). A–D: Anterior clinoidal meningioma. Preoperative MRI revealed a mass in the anterior skull base. The anterior clinoidectomy was performed and Simpson grade I resection was achieved. Complete removal of the tumor was confirmed with postoperative MRI. E–H: Anterior clinoidal meningioma. Preoperative MRI revealed a mass that extended to the frontal and temporal area. ACP was removed and Simpson grade II resection was achieved. Postoperative MRI showed the complete removal of the tumor. I–L: Anterior clinoidal meningioma. The mass in the frontal area was shown with preoperative MRI. ACP was removed and Simpson grade II resection was achieved. Complete removal of the tumor was confirmed with postoperative MRI. M–P: Chondrosarcoma. Preoperative T1-weighted contrast-enhanced MRI revealed a mass that was located in the lateral cavernous sinus and parasella area. The complete removal of the tumor was achieved without temporal lobe retraction under a wide surgical view. Postoperative MRI showed the complete removal of the tumor. Q–T: Trigeminal schwannoma. Preoperative T1-weighted contrast-enhanced MRI revealed a mass that was located in the temporal area. Some portion of the tumor extended to the left side of the optic chiasm. Postoperative MRI showed the complete removal of the tumor. U–X: Trigeminal schwannoma. Preoperative MRI revealed a mass in the lateral cavernous sinus area. The mass was extended to the posterior fossa. The complete removal of the tumor was achieved, as shown on postoperative MRI.
Illustrative case 1. ETOA with SLOR osteotomy for right anterior clinoidal meningioma. A and B: Preoperative MRI revealed a mass in the anterior skull base. C and D: After the SLOR osteotomy was performed, the frontal and temporal bones were drilled out. E–G: After the MOB was coagulated and cut, the anterior clinoidectomy was performed easily and completely. The frontal dura was retracted with a retractor during the procedure. H–J: After a dural incision was made, the tumor marginal dissection was performed. GTR of the tumor was then performed. K and L: The dura origin of the tumor was removed. Simpson grade I resection was achieved. M–O: Complete removal of the ACP and the tumor was confirmed with postoperative CT and MRI. P: An incision below the eyebrow without lateral extension was performed. Copyright Ju Hyung Moon. Published with permission. Figure is available in color online only.
Illustrative case 2. ETOA with SLOR osteotomy for left anterior clinoidal meningioma. A and B: Preoperative MRI revealed a mass in the anterior skull base lesion. C and D: An incision below the eyebrow with 1 cm lateral extension was performed. E: SLOR osteotomy was performed first. F and G: Superior orbital wall, sphenoid bone, and temporal bone were drilled out. H and I: The anterior clinoidectomy was performed easily. The MOB was not cut. J–L: The tumor (dashed line) was directly visible following dural incision that was made from frontal to temporal dura. M–O: The tumor base lesion was cauterized first, and then the tumor marginal dissection was performed. The tumor was removed. The dashed line in panel N shows the area of the ACP dura (original site of tumor). P and Q: Anteromedial extended dural tail was completely removed, and the dural origin of the tumor was cauterized by bipolar cautery. Simpson grade II resection was achieved. R: The SLOR bone flap was fixed with miniplates. S and T: Complete removal of the tumor was confirmed with postoperative MRI. C. = contralateral; F = frontal dura; N = nerve; OFN = olfactory nerve; ON = optic nerve; PO = periorbita; TD = temporal dura. Panels A, B, D, F–H, and J–T: Copyright Ju Hyung Moon. Published with permission. Figure is available in color online only.
All patients with ETOA with SLOR osteotomy were satisfied with the skin incision and surgical outcome, and there were no complications such as CSF leakage or infection.
Illustrative Cases
Case 1
VIDEO 1. Illustrative case 1. ETOA with SLOR osteotomy for right anterior clinoidal meningioma. Copyright Ju Hyung Moon. Published with permission. Click here to view.
After the MOB was cut, the anterior clinoidectomy was performed easily and completely. The tumor was directly identified via a dural incision from the frontal to the temporal dura. The tumor base lesion was coagulated, and then tumor marginal dissection was performed. Gross-total resection (GTR) of the tumor was performed, and then the tumor origin dura was removed. Simpson grade I resection was achieved (Fig. 6). Postoperative MRI showed complete removal of the tumor. The pathological diagnosis was meningothelial meningioma (WHO grade I). The patient recovered without any complication.
Case 2
VIDEO 2. Illustrative case 2. ETOA with SLOR osteotomy for left anterior clinoidal meningioma. Copyright Ju Hyung Moon. Published with permission. Click here to view.
After 3 sides of the anterior process (optic strut, lesser sphenoid wing, orbital roof) were drilled out, the anterior clinoidectomy was performed easily. Because there was sufficient surgical space, the MOB was not cut. After a dural incision was made from the frontal to the temporal dura, we identified the tumor directly. The tumor base lesion was cauterized to handle the vascular supply first, and then the tumor marginal dissection was performed. We identified the posterior margin of the tumor with adhesion to the optic nerve, internal carotid artery (ICA), and middle cerebral artery (MCA). After the brain was carefully retracted with a brain retractor, we performed the delicate dissection. The optic nerve, ICA, and MCA and its branches were not injured during dissection. The anteromedial extended dural tail was completely removed, and the tumor origin dural lesion was cauterized by bipolar cautery. Simpson grade II resection was achieved. The dural layer was sutured with an artificial dura substitute. After the SLOR bone flap was fixed with miniplates, the skin was sutured layer by layer. Complete removal of the tumor and ACP was confirmed with postoperative MRI and CT (Fig. 7). The pathological diagnosis was meningothelial meningioma (WHO grade I). The patient recovered without any complication.
Discussion
Classic craniotomy surgery tends to enable surgical treatment through a smaller craniotomy and minimally invasive surgery.11,13,14 Endoscopic surgery, which started with minimally invasive surgery, is gradually expanding its indication to a wider range of procedure types.15–22 Because of the characteristics of the instrument, endoscopic surgery entails large differences in accessibility and even some differences in the entry site.9 The entry site of the endoscopic endonasal approach, which is performed through the nostrils, is relatively constant, and variation is mainly between uninostril and binostril.23,24 In contrast, research on the ETOA is ongoing for methods to expand entry sites and to secure various entry sites.7,8,25,26 Previous research revealed that even with a small additional entry site space for LOR osteotomy, not only the angle range of the surgical instrument approaching the space but also the surgical freedom to move the instrument was increased.9 Assuming that the existing studies addressed increasing lateral movement, this study investigated a method that can dramatically increase the space by increasing the vertical movement as well (Figs. 1 and 3). One of the main reasons for the limited access area in the existing ETOA is the SOR. The deeper the intracranial depth, the easier it is for the scope to visualize from the bottom to the top view.27,28 However, given that the vertical movement is limited by the SOR, visibility of the more anterior and medial sides, especially in the anterior skull base area close to the skull, is limited (Figs. 1A and 3B).5,29,30
In this study, first through a cadaveric dissection, osteotomy was performed on the SOR and the LOR to examine if the benefits in not only horizontal but also vertical movement can be applied to surgery (Fig. 4). Compared to the conventional ETOA and ETOA with LOR osteotomy, the ETOA with SLOR osteotomy provided an appreciable difference in the vertical attack range and surgical manipulation field (Figs. 3 and 4). In all approaches, the lateral part of the frontal lobe and the temporal lobe areas are relatively easy to access, whereas the accessibility of the anteromedial lesion is decreased in conventional ETOA and ETOA with LOR osteotomy (Fig. 3B). The vertical movement is limited by the SOR, so a wider bony opening of the superior orbital roof and anterior skull base can be difficult.10,31 A view of anteromedial lesions through an endoscope could be possible, but it can be difficult to continue with the surgical procedure due to the limited surgical field. Compared with the ETOA and ETOA with LOR osteotomy, the attack range and surgical freedom of the ETOA with SLOR osteotomy was increased tremendously (Figs. 3 and 4, Supplemental Table 1).
One of the limitations of meningioma surgery via the ETOA is that it is difficult to treat extended tumor masses and the dural origin of the tumor. The surgical accessibility to the olfactory groove, planum sphenoidale, ACP, parasellae, suprasellae, and orbital area is important to permit the selection of the appropriate approach in the treatment of the disease. In the case of anterior clinoidal meningioma, which often extends to planum sphenoidale or tuberculum sellae, it is very difficult to remove such tumors by using conventional ETOA methods. Once the SOR is removed additionally in ETOA with SLOR osteotomy, the approach to the anteromedial skull base can be facilitated by widening the surgical field. ETOA with SLOR osteotomy allows access not only to the medial side of the ipsilateral optic nerve, but also to the medial side of the contralateral optic nerve (Figs. 4Q, 4S, 6I–L, and 7N–Q). Thus, broadening the indication for the ETOA is possible.
Anterior clinoidectomy can be performed via the conventional ETOA and in ETOA with LOR osteotomy. However, gaining access to the upper part of the optic canal is very difficult, so this procedure tends to be implemented with a limited view (Fig. 4I). Additionally, it is difficult to secure space around the ACP (Fig. 4G and I). These nuisances can make the anterior clinoidectomy risky in the ETOA and ETOA with LOR osteotomy. In the case of ETOA with SLOR osteotomy, all 3 structures of the ACP (optic strut, lesser wing of the sphenoid bone, and orbital roof) were easily accessible, so anterior clinoidectomy was performed easily and safely in cadaveric and clinical cases (Figs. 4, 6, and 7). After anterior clinoidectomy, a tumor with extended margin and the tumor origin dural lesion were manipulated successfully in all clinical cases (Simpson grade I or II resection) (Figs. 6 and 7). Given that anterior clinoidectomy, which is an important procedure in anterior cranial fossa surgery, can be easily and safely performed in ETOA with SLOR osteotomy, there will be a variety of diseases that can be treated using this method.
Although the ETOA has many advantages as one of the innovative skull base operative concepts, it still has a limitation of surgical space induced by the nature of the endoscopic surgery. The SLOR osteotomy performed in this study entails an attempt to expand the space in the extradural phase and provide additional space required for delicate procedures in the intradural phase. With this additional procedure, the need for retraction of orbital contents can be reduced significantly, simultaneously allowing for greatly enhanced instrument maneuverability. The expanded working corridor with SLOR osteotomy can be an important advantage in endoscopic surgery for microdissection and neurovascular structures that require very elaborate surgical procedures. The frontal lobe can be an obstacle in the intradural phase of the conventional ETOA as well. In the ETOA, frontal lobe retraction and tumor removal are often performed simultaneously with suction devices or other surgical instruments because there is insufficient space for the application of a retractor. If the frontal lobe can be retracted with alternative instruments in the ETOA, a more delicate dissection and surgical procedure may be possible, similar to open transcranial surgery. Whereas it is difficult to provide space for frontal lobe retraction to allow the use of additional surgical devices in the conventional ETOA or in ETOA with LOR osteotomy, such use is made possible by the ETOA with SLOR osteotomy through the space secured as a result of this procedure (Figs. 4Q, 4S, and 6H–J). As a result of the expanded space secured through brain retraction in the clinical cases, medial expansion of the surgical field and delicate microdissection and surgical procedures were possible without adding force for retraction to suction devices and surgical instruments (Videos 1 and 2). This is thought to play an important role in approaching the frontal base area and helps to reduce injury of adjacent nerves, vessels, and brain tissue, which is of the utmost importance during surgery. It is expected that ETOA with SLOR osteotomy can be applied to the treatment of more diverse disease and target lesions, enabling sophisticated surgical treatment by taking advantage of conventional cranial surgery that permits brain retraction.32,33
Compared to conventional transcranial approaches, including standard supraorbital craniotomy and the pterional or minipterional approach, ETOA with SLOR osteotomy has several advantages for treating lesions around the ACP. Whereas the accessible region of the standard supraorbital craniotomy is limited mainly to the anterior cranial fossa, a wider area in the anterior and middle fossa can be accessed through the ETOA with SLOR osteotomy. With this wider accessibility, removal of tumors extending to the temporal area and extradural anterior clinoidectomy can be accomplished using the ETOA with SLOR osteotomy. Compared to the pterional or minipterional approach, ETOA with SLOR osteotomy is less invasive, the skin incision is smaller, and manipulation of the temporalis muscle is minimal, which permits improved cosmetic results without atrophy of the temporalis muscle. The endoscopic approach, which affords a more panoramic view even with a 0° endoscope, could provide better anatomical visualization of the surgical field and diminish brain retraction and manipulation, which can reduce morbidity in certain patients. The range of visualization could also be improved with the use of an angled endoscope. In addition, the direction of the ETOA with SLOR osteotomy could provide additional ease for anterior clinoidectomy compared to conventional transcranial approaches. The 3 supporting structures of the ACP (optic strut, medial portion of the sphenoid lesser wing, and roof of the optic canal) could be removed in a simple and safe fashion under direct visualization of these structures in the course of approaching the intracranial lesion (Videos 1 and 2). The longitudinal axis of the ACP is parallel to the direction of the approach, which facilitates removal of the ACP while reducing the retraction of adjacent structures.
Nevertheless, ETOA with SLOR osteotomy has some disadvantages compared to transcranial approaches. Depending on the direction of ETOA with SLOR osteotomy, critical neurovascular structures are encountered in the second half of the surgical procedure compared to conventional craniotomy. To prevent injury to posteriorly located neurovascular structures, the anatomy around the tumor should be meticulously investigated using imaging studies including high-resolution MRI and angiography. A careful and delicate technique is necessary during the dissection of the posterior margin of the tumor. The frontal sinus, ethmoid sinus, and sphenoid sinus could be opened during ETOA with SLOR osteotomy in consideration of the area of bone removal. To prevent the opening of these nasal sinuses, tailored craniotomy can be helpful using preoperative imaging studies. If these sinuses are open, the opening is frequently small due to the size of craniotomy, so it can be easily sealed with artificial material or a muscle patch, fat, and sealant. In our study, we encountered the opening of frontal and ethmoid sinuses in 2 cases; both were successfully repaired with fat packing without postoperative CSF leakage. Given the location of the skin incision, there is a possibility of injury to the supraorbital nerve or the frontalis branch of the frontal nerve, although there was no complication related to this in our study. To avoid these injuries, the skin incision should not extend too far in either the medial or lateral direction and skin retraction should be reduced.
Limitations
Given that the clinical implication of the ETOA with SLOR osteotomy cases that are presented here is restricted to small- to medium-sized meningiomas, it is necessary to confirm the applicability to large tumors. In the case of large tumors, the critical neurovascular structures including optic nerve, ICA, MCA, ACA, and their branches can be adhered to or encased by the tumor. Because these critical neurovascular structures are usually located posterior to the tumor for ETOA with SLOR osteotomy, it would be difficult to manipulate them safely. So, this approach may not be feasible for the large tumors encasing the neurovascular structures and should be selected with careful deliberation in such cases. The skin incision and degree of SLOR osteotomy would be tailored specifically according to individual cases. Although there was no cosmetic problem in any clinical cases in this study, the SLOR osteotomy necessitates surgical reconstruction, which may be associated with poor cosmetic outcomes. Careful and delicate bone fixation and wound closure techniques are required to avoid cosmetic issues. In addition, caution should be taken to avoid CSF leakage or infection in cases in which the frontal sinus, ethmoid sinus, or sphenoid sinus might be opened.
Conclusions
We evaluated the clinical feasibility of ETOA with SLOR osteotomy based on a cadaveric study. We believe that ETOA with SLOR osteotomy could be applied to more diverse disease groups for which the conventional ETOA is not appropriate or to cases in which surgical application is difficult. ETOA with SLOR osteotomy might serve as an opportunity to broaden the indication for the ETOA.
Acknowledgments
We deeply appreciate Mr. Jun Ho Kim and Mr. Jong Ho Bang in the Surgical Anatomy Education Center of Yonsei University College of Medicine for their technical support. We also thank Mr. Dong-Su Jang and Ms. Yurie Lee for beautiful work on the figures in this paper. “SevEN” is the Severance Endoscopic Neurosurgery study group. This work was supported by a faculty research grant from the Department of Neurosurgery, Yonsei University College of Medicine (Dr. Moon) and by grants from the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Korea Government (MSIT; grant no. 2018R1C1B5086460 to Dr. J. Lim). The role of the funding organizations was as follows: design and conduct of the study; collection, analysis, and interpretation of the data; and preparation, review, or approval of the manuscript.
Disclosures
The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.
Author Contributions
Conception and design: Moon, J Lim, Sung, Hong. Acquisition of data: all authors. Analysis and interpretation of data: Moon, J Lim, Sung. Drafting the article: Moon, J Lim, Sung. Critically revising the article: Moon, J Lim, Sung. Reviewed submitted version of manuscript: all authors. Approved the final version of the manuscript on behalf of all authors: Moon. Statistical analysis: Moon, J Lim, Sung. Administrative/technical/material support: Moon, J Lim, Sung, Roh, Hong. Study supervision: Moon, J Lim, Sung, Hong.
Supplemental Information
Online-Only Content
Supplemental material is available with the online version of the article.
Supplemental Tables 1 and 2. https://thejns.org/doi/suppl/10.3171/2021.7.JNS21996.
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