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Anatomical Aspects of the Transnasal Endoscopic Access to the Craniovertebral Junction
Journal Pre-proof “Anatomical aspects of the transnasal endoscopic access to the craniovertebral junction.” Shkarubo Alexey Nikolaevich, MD, PhD, Nikolenko Vladimir Nikolaevich, MD, PhD, Chernov Ilia Valerievich, MD, Andreev Dmitry Nikolaevich, MD, Shkarubo Mikhail Alekseevich, MD, Chmutin Kirill Gennadievich, MD, Sinelnikov Mikhail Yegorovich, MD PII:
S1878-8750(19)32440-4
DOI:
https://doi.org/10.1016/j.wneu.2019.09.011
Reference:
WNEU 13309
To appear in:
World Neurosurgery
Received Date: 17 July 2019 Revised Date:
2 September 2019
Accepted Date: 3 September 2019
Please cite this article as: Nikolaevich SA, Nikolaevich NV, Valerievich CI, Nikolaevich AD, Alekseevich SM, Gennadievich CK, Yegorovich SM, “Anatomical aspects of the transnasal endoscopic access to the craniovertebral junction.”, World Neurosurgery (2019), doi: https://doi.org/10.1016/j.wneu.2019.09.011. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Inc. All rights reserved.
Title page. Title: “Anatomical aspects of the transnasal endoscopic access to the craniovertebral junction.”
Authors: Shkarubo Alexey Nikolaevich1,4,5, MD, PhD Nikolenko Vladimir Nikolaevich2,3, MD, PhD Chernov Ilia Valerievich1, MD Andreev Dmitry Nikolaevich1, MD Shkarubo Mikhail Alekseevich1, MD Chmutin Kirill Gennadievich4, MD Sinelnikov Mikhail Yegorovich2, MD
1 - N.N. Burdenko National Medical Research Center of Neurosurgery. Address: 4 TverskayaYamskaya st., 16, Moscow, 125047, Russian Federation. 2 - I.M. Sechenov First Moscow State Medical University of the Ministry of Health of the Russian Federation (Sechenov University). Address: Bolshaya Pirogovskaya st., 19/1, Moscow, 119146, Russian Federation. 3 - Lomonosov Moscow State University. Address: Leninskiye Gori st., 1, Moscow, 119991, Russian Federation. 4 - RUDN University. Address: Mikluho-Maklaya st., 6, Moscow, 117198, Russian Federation 5 - N.N. Priorov Central Institute of Traumatology and Orthopedics. Address: Novospasskiy pereulok, 9, Moscow, 115172, Russian Federation.
Corresponding author: Sinelnikov Mikhail Yegorovich; address - Bolshaya Pirogovskaya st., 6/1, Moscow, 119435, Russian Federation; phone - +79199688587; email: [email protected]. Key words: endoscopic transnasal approach, odontoid resection, craniovertebral junction access, Short title: Anatomy of the transnasal endoscopic neurosurgical access. Financial statement: The work in it’s entirety was funded by the authors. No financial disclosures.
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Previous presentation of work: none.
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Abstract. Interest in endoscopic transnasal access has increased with continued technological advances in endoscopic technology. Objective: The goals of this study were to review the normal anatomy in transnasal endoscopic neurosurgery and outline the anatomical basis for an expanded surgical approach. Defining anatomical aspects of surgical endoscopy helps guide the surgeon by defining normal anatomy of the access vector. Methods: this anatomic study was conducted on 15 adult male cadaver specimen using various microsurgical tools and endoscopic instruments and 1 intraoperative case. The vasculature was injected with colored silicone to aid visualization. Different transnasal approach techniques were used, with angle of endoscope access at 0°, 30°, 45° and 70° accordingly for extensive anatomical mapping. Results: the proximity of critical structures is different in each approach degree, a full understanding of the possible structures to be met during transnasal access is described. As a result of the study, anatomical aspects and important structures were outlined, a surgical protocol was defined for minimal risk access in respect to normal anatomy of the area. Conclusions: thorough knowledge of topographic anatomy of the craniovertebral junction is required for performing minimal-risk surgical intervention in this region. It is important to know all anatomical aspects of the transnasal approach in order to reduce the risk of damage to vital structures. Transnasal endoscopic surgery of the craniovertebral junction is a relatively new direction in neurosurgery, therefore anatomical studies such as the one described in this article are extremely important for development of this access method.
Key words: endoscopic transnasal approach, odontoid resection, craniovertebral junction access, transnasal anatomy.
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Text. Introduction. The craniovertebral junction, which includes the occipital bone, first cervical (C1) and second cervical (C2) vertebrae, as well as the ligamentous apparatus and neurovascular structures is a complex transitional zone between the skull and the upper cervical spine which provides stability and movement of the head
1-4
. Developmental abnormalities,
degenerative diseases, traumatic injuries, and oncological lesions (extradural and intradural) can cause anterior compression of the upper spinal cord and brain stem structures in this area 5. In such cases, anterior access has been the gold standard for approaching the pathological lesions, as it provides direct access to the lower sections of the clivus and the C1- C2 segment of the spine without the need for retraction of neurovascular structures 5. The standard access routes for such interventions are the transoral, transcervical, and transnasal approaches
6,7,8
. The transoral approach is most frequently used and is well described
in literature. However, with the latest technological advances in endoscopic technology, interest in the endoscopic transnasal approach has increased. The anatomical basis for clinical application of this method has been outlined in several studies (Table 1)
1,2,3,5,9-14.
With prior
knowledge of anatomical access and perspectives of this access, we developed and applied clinically a method for expansion of surgical access to facilitate advances in transnalas neurosurgery. Understanding the anatomical aspects of an expanded transnasal access with knowledge of margins and structures met upon intervention provides a basis for future development of important reconstructive procedures and safer interventions. When performing the endoscopic transnasal access, the surgical field is limited by the bony structures of the region (the nasal and palatine bones), which form two arbitrary lines defining the triangular shape of the surgical corridor. The nasopalatine line (connecting the rhinion with the posterior edge of the hard palate – Kassam line), and the nasoclival line (connecting the rhinion and the lower section of the clivus – Shkarubo line)15. This surgical corridor provides access to the entire anterior region of the craniovertebral junction in the median plane 16. Trepanation of the posterior region of the hard palate allows for caudal extension of this approach. Superior extension is possible through trepanation of the lower regions of the clivus (Figure 1). At the same time, when considering access to the C1-C2 vertebrae, the surgical field
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is confined by the eustachian tubes, the medial pterygoid processes, and the clinoid and supraclinoid segments of the internal carotid arteries laterally. Since the transnasal access to the odontoid process is performed through a small incision in the nasopharynx, the risk of infectious complications is considerably reduced due to the lack of exposure to saliva and oropharyngeal bacteria
17,18
. Another advantage, in comparison to the
transoral approach, is a vertical superior to anterior access direction, which allows for better control of bone trepanation stages and better visualization of the ligamentous apparatus of the odontoid process 19. Materials and methods. The study was conducted on 15 adult male cadavers and applied clinically in 1 intraoperative case. In order to visualize the arteries and veins, the vascular structures were infused with colored silicone 20. Tissue dissection was performed using various microsurgical tools with consecutive use of 0°, 30°, 45° and 70° endoscopes. The anatomical findings were mapped and documented accordingly. In each case a transnasal approach to the craniovertebral junction was performed. The structures encountered during the procedure were analyzed and documented. Surgical margins were assessed through subjective analysis and objective capabilities of the endoscope. Results. As a result of the conducted study, important anatomical aspects of surgical intervention during the transnasal endoscopic approach were outlined in regard to existing anatomical basis and expanded access technique. One of the most important aspects of the transnasal endoscopic approach to the structures of the craniovertebral junction (both bony and neurovascular) is the resection of the odontoid process, which allows for anterior decompression of the brain stem in cases of odontoid process invagination and for optimal visualization of the underlying subdural structures on this level (Figure 2). The endoscopic transnasal approach to the craniovertebral junction requires a lower trajectory compared with approaches to the sella turcica 9. Access to the C2 odontoid process begins with the dissection of the soft tissues of the posterior nasopharyngeal wall in the projection of the anterior arch of the C1 vertebra. For this purpose it is recommended to use monopolar cautery and microscissors (Figure 3). Thin prevertebral muscles have an avascular zone in the form of a band along the median line. It can be used for their retraction in the lateral direction or their transposition in the form of a U-shaped flap with subsequent closure of the defect. For adequate access extension, resection of the Page 3 of 22
posterior portion of the nasal septum is advisable, and provides sufficiently more area for surgical manipulation. After decortication of the anterior arch of C1 vertebra, the trepanation stage begins. Trepanation should be performed using a high-speed drill with a diameter of 2-2.5 mm. The linear cuts should be carried out strictly vertically with a maximum offset from the median line within 10-14mm in each direction so as not to damage the vertebral arteries exiting the transverse foramen and lying in the sulcus arteriae vertebralis (Figure 4). It is advisable to remove a fragment of the anterior arch of C1 vertebra in a single block (16-20 mm in size, taking into account the width of the cut lines) for its subsequent use for anterior stabilization of C0-C1 21
. The ligaments between the anterior surface of the odontoid process and the posterior surface
of the anterior arch of C1 vertebra are dissected and cauterized using monopolar cautery. The next step involves decortication of the C2 odontoid process and the upper portion of the C2 body. Then the odontoid process is transected at its base, as close as possible to the body of the C2 vertebra (Figure 5). A step-by-step trepanation of the odontoid process is performed, as it is drilled from the inside towards its posterior cortical wall, which is thinned to the thickness of an eggshell and fragmented using Kerrison rongeurs or separated in a single block from the underlying dura mater. In cases of odontoid process invagination, its removal must be carried out in an extremely delicate manner (due to thinning of the underlying dura mater) in order to avoid its perforation which can lead to cerebrospinal fluid leakage, requiring reconstruction of the resulting defect. The C2 odontoid process is held by a complex ligamentous apparatus, formed by the pterygoid, apical, and cruciate ligaments. The pterygoid ligaments are thin fibrous structures that connect the odontoid process to the occipital condyles. The apical ligament is located at the midline and connects the apex of the dens to the edge of the foramen magnum of the occipital bone. For an effective removal of the odontoid process, all the above ligaments must be transected. If revision of the subdural space (when removing intradural tumors) is necessary, the dura mater is opened using microscissors (Figure 6). During dissection of the dura mater, the underlying neurovascular structures are exposed (Figures 7, 8). A schematic of important surgical access steps is provided in Figure 9.
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Intraoperative visualization was carried out on a female twenty-two-year-old patient, who was admitted with platybasia, C2 odontoid process invagination, and medulla oblongata compression. The first stage of surgical treatment in the form of occipitocervical fusion was performed at another institution in March of 2018. Clinically, the patient presented with recurrent breathing impairment, finger numbness and severe cranialgia. A transnasal endoscopic removal of the invaginated odontoid process was performed with decompression of brain stem structures, the described anatomical structures were assessed intraoperatively and aided in the correct access through transnasal endoscopic approach. Signs of insignificant transient bulbar disorders were noted after surgery. The patient was discharged in satisfactory condition 12 days after surgery. At six month after surgery the above symptoms fully resolved. Discussion. As transnasal endosocopic surgery of the craniovertebral junction is a relatively new approach, the question of possible intraoperative and postoperative complications of this procedure remains quite relevant. The most frequent intraoperative complication is hemorrhage. Therefore, one of the central problems associated with endonasal approaches has to do with achieving adequate and reliable hemostasis. Modern hemostatic agents and instruments intended for endoscopic endonasal surgery, including diamond burs and bipolar cautery, as well as irrigation with a warm solution, allow for effective hemostasis
22
. An equally important
problem is intraoperative cerebrospinal fluid leakage. In cases of removal of extradural lesions, intraoperative CSF leakage is most frequently associated with limitations of two-dimensional visualization, characteristic of endoscopic techniques. In contrast, microscopic techniques used in transoral approaches utilize three-dimensional visualization
23
. According to current data, the
frequency of intraoperative and postoperative CSF leaks in transnasal endoscopic surgery of the craniovertebral junction is approximately 12%, which leads to meningitis in only 1-2% cases due to the use of modern antibiotics 23, 24. In endoscopic transnasal surgery, reconstruction of the bone/dural defect of the craniovertebral junction and the clivus is a challenging task not only because of the dimensions of the defect, but also because of the high pressure of the cerebrospinal fluid, lack of supporting structures, and the force of gravity
25
. The main approach to bone/dural reconstruction in this
region is a combination of free flap transplantation (fat and fascia) and transplantation of flaps on
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feeding pedicles. The approach that is used in the majority of cases is the “triple F” approach (fat, fascia, flap) technique 22,26. Other complications that can be expected in the postoperative period are transient velopharyngeal insufficiency, which is manifested by swallowing and speech disorders (observed in 6% of patients), postoperative nasal bleeding (up to 2% of cases), and difficulty in breathing, which necessitates tracheostomy (up to 2% of cases) 23,24,27. Stabilization of the C1-C2 vertebral segment is one of the most important aspects following transnasal odontoidectomy. Posterior stabilization is most commonly used
28
, but
alternatively, anterior stabilization of the C1-C2 segment with metallic constructions has been clinically applied
29,30,31,32,33,34
. The use of bone autotranplants have also been described as
methods for successful stabilization of the C1-C2 vertebral segment
35
. Stabilization of the
craniovertebral junction with the use of autologous bone grafts is a perspective method, but requires development of specialized instruments for the insertion and fixation of the autologous transplants. Undeniably, transnasal endoscopic surgery of the craniovertebral junction has some disadvantages such as increased duration of surgery and a longer learning curve
36-39
. However,
the use of the proposed technique makes it possible to expand the possibilities of surgical intervention in this complex region and to ensure excellent results of surgical treatment, which are comparable to transoral microsurgical techniques. The main advantages of the transnasal approach over transoral are smaller incision, less chance of infection, no need for oral retraction, less chance of oropharyngeal compromise. Conclusions. Thorough knowledge and understanding of topographic anatomy of the craniovertebral junction is a prerequisite for performing surgical interventions in this region. The proximity of critical structures (brainstem, great vessels) is associated with extremely high risks when performing surgical removal of various pathological lesions in this area. As transnasal endoscopic surgery of the craniovertebral junction is a relatively new surgical direction, anatomical studies such as the one described in this article are in high demand and extremely important for practicing neurosurgeons and for further development of this area of expertise. The transnasal approach has anatomical basis for safe access and provides adequate visualization with possibility of expansion without damage to important structures. The minimality of the Page 6 of 22
incision and intervention in this approach dictates the high risks due to access constriction. Despite notable disadvantages, we believe that further development of the transnasal approach can yield better overall outcome for patients requiring anterior craniocervical surgery.
Disclosure. The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper
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Figure legend.
Figure 1. Accessibility zone of endoscopic transnasal access to the C2 vertebra. 1 – nasopalatine line (Kassam line), 2 – nasoclival line, 3 – nasal bone, 4 – odontoid process, 5 - anterior arch of C2 vertebra, 6 - clivus, 7 - angle between the nasopalatine and nasoclival lines 14-160, 8 - angle between the nasopalatine and nasoclival lines after accessibility zone extension 23-250, 9 - hard palate.
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Figure 2. Outline of the transnasal approach method (A – graphic representation of endoscopic transnasal resection of the odontoid process; B - view of the bony structures of the craniovertebral junction before trepanation of the anterior arch of C1 vertebra and C2 odontoid process; C - resection of the front arch of C1 vertebra; D - resection of C2 odontoid process, lower portions of the clivus, upper portions of the C2 vertebral body).
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Figure 3. Stages of the access to the anterior arch of the C1 vertebra (A – general view of the nasopharynx through the right choana: 1 – posterior nasal septum, 2 – posterior wall of nasopharynx, 3 – posterior portions of hard palate; B – the beginning of the dissection of the posterior wall of the nasopharynx: 1 – microscissors, 2 – Blakesley forceps; C – continued dissection, extended accessibility after posterior nasal septum removal: 1 – remnants of the posterior nasal septum, 2 – microscissors, 3 – Blakesley forceps; D – continued dissection: 1 – Blakesley forceps; E – continued dissection: 1 - prevertebral muscles are exposed; F – finished dissection of soft tissues of the posterior nasopharynx: 1 – front arch of C1 vertebra).
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Figure 4. The stages of trepanation of the anterior arch of C1 vertebra using a highspeed drill and Kerrison rongeurs (A – general view of the bony structures of the anterior craniovertebral junction: 1 - anterior arch of C1 vertebra, 2 - C2 odontoid process, 3 - body of C2 vertebra, 4 - lower portion of the clivus; B - beginning of trepanation of the anterior arch of C1 vertebra: 1 - high-speed drill, 2- line of cut, 3 - C2 odontoid process; C- continued trepanation of the anterior arch of C1 vertebra; D – finishing stage of trepanation of the anterior arch of C1 vertebra and its extraction using Kerrison rongeurs).
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Figure 5. Stages of C2 odontoid process trepanation using a high-speed drill (A beginning of odontoid process trepanation: 1 - high-speed drill, 2 – odontoid process, 3 - body of C2 vertebra; B - continued trepanation of the odontoid process: 1 – odontoid process, 2 - line of the cut, 3 - C2 vertebral body; C – separation and extraction of the odontoid process from the underlying tissues: 1 – odontoid process; D - internal decancellation of the odontoid process: 1 – odontoid process, 2 - lower portion of the clivus, 3 - high-speed drill; E - separation of the odontoid process from the dura mater: 1 – Kerrison rongeurs, 2 – remnants of the odontoid process; F - continued separation of the odontoid process from the dura mater: 1 – remnants of the odontoid process).
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Figure 6. The stages of dura mater opening (A - dissection of the dura mater: 1 - dura mater, 2 - microscissors; B - general view of the subdural structures after opening the dura mater at the level of C1-C2 vertebrae: 1 - spinal cord, 2 - vertebral arteries).
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Figure 7. Intradural neurovascular structures at the level of C1-C2 vertebrae, the spinal cord, both vertebral arteries, and spinal nerves are visualized (A - general view of subdural structures after opening the dura mater at the level of C1-C2 vertebrae: 1 spinal cord, 2 - vertebral arteries, 3 - anterior spinal artery; B - view of subdural structures at the level of C1-C2 vertebrae: 1 - spinal cord, 2 - vertebral arteries, 3 spinal nerve roots; C - view of the left half of the spinal canal: 1 - cerebellar tonsil, 2 left vertebral artery; D - view of the left half of the spinal canal: 1 - cerebellar tonsil, 2 spinal nerves, 3 - posterior inferior cerebellar artery; E - point of the entrance of the left vertebral artery into the subdural space: 1 - left vertebral artery; F - point of the entrance of the left vertebral artery into the subdural space: 1 - left vertebral artery).
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Figure 8. Intradural neurovascular structures at the level of C1-C2 vertebrae (A - view of subdural structures at the level of C1-C2 vertebrae: 1 - spinal cord, 2 - right vertebral artery; B - view of subdural structures at the level of C1-C2 vertebrae: 1 - spinal cord, 2 - right vertebral artery, 3 – cerebellar tonsil).
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Figure 9. Flow chart of transnasal access to craniovertebral junction with important additions on expanded access.
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Table 1. Important anatomical studies on the craniovertebral junction
First author Lopez AJ Menezes AH
Pacca P
Jhawar SS
Cavallo LM
Pillai P
Zhong S
Andrew K. Chan
Abuzayed B
Journal Neurosurg Focus. 2015
Title Anatomy and biomechanics of the craniovertebral junction Childs Nerv Syst. 2008 Anatomy and biomechanics of normal craniovertebral junction and biomechanics of stabilization Acta Neurochir Suppl. The Endoscopic Endonasal Approach to 2019 Craniovertebral Junction Pathologies: Surgical Skills and Anatomical Study J Craniovertebr Junction Craniovertebral junction 360°: A combined microscopic and endoscopic anatomical Spine. 2016 study Childs Nerv Syst. 2007 The extended endoscopic endonasal approach to the clivus and cranio-vertebral junction: anatomical study Neurosurgery. 2009 Endoscopic image-guided transoral approach to the craniovertebral junction: an anatomic study comparing surgical exposure and surgical freedom obtained with the endoscope and the operating microscope. J Craniofac Surg. 2018 Anatomic Study of Craniocervical Junction and Its Surrounding Structures in Endoscopic Transoral-Transpharyngeal Approach Neurosurg Focus. 2016 The endoscopic transoral approach to the craniovertebral junction: an anatomical study with a clinical example Turk Neurosurg. 2009 Extended endoscopic endonasal approach to the anterior cranio-vertebral junction: anatomic study.
Page 22 of 22
First author Lopez AJ
Journal Neurosurg Focus. 2015
Title Anatomy and biomechanics of the craniovertebral junction Menezes AH Childs Nerv Syst. 2008 Anatomy and biomechanics of normal craniovertebral junction and biomechanics of stabilization Pacca P Acta Neurochir Suppl. The Endoscopic Endonasal Approach to 2019 Craniovertebral Junction Pathologies: Surgical Skills and Anatomical Study Jhawar SS J Craniovertebr Junction Craniovertebral junction 360°: A combined microscopic and endoscopic anatomical Spine. 2016 study Cavallo LM Childs Nerv Syst. 2007 The extended endoscopic endonasal approach to the clivus and cranio-vertebral junction: anatomical study Pillai P Neurosurgery. 2009 Endoscopic image-guided transoral approach to the craniovertebral junction: an anatomic study comparing surgical exposure and surgical freedom obtained with the endoscope and the operating microscope. Zhong S J Craniofac Surg. 2018 Anatomic Study of Craniocervical Junction and Its Surrounding Structures in Endoscopic Transoral-Transpharyngeal Approach Andrew K. Chan Neurosurg Focus. 2016 The endoscopic transoral approach to the craniovertebral junction: an anatomical study with a clinical example Abuzayed B Turk Neurosurg. 2009 Extended endoscopic endonasal approach to the anterior cranio-vertebral junction: anatomic study. TABLE 1. Imporant studies on anatomical aspects of transnasal access to craniovertebral junction.
Page 1 of 1
Abbreviations list: C1 – First cervical vertebrae C2 – Second cervical vertebrae CSF – cerebrospinal fluid
S1878-8750(19)32440-4
DOI:
https://doi.org/10.1016/j.wneu.2019.09.011
Reference:
WNEU 13309
To appear in:
World Neurosurgery
Received Date: 17 July 2019 Revised Date:
2 September 2019
Accepted Date: 3 September 2019
Please cite this article as: Nikolaevich SA, Nikolaevich NV, Valerievich CI, Nikolaevich AD, Alekseevich SM, Gennadievich CK, Yegorovich SM, “Anatomical aspects of the transnasal endoscopic access to the craniovertebral junction.”, World Neurosurgery (2019), doi: https://doi.org/10.1016/j.wneu.2019.09.011. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Inc. All rights reserved.
Title page. Title: “Anatomical aspects of the transnasal endoscopic access to the craniovertebral junction.”
Authors: Shkarubo Alexey Nikolaevich1,4,5, MD, PhD Nikolenko Vladimir Nikolaevich2,3, MD, PhD Chernov Ilia Valerievich1, MD Andreev Dmitry Nikolaevich1, MD Shkarubo Mikhail Alekseevich1, MD Chmutin Kirill Gennadievich4, MD Sinelnikov Mikhail Yegorovich2, MD
1 - N.N. Burdenko National Medical Research Center of Neurosurgery. Address: 4 TverskayaYamskaya st., 16, Moscow, 125047, Russian Federation. 2 - I.M. Sechenov First Moscow State Medical University of the Ministry of Health of the Russian Federation (Sechenov University). Address: Bolshaya Pirogovskaya st., 19/1, Moscow, 119146, Russian Federation. 3 - Lomonosov Moscow State University. Address: Leninskiye Gori st., 1, Moscow, 119991, Russian Federation. 4 - RUDN University. Address: Mikluho-Maklaya st., 6, Moscow, 117198, Russian Federation 5 - N.N. Priorov Central Institute of Traumatology and Orthopedics. Address: Novospasskiy pereulok, 9, Moscow, 115172, Russian Federation.
Corresponding author: Sinelnikov Mikhail Yegorovich; address - Bolshaya Pirogovskaya st., 6/1, Moscow, 119435, Russian Federation; phone - +79199688587; email: [email protected]. Key words: endoscopic transnasal approach, odontoid resection, craniovertebral junction access, Short title: Anatomy of the transnasal endoscopic neurosurgical access. Financial statement: The work in it’s entirety was funded by the authors. No financial disclosures.
Page 1 of 2
Previous presentation of work: none.
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Abstract. Interest in endoscopic transnasal access has increased with continued technological advances in endoscopic technology. Objective: The goals of this study were to review the normal anatomy in transnasal endoscopic neurosurgery and outline the anatomical basis for an expanded surgical approach. Defining anatomical aspects of surgical endoscopy helps guide the surgeon by defining normal anatomy of the access vector. Methods: this anatomic study was conducted on 15 adult male cadaver specimen using various microsurgical tools and endoscopic instruments and 1 intraoperative case. The vasculature was injected with colored silicone to aid visualization. Different transnasal approach techniques were used, with angle of endoscope access at 0°, 30°, 45° and 70° accordingly for extensive anatomical mapping. Results: the proximity of critical structures is different in each approach degree, a full understanding of the possible structures to be met during transnasal access is described. As a result of the study, anatomical aspects and important structures were outlined, a surgical protocol was defined for minimal risk access in respect to normal anatomy of the area. Conclusions: thorough knowledge of topographic anatomy of the craniovertebral junction is required for performing minimal-risk surgical intervention in this region. It is important to know all anatomical aspects of the transnasal approach in order to reduce the risk of damage to vital structures. Transnasal endoscopic surgery of the craniovertebral junction is a relatively new direction in neurosurgery, therefore anatomical studies such as the one described in this article are extremely important for development of this access method.
Key words: endoscopic transnasal approach, odontoid resection, craniovertebral junction access, transnasal anatomy.
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Text. Introduction. The craniovertebral junction, which includes the occipital bone, first cervical (C1) and second cervical (C2) vertebrae, as well as the ligamentous apparatus and neurovascular structures is a complex transitional zone between the skull and the upper cervical spine which provides stability and movement of the head
1-4
. Developmental abnormalities,
degenerative diseases, traumatic injuries, and oncological lesions (extradural and intradural) can cause anterior compression of the upper spinal cord and brain stem structures in this area 5. In such cases, anterior access has been the gold standard for approaching the pathological lesions, as it provides direct access to the lower sections of the clivus and the C1- C2 segment of the spine without the need for retraction of neurovascular structures 5. The standard access routes for such interventions are the transoral, transcervical, and transnasal approaches
6,7,8
. The transoral approach is most frequently used and is well described
in literature. However, with the latest technological advances in endoscopic technology, interest in the endoscopic transnasal approach has increased. The anatomical basis for clinical application of this method has been outlined in several studies (Table 1)
1,2,3,5,9-14.
With prior
knowledge of anatomical access and perspectives of this access, we developed and applied clinically a method for expansion of surgical access to facilitate advances in transnalas neurosurgery. Understanding the anatomical aspects of an expanded transnasal access with knowledge of margins and structures met upon intervention provides a basis for future development of important reconstructive procedures and safer interventions. When performing the endoscopic transnasal access, the surgical field is limited by the bony structures of the region (the nasal and palatine bones), which form two arbitrary lines defining the triangular shape of the surgical corridor. The nasopalatine line (connecting the rhinion with the posterior edge of the hard palate – Kassam line), and the nasoclival line (connecting the rhinion and the lower section of the clivus – Shkarubo line)15. This surgical corridor provides access to the entire anterior region of the craniovertebral junction in the median plane 16. Trepanation of the posterior region of the hard palate allows for caudal extension of this approach. Superior extension is possible through trepanation of the lower regions of the clivus (Figure 1). At the same time, when considering access to the C1-C2 vertebrae, the surgical field
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is confined by the eustachian tubes, the medial pterygoid processes, and the clinoid and supraclinoid segments of the internal carotid arteries laterally. Since the transnasal access to the odontoid process is performed through a small incision in the nasopharynx, the risk of infectious complications is considerably reduced due to the lack of exposure to saliva and oropharyngeal bacteria
17,18
. Another advantage, in comparison to the
transoral approach, is a vertical superior to anterior access direction, which allows for better control of bone trepanation stages and better visualization of the ligamentous apparatus of the odontoid process 19. Materials and methods. The study was conducted on 15 adult male cadavers and applied clinically in 1 intraoperative case. In order to visualize the arteries and veins, the vascular structures were infused with colored silicone 20. Tissue dissection was performed using various microsurgical tools with consecutive use of 0°, 30°, 45° and 70° endoscopes. The anatomical findings were mapped and documented accordingly. In each case a transnasal approach to the craniovertebral junction was performed. The structures encountered during the procedure were analyzed and documented. Surgical margins were assessed through subjective analysis and objective capabilities of the endoscope. Results. As a result of the conducted study, important anatomical aspects of surgical intervention during the transnasal endoscopic approach were outlined in regard to existing anatomical basis and expanded access technique. One of the most important aspects of the transnasal endoscopic approach to the structures of the craniovertebral junction (both bony and neurovascular) is the resection of the odontoid process, which allows for anterior decompression of the brain stem in cases of odontoid process invagination and for optimal visualization of the underlying subdural structures on this level (Figure 2). The endoscopic transnasal approach to the craniovertebral junction requires a lower trajectory compared with approaches to the sella turcica 9. Access to the C2 odontoid process begins with the dissection of the soft tissues of the posterior nasopharyngeal wall in the projection of the anterior arch of the C1 vertebra. For this purpose it is recommended to use monopolar cautery and microscissors (Figure 3). Thin prevertebral muscles have an avascular zone in the form of a band along the median line. It can be used for their retraction in the lateral direction or their transposition in the form of a U-shaped flap with subsequent closure of the defect. For adequate access extension, resection of the Page 3 of 22
posterior portion of the nasal septum is advisable, and provides sufficiently more area for surgical manipulation. After decortication of the anterior arch of C1 vertebra, the trepanation stage begins. Trepanation should be performed using a high-speed drill with a diameter of 2-2.5 mm. The linear cuts should be carried out strictly vertically with a maximum offset from the median line within 10-14mm in each direction so as not to damage the vertebral arteries exiting the transverse foramen and lying in the sulcus arteriae vertebralis (Figure 4). It is advisable to remove a fragment of the anterior arch of C1 vertebra in a single block (16-20 mm in size, taking into account the width of the cut lines) for its subsequent use for anterior stabilization of C0-C1 21
. The ligaments between the anterior surface of the odontoid process and the posterior surface
of the anterior arch of C1 vertebra are dissected and cauterized using monopolar cautery. The next step involves decortication of the C2 odontoid process and the upper portion of the C2 body. Then the odontoid process is transected at its base, as close as possible to the body of the C2 vertebra (Figure 5). A step-by-step trepanation of the odontoid process is performed, as it is drilled from the inside towards its posterior cortical wall, which is thinned to the thickness of an eggshell and fragmented using Kerrison rongeurs or separated in a single block from the underlying dura mater. In cases of odontoid process invagination, its removal must be carried out in an extremely delicate manner (due to thinning of the underlying dura mater) in order to avoid its perforation which can lead to cerebrospinal fluid leakage, requiring reconstruction of the resulting defect. The C2 odontoid process is held by a complex ligamentous apparatus, formed by the pterygoid, apical, and cruciate ligaments. The pterygoid ligaments are thin fibrous structures that connect the odontoid process to the occipital condyles. The apical ligament is located at the midline and connects the apex of the dens to the edge of the foramen magnum of the occipital bone. For an effective removal of the odontoid process, all the above ligaments must be transected. If revision of the subdural space (when removing intradural tumors) is necessary, the dura mater is opened using microscissors (Figure 6). During dissection of the dura mater, the underlying neurovascular structures are exposed (Figures 7, 8). A schematic of important surgical access steps is provided in Figure 9.
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Intraoperative visualization was carried out on a female twenty-two-year-old patient, who was admitted with platybasia, C2 odontoid process invagination, and medulla oblongata compression. The first stage of surgical treatment in the form of occipitocervical fusion was performed at another institution in March of 2018. Clinically, the patient presented with recurrent breathing impairment, finger numbness and severe cranialgia. A transnasal endoscopic removal of the invaginated odontoid process was performed with decompression of brain stem structures, the described anatomical structures were assessed intraoperatively and aided in the correct access through transnasal endoscopic approach. Signs of insignificant transient bulbar disorders were noted after surgery. The patient was discharged in satisfactory condition 12 days after surgery. At six month after surgery the above symptoms fully resolved. Discussion. As transnasal endosocopic surgery of the craniovertebral junction is a relatively new approach, the question of possible intraoperative and postoperative complications of this procedure remains quite relevant. The most frequent intraoperative complication is hemorrhage. Therefore, one of the central problems associated with endonasal approaches has to do with achieving adequate and reliable hemostasis. Modern hemostatic agents and instruments intended for endoscopic endonasal surgery, including diamond burs and bipolar cautery, as well as irrigation with a warm solution, allow for effective hemostasis
22
. An equally important
problem is intraoperative cerebrospinal fluid leakage. In cases of removal of extradural lesions, intraoperative CSF leakage is most frequently associated with limitations of two-dimensional visualization, characteristic of endoscopic techniques. In contrast, microscopic techniques used in transoral approaches utilize three-dimensional visualization
23
. According to current data, the
frequency of intraoperative and postoperative CSF leaks in transnasal endoscopic surgery of the craniovertebral junction is approximately 12%, which leads to meningitis in only 1-2% cases due to the use of modern antibiotics 23, 24. In endoscopic transnasal surgery, reconstruction of the bone/dural defect of the craniovertebral junction and the clivus is a challenging task not only because of the dimensions of the defect, but also because of the high pressure of the cerebrospinal fluid, lack of supporting structures, and the force of gravity
25
. The main approach to bone/dural reconstruction in this
region is a combination of free flap transplantation (fat and fascia) and transplantation of flaps on
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feeding pedicles. The approach that is used in the majority of cases is the “triple F” approach (fat, fascia, flap) technique 22,26. Other complications that can be expected in the postoperative period are transient velopharyngeal insufficiency, which is manifested by swallowing and speech disorders (observed in 6% of patients), postoperative nasal bleeding (up to 2% of cases), and difficulty in breathing, which necessitates tracheostomy (up to 2% of cases) 23,24,27. Stabilization of the C1-C2 vertebral segment is one of the most important aspects following transnasal odontoidectomy. Posterior stabilization is most commonly used
28
, but
alternatively, anterior stabilization of the C1-C2 segment with metallic constructions has been clinically applied
29,30,31,32,33,34
. The use of bone autotranplants have also been described as
methods for successful stabilization of the C1-C2 vertebral segment
35
. Stabilization of the
craniovertebral junction with the use of autologous bone grafts is a perspective method, but requires development of specialized instruments for the insertion and fixation of the autologous transplants. Undeniably, transnasal endoscopic surgery of the craniovertebral junction has some disadvantages such as increased duration of surgery and a longer learning curve
36-39
. However,
the use of the proposed technique makes it possible to expand the possibilities of surgical intervention in this complex region and to ensure excellent results of surgical treatment, which are comparable to transoral microsurgical techniques. The main advantages of the transnasal approach over transoral are smaller incision, less chance of infection, no need for oral retraction, less chance of oropharyngeal compromise. Conclusions. Thorough knowledge and understanding of topographic anatomy of the craniovertebral junction is a prerequisite for performing surgical interventions in this region. The proximity of critical structures (brainstem, great vessels) is associated with extremely high risks when performing surgical removal of various pathological lesions in this area. As transnasal endoscopic surgery of the craniovertebral junction is a relatively new surgical direction, anatomical studies such as the one described in this article are in high demand and extremely important for practicing neurosurgeons and for further development of this area of expertise. The transnasal approach has anatomical basis for safe access and provides adequate visualization with possibility of expansion without damage to important structures. The minimality of the Page 6 of 22
incision and intervention in this approach dictates the high risks due to access constriction. Despite notable disadvantages, we believe that further development of the transnasal approach can yield better overall outcome for patients requiring anterior craniocervical surgery.
Disclosure. The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper
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36. Hankinson TC, Grunstein E, Gardner P, Spinks TJ, Anderson RCE. Transnasal odontoid resection followed by posterior decompression and occipitocervical fusion in children with Chiari malformation Type I and ventral brainstem compression. J Neurosurg Pediatr. 2010;5:549-553. 37. Lee A, Sommer D, Reddy K, Murty N, Gunnarsson T. Endoscopic transnasal approach to the craniocervical junction. Skull Base. 2010;20:199-205. 38. Patel AJ, Boatey J, Muns J, Bollo RJ, Whitehead WE, Giannoni CM, et al. Endoscopic endonasal odontoidectomy in a child with chronic type 3 atlantoaxial rotatory fixation: case report and literature review. Childs Nerv Syst. 2012;28:1971-1975. 39. Yu Y, Wang X, Zhang X, Hu F, Gu Y, Xie T, et al. Endoscopic transnasal odontoidectomy to treat basilar invagination with congenital osseous malformations. Eur Spine J. 2013;22:1127-1136.
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Figure legend.
Figure 1. Accessibility zone of endoscopic transnasal access to the C2 vertebra. 1 – nasopalatine line (Kassam line), 2 – nasoclival line, 3 – nasal bone, 4 – odontoid process, 5 - anterior arch of C2 vertebra, 6 - clivus, 7 - angle between the nasopalatine and nasoclival lines 14-160, 8 - angle between the nasopalatine and nasoclival lines after accessibility zone extension 23-250, 9 - hard palate.
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Figure 2. Outline of the transnasal approach method (A – graphic representation of endoscopic transnasal resection of the odontoid process; B - view of the bony structures of the craniovertebral junction before trepanation of the anterior arch of C1 vertebra and C2 odontoid process; C - resection of the front arch of C1 vertebra; D - resection of C2 odontoid process, lower portions of the clivus, upper portions of the C2 vertebral body).
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Figure 3. Stages of the access to the anterior arch of the C1 vertebra (A – general view of the nasopharynx through the right choana: 1 – posterior nasal septum, 2 – posterior wall of nasopharynx, 3 – posterior portions of hard palate; B – the beginning of the dissection of the posterior wall of the nasopharynx: 1 – microscissors, 2 – Blakesley forceps; C – continued dissection, extended accessibility after posterior nasal septum removal: 1 – remnants of the posterior nasal septum, 2 – microscissors, 3 – Blakesley forceps; D – continued dissection: 1 – Blakesley forceps; E – continued dissection: 1 - prevertebral muscles are exposed; F – finished dissection of soft tissues of the posterior nasopharynx: 1 – front arch of C1 vertebra).
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Figure 4. The stages of trepanation of the anterior arch of C1 vertebra using a highspeed drill and Kerrison rongeurs (A – general view of the bony structures of the anterior craniovertebral junction: 1 - anterior arch of C1 vertebra, 2 - C2 odontoid process, 3 - body of C2 vertebra, 4 - lower portion of the clivus; B - beginning of trepanation of the anterior arch of C1 vertebra: 1 - high-speed drill, 2- line of cut, 3 - C2 odontoid process; C- continued trepanation of the anterior arch of C1 vertebra; D – finishing stage of trepanation of the anterior arch of C1 vertebra and its extraction using Kerrison rongeurs).
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Figure 5. Stages of C2 odontoid process trepanation using a high-speed drill (A beginning of odontoid process trepanation: 1 - high-speed drill, 2 – odontoid process, 3 - body of C2 vertebra; B - continued trepanation of the odontoid process: 1 – odontoid process, 2 - line of the cut, 3 - C2 vertebral body; C – separation and extraction of the odontoid process from the underlying tissues: 1 – odontoid process; D - internal decancellation of the odontoid process: 1 – odontoid process, 2 - lower portion of the clivus, 3 - high-speed drill; E - separation of the odontoid process from the dura mater: 1 – Kerrison rongeurs, 2 – remnants of the odontoid process; F - continued separation of the odontoid process from the dura mater: 1 – remnants of the odontoid process).
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Figure 6. The stages of dura mater opening (A - dissection of the dura mater: 1 - dura mater, 2 - microscissors; B - general view of the subdural structures after opening the dura mater at the level of C1-C2 vertebrae: 1 - spinal cord, 2 - vertebral arteries).
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Figure 7. Intradural neurovascular structures at the level of C1-C2 vertebrae, the spinal cord, both vertebral arteries, and spinal nerves are visualized (A - general view of subdural structures after opening the dura mater at the level of C1-C2 vertebrae: 1 spinal cord, 2 - vertebral arteries, 3 - anterior spinal artery; B - view of subdural structures at the level of C1-C2 vertebrae: 1 - spinal cord, 2 - vertebral arteries, 3 spinal nerve roots; C - view of the left half of the spinal canal: 1 - cerebellar tonsil, 2 left vertebral artery; D - view of the left half of the spinal canal: 1 - cerebellar tonsil, 2 spinal nerves, 3 - posterior inferior cerebellar artery; E - point of the entrance of the left vertebral artery into the subdural space: 1 - left vertebral artery; F - point of the entrance of the left vertebral artery into the subdural space: 1 - left vertebral artery).
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Figure 8. Intradural neurovascular structures at the level of C1-C2 vertebrae (A - view of subdural structures at the level of C1-C2 vertebrae: 1 - spinal cord, 2 - right vertebral artery; B - view of subdural structures at the level of C1-C2 vertebrae: 1 - spinal cord, 2 - right vertebral artery, 3 – cerebellar tonsil).
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Figure 9. Flow chart of transnasal access to craniovertebral junction with important additions on expanded access.
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Table 1. Important anatomical studies on the craniovertebral junction
First author Lopez AJ Menezes AH
Pacca P
Jhawar SS
Cavallo LM
Pillai P
Zhong S
Andrew K. Chan
Abuzayed B
Journal Neurosurg Focus. 2015
Title Anatomy and biomechanics of the craniovertebral junction Childs Nerv Syst. 2008 Anatomy and biomechanics of normal craniovertebral junction and biomechanics of stabilization Acta Neurochir Suppl. The Endoscopic Endonasal Approach to 2019 Craniovertebral Junction Pathologies: Surgical Skills and Anatomical Study J Craniovertebr Junction Craniovertebral junction 360°: A combined microscopic and endoscopic anatomical Spine. 2016 study Childs Nerv Syst. 2007 The extended endoscopic endonasal approach to the clivus and cranio-vertebral junction: anatomical study Neurosurgery. 2009 Endoscopic image-guided transoral approach to the craniovertebral junction: an anatomic study comparing surgical exposure and surgical freedom obtained with the endoscope and the operating microscope. J Craniofac Surg. 2018 Anatomic Study of Craniocervical Junction and Its Surrounding Structures in Endoscopic Transoral-Transpharyngeal Approach Neurosurg Focus. 2016 The endoscopic transoral approach to the craniovertebral junction: an anatomical study with a clinical example Turk Neurosurg. 2009 Extended endoscopic endonasal approach to the anterior cranio-vertebral junction: anatomic study.
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First author Lopez AJ
Journal Neurosurg Focus. 2015
Title Anatomy and biomechanics of the craniovertebral junction Menezes AH Childs Nerv Syst. 2008 Anatomy and biomechanics of normal craniovertebral junction and biomechanics of stabilization Pacca P Acta Neurochir Suppl. The Endoscopic Endonasal Approach to 2019 Craniovertebral Junction Pathologies: Surgical Skills and Anatomical Study Jhawar SS J Craniovertebr Junction Craniovertebral junction 360°: A combined microscopic and endoscopic anatomical Spine. 2016 study Cavallo LM Childs Nerv Syst. 2007 The extended endoscopic endonasal approach to the clivus and cranio-vertebral junction: anatomical study Pillai P Neurosurgery. 2009 Endoscopic image-guided transoral approach to the craniovertebral junction: an anatomic study comparing surgical exposure and surgical freedom obtained with the endoscope and the operating microscope. Zhong S J Craniofac Surg. 2018 Anatomic Study of Craniocervical Junction and Its Surrounding Structures in Endoscopic Transoral-Transpharyngeal Approach Andrew K. Chan Neurosurg Focus. 2016 The endoscopic transoral approach to the craniovertebral junction: an anatomical study with a clinical example Abuzayed B Turk Neurosurg. 2009 Extended endoscopic endonasal approach to the anterior cranio-vertebral junction: anatomic study. TABLE 1. Imporant studies on anatomical aspects of transnasal access to craniovertebral junction.
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Abbreviations list: C1 – First cervical vertebrae C2 – Second cervical vertebrae CSF – cerebrospinal fluid