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Exposing the Fundus of the Internal Acoustic Meatus without Entering the Labyrinth Using a Retrosigmoid Approach: Is It Possible?
Accepted Manuscript “Exposing the fundus of the Internal Acoustic Meatus without entering the labyrinth using a retrosigmoid approach. Is it possible?” Alba Scerrati, M.D, Jung-Shun Lee, M.D., M.Sc, Jun Zhang, Ph.D, Mario Ammirati, M.D., M.B.A PII:
S1878-8750(16)30020-1
DOI:
10.1016/j.wneu.2016.03.093
Reference:
WNEU 3916
To appear in:
World Neurosurgery
Received Date: 24 December 2015 Revised Date:
25 March 2016
Accepted Date: 29 March 2016
Please cite this article as: Scerrati A, Lee J-S, Zhang J, Ammirati M, “Exposing the fundus of the Internal Acoustic Meatus without entering the labyrinth using a retrosigmoid approach. Is it possible?”, World Neurosurgery (2016), doi: 10.1016/j.wneu.2016.03.093. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
ACCEPTED MANUSCRIPT Title Page Title: “Exposing the fundus of the Internal Acoustic Meatus without entering the labyrinth using a retrosigmoid approach. Is it possible?”
Author names and affiliations:
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Alba Scerrati, M.D.,1,3 Jung-Shun Lee, M.D., M.Sc.,2,3 Jun Zhang, Ph.D.,4 Mario Ammirati, M.D., M.B.A.3
Institute of Neurosurgery, Catholic University of Rome, Rome, Italy
2
Section of Neurosurgery, Department of Surgery, National Cheng Kung University
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1
3
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Hospital, College of Medicine, National Cheng Kung University, Tainan, Taiwan
Dardinger Microneurosurgical Skull Base Laboratory, Department of Neurological
Surgery, Wexner Medical Center, The Ohio State University, Columbus, Ohio, USA 4
Department of Radiology and Wright Center of Innovation in Biomedical Imaging,
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Wexner Medical Center, The Ohio State University, Columbus, Ohio, USA
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The authors declare this study did not receive any funding and no conflict of interest.
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Corresponding author:
Mario Ammirati, M.D., M.B.A.
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Department of Neurological Surgery, Wexner Medical Center, The Ohio State University,
N1025 Doan Hall, 410 West 10th Avenue, Columbus, Ohio, 43210, USA Telephone: (614) 293-1970 Fax: (614) 293-4024
Email: [email protected]
Keywords: acoustic neuroma; fundus; internal acoustic meatus; innear ear; neuronavigation; retrosigmoid approach; semicircular canals.
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Drilling the Posterior wall of the Internal Acoustic Meatus without entering the labyrinth using a
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retrosigmoid approach. Is it possible?
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Introduction
The use of the retrosigmoid transmeatal approach to completely remove vestibular schwannoma while preserving hearing is still debated in the skull base literature. (16, 22, 27) The middle fossa and translabyrinthine approaches are two others commonly employed approaches to the cerebellopontine angle
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and internal acoustic meatus. (4, 6) The middle fossa approach presents the following advantages: better hearing preservation for size-matched groups of intracanalicular tumors and tumors extending 1 cm or less into the CPA, no cerebellar retraction, enhanced lateral exposure facilitating dissection in a lateral-tomedial direction and better development of tumor arachnoid planes at the fundus. (19) The translabyrinthine approach is preferred in patients with non-serviceable hearing and with large tumors who
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have a low probability of hearing preservation. It offers early identification of the facial nerve in the auditory canal during surgery and cerebellar retraction is not needed. (11) The retrosigmoid approach is considered more versatile, enabling removal of tumors largely independent of size and offering the surgeon
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an improved access to the root entry zone of the acoustic nerve. One of the main criticism has to do with the ability to reach the fundus of the internal acoustic meatus (IAM) while at the same time preserving the labyrinth structures embedded in the posterior meatal wall. (13) Many studies have described anatomical landmarks and techniques for a safe drilling of the posterior meatal wall (5, 12, 13, 15, 26, 29) but few (22) reports deal with this drilling as performed via a retrosigmoid approach or with the use of intraoperative neuronavigation. (33, 34)
Goal of our study was to perform an anatomic, radiologic, image-guided study of the anatomy of the inner
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ear structures embedded in the posterior meatal wall and to describe their relationships to the IAM using a retrosigmoid approach. More importantly we endeavored to integrate all this information in evaluating the usefulness of the retrosigmoid approach in performing extensive labyrinthine sparing drilling of the
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posterior wall of the Internal Auditory Meatus.
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Materials and methods:
Ten surgical dissections were performed bilaterally on 5 fresh cadaver heads. Six titanium microscrews were implanted onto the skull as permanent bone-reference markers. Cadavers were then subjected to highresolution bony computed tomographic (CT) scans (slice thickness, 0.6 mm; contiguous non-overlapping
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slices). The CT scanning was performed at a gantry of 0 degrees, with a scan window diameter of 225 mm and a pixel size of more than 0.44 x 0.44. Before performing the dissections, measurements of landmarks and distances between the important topographic structures of the petrous bone were taken on preoperative CT scan (Fig 1a).
The specimens were divided in 3 categories according to the sigmoid-fundus line (S-F line) (Fig.1b) (34): L
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group in which the most medial extension of the labyrinth is lateral to the S-F line; O group in which is on the S-F line and M group in which is medial.
We defined a ‘safety line’ as a line going at least 1mm parallel to the vestibule, from the fundus (F) to the
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occipital bone (similar to the ‘fundus-labyrinth’ line reported by Mazzoni et al. (28)). Along this line we considered the drilling safe. Intersection of the line on the petrous bone surface was defined as F1. (Fig.1b) The extension of this line on the occipital bone was called F2.
We used a Stryker Navigation System (Kalamazoo, MI) for intraoperative navigation.
Surgical procedure
OR).
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The heads were placed on a Mayfield head-holder in a semisitting position (the same position we use in the
A standard retrosigmoid craniectomy was performed on both sides of each of 5 cadaveric heads. The size of the craniotomy was based on the distance of point F2 from the transverse-sigmoid angle, measured and recorded with navigation parameters on pre-operative CT scans. The dura was incised in a standard fashion
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and a cerebellar retractor was placed in order to simulate operative conditions. The petrous pyramid surface was inspected and its surface landmarks identified and digitized using the neuro-navigation system (Fig 1c); at this time we measured distances between different surface petrous landmarks. The endolymphatic
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sac was identified and preserved. The opening of the IAM started with the drilling of its roof. Afterwards, the posterior wall of the IAM was drilled with a high-speed diamond burr (4mm) under continuous saline irrigation. In this way opening of the semicircular canal can be avoided, although the angle of view is sometimes obscured. This limit can be overcome by endoscopy. Drilling of the posterior wall of the IAM was stopped when, according to the information provided by the navigation system, no more than 1 mm of bone was covering the labyrinthine structures. (Fig.2) Preservation of the integrity of the deep labyrinthine structures was confirmed by microscopic and endoscopic inspection. Afterwards, exposure of the posterior semicircular canal (PSC) was performed following it supero-medially towards the common crus (CC) and superior semicircular canal (SSC). The relationship of the IAM with the position of the semicircular canals was finally shown. (Fig.3)
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Next, the specimens underwent a new CT scan in order to verify the length of the opened IAM and its residual length, the status of the vestibular aqueduct (VA) and of the semicircular canals, and the angle of drilling (Fig.4). In order to verify the accuracy of the neuronavigation, the endolymphatic sac and duct (up to the genu) were removed to open the posterior semicircular canal and to visualize the common crus. Finally the
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superior semicircular canal was partially opened. (Fig. 5)
In this way we were able to clearly identify the same deep landmarks that we had identified on the CT scan. These landmarks were the posterior semicircular canal, just before joining the common crus (aPSC) and at the most posterior – lateral part of its C shaped portion (pPSC), the highest point of the superior semicircular canal (SSC), the beginning of the common crus (CC), the genu of the vestibular aqueduct
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(VA) and the fundus at the midpoint of the transverse crest (F).(Fig.5) The coordinates based on the radiological images were compared with the coordinates of the selected anatomic landmarks coordinates using
the
tip
of
the
navigation
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digitized
tool.
All distances were calculated using the three-dimensional distance formula in which the distance between points (X1, Y1, Z1) and (X2, Y2, Z2) can be defined as:
d = √ (X1 – X2)2 + (Y1 – Y2) 2 + (Z1 – Z2)2 every
distance
mean
value,
standard
deviation
(SD)
and
range
were
calculated.
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For
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Results The anatomic relationships between the internal acoustic meatus (IAM) and inner ear structures are shown
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in figures 3 and 5. The average length of the IAM based on CT scanning was 8.06 mm (SD: 1.70 mm; range 5.56 – 10.80) (Table 1).
The mean distance between posterior lip of the IAM (PIAM) and F1 was 7.473 (SD:0.966 mm; range:9.45
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- 6.22). Others results of the CT scan landmarks are shown in Table 1.
The mean distance between PIAM and opercule of endolymphatic sac (O) was 8.919 (SD:1.354 mm; range:
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6.90 - 10.77 mm). The other anatomic measurements are reported in Table 2.
In 9 sides the internal acoustic meatus was opened all the way to the fundus, without injuring labyrinth structures; in one side the vestibule was opened (Figs.6 and 7). The mean value of the residual bone on the fundus was 0.97 mm ( SD: 1.22 mm; range: 0-3.8). The average length of the internal acoustic meatus that was accessible was 88.95% (SD: 0.124; range 62.93 – 100 %). Our results are shown in Table 3.
the L group.
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4 sides belonged to the O group, while 6 sides belonged to the M group. We didn’t have any specimen in
The best accuracy of the navigation was for the identification of the common crus (CC) with a mean value of 0.73 mm (SD: 0.255 mm ; range: 0.34 - 1.16 mm), of the anterior part of posterior semicircular canal (aPSC) which was 0.73 mm (SD: 0.529 mm; range: 0.25 – 1.65 mm) and of the fundus (F): 0.73 mm (SD:
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0.407 mm ; range:0.22 - 1.5 mm). Other results are shown in Table 4.
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Discussion Surgery involving the temporal bone is always challenging because of the complex anatomy and high vulnerability of its structures. In particular during vestibular schwannoma surgery opening of the IAM is
inadvertent inner ear structures damages. (8, 35)
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often required to achieve complete tumor removal; during this drilling, hearing may be compromised due to
Preoperative estimation of the risk of opening inner ear structures during drilling of the posterior wall of the IAM was attempted by Tatagiba (37) and Yokoama (39). They introduced a “safety line” going from the medial border of the sigmoid sinus to the IAM fundus estimating a low risk for labyrinthine injury when
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the labyrinth was situated lateral to this line. Samii et al. (34)further expanded on this concept by using this line to define 3 groups: ‘L (labyrinth lateral to the line)’, ‘O (labyrinth is on the line)’ and ‘M (labyrinth is
labyrinthine drilling injuries.
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medial to the line)’ (see materials and methods) associated with increasing risk of posterior wall
Some authors questioned the real predictive value of this line.(12, 16) In our study, all specimen belonged to the O or M groups, the medium/high risk groups, still we were able to reach the fundus without damage the labyrinth in 90% of cases.
At the present time robust standardized guidelines to protect inner ear structures during posterior meatal
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drilling are not available. Different anatomic landmarks have been proposed as limits of safe bone removal: posterior semicircular canal (9), probing for the transverse crest (20, 30), the blue line of PSC and VA (10), the last 3 mm of bone next to the fundus assessed by a probe (38). Our data, confirmed previous studies (2, 13, 14, 21)showing that anatomic landmarks, while helpful, are unreliable in avoiding injury to the labyrinth due to their great variability. The preoperative CT scans is invaluable in evaluating the
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relationships between the IAM and the labyrinth and in measuring appropriate distances such as the one between the posterior meatal lip of the IAM and the projection of the vestibular structures on the posterior meatal wall. However these measurements are hardly applied during surgery with the same accuracy as
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they are on the preoperative CT for a variety of reasons, including the difficulties in identifying at surgery the same plan that was used during the CT scan acquisition. Indeed they give more of a general orientation than hard measurements to reach the fundus and preserve inner ear structures. Moreover, Tatagiba et al. in 1996 (37) stated that approximately 3 to 4 mm of bone at the lateral-most wall of the internal acoustic meatus must be left intact to avoid labyrinth injury. The knowledge of the anatomy of the semicircular canals and of their spatial relationship with the petrous bone and the canal can be considered as an additional tool to get a better outcome, together with preoperative imaging, intraoperative neuronavigation and endoscopic assistance. We also found useful in the general initial guidance of our drilling the knowledge of the distances measured on the surface of the petrous bone. Our dissections were
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performed combining all these tools. The results, if compared to previous studies were encouraging; we attained a higher mean percentage (88.95% versus 75.3 %) of exposed internal acoustic meatus resulting in a lower mean residual bone on the fundus (0.97 mm versus 1 mm). (12, 29, 36-38) Indeed, in the majority of cases, an opening of the IAM between 80% and 90 % allows complete removal of the intracanalicular part of the tumor. (38) However the consistency of the tumor should always be kept in mind: indeed the
it from the fundus, even when it is wide exposed.
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softer and the more easily fragmentable the tumor is, the more challenging it may be to completely remove
We are aware that a greater exposure doesn't always result in improved visualization. The variability of the patient petrous bone anatomy is one of the main factors that affect the visualization. However we were able
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to visualize ± 90 % of the IAM by using a combination of microscopic and endoscopic visualization.
The drilling angle is an important parameter to consider because of its influence on the medial extension of
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the craniotomy. The size of the dissection craniotomies was calculated on the preoperative evaluation of CT scan and was tailored on every side evaluating the S-F line and its intersection with the occipital bone (F2). The vestibular aqueduct and the vestibule can be considered as the lateral limit of the drilling space: the closer they are to the IAM, the wider will be the drilling angle and the extension of the craniotomy. Due to the great variability of petrous bone anatomy, the distance between these structures and the IAM is very variable, implying a very variable drilling angle and consequently craniotomies size. We are aware that in real surgery, not every drilling angle (based on the preoperative evaluation of the safety line) could be
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suitable, especially when it would imply a larger craniotomy and an excessive cerebellar retraction, resulting in potential complications. (3) However, it has been estimated that a drilling angle of 60° is safe in clinical situations (12), even if there is no commonly accepted limit of medial cerebellar retraction that is safe. (23) In addition this is a cadaveric study and suffers from all the known limitation of such studies such as, amongst others, brain stiffness and lack of cerebrospinal fluid. Clearly the retraction of an
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embalmed brain is different from that of a living brain and the lack of cerebrospinal fluid does not allow to take advantage of the cerebellar relaxation obtained by opening the posterior fossa basal cisterns.
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In Laine and Palva study(25) a suboccipital craniotomy extending 4 cm from the midline is considered adequate to allow to open the whole IAM without risk to the inner ear. The mean craniotomy size in our study was 46.39 mm (SD:11.812 mm; range: 30.2 – 63.7 mm) and it could be considered suitable for most of the patients. The size showed a great variability. Again, this could be explained by the variability of the drilling angle, strictly dependent on the variable petrous bone inner ear anatomy. Indeed, in Mazzoni et al. study (28), where a 4 x 5 cm retrosigmoid craniotomies were compared to standard ones, the rate of clinical morbidity for cerebellar malacia showed no relationship to the size of craniotomy. Advantages and benefits of neuronavigation are already well established (1, 32). However it could be questionable when applied to the minute structures such as the labyrinthine ones. We used an experimental
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setting similar to the one used by Pillai et al. (32) who in 2008 demonstrated the advantage of integrating image guidance and endoscopy to safely drill the IAM all the way to the fundus. Nevertheless, the novelty of the present study consists in the introduction of the preoperative evaluation of the anatomic and radiologic landmarks and distances measurements useful for an accurate planning of the surgery as well as in the objective evaluation of the IAM opening and inner ear structures integrity using the comparison
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between the pre operative and post operative CT scans. In addition the skeletonization of inner ear structures allowed to better understand their complex anatomy and relationship with the IAM and was essential for the evaluation of the neuronavigation accuracy. Differently than Samii et al. (34) we used tool tip navigation rather than microscope focal point navigation. The tool tip can avoid problem related to focus dependent navigation, even if the introduction of a supplemental instrument during the drilling
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procedure may be uncomfortable. As already shown in Pillai et al. paper (32) we focused our attention on the accuracy of the navigation, that is based on the ability to reach a target in a precise and unbiased way. The accuracy showed no large differences compared to Samii et al. data (0.73 mm, SD: 0.255 mm; versus
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0.71 ± 0.37 mm for PSC). Tool navigation, in term of accuracy, can be absolutely compared to microscopic navigation. However also submillimetric errors may cause an injury to the tiny inner ear structures and an error-free navigation allowing a direct anatomic contouring is still not possible. This could explain the only case we had with an intraoperative injury of the vestibule. Furthermore, in that specimen, a higher accuracy error was recorded.
Our results show that in our experimental settings the retrosigmoid approach may be safely used to reach
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the IAM fundus when neuronavigation is coupled to a general understanding of the 3-D anatomy of the labyrinthine structures embedded in the posterior wall of the IAM. Probably, one of the main concern and limit of our study is the possibility to translate its cadavericlaboratory setting in a real clinical situation. It could not seem practical nor likely that live patients would
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have bone-anchored fiducials placed. Nevertheless, the use of bone-implanted markers in a clinical setting has been already reported in different studies. (1, 7, 17, 18, 24, 31, 34)
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In addition with the up and coming popularization of intraoperative CT scans it would be easy to implant bony fiducials in the operating room after the patient has been anesthetized and then acquire the navigation CT that could be fused to the preoperative MRI. Conclusions
Labyrinth injury can’t be always avoided during surgery of Vestibular Schwannomas; the surgical technique we presented could facilitate the opening of the internal acoustic meatus (IAM) with preservation of inner ear structures. Compared to other anatomic studies that used just temporal bones, we tried to simulate a real intraoperative approach.
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The value of navigation for opening the IAM is still somewhat limited and we think that it has to be strictly integrated with a solid neuroanatomic knowledge. Applying this principle, in our study we were able to expose 88.95 % of its course (mean value) without entering the labyrinth structures in all specimens but one.
acoustic
meatus
fundus
with
preservation
of
inner
ear
structures.
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internal
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the
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The results of our anatomic study confirm the feasibility of the retrosigmoid approach for the exposure of
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Figures legend
Fig. 1 CT scan and anatomic landmarks a) Landmarks identified and digitized on CT scan; b) The yellow line, going from the fundus to the petrous surface parallel to the vestibule represents our ‘safety line’. Its intersection with the pyramid surface is the point F1; the light blue line is the sigmoid-fundus line (see
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materials and methods); c) Landmarks identified and digitized on petrous bone surface; PS: Petrosigmoid Intersection; PR: Petrous Ridge; PIAM: posterior lip of IAM; IIAM: inferior lip of IAM; SIAM: superior lip of IAM; IX: entry point of IX cranial nerve in the jugular foramen; VA: vestibular aqueduct; AFB: acoustic-facial bundle; aPSC: anterior part of the posterior semicircular canal; pPSC: posterior part of the posterior semicircular canal; CC: common crus; VE: vestibule; F: fundus of IAM; F1: projection of the
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fundus on the petrous bone surface along the safety line; S: sigmoid. Adapted from Ammirati et al. (2) with permission from the American Association of Neurological Surgeons.
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Fig.2 Intraoperative navigation. The tip of the navigation tool points to the fundus of the internal acoustic meatus. The relative position is shown on the navigation image.
Fig.3 Bony labyrinth with opened internal acoustic meatus and closed semicircular canals. Left side. Posterior, superior and lateral semicircular canals are skeletonized. The medial edge of the opercule with part of endolymphatic sac is shown. The IAM has been opened to the fundus and its relationship with the semicircular canals can be appreciated. PSC: posterior semicircular canal; SSC: superior semicircular
nerve;
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canal; LSC: lateral semicircular canal; ES: endolymphatic sac; AFB: acoustic-facial bundle; IX: IX cranial
Fig.4 Post dissection CT scan showing the drilling angle (@).
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Fig.5 Same specimen shown in Fig.3. Posterior and superior semicircular canals have been opened in order to show the common crus. The genu of the vestibular aqueduct is visible. The endolymphatic sac is covered by the dura going towards the sigmoid sinus. pPSC: posterior part of the posterior semicircular canal;
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aPSC: anterior part of the posterior semicircular canal; SSC: superior semicircular canal; LSC: lateral semicircular canal; ES: endolymphatic sac; AFB: acoustic-facial bundle; LCN: lower cranial nerves; SS: sigmoid sinus; VA: vestibular aqueduct; CC: common crus. Fig.6 Post dissection CT scans, showing the drilled internal acoustic meatus and the residual bone on the fundus. The red arrow shows the injured vestibule.
Fig.7 Injured vestibule. Right side a) Microscopic view of opened internal acoustic meatus. The injured vestibule is not visible. b) Endoscopic view (4 mm – 30° angled) showing the injured vestibule close to the fundus of the opened internal acoustic meatus.
PSC: posterior semicircular canal; SSC: superior
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semicircular canal; LSC: lateral semicircular canal; AFB: acoustic-facial bundle; LCN: lower cranial
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nerves; V: vestibule
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Max
PIAM – CC
8.988
1.474
11.23
PIAM – VE
8.409
1.377
10.27
PIAM – F
8.056
1.696
10.78
PIAM – F1
7.473
0.966
9.45
PIAM – ES
10.397
2.012
14.46
PIAM – JB
10.281
2.787
PIAM – VA
8.797
1.317
Deviation
Min 6.54
6.01
5.56
6.22
7.52
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Mean
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Standard
Distances
16.74
6.79
11.08
6.70
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PIAM: posterior lip of internal acoustic meatus; CC: common crus; VE: vestibule; F: fundus; F1 projection of fundus on petrous bone surface a long the ‘safety line’; ES: endolymphatic sac; JB: jugular bulb; VA: vestibular
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aqueduct
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Max
Mean
PIAM-O
8.919
1.354
10.77
PIAM-PR
8.418
2.197
11.36
PIAM-PS
29.673
3.323
32.66
IIAM-IX
4.564
1.156
6.35
IIAM-SIAM
6.234
1.505
8.5
Min 6.9
5.09
23.72 2.24
3.81
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Deviation
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PIAM: posterior lip of internal acoustic meatus; O: opercule; PR: petrous ridge; PS: petrosigmoid intersection; IIAM: inferior lip of internal acoustic
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meatus; IX: glossopharyngeal nerve; SIAM: superior lip of internal acoustic
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meatus; C: cerebellum before retraction; C1: cerebellum after retraction
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Bone residual
Standard Deviation
Max
Min
0
0.97
1.220
3.8
% of opened IAM
88.95%
0.124
100%
Angle of drilling (°)
65.91
9.202
79.1
Craniotomy
46.39
11.812
63.7
62.93% 45.4
30.2
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(mm)
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Mean
Standard Deviation
Max
Min 0.25
0.73
0.529
1.65
^pPSC
1.02
0.531
2.11
^SSC
0.88
0.463
1.75
^CC
0.73
0.255
1.16
^VA
0.85
0.304
1.27
^F
0.73
0.407
1.5
0.35
0.39
0.34
0.35
0.22
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^aPSC
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Distances
aPSC: anterior part of posterior semicircular canal; pPSC: posterior part of posterior semicircular canal; SSC: superior semicircular
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canal; CC: common crus; VA: vestibular aqueduct; F: fundus
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Highlights A neuronavigation assisted retrosigmoid approach to the fundus of the IAM is described. The anatomy of the inner ear structures embedded in the petrous bone is shown.
The average length of the IAM accessible was 88.95%.
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The mean residual bone on the fundus was 0.97 mm.
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The presented technique could facilitate the opening of IAM preserving the labyrinth.
ACCEPTED MANUSCRIPT Abbreviations
AFB: acoustic-facial bundle aPSC: anterior part CC: common crus
F: fundus of internal acoustic meatus IAM: internal acoustic meatus IIAM: inferior lip of internal acoustic meatus
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IX: entry point of IX cranial nerve in the jugular foramen LCN: lower cranial nerves
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LSC: lateral semicircular canal O: opercule of endolymphatic sac
PIAM: posterior lip of the internal acoustic meatus
pPSC: posterior part of posterior semicircular canal PSC: posterior semicircular canal
SIAM: superior lip of internal acoustic meatus
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SSC: superior semicircular canal VA: vestibula aqueduct
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VE: vestibule
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ES: endolymphatic sac
S1878-8750(16)30020-1
DOI:
10.1016/j.wneu.2016.03.093
Reference:
WNEU 3916
To appear in:
World Neurosurgery
Received Date: 24 December 2015 Revised Date:
25 March 2016
Accepted Date: 29 March 2016
Please cite this article as: Scerrati A, Lee J-S, Zhang J, Ammirati M, “Exposing the fundus of the Internal Acoustic Meatus without entering the labyrinth using a retrosigmoid approach. Is it possible?”, World Neurosurgery (2016), doi: 10.1016/j.wneu.2016.03.093. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
ACCEPTED MANUSCRIPT Title Page Title: “Exposing the fundus of the Internal Acoustic Meatus without entering the labyrinth using a retrosigmoid approach. Is it possible?”
Author names and affiliations:
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Alba Scerrati, M.D.,1,3 Jung-Shun Lee, M.D., M.Sc.,2,3 Jun Zhang, Ph.D.,4 Mario Ammirati, M.D., M.B.A.3
Institute of Neurosurgery, Catholic University of Rome, Rome, Italy
2
Section of Neurosurgery, Department of Surgery, National Cheng Kung University
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1
3
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Hospital, College of Medicine, National Cheng Kung University, Tainan, Taiwan
Dardinger Microneurosurgical Skull Base Laboratory, Department of Neurological
Surgery, Wexner Medical Center, The Ohio State University, Columbus, Ohio, USA 4
Department of Radiology and Wright Center of Innovation in Biomedical Imaging,
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Wexner Medical Center, The Ohio State University, Columbus, Ohio, USA
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The authors declare this study did not receive any funding and no conflict of interest.
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Corresponding author:
Mario Ammirati, M.D., M.B.A.
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Department of Neurological Surgery, Wexner Medical Center, The Ohio State University,
N1025 Doan Hall, 410 West 10th Avenue, Columbus, Ohio, 43210, USA Telephone: (614) 293-1970 Fax: (614) 293-4024
Email: [email protected]
Keywords: acoustic neuroma; fundus; internal acoustic meatus; innear ear; neuronavigation; retrosigmoid approach; semicircular canals.
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Drilling the Posterior wall of the Internal Acoustic Meatus without entering the labyrinth using a
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retrosigmoid approach. Is it possible?
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Introduction
The use of the retrosigmoid transmeatal approach to completely remove vestibular schwannoma while preserving hearing is still debated in the skull base literature. (16, 22, 27) The middle fossa and translabyrinthine approaches are two others commonly employed approaches to the cerebellopontine angle
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and internal acoustic meatus. (4, 6) The middle fossa approach presents the following advantages: better hearing preservation for size-matched groups of intracanalicular tumors and tumors extending 1 cm or less into the CPA, no cerebellar retraction, enhanced lateral exposure facilitating dissection in a lateral-tomedial direction and better development of tumor arachnoid planes at the fundus. (19) The translabyrinthine approach is preferred in patients with non-serviceable hearing and with large tumors who
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have a low probability of hearing preservation. It offers early identification of the facial nerve in the auditory canal during surgery and cerebellar retraction is not needed. (11) The retrosigmoid approach is considered more versatile, enabling removal of tumors largely independent of size and offering the surgeon
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an improved access to the root entry zone of the acoustic nerve. One of the main criticism has to do with the ability to reach the fundus of the internal acoustic meatus (IAM) while at the same time preserving the labyrinth structures embedded in the posterior meatal wall. (13) Many studies have described anatomical landmarks and techniques for a safe drilling of the posterior meatal wall (5, 12, 13, 15, 26, 29) but few (22) reports deal with this drilling as performed via a retrosigmoid approach or with the use of intraoperative neuronavigation. (33, 34)
Goal of our study was to perform an anatomic, radiologic, image-guided study of the anatomy of the inner
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ear structures embedded in the posterior meatal wall and to describe their relationships to the IAM using a retrosigmoid approach. More importantly we endeavored to integrate all this information in evaluating the usefulness of the retrosigmoid approach in performing extensive labyrinthine sparing drilling of the
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posterior wall of the Internal Auditory Meatus.
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Materials and methods:
Ten surgical dissections were performed bilaterally on 5 fresh cadaver heads. Six titanium microscrews were implanted onto the skull as permanent bone-reference markers. Cadavers were then subjected to highresolution bony computed tomographic (CT) scans (slice thickness, 0.6 mm; contiguous non-overlapping
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slices). The CT scanning was performed at a gantry of 0 degrees, with a scan window diameter of 225 mm and a pixel size of more than 0.44 x 0.44. Before performing the dissections, measurements of landmarks and distances between the important topographic structures of the petrous bone were taken on preoperative CT scan (Fig 1a).
The specimens were divided in 3 categories according to the sigmoid-fundus line (S-F line) (Fig.1b) (34): L
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group in which the most medial extension of the labyrinth is lateral to the S-F line; O group in which is on the S-F line and M group in which is medial.
We defined a ‘safety line’ as a line going at least 1mm parallel to the vestibule, from the fundus (F) to the
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occipital bone (similar to the ‘fundus-labyrinth’ line reported by Mazzoni et al. (28)). Along this line we considered the drilling safe. Intersection of the line on the petrous bone surface was defined as F1. (Fig.1b) The extension of this line on the occipital bone was called F2.
We used a Stryker Navigation System (Kalamazoo, MI) for intraoperative navigation.
Surgical procedure
OR).
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The heads were placed on a Mayfield head-holder in a semisitting position (the same position we use in the
A standard retrosigmoid craniectomy was performed on both sides of each of 5 cadaveric heads. The size of the craniotomy was based on the distance of point F2 from the transverse-sigmoid angle, measured and recorded with navigation parameters on pre-operative CT scans. The dura was incised in a standard fashion
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and a cerebellar retractor was placed in order to simulate operative conditions. The petrous pyramid surface was inspected and its surface landmarks identified and digitized using the neuro-navigation system (Fig 1c); at this time we measured distances between different surface petrous landmarks. The endolymphatic
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sac was identified and preserved. The opening of the IAM started with the drilling of its roof. Afterwards, the posterior wall of the IAM was drilled with a high-speed diamond burr (4mm) under continuous saline irrigation. In this way opening of the semicircular canal can be avoided, although the angle of view is sometimes obscured. This limit can be overcome by endoscopy. Drilling of the posterior wall of the IAM was stopped when, according to the information provided by the navigation system, no more than 1 mm of bone was covering the labyrinthine structures. (Fig.2) Preservation of the integrity of the deep labyrinthine structures was confirmed by microscopic and endoscopic inspection. Afterwards, exposure of the posterior semicircular canal (PSC) was performed following it supero-medially towards the common crus (CC) and superior semicircular canal (SSC). The relationship of the IAM with the position of the semicircular canals was finally shown. (Fig.3)
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Next, the specimens underwent a new CT scan in order to verify the length of the opened IAM and its residual length, the status of the vestibular aqueduct (VA) and of the semicircular canals, and the angle of drilling (Fig.4). In order to verify the accuracy of the neuronavigation, the endolymphatic sac and duct (up to the genu) were removed to open the posterior semicircular canal and to visualize the common crus. Finally the
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superior semicircular canal was partially opened. (Fig. 5)
In this way we were able to clearly identify the same deep landmarks that we had identified on the CT scan. These landmarks were the posterior semicircular canal, just before joining the common crus (aPSC) and at the most posterior – lateral part of its C shaped portion (pPSC), the highest point of the superior semicircular canal (SSC), the beginning of the common crus (CC), the genu of the vestibular aqueduct
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(VA) and the fundus at the midpoint of the transverse crest (F).(Fig.5) The coordinates based on the radiological images were compared with the coordinates of the selected anatomic landmarks coordinates using
the
tip
of
the
navigation
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digitized
tool.
All distances were calculated using the three-dimensional distance formula in which the distance between points (X1, Y1, Z1) and (X2, Y2, Z2) can be defined as:
d = √ (X1 – X2)2 + (Y1 – Y2) 2 + (Z1 – Z2)2 every
distance
mean
value,
standard
deviation
(SD)
and
range
were
calculated.
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Results The anatomic relationships between the internal acoustic meatus (IAM) and inner ear structures are shown
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in figures 3 and 5. The average length of the IAM based on CT scanning was 8.06 mm (SD: 1.70 mm; range 5.56 – 10.80) (Table 1).
The mean distance between posterior lip of the IAM (PIAM) and F1 was 7.473 (SD:0.966 mm; range:9.45
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- 6.22). Others results of the CT scan landmarks are shown in Table 1.
The mean distance between PIAM and opercule of endolymphatic sac (O) was 8.919 (SD:1.354 mm; range:
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6.90 - 10.77 mm). The other anatomic measurements are reported in Table 2.
In 9 sides the internal acoustic meatus was opened all the way to the fundus, without injuring labyrinth structures; in one side the vestibule was opened (Figs.6 and 7). The mean value of the residual bone on the fundus was 0.97 mm ( SD: 1.22 mm; range: 0-3.8). The average length of the internal acoustic meatus that was accessible was 88.95% (SD: 0.124; range 62.93 – 100 %). Our results are shown in Table 3.
the L group.
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4 sides belonged to the O group, while 6 sides belonged to the M group. We didn’t have any specimen in
The best accuracy of the navigation was for the identification of the common crus (CC) with a mean value of 0.73 mm (SD: 0.255 mm ; range: 0.34 - 1.16 mm), of the anterior part of posterior semicircular canal (aPSC) which was 0.73 mm (SD: 0.529 mm; range: 0.25 – 1.65 mm) and of the fundus (F): 0.73 mm (SD:
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0.407 mm ; range:0.22 - 1.5 mm). Other results are shown in Table 4.
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Discussion Surgery involving the temporal bone is always challenging because of the complex anatomy and high vulnerability of its structures. In particular during vestibular schwannoma surgery opening of the IAM is
inadvertent inner ear structures damages. (8, 35)
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often required to achieve complete tumor removal; during this drilling, hearing may be compromised due to
Preoperative estimation of the risk of opening inner ear structures during drilling of the posterior wall of the IAM was attempted by Tatagiba (37) and Yokoama (39). They introduced a “safety line” going from the medial border of the sigmoid sinus to the IAM fundus estimating a low risk for labyrinthine injury when
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the labyrinth was situated lateral to this line. Samii et al. (34)further expanded on this concept by using this line to define 3 groups: ‘L (labyrinth lateral to the line)’, ‘O (labyrinth is on the line)’ and ‘M (labyrinth is
labyrinthine drilling injuries.
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medial to the line)’ (see materials and methods) associated with increasing risk of posterior wall
Some authors questioned the real predictive value of this line.(12, 16) In our study, all specimen belonged to the O or M groups, the medium/high risk groups, still we were able to reach the fundus without damage the labyrinth in 90% of cases.
At the present time robust standardized guidelines to protect inner ear structures during posterior meatal
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drilling are not available. Different anatomic landmarks have been proposed as limits of safe bone removal: posterior semicircular canal (9), probing for the transverse crest (20, 30), the blue line of PSC and VA (10), the last 3 mm of bone next to the fundus assessed by a probe (38). Our data, confirmed previous studies (2, 13, 14, 21)showing that anatomic landmarks, while helpful, are unreliable in avoiding injury to the labyrinth due to their great variability. The preoperative CT scans is invaluable in evaluating the
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relationships between the IAM and the labyrinth and in measuring appropriate distances such as the one between the posterior meatal lip of the IAM and the projection of the vestibular structures on the posterior meatal wall. However these measurements are hardly applied during surgery with the same accuracy as
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they are on the preoperative CT for a variety of reasons, including the difficulties in identifying at surgery the same plan that was used during the CT scan acquisition. Indeed they give more of a general orientation than hard measurements to reach the fundus and preserve inner ear structures. Moreover, Tatagiba et al. in 1996 (37) stated that approximately 3 to 4 mm of bone at the lateral-most wall of the internal acoustic meatus must be left intact to avoid labyrinth injury. The knowledge of the anatomy of the semicircular canals and of their spatial relationship with the petrous bone and the canal can be considered as an additional tool to get a better outcome, together with preoperative imaging, intraoperative neuronavigation and endoscopic assistance. We also found useful in the general initial guidance of our drilling the knowledge of the distances measured on the surface of the petrous bone. Our dissections were
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performed combining all these tools. The results, if compared to previous studies were encouraging; we attained a higher mean percentage (88.95% versus 75.3 %) of exposed internal acoustic meatus resulting in a lower mean residual bone on the fundus (0.97 mm versus 1 mm). (12, 29, 36-38) Indeed, in the majority of cases, an opening of the IAM between 80% and 90 % allows complete removal of the intracanalicular part of the tumor. (38) However the consistency of the tumor should always be kept in mind: indeed the
it from the fundus, even when it is wide exposed.
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softer and the more easily fragmentable the tumor is, the more challenging it may be to completely remove
We are aware that a greater exposure doesn't always result in improved visualization. The variability of the patient petrous bone anatomy is one of the main factors that affect the visualization. However we were able
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to visualize ± 90 % of the IAM by using a combination of microscopic and endoscopic visualization.
The drilling angle is an important parameter to consider because of its influence on the medial extension of
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the craniotomy. The size of the dissection craniotomies was calculated on the preoperative evaluation of CT scan and was tailored on every side evaluating the S-F line and its intersection with the occipital bone (F2). The vestibular aqueduct and the vestibule can be considered as the lateral limit of the drilling space: the closer they are to the IAM, the wider will be the drilling angle and the extension of the craniotomy. Due to the great variability of petrous bone anatomy, the distance between these structures and the IAM is very variable, implying a very variable drilling angle and consequently craniotomies size. We are aware that in real surgery, not every drilling angle (based on the preoperative evaluation of the safety line) could be
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suitable, especially when it would imply a larger craniotomy and an excessive cerebellar retraction, resulting in potential complications. (3) However, it has been estimated that a drilling angle of 60° is safe in clinical situations (12), even if there is no commonly accepted limit of medial cerebellar retraction that is safe. (23) In addition this is a cadaveric study and suffers from all the known limitation of such studies such as, amongst others, brain stiffness and lack of cerebrospinal fluid. Clearly the retraction of an
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embalmed brain is different from that of a living brain and the lack of cerebrospinal fluid does not allow to take advantage of the cerebellar relaxation obtained by opening the posterior fossa basal cisterns.
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In Laine and Palva study(25) a suboccipital craniotomy extending 4 cm from the midline is considered adequate to allow to open the whole IAM without risk to the inner ear. The mean craniotomy size in our study was 46.39 mm (SD:11.812 mm; range: 30.2 – 63.7 mm) and it could be considered suitable for most of the patients. The size showed a great variability. Again, this could be explained by the variability of the drilling angle, strictly dependent on the variable petrous bone inner ear anatomy. Indeed, in Mazzoni et al. study (28), where a 4 x 5 cm retrosigmoid craniotomies were compared to standard ones, the rate of clinical morbidity for cerebellar malacia showed no relationship to the size of craniotomy. Advantages and benefits of neuronavigation are already well established (1, 32). However it could be questionable when applied to the minute structures such as the labyrinthine ones. We used an experimental
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setting similar to the one used by Pillai et al. (32) who in 2008 demonstrated the advantage of integrating image guidance and endoscopy to safely drill the IAM all the way to the fundus. Nevertheless, the novelty of the present study consists in the introduction of the preoperative evaluation of the anatomic and radiologic landmarks and distances measurements useful for an accurate planning of the surgery as well as in the objective evaluation of the IAM opening and inner ear structures integrity using the comparison
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between the pre operative and post operative CT scans. In addition the skeletonization of inner ear structures allowed to better understand their complex anatomy and relationship with the IAM and was essential for the evaluation of the neuronavigation accuracy. Differently than Samii et al. (34) we used tool tip navigation rather than microscope focal point navigation. The tool tip can avoid problem related to focus dependent navigation, even if the introduction of a supplemental instrument during the drilling
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procedure may be uncomfortable. As already shown in Pillai et al. paper (32) we focused our attention on the accuracy of the navigation, that is based on the ability to reach a target in a precise and unbiased way. The accuracy showed no large differences compared to Samii et al. data (0.73 mm, SD: 0.255 mm; versus
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0.71 ± 0.37 mm for PSC). Tool navigation, in term of accuracy, can be absolutely compared to microscopic navigation. However also submillimetric errors may cause an injury to the tiny inner ear structures and an error-free navigation allowing a direct anatomic contouring is still not possible. This could explain the only case we had with an intraoperative injury of the vestibule. Furthermore, in that specimen, a higher accuracy error was recorded.
Our results show that in our experimental settings the retrosigmoid approach may be safely used to reach
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the IAM fundus when neuronavigation is coupled to a general understanding of the 3-D anatomy of the labyrinthine structures embedded in the posterior wall of the IAM. Probably, one of the main concern and limit of our study is the possibility to translate its cadavericlaboratory setting in a real clinical situation. It could not seem practical nor likely that live patients would
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have bone-anchored fiducials placed. Nevertheless, the use of bone-implanted markers in a clinical setting has been already reported in different studies. (1, 7, 17, 18, 24, 31, 34)
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In addition with the up and coming popularization of intraoperative CT scans it would be easy to implant bony fiducials in the operating room after the patient has been anesthetized and then acquire the navigation CT that could be fused to the preoperative MRI. Conclusions
Labyrinth injury can’t be always avoided during surgery of Vestibular Schwannomas; the surgical technique we presented could facilitate the opening of the internal acoustic meatus (IAM) with preservation of inner ear structures. Compared to other anatomic studies that used just temporal bones, we tried to simulate a real intraoperative approach.
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The value of navigation for opening the IAM is still somewhat limited and we think that it has to be strictly integrated with a solid neuroanatomic knowledge. Applying this principle, in our study we were able to expose 88.95 % of its course (mean value) without entering the labyrinth structures in all specimens but one.
acoustic
meatus
fundus
with
preservation
of
inner
ear
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internal
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the
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The results of our anatomic study confirm the feasibility of the retrosigmoid approach for the exposure of
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Figures legend
Fig. 1 CT scan and anatomic landmarks a) Landmarks identified and digitized on CT scan; b) The yellow line, going from the fundus to the petrous surface parallel to the vestibule represents our ‘safety line’. Its intersection with the pyramid surface is the point F1; the light blue line is the sigmoid-fundus line (see
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materials and methods); c) Landmarks identified and digitized on petrous bone surface; PS: Petrosigmoid Intersection; PR: Petrous Ridge; PIAM: posterior lip of IAM; IIAM: inferior lip of IAM; SIAM: superior lip of IAM; IX: entry point of IX cranial nerve in the jugular foramen; VA: vestibular aqueduct; AFB: acoustic-facial bundle; aPSC: anterior part of the posterior semicircular canal; pPSC: posterior part of the posterior semicircular canal; CC: common crus; VE: vestibule; F: fundus of IAM; F1: projection of the
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fundus on the petrous bone surface along the safety line; S: sigmoid. Adapted from Ammirati et al. (2) with permission from the American Association of Neurological Surgeons.
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Fig.2 Intraoperative navigation. The tip of the navigation tool points to the fundus of the internal acoustic meatus. The relative position is shown on the navigation image.
Fig.3 Bony labyrinth with opened internal acoustic meatus and closed semicircular canals. Left side. Posterior, superior and lateral semicircular canals are skeletonized. The medial edge of the opercule with part of endolymphatic sac is shown. The IAM has been opened to the fundus and its relationship with the semicircular canals can be appreciated. PSC: posterior semicircular canal; SSC: superior semicircular
nerve;
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canal; LSC: lateral semicircular canal; ES: endolymphatic sac; AFB: acoustic-facial bundle; IX: IX cranial
Fig.4 Post dissection CT scan showing the drilling angle (@).
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Fig.5 Same specimen shown in Fig.3. Posterior and superior semicircular canals have been opened in order to show the common crus. The genu of the vestibular aqueduct is visible. The endolymphatic sac is covered by the dura going towards the sigmoid sinus. pPSC: posterior part of the posterior semicircular canal;
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aPSC: anterior part of the posterior semicircular canal; SSC: superior semicircular canal; LSC: lateral semicircular canal; ES: endolymphatic sac; AFB: acoustic-facial bundle; LCN: lower cranial nerves; SS: sigmoid sinus; VA: vestibular aqueduct; CC: common crus. Fig.6 Post dissection CT scans, showing the drilled internal acoustic meatus and the residual bone on the fundus. The red arrow shows the injured vestibule.
Fig.7 Injured vestibule. Right side a) Microscopic view of opened internal acoustic meatus. The injured vestibule is not visible. b) Endoscopic view (4 mm – 30° angled) showing the injured vestibule close to the fundus of the opened internal acoustic meatus.
PSC: posterior semicircular canal; SSC: superior
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semicircular canal; LSC: lateral semicircular canal; AFB: acoustic-facial bundle; LCN: lower cranial
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nerves; V: vestibule
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Max
PIAM – CC
8.988
1.474
11.23
PIAM – VE
8.409
1.377
10.27
PIAM – F
8.056
1.696
10.78
PIAM – F1
7.473
0.966
9.45
PIAM – ES
10.397
2.012
14.46
PIAM – JB
10.281
2.787
PIAM – VA
8.797
1.317
Deviation
Min 6.54
6.01
5.56
6.22
7.52
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Mean
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Distances
16.74
6.79
11.08
6.70
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PIAM: posterior lip of internal acoustic meatus; CC: common crus; VE: vestibule; F: fundus; F1 projection of fundus on petrous bone surface a long the ‘safety line’; ES: endolymphatic sac; JB: jugular bulb; VA: vestibular
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aqueduct
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Max
Mean
PIAM-O
8.919
1.354
10.77
PIAM-PR
8.418
2.197
11.36
PIAM-PS
29.673
3.323
32.66
IIAM-IX
4.564
1.156
6.35
IIAM-SIAM
6.234
1.505
8.5
Min 6.9
5.09
23.72 2.24
3.81
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Deviation
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Standard
Distances
PIAM: posterior lip of internal acoustic meatus; O: opercule; PR: petrous ridge; PS: petrosigmoid intersection; IIAM: inferior lip of internal acoustic
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meatus; IX: glossopharyngeal nerve; SIAM: superior lip of internal acoustic
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meatus; C: cerebellum before retraction; C1: cerebellum after retraction
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Bone residual
Standard Deviation
Max
Min
0
0.97
1.220
3.8
% of opened IAM
88.95%
0.124
100%
Angle of drilling (°)
65.91
9.202
79.1
Craniotomy
46.39
11.812
63.7
62.93% 45.4
30.2
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Mean
Standard Deviation
Max
Min 0.25
0.73
0.529
1.65
^pPSC
1.02
0.531
2.11
^SSC
0.88
0.463
1.75
^CC
0.73
0.255
1.16
^VA
0.85
0.304
1.27
^F
0.73
0.407
1.5
0.35
0.39
0.34
0.35
0.22
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^aPSC
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aPSC: anterior part of posterior semicircular canal; pPSC: posterior part of posterior semicircular canal; SSC: superior semicircular
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canal; CC: common crus; VA: vestibular aqueduct; F: fundus
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Highlights A neuronavigation assisted retrosigmoid approach to the fundus of the IAM is described. The anatomy of the inner ear structures embedded in the petrous bone is shown.
The average length of the IAM accessible was 88.95%.
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The mean residual bone on the fundus was 0.97 mm.
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The presented technique could facilitate the opening of IAM preserving the labyrinth.
ACCEPTED MANUSCRIPT Abbreviations
AFB: acoustic-facial bundle aPSC: anterior part CC: common crus
F: fundus of internal acoustic meatus IAM: internal acoustic meatus IIAM: inferior lip of internal acoustic meatus
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IX: entry point of IX cranial nerve in the jugular foramen LCN: lower cranial nerves
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LSC: lateral semicircular canal O: opercule of endolymphatic sac
PIAM: posterior lip of the internal acoustic meatus
pPSC: posterior part of posterior semicircular canal PSC: posterior semicircular canal
SIAM: superior lip of internal acoustic meatus
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SSC: superior semicircular canal VA: vestibula aqueduct
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VE: vestibule
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ES: endolymphatic sac