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Surgical Anatomy of the Human Round Window Region: Implication for Cochlear Endoscopy Through the External Auditory Canal

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Surgical Anatomy of the Human Round Window Region: Implication for Cochlear Endoscopy Through the External Auditory Canal CE: ; ON-16-1; Total nos of Pages: 6; ON-16-1 Otology & Neurotology xx:xx–xx ß 2016, Otology & Neurotology, Inc. Surgical Anatomy of the Human Round Window Region: Implication for Cochlear Endoscopy Through the External Auditory Canal ÃyTakeshi Fujita, Ãy§Jung Eun Shin, zMaryBeth Cunnane, ÃyKyoko Fujita, jjSimon Henein, jjDemetri Psaltis, and ÃyKonstantina M. Stankovic ÃEaton Peabody Laboratories, Department of Otolaryngology, Massachusetts Eye and Ear Infirmary, Boston, Massachusetts; yDepartment of Otology and Laryngology; zDepartment of Radiology, Harvard Medical School, Boston, Massachusetts; §Department of Otorhinolaryngology–Head and Neck Surgery, Konkuk University Medical Center, Seoul, South Korea; and jjSchool of Engineering, E´cole Polytechnique Fe´de´rale de Laussane (EPFL), Lausanne, Switzerland Objective: To enable development of an endoscope for were excluded because of pathology. The opening of the cellular-level optical imaging of the inner ear. RW niche was located posteriorly in six bones (13%), Study Design: A prospective study of 50 cadaveric human inferiorly in 18 bones (39%), and postero-inferiorly in 22 temporal bones to define detailed surgical anatomy of the bones (48%). The angles were not statistically different round window (RW) region and the range of angles among the three orientations of the RW niche. necessary to reach the RW membrane perpendicularly via Conclusions: By correlating measurement from cadaveric the external ear canal. human temporal bones and their CT scans, we defined key Main Outcome Measure: The transcanal angle to the RW parameters necessary for designing an endoscope for intraco- membrane was surgically measured in 3D intact specimens, chlear imaging using a minimally invasive approach through and correlated with the angle calculated from temporal bone the external auditory canal. The excellent correlation computed tomography (CT) scans of the same specimens between the measurement on the CT scan and the actual obtained before and after measurements in situ. shape of the probe that was able to reach the RW through Results: Surgically measured transcanal angles to the RW the ear canal enables selection of the probe using the CT membrane correlated well with the radiographically data. Key Words: Endoscopy—Human temporal bones— measured angles. The angles ranged from 110 to 127 Round window—Sensorineural hearing loss. degrees, with the median of 115 degrees and the middle 50% ranging from 109 to 119 degrees. Four temporal bones Otol Neurotol 37:xxx–xxx, 2016. Sensorineural hearing loss is the most common sen- hearing loss, are very insensitive—they may be normal sory deficit worldwide and it most commonly originates even when 80% of cochlear neurons are missing (2). from the inner ear (1). However, state of the art clinical There is an unmet medical need to develop diagnostic imaging of the inner ear today relies on computed tools to establish cell-specific, early diagnosis of sensor- tomography (CT) scans and magnetic resonance imaging ineural hearing loss while using minimally invasive (MRI), neither of which allows identification of individ- approaches. ual cells in the inner ear. Autopsy specimens are pres- To address that need, we have explored optical imag- ently the only source of information about the cellular ing of the inner ear because optical approaches provide basis of human deafness. Clinical audiograms, which are higher spatial resolution at a lower cost than CT or MRI the current gold standard for establishing the degree of scans. We have demonstrated, in mice, that two photon imaging through the intact round window (RW) provides Address correspondence and reprint requests to Konstantina M. unprecedented resolution of unstained cochlear cells, Stankovic, 243 Charles St Boston, MA 02114, U.S.A.; including sensory hair cells and cochlear neurons, and E-mail: [email protected] their intracellular organelles (3,4). Based on these results Authors Takeshi Fujita and Jung Eun Shin contributed equally. This in mice, we aim to develop an endoscope for optical project was supported by the Bertarelli Foundation (DP, KMS), Wyss Center for Bio and Neuroengineering (DP, KMS), Lauer Tinnitus imaging of the human cochlea through the intact RW to Research Center (KMS), and Nancy Sayles Day Foundation (KMS). establish, for the first time, cellular-level diagnosis The authors disclose no conflicts of interest. of sensorineural hearing loss. A critical part of the 1 Copyright © 2016 Otology & Neurotology, Inc. Unauthorized reproduction of this article is prohibited. CE: ; ON-16-1; Total nos of Pages: 6; ON-16-1 2 T. FUJITA ET AL. endoscope design is detailed understanding of the lateral aspect of the RW niche. The angle formed by Line 1, anatomy of the RW region. To gain that knowledge, drawn in parallel with the skull base, and Line 2, drawn in the we have studied cadaveric human temporal bones surgic- middle of the RW niche, was measured (Fig. 2A). We also ally and radiographically. measured the depth of the RW niche. The measurements were Endoscopic middle ear surgery is known to allow made from a single section of an axial scan reformatted for the angle measurement between the RW membrane and EAC as visualization around the corners of the three-dimension- described above. The distance between the center of the RW ally complex structures of the middle ear and mastoid. membrane to the RW niche entrance was measured (Fig. 1C). Endoscopic otologic surgery has been gaining popularity over the last few decades (5), and new angled instruments Measuring Transcanal Angles to RW Membrane in have been designed for endoscopic cholesteatoma Cadaveric Temporal Bone surgery (6). In earlier reports, endoscopes have been Otomicroscopic examination of the RW region in cadaveric used in the middle ear to evaluate the RW membrane temporal bones was performed using a transcanal tympanotomy in cases of suspected perilymphatic fistula (7,8). Recent approach after elevating a posterior tympanomeatal flap. Four advances in cochlear implant surgery and intratympanic temporal bones were excluded because of adhesive otitis media, drug applications made it necessary to observe closely EAC cholesteatoma or chronic otitis media with a tympanic the anatomical variation of the RW niche and its relations membrane perforation. The angulation of the RW niche open- to other middle ear landmarks (9). Specifically, identify- ing was categorized as inferior if oriented completely down- ing the position of the RW niche is essential in perform- ward, posterior if directly facing the posterior mesotympanum, and postero-inferior if oriented between the first two directions ing cochlear implant surgery through posterior (12). The RW niche was drilled out as described below to tympanotomy (10). It also has been suggested that the expose the RW membrane circumferentially. A straight, 0.69- evaluation of the RW membrane before intratympanic mm-diameter iron wire was inserted through the external drug application is important to ensure that the drug can auditory canal and bent at the tip to reach the RW membrane actually reach the RW membrane (11). perpendicularly (Fig. 3, A and B). The angle of the bent wire To enable intracochlear endoscopy through the RW, was measured, and correlated with the angle calculated from we have studied detailed surgical anatomy of the RW temporal bone CT scans obtained before and after surgical region and have defined the range of angles necessary to measurements. reach the RW membrane perpendicularly via the external ear canal. By correlating direct measurements from Statistical Analysis human cadaveric temporal bones with radiographic One-way ANOVA followed by Bonferroni’s correction for measurements from CT scans of the same specimens, multiple comparisons was used to compare the three groups (designated by angulation of the RW niche opening). Pearson we specify design parameters for a clinical endoscope for correlations were performed to evaluate the association future optical imaging of the human inner ear. To the best between the angles measured in cadaveric temporal bones of our knowledge, the present study is the largest exam- and their corresponding CT scans. All analyses were conducted ination of three-dimensionally intact human temporal using STATA (STATA 11.1, STATA Corp., College Station, bones and their corresponding CT scans. The results TX, U.S.A.). P-values less than 0.05 were considered indicate a possibility to select customized endoscopic statistically significant. probes based on preoperative CT scans. RESULTS MATERIALS AND METHODS The method for measuring the angulation of the RW membrane relative to the external auditory canal is Measuring Angles and Length in CT Scans of depicted on a key axial slice of a temporal bone CT Temporal Bones scan (Fig. 1A). The transcanal angle to the RW mem- Fifty cadaveric human temporal bones with no known oto- brane, measured radiographically, ranged from 103 logic disease were studied. Gender and age of the donors were unknown. The Ethics and Human Studies Committee at the degrees to 124 degrees. Median was 113.5 degrees; Massachusetts Eye and Ear Infirmary granted protocol exemp- 25th percentile was 110 degrees; 75th percentile was tion because our study focused on de-identified autopsy speci- 116 degrees (Fig. 1B). mens. The bones were imaged using high-resolution CT. CT A key sagittal section of a temporal bone CT scan was scans were performed with a multidetector CT scanner (GE used to measure the orientation of the RW niche relative Discovery) using 0.625 mm collimation. The raw data were to the long axis of the external auditory canal (Fig. 2, Aa reconstructed at 0.5 mm intervals. Using a 3-D work station (GE and Ab). A transcanal view of this plane is schematized in Advantage), all scans were reformatted to allow identical Fig. 2Ac. The orientation of the RW niche was catego- horizontal sectioning through a plane that contained key ana- rized as inferior, posterior or postero-inferior, as detailed tomical landmarks: basal turn of the cochlea, RW niche, RW in the methods.
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