Morphologic Analysis of the Scala Tympani Using Synchrotron: Implications for Cochlear Implantation.

OBJECTIVES: To use synchrotron radiation phase-contrast imaging (SR-PCI) to visualize and measure the morphology of the entire cochlear scala tympani (ST) and assess cochlear implant (CI) electrode trajectories. METHODS: SR-PCI images were used to obtain geometric measurements of the cochlear scalar diameter and area at 5-degree increments in 35 unimplanted and three implanted fixed human cadaveric cochleae. RESULTS: The cross-sectional diameter and area of the cochlea were found to decrease from the base to the apex. This study represents a wide variability in cochlear morphology and suggests that even in the smallest cochlea, the ST can accommodate a 0.4 mm diameter electrode up to 720°. Additionally, all lateral wall array trajectories were within the anatomically accommodating insertion zone. CONCLUSION: This is the first study to use SR-PCI to visualize and quantify the entire ST morphology, from the round window to the apical tip, and assess the post-operative trajectory of electrodes. These high-resolution anatomical measurements can be used to inform the angular insertion depth that can be accommodated in CI patients, accounting for anatomical variability. LEVEL OF EVIDENCE: N/A. Laryngoscope, 134:2889-2897, 2024.

Anatomy of the Round Window Region With Relation to Selection of Entry Site Into the Scala Tympani.

OBJECTIVES/HYPOTHESIS: The aim of cochlear implantation is to safely insert an electrode array into the scala tympani (ST) while avoiding damage to surrounding structures. There is disagreement on the optimal way of entering the ST-the round window (RW) approach versus cochleostomy. Regardless of the chosen approach, it is vital to understand the regional anatomy, which is complex, difficult to conceptualize, and rarely dissected in temporal bone courses. The goal of this study was to examine the anatomy of the RW to gain more in-depth knowledge on the local relationships of the anatomical structures and propose an approach for entering the ST in cochlear implant surgery tailored to the encountered anatomy. STUDY DESIGN: Cadaveric prevalence study and expert opinion with literature review. METHODS: Cadaveric temporal bone dissection (n = 13) by the first author assessing the RW anatomy. RESULTS: The round window membrane (RWM) and the osseous spiral lamina (OSL) are curved structures, each with a horizontal and a vertical part. The two horizontal portions are very closely apposed. The relationship between the OSL and the RWM determines the best site for a cochleostomy, which if required is best placed anteroinferiorly to the RWM. The distance between the oval window inferior margin and the RW membrane is less than 2 to 3 mm. The ST initially extends inferiorly and medially to the RW. CONCLUSIONS: The findings of our dissection have implications for cochlear implant surgery in aiming to avoid trauma to the OSL and basilar membrane and aid decision making in choosing the safest surgical approach. LEVEL OF EVIDENCE: 5. Laryngoscope, 131:E598-E604, 2021.

Effect of Scala Tympani Height on Insertion Depth of Straight Cochlear Implant Electrodes.

OBJECTIVE: Studies suggest lateral wall (LW) scala tympani (ST) height decreases apically, which may limit insertion depth. No studies have investigated the relationship of LW ST height with translocation rate or location. STUDY DESIGN: Retrospective review. SETTING: Cochlear implant program at tertiary referral center. SUBJECTS AND METHODS: LW ST height was measured in preoperative images for patients with straight electrodes. Scalar location, angle of insertion depth (AID), and translocation depth were measured in postoperative images. Audiologic outcomes were tracked. RESULTS: In total, 177 ears were identified with 39 translocations (22%). Median AID was 443° (interquartile range [IQR], 367°-550°). Audiologic outcomes (126 ears) showed a small, significant correlation between consonant-nucleus-consonant (CNC) word score and AID (r = 0.20, P = .027), although correlation was insignificant if translocation occurred (r = 0.11, P = .553). Translocation did not affect CNC score (P = .335). AID was higher for translocated electrodes (503° vs 445°, P = .004). Median translocation depth was 381° (IQR, 222°-399°). Median depth at which a 0.5-mm electrode would not fit within 0.1 mm of LW was 585° (IQR, 405°-585°). Median depth at which a 0.5-mm electrode would displace the basilar membrane by ≥0.1 mm was 585° (IQR, 518°-765°); this was defined as predicted translocation depth (PTD). Translocation rate was 39% for insertions deeper than PTD and 14% for insertions shallower than PTD (P = .008). CONCLUSION: AID and CNC are directly correlated for straight electrodes when not translocated. Translocations generally occur around 380° and are more common with deeper insertions due to decreasing LW ST height. Risk of translocation increases significantly after 580°.

An Optimized Robot-Based Technique for Cochlear Implantation to Reduce Array Insertion Trauma.

OBJECTIVE: To compare the intracochlear trauma induced by optimized robot-based and manual techniques with a straight electrode array prototype inserted at different lengths. STUDY DESIGN: Experimental study. SETTING: Robot-based otologic surgery laboratory. SUBJECTS AND METHODS: A prototype array was inserted at different insertion lengths (21 and 25 mm) in 20 temporal bones. The manual insertion was performed with a microforceps. The optimized approach consisted of an optimal axis insertion provided by a robot-based arm controlled by a tracking system, with a constant speed of insertion (0.25 mm/s) achieved by a motorized insertion tool. The electrode position was determined at the level of each electrode by stereomicroscopic cochlea section analysis. RESULTS: A higher number of electrodes correctly located in the scala tympani was associated with the optimized approach ( P = .03, 2-way analysis of variance). Regardless of the insertion technique used, the array inserted at 25 mm allowed complete insertion of the active stimulating portion of the array in all cases. Insertion depth was greater when the array was inserted to 25 mm versus 21 mm ( P < .001, 2-way analysis of variance). The optimized insertion was associated with less trauma than that from manual insertion regardless the length of the inserted array ( P = .04, 2-way analysis of variance). CONCLUSION: Compared with a manual insertion, intracochlear trauma could be reduced with array insertion performed on an optimal axis by using motorized insertion and by applying a constant insertion speed.

Atraumatic Scala Tympani Cochleostomy; Resolution of the Dilemma.

OBJECTIVES: While an accurate placement in cochleostomy is critical to ensure appropriate insertion of the cochlear implant (CI) electrode into the scala tympani (ST), the choice of preferred cochleostomy sites widely varied among experienced surgeons. We present a novel technique for precise yet readily applicable localization of the optimum site for performing ST cochleostomy. MATERIAL AND METHODS: Twenty fresh frozen temporal bones were dissected using the mastoidectomy-posterior tympanotomy approach. Based on the facial nerve and the margins of the round window membrane (RWM), the cochleostomy site was chosen to insert the electrode into the ST while preserving the surrounding intracochlear structures. RESULTS: There is a limited safe area suitable for the ST implantation in the area inferior and anterior to the RWM. There is a higher risk of scala vestibuli (SV) insertion anterior to that area. Posterior to that area, the cochlear aqueduct (CA) and inferior cochlear vein (ICV) are liable for the injury. CONCLUSION: For atraumatic CI, precise and easy localization of the site of cochleostomy play a pivotal role in preserving intracochlear structures. Accurate setting of the vertical and horizontal orientations is mandatory before choosing the site of cochleostomy. The facial nerve and the margins of the RWM offer a very helpful clue for such localization; meanwhile, it is readily identifiable in the surgical field.

The Crista Fenestra and Its Impact on the Surgical Approach to the Scala Tympani during Cochlear Implantation.

OBJECTIVE: The aim of this work was to describe the dimensions of the crista fenestra and determine its presence by means of high-resolution computed tomography (CT) for the purpose of cochlear implantation via the round window approach. METHODS: A series of 10 adult human temporal bones underwent high-resolution CT scanning and were further dissected for microscopic study of the round window niche. RESULTS: In all of the specimens, the round window membrane was fully visualized after the complete removal of bony overhangs. The crista fenestra was identified as a sharp bony crest located in the anterior and inferior borders of the niche; its area ranged from 0.28 to 0.80 mm2 (mean 0.51 ± 0.18). The proportion of the area occupied by the crista fenestra in the whole circumference of the round window ranged from 23 to 50% (mean 36%). We found a moderate positive correlation between the area of the niche and the dimensions of the crista fenestra (Spearman rho: 0.491). In every case, high-resolution CT scanning was unable to determine the presence of the crista fenestra. CONCLUSION: The crista fenestra occupies a variable but expressive area within the bony round window niche. Narrower round window niches tended to house smaller crests. The presence of the crista fenestra is an important obstacle to adequate access to the scala tympani. Nevertheless, a high-resolution CT scan provides no additional preoperative information with regard to its presence for the purpose of surgical access to the scala tympani via the round window niche.

The Effect of Scala Tympani Morphology on Basilar Membrane Contact With a Straight Electrode Array: A Human Temporal Bone Study.

HYPOTHESIS: Scala tympani morphology influences the insertion dynamics and intra-scalar position of straight electrode arrays. BACKGROUND: Hearing preservation is the goal of cochlear implantation with current thin straight electrode arrays. These hug the lateral wall, facilitating full, atraumatic insertions. However, most studies still report some postoperative hearing loss. This study explores the influence of scala tympani morphology on array position relative to the basilar membrane and its possible contribution to postoperative hearing loss. MATERIALS AND METHODS: Twenty-six fresh-frozen human temporal bones implanted with a straight electrode array were three-dimensionally reconstructed from micro-photographic histological sections. Insertion depth and the proximity between the array and basilar membrane were recorded. Lateral wall shape was quantified as a curvature ratio. RESULTS: Insertion depths ranged from 233 to 470 degrees. The mean first point of contact between the array and basilar membrane was 185 degrees; arrays tended to remain in contact with the membrane after first contacting it. Eighty-nine and 93% of arrays that reached the upper basal (>240-360 degrees) and second (>360-720 degrees) turns respectively contacted the basilar membrane in these regions. Scalar wall curvature ratio decreased significantly (the wall became steeper) from the basal to second turns. This shift correlated with a reduced distance between the array and basilar membrane. CONCLUSION: Scala tympani morphology influences the insertion dynamics and intra-scalar position of a straight electrode array. In addition to gross trauma of cochlear structures, contact between the array and basilar membrane and how this impacts membrane function should be considered in hearing preservation cases.

Micro-optical coherence tomography of the mammalian cochlea.

The mammalian cochlea has historically resisted attempts at high-resolution, non-invasive imaging due to its small size, complex three-dimensional structure, and embedded location within the temporal bone. As a result, little is known about the relationship between an individual's cochlear pathology and hearing function, and otologists must rely on physiological testing and imaging methods that offer limited resolution to obtain information about the inner ear prior to performing surgery. Micro-optical coherence tomography (µOCT) is a non-invasive, low-coherence interferometric imaging technique capable of resolving cellular-level anatomic structures. To determine whether µOCT is capable of resolving mammalian intracochlear anatomy, fixed guinea pig inner ears were imaged as whole temporal bones with cochlea in situ. Anatomical structures such as the tunnel of Corti, space of Nuel, modiolus, scalae, and cell groupings were visualized, in addition to individual cell types such as neuronal fibers, hair cells, and supporting cells. Visualization of these structures, via volumetrically-reconstructed image stacks and endoscopic perspective videos, represents an improvement over previous efforts using conventional OCT. These are the first µOCT images of mammalian cochlear anatomy, and they demonstrate µOCT's potential utility as an imaging tool in otology research.

Hydrops of the organ of Corti.

OBJECTIVES: The authors would like to confirm a fluid pathway from the scala tympani to the organ of Corti, and to observe its morphological changes. METHODS: A staining solution for succinic dehydrogenase was perfused with phenazine methosulfate in the scala tympani of living guinea pigs (n = 5) under deep anesthesia. After fixation, the cochleas were eventually embedded in epon. Sections were observed under a light microscope. RESULTS: Blue-stained tissue is indicative of the pathway taken by the solution. The staining solution entered the organ of Corti through Hensen-Deiters' slit. The slit widened and Hensen's cells were pushed laterally. A new space was formed medial to Hensen's cells. Cortilymphatic hydrops developed. CONCLUSION: The Hensen-Deiters' slit is a pathway of a certain staining solution from the scala tympani to inside the organ of Corti of the guinea pig. The influx of the fluid pushes Hensen's cells laterally and upward, resulting in a formation of hydrops of the organ of Corti or cortilymphatic hydrops. The hydrops is observed in animals with experimental perilymphatic fistula and with viral labyrinthitis. At the end stage of the hydrops, only the surface of the organ of Corti remains as a thin layer without any cellular elements.

Round window chamber and fustis: endoscopic anatomy and surgical implications.

The round window region is of critical importance in the anatomy of the middle ear. The aim of this paper is to describe its anatomy from an endoscopic point of view, emphasizing structures that have important surgical implications, in particular the fustis and the subcochlear canaliculus. The fustis, a smooth bony structure that forms the floor of the round window region, is a constant and important structure. It seems to indicate the round window membrane and the correct position of scala tympani. A structure connecting the round window region to the petrous apex, named the subcochlear canaliculus, is also described. A retrospective review of video recordings of endoscopic dissection and surgical procedures, carried out between June 2014 and February 2015, was conducted across two Tertiary university referral centers. A total of 42 dissections were analyzed in the study. We observed the fustis in all the cases and we identify two different anatomical conformations. The subcochlear canaliculus was found in 81.0 %, with a pneumatization direct to the petrous apex in 47.7 %. Conformation and limits of the round window niche may influence the surgical view of the round window membrane. Endoscopic approaches allow a very detailed view, which enables a comprehensive exploration of the round window region. Accurate knowledge of the anatomical relationships of this region has important advantages during middle ear surgery.

Diverse patterns of perilymphatic space enhancement in the rat inner ear after intratympanic injection of two different types of gadolinium: a 9.4-tesla magnetic resonance study.

OBJECTIVE: To compare the quality of perilymphatic enhancement in the rat inner ear after intratympanic injection of two types of gadolinium with a 9.4-tesla micro-MRI. MATERIALS AND METHODS: Gadolinium was injected into the middle ear in 6 Sprague-Dawley rats via the transtympanic route. The left ear was injected with Gd-DO3A-butrol first, and then the right ear was injected with Gd-DOTA. MR images of the inner ear were acquired 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, and 4 h after intratympanic (IT) injection using an Agilent MRI system 9.4T/160/AS. The normalized signal intensity was quantitatively analyzed at the scala vestibuli (SV), scala media, and scala tympani (ST) using a Marosis M-view system. Then the normalized signal intensities (SIs) were compared between the two contrast agents. RESULTS: For Gd-DO3A-butrol, the SI was as low as 1.0-1.5 throughout 1-4 h at the SV and ST of the basal turn. The maximum SI was 1.5 ± 0.5 at the SV (2 h) and 1.3 ± 0.5 at the ST (2 h). For Gd-DOTA, the 1-hour postinjection SI at the basal turn was 2.5 ± 0.5 at the SV, 1.6 ± 0.3 at the ST, and 1.2 ± 0.3 at the scala media. In the apical turn, the maximum SI was reached after 2.5 h. The maximum SI in the apical turn was 1.8 ± 0.4 at the SV (3.5 h), 1.8 ± 0.4 at the ST (4 h), and 1.4 ± 0.3 at the scala media (4 h). CONCLUSIONS: We were able to clearly visualize and separate the ST and SV using IT Gd and 9.4-tesla micro-MRI. We recommend using Gd-DO3A-butrol over Gd-DOTA and to perform the MRI 2.5 h after using IT Gd in the rat inner ear.

Fabrication and evaluation of an improved polymer-based cochlear electrode array for atraumatic insertion.

An atraumatic cochlear electrode array has become indispensable to high-performance cochlear implants such as electric acoustic stimulation (EAS), wherein the preservation of residual hearing is significant. For an atraumatic implantation, we propose and demonstrate a new improved design of a cochlear electrode array based on liquid crystal polymer (LCP), which can be fabricated by precise batch processes and a thermal lamination process, in contrast to conventional wire-based cochlear electrode arrays. Using a thin-film process of LCP-film-mounted silicon wafer and thermal press lamination, we devise a multi-layered structure with variable layers of LCP films to achieve a sufficient degree of basal rigidity and a flexible tip. A peripheral blind via and self-aligned silicone elastomer molding process can reduce the width of the array. Measuring the insertion and extraction forces in a human scala tympani model, we investigate five human temporal bone insertion trials and record electrically evoked auditory brainstem responses (EABR) acutely in a guinea pig model. The diameters of the finalized electrode arrays are 0.3 mm (tip) and 0.75 mm (base). The insertion force with a displacement of 8 mm from a round window and the maximum extraction force are 2.4 mN and 34.0 mN, respectively. The electrode arrays can be inserted from 360° to 630° without trauma at the basal turn. The EABR data confirm the efficacy of the array. A new design of LCP-based cochlear electrode array for atraumatic implantation is fabricated. Verification indicates that foretells the development of an atraumatic cochlear electrode array and clinical implant.

Variations in microanatomy of the human cochlea.

The human cochlea shows considerable interindividual variability in size and morphology. In order to develop atraumatic cochlear implant (CI) electrodes, high-precision details of the variability of human anatomy are required. Sixteen human temporal bones were cut around the cochlea in blocks of approximately 3.5 × 3.5 cm. The bones were scanned by using a Skyscan 1173 micro-computed tomography (µCT) device. Mimics software (Materialise, Leuven, Belgium) was used to segment out the scala tympani (ST) from the µCT images. A three-dimensional surface model of the segmented area was generated for each cochlea. Cross-sectional images were taken and analyzed by custom-designed software in MATLAB. Comparison of different STs showed large variability in cross-sectional diameter (CSD), vertical trajectory, and height of the ST. Relative standard deviations of the CSD were between 9 and 15%. Heights measured at the center of the ST exceeded those in the modiolar and lateral regions of the scala. At the lateral region, the height decreased significantly at the beginning of the second turn. In the vertical trajectory, critical anatomic features were observed, such as dips, vertical jumps, and peaks. Rosenthal's canal (RC) extended to between 560 and 650°. We found a correlation between the length of the RC and that of the ST. The ST was segmented and the internal dimensions measured by using µCT. We observed large dimensional variability between different STs. These differences could have considerable implications for approaches to the design of CI arrays, especially in terms of their ability to preserve residual hearing during insertion of the electrode array.

Cochleostomy versus round window insertions: influence on functional outcomes in electric-acoustic stimulation of the auditory system.

OBJECTIVE: To assess the potential influence of 2 different surgical access routes to scala tympani for hearing preservation cochlear implantation on outcomes. STUDY DESIGN: Retrospective review. SETTING: Tertiary care academic center. PATIENTS: Twenty adult subjects participating in a prospective clinical trial on electric-acoustic stimulation. Subjects underwent cochlear implantation with attempted hearing preservation and subsequent ipsilateral electric-acoustic stimulation of the auditory system. INTERVENTIONS: Eight subjects (40%) were implanted using a round window-related cochleostomy and 12 subjects (60%) via a round window approach. MAIN OUTCOME MEASURES: Postoperative acoustic hearing preservation and speech perception measures were obtained at defined intervals and compared for both groups with and without the use of the external speech processor. RESULTS: The data demonstrate no statistically significant differences in postoperative outcomes for both preservation of residual hearing and unaided and aided speech perception between the cochleostomy and round window groups. CONCLUSION: The results of this investigation suggest that hearing preservation cochlear implantation can be performed either via a round window-related cochleostomy as well as via the round window membrane itself with similar outcomes in terms of both hearing preservation rates as well as speech perception measures.

Evidence of inner ear contribution in bone conduction in chinchilla.

We investigated the contribution of the middle ear to the physiological response to bone conduction stimuli in chinchilla. We measured intracochlear sound pressure in response to air conduction (AC) and bone conduction (BC) stimuli before and after interruption of the ossicular chain at the incudo-stapedial joint. Interruption of the chain effectively decouples the external and middle ear from the inner ear and significantly reduces the contributions of the outer ear and middle ear to the bone conduction response. With AC stimulation, both the scala vestibuli Psv and scala tympani Pst sound pressures drop by 30-40 dB after the interruption. In BC stimulation, Psv decreases after interruption by about 10-20 dB, but Pst is little affected. This difference in the sensitivity of the BC induced Psv and Pst to ossicular interruption is not consistent with a BC response to ossicular motion, but instead suggests a significant contribution of an inner-ear drive (e.g., cochlear fluid inertia or compressibility) to the BC response. This article is part of a special issue entitled "MEMRO 2012".

[Applied anatomy of scala tympani inlet related to cochlear implantation].

OBJECTIVE: To investigate the related parameters of the temporal bone structure for determining the position of implanting electrode into the scala tympani in cochlear implantation surgery through the facial recess and epitympanum approach. METHODS: In a surgical simulation experiment, 20 human temporal bones were studied and measured to determine the related parameters of the temporal bone structure. RESULTS: The distance 5.91∓0.29 mm between the short process of the incus and the round window niche, 2.11∓0.18 mm between the stapes and the round window niche, 6.70∓0.19 mm between the facial nerve in the perpendicular paragraph and the round window niche, 2.22∓0.21 mm from the pyramidal eminence to the round window, and 2.16∓0.14 mm between the stapes and the round window. The minimal distance between the implanting electrode and the vestibular window was 2.12∓0.19 mm. The distance between the cochleariform process and the round window niche was 3.79∓0.17 mm. The position of the cochlear electrode array insertion into the second cochlear turn was 2.25∓0.13 mm under the stapes. The location of the cochlear electrode array insertion into the second cochlear turn was 2.28∓0.20 mm inferior to the pyramidal eminence. CONCLUSION: These parameters provide a reference value to determine the different positions of cochlear electrode array insertion into the scale tympani in different patients.

Temporal bone characterization and cochlear implant feasibility in the common marmoset (Callithrix jacchus).

The marmoset (Callithrix jacchus) is a valuable non-human primate model for studying behavioral and neural mechanisms related to vocal communication. It is also well suited for investigating neural mechanisms related to cochlear implants. The purpose of this study was to characterize marmoset temporal bone anatomy and investigate the feasibility of implanting a multi-channel intracochlear electrode into the marmoset scala tympani. Micro computed tomography (microCT) was used to create high-resolution images of marmoset temporal bones. Cochlear fluid spaces, middle ear ossicles, semicircular canals and the surrounding temporal bone were reconstructed in three-dimensional space. Our results show that the marmoset cochlea is ∼16.5 mm in length and has ∼2.8 turns. The cross-sectional area of the scala tympani is greatest (∼0.8 mm(2)) at ∼1.75 mm from the base of the scala, reduces to ∼0.4 mm(2) at 5 mm from the base, and decreases at a constant rate for the remaining length. Interestingly, this length-area profile, when scaled 2.5 times, is similar to the scala tympani of the human cochlea. Given these dimensions, a compatible multi-channel implant electrode was identified. In a cadaveric specimen, this electrode was inserted ¾ turn into the scala tympani through a cochleostomy at ∼1 mm apical to the round window. The depth of the most apical electrode band was ∼8 mm. Our study provides detailed structural anatomy data for the middle and inner ear of the marmoset, and suggests the potential of the marmoset as a new non-human primate model for cochlear implant research.

Sheep as a large animal model for middle and inner ear implantable hearing devices: a feasibility study in cadavers.

OBJECTIVE: Currently, no large animal model exists for surgical-experimental exploratory analysis of implantable hearing devices. In a histomorphometric study, we sought to investigate whether sheep or pig cochleae are suitable for this purpose and whether device implantation is feasible. METHODS: Skulls of pig and sheep cadavers were examined using high-resolution 128-slice computed tomography (CT) to study anatomic relationships. A cochlear implant and an active middle ear implant could be successfully implanted into the sheep's inner and middle ear, respectively. Correct device placement was verified by CT and histology. The cochlear anatomy of the sheep was further studied by micro-CT and histology. RESULTS: Our investigations indicate that the sheep is a suitable animal model for implantation of implantable hearing devices. The implantation of the devices was successfully performed by access through a mastoidectomy. The histologic, morphologic, and micro-CT study of the sheep cochlea showed that it is highly similar to the human cochlea. The temporal bone of the pig was not suitable for these microsurgical procedures because the middle and inner ear were not accessible owing to distinct soft and fatty tissue coverage of the mastoid. CONCLUSION: The sheep is an appropriate large animal model for experimental studies with implantable hearing devices, whereas the pig is not.

Direct entry of gadolinium into the vestibule following intratympanic applications in Guinea pigs and the influence of cochlear implantation.

Although intratympanic (IT) administration of drugs has gained wide clinical acceptance, the distribution of drugs in the inner ear following IT administration is not well established. Gadolinium (Gd) has been previously used as a marker in conjunction with magnetic resonance imaging (MRI) to visualize distribution in inner ear fluids in a qualitative manner. In the present study, we applied gadolinium chelated with diethylenetriamine penta-acetic acid (Gd-DTPA) to the round window niche of 12 guinea pigs using Seprapack(TM) (carboxlmethylcellulose-hyaluronic acid) pledgets which stabilized the fluid volume in the round window niche. Gd-DTPA distribution was monitored sequentially with time following application. Distribution in normal, unperforated ears was compared with ears that had undergone a cochleostomy in the basal turn of scala tympani and implantation with a silastic electrode. Results were quantified using image analysis software. In all animals, Gd-DTPA was seen in the lower basal scala tympani (ST), scala vestibuli (SV), and throughout the vestibule and semi-circular canals by 1 h after application. Although Gd-DTPA levels in ST were higher than those in the vestibule in a few ears, the majority showed higher Gd-DTPA levels in the vestibule than ST at both early and later time points. Quantitative computer simulations of the experiment, taking into account the larger volume of the vestibule compared to scala tympani, suggest most Gd-DTPA (up to 90%) entered the vestibule directly in the vicinity of the stapes rather than indirectly through the round window membrane and ST. Gd-DTPA levels were minimally affected by the implantation procedure after 1 h. Gd-DTPA levels in the basal turn of scala tympani were lower in implanted animals, but the difference compared to non-implanted ears did not reach statistical significance.

Automatic segmentation of intracochlear anatomy in conventional CT.

Cochlear implant surgery is a procedure performed to treat profound hearing loss. Clinical results suggest that implanting the electrode in the scala tympani, one of the two principal cavities inside the cochlea, may result in better hearing restoration. Segmentation of intracochlear cavities could thus aid the surgeon to choose the point of entry and angle of approach that maximize the likelihood of successful implant insertion, which may lead to more substantial hearing restoration. However, because the membrane that separates the intracochlear cavities is too thin to be seen in conventional in vivo imaging, traditional segmentation techniques are inadequate. In this paper, we circumvent this problem by creating an active shape model with micro CT (µCT) scans of the cochlea acquired ex vivo. We then use this model to segment conventional CT scans. The model is fitted to the partial information available in the conventional scans and used to estimate the position of structures not visible in these images. Quantitative evaluation of our method, made possible by the set of µCTs, results in Dice similarity coefficients averaging 0.75. Mean and maximum surface errors average 0.21 and 0.80 mm.