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.

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°.

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.

[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.

[Applied anatomic study on lamina spiralis below promontory for cochlear implantation]

OBJECTIVE

To provide anatomic data for cochlear implantation, and to find the method of locating lamina spiralis (LS) on the surface of promontory.

METHOD

Microanatomical study was carried out on 30 sides of human temporal bones by observing and measuring lamina spiralis below promontory, including its location, course and adjacent structures.

RESULT

(1) The basal turn of lamina spiralis below promontory can be divided into three segments: the hook segment (1.52 +/- 0.16) mm, the anteroinferior round window segment (3.83 +/- 0.37) mm and the forwarding segment (2.70 +/- 0.36) mm by two hinge points of which one was located at anterior of the junction of superior margin and anterior border of RW, and the other was located at anteroinferior of the round window; (2) The plane of round window anteroinferior segment of LS lay (51.00 +/- 5.97) degrees anteroinferior to horizontal segment of the facial nerve and comparative permanently meet posterior margin of'stapes head. Made posterior margin of stapes head as a fixation point and draw a line on promontory lay (51.00 +/- 45.97) degrees anteroinferior to horizontal segment of the facial nerve. This line can be thought as the projection of anteroinferior round window segment of LS on promontory; (3) The width of scala tympani at cochleostomy site on promontory: width of scala tympani at midpoint of superior margin of round window was (0.36 +/- 0.06) mm; width of scala tympani at midpoint of anterior border of round window was (0.97 +/- 0.14) mm; width of scala tympani at 3 mm point of anteroinferior round window segment was (1.24 +/- 0.21) mm.

CONCLUSION

(1) The basal turn lamina spiralis below promontory can be divided into three segments (the hook segment, the anteroinferior round window segment and the forwarding segment) by two hinge points; (2) The projection of anteroinferior round window segment of LS and the features exhibited in its course provide reference for locating the basal turn scala tympani and offer reliable anatomical basis for minimal invasive intervention during cochlear implantation.

Scala Tympani Cochleostomy II: Topography and Histology

OBJECTIVE

To assess intracochlear trauma using two different round window-related cochleostomy techniques in human temporal bones.

METHODS

Twenty-eight human temporal bones were included in this study. In 21 specimens, cochleostomies were initiated inferior to the round window (RW) annulus. In seven bones, cochleostomies were drilled anterior-inferior to the RW annulus. Limited cochlear implant electrode insertions were performed in 19 bones. In each specimen, promontory anatomy and cochleostomy drilling were photographically documented. Basal cochlear damage was assessed histologically and electrode insertion properties were documented in implanted bones.

RESULTS

All implanted specimens showed clear scala tympani electrode placements regardless of cochleostomy technique. All 21 inferior cochleostomies were atraumatic. Anterior-inferior cochleostomies resulted in various degrees of intracochlear trauma in all seven bones.

CONCLUSION

For atraumatic opening of the scala tympani using a cochleostomy approach, initiation of drilling should proceed from inferior to the round window annulus, with gradual progression toward the undersurface of the lumen. While cochleostomies initiated anterior-inferior to the round window annulus resulted in scala tympani opening, many of these bones displayed varying degrees of intracochlear trauma that may result in hearing loss. When intracochlear drilling is avoided, the anterior bony margin of the cochleostomy remains a significant intracochlear impediment to in-line electrode insertion.

Scalar localization of the electrode array after cochlear implantation: clinical experience using 64-slice multidetector computed tomography

OBJECTIVE

To use the improved resolution available with 64-slice multidetector computed tomography (MDCT) in vivo to localize the cochlear implant electrode array within the basal turn.

STUDY DESIGN

Sixty-four-slice MDCT examinations of the temporal bones were retrospectively reviewed in 17 patients. Twenty-three implants were evaluated.

SETTING

Tertiary referral facility.

PATIENTS

All patients with previous cochlear implantation evaluated at our center between January 2004 and March 2006 were offered a computed tomographic examination as part of the study. In addition, preoperative computed tomographic examinations in patients being evaluated for a second bilateral device were included.

INTERVENTION

Sixty-four-slice MDCT examination of the temporal bones.

MAIN OUTCOME MEASURE

Localization of the electrode array within the basal turn from multiplanar reconstructions of the cochlea.

RESULTS

Twenty-three implants were imaged in 17 patients. We were able to localize the electrode array within the scala tympani within the basal turn in 10 implants. In 3 implants, the electrode array was localized to the scala vestibuli. Migration of the electrode array from scala tympani to scala vestibuli was observed in three implants. Of the 7 implants in which localization of the electrode array was indeterminate, all had disease entities that obscured the definition of the normal cochlear anatomy.

CONCLUSIONS

Sixty-four-slice MDCT with multiplanar reconstructions of the postoperative cochlea after cochlear implantation allows for accurate localization of the electrode array within the basal turn where normal cochlear anatomy is not obscured by the underlying disease process. Correlating the position of the electrode in the basal turn with surgical technique and implant design could be helpful in improving outcomes.

Design and fabrication of multichannel cochlear implants for animal research

The effectiveness of multichannel cochlear implants depends on the activation of perceptually distinct regions of the auditory nerve. Increased information transfer is possible as the number of channels and dynamic range are increased and electrical and neural interaction among channels is reduced. Human and animal studies have demonstrated that specific design features of the intracochlear electrode directly affect these performance factors. These features include the geometry, size, and orientation of the stimulating sites, proximity of the device to spiral ganglion neurons, shape and position of the insulating carrier, and the stimulation mode (monopolar, bipolar, etc.). Animal studies to directly measure the effects of changes in electrode design are currently constrained by the lack of available electrodes that model contemporary clinical devices. This report presents methods to design and fabricate species-specific customizable electrode arrays. We have successfully implanted these arrays in guinea pigs and cats for periods of up to 14 months and have conducted acute electrophysiological experiments in these animals. Modifications enabling long-term intracochlear drug infusion are also described. Studies using these scale model arrays will improve our understanding of how these devices function in human subjects and how we can best optimize future cochlear implants.

Imaging the living inner ear using intravital confocal microscopy

Confocal laser scanning microscopy permits detailed visualization of structures deep within thick fluorescently labeled specimen. This makes it possible to investigate living cells inside intact tissue without prior chemical sample fixation and sectioning. Isolated guinea pig temporal bones have previously been used for confocal experiments in vitro, but tissue deterioration limits their use to a few hours after the death of the animal. In order to preserve the cochlea in an optimal functional and physiological condition, we have developed an in vivo model based on a confocal microscopy approach. Using a ventral surgical approach, the inner ear is exposed in deeply anaesthetized, tracheotomized, living guinea pigs. To label the inner ear structures, scala tympani is perfused via an opening in the basal turn, delivering tissue culture medium with fluorescent vital dyes (RH 795 and calcein AM). An apical opening is made in the bony shell of cochlea to enable visualization using a custom-built objective lens. Intravital confocal microscopy, with preserved blood and nerve supply, may offer an important tool for studying auditory physiology and the pathology of hearing loss. After acoustic overstimulation, shortening and swelling of the sensory hair cells were observed.

The consequences of neural degeneration regarding optimal cochlear implant position in scala tympani: a model approach

Cochlear implant research endeavors to optimize the spatial selectivity, threshold and dynamic range with the objective of improving the speech perception performance of the implant user. One of the ways to achieve some of these goals is by electrode design. New cochlear implant electrode designs strive to bring the electrode contacts into close proximity to the nerve fibers in the modiolus: this is done by placing the contacts on the medial side of the array and positioning the implant against the medial wall of scala tympani. The question remains whether this is the optimal position for a cochlea with intact neural fibers and, if so, whether it is also true for a cochlea with degenerated neural fibers. In this study a computational model of the implanted human cochlea is used to investigate the optimal position of the array with respect to threshold, dynamic range and spatial selectivity for a cochlea with intact nerve fibers and for degenerated nerve fibers. In addition, the model is used to evaluate the predictive value of eCAP measurements for obtaining peri-operative information on the neural status. The model predicts improved threshold, dynamic range and spatial selectivity for the peri-modiolar position at the basal end of the cochlea, with minimal influence of neural degeneration. At the apical end of the array (1.5 cochlear turns), the dynamic range and the spatial selectivity are limited due to the occurrence of cross-turn stimulation, with the exception of the condition without neural degeneration and with the electrode array along the lateral wall of scala tympani. The eCAP simulations indicate that a large P(0) peak occurs before the N(1)P(1) complex when the fibers are not degenerated. The absence of this peak might be used as an indicator for neural degeneration.

Communication between the perilymphatic scalae and spiral ligament visualized by in vivo MRI

We evaluated the transport of Gadolinium-diethylenetriaminepentaacetate-bismethylamide (Gd-DTPA-BMA) through the round window (RW) membrane into the perilymphatic space with 4.7-T MRI in an animal study and 1.5-T MRI in humans. After administration of Gd-DTPA-BMA onto the intact RW membrane of guinea pig, Gd-DTPA-BMA uptake was observed in the basal turn and part of the second turn within 40 min. The scala tympani, scala vestibuli, the fibrous part of the spiral ligament and semicircular canal all showed uptake of Gd-DTPA-BMA. All turns of the cochlea were filled with Gd within 10 min in the perforated RW membrane administration group and within 30 min in the intravenous administration group. In patients who accepted middle ear injection of Gd-DTPA-BMA, uptake was observed within 2 h in the basal turn and semicircular canal. After 12 h the apex did still not show any uptake. Gd-DTPA-BMA is transported from the RW to the semicircular canal, the scala tympani and scala vestibuli without passing the helicotrema.

Stiffness properties for Nucleus standard straight and contour electrode arrays

Trauma and damage during insertion of electrode arrays into the human cochlea are strongly related to the stiffness of the array. The stiffness properties of electrode arrays, which were determined by three-point flexural bending and buckling tests, are reported in this paper. To date there has been limited publication on mechanical properties of these electrode arrays. Previous studies mainly focused on characterizing the stiffness of the tip of the Nucleus straight array with little emphasis on characterizing the stiffness of its whole length. In this study, stiffnesses of the Nucleus straight and contour electrode arrays have been determined along their length. Young's modulus of elasticity of the Nucleus straight array has been found to increase from the tip (182 MPa) to the rear end (491 MPa), whereas the stiffness of the contour array is greatest near the tip (480 MPa) and is fairly uniform in the middle and rear sections of the electrode array (380-400 MPa). Buckling experiments have shown that the contour array has much higher critical buckling load (about four times) than the Nucleus straight array. The results from three-point flexural bending and buckling experiments provide significant data for the development of electrode arrays, from which new array designs with improved flexibility can be developed. The results of stiffness properties are also important input for use in finite element models to predict the trajectories during insertion and to help evaluate the effects of different electrode array designs on damage sustained during insertion.

The topographical anatomy of the round window and related structures for the purpose of cochlear implant surgery

The treatment of total deafness using a cochlear implant has now become a routine medical procedure. The tendency to expand the audiological indications for cochlear stimulation and to preserve the remnants of hearing has brought new problems. The authors have studied the topographical anatomy of the internal structures of the ear in the area where cochleostomy is usually performed and an implant electrode inserted. Ten human temporal bones were obtained from cadavers and prepared in a formalin stain. After dissection of the bone in the area of round and oval windows, the following diameters were measured using a microscope with a scale: the transverse diameters of the cochlear and vestibular scalae at the level of the centre of the round window and 0.5 mm anteriorly to the round window, the distance between the windows and the distances from the end of the spiral lamina to the centre of the round window and to its anterior margin. The width of the cochlear scala at the level of the round window was 1.23 mm, and 0.5 mm anteriorly to the round window membrane it was 1.24 mm. The corresponding diameters for the vestibular scala are 1.34 and 1.27 mm. The distances from the end of the spiral lamina to the centre of the round window and to its anterior margin are 1.26 and 2.06 respectively. The authors noted that the two methods of electrode insertion show a difference of 2 mm in the length of the stimulated spiral lamina. The average total length of the unstimulated lamina is 2.06 and 4.06 in the two situations respectively.

Dimensions of the vestibular and tympanic scalae of the cochlea in selected mammals

The spiral shaped organ of hearing occurs only in mammals. This shape creates good conditions for the acoustic wave inside the cochlea. There are various forms of the cochlea in different species of mammal: the number of turns ranges from 1.5 to 4.5, a fact for which there seems no obvious explanation. In order to become more familiar with the geometry of the cochlear scalae in animals, a microanatomical study was carried out on 40 temporal bones, obtained from four common species of mammal: cat, dog, cattle and macaca. The bones were dissected with the aid of an operation microscope using standard otosurgical equipment, in which their perilymphatic spaces were filled with latex and further prepared in a formalin stain. Each of the rubber molds was removed from the osseous matrix and subsequently manually cut into 1 mm segments. The results, presented in diagrams, indicate that the vestibular and tympanic scalae present alternate dominance in their width and height, as was previously found in a study of humans. The change of this alternation domination appears two to five times on their entire length. The dimensions of the cochlear scalae are to a certain extent proportional to the weight of the animal: the largest were found in cattle and the smallest in the macaca.

Connection between the inner ear and the lymphatic system

OBJECTIVE

The aim of this study was to identify the lymphatic drainage of the inner ear in guinea pigs.

STUDY DESIGN

Prospective study.

METHODS

The prospective study was performed in guinea pigs by injection of keyhole limpet hemocyanin (KLH) into either the right-side scala tympani or the middle ear cavity. The left side was not injected and served as a control. Fifteen minutes after injection, the animals were killed by intracardiac perfusion with paraformaldehyde and tissue specimens (right and left temporal bones, cervical lymph nodes, and the spleen) were collected. The presence of KLH in each specimen was determined by immunohistochemical assay of frozen sections using polyclonal mouse anti-KLH antibodies.

RESULTS

After injection into the middle ear, labeled cells were identified in the parotid, superficial ventral, mandibular, and deep cranial cervical lymph nodes. However, after inner ear injections KLH was present in only the parotid and superficial ventral cervical nodes. The spleen contained KLH-positive cells following injection into either the middle or inner ear, but not all animals contained labeled spleen cells.

CONCLUSIONS

The inner ear has a connection to the lymphatic drainage system. Because fewer lymph nodes contained labeled cells after inner ear injection than after middle ear injection, it is concluded that the inner ear does not simply drain to the middle ear and subsequently to the lymph nodes but seems likely to have its own connections.

[The location of inserting the electrode into the cochlear]

OBJECTIVE

To determine the position of implanting electrode into the scala tympani.

METHOD

The analogous operation of cochlear implantation were made in 50 temporal bones and the anatomic data were measured after the operation.

RESULT

The position of implanting electrode is 2.77 mm below the vestibular window and 0.85 mm anterior to the round window.

CONCLUSION

It suggests that this anatomic parameter provide a reference value to determine the position of cochlear implant electrode into the scala tympani.

Cochlear fluid space dimensions for six species derived from reconstructions of three-dimensional magnetic resonance images

OBJECTIVES

To establish the dimensions and volumes of the cochlear fluid spaces.

STUDY DESIGN

Fluid space volumes, lengths, and cross-sectional areas were derived for the cochleas from six species: human, guinea pig, bat, rat, mouse, and gerbil.

METHODS

Three-dimensional reconstructions of the fluid spaces were made from magnetic resonance microscopy (MRM) images. Consecutive serial slices composed of isotropic voxels (25 microm3) representing the entire volume of fixed, isolated cochleas were obtained. The boundaries delineating the fluid spaces, including Reissner's membrane, were resolved for all specimens, except for the human, in which Reissner's membrane was not consistently resolved. Three-dimensional reconstructions of the endolymphatic and perilymphatic fluid spaces were generated. Fluid space length and variation of cross-sectional area with distance were derived by an algorithm that followed the midpoint of the space along the length of the spiral. The total volume of each fluid space was derived from a voxel count for each specimen.

RESULTS

Length, volume, and cross-sectional areas are provided for six species. In all cases, the length of the endolymphatic fluid space was consistently longer than that of either perilymphatic scala, primarily as a result of a greater radius of curvature. For guinea pig specimens, the measured volumes of the fluid spaces were considerably lower than those suggested by previous reports based on histological data.

CONCLUSIONS

The quantification of cochlear fluid spaces provided by this study will enable the more accurate calculation of drug and other solute movements in fluids of the inner ear during experimental or clinical manipulations.

Dimensions of the human vestibular and tympanic scalae

The cochlear scalae provide a practical access route for the insertion of cochlear implant electrodes. A microanatomical study was carried out on 25 human temporal bones obtained from cadavers. These bones were dissected with the aid of an operation microscope, in which their perilymphatic spaces were filled with coloured latex and further prepared in a formalin stain. Each of the rubber moulds was removed from the osseous matrix using standard otosurgical equipment, and subsequently cut into 1 mm segments. The height and width of the vestibular and tympanic scalae were measured. The results, presented in diagrams, indicate that the vestibular scala is less prominent than the tympanic scala in the basic and middle coil of the cochlea and in the upper coil, they display greater dimensions which could serve as a place for electrode insertion in cochlear implant procedures. In addition, the vestibular and tympanic scalae present alternate dominance in their width and height as corroborated by the calculated coefficients. The results obtained in this study supplement our knowledge of the anatomy of the cochlea thus far lacking a full investigation of the scalae, and could serve as a basis for other studies dealing with the physiology of the organs of hearing.

Comparison of three-dimensional visualization techniques for depicting the scala vestibuli and scala tympani of the cochlea by using high-resolution MR imaging

BACKGROUND AND PURPOSE

Cochlear implantation requires introduction of a stimulating electrode array into the scala vestibuli or scala tympani. Although these structures can be separately identified on many high-resolution scans, it is often difficult to ascertain whether these channels are patent throughout their length. The aim of this study was to determine whether an optimized combination of an imaging protocol and a visualization technique allows routine 3D rendering of the scala vestibuli and scala tympani.

METHODS

A submillimeter T2 fast spin-echo imaging sequence was designed to optimize the performance of 3D visualization methods. The spatial resolution was determined experimentally using primary images and 3D surface and volume renderings from eight healthy subjects. These data were used to develop the imaging sequence and to compare the quality and signal-to-noise dependency of four data visualization algorithms: maximum intensity projection, ray casting with transparent voxels, ray casting with opaque voxels, and isosurface rendering. The ability of these methods to produce 3D renderings of the scala tympani and scala vestibuli was also examined. The imaging technique was used in five patients with sensorineural deafness.

RESULTS

Visualization techniques produced optimal results in combination with an isotropic volume imaging sequence. Clinicians preferred the isosurface-rendered images to other 3D visualizations. Both isosurface and ray casting displayed the scala vestibuli and scala tympani throughout their length. Abnormalities were shown in three patients, and in one of these, a focal occlusion of the scala tympani was confirmed at surgery.

CONCLUSION

Three-dimensional images of the scala vestibuli and scala tympani can be routinely produced. The combination of an MR sequence optimized for use with isosurface rendering or ray-casting algorithms can produce 3D images with greater spatial resolution and anatomic detail than has been possible previously.

Morphology of the unfixed cochlea

Our knowledge of cochlear geometry is based largely upon anatomical observations derived from fixed, dehydrated, embedded and/or sputter-coated material. We have now developed a novel preparation, the hemicochlea, where for the first time living cochlear structures can be observed in situ and from a radial perspective. The experiments were performed on the Mongolian gerbil. Ion substitution experiments suggest that no significant swelling or shrinkage occurs when the preparation is bathed in normal culture medium, so long as calcium concentration is kept at endolymph-like (20 microM) levels. The tectorial membrane-reticular lamina relationship appears to remain well preserved. Hensen's stripe maintains a close relationship with the inner hair cell stereociliary bundle, unless the mechanical coupling becomes disturbed. In addition, standard fixation and/or dehydration procedures are used to quantify changes due to shrinkage artifacts. Various morphometric gradients are examined in unfixed specimens from apical, middle, and basal turns.

Measurement of guinea pig basilar membrane using computer-aided three-dimensional reconstruction system

Cochleas are known to have the ability to analyze a frequency widely, and this ability seems to be owed mostly to the basilar membrane (BM) configuration. However, the relationship between the cochlear frequency-position map and the BM configuration is not clear. Therefore, in this paper, the internal structures of a guinea pig cochlea, especially the BM configuration, were reconstructed and measured using a computer-aided three-dimensional (3-D) reconstruction system. Then, an attempt was made to examine the influence of the BM configuration on the cochlear frequency-position map. The measurement results indicate that the width of the BM increased and its thickness decreased with an increase in the distance from the basal turn towards the apical turn. Theoretical consideration reveals that the wide frequency-position of the cochlea is achieved by not only the BM configuration change along the length of the cochlea but also the change of the Young's modulus of the BM along the length of the cochlea.

[In vivo cochleoscopy through the round window]

The overall aim of the present investigation was to develop a technique for endoscopic investigation of the cochlea. In the experiments reported here, the possible effect of the endoscope-called the "cochleoscope"-on the electrophysiology of the cochlea was investigated by recording the cochlear action potential (CAP) threshold tuning curve from (0.1-34 kHz). The dorsolateral bulla of anesthesized guinea pigs (with ketamine 60 mg/kg and Rompun 12 mg/kg) was opened, after which the cochleoscope was introduced under micromanipulator control through the round window membrane. Three cochleoscopes were used and had diameters of 0.29 mm, 0.7 mm and 0.89 mm, respectively, containing 2000, 3000 and 3000 fibers each. Experiments in 7 animals showed that the cochleoscope did not influence CAP thresholds. Although the present resolution of the endoscopes is limited, the basilar membrane can be clearly distinguished from the osseous spiral lamina. It is anticipated that improved resolution will allow the cochleoscope to be used for diagnostic purposes in cases of sensorineural hearing loss.

The cochlear aqueduct in pediatric temporal bones

The cochlear aqueduct is a bony channel which contains the fibrous periotic duct and connects the perilymphatic space of the basal turn of the cochlea with the subarachnoid space of the posterior cranial cavity. Previous histological studies suggested that patency depended on age, whereas a more recent study showed no statistical correlation between age and patency. To clarify patency in pediatric cochlear aqueducts, we selected 21 temporal bones from 12 infants and children, varying in age from birth to 9 years, in which the cochlear aqueduct was fully visible on one histological section. Photographs were taken for documentation and the length and width of the orifice of the external aperture of the aqueduct at the scala tympani were measured and followed to the internal aperture at the subarachnoid space. The lumen of the duct was examined for mononucleated cells, blood cells and fibrous tissue. Measurements revealed that the mean length of the cochlear aqueduct was 4.6 mm (range, 2.4-10.7 mm), mean width of the external aperture was 484 microm (range, 225-869 microm), and mean width of the internal aperture was 1293 microm (range, 699-2344 microm). The mean diameter of the narrowest part (isthmus) was 151 microm (range, 75-244 microm). In all temporal bones the cochlear aqueduct was patent, with one exception. This latter temporal bone was from a 2-month-old girl with multiple intralabyrinthine anomalies, with the missing cochlear aqueduct believed to be due to an aplasia. Our results support prior measurements of the cochlear aqueduct and demonstrate a short and patent cochlear aqueduct in newborns. With growth, a significant increasing length of the duct was found.

Computer-aided three-dimensional reconstruction in human cochlear maps: measurement of the lengths of organ of Corti, outer wall, inner wall, and Rosenthal's canal

This paper describes the application of computer-aided three-dimensional reconstruction to measurements of the length of the organ of Corti (mean +/- SD, 35.58 +/- 1.41 mm), scala tympani outer wall (40.81 +/- 1.97 mm), scala tympani inner wall (18.29 +/- 1.47 mm), and Rosenthal's canal center (15.98 +/- 1.33 mm) in eight adult male cochleas. The Rosenthal's canal center ranged between 1 3/4 and 2 turns, did not appear to be linearly related to the organ of Corti, and lay near the basal end of the latter. The length of the organ of Corti measured with three-dimensional reconstruction differed by 7.6% +/- 3.2% ("cutting angle difference") from that derived from traditional two-dimensional reconstruction on the plane perpendicular to the plane of section, and by 2.6% +/- 1.7% ("geometric difference") from that derived from two-dimensional reconstruction on the axial plane at right angles to the modiolar axis.

A transparent model of the human scala tympani cavity

A dimensionally accurate clear model of the human scala tympani has been produced to evaluate the insertion and position of clinically applied intracochlear electrodes for electrical stimulation. Replicates of the human scala tympani were made from low melting point metal alloy (LMA) and from polymethylmeth-acrylate (PMMA) resin. The LMA metal casts were embedded in blocks of epoxy and in clear silicone rubber. After removal of the metal alloy, a cavity was produced that accurately models the human scala tympani. Investment casting molds were made from the PMMA scala tympani casts to enable production of multiple LMA casts from which identical models were fabricated. Total dimensional distortion of the LMA casting process was less than 1% in length and 2% in diameter. The models have been successfully integrated into the design process for the iterative development of advanced intracochlear electrode arrays at UCSF. These fabrication techniques are applicable to a wide range of biomedical design problems that require modelling of visually obscured cavities.

Three-dimensional reconstruction of the cochlea from two-dimensional images of optical sections

This paper describes a methodology for three-dimensional (3D) computer-aided reconstruction of the guinea pig cochlea using orthogonal-plane fluorescence optical sectioning. Specimens are sectioned optically, allowing them to remain intact during observation. Equations to correct the data for specimen translation and rotation are developed and 3D reconstructions of the scala tympani, round window membrane, and cochlear aqueduct are presented. The error associated with the reconstruction is estimated to be < 19 microns.

Imaging the cochlea by magnetic resonance microscopy

The isolated, fixed cochlea of the mustached bat was studied with three dimensional magnetic resonance (MR) microscopy. The cochlea of this animal is about 4 mm in diameter and its entire volume was imaged. With the field of view and matrix size used, the volume elements (voxels) making up the volume data set were isotropic 25 x 25 x 25 micron cubes. Three dimensional (3D) MR microscopy based on isotropic voxels has many advantages over commonly used light microscopy: 1) it is non destructive; 2) it is much less time consuming; 3) no dehydration is required and shrinkage is minimized; 4) the data set can be used to create sections in any desired plane; 5) the proper alignment of sections is inherent in the 3D acquisition so that no reference points are required; 6) the entire data set can be viewed from any point of view in a volume rendered image; 7) the data is digital and features can be enhanced by computer image processing; and 8) the isotropic dimensions of the voxels make the data well-suited for structural reconstructions and measurements. Good images of the osseous spiral lamina, spiral ligament, scala tympani, scala vestibuli, and nerve bundles were obtained. The vestibular (Reissner's) membrane was easily identified in the mustached bat and it appears to bulge into the scala vestibuli. The visibility of this structure suggests that MR microscopy would be well-suited for studies of endolymphatic hydrops.

High-resolution magnetic resonance imaging of the human temporal bone

A magnetic resonance imaging (MRI) system (Hitachi, Naka, Japan) with a superconductive magnet running at 2.11 T was used to obtain 2-mm-thick slices of fixed, decalcified and celloidin-embedded human temporal bone. The temporal bone was then sectioned and stained for routine histological evaluation. Both the MR images and the histological sections were in the mid-modiolar slice plane, and comparable images and sections were analyzed to confirm the identity of the inner-ear structures visualized on the MR images. The cochlear duct, scala tympani, scala vestibuli and basement membrane of all three cochlear turns were clearly imaged on MRI. In addition, the vestibule and three semicircular ducts were also clearly seen. This study raises the possibility of some day using MRI for the diagnosis of inner-ear diseases.

Dimensions of the scala tympani in the human and cat with reference to cochlear implants

The width, height, and cross-sectional area of the scala tympani in both the human and cat were measured to provide dimensional information relevant to the design of scala tympani electrode arrays. Both the height and width of the human scala tympani decreased rapidly within the first 1.5 mm from the round window. Thereafter, they exhibit a gradual reduction in their dimension with increasing distance from the round window. The cross-sectional area of the human scala tympani reflects the changes observed in both the height and width. In contrast, the cat scala tympani exhibits a rapid decrease in its dimensions over the first 6 to 8 mm from the round window. However, beyond this point the cat scala tympani also exhibits a more gradual decrease in its dimensions. Finally, the width of the scala tympani, in both human and cat, is consistently greater than the height.

Cochlea in old world mice and rats (Muridae)

Morphometric analysis of the cochlea was performed in wild and laboratory murids: Mus musculus, Apodemus sylvaticus, Rattus rattus, R. norvegicus, NMRI mouse, and Wistar rat. Results are based on light microscopic examination of surface specimens and serial sections and on three-dimensional computer reconstruction. The cochleae have 1.75-2.2 coils. The length of the basilar membrane varies from 6.0 to 12.1 mm. Mean density of outer hair cells ranges between 363 and 411, inner hair cells 98 and 121, neurons 1,230 and 1,760 per 1 mm. Following parameters change from base to apex: basilar membrane width 66.0 (+/- 8.2) to 175.0 (+/- 24.7) microns, basilar membrane thickness 17.0 (+/- 2.6) to 1.9 (+/- 0.1) microns, width of triad of outer hair cells 13.2 (+/- 0.7) to 28.8 (+/- 4.4) microns. The given numbers are mean "murid" values (with respective standard deviations). Maximum of dimensions of scalae is located at 10-15%, that of density of outer hair cells at 65%, density of inner hair cells at 2.8 mm, maximum of innervation density at 40-60% from the base. The following parameters are correlated with pinna size: length and maximum width of basilar membrane, dimensions of scalae, total number of receptors, and probably resolution capabilities. The following parameters are correlated with body size: maximum width of triad of outer hair cells, density and total number of neurons, ratio of neurons to receptors, apicobasal difference in basilar membrane stiffness and width of triad of outer hair cells; inversely proportional is receptor density and ratio of outer to inner hair cells and probably low-frequency cut-off. Thickness, and minimum width of basilar membrane and triad of outer hair cells and probably high-frequency cutoff are species-specific and independent of pinna or body size. The parameters mentioned indicate that the examined murids are acoustically unspecialized mammals and their cochleae approximate the generalized plan for a mammalian cochlea. Differences between domesticated and wild murids are stated.

Segregation of efferent projections to different turns of the guinea pig cochlea

Localized intracochlear injections of the fluorescent retrograde label diamidino yellow were used to investigate the organization of efferent projections from the brainstem to different turns of the cochlea, in the guinea pig. It was found that the location of small neurones within the lateral superior olive ipsilateral to the injection varied in a systematic manner when injections proceeded from base to apex of the cochlea. In addition, a cruder form of cochleotopic organization was present in that most of the large medial system efferent neurones were labelled only after injection into the 3 most basal turns of the cochlea. The decline of medial system efferent innervation proceeding from base to apex was most striking for the contralateral efferent neurones. The details of base to apex innervation density were different for the different nuclei of origin of the medial system, implying the existence of complex subsystems within the medial neurone population.

Electrocochleography for different electrode positions in guinea pig

In 20 guinea pigs the relation between the electrode position on the cochlear surface and the recorded potentials was studied. We found the greatest AP amplitudes at the free lateral part of the cochlea; AP amplitudes decreased when the electrode was moved into the direction of the attachment of the cochlea to the petrous bone. Moving the electrode from the basal turn to the apex the maximal AP amplitude was found at the apex. The most negative SP was always recorded from the apex. This negative SP became smaller when the electrode was moved to the basal turn. Near the round window a small negative SP was recorded for 1 kHz stimuli and a positive SP for most 4 kHz stimuli. Moving the electrode from the free lateral part of the cochlea to the place of attachment to the petrous bone the magnitude of the SP decreased in a fashion similar to the AP amplitude. For 4 kHz stimuli we recorded a range of SP-AP ratios from -0.21 to +0.50 merely by varying the electrode position. Thus the SP-AP ratio highly depends on the electrode position.

Scala tympani measurement

The length and cross-sectional height of the scala tympani are relevant to the design of cochlear implants. The lengths and heights of the scalae tympani in ten pairs of serially sectioned temporal bones were measured by an adaptation of the serial section method of cochlear reconstruction. The study found the middle segments of individual pairs of scalae tympani to be very similar in height, but each pair varied slightly from other pairs. The height decreased overall from the base to the apex, but there was a small expansion at the junction of the basal and middle turns where the interscalar septum originated. The theoretical relationships of different diameter electrodes to the organ of Corti were plotted for one cochlea. The size of the electrode and the path it followed were shown in theory to alter considerably its position in relation to the organ of Corti.

Ultrastructure of the lining of the scala tympani of the bat, Pteronotus parnellii

The cells lining the scala tympani of the cochlea of Pteronotus p. parnellii were studied in whole mount preparations and with light and scanning and transmission electron microscopy. On the basis of structure and location three different cell types were recognized: (1) those lying on the undersurface of the basilar membrane; (2) those covering the internal surface of most of the otic capsule; and (3) those associated with a thick layer of osmiophilic substance and restricted to a specific region in the basal turn. The cells associated with the osmiophilic substance were strikingly different from the other cells; they were relatively rich in organelles and had a Golgi complex which appeared to produce granules which coalesced both intracellularly and extracellularly to form the osmiophilic layer. The function and composition of the osmiophilic substance is unknown but it seems to be unique to Pteronotus parnellii and related subspecies known to have greatly enlarged perilymphatic scalae and unusual hearing capacities associated with Doppler shift compensation sonar.

Dimensions of the scala vestibuli and sectional areas of both scales

Height and width of the scala vestibuli were measured on resin casts at seven different places, beginning at the round window up to 1 3/4 turns of the cochlea. These figures do not decrease continuously but are augmented at certain places. The sectional areas of both scales show the same result according to the transient increases of height and width.

Dimensions of the scala tympani in relation to the diameters of multichannel electrodes

Dimensions of the scala tympani were measured at eight different places of 1 3/4 turns of the cochlea, beginning at the round window. Height and width of the scala tympani do not decrease continuously but show enlargements in some places. The diameter of two types of cochlear implants were compared with the mean heights of the scala tympani. The data provide an anatomical basis for the development of cochlear prosthesis, which could be inserted for a longer distance as accomplished hitherto.

A morphometric study of the cochlea of the little brown bat (Myotis lucifugus)

A detailed morphometric study of the basilar membrane was made from serial sections and graphic reconstructions of the cochlea of three little brown bats. Four distinct morphometric changes were observed within the basilar membrane. First, between 0-1.4 mm from the basal end of the cochlea, there is a rapid increase in width and cross-sectional area of the basilar membrane. Secondly, between 1.4-2.5 mm, there is little change in width of the basilar membrane (its cross-sectional area is at its greatest in this region). Thirdly, between 2.7-3.1 mm, there is a sudden decrease in cross-sectional area concomitant with an increase in the width of the basilar membrane. Finally, between 3.1 mm and the apex, there is a gradual decrease in cross-sectional area concomitant with an increase in the width of the basilar membrane. The magnitudes of the cross-sectional areas of the scalae media and vestibuli decrease from base to apex, but this is not true for the scala tympani. The cross-sectional area of the scala tympani appears to decrease from the base to 0.7 mm, then it increases up to 1.4 mm, and then it decreases to the apex. These morphometric changes in the basilar membrane of the little brown bat are compared to those in other echolocating and non-echolocating mammals. The significance of these changes is discussed in relation to the range of hearing in the little brown bat.