Morphometric Analysis and Linear Measurements of the Scala Tympani and Implications in Cochlear Implant Electrodes

Morphometric Measurements of the Incomplete Partition Type II (IP-II) Cochlea and Implications on Cochlear Implantation

HYPOTHESIS

The objective of this study is to obtain comprehensive morphometric measurements of the incomplete partition type II (IP-II) cochlea to provide a better understanding of intracochlear anatomy and important considerations for electrode selection and insertion.

BACKGROUND

IP-II is the most common bony inner ear malformation that often requires cochlear implantation. Currently, there is significant controversy on electrode selection due to a lack of research that can provide reliable, high-resolution measurements.

METHODS

Three-dimensional reconstructions of the cochlea were made from hematoxylin and eosin-stained slides from 11 archival human temporal bones from 8 adult IP-II patients (one paired) and 2 fetuses. Detailed measurements of the angular and linear length of the spiral ganglion neurons and cochlear duct at the modiolar and lateral wall of the scala tympani as well as cross-sectional areas and vertical height measurements of the scala tympani at 90-degree intervals were measured.

RESULTS

The spiral ganglia neurons terminated at 540.5 ± 45.4 degrees, which corresponded to the beginning of the interscalar septal defect. The corresponding Rosenthal's canal length was 12.75 ± 0.82 mm, and the lateral wall length was 23.95 ± 1.04. The average cochlear duct length was 32.44 mm ± 1.58 mm, corresponding to an average angular distance of 951.6 ± 80 degrees. The modiolar height demonstrated less variation within the scala tympani but was significantly smaller at 0 and 90 degrees compared with the normal cochlea. The lateral wall height was also significantly smaller at 0, 180, and 540 degrees. There was a drastic decrease in lateral wall height at 540 degrees to 0.4 mm, which is smaller than the apical dimension of many electrodes.

CONCLUSION

This is the first study to provide detailed morphometric measurements of the IP-II cochlea including spiral ganglion neuron length and scala tympani height. These measurements directly relate to electrode selection for cochlear implantation.

Hampshire Sheep as a Large-Animal Model for Cochlear Implantation

BACKGROUND

Sheep have been proposed as a large-animal model for studying cochlear implantation. However, prior sheep studies report that the facial nerve (FN) obscures the round window membrane (RWM), requiring FN sacrifice or a retrofacial opening to access the middle-ear cavity posterior to the FN for cochlear implantation. We investigated surgical access to the RWM in Hampshire sheep compared to Suffolk-Dorset sheep and the feasibility of Hampshire sheep for cochlear implantation via a facial recess approach.

METHODS

Sixteen temporal bones from cadaveric sheep heads (ten Hampshire and six Suffolk-Dorset) were dissected to gain surgical access to the RWM via an extended facial recess approach. RWM visibility was graded using St. Thomas' Hospital (STH) classification. Cochlear implant (CI) electrode array insertion was performed in two Hampshire specimens. Micro-CT scans were obtained for each temporal bone, with confirmation of appropriate electrode array placement and segmentation of the inner ear structures.

RESULTS

Visibility of the RWM on average was 83% in Hampshire specimens and 59% in Suffolk-Dorset specimens (p = 0.0262). Hampshire RWM visibility was Type I (100% visibility) for three specimens and Type IIa (> 50% visibility) for seven specimens. Suffolk-Dorset RWM visibility was Type IIa for four specimens and Type IIb (< 50% visibility) for two specimens. FN appeared to course more anterolaterally in Suffolk-Dorset specimens. Micro-CT confirmed appropriate CI electrode array placement in the scala tympani without apparent basilar membrane rupture.

CONCLUSIONS

Hampshire sheep appear to be a suitable large-animal model for CI electrode insertion via an extended facial recess approach without sacrificing the FN. In this small sample, Hampshire specimens had improved RWM visibility compared to Suffolk-Dorset. Thus, Hampshire sheep may be superior to other breeds for ease of cochlear implantation, with FN and facial recess anatomy more similar to humans.

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.

Intracochlear Trauma and Local Ossification Patterns Differ Between Straight and Precurved Cochlear Implant Electrodes

HYPOTHESIS

Trauma to the osseous spiral lamina (OSL) or spiral ligament (SL) during cochlear implant (CI) insertion segregates with electrode type and induces localized intracochlear ossification and fibrosis.

BACKGROUND

The goal of atraumatic CI insertion is to preserve intracochlear structures, limit reactive intracochlear tissue formation, and preserve residual hearing. Previous qualitative studies hypothesized a localized effect of trauma on intracochlear tissue formation; however, quantitative studies failed to confirm this.

METHODS

Insertional trauma beyond the immediate insertion site was histologically assessed in 21 human temporal bones with a CI. Three-dimensional reconstructions were generated and virtually resectioned perpendicular to the cochlear spiral at high resolution. The cochlear volume occupied by ossification or fibrosis was determined at the midpoint of the trauma and compared with regions proximal and distal to this point.

RESULTS

Seven cases, all implanted with precurved electrodes, showed an OSL fracture beyond the immediate insertion site. Significantly more intracochlear ossification was observed at the midpoint of the OSL fracture, compared with the -26 to -18 degrees proximal and 28 to 56 degrees distal to the center. No such pattern was observed for fibrosis. In the 12 cases with a perforation of the SL (9 straight and 3 precurved electrodes), no localized pattern of ossification or fibrosis was observed around these perforations.

CONCLUSION

OSL fractures were observed exclusively with precurved electrodes in this study and may serve as a nidus for localized intracochlear ossification. Perforation of the SL, in contrast, predominantly occurred with straight electrodes and was not associated with localized ossification.