Variance of angular insertion depths in free-fitting and perimodiolar cochlear implant electrodes

Objective: To assess the variance in cochlear implant electrode insertion depth in degrees around the modiolus (angular insertion depth) in free-fitting and perimodiolar electrode arrays. Materials and Methods: Twenty-eight fresh human temporal bones were implanted with free-fitting cochlear implant electrodes, and 18 bones were implanted using perimodiolar electrode arrays. Specimens were embedded, and 2-dimensional radiographs were obtained to assess angular insertion depths. Histologic serial sections of undecalcified bones were then evaluated to analyze intracochlear electrode positions. Finally, linear surgical insertion depths (in millimeters) were correlated with angular insertion depth (degrees around the modiolus). Results: A moderate variance of angular insertion depth was documented for both free-fitting and perimodiolar electrode arrays. Full insertions into the scala tympani ranged from 540 to 630 degrees with free-fitting arrays and from 270 to 375 degrees with perimodiolar electrodes. In free-fitting devices, a linear relationship between linear (in millimeters) and angular (degrees) insertion depths was observed. Insertions into scala vestibuli were observed in 9 of 28 and 5 of 18 of the specimens for free-fitting and perimodiolar electrodes, respectively. Additionally, scala vestibuli insertions showed greater angular insertion depths when compared with scala tympani implantations. Conclusion: Variances in angular insertion depths seem to be moderate and similar in free-fitting and perimodiolar electrode arrays. Scala vestibuli insertions showed greater angular insertion depths than comparable insertions into the scala tympani. In perimodiolar electrodes, angular insertion depths equal or greater than 390 degrees suggested scala vestibuli placement.