Variations in Microanatomy of the Human Modiolus: Implications for Cochlear Implants

Human cochlear anatomy is highly variable. The phenomenon has been first described qualitatively, followed by a quantitative variability assessment with detailed anatomical models of the human cochlea. However, all previous work focused on lateral cochlear wall. Few information is available on the variability of the modiolar wall. Modiolar variability, likely determined by variability in the spiral ganglion, provides key information on when during ontogenesis the individual cochlear morphology is established: before and/or after neuronal structures are formed. In the present study we analyzed 108 corrosion casts, 95 clinical cone beam computer tomographies and 15 µCTs of human cochleae and observed modiolar variability of similar and larger extent than the lateral wall variability. Lateral wall measures correlated with modiolar wall measures significantly. ~49% of the variability has a common cause, very likely established already during the time when the spiral ganglion is formed. Proximity of other neuronal and vascular structures, defining the remaining variability in scalar spaces, are determined later in ontogenesis, when the scalae are formed. The present data further allows implications for perimodiolar cochlear implants and their tip fold-overs. In particular, the data demonstrate that tip fold-overs of preformed implants likely result from the morphology of the modiolus (with radius changing from base to apex), and that optimal cochlear implantation of perimodiolar arrays cannot be guaranteed without an individualized surgical technique.