Max Fröhlich ^1,2* Jaro Deutz ^1,3 Matthias Wangenheim ³ Thomas S. Rau ² Andrej Kral ² Thomas Lenarz ² Daniel Schurzig ^1,2

• ¹ MED-EL Research Center, Hannover, Germany
• ² Department of Otolaryngology, Head and Neck Surgery, Hannover Medical School, Hannover, Germany
• ³ Institute of Dynamic and Vibration Research, Leibniz University Hannover, Hannover, Germany

Objectives: Despite the success of cochlear implant (CI) surgery for hearing restoration, reducing CI electrode insertion forces is an ongoing challenge with the goal to further reduce post implantation hearing loss. While research in this field shows that both friction and quasistatic pressure forces occur during CI insertion, there is a lack of studies distinguishing between these origins. The present study was conducted to analyze the contribution of both force phenomena during automated CI insertion.Methods: Five MED-EL FLEX28 CI electrode arrays were inserted into both a regular and uncoiled version of the same average scala tympani (ST). Both ST models had a pressure release hole at the apical end which was kept open or closed to quantify pressure forces. ST models were filled with different sodium dodecyl sulfate (SDS) lubricants (1%, 5%, 10% SDS:water). The viscosity of lubricants was determined using a rheometer. Insertions were conducted with velocities ranging from vs = 0.125 mm/s to 2.0 mm/s Results: Viscosity of SDS lubricants at 20°C was between 1.28, 1.96, and 2.51 mPas for 1%, 5%, 10% SDS respectively which lies well within the values reported for human perilymph. In the uncoiled ST model, forces stayed within the noise floor (maximum: 0.049*10 -3 N ± 1.5*10 -3 N), indicating minimal contribution frorm quasistatic pressure. Conversely, forces using the regular, coiled ST model were at least an order of magnitude larger (minimum: Fmax = 28.95*10 -3 N, v = 1 mm/s, 10% SDS), confirming that friction forces are the main contributor to total insertion forces. An N-way ANOVA revealed that both lubricant viscosity and insertion speed significantly reduce insertion forces (p<.001).This study demonstrates for the first time that at realistic perilymph viscosities, quasistatic pressure forces minimally affect the total insertion force profile during insertion. Mixed friction is the main determinant, which significantly decrease with increased insertion speeds. This suggests that in clinical settings with similar ST geometries and surgical preparation, quasistatic pressure plays a subordinate role.Moreover, the findings indicate that managing the hydrodynamics of the cochlear environment, possibly through pre-surgical preparation or the use of specific lubricants, could effectively reduce insertion forces.