Three-dimensional force profile during cochlear implantation depends on individual geometry and insertion trauma

Objectives: To preserve the acoustic hearing, cochlear implantation has to be as atraumatic as possible. Therefore, understanding the impact of the cochlear geometry on insertion forces and intracochlear trauma might help to adapt and improve the electrode insertion and reduce the probability of intracochlear trauma. Design: The study was conducted on 10 fresh-frozen human temporal bones. The inner ear was removed from the temporal bone. The bony capsule covering the scala vestibuli was removed and the dissected inner ear was mounted on the three-dimensional (3D) force measurement system (Agilent technologies, Nano UTM, Santa Clare, CA). A lateral wall electrode array was inserted, and the forces were recorded in three dimensions with a sensitivity of 2 μN. Afterwards, the bones were scanned using a Skyscan 1173 micro-computed tomography (micro- CT). The obtained 3D force profiles were correlated with the videos of the insertions recorded through the microscope, and the micro-CT images. Results: A correlation was found between intracochlear force profiles measured in three different directions with intracochlear trauma detected with micro-CT imaging. The angle of insertion and the cochlear geometry had a significant impact on the electrode array insertion forces and possible insertion trauma. Intracochlear trauma occurred frequently within the first 180° from the round window, where buckling of the proximal part of the electrode carrier inside the cochlea, and rupturing of the spiral ligament was observed. Conclusions: The combination of the 3D force measurement system and micro-CT can be used to characterize the mechanical behavior of a CI electrode array and some forms of insertion trauma. Intracochlear trauma does not always correlate with higher force amplitudes, but rather with an abrupt change of force directions.