True keyhole cochlear implant surgery

The ultimate goal of robot-assisted cochlear implant surgery has always been to reach the round window in a minimally invasive manner by following a trajectory of minimal trauma. >15 years have passed since the start of the development of a robotic system for cochlear implantation. Weber et al. published a study in 2017 of a fully functional robotic system (HEARO®, CAScination, Bern, Switzerland) assisting in cochlear implant surgery achieving autonomous middle ear access [1]. Since then Topsakal et al. published a study in 2022 achieving the next step of robot-assisted cochlear implant surgery reporting successful autonomous inner ear access in 22 out of 25 patients [2]. No difference in audiological outcome has been reported compared to conventional cochlear implantation [3]. The required accuracy and sensitivity during cochlear implantation is already at the limits of human dexterity. With electrode arrays becoming smaller in the future, the need for robotic accuracy and reliability will become necessary. In recent years, the HEARO-procedure has become a European staple with the procedure being done in different centres.

The combination of image guided and robot assisted surgery is crucial for this form of minimal invasive key hole surgery to be achieved. Using imaging and the latest version of a dedicated planning software (OTOPLAN® CAScination, Bern, Switzer-land, version 3.1), a safe trajectory to the round window is planned. The cochlear duct length is also measured as to calculate when to stop electrode array insertion. RACIS allows the calculation and performance of better electrode insertion angles compared to conventional surgery. Studies have shown that the best trajectory is a trajectory in which the insertion of the electrode array is parallel to the basal turn of the cochlea for as long as possible. To follow this trajectory, one would have to drill through crucial structures such as the facial nerve. When planning the trajectory we naturally deviate from this so called best or ideal trajectory. These deviations out of this ideal trajectory have been defined as the in-plane and out-plane angle. The in-plane angle is in the plane of the basal turn, the out-plane angle is orthogonal to this plane [4,5]. The trajectory chosen will be one in which the insertion is parallel to the basal turn of the cochlea for as long as possible and as lateral as possible in the scala tympani. During robot-assisted cochlear implant surgery, a trajectory is thus made in which a tunnel of 1.8 mm through the mastoid and facial recess is drilled for middle ear access and a 1.0 mm tunnel through the canonus (bony overhang) of the round window is milled for inner ear access [2]. Until now, all cases required round window visualisation via the external auditory canal by folding over the tympanic membrane for electrode array insertion.

After the largest series of 33 cases we report an update on the newest developments. More specifically experiences with the first case of electrode array insertion without the opening of a tympanomeatal flap are highlighted. Insights in screening of patients and a greater understanding of the correlation between the insertion angles, the spatial orientation of the cochlea and the ease of electrode insertion has made a standardisation of this procedure a reality.

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