Supplemental content 1 : Surgical planning
An ideal insertion trajectory was planned in two phases according to a previously reported method (1) . First, the auditory canal, ossicles, facial nerve and chorda tympani were segmented and the corresponding 3D surface models were generated using a custom planning software, OtoPlan (2,3). In the second phase, the basal turn of the cochlea was segmented in a medical image analysis software (Amira 5, FEI Visualization Sciences Group, Düsseldorf, Germany). Next, a local coordinate system at the cochlea was established using a previously established set of landmarks at the round window, modiolus and apex center (1). This method assumes that the basilar membrane lies in the basilar plane defined by the cochlear coordinate system. Ideal trajectories are then calculated from tangents to a spline approximating the center of the scala tympani in the basal turn (see Wimmer et al. (1) for more details).
As the ideal trajectory (i.e. that orientated directly along the basal turn of the scala tympani) passes close to, or crosses, the mastoid part of the facial nerve in most cases, an alternative trajectory was defined preoperatively by the surgeon using the Otoplan software. The distances between the trajectory of the DCA tunnel and the facial nerve, the chorda tympani, the posterior wall of the external auditory canal and the ossicles were calculated on the 3D model for each trajectory tested. Ideally, to minimize the risk of basilar membrane collision or tearing of the lateral wall, the insertion must be aligned with the center of scala tympani. Deviation from the ideal trajectory towards the modiolus or the lateral wall is defined as the angle ε (see Supplemental data Figure), and deviation towards the basilar membrane or the bottom of the scala tympani is defined as the angle δ (see Supplemental data Figure). The ideal insertion aligns with null angles. Negative angles will drive the electrode array to the basilar membrane or the modiolus, whereas positive angle will drive it to the lateral wall of the scala.
The position of the cochleostomy was chosen in order to optimize δ and ε angles, as well as the distances to the critical structures. The position of the cochleostomy lay below the virtual line representing the basilar membrane in order to place the cochleostomy anteroinferiorly as suited for scala tympani insertion. The distance of the cochleostomy from the round window was defined as the angular distance between 0° (center of the round window) and 20° (more anteriorly) by 2° angle steps (defined as θC, see Supplemental data Figure).
Ultimately, the final plan and alignment of the trajectory was performed by an experienced ENT surgeon with the goal of minimizing δ and ε angles, as well as maintaining a distance of higher than 0.3mm for the facial nerve for security. In anatomically difficult cases, a sacrifice of the chorda tympani was chosen to avoid any risk to the facial nerve.
Supplemental data Figure : Planning of direct cochlear access (DCA) trajectory
The ideal trajectory (white line) represents the line passing through the center of the scala tympani in the basal turn and the position chosen for the cochleostomy. In plane deviation between the alternative trajectory (black dashed line) chosen by the surgeon for the drilling and the ideal trajectory is represented as the e angle (see Supplemental data Figure A), whereas out of plane deviation is denoted with angle d (see Supplemental data Figure B). The site of the cochleostomy is chosen amongst several positions (red dots, see Supplemental data Figure C) defined by the angle qc, which is the angular distance between 0° (center of the round window) and 20° (more anteriorly) by 2° steps (see Supplemental data Figure C). The choice of the site of the cochleostomy and of the trajectory aims at reducing d and e angles, while increasing the distance between the DCA and the facial nerve (FN), the chorda tympani (ChT), the ossicles (OS), and the ear canal (not shown on the figure). (Lab : labyrinth)
1. Wimmer W, Venail F, Williamson Tet al. Semiautomatic cochleostomy target and insertion trajectory planning for minimally invasive cochlear implantation. Biomed Res Int 2014;2014:596498.
2. Gerber N, Bell B, Gavaghan Ket al. Surgical planning tool for robotically assisted hearing aid implantation. Int J Comput Assist Radiol Surg 2014;9:11-20.
3. Gerber N, Gavaghan KA, Bell BJet al. High-accuracy patient-to-image registration for the facilitation of image-guided robotic microsurgery on the head. IEEE Trans Biomed Eng 2013;60:960-8.