Semiautomatic cochleostomy target and insertion trajectory planning for minimally invasive cochlear implantation

Abstract

A major component of minimally invasive cochlear implantation is atraumatic scala tympani (ST) placement of the electrode array. This work reports on a semiautomatic planning paradigm that uses anatomical landmarks and cochlear surface models for cochleostomy target and insertion trajectory computation. The method was validated in a human whole head cadaver model (n = 10 ears). Cochleostomy targets were generated from an automated script and used for consecutive planning of a direct cochlear access (DCA) drill trajectory from the mastoid surface to the inner ear. An image-guided robotic system was used to perform both, DCA and cochleostomy drilling. Nine of 10 implanted specimens showed complete ST placement. One case of scala vestibuli insertion occurred due to a registration/drilling error of 0.79 mm. The presented approach indicates that a safe cochleostomy target and insertion trajectory can be planned using conventional clinical imaging modalities, which lack sufficient resolution to identify the basilar membrane.

Figure 1

Landmark identification of a right human cochlea using cone beam CT data. (a) Oblique axial slice corresponding to the 0° reference plane (red line in (b), as defined in [18]). The RW center adjacent to the bony overhang (R), the inner wall border at the RW (I), the center of the modiolus in the basal turn (C), and the apical center of the modiolus (A) are used to define a local cochlear coordinate system for further computations. (b) Oblique coronal slice of the basal turn (blue line in a) and corresponding in-plane landmark positions.

Figure 2

Illustration of the automatic cochleostomy target/insertion trajectory computation algorithm. (a) Based on the landmarks (black circles), a local cochlear coordinate system is computed. As an assumption, the x-y plane is defined as the location of the basilar membrane. The surface model of the cochlea is truncated to the first half turn of the ST. (b) Radial cross sections are computed starting at the RW (0° reference). The center of gravity is estimated (red circles) based on the extracted vertices (black dots) for each cross section. (c) The centroid line (red line) is fitted with the data points, representing the mid-scala course of ST. For a specified range, the tangents of the centroid line are computed, defining the optimal insertion trajectories (blue lines) and the corresponding cochleostomy targets (diamonds) at the angular position θ _C.

Figure 3

The distance between the approximated position of the basilar membrane (as computed with the landmark based approach) and its actual position in the corresponding micro-CT data (blue line) of 5 human cochleae is shown. An average error of 0.22 mm was observed in the region used for insertion trajectory computation (60° ≥ θ ≥ 45°).

Figure 4

Intervention planning for minimally invasive CI surgery in a dedicated software tool [21]. Visualization of the segmented posterior wall of the external auditory canal (EAC), the facial nerve (FN), the chorda tympani (ChT), the ossicles (OS), and the bony labyrinth (L). The planned trajectory (Tr) and the ideal trajectory as computed by the algorithm (broken-dotted line) are shown. (a) Planning situation from an inferior view; the angle δ describes the offset between the planned trajectory and the ideal trajectory with respect to the basal turn of the cochlea for a given cochleostomy target. Note that the ideal trajectory is running through the facial nerve. (b) The same plan as seen from an anterior view; the offset between the planned and the computed ideal trajectory in the basal turn plane is described by the angle ε.

Figure 5

Three-dimensional virtual view of the promontory (a)–(c) and corresponding endoscopic photo documentation (d)–(g) during cochleostomy drilling and array insertion in specimen 2R. The facial nerve (FN), the stapes (St), the long process of the incus (In), and the malleus (Ma) provide orientation landmarks. (a) The planned trajectory (Tr) and the computed ideal trajectory (IdTr) are shown. The cochleostomy (dotted semicircle) is aimed at drilling through the RW bony overhang (black star). (b) View of the promontory after cochleostomy with corresponding drilled trajectory (DrTr). (c) Transparent view of the promontory after insertion of the electrode array (EA). The cochlea (Co) and the centroid line as computed by the algorithm (arrow) are shown. (d) Promontory prior to cochleostomy drilling (dotted semicircle) at the RW bony overhang (black star). (e) Cochleostomy drilling with a 1 mm diamond burr (D). (f) Promontory with cochleostomy (arrow). (g) After insertion of the electrode array (EA).

Figure 6

Radiological evaluation of the insertion outcome in axial cone beam computed tomography slices. (a) Left cochlea with scala vestibuli insertion caused by a target drilling error of 0.79 mm orientated anteriorly (specimen 1L). (b) Complete ST insertion in a left cochlea (specimen 4L).