Robotic Milling of Electrode Lead Channels During Cochlear Implantation in an ex-vivo Model

Abstract

Objective: Robotic cochlear implantation is an emerging surgical technique for patients with sensorineural hearing loss. Access to the middle and inner ear is provided through a small-diameter hole created by a robotic drilling process without a mastoidectomy. Using the same image-guided robotic system, we propose an electrode lead management technique using robotic milling that replaces the standard process of stowing excess electrode lead in the mastoidectomy cavity. Before accessing the middle ear, an electrode channel is milled robotically based on intraoperative planning. The goal is to further standardize cochlear implantation, minimize the risk of iatrogenic intracochlear damage, and to create optimal conditions for a long implant life through protection from external trauma and immobilization in a slight press fit to prevent mechanical fatigue and electrode migrations. Methods: The proposed workflow was executed on 12 ex-vivo temporal bones and evaluated for safety and efficacy. For safety, the difference between planned and resulting channels were measured postoperatively in micro-computed tomography, and the length outside the planned safety margin of 1.0 mm was determined. For efficacy, the channel width and depth were measured to assess the press fit immobilization and the protection from external trauma, respectively. Results: All 12 cases were completed with successful electrode fixations after cochlear insertions. The milled channels stayed within the planned safety margins and the probability of their violation was lower than one in 10,000 patients. Maximal deviations in lateral and depth directions of 0.35 and 0.29 mm were measured, respectively. The channels could be milled with a width that immobilized the electrode leads. The average channel depth was 2.20 mm, while the planned channel depth was 2.30 mm. The shallowest channel depth was 1.82 mm, still deep enough to contain the full 1.30 mm diameter of the electrode used for the experiments. Conclusion: This study proposes a robotic electrode lead management and fixation technique and verified its safety and efficacy in an ex-vivo study. The method of image-guided robotic bone removal presented here with average errors of 0.2 mm and maximal errors below 0.5 mm could be used for a variety of other otologic surgical procedures.

Keywords: electrode fixation; electrode lead channel; ex-vivo human cephalic study; image-guidance; patient-specific planning; robotic cochlear implantation; robotic milling; robotic surgery.

Figure 1

The proposed concept for robotic milling of electrode lead channels during robotic cochlear implantation. (A) Surgical site of a left ear with its pinna (1) after the incision and placement of four fiducial screws (2) in which center the middle ear access hole (3) has been drilled. The electrode lead lies within the milled channel (4) and channel widenings (5) while the receiver-stimulator (6) is placed in a subperiosteal pocket. The patient marker attachment tripod (7) is fixed to the skull using a fifth screw. (B) Cross-sectional view of the immobilization of the electrode (8) in the channel with a slight press fit. (C) Cross-sectional view of the access point preparation (9) for the planned middle ear access drill hole (10). The sharp edge between the drill hole and the channel has been rounded off to create a smooth tunnel-to-surface transition (11).

Figure 2

Conceptual workflow of the proposed electrode lead management technique. During middle ear and inner ear access planning, there is a possible concurrency when the structure virtualization can be completed. The results of both processes are required for electrode lead channel planning. Workflow elements that are developed and investigated in this work are marked in green color.

Figure 3

Left: The custom-developed surgery planning software, implemented in Blender. The visualized objects are the HEARO Patient Marker Attachment tripod (1), the silicone template of the receiver-stimulator (2), the four HEARO Fiducial Screws (3), the end-effector with the milling cutter (4) in the planned electrode lead channel widening (5), and the planned drill access hole to the middle ear (6). All these structures reside on the patient's reconstructed temporal bone, onto which a red-green map is overlayed showing where the bone thickness is sufficient to place a channel plus safety margin (7). Right: A possible user interface implementation for the planning workflow (8).

Figure 4

The HEARO robotic surgical system, consisting of the five-axis HEARO Robot (1) with a tracked HEARO Drill end-effector on the quick-release wrist mount (2), the cylindrical milling cutter (3), a high-precision tracking camera (4), and a carbon-fiber headrest with air-pressure cushions (5) under a draped specimen with a patient marker attached (6). The navigation software displayed on the draped screen guides the surgeon through the procedure (7). On the top right, a close-up of the end-effector with the milling cutter, and on the bottom right the robotic system in the process of milling a channel during the ex-vivo study.

Figure 5

The process for the analysis of the milled channels. (A) The reconstructed surface of the resulting electrode channel from the micro-CT. (B) This reconstructed surface was sampled radially to the approximate channel center line, resulting in (C) a resampled surface containing only the channel, which was then classified into top left, top right, left wall, right wall and bottom. (D) The measurements through these classifications were displayed as a simplified version of the channel, where CD stands for channel depth, CW for channel width, DD for depth displacement, and LD for lateral displacement.

Figure 6

For each subject, the top image depicts the planned path (orange) over the reconstructed temporal bone surface from the micro-CT (uCT) scan, containing the resulting electrode lead channel. The second image is a photo of the postoperative result after electrode insertion. The four following graphs show the lateral displacement (LD), the depth displacement (DD), the channel width error (CWE), and the channel depth error (CDE), all on a scale from −1 to 1 mm, which corresponds to the chosen safety margin. The safety margins were respected in all endpoints and cases.

Figure 7

Cross section of the resulting channel with an embedded electrode lead cut for visualization. The electrode lead lies completely below the surface of the temporal bone and is held in place through the slight press fit between the channel walls.

Figure 8

Example photos of the resulting electrode lead management. (A) The resulting electrode channel with the attached patient marker on the tripod attachment. (B) Close-up of the electrode lead embedded within the resulting channel, in another specimen. (C) A microscope image through the external auditory canal after the elevation of the tympanomeatal flap. It shows the tapered neck of the electrode array, which is being inserted into the cochlea at the round window.

Figure 9

Histograms of the measured endpoints (i.e., lateral displacement, depth displacement, channel width error, channel depth error) in blue, the estimated distribution in red, the segmentation error distribution in green (65) where applicable, and the safety margins as red zones at the sides. The measured length is the length along which the individual measurements were carried out in the resulting channel and widening.

Figure 10

Time spent for the electrode lead management during robotic cochlear implantation in this ex-vivo study.