Towards robotic micro-milling for lateral skull base surgery

Hermann, Jan (2021). Towards robotic micro-milling for lateral skull base surgery. (Dissertation, University of Bern, Faculty of Medicine)

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Abstract
Robotic cochlear implantation is an emerging surgical technique for patients with sensorineural hearing loss. There is a clinical need to decrease variations in audiological outcomes and preservation of residual hearing. It is hypothesized that highly consistent cochlear insertion of the electrode array can only be achieved on the basis of a thorough understanding and standardized control of insertion angles specifically and the insertion process in general. Surgical access for electrode array insertion into the hearing organ is created through robotic image-guided drilling of a small-diameter hole from the temporal bone surface directly to the cochlea, as opposed to the manual approach where a mastoidectomy cavity is milled. This mastoidectomy cavity posterior to the external auditory canal provides manipulation space for the surgeon to insert the electrode through the facial recess into the cochlea, but afterwards also to store the surplus electrode lead, where it can be stabilized with various techniques to prevent electrode migrations and fatigue breaks. To date, no such technique has been proposed for the electrode lead fixation for robotic cochlear implantation.
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 the middle ear is accessed, a channel is milled robotically based on intraoperative planning. This channel is designed to protect the electrode lead from external trauma and immobilize it in a slight press fit to prevent mechanical fatigue and electrode migrations. The proposed workflow minimizes the risk of iatrogenic intracochlear damage. Robotic execution, next to the robotic middle and inner ear access and potentially robotic insertion, further standardizes cochlear implantation and aims to create optimal conditions for a long implant life.
This thesis describes this concept for electrode lead management in the case of robotic cochlear implantation, as well as its implementation and validation. Experiments are presented to first identify suitable milling parameters in a bovine bone model, then verify the feasibility of the proposed approach in technical phantom and ex-vivo studies, and finally demonstrate the safety and efficacy in human ex-vivo full-head specimens. The introduction of the proposed electrode lead management into clinics is the subject of future work.
Hypothesis: In this thesis, it is hypothesized that robotic electrode lead management for robotic cochlear implantation is safe and effective, potentially leading to better consistency of outcomes and longer implant life. Further, that such a robotic approach to electrode lead management can be carried out in clinics with existing technology, meaning commercially available medical robots, clinically available CT scanners, and touchscreen-based planning methods.
Methods: After verification in technical phantoms and bovine bone models, the proposed workflow was executed on twelve ex-vivo full-head specimens 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 along which the resulting channel exceeds the planned safety margin of 1.0 mm was determined. The planning ensures that the channel plus safety margins do not intersect with any essential anatomical structure. For efficacy, the channel width and depth were measured to assess the press fit immobilization and the protection from external trauma, respectively.
Results: All twelve cases were completed with successful electrode lead fixations after cochlear insertion. The milled channels stayed within the planned safety margin. The root mean square error in the lateral and depth directions were 0.11 mm and 0.08 mm, respectively, while maximal deviations of 0.35 mm and 0.29 mm were measured. The probability of safety margin violations was lower than 1 in 10’000 patients. 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 thesis investigated robotic milling on the specific use case of surplus electrode lead management during robotic cochlear implantation, and verified a proposed approach as safe and effective in an ex-vivo model. The approach follows a concept of a non-intersecting electrode lead channel on the temporal bone surface, intraoperatively planned while taking the virtualized surgical site into account and executed with a high-accuracy robotic system. It is designed to provide a standardized, reproducible way of protecting the electrode lead from external trauma and mechanical fatigue due to micro-movements, and to prevent electrode migrations and iatrogenic intracochlear damage. Thus, another element has been added to the robotic cochlear implantation procedure, already consisting of robotic middle and inner ear access and robotic electrode array insertion. We could show robotic milling on bone in a compliant headrest with average errors below 0.2 mm, and maximal errors below 0.5 mm. To our knowledge, this is the most accurate medical robot capable of creating free-form cavities in bone to date, potentially enabling unprecedented surgeries in the lateral skull base.

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