Introduction
The 3D exoscope has gradually gained recognition due to its high-definition imaging and ergonomic features.Reference Zhang, Wang, Zhang, Liu, Xiao and Xia1 It has been effectively applied in middle ear and lateral skull base surgeries.Reference Smith, Kozin, Kanumuri, Barber, Backous and Flávio Nogueira2
In cochlear implantation (CI), the exoscope offers distinct advantages. During mastoidectomy and facial recess (FR) fenestration, its superior imaging quality enables earlier visualization of the facial nerve and chorda tympani after thinning the bone shell over the facial nerve, a process known as “eggshelling”, compared to traditional microscopes, thus enhancing surgical safety. Additionally, the exoscope’s ergonomic design proves advantageous during well drilling for receiver-stimulator placement. Unlike the microscope, which forces the surgeon into uncomfortable positions, the exoscope allows for observation on a display screen, making the procedure more comfortable and efficient.
However, the exoscope is not without its limitations in CI procedures. When reaching narrow surgical corridors, brightness and image clarity tend to decrease.Reference Rubini, Di Gioia and Marchioni3–Reference Ally, Kullar, Mochloulis and Vijendren5 It is generally recognized that observation of the round window niche (RWN) can be problematic in up to 50 per cent of cases when using the exoscopic approach via the FR.Reference Rusetsky, Chuchueva, Meytel, Malyavina and Farikov6
To address the challenges in observing the RWN during exoscopic CI, we have developed an assisted endoscopic underwater drilling technique as depicted in Figure 1.
The assisted endoscopic underwater drilling technique.
Relevant surgical anatomy
The posterior tympanotomy, or FR approach, is the standard technique for cochlear implantation. This method relies on detailed anatomical knowledge of the facial nerve’s course and the FR. The FR is the space defined by the fossa incudis, the vertical segment of the facial nerve and the chorda tympani (Figure 2A).
The surgical procedure. (A) The RWN observed through FR under exoscope. (B, C) The RWN (B) and RWM (C) observed under endoscope with saline irrigation. (D) Electrode insertion under exoscope. RWM, round window membrane; RWN, round window niche.
Upon completion of the posterior tympanotomy, the RWN becomes visible (Figure 2B). Located within this niche, the round window membrane (RWM) is typically positioned in the coronal plane (Figure 2C). Electrode insertion into the cochlea is generally performed through the round window. To ensure full exposure of the RWM, the bony overhang covering the RWN must be meticulously drilled away until the membrane is fully visible.
Description of the technique
Before surgery, a surgical drape equipped with a fluid collection pouch is applied to prevent leakage of perfusion fluid.
Initially, under the exoscope, a mastoidectomy is performed, followed by the creation of the bony well for the receiver/stimulator. After the FR is opened and the RWN exposed, the procedure shifts to the endoscope to enhance visibility through the FR route. During the removal of the bony overhang, saline is administered through a pressure infusion bag or irrigation pump, continuously flushing out bone dust and blood from the middle ear cavity to maintain a clear view. This method, known as underwater drilling, was first introduced by Nishiike et al. in endoscopic cholesteatoma surgery.Reference Nishiike, Imai, Uetsuka, Michiba, Ashida and Otami7 The suction tip is strategically placed in the mastoid cavity to prevent fluid overflow (Figure 1).
Transitioning from the exoscope to the endoscope highlights the discrepancy in the clarity of the RWN (Figures 2A and 2B). The endoscope is managed by the surgeon in their non-dominant hand, with the drill in the dominant hand. The RWN should be drilled at the anterior, superior and posterior bony-free margins to adequately expose the RWM (Figures 2B and 2C). Once the RWM is fully exposed, the endoscope is withdrawn and the exoscope is reintroduced. Aspirate and remove the irrigated fluid, then carefully detach the RWM with an angled pick. Finally, the electrodes are introduced at an angle pointing towards the basal turn of the scala tympani (Figure 2D). The opening of the round window is sealed with periosteum wrapped around the electrodes.
Indications
The indications for exoscopic CI assisted with endoscopic underwater technique are the same as that for conventional microscopic CI.
Limitations
During exoscopic CI, visibility at the RWN is often suboptimal. While this issue can be ameliorated by switching to an endoscope, which offers a clearer view of the structures at the RWN, this transition introduces several challenges. The use of an endoscope not only increases the complexity of the procedure but also extends the surgery time. Additionally, performing the removal of the bony overhang at the RWN using the underwater technique under endoscopic guidance requires extra preparation and equipment, adding further inconvenience to the surgical process. These factors combine to complicate the surgical workflow and increase the overall demands on the surgical team.
How to avoid complications
Utilizing the exoscope to expose the facial nerve and chorda tympani can significantly decrease the risk of damaging these critical structures due to its enhanced clarity. Nevertheless, when employing underwater techniques to remove the bony overhang at the RWN, it is crucial to take precautions to prevent inadvertently opening the RWM.Reference Nishiike, Imai, Uetsuka, Michiba, Ashida and Otami7 One effective strategy to mitigate this risk is to reduce the speed of the drill during this delicate part of the procedure. This adjustment helps ensure more controlled drilling, minimizing the potential for accidental damage.
Specific peri-operative considerations
Peri-operative care should align with the standards established for microscopic CI.
Specific information
CI procedures were performed on patients with severe to profound sensorineural hearing loss by the senior surgeon, Yong Cui, using the exoscope assisted by the endoscopic underwater technique described here. This method resulted in operation times that are longer compared to conventional microscopic CI. The extended duration can be attributed to the learning curve and the additional time required to switch to the endoscope for underwater bone drilling. Despite these factors, due to its enhanced safety and ergonomic advantages, this technique remains a highly favorable choice. Given the ultra-high resolution of the exoscope, combined with the endoscope’s superior visibility of the RWN via the FR and the benefits of underwater bone drilling, we continue to recommend this innovative approach as a valuable alternative to traditional microscopic CI.
Key points
1. Use the ultra-high resolution of the exoscope to complete the mastoidectomy and drilling of the implant bed. The exoscope’s capability to provide low-angle views is particularly beneficial during these steps.
2. When performing FR fenestration, expand the area as much as possible without risking injury to the facial nerve and chorda tympani.
3. Use forceps or scissors to thoroughly clear the residual edges of the posterior tympanum mucosa, preventing them from floating in the fluid and obstructing the view during irrigation.
4. Properly manage bleeding from capillaries around the RWN before enlarging it to prevent blood from obscuring the field during the drilling of the bony overhang.
5. While grinding the bony overhang of the RWN, maintain continuous saline irrigation to ensure a consistently clear visual field. This can be achieved using a pressurized system, such as an irrigation pump or a pressure infusion bag.
6. Drill the bony overhang as slowly as possible to maintain precision and control, reducing the risk of damaging the RWM.
7. Before rupturing the RWM, aspirate the saline to prevent bone dust and/or blood from entering the scala tympani.
8. Continuously apply dexamethasone drops during the detachment of the RWM and the insertion of the electrode to reduce inflammation and enhance healing.
• The 3D exoscope provides superior imaging and ergonomic advantages in cochlear implantation, especially during mastoidectomy and facial nerve recess fenestration
• Introduces an endoscopic underwater technique to overcome visibility limitations of the exoscope during round window niche manipulation
• Provides practical guidelines for optimizing the procedure, including managing bleeding, controlling drill speed and using dexamethasone drops to reduce inflammation
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/S0022215125103010.
Funding
This study was funded by the Bureau of Science and Technology of Ganzhou Municipality (GZ2024YLJ08) and the Guangzhou Municipal Science and Technology Bureau, Natural Science Foundation of Guangzhou Municipality (2025A04J4749).
Disclosure of potential conflicting interests
The authors declare that they have no conflicts of interest, financial or non-financial, directly or indirectly related to the work described in this manuscript.
Research involving human participants and/or animals
This article describes a clinical technique involving human participants. All procedures performed in the reported cases were in accordance with the ethical standards of the institutional review board of Ganzhou Hospital of Guangdong Provincial People’s Hospital (Ganzhou Municipal Hospital) (approval no. LW2024014H) and with the 1964 Helsinki Declaration and its later amendments. No animal studies were conducted in this work.
Informed consent
Informed consent was obtained from all individual participants included in the study for the use of their anonymized clinical data and images. For retrospective case descriptions where direct consent could not be obtained, a waiver was granted by the institutional ethics committee due to the non-interventional and anonymized nature of the data.
Contributions of each author
Yong Cui and Bin Xiong contributed equally to this work. Sida Huang: Conducted data curation and authored the original draft of the manuscript. Bin Xiong: Assisted in data curation and managed research resources. Yong Cui: Led the investigation, supervised the study, performed the cochlear implantation surgery and contributed to the original manuscript writing.
