Characterizing the size of the target region for atraumatic opening of the cochlea through the facial recess.

Surgical treatment with a cochlear implant (CI) for hearing rehabilitation requires a highly accurate and personalized opening of the inner ear (cochlea) to protect the delicate intra-cochlear fine structures, whose functional integrity needs to be maintained to preserve residual hearing. Spatial orientation within the complex anatomy of the lateral skull base during the procedure is a highly demanding task for the surgeon. In order to reduce risk of facial nerve palsy and loss of residual hearing as well as to establish minimally invasive CI surgery (minCIS), image-guided procedures incorporating surgical assistance systems are under development. However, there is a lack of an accuracy threshold value or range that such a system needs to fulfill to be considered sufficiently accurate for atraumatic opening of the inner ear. In this study, high resolution three-dimensional (3D) morphological images of eight human temporal bone specimens were manually segmented to build anatomical models of the human inner ear including all surgically relevant intra-cochlear structures as well as the facial recess. These 3D models were used to plan the surgical access path to the basal turn of the cochlea using the mastoidectomy posterior tympanotomy approach (MPTA). Therefore, custom-made image-processing software was developed to perform both path planning and identification of the valid target region- i.e., the largest possible region for atraumatic opening of the scala tympani. The developed 3D models provide visualization of the complex and variable anatomy of the basal portion of the human cochlear duct (also known as cochlear "hook region") as well as its spatial relationship to the facial recess. Their spatial arrangement directly impacts the accessibility of the hook region and limits the entry direction into scala tympani. The average diameter of the target region was found to be 1.56 mm ±â€¯0.10 mm (range: 1.43 to 1.72 mm). The anatomic variability and the need for a high safety level of at least 95% for hearing preservation CI surgery lead to a remaining safety margin of approximately 0.3 mm. In the future, this accuracy threshold value can serve as benchmark during the pre-clinical evaluation of image-guidance technologies to allow for highly accurate CI surgery.

Feasibility Assessment of Optical Coherence Tomography-Guided Laser Labeling in Middle Cranial Fossa Approach.

INTRODUCTION: Different approaches have been developed to find the position of the internal auditory canal (IAC) in middle cranial fossa approach. A feasibility study was performed to investigate the combination of cone beam computed tomography (CBCT), optical coherence tomography (OCT), and laser ablation to assist a surgeon in a middle cranial fossa approach by outlining the internal auditory canal (IAC). MATERIALS AND METHODS: A combined OCT laser setup was used to outline the position of IAC on the surface of the petrous bone in cadaveric semi-heads. The position of the hidden structures, such as IAC, was determined in MATLAB software using an intraoperative CBCT scan. Four titanium spheres attached to the edge of the craniotomy served as reference markers visible in both CBCT and OCT images in order to transfer the plan to the patient. The integrated erbium-doped yttrium aluminum garnet laser was used to mark the surface of the bone by shallow ablation under OCT-based navigation before the surgeon continued the operation. RESULT: The technical setup was feasible, and the laser marking of the border of the IAC was performed with an overall accuracy of 300 µm. The depth of each ablation phase was 300 µm. The marks indicating a safe path supported the surgeon in the surgery. CONCLUSION: The technique investigated in the present study could decrease the surgical risks for the mentioned structures and improve the pace and precision of operation.