Introduction: For successful cochlear implantation in difficult ears, image guided navigation systems can help identify surgical landmarks or confirm the surgeon's anatomical knowledge. In this pilot case study, exact navigation based on intraoperative CT scanning was investigated and helped confirm important and necessary landmarks, such as the facial nerve, cochlea and intracochlear structures, and at least adequate placement of a straight electrode array.
Material and methods: Intraoperative imaging was performed on a 40-slice sliding-gantry CT scanner (Siemens SOMATOM Sensation 40 Open) with an expanded gantry bore (82 cm). Raw image data were reconstructed with a slice thickness and increment of 0.6 mm and were imported to a frameless infrared-based navigation station (BrainLAB VectorVision Sky). In a preoperative accuracy and feasibility study, a phantom skull was scanned and registered five times by the navigation system. Based on the encouraging results, the system was then applied to a male patient with post-traumatic sensorineural hearing loss. The intraoperative target positioning error was measured by a "blinded" colleague who defined the distance of the pointer from different sections of the facial nerve without seeing the intraoperative field.
Results: The average deviation in the phantom skull was 0.91 mm (SD 0.27 mm) on the mastoid, 1.01 mm (SD 0.21 mm) on the round window, and 0.9 mm (SD 0.18 mm) on the inner ear canal. Surgery could be performed without major complications. The distance of the pointer from the facial nerve could be defined exactly using navigation in ten measurements. The cochleostomy and electrode insertion were performed with the aid of navigation. After insertion, direct intraoperative control of the electrode position was achieved by means of a low-dose CT scan. Two months postoperatively, the patient had a satisfactory open-set speech understanding of 85%.
Conclusion: With the use of intraoperative acquisition of CT images (or digital volume tomography devices) and automatic volumetric registration for navigation, surgical precision can be improved, thereby allowing successful cochlear implant surgery in patients with complex malformations or who have undergone multiple previous ear surgeries and consequently lack anatomical landmarks. Our study clearly shows that this high-technology combination is superior to other registration methods in terms of accuracy and precision. Further investigations should aim at developing automatic segmentation and applications for minimally invasive surgery of the lateral skull base.