Determination of a Facial Nerve Safety Zone for Navigated Temporal Bone Surgery

BACKGROUND: Transtemporal approaches require surgeons to drill the temporal bone to expose target lesions while avoiding the critical structures within it, such as the facial nerve and other neurovascular structures. We envision a novel protective neuronavigation system that continuously calculates the drill tip-to-facial nerve distance intraoperatively and produces audiovisual warnings if the surgeon drills too close to the facial nerve. Two major problems need to be solved before such a system can be realized. OBJECTIVE: To solve the problems of (1) facial nerve segmentation and (2) calculating a safety zone around the facial nerve in relation to drill-tip tracking inaccuracies. METHODS: We developed a new algorithm called NerveClick for semiautomatic segmentation of the intratemporal facial nerve centerline from temporal bone computed tomography images. We evaluated NerveClick's accuracy in an experimental setting of neuro-otologic and neurosurgical patients. Three neurosurgeons used it to segment 126 facial nerves, which were compared with the gold standard: manually segmented facial nerve centerlines. The centerlines are used as a central axis around which a tubular safety zone is built. The zone's thickness incorporates the drill tip tracking errors. The system will warn when the tracked tip crosses the safety zone. RESULTS: Neurosurgeons using NerveClick could segment facial nerve centerlines with a maximum error of 0.44 ± 0.23 mm (mean ± standard deviation) on average compared with manual segmentations. CONCLUSION: Neurosurgeons using our new NerveClick algorithm can robustly segment facial nerve centerlines to construct a facial nerve safety zone, which potentially allows timely audiovisual warnings during navigated temporal bone drilling despite tracking inaccuracies.