Prospective Validation of Facial Nerve Monitoring to Prevent Nerve Damage During Robotic Drilling

Abstract

Facial nerve damage has a detrimental effect on a patient's life, therefore safety mechanisms to ensure its preservation are essential during lateral skull base surgery. During robotic cochlear implantation a trajectory passing the facial nerve at <0.5 mm is needed. Recently a stimulation probe and nerve monitoring approach were developed and introduced clinically, however for patient safety no trajectory was drilled closer than 0.4 mm. Here we assess the performance of the nerve monitoring system at closer distances. In a sheep model eight trajectories were drilled to test the setup followed by 12 trajectories during which the ENT surgeon relied solely on the nerve monitoring system and aborted the robotic drilling process if intraoperative nerve monitoring alerted of a distance <0.1 mm. Microcomputed tomography images and histopathology showed prospective use of the technology prevented facial nerve damage. Facial nerve monitoring integrated in a robotic system supports the surgeon's ability to proactively avoid damage to the facial nerve during robotic drilling in the mastoid.

Keywords: monopolar and bipolar; nerve stimulation electrode; neurophysiology monitoring; robotic cochlear implantation; robotic surgery.

Figure 1

Robotic cochlear implantation supported by facial nerve monitoring. Robotic drilling is performed from the surface of the mastoid to a first safety point 3 mm before the facial nerve (yellow) and the chorda tympani nerve (pink). The robot drill is retracted and the stimulating probe with monopolar and bipolar configurations (Bipolar 1,2,3) is inserted to the end of the trajectory, following application of a stimulation intensity ramp (0.2–2 mA, 4 Hz, 250 μs). The minimum intensity that elicits a muscle evoked potential above threshold is registered for each configuration. Different stimulus thresholds are expected to provide different prediction of distance to the facial nerve. A stimulation intensity above 0.35 mA (bipolar configuration) suggest sufficient facial nerve distance to ensure structural preservation of the facial nerve (e.g., 1 mA stimulus threshold bipolar B1).

Figure 2

(a) Trajectories (n = 8) plan relative to the facial nerve. (b) Schematic representation: five measurement points planned relative to the origin of the facial nerve in the drill axis (O_FN) for the lateral trajectories. Five measurement points planned relative to the facial nerve canal (FC) in the frontal trajectories. Lateral distance (LD) planned between 0.00 mm and 0.5 mm. Drill trajectory to nerve closest distance defined as CD. (c) Stimulation probe, same diameter as the drill, four different stimulation configurations, bipolar 1 (red), bipolar 2 (dark blue), bipolar 3 (blue) and monopolar (light blue). The monopolar configuration is measured with needle anode near the sternum.

Figure 3

Nerve monitoring workflow: using a surgical robot and a stimulation probe, the surgeon follows this workflow to determine if it is safe to continue to drill or if she should stop. EMG data of only one EMG channel and only for stimulation electrode configuration Bipolar 1 is represented.

Figure 4

Representative trajectories of the training subject, stimulation thresholds of the FNM (bottom row), with the decision thresholds and the decisive electrode configuration marked in red, and electrode configurations Monopolar, Bipolar 2 and Bipolar 3 (blue). μCT (middle row), drill outlined in turquoise, measurement points marked red, facial nerve outlined in yellow. Histology (top row) with H&E staining, facial nerve assessment marked with a star. (a) 1.0: lateral distance > 0.4 mm, no warning from system. (b) 1.1: lateral distance < 0.0 mm, stimulus threshold below 0.35 mA indicates an emitted system warning at the first FNM point. (c) 1.5: frontal trajectory with final nerve distance < 0.1 mm, the system warned at the deepest FNM point.

Figure 5

Stimulation Thresholds of the FNM (bottom row), with the decision thresholds and the decisive electrode configuration marked in red, bipolar 2,3 and monopolar (blue). μCT (middle row), drill (turquois), measurement points marked green/red (go/stop) depending on the FNM assessment, facial nerve (yellow). CD: closest distance between drill and facial nerve in mm in each measurement point. Histology (top row) with H&E staining, Facial nerve assessment marked with a star. (a–c) Lateral trajectories with FN distance ≥ 0.2 mm; (d–g) Lateral trajectories with FN distance ≤ 0.1 mm; (h–l) Frontal trajectories with FN distance ≤ 0.1 mm.

Figure 6

Histopathology of lateral trajectories at critically low distance to the fallopian channel (facial nerve canal). Decalcified mastoid bone slices with hematoxylin and eosin staining. (a) Trajectory 2.3–Facial nerve in close proximity to the drilled hole but without visible destruction of the nerve fascicles. (b) Trajectory 2.4 with 6 μm distance (measured in μCT) between drill and fallopian canal, the black arrow indicating intact facial nerve fascicles. (c) Trajectory 3.0 with 10 μm distance between drill and fallopian channel and with intact facial nerve fascicles. (d) Trajectory 3.4 with facial nerve not visible (due to a technical artefact) and an intact epineurium.

Figure 7

(a) Scatter plot of the stimulation thresholds and the corresponding measured distances to the facial nerve, for subjects 1,2, and 3. (b) Zoom into the lower stimulation thresholds, (c) number of points closer to the facial nerve than 0.1 mm which generate a CMAP at a given stimulus threshold, percentage only shown for Bipolar 1 (d) Cumulative percentage—percentage of points closer to the facial nerve than 0.1 mm recognized by a given stimulus threshold.

Figure 8

For different stimulation threshold levels, given a CMAP response >100 μV with a positive predictive value PPV > 95%, the trajectory will be found in the distance range depicted in colors. For example, with a positive response measured in B1 at 0.5 mA, the distance between the trajectory and the facial nerve is <0.3 mm.