Clinical validation study of percutaneous cochlear access using patient-customized microstereotactic frames

Abstract

Objective: Percutaneous cochlear implant (PCI) surgery consists of drilling a single trough from the lateral cranium to the cochlea avoiding vital anatomy. To accomplish PCI, we use a patient-customized microstereotactic frame, which we call a "microtable" because it consists of a small tabletop sitting on legs. The orientation of the legs controls the alignment of the tabletop such that it is perpendicular to a specified trajectory.

Study design: Prospective.

Setting: Tertiary referral center.

Patients: Thirteen patients (18 ears) undergoing traditional cochlear implant surgery.

Interventions: With institutional review board approval, each patient had 3 fiducial markers implanted in bone surrounding the ear. Temporal bone computed tomographic scans were obtained, and the markers were localized, as was vital anatomy. A linear trajectory from the lateral cranium through the facial recess to the cochlea was planned. A microtable was fabricated to follow the specified trajectory.

Main outcome measures: After mastoidectomy and posterior tympanotomy, accuracy of trajectories was validated by mounting the microtables on the bone-implanted markers and then passing sham drill bits across the facial recess to the cochlea. The distance from the drill to vital anatomy was measured.

Results: Microtables were constructed on a computer-numeric-control milling machine in less than 5 minutes each. Successful access across the facial recess to the cochlea was achieved in all 18 cases. The mean +/- SD distance was 1.20 +/- 0.36 mm from midportion of the drill to the facial nerve and 1.25 +/- 0.33 mm from the chorda tympani.

Conclusion: These results demonstrate the feasibility of PCI access using customized microstereotactic frames.

Figure 1

Customized StarFix™ micro-stereotactic frame mounted on bone-implanted markers for percutaneous cochlear implantation (PCI) validation.

Figure 2

Microtable. Three spherical fiducial markers are attached to the patient via anchors. The tabletop is elevated above the spherical fiducial markers using legs that are countersunk to orient the tabletop perpendicularly to the trajectory (shown in red). Coupling mechanism between spherical fiducial marker and table leg is shown in the inset. Twisting the thumbscrew tightens the grippers, thereby fixing the leg to the marker.

Figure 3

Approximate locations for the three bone-implanted markers.

Figure 4

Intraoperative CT scan acquired using the Xoran xCAT flat-panel scanner after anchor implantation.

Figure 5

Construction of microtable on computer-numeric-control (CNC) machine.

Figure 6

Microtable mounted on the patient for PCI validation. Sham drill bit is passed in the center hole and endoscope used to photodocument trajectory.

Figure 7

Measurement technique for estimating the distance from the centerline of the drill trajectory to facial nerve and chorda tympani. On the intraoperative endoscopic photograph a line is drawn over the drill bit, which provides a scale to the true physical size - 2 mm in this case. The distance from the edge of the drill to the chorda tympani and the facial nerve are then digitally drawn and calibrated.

Figure 8

Location of Cochleostomy. Shown in Panel A is the cochleostomy site as selected by the surgeon by drilling off the round window overhang. Panel B shows the 1 mm sham drill bit targeting this location.