Clinical validation of percutaneous cochlear implant surgery: initial report

A robot-guided minimally invasive approach for cochlear implant surgery: preliminary results of a temporal bone study

PURPOSE

The aim of this study was to create an access canal to the inner ear, by drilling, and perform the cochleostomy for cochlear implant surgery using robot guidance.

METHODS

A robot, a surgical drill and an Image-Guided Surgery (IGS) system were combined in a closed-loop setup. Ten temporal bones were scanned at the planning stages of the procedure. The robot guided the drill along the preplanned trajectory and created the approach. Postoperative scans were obtained.

RESULTS

The cochleostomy was performed completely in nine out of ten cases. This did not prove possible for one of the specimens, the target site selected being in too superficial a location in relation to the round window. No violation of the facial nerve took place, although the chorda tympani nerve was violated in one case and the stapes in two. It was obvious during preoperative planning that these structures would be violated, but this was accepted in order to maintain a safety margin from the facial nerve. No other unforeseen damage occurred.

CONCLUSIONS

This preliminary study suggests that robot-guided drilling of a minimally invasive approach to the cochlea might be feasible, but further improvements are necessary before any clinical application becomes possible. Where the width of the facial recess is less than 2.5 mm, the chorda tympani nerve and the ossicles are at risk.

[Surgical technique in cochlear implantation]

The cochlear implant (CI) has become a standard option for treating prelingually deaf children. But postlingual late deafness in adults is becoming increasingly common. In addition, hybrid implantation with a CI and a hearing aid in the same ear has come into focus, which demands a soft insertion technique that spares the apical parts of the cochlea. Also, the chorda tympani should be saved, especially in bilateral implantations, which are gaining importance because improved speech discrimination in noisy conditions is seen as proven today. Control of the electrode position intraoperatively with intraoperative computed tomography can further increase the safety and reliability of the position. The position and length of the skin incision is a more aesthetic issue. Future developments will include fully implantable CIs and navigation-assisted, minimally invasive drilling of a hole from the surface of the skull into the cochlea. Bioactive, neurotrophic-drug-releasing electrode designs for improved and sustainable connectivity to the neurons may become applicable.

Automatic identification and 3D rendering of temporal bone anatomy

HYPOTHESIS

Using automated methods, vital anatomy of the middle ear can be identified in computed tomographic (CT) scans and used to create 3-dimensional (3D) renderings.

BACKGROUND

Although difficult to master, clinicians compile 2D data from CT scans to envision 3D anatomy. Computer programs exist that can render 3D surfaces but are limited in that ear structures, for example, the facial nerve, can only be visualized after time-intensive manual identification for each scan. Here, we present results from novel computer algorithms that automatically identify temporal bone anatomy (external auditory canal, ossicles, labyrinth, facial nerve, and chorda tympani).

METHODS

An atlas of the labyrinth, ossicles, and auditory canal was created by manually identifying the structures in a "normal" temporal bone CT scan. Using well-accepted techniques, these structures were automatically identified in (n = 14) unknown CT images by deforming the atlas to match the unknown volumes. Another automatic localization algorithm was implemented to identify the position of the facial nerve and chorda tympani. Results were compared with manual identification by measuring false-positive and false-negative error.

RESULTS

The labyrinth, ossicles, and auditory canal were identified with mean errors less than 0.5 mm. The mean errors in facial nerve and chorda tympani identification were less than 0.3 mm.

CONCLUSION

Automated identification of temporal bone anatomy is achievable. The presented combination of techniques was successful in accurately identifying temporal bone anatomy. These results were obtained in less than 10 minutes per patient scan using standard computing equipment.

Percutaneous access to the petrous apex in vitro using customized micro-stereotactic frames based on image-guided surgical technology

Conclusion. Our study demonstrates (in cadavers) the ability to obtain a minimally invasive approach to access the petrous apex using patient-customized micro-stereotactic frames based on pre-intervention radiographic studies. Objective. To conduct in vitro studies to demonstrate the feasibility of percutaneous petrous apex access using customized, bone-mounted, micro-stereotactic frames. Methods. Cadaveric temporal bone specimens (n=10) were affixed with three bone-implanted fiducial markers. CT scans were obtained and used in planning, in reference to the fiducial markers, a straight transmastoid infralabyrinthine trajectory from the mastoid surface to the petrous apex without violating the basal turn of the cochlea or the carotid artery. A drill press was mounted on the customized frame and used to guide a 2 mm drill bit on the desired trajectory. The course of the drill bit and its relationship to surrounding vital anatomy (cochlea, carotid artery, facial nerve, and internal jugular vein) were determined by repeat CT scanning. Results. In 10 of 10 specimens, the drill bit trajectory was accurate with clearance (mean+/-standard deviation in mm) from the cochlea, facial nerve, carotid artery, and jugular vein of 3.43+/-1.57, 3.14+/-1.15, 4.57+/-1.52, and 6.05+/-2.98, respectively.

Penetration of CO2 laser into the otic capsule using a hand-held, flexible-fiber delivery system

BACKGROUND AND OBJECTIVE

Recently, a new, flexible-fiber, CO2 laser delivery system has been FDA-cleared for clinical use. However, for otologic surgery, no data have been reported correlating power settings to depth of penetration into the otic capsule-the bone that covers the inner ear. This was the goal of our study.

STUDY DESIGN/MATERIALS AND METHODS

Eight cadaveric temporal bones were procured as per our institution's protocols. For each specimen, nine different laser holes were burned into the otic capsule using the flexible-fiber CO2 laser delivery system. Power settings were varied from 10 to 20 W in 2 W increments, and duration of exposure was 100, 200, 300, 400, or 600 milliseconds. Each setting (power and duration) was tested on two specimens. Following laser exposure, each specimen was scanned in a microCT scanner, and the depth of penetration measured from these images.

RESULTS

Of the 72 laser shots, 8 were excluded due to double hits (4), oblique hits (3), or complete penetration (1). After excluding these 8, bone penetration was found to vary from 160 to 670 microm based on power and time settings. Spearman analysis on ranked data showed that time had a greater impact on depth than power. The correlation coefficients for time and power were 0.84 (P = 0.013) and 0.40 (P<0.001), respectively.

CONCLUSION

The flexible-fiber CO2 laser is effective for otic capsule ablation in this model. High power setting and long pulse duration can lead to complete penetration of the otic capsule potentially causing damage of underlying structures such as the facial nerve, horizontal semicircular canal, and cochlea.

Customized, rapid-production microstereotactic table for surgical targeting: description of concept and in vitro validation

PURPOSE

To introduce a novel microstereotactic frame, called the Microtable, consisting of a tabletop that mounts on bone-implanted spherical markers. The microtable is customized for individual patient anatomy to guide a surgical instrument to a specified target.

METHODS

Fiducial markers are bone-implanted, and CT scanning is performed. A microtable is custom-designed for the location of the markers and the desired surgical trajectory and is constructed using a computer-numerical-control machine. Validation studies were performed on phantoms with geometry similar to that for cochlear implant surgery. Two designs were tested with two different types of fiducial markers.

RESULTS

Mean targeting error of the microtables for the two designs were 0.37 +/- 0.18 and 0.60 +/- 0.21 mm (n = 5). Construction of each microtable required approximately 6 min.

CONCLUSIONS

The new frame achieves both high accuracy and rapid fabrication. We are currently using the microtable for clinical testing of the concept of percutaneous cochlear implant surgery.

Automatic segmentation of the facial nerve and chorda tympani in CT images using spatially dependent feature values

In cochlear implant surgery, an electrode array is permanently implanted in the cochlea to stimulate the auditory nerve and allow deaf people to hear. A minimally invasive surgical technique has recently been proposed-percutaneous cochlear access-in which a single hole is drilled from the skull surface to the cochlea. For the method to be feasible, a safe and effective drilling trajectory must be determined using a preoperative CT. Segmentation of the structures of the ear would improve trajectory planning safety and efficiency and enable the possibility of automated planning. Two important structures of the ear, the facial nerve and the chorda tympani, are difficult to segment with traditional methods because of their size (diameters as small as 1.0 and 0.3 mm, respectively), the lack of contrast with adjacent structures, and large interpatient variations. A multipart, model-based segmentation algorithm is presented in this article that accomplishes automatic segmentation of the facial nerve and chorda tympani. Segmentation results are presented for ten test ears and are compared to manually segmented surfaces. The results show that the maximum error in structure wall localization is approximately 2 voxels for the facial nerve and the chorda, demonstrating that the method the authors propose is robust and accurate.

Conception and design of an automated insertion tool for cochlear implants

Cochlear implants (CI) are electronic devices incorporating an electrode inserted into the human cochlea for direct electric stimulation of the auditory nerve. The implantation has become the standard treatment for patients with severe-to-profound sensorineural loss not aidable with conventional hearing aids. The state of the art operative technique is a facial recess approach to the middle ear, following the opening of the scala tympani (cochleostomy) and insertion of the electrode array. The facial recess approach is applicable only by experienced surgeons and optimal CI results primarily depend on optimal electrode placement and minimal traumatic insertion. This also requires a certain amount of experience. Additionally several groups work on minimally-invasive approaches to the cochlea, resulting in the necessity to insert the implant via a keyhole access, which is not applicable with current techniques. This paper presents a mechatronic device for an automated insertion of the electrode array of a cochlear implant system. Being designed especially for minimally-invasive approaches, the tool is also applicable for regular facial recess approaches. Moreover the device allows reliable and repeatable insertion studies at synthetic models or cadaver specimen. The functionality of the tool is proofed with first experiments on a synthetic model.