Implanting straight into cochlea risks the facial nerve: a Cartesian coordinate study

Surgical approach to the facial recess influences the acceptable trajectory of cochlear implantation electrodes

PURPOSE

To provide practical guidance to the operative surgeon by mapping the location, where acceptable straight-line virtual cochlear implant electrode trajectories intersect the facial recess. In addition, to investigate the influence of facial recess preparation, virtual electrode width and surgical approach to the cochlea on these available trajectories.

METHODS

The study was performed on imaging data from eight cadaveric temporal bones within the University of Melbourne Virtual Reality (VR) Temporal Bone Surgery Simulator. The facial recess was opened to varying degrees, and acceptable trajectory vectors with varying diameters were calculated for electrode insertions via cochleostomy or round window membrane (RWM). The percentage of acceptable insertion vectors through each location of the facial recess was visually represented using heatmaps.

RESULTS

Seven of the eight bones allowed for acceptable vector trajectories via both cochleostomy and RWM approaches. These acceptable trajectories were more likely to lie superiorly within the facial recess for insertion via the round window, and inferiorly for insertion via cochleostomy. Cochleostomy insertions required a greater degree of preparation and skeletonisation of the junction of the facial nerve and chorda tympani within the facial recess. The width of the virtual electrode had only marginal impact on the availability of acceptable trajectories. Heatmaps emphasised the intimate relationship the acceptable trajectories have with the facial nerve and chorda tympani.

CONCLUSION

These findings highlight the differences in the acceptable straight-line trajectories for electrodes when implanted via the round window or cochleostomy. There were notable exceptions to both surgical approaches, likely explained by the variation of hook region anatomy. The methodology used in this study holds promise for translation to patient specific surgical planning.

Radiologic anatomy of the round window relevant to cochlear implantation and inner ear drug delivery

Objective

To determine anatomic relationships and variation of the round window membrane to bony surgical landmarks on computed tomography.

Study design

Retrospective imaging review.

Methods

100 temporal bone images were evaluated. Direct measurements were obtained for membrane position. Vector distances and angulation from umbo and bony annulus were calculated from image viewer software coordinates.

Results

The angle of round window membrane at junction with cochlear basal turn was (42.1 ± 8.6)°. The membrane's position relative to plane of the facial nerve through facial recess was (14.7 ± 5.2)° posterior from a reference line drawn through facial recess to carotid canal. Regarding transtympanic drug delivery, the round window membrane was directed 4.1 mm superiorly from the inferior annulus and 5.4 mm anteriorly from the posterior annulus. The round window membrane on average was angled superiorly from the inferior annulus (77.1 ± 27.9)° and slightly anteriorly from the posterior annulus (19.1 ± 11.1°). The mean distance of round window membrane from umbo was 4 mm and posteriorly rotated 30° clockwise from a perpendicular drawn from umbo to inferior annulus towards posterior annulus. Together, these measurements approximate the round window membrane in the tympanic membrane's posteroinferior quadrant.

Conclusions

These radiologic measurements demonstrate normal variations seen in round window anatomy relative to facial recess approach and bony tympanic annulus, providing a baseline to assess round window insertion for cochlear implantation and outlines anatomic factors affecting transtympanic drug delivery.

Spatial orientation of the adult cochlea: rotation, tilt, and angle theta 3

OBJECTIVE

Quantitative description in adult crania of (1) angular orientation of the basal turn of the cochlea relative to the sagittal (termed "rotation") and Frankfort horizontal (termed "tilt") planes, and angle theta 3 [angular relationship of the line defined by the cochlea's spiral center and cochlear (round) window, to the cochlear window]; (2) orientation of the cochlea relative to the plane defined by the horizontal and vertical portions of the facial nerve; (3) orientation of the basal turn of the cochlea relative to the plane of the posterior semicircular canal; and (4) the association of these orientations with the extent of mastoid pneumatization.

METHODS

Postmortem material analysis. From 41 bequeathed anatomical ear-normal cadaveric cranial, high-resolution CT scans were performed of the five crania with the largest and the five with the smallest mastoids. Eleven points in three-dimensional Cartesian space were appointed and studied with the software program FIJI.

RESULTS

The median angle values (and ranges) for right ears were: "rotation" 52° (range 47-61); and, "tilt" 84° (79-89). The planes of the cochlear basal turn and facial nerve approximated superimposition: median 15° (2-19). Angle theta 3 for right ears was median 40° (28-44). Bilateral symmetry was found for the relationships between the planes. However, no association of any planar relationship with mastoid pneumatization was suggested.

CONCLUSION

Considering the range of angles found in clinically normal adult specimens, spatial orientation of the cochlea may explain some of the difficulties in implantation.