Computer-aided 3-D temporal bone anatomy for cochlear implant surgery.

To define anatomical relationships important in cochlear implantation, computer-aided three-dimensional reconstruction and measurement of middle and inner ear structures in six normal temporal bones were performed. Our findings were as follows: 1. When viewed from the posterior hypotympanotomy (facial recess) approach, the inferior 10% to 30% of the round window (RW) membrane was visible in only half the cases. 2. The most inferior portion of the basal turn of the scala tympani was not only inferior but also slightly anteriorly behind the RW membrane in more than half the cases. 3. The shortest distances from the aperture of the RW niche and from the margin of the RW to the stapes head were 2.38 +/- 0.33 and 2.15 +/- 0.22 mm, respectively. 4. The distance between the RW and the most inferior portion of the basal turn scala tympani was 5.15 +/- 0.34 mm. 5. The direction of the electrode advancement lay at a sharp angle to the inferior part of the RW (mean 31.9 +/- 3.6 degrees). 6. The direction from the RW to the most inferior portion of the basal turn scala tympani lay 16.0 +/- 5.3 degrees anteroinferior to the direction of the advancement of the electrode to the RW. 7. The distance between the margin of the RW and the basilar membrane of the cochlea was 0.58 +/- 0.10 mm at the superior aspect of the RW, and was 1.23 +/- 0.12 mm at the lateral aspect of the RW. 8. Dissecting away less than 1 mm (mean 0.7 +/- 0.27 mm) of the RW margin inferiorly or inferolaterally was enough to permit straight insertion of the electrode in most cases.