Variations in microanatomy of the human cochlea

Abstract

The human cochlea shows considerable interindividual variability in size and morphology. In order to develop atraumatic cochlear implant (CI) electrodes, high-precision details of the variability of human anatomy are required. Sixteen human temporal bones were cut around the cochlea in blocks of approximately 3.5 × 3.5 cm. The bones were scanned by using a Skyscan 1173 micro-computed tomography (μCT) device. Mimics software (Materialise, Leuven, Belgium) was used to segment out the scala tympani (ST) from the μCT images. A three-dimensional surface model of the segmented area was generated for each cochlea. Cross-sectional images were taken and analyzed by custom-designed software in MATLAB. Comparison of different STs showed large variability in cross-sectional diameter (CSD), vertical trajectory, and height of the ST. Relative standard deviations of the CSD were between 9 and 15%. Heights measured at the center of the ST exceeded those in the modiolar and lateral regions of the scala. At the lateral region, the height decreased significantly at the beginning of the second turn. In the vertical trajectory, critical anatomic features were observed, such as dips, vertical jumps, and peaks. Rosenthal's canal (RC) extended to between 560 and 650°. We found a correlation between the length of the RC and that of the ST. The ST was segmented and the internal dimensions measured by using μCT. We observed large dimensional variability between different STs. These differences could have considerable implications for approaches to the design of CI arrays, especially in terms of their ability to preserve residual hearing during insertion of the electrode array.

Keywords: cochlear implant; hearing preservation; individualized therapy; scala tympani; μCT.

Figure 1

A,B: The μCT image of a fluid-filled (A) and air-filled (B) human cochlea. Resolution: 10 μm. The fluid-filled image is noisier due to the minimal difference of the linear attenuation coefficient of the perilymph and the soft tissue (A). In the air-filled image, the fine structures (ST, scala tympani; SV, scala vestibuli; RC, Rosenthal's canal; SL, spiral ligament; BM, basilar membrane, and OSL, osseous spiral lamina) are clearly visible. The extremely thin Reissner's membrane, which separates the scala media (SM) and SV, is not visible (B). Scale bar = 1 mm in A,B.

Figure 2

A: Cross sections taken perpendicular to the yellow line along the scala tympani, every 0.1 mm. B: The red lines show the contour of the segmented area, and the green line shows the largest circle for that segment. The images were exported to MATLAB and analyzed further. Scale bar = 1 mm.

Figure 3

A,B: Measurement of the width and length of the cochlear base (A) and the cochlear axis height (B).

Figure 4

Correlation between length of the cochlear base (largest distance from the round window, through the central axis, to the lateral wall) and width of the cochlear base (perpendicular distance at the central axis); red line indicates the linear fit.

Figure 5

Correlation between Rosenthal's canal length and the length along the scala tympani (ST) for the first two turns; red line indicates the linear fit. The length of the ST is measured along its center.

Figure 6

A–C: Cross-sectional view of two cochleae in coronal (A), sagittal (C), and cochlear views (B). D: Three-dimensional surface reconstruction of the scala tympani with the largest (beige, TB11), and smallest (pink, TB06) cross-sectional diameter observed in 16 temporal bones (see Fig. 8). The large dimensional variability between the largest and smallest scala tympani is clearly visible. CA, cochlear axis; RW, round window. Scale bar = 1 mm.

Figure 7

Mean height of the scala tympani (solid lines) with ± 1 standard deviation (dotted lines) as a function of angular distance. The largest height is at the center (green) of the scala tympani, where the modiolar height (black) remained nearly constant, and the lateral height (red) decreased markedly after 450°. At the end of the second turn (600–720°), the mean modiolar height exceeds the mean central height.

Figure 8

The average cross-sectional diameter (aCSD; black) and individual cross-sectional diameter (CSD; colored) as a function of the angular distance based on the largest circle that could fit into the cochlear scala for each segment. The aCSD increased within the first 20°, and decreased until the end of the second cochlear turn.

Figure 9

The average sectional area (aSA; black-dotted) and individual sectional area (SA; colored) as a function of the angular distance. The aSA increased within the first 20°, and subsequently continuously decreased until the end of the second cochlear turn. RW, round window.

The average horizontal position of the largest circle of the scala tympani (ST). Horizontal distance from the center of the circle (C) to the lateral edge (L) was used to calculate the relative position of the circle. The largest circle was mainly located at the center-modiolar region of the ST.

Figure 11

The average and the individual distance along the scala tympani (measured at the central and lateral position) as a function of the angular distance. The gray vertical lines indicate ± 1 standard deviation.

Figure 12

Vertical trajectory of different scalae tympani (ST). Relative position (relative to round window [RW]) of the largest circle's vertical coordinate (z-axis) is plotted (black line) against the distance along the ST (starting from the inferior edge of the RW). The orange lines mark the limits (i.e., the diameter) of the different ST. Bottom right: Schematic representation of a cochlea to visualize the angular position for each characteristic point in individual cochleae.

Figure 13

The mean vertical trajectory curve for each category. Critical anatomic features were found at individually different positions along the scala tympani (ST). Here, the rollercoaster category appears critical for cochlear implantation, as it can force the electrode array initially into the downward direction and subsequently into the upward direction. This may lead to damage to the basilar membrane and penetration into the scala vestibuli at the 180° position. The sloping category has a critical position at 12 mm from the round window (RW), where a distinct peak in the vertical direction may favor basilar membrane damage. The intermediate category would have a probability of damage highest at 20 mm from the RW.