Analysis of Vestibular Labyrinthine Geometry and Variation in the Human Temporal Bone

Abstract

Stable posture and body movement in humans is dictated by the precise functioning of the ampulla organs in the semi-circular canals. Statistical analysis of the interrelationship between bony and membranous compartments within the semi-circular canals is dependent on the visualization of soft tissue structures. Thirty-one human inner ears were prepared, post-fixed with osmium tetroxide and decalcified for soft tissue contrast enhancement. High resolution X-ray microtomography images at 15 μm voxel-size were manually segmented. This data served as templates for centerline generation and cross-sectional area extraction. Our estimates demonstrate the variability of individual specimens from averaged centerlines of both bony and membranous labyrinth. Centerline lengths and cross-sectional areas along these lines were identified from segmented data. Using centerlines weighted by the inverse squares of the cross-sectional areas, plane angles could be quantified. The fit planes indicate that the bony labyrinth resembles a Cartesian coordinate system more closely than the membranous labyrinth. A widening in the membranous labyrinth of the lateral semi-circular canal was observed in some of the specimens. Likewise, the cross-sectional areas in the perilymphatic spaces of the lateral canal differed from the other canals. For the first time we could precisely describe the geometry of the human membranous labyrinth based on a large sample size. Awareness of the variations in the canal geometry of the membranous and bony labyrinth would be a helpful reference in designing electrodes for future vestibular prosthesis and simulating fluid dynamics more precisely.

Keywords: centerlines; inner ear; membranous labyrinth; microCT; semi-circular canals; vestibular labyrinth.

Figure 1

Extraction of cross-sectional images using a Region of Interest (ROI) box. The bony labyrinth is shown in transparent yellow and the membranous labyrinth in red. The green line indicates the centroid of the bony labyrinth, which serves as the trajectory for the slice, shown in black. A ROI box then crops the slice to a reasonable size.

Figure 2

Different artifacts in the membranous labyrinth. Shown are detached superior (1) and lateral (2) membranous ducts, a partially collapsed superior ampulla (3) and a disrupted common crus (4 and 5). Segments of specimens which appeared similar where rated according to the four point scale. The posterior duct in this specimen appears to be detached in this view, but is actually just not attached to the centrifugal side of the bony canal.

Figure 3

Centerlines of 31 manually segmented SCC membranous labyrinths aligned along Reid's coordinate system. Average centerlines of all the specimens in bold (red) for the membranous labyrinth and dashed lines (green) for the bony labyrinth. (A) Centerlines of the membranous labyrinth on the sagittal plane. (B) Centerlines of the membranous labyrinth on the coronal plane. (C) Centerlines of the membranous labyrinth on the transverse plane.

Figure 4

Perpendicular distances between the paths of centroids of the bony and membranous labyrinth toward the ampulla. Shown are the distances in the posterior SCC (A), superior SCC (B), lateral SCC (C), and the common crus (D). The average distance is represented by the red line (bold). Light gray areas indicate the values lying in the range of the minimum and maximum distances. The dark gray area represents the values in between the first and the third quartile. The blue line marks the mean locus where the ampulla arises. Within the red hatched area the sample size is reduced due to the presence of the regions from the adjacent SCC or ampulla.

Figure 5

Cross-sectional area of the perilymphatic spaces stacked on the centerline of the bony labyrinth. Shown are the average cross-sectional area of the perilymphatic spaces in the posterior SCC (A), superior SCC (B), lateral SCC (C), and the common crus (D). The average area is represented by the red line (bold). Light gray areas indicate the values lying in the range of the minimum and maximum areas. The dark gray area represents the values in between the first and the third quartile. The blue line marks the mean locus where the ampulla arises. Within the red hatched area the sample size is reduced due to the presence of the regions from the adjacent SCC or ampulla.

Figure 6

Cross-sectional area of the membranous labyrinths arrayed on the centerline of the bony labyrinth toward the ampulla. Shown are the average cross-sectional area of the membranous labyrinth in the posterior SCC (A), superior SCC (B), lateral SCC (C), and the common crus (D). The average area is represented by the red line (bold). Light gray areas indicate the values lying in the range of the minimum and maximum areas. The dark gray area represents the values in between the first and the third quartile. The blue line marks the mean locus where the ampulla arises. Within the red hatched area the sample size is reduced due to the presence of the regions from the adjacent SCC or ampulla. Note that the small dip in the red graph at ~95% centerline length represents the ampullary crest.

Figure 7

Superimposed membranous labyrinths in the lateral canal of two sample datasets. The comparison of two selected datasets (yellow and red) depicts the bulging of the SCC ending opposing the ampulla (yellow dataset) present in nearly 30% of cases.

Figure 8

Ratio of the cross-sectional areas of the membranous labyrinth to the bony labyrinth toward the ampulla. Shown are the area ratios of the posterior SCC (A), superior SCC (B), lateral SCC (C), and the common crus region (D). The average ratio is represented by the red line (bold). Light gray areas indicate the values lying in the range of the minimum and maximum areas. The dark gray area represents the values in between the first and the third quartile. The blue line marks the mean locus where the ampulla arises. Within the red hatched area the sample size is reduced due to the presence of the regions from the adjacent SCC or ampulla. Along slender parts of SCCs only about 10% of the bony labyrinth area is occupied by the membranous labyrinth till the ampulla starts to widen.