Characterization of stapes anatomy: investigation of human and guinea pig

Abstract

The accuracy of any stapes model relies on the accuracy of the anatomical information upon which it is based. In many previous models and measurements of the stapes, the shape of the stapes has been considered as symmetric with respect to the long and short axes of the footplate. Therefore, the reference frame has been built based upon this assumption. This study aimed to provide detailed anatomical information on the dimensions of the stapes, including its asymmetries. High-resolution microcomputed tomography data from 53 human stapes and 11 guinea pig stapes were collected, and their anatomical features were analyzed. Global dimensions of the stapes, such as the size of the footplate, height, and volume, were compared between human and guinea pig specimens, and asymmetric features of the stapes were quantitatively examined. Further, dependence of the stapes dimensions on demographic characteristics of the subjects was explored. The height of the stapes relative to the footplate size in the human stapes was found to be larger than the corresponding value in guinea pig. The stapes showed asymmetry of the footplate with respect to the long axis and offset of the stapes head from the centroid of the medial surface of the footplate for both humans and guinea pigs. The medial surface of the footplate was curved, and the longitudinal arches of the medial surface along the long axis of the footplate were shaped differently between humans and guinea pigs. The dimension of the footplate was gender-dependent, with the size greater in men than in women.

FIG. 1

Segmented bony parts of the human stapes on a slice image for scans A with the 10.5-μm resolution and B with the 5-μm resolution. The white parts represent the selected bony parts and numbers on the axes indicate the numbers of the pixels. The same grayscale range of 300–1,000 (1,000 is the maximum attenuation and 0 is no attenuation) was selected for both the scan resolutions.

FIG. 2

References for the stapes dimensions. A Lateral view and B inferior view. a long length of footplate, b short length of footplate, h total height of the stapes, h _c height of the center of the mass. Other references not shown in the figure are as follows: A _FT footplate area (medial surface), A _{FT_proj} projected footplate area (to x–y plane), d _{FT_eq} equivalent diameter of the footplate area, d _{FT_eq_proj} equivalent diameter of the projected footplate area, V _ST volume of the stapes. H represents the centroid of the stapes head surface with an offset from the origin O of the intrinsic frame (H _offset) and O′ represents the center of mass of the stapes. The annular rim of the footplate is shown in red.

FIG. 3

References for the asymmetry of the stapes footplate. The outline of the footplate (red) was also obtained to diversity of the outline shape across the TBs.

FIG. 4

Relative position of the three principal axes in the intrinsic reference frame (n = 28 for human stapes and n = 9 for human stapes. A Principal frame (x′y′z′, red) and the intrinsic frame (xyz, black) in a human stapes, B position of x′-axis relative to x-axis, C position of y′-axis relative to y-axis, and D position of z′-axis relative to z-axis. (x ₁, y ₁, z ₁), (x ₂, y ₂, z ₂), and (x ₃, y ₃, z ₃) represent coordinates of the unit vectors of the x′-, y′-, and z′-axes, respectively, in the intrinsic frame.

FIG. 5

Surface profiles (medial surface) of the footplate in the human stapes. A Deviation from the fitted plane (x–y plane), B longitudinal arch of the medial surface along the long axis (x-axis), C longitudinal arch of the surface along the short axis (y-axis), and D lateral and medial boundary of the surface. The longitudinal and transverse arches were obtained from 29 human stapes, and average arches were calculated.

FIG. 6

Surface profiles (medial surface) of the footplate in the guinea pig stapes. A Deviation from the fitted plane (x–y plane), B longitudinal arch of the medial surface along the long axis (x-axis), C longitudinal arch of the surface along the short axis (y-axis), and D lateral and medial boundary of the surface. The longitudinal and transverse arches were obtained from 29 human stapes, and average arches were calculated.

FIG. 7

Outlines of the medial surface of the footplate (corresponds to red in Fig. 3) on the x–y plane A in the human stapes (n = 53) and B in the guinea pig stapes (n = 11).

FIG. 8

Offset of the stapes head from the origin A in intrinsic frame (n = 32 for humans and n = 11 for guinea pigs) and B in principal frame (n = 28 for humans and n = 9 for guinea pigs). In the intrinsic frame, (x _H/z _H, y _H/z _H) = (0.073 ± 0.075, −0.063 ± 0.072) for humans and (x _H/z _H, y _H/z _H) = (0.081 ± 0.034, −0.119 ± 0.023) for guinea pigs. In the principal frame, (x′ _H/z′ _H, y′ _H/z′ _H) = (0.029 ± 0.088, 0.039 ± 0.048) for humans and (x′ _H/z′ _H, y′ _H/z′ _H) = (−0.160 ± 0.085, −0.019 ± 0.031) for guinea pigs.

FIG. 9

Thickness map of the annular rim of the footplate measured in A three human and B three guinea pig stapes.

FIG. 10

Dependence of footplate area (medial surface) on age, height, body weight, and brain weight. The correlation of the footplate area with age, height, body weight, and brain weight was examined with linear regression, and the obtained p values were 0.646, 0.184, 0.737, and 0.157, respectively, indicating no significant correlation.

FIG. 11

Four types of the medial surfaces of the footplate that are different from the general profiles of the medial surface. A Nonobvious medial peak in the posterior region, B nonobvious medial peak in the anterior region, C a small or nonobvious lateral peak around the center of the surface, and D no protruding at the anterior inferior edge.