Lexicon for classifying ear-canal shapes

Abstract

The ear canal is usually described as an S-shaped funnel. In attempting to classify ear-canal shapes obtained from point clouds digitized from molds of 300 ears, the problem of designing criteria for distinguishing and organizing the canal shapes arose. In this work, we extracted features inspired by the S-shape characteristic (critical point, maximum, minimum, twist, writhe, translation, rotation) and, through them, introduced 14 types of ear-canal shapes. This classification allowed comparison of ears within a type and of ears between different types. It expanded our range of descriptors of canal shapes and unlocked perspectives for applications.

Figure 1

Schematic diagram of the initial elements used to describe the shape of the ear canal.

Figure 2

(a) As the mobile frame moves along the medial line to a given elliptic slice, the coordinates (X₀, Y₀, Z₀) of its center are measured from the stationary XYZ reference frame outside the ear. Note that the origin of the XYZ frame is not generally at the entrance of the canal. (b) The Euler angles of a slice are determined from the frame $X{}^{\prime }Y{}^{\prime }Z{}^{\prime },$ which is the parallel transport of the XYZ reference frame to the center of the slice.

Figure 3

Data on ear 050 obtained via the above analysis.

Figure 4

Schematic trends of the orientation of the normal (arrow) to the slices as one goes into the canal (increasing Z). (a) On the left, $\theta$ decreases monotonically, (b) while on the right, it reaches a minimum and increasing further on. Note that due to the arbitrariness of the orientation of the XYZ axes outside the ear, the minimum of the graphs need not be at $\theta =0$. In fact, it generally is not.

Figure 5

A sample of Type-1 (left) ears. All lengths for graphs are in mm.

Figure 6

Ear 156 shows greater twist than ear 536.

Figure 7

A sample of Type-2 (left) ears. The symbol $\leftrightarrow$ indicates that both critical points occur on the same slice.

Figure 8

A sample of Type-3 (left) ears.

Figure 9

A sample of Type 4 (left) ears.

Figure 10

A sample of Type 5 (left) ears.

Figure 11

Type 6 left ear 034. The green ellipse has large axes which might be due to its being outside the canal.

Figure 12

Type 7 left ear 185. Notice the ear contracting toward the end.

Figure 13

A sample of Type-1 (right) ears.

Figure 14

A sample of Type-2 (right) ears.

Figure 15

A sample of Type-3 (right) ears.

Figure 16

A sample of Type-4 (right) ears.

Figure 17

A sample of Type-5 (right) ears.

Figure 18

Type-6 right ear 086.

Figure 19

Type-7 right ear 050.