Design Process

Overview

A major component of the project was designing the 3D model that was used to print the ear. Due to the ear’s organic form and nonplanar structure, the initial plan which used geometric extrusions in PTC Creo was discarded in favor of creating a polygonal mesh with Autodesk Maya.

Polygon meshes consist of a large number of 3- or 4-sided faces with the vertices arranged in 3-dimensional space to define the form of the structure. Edges commonly follow contour lines and the faces themselves are often of similar size. A “rough” model, in which the form is constructed from a small number of polygons (generally fewer than 200) can then be “smoothed” by subdividing the existing polygons to create a more rounded, realistic look. Smoothing often removes small details such as sharp edges or low ridges, which may not be beneficial depending on the design of the object.

However, an ear does not have such tiny details that this became a problem. Maya also allows images to be set as reference planes which can then be traced over, and an image containing multiple views of the same ear model was obtained for this purpose [3]. Because the reference image used a single ear to create the different viewpoints it eliminated the uneven scaling and shape that would have occurred had the reference images been of different ears. As it was, the model often looked skewed when viewed from different angles at the beginning, due to the difficulty of moving a point in 3D space on a 2D monitor.

Design Process

Preliminary polygonal mesh (green) outlining some of the major loops of the ear, as viewed from the plane of the ear itself.

To aid in positioning the vertices properly much of the outlining was performed using a planar mesh and refraining from rotating the model (Fig. 1). Only after most of the edges were aligned with the contours of the “front” view (week 4) was the model view shifted and a perpendicular perspective adopted to aid in extruding the vertices along the z-axis (Fig. 2).

 
Initial model of ear as seen from behind 

The model was refined from the outside working inwards, by moving vertices to correspond either to contour lines visible on the reference images or to evenly spaced and aligned gaps between these points. Parts of the ear such as the ear canal and those areas not directly visible in the photographs were instead based upon real-life observation. 3.2 Refining and Printing By the end of week 6, the polygon mesh was completed and smoothed for the first time. When viewed from any angle the model was immediately recognizable as a generic ear (Fig. 3). The model extended beyond the ear itself (Fig. 4) to allow for variations in the severity of a patient’s injury, and the extra material is able to be cut to fit the patient.

 

Model of ear after one smoothing, from two different views. 

Printing

The polygon mesh was then closed to create a solid structure by merging a planar mesh to the ends of the ear model. The solid was then checked for bad geometry (reversed normals, naked edges, concave faces, etc) and these imperfections corrected. As Maya cannot create the .stl files needed for a 3D printer, the model was exported to .stl format using the program Rhinoceros 3D. The first smoothing still resulted in a rough model (Fig. 5), so the model was smoothed again. A total of four smoothings in Maya ended up being needed, before the model was approved for printing. The final object has dimensions of 4.91 x 5.66 x 2.23 cm, which is within the size range of a normal adult ear.

 

Ear model after one and three levels of smoothing.