Endoprosthesis Modelling with 3D Printing Technology

Endoprosthesis Modelling with 3D Printing Technology

Supported by sophisticated CAD/CAM software modern manufacturing technologies create new possibilities of modelling even most complicated shapes with high precision. Medical application for 3D Printing is particularly interesting. Additive manufacturing technology can be used to print, at low cost, faithful models of human organs serving as teaching aids for future doctors.

Three-dimensional models featuring high precision, acquired based on medical imaging (e.g. CT) of a specific patient, can be helpful when preparing for an operation. Endoprosthesis printed for an individual patient can be yet more useful even during a clinical trial.

Shell modelling
Mesh polygon is considered a very convenient tool in multi-dimensional object modelling. By joining edges of at least two polygons one can model geometric figures in two-dimensional space as well as volumetric objects.

Mesh polygon is described in two important parameters: the type of polygons and the way they are joined. They generate a large amount of mesh which is important from the practical point of view. When choosing mesh used to create a shell, which is useful in medicine, we take a few criteria into consideration. The first criterion is the simplicity of geometrical arrangement which makes computer-aided implementation of the model easier. The second criterion is ensuring the stiffness of the construction and the third one is – economic criterion. The simplest mesh is the rectangular (square) one, triangular mesh is characterized by high stiffness, whereas the most economic mesh is the hexagonal one.

An important economic indicator of 3D Printing is the amount of printing material needed to print the mesh covering the surface. It is estimated by measuring the circuit of single mesh element covering unit area. The hexagonal mesh is very economical, the smallest amount of material is used to create it (similarly as in the case of honeycomb). As it turns out, each division of area with similar field has circuit size at least like the size of the mesh of regular hexagons (T. Hales, 1991).

In order to obtain openwork shell structure one needs to project flat mesh on the shell surface. It is a two-stage projection. The first stage is the projection of a flat mesh on a regular surface e.g. cylindrical surface. The second stage is the projection of cylinder mesh nodes on dedicated surface.

Porous volumetric objects modelling
Openwork volumetric objects modelling can be done in many ways. One of them is to use the method of hermetic infill of the area with the use of wireframe models, polyhedral solid by copying the elementary cell. As it turns out, it is not easy to find solids of such shape at all. Another solid infill is tetrakaidekahedron. It is, however, possible to create spatial mesh based on a cube to achieve continuous mesh after copying.
Diamond structure can be easily computer-generated, it leans on vertices of regular tetrahedron and its geometric center of gravity. When copying that model we generate a structure with nodes joined by 4 edges.

3D Printing technology with the use of STL files
Printing geometric objects is a new technology. Its active development is backed by a few advantages. The most important ones are:

• it enables the creation of shapes often impossible to achieve with different technologies,
• it substitutes for a few (a dozen or so) “traditional” technological processes,
• it is a relatively cheap technology (e.g. with the use of plastic materials),
• it can be realized with the use of unusual materials.

The printing is realized in one technological process and it consists in placing paths layer by layer so as to create desired geometric model. During printing process functions need to be defined – support functions of n technological processes. The final effect of created element depends on the quality of these activities, the equipment (printer) and the cooperating software.

Geometric model is most commonly saved in STL format because of its simplicity. STL standard assumes modelling with triangular mesh, where apart from constituent triangles there are no connections. The neighboring triangles (on the mesh) can be placed far from each other in the STL file. What is more, the information is copied – the same edge can appear in a few triangles. Processing STL file in order to create a stream of control data with a 3D Printer is a complicated algorithmic process and it includes:

• geometric data import,
• geometric model repairing and optimizing (many devices generate objects with faulty mesh),
• geometric processing of the object’s edge,
• entering infill into the object or its section,
• supports, bridges and scaffoldings planning (considering material adaptability),
• kinematic calculation of polyline in order to create G-code (machine language used to program movement),
• processing the G-code into machine language.

It is worth mentioning that SOFTSHAPER software by Verashape allows the user to process files with the amount of triangles bigger than 200 mln and it is the only software of this class created in Poland.

It is easy to imagine the vast potential of medical application for 3D Printing technology. This paper presents merely a small part of such possibilities in the scope of geometric modelling and openwork structures printing as well as shells and volumetric objects.

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