**2. Material and methods**

#### **2.1 Mechanical properties of the material**

For this study, have been selected maxillary complete dentures from different acrylic resins obtained by two technologies: heat-curing material - Meliodent (Heraeus Kulzer, Senden, Germany) and light curing material - Eclipse Resin System (Dentsply International Inc.- DeguDent GmbH, Hanau Germany). The dentures were non-destructively evaluated using an Olympus stereomicroscope, Type SZX7, locating the defects and micro-cracks resulted from their technology, fig. 1-a, b. In most of dentures assessed, defects were located in the area indicated in Figure 1. Using an image processing system, QuickphotoMicro 2.2, was made an assessment of defects size, fig. 1-b. In the next step of the study, was performed an experimental program to determine the mechanical properties of the materials involved in the analysis. The materials have been prepared in accordance to the manufacturer's recommendations, Table 1. The mechanical properties were determined by tensile tests on a testing machine, model Zwick Roell (Zwick GmbH & Co. KG, Ulm, Germany), according to ISO 527 standard and bending tests according to ASTM D 790 standard. The test results showed a brittle fracture behavior, which may indicate some vulnerabilty of this materials in the presence of defects.

Fig. 1. Non-destructive evaluation of maxillary complete dentures: a) locating defects in denture; b) measurement of localised defects.

264 Reverse Engineering – Recent Advances and Applications

Based on FEM analysis have been investigated the stress distribution and structural integrity of a maxillary complete denture. The study focused on fracture resistance evaluation of dentures, in the presence of structural defects of materials, which initiates denture's cracking or fracture, before the estimated lifetime. Also, was analysed, through defectoscopy method, the porosity degree of dentures depending on the material they are made, and the influence of

For this study, have been selected maxillary complete dentures from different acrylic resins obtained by two technologies: heat-curing material - Meliodent (Heraeus Kulzer, Senden, Germany) and light curing material - Eclipse Resin System (Dentsply International Inc.- DeguDent GmbH, Hanau Germany). The dentures were non-destructively evaluated using an Olympus stereomicroscope, Type SZX7, locating the defects and micro-cracks resulted from their technology, fig. 1-a, b. In most of dentures assessed, defects were located in the area indicated in Figure 1. Using an image processing system, QuickphotoMicro 2.2, was made an assessment of defects size, fig. 1-b. In the next step of the study, was performed an experimental program to determine the mechanical properties of the materials involved in the analysis. The materials have been prepared in accordance to the manufacturer's recommendations, Table 1. The mechanical properties were determined by tensile tests on a testing machine, model Zwick Roell (Zwick GmbH & Co. KG, Ulm, Germany), according to ISO 527 standard and bending tests according to ASTM D 790 standard. The test results showed a brittle fracture behavior, which may indicate some vulnerabilty of this materials in

the defect size and location in denture, on the stress and strain state.

**2. Material and methods** 

the presence of defects.

The area with high density of defects

**2.1 Mechanical properties of the material** 

denture; b) measurement of localised defects.

a) b)

Fig. 1. Non-destructive evaluation of maxillary complete dentures: a) locating defects in


Table 1. The characteristics of acrylic resins included in this study

#### **2.2 Geometric modeling of the upper complete denture**

Finite element analysis was performed on geometric models, resulted after the complete dentures' 3D scanning (with 3D laser scanner LPX1200, Roland) and image processing by ,,reverse engineering'', taking into consideration the located defects. A thin layer of green dye (Okklean, Occlusion spray, DFS, Germany) was sprayed on the surface of the denture to increase its contrast for scanning. The denture was positioned on the rotating table of the 3D scanner and scanned with a scanning pitch of 0.1 x 0.1 mm.

Scanning technique used is that of triangulation which is based on using a 3D non-contact active scanner. Non-contact active scanners emit some kind of radiation or light and exploit a camera to look for the location of the laser dot. Depending on how far away the laser strikes a surface, the laser dot appears at different places in the camera's field of view. This technique is called triangulation because the laser dot, the camera and the laser emitter form a triangle, fig. 2.

Fig. 2. Principle of laser triangulation sensor

The results of scanning process may be a point cloud (fig. 3) or a polygon mesh (fig. 4) having the shape of the scanned object. In a polygonal representation, the registered points are connected by straight edges forming a network of small plane triangular facets. After 3D scanning, the point cloud or polygon mesh are processed by reverse engineering technique and converted into a solid geometric model, fig. 5.

Reverse Engineering and FEM Analysis

Point Cloud Processing & Polygonization

Registration & Merge

Polygon Optimization & Modeling

Fig. 5. The work process of reverse engineering technique

meshes, refining the surface by smoothing and fill holes.

can be the basis for subsequent realization of the surfaces.

continuous (Pixform Pro, 2004).

conversion into a solid model, fig. 9.

Point cloud

Polygon mesh

for Mechanical Strength Evaluation of Complete Dentures: A Case Study 267

gone through a set of operations in the order they are listed: cleaning abnormal polygon

The smooth operation smoothes the surface of the polygonal model by changing the coordinates of the input vertices. The operation can be done automatically and provides three methods: Laplacian, Loop and Curvature. The Laplacian method is a tool for enhance global smoothness wile Loop method is a kind of local smoothing tool for keeping details of model. The Curvature method is used for curvature based smoothing. For this work was chosen the Laplacian smoothing method because it has the smallest deviations from the scanned model. Another important operation in polygon mesh optimization is to fill holes in a model that may have been introduced during the scanning process. This operation can be done automatically or manually and constructs a polygonal structure to fill the hole, and both the hole and the surrounding region are remeshed so the polygonal layout is organized and

The result of polygon mesh optimization stage is a fully closed model (fig. 6) ready to

The surface creation process begins by laying down curves directly on the polygonal model to define the different surfaces to be created. The curves network created on model (fig. 7)

Once the curves network is created, the model is ready to generate NURBS surfaces (fig. 8). This can be done automatically or manual. Automatic surface generation doesn't need to draw a curve, while manual surface generation can completely maintain the flow line of the original polygon surface. Manual generation of surfaces is related to the network of curves. For this case, because the scanned denture has a complex geometry, was chosen automatic generation of NURBS surfaces on the polygonal model. To obtain the geometric model of the maxillary complete denture the NURBS surfaces network was exported in initial graphics exchange specification (IGES) format and then imported into Solid Works 2007 for

generate NURBS (Non-uniform Rational B-Spline) curves or surfaces network.

**Reverse engineering** 

Feature Curve Generation

NURBS Curve Network

Data Export

NURBS Surface Network

> Data Export

Polygon-to-NURBS

Fig. 3. The point cloud resulted from 3D scanning of the maxillary denture

The first step in converting a point cloud is processing and polygonization. This stage may include operations such as:


If an object is scanned from several angles and resulting in more scanned surfaces, they must be registered and merged in a single polygon mesh.

After 3D scanning of a maxillary complete denture, a point cloud (fig. 3) was imported into the Pixform Pro software (INUS Technology, Seoul, Korea), and processed to create a fully closed polygon mesh. To create a continuous polygon mesh the point cloud of the scanned denture was triangulated and converted into a polygonal model. This polygonal model has 266 Reverse Engineering – Recent Advances and Applications

Fig. 3. The point cloud resulted from 3D scanning of the maxillary denture

Fig. 4. The polygon mesh resulted from 3D scanning of the maxillary complete denture

include operations such as:

errors, this can be done automatically;

points are too close or overlapped each other;

must be registered and merged in a single polygon mesh.

The first step in converting a point cloud is processing and polygonization. This stage may



If an object is scanned from several angles and resulting in more scanned surfaces, they

After 3D scanning of a maxillary complete denture, a point cloud (fig. 3) was imported into the Pixform Pro software (INUS Technology, Seoul, Korea), and processed to create a fully closed polygon mesh. To create a continuous polygon mesh the point cloud of the scanned denture was triangulated and converted into a polygonal model. This polygonal model has


Fig. 5. The work process of reverse engineering technique

gone through a set of operations in the order they are listed: cleaning abnormal polygon meshes, refining the surface by smoothing and fill holes.

The smooth operation smoothes the surface of the polygonal model by changing the coordinates of the input vertices. The operation can be done automatically and provides three methods: Laplacian, Loop and Curvature. The Laplacian method is a tool for enhance global smoothness wile Loop method is a kind of local smoothing tool for keeping details of model. The Curvature method is used for curvature based smoothing. For this work was chosen the Laplacian smoothing method because it has the smallest deviations from the scanned model.

Another important operation in polygon mesh optimization is to fill holes in a model that may have been introduced during the scanning process. This operation can be done automatically or manually and constructs a polygonal structure to fill the hole, and both the hole and the surrounding region are remeshed so the polygonal layout is organized and continuous (Pixform Pro, 2004).

The result of polygon mesh optimization stage is a fully closed model (fig. 6) ready to generate NURBS (Non-uniform Rational B-Spline) curves or surfaces network.

The surface creation process begins by laying down curves directly on the polygonal model to define the different surfaces to be created. The curves network created on model (fig. 7) can be the basis for subsequent realization of the surfaces.

Once the curves network is created, the model is ready to generate NURBS surfaces (fig. 8). This can be done automatically or manual. Automatic surface generation doesn't need to draw a curve, while manual surface generation can completely maintain the flow line of the original polygon surface. Manual generation of surfaces is related to the network of curves.

For this case, because the scanned denture has a complex geometry, was chosen automatic generation of NURBS surfaces on the polygonal model. To obtain the geometric model of the maxillary complete denture the NURBS surfaces network was exported in initial graphics exchange specification (IGES) format and then imported into Solid Works 2007 for conversion into a solid model, fig. 9.

Reverse Engineering and FEM Analysis

Fig. 9. The solid geometric model of complete denture

mm and 1 mm depth, in the area indicated in figure 1.

applied supports on surfaces shown in fig. 15.

the surface is determined by control points.

**2.3 The FEM analysis** 

1 mm.

for Mechanical Strength Evaluation of Complete Dentures: A Case Study 269

The NURBS tools (curves and surfaces) are commonly used in computer-aided design (CAD) and also found in various 3D modeling and animation software packages. They allow representation of geometrical shapes in a compact form. NURBS surfaces are functions of two parameters mapping to a surface in three-dimensional space. The shape of

Using the FEM software package, ABAQUS v6.9.3, on the geometric model of complete denture, was performed an analysis of the stress and strain field, taking into consideration different located defects. Based on non-destructive evaluation were carried out four models of analysis. At each model was considered a defect as a material hole with a diameter of 2

In the first model the fault was introduced near the midlle of the denture thickness, fig. 10. In the second and third model the faults were considered near the top surface of the denture and bottom respectively, fig. 11 and 12, and the fourth model shows a situation with a fault located in the thickness of the denture, with an irregular shape and a total surface twice that the faults defects of previous models, fig. 13. The fault depth of the fourth model is about

All four models were analyzed in two situations, in the first case we consider that the material of maxillary denture is Eclipse and the second case when the material is Meliodent. The maximum force of mastication at a patient with complete denture is between 60-80 N (Zarb G, 1997). For this study was considered a mastication force of 70 N distributed on palatal cusps of the upper teeth, fig. 14. The areas with distributed force are about 46.666 mm2 and the result is a normal pressure of 1.5 MPa. To fix the model, there have been

The supports from denture channel allow a 0.2 mm displacement in vertical direction and stop the displacements in horizontal plane. Also, the supports from palatal vault allow a 0.1 mm displacement in vertical direction and stop the displacements in horizontal plane. Allowed displacements were considered to replace the deformations of the oral mucosa.

Fig. 6. A closed polygon mesh resulted after the optimization stage

Fig. 7. The NURBS curve network generated on polygonal model

Fig. 8. The NURBS surface network generated on polygonal model

Fig. 9. The solid geometric model of complete denture

The NURBS tools (curves and surfaces) are commonly used in computer-aided design (CAD) and also found in various 3D modeling and animation software packages. They allow representation of geometrical shapes in a compact form. NURBS surfaces are functions of two parameters mapping to a surface in three-dimensional space. The shape of the surface is determined by control points.

#### **2.3 The FEM analysis**

268 Reverse Engineering – Recent Advances and Applications

Fig. 6. A closed polygon mesh resulted after the optimization stage

Fig. 7. The NURBS curve network generated on polygonal model

Fig. 8. The NURBS surface network generated on polygonal model

Using the FEM software package, ABAQUS v6.9.3, on the geometric model of complete denture, was performed an analysis of the stress and strain field, taking into consideration different located defects. Based on non-destructive evaluation were carried out four models of analysis. At each model was considered a defect as a material hole with a diameter of 2 mm and 1 mm depth, in the area indicated in figure 1.

In the first model the fault was introduced near the midlle of the denture thickness, fig. 10. In the second and third model the faults were considered near the top surface of the denture and bottom respectively, fig. 11 and 12, and the fourth model shows a situation with a fault located in the thickness of the denture, with an irregular shape and a total surface twice that the faults defects of previous models, fig. 13. The fault depth of the fourth model is about 1 mm.

All four models were analyzed in two situations, in the first case we consider that the material of maxillary denture is Eclipse and the second case when the material is Meliodent.

The maximum force of mastication at a patient with complete denture is between 60-80 N (Zarb G, 1997). For this study was considered a mastication force of 70 N distributed on palatal cusps of the upper teeth, fig. 14. The areas with distributed force are about 46.666 mm2 and the result is a normal pressure of 1.5 MPa. To fix the model, there have been applied supports on surfaces shown in fig. 15.

The supports from denture channel allow a 0.2 mm displacement in vertical direction and stop the displacements in horizontal plane. Also, the supports from palatal vault allow a 0.1 mm displacement in vertical direction and stop the displacements in horizontal plane. Allowed displacements were considered to replace the deformations of the oral mucosa.

Reverse Engineering and FEM Analysis

Fig. 14. The applied pressure on palatal cusps

for a total of 243032 elements and 49154 nodes.

Fig. 15. Applied boundary conditions on complete denture model

for Mechanical Strength Evaluation of Complete Dentures: A Case Study 271

Fig. 13. The fourth analysis model: the fault is considered in the thickness of the denture

u1=0; u2=0; u3=0.2

u1=0; u2=0; u3=0.1

All the analysis models have been meshed in tetrahedral finite elements, C3D4, accounting

Fig. 10. The first analysis model: the fault is considered near the midlle of the denture thickness

Fig. 11. The second analysis model: the fault is considered near the top surface of the denture

Fig. 12. The third analysis model: the fault is considered near the bottom of the denture

270 Reverse Engineering – Recent Advances and Applications

Fig. 10. The first analysis model: the fault is considered near the midlle of the denture

Fig. 11. The second analysis model: the fault is considered near the top surface of the

Fig. 12. The third analysis model: the fault is considered near the bottom of the denture

thickness

denture

Fig. 13. The fourth analysis model: the fault is considered in the thickness of the denture

Fig. 14. The applied pressure on palatal cusps

Fig. 15. Applied boundary conditions on complete denture model

All the analysis models have been meshed in tetrahedral finite elements, C3D4, accounting for a total of 243032 elements and 49154 nodes.

Reverse Engineering and FEM Analysis

denture with Eclipse material

for Mechanical Strength Evaluation of Complete Dentures: A Case Study 273

Fig. 17. The Maximum Principal Stress around the defect in the first analysis model for

Fig. 18. The Maximum Principal Stress (a) and Maximum Principal Strain (b) state in the first

Fig. 19. The Maximum Principal Stress (a) and Maximum Principal Strain (b) state in the

a) b)

a) b)

second analysis model for denture with Eclipse material

analysis model for denture with Meliodent material
