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ORIGINAL ARTICLES

abstract

Received for publication: Jul. 07, 2007. Revised: Dec. 19, 2007.

rEZUMat

1 Department of Dental Technology, 2 Department of Prosthetic Dentistry, Faculty of Dental Medicine, Victor Babes University of Medicine and Pharmacy, Timisoara

Correspondence to:Liliana Sandu, 6 Socrate Str., 300552 Timisoara, Tel. +40-722-310299Email: lilianasandu@gmail.com

INtrODUctION

The requirements imposed at this time considering the modern calculation of the dental restorations design, as well the manner of taking decisions in case of their life duration assessing, need good knowledge in the field of biomechanics. The permanent problem

LasEr scaNNING aPPLIcatIONs FOr tHrEE-DIMENsIONaL tEEtH MODELs rEcONstrUctION

Liliana Sandu1, Florin Topala2, Cristina Bortun1, Sorin Porojan1

Introduction: A three-dimensional scanner is a device that analyzes an object to collect data on its shape, in order to construct three-dimensional models. These can be used in a wide variety of applications. In dentistry laser scanning was introduced through CAD/CAM systems. Purpose: The aim of the study was to create digital three-dimensional teeth models using laser scanning. Material and methods: Different normal plaster teeth were scanned rotary and from a single plane using a laser scanner PLX 1200. The scanning time was about 10 minutes. The resolution or accuracy was 100 m. These scans, from different directions were required to obtain information about all sides of the teeth. After that, the scans were brought in a common reference system, a process that is usually called alignment, and then merged to create a complete model. The three-dimensional laser allowed to create a point cloud of geometric samples on the surface of the teeth. These points were then used to extrapolate the shape of the object, process called reconstruction. Adjacent points were connected in order to create a continuous surface. Using this, solids were generated. Results: Three-dimensional teeth models obtained after laser scanning have a proper design to be used for didactic and basic research applications. Conclusions: Construction of three-dimensional models using laser scanning involves a lot of working steps and specific knowledge. They can be used in a wide variety of applications, for finite element analyses, for computer aided machines and for didactic purpose. Key Words: laser scanner, three-dimensional teeth models, reconstruction

Introducere: Scanner-ul tridimensional este un aparat care analizeaz\ un obiect pentru a colecta date despre forma sa, n vederea realiz\rii de modele tridimensionale. Acestea pot fi folosite pentru o varietate mare de aplica]ii. n medicina dentar\ scanarea laser a fost introdus\ prin intermediul sistemelor CAD/CAM. Scop: Scopul studiului a fost de a crea modele tridimensionale ale din]ilor, utiliznd scanarea laser. Material [i metode: Modele de din]i normali din gips au fost scanate rotativ [i dintr-un singur plan, utiliznd scannerul laser PLX 1200. Timpul de scanare a fost de aproximativ 10 minute. Rezolu]ia sau acurate]ea a fost de 100 m. Scan\ri din diferite direc]ii au fost necesare pentru pentru a ob]ine informa]ii asupra tuturor fe]elor din]ilor. Dup\ aceea scan\rile au fost aduse ntr-un sistem de referin]\ comun, proces numit aliniere [i apoi unite pentru a crea un model complet. Laserul tridimensional a permis crearea unui nor de puncte ale modelelor geometrice pe suprafe]ele din]ilor. Aceste puncte au fost utilizate pentru a extrapola forma obiectului, proces numit reconstruc]ie.Punctele adiacente au fost conectate pentru a crea suprafe]e continue. Au fost generate solide prin utilizarea acestora. Rezultate: Modelele tridimensionale ale din]ilor ob]inute dup\ scanarea laser au un design optim pentru a fi folosite pentru aplica]ii didactice [i de cerectare fundamental\. Concluzii: Construc]ia de modele tridimensionale utiliznd scanarea laser implic\ o serie de etape de lucru [i cuno[tin]e specifice. Ele pot fi utilizate pentru o varietate mare de aplica]ii, pentru analiza cu elemente finite, pentru sisteme de frezaj computerizat [i n scop didactic. Cuvinte cheie: scanner laser, modele tridimensionale ale din]ilor, reconstruc]ie

is the choice of the prosthetic restorations geometry and dimensions, so that they satisfy functional needs, without touching the yielding stage, alteration of the initial shape or having nocive effects on the remaining teeth.

The study of complex structures, like teeth and dental restorations can be made just bounded availing analytical methods of calculation. At present modern design calculations are required. The situation is a direct consequence of the advances in the field of computers, both in hardware and software. Modern design and evaluation calculus of prosthetic restorations in order to obtain an adequate framework strength cannot be imagined without using numerical simulations. Analyses in this field continuously advance both in three-dimensional modeling, computer aided design and numerical simulation methods.

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The finite element method is the most popular method of numerical simulation, at this moment. It penetrated the field of dentistry, for the necessity of filling the gap of the prosthetic activity, beginning from the design and up to the final stage. It can be claimed that the finite element method, that evolved beginning with the 60’s of the XXth century, put at our disposal a formidable tool of numerical investigation, that theoretically doesn’t have any limits or barriers.1 The only problems we are up against yet at this time are relative to the modeling, conditions impositions that approaching the real situation at one side and at the other side to the calculation power and speed we get access.

Sophisticated 3D models are required to better understand the mechanical behavior of teeth structures and prosthetic dental restorations.2-4 Direct comparison of experimental and theoretical results in biomechanical studies requires a careful reconstruction of the object in order to achieve congruencies for validation.5-9

In dentistry numerical simulations were used for the first time three decades ago, and since then, the applications were continuously diversified. The research evolution has to be watched in relation to the advances extended in the field of electronic computers. Finite element analysis is a numeric analising method for the states of stresses and displacements. It penetrated dentistry with biomechanical analysis of some intact and restored teeth.10 Analyses in this field advanced continuously, regarding both modeling, and analysing methods.

PUrPOsE

The aim of the study was to create digital three-dimensional teeth models using laser scanning in order to develop applications for didactic and basic research use, to design and optimize prosthetic restorations.

MatErIaLs aND MEtHODs

In order to develop applications for didactic and basic research use, to design and optimize dental restorations, detailed 3D models of the teeth and dentures components are required. These can be obtained by reconstructions after 3D scanings and computed tomographies.

Nonuniform rational B-spline (NURBS) is a mathematical model used through CAD/CAM systems for computer aided design and manufacturing of prosthetic restorations. For most situations, a single

scan will not produce a complete model of the object. Multiple scans, from many different directions are usually required to obtain information about all sides of the object. Therefore different normal plaster teeth were scanned using a laser scanner PLX 1200. The scanning time was about 10 minutes. The resolution or accuracy was 100 μm. These scans were brought in a common reference system, a process that is usually called alignment, and then merged to create a complete model. (Fig. 1)

Resulted files were imported in LeiosMesh program, were the 24631 point clouds from the teeth surfaces were cleaned and assembled. (Fig. 2) The collected data were used to construct three dimensional models. These points were used to extrapolate the shape of the subject, a process called reconstruction. (Fig. 3)

The parts resulted from the scanning were assembled by the superposition of the points. Reconstruction involves finding and connecting adjacent points in order to create continuous surfaces. (Fig. 4) Because of the complex teeth geometry, a nonparametric program was chosen for modeling (Rhinoceros NURBS).

Figure 1. Complete model of a premolar, after scans alignment.

Figure 2. Point clouds from the teeth surface.

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Figure 3. Reconstruction of the tooth shape.

Figure 4. Continuous surface of the tooth.

rEsULts

The generation of NURBS surfaces was done manually using the path toolbar button. (Fig. 5) The grid lines number was 25, control points number 11, smoothing parameter 1e-006. The resulted file was imported in Rhinoceros, where the open surfaces were closed. In this way the solids were generated. (Fig. 6) NURBS modelled resulted solids have a properly designed morphology and can be used for a wide variety of applications.

The working methods are laboriously, they involve a lot of working steps and specific knowledge. They can be used in a wide variety of applications, for finite element analyses, for computer aided machines and for didactic purpose.

The faithfulness of the solids depends on the structural complexity, the aggrandizement degree and the image processing procedures.

The described method can generate detailed 3D models of a tooth that can readily be used to develop applications for didactic and basic research use, to design and optimize prosthetic restorations.

Figure 5. Meshing of the teeth surface.

Figure 6. Surface reconstruction.

Figure 7. External surface used for solid generation.

DIscUssIONs

Although CAD/CAM methods for producing fixed restorations are of increasing interest, little information has been published about their mode of operation and functionality.11 At this time a lot of devices for digitizing model surfaces for dental CAD/CAM applications are available in dentistry.12-16

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Figure 8. Three dimensional model of a premolar.

Figure 9. NURBS modeled resulted solid.

The described method is open and doesn’t depend on a specific scanner or system. The results depend only on the scanning accuracy and the file types resulted after scanning.

The precision of the models obtained by three dimensional scanning depends on the working procedures, from scaning to volume generation, is higher than those obtained using other modeling methods. (Figs. 10, 11)

Figure 10. Defects resulted during scanning.

Figure 11. Defects resulted during surface generation.

The reconstruction methods using laser scaning or micro-CT data can generate detailed and valid 3D models of teeth and dental restorations. This method is rapid and can readily be used for other medical (and dental) applications.2,3,9

cONcLUsIONs

Construction of three-dimensional models using laser scanning involves a lot of working steps and specific knowledge: object preparation, scanning, importing of the point clouds file into a program in order to generate NURBS surfaces, registration and merge of the point clouds in a single model, converting the point cloud into a triangulated mesh, editing and processing of the mesh, reconstruction of the model by NURBS path.

The described method can generate detailed 3D models of a tooth that can be used to develop applications for didactic and basic research use, to design and optimize prosthetic restorations.

rEFErENcEs

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4. Romeed SA, Fok SL, Wilson NH. A comparison of 2D and 3D finite element analysis of a restored tooth. J Oral Rehabil 2006;33(3):209-15.

5. Shimizu Y, Usui K, Araki K, et al. Study of finite element modeling from CT images. Dent Mater J 2005;24(3):447-55.

6. Li W, Swain MV, Li Q, et al. Towards automated 3D finite element modeling of direct fiber reinforced composite dental bridge. J Biomed Mater Res B Appl Biomater 2005;74(1):520-8.

7. Rahimi A, Keilig L, Bendels G, et al. 3D reconstruction of dental specimens from 2D histological images and microCT-scans. Comput Methods Biomech Biomed Engin 2005;8(3):167-76.

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10. Ausiello P, Apicella A, Davidson CL, et al. 3D - finite element analysis of cusp movements in a human upper premolar, restored with adhesive resin-based composites. J Biomech 2001;34:1269-77.

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