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orIgInal arTIClES

Multiple possibilities in detection of Materials defects in ceraMic fixed partial prostheses. nuMerical siMulation and photoelasticity Methods eMployed for evaluation and prognostic.

Cosmin Sinescu1, Meda Negrutiu1, Ciprian Ionita2, Radu Negru3, Adrian Bradu4, Daniela Maria Pop1, Florin Topala1, Liviu Marsavina3, Mihai Rominu1, George Dobre4, Adrian Podoleanu4

reZuMatIntroducere. In literatura de specialitate există citate mai multe metode dedicate evaluarii adaptării marginale a obturaţiilor din compozit de clasa a V-a. Testele de penetrare a unor coloranţi (testări de microleakage) sunt cele mai utilizate datorită simplicitătii tehnicii şi a preţului de cost redus. Dezavantajul major al acestei metode rezidă în secţionarea probelor şi evaluarea selectivă a infiltraţiei colorantului. In studiile realizate anterior de colectivul de autori au fost demonstrate avantajele utilizării tomografiei optice coerente ca metodă de investigare în acest domeniu de cercetare. Scopul studiului de faţă este acela de a evidenţia stratul de adeziv în ideea facilitării interpretărilor. Material şi metodă. Tomografia optică coerentă în mod en face a fost utilizată ca metodă de evaluare a 147 obturaţii de clasa V în calitate de probe. Cavităţile standard de clasa V au fost preparate în dinţi umani extraşi şi apoi cavităţile au fost obturate cu compozit Premise (Kerr). Probele au fost termociclate. Evaluarea interfeţelor a fost realizată utilizând tomografia optică coerentă în mod en face (OCT) combinată cu microscopia confocală (CM). OCT a lucrat la 1300 nm iar CM la 970 nm. Rezultate. OCT combinat cu CM au permis identificarea unor probleme de adaptare marginală a obturaţilor de compozit precum şi defecte de material în masa obturaţilor. Concluzii. OCT are numeroase avantaje care îl recomandă şi justifică utilizarea acestei metode în cadrul investigării cavităţii bucale. Această metodă atinge o rezoluţie de profunzime de până la 1 micron. Utilizarea unui adeziv îmbunătăţit facilitează interpretarea imagistică.Cuvinte cheie: tomografie optică coerentă, cavități de clasa a V-a, interfețe, microscopie confocală, medicină dentară.

abstractIntroduction. For the evaluation of marginal adaptation in class V composite restorations, several methods have been developed (bacterial penetration, fluid transport, clarification, penetration of radioisotopes, electrochemical methods and gas chromatography). Dye penetration tests (microleakage tests) however, are to be the most widely used due to their ease of application and low costs. Their main disadvantage is that the samples are sectioned. In previous works the advantages of using en face optical coherence tomography investigation in this research direction were demonstrated. The purpose of this study is to increase the scattering of the adhesive layers in order to facilitate the interpretations. Material and Methods. En Face Optical Coherence Tomography was employed to investigate 147 class V cavities samples. Standardized class V cavities, prepared in human extracted teeth, were filled with Premise (Kerr) composite. The specimens were thermo cycled. The interfaces were examined by Optical Coherence Tomography method (OCT) combined with the confocal microscopy. The optical configuration uses two single mode directional couplers with a superluminiscent diode as the source at 1300 nm. The scanning procedure is similar to that used in any confocal microscope, where the fast scanning is en-face (line rate) and the depth scanning is much slower (at the frame rate) Results. Marginal adaptation at the interfaces and gaps inside the composite resin materials were identified by means of optical coherence tomography. Exploration of the recent advances in OCT in terms of different excitation wavelengths and wider bandwidths can lead to state-of-the-art imaging systems in odontology. Conclusion. OCT has numerous advantages which justify its use in the oral cavity in comparison with conventional dental imaging. OCT can achieve the best depth resolution of all known methods (in principle one micron if the source exhibits a sufficiently wide spectrum) and is safe. It is important to use an improved adhesive in order to be spotted on the imaging investigation. Key words: optical coherence tomography, class V cavities, interfaces, confocal microscopy, dentistry

Received for publication: Oct 15, 2010. Revised: Dec. 15, 2009.

1University of Medicine and Pharmacy Victor Babes Timisoara, Romania2State University of New York at Buffalo, Buffalo, New York3Politehnica University, Romania4University of Kent, UK

Correspondence to:Cosmin SinescuRomania, 300180 Timisoara, Bd. Iuliu Maniu nr 1 sc A et 1 ap 1Phone: 0722280132E-mail: minosinescu@yahoo.com

introduction

Ceramics have the advantages of esthetics, biocompatibility, and mimicry of natural enamel properties. However, most are custom fabricated into dental restorations and contain porosity and/or stress risers that lead to cracks. There still is a clear need for more crack-resistant or crack-tolerant designs in dental ceramic engineering. Both pathways are being explored. Crack resistant materials include zirconia

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and alumina cores. Both have interesting potential. Yet, these approaches introduce other problems such as alloying esthetic ceramic veneers or creating strong attachment opportunities for dental cements or bonding systems. There is ever-increasing interest and research on options for crack-tolerant designs. This involves identification of new mechanisms to arrest cracks or slow down their propagation rates. Layered ceramics have potential because different layers can be designed to have different properties that produce crack blunting. Textured ceramics (woven ceramic microstructure motifs) create local variations in properties that discourage crack formation and make growth very difficult. Surface film modifications or surface retreatment such as sandblasting also may be used to eliminate fabrication defects that otherwise might generate cracks. The key challenges for improved dental ceramic systems seem to be crack-tolerance, low cost, bond ability, and excellent esthetic characterization. These goals involve materials design more than fabrication limitations. Major dental laboratories now provide full-service ceramic processing options; so many cost concerns for expensive processing equipment have disappeared when considering the potential for new processing technologies for dental restorations. Novel ceramic textures and micro structural control can be produced by rapid prototyping, stereo lithography, and other printing techniques. Their potential has not yet been realized.

Promoting ceramics in dentistry is based on the quality of biocompatibility and superior aesthetics of these materials.1 Among other qualities of ceramics, poor mechanical stability and its resistance limit its indications. In the 1990s only full ceramic fixed partial prostheses small area (maximum 3 items) were recommended to address prosthetics in the front. At present several types of ceramics, such as glass infiltrated alumina (In Ceram Alumina, In Ceram Zirconia), vitreous ceramics (Empress 2) or high resistance ceramics can be used to achieve full ceramic fixed partial prostheses for aesthetic troubleshooting and resistance in the front and side.2-4 Ceramics based on aluminum oxide (alumina ceramic) presents a fracture resistance between 236 and 600 MPa.5, 6 Fracture lines appeared on these types of ceramics are carried out at the interface between the capacity of aluminum oxide and ceramic masses subsequently applied. Length of sea lines of fracture and friction of ceramic fragments obtained after the initiation of the fracture line are responsible for initial energy dissipation.7 Sorensen et al reported at three years a failure rate of 0% for the front, 11% to 24% for the premolar and molar area,

in terms of achieving a minimum of 4 mm connector in occlusal-gingival shaft and 3 mm shaft oral-lingual.8 On the other hand, Vult von Steern et al reported a success rate after 5 years for fixed partial prostheses in the posterior three elements.9,10 Zirconia ceramic presented a fracture resistance between 421 and 800 MPa. The applications of these ceramics are directed to CAD / CAM systems, like Cerec, Sirona, namely to electrophoresis systems.11 Strength of Zirconia ceramic type materials is attributed to phase transformation mechanism of the mass of material. The dimensions of connectors have been designed to bucolingual axis and 4 mm 5 mm occlusal-gingival axis in the molar and premolar region on the axis of 3 mm and 4 mm oral-lingual axis occlusal-gingival. Embedded systems type ceramic Procera (Nobel Biocare) presents a resistance between 487 and 699 MPa for a minimum size of the connectors 3 mm high and 2 mm wide. Glassy ceramics (IPS Empress 2, Ivoclar Vivadent) hold between 300 and 400 MPa for a connector to 4mm shaft occlusal-gingival and 4 mm shaft oral-lingual.12 Clinical studies highlighted a success rate situated between 73 and 93%. In 2004, Esquivel-Upshaw et al reported a failure rate of 7% after 2 years. Zimmer et al have found a failure rate of 27.6% after 3 years and after 5 years, a success rate of 70%.13 Ceramics based on zirconium dioxide having strengths of 1192 N, that is 2.6 times higher than alumina ceramic and 2.1 times higher than the Empress 2, a connector section of 6.9 mm2. B. Taskonak, J.J. Mecholsky, Jr., and K.J. Anusavice have started, in 2006, research on the causes of fractures occurring in full ceramic fixed partial dentures.14 The results were made to how the existence of existing defects in material thickness prosthetic concentrators generate power during chewing with the initiation of fracture lines.

The aim of this paper is to present some of the methods that allow to detect the ceramic defects and to evaluate the prognostic of them by using dedicated methods.

Materials and Methods

En Face Optical Coherence Tomography (OCT) and Micro CT technologies were employed to detect the materials defects inside the prostheses.15-20 Time Domain system was used for the OCT technology. The OCT system, equipped with a confocal channel at 970 nm, uses a high NA interface optics allowing 1 mm image size.21-23 The configuration of the second system, as shown in Fig. 1, uses two single mode directional couplers.24 Light from the SLD source is injected into the system via the directional coupler DC1 which splits

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the light towards the two arms of the interferometer, the probing and the reference arm respectively. The probing beam is reflected by the dichroic beam-splitter BS1 and then sent via the galvanometer scanners SX and SY to the sample.24 Two telescopes incorporated between these elements conveniently alter the diameter of the beam in order to match the aperture of different elements in the probing path and convey a probing beam of around 8 mm in diameter through the microscope objective MO’s pupil plane.24 Hence, a lateral resolution of around 2 µm in the confocal channel could be achieved. A transversal resolution better than 5 microns is obtained in the OCT channel. Light back-scattered by the sample passes a second time through the object arm and is guided towards the single mode directional coupler DC2 via DC1 where it interferes with that coming from the reference arm. Both output fibers from DC2 are connected to two pin photo-detectors in a balanced photo-detection unit. A computer driven translation stage (TS) is used to construct B-scan images by stopping the frame scanner and moving TS along the optical axis of the reference beam.

Figure 1. En-face OCT at 1300 nm/confocal at 970 nm system. SLD= superluminescent diode, SX, SY: X and Y scanners; IMG = index matching gel; APD: avalanche photodiode; L1, L2, L3, L4: lenses: MO1-5: microscope objectives; PD1, 2: pin photodetectors; BS1,2: beamsplitters; LPF: low pass filter; PM: polarization.

The scanning procedure is similar to that used in any confocal microscope, where the fast scanning is en-face (line rate) and the depth scanning is much slower (at the frame rate). The en-face scans provide an instant comparison to the familiar sight provided by direct view or by a conventional microscope. Features

seen with the naked eye can easily be compared with features hidden in depth. Sequential and rapid switching between the en-face regime and the cross-section regime, specific for the en-face OCT systems, represents a significant advantage in the non-invasive imaging as images with different orientations can be obtained using the same system.

Figure 2. a. Picture of the MAF. b. Schematic diagram of the MAF.

Micro-computed tomography (micro-CT) systems have better resolution (10 lp/mm) but to date have relied either on rotating object imaging or small bore geometry for small animal imaging, and thus are not used for clinical imaging (Fig. 2). The authors from Toshiba Stroke Research Center reported the development and use of a 3D rotational micro-angiography (RMA) system created by mounting a micro-angiographic fluoroscope (MAF)[35 µm pixel, resolution >10 lp/mm, field of view (FOV)=3.6 cm] on a standard clinical gantry (Infinix, Model RTP12303J-G9E, Toshiba Medical Systems Corp., Tustin, CA). RA image sequences are obtained using the MAF and reconstructed. To eliminate artifacts due to image truncation, lower-dose (compared to MAF acquisition) full-FOV (FFOV) FPD RA sequences (194 µm pixel, FOV=20 cm) were also obtained to complete the missing data.

Numerical simulation investigations using finite elements method (FEM) were developed in order to study the stress distribution in the fixed partial dentures in the masticator environment. The virtual models of the considerate fixed partial dentures were developed using manual design and scanners (mechanical and laser scanners). For all cases the obtained results pointed out the most tensioned zones for each dental construct which represented the targeted investigation zones for the OCT technique.

After the detection of the materials defects the numerical simulation method was used in order to estimate the effect of the masticatory forces on the fractures propagations in the ceramic materials. The numerical simulation was performed in FEM using COSMOS/M software package. The discretization of the structure was performed in an automatic mode

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using a triangular finite element (TRIANG, as a 3- to 6-node triangular, two-dimensional element for plane stress, plane strain, or ax symmetric structural and thermal models). In order to simulate the cracking of metal ceramic fixed partial dentures a fracture analysis code FRANC2D/L was used. The photo elasticity method was used in order to check the accuracy of the tensions obtained with numerical simulation method.

23 ceramics fixed partial dentures was investigated using Optical Coherence Tomography as a non invasive method. 218 slices were used for each set of investigation. The distance between the slices was 20 microns. A 3D reconstruction was made for each investigation in order to evaluate the position and the magnitude of the ceramic defect.

results

OCT images from different ceramic fixed partial prostheses are shown below. Using incisal scanning, we found many pores which can cause possible fractures of the investigated dental bridges due to its dimensions and its positions. All the pores depicted below are deep in the dental ceramic material; therefore it will be hard to be detected via normal visual inspection. The detected defects have a large volume highly capable to generate fracture lines in the proximal or almost superficial on the occlusal area, leading to the failure of the prosthetic treatment. The detection of these defects before inserting the prostheses allows all the corrections in order to avoid the fracture of the ceramic component. For a better understanding of the ceramic defect spreading a three dimensional reconstruction can be develop.

The defects observed with the OCT technology (Fig. 3 and Fig.4) were subsequently investigated with the Micro-CT method in order to prove their presences at the location identified with the first method. (Fig. 5)

The numerical simulation was performed in FEM using COSMOS/M software package. For the investigation only the tooth (canine 3.3) with the depicted ceramic defect was considered. In order to simplify the numerical analysis, considering the mesio-distal strain on this tooth are zero, the numerical simulation become a plane strain problem. The material properties of the considered materials were taken from the material library of the software: E=2.21e5 MPa (Young’s modulus) and ν=0.22 (Poisson’s Ratio) for the ceramic material.

The boundary conditions were imposed as fixed on the inner part of the metallic infrastructure and the pressure normal on the occlusal surface, constant in value (p = 1 MPa) on the strict occlusal area and

linear on the two occluso-proximal areas (the pressure decrease from 1 MPa to zero).

Figure 3. (up) Photo of an all ceramic crown produce by pressed ceramic technology. (down). OCT C-scan image acquired at 1300 nm at 31x9.5 microns depth. At this depth, a large gap is seen between the ceramic layers. All ceramic crown (pressed ceramic technology). a: crown; b: defect inside the ceramic layers.

The maximum tensile stresses occurred on the vertical diameter of the defect could initiate fracture lines towards occlusal surface and/or metal infrastructure. Starting from these points a simulation of crack initiation and propagation was carried out for two cases: when the crack initiates on the upper point and growths toward occlusal surface, and crack initiates on the lower point and propagates towards the infrastructure. The crack propagation simulation was performed according with the maximum tensile stress criteria of Sih and Erdogan, and a remesh and fill algorithms were used to extend the crack.25,26 A 0.05 mm crack increment was considered. The stress intensity factors were calculated based on displacement correlation method.

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Figure 4. All ceramic crown (pressed ceramic technology) with the defect (black arrow) inside the ceramic layers after the metallographic investigation.

Figure 5. The same defect detected by Micro – CT method inside the ceramic layers.

The photo elasticity method allows evaluating the possibility of the fracture initiation and the contour of the morphology of those fractures.

discussions and conclusions

Ceramics are difficult to form into complicated geometries using high-temperature processes in a cost-effective manner in small dental laboratories. Other processes are well suited for custom operations. Hot-isostatic-pressing (HIP) has great advantages for creating standard shapes in a reusable mold, such as

prepable zirconia abutments for implants. For custom prostheses (crowns and bridges), it is currently more practical to rely on milling operations or molding operations to form dental shapes. CAD/CAM ceramic materials provide a unique option to start with almost defect-free material, but they don’t provide flexibility to regionally customize esthetics or other properties for a restoration. That is a large part of the reason that CAD/CAM has not replaced much of traditional ceramic fabrication technology. No alternative yet competes with the esthetic result of dental porcelain being layered by an artistic ceramic technician to fully characterize a restoration. While one can speculate that this is possible, this is not currently an option. When this is true, then CAD/CAM might have much grander appeal.

The detection of ceramic defects before oral inserting the prostheses allows all the corrections in order to avoid the fracture of the ceramic component. The fractures that occur within the structure of these prostheses were motivated by the elasticity module of the ceramics and by the defects within the ceramic layers. Early detection of substance defects within these layers allows for optimal corrections before inserting them and applying masticatory stress together with reduction of fractures. Also some of the defects are situated superficial enough and cervical, namely in the maximum tension area recorded during mastication with high risks of fracture at this level.

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