Three-dimensional finite element modelling of all-ceramic restorations based on micro-CT

In Silico Finite Element Analysis of Implant-Supported CAD-CAM Resin Composite Crowns

PURPOSE

The aim of this study was to evaluate the mechanical behavior of an implant-supported crown made using computer-aided design and computer-aided manufacturing (CAD-CAM) resin composite (RC) blocks in the posterior region.

MATERIAL AND METHODS

Four commercially available CAD-CAM RC blocks were used in this study: Cerasmart 300 (CS300; GC, Tokyo, Japan), Katana Avencia P Block (KAP; Kuraray Noritake Dental, Niigata, Japan); KZR HR3 Gamma Theta (HR3; Yamakin, Osaka, Japan), and Estelite P block (ESP; Tokuyama Dental, Tokyo, Japan). Katana Zirconia STML (ST; Kuraray Noritake Dental) was used as the control group. The elastic moduli of each material were determined by a three-point bending test. After the CAD models were designed, two different loading scenarios (oblique, vertical) were created. 3D finite element analysis was conducted with the prepared models.

RESULTS

The elastic modulus of the material utilized for the implant restorations did not cause any change in the stresses transmitted to the implant or peripheral bone. An important difference was detected in the abutment-crown junction area. The minimum von Mises value at the abutment-crown interface was obtained in ST, which has the closest elastic modulus to the titanium abutment.

CONCLUSIONS

The 3D finite element model designed in this study was used to demonstrate that implant-supported crowns fabricated with four different CAD-CAM RCs showed no critical stress concentrations in the bone or implant under all loading conditions. These results suggest that CAD-CAM RC blocks could be used as an alternative material for implant-supported restorations in the posterior region in terms of stress distribution.

[Stress in an implant-supported unitary fixed partial prosthesis with different materials in the first lower premolar through finite elements]

AIM

To analyze stress in a metal-ceramic, zirconia and lithium disilicate implant-supported unitary fixed partial prosthesis in the first lower premolar through finite element analysis at a 500 N force.

MATERIALS AND METHODS

Three study models were carried out, metal-ceramic, lithium disilicate and zirconium implant-supported crowns in the first lower premolar. The dental implant was made of titanium grade 5 based on the Bolt® model of UniDentalDirect with internal grooved connection (18 grooves) and the implant had a size of 11,0 x 4,5 mm, preformed abutment and integrated screw. The three designs had vertical and oblique (45°) forces applications at 500 N. The geometric modeling was performed with the SolidWorks® 2017 program and the results were obtained through the Von mises analysis using the CosmoWorks®2017 program.

RESULTS

The lowest value of maximum stress on crown level, under vertical and oblique forces, was found in the lithium disilicate crown with 21,9 MPa and 33,2 MPa, and with a minimum difference with the zirconium crown with 22,1 MPa and 35,1 MPa; on the abutment level, the zirconium crown had the lowest value of maximum stress with 18,6 MPa and 28,1 MPa; at the screw level, there were no significant differences.

CONCLUSION

Metal-ceramic, lithium disilicate, and zirconia crowns proved to be materials of good compressive and tensile strength, but it was concluded that the zirconia crown design generated lower overall stress.

Comparative Evaluation of Stress Acting on Abutment, Bone, and Connector of Different Designs of Acid-Etched Resin-Bonded Fixed Partial Dentures: Finite Element Analysis

Background: Finite element analysis (FEA) is one of the best methods for evaluating the stress distribution of restorations, such as fixed partial dentures. The development of resin cement has transformed prosthesis bonding and retention properties. Resin-bonded fixed partial dentures (RBFPD) have been considered minimally invasive treatment options for the prosthetic rehabilitation of single missing teeth.

Objectives: The aim of this study was to evaluate the stress load and distribution in four different designs of acid-etched RBFPDs using FEA.

Materials and Methods: The designs included standard tooth preparation principles and additional features. The first premolar and first molar abutments replaced the missing second premolar. Designs 1, 2, 3, and 4 included (1) lingual wings and occlusal rests; (2) wings and proximal slices; (3) wings, rests, and grooves; and (4) wings, rests, grooves, and occlusal coverage. The prepared models were restored with RBFPDs. A load of 100 N was applied to the central groove of the pontic to simulate occlusal forces. The materials used in the models were considered to be isotropic, homogeneous, and linearly elastic. FEA was used to reveal stresses acting on the abutment, bone, and connector in all prosthesis designs.

Results: The stresses transmitted to the abutment and bones were lowest for design 3, using wings, rests, and grooves. The stresses acting on the connector were the weakest in design 2. The stresses transmitted to the abutment and bone were highest in designs 1 and 4. The stresses transmitted to the connector were highest in design 3.

Conclusion: The wings, rests, and grooves design is possibly the ideal and conservative tooth preparation design to receive a posterior RBFPD. This design transmits less stress to the abutments and less bone resorption in the FEA. It is most likely to be successful in the clinical provision and ensures the longevity of the prosthesis.

Assessment of the survival and success rates of lithium disilicate crowns after different surface finishing procedures: An in vitro study

STATEMENT OF PROBLEM

Evidence is limited for the impact of clinical adjustments and polishing on the longevity of glazed lithium disilicate restorations.

PURPOSE

The purpose of this in vitro study was to evaluate the influence of surface finishing on the survival and success rates of lithium disilicate restorations based on fatigue resistance and failure mode.

MATERIAL AND METHODS

Lithium disilicate (IPS e.max CAD) maxillary premolar crowns (N=54) were cemented on a dentin analog. The restorations were divided into 3 groups: overglaze (OG), abrasion (GA), and abrasion and polishing (AP). The crowns were submitted to cyclic fatigue in 37 ^oC water at 100 N and 2 Hz in 2 lifetimes. The load was applied to the occlusal surface by using anatomic pistons to simulate a clinical tripod occlusal contact. After cycling, the crowns were examined for failure (cracking, chipping, or catastrophic fractures) under optical and scanning electron microscopy. Cracking was considered either a structural failure (success analysis) or a survival (clinical criteria - survival analysis). Data were analyzed by using the log rank Kaplan-Meier and Holm-Sidak tests (α=.05).

RESULTS

Surface finishing had no influence on the structural integrity of lithium disilicate, with similar success rates (P=.720). The calculated survival rate was higher for AP than that for other groups (P=.028). Cracking was found for GA and AP crowns, mostly initiating from the external surface. Chipping occurred in all experimental groups, and AP crowns did not show catastrophic failures.

CONCLUSIONS

Although surface treatments had no influence on the success of lithium disilicate, polishing showed a positive effect on the survival rate of the crowns based on the clinical implications of cracking (no need for replacement).

Noncontrast Computed Tomography Markers as Predictors of Revised Hematoma Expansion in Acute Intracerebral Hemorrhage

Background Noncontrast computed tomography (NCCT) markers are the emerging predictors of hematoma expansion in intracerebral hemorrhage. However, the relationship between NCCT markers and the dynamic change of hematoma in parenchymal tissues and the ventricular system remains unclear. Methods and Results We included 314 consecutive patients with intracerebral hemorrhage admitted to our hospital from July 2011 to May 2017. The intracerebral hemorrhage volumes and intraventricular hemorrhage (IVH) volumes were measured using a semiautomated, computer-assisted technique. Revised hematoma expansion (RHE) was defined by incorporating the original definition of hematoma expansion into IVH growth. Receiver operating characteristic curve analysis was used to compare the performance of the NCCT markers in predicting the IVH growth and RHE. Of 314 patients in our study, 61 (19.4%) had IVH growth and 93 (23.9%) had RHE. After adjustment for potential confounding variables, blend sign, black hole sign, island sign, and expansion-prone hematoma could independently predict IVH growth and RHE in the multivariate logistic regression analysis. Expansion-prone hematoma had a higher predictive performance of RHE than any single marker. The diagnostic accuracy of RHE in predicting poor prognosis was significantly higher than that of hematoma expansion. Conclusions The NCCT markers are independently associated with IVH growth and RHE. Furthermore, the expansion-prone hematoma has a higher predictive accuracy for prediction of RHE and poor outcome than any single NCCT marker. These findings may assist in risk stratification of NCCT signs for predicting active bleeding.

3D finite element model based on CT images of tooth

Aim: This study aimed the description of a protocol to acquire a 3D finite element (FE) model of a human maxillary central incisor tooth restored with ceramic crowns with enhanced geometric detail through an easy-to-use and low-cost concept and validate it through finite element analysis (FEA). Methods: A human maxillary central incisor was digitalized using a Cone Beam Computer Tomography (CBCT) scanner. The resulted tooth CBCT DICOM files were imported into a free medical imaging software (Invesalius) for 3D surface/geometric reconstruction in stereolithographic file format (STL). The STL file was exported to a computer-aided-design (CAD) software (SolidWorks), converted into a 3D solid model and edited to simulate different materials for full crown restorations. The obtained model was exported into a FEA software to evaluate the influence of different core materials (zirconia - Zr, lithium disilicate - Ds or palladium/silver - Ps) on the mechanical behavior of the restorations under a 100 N applied to the palatal surface at 135 degrees to the long axis of the tooth, followed by a load of 25.5 N perpendicular to the incisal edge of the crown. The quantitative and qualitative analysis of maximum principal stress (ceramic veneer) and maximum principal strain (core) were obtained. Results: The Zr model presented lower stress and strain concentration in the ceramic veneer and core than Ds and Ps models. For all models, the stresses were concentrated in the external surface of the veneering ceramic and strains in the internal surface of core, both near to the loading area. Conclusion: The described procedure is a quick, inexpensive and feasible protocol to obtain a highly detailed 3D FE model, and thus could be considered for future 3D FE analysis. The results of numerical simulation confirm that stiffer core materials result in a reduced stress concentration in ceramic veneer.

Intraoral repair of a chipped porcelain-zirconia restoration

OBJECTIVE

Ceramic fracture is an undesirable outcome of the rehabilitation with fixed partial dentures (FPD), mainly because it may involve additional cost and clinical time for intraoral repair or replacement of the restoration. This clinical report describes a 5 years survival intraoral repair of a chipped porcelain veneered zirconia framework restoration using a resin-based composite.

CLINICAL CONSIDERATIONS

A FPD of porcelain veneered zirconia was made. After 18 months, the FPD presented a porcelain chip (porcelain fracture without exposure to the zirconia structure) on the buccal side of the pontic. An epoxy resin replica of the fractured surface was obtained and was examined under scanning electron microscopy. Fracture origin was found at the cervical area of the pontic. Intraoral repair by bonding the chipped fragment back in place was performed. After 15 days, the porcelain fragment debonded without patient knowledge and the fragment was lost. Then, intraoral repair using composite resin to restore the fractured area was performed and is still in function to date.

CONCLUSIONS

Based on the 5-years survival of the performed intraoral repair, the composite resin reconstruction technique has shown to be an adequate alternative treatment for fractured FPD.

CLINICAL SIGNIFICANCE

A resin composite repair of the fracture site can be performed in one clinical session, using much less time and cost than for the replacement of FPD. This clinical case survived 5 years to date.

In silico non-linear dynamic analysis reflecting in vitro physical properties of CAD/CAM resin composite blocks

PURPOSE

The aim of this study was to assess the validity of in silico models of three-point bending tests to reflect in vitro physical properties obtained from three commercially available computer-aided design/computer-aided manufacturing (CAD/CAM) resin composite blocks and demonstrate notchless triangular prism analysis with those properties.

MATERIAL AND METHODS

Three types of commercially available CAD/CAM resin composite blocks were used: Cerasmart 300 (CS300; GC, Tokyo, Japan), Katana Avencia P Block (AP; Kuraray Noritake Dental, Tokyo, Japan), and KZR CAD HR3 Gamma Theta (GT; Yamakin, Osaka, Japan). In vitro/in silico three-point bending tests were conducted to obtain elastic modulus and fracture strain for non-linear dynamic finite element analysis (n = 10/each). Fractured surfaces of specimens after in vitro NTP tests were observed, and the fracture toughness of each CAD/CAM resin composite was obtained by in silico NTP analysis.

RESULTS

Both in vitro and in silico load-displacement curves obtained from three-point bending tests were significantly correlated (p < 0.05). The elastic moduli of CS300, AP, and GT were 8.0 GPa, 10.0 GPa, and 9.0 GPa, respectively. The fracture toughness values obtained from in silico NTP analysis of CS300, AP, and GT were 5.057 MPa m^1/2, 4.193 MPa m^1/2, and 4.880 MPa m^1/2, respectively. There was no significant difference in the length of the stable region among the three CAD/CAM resin composites (p = 0.09).

CONCLUSIONS

The in silico approach established in this study showed acceptable reflection of in vitro physical properties and will be useful for assessing fracture toughness related to the longevity of CAD/CAM resin composites without wastage of materials.

Edge chipping test in dentistry: A comprehensive review

OBJECTIVE

Literature on edge chipping test applied to dental materials and structures has been systematically reviewed with regard to the evaluation methods and parameters used.

DATA

A systematic search of the literature retrieved 3484 relevant studies. After removing duplicates, 1848 records were screened by titles and abstracts and 1797 were excluded; 51 papers were assessed full text for eligibility. Twenty papers were included in this study and they were organized according to the dental materials and structures tested as follows: 2 studies on human tooth, 9 on dental ceramics, 5 on polymer-based composites, and 4 studies evaluated both ceramic and polymer-based materials.

SOURCES

MEDLINE/PubMed, Scopus and Web of Science databases were searched up to June 2019 without restriction on date and language.

STUDY SELECTION

In vitro studies using edge chipping test on dental materials and structures were included.

CONCLUSIONS

Different methods have been used for edge chipping test, regardless of reported parameters. There is significant evidence that edge chipping test is a relevant approach to predict chipping behavior of dental materials and tooth tissues because chips produced from most edge chipping studies are similar to clinically reported chipping failures.

In vitro evaluation of fracture resistance and cyclic fatigue resistance of computer-aided design-on and hand-layered zirconia crowns following cementation on epoxy dies

Aim

This in vitro study was to compare the fracture resistance and cyclic fatigue resistance of hand-layered zirconia crowns and computer-aided design (CAD)-on crowns (lithium disilicate with zirconium oxide).

Settings and Design

Comparative -Invitro study design.

Materials and Methods

All ceramic crown preparation was done on a mandibular molar ivorine tooth, impression made, and duplicated. Twenty hand-layered zirconia crowns and twenty CAD-on crowns were fabricated using CAD/computer-aided manufacturing (CAD/CAM) technique. All crowns were cemented to their respective dies using resin cement for evaluating fracture resistance and cyclic fatigue resistance using universal testing machine.

Statistical Analysis Used

Independent samples t-test, Mann-Whitney U-test, and Shapiro-Wilk test were used.

Results

The mean fracture resistance of CAD-on crowns (2660.50 ± 501.303 N) was significantly more than that of hand-layered zirconia crowns (1963.60 ± 452.895 N) (independent samples t-test, P < 0.023). Cyclic fatigue resistance test results showed that the mean number of cycles before failure for hand-layered zirconia crowns was 175,297 and for CAD-on crowns was 212,097 (Mann-Whitney U-test, P < 0.012).

Conclusion

CAD-on crowns were found to have significantly higher fracture resistance and cyclic fatigue resistance properties than hand-layered zirconia crowns.

Fracture Load and Failure Mode of CAD-on Ceramic Structures

Abstract This study evaluated the fracture load (Lf) and the failure mode of CAD-on (Ivoclar Vivadent) ceramic structures, testing the hypotheses that Lf of multilayer structures is governed by the veneering ceramic strength and that chipping is more frequent in multilayer than monolithic structures. Disc-shaped specimens were fabricated as follows: CAD-on- trilayer structure composed of Y-TZP (yttria stabilized tetragonal zirconia polycrystal- IPS e.max ZirCAD) infrastructure, fusion glass-ceramic (IPS e.max CAD Crystall/Connect) and lithium disilicate-based glass-ceramic (IPS e.max CAD); YLD- bilayer structure composed of Y-TZP infrastructure and fluorapatite layering ceramic (IPS e.max Ceram); LDC- monolithic lithium-disilicate glass-ceramic (IPS e.max CAD); and YZW- monolithic Y-TZP (Zenostar Zr Translucent). The specimens were loaded in compression until failure and fracture surfaces were evaluated using fractographic principles. Lf values were statistically analyzed using the Weibull statistics, Kruskal-Wallis and Dunn tests (a= 0.05). YZW (1329 N) and CAD-on (1085 N) showed the greatest Lf median values, followed by YLD (832 N) and LDC (421 N). All monolithic structures (LDC and YZW) fractured catastrophically and all YLD structures failed by chipping. The CAD-on technique seems to be a very promising fabrication process because it showed high Lf, similar to monolithic zirconia, and small chipping rate.

Influence of veneer pore defects on fracture behavior of bilayered lithium disilicate glass-ceramic crowns

OBJECTIVE

To identify the conditions under which fabrication pore defects within veneering porcelain in bilayered lithium disilicate glass-ceramic (LDG) crowns will influence and jeopardize the mechanical integrity of the structure.

METHODS

Thirty standardized molar crowns (IPS e.max Press) were fabricated and microCT scanned to 3D-analyze the size, morphology and distribution of pores in veneering porcelain, followed by in vitro fracture test and SEM fractographic observation. Finite element analysis (FEA) of the microCT reconstructed models was used to evaluate the stress state.

RESULTS

The volumes of pores in samples ranged from 3241μm³ to 1.29×10⁹μm³ with the equivalent radius between 10μm to 680μm. Deviation of sphericity of pores ranged from 0.10 to 0.81 and the average of 99.97% pores was near 0.63. For the smaller pores their distribution tended to be uniform, while the larger pores were irregular with elongated ellipsoidal form and located at or near the veneer-core interface. During wedge loading blunt contact fracture testing 21 crowns failed from the fissure on the occlusal surface, of which 16 failed from surface or near surface pores, 2 from the midpoint of the oblique ridge, and 7 from larger interfacial pores. FEA analysis indicated that defects were detrimental to veneer integrity only in regions of tensile stress and where the pore radius associated with crack initiation ranged from 30 to 50μm. Pore morphology appeared to have only a minor effect on fracture.

SIGNIFICANCE

Within the limitation of the microCT resolution and FEA, it suggests that pores radius large than 30-50μm and located in the tensile stress area like grooves and fissures on the occlusal surface or near surface as well as cervical margins of veneering porcelain will jeopardize the bilayered structure and mechanical integrity of LDG.

Approximate relative fatigue life estimation methods for thin-walled monolithic ceramic crowns

OBJECTIVES

The objective is to establish an approximate relative fatigue life estimation method under simulated mastication load for thin-walled monolithic restorations.

METHODS

Experimentally measured fatigue parameters of fluormica, leucite, lithium disilicate and yttrium-stabilized zirconia in the existing literature were expressed in terms of the maximum cyclic stress and stress corresponding to initial crack size prior to N number of loading cycles to assess their differences. Assuming that failures mostly originate from high stress region, an approximate restoration life method was explored by ignoring the multi-axial nature of stress state. Experiments utilizing a simple trilayer restoration model with ceramic LD were performed to test the model validity.

RESULTS

Ceramic fatigue was found to be similar for clinically relevant loading range and mastication frequency, resulting in the development of an approximate fatigue equation that is universally applicable to a wide range of dental ceramic materials. The equation was incorporated into the approximate restoration life estimation, leading to a simple expression in terms of fast fracture parameters, high stress area ΔA, the high stress averaged over ΔA and N. The developed method was preliminarily verified by the experiments. The impact of fast fracture parameters on the restoration life was separated from other factors, and the importance of surface preparation was manifested in the simplified equation. Both the maximum stress and the area of high stress region were also shown to play critical roles.

SIGNIFICANCE

While nothing can replace actual clinical studies, this method could provide a reasonable preliminary estimation of relative restoration life.

Fatigue resistance of all-ceramic fixed partial dentures - Fatigue tests and finite element analysis

OBJECTIVE

To estimate the fatigue resistance of a new translucent zirconia material in comparison to lithium disilicate for 3-unit fixed partial dentures (FPDs).

METHODS

Eighteen 3-unit FPDs (replacement of first upper molar) with a connector size of 4mm×4mm were dry milled with a five-axis milling machine (Zenotec Select, Wieland, Germany) using discs made of a new translucent zirconia material (IPS e.max ZirCAD MT, Ivoclar Vivadent). Another 9 FPDs with a reduced connector size (3mm×4mm) were milled. The zirconia FPDs were sintered at 1500°C. For a comparison, 9 FPDs were made of IPS e.max Press, using the same dimensions. These IPS e.max Press FPDs were ground from a wax disc (Wieland), invested and pressed at 920°C. All FPDs were glazed twice. The FPDs were adhesively luted to PMMA dies with Multilink Automix. Dynamic cyclic loading was carried out on the molar pontic using Dyna-Mess testing machines (Stolberg, Germany) with 2×10⁶ cycles at 2Hz in water (37°C). Two specimens per group and load were subjected to decreasing load levels (at least 4) until the two specimens no longer showed any failures. Another third specimen was subjected to this load to confirm the result. All the specimens were evaluated under a stereo microscope (20× magnification). The number of cycles reached before observing a failure, and their dependence on the load and on the material, were modeled, using a Weibull model. This made it possible to estimate the fatigue resistance as the maximum load for which one would observe less than 1% failure after 2×10⁶ cycles. In addition to the experimental study, Finite Element Modeling (FEM) simulations were conducted to predict the force to failure for IPS e.max ZirCAD MT and IPS e.max Press with a reduced cross-section of the connectors.

RESULTS

The failure mode of the zirconia FPDs was mostly the fracture of the distal connector, whereas the failure mode of the lithium disilicate FPDs observed to be the fracture of the connectors or multiple cracks of the pontic. The fatigue resistance with 1% fracture probability was estimated to be 488N for the IPS e.max ZirCAD MT FPDs (453N for repeated test), 365N for IPS e.max ZirCAD MT FPDs with reduced connector size and 286N for the e.max Press FPDs. All three IPS e.max ZirCAD groups statistically performed significantly better than IPS e.max Press (p<0.001). On the other hand, no significant difference could be established between the two IPS e.max ZirCAD MT3 groups with a 4mm×4mm connector size (p>0.05). The allowable maximum principal stress (σ_max) which did not lead to failure during fatigue testing for IPS e.max ZirCAD MT3 was calculated between 208MPa and 223MPa for FPDs with 4mm×4mm connectors for 2×10⁶ cycles. This value could also be verified for the FPDs of the same material with 3mm×4mm connectors. On the other hand fatigue strength in terms of σ_max at 2×10⁶ cycles of IPS e.max Press was calculated to be between 78 and 90MPa.

SIGNIFICANCE

The fatigue resistance of the translucent zirconia 3-unit FPDs was about 60-70% higher than that of the lithium disilicate 3-unit FPDs, which may justify their use for molar replacements, provided that a minimal connector size of 4mm×4mm is observed. Even with a limited number of specimens (n=9) per group it was possible to statistically differentiate between the tested groups.

Evaluation of Interfacial Gap Volume of Two Low-shrinkage Composites Using Micro-Computed Tomography

OBJECTIVES

To investigate the efficacy of X-ray micro-computed tomography (μCT) in the detection and quantification of interfacial gap formation in standardized Class I and Class II resin composite restorations, to compare the interfacial gaps for two low-shrinkage resin composites with a methacrylate composite material, and to determine any correlation between the cavity configuration factor (C-factor) and the volume of gaps formed.

METHODS AND MATERIALS

Sixty standardized Class I and Class II cavities were prepared and divided into six groups. Three types of composites, with their recommended self-etching adhesive systems, were used: Filtek Z250 XT; Estelite Sigma Quick; and Filtek P90. Each of the composite materials was placed in 10 Class I and 10 Class II cavities. The specimens were digitized using Skyscan 1172 μCT. They were examined for gap volume measurements, the thickness of the adhesive layer, and location of interfacial gaps.

RESULTS

There was a significant difference in the mean gap volume percentages of the three materials. The gap volume percent of Estelite Sigma quick was significantly lower than that of Filtek P90. No significant difference in the mean gap volume percentages of Class I and Class II restorations was found, except for Estelite Sigma Quick, in which the Class I gap volume percentage was higher than that of the Class II restorations.

CONCLUSIONS

μCT is an efficacious tool for the measurement of volumetric gaps formed at the tooth/restoration interface and for the evaluation of the adhesive layer. The differences in the C-factor do not always have a pronounced effect on the gap volumes of low-shrinkage composites.

Survival Predictions of Ceramic Crowns Using Statistical Fracture Mechanics

This work establishes a survival probability methodology for interface-initiated fatigue failures of monolithic ceramic crowns under simulated masticatory loading. A complete 3-dimensional (3D) finite element analysis model of a minimally reduced molar crown was developed using commercially available hardware and software. Estimates of material surface flaw distributions and fatigue parameters for 3 reinforced glass-ceramics (fluormica [FM], leucite [LR], and lithium disilicate [LD]) and a dense sintered yttrium-stabilized zirconia (YZ) were obtained from the literature and incorporated into the model. Utilizing the proposed fracture mechanics-based model, crown survival probability as a function of loading cycles was obtained from simulations performed on the 4 ceramic materials utilizing identical crown geometries and loading conditions. The weaker ceramic materials (FM and LR) resulted in lower survival rates than the more recently developed higher-strength ceramic materials (LD and YZ). The simulated 10-y survival rate of crowns fabricated from YZ was only slightly better than those fabricated from LD. In addition, 2 of the model crown systems (FM and LD) were expanded to determine regional-dependent failure probabilities. This analysis predicted that the LD-based crowns were more likely to fail from fractures initiating from margin areas, whereas the FM-based crowns showed a slightly higher probability of failure from fractures initiating from the occlusal table below the contact areas. These 2 predicted fracture initiation locations have some agreement with reported fractographic analyses of failed crowns. In this model, we considered the maximum tensile stress tangential to the interfacial surface, as opposed to the more universally reported maximum principal stress, because it more directly impacts crack propagation. While the accuracy of these predictions needs to be experimentally verified, the model can provide a fundamental understanding of the importance that pre-existing flaws at the intaglio surface have on fatigue failures.

Effect of cement space on stress distribution in Y-TZP based crowns

OBJECTIVE

To evaluate the stress distribution in bi-layered Y-TZP based crowns, according to the occlusal internal spacing between coping and abutment.

METHODS

Twelve premolar shaped Y-TZP copings were made by a CAD/CAM system and seated on an abutment to evaluate the internal fit at the occlusal third using micro-CT images. Considering the fitting range obtained experimentally, two 3D finite element models, consisting on bone tissue, a titanium implant, a zirconia abutment, cement layer and a bi-layered Y-TZP ceramic crown were constructed based on the micro-CT images, one corresponding to the thinnest cement space and other representing the specimen with the thickest cement space obtained experimentally. A 250N axial load was applied at the center of the occlusal surface of the crown (≅0,8mm² area) and the first principal stress distribution was plotted and analyzed.

RESULTS

The greatest maximum principal stress occurred within the veneer ceramic right below the site of loading. The thickest cement model showed higher stress concentration at the center of occlusal surface of veneer and the center of occlusal internal surface of coping.

SIGNIFICANCE

Knowledge of stress distribution in ceramic crowns with different cement thicknesses will help clinicians to properly adjust crown fit, in seeking to avoid porcelain fractures.

Micro-CT Evaluation of Ceramic Inlays: Comparison of the Marginal and Internal Fit of Five and Three Axis CAM Systems with a Heat Press Technique

OBJECTIVES

To evaluate the marginal and internal adaptation of CAD/CAM lithium-disilicate inlay restorations fabricated by two milling systems (Five and Three-axis), and a traditional heat-press technique.

METHODS

Fifteen premolar teeth with an MOD cavity preparation were fabricated. Lithium-disilicate inlay restorations were obtained by three fabrication techniques and fitted to their dies (n = 15/gp) as follows: Group-1, three-axis milling system, Group-2, five-axis milling system, Group-3, conventional heat-press technique. Gaps were evaluated by X-ray microtomography. Marginal gap (MG), occlusal-marginal gap (OMG), proximal-marginal gap (PMG), gingival-marginal gap (GMG), absolute marginal discrepancy (AMD), axial-internal gap (AIG), and occlusal-internal gap (OIG) were evaluated at 120 different points per inlay. Data were analyzed using repeated measures ANOVA. Pairwise comparisons were conducted for post-hoc testes and the Bonferroni correction was used to adjust for multiple comparisons (α = 0.007).

RESULTS

The heat-press group demonstrated significantly smaller mean-values amongst all outcomes compared with CAD/CAM groups except for GMG, where there was no statistically significant difference between groups in the ANOVA (p = 0.042). Within the CAD/CAM groups, the five-axis group showed significantly lower OMG mean-value compared with the three-axis group p < 0.001, and lower AIG mean-value compared with the three-axis group p < 0.001. There was no significant difference between the five-axis and the three-axis groups' AMD, MG, PMG, and OIG locations.

CONCLUSION

Different fabrication techniques affected the marginal and internal adaptation of ceramic inlay restorations. The heat-press group showed the best marginal and internal adaptation results; however, in every group, all samples were within the clinically acceptable MG limit (100 μm).

CLINICAL SIGNIFICANCE

The marginal fit and internal adaptation of inlay ceramic restorations fabricated by a five-axis milling system have not been tested or compared with those fabricated by three-axis machines and the conventional heat-press method. The preferred method of inlay fabrication, whether in the lab or chair side, may be influenced by the results of this study and could affect future clinical decision-making. (J Esthet Restor Dent 29:49-58, 2017).

Effect of geometry on deformation of anterior implant-supported zirconia frameworks: An in vitro study using digital image correlation

PURPOSE

To evaluate the effect of geometry on the displacement and the strain distribution of anterior implant-supported zirconia frameworks under static load using the 3D digital image correlation method.

METHODS

Two groups (n=5) of 4-unit zirconia frameworks were produced by CAD/CAM for the implant-abutment assembly. Group 1 comprised five straight configuration frameworks and group 2 consisted of five curved configuration frameworks. Specimens were cemented and submitted to static load up to 200N. Displacements were captured with two high-speed photographic cameras and analyzed with video correlation system in three spacial axes U, V, W. Statistical analysis was made using the nonparametric Mann-Whitney test.

RESULTS

Up to 150N loads, the vertical displacements (V axis) were statistically higher for curved frameworks (-267.83±23.76μm), when compared to the straight frameworks (-120.73±36.17μm) (p=0.008), as well as anterior displacements in the W transformed axis (589.55±64.51μm vs 224.29±50.38μm for the curved and straight frameworks), respectively (p=0.008). The mean von Mises strains over the surface frameworks were statistically higher for the curved frameworks under any load.

CONCLUSION

Within the limitations of this in vitro study, it is possible to conclude that the geometric configuration influences the deformation of 4-unit anterior frameworks under static load. The higher strain distribution and micro-movements of the curved frameworks reflect less rigidity and increased risk of fractures associated to FPDs.

3D statistical failure analysis of monolithic dental ceramic crowns

For adhesively retained ceramic crown of various types, it has been clinically observed that the most catastrophic failures initiate from the cement interface as a result of radial crack formation as opposed to Hertzian contact stresses originating on the occlusal surface. In this work, a 3D failure prognosis model is developed for interface initiated failures of monolithic ceramic crowns. The surface flaw distribution parameters determined by biaxial flexural tests on ceramic plates and point-to-point variations of multi-axial stress state at the intaglio surface are obtained by finite element stress analysis. They are combined on the basis of fracture mechanics based statistical failure probability model to predict failure probability of a monolithic crown subjected to single-cycle indentation load. The proposed method is verified by prior 2D axisymmetric model and experimental data. Under conditions where the crowns are completely bonded to the tooth substrate, both high flexural stress and high interfacial shear stress are shown to occur in the wall region where the crown thickness is relatively thin while high interfacial normal tensile stress distribution is observed at the margin region. Significant impact of reduced cement modulus on these stress states is shown. While the analyses are limited to single-cycle load-to-failure tests, high interfacial normal tensile stress or high interfacial shear stress may contribute to degradation of the cement bond between ceramic and dentin. In addition, the crown failure probability is shown to be controlled by high flexural stress concentrations over a small area, and the proposed method might be of some value to detect initial crown design errors.

Reliability of FEA on the Results of Mechanical Properties of Materials

The present study evaluated the reliability of FEA on the results of different mechanical properties (E and v) of materials. Two 3D models of a maxillary canine with endodontic treatment, intracanal post, composite resin core and restored with porcelain-fused-to-metal crown were generated according to micro-CT images. Two groups with different E and ν values for porcelain, metal coping alloy, resin cement and composite resin were established. The materials' properties for group GL were based on literature data, while for group GIE the impulse excitation technique was used. A load of 180 N was applied at 45° on the incisal third of the lingual surface of the canine tooth. All models were supported by the periodontal ligament (x=y=z=0). The von Mises stress (VMS) was calculated. The stress values revealed differences between the groups for both VMS distribution and value. The porcelain (GL: 5.966 MPa; GIE: 7.478 MPa), metal coping (GL: 3.811 MPa; GIE: 0.973 MPa) and core (GL: 4.771 MPa; GIE: 0.026 MPa) were significantly affected. In conclusion, this study showed that the determination of mechanical properties (E and ν) of materials is essential for the reliability on the results of FEA.

Effect of ceramic infrastructure on the failure behavior and stress distribution of fixed partial dentures

OBJECTIVES

The effect of the ceramic infrastructure (IS) on the failure behavior and stress distribution of fixed partial dentures (FPDs) was evaluated.

METHODS

Twenty FPDs with a connector cross-section of 16 mm(2) were produced for each IS and veneered with porcelain: (YZ) Vita In-Ceram YZ/Vita VM9 porcelain; (IZ) Vita In-Ceram Zirconia/Vita VM7 porcelain; (AL) Vita In-Ceram AL/Vita VM7 porcelain. Two experimental conditions were evaluated (n = 10). For control specimens, load was applied in the center of the pontic at 0.5 mm/min until failure, using a universal testing machine, in 37°C deionized water. For mechanical cycling (MC) specimens, FPDs were subjected to MC (2 Hz, 140 N, 10(6) cycles) and subsequently tested as described for the control group. For YZ, an extra group of 10 FPDs were built with a connector cross-section of 9 mm(2) and tested until failure. Fractography and FEA were performed. Data were analyzed by ANOVA and Tukey's test (α = 0.05).

RESULTS

YZ16 showed the greatest fracture load mean value, followed by YZ16-MC. Specimens from groups YZ9, IZ16, IZ16-MC, AL16 and AL16-MC showed no significant difference for the fracture load.

SIGNIFICANCE

The failure behavior and stress distribution of FPDs was influenced by the type of IS. AL and IZ FPDs showed similar fracture load values but different failure modes and stress distribution. YZ showed the best mechanical behavior and may be considered the material of choice to produce posterior FPDs as it was possible to obtain a good mechanical performance even with a smaller connector dimension (9 mm(2)).

Effect of crown-to-implant ratio on peri-implant stress: a finite element analysis

The aim of this study was to evaluate stress distribution in the fixation screws and bone tissue around implants in single-implant supported prostheses with crowns of different heights (10, 12.5, 15 mm - crown-to-implant ratio 1:1, 1.25:1, 1.5:1, respectively). It was designed using three 3-D models. Each model was developed with a mandibular segment of bone block including an internal hexagon implant supporting a screw-retained, single metal-ceramic crown. The crown height was set at 10, 12.5, and 15 mm with crown-to-implant ratio of 1:1, 1.25:1, 1.5:1, respectively. The applied forces were 200N (axial) and 100 N (oblique). The increase of crown height showed differences with the oblique load in some situations. By von Mises' criterion, a high stress area was concentrated at the implant/fixation screw and abutment/implant interfaces at crown-to-implant ratio of 1:1, 1.25:1, 1.5:1, respectively. Using the maximum principal criteria, the buccal regions showed higher traction stress intensity, whereas the distal regions showed the largest compressive stress in all models. The increase of C/I ratio must be carefully evaluated by the dentist since the increase of this C/I ratio is proportional to the increase of average stress for both screw fixation (C/I 1:1 to 1:1.25 ratio=30.1% and C/I 1:1 to 1:1.5 ratio=46.3%) and bone tissue (C/I 1:1 to 1:1.25 ratio=30% and C/I 1:1 to 1:1.5 ratio=51.5%).

3D Finite element analysis of functionally graded multilayered dental ceramic cores

This study aimed at investigating and establishing stress distributions in graded multilayered zirconia/alumina ceramic cores and at veneer-core-cement-dentin interfaces, using finite element analysis (FEA), to facilitate the structural design of ceramic cores through computer modeling. An intact maxillary premolar was digitized using CT scanning. An imaging software, Mimics, was used to reconstruct 3D models based on computed tomography (CT) data saved in DICOM format. Eight different 3D models were created for FEA, where each 3D model was meshed and its bottom boundaries constrained. A static load was applied in the oblique direction. The materials were assumed to be isotropic and homogeneous. Highest von Mises stress values were found in areas directly below the load application point, and stress gradually decreased in occlusal loading direction from the external surface toward the dentin. Stress levels occurring at veneer-ceramic core-cement-dentin interfaces were shown to be lower in multilayered ceramic cores than in single-layer models.