Influence of the veneer-framework interface on the mechanical behavior of ceramic veneers: A nonlinear finite element analysis Priscilla Cardoso Lazari, DDS, MSc,a Bruno Salles Sotto-Maior, DDS, MSc, PhD,b Eduardo Passos Rocha, DDS, MSc, PhD,c Germana de Villa Camargos, DDS, MSc,d and Altair Antoninha Del Bel Cury, DDS, MSc, PhDe Piracicaba Dental School, State University of Campinas, Piracicaba, São Paulo, Brazil; Juiz de Fora Dental School, Federal University of Juiz de Fora, Minas Gerais, Brazil, Araçatuba Dental School, São Paulo State University, São Paulo, Brazil Statement of problem. The chipping of ceramic veneers is a common problem for zirconia-based restorations and is due to the weak interface between both structures. Purpose. The purpose of this study was to evaluate the mechanical behavior of ceramic veneers on zirconia and metal frameworks under 2 different bond-integrity conditions. Material and methods. The groups were created to simulate framework-veneer bond integrity with the crowns partially debonded (frictional coefficient, 0.3) or completely bonded as follows: crown with a silver-palladium framework cemented onto a natural tooth, ceramic crown with a zirconia framework cemented onto a natural tooth, crown with a silver-palladium framework cemented onto a morse taper implant, and ceramic crown with a zirconia framework cemented onto a morse taper implant. The test loads were 49 N applied to the palatal surface at 45 degrees to the long axis of the crown and 25.5 N applied perpendicular to the incisal edge of the crown. The maximum principal stress, shear stress, and deformation values were calculated for the ceramic veneer; and the von Mises stress was determined for the framework. Results. Veneers with partial debonding to the framework (frictional coefficient, 0.3) had greater stress concentrations in all structures compared with the completely bonded veneers. The metal ceramic crowns experienced lower stress values than ceramic crowns in models that simulate a perfect bond between the ceramic and the framework. Frameworks cemented to a tooth exhibited greater stress values than frameworks cemented to implants, regardless of the material used. Conclusion. Incomplete bonding between the ceramic veneer and the prosthetic framework affects the mechanical performance of the ceramic veneer, which makes it susceptible to failure, independent of the framework material or complete crown support. (J Prosthet Dent 2014;-:---)
Clinical Implications Failures in the ceramic veneer-framework interface in zirconia crowns are common and can influence the biomechanical behavior of the ceramic veneer, increasing chipping or fractures.
This study was supported by the São Paulo Research Foundation grant no. 2011/03555-8). a
Graduate student, Department of Prosthodontics and Periodontology, Piracicaba Dental School, State University of Campinas. Professor, Department of Restorative Dentistry, Juiz de Fora Dental School, Federal University of Juiz de Fora. c Professor, Department of Dental Materials and Prosthodontics, Araçatuba Dental School, São Paulo State University. d Graduate student, Department of Prosthodontics and Periodontology, Piracicaba Dental School, State University of Campinas. e Professor, Department of Prosthodontics and Periodontology, Piracicaba Dental School, State University of Campinas. b
Lazari et al
2
Volume Because of its stiffness and flexural strength, yttria-stabilized tetragonal polycrystalline zirconia (Y-TZP) has been used as a framework material in dental prostheses in an effort to improve the performance of metal-free crowns.1-5 The material also has excellent properties in terms of esthetics, radiopacity, and biocompatibility.6 Clinical trials have demonstrated the potential of zirconia to enhance the performance of prostheses7 because the tensile strength of metal-free crowns depends on the properties of the material used to fabricate the framework.8 However, metalfree prostheses have been associated with a high complication rate.7-9 Most of the reported failures are due to fracture, wear, or chipping of the ceramic veneer,7,9-13 with fractures occurring in 6% to 15% of restorations after 3 to 5 years.14,15 This failure rate is high compared with that of metal ceramic prostheses, which have a 4% failure rate over 10 years. The greater number of failures in metal-free prostheses is due to the weaker interface between the zirconia structure and the ceramic veneer.16 The fragility of this interface is related to the difference in the thermal expansion coefficients (CTE) of the ceramics,6,16,17 and proper matching of CTEs is essential to prevent failure after porcelain firing. Incompatible CTEs are aggravated by improper burning or rapid cooling because the low thermal conductivity of zirconia can cause excessive tensile stresses in the feldspathic ceramic veneer.8,18-21 The stresses generated in ceramic veneers in the presence of an incomplete bond between the veneer and the zirconia framework have not been extensively examined.6,22 In the single study in which simulated failures at this interface were investigated, the stresses in the ceramic veneer were up to 12 times greater than those under conditions of perfect bonding between the materials.23 Another aspect to consider is the structure that supports the restoration because the stresses depend on whether the restoration is on a natural tooth or an osseointegrated implant.24 Implants and abutments have much greater elastic
moduli than natural teeth,25 which results in a greater stress concentration and a greater likelihood of failure if the bond between the 2 materials is incomplete. Failure rates in metal ceramic prostheses are higher when they are mounted on implanted retainers compared with those mounted on natural teeth.26 The purpose of this study was to use 3-dimensional nonlinear finite element analysis to evaluate the mechanical behavior of ceramic veneers, partially debonded or completely bonded, with zirconia or palladium-silver frameworks on natural teeth and implants. The hypothesis was that the partially debonded ceramic veneer and zirconia framework would have greater stresses in the ceramic veneer compared with those in the completely bonded veneers, independent of the framework material or crown support.
MATERIAL AND METHODS Models of a complete crown on a central incisor were constructed in which the crown was supported by either a natural tooth or an implant. The factors studied included the frictional coefficient (0.3 or completely bonded), which is used to describe the condition of the bond between the veneer and the framework, the framework material (zirconia or palladium and/or silver), and the type of support (tooth or implant). The groups were created with the crowns partially debonded
The Journal of Prosthetic Dentistry
-
Issue
-
(frictional coefficient, 0.3) or completely bonded as follows: C, crown with a silver-palladium framework cemented onto a natural tooth; Cz, ceramic crown with a zirconia framework cemented onto a natural tooth; Ci, crown with a silver-palladium framework cemented onto a morse taper implant; Czi, ceramic crown with a zirconia framework cemented onto a morse taper implant. Oblique loads were applied and analyzed with nonlinear finite element analysis software to determine the maximum principal stress, shear stress, and deformation for the veneer, and the von Mises stress for the frameworks.
Finite element model design This study was approved by the human research ethics committee at the Faculty of Dentistry of Aracatuba, São Paulo State University. Computerized tomographic (CT) images of a human edentulous maxilla were used to construct the virtual model. The CT images were exported to MIMICS 13.1 software (Materialise) for 3-dimensional model construction. The resulting model was exported to the SolidWorks 2012 software (Dassault Systèmes SolidWorks Corp) for geometry simplification and design refinement. The crown cemented onto a tooth was modeled by using images from a CT of a central incisor. The images were transformed into dicon solid models with InVesalius software (CTI). All tooth structures
1 Tooth model with ceramic veneer and framework, cement layer, dentin, dental pulp, and periodontal ligament.
Lazari et al
-
2014
3
(enamel, crown and root dentin, dental pulp, and periodontal ligament) were included in the solid model. With SolidWorks 2012 software, the tooth was reduced to 2.0 mm in thickness on the buccal and lingual surfaces and 3.0 mm in thickness on the incisal surface. The ceramic veneer and framework thicknesses were 2.0 mm and 0.4 mm, respectively (Fig. 1). The crown cemented onto implants was modeled by using the same crown cemented onto a tooth. The computer-aided design models of the implant (Titamax Ex, 4 mm 13 mm; Neodent) and prosthetic platform were obtained from the manufacturer (Fig. 2). All crowns were cemented onto the tooth or abutment with resin cement (Panavia; Kuraray) in a 0.09-mm-thick layer.
2 Implant model with ceramic veneer and framework, abutment, cement layer, screw, and implant.
Material properties The computer-aided design models were exported to Ansys Workbench 14.0 FEA software (Swanson Analysis Inc) as Initial Graphics Exchange Specification files. All structures were considered isotropic and homogeneous. Material properties such as the elastic modulus and Poisson ratio were obtained from the literature (Table I). The mesh was constructed through convergence of analysis (5%), which was determined in all models by using a tetrahedral element of 0.8 mm in size. The models had 29 665 (C and Cz) and 68 673 (Ci and Czi) elements. The numbers of nodes were 55 651 (C and Cz) and 121 061 (Ci and Czi) (Fig. 3).
Table I.
Mechanical properties of materials
Materials
Elastic Modulus (GPa)
Poisson Ratio
Reference No.
Palladium-silver
95
0.33
35
Ceramic veneer
70
0.19
36
Zirconia
205
0.22
36
Dentin
20
0.31
37
Pulp
0.002
0.45
38
Periodontal ligament
0.0689
0.45
39
Resin cement
18.3
0.33
40
Cortical bone
13.6
0.26
35
Trabecular bone
1.36
0.31
35
Implant (titanium)
110
0.33
35
Interface conditions All structures were considered bonded except the veneer-framework interface. This interface was considered perfectly bonded (linear) or with a frictional coefficient of 0.3 (nonlinear).23 Contact nonlinearity allowed both faces to separate in their normal direction or slide in the tangential direction.
Loading and boundary conditions The models were defined by fixing the mesial and distal exterior surfaces of
Lazari et al
3 Models with tetrahedral element of 0.8 mm.
4
Volume the bone segment in all directions. The models were loaded by using an initial load distribution of 49 N26 applied to the lingual surface at an angle of 45 degrees from the long axis of the tooth, followed by an axial load of 25.5 N27 distributed across the incisor face (Fig. 7). The maximum principal stress, shear stress, and deformation for the ceramic veneer and the von Mises stress for the framework were obtained.
Table II.
Issue
Frictional Coefficient
Maximum Principal Stress (MPA)
Shear Stress (MPA)
C
0.3
602.06
140.22
1.0
35.464
7.2781
0.3
674.48
159.39
1.0
33.374
6.9906
0.3
346.12
152.93
1.0
41.281
7.1687
0.3
355.91
162.48
1.0
40.619
7.086
Cz Ci Czi
-
Maximum principal stress and shear stress for ceramic veneer
Model
RESULTS The maximum principal stress and shear stress values in the ceramic veneer under load are presented in Table II. When the veneer-framework bond was incomplete (frictional coefficient, 0.3), the stress concentration was greater in all structures, regardless of the framework material or crown support. The stresses were always located in the region of load application (middle third of the palate and incisal edge of the crown) (Fig. 4). Analogous to the maximum principal stress, the greatest shear stress (162.48 MPa) occurred in the Czi model on the inside of the ceramic veneer near the veneerframework interface (Fig. 5). Deformation of the ceramic veneer was greater in the models with a frictional coefficient of 0.3 (C, 0.036 mm; Cz, 0.035 mm; Ci, 0.042 mm; and Czi, 0.042 mm). In completely bonded models, those mounted on teeth (C and Cz) underwent deformations 3 times less than in the poorly bonded models,
-
C, crown with a silver-palladium framework cemented onto a natural tooth; Cz, ceramic crown with a zirconia framework cemented onto a natural tooth; Ci, crown with a silver-palladium framework cemented onto a morse taper implant; Czi, ceramic crown with a zirconia framework cemented onto a morse taper implant.
whereas those mounted on implants (Ci and Czi) experienced deformations approximately one-half as great as in the poorly bonded models. The framework results are depicted in Table III. The stresses in frameworks cemented to teeth were greater than those in frameworks cemented to implants. This effect was more evident in models with partial debonding between the structures (Fig. 6).
DISCUSSION Ceramic systems are widely used to achieve excellent esthetic restorations, and they need to be covered by feldspathic ceramic28 to decrease the opacity of the framework.28 Zirconia, as a framework material, allows a higher
metal-free restoration resistance; however, it is clear that failures in the interface of the zirconia and ceramic veneer can reduce clinical success. Results of several studies have indicated that the veneer-framework bonding interface is the primary cause of failure, which can be due to the difference in the CTEs of the 2 materials.17,20,29 The incompatibility of the CTEs is aggravated by improper burning or fast cooling due to the low thermal conductivity of zirconia, which can promote tensile stresses in the veneer and the eventual failure of the restoration. In the present study, the failure (weak veneer-framework interface bonding) was simulated by a frictional contact of 0.3 through the nonlinear finite element method. This coefficient
4 Maximum principal stress distribution on ceramic veneer.
The Journal of Prosthetic Dentistry
Lazari et al
-
2014
5
5 Shear stress (MPa) for Czi (ceramic crown with a zirconia framework cemented onto a morse taper implant) model was located at internal ceramic veneer-framework interface.
has previously been used, and a 12-fold increase in stress in ceramic crowns due to poor veneer-framework interface was reported,23 similar to the present findings. When considering a flexural strength for feldspathic porcelain of 40 MPa,30 inadequate bonding can result in stress values greater than the strength of the material. The flaws propagate from ceramic with low resistance and modulus of elasticity values to ceramic with higher resistance and modulus of elasticity values31,32 Higher stress values were found in the ceramic veneers, and lower stress values were found in the framework, regardless of the material.31 Moreover, a thin layer of ceramic veneer fired upon a framework significantly reduces the resistance of the
Table III.
bilayer crown compared with monolithic restoration.28 Although the critical load for the fracture is strongly influenced by the total thickness of the crown and is less influenced by the proportion of the veneer-framework thickness, the ceramic veneer is still the weakest link, which compromises the strength of the bilayer system.28 When considering the qualitative analysis of these results, the stress distributions appeared to be similar among all groups and were concentrated at the location of the applied load. The frictional coefficient affected the stress values but not the stress distribution (Fig. 1).23 The shear stress was calculated to evaluate the effect of the veneerframework interface contact type:
von Mises stress for frameworks
Model
Frictional Coefficient
von Mises stress
C
0.3
868.03
1.0
18.63
0.3
1050.3
1.0
25.469
0.3
167.23
1.0
17.283
0.3
182.88
1.0
25.118
Cz Ci Czi
C, crown with silver-palladium framework cemented onto natural tooth; Cz, ceramic crown with zirconia framework cemented onto natural tooth; Ci, crown with silver-palladium framework cemented onto morse taper implant; Czi, ceramic crown with zirconia framework cemented onto morse taper implant.
Lazari et al
frictional (0.3) or bonded. Increasing the friction between the materials allows tangential microdetachment, which promotes higher stress levels compared with a perfectly bonded interface.33 Similar behavior was observed in the deformation values for the veneer, which increased 3- to 4-fold in models with less-effective bonding. In the results, the metal ceramic restorations with a bonded veneer-framework interface exhibited greater stress values compared with zirconia restorations, similar to the in vitro findings by Augstin-Panadero et al.32 However, zirconia restorations can fracture under low stress values and more frequently than metal ceramic restorations due to poor bonding on the veneer-zirconia interface.34 The stress concentration regions were similar for the 2 loading conditions and all framework materials, in agreement with previous studies.23 Frameworks cemented on implants exhibit higher stress concentrations due to the high elastic modulus of implants compared with teeth.25 The periodontal ligament improves the distribution of stresses from occlusal forces. However, a greater stress concentration in frameworks cemented on teeth was observed compared with ones cemented on implants. This result may be explained by the framework geometry discrepancy between the crown support types. The teeth-supported frameworks were modeled with a uniform thickness of 0.4 mm, although the
6
Volume
-
Issue
-
6 von Mises stress for framework with frictional coefficient of 0.3.
7 Loading was performed in 2 steps: oblique load (45 degrees) of 49 N was applied to incisal third of lingual crown (A), and 25.5 N was applied perpendicularly to incisor crown (B).
implant-supported frameworks were thicker, which may have significantly reduced stresses. Further studies that compare different framework thicknesses would be beneficial to confirm the stress concentrations in thinner structures and the actual role of the periodontal ligament in stress distribution. The nonlinear contact increased tensile stresses in the framework, although the increased stresses were not sufficient to compromise the strength of the materials.5 Finally, the methodology focused only on static analysis. Exploring other factors, for example, thermal and mechanical fatigue, should be performed to explain the differences between the results of the present study and those observed in the literature. Finite element analysis was used because simulating the 2 veneer-framework interface conditions was impossible in an in vitro test. The nonlinear contact condition was used to simulate
The Journal of Prosthetic Dentistry
pore formation or blisters on this interface. These failures can be minimized by following the manufacturer’s instructions for ceramic heating and cooling cycles.
CONCLUSION Within the limitations of this study, it was concluded that the bond integrity between the ceramic veneer and the framework affects the mechanical performance of the ceramic veneer, regardless of the framework material or crown support.
REFERENCES 1. Lawn BR, Deng Y, Thompson VP. Use of contact testing in the characterization and design of all-ceramic crownlike layer structures: a review. J Prosthet Dent 2001;86: 495-510. 2. Kelly JR, Denry I. Stabilized zirconia as a structural ceramic: an overview. Dent Mater 2008;24:289-98.
Lazari et al
-
2014
3. Piconi C, Maccauro G. Zirconia as a ceramic biomaterial. Biomaterials 1999;20:1-25. 4. Abduo J, Lyons K, Swain M. Fit of zirconia fixed partial denture: a systematic review. J Oral Rehabil 2010;37:866-76. 5. Borba M, De Araújo MD, De Lima E, Yoshimura HN, Cesar PF, Griggs JA, et al. Flexural strength and failure modes of layered ceramic structures. Dent Mater 2011;27: 1259-66. 6. Vagkopoulou T, Koutayas SO, Koidis P, Strub JR. Zirconia in dentistry: part 1. Discovering the nature of an upcoming bioceramic. Eur J Esthet Dent 2009;4:130-51. 7. Vult von Steyern P. All-ceramic fixed partial dentures. Studies on aluminum oxide and zirconium dioxide-based ceramic systems. Swed Dent J Suppl 2005;(173):1-69. 8. Guazzato M, Proos K, Quach L, Swain MV. Strength, reliability and mode of fracture of bilayered porcelain/zirconia (Y-TZP) dental ceramics. Biomaterials 2004;25:5045-52. 9. Roediger M, Gersdorff N, Huels A, Rinke S. Prospective evaluation of zirconia posterior fixed partial dentures: four-year clinical results. Int J Prosthodont 2010;23:141-8. 10. Denry I, Kelly JR. State of the art of zirconia for dental applications. Dent Mater 2008;24: 299-307. 11. Heintze SD, Cavalleri A, Zellweger G, Büchler A, Zappini G. Fracture frequency of all-ceramic crowns during dynamic loading in a chewing simulator using different loading and luting protocols. Dent Mater 2008;24: 1352-61. 12. Larsson C, Vult von Steyern P, Sunzel B, Nilner K. All-ceramic two- to five-unit implant-supported reconstructions. A randomized, prospective clinical trial. Swed Dent J 2006;30:45-53. 13. Raigrodski AJ, Chiche GJ, Potiket N, Hochstedler JL, Mohamed SE, Billiot S, et al. The efficacy of posterior three-unit zirconium-oxide-based ceramic fixed partial dental prosthesis: a prospective clinical pilot study. J Prosthet Dent 2006;96: 237-44. 14. Sailer I, Fehér A, Filser F, Gauckler LJ, Lüthy H, Hämmerle CHF. Five-year clinical results of zirconia frameworks for posterior fixed partial dentures. Int J Prosthodont 2007;20:383-8. 15. Sorrentino R, De Simone G, Tetè S, Russo S, Zarone F. Five-year prospective clinical study of posterior three-unit zirconia-based fixed dental prosthesis. Clin Oral Investig 2012;16: 977-85. 16. Aboushelib MN, Kleverlaan CJ, Feilzer AJ. Microtensile bond strength of different components of core veneered all-ceramic restorations. Part II: zirconia veneering ceramics. Dent Mater 2006;22:857-63. 17. Fischer J, Stawarzcyk B, Trottmann A, Hämmerle CHF. Impact of thermal misfit on shear strength of veneering ceramic/ zirconia composites. Dent Mater 2009; 25:419-23.
Lazari et al
7 18. Tinschert J, Natt G, Mautsch W, Augthun M, Spiekermann H. Fracture resistance of lithium disilicate-, alumina-, and zirconiabased three-unit fixed partial dentures: a laboratory study. Int J Prosthodont 2001;14: 231-8. 19. Benetti P, Della Bona A, Kelly JR. Evaluation of thermal compatibility between core and veneer dental ceramics using shear bond strength test and contact angle measurement. Dent Mater 2010;26:743-50. 20. Komine F, Saito A, Kobayashi K, Koizuka M, Koizumi H, Matsumura H. Effect of cooling rate on shear bond strength of veneering porcelain to a zirconia ceramic material. J Oral Sci 2010;52:647-52. 21. Bulpakdi P, Taskonak B, Yan J, Mecholsky JJ. Failure analysis of clinically failed all-ceramic fixed partial dentures using fractal geometry. Dent Mater 2009;25:634-40. 22. De Jager N, De Kler M, Van der Zel JM. The influence of different core material on the FEA-determined stress distribution in dental crowns. Dent Mater 2006;22: 234-42. 23. Rocha EP, Anchieta RB, Freitas AC, De Almeida EO, Cattaneo PM, Chang Ko C. Mechanical behavior of ceramic veneer in zirconia-based restorations: a 3- dimensional finite element analysis using microcomputed tomography data. J Prosthet Dent 2011;105: 14-20. 24. Pjetursson BE, Brägger U, Lang NP, Zwahlen M. Comparison of survival and complication rates of tooth-supported fixed dental prostheses (FDPs) and implantsupported FDPs and single crowns (SCs). Clin Oral Implants Res 2007;18 (suppl 3): 97-113. 25. Burak Özcelik T, Ersoy E, Yilmaz B. Biomechanical evaluation of tooth- and implantsupported fixed dental prostheses with various nonrigid connector positions: a finite element analysis. J Prosthodont 2011;20: 16-28. 26. Att W, Yajima N-D, Wolkewitz M, Witkowski S, Strub JR. Influence of preparation and wall thickness on the resistance to fracture of zirconia implant abutments. Clin Implant Dent Relat Res 2012;14(suppl 1): 196-203. 27. Caglar A, Bal BT, Aydin C, Yilmaz H, Ozkan S. Evaluation of stresses occurring on three different zirconia dental implants: three-dimensional finite element analysis. Int J Oral Maxillofac Implants 2009;25:95-103. 28. Lin W-S, Ercoli C, Feng C, Morton D. The effect of core material, veneering porcelain, and fabrication technique on the biaxial flexural strength and Weibull analysis of selected dental ceramics. J Prosthodont 2012;21:353-62. 29. Fischer J, Stawarczyk B, Tomic M, Strub JR, Hämmerle CHF. Effect of thermal misfit between different veneering ceramics and zirconia frameworks on in vitro fracture load of single crowns. Dent Mater 2007;26:766-72.
30. Kim B, Zhang Y, Pines M, Thompson VP. Fracture of porcelain-veneered structures in fatigue. J Dent Res 2007;86:142-6. 31. Isgrò G, Pallav P, Van der Zel JM, Feilzer AJ. The influence of the veneering porcelain and different surface treatments on the biaxial flexural strength of a heat-pressed ceramic. J Prosthet Dent 2003;90:465-73. 32. Augstin-Panadero R, Fons-Font A, RomanRodriguez JL, Granell-Ruiz M, del RioHighsmith J, Sola-Ruiz MF. Zirconia versus metal: a preliminary comparative analysis of ceramic veneer behavior. Int J Prosthodont 2012;25:294-300. 33. Taskonak B, Yan J, Mecholsky JJ, Sertgöz A, Koçak A. Fractographic analyses of zirconiabased fixed partial dentures. Dent Mater 2008;24:1077-82. 34. Silva NRF, Bonfante E, Rafferty BT, Zavanelli R, Martins LL, Rekow ED, et al. Conventional and modified veneered zirconia vs. metalloceramic: fatigue and finite element analysis. J Prosthodont 2012;21:433-9. 35. Cruz M, Wassall T, Toledo EM. Da Silva Barra LP, Cruz S. Finite element stress analysis of dental prostheses supported by straight and angled implants. Int J Oral Maxillofac Implants 2009;24:391-403. 36. Coelho PG, Bonfante EA, Silva NRF, Rekow ED, Thompson VP. Laboratory simulation of Y-TZP all-ceramic crown clinical failures. J Dent Res 2009;88:382-6. 37. Dejak B, Mlotkowski A. Three-dimensional finite element analysis of strength and adhesion of composite resin versus ceramic inlays in molars. J Prosthet Dent 2008;99: 131-40. 38. Lin CL, Chang CH, Wang CH, Ko CC, Lee HE. Numerical investigation of the factors affecting interfacial stresses in an MOD restored tooth by auto-meshed finite element method. J Oral Rehabil 2001;28: 517-25. 39. Asmussen E, Peutzfeldt A, Sahafi A. Finite element analysis of stresses in endodontically treated, dowel-restored teeth. J Prosthet Dent 2005;94:321-9. 40. Li LL, Wang ZY, Bai ZC, Mao Y, Gao B, Xin HT, et al. Three-dimensional finite element analysis of weakened roots restored with different cements in combination with titanium alloy posts. Chin Med J 2006;119: 305-11. Corresponding author: Dr Altair Antoninha Del Bel Cury University of Campinas Avenida Limeira, 901 Piracicaba, São Paulo, 13414-903 BRAZIL E-mail: [email protected] Copyright ª 2014 by the Editorial Council for The Journal of Prosthetic Dentistry.
Clinical Implications Failures in the ceramic veneer-framework interface in zirconia crowns are common and can influence the biomechanical behavior of the ceramic veneer, increasing chipping or fractures.
This study was supported by the São Paulo Research Foundation grant no. 2011/03555-8). a
Graduate student, Department of Prosthodontics and Periodontology, Piracicaba Dental School, State University of Campinas. Professor, Department of Restorative Dentistry, Juiz de Fora Dental School, Federal University of Juiz de Fora. c Professor, Department of Dental Materials and Prosthodontics, Araçatuba Dental School, São Paulo State University. d Graduate student, Department of Prosthodontics and Periodontology, Piracicaba Dental School, State University of Campinas. e Professor, Department of Prosthodontics and Periodontology, Piracicaba Dental School, State University of Campinas. b
Lazari et al
2
Volume Because of its stiffness and flexural strength, yttria-stabilized tetragonal polycrystalline zirconia (Y-TZP) has been used as a framework material in dental prostheses in an effort to improve the performance of metal-free crowns.1-5 The material also has excellent properties in terms of esthetics, radiopacity, and biocompatibility.6 Clinical trials have demonstrated the potential of zirconia to enhance the performance of prostheses7 because the tensile strength of metal-free crowns depends on the properties of the material used to fabricate the framework.8 However, metalfree prostheses have been associated with a high complication rate.7-9 Most of the reported failures are due to fracture, wear, or chipping of the ceramic veneer,7,9-13 with fractures occurring in 6% to 15% of restorations after 3 to 5 years.14,15 This failure rate is high compared with that of metal ceramic prostheses, which have a 4% failure rate over 10 years. The greater number of failures in metal-free prostheses is due to the weaker interface between the zirconia structure and the ceramic veneer.16 The fragility of this interface is related to the difference in the thermal expansion coefficients (CTE) of the ceramics,6,16,17 and proper matching of CTEs is essential to prevent failure after porcelain firing. Incompatible CTEs are aggravated by improper burning or rapid cooling because the low thermal conductivity of zirconia can cause excessive tensile stresses in the feldspathic ceramic veneer.8,18-21 The stresses generated in ceramic veneers in the presence of an incomplete bond between the veneer and the zirconia framework have not been extensively examined.6,22 In the single study in which simulated failures at this interface were investigated, the stresses in the ceramic veneer were up to 12 times greater than those under conditions of perfect bonding between the materials.23 Another aspect to consider is the structure that supports the restoration because the stresses depend on whether the restoration is on a natural tooth or an osseointegrated implant.24 Implants and abutments have much greater elastic
moduli than natural teeth,25 which results in a greater stress concentration and a greater likelihood of failure if the bond between the 2 materials is incomplete. Failure rates in metal ceramic prostheses are higher when they are mounted on implanted retainers compared with those mounted on natural teeth.26 The purpose of this study was to use 3-dimensional nonlinear finite element analysis to evaluate the mechanical behavior of ceramic veneers, partially debonded or completely bonded, with zirconia or palladium-silver frameworks on natural teeth and implants. The hypothesis was that the partially debonded ceramic veneer and zirconia framework would have greater stresses in the ceramic veneer compared with those in the completely bonded veneers, independent of the framework material or crown support.
MATERIAL AND METHODS Models of a complete crown on a central incisor were constructed in which the crown was supported by either a natural tooth or an implant. The factors studied included the frictional coefficient (0.3 or completely bonded), which is used to describe the condition of the bond between the veneer and the framework, the framework material (zirconia or palladium and/or silver), and the type of support (tooth or implant). The groups were created with the crowns partially debonded
The Journal of Prosthetic Dentistry
-
Issue
-
(frictional coefficient, 0.3) or completely bonded as follows: C, crown with a silver-palladium framework cemented onto a natural tooth; Cz, ceramic crown with a zirconia framework cemented onto a natural tooth; Ci, crown with a silver-palladium framework cemented onto a morse taper implant; Czi, ceramic crown with a zirconia framework cemented onto a morse taper implant. Oblique loads were applied and analyzed with nonlinear finite element analysis software to determine the maximum principal stress, shear stress, and deformation for the veneer, and the von Mises stress for the frameworks.
Finite element model design This study was approved by the human research ethics committee at the Faculty of Dentistry of Aracatuba, São Paulo State University. Computerized tomographic (CT) images of a human edentulous maxilla were used to construct the virtual model. The CT images were exported to MIMICS 13.1 software (Materialise) for 3-dimensional model construction. The resulting model was exported to the SolidWorks 2012 software (Dassault Systèmes SolidWorks Corp) for geometry simplification and design refinement. The crown cemented onto a tooth was modeled by using images from a CT of a central incisor. The images were transformed into dicon solid models with InVesalius software (CTI). All tooth structures
1 Tooth model with ceramic veneer and framework, cement layer, dentin, dental pulp, and periodontal ligament.
Lazari et al
-
2014
3
(enamel, crown and root dentin, dental pulp, and periodontal ligament) were included in the solid model. With SolidWorks 2012 software, the tooth was reduced to 2.0 mm in thickness on the buccal and lingual surfaces and 3.0 mm in thickness on the incisal surface. The ceramic veneer and framework thicknesses were 2.0 mm and 0.4 mm, respectively (Fig. 1). The crown cemented onto implants was modeled by using the same crown cemented onto a tooth. The computer-aided design models of the implant (Titamax Ex, 4 mm 13 mm; Neodent) and prosthetic platform were obtained from the manufacturer (Fig. 2). All crowns were cemented onto the tooth or abutment with resin cement (Panavia; Kuraray) in a 0.09-mm-thick layer.
2 Implant model with ceramic veneer and framework, abutment, cement layer, screw, and implant.
Material properties The computer-aided design models were exported to Ansys Workbench 14.0 FEA software (Swanson Analysis Inc) as Initial Graphics Exchange Specification files. All structures were considered isotropic and homogeneous. Material properties such as the elastic modulus and Poisson ratio were obtained from the literature (Table I). The mesh was constructed through convergence of analysis (5%), which was determined in all models by using a tetrahedral element of 0.8 mm in size. The models had 29 665 (C and Cz) and 68 673 (Ci and Czi) elements. The numbers of nodes were 55 651 (C and Cz) and 121 061 (Ci and Czi) (Fig. 3).
Table I.
Mechanical properties of materials
Materials
Elastic Modulus (GPa)
Poisson Ratio
Reference No.
Palladium-silver
95
0.33
35
Ceramic veneer
70
0.19
36
Zirconia
205
0.22
36
Dentin
20
0.31
37
Pulp
0.002
0.45
38
Periodontal ligament
0.0689
0.45
39
Resin cement
18.3
0.33
40
Cortical bone
13.6
0.26
35
Trabecular bone
1.36
0.31
35
Implant (titanium)
110
0.33
35
Interface conditions All structures were considered bonded except the veneer-framework interface. This interface was considered perfectly bonded (linear) or with a frictional coefficient of 0.3 (nonlinear).23 Contact nonlinearity allowed both faces to separate in their normal direction or slide in the tangential direction.
Loading and boundary conditions The models were defined by fixing the mesial and distal exterior surfaces of
Lazari et al
3 Models with tetrahedral element of 0.8 mm.
4
Volume the bone segment in all directions. The models were loaded by using an initial load distribution of 49 N26 applied to the lingual surface at an angle of 45 degrees from the long axis of the tooth, followed by an axial load of 25.5 N27 distributed across the incisor face (Fig. 7). The maximum principal stress, shear stress, and deformation for the ceramic veneer and the von Mises stress for the framework were obtained.
Table II.
Issue
Frictional Coefficient
Maximum Principal Stress (MPA)
Shear Stress (MPA)
C
0.3
602.06
140.22
1.0
35.464
7.2781
0.3
674.48
159.39
1.0
33.374
6.9906
0.3
346.12
152.93
1.0
41.281
7.1687
0.3
355.91
162.48
1.0
40.619
7.086
Cz Ci Czi
-
Maximum principal stress and shear stress for ceramic veneer
Model
RESULTS The maximum principal stress and shear stress values in the ceramic veneer under load are presented in Table II. When the veneer-framework bond was incomplete (frictional coefficient, 0.3), the stress concentration was greater in all structures, regardless of the framework material or crown support. The stresses were always located in the region of load application (middle third of the palate and incisal edge of the crown) (Fig. 4). Analogous to the maximum principal stress, the greatest shear stress (162.48 MPa) occurred in the Czi model on the inside of the ceramic veneer near the veneerframework interface (Fig. 5). Deformation of the ceramic veneer was greater in the models with a frictional coefficient of 0.3 (C, 0.036 mm; Cz, 0.035 mm; Ci, 0.042 mm; and Czi, 0.042 mm). In completely bonded models, those mounted on teeth (C and Cz) underwent deformations 3 times less than in the poorly bonded models,
-
C, crown with a silver-palladium framework cemented onto a natural tooth; Cz, ceramic crown with a zirconia framework cemented onto a natural tooth; Ci, crown with a silver-palladium framework cemented onto a morse taper implant; Czi, ceramic crown with a zirconia framework cemented onto a morse taper implant.
whereas those mounted on implants (Ci and Czi) experienced deformations approximately one-half as great as in the poorly bonded models. The framework results are depicted in Table III. The stresses in frameworks cemented to teeth were greater than those in frameworks cemented to implants. This effect was more evident in models with partial debonding between the structures (Fig. 6).
DISCUSSION Ceramic systems are widely used to achieve excellent esthetic restorations, and they need to be covered by feldspathic ceramic28 to decrease the opacity of the framework.28 Zirconia, as a framework material, allows a higher
metal-free restoration resistance; however, it is clear that failures in the interface of the zirconia and ceramic veneer can reduce clinical success. Results of several studies have indicated that the veneer-framework bonding interface is the primary cause of failure, which can be due to the difference in the CTEs of the 2 materials.17,20,29 The incompatibility of the CTEs is aggravated by improper burning or fast cooling due to the low thermal conductivity of zirconia, which can promote tensile stresses in the veneer and the eventual failure of the restoration. In the present study, the failure (weak veneer-framework interface bonding) was simulated by a frictional contact of 0.3 through the nonlinear finite element method. This coefficient
4 Maximum principal stress distribution on ceramic veneer.
The Journal of Prosthetic Dentistry
Lazari et al
-
2014
5
5 Shear stress (MPa) for Czi (ceramic crown with a zirconia framework cemented onto a morse taper implant) model was located at internal ceramic veneer-framework interface.
has previously been used, and a 12-fold increase in stress in ceramic crowns due to poor veneer-framework interface was reported,23 similar to the present findings. When considering a flexural strength for feldspathic porcelain of 40 MPa,30 inadequate bonding can result in stress values greater than the strength of the material. The flaws propagate from ceramic with low resistance and modulus of elasticity values to ceramic with higher resistance and modulus of elasticity values31,32 Higher stress values were found in the ceramic veneers, and lower stress values were found in the framework, regardless of the material.31 Moreover, a thin layer of ceramic veneer fired upon a framework significantly reduces the resistance of the
Table III.
bilayer crown compared with monolithic restoration.28 Although the critical load for the fracture is strongly influenced by the total thickness of the crown and is less influenced by the proportion of the veneer-framework thickness, the ceramic veneer is still the weakest link, which compromises the strength of the bilayer system.28 When considering the qualitative analysis of these results, the stress distributions appeared to be similar among all groups and were concentrated at the location of the applied load. The frictional coefficient affected the stress values but not the stress distribution (Fig. 1).23 The shear stress was calculated to evaluate the effect of the veneerframework interface contact type:
von Mises stress for frameworks
Model
Frictional Coefficient
von Mises stress
C
0.3
868.03
1.0
18.63
0.3
1050.3
1.0
25.469
0.3
167.23
1.0
17.283
0.3
182.88
1.0
25.118
Cz Ci Czi
C, crown with silver-palladium framework cemented onto natural tooth; Cz, ceramic crown with zirconia framework cemented onto natural tooth; Ci, crown with silver-palladium framework cemented onto morse taper implant; Czi, ceramic crown with zirconia framework cemented onto morse taper implant.
Lazari et al
frictional (0.3) or bonded. Increasing the friction between the materials allows tangential microdetachment, which promotes higher stress levels compared with a perfectly bonded interface.33 Similar behavior was observed in the deformation values for the veneer, which increased 3- to 4-fold in models with less-effective bonding. In the results, the metal ceramic restorations with a bonded veneer-framework interface exhibited greater stress values compared with zirconia restorations, similar to the in vitro findings by Augstin-Panadero et al.32 However, zirconia restorations can fracture under low stress values and more frequently than metal ceramic restorations due to poor bonding on the veneer-zirconia interface.34 The stress concentration regions were similar for the 2 loading conditions and all framework materials, in agreement with previous studies.23 Frameworks cemented on implants exhibit higher stress concentrations due to the high elastic modulus of implants compared with teeth.25 The periodontal ligament improves the distribution of stresses from occlusal forces. However, a greater stress concentration in frameworks cemented on teeth was observed compared with ones cemented on implants. This result may be explained by the framework geometry discrepancy between the crown support types. The teeth-supported frameworks were modeled with a uniform thickness of 0.4 mm, although the
6
Volume
-
Issue
-
6 von Mises stress for framework with frictional coefficient of 0.3.
7 Loading was performed in 2 steps: oblique load (45 degrees) of 49 N was applied to incisal third of lingual crown (A), and 25.5 N was applied perpendicularly to incisor crown (B).
implant-supported frameworks were thicker, which may have significantly reduced stresses. Further studies that compare different framework thicknesses would be beneficial to confirm the stress concentrations in thinner structures and the actual role of the periodontal ligament in stress distribution. The nonlinear contact increased tensile stresses in the framework, although the increased stresses were not sufficient to compromise the strength of the materials.5 Finally, the methodology focused only on static analysis. Exploring other factors, for example, thermal and mechanical fatigue, should be performed to explain the differences between the results of the present study and those observed in the literature. Finite element analysis was used because simulating the 2 veneer-framework interface conditions was impossible in an in vitro test. The nonlinear contact condition was used to simulate
The Journal of Prosthetic Dentistry
pore formation or blisters on this interface. These failures can be minimized by following the manufacturer’s instructions for ceramic heating and cooling cycles.
CONCLUSION Within the limitations of this study, it was concluded that the bond integrity between the ceramic veneer and the framework affects the mechanical performance of the ceramic veneer, regardless of the framework material or crown support.
REFERENCES 1. Lawn BR, Deng Y, Thompson VP. Use of contact testing in the characterization and design of all-ceramic crownlike layer structures: a review. J Prosthet Dent 2001;86: 495-510. 2. Kelly JR, Denry I. Stabilized zirconia as a structural ceramic: an overview. Dent Mater 2008;24:289-98.
Lazari et al
-
2014
3. Piconi C, Maccauro G. Zirconia as a ceramic biomaterial. Biomaterials 1999;20:1-25. 4. Abduo J, Lyons K, Swain M. Fit of zirconia fixed partial denture: a systematic review. J Oral Rehabil 2010;37:866-76. 5. Borba M, De Araújo MD, De Lima E, Yoshimura HN, Cesar PF, Griggs JA, et al. Flexural strength and failure modes of layered ceramic structures. Dent Mater 2011;27: 1259-66. 6. Vagkopoulou T, Koutayas SO, Koidis P, Strub JR. Zirconia in dentistry: part 1. Discovering the nature of an upcoming bioceramic. Eur J Esthet Dent 2009;4:130-51. 7. Vult von Steyern P. All-ceramic fixed partial dentures. Studies on aluminum oxide and zirconium dioxide-based ceramic systems. Swed Dent J Suppl 2005;(173):1-69. 8. Guazzato M, Proos K, Quach L, Swain MV. Strength, reliability and mode of fracture of bilayered porcelain/zirconia (Y-TZP) dental ceramics. Biomaterials 2004;25:5045-52. 9. Roediger M, Gersdorff N, Huels A, Rinke S. Prospective evaluation of zirconia posterior fixed partial dentures: four-year clinical results. Int J Prosthodont 2010;23:141-8. 10. Denry I, Kelly JR. State of the art of zirconia for dental applications. Dent Mater 2008;24: 299-307. 11. Heintze SD, Cavalleri A, Zellweger G, Büchler A, Zappini G. Fracture frequency of all-ceramic crowns during dynamic loading in a chewing simulator using different loading and luting protocols. Dent Mater 2008;24: 1352-61. 12. Larsson C, Vult von Steyern P, Sunzel B, Nilner K. All-ceramic two- to five-unit implant-supported reconstructions. A randomized, prospective clinical trial. Swed Dent J 2006;30:45-53. 13. Raigrodski AJ, Chiche GJ, Potiket N, Hochstedler JL, Mohamed SE, Billiot S, et al. The efficacy of posterior three-unit zirconium-oxide-based ceramic fixed partial dental prosthesis: a prospective clinical pilot study. J Prosthet Dent 2006;96: 237-44. 14. Sailer I, Fehér A, Filser F, Gauckler LJ, Lüthy H, Hämmerle CHF. Five-year clinical results of zirconia frameworks for posterior fixed partial dentures. Int J Prosthodont 2007;20:383-8. 15. Sorrentino R, De Simone G, Tetè S, Russo S, Zarone F. Five-year prospective clinical study of posterior three-unit zirconia-based fixed dental prosthesis. Clin Oral Investig 2012;16: 977-85. 16. Aboushelib MN, Kleverlaan CJ, Feilzer AJ. Microtensile bond strength of different components of core veneered all-ceramic restorations. Part II: zirconia veneering ceramics. Dent Mater 2006;22:857-63. 17. Fischer J, Stawarzcyk B, Trottmann A, Hämmerle CHF. Impact of thermal misfit on shear strength of veneering ceramic/ zirconia composites. Dent Mater 2009; 25:419-23.
Lazari et al
7 18. Tinschert J, Natt G, Mautsch W, Augthun M, Spiekermann H. Fracture resistance of lithium disilicate-, alumina-, and zirconiabased three-unit fixed partial dentures: a laboratory study. Int J Prosthodont 2001;14: 231-8. 19. Benetti P, Della Bona A, Kelly JR. Evaluation of thermal compatibility between core and veneer dental ceramics using shear bond strength test and contact angle measurement. Dent Mater 2010;26:743-50. 20. Komine F, Saito A, Kobayashi K, Koizuka M, Koizumi H, Matsumura H. Effect of cooling rate on shear bond strength of veneering porcelain to a zirconia ceramic material. J Oral Sci 2010;52:647-52. 21. Bulpakdi P, Taskonak B, Yan J, Mecholsky JJ. Failure analysis of clinically failed all-ceramic fixed partial dentures using fractal geometry. Dent Mater 2009;25:634-40. 22. De Jager N, De Kler M, Van der Zel JM. The influence of different core material on the FEA-determined stress distribution in dental crowns. Dent Mater 2006;22: 234-42. 23. Rocha EP, Anchieta RB, Freitas AC, De Almeida EO, Cattaneo PM, Chang Ko C. Mechanical behavior of ceramic veneer in zirconia-based restorations: a 3- dimensional finite element analysis using microcomputed tomography data. J Prosthet Dent 2011;105: 14-20. 24. Pjetursson BE, Brägger U, Lang NP, Zwahlen M. Comparison of survival and complication rates of tooth-supported fixed dental prostheses (FDPs) and implantsupported FDPs and single crowns (SCs). Clin Oral Implants Res 2007;18 (suppl 3): 97-113. 25. Burak Özcelik T, Ersoy E, Yilmaz B. Biomechanical evaluation of tooth- and implantsupported fixed dental prostheses with various nonrigid connector positions: a finite element analysis. J Prosthodont 2011;20: 16-28. 26. Att W, Yajima N-D, Wolkewitz M, Witkowski S, Strub JR. Influence of preparation and wall thickness on the resistance to fracture of zirconia implant abutments. Clin Implant Dent Relat Res 2012;14(suppl 1): 196-203. 27. Caglar A, Bal BT, Aydin C, Yilmaz H, Ozkan S. Evaluation of stresses occurring on three different zirconia dental implants: three-dimensional finite element analysis. Int J Oral Maxillofac Implants 2009;25:95-103. 28. Lin W-S, Ercoli C, Feng C, Morton D. The effect of core material, veneering porcelain, and fabrication technique on the biaxial flexural strength and Weibull analysis of selected dental ceramics. J Prosthodont 2012;21:353-62. 29. Fischer J, Stawarczyk B, Tomic M, Strub JR, Hämmerle CHF. Effect of thermal misfit between different veneering ceramics and zirconia frameworks on in vitro fracture load of single crowns. Dent Mater 2007;26:766-72.
30. Kim B, Zhang Y, Pines M, Thompson VP. Fracture of porcelain-veneered structures in fatigue. J Dent Res 2007;86:142-6. 31. Isgrò G, Pallav P, Van der Zel JM, Feilzer AJ. The influence of the veneering porcelain and different surface treatments on the biaxial flexural strength of a heat-pressed ceramic. J Prosthet Dent 2003;90:465-73. 32. Augstin-Panadero R, Fons-Font A, RomanRodriguez JL, Granell-Ruiz M, del RioHighsmith J, Sola-Ruiz MF. Zirconia versus metal: a preliminary comparative analysis of ceramic veneer behavior. Int J Prosthodont 2012;25:294-300. 33. Taskonak B, Yan J, Mecholsky JJ, Sertgöz A, Koçak A. Fractographic analyses of zirconiabased fixed partial dentures. Dent Mater 2008;24:1077-82. 34. Silva NRF, Bonfante E, Rafferty BT, Zavanelli R, Martins LL, Rekow ED, et al. Conventional and modified veneered zirconia vs. metalloceramic: fatigue and finite element analysis. J Prosthodont 2012;21:433-9. 35. Cruz M, Wassall T, Toledo EM. Da Silva Barra LP, Cruz S. Finite element stress analysis of dental prostheses supported by straight and angled implants. Int J Oral Maxillofac Implants 2009;24:391-403. 36. Coelho PG, Bonfante EA, Silva NRF, Rekow ED, Thompson VP. Laboratory simulation of Y-TZP all-ceramic crown clinical failures. J Dent Res 2009;88:382-6. 37. Dejak B, Mlotkowski A. Three-dimensional finite element analysis of strength and adhesion of composite resin versus ceramic inlays in molars. J Prosthet Dent 2008;99: 131-40. 38. Lin CL, Chang CH, Wang CH, Ko CC, Lee HE. Numerical investigation of the factors affecting interfacial stresses in an MOD restored tooth by auto-meshed finite element method. J Oral Rehabil 2001;28: 517-25. 39. Asmussen E, Peutzfeldt A, Sahafi A. Finite element analysis of stresses in endodontically treated, dowel-restored teeth. J Prosthet Dent 2005;94:321-9. 40. Li LL, Wang ZY, Bai ZC, Mao Y, Gao B, Xin HT, et al. Three-dimensional finite element analysis of weakened roots restored with different cements in combination with titanium alloy posts. Chin Med J 2006;119: 305-11. Corresponding author: Dr Altair Antoninha Del Bel Cury University of Campinas Avenida Limeira, 901 Piracicaba, São Paulo, 13414-903 BRAZIL E-mail: [email protected] Copyright ª 2014 by the Editorial Council for The Journal of Prosthetic Dentistry.
