A three-dimensional finite element study on anterior laminate veneers with different incisal preparations Zhongjie Li, BDS, MSD,a Zheng Yang, DDS, PhD,b Ling Zuo, BDS, MSD,c and Yukun Meng, DDS, PhDd West China School of Stomatology, Sichuan University, Chengdu, Sichuan, China Statement of problem. Mechanical properties are important in the long-term success of restorations, but whether different incisal preparations can affect the behavior of veneers remains controversial. Purpose. The purpose of this study was to evaluate the influence of different preparation designs on stress distribution in a maxillary incisor restored with veneers and with regard to different restorative materials and loading conditions. Material and methods. Based on the cone beam computed tomography scanning of a maxillary incisor, 3-dimensional finite element models for 2 different designs were developed. A static load of 50 N was applied with angulations of 60 degrees and 125 degrees to the longitudinal axis at the level of the incisal margin, simulating functional movements. Both porcelain laminate veneer and resin composite veneer were considered. The maximum stress values and stress distribution of the veneer, cement layer, and tooth structure were calculated and analyzed. Results. The maximum stress values in the veneer and tooth were higher with the butt-joint design. Stresses were distributed more uniformly in the cement layer in the palatal chamfer design for porcelain laminate veneers, whereas a better stress distribution under protrusive movement was observed in the butt-joint design for composite resin veneers. Conclusions. The palatal chamfer design for porcelain laminate veneers tolerated stress better, whereas the butt-joint design was favored for composite resin veneers, particularly under protrusive movement. (J Prosthet Dent 2014;-:---)
Clinical Implications The choice of designs for anterior laminate veneers might be based on the restorative materials and the individual’s occlusion. For ceramic veneers, the incisal-overlap design with a palatal chamfer should be considered first, whereas the incisal overlap with a butt joint might be better when composite resin veneers are chosen. During the past 2 decades, porcelain laminate veneers have proved to be a reliable and successful technique for discolored, malformed, worn, or fractured teeth, especially in visible areas.1 A cumulative success rate of approximately 93% was reported after a 15-year retrospective clinical observation,2 and the relative success rate without minor alterations was recorded at 85% and 72% for 2 preparation a
designs in a 5-year prospective study.3 The most frequent failure associated with veneers was fracture and debonding, in which unfavorable occlusion and articulation play an important role.1 The incisal margin and cervical area were reported to be the most likely regions to fail.4-6 Therefore, the mechanical properties were important to long-term clinical success. Theoretically, veneers should be subjected to
Postgraduate student, Department of Prosthodontics. Associate Professor, Department of Temporomandibular Joint. c Graduate student, Department of Prosthodontics. d Associate Professor, Department of Prosthodontics. b
Li et al
minimal occlusal load and only used to restore esthetics, not function.7 However, Friedman8 held a different opinion and argued that veneers with appropriate incisal length could also provide valid anterior guidance. As a result, identifying factors that could be used to improve the mechanical behavior of veneers is important, especially the most appropriate preparation design.
2
Volume Tooth preparation is advocated for porcelain veneers to control overcontouring, stress distribution, and technical ease of handling, although a 0.4- to 0.6-mm reduction would cause inevitable dentin exposure in incisors.9-11 No consensus has been reached regarding the geometric effect on the mechanical behavior of veneer.4,12-16 Four widely accepted designs are the ‘window preparation,’ which is limited in the labial surface and does not involve the incisal edge, the ‘feather preparation,’ which covers the entire labial surface with a thin layer up to the incisal edge, and the ‘incisal overlap preparation,’ which involves the preparation of the incisal edge. The incisal overlap preparation can be divided into 2 categories, depending on the configuration of the incisal area, either with a palatal chamfer or only an incisal reduction (butt joint).4,13,17 A number of in vitro studies have indicated no difference in strength, whether or not incisal reduction was performed.4,12-14 However, results of other studies showed that stress concentration was reduced with incisal reduction.15,18 Because the translucency of veneers can be improved by incisal reduction, the ‘incisal overlap’ is generally recommended.19 Whether any difference in mechanical properties exists between the 2 incisal configurations described in the ‘incisal overlap preparation’ is not clear. A palatal chamfer was believed to be necessary to strengthen ceramic veneers.20 However, Castelnuovo et al4 reported a significant difference in fracture load between the preparation with a 1mm palatal chamfer and that with an incisal butt joint, which indicates that the butt-joint type was stronger. Stappert et al14 demonstrated no significant difference between the two. Evidence from a 2-dimensional finite element analysis (FEA), however, also showed no difference in stress distribution between preparations with a butt joint and with a palatal minichamfer.16 Clinical studies with ceramic veneers have shown that different preparation did not influence the survival rate. In a 2.5-year study, Meijering et al21 found
no significant difference in survival rate between the ‘window’ type and the ‘incisal overlap’ type. Magne et al22 observed no difference in survival between the butt-joint incisal configuration and the incisal preparation with a palatal chamfer at a 4.5-year recall. In a 5-year prospective study, Guess and Stappert3 reported no significant difference in either survival rate or relative success rate between the 2 preparation designs. Nevertheless, when considering the relatively small sample size, results from a systematic review or meta-analysis would be more convincing, neither of which has yet been published. Therefore, a further detailed examination was needed, especially of the different configurations of the incisal overlap. In addition, almost all of the recent in vitro studies tested ceramic laminate veneers with few studies of composite resin veneers. Although these restorations were reported to have a relatively lower survival rate23 and to be less satisfactory to patients,21 some clinicians have reported good clinical outcomes.24 Because of the convenience and improved properties, newly formulated composite resin veneers may be a favorable option25 and also should be studied. Analyzing stress distribution in teeth or restorations is difficult because of the different materials and complex geometries involved.15 One of the most powerful and effective tools is FEA.26 Zarone et al15 and Sorrentino et al27 successfully applied 3-dimensional (3D) FEA to incisal overlapped veneer restorations, but only the type with a palatal chamfer. Magne and Douglas16 conducted a 2-dimensionaleFEA on the incisal preparations both with a palatal chamfer and with a butt joint. However, the veneer restoration was a 3D structure, and, although the 2dimensional model considered the most important buccolingual plane, considerable information was not included. The purpose of this study is to assess the influence of different incisal preparations on the mechanical behavior of anterior laminate veneers by means of 3D-FEA. Different loading
The Journal of Prosthetic Dentistry
-
Issue
-
angles and restorative materials were considered.
MATERIAL AND METHODS This study was approved by the medical ethics committee of West China Hospital of Stomatology, Sichuan University. An extracted human maxillary incisor was used to develop the 3D finite element model. With cone-beam computed tomography (CT) (3D Accuitomo XYZ Slice View Tomograph; J. Morita Mfg Corp), a total of 161 slices were obtained after scanning, with the interval space of 0.25 mm. The scanned profiles were assembled in a 3D solid model with software (Mimics 10.01; Materialise Group; Rapidform 2004; Inus Technology Inc). Enamel, dentin, and dental pulp were included in the solid model. An average thickness of 0.2 mm of periodontal ligament and 0.7 mm of alveolar bone (taken as cancellous bone) was assumed around the root surface. A second modeling step was performed to obtain the veneer restored incisors. Two different types were tested: the butt joint design and the palatal chamfer design. The solid model was modified by simulating a clinical preparation protocol (average 0.5-mm buccal and proximal reduction, cervical margin placed 1.0 mm coronal to the cementoenamel junction) and chamfer made for all finish lines. The removed part of the crown was assumed to be the veneer; a cement layer with a thickness of 0.1 mm was built on the inner surface of the veneer. Two different incisal designs were formed; one was a 1.0-mm incisal reduction without a palatal chamfer (butt-joint design) and the other was a 1.0-mm incisal reduction with a 0.5mm wide palatal chamfer that extended 1.0 mm gingivally (palatal chamfer design). A finite element model was obtained by importing the solid models into Abaqus 6.9 (Dassault Systèmes Simulia Corp) by using a tetrahedron format. The volumes were redefined in the new environment and meshed with tetrahedron with 3 degrees of freedom.
Li et al
-
2014
3 Mechanical properties of tissues and materials
Table I.
Tissues and Materials/Study
Elastic Modulus (MPa)
Poisson Ratio
Enamel/Zarone et al15
84 100
0.33
14 700
0.31
69
0.45
Dentin/Zarone et al
15 28
Periodontal ligament/Rocha et al Cancellous bone/Sorrentino et al
27
10 700
29
2
Pulp/Chander and Padmanabhan Resin cement/Chang et al
30
0.3 0.45
6000
0.3
Ceramics/Chang30; Albakry and Guazzato,31
67 000
0.22
Composite resin/Chuang et al32; Papadogiannis et al33
14 740
0.33
1 Two loading angles with longitudinal axis of tooth simulated intercuspal position (60 degrees) and protrusive movement (125 degrees).
Table II.
The accuracy of this model was checked by convergence tests. Such models consist of 4 main volumes (the geometrical dentin and enamel volumes, the cement layer with a thickness of 0.1 mm, and the veneer structure). Particular attention was paid to refining of the mesh that resulted from the convergence tests at the veneer-cement layer-tooth interfaces. Different material properties (elastic modulus and Poisson ratio) were coupled with the material volumes defined (Table I). The following assumptions were made: the complete bonding between the veneer, cement layer, and tooth was considered; the restriction type between the cement layer, tooth structure, and periodontal ligament was tied; both dentin and periodontal ligament were assumed to be elastic isotropic materials; and the boundary condition of the alveolar bone was set to be encastre
Description of 3-dimensionalefinite element models for basic tested
Mod A
Load Application Angle (degrees)
Model No.
Veneer Material
A1
Ceramics
60
A2
Ceramics
125
A3
Composite resins
60
A4
Composite resins
125
Li et al
(immobilized with restriction of 6 degrees of freedom). A 50 N load was applied on the palatal surface of the incisal edge at angles of 60 degrees and 125 degrees with the incisor longitudinal axis, which simulated intercuspal and protrusive movement (Fig. 1). A structural linear static analysis had been performed to evaluate the stress distribution in the critical regions. The complex stress states and stress redistribution at the interfaces between the veneer, cement, and tooth structure were analyzed by maximum principle stresses or the Von Mises criteria. The butt-joint design was assigned as Mod A and the palatal chamfer design as Mod B. Eight tested models were formed (Tables II, III) with different materials and loading angles. To compare stress distribution between Mod A and Mod B, the 8 tested models were divided into 4 groups with the same restorative material and loading angle, and the values for both models were scaled to the maximum values in Mod B.
RESULTS Two basic finite element models were established. The elements and nodes of each model are listed in Table IV. The recorded maximum principle stress are shown in Table V. Stress distribution in the veneer restoration for Mod A and Mod B were similar (Fig. 2). The concentration was observed in 2 locations, one in the mid third of the incisal margin, and the other in the mid third of the labial third area. Moreover, stresses were distributed in a larger area under 60-degree loading than under 125-degree loading, and stresses in the ceramic veneers demonstrated a better distribution pattern. Stress concentrations in the cement layer were similar to that in the veneer, but slight differences between 2 designs existed in the incisal area (Fig. 3). Concentration occurred at the flat incisal area in the butt-joint design and at the palatal chamfer in the palatal chamfer design. Comparatively, the palatal chamfer design demonstrated a
4
Volume
Table III.
Description of 3-dimensionalefinite element models for basic tested
Mod B
Model No.
Veneer Material
Load Application Angle (degrees)
B1
Ceramics
60
B2
Ceramics
125
B3
Composite resins
60
B4
Composite resins
125
Table IV.
Elements and nodes of 3-dimensionalefinite element models
Mod A Nodes Elements
Total
35 640
27 747
184 409
151 364
1381
1064
183 028
150 318
Triangular elements (cement layer and periodontal ligament) Tetrahedral elements (structures except the 2 above)
Table V.
Mod B
Maximum principle stress values
Model No.
Veneer Restoration (MPa)
Cement Layer (MPa)
Tooth Structure (MPa)
A1
174.75
1.23
65.68
A2
207.70
1.79
75.56
A3
150.45
3.56
138.10
A4
178.11
3.25
142.21
B1
82.77
1.25
35.53
B2
94.82
1.68
39.65
B3
69.88
4.56
53.72
B4
83.68
5.95
80.12
more uniform stress distribution in the incisal area under 60-degree loading, and the butt-joint design showed a better stress distribution in the incisal area under protrusive movement. Stress distribution in the tooth structures for Mod A and Mod B was similar (Fig. 4), with concentrations in the incisal mid third area and labial and lingual cervical areas. Higher stresses in the lingual concavity were found in the palatal chamfer design. Stresses were distributed uniformly in the labial and lingual cervical areas under 60-degree loading. As for restorative material, stresses were
more concentrated for composite resin veneer restorations.
DISCUSSION The 2 incisal preparations, namely the butt-joint design and palatal chamfer design demonstrated different mechanic behaviors with different restorative materials and loading angles. Cone beam CT demonstrated a promising application in establishing 3D models for dental tissues because it is noninvasive and can better differentiate dental structures. Scanning with a
The Journal of Prosthetic Dentistry
-
Issue
-
traditional CT and micro CT also was performed, but the images were either too obscure or accompanied by unavoidable noise and were not used. The results for maximum principle stresses showed that the palatal chamfer design decreases the stress in both the veneer and tooth structure regardless of the restorative material and occlusal conditions, whereas the protrusive movement produces higher stresses than the intercuspal position (Table V), as observed by other studies.15 The stress difference between the veneer and tooth structure (Table V) indicated that ceramics acted as a barrier during functional movements, by absorbing most stress and protecting underlying dental tissues. As for composite resin veneers, stresses were transmitted to the cement layer and the tooth. The results of stress distribution indicated that the 2 designs showed a similar stress distribution both in the veneer and tooth structure; the cement layer was a key region for biomechanical behaviors, as found by another study.1 Stresses in the cement layer is more uniformly distributed for the ceramic restored palatal chamfer design under occlusal contacts or for composite resin restored butt-joint design under protrusive movement. Previous 3D-FEA studies reported that the palatal chamfer and incisal area were stress concentrated.15,27 However, in this study, stress concentration also was observed in the labial cervical third, which has not been previously reported. This was probably due to the modeling process. Previous model constructions have been based on a stone cast in which dentin exposure was not observed after preparation. In this study, a human maxillary incisor was used. After simulated preparation, dentin was exposed mainly in the labial third but also at the incisal margin and proximal surface (Fig. 5). Due to differences in elastic moduli, stresses were more likely to accumulate in the transitional region, especially under occlusal contacts. As reported, dentin exposure during tooth preparation for a veneer ranged from 15% to
Li et al
-
2014
5
2 Stress distribution in veneer structure. A, Ceramic veneer. B, Composite resin veneer.
Li et al
6
Volume
-
Issue
-
3 Stress distribution in cement layer. A, Restored with ceramic veneer. B, Restored with composite resin veneer.
The Journal of Prosthetic Dentistry
Li et al
-
2014
7
4 Stress distribution in tooth structure in facial and palatal views. A, Ceramic veneer under 60-degree loading. B, Ceramic veneer under 125-degree loading. C, Composite resin veneer under 60-degree loading. D, Composite resin veneer under 125-degree loading.
Li et al
8
Volume
-
Issue
-
5 Dentin exposures in tooth structure, labial and proximal green areas, which indicate exposed dentin, and circles, which indicate different proximal preparation.
almost 50%,9,10 with the highest incidence in the cervical and proximal areas. Maxillary incisors have been found to have a facial enamel thickness of 0.31 0.01mm, at 1 mm above the cementoenamel junction which decreases after age 50 years.11 Therefore, simulation in this study was closer to clinical situations. In vitro mechanical experiments showed that failure in the form of fracture or debonding could be expected at the incisal margin and labial cervical area.4,5 Stappert et al6 also observed that specimens fractured at root level under load after aging in a chewing simulator, a result in accordance with the stress concentration of the tooth structure in this study. Because the viscoelasticity of the periodontal ligament was not considered, stresses in the cervical areas of the tooth might be absorbed in the clinical situation. The removal of contact points, performed mainly in patients with severe discoloration or malformation, would increase the restored volume. In the
present study, the butt-joint design preserved contact points, whereas the palatal chamfer design, to form a sound finish line in the proximal surface (Fig. 5), did not. Moreover, the preparation of a palatal chamfer also increased the volume of the veneer. As a result, a larger restoration might distribute stress more uniformly. Castelnuovo et al4 observed that the strength of a butt-joint design with 2-mm incisal reduction was significantly higher than that of a palatal chamfer design with 1-mm incisal reduction because of the thickness of the incisal ceramic. Further study is needed to analyze the butt-joint design with more incisal and proximal reduction. Based on the present results, a palatal chamfer design might be a better choice for porcelain veneers because it has a lower maximum principle stress, a more uniform stress distribution in the cement layer, and a high clinical success rate.21 However, this design might weaken the thin incisal ledge when composite resins are used
The Journal of Prosthetic Dentistry
because the load is directly transmitted to dental tissues, particularly under protrusive movement. The butt-joint preparation is simpler and less time consuming. A palatal finish line on a flat surface might be better reproduced, which makes it easier for technicians to fabricate the veneers. Moreover, insertion of veneers could be easier for clinicians. The results did not support the use of the butt-joint design in porcelain veneers, but its use for composite resin veneers is worth further study. As a numerical tool, the FEA did not consider all factors encountered in the oral environment, for example, the nonlinear viscoelastic properties of the periodontal ligament and the anisotropic character of dentin, both of which require a large number of experimental data. Despite the limitations of the method, the results still provided an insight into the 2 designs and should be considered along with clinical and experimental studies. Currently, no direct clinical data are available on this issue, and more experimental and
Li et al
-
2014
clinical evidence is needed for clinical decisions.
CONCLUSION Within the limitations of the present study, the following conclusions were drawn. The butt-joint and palatal chamfer design demonstrated different mechanical behaviors with regard to different restorative materials and loading angles; the palatal chamfer design for porcelain laminate veneers tolerated stress distribution better than the butt-joint design. The butt-joint design is favored for composite resin veneers, particularly under protrusive movement.
REFERENCES 1. Peumans M, Meerbeek BV, Lambrechts P, Vanherle G. Porcelain veneers: a review of the literature. J Dent 2000;28:163-77. 2. Friedman MJ. A 15-year review of porcelain laminate veneersda clinician’s observations. Compend Contin Educ Dent 1998;19:625-36. 3. Guess PC, Stappert CFJ. Midterm results of a 5-year prospective clinical investigation of extended ceramic veneers. Dent Mater 2008;24:804-13. 4. Castelnuovo J, Tjan AHL, Phillips K, Nicholls JI, Kois JC. Fracture load and mode of failure of ceramic veneers with different preparations. J Prosthet Dent 2000;83:171-80. 5. Hahn P, Gustav M, Hellwig E. An in vitro assessment of the strength of porcelain veneers. J Oral Rehabil 2000;27:1024-9. 6. Stappert CFJ, Stathopoulou N, Gerds T, Strub JR. Survival rate and fracture strength of maxillary incisors, restored with different kinds of full veneers. J Oral Rehabil 2005;32:266-72. 7. Toh CG, Setcos JC, Weinstein AR. Indirect dental laminate veneersdan overview. J Dent 1987;15:117-24. 8. Friedman M. Multiple potential of etched porcelain laminate veneers. J Am Dent Assoc 1987:83E-7E. 9. Cherukara GP, Davis GR, Seymour KG, Zou L, Samarawichrama DYD. Dentin exposure in tooth preparations for porcelain veneers: a pilot study. J Prosthet Dent 2005;94: 414-20. 10. Nattress BR, Youngson CC, Patterson CJW, Martin DM, Ralph JP. An in vitro assessment of tooth preparation for porcelain veneer restorations. J Dent 1995;23:165-70.
Li et al
9 11. Atsu SS, Aka PS, Kucukesmen HC, Kilicarslan MA, Atakan C. Age-related changes in tooth enamel as measured by electron microscopy: implications for porcelain laminate veneers. J Prosthet Dent 2005;94:336-41. 12. Hui KK, Williams B, Davis EH, Holt RD. A comparative assessment of the strengths of porcelain veneers for incisor teeth dependent on their design characteristics. Br Dent J 1991;171:51-5. 13. Zarone F, Epifania E, Leone G, Sorrentino R, Ferrari M. Dynamometric assessment of porcelain veneers related to tooth preparations: a comparison between two techniques. J Prosthet Dent 2006;95: 354-63. 14. Stappert CFJ, Ozden U, Gerds T, Strub JR. Longevity and failure load of ceramic veneers with different preparation designs after exposure to masticatory simulation. J Prosthet Dent 2005;94:132-9. 15. Zarone F, Apicella D, Sorrentino R, Ferro V, Aversa R, Apicella A. Influence of preparation designs on stress distribution in maxillary central incisors restored by means of alumina porcelain: a 3-D finite element analysis. Dent Mater 2005;21:1178-88. 16. Magne P, Douglas WH. Design optimization and evolution of bonded ceramics for the anterior dentition a finite-element analysis. Quintessence Int 1999;30:661-72. 17. Walls AWG, Steele JG, Wassell RW. Crowns and other extra-coronal restorations: porcelain laminate veneers. Br Dent J 2002;193: 73-6, 79-82. 18. Highton R, Caputo AA, Matyas J. A photoelastic study of stresses on porcelain laminate preparations. J Prosthet Dent 1987;58:157-61. 19. Weinberg LA. Tooth preparation for porcelain laminates. N Y State Dent J 1989;55: 25-8. 20. Sheets CG, Taniguchi T. Advantages and limitations in the use of porcelain veneer restorations. J Prosthet Dent 1990;64: 406-11. 21. Meijering AC, Creugers NHJ, Roeters FJ, Mulder J. Survival of three types of veneer restorations in a clinical trial: a 2.5-year interim evaluation. J Dent 1998;26:563-8. 22. Magne P, Perroud R, Hodges JS, Belser UC. Clinical performance of novel-design porcelain veneers for the recovery of coronal volume and length. Int J Periodontics Restorative Dent 2000;20:441-57. 23. Kreulen CM, Creugers NHJ, Meijering AC. Meta-analysis of anterior veneer restorations in clinical studies. J Dent 1998;26:345-53. 24. Mangani F, Cerutti A, Putignano A, Bollero R, Madini L. Clinical approach to anterior adhesive restorations using composite resin veneers. Eur J Esthet Dent 2007;2:188-209.
25. Nalbandian S, Millar BJ. The effect of veneers on cosmetic improvement. Br Dent J 2009;207:72-3. 26. Ausiello P, Apicella A, Davidson CL, Rengo S. 3D-finite element analyses of cusp movements in a human upper premolar, restored with adhesive resin-based composites. J Biomech 2001;34:1269-77. 27. Sorrentino R, Apicella D, Riccio C, Gherlone E, Zarone F, Aversa R, et al. Nonlinear viscoelastic finite element analysis of different porcelain veneers configuration. J Biomed Mater Res B Appl Biomater 2009;91:727-36. 28. Rocha EP, Anchieta RB, Freitas AC, Almeida EO, Cattaneo PM, Ko CC. 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. 29. Chander NG, Padmanabhan TV. Finite element stress analysis of diastema closure with ceramic laminate veneers. J Prosthodont 2009;18:577-81. 30. Chang YH, Lin WH, Kuo WC, Chang CY, Lin CL. Mechanical interactions of cuspalcoverage designs and cement thickness in a cusp-replacing ceramic premolar restoration: a finite element study. Med Bioi Eng Comput 2009;47:367-74. 31. Albakry M, Guazzato M, Swain MV. Biaxial flexural strength, elastic moduli, and x-ray diffraction characterization of three pressable all-ceramic materials. J Prosthet Dent 2003;89:374-80. 32. Chuang SF, Chen TY, Chang CH. Application of digital image correlation method to study dental composite shrinkage. Strain 2008;44:231-8. 33. Papadogiannis DY, Lakes RS, Papadogiannis Y, Palaghias G, Helvatjoglu-Antoniades M. The effect of temperature on the viscoelastic properties of nano-hybrid composites. Dent Mater 2008;24:257-66.
Corresponding author: Dr Yukun Meng West China School of Stomatology Sichuan University No. 14, 3rd section, Renmin South Rd Chengdu, Sichuan CHINA E-mail: [email protected] Acknowledgments The authors thank Professor Zhan Liu and Professor Wentao Jiang, and their team at Biomechanical Engineering Key Laboratory of Sichuan Province, Sichuan University, for their excellent help in finite element analysis. Copyright ª 2014 by the Editorial Council for The Journal of Prosthetic Dentistry.
Clinical Implications The choice of designs for anterior laminate veneers might be based on the restorative materials and the individual’s occlusion. For ceramic veneers, the incisal-overlap design with a palatal chamfer should be considered first, whereas the incisal overlap with a butt joint might be better when composite resin veneers are chosen. During the past 2 decades, porcelain laminate veneers have proved to be a reliable and successful technique for discolored, malformed, worn, or fractured teeth, especially in visible areas.1 A cumulative success rate of approximately 93% was reported after a 15-year retrospective clinical observation,2 and the relative success rate without minor alterations was recorded at 85% and 72% for 2 preparation a
designs in a 5-year prospective study.3 The most frequent failure associated with veneers was fracture and debonding, in which unfavorable occlusion and articulation play an important role.1 The incisal margin and cervical area were reported to be the most likely regions to fail.4-6 Therefore, the mechanical properties were important to long-term clinical success. Theoretically, veneers should be subjected to
Postgraduate student, Department of Prosthodontics. Associate Professor, Department of Temporomandibular Joint. c Graduate student, Department of Prosthodontics. d Associate Professor, Department of Prosthodontics. b
Li et al
minimal occlusal load and only used to restore esthetics, not function.7 However, Friedman8 held a different opinion and argued that veneers with appropriate incisal length could also provide valid anterior guidance. As a result, identifying factors that could be used to improve the mechanical behavior of veneers is important, especially the most appropriate preparation design.
2
Volume Tooth preparation is advocated for porcelain veneers to control overcontouring, stress distribution, and technical ease of handling, although a 0.4- to 0.6-mm reduction would cause inevitable dentin exposure in incisors.9-11 No consensus has been reached regarding the geometric effect on the mechanical behavior of veneer.4,12-16 Four widely accepted designs are the ‘window preparation,’ which is limited in the labial surface and does not involve the incisal edge, the ‘feather preparation,’ which covers the entire labial surface with a thin layer up to the incisal edge, and the ‘incisal overlap preparation,’ which involves the preparation of the incisal edge. The incisal overlap preparation can be divided into 2 categories, depending on the configuration of the incisal area, either with a palatal chamfer or only an incisal reduction (butt joint).4,13,17 A number of in vitro studies have indicated no difference in strength, whether or not incisal reduction was performed.4,12-14 However, results of other studies showed that stress concentration was reduced with incisal reduction.15,18 Because the translucency of veneers can be improved by incisal reduction, the ‘incisal overlap’ is generally recommended.19 Whether any difference in mechanical properties exists between the 2 incisal configurations described in the ‘incisal overlap preparation’ is not clear. A palatal chamfer was believed to be necessary to strengthen ceramic veneers.20 However, Castelnuovo et al4 reported a significant difference in fracture load between the preparation with a 1mm palatal chamfer and that with an incisal butt joint, which indicates that the butt-joint type was stronger. Stappert et al14 demonstrated no significant difference between the two. Evidence from a 2-dimensional finite element analysis (FEA), however, also showed no difference in stress distribution between preparations with a butt joint and with a palatal minichamfer.16 Clinical studies with ceramic veneers have shown that different preparation did not influence the survival rate. In a 2.5-year study, Meijering et al21 found
no significant difference in survival rate between the ‘window’ type and the ‘incisal overlap’ type. Magne et al22 observed no difference in survival between the butt-joint incisal configuration and the incisal preparation with a palatal chamfer at a 4.5-year recall. In a 5-year prospective study, Guess and Stappert3 reported no significant difference in either survival rate or relative success rate between the 2 preparation designs. Nevertheless, when considering the relatively small sample size, results from a systematic review or meta-analysis would be more convincing, neither of which has yet been published. Therefore, a further detailed examination was needed, especially of the different configurations of the incisal overlap. In addition, almost all of the recent in vitro studies tested ceramic laminate veneers with few studies of composite resin veneers. Although these restorations were reported to have a relatively lower survival rate23 and to be less satisfactory to patients,21 some clinicians have reported good clinical outcomes.24 Because of the convenience and improved properties, newly formulated composite resin veneers may be a favorable option25 and also should be studied. Analyzing stress distribution in teeth or restorations is difficult because of the different materials and complex geometries involved.15 One of the most powerful and effective tools is FEA.26 Zarone et al15 and Sorrentino et al27 successfully applied 3-dimensional (3D) FEA to incisal overlapped veneer restorations, but only the type with a palatal chamfer. Magne and Douglas16 conducted a 2-dimensionaleFEA on the incisal preparations both with a palatal chamfer and with a butt joint. However, the veneer restoration was a 3D structure, and, although the 2dimensional model considered the most important buccolingual plane, considerable information was not included. The purpose of this study is to assess the influence of different incisal preparations on the mechanical behavior of anterior laminate veneers by means of 3D-FEA. Different loading
The Journal of Prosthetic Dentistry
-
Issue
-
angles and restorative materials were considered.
MATERIAL AND METHODS This study was approved by the medical ethics committee of West China Hospital of Stomatology, Sichuan University. An extracted human maxillary incisor was used to develop the 3D finite element model. With cone-beam computed tomography (CT) (3D Accuitomo XYZ Slice View Tomograph; J. Morita Mfg Corp), a total of 161 slices were obtained after scanning, with the interval space of 0.25 mm. The scanned profiles were assembled in a 3D solid model with software (Mimics 10.01; Materialise Group; Rapidform 2004; Inus Technology Inc). Enamel, dentin, and dental pulp were included in the solid model. An average thickness of 0.2 mm of periodontal ligament and 0.7 mm of alveolar bone (taken as cancellous bone) was assumed around the root surface. A second modeling step was performed to obtain the veneer restored incisors. Two different types were tested: the butt joint design and the palatal chamfer design. The solid model was modified by simulating a clinical preparation protocol (average 0.5-mm buccal and proximal reduction, cervical margin placed 1.0 mm coronal to the cementoenamel junction) and chamfer made for all finish lines. The removed part of the crown was assumed to be the veneer; a cement layer with a thickness of 0.1 mm was built on the inner surface of the veneer. Two different incisal designs were formed; one was a 1.0-mm incisal reduction without a palatal chamfer (butt-joint design) and the other was a 1.0-mm incisal reduction with a 0.5mm wide palatal chamfer that extended 1.0 mm gingivally (palatal chamfer design). A finite element model was obtained by importing the solid models into Abaqus 6.9 (Dassault Systèmes Simulia Corp) by using a tetrahedron format. The volumes were redefined in the new environment and meshed with tetrahedron with 3 degrees of freedom.
Li et al
-
2014
3 Mechanical properties of tissues and materials
Table I.
Tissues and Materials/Study
Elastic Modulus (MPa)
Poisson Ratio
Enamel/Zarone et al15
84 100
0.33
14 700
0.31
69
0.45
Dentin/Zarone et al
15 28
Periodontal ligament/Rocha et al Cancellous bone/Sorrentino et al
27
10 700
29
2
Pulp/Chander and Padmanabhan Resin cement/Chang et al
30
0.3 0.45
6000
0.3
Ceramics/Chang30; Albakry and Guazzato,31
67 000
0.22
Composite resin/Chuang et al32; Papadogiannis et al33
14 740
0.33
1 Two loading angles with longitudinal axis of tooth simulated intercuspal position (60 degrees) and protrusive movement (125 degrees).
Table II.
The accuracy of this model was checked by convergence tests. Such models consist of 4 main volumes (the geometrical dentin and enamel volumes, the cement layer with a thickness of 0.1 mm, and the veneer structure). Particular attention was paid to refining of the mesh that resulted from the convergence tests at the veneer-cement layer-tooth interfaces. Different material properties (elastic modulus and Poisson ratio) were coupled with the material volumes defined (Table I). The following assumptions were made: the complete bonding between the veneer, cement layer, and tooth was considered; the restriction type between the cement layer, tooth structure, and periodontal ligament was tied; both dentin and periodontal ligament were assumed to be elastic isotropic materials; and the boundary condition of the alveolar bone was set to be encastre
Description of 3-dimensionalefinite element models for basic tested
Mod A
Load Application Angle (degrees)
Model No.
Veneer Material
A1
Ceramics
60
A2
Ceramics
125
A3
Composite resins
60
A4
Composite resins
125
Li et al
(immobilized with restriction of 6 degrees of freedom). A 50 N load was applied on the palatal surface of the incisal edge at angles of 60 degrees and 125 degrees with the incisor longitudinal axis, which simulated intercuspal and protrusive movement (Fig. 1). A structural linear static analysis had been performed to evaluate the stress distribution in the critical regions. The complex stress states and stress redistribution at the interfaces between the veneer, cement, and tooth structure were analyzed by maximum principle stresses or the Von Mises criteria. The butt-joint design was assigned as Mod A and the palatal chamfer design as Mod B. Eight tested models were formed (Tables II, III) with different materials and loading angles. To compare stress distribution between Mod A and Mod B, the 8 tested models were divided into 4 groups with the same restorative material and loading angle, and the values for both models were scaled to the maximum values in Mod B.
RESULTS Two basic finite element models were established. The elements and nodes of each model are listed in Table IV. The recorded maximum principle stress are shown in Table V. Stress distribution in the veneer restoration for Mod A and Mod B were similar (Fig. 2). The concentration was observed in 2 locations, one in the mid third of the incisal margin, and the other in the mid third of the labial third area. Moreover, stresses were distributed in a larger area under 60-degree loading than under 125-degree loading, and stresses in the ceramic veneers demonstrated a better distribution pattern. Stress concentrations in the cement layer were similar to that in the veneer, but slight differences between 2 designs existed in the incisal area (Fig. 3). Concentration occurred at the flat incisal area in the butt-joint design and at the palatal chamfer in the palatal chamfer design. Comparatively, the palatal chamfer design demonstrated a
4
Volume
Table III.
Description of 3-dimensionalefinite element models for basic tested
Mod B
Model No.
Veneer Material
Load Application Angle (degrees)
B1
Ceramics
60
B2
Ceramics
125
B3
Composite resins
60
B4
Composite resins
125
Table IV.
Elements and nodes of 3-dimensionalefinite element models
Mod A Nodes Elements
Total
35 640
27 747
184 409
151 364
1381
1064
183 028
150 318
Triangular elements (cement layer and periodontal ligament) Tetrahedral elements (structures except the 2 above)
Table V.
Mod B
Maximum principle stress values
Model No.
Veneer Restoration (MPa)
Cement Layer (MPa)
Tooth Structure (MPa)
A1
174.75
1.23
65.68
A2
207.70
1.79
75.56
A3
150.45
3.56
138.10
A4
178.11
3.25
142.21
B1
82.77
1.25
35.53
B2
94.82
1.68
39.65
B3
69.88
4.56
53.72
B4
83.68
5.95
80.12
more uniform stress distribution in the incisal area under 60-degree loading, and the butt-joint design showed a better stress distribution in the incisal area under protrusive movement. Stress distribution in the tooth structures for Mod A and Mod B was similar (Fig. 4), with concentrations in the incisal mid third area and labial and lingual cervical areas. Higher stresses in the lingual concavity were found in the palatal chamfer design. Stresses were distributed uniformly in the labial and lingual cervical areas under 60-degree loading. As for restorative material, stresses were
more concentrated for composite resin veneer restorations.
DISCUSSION The 2 incisal preparations, namely the butt-joint design and palatal chamfer design demonstrated different mechanic behaviors with different restorative materials and loading angles. Cone beam CT demonstrated a promising application in establishing 3D models for dental tissues because it is noninvasive and can better differentiate dental structures. Scanning with a
The Journal of Prosthetic Dentistry
-
Issue
-
traditional CT and micro CT also was performed, but the images were either too obscure or accompanied by unavoidable noise and were not used. The results for maximum principle stresses showed that the palatal chamfer design decreases the stress in both the veneer and tooth structure regardless of the restorative material and occlusal conditions, whereas the protrusive movement produces higher stresses than the intercuspal position (Table V), as observed by other studies.15 The stress difference between the veneer and tooth structure (Table V) indicated that ceramics acted as a barrier during functional movements, by absorbing most stress and protecting underlying dental tissues. As for composite resin veneers, stresses were transmitted to the cement layer and the tooth. The results of stress distribution indicated that the 2 designs showed a similar stress distribution both in the veneer and tooth structure; the cement layer was a key region for biomechanical behaviors, as found by another study.1 Stresses in the cement layer is more uniformly distributed for the ceramic restored palatal chamfer design under occlusal contacts or for composite resin restored butt-joint design under protrusive movement. Previous 3D-FEA studies reported that the palatal chamfer and incisal area were stress concentrated.15,27 However, in this study, stress concentration also was observed in the labial cervical third, which has not been previously reported. This was probably due to the modeling process. Previous model constructions have been based on a stone cast in which dentin exposure was not observed after preparation. In this study, a human maxillary incisor was used. After simulated preparation, dentin was exposed mainly in the labial third but also at the incisal margin and proximal surface (Fig. 5). Due to differences in elastic moduli, stresses were more likely to accumulate in the transitional region, especially under occlusal contacts. As reported, dentin exposure during tooth preparation for a veneer ranged from 15% to
Li et al
-
2014
5
2 Stress distribution in veneer structure. A, Ceramic veneer. B, Composite resin veneer.
Li et al
6
Volume
-
Issue
-
3 Stress distribution in cement layer. A, Restored with ceramic veneer. B, Restored with composite resin veneer.
The Journal of Prosthetic Dentistry
Li et al
-
2014
7
4 Stress distribution in tooth structure in facial and palatal views. A, Ceramic veneer under 60-degree loading. B, Ceramic veneer under 125-degree loading. C, Composite resin veneer under 60-degree loading. D, Composite resin veneer under 125-degree loading.
Li et al
8
Volume
-
Issue
-
5 Dentin exposures in tooth structure, labial and proximal green areas, which indicate exposed dentin, and circles, which indicate different proximal preparation.
almost 50%,9,10 with the highest incidence in the cervical and proximal areas. Maxillary incisors have been found to have a facial enamel thickness of 0.31 0.01mm, at 1 mm above the cementoenamel junction which decreases after age 50 years.11 Therefore, simulation in this study was closer to clinical situations. In vitro mechanical experiments showed that failure in the form of fracture or debonding could be expected at the incisal margin and labial cervical area.4,5 Stappert et al6 also observed that specimens fractured at root level under load after aging in a chewing simulator, a result in accordance with the stress concentration of the tooth structure in this study. Because the viscoelasticity of the periodontal ligament was not considered, stresses in the cervical areas of the tooth might be absorbed in the clinical situation. The removal of contact points, performed mainly in patients with severe discoloration or malformation, would increase the restored volume. In the
present study, the butt-joint design preserved contact points, whereas the palatal chamfer design, to form a sound finish line in the proximal surface (Fig. 5), did not. Moreover, the preparation of a palatal chamfer also increased the volume of the veneer. As a result, a larger restoration might distribute stress more uniformly. Castelnuovo et al4 observed that the strength of a butt-joint design with 2-mm incisal reduction was significantly higher than that of a palatal chamfer design with 1-mm incisal reduction because of the thickness of the incisal ceramic. Further study is needed to analyze the butt-joint design with more incisal and proximal reduction. Based on the present results, a palatal chamfer design might be a better choice for porcelain veneers because it has a lower maximum principle stress, a more uniform stress distribution in the cement layer, and a high clinical success rate.21 However, this design might weaken the thin incisal ledge when composite resins are used
The Journal of Prosthetic Dentistry
because the load is directly transmitted to dental tissues, particularly under protrusive movement. The butt-joint preparation is simpler and less time consuming. A palatal finish line on a flat surface might be better reproduced, which makes it easier for technicians to fabricate the veneers. Moreover, insertion of veneers could be easier for clinicians. The results did not support the use of the butt-joint design in porcelain veneers, but its use for composite resin veneers is worth further study. As a numerical tool, the FEA did not consider all factors encountered in the oral environment, for example, the nonlinear viscoelastic properties of the periodontal ligament and the anisotropic character of dentin, both of which require a large number of experimental data. Despite the limitations of the method, the results still provided an insight into the 2 designs and should be considered along with clinical and experimental studies. Currently, no direct clinical data are available on this issue, and more experimental and
Li et al
-
2014
clinical evidence is needed for clinical decisions.
CONCLUSION Within the limitations of the present study, the following conclusions were drawn. The butt-joint and palatal chamfer design demonstrated different mechanical behaviors with regard to different restorative materials and loading angles; the palatal chamfer design for porcelain laminate veneers tolerated stress distribution better than the butt-joint design. The butt-joint design is favored for composite resin veneers, particularly under protrusive movement.
REFERENCES 1. Peumans M, Meerbeek BV, Lambrechts P, Vanherle G. Porcelain veneers: a review of the literature. J Dent 2000;28:163-77. 2. Friedman MJ. A 15-year review of porcelain laminate veneersda clinician’s observations. Compend Contin Educ Dent 1998;19:625-36. 3. Guess PC, Stappert CFJ. Midterm results of a 5-year prospective clinical investigation of extended ceramic veneers. Dent Mater 2008;24:804-13. 4. Castelnuovo J, Tjan AHL, Phillips K, Nicholls JI, Kois JC. Fracture load and mode of failure of ceramic veneers with different preparations. J Prosthet Dent 2000;83:171-80. 5. Hahn P, Gustav M, Hellwig E. An in vitro assessment of the strength of porcelain veneers. J Oral Rehabil 2000;27:1024-9. 6. Stappert CFJ, Stathopoulou N, Gerds T, Strub JR. Survival rate and fracture strength of maxillary incisors, restored with different kinds of full veneers. J Oral Rehabil 2005;32:266-72. 7. Toh CG, Setcos JC, Weinstein AR. Indirect dental laminate veneersdan overview. J Dent 1987;15:117-24. 8. Friedman M. Multiple potential of etched porcelain laminate veneers. J Am Dent Assoc 1987:83E-7E. 9. Cherukara GP, Davis GR, Seymour KG, Zou L, Samarawichrama DYD. Dentin exposure in tooth preparations for porcelain veneers: a pilot study. J Prosthet Dent 2005;94: 414-20. 10. Nattress BR, Youngson CC, Patterson CJW, Martin DM, Ralph JP. An in vitro assessment of tooth preparation for porcelain veneer restorations. J Dent 1995;23:165-70.
Li et al
9 11. Atsu SS, Aka PS, Kucukesmen HC, Kilicarslan MA, Atakan C. Age-related changes in tooth enamel as measured by electron microscopy: implications for porcelain laminate veneers. J Prosthet Dent 2005;94:336-41. 12. Hui KK, Williams B, Davis EH, Holt RD. A comparative assessment of the strengths of porcelain veneers for incisor teeth dependent on their design characteristics. Br Dent J 1991;171:51-5. 13. Zarone F, Epifania E, Leone G, Sorrentino R, Ferrari M. Dynamometric assessment of porcelain veneers related to tooth preparations: a comparison between two techniques. J Prosthet Dent 2006;95: 354-63. 14. Stappert CFJ, Ozden U, Gerds T, Strub JR. Longevity and failure load of ceramic veneers with different preparation designs after exposure to masticatory simulation. J Prosthet Dent 2005;94:132-9. 15. Zarone F, Apicella D, Sorrentino R, Ferro V, Aversa R, Apicella A. Influence of preparation designs on stress distribution in maxillary central incisors restored by means of alumina porcelain: a 3-D finite element analysis. Dent Mater 2005;21:1178-88. 16. Magne P, Douglas WH. Design optimization and evolution of bonded ceramics for the anterior dentition a finite-element analysis. Quintessence Int 1999;30:661-72. 17. Walls AWG, Steele JG, Wassell RW. Crowns and other extra-coronal restorations: porcelain laminate veneers. Br Dent J 2002;193: 73-6, 79-82. 18. Highton R, Caputo AA, Matyas J. A photoelastic study of stresses on porcelain laminate preparations. J Prosthet Dent 1987;58:157-61. 19. Weinberg LA. Tooth preparation for porcelain laminates. N Y State Dent J 1989;55: 25-8. 20. Sheets CG, Taniguchi T. Advantages and limitations in the use of porcelain veneer restorations. J Prosthet Dent 1990;64: 406-11. 21. Meijering AC, Creugers NHJ, Roeters FJ, Mulder J. Survival of three types of veneer restorations in a clinical trial: a 2.5-year interim evaluation. J Dent 1998;26:563-8. 22. Magne P, Perroud R, Hodges JS, Belser UC. Clinical performance of novel-design porcelain veneers for the recovery of coronal volume and length. Int J Periodontics Restorative Dent 2000;20:441-57. 23. Kreulen CM, Creugers NHJ, Meijering AC. Meta-analysis of anterior veneer restorations in clinical studies. J Dent 1998;26:345-53. 24. Mangani F, Cerutti A, Putignano A, Bollero R, Madini L. Clinical approach to anterior adhesive restorations using composite resin veneers. Eur J Esthet Dent 2007;2:188-209.
25. Nalbandian S, Millar BJ. The effect of veneers on cosmetic improvement. Br Dent J 2009;207:72-3. 26. Ausiello P, Apicella A, Davidson CL, Rengo S. 3D-finite element analyses of cusp movements in a human upper premolar, restored with adhesive resin-based composites. J Biomech 2001;34:1269-77. 27. Sorrentino R, Apicella D, Riccio C, Gherlone E, Zarone F, Aversa R, et al. Nonlinear viscoelastic finite element analysis of different porcelain veneers configuration. J Biomed Mater Res B Appl Biomater 2009;91:727-36. 28. Rocha EP, Anchieta RB, Freitas AC, Almeida EO, Cattaneo PM, Ko CC. 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. 29. Chander NG, Padmanabhan TV. Finite element stress analysis of diastema closure with ceramic laminate veneers. J Prosthodont 2009;18:577-81. 30. Chang YH, Lin WH, Kuo WC, Chang CY, Lin CL. Mechanical interactions of cuspalcoverage designs and cement thickness in a cusp-replacing ceramic premolar restoration: a finite element study. Med Bioi Eng Comput 2009;47:367-74. 31. Albakry M, Guazzato M, Swain MV. Biaxial flexural strength, elastic moduli, and x-ray diffraction characterization of three pressable all-ceramic materials. J Prosthet Dent 2003;89:374-80. 32. Chuang SF, Chen TY, Chang CH. Application of digital image correlation method to study dental composite shrinkage. Strain 2008;44:231-8. 33. Papadogiannis DY, Lakes RS, Papadogiannis Y, Palaghias G, Helvatjoglu-Antoniades M. The effect of temperature on the viscoelastic properties of nano-hybrid composites. Dent Mater 2008;24:257-66.
Corresponding author: Dr Yukun Meng West China School of Stomatology Sichuan University No. 14, 3rd section, Renmin South Rd Chengdu, Sichuan CHINA E-mail: [email protected] Acknowledgments The authors thank Professor Zhan Liu and Professor Wentao Jiang, and their team at Biomechanical Engineering Key Laboratory of Sichuan Province, Sichuan University, for their excellent help in finite element analysis. Copyright ª 2014 by the Editorial Council for The Journal of Prosthetic Dentistry.
