Finite element analysis of stress distribution in ceramic crowns fabricated with different tooth preparation designs Perihan Oyar, DDS,a Mutahhar Ulusoy, DDS,b and Gürcan Eskitas¸çıo glu, DDSc School of Health Service, Dental Prosthetics Technology, Hacettepe University, Ankara, Turkey; University of Near East, Faculty of Dentistry, Cyprus, Turkey; Gazi University, Faculty of Dentistry, Ankara, Turkey Statement of problem. Information about the effect of occlusal preparation designs on the stress distribution in different ceramic crowns and the prepared tooth is limited. Purpose. The purpose of this study was to investigate the effects of anatomic and nonanatomic occlusal preparation designs on the stress distribution in ceramic crowns, teeth, and bone. Material and methods. Finite element analysis was performed on models of a mandibular second premolar. A load of 400 N was applied to the models to test ceramic materials (In-Ceram, Empress Esthetic) and occlusal preparation (anatomic, nonanatomic) designs. Results. The lowest stress value occurred in the core material in the Empress Esthetic model prepared with the nonanatomic occlusal preparation design. In all groups, higher stress values were found to be concentrated in the lingual half of the dentin. Lower stress values were located near the apex of the pulp tissue and bony tissue that surround the root apex. Conclusions. Differences in preparation designs did not result in differences in the distribution or amount of stress in pulp, dentin, or bone. The use of different ceramic materials resulted in no differences in the amount or distribution of stress in pulp and bone. The use of a crown with a high elastic modulus led to increases in stress values in the restoration and the dentin margin, and decreases in stress values in the occlusal surface of the dentin. The nonanatomic design can be recommended as a favorable preparation design for Empress Esthetic ceramic. (J Prosthet Dent 2014;-:---)
Clinical Implications The choice of the preparation design and the elastic modulus of the crown materials can affect the stresses within the restoration-tooth complex.
Metal ceramic restorations offer successful long-term outcomes and have been the criterion standard in prosthetic dentistry for decades. However, increasing demands for improved esthetics have led to the development of tooth-colored restorative materials. Among these materials are InCeram (Vita Zahnfabrik) and Empress a
Esthetic ceramic (Ivoclar Vivadent). InCeram is composed of an infiltrated core veneered with a feldspathic porcelain. The core is composed of either aluminum oxide or spinel, a mixture of aluminum oxide and magnesium oxide. An initially porous structure, the core is subsequently infiltrated with molten glass. Glass infiltrated core veneered
with a feldspathic porcelain used for single- and multiple-unit partial fixed dental prostheses in the anterior region and for single units in the posterior region. Lithium disilicate-reinforced glass ceramic (Empress Esthetic ceramic) is heat pressed by using the lost-wax technique. This ceramic material was introduced for use in single-unit
Assistant Professor, School of Health Service, Dental Prosthetics Technology, Hacettepe University. Professor, Department of Prosthodontics, University of Near East, Faculty of Dentistry. c Professor, Department of Prosthodontics, Gazi University, Faculty of Dentistry. b
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Volume restorations and for 3-unit partial fixed prostheses in the anterior region that extends to the second premolar.1-4 Ceramic crowns are complex, and their performance depends on many factors, including the shape and thickness of the core and veneer porcelain, laboratory processing methods, elastic modulus of the restoration components, framework design, tooth preparation, and the amount of residual tooth structure that remains after preparation. The load-bearing capability of a prosthetic restoration depends not only on the fracture resistance of the material but also on the preparation of a suitable design with adequate material thickness.4-6 Each step in the toothpreparation process requires careful attention to ensure the success of subsequent procedures. Proper preparation can affect the load-bearing capability of restoration.7,8 In addition, the shape of the preparation may also be responsible for the stress state in the ceramic crownetooth complex.9 Other investigators have recommended minimal preparation designs for restorations9,10; however, adequate preparation can lead to lower stress in the restoration because of the increased restoration thickness.11-13 Stress analysis is a useful tool for predicting the physical responses of a system, for example, the most likely fracture location. Much has been reported on the magnitude and distribution of stress associated with various types of restorations.8,9,14-18 Ceramic premolar crowns have been subjected to 2-dimensional9,14,15 and axisymmetric9 finite element analysis (FEA), strain gauge,14 and fracture testing.17,18 A nonanatomic occlusal reduction is sometimes performed as a flat reduction that disregards the anatomic form of the cusps.9 Although the success of partial fixed prostheses is strongly affected by tooth preparation,5-9 few studies have examined how different occlusal preparation designs affect the stress distribution in ceramic crowns. In addition, few reports exist of the amount of residual tooth structure that remains after prosthetic preparation.19
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1 Two-dimensional mathematical model of central buccolingual section of mandibular second premolar tooth (anatomic occlusal preparation).
2 Two-dimensional mathematical model of central buccolingual section of mandibular second premolar tooth (nonanatomic occlusal preparation).
Therefore, the present study examined and compared the effects of 2 different tooth preparation designs on the stress distribution in 2 different ceramic crowns, tooth, and bone.
Structures Inc) to display stress values. By using the geometry described by Nelson,20 four 2-dimensional mathematical models of a mandibular second premolar tooth were generated (Figs. 1, 2). Model In-Ceram/A consisted of In-Ceram (Vita Zahnfabrik) with an anatomic preparation design (occlusal reduction performed in line with the anatomic form of the cusps). Model In-Ceram/N consisted of InCeram with a nonanatomic preparation design (flat occlusal reduction). Model Empress Esthetic/A consisted of Empress Esthetic (Ivoclar-Vivadent)
MATERIAL AND METHODS The FEA was conducted on a personal computer (Pentium IV processor, 512 MB RAM, 70 GB hard-disk; Philips) with a structural analysis program (SAP90; Computers and Structures Inc), and outputs were transferred to a program (SAPLOT; Computers and
Mechanical properties of oral tissues and prosthetic materials in finite element analysis evaluations
Table I.
Materials and Oral Tissues
Modulus of Elasticity (MPa)
Bone21,23 Periodontal ligament
15
13
Dentin
Poisson Ratio (n)
13.7
0.30
69
0.45
18
0.33
24
2
0.45
In-Ceram22
364
0.33
96
0.25
60
0.23
Pulp
Empress Esthetic (core material)a Empress Esthetic (veneer material)
a
a
Data obtained from Ivoclar Vivadent.
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Table II.
Values of stress within restorations (MPa)
Restoration Area
In-Ceram (A)
In-Ceram (N)
Empress Esthetic (A)
Empress Esthetic (N)
1
40-60
40-60
20-40
20-40
2
100-140
100-140
60-80
60-80
3
80-120
80-120
60-80
60-80
4
80-100
80-100
40-60
40-60
5
40-60
40-60
20-40
20-40
6
40-80
40-80
20-40
20-40
7
192
191
197
189
Maximum stress on central developmental groove
A, anatomic; N, nonanatomic; Area 1, external lingual surface; Area 2, internal lingual surface; Area 3, lingual margin; Area 4, internal buccal surface; Area 5, external buccal surface; Area 6, buccal margin; Area 7, central developmental groove.
with an anatomic preparation design. Model Empress Esthetic/N consisted of Empress Esthetic with a nonanatomic preparation design. The crown design featured a core thickness of 0.5 mm, a porcelain thickness of 1.5 mm, a 1-mm shoulder finish line, and 6 degrees of total occlusal convergence.13 Each mathematical model consisted of 885 elements with 931 nodes, and x and y coordinates were identified for each model. On all models, a simulated distributed load of 400 N (total load) was
applied to the centric stop points on the tip of the buccal cusp (node 914, 200 N) and to the central developmental groove (node 881, 200 N) in centric occlusion. FEA was conducted with the boundary of each model defined by the final element on the x-axis, which was assumed to be fixed. The thickness of the cement layer was ignored, and it was assumed that enamel tissue had been completely removed during preparation. All materials were assumed to be linear elastic, homogeneous, and isotropic.
The modulus of elasticity of oral tissue and crown materials and the Poisson ratio were defined according to the literature (Table I).13,15,21-24
3 Distribution of stress within restoration (In-Ceram/A). Highest stress values are marked in white; lowest stress values are marked in lilac.
4 Distribution of stress within restoration (In-Ceram/N). Highest stress values are marked in white; lowest stress values are marked in lilac.
5 Distribution of stress within restoration (Empress Esthetic/A). Highest stress values are marked in white; lowest stress values are marked in lilac.
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RESULTS Stress values for restorations are given in Table II. Stress distribution and localization within In-Ceram/A and InCeram/N restorations were similar (Figs. 3, 4), with maximum values of 192 MPa in the In-Ceram/A specimen (Fig. 3) and 191 MPa in the In-Ceram/
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6 Distribution of stress within restoration (Empress Esthetic/N). Highest stress values are marked in white; lowest stress values are marked in lilac.
7 Distribution of stress within dentin (In-Ceram/A). Highest stress values are marked in white; lowest stress values are marked in lilac.
8 Distribution of stress within dentin (In-Ceram/N). Highest stress values are marked in white; lowest stress values are marked in lilac.
N specimen (Fig. 4) found on the central developmental groove (node 881). Stress distribution and localization within the Empress Esthetic/A and Empress Esthetic/N restorations also were similar (Figs. 5, 6). Maximum stress values of 197 MPa in the Empress Esthetic/A (Fig. 5) specimen and of 189 MPa in the Empress Esthetic/N (Fig. 6) were found on the central developmental groove (node 881). Stress values for dentin tissue are given in Table III. Stress distribution within dentin tissue was similar for InCeram/A and In-Ceram/N (Figs. 7, 8),
with low stress values found at the occlusal and buccal surfaces. The highest stress values in the dentin tissue were found on the lingual pulp horn in both In-Ceram/A (90 MPa) (Fig. 7) and In-Ceram/N (86 MPa) (Fig. 8). Stress distribution within dentin tissue also was similar for Empress Esthetic/A and Empress Esthetic/N (Figs. 9, 10), with low stress values found at the occlusal and buccal surfaces. The highest stress values in the dentin tissue were found on the lingual pulp horn in both Empress Esthetic/A (124 MPa) (Fig. 9) and Empress Esthetic/N (121 MPa)
(Fig. 10). Low stress values were located in the apex of the pulp tissue (Fig. 11) and were similar in all groups. Low stress values were located in the bone (Fig. 12) that surrounds the root apex and were similar in all groups.
Table III.
DISCUSSION The present study examined the effects of occlusal preparation design on the stress distribution in ceramic crowns and found stress distribution and localization within restorations to be similar for both designs and
Values of stress within dentin tissues (MPa)
In-Ceram (A)
In-Ceram (N)
1
27-45
27-45
39-65
39-65
2
27-90
27-86
39-124
39-121
3
45-54
45-54
39-52
39-52
4
27-36
27-36
23-26
23-26
5
36-63
36-63
39-65
39-65
6
27-36
27-36
35-39
35-39
7
27-30
27-30
26-39
26-39
Dentin Area
Empress Esthetic (A)
Empress Esthetic(N)
Maximum stress in dentin tissue on the lingual pulp horn
A, anatomic; N, nonanatomic; Area 1, dentin tissue on the buccal pulp horn; Area 2, dentin tissue on the lingual pulp horn; Area 3, lingual margin; Area 4, buccal margin; Area 5, lingual root surface; Area 6, buccal root surface; Area 7, apex.
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9 Distribution of stress within dentin (Empress Esthetic/A). Highest stress values are marked in white; lowest stress values are marked in lilac.
11 Distribution of stress within pulp tissue (In-Ceram/A). Highest stress values are marked in white; lowest stress values are marked in lilac.
materials tested. Stress values in the core and porcelain material were lower in the Empress Esthetic groups than in the In-Ceram groups, which can be attributed to the lower Poisson ratio and modulus of elasticity of Empress
Esthetic when compared with InCeram. It is possible to conclude from this finding that stress is directly proportional to the modulus of elasticity of the porcelain material. The use of Empress Esthetic material led to stress
10 Distribution of stress within dentin (Empress Esthetic/N). Highest stress values are marked in white; lowest stress values are marked in lilac.
12 Distribution of stress within bone (In-Ceram/A). Highest stress values are marked in white; lowest stress values are marked in lilac.
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reduction (compared with In-Ceram) in buccal core (66%-75%), in lingual core (66%-100%), and in porcelain material (50%-100%). In addition, the use of Empress Esthetic material led to stress reduction in the buccal margin of restoration (approximately 100%) and lingual margin of restoration (approximately 33%-50%). Maximum stress values in the restoration were lower in the nonanatomic occlusal preparation designs than in the anatomic occlusal preparation designs for both In-Ceram (approximately 0.5%) and Empress Esthetic (approximately 4.2%). Based on these findings, stress in the restoration decreased as a result of an increase in the thickness of the coping material in the nonanatomic occlusal preparation designs. The results of the present study are similar to other studies. Oyar et al13 compared the anatomic and nonanatomic occlusal preparation designs of metal ceramic crowns and demonstrated that nonanatomic designs were advantageous for the porcelain structure. Shahrbaf et al9 found that the flat design of occlusal tooth preparation demonstrated a more conservative stress state in any individual structures in comparison with the anatomic occlusal preparation design for machined ceramic adhesively cemented complete coverage crowns. El-Ebrashi et al11 indicated that partial coverage restorations with a chamfer margin and flat occlusal preparation were structurally stronger than those with knife-edge margins and anatomic occlusal protection. In addition, El-Ebrashi et al12 reported that the stress concentration for a flat reduction of the cusp in gold inlay and onlay restorations was 40% less than that of anatomic reduction of the cusp because of the increase in the thickness of the restoration. Broderson25 described the disadvantages for coverage of the functional cusps and a butt joint preparation design, which creates a high number of stress points in the ceramic restoration and may lead to fracture. Skouridou et al10 concluded that the application of minimally prepared ceramic crowns
6
Volume either with the form of complete coverage crowns or ultrathin occlusal veneers had advantages and could benefit certain clinical treatments. The present study found stress distribution and localization within dentin tissue to be similar for all models tested. Maximum stresses in dentin tissues were located on the lingual pulp horn. The amount and distribution of stress in the dentin tissue except for stress values occurred on the lingual pulp horn and did not vary with changes in occlusal preparation design for either In-Ceram or Empress Esthetic. These results are in disagreement with those of Shahrbaf et al,9 who indicated that the design of the occlusal preparation affected the stress values of the dentin. The flat occlusal preparation (1.2 mm) design demonstrated a more benign stress distribution in dentin than the anatomic design. This discrepancy that may be attributable to the differences in the occlusal reduction (1.2 mm compared with 2 mm occlusal reduction). Oyar et al13 demonstrated that anatomic occlusal preparation designs were advantageous for stress distribution in the dentin tissue for metal ceramic crowns. In the present study, the use of different ceramic materials affected the stress values at the buccal and lingual margins. Empress Esthetic material caused stress reduction (compared with In-Ceram) at the buccal margin (approximately 4%15%) and lingual margin (approximately 17%-38%) of the restoration. In the present study, stress values in dentin on the lingual pulp horn were higher (approximately 40%-45%) in the Empress Esthetic models than in the In-Ceram models. Excessive stress may be harmful to vital pulp tissue,26,27 which suggests that porcelain materials with a low modulus of elasticity are unsuitable teeth with short crown lengths. The concentration of stress in the lingual half of the tooth and the higher stress values in the lingual pulp horn compared with other dentin tissue can be attributed to the anatomy of the pulp. Pulp anatomy also can
account for the cervical inclination of stress in the lingual pulp horn rather than in the buccal pulp horn. The use of different ceramic materials and different preparation designs resulted in no differences in the amount or distribution of stress in pulp or bone. This current study has some limitations, including the type of testing used, which does not accurately simulate intraoral conditions. Intraorally, teeth are subjected to cyclic loading through mastication.28 This study assumed that all the materials were linearly elastic, homogeneous, and isotropic, and that the bonds between the ceramic crown and tooth structure were ideal. In fact, tooth structure is neither homogeneous nor isotropic.29 In addition, FEA was performed on 2-dimensional mathematical models of the mandibular second premolar. The results of this study must be regarded as merely indicative of clinical behavior, and further experimental and clinical testing are necessary.
CONCLUSIONS Within the limitations of this study, with the exception of maximum stress values in dentin tissue, differences in occlusal preparation designs did not result in differences in the distribution or amount of stress in pulp, dentin, or bone. The use of different ceramic materials resulted in no differences in the amount or distribution of stress in pulp or bone. The use of a crown with a high elastic modulus led to increases in stress values (compared with the crown with a low elastic modulus) in restoration (33%-100%), increases in stress values in margins (4%-38%) of dentin, and decreases in stress values in occlusal surface (40%-45%) of dentin. The nonanatomic design could be recommended as a favorable preparation design for Empress Esthetic ceramic material.
REFERENCES 1. Donovan TE. Porcelain-fused-to-metal (PFM) alternatives. J Esthet Restor Dent 2009;21:4-6.
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2. Näpänkangas R, Raustia A. Twenty-year follow-up of metal-ceramic single crowns: a retrospective study. Int J Prosthodont 2008;21:307-11. 3. Kelsey WP, Cavel T, Blankenau RJ, Barkmeier WW, Wilwerding TM, Latta MA. 4-year clinical study of castable ceramic crowns. Am J Dent 1995;8:259-62. 4. Reich S, Petschelt A, Lohbauer U. The effect of finish line preparation and layer thickness on the failure load and fractography of ZrO2 copings. J Prosthet Dent 2008;99:369-76. 5. Rekow D, Zhang Y, Thompson V. Can material properties predict survival of allceramic posterior crowns? Compend Contin Educ Dent 2007;28:362-8. 6. Thompson VP, Rekow DE. Dental ceramics and the molar crown testing ground. J Appl Oral Sci 2004;12:26-36. 7. Selna LG, Shillingburg HT, Kerry PA. Finite element analysis of dental structures: axisymmetric and plane stress idealizations. J Biomed Mater Res 1975;9:237-52. 8. Morin DL, Cross M, Voller VR, Douglas WH, Delong R. Biophysical stress analysis of restored teeth: modelling and analysis. Dent Mater 1988;4:77-84. 9. Shahrbaf S, vanNoort R, Mirzakouchaki B, Ghassemieh E. Effect of the crown design and interface lute parameters on the stress-state of a machined crown-tooth system: a finite element analysis. Dent Mater 2013;29:e123-31. 10. Skouridou N, Pollington S, Rosentritt M, Tsitroou E. Fracture strength of minimally prepared all-ceramic CEREC crowns after simulating 5 years of service. Dent Mater 2013;29:e70-7. 11. El-Ebrashi Mk, Craig RG, Peyton FA. Experimental stress analysis of dental restorations. Part VII. Structural design and stress analysis of fixed partial dentures. J Prosthet Dent 1970;23:177-86. 12. El-Ebrashi MK, Craig RG, Peyton FA. Experimental stress analysis of dental restorations. Part V. The concept of occlusal reduction and pins. J Prosthet Dent 1969;22:565-77. 13. Oyar P, Ulusoy M, Eskitascioglu G. Finite element analysis of stress distribution of 2 different tooth preparation designs in porcelain-fused-to-metal crowns. Int J Prosthodont 2006;19:85-91. 14. Proos KA, Swain MV, Ironside J, Steven GP. Finite element analysis studies of all-ceramic crown on a first premolar. Int J Prosthodont 2002;15:404-12. 15. Kamposiora P, Papavasillious G, Bayne SC, Felton DA. Finite element analysis estimates of cement microfracture under complete veneer crowns. J Prosthet Dent 1994;71:435-41. 16. Morin DL, Douglas WH, Cross M, Delong R. Biophysical stress analysis of restored teeth. Experimental strain measurement. Dent Mater 1994;4:41-8. 17. Neiva G, Yaman P, Dennison JB, Razzoog ME, Lang BR. Resistance to fracture of three all-ceramic systems. J Esthet Dent 1998;10:60-6. 18. Yoshinari M, Dérand T. Facture strength of all-ceramic crowns. Int J Prosthodont 1994;7: 329-38.
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19. Davis GR, Tayeb RA, Seymour KG, Cherukara GP. Quantification of residual dentine thickness following crown preparation. J Dent 2012;50:571-6. 20. Nelson SJ. Wheeler’s dental anatomy, physiology and occlusion. 9th ed. St Louis: Saunders; 2009. p. 223-54. 21. Melo C, Matsushita Y, Kayona K, Hirowatari H, Suetsugu T. Comparative stress analyses of fixed free-end osseointegrated prostheses using the finite element method. J Oral Implantol 1995;2: 290-4. 22. Papavasiliou G, Tripodakis APD, Kamposiora P, Strub JR, Bayne SC. Finite element analysis of ceramic abutmentrestoration combination for osseointegrated implants. Int J Prosthodont 1996;9: 254-60.
7 23. Lin CL, Chang YG, Chang CY, Pai CA, Huang SF. Finite element and Weibull analyses to estimate failure risk in the ceramic endocrown and classical crown for endodontically treated maxillary premolar. Eur J Oral Sci 2010;118:87-93. 24. Jian W, Yongchun G, Longxing N. Stress distribution in molars restored with inlays or onlays with or without endodontic treatment: a three-dimensional finite element analysis. J Prosthet Dent 2010;103:6-12. 25. Broderson SP. Complete-crown and partialcoverage tooth preparation designs for bonded cast ceramic restorations. Quintessence Int 1994;25:535-9. 26. Kvinnsland S, Heyeraas K, Ofjord ES. Effect of experimental tooth movement on periodontal and pulpal blood flow. Eur J Orthod 1989;11:200-5.
27. Paphangkorakit J, Osborn JW. The effect of pressure on a maximum incisal bite force in man. Archs Oral Biol 1997;42:11-7. 28. Giovani AR, Vansan LP, Neto MDS, Paulino SM. In vitro fracture resistance of glass-fiber and cast metal posts with different length. J Prosthet Dent 2009;101:183-8. 29. Suzuki C, Miura H, Okada D, Komada W. Investigation of stress distribution in roots restored with different crown materials and luting agents. Dent Mater J 2008;27:229-36. Corresponding author: Dr Perihan Oyar Hacettepe University Vocational School of Health Dis-Protez Teknolojisi, D-Blok, 3. Kat, 06100 Sihhiye-Ankara TURKEY E-mail: [email protected] Copyright ª 2014 by the Editorial Council for The Journal of Prosthetic Dentistry.
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Clinical Implications The choice of the preparation design and the elastic modulus of the crown materials can affect the stresses within the restoration-tooth complex.
Metal ceramic restorations offer successful long-term outcomes and have been the criterion standard in prosthetic dentistry for decades. However, increasing demands for improved esthetics have led to the development of tooth-colored restorative materials. Among these materials are InCeram (Vita Zahnfabrik) and Empress a
Esthetic ceramic (Ivoclar Vivadent). InCeram is composed of an infiltrated core veneered with a feldspathic porcelain. The core is composed of either aluminum oxide or spinel, a mixture of aluminum oxide and magnesium oxide. An initially porous structure, the core is subsequently infiltrated with molten glass. Glass infiltrated core veneered
with a feldspathic porcelain used for single- and multiple-unit partial fixed dental prostheses in the anterior region and for single units in the posterior region. Lithium disilicate-reinforced glass ceramic (Empress Esthetic ceramic) is heat pressed by using the lost-wax technique. This ceramic material was introduced for use in single-unit
Assistant Professor, School of Health Service, Dental Prosthetics Technology, Hacettepe University. Professor, Department of Prosthodontics, University of Near East, Faculty of Dentistry. c Professor, Department of Prosthodontics, Gazi University, Faculty of Dentistry. b
Oyar et al
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Volume restorations and for 3-unit partial fixed prostheses in the anterior region that extends to the second premolar.1-4 Ceramic crowns are complex, and their performance depends on many factors, including the shape and thickness of the core and veneer porcelain, laboratory processing methods, elastic modulus of the restoration components, framework design, tooth preparation, and the amount of residual tooth structure that remains after preparation. The load-bearing capability of a prosthetic restoration depends not only on the fracture resistance of the material but also on the preparation of a suitable design with adequate material thickness.4-6 Each step in the toothpreparation process requires careful attention to ensure the success of subsequent procedures. Proper preparation can affect the load-bearing capability of restoration.7,8 In addition, the shape of the preparation may also be responsible for the stress state in the ceramic crownetooth complex.9 Other investigators have recommended minimal preparation designs for restorations9,10; however, adequate preparation can lead to lower stress in the restoration because of the increased restoration thickness.11-13 Stress analysis is a useful tool for predicting the physical responses of a system, for example, the most likely fracture location. Much has been reported on the magnitude and distribution of stress associated with various types of restorations.8,9,14-18 Ceramic premolar crowns have been subjected to 2-dimensional9,14,15 and axisymmetric9 finite element analysis (FEA), strain gauge,14 and fracture testing.17,18 A nonanatomic occlusal reduction is sometimes performed as a flat reduction that disregards the anatomic form of the cusps.9 Although the success of partial fixed prostheses is strongly affected by tooth preparation,5-9 few studies have examined how different occlusal preparation designs affect the stress distribution in ceramic crowns. In addition, few reports exist of the amount of residual tooth structure that remains after prosthetic preparation.19
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Issue
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1 Two-dimensional mathematical model of central buccolingual section of mandibular second premolar tooth (anatomic occlusal preparation).
2 Two-dimensional mathematical model of central buccolingual section of mandibular second premolar tooth (nonanatomic occlusal preparation).
Therefore, the present study examined and compared the effects of 2 different tooth preparation designs on the stress distribution in 2 different ceramic crowns, tooth, and bone.
Structures Inc) to display stress values. By using the geometry described by Nelson,20 four 2-dimensional mathematical models of a mandibular second premolar tooth were generated (Figs. 1, 2). Model In-Ceram/A consisted of In-Ceram (Vita Zahnfabrik) with an anatomic preparation design (occlusal reduction performed in line with the anatomic form of the cusps). Model In-Ceram/N consisted of InCeram with a nonanatomic preparation design (flat occlusal reduction). Model Empress Esthetic/A consisted of Empress Esthetic (Ivoclar-Vivadent)
MATERIAL AND METHODS The FEA was conducted on a personal computer (Pentium IV processor, 512 MB RAM, 70 GB hard-disk; Philips) with a structural analysis program (SAP90; Computers and Structures Inc), and outputs were transferred to a program (SAPLOT; Computers and
Mechanical properties of oral tissues and prosthetic materials in finite element analysis evaluations
Table I.
Materials and Oral Tissues
Modulus of Elasticity (MPa)
Bone21,23 Periodontal ligament
15
13
Dentin
Poisson Ratio (n)
13.7
0.30
69
0.45
18
0.33
24
2
0.45
In-Ceram22
364
0.33
96
0.25
60
0.23
Pulp
Empress Esthetic (core material)a Empress Esthetic (veneer material)
a
a
Data obtained from Ivoclar Vivadent.
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Table II.
Values of stress within restorations (MPa)
Restoration Area
In-Ceram (A)
In-Ceram (N)
Empress Esthetic (A)
Empress Esthetic (N)
1
40-60
40-60
20-40
20-40
2
100-140
100-140
60-80
60-80
3
80-120
80-120
60-80
60-80
4
80-100
80-100
40-60
40-60
5
40-60
40-60
20-40
20-40
6
40-80
40-80
20-40
20-40
7
192
191
197
189
Maximum stress on central developmental groove
A, anatomic; N, nonanatomic; Area 1, external lingual surface; Area 2, internal lingual surface; Area 3, lingual margin; Area 4, internal buccal surface; Area 5, external buccal surface; Area 6, buccal margin; Area 7, central developmental groove.
with an anatomic preparation design. Model Empress Esthetic/N consisted of Empress Esthetic with a nonanatomic preparation design. The crown design featured a core thickness of 0.5 mm, a porcelain thickness of 1.5 mm, a 1-mm shoulder finish line, and 6 degrees of total occlusal convergence.13 Each mathematical model consisted of 885 elements with 931 nodes, and x and y coordinates were identified for each model. On all models, a simulated distributed load of 400 N (total load) was
applied to the centric stop points on the tip of the buccal cusp (node 914, 200 N) and to the central developmental groove (node 881, 200 N) in centric occlusion. FEA was conducted with the boundary of each model defined by the final element on the x-axis, which was assumed to be fixed. The thickness of the cement layer was ignored, and it was assumed that enamel tissue had been completely removed during preparation. All materials were assumed to be linear elastic, homogeneous, and isotropic.
The modulus of elasticity of oral tissue and crown materials and the Poisson ratio were defined according to the literature (Table I).13,15,21-24
3 Distribution of stress within restoration (In-Ceram/A). Highest stress values are marked in white; lowest stress values are marked in lilac.
4 Distribution of stress within restoration (In-Ceram/N). Highest stress values are marked in white; lowest stress values are marked in lilac.
5 Distribution of stress within restoration (Empress Esthetic/A). Highest stress values are marked in white; lowest stress values are marked in lilac.
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RESULTS Stress values for restorations are given in Table II. Stress distribution and localization within In-Ceram/A and InCeram/N restorations were similar (Figs. 3, 4), with maximum values of 192 MPa in the In-Ceram/A specimen (Fig. 3) and 191 MPa in the In-Ceram/
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6 Distribution of stress within restoration (Empress Esthetic/N). Highest stress values are marked in white; lowest stress values are marked in lilac.
7 Distribution of stress within dentin (In-Ceram/A). Highest stress values are marked in white; lowest stress values are marked in lilac.
8 Distribution of stress within dentin (In-Ceram/N). Highest stress values are marked in white; lowest stress values are marked in lilac.
N specimen (Fig. 4) found on the central developmental groove (node 881). Stress distribution and localization within the Empress Esthetic/A and Empress Esthetic/N restorations also were similar (Figs. 5, 6). Maximum stress values of 197 MPa in the Empress Esthetic/A (Fig. 5) specimen and of 189 MPa in the Empress Esthetic/N (Fig. 6) were found on the central developmental groove (node 881). Stress values for dentin tissue are given in Table III. Stress distribution within dentin tissue was similar for InCeram/A and In-Ceram/N (Figs. 7, 8),
with low stress values found at the occlusal and buccal surfaces. The highest stress values in the dentin tissue were found on the lingual pulp horn in both In-Ceram/A (90 MPa) (Fig. 7) and In-Ceram/N (86 MPa) (Fig. 8). Stress distribution within dentin tissue also was similar for Empress Esthetic/A and Empress Esthetic/N (Figs. 9, 10), with low stress values found at the occlusal and buccal surfaces. The highest stress values in the dentin tissue were found on the lingual pulp horn in both Empress Esthetic/A (124 MPa) (Fig. 9) and Empress Esthetic/N (121 MPa)
(Fig. 10). Low stress values were located in the apex of the pulp tissue (Fig. 11) and were similar in all groups. Low stress values were located in the bone (Fig. 12) that surrounds the root apex and were similar in all groups.
Table III.
DISCUSSION The present study examined the effects of occlusal preparation design on the stress distribution in ceramic crowns and found stress distribution and localization within restorations to be similar for both designs and
Values of stress within dentin tissues (MPa)
In-Ceram (A)
In-Ceram (N)
1
27-45
27-45
39-65
39-65
2
27-90
27-86
39-124
39-121
3
45-54
45-54
39-52
39-52
4
27-36
27-36
23-26
23-26
5
36-63
36-63
39-65
39-65
6
27-36
27-36
35-39
35-39
7
27-30
27-30
26-39
26-39
Dentin Area
Empress Esthetic (A)
Empress Esthetic(N)
Maximum stress in dentin tissue on the lingual pulp horn
A, anatomic; N, nonanatomic; Area 1, dentin tissue on the buccal pulp horn; Area 2, dentin tissue on the lingual pulp horn; Area 3, lingual margin; Area 4, buccal margin; Area 5, lingual root surface; Area 6, buccal root surface; Area 7, apex.
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9 Distribution of stress within dentin (Empress Esthetic/A). Highest stress values are marked in white; lowest stress values are marked in lilac.
11 Distribution of stress within pulp tissue (In-Ceram/A). Highest stress values are marked in white; lowest stress values are marked in lilac.
materials tested. Stress values in the core and porcelain material were lower in the Empress Esthetic groups than in the In-Ceram groups, which can be attributed to the lower Poisson ratio and modulus of elasticity of Empress
Esthetic when compared with InCeram. It is possible to conclude from this finding that stress is directly proportional to the modulus of elasticity of the porcelain material. The use of Empress Esthetic material led to stress
10 Distribution of stress within dentin (Empress Esthetic/N). Highest stress values are marked in white; lowest stress values are marked in lilac.
12 Distribution of stress within bone (In-Ceram/A). Highest stress values are marked in white; lowest stress values are marked in lilac.
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reduction (compared with In-Ceram) in buccal core (66%-75%), in lingual core (66%-100%), and in porcelain material (50%-100%). In addition, the use of Empress Esthetic material led to stress reduction in the buccal margin of restoration (approximately 100%) and lingual margin of restoration (approximately 33%-50%). Maximum stress values in the restoration were lower in the nonanatomic occlusal preparation designs than in the anatomic occlusal preparation designs for both In-Ceram (approximately 0.5%) and Empress Esthetic (approximately 4.2%). Based on these findings, stress in the restoration decreased as a result of an increase in the thickness of the coping material in the nonanatomic occlusal preparation designs. The results of the present study are similar to other studies. Oyar et al13 compared the anatomic and nonanatomic occlusal preparation designs of metal ceramic crowns and demonstrated that nonanatomic designs were advantageous for the porcelain structure. Shahrbaf et al9 found that the flat design of occlusal tooth preparation demonstrated a more conservative stress state in any individual structures in comparison with the anatomic occlusal preparation design for machined ceramic adhesively cemented complete coverage crowns. El-Ebrashi et al11 indicated that partial coverage restorations with a chamfer margin and flat occlusal preparation were structurally stronger than those with knife-edge margins and anatomic occlusal protection. In addition, El-Ebrashi et al12 reported that the stress concentration for a flat reduction of the cusp in gold inlay and onlay restorations was 40% less than that of anatomic reduction of the cusp because of the increase in the thickness of the restoration. Broderson25 described the disadvantages for coverage of the functional cusps and a butt joint preparation design, which creates a high number of stress points in the ceramic restoration and may lead to fracture. Skouridou et al10 concluded that the application of minimally prepared ceramic crowns
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Volume either with the form of complete coverage crowns or ultrathin occlusal veneers had advantages and could benefit certain clinical treatments. The present study found stress distribution and localization within dentin tissue to be similar for all models tested. Maximum stresses in dentin tissues were located on the lingual pulp horn. The amount and distribution of stress in the dentin tissue except for stress values occurred on the lingual pulp horn and did not vary with changes in occlusal preparation design for either In-Ceram or Empress Esthetic. These results are in disagreement with those of Shahrbaf et al,9 who indicated that the design of the occlusal preparation affected the stress values of the dentin. The flat occlusal preparation (1.2 mm) design demonstrated a more benign stress distribution in dentin than the anatomic design. This discrepancy that may be attributable to the differences in the occlusal reduction (1.2 mm compared with 2 mm occlusal reduction). Oyar et al13 demonstrated that anatomic occlusal preparation designs were advantageous for stress distribution in the dentin tissue for metal ceramic crowns. In the present study, the use of different ceramic materials affected the stress values at the buccal and lingual margins. Empress Esthetic material caused stress reduction (compared with In-Ceram) at the buccal margin (approximately 4%15%) and lingual margin (approximately 17%-38%) of the restoration. In the present study, stress values in dentin on the lingual pulp horn were higher (approximately 40%-45%) in the Empress Esthetic models than in the In-Ceram models. Excessive stress may be harmful to vital pulp tissue,26,27 which suggests that porcelain materials with a low modulus of elasticity are unsuitable teeth with short crown lengths. The concentration of stress in the lingual half of the tooth and the higher stress values in the lingual pulp horn compared with other dentin tissue can be attributed to the anatomy of the pulp. Pulp anatomy also can
account for the cervical inclination of stress in the lingual pulp horn rather than in the buccal pulp horn. The use of different ceramic materials and different preparation designs resulted in no differences in the amount or distribution of stress in pulp or bone. This current study has some limitations, including the type of testing used, which does not accurately simulate intraoral conditions. Intraorally, teeth are subjected to cyclic loading through mastication.28 This study assumed that all the materials were linearly elastic, homogeneous, and isotropic, and that the bonds between the ceramic crown and tooth structure were ideal. In fact, tooth structure is neither homogeneous nor isotropic.29 In addition, FEA was performed on 2-dimensional mathematical models of the mandibular second premolar. The results of this study must be regarded as merely indicative of clinical behavior, and further experimental and clinical testing are necessary.
CONCLUSIONS Within the limitations of this study, with the exception of maximum stress values in dentin tissue, differences in occlusal preparation designs did not result in differences in the distribution or amount of stress in pulp, dentin, or bone. The use of different ceramic materials resulted in no differences in the amount or distribution of stress in pulp or bone. The use of a crown with a high elastic modulus led to increases in stress values (compared with the crown with a low elastic modulus) in restoration (33%-100%), increases in stress values in margins (4%-38%) of dentin, and decreases in stress values in occlusal surface (40%-45%) of dentin. The nonanatomic design could be recommended as a favorable preparation design for Empress Esthetic ceramic material.
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2. Näpänkangas R, Raustia A. Twenty-year follow-up of metal-ceramic single crowns: a retrospective study. Int J Prosthodont 2008;21:307-11. 3. Kelsey WP, Cavel T, Blankenau RJ, Barkmeier WW, Wilwerding TM, Latta MA. 4-year clinical study of castable ceramic crowns. Am J Dent 1995;8:259-62. 4. Reich S, Petschelt A, Lohbauer U. The effect of finish line preparation and layer thickness on the failure load and fractography of ZrO2 copings. J Prosthet Dent 2008;99:369-76. 5. Rekow D, Zhang Y, Thompson V. Can material properties predict survival of allceramic posterior crowns? Compend Contin Educ Dent 2007;28:362-8. 6. Thompson VP, Rekow DE. Dental ceramics and the molar crown testing ground. J Appl Oral Sci 2004;12:26-36. 7. Selna LG, Shillingburg HT, Kerry PA. Finite element analysis of dental structures: axisymmetric and plane stress idealizations. J Biomed Mater Res 1975;9:237-52. 8. Morin DL, Cross M, Voller VR, Douglas WH, Delong R. Biophysical stress analysis of restored teeth: modelling and analysis. Dent Mater 1988;4:77-84. 9. Shahrbaf S, vanNoort R, Mirzakouchaki B, Ghassemieh E. Effect of the crown design and interface lute parameters on the stress-state of a machined crown-tooth system: a finite element analysis. Dent Mater 2013;29:e123-31. 10. Skouridou N, Pollington S, Rosentritt M, Tsitroou E. Fracture strength of minimally prepared all-ceramic CEREC crowns after simulating 5 years of service. Dent Mater 2013;29:e70-7. 11. El-Ebrashi Mk, Craig RG, Peyton FA. Experimental stress analysis of dental restorations. Part VII. Structural design and stress analysis of fixed partial dentures. J Prosthet Dent 1970;23:177-86. 12. El-Ebrashi MK, Craig RG, Peyton FA. Experimental stress analysis of dental restorations. Part V. The concept of occlusal reduction and pins. J Prosthet Dent 1969;22:565-77. 13. Oyar P, Ulusoy M, Eskitascioglu G. Finite element analysis of stress distribution of 2 different tooth preparation designs in porcelain-fused-to-metal crowns. Int J Prosthodont 2006;19:85-91. 14. Proos KA, Swain MV, Ironside J, Steven GP. Finite element analysis studies of all-ceramic crown on a first premolar. Int J Prosthodont 2002;15:404-12. 15. Kamposiora P, Papavasillious G, Bayne SC, Felton DA. Finite element analysis estimates of cement microfracture under complete veneer crowns. J Prosthet Dent 1994;71:435-41. 16. Morin DL, Douglas WH, Cross M, Delong R. Biophysical stress analysis of restored teeth. Experimental strain measurement. Dent Mater 1994;4:41-8. 17. Neiva G, Yaman P, Dennison JB, Razzoog ME, Lang BR. Resistance to fracture of three all-ceramic systems. J Esthet Dent 1998;10:60-6. 18. Yoshinari M, Dérand T. Facture strength of all-ceramic crowns. Int J Prosthodont 1994;7: 329-38.
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7 23. Lin CL, Chang YG, Chang CY, Pai CA, Huang SF. Finite element and Weibull analyses to estimate failure risk in the ceramic endocrown and classical crown for endodontically treated maxillary premolar. Eur J Oral Sci 2010;118:87-93. 24. Jian W, Yongchun G, Longxing N. Stress distribution in molars restored with inlays or onlays with or without endodontic treatment: a three-dimensional finite element analysis. J Prosthet Dent 2010;103:6-12. 25. Broderson SP. Complete-crown and partialcoverage tooth preparation designs for bonded cast ceramic restorations. Quintessence Int 1994;25:535-9. 26. Kvinnsland S, Heyeraas K, Ofjord ES. Effect of experimental tooth movement on periodontal and pulpal blood flow. Eur J Orthod 1989;11:200-5.
27. Paphangkorakit J, Osborn JW. The effect of pressure on a maximum incisal bite force in man. Archs Oral Biol 1997;42:11-7. 28. Giovani AR, Vansan LP, Neto MDS, Paulino SM. In vitro fracture resistance of glass-fiber and cast metal posts with different length. J Prosthet Dent 2009;101:183-8. 29. Suzuki C, Miura H, Okada D, Komada W. Investigation of stress distribution in roots restored with different crown materials and luting agents. Dent Mater J 2008;27:229-36. Corresponding author: Dr Perihan Oyar Hacettepe University Vocational School of Health Dis-Protez Teknolojisi, D-Blok, 3. Kat, 06100 Sihhiye-Ankara TURKEY E-mail: [email protected] Copyright ª 2014 by the Editorial Council for The Journal of Prosthetic Dentistry.
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