The effects of post-core and crown material and luting agents on stress distribution in tooth restorations P. Oyar, DDS, PhD Dental Prosthetics Technology Programme, School of Health Services, Hacettepe University, Ankara, Turkey Statement of problem. Cement microfracture, post-and-core dislodgement, and tooth fracture are related to the mechanical properties and deformation of restorations. Purpose. The purpose of this study was to determine which combinations of post-and-core cements provide the most favorable stress distribution upon loading. Material and methods. Three-dimensional models of teeth were created with the ANSYS program to simulate the different materials used for metal ceramic crowns (nickel-chromium, gold-palladium), posts and cores (Ti, Ni-Cr, Au-Pd), and cement (glass ionomer, composite resin, zinc phosphate, polycarboxylate, Panavia). Models were divided into 2 groups according to the alloys used in the crown restorations. A simulated masticatory force of 400 N was applied to the occlusal surface at a 45-degree inclination in the linguolabial direction to the long axis of the tooth, and von Mises equivalent stress values were calculated. Results. The Ni-Cr metal ceramic crown/Au-Pd post-and-core/glass ionomer cement had the highest residual root von Mises equivalent stress value, whereas the Ni-Cr metal ceramic crown/Ni-Cr post-and-core/glass ionomer cement had the highest post stress value and the Ni-Cr metal ceramic crown/Au-Pd post-and-core/zinc phosphate cement had the highest cement stress value. For each post-and-core alloy, the stress values in the post and core were higher with Au-Pd metal ceramic crowns than with Ni-Cr metal ceramic crowns. The post-and-core material affected the amount of deformation. Conclusions. The use of a post-and-core material with a lower elastic modulus and a cement with a higher elastic modulus led to a reduction in deformation in the residual root, cement, and post and core, and a reduction in stress in the post and core. The Ni-Cr metal ceramic crown/Au-Pd post-and-core/zinc phosphate cement or Panavia may therefore be favorable for post-and-core restorations. (J Prosthet Dent 2014;-:---)
Clinical Implications The stress values of teeth with restorations came closest to those of unrestored teeth when materials with a low elastic modulus were used for posts, whereas the use of a luting cement with a high elastic modulus resulted in higher stress concentrations in the cement. Considering that stress minimization is desirable in the restoration of teeth with thin root walls, the use of post materials with a low elastic modulus, for example, Au-Pd alloys and zinc phosphate cement or Panavia, may be appropriate. Posts and cores are used to reconstruct material lost to caries, fracture, or endodontic access and provide retention and stability for artificial crowns. Postand-core materials and fabrication techniques vary in accordance with the amount of lost mineralized structure.1 Assistant Professor.
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Post size and shape, tooth preparation design, and luting agent all affect tooth resistance.1,2 The prognosis of post-and-core restored teeth may be improved by bonding the post to the tooth structure in order to increase post retention and reinforce tooth structure.3
Several post materials have been used in post-and-core restorations. Increases in the elastic modulus of the post material have been shown to decrease the stress on the root surface.4-6 Thus, some authors have recommended using a post with a high
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Volume modulus of elasticity.4-6 However, others have recommended posts with a modulus of elasticity similar to that of dentin. Moreover, while some authors have stated that cast posts ensure better tooth fracture resistance than fiber-reinforced posts,7,8 others have reported that teeth restored with fiber posts and composite resin core materials have a higher fracture resistance than those restored with cast post and cores.9,10 The selection of the incorrect luting agent can significantly affect the longevity of post-and-core restorations.11 Glass ionomer, composite resin, zinc phosphate, and polycarboxylate cement are most commonly used to cement endodontic posts.12,13 For decades, zinc phosphate cement has been the gold standard for cementing cast post and cores, although resin luting agents have recently been studied as alternatives.13-16 Resin cements have been reported to aid resistance to fracture compared to brittle and nonbonding zinc phosphate cement and are recommended for posts in thin-walled roots.4 The use of composite resin cements lowers the risk of fracture and loss of retention.17 Resin-modified glass ionomer cement is also commonly used in post fixation, but not all researchers agree that this material is appropriate.18 Although no intermediate layer exists between the enamel and dentin in natural teeth, in clinical restorations, a luting agent must be used to cement the crown to the remaining natural tooth. The presence of a luting agent gives rise to differences in stress distribution between natural and endodontically treated teeth. Obtaining stress distributions close to those of natural teeth is an important consideration when the clinician is choosing crown material and luting agents.19 Given their role in transmitting and diffusing stress between the post and dentin, bonding agents should be selected not only for their retentive characteristics but also for their ability to resist damage under stress. The elastic modulus and mechanical characteristics of cements significantly affect the
transmission and diffusion of stress. Previous studies have demonstrated that the elastic modulus is an important parameter to consider in selecting a luting agent.20,21 Widely used as a basic research tool in dentistry, finite element analysis (FEA) illustrates internal stresses and is helpful in predicting failure. FEA should be used as an initial step and an aid for planning further laboratory tests and clinical studies.3,22,23 Although post-and-core restorations are fabricated with 3 distinct material systems (post and core, luting agent, crown), the interaction among these systems during function has not been thoroughly researched,4 and there is little understanding of how the combination of post material and luting agent affects stress distribution in residual roots. This study used FEA to evaluate stresses in the post-and-core restorations of posterior teeth fabricated with different post-and-core and crown materials and different cements. The purpose of the study was to identify suitable combinations for post-andcore applications in terms of fracture avoidance.
METHOD AND MATERIALS A mandibular second premolar was scanned with computed tomography to obtain 1-mm-thick slices of tooth tissue, and the data were combined with data from a textbook24 to create 3-dimensional finite element models
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with computer-aided design software (Inventor-AutoCAD 2010; Autodesk). The contours of all cross sections of tooth enamel, dentin, and pulp boundaries were delineated, and the drawing was adapted so that the dimensions of the final model were as close as possible to those of the mandibular second premolar found in the textbook. Data were exported in initial graphics exchange specification (IGES) format to FEA software (ANSYS 11.0 Inc) for stress distribution analysis. The final mesh comprised 51 508 nodes and 27 009 elements (Fig. 1). In total, 30 three-dimensional finite element models comprising bone, apical root canal filling, metal alloy post and core, metal ceramic crown, and luting agent were fabricated. Models were divided into 2 groups, Ni-Cr and Au-Pd (n¼15 for each group), according to the alloys used in the metal ceramic crown restoration. Each group was subdivided according to the alloy used for the post and core, gold-palladium (JF Jelenko), nickelchromium (Remanium CS), and titanium (Essential Dental Systems) and further divided according to post cementation-glass ionomer cement (Ketac CEM; 3M ESPE), composite resin cement (Filtek Supreme, Shade A2B; 3M ESPE), zinc phosphate cement (Poscal; VOCO GmbH), polycarboxylate cement (Durelon, Maxicap; 3M ESPE), and Panavia (Kuraray). Each model also included gutta percha (Coltene/Whaledent Inc) to fill the
1 Finite element meshes for mandibular second premolar.
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apical third of the root canal and zinc phosphate cement to lute the metal ceramic crown. The crowns were modeled with a metal thickness of 0.5 mm, a porcelain thickness (Klema) of 1.5 mm, and a cement thickness of 100 mm. For each model, the application of a simulated 400 N load to the centric occlusion stop of the occlusal surface in centric occlusion at a 45-degree inclination in a linguolabial direction to the long axis of the tooth and the von Mises stress data were analyzed. The models were constrained in all directions by the outline of the bone surface. All structures were assumed to be linearly elastic, homogeneous, and isotropic, except for the periodontal ligament and cortical bone, which were ignored. Ideal adherence between structures (metalceramic-cement, cement-core, corepost, post-cement, and cement-dentin interfaces) was assumed. The Poisson ratio (n) and modulus of elasticity (E) of the oral tissue, crown and post materials, and cements were determined from the literature 6,19,21,25-31 and are
Table I.
Mechanical properties of materials and tooth
Material
Modulus of Elasticity (GPa)
Poisson Ratio
1.37
0.30
Bone6,21,25 6,21,25
18.6
0.31
19
80
0.30
2
0.45
Gutta percha30,31
0.14
0.45
69
0.30
205
0.33
Dentin
Enamel 29
Pulp
Feldspathic porcelain25 Ni-Cr
29 29
Au-Pd
89.5
0.33
Ti25
112
0.33
4
0.35
5.11
0.35
8.3
0.24
22.4
0.35
18.6
0.28
Glass ionomer cement21 Polycarboxylate cement
21
Composite resin cement26 Zinc phosphate cement
21
27,28
Panavia
listed in Table I. A sound tooth was also modeled as a control.
RESULTS The maximum von Mises equivalent stresses and deformation values for
Ti, Ni-Cr, and Au-Pd post and cores are listed in Tables II to IV. The distribution of the von Mises equivalent stress and deformation values are shown in Figures 2 to 8. Stress concentrations were highest in the coronal third of the facial surface
Maximum von Mises equivalent stress values and deformation in residual root (dentin), cement, post-and-core and sound tooth in Ti post-and-core models
Table II.
Von Mises (MPa)
Deformation (mm)
Model
Region
Ni-Cr
Au-Pd
Ni-Cr
Au-Pd
1
Dentin
26.666
25.383
9.4410-4
9.3910-4
4.8836
5.1909
9.6610-4
9.6310-4
20.561
23.77
9.55 10
9.6510-4
26.88
25.514
9.5210-4
9.4810-4
Composite cement
2.272
2.3843
9.7310-4
9.6910-4
Ti post and core
22.329
25.813
9.6710-4
9.7810-4
26.979
25.558
9.5510
-4
9.5010-4
Glass ionomer cement
1.0495
1.10209
9.7410-4
9.7110-4
Ti post and core
22.831
26.41
9.7110-4
9.8410-4
26.727
25.426
9.4610-4
9.4210-4
-4
9.6510-4
Zinc phosphate cement Ti post and core 2
3
4
5
Dentin
Dentin
Dentin Panavia
4.4743
4.7217
9.5810
Ti post and core
21.003
24.277
9.5810-4
9.6810-4
26.946
25.538
9.5410-4
9.5010-4
1.3219
1.391
9.7310-4
9.6910-4
-4
9.8110-4
Dentin Polycarboxylate cement Ti post and core
Sound tooth
Dentin Pulp
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22.738
26.301
9.6910
31.973
9.9610-4
17.9110-3
2.4110-3
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Table III. Maximum von Mises equivalent stress values and deformation in residual root (dentin), cement, post and core, and sound tooth in Ni-Cr post-and-core models Von Mises (MPa) Model
Region
6
Dentin
7
8
9
Ni-Cr
Au-Pd
Ni-Cr
Au-Pd
25.502
24.483
9.7110-4
9.7610-4
Zinc phosphate cement
4.4904
4.6606
9.9910-4
10.0810-4
Ni-Cr post and core
35.251
35.804
9.9110-4
10.1010-4
25.706
24.581
9.7610
-4
9.8210-4
Composite cement
2.041
2.1056
10.0510-4
10.1310-4
Ni-Cr post and core
36.0301
36.657
10.0210-4
10.2210-4
25.753
24.617
9.7910-4
9.8410-4
-4
10.1110-4
Dentin
Dentin Glass ionomer cement
0.92292
0.96779
10.0410
Ni-Cr post and core
36.309
37.117
10.0710-4
10.2810-4
Dentin
25.637
24.516
9.7310-4
9.7810-4
Panavia
4.1009
4.2311
10.0210-4
10.1310-4
35.271
35.846
9.9410
-4
10.1010-4
25.726
24.599
9.7810-4
9.8310-4
Polycarboxylate cement
1.1761
1.2231
10.0310-4
10.1010-4
Ni-Cr post and core
36.422
37.061
10.0510-4
10.2610-4
Ni-Cr post and core 10
Deformation (mm)
Dentin
Table IV. Maximum von Mises equivalent stress values and deformation in residual root (dentin), cement, post-and-core and sound tooth in Au-Pd post-and-core models Von Mises (MPa)
Deformation (mm)
Model
Region
Ni-Cr
Au-Pd
Ni-Cr
Au-Pd
11
Dentin
27.136
25.701
9.4910-4
9.2310-4
5.0212
5.3894
9.5310-4
9.4410-4
17.97
20.958
9.3910
-4
9.4410-4
27.387
25.845
9.5410-4
9.3310-4
Composite cement
2.3711
2.4995
9.6110-4
9.5210-4
Au-Pd post and core
20.011
23.252
9.5110-4
9.5810-4
27.477
25.896
9.5610
-4
9.3610-4
Glass ionomer cement
1.1007
1.1585
9.6210-4
9.5410-4
Au-Pd post and core
20.681
24.04
9.5610-4
9.6410-4
-4
9.2610-4
Zinc phosphate cement Au-Pd post and core 12
13
14
15
Dentin
Dentin
Dentin
27.19
25.74
9.5010
Panavia
4.6349
4.9139
9.5610-4
9.4710-4
Au-Pd post and core
18.05
21.511
9.4210-4
9.4810-4
27.459
25.871
9.5510-4
9.3410-4
-4
9.5210-4 9.6110-4
Dentin Polycarboxylate cement
1.3859
1.4634
9.6010
Au-Pd post and core
20.523
23.859
9.5410-4
of the root, the coronal third of the post, and the cement in all models tested. For all models in the Ni-Cr group, stress values increased with
increases in the elasticity modulus of the cement. The highest von Mises equivalent stress values were found in the residual root of the Au-Pd
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post-and-core/glass ionomer cement model of the Ni-Cr group (model 13, Table IV), in the cement of the Au-Pd post-and-core/zinc phosphate cement
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2 Von Mises equivalent stresses distribution in cements for Au-Pd post and core in Ni-Cr group after occlusal loading. A, Zinc phosphate cement. B, Composite resin cement. C, Glass ionomer cement. D, Panavia. E, Polycarboxylate cement. Highest von Mises equivalent stresses are marked in red; lowest von Mises equivalent stresses are marked in dark blue.
3 Von Mises equivalent stresses distribution in residual root for Ni-Cr post-and-core in Ni-Cr group after occlusal loading. A, Zinc phosphate cement. B, Composite resin cement. C, Glass ionomer cement. D, Panavia. E, Polycarboxylate cement. Highest von Mises equivalent stresses are marked in red; lowest von Mises equivalent stresses are marked in dark blue.
4 Von Mises equivalent stresses distribution in posts for zinc phosphate cement in Ni-Cr group after occlusal loading. A, Ti post and core. B, Ni-Cr post and core. C, Au-Pd post and core. Highest von Mises equivalent stresses are marked in red; lowest von Mises equivalent stresses are marked in dark blue. model of the Ni-Cr group (model 11, Table IV, Fig. 2a), and in the post of the Ni-Cr post-and-core/glass ionomer cement model of the Ni-Cr group (model 8). The lowest von Mises equivalent stress values were found in
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the residual root of the Ni-Cr post-andcore/zinc phosphate cement model of the Ni-Cr group (model 6, Table III, Fig. 3a), in the cement of the Ni-Cr post-and-core/glass ionomer cement model of the Ni-Cr group (model 8),
and in the post of the Au-Pd post-andcore/zinc phosphate cement model of the Ni-Cr group (model 11, Fig. 4c). Deformation in the residual root, cement, and post increased with a decrease in the modulus of elasticity of the cement in all models of the Ni-Cr group, regardless of the post-and-core alloy. Residual root, cement, and post deformation was higher in the Ni-Cr post-and-core model than in the AuPd and Ti post-and-core models of the Ni-Cr group (Tables II-IV). Deformation in the different models was shown in Figures 5 and 7. The von Mises equivalent stress values of the residual root were lower in each post-and-core alloy in the Au-Pd group when compared to the same post-and-core alloy in the Ni-Cr group, whereas cement and post stress values were slightly higher in the Au-Pd group than in the Ni-Cr group. Stress values
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Volume were not affected in the Ni-Cr post-andcore models. Differences in all stress values were observed between the AuPd post-and-core models of the Ni-Cr group and the Au-Pd group (Tables IIIV). The deformation of the residual root, cement, and post of the Ni-Cr post-and-core models was higher in
the Au-Pd group than in the Ni-Cr group, whereas the residual root deformation was lower and the post deformation higher in the Au-Pd and Ti post-and-core models in the Au-Pd group than in the Ni-Cr group. The difference in residual root deformation was particularly notable in the Au-Pd
5 Deformation in posts for zinc phosphate cement in Ni-Cr group after occlusal loading. A, Ti post and core. B, Ni-Cr post and core. C, Au-Pd post and core. Highest von Mises equivalent stresses are marked in red; lowest von Mises equivalent stresses are marked in dark blue.
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(2-3 MPa) post-and-core (Tables II-IV).
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DISCUSSION This study analyzed the effects of post-and-core, crown, and cement composition on von Mises stress values, which are widely used as indicators of failure.22 An increase in the elastic modulus of the luting agent was found to result in a slight decrease in stress concentrations in the cervical area. This result is consistent with that of previous studies.19,32 A higher stress value is a strong indication of a greater possibility of failure, making it important to reduce stress in the cervical area. Previous studies have also shown that by using a luting agent with a modulus of elasticity similar to that of dentin, excessive stress in the cervical area can be avoided.19,32 Mezzomo et al,1 however, found that the type of luting cement used was not a determining
6 Deformation in cements for Au-Pd post-and-core in Ni-Cr group after occlusal loading. A, Ti post and core. B, Ni-Cr post and core. C, Au-Pd post and core. Highest von Mises equivalent stresses are marked in red; lowest von Mises equivalent stresses are marked in dark blue.
7 Deformation in residual root for Ni-Cr post and core in Ni-Cr group after occlusal loading. A, Ti post and core. B, Ni-Cr post and core. C, Au-Pd post and core. Highest von Mises equivalent stresses are marked in red; lowest von Mises equivalent stresses are marked in dark blue.
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8 Von Mises equivalent stresses distribution in sound tooth (unrestored tooth). A, Dentin. B, Pulp. Highest von Mises equivalent stresses are marked in red; lowest von Mises equivalent stresses are marked in dark blue. factor in the physical resistance to fracture of teeth restored with cast gold post and cores. Naumann et al33 and Aggarwal et al34 found that resin cement improves the fracture resistance of posts compared to zinc phosphate or glass ionomer cement, but an adhesive technique makes it difficult to clean the smear layer from the root and to remove the moisture from the root.34 In a 10-year retrospective study, Balkenhol et al35 found that posts and cores inserted with zinc phosphate cement survived best, while post and cores inserted with glass ionomer cement had a higher risk of failure. Spazzin et al36 found that cements with high elastic modulus caused higher concentrations of stress within the cement layer. This result is consistent with that of the present study. In the present study, a 2-fold increase in the elastic modulus of cement resulted in an approximately 2-fold increase in the stress concentrations in the cement. An increase in the elastic modulus of the cement led to very slight decreases in the stress values (approximately 0.36%) of residual roots and slight decreases (approximately 2.24%) in the stress values of posts. Reductions may have been because nonrigid materials absorb more stress than rigid materials, thereby transmitting less stress to the adjacent material. When compared to the other cements tested, zinc phosphate cement was found to result in higher stress to the bonding
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layer and lower stress to the residual root. Zinc phosphate cement and Panavia have a higher elastic modulus than the other luting agents examined in this study but a similar elastic modulus of dentin tissue. A luting agent with a similar rigidity to dentin may be a better choice for the post-and-core restoration in clinical practice because of lower stress in the residual root and lower deformation in the cement layer. Clinicians should remember that the cementation technique may affect the fracture resistance of restored teeth. Residues within a post space, bubbles within the cement layer, and excessive seating pressure can cause stress concentration within the root and predispose it to fracture.12 According to Cailleteau et al,37 the magnitude of stress at the dentin-post interface may be associated with the initiation of dentin cracking or even dentin fracture. The higher the elastic modulus, the greater the stiffness of the cement, and stiffness can cause an increase in the concentration of stress at the cement-dentin interface.18 The use of an alloy with a high elastic modulus has been shown to result in the almost complete transmission of stress to dentin because of the low deformation and absorption capacity associated with a high elastic modulus.1 However, in the present study, the post material with the lowest elastic modulus exhibited low levels of deformation and generated a high level of stress at the
cement (approximately 12% to 19%) and the residual root (approximately 4% to 8%) interface. As the elastic modulus of the cement decreased, deformation increased. This result is consistent with that of previous studies,5,6,8,38 which indicated that decreasing the elastic modulus of the post material increased the stress on the root surface. A high level of deformation within the cement may lead to cracking, bond failure, and even mobility of the post. In addition, Soares et al18 have shown that a high degree of flexibility may result in infiltration and undiagnosed caries if the post flexion remains undetected. The mechanical behavior of endodontically treated teeth restored with posts is affected by the characteristics of the interfaces and the rigidity of the materials. The biomechanical properties of a restorative system significantly affect its deformation and stressabsorbing capacity. Placing an endodontic post, regardless of its composition, fills the post space with a material whose stiffness varies from that of pulp, making it impossible to replicate the stress distribution of the original tooth.27 The present study found the use of a Ni-Cr metal ceramic crown had a greater effect on stress concentrations in the post-and-core material with a lower elastic modulus than the postand-core material with a higher elastic modulus. Increases in the elastic modulus of the metal ceramic crown resulted in decreases in post-and-core stress values (approximately 1.5% to 15.5%), increases in residual root stress values (approximately 4% to 8%), and decreases in cement stress values (approximately 4% to 7%); however, the type of metal alloy used in the metal ceramic crown had little effect on the amount of deformation in the residual root, cement, and post-and-core material. These results are consistent with those of Suzuki et al,19 who indicated that different combinations of crown materials and luting agents did not affect the magnitude of von Mises stress around the post or root apex.
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Volume In the present study, the use of postand-core materials with a higher elastic modulus led to stress reduction (approximately 4% to 8%) in the residual root. Several studies confirm this result.5,6,38,39 The use of a post-andcore material with a lower elastic modulus led to stress reduction (approximately 75% to 95%) in post and cores. All models demonstrated the lowest stress value for the combination of post and core and cement with Ni-Cr metal ceramic crown/Au-Pd post-andcore/glass ionomer cement, whereas the lowest deformation value was observed in the Au-Pd metal ceramic crown/Au-Pd post-and-core/zinc phosphate cement. The lowest stress value for the residual root was observed in the Au-Pd metal ceramic crown/Ni-Cr post-and-core/zinc phosphate cement, and the lowest deformation value was also observed in the Au-Pd metal ceramic crown/Au-Pd post-and-core/ zinc phosphate cement. In conclusion, the use of a post-and-core material with a lower elastic modulus and cement with a higher elastic modulus led to a reduction in deformation in the residual root, cement, and post-andcore material and a reduction in stress in the post-and-core material. The present study recorded stress values for the residual root and pulp tissue in sound teeth to be 31.973 MPa and 17.9110-3 MPa. Among the models evaluated, those with Ni-Cr metal ceramic crowns/Au-Pd postand-core/glass ionomer cement produced stress values closest to those of sound teeth. In this respect, the use of material with a low elastic modulus for posts and cements may be appropriate for the post-and-core restoration, although it must be kept in mind that a cement with a low elastic modulus may undergo deformation more easily than one with a high elastic modulus. In all the models evaluated, a decrease in residual root stress was observed with zinc phosphate cement and Panavia, regardless of the post-and-core material used. This suggests that a combination of Au-Pd post-and-core and zinc phosphate cement or Panavia (which
has a modulus of elasticity similar to that of dentin) may help to reinforce root walls and may therefore be suitable for restorations of teeth with thin root walls. This current study was limited in that the type of testing used did not accurately simulate intraoral conditions. Intraorally, teeth are subjected to cyclic loading through mastication and are immersed in a wet environment that is subject to chemical and thermal changes.40 Furthermore, the interfaces of the structures were assumed to be perfect and continuous, with a 100 mm cement layer; this is not so in clinical practice. The clinical bonding of luting cement is dependent on many factors such as the restoration surface, etching, polymerization, and contamination with saliva, water, and blood.19 In addition, this study assumed that all materials were linearly elastic, homogeneous, and isotropic and that bonds between post and cores and tooth structure were ideal. In fact, tooth structure is neither homogeneous nor isotropic.19 Given that computer simulations of post-and-core applications are unable to account for all factors encountered in the oral environment,41 the results of this study must be regarded as merely indicative of clinical behavior, and further experimental and clinical tests are necessary.
CONCLUSIONS Within the limitations of this study, the following conclusions were drawn: 1. The use of Au-Pd post-and-core material and the use of zinc phosphate cement led to deformation reduction in the residual root (approximately 2.5%), cement (approximately 5%), and post-and-core material (approximately 5.5%) and to stress reduction (approximately 75% to 95%) in post-and-core material. The combination of Au-Pd post-and-core/zinc phosphate cement or Panavia may therefore be favorable for post-and-core restorations. 2. Differences in the metal alloy used in the metal ceramic crown had little
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effect on the amount of deformation in the residual root, cement, and postand-core material; however, the use of a Ni-Cr metal ceramic crown led to increases in residual root stress values (approximately 4% to 8%), decreases in post-and-core stress values (approximately 1.5% to 15.5%), and decreases in cement stress values (approximately 4% to 7%). A Ni-Cr metal ceramic crown may therefore be appropriate for post-and-core restorations.
REFERENCES 1. Mezzomo E, Massa F, Suzuki RM. Fracture resistance of teeth restored with 2 different post-and-core designs with 2 different luting cements: an in vitro study. Part II. Quintessence Int 2006;37:477-84. 2. Shillingburg HT, Hobo S, Whitsett LD, Jacobi R, Brackett SE. Fundamentals of fixed prosthodontics. 4th ed. Chicago: Quintessence; 2012. p. 203-8. 3. Fernandes AS, Shetty S, Ivy Coutinho I. Factors determining post selection: a literature review. J Prosthet Dent 2003; 90:556-62. 4. Balkaya MC, Birdal IS. Effect of resin-based materials on fracture resistance of endodontically treated thin-walled teeth. J Prosthet Dent 2013;109:296-303. 5. Dejak B, Młotkowski A. Finite element analysis of strength and adhesion of cast posts compared to glass fiber-reinforced composite resin posts in anterior teeth. J Prosthet Dent 2011;105:115-26. 6. Asmussen E, Peutzfeld A, Sahafi A. Finite element analysis of stress in endodontically treated, dowel-restored. J Prosthet Dent 2005;94:321-9. 7. Dallari A, Rovatti L. Six years of in vitro/in vivo experience with Composipost. Compend Contin Educ Dent 1996;17:857-63. 8. Pierrisnard L, Bohin F, Renault P, Barquins M. Coronal-radicular reconstruction of pulpless teeth: a mechanical study using finite element analysis. J Prosthet Dent 2002;88:442-8. 9. Rosentritt M, Sikora M, Behr M, Handel G. In vitro fracture resistance and marginal adaptation of metallic and tooth-coloured post systems. J Oral Rehabil 2004;31:675-81. 10. Gonzalez-Lluch C, Rodriguez-Cervantes PJ, Sancho-Bru JL, Perez-Gonzalez A, Barjau Escribano A, Vergara-Monedero M, et al. Influence of material and diameter of prefabricated posts on maxillary central incisors restored with crown. J Oral Rehabil 2009;36: 737-47. 11. Hill EE, Lott J. A clinical focused discussion of luting materials. Aust Dent J 2011;56: 67-76. 12. Fernandes AS, Dessai GS. Factors affecting the fracture resistance of post-core reconstructured teeth: a review. Int J Prosthodont 2001;14:355-63.
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13. Morgano SM, Brackett SE. Foundation restorations in fixed prosthodontics: current knowledge and future needs. J Prosthet Dent 1999;82:643-57. 14. Balbosh A, Ludwig K, Kern M. Comparison of titanium dowel restoration using four different luting agents. J Prosthet Dent 2005;94:227-33. 15. Calheiros FC, Braga RP, Kawano Y, Ballester RY. Relationship between contraction stress and degree of conversion in restorative composites. Dent Mater 2004; 20:939-46. 16. Mendoza DB, Eakle WS, Kahl EA, Ho R. Root reinforcement with a resin-bonded preformed post. J Prosthet Dent 1997; 78:10-4. 17. Reid LC, Kazemi RB, Meiers JC. Effect of fracture testing on core integrity and post microleakage of teeth restored with different post systems. J Endod 2003;29:125-31. 18. Soares CJ, Raposo LH, Soares PV, SantosFilho PC, Menezes MS, Soares PB, et al. Effect of different cements on the biomechanical behavior of teeth restored with cast dowel-and-cores in vitro and FEA analysis. J Prosthodont 2010;19:130-7. 19. 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. 20. Fu G, Deng F, Wang L, Ren A. The threedimension finite element analysis of stress in posterior tooth residual root restored with postcore crown. Dent Traumatol 2010;26: 64-9. 21. Li LL, Wang ZY, Bai ZC, Mao Y, Gao B, Xin HT, et al. Three-dimensional finite element analysis of weakened roots restored with different cements in combination with titanium alloy posts. Chin Med J 2006;119: 305-11. 22. Isidor F, Odman P, Brøndum K. Intermittent loading of teeth restored using prefabricated carbon fiber posts. Int J Prosthodont 1996;9: 131-6. 23. Romeed SA, Dunne SM. Stress analysis of different post-luting systems: a threedimensional finite element analysis. Aust Dent J 2013;58:82-8. 24. Ash MN, Nelson SJ. Wheeler’s dental anatomy, physiology and occlusion. 8th ed. St Louis: Saunders; 2003. p. 251-61.
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9 25. Silva NR, Castro CG, Santos-Filho PC, Silva GR, Campos R, Soares PV, et al. Influence of different post design and composition on stress distribution in maxillary central incisor: finite element analysis. Indian J Dent Res 2009;20:153-8. 26. Ho MH, Lee SY, Chen HH, Lee MC. Threedimensional finite element analysis of the effects of post stress distribution in dentin. J Prosthet Dent 1994;72:367-72. 27. Zorano F, Sorrentino R, Apicella D, Valentino B, Ferrari M, Aversa R, et al. Evaluation of the biomechanical behavior of maxillary central incisors restored by means of endocrowns compared to a natural tooth: a 3D static linear finite elements analysis. Dent Mater 2006;22:1035-44. 28. Lanza A, Aversa R, Rengo S, Apicella D, Apicella A. 3D FEA of cemented steel, glass and carbon posts in a maxillary incisor. Dent Mater 2005;21:709-15. 29. Oyar P, Ulusoy M, Eskitascioglu. 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. 30. Ruse ND. Propagation of erroneous data for the modulus of elasticity of periodontal ligament and gutta percha in FEM/FEA paper: a story of broken links. Dent Mater 2008;24: 1717-9. 31. Mattos CMA, Las Casas EB, Dutra IGR, Sousa HA, Guerra SMG. Numerical analysis of the biomechanical behaviour of a weakened root after adhesive reconstruction and postcore rehabilitation. J Dent 2012;40:423-32. 32. Suzuki S, Nagai E, Taira Y, Minesaki Y. In vitro wear of indirect composite restorates. J Prosthet Dent 2002;88:431-6. 33. Naumann M, Sterzenbach G, Rosentritt M, Beuer F, Frankenberger R. Is adhesive cementation of endodontic posts necessary? J Endod 2008;34:1006-10. 34. Aggarwal R, Gupta S, Tandan A, Gupta NK, Dwivedi R, Aggarwal R. Comparative evaluation of fracture resistance of various post systems using different luting agents under tangential loading. J Oral Biol Craniofac Res 2013;3:63-7. 35. Balkenhol M, Wöstmann B, Rein C, Ferger P. Survival time of cast post and cores: a 10year retrospective study. J Dent 2007;35: 50-8.
36. Spazzin AO, Galasfassi D, de Meira-Jûnior AD, Braz R, Garbin CA. Influence of post-and-resin cement on stress distribution of maxillary central incisors restored with direct resin composite. Oper Dent 2009;34:223-9. 37. Cailleteau JG, Rieger MR, Akin JE. A comparison of intracanal stresses in a postrestored tooth utilizing the finite element method. J Endod 1992;18:540-4. 38. Okamoto K, Ino T, Iwase N, Shimizu E, Suzuki M, Satoh G, et al. Three-dimensional finite element analysis of stress distribution in composite resin cores with fiber posts of varying diameters. Dent Mater J 2008;27: 49-55. 39. Pegoretti A, Fambri L, Zappini G, Bianchetti M. Finite element analysis of a glass fibre reinforced composite endodontic post. Biomaterials 2002; 23:2667-82. 40. 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. 41. Sirimai S, Riis DN, Morgano S. An in vitro study of fracture resistance and the incidence of vertical root fracture of pulpless teeth restored with six post and core system. J Prosthet Dent 1999;81: 262-9. Corresponding author: Dr Perihan Oyar Dental Prosthetics Technology Programme School of Health Services Hacettepe University D-Blok, 3. Kat 06100 Sıhhıye-Ankara TURKEY E-mail: [email protected] Copyright ª 2014 by the Editorial Council for The Journal of Prosthetic Dentistry.
Clinical Implications The stress values of teeth with restorations came closest to those of unrestored teeth when materials with a low elastic modulus were used for posts, whereas the use of a luting cement with a high elastic modulus resulted in higher stress concentrations in the cement. Considering that stress minimization is desirable in the restoration of teeth with thin root walls, the use of post materials with a low elastic modulus, for example, Au-Pd alloys and zinc phosphate cement or Panavia, may be appropriate. Posts and cores are used to reconstruct material lost to caries, fracture, or endodontic access and provide retention and stability for artificial crowns. Postand-core materials and fabrication techniques vary in accordance with the amount of lost mineralized structure.1 Assistant Professor.
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Post size and shape, tooth preparation design, and luting agent all affect tooth resistance.1,2 The prognosis of post-and-core restored teeth may be improved by bonding the post to the tooth structure in order to increase post retention and reinforce tooth structure.3
Several post materials have been used in post-and-core restorations. Increases in the elastic modulus of the post material have been shown to decrease the stress on the root surface.4-6 Thus, some authors have recommended using a post with a high
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Volume modulus of elasticity.4-6 However, others have recommended posts with a modulus of elasticity similar to that of dentin. Moreover, while some authors have stated that cast posts ensure better tooth fracture resistance than fiber-reinforced posts,7,8 others have reported that teeth restored with fiber posts and composite resin core materials have a higher fracture resistance than those restored with cast post and cores.9,10 The selection of the incorrect luting agent can significantly affect the longevity of post-and-core restorations.11 Glass ionomer, composite resin, zinc phosphate, and polycarboxylate cement are most commonly used to cement endodontic posts.12,13 For decades, zinc phosphate cement has been the gold standard for cementing cast post and cores, although resin luting agents have recently been studied as alternatives.13-16 Resin cements have been reported to aid resistance to fracture compared to brittle and nonbonding zinc phosphate cement and are recommended for posts in thin-walled roots.4 The use of composite resin cements lowers the risk of fracture and loss of retention.17 Resin-modified glass ionomer cement is also commonly used in post fixation, but not all researchers agree that this material is appropriate.18 Although no intermediate layer exists between the enamel and dentin in natural teeth, in clinical restorations, a luting agent must be used to cement the crown to the remaining natural tooth. The presence of a luting agent gives rise to differences in stress distribution between natural and endodontically treated teeth. Obtaining stress distributions close to those of natural teeth is an important consideration when the clinician is choosing crown material and luting agents.19 Given their role in transmitting and diffusing stress between the post and dentin, bonding agents should be selected not only for their retentive characteristics but also for their ability to resist damage under stress. The elastic modulus and mechanical characteristics of cements significantly affect the
transmission and diffusion of stress. Previous studies have demonstrated that the elastic modulus is an important parameter to consider in selecting a luting agent.20,21 Widely used as a basic research tool in dentistry, finite element analysis (FEA) illustrates internal stresses and is helpful in predicting failure. FEA should be used as an initial step and an aid for planning further laboratory tests and clinical studies.3,22,23 Although post-and-core restorations are fabricated with 3 distinct material systems (post and core, luting agent, crown), the interaction among these systems during function has not been thoroughly researched,4 and there is little understanding of how the combination of post material and luting agent affects stress distribution in residual roots. This study used FEA to evaluate stresses in the post-and-core restorations of posterior teeth fabricated with different post-and-core and crown materials and different cements. The purpose of the study was to identify suitable combinations for post-andcore applications in terms of fracture avoidance.
METHOD AND MATERIALS A mandibular second premolar was scanned with computed tomography to obtain 1-mm-thick slices of tooth tissue, and the data were combined with data from a textbook24 to create 3-dimensional finite element models
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with computer-aided design software (Inventor-AutoCAD 2010; Autodesk). The contours of all cross sections of tooth enamel, dentin, and pulp boundaries were delineated, and the drawing was adapted so that the dimensions of the final model were as close as possible to those of the mandibular second premolar found in the textbook. Data were exported in initial graphics exchange specification (IGES) format to FEA software (ANSYS 11.0 Inc) for stress distribution analysis. The final mesh comprised 51 508 nodes and 27 009 elements (Fig. 1). In total, 30 three-dimensional finite element models comprising bone, apical root canal filling, metal alloy post and core, metal ceramic crown, and luting agent were fabricated. Models were divided into 2 groups, Ni-Cr and Au-Pd (n¼15 for each group), according to the alloys used in the metal ceramic crown restoration. Each group was subdivided according to the alloy used for the post and core, gold-palladium (JF Jelenko), nickelchromium (Remanium CS), and titanium (Essential Dental Systems) and further divided according to post cementation-glass ionomer cement (Ketac CEM; 3M ESPE), composite resin cement (Filtek Supreme, Shade A2B; 3M ESPE), zinc phosphate cement (Poscal; VOCO GmbH), polycarboxylate cement (Durelon, Maxicap; 3M ESPE), and Panavia (Kuraray). Each model also included gutta percha (Coltene/Whaledent Inc) to fill the
1 Finite element meshes for mandibular second premolar.
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apical third of the root canal and zinc phosphate cement to lute the metal ceramic crown. The crowns were modeled with a metal thickness of 0.5 mm, a porcelain thickness (Klema) of 1.5 mm, and a cement thickness of 100 mm. For each model, the application of a simulated 400 N load to the centric occlusion stop of the occlusal surface in centric occlusion at a 45-degree inclination in a linguolabial direction to the long axis of the tooth and the von Mises stress data were analyzed. The models were constrained in all directions by the outline of the bone surface. All structures were assumed to be linearly elastic, homogeneous, and isotropic, except for the periodontal ligament and cortical bone, which were ignored. Ideal adherence between structures (metalceramic-cement, cement-core, corepost, post-cement, and cement-dentin interfaces) was assumed. The Poisson ratio (n) and modulus of elasticity (E) of the oral tissue, crown and post materials, and cements were determined from the literature 6,19,21,25-31 and are
Table I.
Mechanical properties of materials and tooth
Material
Modulus of Elasticity (GPa)
Poisson Ratio
1.37
0.30
Bone6,21,25 6,21,25
18.6
0.31
19
80
0.30
2
0.45
Gutta percha30,31
0.14
0.45
69
0.30
205
0.33
Dentin
Enamel 29
Pulp
Feldspathic porcelain25 Ni-Cr
29 29
Au-Pd
89.5
0.33
Ti25
112
0.33
4
0.35
5.11
0.35
8.3
0.24
22.4
0.35
18.6
0.28
Glass ionomer cement21 Polycarboxylate cement
21
Composite resin cement26 Zinc phosphate cement
21
27,28
Panavia
listed in Table I. A sound tooth was also modeled as a control.
RESULTS The maximum von Mises equivalent stresses and deformation values for
Ti, Ni-Cr, and Au-Pd post and cores are listed in Tables II to IV. The distribution of the von Mises equivalent stress and deformation values are shown in Figures 2 to 8. Stress concentrations were highest in the coronal third of the facial surface
Maximum von Mises equivalent stress values and deformation in residual root (dentin), cement, post-and-core and sound tooth in Ti post-and-core models
Table II.
Von Mises (MPa)
Deformation (mm)
Model
Region
Ni-Cr
Au-Pd
Ni-Cr
Au-Pd
1
Dentin
26.666
25.383
9.4410-4
9.3910-4
4.8836
5.1909
9.6610-4
9.6310-4
20.561
23.77
9.55 10
9.6510-4
26.88
25.514
9.5210-4
9.4810-4
Composite cement
2.272
2.3843
9.7310-4
9.6910-4
Ti post and core
22.329
25.813
9.6710-4
9.7810-4
26.979
25.558
9.5510
-4
9.5010-4
Glass ionomer cement
1.0495
1.10209
9.7410-4
9.7110-4
Ti post and core
22.831
26.41
9.7110-4
9.8410-4
26.727
25.426
9.4610-4
9.4210-4
-4
9.6510-4
Zinc phosphate cement Ti post and core 2
3
4
5
Dentin
Dentin
Dentin Panavia
4.4743
4.7217
9.5810
Ti post and core
21.003
24.277
9.5810-4
9.6810-4
26.946
25.538
9.5410-4
9.5010-4
1.3219
1.391
9.7310-4
9.6910-4
-4
9.8110-4
Dentin Polycarboxylate cement Ti post and core
Sound tooth
Dentin Pulp
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22.738
26.301
9.6910
31.973
9.9610-4
17.9110-3
2.4110-3
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Table III. Maximum von Mises equivalent stress values and deformation in residual root (dentin), cement, post and core, and sound tooth in Ni-Cr post-and-core models Von Mises (MPa) Model
Region
6
Dentin
7
8
9
Ni-Cr
Au-Pd
Ni-Cr
Au-Pd
25.502
24.483
9.7110-4
9.7610-4
Zinc phosphate cement
4.4904
4.6606
9.9910-4
10.0810-4
Ni-Cr post and core
35.251
35.804
9.9110-4
10.1010-4
25.706
24.581
9.7610
-4
9.8210-4
Composite cement
2.041
2.1056
10.0510-4
10.1310-4
Ni-Cr post and core
36.0301
36.657
10.0210-4
10.2210-4
25.753
24.617
9.7910-4
9.8410-4
-4
10.1110-4
Dentin
Dentin Glass ionomer cement
0.92292
0.96779
10.0410
Ni-Cr post and core
36.309
37.117
10.0710-4
10.2810-4
Dentin
25.637
24.516
9.7310-4
9.7810-4
Panavia
4.1009
4.2311
10.0210-4
10.1310-4
35.271
35.846
9.9410
-4
10.1010-4
25.726
24.599
9.7810-4
9.8310-4
Polycarboxylate cement
1.1761
1.2231
10.0310-4
10.1010-4
Ni-Cr post and core
36.422
37.061
10.0510-4
10.2610-4
Ni-Cr post and core 10
Deformation (mm)
Dentin
Table IV. Maximum von Mises equivalent stress values and deformation in residual root (dentin), cement, post-and-core and sound tooth in Au-Pd post-and-core models Von Mises (MPa)
Deformation (mm)
Model
Region
Ni-Cr
Au-Pd
Ni-Cr
Au-Pd
11
Dentin
27.136
25.701
9.4910-4
9.2310-4
5.0212
5.3894
9.5310-4
9.4410-4
17.97
20.958
9.3910
-4
9.4410-4
27.387
25.845
9.5410-4
9.3310-4
Composite cement
2.3711
2.4995
9.6110-4
9.5210-4
Au-Pd post and core
20.011
23.252
9.5110-4
9.5810-4
27.477
25.896
9.5610
-4
9.3610-4
Glass ionomer cement
1.1007
1.1585
9.6210-4
9.5410-4
Au-Pd post and core
20.681
24.04
9.5610-4
9.6410-4
-4
9.2610-4
Zinc phosphate cement Au-Pd post and core 12
13
14
15
Dentin
Dentin
Dentin
27.19
25.74
9.5010
Panavia
4.6349
4.9139
9.5610-4
9.4710-4
Au-Pd post and core
18.05
21.511
9.4210-4
9.4810-4
27.459
25.871
9.5510-4
9.3410-4
-4
9.5210-4 9.6110-4
Dentin Polycarboxylate cement
1.3859
1.4634
9.6010
Au-Pd post and core
20.523
23.859
9.5410-4
of the root, the coronal third of the post, and the cement in all models tested. For all models in the Ni-Cr group, stress values increased with
increases in the elasticity modulus of the cement. The highest von Mises equivalent stress values were found in the residual root of the Au-Pd
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post-and-core/glass ionomer cement model of the Ni-Cr group (model 13, Table IV), in the cement of the Au-Pd post-and-core/zinc phosphate cement
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2 Von Mises equivalent stresses distribution in cements for Au-Pd post and core in Ni-Cr group after occlusal loading. A, Zinc phosphate cement. B, Composite resin cement. C, Glass ionomer cement. D, Panavia. E, Polycarboxylate cement. Highest von Mises equivalent stresses are marked in red; lowest von Mises equivalent stresses are marked in dark blue.
3 Von Mises equivalent stresses distribution in residual root for Ni-Cr post-and-core in Ni-Cr group after occlusal loading. A, Zinc phosphate cement. B, Composite resin cement. C, Glass ionomer cement. D, Panavia. E, Polycarboxylate cement. Highest von Mises equivalent stresses are marked in red; lowest von Mises equivalent stresses are marked in dark blue.
4 Von Mises equivalent stresses distribution in posts for zinc phosphate cement in Ni-Cr group after occlusal loading. A, Ti post and core. B, Ni-Cr post and core. C, Au-Pd post and core. Highest von Mises equivalent stresses are marked in red; lowest von Mises equivalent stresses are marked in dark blue. model of the Ni-Cr group (model 11, Table IV, Fig. 2a), and in the post of the Ni-Cr post-and-core/glass ionomer cement model of the Ni-Cr group (model 8). The lowest von Mises equivalent stress values were found in
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the residual root of the Ni-Cr post-andcore/zinc phosphate cement model of the Ni-Cr group (model 6, Table III, Fig. 3a), in the cement of the Ni-Cr post-and-core/glass ionomer cement model of the Ni-Cr group (model 8),
and in the post of the Au-Pd post-andcore/zinc phosphate cement model of the Ni-Cr group (model 11, Fig. 4c). Deformation in the residual root, cement, and post increased with a decrease in the modulus of elasticity of the cement in all models of the Ni-Cr group, regardless of the post-and-core alloy. Residual root, cement, and post deformation was higher in the Ni-Cr post-and-core model than in the AuPd and Ti post-and-core models of the Ni-Cr group (Tables II-IV). Deformation in the different models was shown in Figures 5 and 7. The von Mises equivalent stress values of the residual root were lower in each post-and-core alloy in the Au-Pd group when compared to the same post-and-core alloy in the Ni-Cr group, whereas cement and post stress values were slightly higher in the Au-Pd group than in the Ni-Cr group. Stress values
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Volume were not affected in the Ni-Cr post-andcore models. Differences in all stress values were observed between the AuPd post-and-core models of the Ni-Cr group and the Au-Pd group (Tables IIIV). The deformation of the residual root, cement, and post of the Ni-Cr post-and-core models was higher in
the Au-Pd group than in the Ni-Cr group, whereas the residual root deformation was lower and the post deformation higher in the Au-Pd and Ti post-and-core models in the Au-Pd group than in the Ni-Cr group. The difference in residual root deformation was particularly notable in the Au-Pd
5 Deformation in posts for zinc phosphate cement in Ni-Cr group after occlusal loading. A, Ti post and core. B, Ni-Cr post and core. C, Au-Pd post and core. Highest von Mises equivalent stresses are marked in red; lowest von Mises equivalent stresses are marked in dark blue.
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(2-3 MPa) post-and-core (Tables II-IV).
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DISCUSSION This study analyzed the effects of post-and-core, crown, and cement composition on von Mises stress values, which are widely used as indicators of failure.22 An increase in the elastic modulus of the luting agent was found to result in a slight decrease in stress concentrations in the cervical area. This result is consistent with that of previous studies.19,32 A higher stress value is a strong indication of a greater possibility of failure, making it important to reduce stress in the cervical area. Previous studies have also shown that by using a luting agent with a modulus of elasticity similar to that of dentin, excessive stress in the cervical area can be avoided.19,32 Mezzomo et al,1 however, found that the type of luting cement used was not a determining
6 Deformation in cements for Au-Pd post-and-core in Ni-Cr group after occlusal loading. A, Ti post and core. B, Ni-Cr post and core. C, Au-Pd post and core. Highest von Mises equivalent stresses are marked in red; lowest von Mises equivalent stresses are marked in dark blue.
7 Deformation in residual root for Ni-Cr post and core in Ni-Cr group after occlusal loading. A, Ti post and core. B, Ni-Cr post and core. C, Au-Pd post and core. Highest von Mises equivalent stresses are marked in red; lowest von Mises equivalent stresses are marked in dark blue.
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8 Von Mises equivalent stresses distribution in sound tooth (unrestored tooth). A, Dentin. B, Pulp. Highest von Mises equivalent stresses are marked in red; lowest von Mises equivalent stresses are marked in dark blue. factor in the physical resistance to fracture of teeth restored with cast gold post and cores. Naumann et al33 and Aggarwal et al34 found that resin cement improves the fracture resistance of posts compared to zinc phosphate or glass ionomer cement, but an adhesive technique makes it difficult to clean the smear layer from the root and to remove the moisture from the root.34 In a 10-year retrospective study, Balkenhol et al35 found that posts and cores inserted with zinc phosphate cement survived best, while post and cores inserted with glass ionomer cement had a higher risk of failure. Spazzin et al36 found that cements with high elastic modulus caused higher concentrations of stress within the cement layer. This result is consistent with that of the present study. In the present study, a 2-fold increase in the elastic modulus of cement resulted in an approximately 2-fold increase in the stress concentrations in the cement. An increase in the elastic modulus of the cement led to very slight decreases in the stress values (approximately 0.36%) of residual roots and slight decreases (approximately 2.24%) in the stress values of posts. Reductions may have been because nonrigid materials absorb more stress than rigid materials, thereby transmitting less stress to the adjacent material. When compared to the other cements tested, zinc phosphate cement was found to result in higher stress to the bonding
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layer and lower stress to the residual root. Zinc phosphate cement and Panavia have a higher elastic modulus than the other luting agents examined in this study but a similar elastic modulus of dentin tissue. A luting agent with a similar rigidity to dentin may be a better choice for the post-and-core restoration in clinical practice because of lower stress in the residual root and lower deformation in the cement layer. Clinicians should remember that the cementation technique may affect the fracture resistance of restored teeth. Residues within a post space, bubbles within the cement layer, and excessive seating pressure can cause stress concentration within the root and predispose it to fracture.12 According to Cailleteau et al,37 the magnitude of stress at the dentin-post interface may be associated with the initiation of dentin cracking or even dentin fracture. The higher the elastic modulus, the greater the stiffness of the cement, and stiffness can cause an increase in the concentration of stress at the cement-dentin interface.18 The use of an alloy with a high elastic modulus has been shown to result in the almost complete transmission of stress to dentin because of the low deformation and absorption capacity associated with a high elastic modulus.1 However, in the present study, the post material with the lowest elastic modulus exhibited low levels of deformation and generated a high level of stress at the
cement (approximately 12% to 19%) and the residual root (approximately 4% to 8%) interface. As the elastic modulus of the cement decreased, deformation increased. This result is consistent with that of previous studies,5,6,8,38 which indicated that decreasing the elastic modulus of the post material increased the stress on the root surface. A high level of deformation within the cement may lead to cracking, bond failure, and even mobility of the post. In addition, Soares et al18 have shown that a high degree of flexibility may result in infiltration and undiagnosed caries if the post flexion remains undetected. The mechanical behavior of endodontically treated teeth restored with posts is affected by the characteristics of the interfaces and the rigidity of the materials. The biomechanical properties of a restorative system significantly affect its deformation and stressabsorbing capacity. Placing an endodontic post, regardless of its composition, fills the post space with a material whose stiffness varies from that of pulp, making it impossible to replicate the stress distribution of the original tooth.27 The present study found the use of a Ni-Cr metal ceramic crown had a greater effect on stress concentrations in the post-and-core material with a lower elastic modulus than the postand-core material with a higher elastic modulus. Increases in the elastic modulus of the metal ceramic crown resulted in decreases in post-and-core stress values (approximately 1.5% to 15.5%), increases in residual root stress values (approximately 4% to 8%), and decreases in cement stress values (approximately 4% to 7%); however, the type of metal alloy used in the metal ceramic crown had little effect on the amount of deformation in the residual root, cement, and post-and-core material. These results are consistent with those of Suzuki et al,19 who indicated that different combinations of crown materials and luting agents did not affect the magnitude of von Mises stress around the post or root apex.
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Volume In the present study, the use of postand-core materials with a higher elastic modulus led to stress reduction (approximately 4% to 8%) in the residual root. Several studies confirm this result.5,6,38,39 The use of a post-andcore material with a lower elastic modulus led to stress reduction (approximately 75% to 95%) in post and cores. All models demonstrated the lowest stress value for the combination of post and core and cement with Ni-Cr metal ceramic crown/Au-Pd post-andcore/glass ionomer cement, whereas the lowest deformation value was observed in the Au-Pd metal ceramic crown/Au-Pd post-and-core/zinc phosphate cement. The lowest stress value for the residual root was observed in the Au-Pd metal ceramic crown/Ni-Cr post-and-core/zinc phosphate cement, and the lowest deformation value was also observed in the Au-Pd metal ceramic crown/Au-Pd post-and-core/ zinc phosphate cement. In conclusion, the use of a post-and-core material with a lower elastic modulus and cement with a higher elastic modulus led to a reduction in deformation in the residual root, cement, and post-andcore material and a reduction in stress in the post-and-core material. The present study recorded stress values for the residual root and pulp tissue in sound teeth to be 31.973 MPa and 17.9110-3 MPa. Among the models evaluated, those with Ni-Cr metal ceramic crowns/Au-Pd postand-core/glass ionomer cement produced stress values closest to those of sound teeth. In this respect, the use of material with a low elastic modulus for posts and cements may be appropriate for the post-and-core restoration, although it must be kept in mind that a cement with a low elastic modulus may undergo deformation more easily than one with a high elastic modulus. In all the models evaluated, a decrease in residual root stress was observed with zinc phosphate cement and Panavia, regardless of the post-and-core material used. This suggests that a combination of Au-Pd post-and-core and zinc phosphate cement or Panavia (which
has a modulus of elasticity similar to that of dentin) may help to reinforce root walls and may therefore be suitable for restorations of teeth with thin root walls. This current study was limited in that the type of testing used did not accurately simulate intraoral conditions. Intraorally, teeth are subjected to cyclic loading through mastication and are immersed in a wet environment that is subject to chemical and thermal changes.40 Furthermore, the interfaces of the structures were assumed to be perfect and continuous, with a 100 mm cement layer; this is not so in clinical practice. The clinical bonding of luting cement is dependent on many factors such as the restoration surface, etching, polymerization, and contamination with saliva, water, and blood.19 In addition, this study assumed that all materials were linearly elastic, homogeneous, and isotropic and that bonds between post and cores and tooth structure were ideal. In fact, tooth structure is neither homogeneous nor isotropic.19 Given that computer simulations of post-and-core applications are unable to account for all factors encountered in the oral environment,41 the results of this study must be regarded as merely indicative of clinical behavior, and further experimental and clinical tests are necessary.
CONCLUSIONS Within the limitations of this study, the following conclusions were drawn: 1. The use of Au-Pd post-and-core material and the use of zinc phosphate cement led to deformation reduction in the residual root (approximately 2.5%), cement (approximately 5%), and post-and-core material (approximately 5.5%) and to stress reduction (approximately 75% to 95%) in post-and-core material. The combination of Au-Pd post-and-core/zinc phosphate cement or Panavia may therefore be favorable for post-and-core restorations. 2. Differences in the metal alloy used in the metal ceramic crown had little
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effect on the amount of deformation in the residual root, cement, and postand-core material; however, the use of a Ni-Cr metal ceramic crown led to increases in residual root stress values (approximately 4% to 8%), decreases in post-and-core stress values (approximately 1.5% to 15.5%), and decreases in cement stress values (approximately 4% to 7%). A Ni-Cr metal ceramic crown may therefore be appropriate for post-and-core restorations.
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