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Zirconia-based dental crown to support a removable partial denture: a three-dimensional finite element analysis using contact elements and micro-CT data a
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Eduardo Passos Rocha , Rodolfo Bruniera Anchieta , Erika Oliveira de Almeida , Amilcar a
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Chagas Freitas Jr , Ana Paula Martini , Bruno Sales Sotto-Maior , Marco Antonio Luersen & d
Ching Chang Ko a
Department of Dental Materials and Prosthodontics, Araçatuba Dental School, UNESP – Sao Paulo State University, Araçatuba, Brazil b
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Department of Prosthodontics and Periodontology, UNICAMP – State University of Campinas, Piracicaba, Brazil c
Department of Mechanical Engineering, Technical Federal University of Parana, UTFPR, Curitiba, Brazil d
Department of Orthodontics, The University of North Carolina at Chapel Hill, Chapel Hill, NC, USA Published online: 21 Oct 2014.
To cite this article: Eduardo Passos Rocha, Rodolfo Bruniera Anchieta, Erika Oliveira de Almeida, Amilcar Chagas Freitas Jr, Ana Paula Martini, Bruno Sales Sotto-Maior, Marco Antonio Luersen & Ching Chang Ko (2014): Zirconia-based dental crown to support a removable partial denture: a three-dimensional finite element analysis using contact elements and micro-CT data, Computer Methods in Biomechanics and Biomedical Engineering, DOI: 10.1080/10255842.2014.951927 To link to this article: http://dx.doi.org/10.1080/10255842.2014.951927
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Computer Methods in Biomechanics and Biomedical Engineering, 2014 http://dx.doi.org/10.1080/10255842.2014.951927
Zirconia-based dental crown to support a removable partial denture: a three-dimensional finite element analysis using contact elements and micro-CT data Eduardo Passos Rochaa*, Rodolfo Bruniera Anchietaa, Erika Oliveira de Almeidaa, Amilcar Chagas Freitas Jra, Ana Paula Martinia, Bruno Sales Sotto-Maiorb, Marco Antonio Luersenc and Ching Chang Kod Department of Dental Materials and Prosthodontics, Arac atuba Dental School, UNESP – Sao Paulo State University, Arac atuba, Brazil; bDepartment of Prosthodontics and Periodontology, UNICAMP – State University of Campinas, Piracicaba, Brazil; c Department of Mechanical Engineering, Technical Federal University of Parana, UTFPR, Curitiba, Brazil; dDepartment of Orthodontics, The University of North Carolina at Chapel Hill, Chapel Hill, NC, USA
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(Received 16 July 2014; accepted 3 August 2014) Veneer fracture is the most common complication in zirconia-based restorations. The aim of this study was to evaluate the mechanical behavior of a zirconia-based crown in a lower canine tooth supporting removable partial denture (RPD) prosthesis, varying the bond quality of the veneer/coping interface. Microtomography (mCT) data of an extracted left lower canine were used to build the finite element model (M) varying the core material (gold core – MAu; zirconia core – MZi) and the quality of the veneer/core interface (complete bonded – MZi; incomplete bonded – MZi-NL). The incomplete bonding condition was only applied for zirconia coping by using contact elements (Target/Contact) with 0.3 frictional coefficients. Stress fields were obtained using Ansys Workbench 10.0. The loading condition (L ¼ 1 N) was vertically applied at the base of the RPD prosthesis metallic support towards the dental apex. Maximum principal (smax) and von Mises equivalent (svM) stresses were obtained. The smax (MPa) for the bonded condition was similar between gold and zirconia cores (MAu, 0.42; MZi, 0.40). The incomplete bonded condition (MZi-NL) raised smax in the veneer up to 800% (3.23 MPa) in contrast to the bonded condition. The peak of svM increased up to 270% in the MZi-NL. The incomplete bond condition increasing the stress in the veneer/zirconia interface. Keywords: removable partial denture; zirconium oxide; fixed partial denture; finite element analysis
Introduction The tooth support is essential for removable partial denture (RPD), regardless of the number of absent teeth. In order for the support to have an efficient function, it is ideal that the tooth be healthy, or if restored, it should be sufficiently preserved to receive stresses without risks of crown or root fractures that lead to teeth and prosthesis loss (McCracken 2004). There are clinical situations in which the structural condition of the remaining dental crown demands previous fabrication of a fixed partial prosthesis or crown to support the RPD. This procedure has been traditionally carried out with metal-ceramic fixed prosthesis, as it restores the supporting tooth ensuring the necessary stress resistance (Sailer et al. 2006, 2007; Thongthammachat-Thavornthanasarn 2007). In addition, the biomechanical aspects of the RPD, such as the need of stability, retention and one single insertion axis may be improved as rest seats and guiding planes may be incorporated to the total crown with metallic core. Actually, different ceramic systems with greater aesthetics and biocompatibility (Guo 1999; Tinschert et al. 2001; Rekow and Thompson 2007) are available for fabricating total crowns, the literature has presented RPDs
*Corresponding author. Email: [email protected] q 2014 Taylor & Francis
with supporting-teeth restored with metal-free total crowns constituted by aluminum or zirconium oxide cores, without damage to the tooth supporting capacity (Kancyper et al. 2000; Carracho and Razzoog 2006; Marchack et al. 2007; Fischer et al. 2009). However, the safety of the procedure using zirconia-based restorations has been largely questioned, due to clinical and laboratory behavior of the ceramic veneer over the zirconia core, which has been target of criticism, based on the type and incidence of failure reported in the literature (Raigrodski et al. 2006; Santana et al. 2009; Aboushelib et al. 2008; Kohorst et al. 2008; Farga-Nin˜oles et al. 2013), and this failure results, in most cases, in a replacement of restoration (Rafferty et al. 2010). It is unclear whether failures occur due to zirconia’s high elasticity modulus, which intensifies crack propagation starting from small flaws in the ceramic body (Rekow et al. 2006; Coelho et al. 2009), or due to thermal expansion coefficient differences between zirconia and ceramic, impairing ceramic veneer stability (Walter et al. 1999; Raigrodski et al. 2006; Denry and Kelly 2008). In addition, it might be supposed that these failures occur due to inadequate bonding between the veneer and zirconia core; however, this is an inconclusive aspect as
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failures are considered material dependent (Proos et al. 2002; Sailer et al. 2007; Dittmer et al. 2009). As regards to a crown with zirconia core to support a RPD, the aspects mentioned above are also inconclusive due to the lack of studies that evaluate the ceramic veneer behavior considering the specific profile that the core takes supporting biting function. The aim of this study was to evaluate the mechanical behavior of a zirconia-based crown in a lower canine tooth to support a RPD prosthesis, varying the bond quality of the veneer/core interface by the finite element method.
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Materials and methods This study was approved by the Human Research Ethics Committee at the Faculty of Dentistry of Arac atuba, Sa˜o
Paulo State University (UNESP). A maxillary central incisor from the human tooth bank was used to develop the models. Microtomographic images (mCT) of a left mandibular canine were obtained to build solid models for the present study. A total of 720 sections were obtained by scanning (CT40, Scanco Medical, Bassersdorf, Switzerland), 82 serial sections were used to rebuild the tooth in the software (SolidWorks, Dassault Syste`mes, SolidWorks Corp., Concord, MA). A geometric model was created with all structures inherent to the canine (enamel, dentin, and pulp chamber), except the cement layer. From the geometric model, the tooth was prepared for a fixed prosthesis complete crown (Figure 1). The buccal, lingual, and proximal reductions were approximately
Figure 1. (A) Canine tooth with dental reductions for fixed total crown prosthesis ((a) proximal view; (b) vestibular view; (c) incisal view). (B) View of prepared tooth and modified coping positioned (a), ceramic veneer (b), and metallic support positioned over crown (c).
Computer Methods in Biomechanics and Biomedical Engineering Table 1.
Mechanical properties of materials.
Material Zirconiun (Farga-Nin˜oles et al. 2013) Feldspathic ceramic (Farga-Nin˜oles et al. 2013) Chromium – cobalt alloy (Shillingburg et al. 1997) Gold (Hickel and Manhart 2001) Enamel (Taskonak et al. 2008) Dentin (Adanir and Belli 2007) Pulp (Magne et al. 2002) Periodontal ligament (Adanir and Belli 2007; Dejak and Mlotkowski 2008) Resin cement (Lin et al. 2001)
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1 mm, with 1.5-mm incisal reduction, and convergence of vertical walls at 68, with vestibular wear in two-planes (Shillingburg et al. 1997; Couegnat et al. 2006). The model had a large chamfer finish line. From this prepared model, three experimental models were created (Table 1). All models presented were modified core by simulation of lingual cingulum for rest seat: MAu, model restored with a metal-ceramic complete crown with gold core, with approximately 0.5- and 1.0-mm thickness in axial walls and incisal border, respectively, lingual cingulum rest seat covered by ceramic perfectly bonded to core as well as CoCr metallic RPD support positioned over rest seat; MZi, similar to MAu model, although with zirconia core perfectly bonded to veneer ceramic; MZi-NL, similar to MZi, although with zirconia/veneer interface showing contact elements of the type Target/Contact with 0.3 frictional coefficient allowing separation and sliding movements to characterize imperfect bond condition (Figure 2; O’Brien 2002).
Elastic modulus (GPa)
Poisson’s ratio
205 70 218 89.5 80 18.6 2 6.89 £ 1025 18.3
0.22 0.19 0.31 0.33 0.30 0.31 0.45 0.45 0.30
All models had comparable aspects in terms of a 50-mm layer of cementing agent (Panavia F, Kuraray Medical, Inc., Okayama, Japan) (Figure 1), and a periodontal ligament thickness of 0.25 mm were uniformly distributed around the canine root (Figure 2). Other comparable aspect was a homogeneous ceramic thickness of 0.45 mm were applied on the zirconia and gold cores, due to the uniformity of the ceramics which is a basic principle for maximum resistance (Anusavice 1997; Hickel and Manhart 2001; Sadowsky 2006; Rekow and Thompson 2007; Aboushelib et al. 2008; Taskonak et al. 2008). The models were developed in the SolidWorks 2010 software (SolidWorks Corp.) and exported to the finite element program (Ansys Workbench10.0, Swanson Analysis Inc., Houston, PA) in.igs (Initial Graphics Exchange Specification) format for numerical analysis. Mechanical properties (elastic modulus, E and Poisson’s ratio, n) were obtained from the literature
Figure 2. (a) View of contact interface between veneer and zirconia coping. Contact element of the type target and contact for zirconia surface and veneer surface, respectively. (b) Finite elements mesh. (c) Boundary condition applied to periodontal ligament surface (x ¼ y ¼ z ¼ 0). (d) Loading applied to metallic support.
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(Table 1; Yettram et al. 1977; Lin et al. 2001; Magne et al. 2002; O’Brien 2002; Lanza et al. 2005; Adanir and Belli 2007; Dejak and Mlotkowski 2008; Coelho et al. 2009) and MZi and MAu were considered to be homogeneous, isotropic, and linear elastic. All structures of these models were considered perfectly bonded (Asmussen et al. 2005; Sorrentino et al. 2007). An incomplete bond between the zirconia core and ceramic veneer was simulated for the MZi-NL model by allowing separation and sliding movements with a frictional coefficient of 0.3 (Tillitson et al. 1971). The finite element mesh generation was carried out according to convergence of analysis (6%; Huang et al. 2008) showing 0.2-mm tetrahedral elements. A total of 46,000 elements and 80,000 nodes were obtained in all models (Figure 2). The external surface of the periodontal ligament was fixed on x, y, and z coordinates for all models (x ¼ y ¼ z ¼ 0). A vertical loading of 1 N was applied to the metallic support base in contact with the ceramic to simulate the RPD load over the tooth (Rocha et al. 2011). The maximum principal stress (smax) and equivalent von Mises (svM) stress values were obtained for all models. According to Dejak and Mlotkowski (2008) the maximum principal stress is adequate to establish success predictability in friable materials (Anchieta et al. 2007; Martini et al. 2009) such as dental ceramics. To evaluate the stress on the RPD metallic support in contact with the ceramic, the von Mises stress was obtained due to the metallic materials which show a ductile behavior (Cattaneo et al. 2005).
Results The complete and incomplete conditions showed distinct maximum principal stress values for the ceramic veneer. For the gold core model (MAu), the smax was 0.42 MPa for the ceramic, similar to that obtained for the zirconia core (MZi, 0.40 MPa). When an incomplete bonded interface was considered between the veneer and the zirconia core (MZi-NL), the tensile stress occurred in the veneer located close to the angle between the RPD metallic support and minor connector (Figure 3), increased over 800% (3.23 MPa) (Figures 4– 6). The sliding movements, influenced by the non-linear analysis, make the von Mises stress concentration higher for the minor connector, with a difference of 270% between the two zirconia based restorations (MZi and MZi-NL) (Figures 4 – 6).
Discussion The previous experimental studies evaluated the mechanical behavior of the ceramic veneer under influence of
the supporting material. These studies were carried out according to protocols based on simplified models and obtained results that contributed to the improvement of materials that aid clinical applications (Carracho and Razzoog 2006; Sadowsky 2006). The present study employed computational methods to obtain data on the mechanical behavior of a zirconia-based crown supporting RPD prosthesis, which are difficult to reproduce in a physical model. It is quite impossible to employ laboratory analysis to get mechanical behavior data about the ceramic element in multiple layers and uniform dimension, based on different systems, besides the cementing of elements over a tooth with identical dimensions and structural characteristics, with a metallic support directing stresses over the crown. The results of the present study are in agreement with literature data on the incidence of failures in zirconia crowns. Chipping or complete delamination of the veneer ceramic has been reported (Marchack et al. 2007; Sailer et al. 2007). The understanding of such occurrence demands further studies, although the literature points out the incompatibility between thermal expansion coefficients as a strong cause of veneer fracturing (Kancyper et al. 2000). In addition, other possible aspects that may also influence failures are not sufficiently clear yet, such as the bond strength between the zirconia and the veneer, as tensile stress is observed in the veneer/zirconia interface (Fischer et al. 2009), which have been pointed out as consequence of thermal incompatibility (Fischer et al. 2009). Finally, structural veneer failures have occurred even in ceramic systems considered thermally compatible with the ceramic veneer (Marchack et al. 2007). Similar failures have not been observed in metal-based prosthesis, according to the incidence and frequency in which they occur in prosthesis with zirconia core (Magne et al. 2002). In another study, the authors performed by a numerical simulation, the reduced bond between the zirconia and ceramic veneer induced more the stress concentration at the veneer layer in the finish line than when a perfect bond condition was simulated in an incisor crown (Rocha et al. 2011). It is worth mentioning the influence of the zirconia itself on the propagation of flaws from micro-porosities inherent to the ceramic body fabricated by the stratification technique, which suggests that veneers obtained by overpressing techniques may show better mechanical response in face of stress incidence (Taskonak et al. 2008), as well as bond quality between the veneer and the zirconia core (Aboushelib et al. 2008; Taskonak et al. 2008). Regarding the most or least influencing factor, it is true that the veneer is at risk of structural damage in case of failure between the veneer and the zirconia core. Structurally, the veneer represented in the present study suggests the use of ceramics obtained by overpressing technique, as the ceramic body shows no
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Figure 3. (a) Distribution of maximum principal stress (smax) of veneer for gold core model (MAu), (b) zirconia core with complete bond (MZi), and (c) zirconia core with incomplete bond (MZi-NL).
imperfection or micro-porosity inherent to the layering technique (Aboushelib et al. 2008). Although tensile stresses in imperfect bonded model indicate greater risk of structural failure, there are no clinical or experimental data to confirm this assumption, especially due to the difficulty in clinically or experimentally measuring the bond strength between the veneer and
the zirconia, according to literature data. It is worth highlighting that part of the failures reported by previous studies was justified by the inadequate ratios between the veneer and the zirconia cores (Couegnat et al. 2006). This is due to the fact that the zirconia allows getting thinner cores, favoring less dental reduction for tooth preparation. In contrast, it does not permit denying the proportion
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Figure 4. Maximum stress concentrations for maximum principal and equivalent von Mises stresses (smax ¼ S1 and svM ¼ VM), (smax ¼ S1 and svM ¼ VM) in all models (MAu, MZi, MZi-NL).
Figure 5.
Peak of maximum principal stress (smax) values for veneer in all models (MAu, MZi, MZi-NL).
between the core and veneer thicknesses. The ceramic resistance to fracture follows the same principle adopted for metal-based crowns, in which the uniformity of the veneer layer influences load resistance (Anusavice 1997; Aboushelib et al. 2008; Taskonak et al. 2008). It is worth considering that although the success of metal-ceramic crowns is supported by literature (Sailer et al. 2007), biocompatibility and esthetical properties have guided the scientific efforts for improving metal-free systems (Lin et al. 2001; Carracho and Razzoog 2006; Sadowsky 2006), which makes the association with RPD predictable. Nevertheless, this RPD association should be less diversified than that applied to conventional fixed prostheses with metal core, as some biomechanical concepts cannot be fully considered due to the zirconia limitations. It is known that zirconia undergoes phase
structural change in the presence of water, reducing its flexural resistance and becoming weakened by crystal degradation (Kohorst et al. 2008). Considering that a guide plane, or fabrication of an attachment, would continuously expose the zirconia surface to the moist oral environment, and this would likely result in a failure scenario. In line with this issue, Carracho and Razzoog (2006) reported a clinical case on the association of a RPD with zirconia-based fixed partial denture elements. The elements adjacent to the prosthetic area received crowns with rest seats and proximal guide planes. The surface of the guide planes remained in zirconia, which was polished, and the rest seats preparation suggested zirconia exposure. Two aspects may have been potentially able to reduce zirconia durability in that clinical study (Rekow et al.
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Figure 6.
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Peak of equivalent von Mises stress values for RPDP metallic support in all models (MAu, MZi, MZi-NL).
2006) zirconia exposure to the moist oral environment and its surface finishing which may have led to early flaws (Kohorst et al. 2008). In the present study, the rest seats were covered with ceramic, as this is the likely protocol to be adopted if this association occurs clinically, aiming at zirconia preservation. Another aspect that was not addressed in the present study but should influence the characteristics of the zirconia core in association with RPD is the use of an additional clasp. The presence of a retention arm may imply in more stress on the veneer by the friction of the clasp tip during every RPD insertion and removal movements, or even during rotation on the crown in cases of free-end RPD. In any situation, the veneer should provide enough retention for the clasp, being adequately
supported by the zirconia core according to the volume of ceramic material employed (Kancyper et al. 2000; Carracho and Razzoog 2006; Marchack et al. 2007). Finally, the issues involving stability of the ceramic veneer covering the zirconia should be further investigated and clarified, such as the bond quality between these materials. It is considered that in a deficient adherence scenario between the ceramic veneer/zirconia core, the first will be at structural risk, limiting its indication. Conclusions The incomplete bond between the ceramic veneer/zirconia interface significantly increased risks of structural failure on the veneer. When a complete bond between zirconia
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core and ceramic veneer was assumed, the all-ceramic crown showed similar stress concentration than gold-based crown.
Funding This study was supported by the Sa˜o Paulo Research Foundation (FAPESP, Brazil) [grant number 2008/00209-9]; Foundation for the Coordination of Higher Education and Graduate Training (CAPES, Brazil) [grant number BEX 2325-05-5].
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Zirconia-based dental crown to support a removable partial denture: a three-dimensional finite element analysis using contact elements and micro-CT data a
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Eduardo Passos Rocha , Rodolfo Bruniera Anchieta , Erika Oliveira de Almeida , Amilcar a
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Chagas Freitas Jr , Ana Paula Martini , Bruno Sales Sotto-Maior , Marco Antonio Luersen & d
Ching Chang Ko a
Department of Dental Materials and Prosthodontics, Araçatuba Dental School, UNESP – Sao Paulo State University, Araçatuba, Brazil b
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Department of Prosthodontics and Periodontology, UNICAMP – State University of Campinas, Piracicaba, Brazil c
Department of Mechanical Engineering, Technical Federal University of Parana, UTFPR, Curitiba, Brazil d
Department of Orthodontics, The University of North Carolina at Chapel Hill, Chapel Hill, NC, USA Published online: 21 Oct 2014.
To cite this article: Eduardo Passos Rocha, Rodolfo Bruniera Anchieta, Erika Oliveira de Almeida, Amilcar Chagas Freitas Jr, Ana Paula Martini, Bruno Sales Sotto-Maior, Marco Antonio Luersen & Ching Chang Ko (2014): Zirconia-based dental crown to support a removable partial denture: a three-dimensional finite element analysis using contact elements and micro-CT data, Computer Methods in Biomechanics and Biomedical Engineering, DOI: 10.1080/10255842.2014.951927 To link to this article: http://dx.doi.org/10.1080/10255842.2014.951927
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Computer Methods in Biomechanics and Biomedical Engineering, 2014 http://dx.doi.org/10.1080/10255842.2014.951927
Zirconia-based dental crown to support a removable partial denture: a three-dimensional finite element analysis using contact elements and micro-CT data Eduardo Passos Rochaa*, Rodolfo Bruniera Anchietaa, Erika Oliveira de Almeidaa, Amilcar Chagas Freitas Jra, Ana Paula Martinia, Bruno Sales Sotto-Maiorb, Marco Antonio Luersenc and Ching Chang Kod Department of Dental Materials and Prosthodontics, Arac atuba Dental School, UNESP – Sao Paulo State University, Arac atuba, Brazil; bDepartment of Prosthodontics and Periodontology, UNICAMP – State University of Campinas, Piracicaba, Brazil; c Department of Mechanical Engineering, Technical Federal University of Parana, UTFPR, Curitiba, Brazil; dDepartment of Orthodontics, The University of North Carolina at Chapel Hill, Chapel Hill, NC, USA
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(Received 16 July 2014; accepted 3 August 2014) Veneer fracture is the most common complication in zirconia-based restorations. The aim of this study was to evaluate the mechanical behavior of a zirconia-based crown in a lower canine tooth supporting removable partial denture (RPD) prosthesis, varying the bond quality of the veneer/coping interface. Microtomography (mCT) data of an extracted left lower canine were used to build the finite element model (M) varying the core material (gold core – MAu; zirconia core – MZi) and the quality of the veneer/core interface (complete bonded – MZi; incomplete bonded – MZi-NL). The incomplete bonding condition was only applied for zirconia coping by using contact elements (Target/Contact) with 0.3 frictional coefficients. Stress fields were obtained using Ansys Workbench 10.0. The loading condition (L ¼ 1 N) was vertically applied at the base of the RPD prosthesis metallic support towards the dental apex. Maximum principal (smax) and von Mises equivalent (svM) stresses were obtained. The smax (MPa) for the bonded condition was similar between gold and zirconia cores (MAu, 0.42; MZi, 0.40). The incomplete bonded condition (MZi-NL) raised smax in the veneer up to 800% (3.23 MPa) in contrast to the bonded condition. The peak of svM increased up to 270% in the MZi-NL. The incomplete bond condition increasing the stress in the veneer/zirconia interface. Keywords: removable partial denture; zirconium oxide; fixed partial denture; finite element analysis
Introduction The tooth support is essential for removable partial denture (RPD), regardless of the number of absent teeth. In order for the support to have an efficient function, it is ideal that the tooth be healthy, or if restored, it should be sufficiently preserved to receive stresses without risks of crown or root fractures that lead to teeth and prosthesis loss (McCracken 2004). There are clinical situations in which the structural condition of the remaining dental crown demands previous fabrication of a fixed partial prosthesis or crown to support the RPD. This procedure has been traditionally carried out with metal-ceramic fixed prosthesis, as it restores the supporting tooth ensuring the necessary stress resistance (Sailer et al. 2006, 2007; Thongthammachat-Thavornthanasarn 2007). In addition, the biomechanical aspects of the RPD, such as the need of stability, retention and one single insertion axis may be improved as rest seats and guiding planes may be incorporated to the total crown with metallic core. Actually, different ceramic systems with greater aesthetics and biocompatibility (Guo 1999; Tinschert et al. 2001; Rekow and Thompson 2007) are available for fabricating total crowns, the literature has presented RPDs
*Corresponding author. Email: [email protected] q 2014 Taylor & Francis
with supporting-teeth restored with metal-free total crowns constituted by aluminum or zirconium oxide cores, without damage to the tooth supporting capacity (Kancyper et al. 2000; Carracho and Razzoog 2006; Marchack et al. 2007; Fischer et al. 2009). However, the safety of the procedure using zirconia-based restorations has been largely questioned, due to clinical and laboratory behavior of the ceramic veneer over the zirconia core, which has been target of criticism, based on the type and incidence of failure reported in the literature (Raigrodski et al. 2006; Santana et al. 2009; Aboushelib et al. 2008; Kohorst et al. 2008; Farga-Nin˜oles et al. 2013), and this failure results, in most cases, in a replacement of restoration (Rafferty et al. 2010). It is unclear whether failures occur due to zirconia’s high elasticity modulus, which intensifies crack propagation starting from small flaws in the ceramic body (Rekow et al. 2006; Coelho et al. 2009), or due to thermal expansion coefficient differences between zirconia and ceramic, impairing ceramic veneer stability (Walter et al. 1999; Raigrodski et al. 2006; Denry and Kelly 2008). In addition, it might be supposed that these failures occur due to inadequate bonding between the veneer and zirconia core; however, this is an inconclusive aspect as
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failures are considered material dependent (Proos et al. 2002; Sailer et al. 2007; Dittmer et al. 2009). As regards to a crown with zirconia core to support a RPD, the aspects mentioned above are also inconclusive due to the lack of studies that evaluate the ceramic veneer behavior considering the specific profile that the core takes supporting biting function. The aim of this study was to evaluate the mechanical behavior of a zirconia-based crown in a lower canine tooth to support a RPD prosthesis, varying the bond quality of the veneer/core interface by the finite element method.
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Materials and methods This study was approved by the Human Research Ethics Committee at the Faculty of Dentistry of Arac atuba, Sa˜o
Paulo State University (UNESP). A maxillary central incisor from the human tooth bank was used to develop the models. Microtomographic images (mCT) of a left mandibular canine were obtained to build solid models for the present study. A total of 720 sections were obtained by scanning (CT40, Scanco Medical, Bassersdorf, Switzerland), 82 serial sections were used to rebuild the tooth in the software (SolidWorks, Dassault Syste`mes, SolidWorks Corp., Concord, MA). A geometric model was created with all structures inherent to the canine (enamel, dentin, and pulp chamber), except the cement layer. From the geometric model, the tooth was prepared for a fixed prosthesis complete crown (Figure 1). The buccal, lingual, and proximal reductions were approximately
Figure 1. (A) Canine tooth with dental reductions for fixed total crown prosthesis ((a) proximal view; (b) vestibular view; (c) incisal view). (B) View of prepared tooth and modified coping positioned (a), ceramic veneer (b), and metallic support positioned over crown (c).
Computer Methods in Biomechanics and Biomedical Engineering Table 1.
Mechanical properties of materials.
Material Zirconiun (Farga-Nin˜oles et al. 2013) Feldspathic ceramic (Farga-Nin˜oles et al. 2013) Chromium – cobalt alloy (Shillingburg et al. 1997) Gold (Hickel and Manhart 2001) Enamel (Taskonak et al. 2008) Dentin (Adanir and Belli 2007) Pulp (Magne et al. 2002) Periodontal ligament (Adanir and Belli 2007; Dejak and Mlotkowski 2008) Resin cement (Lin et al. 2001)
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1 mm, with 1.5-mm incisal reduction, and convergence of vertical walls at 68, with vestibular wear in two-planes (Shillingburg et al. 1997; Couegnat et al. 2006). The model had a large chamfer finish line. From this prepared model, three experimental models were created (Table 1). All models presented were modified core by simulation of lingual cingulum for rest seat: MAu, model restored with a metal-ceramic complete crown with gold core, with approximately 0.5- and 1.0-mm thickness in axial walls and incisal border, respectively, lingual cingulum rest seat covered by ceramic perfectly bonded to core as well as CoCr metallic RPD support positioned over rest seat; MZi, similar to MAu model, although with zirconia core perfectly bonded to veneer ceramic; MZi-NL, similar to MZi, although with zirconia/veneer interface showing contact elements of the type Target/Contact with 0.3 frictional coefficient allowing separation and sliding movements to characterize imperfect bond condition (Figure 2; O’Brien 2002).
Elastic modulus (GPa)
Poisson’s ratio
205 70 218 89.5 80 18.6 2 6.89 £ 1025 18.3
0.22 0.19 0.31 0.33 0.30 0.31 0.45 0.45 0.30
All models had comparable aspects in terms of a 50-mm layer of cementing agent (Panavia F, Kuraray Medical, Inc., Okayama, Japan) (Figure 1), and a periodontal ligament thickness of 0.25 mm were uniformly distributed around the canine root (Figure 2). Other comparable aspect was a homogeneous ceramic thickness of 0.45 mm were applied on the zirconia and gold cores, due to the uniformity of the ceramics which is a basic principle for maximum resistance (Anusavice 1997; Hickel and Manhart 2001; Sadowsky 2006; Rekow and Thompson 2007; Aboushelib et al. 2008; Taskonak et al. 2008). The models were developed in the SolidWorks 2010 software (SolidWorks Corp.) and exported to the finite element program (Ansys Workbench10.0, Swanson Analysis Inc., Houston, PA) in.igs (Initial Graphics Exchange Specification) format for numerical analysis. Mechanical properties (elastic modulus, E and Poisson’s ratio, n) were obtained from the literature
Figure 2. (a) View of contact interface between veneer and zirconia coping. Contact element of the type target and contact for zirconia surface and veneer surface, respectively. (b) Finite elements mesh. (c) Boundary condition applied to periodontal ligament surface (x ¼ y ¼ z ¼ 0). (d) Loading applied to metallic support.
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(Table 1; Yettram et al. 1977; Lin et al. 2001; Magne et al. 2002; O’Brien 2002; Lanza et al. 2005; Adanir and Belli 2007; Dejak and Mlotkowski 2008; Coelho et al. 2009) and MZi and MAu were considered to be homogeneous, isotropic, and linear elastic. All structures of these models were considered perfectly bonded (Asmussen et al. 2005; Sorrentino et al. 2007). An incomplete bond between the zirconia core and ceramic veneer was simulated for the MZi-NL model by allowing separation and sliding movements with a frictional coefficient of 0.3 (Tillitson et al. 1971). The finite element mesh generation was carried out according to convergence of analysis (6%; Huang et al. 2008) showing 0.2-mm tetrahedral elements. A total of 46,000 elements and 80,000 nodes were obtained in all models (Figure 2). The external surface of the periodontal ligament was fixed on x, y, and z coordinates for all models (x ¼ y ¼ z ¼ 0). A vertical loading of 1 N was applied to the metallic support base in contact with the ceramic to simulate the RPD load over the tooth (Rocha et al. 2011). The maximum principal stress (smax) and equivalent von Mises (svM) stress values were obtained for all models. According to Dejak and Mlotkowski (2008) the maximum principal stress is adequate to establish success predictability in friable materials (Anchieta et al. 2007; Martini et al. 2009) such as dental ceramics. To evaluate the stress on the RPD metallic support in contact with the ceramic, the von Mises stress was obtained due to the metallic materials which show a ductile behavior (Cattaneo et al. 2005).
Results The complete and incomplete conditions showed distinct maximum principal stress values for the ceramic veneer. For the gold core model (MAu), the smax was 0.42 MPa for the ceramic, similar to that obtained for the zirconia core (MZi, 0.40 MPa). When an incomplete bonded interface was considered between the veneer and the zirconia core (MZi-NL), the tensile stress occurred in the veneer located close to the angle between the RPD metallic support and minor connector (Figure 3), increased over 800% (3.23 MPa) (Figures 4– 6). The sliding movements, influenced by the non-linear analysis, make the von Mises stress concentration higher for the minor connector, with a difference of 270% between the two zirconia based restorations (MZi and MZi-NL) (Figures 4 – 6).
Discussion The previous experimental studies evaluated the mechanical behavior of the ceramic veneer under influence of
the supporting material. These studies were carried out according to protocols based on simplified models and obtained results that contributed to the improvement of materials that aid clinical applications (Carracho and Razzoog 2006; Sadowsky 2006). The present study employed computational methods to obtain data on the mechanical behavior of a zirconia-based crown supporting RPD prosthesis, which are difficult to reproduce in a physical model. It is quite impossible to employ laboratory analysis to get mechanical behavior data about the ceramic element in multiple layers and uniform dimension, based on different systems, besides the cementing of elements over a tooth with identical dimensions and structural characteristics, with a metallic support directing stresses over the crown. The results of the present study are in agreement with literature data on the incidence of failures in zirconia crowns. Chipping or complete delamination of the veneer ceramic has been reported (Marchack et al. 2007; Sailer et al. 2007). The understanding of such occurrence demands further studies, although the literature points out the incompatibility between thermal expansion coefficients as a strong cause of veneer fracturing (Kancyper et al. 2000). In addition, other possible aspects that may also influence failures are not sufficiently clear yet, such as the bond strength between the zirconia and the veneer, as tensile stress is observed in the veneer/zirconia interface (Fischer et al. 2009), which have been pointed out as consequence of thermal incompatibility (Fischer et al. 2009). Finally, structural veneer failures have occurred even in ceramic systems considered thermally compatible with the ceramic veneer (Marchack et al. 2007). Similar failures have not been observed in metal-based prosthesis, according to the incidence and frequency in which they occur in prosthesis with zirconia core (Magne et al. 2002). In another study, the authors performed by a numerical simulation, the reduced bond between the zirconia and ceramic veneer induced more the stress concentration at the veneer layer in the finish line than when a perfect bond condition was simulated in an incisor crown (Rocha et al. 2011). It is worth mentioning the influence of the zirconia itself on the propagation of flaws from micro-porosities inherent to the ceramic body fabricated by the stratification technique, which suggests that veneers obtained by overpressing techniques may show better mechanical response in face of stress incidence (Taskonak et al. 2008), as well as bond quality between the veneer and the zirconia core (Aboushelib et al. 2008; Taskonak et al. 2008). Regarding the most or least influencing factor, it is true that the veneer is at risk of structural damage in case of failure between the veneer and the zirconia core. Structurally, the veneer represented in the present study suggests the use of ceramics obtained by overpressing technique, as the ceramic body shows no
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Figure 3. (a) Distribution of maximum principal stress (smax) of veneer for gold core model (MAu), (b) zirconia core with complete bond (MZi), and (c) zirconia core with incomplete bond (MZi-NL).
imperfection or micro-porosity inherent to the layering technique (Aboushelib et al. 2008). Although tensile stresses in imperfect bonded model indicate greater risk of structural failure, there are no clinical or experimental data to confirm this assumption, especially due to the difficulty in clinically or experimentally measuring the bond strength between the veneer and
the zirconia, according to literature data. It is worth highlighting that part of the failures reported by previous studies was justified by the inadequate ratios between the veneer and the zirconia cores (Couegnat et al. 2006). This is due to the fact that the zirconia allows getting thinner cores, favoring less dental reduction for tooth preparation. In contrast, it does not permit denying the proportion
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Figure 4. Maximum stress concentrations for maximum principal and equivalent von Mises stresses (smax ¼ S1 and svM ¼ VM), (smax ¼ S1 and svM ¼ VM) in all models (MAu, MZi, MZi-NL).
Figure 5.
Peak of maximum principal stress (smax) values for veneer in all models (MAu, MZi, MZi-NL).
between the core and veneer thicknesses. The ceramic resistance to fracture follows the same principle adopted for metal-based crowns, in which the uniformity of the veneer layer influences load resistance (Anusavice 1997; Aboushelib et al. 2008; Taskonak et al. 2008). It is worth considering that although the success of metal-ceramic crowns is supported by literature (Sailer et al. 2007), biocompatibility and esthetical properties have guided the scientific efforts for improving metal-free systems (Lin et al. 2001; Carracho and Razzoog 2006; Sadowsky 2006), which makes the association with RPD predictable. Nevertheless, this RPD association should be less diversified than that applied to conventional fixed prostheses with metal core, as some biomechanical concepts cannot be fully considered due to the zirconia limitations. It is known that zirconia undergoes phase
structural change in the presence of water, reducing its flexural resistance and becoming weakened by crystal degradation (Kohorst et al. 2008). Considering that a guide plane, or fabrication of an attachment, would continuously expose the zirconia surface to the moist oral environment, and this would likely result in a failure scenario. In line with this issue, Carracho and Razzoog (2006) reported a clinical case on the association of a RPD with zirconia-based fixed partial denture elements. The elements adjacent to the prosthetic area received crowns with rest seats and proximal guide planes. The surface of the guide planes remained in zirconia, which was polished, and the rest seats preparation suggested zirconia exposure. Two aspects may have been potentially able to reduce zirconia durability in that clinical study (Rekow et al.
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Figure 6.
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Peak of equivalent von Mises stress values for RPDP metallic support in all models (MAu, MZi, MZi-NL).
2006) zirconia exposure to the moist oral environment and its surface finishing which may have led to early flaws (Kohorst et al. 2008). In the present study, the rest seats were covered with ceramic, as this is the likely protocol to be adopted if this association occurs clinically, aiming at zirconia preservation. Another aspect that was not addressed in the present study but should influence the characteristics of the zirconia core in association with RPD is the use of an additional clasp. The presence of a retention arm may imply in more stress on the veneer by the friction of the clasp tip during every RPD insertion and removal movements, or even during rotation on the crown in cases of free-end RPD. In any situation, the veneer should provide enough retention for the clasp, being adequately
supported by the zirconia core according to the volume of ceramic material employed (Kancyper et al. 2000; Carracho and Razzoog 2006; Marchack et al. 2007). Finally, the issues involving stability of the ceramic veneer covering the zirconia should be further investigated and clarified, such as the bond quality between these materials. It is considered that in a deficient adherence scenario between the ceramic veneer/zirconia core, the first will be at structural risk, limiting its indication. Conclusions The incomplete bond between the ceramic veneer/zirconia interface significantly increased risks of structural failure on the veneer. When a complete bond between zirconia
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core and ceramic veneer was assumed, the all-ceramic crown showed similar stress concentration than gold-based crown.
Funding This study was supported by the Sa˜o Paulo Research Foundation (FAPESP, Brazil) [grant number 2008/00209-9]; Foundation for the Coordination of Higher Education and Graduate Training (CAPES, Brazil) [grant number BEX 2325-05-5].
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