d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 234–241
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Fracture strength of machined ceramic crowns as a function of tooth preparation design and the elastic modulus of the cement Shirin Shahrbaf a,∗ , Richard van Noort a , Behnam Mirzakouchaki b , Elaheh Ghassemieh c , Nicolas Martin a a
The University of Sheffield, Academic Unit of Restorative Dentistry, The School of Clinical Dentistry, Claremont Crescent, Sheffield, UK b Tabriz University of Medical Sciences, Tabriz Dental School, Orthodontic Department, Tabriz, Iran c School of Mechanical and Aerospace Engineering, Queen’s University of Belfast, Ashby Building, Room 3020, Stranmillis Road, Belfast, BT9 5AH, UK
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. To determine, by means of static fracture testing the effect of the tooth preparation
Received 11 February 2013
design and the elastic modulus of the cement on the structural integrity of the cemented
Received in revised form
machined ceramic crown-tooth complex.
21 August 2013
Methods. Human maxillary extracted premolar teeth were prepared for all-ceramic crowns
Accepted 25 November 2013
using two preparation designs; a standard preparation in accordance with established protocols and a novel design with a flat occlusal design. All-ceramic feldspathic (Vita MK II) crowns were milled for all the preparations using a CAD/CAM system (CEREC-3). The machined all-
Keywords:
ceramic crowns were resin bonded to the tooth structure using one of three cements with
Maxillary premolar
different elastic moduli: Super-Bond C&B, Rely X Unicem and Panavia F 2.0. The specimens
CAD/CAM
were subjected to compressive force through a 4 mm diameter steel ball at a crosshead speed
Vita MK II
of 1 mm/min using a universal test machine (Loyds Instrument Model LRX.). The load at the
Anatomic design
fracture point was recorded for each specimen in Newtons (N). These values were compared
Flat design
to a control group of unprepared/unrestored teeth.
Elastic modulus
Results. There was a significant difference between the control group, with higher fracture
Resin luting agent
strength, and the cemented samples regardless of the occlusal design and the type of resin
Mechanical test
cement. There was no significant difference in mean fracture load between the two designs of occlusal preparation using Super-Bond C&B. For the Rely X Unicem and Panavia F 2.0 cements, the proposed preparation design with a flat occlusal morphology provides a system with increased fracture strength. Significance. The proposed novel flat design showed less dependency on the resin cement selection in relation to the fracture strength of the restored tooth. The choice of the cement resin, with respect to its modulus of elasticity, is more important in the anatomic design than in the flat design. © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗ Corresponding author at: Academic Unit of Restorative Dentistry, The School of Clinical Dentistry, Claremont Crescent, Sheffield, S10 2TA, UK. Tel.: +44 114 2717932; fax: +44 114 2265484. E-mail address: s.shahrbaf@sheffield.ac.uk (S. Shahrbaf). 0109-5641/$ – see front matter © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dental.2013.11.010
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 234–241
1.
Introduction
A major disadvantage of dental ceramic materials relates to their brittle nature owing to their atomic bonds that inhibits the atomic planes to slide apart when subjected to load [1]. The applied stresses resulting from masticatory loads are concentrated on the inherent flaws that exist in the ceramic, which have the ability to amplify an applied stress leading to rapid crack propagation that can in turn lead to brittle fracture of the ceramics [2]. As a result, it is probable that, ceramic materials fracture at a fraction of their theoretical strength due to the flaws’ stress-raising effect [3,4]. It has been shown that porosities and microcracks are the sites of fracture initiation [5]. The inherent properties of the ceramic crown, as a standalone item, are of limited interest as it is the overall strength of restored tooth-crown complex that is clinically relevant. The materials used, their geometrical configuration and the manner in which they are integrated and joined to each other and to the dental tissues determine the ability of the restored tooth to withstand the occlusal stresses placed upon them. Thus, in addition to the properties of the ceramic material, the performance of the other constituent parts of the compound system should be considered, principally: the geometrical configuration of the crown (thickness and occlusal cuspal morphology); the quality of the established bond between the all-ceramic crown, the resin cement and the dentin structure; the characteristics of the adhesive lute (dimensions and elastic modulus) and the amount and quality of the remaining dentin structure (support and preservation of pulp vitality). Moreover, while some ceramic systems have truly impressive fracture strength properties (e.g. zirconia core crowns), this is at the expense of aggressive tooth preparations (1.5 mm shoulder and 3 mm occlusal reduction) that compromise the amount of remaining dentin support and pulp vitality. Considering the ceramic material per se, a wide range of ceramic systems are currently available to select from, based on an equally wide range of fabrication technologies. These ceramics, have been shown to have a fracture strength value that should resist normal functional occlusal loads (150–665 N) [6]; ranging from 772.3 N for machined feldspathic ceramics to 1000 N for zirconia machined crowns [7,8]. Machinable ceramics, using CAD/CAM technology, are of interest as they are homogenous and stronger, than conventional sintered porcelains where voids, flaws, and cracks are reduced to minimum and the effects of distortion or shrinkage have been omitted [9]. Moreover, CAD/CAM systems, increase production predictability and reduce the working process and the production cost. The configuration of the crown (wall and occlusal thickness), as advocated by the manufacturers, is designed with an element of built-in ‘insurance’ (over-compensation) so that its fracture strength is optimized as a stand-alone item, with less regard for other elements of the crown-tooth complex. This increase in the thickness and overall dimensions of the ceramic walls is undertaken at the expense of conservation of tooth structure and preservation of tooth vitality. The actual geometry of the crown, in particular the shape of the tooth preparation design is based on historical empirical design
235
configurations for non-adhesive full coverage crowns as advocated for cast metal and ceramo-metal restorations [10,11]. To date, little consideration has been given to the performance of all-ceramic adhesive crowns as part of a restored tooth-crown complex and how this can be optimized for the preservation of tooth structure. There is evidence to suggest that the geometry of the crown and the stiffness distribution within it also appears to have an effect on the distribution of the stresses within the tooth-crown complex [12,13]. The propagation of cracks in the ceramic crown is affected by the support offered from the underlying tougher and more elastic structures; the cement and the underlying dentin. The adhesive nature of resin-based cements has the effect of covering the internal surfaces of microcracks and small defects of the ceramic restorations; microcracks are thus blunted and inhibited from propagating [14]. In this way, evidence suggests that resin-based adhesive luting agents that bond to the tooth structure and the ceramic restoration can increase the fracture resistance of the restoration complex [15,16]. The overall dimensions and physical characteristics of the adhesive lute may also play an important role in stress distribution and crack propagation within the overlying ceramic restoration [17]. However, while various resin cements have been advocated for cementation of all-ceramic crowns there is neither guidance nor consensus in the literature regarding the ideal parameters for optimum performance throughout the tooth restored with an all-ceramic crown. Thickness of the cement layer on the stress levels within the crown depends on the nature of the cement; for a thick cement lute, the stress development is faster if this is a glass ionomer and slower with a resin cement [18]; which may be on account of the difference of the elastic moduli of these materials. Also, for an elastic resin-based lute the thickness is only an important determining parameter of fracture strength of machined ceramics when this exceeds 300 m [17,19]. Concerning the elastic modulus of the lute, the fracture resistance of ceramic crowns is highly influenced by the high elastic modulus substrates [20]. Moreover, high elastic modulus resin cements, assuming that they have acceptable viscosity and thickness, have the best performance in terms of all-ceramic crown survival [21]. It is a more clinically relevant research question to consider how these parameters, in isolation or combined, affect the fracture strength of an adhesively bonded ceramic crowntooth system with due consideration to the preservation of tooth structure. The aim of this in vitro investigation was to determine, by means of static fracture testing the effect of tooth preparation design and the elastic modulus of the cement on the structural integrity of the cemented machined ceramic crowntooth complex. The effect of the cement elastic modulus and the tooth preparation design on the stress state of this same ceramic crown-tooth complex has been investigated by the authors in an earlier study by means of Finite Element Analysis (FEA) [22]. The FEA study revealed significant differences in the stress state that occurs in the crown-tooth complex as a result of both the crown design and the elastic modulus of the cement. A machined feldspathic ceramic has been selected as a popular machinable ceramic with a low documented fracture
236
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 234–241
The “Correlation mode” of the CEREC CAD/CAM (Software version v3.6; SIRONA Dental Systems) was used to replicate the occlusal morphology of the unprepared teeth for the CEREC crowns. All teeth were coated with an optical scanning powder (CEREC powder VITA, Zahnfabric, Germany) before tooth preparation. The samples were fixed to a special base plate provided by the SIRONA Company and an optical impression of the tooth was taken with the inEos laser scanner (SIRONA Dental Systems, GmbH). The models for the different preparations were correlated by placing notches on the acrylic base
Panavia F 2.0 Kuraray Medical, Inc. Dual-polymerized phosphate-modified resin cement 18.3 [24] 1.8 [24]
Crown fabrication
Elastic modulus of cement (GPa)
2.2.
Cement chemistry
Ethical approval was obtained from the University of Sheffield for this study. Extracted human maxillary premolar teeth were selected for the purpose of this investigation. The teeth were cleaned of any calculus deposits and soft tissue with hand scalers and stored in 0.9% normal saline solution. The teeth were measured in Bucco-Lingual (BL, 9.46 ± 0.11), OcclusoGingival (OG, 5.12 ± 0.09), and Mesio-distal (MD, 7.44 ± 0.10) directions using a digital calliper (Mitutoyo, Japan), and the teeth with the dimensions beyond these ranges were excluded from the study. Seventy caries free and crack free teeth with the accepted dimensions were implemented. The teeth were randomly divided into seven groups of 10 teeth each (n = 10). Ten unprepared and unrestored teeth were kept and tested as a control group (n = 10). The remaining 60 teeth were assigned to two equal groups (n = 30), according to the occlusal reduction design: Group #1 Anatomic reduction and Group #2 Flat occlusal reduction (Table 1). Each group of 30 was further sub-divided into three groups (n = 10) in accordance with three different cementation protocols (Table 2). All the specimens were mounted vertically in cylindrical moulds with the roots set in acrylic resin up to the amelo-cemental junction and the long axis of the crown perpendicular to the horizontal plane. Two different designs of preparation were chosen for this experiment: 1. A standard anatomic occlusal reduction for Group #1 (Fig. 1a) and 2. A flat occlusal preparation for Group #2, in accordance with the guidelines detailed in Table 1.
Rely X Unicem 3M ESPE USA Dual-polymerized self-adhesive universal resin cement 8 [25]
Tooth preparation
Cement material
2.1.
Dentin bonding
Materials and methods
Table 2 – Sub-groups according to cementation protocols.
2.
Rely X Unicem (Group RX)
strength that can be reproduced in a predictable manner; this way enabling an assessment of the effect of the tooth preparation geometry, the occlusal ceramic thickness and cement modulus, assuming an adhesive interface.
Clearfil Porcelain Bond Activator Kuraray Medical, Inc.
Axial reduction Finishing line Taper
Rely X Ceramic Primer 3M ESPE USA
1.2 mm from occlusal center 1.2–1.5 mm 1 mm shoulder 12◦
Silane (Ceramic primer)
2 mm from occlusal center 1.2–1.5 mm 1 mm shoulder 12◦
Vita Ceramics Etch Vita ceramics etch, Vita Zahnfabrik, Bad Sackingen, Germany Porcelain Liner M Sun Medical, Tokyo, Japan Heliobond Ivoclar Vivadent Super-Bond C&B Sun Medical, Tokyo, Japan Self-cure dental adhesive resin cement
Occlusal
Hydrofluoric acid gel (5%)
Flat occlusal reduction
Super-Bond C&B (Group SB)
Anatomic occlusal reduction
Group
Area of reduction
Panavia F 2.0 (Group PN)
Table 1 – Preparation guidelines.
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 234–241
237
correct crown morphology. The marginal fit of the crowns was measured using a traveling microscope (MitutoyoTM ) at 30× magnification at four different points in the middle of the buccal, lingual, mesial, and distal wall. Only one crown fell outside these parameters and a replacement was milled. The ceramic thickness for each crown was standardized so that the ceramic thickness at the central occlusal fossae for Group #1 was verified to have a thickness of 2 mm and for Group #2 this was1.2 mm. The milled crowns were glazed using an Akzent glazing kit (Vita Zahnfabrik, Bad Sackingen, Germany) to increase the strength of the ceramic following the manufacturer’s recommendations of pre-drying at a temperature of 600 ◦ C, followed by a temperature increase at the rate of 58 ◦ C/min with closing time of 6 min and a final firing temperature of 950 ◦ C with a holding time of 1 min.
2.3.
Fig. 1 – a: Anatomic design of tooth preparation for all-ceramic crown. b: Flat design of tooth preparation for all-ceramic crown.
to allow reproducing the original tooth morphology for the milled crown. After completion of the preparations, the specimens were powdered and scanned in the same manner as the unprepared ones. Before designing the crowns, the spacer value in the CEREC software design parameters was set to 30 m [23]. The restorations were designed using the CEREC inLab software version 3.6. The “Correlation mode” was selected for the purpose of designing the crown. The crown for the anatomic preparation was designed with 2 mm thickness over whole of the occlusal ceramic thickness (over the central fissure and over the cusps). In the flat design, the thickness of ceramic in the central fissure was 1.2 mm and 2 mm over the cusps. The thickness of the crowns was standardized with the software, so that crowns of the same preparation design would have the same dimensions. The “Endo mode” was selected for the milling process using the cylindrical bur (REF 58 55 734) and the step bur (REF 60 89 010) according to the manufacturer’s instructions. The Vita Mark II feldspathic ceramic blocks for CEREC/inLab (shade A 2, Vita Mark II, 2M2C I12, 12 mm long, Vita Zahnfabrik, Bad Sackingen, Germany) were used for milling. The cutting diamond burs were changed after milling 10 crowns (five for each group); and the milling unit was calibrated using the CEREC calibration kit at the beginning of the study and whenever the computer software requested it. Each milled crown was evaluated by means of visual examination (with loupes, ×4 magnification) to exclude any unfavorable restorations from the study. The inclusion criteria were: marginal gap of 0.05) (Table 6). However, within each group, the samples restored with Super-Bond C&B demonstrated a significant difference with the other specimens (Anatomic, p = 0.006; Flat, p = 0.019) as shown in Fig. 6.
5.
Discussion
Much of the focus of research on the fracture strength of the tooth-ceramic crown complex has been on the ceramic
itself, with little attention given to the effect of the preparation design and the elastic modulus of the supporting cement. This study sought to address the specific research question of the effect of crown preparation design and elastic modulus of the cement on the fracture strength of the crown-tooth complex. A static load fracture test has been employed as this is considered to be appropriate for this primary question, without the addition of confounding variables brought from fatigue testing. This static load has an occlusal/axial vector, which is considered to be the most common method for assessing structural integrity [7,28,29]. The cements were selected in accordance with their elastic modulus values. Two of the cements had elastic moduli values of 1.8–18.3 GPa, which were set as the min and max values found in commercially available materials. A third cement with an intermediate value of 8 GPa was also selected to provide a more even range. A new preparation, with a more conservative occlusal reduction, is proposed and compared to the widely accepted traditional anatomical design. The effect of occlusal preparation design showed that the flat design had the higher fracture strength for each cementation group. The highest value for the fracture strength was recorded for the flat design in the Panavia F 2.0 cemented specimens [833 N (±147.5)]. The lowest fracture strength value was recorded for the anatomic design crowns cemented with the Rely X Unicem [407.7 N
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d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 234–241
(±102.7)]. The statistical analysis of the results showed that there was a statistically significant difference between the two preparation groups when Rely X Unicem and Panavia F 2.0 cements were used while there was no statistical difference when the Super-Bond C&B cement was used. A possible explanation of the difference observed between preparation designs in the Panavia F 2.0, Rely X Unicem and Super-Bond C&B cement groups could be related to the viscosity and the fracture strength of the cement itself. However, considering the differences between groups and as the aforementioned parameters were fixed in both designs, the variability in thickness of cement is considered to be a more probable cause. For the flat occlusal reduction design, the interface adaptation was more homogenous than that of the anatomic occlusal reduction [30]. It is assumed that the CAD/CAM burs are not able to mill the complex internal morphology of the anatomic design due to the complex morphology internally. The proposed flat design of occlusal preparation demonstrated a more conservative tooth tissue reduction [30]. Moreover, a finite element analysis study undertaken by the authors has shown that the stress values in the cement and dentin structures of the flat design of occlusal reduction was more benign and homogenous stress state in the cement and dentin layers in comparison with the anatomic design [30]. In this study, the correlation between preparation design and elastic modulus was assessed but not the effect of other possible but subtler contributing factors such as the mechanism of bonding with the dentin and the ceramic. While there is a distinct variation in the bonding mechanisms achieved by these adhesive lute systems, their mode of action and performance is well documented and considered to be clinically acceptable [31–34]. Inclusion of this parameter and the possible effect of time-dependent adhesion degradation would warrant consideration in a longer-term fatigue study. In terms of fracture mode, none of the specimens demonstrated a complete separation of the cement from the dentin or the ceramic, suggesting that the failure was not due to an adhesive failure of the cement–dentin or cement–ceramic interfaces. The crowns cemented with Super-Bond C&B there was no significant difference in the fracture mode between the two occlusal designs preparations, but both did exhibit a higher incidence of codes 4 and 5, indicating a simultaneous catastrophic fracture of all the substrates in the crown-tooth complex. The different behavior of Super-Bond C&B to the other two cements may be attributed to its particularly low elastic modulus that suggests that there is a more rapid distribution of stresses to the underlying dentin. These results for Super-Bond C&B are in accordance with the study of Attia and Kern that reported a mean fracture strength of 680.5 N (±183.5) for the restored tooth with machined Vita Mark II ceramic blocks cemented with Super-Bond C&B [35]. They also used natural maxillary premolar teeth, with a 2 mm anatomic design occlusal reduction. An all-ceramic crown preparation is an aggressive approach in terms of tooth reduction, so that the resultant fracture strength in all specimens was significantly lower compared to that of the natural teeth. It has been suggested that the restorations routinely experience masticatory forces ranging from 150 N to 665 N depending upon the location of tooth
[6]. The average occlusal force applied to maxillary premolars is about 300 N [36]. Hence according to this study the three different adhesive luting agents used to cement Vita Mark II CAD/CAM ceramic crowns would have been able to sustain masticatory forces. In a study carried out by Zahran et al., the mean fracture load of Vita Mark II machined ceramic crown cemented with Panavia F 2.0 was 1272 N (±109) [37]. In this study the value of 833.4 N (±147.5) was found as the mean fracture load for the flat occlusal reduction design cemented with Panavia F 2.0. These differences may be related to the difference in methodology, Zahran et al. used the molar typodont teeth with 1.5 mm flat occlusal reduction, while the natural tooth has been used in the current study with a 1.2 mm flat occlusal reduction. The results of this investigation, suggest that with machined ceramic crowns, a flat occlusal preparation design is a more benign configuration. The effect of the elastic modulus of the cement is not clear, but does appear to be related to the geometric configuration of the internal surface of the crown. Static fracture testing, as used in this investigation, is able to discriminate between the different crown-tooth systems, suggesting the potential for more conservative occlusal reduction preparations and the need to consider the stiffness of the cement. Further extrapolations are not possible from this static fracture-test investigation. The effect of fatigue is considered as a contributing factor to predict the long-term success of a restoration reliably [38]. The use of a chewing simulator test that enables an analysis of fatigue as a simulation of the oral environment would be an appropriate further investigative stage to further our understanding of the performance of these systems prior to in vivo investigations.
6.
Conclusions
This investigation showed that both the design of the crown preparation and the elastic modulus of the lute cement have an effect on the structural integrity of the restored tooth-crown complex. The teeth restored with crowns of a flat occlusal preparation design exhibited the highest fracture strength and showed the least dependency on the resin cement selection. The flat occlusal preparation design together with the highest modulus cement (Panavia F 2.0; Kuraray Medical, Inc.) revealed the greatest structural integrity of the restored system.
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Fracture strength of machined ceramic crowns as a function of tooth preparation design and the elastic modulus of the cement Shirin Shahrbaf a,∗ , Richard van Noort a , Behnam Mirzakouchaki b , Elaheh Ghassemieh c , Nicolas Martin a a
The University of Sheffield, Academic Unit of Restorative Dentistry, The School of Clinical Dentistry, Claremont Crescent, Sheffield, UK b Tabriz University of Medical Sciences, Tabriz Dental School, Orthodontic Department, Tabriz, Iran c School of Mechanical and Aerospace Engineering, Queen’s University of Belfast, Ashby Building, Room 3020, Stranmillis Road, Belfast, BT9 5AH, UK
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. To determine, by means of static fracture testing the effect of the tooth preparation
Received 11 February 2013
design and the elastic modulus of the cement on the structural integrity of the cemented
Received in revised form
machined ceramic crown-tooth complex.
21 August 2013
Methods. Human maxillary extracted premolar teeth were prepared for all-ceramic crowns
Accepted 25 November 2013
using two preparation designs; a standard preparation in accordance with established protocols and a novel design with a flat occlusal design. All-ceramic feldspathic (Vita MK II) crowns were milled for all the preparations using a CAD/CAM system (CEREC-3). The machined all-
Keywords:
ceramic crowns were resin bonded to the tooth structure using one of three cements with
Maxillary premolar
different elastic moduli: Super-Bond C&B, Rely X Unicem and Panavia F 2.0. The specimens
CAD/CAM
were subjected to compressive force through a 4 mm diameter steel ball at a crosshead speed
Vita MK II
of 1 mm/min using a universal test machine (Loyds Instrument Model LRX.). The load at the
Anatomic design
fracture point was recorded for each specimen in Newtons (N). These values were compared
Flat design
to a control group of unprepared/unrestored teeth.
Elastic modulus
Results. There was a significant difference between the control group, with higher fracture
Resin luting agent
strength, and the cemented samples regardless of the occlusal design and the type of resin
Mechanical test
cement. There was no significant difference in mean fracture load between the two designs of occlusal preparation using Super-Bond C&B. For the Rely X Unicem and Panavia F 2.0 cements, the proposed preparation design with a flat occlusal morphology provides a system with increased fracture strength. Significance. The proposed novel flat design showed less dependency on the resin cement selection in relation to the fracture strength of the restored tooth. The choice of the cement resin, with respect to its modulus of elasticity, is more important in the anatomic design than in the flat design. © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗ Corresponding author at: Academic Unit of Restorative Dentistry, The School of Clinical Dentistry, Claremont Crescent, Sheffield, S10 2TA, UK. Tel.: +44 114 2717932; fax: +44 114 2265484. E-mail address: s.shahrbaf@sheffield.ac.uk (S. Shahrbaf). 0109-5641/$ – see front matter © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dental.2013.11.010
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1.
Introduction
A major disadvantage of dental ceramic materials relates to their brittle nature owing to their atomic bonds that inhibits the atomic planes to slide apart when subjected to load [1]. The applied stresses resulting from masticatory loads are concentrated on the inherent flaws that exist in the ceramic, which have the ability to amplify an applied stress leading to rapid crack propagation that can in turn lead to brittle fracture of the ceramics [2]. As a result, it is probable that, ceramic materials fracture at a fraction of their theoretical strength due to the flaws’ stress-raising effect [3,4]. It has been shown that porosities and microcracks are the sites of fracture initiation [5]. The inherent properties of the ceramic crown, as a standalone item, are of limited interest as it is the overall strength of restored tooth-crown complex that is clinically relevant. The materials used, their geometrical configuration and the manner in which they are integrated and joined to each other and to the dental tissues determine the ability of the restored tooth to withstand the occlusal stresses placed upon them. Thus, in addition to the properties of the ceramic material, the performance of the other constituent parts of the compound system should be considered, principally: the geometrical configuration of the crown (thickness and occlusal cuspal morphology); the quality of the established bond between the all-ceramic crown, the resin cement and the dentin structure; the characteristics of the adhesive lute (dimensions and elastic modulus) and the amount and quality of the remaining dentin structure (support and preservation of pulp vitality). Moreover, while some ceramic systems have truly impressive fracture strength properties (e.g. zirconia core crowns), this is at the expense of aggressive tooth preparations (1.5 mm shoulder and 3 mm occlusal reduction) that compromise the amount of remaining dentin support and pulp vitality. Considering the ceramic material per se, a wide range of ceramic systems are currently available to select from, based on an equally wide range of fabrication technologies. These ceramics, have been shown to have a fracture strength value that should resist normal functional occlusal loads (150–665 N) [6]; ranging from 772.3 N for machined feldspathic ceramics to 1000 N for zirconia machined crowns [7,8]. Machinable ceramics, using CAD/CAM technology, are of interest as they are homogenous and stronger, than conventional sintered porcelains where voids, flaws, and cracks are reduced to minimum and the effects of distortion or shrinkage have been omitted [9]. Moreover, CAD/CAM systems, increase production predictability and reduce the working process and the production cost. The configuration of the crown (wall and occlusal thickness), as advocated by the manufacturers, is designed with an element of built-in ‘insurance’ (over-compensation) so that its fracture strength is optimized as a stand-alone item, with less regard for other elements of the crown-tooth complex. This increase in the thickness and overall dimensions of the ceramic walls is undertaken at the expense of conservation of tooth structure and preservation of tooth vitality. The actual geometry of the crown, in particular the shape of the tooth preparation design is based on historical empirical design
235
configurations for non-adhesive full coverage crowns as advocated for cast metal and ceramo-metal restorations [10,11]. To date, little consideration has been given to the performance of all-ceramic adhesive crowns as part of a restored tooth-crown complex and how this can be optimized for the preservation of tooth structure. There is evidence to suggest that the geometry of the crown and the stiffness distribution within it also appears to have an effect on the distribution of the stresses within the tooth-crown complex [12,13]. The propagation of cracks in the ceramic crown is affected by the support offered from the underlying tougher and more elastic structures; the cement and the underlying dentin. The adhesive nature of resin-based cements has the effect of covering the internal surfaces of microcracks and small defects of the ceramic restorations; microcracks are thus blunted and inhibited from propagating [14]. In this way, evidence suggests that resin-based adhesive luting agents that bond to the tooth structure and the ceramic restoration can increase the fracture resistance of the restoration complex [15,16]. The overall dimensions and physical characteristics of the adhesive lute may also play an important role in stress distribution and crack propagation within the overlying ceramic restoration [17]. However, while various resin cements have been advocated for cementation of all-ceramic crowns there is neither guidance nor consensus in the literature regarding the ideal parameters for optimum performance throughout the tooth restored with an all-ceramic crown. Thickness of the cement layer on the stress levels within the crown depends on the nature of the cement; for a thick cement lute, the stress development is faster if this is a glass ionomer and slower with a resin cement [18]; which may be on account of the difference of the elastic moduli of these materials. Also, for an elastic resin-based lute the thickness is only an important determining parameter of fracture strength of machined ceramics when this exceeds 300 m [17,19]. Concerning the elastic modulus of the lute, the fracture resistance of ceramic crowns is highly influenced by the high elastic modulus substrates [20]. Moreover, high elastic modulus resin cements, assuming that they have acceptable viscosity and thickness, have the best performance in terms of all-ceramic crown survival [21]. It is a more clinically relevant research question to consider how these parameters, in isolation or combined, affect the fracture strength of an adhesively bonded ceramic crowntooth system with due consideration to the preservation of tooth structure. The aim of this in vitro investigation was to determine, by means of static fracture testing the effect of tooth preparation design and the elastic modulus of the cement on the structural integrity of the cemented machined ceramic crowntooth complex. The effect of the cement elastic modulus and the tooth preparation design on the stress state of this same ceramic crown-tooth complex has been investigated by the authors in an earlier study by means of Finite Element Analysis (FEA) [22]. The FEA study revealed significant differences in the stress state that occurs in the crown-tooth complex as a result of both the crown design and the elastic modulus of the cement. A machined feldspathic ceramic has been selected as a popular machinable ceramic with a low documented fracture
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The “Correlation mode” of the CEREC CAD/CAM (Software version v3.6; SIRONA Dental Systems) was used to replicate the occlusal morphology of the unprepared teeth for the CEREC crowns. All teeth were coated with an optical scanning powder (CEREC powder VITA, Zahnfabric, Germany) before tooth preparation. The samples were fixed to a special base plate provided by the SIRONA Company and an optical impression of the tooth was taken with the inEos laser scanner (SIRONA Dental Systems, GmbH). The models for the different preparations were correlated by placing notches on the acrylic base
Panavia F 2.0 Kuraray Medical, Inc. Dual-polymerized phosphate-modified resin cement 18.3 [24] 1.8 [24]
Crown fabrication
Elastic modulus of cement (GPa)
2.2.
Cement chemistry
Ethical approval was obtained from the University of Sheffield for this study. Extracted human maxillary premolar teeth were selected for the purpose of this investigation. The teeth were cleaned of any calculus deposits and soft tissue with hand scalers and stored in 0.9% normal saline solution. The teeth were measured in Bucco-Lingual (BL, 9.46 ± 0.11), OcclusoGingival (OG, 5.12 ± 0.09), and Mesio-distal (MD, 7.44 ± 0.10) directions using a digital calliper (Mitutoyo, Japan), and the teeth with the dimensions beyond these ranges were excluded from the study. Seventy caries free and crack free teeth with the accepted dimensions were implemented. The teeth were randomly divided into seven groups of 10 teeth each (n = 10). Ten unprepared and unrestored teeth were kept and tested as a control group (n = 10). The remaining 60 teeth were assigned to two equal groups (n = 30), according to the occlusal reduction design: Group #1 Anatomic reduction and Group #2 Flat occlusal reduction (Table 1). Each group of 30 was further sub-divided into three groups (n = 10) in accordance with three different cementation protocols (Table 2). All the specimens were mounted vertically in cylindrical moulds with the roots set in acrylic resin up to the amelo-cemental junction and the long axis of the crown perpendicular to the horizontal plane. Two different designs of preparation were chosen for this experiment: 1. A standard anatomic occlusal reduction for Group #1 (Fig. 1a) and 2. A flat occlusal preparation for Group #2, in accordance with the guidelines detailed in Table 1.
Rely X Unicem 3M ESPE USA Dual-polymerized self-adhesive universal resin cement 8 [25]
Tooth preparation
Cement material
2.1.
Dentin bonding
Materials and methods
Table 2 – Sub-groups according to cementation protocols.
2.
Rely X Unicem (Group RX)
strength that can be reproduced in a predictable manner; this way enabling an assessment of the effect of the tooth preparation geometry, the occlusal ceramic thickness and cement modulus, assuming an adhesive interface.
Clearfil Porcelain Bond Activator Kuraray Medical, Inc.
Axial reduction Finishing line Taper
Rely X Ceramic Primer 3M ESPE USA
1.2 mm from occlusal center 1.2–1.5 mm 1 mm shoulder 12◦
Silane (Ceramic primer)
2 mm from occlusal center 1.2–1.5 mm 1 mm shoulder 12◦
Vita Ceramics Etch Vita ceramics etch, Vita Zahnfabrik, Bad Sackingen, Germany Porcelain Liner M Sun Medical, Tokyo, Japan Heliobond Ivoclar Vivadent Super-Bond C&B Sun Medical, Tokyo, Japan Self-cure dental adhesive resin cement
Occlusal
Hydrofluoric acid gel (5%)
Flat occlusal reduction
Super-Bond C&B (Group SB)
Anatomic occlusal reduction
Group
Area of reduction
Panavia F 2.0 (Group PN)
Table 1 – Preparation guidelines.
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 234–241
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correct crown morphology. The marginal fit of the crowns was measured using a traveling microscope (MitutoyoTM ) at 30× magnification at four different points in the middle of the buccal, lingual, mesial, and distal wall. Only one crown fell outside these parameters and a replacement was milled. The ceramic thickness for each crown was standardized so that the ceramic thickness at the central occlusal fossae for Group #1 was verified to have a thickness of 2 mm and for Group #2 this was1.2 mm. The milled crowns were glazed using an Akzent glazing kit (Vita Zahnfabrik, Bad Sackingen, Germany) to increase the strength of the ceramic following the manufacturer’s recommendations of pre-drying at a temperature of 600 ◦ C, followed by a temperature increase at the rate of 58 ◦ C/min with closing time of 6 min and a final firing temperature of 950 ◦ C with a holding time of 1 min.
2.3.
Fig. 1 – a: Anatomic design of tooth preparation for all-ceramic crown. b: Flat design of tooth preparation for all-ceramic crown.
to allow reproducing the original tooth morphology for the milled crown. After completion of the preparations, the specimens were powdered and scanned in the same manner as the unprepared ones. Before designing the crowns, the spacer value in the CEREC software design parameters was set to 30 m [23]. The restorations were designed using the CEREC inLab software version 3.6. The “Correlation mode” was selected for the purpose of designing the crown. The crown for the anatomic preparation was designed with 2 mm thickness over whole of the occlusal ceramic thickness (over the central fissure and over the cusps). In the flat design, the thickness of ceramic in the central fissure was 1.2 mm and 2 mm over the cusps. The thickness of the crowns was standardized with the software, so that crowns of the same preparation design would have the same dimensions. The “Endo mode” was selected for the milling process using the cylindrical bur (REF 58 55 734) and the step bur (REF 60 89 010) according to the manufacturer’s instructions. The Vita Mark II feldspathic ceramic blocks for CEREC/inLab (shade A 2, Vita Mark II, 2M2C I12, 12 mm long, Vita Zahnfabrik, Bad Sackingen, Germany) were used for milling. The cutting diamond burs were changed after milling 10 crowns (five for each group); and the milling unit was calibrated using the CEREC calibration kit at the beginning of the study and whenever the computer software requested it. Each milled crown was evaluated by means of visual examination (with loupes, ×4 magnification) to exclude any unfavorable restorations from the study. The inclusion criteria were: marginal gap of 0.05) (Table 6). However, within each group, the samples restored with Super-Bond C&B demonstrated a significant difference with the other specimens (Anatomic, p = 0.006; Flat, p = 0.019) as shown in Fig. 6.
5.
Discussion
Much of the focus of research on the fracture strength of the tooth-ceramic crown complex has been on the ceramic
itself, with little attention given to the effect of the preparation design and the elastic modulus of the supporting cement. This study sought to address the specific research question of the effect of crown preparation design and elastic modulus of the cement on the fracture strength of the crown-tooth complex. A static load fracture test has been employed as this is considered to be appropriate for this primary question, without the addition of confounding variables brought from fatigue testing. This static load has an occlusal/axial vector, which is considered to be the most common method for assessing structural integrity [7,28,29]. The cements were selected in accordance with their elastic modulus values. Two of the cements had elastic moduli values of 1.8–18.3 GPa, which were set as the min and max values found in commercially available materials. A third cement with an intermediate value of 8 GPa was also selected to provide a more even range. A new preparation, with a more conservative occlusal reduction, is proposed and compared to the widely accepted traditional anatomical design. The effect of occlusal preparation design showed that the flat design had the higher fracture strength for each cementation group. The highest value for the fracture strength was recorded for the flat design in the Panavia F 2.0 cemented specimens [833 N (±147.5)]. The lowest fracture strength value was recorded for the anatomic design crowns cemented with the Rely X Unicem [407.7 N
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d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 234–241
(±102.7)]. The statistical analysis of the results showed that there was a statistically significant difference between the two preparation groups when Rely X Unicem and Panavia F 2.0 cements were used while there was no statistical difference when the Super-Bond C&B cement was used. A possible explanation of the difference observed between preparation designs in the Panavia F 2.0, Rely X Unicem and Super-Bond C&B cement groups could be related to the viscosity and the fracture strength of the cement itself. However, considering the differences between groups and as the aforementioned parameters were fixed in both designs, the variability in thickness of cement is considered to be a more probable cause. For the flat occlusal reduction design, the interface adaptation was more homogenous than that of the anatomic occlusal reduction [30]. It is assumed that the CAD/CAM burs are not able to mill the complex internal morphology of the anatomic design due to the complex morphology internally. The proposed flat design of occlusal preparation demonstrated a more conservative tooth tissue reduction [30]. Moreover, a finite element analysis study undertaken by the authors has shown that the stress values in the cement and dentin structures of the flat design of occlusal reduction was more benign and homogenous stress state in the cement and dentin layers in comparison with the anatomic design [30]. In this study, the correlation between preparation design and elastic modulus was assessed but not the effect of other possible but subtler contributing factors such as the mechanism of bonding with the dentin and the ceramic. While there is a distinct variation in the bonding mechanisms achieved by these adhesive lute systems, their mode of action and performance is well documented and considered to be clinically acceptable [31–34]. Inclusion of this parameter and the possible effect of time-dependent adhesion degradation would warrant consideration in a longer-term fatigue study. In terms of fracture mode, none of the specimens demonstrated a complete separation of the cement from the dentin or the ceramic, suggesting that the failure was not due to an adhesive failure of the cement–dentin or cement–ceramic interfaces. The crowns cemented with Super-Bond C&B there was no significant difference in the fracture mode between the two occlusal designs preparations, but both did exhibit a higher incidence of codes 4 and 5, indicating a simultaneous catastrophic fracture of all the substrates in the crown-tooth complex. The different behavior of Super-Bond C&B to the other two cements may be attributed to its particularly low elastic modulus that suggests that there is a more rapid distribution of stresses to the underlying dentin. These results for Super-Bond C&B are in accordance with the study of Attia and Kern that reported a mean fracture strength of 680.5 N (±183.5) for the restored tooth with machined Vita Mark II ceramic blocks cemented with Super-Bond C&B [35]. They also used natural maxillary premolar teeth, with a 2 mm anatomic design occlusal reduction. An all-ceramic crown preparation is an aggressive approach in terms of tooth reduction, so that the resultant fracture strength in all specimens was significantly lower compared to that of the natural teeth. It has been suggested that the restorations routinely experience masticatory forces ranging from 150 N to 665 N depending upon the location of tooth
[6]. The average occlusal force applied to maxillary premolars is about 300 N [36]. Hence according to this study the three different adhesive luting agents used to cement Vita Mark II CAD/CAM ceramic crowns would have been able to sustain masticatory forces. In a study carried out by Zahran et al., the mean fracture load of Vita Mark II machined ceramic crown cemented with Panavia F 2.0 was 1272 N (±109) [37]. In this study the value of 833.4 N (±147.5) was found as the mean fracture load for the flat occlusal reduction design cemented with Panavia F 2.0. These differences may be related to the difference in methodology, Zahran et al. used the molar typodont teeth with 1.5 mm flat occlusal reduction, while the natural tooth has been used in the current study with a 1.2 mm flat occlusal reduction. The results of this investigation, suggest that with machined ceramic crowns, a flat occlusal preparation design is a more benign configuration. The effect of the elastic modulus of the cement is not clear, but does appear to be related to the geometric configuration of the internal surface of the crown. Static fracture testing, as used in this investigation, is able to discriminate between the different crown-tooth systems, suggesting the potential for more conservative occlusal reduction preparations and the need to consider the stiffness of the cement. Further extrapolations are not possible from this static fracture-test investigation. The effect of fatigue is considered as a contributing factor to predict the long-term success of a restoration reliably [38]. The use of a chewing simulator test that enables an analysis of fatigue as a simulation of the oral environment would be an appropriate further investigative stage to further our understanding of the performance of these systems prior to in vivo investigations.
6.
Conclusions
This investigation showed that both the design of the crown preparation and the elastic modulus of the lute cement have an effect on the structural integrity of the restored tooth-crown complex. The teeth restored with crowns of a flat occlusal preparation design exhibited the highest fracture strength and showed the least dependency on the resin cement selection. The flat occlusal preparation design together with the highest modulus cement (Panavia F 2.0; Kuraray Medical, Inc.) revealed the greatest structural integrity of the restored system.
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