Shingo Kamio Futoshi Komine Kohei Taguchi Taro Iwasaki Markus B. Blatz Hideo Matsumura
Authors’ affiliations: Shingo Kamio, Futoshi Komine, Kohei Taguchi, Taro Iwasaki, Hideo Matsumura, Department of Fixed Prosthodontics, Nihon University School of Dentistry, Tokyo, Japan Markus B. Blatz, Department of Preventive and Restorative Sciences, University of Pennsylvania School of Dental Medicine, Philadelphia, PA, USA Corresponding author: Futoshi Komine, DDS, PhD Nihon University School of Dentistry, Department of Fixed Prosthodontics 1-8-13, Kanda-Surugadai, Chiyoda-Ku, Tokyo 101-8310, Japan Tel.: +81 3 3219 8145 Fax: +81 3 3219 8351 e-mail: [email protected]
Effects of framework design and layering material on fracture strength of implant-supported zirconia-based molar crowns
Key words: dental implant, fracture strength, framework design, indirect composite material,
zirconia ceramics Abstract Objectives: To evaluate the effects of framework design and layering material on the fracture strength of implant-supported zirconia-based molar crowns. Material and methods: Sixty-six titanium abutments (GingiHue Post) were tightened onto dental implants (Implant Lab Analog). These abutment–implant complexes were randomly divided into three groups (n = 22) according to the design of the zirconia framework (Katana), namely, uniform-thickness (UNI), anatomic (ANA), and supported anatomic (SUP) designs. The specimens in each design group were further divided into two subgroups (n = 11): zirconia-based all-ceramic restorations (ZAC group) and zirconia-based restorations with an indirect composite material (Estenia C&B) layered onto the zirconia framework (ZIC group). All crowns were cemented on implant abutments, after which the specimens were tested for fracture resistance. The data were analyzed with the Kruskal–Wallis test and the Mann–Whitney U-test with the Bonferroni correction (a = 0.05). Results: The following mean fracture strength values (kN) were obtained in UNI design, ANA design, and SUP design, respectively: Group ZAC, 3.78, 6.01, 6.50 and Group ZIC, 3.15, 5.65, 5.83. In both the ZAC and ZIC groups, fracture strength was significantly lower for the UNI design than the other two framework designs (P = 0.001). Fracture strength did not significantly differ (P > 0.420) between identical framework designs in the ZAC and ZIC groups. Conclusions: A framework design with standardized layer thickness and adequate support of veneer by zirconia frameworks, as in the ANA and SUP designs, increases fracture resistance in implant-supported zirconia-based restorations under conditions of chewing attrition. Indirect composite material and porcelain perform similarly as layering materials on zirconia frameworks.
Date: Accepted 29 June 2014 To cite this article: Kamio S, Komine F, Taguchi K, Iwasaki T, Blatz MB, Matsumura H. Effects of framework design and layering material on fracture strength of implant-supported zirconiabased molar crowns. Clin. Oral Impl. Res. 26, 2015; 1407–1413 doi: 10.1111/clr.12468
All-ceramic restorations are widely used in clinical practice, due to their excellent esthetic properties. Zirconium dioxide (zirconia) is the strongest and toughest of all dental ceramics (flexural strength, >1 GPa; fracture toughness, KIC = 9–10 MN/m3/2) (Christel et al. 1989) and meets the mechanical requirements for high-stress-bearing posterior restorations. Zirconia has been in clinical use for more than 10 years, and numerous clinical studies have found that it is a satisfactory framework material, with good medium- and long-term stability, in the production of crowns and fixed dental prostheses (Vult von Steyern et al. 2005; Edelhoff et al. 2008; Tinschert et al. 2008; Sailer et al. 2009a; Roediger et al. 2010). However, the rate of chipping/fracture of layering porcelain, the
© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd
most common cause of technical complications, in tooth-supported fixed dental prostheses is 6–25% for zirconia-based restorations (Vult von Steyern et al. 2005; Edelhoff et al. 2008; Tinschert et al. 2008; Sailer et al. 2009a; Roediger et al. 2010). The results of clinical studies confirm the high reliability of zirconia for abutments and framework materials in implant-supported restorations (Larsson et al. 2006; Nothdurft & Pospiech 2009; Sailer et al. 2009b; Larsson & Vult von Steyern 2010). However, the chipping rate for layering porcelain in implantsupported zirconia-based restorations (10– 40%) (Larsson et al. 2006; Nothdurft & Pospiech 2009; Sailer et al. 2009b; Larsson & Vult von Steyern 2010) is higher than that in toothsupported restorations. This difference might
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be due to the absence of the periodontal ligament around the dental implants. Occlusal stress can be greater on implant-supported restorations than on tooth-supported restorations and is conducted directly to the bone (H€ ammerle et al. 1995; C ß iftcßi & Canay 2000). Several techniques have been developed to address potential chipping of layering porcelain, such as the “overpressing technique” (which improves the homogeneity and density of layering materials) (Beuer et al. 2010; Chaar et al. 2013), use of an anatomically designed framework with appropriate porcelain thickness (Marchack et al. 2008; Larsson et al. 2012), and application of an indirect composite material as an alternative to layering porcelain (Kobayashi et al. 2009; Taguchi et al. 2014). In vitro studies confirmed that the bond strength between indirect composite material and a zirconia framework was potentially sufficient to survive clinical application of the restorations (Kobayashi et al. 2009; Komine et al. 2013). In addition, the fracture resistance of implant-supported zirconia-based restorations layered with indirect composite (ZIC) is equivalent to that of zirconia-based all-ceramic (ZAC) restorations (Taguchi et al. 2014). The effects of framework design on fracture strength of ZAC molar restorations were investigated in previous in vitro studies (Lorenzoni et al. 2010; Kokubo et al. 2011; Larsson et al. 2012). Framework design affected the fracture strength of ZAC molar crowns (Kokubo et al. 2011; Larsson et al. 2012). Moreover, fracture strength was significantly higher for ZAC restorations with an anatomically designed framework than for those with a uniform framework thickness (Larsson et al. 2012). Proper framework support of layering porcelain reduced adverse effects associated with the use of mechanically weaker layering porcelain. However, the effects of framework design on fracture strength in ZIC restorations have not been studied in detail. The purpose of this study was to evaluate the effects of framework design and layering material on the fracture strength of implantsupported zirconia-based molar crowns. The working hypotheses were that fracture strength would be significantly affected by framework design and that there would be differences in fracture strength between restorations with different layering materials.
Material and methods The materials assessed in this study are listed in Table 1. Sixty-six dental implants
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Table 1. Materials assessed in this study Material (Brand name)
Lot no.
Component
Manufacturer
Implant Implant Lab Analog
885142
Ti 99%
Biomet 3i, Palm Beach Gardens, FL, USA
846929
Ti 99%
Biomet 3i
Abutment GingiHue Post WPP572G Abutment screw Titanium Square UniScrew UNIST Zirconia ceramic material Noritake Katana Zirconia Indirect composite material Estenia C&B
1039039
Biomet 3i
00044D, 00042D, 00086A
Feldspathic porcelain for zirconia Cerabien ZR 026626, 032027, 029649 Priming agents Estenia Opaque Primer
00172A
Glass-ionomer cement Ketac Cem Easymix
448093
with a diameter of 5.0 mm (Implant Lab Analog; Biomet 3i, Palm Beach Gardens, FL, USA) were used for replacement of a missing mandibular first molar. Clinical conditions were simulated by embedding all implants in plastic specimen holders (Plastic Ring; inner diameter, 2.0 cm; Sankei, Tokyo, Japan) at an angle perpendicular to the horizontal plane, using autopolymerizing acrylic resin (Technovit 4000; Heraeus Kulzer, Wehrheim, Germany) (Strub & Gerds 2003). The implant body was covered with the autopolymerizing acrylic resin up to the first thread. The elastic modulus of the resin was 12 GPa, that is, approximately that of human bone (18 GPa). Titanium abutments (GingiHue Post WPP572G; Biomet 3i) of standardized shape (platform diameter, 5.0 mm; abutment width, 7.5 mm; width of rounded-shoulder finish line, 0.8 mm; height, 7.0 mm above shoulder) were placed onto the implants. A torque control system (Torque Driver HTD-C; Biomet 3i) was used to tighten the abutments to 32N with a titanium screw (Titanium Square UniScrew UNIST; Biomet 3i), according to the manufacturer’s instructions. Diamond rotary cutting instruments (Bur No.106RD; Shofu Inc., Kyoto, Japan) with water-spray application (Presto Aqua; Nakanishi Inc., Kanuma, Japan) were used to prepare abutments with 1.0 mm of occlusal reduction (definitive total height of 6.0 mm). The abutment height was standardized with a polyvinyl siloxane index (Lab Silicone; Shofu Inc.). The abutments were later polished using silicone wheels (Silicone Wheel P Type; Shofu Inc.).
ZrO2 94.4%, Y2O3 5.4%
Kuraray Noritake Dental Inc., Tokyo, Japan
OA2, DA2, E1
Kuraray Noritake Dental Inc.
SBA2, A2B, E2
Kuraray Noritake Dental Inc.
MDP, monomer solvent
Kuraray Noritake Dental Inc. 3M ESPE, St. Paul, MN, USA
A total of 66 abutment–implant complexes were randomly divided into three groups (n = 22 each) according to the design of the zirconia framework, namely, uniform-thickness (UNI), anatomic (ANA), and supported anatomic (SUP) designs (Fig. 1). All frameworks for the implant-supported restorations were fabricated from presintered zirconia blanks (Noritake Katana Zirconia Frame; Kuraray Noritake Dental Inc., Tokyo, Japan) with a commercial dental computer-aided design and computer-aided manufacturing (CAD/CAM) system (Katana; Kuraray Noritake Dental Inc.). For the UNI design, the thickness of the zirconia framework was uniform, 0.5 mm in both axial and occlusal areas (Fig. 1a). The abutments were scanned with a measurement device (Dental Scanner SC-3; Kuraray Noritake Dental Inc.). Scanned data were converted into CAD data. The designs did not include a cement space at the finish line, but a cement space of 40 lm at the axial and occlusal surfaces of the abutment. Design data were converted and sent to the processing machine. The frameworks were milled with a milling device (Katana DWX-50N; Kuraray Noritake Dental Inc.) and sintered to full density in a heat furnace (Katana F-1; Kuraray Noritake Dental Inc.) at 1375°C for 90 min. The ANA design had a standardized layering thickness (1.2 mm) on the frameworks as an anatomic design (Fig. 1b). Full-contour waxing (Inlay Wax Medium; GC Corp., Tokyo, Japan) and cut back, as in metal–ceramic restorations, were used to obtain uni-
© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd
Kamio et al Strength of zirconia crown for implant
(a)
(b)
(c)
Fig. 1. Cross sections of framework designs for a single-tooth implant-supported restoration. a: UNI design, b: ANA design, and c: SUP design. (unit: mm).
form and adequate occlusal veneer thickness. Double scanning was performed with a measurement device (Dental Scanner SC-3; Kuraray Noritake Dental Inc.) to create and merge the datasets from the abutment and wax pattern. The merged data were then transmitted to the milling device, and the coping was fabricated, as described above. The SUP design was similar to the ANA design and a supportive design at high palatal shoulder (5.0 mm) with a horizontal butt joint, for application of the layering material (Fig. 1c). These zirconia frameworks were fabricated in a manner similar to that described for the ANA design. Calipers (Measuring Device 2; YDM, Tokyo, Japan) were used to verify that the thickness of all zirconia frameworks corresponded to the predetermined thicknesses in each group. The zirconia frameworks were then placed on the implant abutments, and one of the investigators inspected the adaptation with a sharp probe (Single-end Explorer; YDM) and polyvinyl siloxane disclosing medium (Fit Checker; GC Corp.) under an optical
microscope (Stemi DV4; Carl Zeiss Co., Ltd., Jena, Germany). The layered surface of zirconia frameworks was airborne particle abraded with 50-lm aluminum oxide particles (Hi-Aluminas; Shofu Inc.) at a pressure of 0.2 MPa and a distance of 10 mm for 20 s. Each framework design was then further divided into two subgroups (n = 11 each): the zirconia-based all-ceramic restorations (ZAC group) and zirconia-based restorations with an indirect composite material layered onto the zirconia frameworks (ZIC group). ZAC group
The zirconia frameworks were layered with feldspathic porcelain (Cerabien ZR; Kuraray Noritake Dental Inc.), and a special index (K854-02-000E; Tokyo Giken Inc., Tokyo, Japan) was used to standardize the shape and dimensions of the restorations (Fig. 2) (Nelson & Ash 2010). The layering feldspathic porcelain and all firing procedures were performed with opaque porcelain (SBA2), dentin shade (A2B), and enamel shade
Fig. 2. A photograph of the index used to standardize the dimensions of restorations.
© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd
(E2), according to the manufacturer’s recommendations. Each layering porcelain powder was mixed with the corresponding manufacturer’s liquid (Meister Liquid; Kuraray Noritake Dental Inc.). The resulting porcelain slurry was layered on the frameworks and condensed by vibration with an ultrasonic vibrator (Ceracon II; Shofu Inc.). The restorations were fired in a SingleMat Porcelain Furnace (Shofu Inc.), using the exact procedure recommended by the manufacturer (Table 2). To obtain a satisfactory shape, a second application of porcelain was used to compensate for firing shrinkage. After porcelain firing, the thickness of restorations was confirmed with calipers and a silicone index. Then, all restorations were glazed in the furnace (Table 2). The finished restorations were placed on the abutments to verify adaptation, as described above. ZIC group
The zirconia frameworks were layered with an indirect composite material (Estenia C&B; Kuraray Noritake Dental Inc.), with the same index used for the ZAC group. First, the zirconia surface was treated with Estenia Opaque Primer (Kuraray Noritake Dental Inc.), according to the manufacturer’s recommendations. As an additional bonding agent, two thin layers of opaque material (Estenia C&B Body Opaque OA2; Kuraray Noritake Dental Inc.) were applied to the frameworks and light-cured for 90 s in a laboratory light-polymerization unit (a-light II; J. Morita Corp., Suita, Japan). Then, a special index was used to layer the dentin shade (Estenia C&B Dentin DA2; Kuraray Noritake Dental Inc.) and enamel shade (Estenia C&B Enamel E1; Kuraray Noritake Dental Inc.) of the composite material onto the frameworks (Fig. 2). The restorations were then light-cured in a laboratory light-polymerization unit for 5 min and polymerized in a heat oven (KL-310; J. Morita Corp.) at 110°C for 15 min. To achieve identical dimensions, the calipers and the silicone index were used to confirm the thickness and shape of restorations. All restorations were polished with the companion polishing accessory (Polishing Instrument Kit; Kuraray Noritake Dental Inc.). Adaptation of the restorations was later confirmed with the same procedure used for the ZAC group specimens. The intaglio surfaces of the restorations and the retention surface of the implant abutments were airborne particle abraded with 50-lm aluminum oxide particles at a pressure of 0.2 MPa for 10 s. All restorations were then placed onto the implant abutments with
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Table 2. Firing schedules of feldspathic porcelain based on manufacturer recommendations Predrying Heating rate Feldspathic porcelain Shade Base (SBA2) Body (A2B) and Enamel (E1) Glaze
Firing temperature
Holding time
Cooling time
Temperature (°C)
Time (min)
(°C/min)
(°C)
(min)
(min)
600 600
5 8
45 45
930 935
1 1
4 4
600
5
50
930
0.5
4
fracture or framework fracture. After testing, randomly selected specimens were sputtered with osmium (HPC-IS; Vacuum Device Inc., Mito, Japan) for 30 s and observed with a scanning electron microscope (SEM; S-4300; Hitachi High Technologies Co. Ltd., Tokyo, Japan) operated at 15 kV.
Results
Fig. 3. A photograph of the apparatus used for fracture resistance testing shows the position of the stainless steel ball and tin foil at loading onset.
a glass-ionomer cement (Ketac Cem Easymix; 3M ESPE AG, Seefeld, Germany), according to the manufacturer’s instructions. To simulate finger pressure during restoration placement, a static load of 30N was applied to the occlusal surface for 7 min. Excess cement was removed from the margins with a dental explorer. All specimens were then stored in distilled water at 37°C for 24 h before fracture resistance testing. All abutment–crown specimens were mounted on a universal testing machine (Type 5567; Instron Corp., Canton, MA, USA), and a stainless steel ball (diameter, 6.0 mm) was placed on the occlusal surface of the restorations to simulate application of clinical occlusal force. Axial loading with a spherical indenter of 5–10 mm in the lateral dimension, which is in the same scale as a molar crown, is recommended to evaluate fatigue and fracture resistance in ceramic restorations (Kim et al. 2008; Alhasanyah et al. 2013). Such loading to brittle layer on compliant substrate exhibits three crack modes: occlusal surface outer cone cracks, inner cone cracks, and radial cracks of interface between veneer and framework materials (Kim et al. 2008). To equally distribute occlusal force, tin foil (Dentaurum; thickness, 1.0 mm; Ispringen, Germany) was seated in between the specimen and the stainless steel ball. Loads were applied along the load axis
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of the specimens with a crosshead speed of 0.5 mm/min until fracture (Fig. 3). The compressive load required to cause fracture (N) was recorded for each specimen. In addition, the compressive load was graphically recorded as a load-deflection curve. The break detector level was set at a 10% loss of maximum force, as in previous studies (Lehmann et al. 2004). Data from fracture resistance testing were primarily analyzed with the Levene test, using statistical analysis software (SPSS version 19.0; SPSS, Inc., Chicago, IL, USA). When the Levene test did not show equality of variance, the Kruskal–Wallis test (SPSS version 19.0) was performed to evaluate the difference among framework design variations. On the basis of the results of the Kruskal–Wallis test, the Mann–Whitney U-test with the Bonferroni correction for multiple comparisons (SPSS version 19.0) was additionally used to compare differences among the three groups for an identical layering material. Differences between the ZAC and ZIC groups for an identical framework design were analyzed with the Mann–Whitney U-test (SPSS version 19.0). The significance level was set at 0.05 in all analyses. After fracture resistance testing, an optical microscope (Stemi DV4; Carl Zeiss Co.) at 932 magnification was used to determine fracture mode, which was classified as layer
Fracture strength values and the results of statistical analysis are shown in Table 3. Mean fracture strength was 3.78–6.50 kN in the ZAC group and 3.15–5.83 kN in the ZIC group. In both the ZAC and ZIC groups, fracture strength was significantly lower for the UNI design than for the other two framework designs (P = 0.001). The Mann–Whitney U-test showed no significant difference between the ZAC and ZIC groups for any framework design (P > 0.420). Table 4 shows the results of fracture mode assessment after fracture resistance testing. In ZAC group specimens, the predominant fracture mode was catastrophic framework fracture for all framework designs. In contrast, approximately equal numbers of ZIC group specimens exhibited layer fracture and framework fracture. For UNI design in both ZAC and ZIC groups, the fracture of layering materials was observed from occlusal surface to crown margin. On the other hand, the marginal ridge and cusp of the specimens mainly fractured in the ANA and SUP designs. Fig. 4a shows scratches produced by the milling bar on the surface of machine-milled zirconia frameworks. Airborne particle abrasion of the zirconia surface created an irregular, roughened surface (Fig. 4b). Fig. 4c,d show representative SEM images of the fractured surface of ZAC and ZIC group specimens, respectively. The specimens that developed layer fractures exhibited a combination of adhesive and cohesive fracture. These SEM images also show remnants of feldspathic porcelain and indirect composite material.
Discussion The results confirm the first working hypothesis, that is, that fracture strength would be significantly affected by the framework design of zirconia-based restorations. The fracture loads for the UNI framework design were significantly lower than those for the ANA and SUP designs in both the ZAC and ZIC groups. However, there were no statisti-
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Kamio et al Strength of zirconia crown for implant
Table 3. Results of fracture strength (kN) testing Group
Design
Minimum
Maximum
Median
Mean
IQR*
ZAC
UNI ANA SUP UNI ANA SUP
2.98 4.54 5.48 1.94 4.44 4.12
4.82 7.52 7.17 3.93 6.97 8.48
3.93 5.61 6.57 3.60 5.73 5.37
3.78 6.01 6.50 3.15 5.65 5.83
[3.14; [5.29; [6.06; [2.63; [5.52; [4.80;
ZIC
Category† 4.26] 6.96] 7.02] 3.70] 5.88] 6.94]
A, a B, b B, c C, a D, b D, c
ZAC, zirconia-based all-ceramic restorations; ZIC, zirconia-based restorations with an indirect composite material layered onto a zirconia framework; UNI, uniform-thickness design; ANA, anatomic design, SUP, supported anatomic design. *Interquartile range. †Identical uppercase letters indicate that the values are not statistically different between each framework design for the identical layering material (Mann–Whitney U-test with a Bonferroni correction, P > 0.05). Identical lowercase letters indicate that the values are not statistically different between the ZAC and ZIC groups for the identical framework design (Mann–Whitney U-test, P > 0.05).
Table 4. Fracture mode after fracture resistance testing Group
Design
Layer fracture (fracture locations)
Framework fracture
ZAC
UNI ANA SUP UNI ANA SUP
3 2 0 6 6 6
8 9 11 5 5 5
ZIC
(from occlusal surface to crown margin; 3) (marginal ridge; 1, cusp; 1) (from occlusal surface to crown margin; 6) (marginal ridge; 3, cusp; 3) (marginal ridge; 4, cusp; 2)
ZAC, zirconia-based all-ceramic restorations; ZIC, zirconia-based restorations with an indirect composite material layered onto a zirconia framework; UNI, uniform-thickness design; ANA, anatomic design; SUP, supported anatomic design.
(a)
(b)
(c)
(d)
Fig. 4. SEM image of representative zirconia surfaces (original magnification 9 600): (a) as milled, (b) after airborneparticle abrasion, (c) a ZAC group specimen after fracture testing, and (d) a ZIC group specimen after fracture testing. Z: zirconia framework, P: porcelain, I: indirect composite.
cally significant differences in fracture load between the ZAC and ZIC groups for the same framework design. Thus, the second hypothesis, that there would be difference in
fracture strength between restorations with different layering materials, was rejected. The mean fracture strength of the zirconiabased restorations exceeded 3.0 kN in all
© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd
groups, which is higher than the reported physiologic maximal posterior masticatory force of 0.88 kN (Bates et al. 1975; Kiliaridis et al. 1993). Therefore, the tested zirconiabased restorations are unlikely to fracture and can potentially withstand physiologic occlusal forces. This study investigated the effect of framework design in improving support of layering material in implant-supported zirconia-based molar restorations. The results indicate that when zirconia frameworks are anatomically designed with a standardized veneer layer thickness, the fracture loads of both ZAC and ZIC group specimens are much greater than those for zirconia frameworks with a uniform thickness. This finding supports the results of previous studies, which reported that the fracture load for zirconia-based allceramic restorations with anatomically designed frameworks was higher than that for a framework of uniform thickness (Kokubo et al. 2011; Larsson et al. 2012). This is likely due to the thinner layer of layering material and adequate support of that layer by the zirconia framework at the occlusal surface with an anatomically designed framework (Rosentritt et al. 2009; Larsson et al. 2012). Standardized thickness and adequate support of layering materials by the zirconia framework, as in designs ANA and SUP, may increase fracture resistance in zirconia-based restorations under conditions of chewing attrition. Zirconia ceramics have low thermal diffusivity; thus, the presence of thicker layering porcelain on the zirconia framework could adversely affect fracture strength, due to the higher probability of critical flaws (Swain 2009; Larsson et al. 2012). In addition, palatal support (e.g., in the SUP design), which is similar to the framework of metalceramic restorations, might not affect fracture strength in the ZAC and ZIC groups. Thus, standardized thickness and adequate support of layering materials by the zirconia framework are critical. The fracture strength of zirconia-based indirect composite-layered crowns (ZIC) was comparable to that of zirconia-based all-ceramic crowns (ZAC) for all framework designs. This finding suggests that indirect composite material performs similar to feldspathic porcelain as a layering material on zirconia frameworks and can therefore be recommended as an alternative to feldspathic porcelain. Although few studies have compared the fracture strength of these two types of zirconia-based restorations, the present results are supported by past findings (Oderich et al. 2012; Taguchi et al. 2014). They
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are due to the difference in the modulus of elasticity between porcelain and composite materials. The elastic modulus of composite material is lower than that of porcelain (C ß iftcßi & Canay 2000). Thus, indirect composite material layered on zirconia frameworks may reduce impact force under a static load. Regarding fracture mode, most ZAC group specimens exhibited complete fracture of the zirconia framework and layer porcelain. In contrast, fracture of the layer composite material accounted for over half of fractures in the ZIC group specimens. This difference is likely due to the strength of feldspathic porcelain and its strong bond to zirconia frameworks (Kobayashi et al. 2009; Saito et al. 2010). Indirect composite materials are widely used in fixed prostheses, including implantsupported restorations. The present approach is interesting because indirect composite materials have not been advocated as layering materials on zirconia frameworks for implantsupported restorations. However, clinical use of composite resin materials on the occlusal surface may cause several long-term complications, such as fracture or wear, esthetic
defects, and loosening of occlusal implant screws (Lindquist et al. 1987; Carlson & Carlsson 1994; Lekholm et al. 1994). The zirconia frameworks of SUP design in the present study were uncovered with the layering materials, exposing the zirconia surface to moisture in the mouth. It is known that zirconia ceramic is prone to low temperature degradation (LTD) in the presence of water or moisture (Lughi & Sergo 2010). The presence of hydrothermal stress such as water, blood, and synovial fluids over a long period of time induces the tetragonal-to-monoclinic transformation. Although there is no accepted mechanism to explain the LTD so far, the LTD reduces the mechanical properties of zirconia as the excess volume is not compensated by crack space and causes micro- and macrocracking (Lughi & Sergo 2010). On the other hand, conflicting findings suggesting that the presence of moisture was not identified as having a detrimental effect on the mechanical properties have been reported (Curtis et al. 2006; Papanagiotou et al. 2006; Alghazzawi et al. 2012). In the present study, the tested specimens were not exposed to artificial aging or dynamic loading, which could be regarded
Acknowledgements: This study was supported in part by JSPS KAKENHI Grant Number 24592933, a Grant from the Sato Fund, Nihon University School of Dentistry (2013 and 2014), and a Grant from the Promotion and Mutual Aid Corporation for Private Schools of Japan (2013 and 2014).
C ß iftcßi, Y. & Canay, S. (2000) The effect of veneering materials on stress distribution in implant-supported fixed prosthetic restorations. The International Journal of Oral & Maxillofacial Implants 15: 571–582. Curtis, A.R., Wright, A.J. & Fleming, G.J. (2006) The influence of surface modification techniques on the performance of a Y-TZP dental ceramic. Journal of Dentistry 34: 195–206. Edelhoff, D., Florian, B., Florian, W. & Johnen, C. (2008) HIP zirconia fixed partial dentures–clinical results after 3 years of clinical service. Quintessence International 39: 459–471. H€ammerle, C.H., Wagner, D., Br€agger, U., Lussi, A., Karayiannis, A., Joss, A. & Lang, N.P. (1995) Threshold of tactile sensitivity perceived with dental endosseous implants and natural teeth. Clinical Oral Implants Research 6: 83–90. Kiliaridis, S., Kjellberg, H., Wenneberg, B. & Engstr€ om, C. (1993) The relationship between maximal bite force, bite force endurance, and facial morphology during growth. A cross-sectional study. Acta Odontologica Scandinavica 51: 323– 331. Kim, J.H., Kim, J.W., Myoung, S.W., Pines, M. & Zhang, Y. (2008) Damage maps for layered ceramics under simulated mastication. Journal of Dental Research 87: 671–675. Kobayashi, K., Komine, F., Blatz, M.B., Saito, A., Koizumi, H. & Matsumura, H. (2009) Influence of priming agents on the short-term bond strength of an indirect composite veneering material to zirconium dioxide ceramic. Quintessence International 40: 545–551.
Kokubo, Y., Tsumita, M., Kano, T. & Fukushima, S. (2011) The influence of zirconia coping designs on the fracture load of all-ceramic molar crowns. Dental Materials Journal 30: 281–285. Komine, F., Kobayashi, K., Blatz, M.B., Fushiki, R., Koizuka, M., Taguchi, K. & Matsumura, H. (2013) Durability of bond between an indirect composite veneering material and zirconium dioxide ceramics. Acta Odontologica Scandinavica 71: 457–463. Larsson, C., El Madhoun, S., Wennerberg, A. & Vult von Steyern, P. (2012) Fracture strength of yttria-stabilized tetragonal zirconia polycrystals crowns with different design: an in vitro study. Clinical Oral Implants Research 23: 820–826. Larsson, C. & Vult von Steyern, P. (2010) Five-year follow-up of implant-supported Y-TZP and ZTA fixed dental prostheses. A randomized, prospective clinical trial comparing two different material systems. The International Journal of Prosthodontics 23: 555–561. Larsson, C., Vult von Steyern, P., Sunzel, B. & Nilner, K. (2006) All-ceramic two- to five-unit implant-supported reconstructions. A randomized, prospective clinical trial. Swedish Dental Journal 30: 45–53. Lehmann, F., Eickemeyer, G. & Rammelsberg, P. (2004) Fracture resistance of metal-free composite crowns-effects of fiber reinforcement, thermal cycling, and cementation technique. The Journal of Prosthetic Dentistry 92: 258–264. Lekholm, U., Steenberghe, D.V., Herrmann, I., Bolender, C., Folmer, T., Gunne, J., Henry, P., Higuchi, K., Laney, W.R. & Linden, U. (1994)
as a study limitation. Therefore, further laboratory studies and clinical trials are necessary before definitive conclusions can be drawn regarding the long-term performance of implant-supported, zirconia-based, indirect composite-layered crowns. Within the limitations of this in vitro study, the present results suggest that framework designs with a standardized thickness and adequate support of layering materials by the zirconia framework increase fracture resistance in implant-supported zirconiabased restorations under conditions of chewing attrition. Furthermore, indirect composite material performs similar to feldspathic porcelain as a layering material on zirconia frameworks and should be considered an alternative.
References Alghazzawi, T.F., Lemons, J., Liu, P.R., Essig, M.E., Bartolucci, A.A. & Janowski, G.M. (2012) Influence of low-temperature environmental exposure on the mechanical properties and structural stability of dental zirconia. Journal of Prosthodontics 21: 363–369. Alhasanyah, A., Vaidyanathan, T.K. & Flinton, R.J. (2013) Effect of core thickness differences on postfatigue indentation fracture resistance of veneered zirconia crowns. Journal of Prosthodontics 22: 383–390. Bates, J.F., Stafford, G.D. & Harrison, A. (1975) Masticatory function -a review of the literature (II) Speed of movement of the mandibular, rate of chewing and forces developed in chewing. Journal of Oral Rehabilitation 2: 349–361. Beuer, F., Stimmelmayr, M., Gernet, W., Edelhoff, D., G€ uh, J.F. & Naumann, M. (2010) Prospective study of zirconia-based restorations: 3-year clinical results. Quintessence International 41: 631– 637. Carlson, B. & Carlsson, G.E. (1994) Prosthodontic complications in osseointegrated dental implant treatment. The International Journal of Oral & Maxillofacial Implants 9: 90–94. Chaar, M.S., Witkowski, S., Strub, J.R. & Att, W. (2013) Effect of veneering technique on the fracture resistance of zirconia fixed dental prostheses. Journal of Oral Rehabilitation 40: 51–59. Christel, P., Meunier, A., Heller, M., Torre, J.P. & Peille, C.N. (1989) Mechanical properties and short-term in-vivo evaluation of yttrium-oxidepartially-stabilized zirconia. Journal of Biomedical Materials Research 23: 45–61.
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Osseointegrated implants in the treatment of partially edentulous jaws: a prospective 5-year multicenter study. The International Journal of Oral & Maxillofacial Implants 6: 627–635. Lindquist, L.W., Carlsson, G.E. & Glantz, P.O. (1987) Rehabilitation of the edentulous mandible with a tissue-integrated fixed prosthesis: a sixyear longitudinal study. Quintessence International 18: 89–96. Lorenzoni, F.C., Martins, L.M., Silva, N.R., Coelho, P.G., Guess, P.C., Bonfante, E.A., Thompson, V.P. & Bonfante, G. (2010) Fatigue life and failure modes of crowns systems with a modified framework design. Journal of Dentistry 38: 626–634. Lughi, V. & Sergo, V. (2010) Low temperature degradation -aging- of zirconia: a critical review of the relevant aspects in dentistry. Dental Materials 26: 807–820. Marchack, B.W., Futatsuki, Y., Marchack, C.B. & White, S.N. (2008) Customization of milled zirconia copings for all-ceramic crowns: a clinical report. The Journal of Prosthetic Dentistry 99: 169–173. Nelson, S.J. & Ash, M.M. (2010) Wheeler’s Dental anatomy, physiology, and occlusion. In: The Permanent Mandibular Molars, 9th edition, 189–207. Philadelphia: Saunders. Nothdurft, F.P. & Pospiech, P.R. (2009) Zirconium dioxide implant abutments for posterior singletooth replacement: first results. Journal of Periodontology 80: 2065–2072.
Oderich, E., Boff, L.L., Cardoso, A.C. & Magne, P. (2012) Fatigue resistance and failure mode of adhesively restored custom implant zirconia abutments. Clinical Oral Implants Research 23: 1360–1368. Papanagiotou, H.P., Morgano, S.M., Giordano, R.A. & Pober, R. (2006) In vitro evaluation of low-temperature aging effects and finishing procedures on the flexural strength and structural stability of Y-TZP dental ceramics. The Journal of Prosthetic Dentistry 96: 154–164. Roediger, M., Gersdorff, N., Huels, A. & Rinke, S. (2010) Prospective evaluation of zirconia posterior fixed partial dentures: four-year clinical results. The International Journal of Prosthodontics 23: 141–148. Rosentritt, M., Steiger, D., Behr, M., Handel, G. & Kolbeck, C. (2009) Influence of substructure design and spacer settings on the in vitro performance of molar zirconia crowns. Journal of Dentistry 37: 978–983. Sailer, I., Gottner, J., K€anel, S. & H€ammerle, C.H. (2009a) Randomized controlled clinical trial of zirconia-ceramic and metal-ceramic posterior fixed dental prostheses: a 3-year follow-up. The International Journal of Prosthodontics 22: 553– 560. Sailer, I., Philipp, A., Zembic, A., Pjetursson, B.E., H€ammerle, C.H. & Zwahlen, M. (2009b) A systematic review of the performance of ceramic and metal implant abutments supporting fixed
© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd
implant reconstructions. Clinical Oral Implants Research 20(Suppl 4): 4–31. Saito, A., Komine, F., Blatz, M.B. & Matsumura, H. (2010) A comparison of bond strength of layered veneering porcelains to zirconia and metal. The Journal of Prosthetic Dentistry 104: 247– 257. Strub, J.R. & Gerds, T. (2003) Fracture strength and failure mode of five different single-tooth implant-abutment combinations. The International Journal of Prosthodontics 16: 167–171. Swain, M.V. (2009) Unstable cracking (chipping) of veneering porcelain on all-ceramic dental crowns and fixed partial dentures. Acta Biomaterialia 5: 1668–1677. Taguchi, K., Komine, F., Fushiki, R., Blatz, M.B., Kamio, S. & Matsumura, H. (2014) Fracture resistance of single-tooth implant-supported zirconia-based indirect composite-layered molar restorations. Clinical Oral Implants Research 25: 983–991. Tinschert, J., Schulze, K.A., Natt, G., Latzke, P., Heussen, N. & Spiekermann, H. (2008) Clinical behavior of zirconia-based fixed partial dentures made of DC-Zirkon: 3-year results. The International Journal of Prosthodontics 21: 217–222. Vult von Steyern, P., Carlson, P. & Nilner, K. (2005) All-ceramic fixed partial dentures designed according to the DC-Zirkon technique. A 2-year clinical study. Journal of Oral Rehabilitation 32: 180–187.
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Authors’ affiliations: Shingo Kamio, Futoshi Komine, Kohei Taguchi, Taro Iwasaki, Hideo Matsumura, Department of Fixed Prosthodontics, Nihon University School of Dentistry, Tokyo, Japan Markus B. Blatz, Department of Preventive and Restorative Sciences, University of Pennsylvania School of Dental Medicine, Philadelphia, PA, USA Corresponding author: Futoshi Komine, DDS, PhD Nihon University School of Dentistry, Department of Fixed Prosthodontics 1-8-13, Kanda-Surugadai, Chiyoda-Ku, Tokyo 101-8310, Japan Tel.: +81 3 3219 8145 Fax: +81 3 3219 8351 e-mail: [email protected]
Effects of framework design and layering material on fracture strength of implant-supported zirconia-based molar crowns
Key words: dental implant, fracture strength, framework design, indirect composite material,
zirconia ceramics Abstract Objectives: To evaluate the effects of framework design and layering material on the fracture strength of implant-supported zirconia-based molar crowns. Material and methods: Sixty-six titanium abutments (GingiHue Post) were tightened onto dental implants (Implant Lab Analog). These abutment–implant complexes were randomly divided into three groups (n = 22) according to the design of the zirconia framework (Katana), namely, uniform-thickness (UNI), anatomic (ANA), and supported anatomic (SUP) designs. The specimens in each design group were further divided into two subgroups (n = 11): zirconia-based all-ceramic restorations (ZAC group) and zirconia-based restorations with an indirect composite material (Estenia C&B) layered onto the zirconia framework (ZIC group). All crowns were cemented on implant abutments, after which the specimens were tested for fracture resistance. The data were analyzed with the Kruskal–Wallis test and the Mann–Whitney U-test with the Bonferroni correction (a = 0.05). Results: The following mean fracture strength values (kN) were obtained in UNI design, ANA design, and SUP design, respectively: Group ZAC, 3.78, 6.01, 6.50 and Group ZIC, 3.15, 5.65, 5.83. In both the ZAC and ZIC groups, fracture strength was significantly lower for the UNI design than the other two framework designs (P = 0.001). Fracture strength did not significantly differ (P > 0.420) between identical framework designs in the ZAC and ZIC groups. Conclusions: A framework design with standardized layer thickness and adequate support of veneer by zirconia frameworks, as in the ANA and SUP designs, increases fracture resistance in implant-supported zirconia-based restorations under conditions of chewing attrition. Indirect composite material and porcelain perform similarly as layering materials on zirconia frameworks.
Date: Accepted 29 June 2014 To cite this article: Kamio S, Komine F, Taguchi K, Iwasaki T, Blatz MB, Matsumura H. Effects of framework design and layering material on fracture strength of implant-supported zirconiabased molar crowns. Clin. Oral Impl. Res. 26, 2015; 1407–1413 doi: 10.1111/clr.12468
All-ceramic restorations are widely used in clinical practice, due to their excellent esthetic properties. Zirconium dioxide (zirconia) is the strongest and toughest of all dental ceramics (flexural strength, >1 GPa; fracture toughness, KIC = 9–10 MN/m3/2) (Christel et al. 1989) and meets the mechanical requirements for high-stress-bearing posterior restorations. Zirconia has been in clinical use for more than 10 years, and numerous clinical studies have found that it is a satisfactory framework material, with good medium- and long-term stability, in the production of crowns and fixed dental prostheses (Vult von Steyern et al. 2005; Edelhoff et al. 2008; Tinschert et al. 2008; Sailer et al. 2009a; Roediger et al. 2010). However, the rate of chipping/fracture of layering porcelain, the
© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd
most common cause of technical complications, in tooth-supported fixed dental prostheses is 6–25% for zirconia-based restorations (Vult von Steyern et al. 2005; Edelhoff et al. 2008; Tinschert et al. 2008; Sailer et al. 2009a; Roediger et al. 2010). The results of clinical studies confirm the high reliability of zirconia for abutments and framework materials in implant-supported restorations (Larsson et al. 2006; Nothdurft & Pospiech 2009; Sailer et al. 2009b; Larsson & Vult von Steyern 2010). However, the chipping rate for layering porcelain in implantsupported zirconia-based restorations (10– 40%) (Larsson et al. 2006; Nothdurft & Pospiech 2009; Sailer et al. 2009b; Larsson & Vult von Steyern 2010) is higher than that in toothsupported restorations. This difference might
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be due to the absence of the periodontal ligament around the dental implants. Occlusal stress can be greater on implant-supported restorations than on tooth-supported restorations and is conducted directly to the bone (H€ ammerle et al. 1995; C ß iftcßi & Canay 2000). Several techniques have been developed to address potential chipping of layering porcelain, such as the “overpressing technique” (which improves the homogeneity and density of layering materials) (Beuer et al. 2010; Chaar et al. 2013), use of an anatomically designed framework with appropriate porcelain thickness (Marchack et al. 2008; Larsson et al. 2012), and application of an indirect composite material as an alternative to layering porcelain (Kobayashi et al. 2009; Taguchi et al. 2014). In vitro studies confirmed that the bond strength between indirect composite material and a zirconia framework was potentially sufficient to survive clinical application of the restorations (Kobayashi et al. 2009; Komine et al. 2013). In addition, the fracture resistance of implant-supported zirconia-based restorations layered with indirect composite (ZIC) is equivalent to that of zirconia-based all-ceramic (ZAC) restorations (Taguchi et al. 2014). The effects of framework design on fracture strength of ZAC molar restorations were investigated in previous in vitro studies (Lorenzoni et al. 2010; Kokubo et al. 2011; Larsson et al. 2012). Framework design affected the fracture strength of ZAC molar crowns (Kokubo et al. 2011; Larsson et al. 2012). Moreover, fracture strength was significantly higher for ZAC restorations with an anatomically designed framework than for those with a uniform framework thickness (Larsson et al. 2012). Proper framework support of layering porcelain reduced adverse effects associated with the use of mechanically weaker layering porcelain. However, the effects of framework design on fracture strength in ZIC restorations have not been studied in detail. The purpose of this study was to evaluate the effects of framework design and layering material on the fracture strength of implantsupported zirconia-based molar crowns. The working hypotheses were that fracture strength would be significantly affected by framework design and that there would be differences in fracture strength between restorations with different layering materials.
Material and methods The materials assessed in this study are listed in Table 1. Sixty-six dental implants
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Table 1. Materials assessed in this study Material (Brand name)
Lot no.
Component
Manufacturer
Implant Implant Lab Analog
885142
Ti 99%
Biomet 3i, Palm Beach Gardens, FL, USA
846929
Ti 99%
Biomet 3i
Abutment GingiHue Post WPP572G Abutment screw Titanium Square UniScrew UNIST Zirconia ceramic material Noritake Katana Zirconia Indirect composite material Estenia C&B
1039039
Biomet 3i
00044D, 00042D, 00086A
Feldspathic porcelain for zirconia Cerabien ZR 026626, 032027, 029649 Priming agents Estenia Opaque Primer
00172A
Glass-ionomer cement Ketac Cem Easymix
448093
with a diameter of 5.0 mm (Implant Lab Analog; Biomet 3i, Palm Beach Gardens, FL, USA) were used for replacement of a missing mandibular first molar. Clinical conditions were simulated by embedding all implants in plastic specimen holders (Plastic Ring; inner diameter, 2.0 cm; Sankei, Tokyo, Japan) at an angle perpendicular to the horizontal plane, using autopolymerizing acrylic resin (Technovit 4000; Heraeus Kulzer, Wehrheim, Germany) (Strub & Gerds 2003). The implant body was covered with the autopolymerizing acrylic resin up to the first thread. The elastic modulus of the resin was 12 GPa, that is, approximately that of human bone (18 GPa). Titanium abutments (GingiHue Post WPP572G; Biomet 3i) of standardized shape (platform diameter, 5.0 mm; abutment width, 7.5 mm; width of rounded-shoulder finish line, 0.8 mm; height, 7.0 mm above shoulder) were placed onto the implants. A torque control system (Torque Driver HTD-C; Biomet 3i) was used to tighten the abutments to 32N with a titanium screw (Titanium Square UniScrew UNIST; Biomet 3i), according to the manufacturer’s instructions. Diamond rotary cutting instruments (Bur No.106RD; Shofu Inc., Kyoto, Japan) with water-spray application (Presto Aqua; Nakanishi Inc., Kanuma, Japan) were used to prepare abutments with 1.0 mm of occlusal reduction (definitive total height of 6.0 mm). The abutment height was standardized with a polyvinyl siloxane index (Lab Silicone; Shofu Inc.). The abutments were later polished using silicone wheels (Silicone Wheel P Type; Shofu Inc.).
ZrO2 94.4%, Y2O3 5.4%
Kuraray Noritake Dental Inc., Tokyo, Japan
OA2, DA2, E1
Kuraray Noritake Dental Inc.
SBA2, A2B, E2
Kuraray Noritake Dental Inc.
MDP, monomer solvent
Kuraray Noritake Dental Inc. 3M ESPE, St. Paul, MN, USA
A total of 66 abutment–implant complexes were randomly divided into three groups (n = 22 each) according to the design of the zirconia framework, namely, uniform-thickness (UNI), anatomic (ANA), and supported anatomic (SUP) designs (Fig. 1). All frameworks for the implant-supported restorations were fabricated from presintered zirconia blanks (Noritake Katana Zirconia Frame; Kuraray Noritake Dental Inc., Tokyo, Japan) with a commercial dental computer-aided design and computer-aided manufacturing (CAD/CAM) system (Katana; Kuraray Noritake Dental Inc.). For the UNI design, the thickness of the zirconia framework was uniform, 0.5 mm in both axial and occlusal areas (Fig. 1a). The abutments were scanned with a measurement device (Dental Scanner SC-3; Kuraray Noritake Dental Inc.). Scanned data were converted into CAD data. The designs did not include a cement space at the finish line, but a cement space of 40 lm at the axial and occlusal surfaces of the abutment. Design data were converted and sent to the processing machine. The frameworks were milled with a milling device (Katana DWX-50N; Kuraray Noritake Dental Inc.) and sintered to full density in a heat furnace (Katana F-1; Kuraray Noritake Dental Inc.) at 1375°C for 90 min. The ANA design had a standardized layering thickness (1.2 mm) on the frameworks as an anatomic design (Fig. 1b). Full-contour waxing (Inlay Wax Medium; GC Corp., Tokyo, Japan) and cut back, as in metal–ceramic restorations, were used to obtain uni-
© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd
Kamio et al Strength of zirconia crown for implant
(a)
(b)
(c)
Fig. 1. Cross sections of framework designs for a single-tooth implant-supported restoration. a: UNI design, b: ANA design, and c: SUP design. (unit: mm).
form and adequate occlusal veneer thickness. Double scanning was performed with a measurement device (Dental Scanner SC-3; Kuraray Noritake Dental Inc.) to create and merge the datasets from the abutment and wax pattern. The merged data were then transmitted to the milling device, and the coping was fabricated, as described above. The SUP design was similar to the ANA design and a supportive design at high palatal shoulder (5.0 mm) with a horizontal butt joint, for application of the layering material (Fig. 1c). These zirconia frameworks were fabricated in a manner similar to that described for the ANA design. Calipers (Measuring Device 2; YDM, Tokyo, Japan) were used to verify that the thickness of all zirconia frameworks corresponded to the predetermined thicknesses in each group. The zirconia frameworks were then placed on the implant abutments, and one of the investigators inspected the adaptation with a sharp probe (Single-end Explorer; YDM) and polyvinyl siloxane disclosing medium (Fit Checker; GC Corp.) under an optical
microscope (Stemi DV4; Carl Zeiss Co., Ltd., Jena, Germany). The layered surface of zirconia frameworks was airborne particle abraded with 50-lm aluminum oxide particles (Hi-Aluminas; Shofu Inc.) at a pressure of 0.2 MPa and a distance of 10 mm for 20 s. Each framework design was then further divided into two subgroups (n = 11 each): the zirconia-based all-ceramic restorations (ZAC group) and zirconia-based restorations with an indirect composite material layered onto the zirconia frameworks (ZIC group). ZAC group
The zirconia frameworks were layered with feldspathic porcelain (Cerabien ZR; Kuraray Noritake Dental Inc.), and a special index (K854-02-000E; Tokyo Giken Inc., Tokyo, Japan) was used to standardize the shape and dimensions of the restorations (Fig. 2) (Nelson & Ash 2010). The layering feldspathic porcelain and all firing procedures were performed with opaque porcelain (SBA2), dentin shade (A2B), and enamel shade
Fig. 2. A photograph of the index used to standardize the dimensions of restorations.
© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd
(E2), according to the manufacturer’s recommendations. Each layering porcelain powder was mixed with the corresponding manufacturer’s liquid (Meister Liquid; Kuraray Noritake Dental Inc.). The resulting porcelain slurry was layered on the frameworks and condensed by vibration with an ultrasonic vibrator (Ceracon II; Shofu Inc.). The restorations were fired in a SingleMat Porcelain Furnace (Shofu Inc.), using the exact procedure recommended by the manufacturer (Table 2). To obtain a satisfactory shape, a second application of porcelain was used to compensate for firing shrinkage. After porcelain firing, the thickness of restorations was confirmed with calipers and a silicone index. Then, all restorations were glazed in the furnace (Table 2). The finished restorations were placed on the abutments to verify adaptation, as described above. ZIC group
The zirconia frameworks were layered with an indirect composite material (Estenia C&B; Kuraray Noritake Dental Inc.), with the same index used for the ZAC group. First, the zirconia surface was treated with Estenia Opaque Primer (Kuraray Noritake Dental Inc.), according to the manufacturer’s recommendations. As an additional bonding agent, two thin layers of opaque material (Estenia C&B Body Opaque OA2; Kuraray Noritake Dental Inc.) were applied to the frameworks and light-cured for 90 s in a laboratory light-polymerization unit (a-light II; J. Morita Corp., Suita, Japan). Then, a special index was used to layer the dentin shade (Estenia C&B Dentin DA2; Kuraray Noritake Dental Inc.) and enamel shade (Estenia C&B Enamel E1; Kuraray Noritake Dental Inc.) of the composite material onto the frameworks (Fig. 2). The restorations were then light-cured in a laboratory light-polymerization unit for 5 min and polymerized in a heat oven (KL-310; J. Morita Corp.) at 110°C for 15 min. To achieve identical dimensions, the calipers and the silicone index were used to confirm the thickness and shape of restorations. All restorations were polished with the companion polishing accessory (Polishing Instrument Kit; Kuraray Noritake Dental Inc.). Adaptation of the restorations was later confirmed with the same procedure used for the ZAC group specimens. The intaglio surfaces of the restorations and the retention surface of the implant abutments were airborne particle abraded with 50-lm aluminum oxide particles at a pressure of 0.2 MPa for 10 s. All restorations were then placed onto the implant abutments with
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Table 2. Firing schedules of feldspathic porcelain based on manufacturer recommendations Predrying Heating rate Feldspathic porcelain Shade Base (SBA2) Body (A2B) and Enamel (E1) Glaze
Firing temperature
Holding time
Cooling time
Temperature (°C)
Time (min)
(°C/min)
(°C)
(min)
(min)
600 600
5 8
45 45
930 935
1 1
4 4
600
5
50
930
0.5
4
fracture or framework fracture. After testing, randomly selected specimens were sputtered with osmium (HPC-IS; Vacuum Device Inc., Mito, Japan) for 30 s and observed with a scanning electron microscope (SEM; S-4300; Hitachi High Technologies Co. Ltd., Tokyo, Japan) operated at 15 kV.
Results
Fig. 3. A photograph of the apparatus used for fracture resistance testing shows the position of the stainless steel ball and tin foil at loading onset.
a glass-ionomer cement (Ketac Cem Easymix; 3M ESPE AG, Seefeld, Germany), according to the manufacturer’s instructions. To simulate finger pressure during restoration placement, a static load of 30N was applied to the occlusal surface for 7 min. Excess cement was removed from the margins with a dental explorer. All specimens were then stored in distilled water at 37°C for 24 h before fracture resistance testing. All abutment–crown specimens were mounted on a universal testing machine (Type 5567; Instron Corp., Canton, MA, USA), and a stainless steel ball (diameter, 6.0 mm) was placed on the occlusal surface of the restorations to simulate application of clinical occlusal force. Axial loading with a spherical indenter of 5–10 mm in the lateral dimension, which is in the same scale as a molar crown, is recommended to evaluate fatigue and fracture resistance in ceramic restorations (Kim et al. 2008; Alhasanyah et al. 2013). Such loading to brittle layer on compliant substrate exhibits three crack modes: occlusal surface outer cone cracks, inner cone cracks, and radial cracks of interface between veneer and framework materials (Kim et al. 2008). To equally distribute occlusal force, tin foil (Dentaurum; thickness, 1.0 mm; Ispringen, Germany) was seated in between the specimen and the stainless steel ball. Loads were applied along the load axis
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of the specimens with a crosshead speed of 0.5 mm/min until fracture (Fig. 3). The compressive load required to cause fracture (N) was recorded for each specimen. In addition, the compressive load was graphically recorded as a load-deflection curve. The break detector level was set at a 10% loss of maximum force, as in previous studies (Lehmann et al. 2004). Data from fracture resistance testing were primarily analyzed with the Levene test, using statistical analysis software (SPSS version 19.0; SPSS, Inc., Chicago, IL, USA). When the Levene test did not show equality of variance, the Kruskal–Wallis test (SPSS version 19.0) was performed to evaluate the difference among framework design variations. On the basis of the results of the Kruskal–Wallis test, the Mann–Whitney U-test with the Bonferroni correction for multiple comparisons (SPSS version 19.0) was additionally used to compare differences among the three groups for an identical layering material. Differences between the ZAC and ZIC groups for an identical framework design were analyzed with the Mann–Whitney U-test (SPSS version 19.0). The significance level was set at 0.05 in all analyses. After fracture resistance testing, an optical microscope (Stemi DV4; Carl Zeiss Co.) at 932 magnification was used to determine fracture mode, which was classified as layer
Fracture strength values and the results of statistical analysis are shown in Table 3. Mean fracture strength was 3.78–6.50 kN in the ZAC group and 3.15–5.83 kN in the ZIC group. In both the ZAC and ZIC groups, fracture strength was significantly lower for the UNI design than for the other two framework designs (P = 0.001). The Mann–Whitney U-test showed no significant difference between the ZAC and ZIC groups for any framework design (P > 0.420). Table 4 shows the results of fracture mode assessment after fracture resistance testing. In ZAC group specimens, the predominant fracture mode was catastrophic framework fracture for all framework designs. In contrast, approximately equal numbers of ZIC group specimens exhibited layer fracture and framework fracture. For UNI design in both ZAC and ZIC groups, the fracture of layering materials was observed from occlusal surface to crown margin. On the other hand, the marginal ridge and cusp of the specimens mainly fractured in the ANA and SUP designs. Fig. 4a shows scratches produced by the milling bar on the surface of machine-milled zirconia frameworks. Airborne particle abrasion of the zirconia surface created an irregular, roughened surface (Fig. 4b). Fig. 4c,d show representative SEM images of the fractured surface of ZAC and ZIC group specimens, respectively. The specimens that developed layer fractures exhibited a combination of adhesive and cohesive fracture. These SEM images also show remnants of feldspathic porcelain and indirect composite material.
Discussion The results confirm the first working hypothesis, that is, that fracture strength would be significantly affected by the framework design of zirconia-based restorations. The fracture loads for the UNI framework design were significantly lower than those for the ANA and SUP designs in both the ZAC and ZIC groups. However, there were no statisti-
© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd
Kamio et al Strength of zirconia crown for implant
Table 3. Results of fracture strength (kN) testing Group
Design
Minimum
Maximum
Median
Mean
IQR*
ZAC
UNI ANA SUP UNI ANA SUP
2.98 4.54 5.48 1.94 4.44 4.12
4.82 7.52 7.17 3.93 6.97 8.48
3.93 5.61 6.57 3.60 5.73 5.37
3.78 6.01 6.50 3.15 5.65 5.83
[3.14; [5.29; [6.06; [2.63; [5.52; [4.80;
ZIC
Category† 4.26] 6.96] 7.02] 3.70] 5.88] 6.94]
A, a B, b B, c C, a D, b D, c
ZAC, zirconia-based all-ceramic restorations; ZIC, zirconia-based restorations with an indirect composite material layered onto a zirconia framework; UNI, uniform-thickness design; ANA, anatomic design, SUP, supported anatomic design. *Interquartile range. †Identical uppercase letters indicate that the values are not statistically different between each framework design for the identical layering material (Mann–Whitney U-test with a Bonferroni correction, P > 0.05). Identical lowercase letters indicate that the values are not statistically different between the ZAC and ZIC groups for the identical framework design (Mann–Whitney U-test, P > 0.05).
Table 4. Fracture mode after fracture resistance testing Group
Design
Layer fracture (fracture locations)
Framework fracture
ZAC
UNI ANA SUP UNI ANA SUP
3 2 0 6 6 6
8 9 11 5 5 5
ZIC
(from occlusal surface to crown margin; 3) (marginal ridge; 1, cusp; 1) (from occlusal surface to crown margin; 6) (marginal ridge; 3, cusp; 3) (marginal ridge; 4, cusp; 2)
ZAC, zirconia-based all-ceramic restorations; ZIC, zirconia-based restorations with an indirect composite material layered onto a zirconia framework; UNI, uniform-thickness design; ANA, anatomic design; SUP, supported anatomic design.
(a)
(b)
(c)
(d)
Fig. 4. SEM image of representative zirconia surfaces (original magnification 9 600): (a) as milled, (b) after airborneparticle abrasion, (c) a ZAC group specimen after fracture testing, and (d) a ZIC group specimen after fracture testing. Z: zirconia framework, P: porcelain, I: indirect composite.
cally significant differences in fracture load between the ZAC and ZIC groups for the same framework design. Thus, the second hypothesis, that there would be difference in
fracture strength between restorations with different layering materials, was rejected. The mean fracture strength of the zirconiabased restorations exceeded 3.0 kN in all
© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd
groups, which is higher than the reported physiologic maximal posterior masticatory force of 0.88 kN (Bates et al. 1975; Kiliaridis et al. 1993). Therefore, the tested zirconiabased restorations are unlikely to fracture and can potentially withstand physiologic occlusal forces. This study investigated the effect of framework design in improving support of layering material in implant-supported zirconia-based molar restorations. The results indicate that when zirconia frameworks are anatomically designed with a standardized veneer layer thickness, the fracture loads of both ZAC and ZIC group specimens are much greater than those for zirconia frameworks with a uniform thickness. This finding supports the results of previous studies, which reported that the fracture load for zirconia-based allceramic restorations with anatomically designed frameworks was higher than that for a framework of uniform thickness (Kokubo et al. 2011; Larsson et al. 2012). This is likely due to the thinner layer of layering material and adequate support of that layer by the zirconia framework at the occlusal surface with an anatomically designed framework (Rosentritt et al. 2009; Larsson et al. 2012). Standardized thickness and adequate support of layering materials by the zirconia framework, as in designs ANA and SUP, may increase fracture resistance in zirconia-based restorations under conditions of chewing attrition. Zirconia ceramics have low thermal diffusivity; thus, the presence of thicker layering porcelain on the zirconia framework could adversely affect fracture strength, due to the higher probability of critical flaws (Swain 2009; Larsson et al. 2012). In addition, palatal support (e.g., in the SUP design), which is similar to the framework of metalceramic restorations, might not affect fracture strength in the ZAC and ZIC groups. Thus, standardized thickness and adequate support of layering materials by the zirconia framework are critical. The fracture strength of zirconia-based indirect composite-layered crowns (ZIC) was comparable to that of zirconia-based all-ceramic crowns (ZAC) for all framework designs. This finding suggests that indirect composite material performs similar to feldspathic porcelain as a layering material on zirconia frameworks and can therefore be recommended as an alternative to feldspathic porcelain. Although few studies have compared the fracture strength of these two types of zirconia-based restorations, the present results are supported by past findings (Oderich et al. 2012; Taguchi et al. 2014). They
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are due to the difference in the modulus of elasticity between porcelain and composite materials. The elastic modulus of composite material is lower than that of porcelain (C ß iftcßi & Canay 2000). Thus, indirect composite material layered on zirconia frameworks may reduce impact force under a static load. Regarding fracture mode, most ZAC group specimens exhibited complete fracture of the zirconia framework and layer porcelain. In contrast, fracture of the layer composite material accounted for over half of fractures in the ZIC group specimens. This difference is likely due to the strength of feldspathic porcelain and its strong bond to zirconia frameworks (Kobayashi et al. 2009; Saito et al. 2010). Indirect composite materials are widely used in fixed prostheses, including implantsupported restorations. The present approach is interesting because indirect composite materials have not been advocated as layering materials on zirconia frameworks for implantsupported restorations. However, clinical use of composite resin materials on the occlusal surface may cause several long-term complications, such as fracture or wear, esthetic
defects, and loosening of occlusal implant screws (Lindquist et al. 1987; Carlson & Carlsson 1994; Lekholm et al. 1994). The zirconia frameworks of SUP design in the present study were uncovered with the layering materials, exposing the zirconia surface to moisture in the mouth. It is known that zirconia ceramic is prone to low temperature degradation (LTD) in the presence of water or moisture (Lughi & Sergo 2010). The presence of hydrothermal stress such as water, blood, and synovial fluids over a long period of time induces the tetragonal-to-monoclinic transformation. Although there is no accepted mechanism to explain the LTD so far, the LTD reduces the mechanical properties of zirconia as the excess volume is not compensated by crack space and causes micro- and macrocracking (Lughi & Sergo 2010). On the other hand, conflicting findings suggesting that the presence of moisture was not identified as having a detrimental effect on the mechanical properties have been reported (Curtis et al. 2006; Papanagiotou et al. 2006; Alghazzawi et al. 2012). In the present study, the tested specimens were not exposed to artificial aging or dynamic loading, which could be regarded
Acknowledgements: This study was supported in part by JSPS KAKENHI Grant Number 24592933, a Grant from the Sato Fund, Nihon University School of Dentistry (2013 and 2014), and a Grant from the Promotion and Mutual Aid Corporation for Private Schools of Japan (2013 and 2014).
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