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In vitro investigation of fracture load and aging resistance of high-speed sintered monolithic tooth-borne zirconia crowns
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Original article
In vitro investigation of fracture load and aging resistance of high-speed sintered monolithic tooth-borne zirconia crowns Takashi Nakamuraa,b,* , Yoshiro Nakanoa , Hirofumi Usamia , Shinya Okamuraa , Kazumichi Wakabayashia , Hirofumi Yatania a b
Department of Fixed Prosthodontics, Osaka University Graduate School of Dentistry, 1-8 Yamadaoka, Suita 565-0871, Japan Department of Oral Health Sciences, Otemae College, 6-42 Ochayasho-cho, Nishinomiya, Hyogo 6628552, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 October 2018 Received in revised form 27 June 2019 Accepted 3 July 2019 Available online xxx
Objectives: The purpose of this study was to evaluate the fit, fracture load and aging resistance of the monolithic zirconia tooth-borne crowns with conventional and high-speed sintering. Methods: The Y-TZP block was machined and sintered with conventional and high-speed sintering furnace. The marginal and internal gap between the crown and abutment was measured using a microscope and a fit checking material. A total of 28 crowns were further divided into an undegraded and a degraded group. An accelerated aging test was carried out on the degraded group. The crown was cemented and a fracture resistance was tested. X-ray diffraction was used to evaluate the crystalline structure. The data were analyzed with Student’s t-test, and a one-way ANOVA and Tukey’s multiple comparison test. Results: There was no significant difference in mean marginal gap between the two groups. The mean internal gap was significantly greater in the speed sintering than in the conventional sintering (P <0.001). The mean fracture load of the conventional sintering crowns was not significantly different from that of speed sintering crowns after aging. The occurrence of monoclinic crystals of degraded crown was significantly higher than that of undegraded crown both in the conventional (P <0.001) and speedsintering group (P <0.001). Conclusions: It was concluded that the monolithic zirconia crowns produced by high-speed sintering showed no significant difference in the marginal gap and the fracture load after aging compared to conventional sintering. Therefore, the high-speed sintering seems a valid method of producing toothborne monolithic zirconia crowns. © 2019 Japan Prosthodontic Society. Published by Elsevier Ltd. All rights reserved.
Keywords: Zirconia Crown Speed sintering Fracture Load Degradation
1. Introduction Translucent yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) has better translucency than the conventional Y-TZP used in frames, and is now frequently used in the manufacture of monolithic zirconia restorations [1]. These monolithic zirconia restorations do not require a veneering ceramic, and they are easy to manufacture and resistant to fracture [2]. Additional advantages of monolithic zirconia restorations in comparison with conventional all-ceramic restorations are that they reduce the amount of
* Corresponding author at: Department of Oral Health Sciences, Otemae College, 6-42 Ochayasho-cho, Nishinomiya, Hyogo 6628552, Japan. E-mail addresses: [email protected], [email protected] (T. Nakamura), [email protected] (Y. Nakano), [email protected] (H. Usami), [email protected] (S. Okamura), [email protected] (K. Wakabayashi), [email protected] (H. Yatani).
tooth removal required on the abutment teeth, and cause little wear on the opposing teeth [3,4]. However, following the milling of a partially sintered Y-TZP block, monolithic zirconia restorations require 2–5 h of sintering at 1350–1550 C [5]. Additionally, conventional Y-TZP used in frames contains approximately 0.25% alumina to control degradation [6], whereas highly translucent Y-TZP contains very little alumina [7]. There is therefore a concern that monolithic zirconia restorations may be prone to low-temperature degradation caused by the phase transition that is characteristic of Y-TZP [8]. Recently, a chairside furnace for zirconia using high-frequency induction heating was commercialized, making it possible to sinter monolithic tooth-borne zirconia crowns made of translucent Y-TZP in a short amount of time (10–30 min) [9]. Combining this furnace with the same manufacturer’s intraoral scanner and milling machine has made one-day treatment with monolithic zirconia crowns possible. However, the fracture resistance of monolithic
https://doi.org/10.1016/j.jpor.2019.07.003 1883-1958/ © 2019 Japan Prosthodontic Society. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: T. Nakamura, et al., In vitro investigation of fracture load and aging resistance of high-speed sintered monolithic tooth-borne zirconia crowns, J Prosthodont Res (2019), https://doi.org/10.1016/j.jpor.2019.07.003
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Fig. 1. Titanium abutment tooth.
zirconia crowns sintered in a short amount of time after aging could differ from that of crowns produced using a conventional sintering schedule. The aim of this study was to evaluate the fracture resistance of monolithic zirconia crowns after aging using the same translucent Y-TZP under two different conditions – high-speed sintering using an intraoral scanner and an induction heating furnace, and long sintering using a dental laboratory scanner and a laboratory furnace. The null hypothesis is that high-speed sintering is not effective in keeping the fracture resistance of monolithic zirconia crowns after aging. 2. Materials and methods 2.1. Abutment tooth specimens An epoxy abutment tooth for a full metal crown on the maxillary right first molar was used. The angles were rounded for use with a monolithic zirconia crown and a 0.5 mm width chamfer was created around the entire circumference. The prepared epoxy abutment was scanned with a laboratory scanner and Stereolithography (STL) data of the abutment was created using the dental computer aided design (CAD) system (S-WAVE D2000, Shofu, Kyoto, Japan). A processing pass was made using computer aided manufacturing (CAM) software (DentMILL 2013 R3, Delcam, Birmingham, England) based on the STL data, and a titanium disk was milled. A total of 28 titanium abutment teeth with the same shape were milled with a 5-axis milling machine (RXP500DSC, Roeders, Soltau, Germany) (Fig. 1). An experimental flow chart is shown in Fig. 2. 2.2. Crown specimens The titanium abutment tooth was measured using a dental laboratory scanner (inEos X5, Dentsply Sirona, Bensheim, Germany), and a monolithic zirconia crown with the thinnest part of the occlusal surface measuring 0.8 mm thick, and the thinnest part of the tooth cervix measuring 0.5 mm was designed using CAD software (CEREC inLab softwareV4.2, Dentsply Sirona). The cement space was set to 70 mm. The zirconia block used was a commercially available, translucent, partially sintered Y-TZP block for monolithic crowns (inCoris TZI, Dentsply Sirona) of shade F0 (no color). The Y-TZP block was machined using a dental laboratory milling machine (CEREC MC XL, Dentsply Sirona) and sintered at 1510 C in a laboratory furnace (inFire HTC Speed, Dentsply Sirona) according to the schedule specified by the manufacturer, to
produce 14 crowns (conventional sintering group). The total sintering time was 220 min. Double scanning of the crown produced by conventional sintering and the titanium abutment tooth was performed using an intraoral scanner (CEREC Omnicam, Dentsply Sirona), and the design of the zirconia crown was determined using CAD software (CEREC software 4.3., Dentsply Sirona). The thickness of the thinnest part of the occlusal surface was set to 0.8 mm, and the cement space was set to 70 mm. The zirconia block used was the same partially sintered block as in the conventional sintering group (inCoris TZI). The block was machined using a milling machine (CEREC inLab MC X5, Dentsply Sirona) and sintered using a speed sintering furnace (CEREC Speedfire, Dentsply Sirona), according to the schedule assigned by the software, to produce 14 crowns (speed sintering group). The sintering temperature displayed on the screen was 1580 C, and the sintering time was 15 min. Polishing was not carried out after sintering in either of the two groups. 2.3. Density and amount of marginal/internal gap Using a density determination kit (AD-1653, A&D, Tokyo, Japan), the bulk density of the completed crowns was measured using Archimedes’ method. Next, marks were made in three places at the margin on the buccal, lingual, mesial and distal sides of the abutment tooth (a total of 12 points). With the crown installed on the abutment tooth, the marginal gap was measured at each of the 12 measurement sites using a microscope (VH-Z100UR, KEYENCE, Osaka, Japan), and mean values were determined. The internal gap was measured using a fit checking material (Fitchecker 2, GC, Tokyo, Japan) using a previously reported method [10]. The mixed fit checking material was placed inside the crown, which was then installed on the abutment tooth and secured with a force of 9.6 N using a clothespin (Daiya, Tokyo, Japan). The excess was wiped away and, after hardening, the fit checking material was removed and its mass was measured using an electronic balance (AEL-200, Shimadzu, Kyoto, Japan). The mean film thickness was calculated from the mass and density of the fit checking material and the surface area of the abutment tooth, and the mean film thickness was taken as the size of the internal gap. The surface area of the abutment tooth was determined with software (MiniMagic 2.0, Materialize, Yokohama, Japan) that analyzed the STL data for the shape of the abutment tooth scanned using a dental model scanner (inEos X5, Dentsply Sirona). The following equation was used in the calculation:
Please cite this article in press as: T. Nakamura, et al., In vitro investigation of fracture load and aging resistance of high-speed sintered monolithic tooth-borne zirconia crowns, J Prosthodont Res (2019), https://doi.org/10.1016/j.jpor.2019.07.003
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Fig. 2. Flaw chart of the experiment.
Amount of internal gap (mm) = mass (mg) / density (1.15 mg/mm3) / surface area (175.4 mm2). SPSS Statistics 17.0 (IBM Japan, Tokyo, Japan) was used for the statistical analysis. Student’s t-test was performed to determine the density, the marginal gap, and the internal gap. The significance level was set to 5%. 2.4. Change in fracture load using accelerated aging test The crown specimens in the conventional and speed sintering groups were divided further into undegraded and degraded groups (7 specimens in each group). A 10 h accelerated degradation test was performed on the degraded group in an underwater environment at 134 C and 2 atm, based on the standard for Y-TZP used for surgical implants (ISO 13356) [11], by placing distilled water into a decomposition vessel, inserting the specimen, and using a constant temperature oven (MOV-112, Sanyo, Osaka, Japan). The atmospheric pressure was set by calculating the volume of water and the volume of saturated water vapor based on the combined gas law. Using alumina powder with an average grain size of 50 mm, sandblasting was performed on the internal surface of the crown for 15 s at a pressure of 0.2 MPa and a distance of 10 mm. The crown was cemented to the abutment tooth using a selfadhesive resin cement (SA Luting, Kuraray Noritake Dental, Tokyo, Japan) and photoirradiation was performed for 10 s at a distance of 10 mm from the occlusal surface using a light curing device (Pencure, Morita, Osaka, Japan). After the removal of excess cement, the crown was secured for 30 min with a force of 9.6 N using a clothespin. Following cementation, the specimen was stored for 24 h at room temperature before a fracture test was carried out. The fracture load of the two types of crown in both undegraded and degraded states was found by performing a fracture test using a universal testing machine (INSTRON5582, INSTRON, Norwood, MA, USA). A 7-mm diameter stainless steel ball was interposed on the central part of the occlusal surface of the crown, and a load was applied in a vertical direction at a crosshead speed of 0.5 mm/min. One-way analysis of variance and Tukey’s multiple comparison test were performed for the fracture loads (SPSS Statistics 17.0). The significance level was set to 5%. 2.5. Crystalline structure X-ray diffraction was performed on the fragments after fracture testing, and the crystalline structure of each specimen was evaluated. Using an x-ray diffractometer (D8 ADVANCE, Bruker, Billerica, MA, USA), measurements were carried out by the 2u/u method with a range of 20 to 60 and a step size of 0.05 . The monoclinic ratio, an indicator of zirconia degradation, was calculated using the Garvie-Nicholson method [12].
One-way analysis of variance and Tukey’s multiple comparison test were performed for the monoclinic content (SPSS Statistics 17.0). The significance level was set to 5%. 3. Results There was no significant difference in the average bulk density of the crown specimens between the conventional and speed sintering groups, with an average bulk density of 6.00 0.03 g/cm3 in the conventional sintering group and 5.99 0.02 g/cm3 in the speed sintering group. The mean marginal gap was 61.5 9.0 mm in the conventional sintering group and 68.2 10.1 mm in the speed sintering group, showing no significant difference between the two groups. The mean internal gap was significantly greater (P < 0.001) in the speed sintering group (205.0 31.4 mm) than in the conventional sintering group (160.1 22.9 mm) (Fig. 3). In the fracture tests, it was observed that several large fragments separated from the abutment tooth in the crown specimens (Fig. 4). All of the crown specimens exhibited the same pattern of fracture. In the conventional sintering group, the mean fracture loads of the undegraded crowns and degraded crowns were 10,610 1211 N and 7845 833 N, respectively. In the speed sintering group, the mean fracture loads of the undegraded crowns and degraded crowns were 8300 759 N and 7173 788 N, respectively (Fig. 5). Only the fracture load of the undegraded crown specimens in the conventional sintering group was significantly greater (P < 0.001) than the other three conditions, i.e., degraded crowns in the conventional sintering group, and undegraded and degraded crowns in the speed sintering group. In the conventional sintering group, the occurrence of monoclinic crystals, an indicator of degradation, was 3.7 2.3% and 33.4 6.9% in the undegraded and degraded crowns, respectively, showing significantly higher occurrence in the degraded crowns (P < 0.001) (Fig. 6). In the speed sintering group, the occurrence of monoclinic crystals was also significantly higher in the degraded crowns (P < 0.001), with 6.7 4.7% and 37.3 13.7% in the undegraded and degraded crowns, respectively. No significant difference was observed in the occurrence of monoclinic crystals between the undegraded crowns of the conventional sintering group and the speed sintering group, or between the degraded crowns of the conventional sintering group and the speed sintering group. 4. Discussion The high-speed sintering furnace used in this study made use of induction heating by electromagnetic waves. This induction furnace was designed for chairside use and, unlike a conventional electric furnace, it does not allow multiple crowns to be sintered at the same time. Following measurement of the abutment tooth by
Please cite this article in press as: T. Nakamura, et al., In vitro investigation of fracture load and aging resistance of high-speed sintered monolithic tooth-borne zirconia crowns, J Prosthodont Res (2019), https://doi.org/10.1016/j.jpor.2019.07.003
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Fig. 3. Average marginal and internal gap of conventional sintering group specimens (CS) and speed sintering group specimens (SS). There is no significant difference between two groups. The average internal gap was significantly greater SS group than CS group (P < 0.001).
Fig. 4. The crown specimen after the fracture test. Several large fragments separated from the titanium abutment.
an intraoral scanner, a single crown is designed by the software, the temperature and time are altered according to the volume and shape of the crown, and the crown is sintered [9]. Additionally, the types of zirconia blocks that can be used in an induction furnace are limited. In this study, to examine differences resulting from the sintering method, we used the same translucent zirconia blocks to produce speed sintered crowns and to sinter crowns conventionally with a dental laboratory system. The block (inCoris TZI) was translucent zirconia containing 3 mol% yttria (3Y-TZP), which is a material that has long been used in monolithic zirconia restorations [13]. The bulk density of the crowns produced in this study was 5.9–6.0 g/cm3 in the conventional and speed sintering groups, which is close to the density of fully sintered dental 3Y-TZP (6.05–6.07 g/cm3) [14]. This result led us to infer that even though the sintering speed employed in this study was high, it achieved almost full sintering of
the 3Y-TZP, similar to conventional sintering. The finding that there was no difference in density between 3Y-TZP sintered rapidly or sintered conventionally was consistent with the result of highspeed sintering using microwaves [15]. In vitro studies on the amount of marginal gap in monolithic zirconia crowns have reported results in the range of 60.7–63.1 mm [16], and 68.7–77.9 mm [17]. The mean marginal gap in the two types of crowns produced in this study, at 61.5–68.2 mm, did not differ greatly from that of previous reports, and we consider this to be a gap size that does not present clinical problems. Additionally, sintering time has been reported to have no effect on the amount of marginal gap [18], and this study also found no difference in the amount of marginal gap as a result of the different sintering times. The mean internal gap inside the crown was larger in the speed sintering group (205.0 31.4 mm) than in the conventional sintering group (160.1 22.9 mm). The size of the internal gap in a CAD/CAM glass ceramic crown measured using the same method was reported to be 112–162 mm [10], and this is similar to the conventional sintering group in this study. The cement space was set to 70 mm in both the conventional sintering group and the speed sintering group; however, there was a significant difference in the mean internal gap inside the crown. A study using micro computed tomography reported that the fit of Y-TZP copings differed depending on the software [19]. In this study, we used different software for the conventional sintering group and the speed sintering group, and the fact that the size of the internal gap differed could be attributed to difference in the software. The fracture loads of crowns without aging ranged from 8300 N (speed sintering group) to 10,610 N (conventional sintering group). The fracture loads of high-strength glass ceramic lithium disilicatereinforced monolithic molar crowns has been reported as 2027 N [20] – 3147 N [21] for an occlusal surface thickness of 1.5 mm. Additionally, the fracture load of monolithic zirconia molar crowns with an occlusal surface thickness of 0.5 mm has been reported as 5558 N [22]. An accurate comparison is not possible because the crown shapes and loading conditions differ, but as in the previous report [21], the monolithic zirconia molar crown fracture loads obtained in this study appear to be much higher than those of
Please cite this article in press as: T. Nakamura, et al., In vitro investigation of fracture load and aging resistance of high-speed sintered monolithic tooth-borne zirconia crowns, J Prosthodont Res (2019), https://doi.org/10.1016/j.jpor.2019.07.003
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Fig. 5. Fracture load of conventional sintering group specimens (CS) and speed sintering group specimens (SS). The same superscript letters were not significantly different.
Fig. 6. Monoclinic volume fraction of conventional sintering group specimens (CS) and speed sintering group specimens (SS). The same superscript letters were not significantly different.
monolithic glass ceramic molar crowns. The fracture loads of the speed sintered crowns were lower than those of the conventionally sintered crowns. It has been reported that when 3Y-TZP is sintered rapidly by spark plasma sintering, with a temperature increase of 500 C per minute and then held at 1600 C for 2 min, pores appear inside the zirconia sample and its fracture toughness declines [23], unlike when the temperature is increased by 50 C per minute. In this study, the total sintering time of the speed sintering was 15 min and, compared with conventional sintering, the rate of temperature increase was much faster and the sintering temperature, at 1580 C, was also higher. Therefore, in the speed sintering group, it is possible that a decline in fracture toughness caused by the pores observed in the previous report [23] led to a decrease in the crown fracture load. In this study, we investigated low-temperature degradation of the crowns using accelerated aging. Storage for 1 h in water at 134 C and 2 atm is said to be equivalent to 3–4 years in water at 37 C [24], so this study simulates several decades of intraoral use.
In the standard for Y-TZP used for biological implants, as an indicator of degradation, it is required that the monoclinic ratio does not exceed 25% after 5 h in water at 134 C and 2 atm [11]. 3Y-TZP containing approximately 3 mol% yttria is often used in dentistry. The zirconia block used in this study was also 3Y-TZP, but in order to enhance translucency, it contained very little alumina. It has been reported that the monoclinic ratio of ordinary dental 3Y-TZP containing alumina remains below 10% even after immersion in water for 10 h at 134 C and 2 atm following machining and grinding [25]. The monoclinic ratio after immersion for 10 h was high (33–37%). Considering the results, we infer that the 3Y-TZP was degraded. The large increase in the monoclinic ratio in the accelerated aging samples appears to be due to the fact that the translucent 3Y-TZP used contained very little alumina, which effectively controls degradation [26]. We consider the significant decrease in crown fracture load in the conventional sintering group following accelerated aging to be the effect of material degradation. Dental Y-TZP is reported to show no change in bending strength when its monoclinic ratio increases from 30% to 40% in accelerated aging [27,28]. In this study, the monoclinic ratio in the speed sintering group was also above 30% after accelerated aging, and yet there was no significant difference in crown fracture load compared with the crowns without aging. Degradation of Y-TZP progresses from the outer layer, and it is reported to progress to approximately 60 mm even when accelerated aging with 100 h in water at 134 C [28] is performed. We infer that the degradation of Y-TZP did not have a major effect on the crown fracture load. The crown fracture loads of both groups after accelerated aging were still high (7100–7800 N) and were not statistically significant. The null hypothesis was rejected. Given that the maximum bite force in humans is approximately 260–740 N [29], we consider the possibility of crown fracture to be extremely low, even if the Y-TZP were to degrade after long use. This in vitro study has some limitations. In the oral cavity, cyclic loading is applied to the occlusal surface of crowns; however, in this study, a static load has been applied. In the accelerated aging test, only an experimental crown was degraded, and the influence of an abutment tooth or luting cement was not considered. Further studies that replicate true oral conditions should be considered.
Please cite this article in press as: T. Nakamura, et al., In vitro investigation of fracture load and aging resistance of high-speed sintered monolithic tooth-borne zirconia crowns, J Prosthodont Res (2019), https://doi.org/10.1016/j.jpor.2019.07.003
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5. Conclusion Within the limitations of this study, we found that monolithic zirconia crowns produced by high-speed sintering showed no major difference in density or fit compared with those produced by conventional sintering, and also showed no significant difference in fracture load after accelerated aging. These results suggest that chairside high-speed sintering is a valid method of producing monolithic zirconia crowns. References [1] Miyazaki T, Nakamura T, Matsumura H, Ban S, Kobayashi T. Current status of zirconia restoration. J Prosthodont Res 2013;57:236–61. [2] Ghodsi S, Jafarian Z. A review on translucent zirconia. Eur J Prosthodont Restor Dent 2018;26:62–74. [3] Zesewitz TF, Knauber AW, Northdurft FP. Fracture resistance of a selection of fullcontour all-ceramic crowns: an in vitro study. Int J Prosthodont 2014;27:264–6. [4] Passos SP, Torrealba Y, Major P, Linke B, Flores-Mir C, Nychka JA. In vitro wear behavior of zirconia opposing enamel: a systematic review. J Prosthodont 2014;23:593–601. [5] Abd El-Ghany OS, Sherief AH. Zirconia based ceramics, some clinical and biological aspects: review. Future Dent J 2016;2:55–64. [6] Hallmann L, Ulmer P, Reusser E, Louvel M, Hämmerle CHF. Effect of dopants and sintering temperature on microstructure and low temperature degradation of dental Y-TZP-zirconia. J Eur Ceram Soc 2012;32:4091–104. [7] Zhang Y. Making yttria-stabilized tetragonal zirconia translucent. Dent Mater 2014;30:1195–203. [8] Lawson S. Environmental degradation of zirconia ceramics. J Eur Ceram Soc 1995;15:485–502. [9] Wiedhahn K, Fritzsche G, Wiedhahn C, Schenk O. Zirconia crowns - the new standard for single-visit dentistry? Int J Comput Dent 2016;19:9–26. [10] Nakamura T, Dei N, Kojima T, Wakabayashi K. Marginal and internal fit of Cerec 3 CAD/CAM all-ceramic crowns. Int J Prosthodont 2003;16:244–8. [11] International Standard Organization No.13356. Implants for surgery -Ceramic materials based on yttria-stabilized tetragonal zirconia (Y-TZP). Geneva: International Organizanization for Standardization; 2015. [12] Garvie RC, Nicholson PS. Phase analysis in zirconia systems. J Am Cerm Soc 1972;55:303–5. [13] Vichi A, Carrabba M, Paravina R, Ferrari M. Translucency of ceramic materials for CEREC CAD/CAM system. J Esthet Restor Dent 2014;26:224–31.
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Please cite this article in press as: T. Nakamura, et al., In vitro investigation of fracture load and aging resistance of high-speed sintered monolithic tooth-borne zirconia crowns, J Prosthodont Res (2019), https://doi.org/10.1016/j.jpor.2019.07.003
journal of prosthodontic research xxx (2019) xxx–xxx
Contents lists available at ScienceDirect
Journal of Prosthodontic Research journal homepage: www.elsevier.com/locate/jpor
Original article
In vitro investigation of fracture load and aging resistance of high-speed sintered monolithic tooth-borne zirconia crowns Takashi Nakamuraa,b,* , Yoshiro Nakanoa , Hirofumi Usamia , Shinya Okamuraa , Kazumichi Wakabayashia , Hirofumi Yatania a b
Department of Fixed Prosthodontics, Osaka University Graduate School of Dentistry, 1-8 Yamadaoka, Suita 565-0871, Japan Department of Oral Health Sciences, Otemae College, 6-42 Ochayasho-cho, Nishinomiya, Hyogo 6628552, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 October 2018 Received in revised form 27 June 2019 Accepted 3 July 2019 Available online xxx
Objectives: The purpose of this study was to evaluate the fit, fracture load and aging resistance of the monolithic zirconia tooth-borne crowns with conventional and high-speed sintering. Methods: The Y-TZP block was machined and sintered with conventional and high-speed sintering furnace. The marginal and internal gap between the crown and abutment was measured using a microscope and a fit checking material. A total of 28 crowns were further divided into an undegraded and a degraded group. An accelerated aging test was carried out on the degraded group. The crown was cemented and a fracture resistance was tested. X-ray diffraction was used to evaluate the crystalline structure. The data were analyzed with Student’s t-test, and a one-way ANOVA and Tukey’s multiple comparison test. Results: There was no significant difference in mean marginal gap between the two groups. The mean internal gap was significantly greater in the speed sintering than in the conventional sintering (P <0.001). The mean fracture load of the conventional sintering crowns was not significantly different from that of speed sintering crowns after aging. The occurrence of monoclinic crystals of degraded crown was significantly higher than that of undegraded crown both in the conventional (P <0.001) and speedsintering group (P <0.001). Conclusions: It was concluded that the monolithic zirconia crowns produced by high-speed sintering showed no significant difference in the marginal gap and the fracture load after aging compared to conventional sintering. Therefore, the high-speed sintering seems a valid method of producing toothborne monolithic zirconia crowns. © 2019 Japan Prosthodontic Society. Published by Elsevier Ltd. All rights reserved.
Keywords: Zirconia Crown Speed sintering Fracture Load Degradation
1. Introduction Translucent yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) has better translucency than the conventional Y-TZP used in frames, and is now frequently used in the manufacture of monolithic zirconia restorations [1]. These monolithic zirconia restorations do not require a veneering ceramic, and they are easy to manufacture and resistant to fracture [2]. Additional advantages of monolithic zirconia restorations in comparison with conventional all-ceramic restorations are that they reduce the amount of
* Corresponding author at: Department of Oral Health Sciences, Otemae College, 6-42 Ochayasho-cho, Nishinomiya, Hyogo 6628552, Japan. E-mail addresses: [email protected], [email protected] (T. Nakamura), [email protected] (Y. Nakano), [email protected] (H. Usami), [email protected] (S. Okamura), [email protected] (K. Wakabayashi), [email protected] (H. Yatani).
tooth removal required on the abutment teeth, and cause little wear on the opposing teeth [3,4]. However, following the milling of a partially sintered Y-TZP block, monolithic zirconia restorations require 2–5 h of sintering at 1350–1550 C [5]. Additionally, conventional Y-TZP used in frames contains approximately 0.25% alumina to control degradation [6], whereas highly translucent Y-TZP contains very little alumina [7]. There is therefore a concern that monolithic zirconia restorations may be prone to low-temperature degradation caused by the phase transition that is characteristic of Y-TZP [8]. Recently, a chairside furnace for zirconia using high-frequency induction heating was commercialized, making it possible to sinter monolithic tooth-borne zirconia crowns made of translucent Y-TZP in a short amount of time (10–30 min) [9]. Combining this furnace with the same manufacturer’s intraoral scanner and milling machine has made one-day treatment with monolithic zirconia crowns possible. However, the fracture resistance of monolithic
https://doi.org/10.1016/j.jpor.2019.07.003 1883-1958/ © 2019 Japan Prosthodontic Society. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: T. Nakamura, et al., In vitro investigation of fracture load and aging resistance of high-speed sintered monolithic tooth-borne zirconia crowns, J Prosthodont Res (2019), https://doi.org/10.1016/j.jpor.2019.07.003
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Fig. 1. Titanium abutment tooth.
zirconia crowns sintered in a short amount of time after aging could differ from that of crowns produced using a conventional sintering schedule. The aim of this study was to evaluate the fracture resistance of monolithic zirconia crowns after aging using the same translucent Y-TZP under two different conditions – high-speed sintering using an intraoral scanner and an induction heating furnace, and long sintering using a dental laboratory scanner and a laboratory furnace. The null hypothesis is that high-speed sintering is not effective in keeping the fracture resistance of monolithic zirconia crowns after aging. 2. Materials and methods 2.1. Abutment tooth specimens An epoxy abutment tooth for a full metal crown on the maxillary right first molar was used. The angles were rounded for use with a monolithic zirconia crown and a 0.5 mm width chamfer was created around the entire circumference. The prepared epoxy abutment was scanned with a laboratory scanner and Stereolithography (STL) data of the abutment was created using the dental computer aided design (CAD) system (S-WAVE D2000, Shofu, Kyoto, Japan). A processing pass was made using computer aided manufacturing (CAM) software (DentMILL 2013 R3, Delcam, Birmingham, England) based on the STL data, and a titanium disk was milled. A total of 28 titanium abutment teeth with the same shape were milled with a 5-axis milling machine (RXP500DSC, Roeders, Soltau, Germany) (Fig. 1). An experimental flow chart is shown in Fig. 2. 2.2. Crown specimens The titanium abutment tooth was measured using a dental laboratory scanner (inEos X5, Dentsply Sirona, Bensheim, Germany), and a monolithic zirconia crown with the thinnest part of the occlusal surface measuring 0.8 mm thick, and the thinnest part of the tooth cervix measuring 0.5 mm was designed using CAD software (CEREC inLab softwareV4.2, Dentsply Sirona). The cement space was set to 70 mm. The zirconia block used was a commercially available, translucent, partially sintered Y-TZP block for monolithic crowns (inCoris TZI, Dentsply Sirona) of shade F0 (no color). The Y-TZP block was machined using a dental laboratory milling machine (CEREC MC XL, Dentsply Sirona) and sintered at 1510 C in a laboratory furnace (inFire HTC Speed, Dentsply Sirona) according to the schedule specified by the manufacturer, to
produce 14 crowns (conventional sintering group). The total sintering time was 220 min. Double scanning of the crown produced by conventional sintering and the titanium abutment tooth was performed using an intraoral scanner (CEREC Omnicam, Dentsply Sirona), and the design of the zirconia crown was determined using CAD software (CEREC software 4.3., Dentsply Sirona). The thickness of the thinnest part of the occlusal surface was set to 0.8 mm, and the cement space was set to 70 mm. The zirconia block used was the same partially sintered block as in the conventional sintering group (inCoris TZI). The block was machined using a milling machine (CEREC inLab MC X5, Dentsply Sirona) and sintered using a speed sintering furnace (CEREC Speedfire, Dentsply Sirona), according to the schedule assigned by the software, to produce 14 crowns (speed sintering group). The sintering temperature displayed on the screen was 1580 C, and the sintering time was 15 min. Polishing was not carried out after sintering in either of the two groups. 2.3. Density and amount of marginal/internal gap Using a density determination kit (AD-1653, A&D, Tokyo, Japan), the bulk density of the completed crowns was measured using Archimedes’ method. Next, marks were made in three places at the margin on the buccal, lingual, mesial and distal sides of the abutment tooth (a total of 12 points). With the crown installed on the abutment tooth, the marginal gap was measured at each of the 12 measurement sites using a microscope (VH-Z100UR, KEYENCE, Osaka, Japan), and mean values were determined. The internal gap was measured using a fit checking material (Fitchecker 2, GC, Tokyo, Japan) using a previously reported method [10]. The mixed fit checking material was placed inside the crown, which was then installed on the abutment tooth and secured with a force of 9.6 N using a clothespin (Daiya, Tokyo, Japan). The excess was wiped away and, after hardening, the fit checking material was removed and its mass was measured using an electronic balance (AEL-200, Shimadzu, Kyoto, Japan). The mean film thickness was calculated from the mass and density of the fit checking material and the surface area of the abutment tooth, and the mean film thickness was taken as the size of the internal gap. The surface area of the abutment tooth was determined with software (MiniMagic 2.0, Materialize, Yokohama, Japan) that analyzed the STL data for the shape of the abutment tooth scanned using a dental model scanner (inEos X5, Dentsply Sirona). The following equation was used in the calculation:
Please cite this article in press as: T. Nakamura, et al., In vitro investigation of fracture load and aging resistance of high-speed sintered monolithic tooth-borne zirconia crowns, J Prosthodont Res (2019), https://doi.org/10.1016/j.jpor.2019.07.003
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Fig. 2. Flaw chart of the experiment.
Amount of internal gap (mm) = mass (mg) / density (1.15 mg/mm3) / surface area (175.4 mm2). SPSS Statistics 17.0 (IBM Japan, Tokyo, Japan) was used for the statistical analysis. Student’s t-test was performed to determine the density, the marginal gap, and the internal gap. The significance level was set to 5%. 2.4. Change in fracture load using accelerated aging test The crown specimens in the conventional and speed sintering groups were divided further into undegraded and degraded groups (7 specimens in each group). A 10 h accelerated degradation test was performed on the degraded group in an underwater environment at 134 C and 2 atm, based on the standard for Y-TZP used for surgical implants (ISO 13356) [11], by placing distilled water into a decomposition vessel, inserting the specimen, and using a constant temperature oven (MOV-112, Sanyo, Osaka, Japan). The atmospheric pressure was set by calculating the volume of water and the volume of saturated water vapor based on the combined gas law. Using alumina powder with an average grain size of 50 mm, sandblasting was performed on the internal surface of the crown for 15 s at a pressure of 0.2 MPa and a distance of 10 mm. The crown was cemented to the abutment tooth using a selfadhesive resin cement (SA Luting, Kuraray Noritake Dental, Tokyo, Japan) and photoirradiation was performed for 10 s at a distance of 10 mm from the occlusal surface using a light curing device (Pencure, Morita, Osaka, Japan). After the removal of excess cement, the crown was secured for 30 min with a force of 9.6 N using a clothespin. Following cementation, the specimen was stored for 24 h at room temperature before a fracture test was carried out. The fracture load of the two types of crown in both undegraded and degraded states was found by performing a fracture test using a universal testing machine (INSTRON5582, INSTRON, Norwood, MA, USA). A 7-mm diameter stainless steel ball was interposed on the central part of the occlusal surface of the crown, and a load was applied in a vertical direction at a crosshead speed of 0.5 mm/min. One-way analysis of variance and Tukey’s multiple comparison test were performed for the fracture loads (SPSS Statistics 17.0). The significance level was set to 5%. 2.5. Crystalline structure X-ray diffraction was performed on the fragments after fracture testing, and the crystalline structure of each specimen was evaluated. Using an x-ray diffractometer (D8 ADVANCE, Bruker, Billerica, MA, USA), measurements were carried out by the 2u/u method with a range of 20 to 60 and a step size of 0.05 . The monoclinic ratio, an indicator of zirconia degradation, was calculated using the Garvie-Nicholson method [12].
One-way analysis of variance and Tukey’s multiple comparison test were performed for the monoclinic content (SPSS Statistics 17.0). The significance level was set to 5%. 3. Results There was no significant difference in the average bulk density of the crown specimens between the conventional and speed sintering groups, with an average bulk density of 6.00 0.03 g/cm3 in the conventional sintering group and 5.99 0.02 g/cm3 in the speed sintering group. The mean marginal gap was 61.5 9.0 mm in the conventional sintering group and 68.2 10.1 mm in the speed sintering group, showing no significant difference between the two groups. The mean internal gap was significantly greater (P < 0.001) in the speed sintering group (205.0 31.4 mm) than in the conventional sintering group (160.1 22.9 mm) (Fig. 3). In the fracture tests, it was observed that several large fragments separated from the abutment tooth in the crown specimens (Fig. 4). All of the crown specimens exhibited the same pattern of fracture. In the conventional sintering group, the mean fracture loads of the undegraded crowns and degraded crowns were 10,610 1211 N and 7845 833 N, respectively. In the speed sintering group, the mean fracture loads of the undegraded crowns and degraded crowns were 8300 759 N and 7173 788 N, respectively (Fig. 5). Only the fracture load of the undegraded crown specimens in the conventional sintering group was significantly greater (P < 0.001) than the other three conditions, i.e., degraded crowns in the conventional sintering group, and undegraded and degraded crowns in the speed sintering group. In the conventional sintering group, the occurrence of monoclinic crystals, an indicator of degradation, was 3.7 2.3% and 33.4 6.9% in the undegraded and degraded crowns, respectively, showing significantly higher occurrence in the degraded crowns (P < 0.001) (Fig. 6). In the speed sintering group, the occurrence of monoclinic crystals was also significantly higher in the degraded crowns (P < 0.001), with 6.7 4.7% and 37.3 13.7% in the undegraded and degraded crowns, respectively. No significant difference was observed in the occurrence of monoclinic crystals between the undegraded crowns of the conventional sintering group and the speed sintering group, or between the degraded crowns of the conventional sintering group and the speed sintering group. 4. Discussion The high-speed sintering furnace used in this study made use of induction heating by electromagnetic waves. This induction furnace was designed for chairside use and, unlike a conventional electric furnace, it does not allow multiple crowns to be sintered at the same time. Following measurement of the abutment tooth by
Please cite this article in press as: T. Nakamura, et al., In vitro investigation of fracture load and aging resistance of high-speed sintered monolithic tooth-borne zirconia crowns, J Prosthodont Res (2019), https://doi.org/10.1016/j.jpor.2019.07.003
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Fig. 3. Average marginal and internal gap of conventional sintering group specimens (CS) and speed sintering group specimens (SS). There is no significant difference between two groups. The average internal gap was significantly greater SS group than CS group (P < 0.001).
Fig. 4. The crown specimen after the fracture test. Several large fragments separated from the titanium abutment.
an intraoral scanner, a single crown is designed by the software, the temperature and time are altered according to the volume and shape of the crown, and the crown is sintered [9]. Additionally, the types of zirconia blocks that can be used in an induction furnace are limited. In this study, to examine differences resulting from the sintering method, we used the same translucent zirconia blocks to produce speed sintered crowns and to sinter crowns conventionally with a dental laboratory system. The block (inCoris TZI) was translucent zirconia containing 3 mol% yttria (3Y-TZP), which is a material that has long been used in monolithic zirconia restorations [13]. The bulk density of the crowns produced in this study was 5.9–6.0 g/cm3 in the conventional and speed sintering groups, which is close to the density of fully sintered dental 3Y-TZP (6.05–6.07 g/cm3) [14]. This result led us to infer that even though the sintering speed employed in this study was high, it achieved almost full sintering of
the 3Y-TZP, similar to conventional sintering. The finding that there was no difference in density between 3Y-TZP sintered rapidly or sintered conventionally was consistent with the result of highspeed sintering using microwaves [15]. In vitro studies on the amount of marginal gap in monolithic zirconia crowns have reported results in the range of 60.7–63.1 mm [16], and 68.7–77.9 mm [17]. The mean marginal gap in the two types of crowns produced in this study, at 61.5–68.2 mm, did not differ greatly from that of previous reports, and we consider this to be a gap size that does not present clinical problems. Additionally, sintering time has been reported to have no effect on the amount of marginal gap [18], and this study also found no difference in the amount of marginal gap as a result of the different sintering times. The mean internal gap inside the crown was larger in the speed sintering group (205.0 31.4 mm) than in the conventional sintering group (160.1 22.9 mm). The size of the internal gap in a CAD/CAM glass ceramic crown measured using the same method was reported to be 112–162 mm [10], and this is similar to the conventional sintering group in this study. The cement space was set to 70 mm in both the conventional sintering group and the speed sintering group; however, there was a significant difference in the mean internal gap inside the crown. A study using micro computed tomography reported that the fit of Y-TZP copings differed depending on the software [19]. In this study, we used different software for the conventional sintering group and the speed sintering group, and the fact that the size of the internal gap differed could be attributed to difference in the software. The fracture loads of crowns without aging ranged from 8300 N (speed sintering group) to 10,610 N (conventional sintering group). The fracture loads of high-strength glass ceramic lithium disilicatereinforced monolithic molar crowns has been reported as 2027 N [20] – 3147 N [21] for an occlusal surface thickness of 1.5 mm. Additionally, the fracture load of monolithic zirconia molar crowns with an occlusal surface thickness of 0.5 mm has been reported as 5558 N [22]. An accurate comparison is not possible because the crown shapes and loading conditions differ, but as in the previous report [21], the monolithic zirconia molar crown fracture loads obtained in this study appear to be much higher than those of
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Fig. 5. Fracture load of conventional sintering group specimens (CS) and speed sintering group specimens (SS). The same superscript letters were not significantly different.
Fig. 6. Monoclinic volume fraction of conventional sintering group specimens (CS) and speed sintering group specimens (SS). The same superscript letters were not significantly different.
monolithic glass ceramic molar crowns. The fracture loads of the speed sintered crowns were lower than those of the conventionally sintered crowns. It has been reported that when 3Y-TZP is sintered rapidly by spark plasma sintering, with a temperature increase of 500 C per minute and then held at 1600 C for 2 min, pores appear inside the zirconia sample and its fracture toughness declines [23], unlike when the temperature is increased by 50 C per minute. In this study, the total sintering time of the speed sintering was 15 min and, compared with conventional sintering, the rate of temperature increase was much faster and the sintering temperature, at 1580 C, was also higher. Therefore, in the speed sintering group, it is possible that a decline in fracture toughness caused by the pores observed in the previous report [23] led to a decrease in the crown fracture load. In this study, we investigated low-temperature degradation of the crowns using accelerated aging. Storage for 1 h in water at 134 C and 2 atm is said to be equivalent to 3–4 years in water at 37 C [24], so this study simulates several decades of intraoral use.
In the standard for Y-TZP used for biological implants, as an indicator of degradation, it is required that the monoclinic ratio does not exceed 25% after 5 h in water at 134 C and 2 atm [11]. 3Y-TZP containing approximately 3 mol% yttria is often used in dentistry. The zirconia block used in this study was also 3Y-TZP, but in order to enhance translucency, it contained very little alumina. It has been reported that the monoclinic ratio of ordinary dental 3Y-TZP containing alumina remains below 10% even after immersion in water for 10 h at 134 C and 2 atm following machining and grinding [25]. The monoclinic ratio after immersion for 10 h was high (33–37%). Considering the results, we infer that the 3Y-TZP was degraded. The large increase in the monoclinic ratio in the accelerated aging samples appears to be due to the fact that the translucent 3Y-TZP used contained very little alumina, which effectively controls degradation [26]. We consider the significant decrease in crown fracture load in the conventional sintering group following accelerated aging to be the effect of material degradation. Dental Y-TZP is reported to show no change in bending strength when its monoclinic ratio increases from 30% to 40% in accelerated aging [27,28]. In this study, the monoclinic ratio in the speed sintering group was also above 30% after accelerated aging, and yet there was no significant difference in crown fracture load compared with the crowns without aging. Degradation of Y-TZP progresses from the outer layer, and it is reported to progress to approximately 60 mm even when accelerated aging with 100 h in water at 134 C [28] is performed. We infer that the degradation of Y-TZP did not have a major effect on the crown fracture load. The crown fracture loads of both groups after accelerated aging were still high (7100–7800 N) and were not statistically significant. The null hypothesis was rejected. Given that the maximum bite force in humans is approximately 260–740 N [29], we consider the possibility of crown fracture to be extremely low, even if the Y-TZP were to degrade after long use. This in vitro study has some limitations. In the oral cavity, cyclic loading is applied to the occlusal surface of crowns; however, in this study, a static load has been applied. In the accelerated aging test, only an experimental crown was degraded, and the influence of an abutment tooth or luting cement was not considered. Further studies that replicate true oral conditions should be considered.
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5. Conclusion Within the limitations of this study, we found that monolithic zirconia crowns produced by high-speed sintering showed no major difference in density or fit compared with those produced by conventional sintering, and also showed no significant difference in fracture load after accelerated aging. These results suggest that chairside high-speed sintering is a valid method of producing monolithic zirconia crowns. References [1] Miyazaki T, Nakamura T, Matsumura H, Ban S, Kobayashi T. Current status of zirconia restoration. J Prosthodont Res 2013;57:236–61. [2] Ghodsi S, Jafarian Z. A review on translucent zirconia. Eur J Prosthodont Restor Dent 2018;26:62–74. [3] Zesewitz TF, Knauber AW, Northdurft FP. Fracture resistance of a selection of fullcontour all-ceramic crowns: an in vitro study. Int J Prosthodont 2014;27:264–6. [4] Passos SP, Torrealba Y, Major P, Linke B, Flores-Mir C, Nychka JA. In vitro wear behavior of zirconia opposing enamel: a systematic review. J Prosthodont 2014;23:593–601. [5] Abd El-Ghany OS, Sherief AH. Zirconia based ceramics, some clinical and biological aspects: review. Future Dent J 2016;2:55–64. [6] Hallmann L, Ulmer P, Reusser E, Louvel M, Hämmerle CHF. Effect of dopants and sintering temperature on microstructure and low temperature degradation of dental Y-TZP-zirconia. J Eur Ceram Soc 2012;32:4091–104. [7] Zhang Y. Making yttria-stabilized tetragonal zirconia translucent. Dent Mater 2014;30:1195–203. [8] Lawson S. Environmental degradation of zirconia ceramics. J Eur Ceram Soc 1995;15:485–502. [9] Wiedhahn K, Fritzsche G, Wiedhahn C, Schenk O. Zirconia crowns - the new standard for single-visit dentistry? Int J Comput Dent 2016;19:9–26. [10] Nakamura T, Dei N, Kojima T, Wakabayashi K. Marginal and internal fit of Cerec 3 CAD/CAM all-ceramic crowns. Int J Prosthodont 2003;16:244–8. [11] International Standard Organization No.13356. Implants for surgery -Ceramic materials based on yttria-stabilized tetragonal zirconia (Y-TZP). Geneva: International Organizanization for Standardization; 2015. [12] Garvie RC, Nicholson PS. Phase analysis in zirconia systems. J Am Cerm Soc 1972;55:303–5. [13] Vichi A, Carrabba M, Paravina R, Ferrari M. Translucency of ceramic materials for CEREC CAD/CAM system. J Esthet Restor Dent 2014;26:224–31.
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Please cite this article in press as: T. Nakamura, et al., In vitro investigation of fracture load and aging resistance of high-speed sintered monolithic tooth-borne zirconia crowns, J Prosthodont Res (2019), https://doi.org/10.1016/j.jpor.2019.07.003