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ORIGINAL ARTICLE
Year : 2018  |  Volume : 21  |  Issue : 3  |  Page : 380-387

In vitro Fracture strength and hardness of different computer-aided design/computer-aided manufacturing inlays


1 Department of Restorative Treatment, Faculty of Dentistry, Ataturk University, Erzurum, Turkey
2 Department of Mechanical Engineering, Faculty of Engineering, Erzincan University, Erzincan, Turkey
3 Cumhuriyet District, Tepebasi/Eskisehir, Turkey

Date of Acceptance17-Mar-2017
Date of Web Publication09-Mar-2018

Correspondence Address:
Dr. O Sagsoz
Department of Restorative Treatment, Faculty of Dentistry, Ataturk University, Erzurum
Turkey
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/njcp.njcp_58_17

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   Abstract 


Objective: The purpose of this study was to examine the fracture strength and surface microhardness of computer-aided design/computer-aided manufacturing (CAD/CAM) materials in vitro. Materials and Methods: Mesial-occlusal-distal inlays were made from five different CAD/CAM materials (feldspathic ceramic, CEREC blocs; leucite-reinforced ceramic, IPS Empress CAD; resin nano ceramic, 3M ESPE Lava Ultimate; hybrid ceramic, VITA Enamic; and lithium disilicate ceramic, IPS e.max CAD) using CEREC 4 CAD/CAM system. Samples were adhesively cemented to metal analogs with a resin cement (3M ESPE, U200). The fracture tests were carried out with a universal testing machine. Furthermore, five samples were prepared from each CAD/CAM material for micro-Vickers hardness test. Data were analyzed with statistics software SPSS 20 (IBM Corp., New York, USA). Results: Fracture strength of lithium disilicate inlays (3949 N) was found to be higher than other ceramic inlays (P < 0.05). There was no difference between other inlays statistically (P > 0.05). The highest micro-Vickers hardness was measured in lithium disilicate samples, and the lowest was in resin nano ceramic samples. Conclusion: Fracture strength results demonstrate that inlays can withstand the forces in the mouth. Statistical results showed that fracture strength and micro-Vickers hardness of feldspathic ceramic, leucite-reinforced ceramic, and lithium disilicate ceramic materials had a positive correlation.

Keywords: Ceramics, computer-aided design/computer-aided manufacturing, fracture strength, inlays, micro-Vickers hardness


How to cite this article:
Sagsoz O, Yildiz M, Hojjat Ghahramanzadeh A S, Alsaran A. In vitro Fracture strength and hardness of different computer-aided design/computer-aided manufacturing inlays. Niger J Clin Pract 2018;21:380-7

How to cite this URL:
Sagsoz O, Yildiz M, Hojjat Ghahramanzadeh A S, Alsaran A. In vitro Fracture strength and hardness of different computer-aided design/computer-aided manufacturing inlays. Niger J Clin Pract [serial online] 2018 [cited 2019 Nov 22];21:380-7. Available from: http://www.njcponline.com/text.asp?2018/21/3/380/226979




   Introduction Top


Ceramic materials were first used at the end of the 1700s for dental purposes. The first all-ceramic restoration was introduced by Land in 1889.[1] In the first years, popularity of ceramic materials was mostly dependent on their superior esthetic properties and attainability. On the other hand, their fragile nature limited the usage.[2] The brittle nature of the ceramics may depend on the inherent flaws within the material. In addition, external loading causes fracture which can be propagated by cracks starting at inherent flaws.[3] This phenomenon is similar to a theory explained by Griffith, about glass fracture that occurs earlier than it should be. In this theory, microcracks existing in the structure of glass or formed subsequently cause material weakening and fracture under lower loads. Due to inadequate fracture resistances causing early failure of primitive ceramics, investigations have been made to improve the material's mechanical properties.[3]

Ceramics have superior properties to other materials such as providing good heat insulation, biocompatibility, inertness, and showing great esthetics;[4] however, ceramics are still prone to fracture in mouth conditions due to their structural features.[5] The reinforcements made in recent years are controversial whether they are adequate or not. Ceramic materials could be just used for single restorations or their usage for fixed partial dentures could be with appropriate substructures. Another disadvantage is the necessity of indirect application of ceramic materials, which causes clinical procedures to take longer time than direct restorations. However, there are benefits, such as eliminating the risks of polymerization shrinkage, micro-leakage, and postoperative sensitivity, which can be caused by direct composite restorations. In addition, dental CAD/CAM systems allow chairside restorations to end at one appointment, shortening the treatment period. Furthermore, ceramic blocks fabricated for CAD/CAM systems are more homogeneous and contain fewer flaws and cracks in comparison with dental laboratory-processed ceramics.[5]

Ceramics used in CAD/CAM systems should be milled fast; finishing procedures such as polishing, glazing, and coloring should be done easily; these ceramics should be resistant to the milling process; they should prepare clinically acceptable restorations; and they should not cause excessive wear to opposing teeth. Glass ceramics reinforced with leucite and lithium disilicate have recently been developed.[6],[7] In addition to these, resin nano ceramic and hybrid ceramic CAD/CAM blocks where composite and ceramic materials are used together have also been put on the market. The main purpose of these developments is to prepare restorations that are long-lasting and resistant to forces in mouth, without compromising on esthetics.

Fracture tests of ceramic materials are performed with shear tests, three or four-point bending tests, fracture toughness tests, and fractography by preparing geometric samples. In these test methods, measurements are carried out with simple geometric samples and may not exactly represent clinical conditions. Ceramic restorations can also be tested in terms of their fracture resistance in the anatomic form they are used in clinic. This approach can be more useful for determining the behavior of the material.[8] This test is performed on samples such as inlay, crown, or bridge fractured under load. However, it is difficult to compare the study results with each other since each study can be designed in a different way in this test method.[9] Despite this, a material that is recently put on the market can be compared to materials used in previous studies by being tested in similar laboratory conditions.[10] Even this method cannot provide exact clinic conditions, so there may be differences.[11]

In addition to fracture resistance tests, surface hardness tests are also useful for obtaining information about mechanical properties of materials. In literature, there are only a few studies examining the relation between the fracture strength and surface hardness of CAD/CAM materials.

Newly developed ceramic materials can be both milled in laboratory and used in CAD/CAM systems. The purpose of this study is to evaluate the fracture strength of inlays and micro-Vickers hardness of samples prepared with prefabricated CAD/CAM materials that have different contents. Here are the null hypotheses: fracture strength would not vary among ceramics that have different contents. Surface hardness of ceramics would not be related to fracture strength.


   Materials and Methods Top


In this study, the fracture strength and surface hardness measurements of samples, prepared from five different CAD/CAM blocks, were tested in vitro. CAD/CAM materials used in this study are shown in [Table 1].
Table 1: Computer-aided design/computer-aided manufacturing materials used in this study and their compositions

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First, an inlay cavity was prepared before metal analogs shaped for fracture strength tests. A mesial-occlusal-distal inlay cavity was prepared in an upper right second premolar tooth that was made of solid plastic (Frasaco GmbH, Tettnang, Germany) with an aerator under water cooling. The sizes of the tooth used were 6 mm in the mesiodistal direction and 8 mm in the vestibulopalatinal direction, and the length of the crown was 7.5 mm. The inlay cavity was prepared as the distance from the deepest point (fissure; anatomical area in the middle of the restoration between tooth cusps) to the cavity floor (the biggest horizontal area lying between shoulders) was 1.5 mm; the size of the proximal surfaces in the vestibulopalatinal direction was 4 mm; the shortest isthmus distance in the vestibulopalatinal direction was 4 mm on the occlusal surface; the width of the shoulder was 1.5 mm; the depth of the shoulder was 1.5 mm from the cavity floor; and the cavosurface angle from the cavity base to the occlusal surface was 6° [Figure 1].
Figure 1: Schematic view of the inlay cavity

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Fifty wax samples were prepared after taking impressions separately for each sample with silicone-based impression material (Elite double, Zhermack, Rovigo, Italy) from the prepared upper second premolar tooth. Wax samples were taken to investment, and Co-Cr casting and leveling were performed. The roots of the obtained metal analogs were embedded in cold acrylic in a distance of 2 mm to the cementoenamel junction.

In this study, five different CAD/CAM ceramic blocks were used. Ten ceramic samples were prepared on ten metal analogs allocated for each group (n = 10 and n = 50).

Optical impression of metal analogs was taken with CEREC Bluecam (Sirona Dental Systems Gmbh, Bensheim, Germany). Before this process, a reflective spray (CEREC Optispray, Sirona Systems Gmbh, Bensheim, Germany) was used to make the analogs suitable for the impression process. The optical impression was processed with CEREC 4 software (Sirona Dental Systems Gmbh, Bensheim, Germany) and a model was obtained in the software. Cement thickness (90 μm), preparation margins, and restoration insertion axis were determined, and the restoration was formed on this model. And then, fifty restoration milling processes were done with CEREC MCXL milling device [Figure 2].
Figure 2: Creating an inlay. (a) Reflective spray, (b) optical impression with CEREC BlueCam, (c) tooth model, (d) drawing margins on the model, (e) inlay model in the cavity, (f) inlay model in the computer-aided design/computer-aided manufacturing block, (g) milling, (h) inlay milled from a computer-aided design/computer-aided manufacturing block

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The adaptation of each of the fifty inlays was checked, and they were cemented with a dual cure resin cement (RelyX U200, 3M ESPE, USA). Then, excess resin cements were removed. After the samples were kept under the load of 50 N for 5 min, the cementation process was ended. Cemented inlays were stored in distilled water at 37°C for 1 week.

Shimadzu AG-IS 1000 (Shimadzu Corporation, Kyoto, Japan) universal test device was used for the fracture strength tests. Before the fracture strength tests, the samples were fixed to the lower jaw of the test device in a way that the load would be applied vertically. With the round chrome cusp in diameter of 3.5 mm fixed to the mobile upper jaw of the device, load was applied over the central fossa and cusp slopes of the teeth [Figure 3] and [Figure 4].
Figure 3: Fracture strength test

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Figure 4: Areas (blue color) showing the loading indenter placed on inlays

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The cross-head speed of the chrome cusp was arranged to be 1 mm/min. According to the graphics showing the rates which were displayed on the computer monitor simultaneously with the test, when the increasing load suddenly dropped to zero, it was presumed that the fracture occurred and the test was ended. The highest values were recorded as fracture strength values.

To measure the surface hardness of the materials, micro-Vickers hardness test was carried out. Five samples in the shape of rectangular prism were obtained from the ceramic blocks in each group. These samples were cut from the blocks with a low-speed diamond saw (Isomet 1000, Buehler, Lake Bluff, IL, USA). The obtained samples were wet ground with 800-, 1200-, and 2000- silicon carbide paper. The samples were tested under 100 g load for 10 s with a micro-Vickers hardness tester (Future Tech FM 800e, Future Tech Corp, Tokyo, Japan).

Data were analyzed with one-way ANOVA and Tukey's post hoc test (α =0.05). The relation between fracture strength and hardness was tested with Pearson's correlation. Two groups were created for correlation analysis (Gcer: feldspathic ceramic, leucite-reinforced ceramic, and lithium disilicate ceramic; Gcom: Resin nano ceramic and hybrid ceramic) because of the different material properties.


   Results Top


[Table 2] summarizes the mean fracture strength and micro-Vickers hardness of CAD/CAM materials.
Table 2: Mean fracture strength, micro-Vickers hardness and standard deviations of computer-aided design/computer-aided manufacturing materials

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Fracture graphics are shown in [Figure 5], and fractured inlays are seen in [Figure 6].
Figure 5: Fracture strength-stroke graphic

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Figure 6: Fractured samples

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The fracture strength of lithium disilicate (IPS e.max CAD) inlays was statistically higher than other inlays (P< 0.05). There was no difference found between other groups.

The micro-Vickers hardness of lithium disilicate (IPS e.max CAD) samples was higher than other samples. The micro-Vickers hardness of resin nano ceramic (Lava Ultimate) samples was significantly lower than other samples. There was no significant difference found between feldspar (CEREC Blocs) and leucite-reinforced (IPS Empress) samples.

There was a positive correlation (r = 0.653) between fracture strength and hardness in Gcer (P< 0.01). The correlation was poor in Gcom (r = 0.084, P = 0.725).


   Discussion Top


In vitro experiments used for the failure analysis are methods that make significant contributions to the development of restorative procedures.[12],[13] Experimental fracture tests are used to convert the fracture resistance of the restorative materials to numeric data.[13],[14] In these tests, generally fracture forces exceeding chewing forces that occur in the stomatognathic system.[15],[16] In addition, high forces that occur during the experiment can be associated with the force concentrated on one tooth while a person is chewing a solid substance.[14]

The reason why natural teeth were not used in the study was that they had disadvantages (structural variations, patient's age, and waiting time after extraction) affecting the standardization in a negative way.[17],[18] To prevent from the differences, those occur in the preparation and in the loading direction, identical analogs, and loading conditions were used in all samples. To do this, metal analogs were prepared instead of natural teeth in the fracture test. Accordingly, used metal analogs provided equal conditions for different ceramic materials to be compared. At the same time, the use of these analogs was considered to be suitable because the load was applied only on inlay restorations and these analogs gave enough support to these restorations.

Rekow and Thompson[19] reported that the cement thickness can vary between 20 and 200 μm. Furthermore, Liu et al.[20] showed that the ideal cement thickness is 90 μm so that the least stress formation occurs in the restoration. In the present study, the cement thickness was determined as 90 μm using CEREC 4 software in all groups. On the other hand, it has been claimed that cement thickness has inferior importance in overloading fracture tests.[20]

Different test devices were used so as to obtain fractures similar to fractures in vivo. Dietschi et al.[21] concluded that applying compression loads on the occlusal surface of the ceramic inlay with a small sphere was the most appropriate method to obtain fractures similar to fractures in vivo. In literature, metal antagonists in diameter of 24 mm and in the shape of a sphere or a round cusp were used to measure the fracture resistance of the premolar teeth.[21],[22],[23],[24] However, in this study, a metal antagonist cusp in diameter of 3.5 mm, imitating a premolar tooth cusp, was used as in the study of Yıldız et al.[23]

In the light of the obtained data, it was found that statistically the fracture resistance of ceramic inlays prepared with lithium disilicate (IPS e.max CAD) was significantly higher than the inlays prepared with other materials (P< 0.05). No significant difference was discovered among feldspathic ceramic (CEREC blocs), leucite-reinforced ceramic (IPS Empress), and two different resin nano ceramics (3M ESPE Lava Ultimate and VITA Enamic) (P > 0.05). The success of IPS e.max CAD inlays can be associated with lithium disilicate crystals they involve because these crystals have the characteristics of preventing fracture formation and progress.[25] The least mean fracture strength was in IPS Empress (1942.04 N) and is superior from the average occlusal force in vivo that is about 212 N.[26]

In literature, there were not many studies about the fracture resistance of resin nano ceramic and hybrid ceramic inlays. The modulus of elasticity of these materials is close to composite materials that are used in dentistry. Accordingly, we can talk about studies that compare ceramic materials to composite materials. Similar to our findings, Liu et al.[27] compared the fracture resistance of feldspathic ceramic (VITA Mark II) prepared with CAD/CAM and composite (3M ESPE MZ100) inlays and did not find a significant difference between the two. Costa et al.[28] found out a statistically significant difference between lithium disilicate ceramic (IPS e.max Pres) inlays and composite (Signum ceramis) inlays in terms of fracture resistance. Magne et al.[29] researched the fracture resistance of CAD/CAM inlays under increasing sudden loadings and discovered that lithium ceramic (IPS e.max CAD) was more resistant than composite (3M ESPE MZ100) and feldspathic ceramics (VITA Mark II). In contrast to our research findings, they concluded that composite inlays were more resistant to fractures than inlays prepared with feldspathic ceramic.

Clausen et al.[30] compared the fracture resistance of full ceramic crowns and found out that lithium disilicate ceramic (IPS e.max Press) crowns were more resistant than leucite-reinforced ceramic (IPS Empress esthetic) crowns. Chen et al.[31] indicated that leucite-reinforced ceramic (ProCAD) crowns had similar fracture resistance to feldspathic ceramic (VITA Mark II) crowns. Homaei et al.[32] investigated the fatigue resistance of lithium disilicate ceramic (IPS e.max CAD) and hybrid ceramic (VITA Enamic) and found that the lithium disilicate ceramic was more resistant. The differences among the materials used in those studies are consistent with the results of our study. Carvalho et al.[33] in a study conducted that the fracture resistance of lithium disilicate ceramic (IPS e.max CAD) and resin nano ceramic crowns (3M ESPE Lava Ultimate) was higher than the fracture resistance of feldspathic ceramic crowns (VITA Mark II). However, unlike the results of our study, it was stated that there was no significant difference between lithium disilicate ceramic crowns and resin nano ceramic crowns. In another study which resulted different from the current study, fracture strength of ultrathin occlusal veneer restorations was compared. The fracture strength of LAVA Ultimate was found significantly higher than Enamic.[34]

The surface hardness is a resistance indicator of a material to external forces[35] and has been used to characterize the mechanical properties of materials. Homaei et al.[36] found that the Vickers hardness of lithium disilicate ceramic - e.max CAD (676.7 Hv) was higher than hybrid ceramic - VITA Enamic (261.7 Hv), well matched with the current study. Lauvahutanon et al.[35] investigated the Vickers hardness of CAD/CAM blocks and reported significant differences between feldspathic ceramic (Mark II, VITA), resin nano ceramic (Lava Ultimate, 3M ESPE), and hybrid ceramic (Enamic, VITA); similar with the current study. Mean Vickers hardness (Hv) values were 454.8, 97.9, and 189.8, respectively. Furthermore, Mormann et al.[37] studied the Martens hardness of the same materials. They have found that lithium disilicate ceramic (IPS e.max) hardness was significantly higher than others that well matched with our results. There was significant difference found between leucite-reinforced ceramic (IPS Empress) and feldspathic ceramic (Mark II, VITA) and no significant difference between resin nano ceramic (Lava Ultimate, 3M ESPE) and hybrid ceramic (Enamic, VITA). These results are contradictory with our results and could be explained using different hardness measurement methods.

The homogeneity of prefabricated CAD/CAM blocks means that mechanical properties, such as strength, of entire inlay structure could be estimated using surface hardness values.[38] Accordingly, the relation between fracture strength and surface hardness was tested, and the null hypothesis for Gcer was rejected. The results show that the fracture strength of the materials is increasing with the increase of the hardness value among Gcer and Gcomp.

Understanding the relationship between the hardness and strength of materials is very important for a variety of reasons.[39] Hardness testing has some advantages over strength tests (fast, inexpensive, non-destructive). Furthermore, hardness testing is often the only choice for small scale materials that are insufficient for strength tests. Having a reliable relationship with strength makes the hardness results more crucial.[40]

There is a ratio between strength and hardness, which are important properties of materials. This ratio is exact for metal and some metallic glasses but increases in ceramic and decreases in ductile materials. The lower ratio in materials with relatively high ductility is caused by insufficient hardening under hardness test.[38] The ratio difference is also observed in the present study between Gcer and Gcomp.

When Gcer and Gcom were investigated separately, a positive correlation was found in Gcer and no correlation was found in Gcom. This time, it should be noted that besides the difference in material properties, the study design could produce such results. Since the elastic properties of composite materials are less than ceramic materials, higher stress will be transferred to their substructure and lower stress will be formed in the restoration.[28],[41] Metal analogs could be exposed to higher external forces and withstand in Gcom during fracture strength tests. In Gcer, there was a larger stress in inlays, withstanding the forces themselves. The poor correlation in Gcom could be explained as inlays were exposed to forces together with metal analogs in fracture strength tests. However, in hardness tests, the load was applied to only surface of the materials.

Fatigue occurrence would be observed in inlay materials under continuous forces in mouth and there could be fractures under lower forces. In addition, the change of temperature in mouth could also affect the fracture resistance. Studies considering these issues will show the effect of conditions in mouth in a better way.


   Conclusion Top


The results obtained from this study:

  1. In in vitro experiments, the fracture resistance of lithium disilicate ceramic inlays was found to be significantly higher than the fracture resistance of other ceramics. It was found out that reinforcing the ceramic structure with leucite or adding resin to the structure did not affect the fracture resistance
  2. The measured values of the surface hardness and fracture resistance were in direct proportion in Gcer. The correlation was poor in Gcom
  3. According to the findings, it was found that all ceramics tested were resistant to the normal forces that occur in mouth.


Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
 
 
    Tables

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