Indian Journal of Dental Research

ORIGINAL RESEARCH
Year
: 2013  |  Volume : 24  |  Issue : 6  |  Page : 653--658

The effect of thermocycling on fracture toughness and hardness of different core build up materials


GS Shanthala1, Mathew K Xavier2,  
1 Department of Prosthodontics, Dr. DY Patil Dental College and Hospital, Mahesh Nagar, Pimpri, Pune, Maharashtra, India
2 Department of Prosthodontics, College of Dental Sciences, Davangere, Karnataka, India

Correspondence Address:
G S Shanthala
Department of Prosthodontics, Dr. DY Patil Dental College and Hospital, Mahesh Nagar, Pimpri, Pune, Maharashtra
India

Abstract

Introduction: Core build up materials are routinely used to restore grossly decayed teeth and in the oral environment they are subjected to changes in the temperature due to consumption of hot and cold food. Aims: The purpose of this study was to determine the effect of thermocycling on the fracture toughness and hardness of 5 core build up materials. Materials and Methods: Fifteen specimens were prepared for each of the following materials: DPI alloy, Miracle-mix, Vitremer, Fuji II LC and Photocore. American Standard for Testing Materials guidelines were used for the preparation of single-edge notch, bar-shaped specimens. Ten specimens of each material were thermocycled for 2000 cycles and the other 5 specimens were not thermocycled (non-thermocycled group). All specimens were subjected to 3-point bending in a universal testing machine. The load at fracture was recorded and the fracture toughness (K IC ) was calculated. Vickers hardness test was conducted on the thermocycled and non-thermocycled group specimens. Results: Photocore had the highest mean K IC in both thermocycled and non-thermocycled groups. Miracle-mix demonstrated the lowest mean fracture toughness (K IC ) for both thermocycled and non-thermocycled groups. By applying Mann Whitney SQUSQ test the Vickers hardness value in all materials used in the study is highly superior in non-thermocycled group as compared to thermocycled group (P < 0.01). Non-thermocycled Photocore showed highest hardness values of 87.93. Vitremer had lowest hardness of 40.48 in thermocycled group. Conclusion: Thermocycling process negatively affected the fracture toughness and hardness of the core build-up materials.



How to cite this article:
Shanthala G S, Xavier MK. The effect of thermocycling on fracture toughness and hardness of different core build up materials.Indian J Dent Res 2013;24:653-658


How to cite this URL:
Shanthala G S, Xavier MK. The effect of thermocycling on fracture toughness and hardness of different core build up materials. Indian J Dent Res [serial online] 2013 [cited 2020 Sep 20 ];24:653-658
Available from: http://www.ijdr.in/text.asp?2013/24/6/653/127603


Full Text

A common problem facing dentists in clinical situations is the "build up" of tooth structure with a core material after fracture or extensive caries removal. [1] Fracture toughness is described as the tendency of a brittle material to resist catastrophic crack propagation. [2] In the oral environment, restorative materials are subjected to a wide variety of temperatures. [3] Thermocycling of selected samples is done to artificially age some specimens by creating a temperature difference. [4] The aim of this study was to determine thermocycling effect on fracture toughness and hardness of core build up materials.

 Materials and Methods



Following materials [Table 1] and [Figure 1], instruments and equipments [Figure 2] were utilized for determining the effect of thermocycling on fracture toughness and hardness of 5 core build-up materials.{Table 1}{Figure 1}{Figure 2}

Preparation of the specimen

The specimens were prepared using a brass mould. Dimensions of the specimen were 2 × 4× 20 mm with a 2 mm length and 2 mm depth notch on one edge. This notch was formed using a new no. 15 surgical blade (Swann-Morton, Sheffield, England). The prepared specimens confirmed to the American standard for testing materials (ASTM) guidelines for the single edge notch bar-shaped specimen-standard E-399. [2] A total of 75 specimens were prepared [Figure 3]. All dimensional measurements of the specimens were made with a digital caliper.{Figure 3}

Distribution of specimens

A total of 75 specimens were tested.

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Thermocycling

Thermocycling was carried out in two stainless steel water baths filled with deionized water [Figure 4]. A digital thermostat controlled the temperature of the hot water bath at 60°C (±2°C). Cold water bath was controlled by the addition of ice to maintain the water bath at 3°C (±2°C). Specimens selected for thermocycling were subjected to 2000 cycles. Immersion time in each water bath was 30 seconds per cycle.{Figure 4}

Testing of specimens for fracture toughness

A universal testing machine was allowed to warm up for 1 h before routine calibration and use. A compression load cell (1 to 50 kg) was used at a 10-kg full-scale range. The specimens were placed on a 2-point supporting fixture with a span of 16 mm between the supports. This allowed the testing of specimens in a 3-point bending mode as shown in [Figure 5]. The load was applied over the specimen notch with a loading point attached to the cross head at a cross-head speed of 0.51 mm/min. A schematic diagram of load applied on single edge notch specimen is shown in the [Figure 6]. The load at which fracture of the specimens occurred was recorded.{Figure 5}{Figure 6}

The fracture toughness was calculated by using following equation 2

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Microhardness

Microhardness testing will be carried out for each specimen immediately after fracture toughness testing. The specimens were polished using pumice and rubber cup. Vickers hardness test was performed at 200 g load using Microhardness testing machine - Zwick/Roell ZHV Indentec [Figure 7]. 2-3 indentations were made in each specimen and the mean of indentations was calculated. [5] Vickers hardness values obtained for amalgam thermocycled and non-thermocycled groups are shown in [Figure 8] and [Figure 9].{Figure 7}{Figure 8}{Figure 9}

Statistical analysis

Data was analyzed by descriptive statistics such as mean, SD etc., A non-parametric test Mann-Whitney 'U' test for independent samples was applied to find out which material is better/superior than the other. The statistical software namely SYSTAT version 12 was applied to analyze the data under study.

 Results



Fracture toughness test: The mean fracture toughness and standard deviation of both non-thermocycled and thermocycled specimens are listed in [Table 2]. Results of Mann-Whitney 'U" test for fracture toughness indicated that mean fracture toughness is highly superior in non-thermocycled group as compared to thermocycled group (P < 0.01). Graph 1 shows the comparison of fracture toughness values between non-thermocycled and thermocyled groups for different materials.{Table 2}

[INLINE:3]

Microhardness test

The mean and standard deviation of the Vickers hardness values for both non-thermocycled and thermocycled groups are listed in [Table 3]. By applying Mann-Whitney 'U" test, the Vickers hardness value in all materials used in the study is highly superior in non-thermocycled group as compared to thermocycled group (P < 0.01). Non-thermocycled photocore showed highest hardness values of 87.93. Vitremer had lowest hardness of 40.48 in thermocycled group. Graph 2 depicts the difference in hardness values for non-thermocycled and thermocycled groups among various core build-up materials.{Table 3}

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 Discussion



The data are discussed in two parts. In part I, effect of thermocycling on fracture toughness is discussed, and in part II effect of thermocycling on hardness of core build-up materials is considered.

Part I: The amalgam chosen for this study was a typical, high-copper alloy used routinely in clinical dentistry. Of the many variables which may influence the fracture toughness of amalgam alloy are alloy composition, particle morphology, and mercury/alloy ratio. It has been shown that fracture toughness decreases as the silver content decreases. High copper amalgams have lower fracture toughness (K IC ) when compared to low copper amalgams. The increased addition of copper produces a proportional reduction in toughness. [6]

Miracle mix used in this study is a reinforced glass ionomer. This glass ionomer material is a mixture of Fuji II powder and amalgam alloy powder in a ratio of 7:1. [7]

Vitremer is a tri-cure resin-modified glass ionomer cement which, utilizes a microencapsulated water-soluble ascorbic acid/potassium persulfate redox catalyst system in order to obtain a chemical-cure, free-radical methacrylate setting reaction in addition to its light cure and glass-ionomer setting modes (Mitra and Mitra 1992). Fuji II LC is a tri-cure resin-modified glass-ionomer cement, which contains the more conventional peroxide/tertiary amine (or sulfinic salt) catalyst accelerator system (Akahane et al. 1991). [8]

It has been found that fracture toughness of composites was related to filler composition and degree of conversion in the composite resins. It is suggested that crack propagation in dental composite resins is mainly through the matrix, but occasionally cleavage or de-bonding of the larger filler particles in the composite occurs as well. [9]

In 1985, Lloyd and Adamson determined the fracture toughness of various high-copper amalgams. The K IC value recorded for Valiant was 1.291 ± 0.03 MN. m−1.5 , which matched closely with the value obtained in this study for non-thermocycled amalgam specimens. In 1990, research was conducted on Glass ionomers by Lloyd and Butchart. [10] This study found that Miracle mix had a fracture toughness of 0.21 ± 0.05 MN. m−1.5 . The K IC value obtained for Miracle mix thermocycled specimen in the present study was equivalent with that obtained by Lloyd and Butchart study. Medina Tirado et al., found the Vitremer K IC values as 0.82 ± 0.11 MN.m−1.5 (non-thermocycled group) and 0.66 ± 0.27 MN.m−1.5 (thermocycled group). [2] These values were consistent with the present study. Bonilla and Mardirossian found the K IC value of Vitremer to be 0.75 ± 0.06 MN.m−1.5 . [11]

The mean value obtained for thermocycled amalgam was significantly lower than that for non-thermocycled specimens. This decrease in fracture toughness can be attributed to

Increased corrosion of the materialA change in the matrix that allowed for lower energy fracture pathways or a change in the coefficient of thermal expansion. [2]

Fuji II LC showed significant difference between thermocycled and non-thermocycled groups and this is because glass ionomers are very sensitive to moisture to which they are exposed. Cattani Lorente et al., in 1999 studied the effect of water on physical properties of resin-modified glass-ionomer cements. [12] Their study revealed that resin-modified GIC absorbed rather high amounts of water. A correlation was established between decrease in their physical properties and the water uptake. Therefore water along with high temperature difference during thermocycling caused a relative decrease in fracture toughness values of Fuji II LC and Vitremer (resin-modified glass-ionomer). Miracle mix mixture exhibited highly significant difference between thermocycled and non-thermocycled group. This can be again correlated with its glass ionomer composition.

Photocore showed highly significant differences between thermocycled and non-thermocycled groups. This is because fracture toughness (K IC ) of dental composites was found to reduce if stored in water. [3]

Part II: Results of Vickers hardness test indicated that non-thermocycled Photocore (87.93 ± 2.4) and amalgam (85.86 ± 5.31) had highest hardness values. A study by Pedigoni ML et al., found the Vickers hardness value of amalgam as 71.4, which was similar to the values obtained in this study. [13] Miracle mix which was lowest in fracture toughness showed higher hardness than resin-modified GIC i.e., Vitremer and Fuji II LC. Among resin-modified GIC, Vitremer showed lowest hardness values than Fuji II LC. In 1996, Uno S. et al. found that Fuji II LC had HV of 66.1 ± 1.6 and Vitremer about 62.9 ± 1.3 at 1 month storage in water and the load applied was 1N. [14] Slightly lower hardness values obtained in this study for Fuji II LC and Vitremer may be attributed to difference in load (200 g used in this study). In another study, Fuji II LC had 100 HV and Vitremer showed 75 HV under 100 g load. [12] Microhardness of GIC stored in water was decreased due to softening effect on it. [15]

Photocore contains silanated glass powder and silanated barium glass powder, which are not found in other composites. Glass and its translucency can cause a high depth of curing and hardness for the composite. Review of literature showed Photocore Vickers hardness value as 77.742 ± 0.692, which can be correlated to Vickers hardness value of thermocycled group in this study. [16]

If the final restoration is a full crown with margins on tooth structure, the effect of thermal changes and moisture on the core material may be diminished. However, preliminary failure of luting cement caused by cyclic loading is clinically undetectable and allows leakage between the restoration and the tooth. Once the seal is broken, moisture contamination and thermal changes may play an important role in core degradation. [2] Direct core materials that maintain high fracture toughness and hardness after thermocycling are preferable and offer an extra margin of safety.

 Conclusion



Photocore (resin composite) and DPI alloy (amalgam) showed the highest fracture toughness of all the materials tested under both thermocycled and non-thermocycled conditions. Miracle mix had the lowest fracture toughness valuePhotocore and DPI alloy had similar Vickers hardness values. While Vitremer had lowest hardness values among all the other materials tested.

The results indicated that thermocycling negatively affected the fracture toughness and hardness of core build-up materials

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