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Journal of Applied Oral Science

Print version ISSN 1678-7757

J. Appl. Oral Sci. vol.19 no.2 Bauru Mar./Apr. 2011

http://dx.doi.org/10.1590/S1678-77572011000200003 

ORIGINAL ARTICLES

 

Flexural strength of fluorapatite-leucite and fuorapatite porcelains exposed to erosive agents in cyclic immersion

 

 

Peerapong JunpoomI; Boonlert KukiattrakoonII; Chanothai HengtrakoolII

IDDS, MSc, Section of Dental Public Health, Huay Yod Hospital, Trang, Thailand
IIDDS, MSc, Associate Professor, Department of Conservative Dentistry, Faculty of Dentistry, Prince of Songkla University, Hat Yai, Songkhla, Thailand
IIIDDS, MSc, PhD, Assistant Professor, Department of Conservative Dentistry, Faculty of Dentistry, Prince of Songkla University, Hat Yai, Songkhla, Thailand

Corresponding address

 

 


ABSTRACT

OBJECTIVE: The aim of this study was to evaluate the fexural strength of two porcelain materials (IPS d.SIGN and IPS e.max Ceram) exposed to erosive agents.
MATERIAL AND METHODS: One hundred and twenty bar-shaped specimens were made from each of fuorapatite-leucite porcelain (IPS d.SIGN) and fuorapatite porcelain (IPS e.max Ceram) and divided into 8 groups of 15 specimens each. Six groups were alternately immersed in the following storage agents for 30 min: deionized water (control), citrate buffer solution, pineapple juice, green mango juice, cola soft drink and 4% acetic acid. Then, they were immersed for 5 min in deionized water at 37ºC. Seven cycles were completed, totalizing 245 min. A 7th group was continuously immersed in 4% acetic acid at 80ºC for 16 h. The final, 8th, group was stored dry at 37ºC for 245 min. Three-point bending tests were performed in a universal testing machine. The data were analyzed statistically by 2-way ANOVA, Tukey's HSD test and t-test at signifcance level of 0.05.
RESULTS: The fexural strengths of all groups of each porcelain after exposure to erosive agents in cyclic immersion did not differ signifcantly (p>0.05). For both types of porcelain, dry storage at 37ºC yielded the highest fexural strength, though without signifcant difference from the other groups (p>0.05). The fexural strengths of all groups of fuorapatite porcelains were signifcantly higher (p<0.05) than those of the fuorapatite-leucite porcelains.
CONCLUSIONS: This study demonstrated that the erosive agents evaluated did not affect the fexural strength of the tested dental porcelains.

Key words: Dental porcelain. Erosion. Immersion. Juices. Soft drinks.


 

 

INTRODUCTION

Porcelains are highly esthetic materials extensively used in dentistry to construct various types of restorations and prostheses such as porcelain fused to metal crowns, veneers, inlays, onlays and all ceramic restorations. They fulfll the esthetic and functional demands of the patients by their superior properties when compared to other restorative materials26. The new glass ceramics (IPS d.SIGN; Ivoclar Vivadent AG, Schaan, Liechtenstein) have become popular for porcelain-fused-to-metal restorations. IPS d.SIGN is a new type feldspathic based porcelain containing dispersed fuorapatite and leucite crystals in a feldspathic glassy matrix10. The leucite crystals (<3 µm) present in the IPS d.SIGN porcelain also contribute to the overall strength28. Recently, the new all ceramic systems (IPS e.max; Ivoclar Vivadent AG,) have been introduced into the market. IPS e.max Ceram is a veneering porcelain of this system which is a feldspathic-based porcelain having a microstructure unlike IPS d.SIGN. This porcelain only consists of dispersed fuorapatite crystals in a feldspathic glassy matrix; thus, having a microstructure unlike that of any other commercially available dental porcelains28. Fluorapatite crystals, 2-5 µm in length and 300 nm in diameter of needle-like morphology, are known to be contained in natural bone and teeth. These very small crystals in dental microstructures result in very special optical properties such as translucence and opalescence, which also result the same properties as in dental restorations11.

Despite the outstanding esthetic quality of the porcelains, the most serious problem of this material is its susceptibility to fractures9,15. Porcelains exhibit inherent faws or defects on the surface and body. These characteristics impair their physical properties such as surface roughness, surface hardness, strength3,6and infuence the clinical success and failure of porcelain restorations7. Crack propagation and degradation of dental porcelains occur when porcelains are exposed to aqueous solutions or erosive agents1. These phenomenons take place as a result of selective leaching of alkaline ions. Alkaline metal ions are far less stable in the glass phase than in the crystalline phase1. Therefore, some alkaline ions in porcelains were leached after being exposed to acidic solutions17-19,24. Variation in pH, solution chemistry, wear and mechanical load makes the oral cavity a complex environment1. Environmental conditions may also damage resistance to surface and bulk degradation of porcelains. Consequences of porcelain degradation include coarseness of exposed surface4,24, promoting plaque accumulation1,2,4,24 and wear to antagonist materials1. Furthermore, increasing of surface roughness of porcelains may decrease strength9,15.

Many people frequently consume acidic food, sour fruits and drinks. This consuming habit relates to a high incidence of dental erosion14,16,23,27. The potential erosive effect of these acidic food and beverages on enamel occurs primarily by the dissolution of apatite crystals12,16,20. However, their effect on the porcelain restorations has not been clearly documented. Therefore, the present in vitro study was designed to evaluate changes of the fexural strength and surface of fuorapatite-leucite and fuorapatite porcelains after being exposed to erosive agents (pineapple juice, green mango juice, coca soft drink, citrate buffer solution and 4% acetic acid) in cyclic immersion. The null hypothesis was that there was no signifcant difference in fexural strength of each type of dental porcelain tested after being exposed to erosive agents.

 

MATERIAL AND METHODS

Specimen Preparation

Two commercial dentin shade A3 porcelain powders were used: IPS d.SIGN and IPS e.max Ceram (Ivoclar Vivadent AG) (Figure 1). IPS d.SIGN and IPS e.max Ceram are indicated to be used as veneering porcelain for porcelain fused to metal and all ceramic restorations, respectively. One hundred and twenty bar specimens from each of the 2 porcelains were fabricated using the 26.0X6.0X3.0 mm silicone mold (Provil novo putty; Heraeus Kulzer GmbH, D-63450 Hanau, Germany). The porcelain powders were mixed with deionized water, flled in the silicone mold and condensed with a condenser (Ceramosonic II; Shofu Inc, Higashiyama-ku, Kyoto, Japan). The specimens were then fred according to the manufacturer's instructions (Table 1). After fring, the specimens were polished (model Phoenix 4000; Buehler GmbH, 40599 Düsseldorf, Germany) under running water using 600- and 1,200-grit silicon carbide paper (3M ESPE, St. Paul, MN, USA) to the dimensions of 25.0x5.0x2.0 mm, following the guidelines of the ISO 6872 standard13. Then, the specimens were ultrasonically cleaned in distilled water for 10 min, and subjected to selfglazing according to the manufacturer's instructions (Table 1).

Erosive Agents Exposure

The porcelain bars were divided into 8 groups of 15 specimens each. Subsequently, the specimens were alternately immersed in 25 mL of an erosive agent for 30 min and in 25 mL of deionized water for 5 min for 7 cycles at 37ºC. This amount of erosive agent (25 mL) was a suffcient volume to completely cover the specimen. In order to maintain the original pH level of the erosive agent, the agents were refreshed every cycle throughout the experiment. The same protocol was used with different types of 5 erosive solutions included in the study (citrate buffer solution, pineapple juice, green mango juice, cola soft drink and 4% acetic acid; see Figure 2) and deionized water (control). The specimens' immersion protocol simulated an individual eating acidic food, sour fruits and drinks. Total immersion time was 245 min. Seventh group was continuously immersed in 4% acetic acid at 80ºC for 16 h (as modifed from ISO 687213) in order to examine the extensive effect which could occur. A 8thgroup was kept dry at 37ºC for 245 min in order to compare the effect of moisture condition. After the immersion sequence was completed, the specimens were rinsed with deionized water, blotted dry and subjected to fexural strength testing.

Flexural Strength Measurements

The fexural strength was measured with the universal testing machine (model LRX-plus; Ametek Lloyd Instruments, Farnborough, Hampshire, UK). Bar-shaped specimens were centered and placed on two steel spheres (1.6 mm in diameter) of a supporter part positioned 12 mm apart from each other. Three point bending tests were carried out using a 250 N load cell at crosshead speed 0.25 mm/min. The load at failure was recorded in Newton and converted to fexural strength in MPa (3WL/2BD²; W=failure load, L=span length, B=specimen's width, and D=specimen's thickness).

Statistical Analysis

Two-way ANOVA was analyzed to measure statistically signifcant differences among the types of erosive agents and the type of porcelains after being exposed to erosive agents. Tukey's Honestly Signifcant Difference (HSD) tests were used for post hoc comparisons (a=0.05). The t-test was used for comparing the fexural strength between the two types of porcelain for each erosive agent (a=0.05).

 

RESULTS

The fexural strength values of the two types of porcelain were showed in Table 2. ANOVA results showed that the interaction between the two variables (type of porcelain and erosive agent) found statistically signifcant difference (p=0.02). Between the two dental porcelains, a statistically signifcant difference was also found (p=0.01), but none was found among the types of erosive agents (p=0.46).

When comparing the mean fexural strength values between the porcelains for each group, the results of the t-test showed that all IPS e.max Ceram groups yielded a signifcantly higher mean fexural strength (p<0.05) than that of IPS d.SIGN groups.

 

DISCUSSION

The results of this study support acceptance of the null hypothesis, as the fexural strength of both porcelains was not affected by the erosive agents. It is noticed that the highest fexural strength was found in the dry condition group (stored at 37ºC) of both porcelains and decreased in all groups when the porcelains were immersed in erosive agents as well as in water. The possible explanation for these results could be the effect of glazing in determining fexural strength values.

Porcelains, by nature, exhibit inherent faws or defects on their surface and internal body. These faws could impair their physical properties. However, the surface faws are covered by the glazes, either self-glazing or overglazing. Re-fring the porcelain prior to fnal restoration produces a self-glaze layer. This layer may increase the strength of the porcelain restoration from two possible mechanisms8. Firstly, when the restoration is heated, the self-glaze layer flls in surface faws, reducing their depth and blunting the faw tips. This should increase strength because, for given porcelains, strength increases with decreasing sharpness and faw depth. Secondly, for feldspathicbased porcelains, the self-glaze layer has a lower coeffcient of thermal expansion than the leuciterich interior. This places the outer surface in compression when cooled. The compressive stress state diminishes the local tensile stress produced from applied loading at surface faws, thereby needing application of increased load to initiate faw propagation from the external surface.

The IPS e.max Ceram had higher flexural strength than the IPS d.SIGN in all groups. A possible explanation for this result could be the microstructure of these porcelains11. The IPS d.SIGN, feldspathic-based porcelain, is unique and distinct from other porcelains since its microstructure consists of fuorapatite crystal phases in addition to having leucite particles in a feldspathic glassy matrix10, while the IPS e.max Ceram consists of only dispersed fuorapatite crystals in a feldspathic glassy matrix28. In feldspathic-based porcelains, the leucite particles contract more than the surrounding glass upon cooling. Above a critical particle size, the stresses created during cooling can induce microcracks circumferential to the leucite particles22. Previous studies have documented that the size of leucite particles in feldspathic porcelain increases during heat treatment within the normal porcelain fring range5,21. This can increase the probability of microcracking22. It is possible that microcracking occurred during the self-glaze treatment. In contrast to fuorapatite porcelains, the fuorapatite phase particles are needle-like and contribute to high fexural strength as well as high chemical durability11.

The erosive agents used in this present study, pineapple juice and green mango juice, are favorite sour fruit juices in many Asia countries. They consist of citric acid and other organic acid12,14,16, which give an acidic pH. However, in the present study, these juices did not affect the flexural strength of the tested porcelain after immersion, which do not agree with the fndings of previous studies that showed an impact of acidic agents on porcelains17-19,24. This study was a short-term experiment and could be the reason to explain why there was no signifcant difference among the acidic agents and this aspect should be explored. So, a long-term evaluation of the effect of erosive agents on porcelains is required.

It must be noted that there are some limitations to this present study. This study did not consider the different conditions found in the oral environment. For example, the presence of water, temperature change, the pH level and the role of saliva25 in the oral cavity may considerably infuence strengths of restorations. In addition, the present study evaluated only fuorapatite-leucite and fuorapatite porcelains. Further studies are required to investigate the effect on other porcelains.

 

CONCLUSIONS

Within the limitations of this study, the fexural strengths of the porcelains (fuorapatite-leucite and fuorapatite porcelains) after exposure to erosive agents in cyclic immersion were not signifcantly different. For both types of porcelain, dry storage at 37ºC yielded the highest fexural strength, though without signifcant difference from the other groups. The fexural strengths of all groups of fuorapatite porcelains were signifcantly higher than those of the fuorapatite-leucite porcelains.

 

ACKNOWLEDGEMENTS

This study is supported in part by a grant from the Graduate School, Prince of Songkla University.

 

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Corresponding address:
Assoc. Prof. Boonlert Kukiattrakoon
Department of Conservative Dentistry - Faculty of Dentistry - Prince of Songkla University
Hat Yai - Songkhla - Thailand
Phone: +66-74-28-7703 - Fax: +66-74-429877
e-mail: boonlert.k@psu.ac.th

Received: June 17, 2009
Modifcation: January 23, 2010
Accepted: February 10, 2010