Fracture Toughness, Surface Roughness and Fluoride Release of Glass Ionomers after Immersion in Athletic Drinks

Objectives: This study was conducted to evaluate the effect of sport and energy drinks on conventional and resin-modified glass ionomer restorative materials regarding fracture toughness, surface roughness and fluoride release. Methods: The restorative materials used were conventional and resin-modified glass ionomers. Sport drinks used were Gatorade Perform 02 and Pocari sweat, while the energy drinks were Red Bull and Power Horse. Specimens were prepared and divided into five groups according to the immersion media (distilled water, two sport drinks and two energy drinks) for 1 and 7 days. The fracture toughness was determined using threepoint bending method. Surface roughness was measured using surface profilometer. Fluoride release was determined using a conventional ion chromatograph testing unit. The data were analyzed using three-way ANOVA and Least Significant Difference test. For comparison between the two materials under each condition, t-test was used. Results: There was no significant difference in fracture toughness between sport, energy drinks and distilled water at the different time intervals except for conventional glass ionomer after 7 days. Resin-modified glassionomer exhibited smoother surfaces more than conventional one in sport and energy drinks after 1 day. After 7 days, both conventional and resin-modified glass ionomers showed greater surface roughness. Both conventional and resin-modified glass ionomers release more fluoride in acidic beverages than distilled water. Conclusions: The effect of sport and energy drinks on the fracture toughness may depend on the composition and acidity of drink. Fluoride release increased with the consumption of sport and energy drinks. DOI : 10.14302/issn.2473-1005.jdoi-14-413 Corresponding Author: Ibrahim M. Hamouda, Faculty of Dentistry, Mansoura University, Mansoura, Egypt. Faculty of Dentistry, Umm Al Qura University, Makkah, Saudi Arabia. Imh100@hotmail.com Mobile: 966542812148


Introduction
Fluid replacement drinks or carbohydrate-electrolyte beverages may be one of the most researched sports nutrition topics ever and accompanying this high volume of research are continually evolving recommendation. 1 Sport drinks were developed in the United States in 1960s when the University of Florida Gators began drinking a formulation of carbohydrate and electrolytes to enhance their performance and prevent dehydration.
Most marketing for these beverages is now aimed at the nonathletic. 2 Sport drinks are popular worldwide, but the various products differ little in their composition. They contain 6% to 8% carbohydrates, with the principal carbohydrates being glucose, fructose, sucrose, and synthetic maltodextrins. All contain small amounts of electrolytes, including sodium, potassium and chloride, to improve palatability and help maintain the fluid/ electrolyte balance. The purpose of sport drinks is to prevent dehydration, to provide carbohydrates to boost energy, to supply electrolytes that can replace those lost via perspiration. 3 In 2006, nearly 500 new brands of energy drinks were introduced and more than 7 million adolescents reported that they have consumed an energy drink. The difference between sport and energy drinks that sport drinks tend to be caffeine free, but energy drinks are loaded with caffeine. Energy drinks also tend to have a higher carbohydrate content (9% to 10%) than do sport drinks. 2 The dental status of athletes who consume these acidic beverages is little considered. These beverages have an erosive effect and risks to dental health. 4 Clinical performance of filling materials is affected by erosion as well. 5 Glass ionomer restorative materials have a number of unique properties, including adhesion to tooth structure, biological compatibility, and anticariogenic properties due to their fluoride release. 6 The ability of restorative dental materials to withstand the functional force and exposure to various media in the mouth is an important requirement for their clinical performance for considerable period of time. However, although these materials are tested for strength, they are rarely tested following storage in a kind of aqueous media found in the mouth.Instead, they are tested after being stored in deionized water of high purity. 7 For ionic restorative materials, such as glass ionomer, this storage regime may be inappropriate. These restorative materials have recently been shown to interact with various aqueous media. For example; in saliva, they undergone a surface reaction that led to precipitation of calcium and phosphate ions into the outermost layer. 7,8 In acidic conditions, matrix forming ions were found to be released into solution as part of a process of buffering the medium. 9 It was found that glass ionomer in orange and apple juice underwent severe erosion and loss of strength.
This was attributed to the presence of carboxylic acids such as citric and malic acids in these fruit juices, which are capable of chelating with cement-forming ions, such as calcium, to yield soluble products. 7  these materials, the nature of the storage medium is important. So, the null hypothesis of this study was sport and energy drinks will negatively affect the properties of glass ionomer restorative materials.

Materials and Methods
The materials used in this study are listed in Tables  The pH of each storage medium was determined before immersion of the specimens.

Determination of Fracture Toughness:
A total number of hundred and forty notched specimens were prepared, seventy specimens for each glass ionomer. Specimens were prepared in a stainlesssteel split mould (25 mm length × 2.5 mm thickness × 5 mm width). The mold was notched ( 0.5 mm width and 2.5 mm depth). 11 The mixed cement was condensed into the mold, pressed between matrix strips and glass plates under load for 10 min. The light-cured glass ionomer, the specimens were light-cured at each surface using an overlapped technique for 40 s using a visible light curing unit at 320 mW/cm 2 (Visilux II; 3M, St Paul, USA). After approximately one hour in a humidor, each specimen was removed from its mold. 11,12 The specimens were divided as mentioned before (n = 7/ group for each test period).
The specimens were immersed in 5 mL of the testing medium and stored at 37 o C. Specimens were tested after 24 h and after one week from the start of immersion. The storage medium was changed daily.
Fracture toughness was determined using three-point bending method according to the procedures outlined in ASTM E399-90. 13 The test was done using a computer- Where: P Q is the peak load (kN), B is the specimen thickness (cm), S is the span length (cm), W is the specimen width (cm), a is the crack length (cm) and f(a/ W) is a function of a/W.

Determination of Surface Roughness:
A total number of fifty disc-shaped specimens, twenty five for each restorative material, were fabricated in a split Teflon mould (10 mm diameter × 2 mm thickness. The cement paste was packed into the mold that was placed on a microscope slide. A second slide was placed over the mold and light hand pressure applied to enable the excess material to flow out of the mold through the slit. Resin-modified glass ionomer specimens were light were divided into five equal groups (n = 5/group) according to the storage medium as mentioned before.
The specimens were immersed in 5 mL of the testing medium and stored at 37 o C. Surface roughness was measured after 24 h and one week from the start of immersion. The storage medium was changed daily.
Surface roughness was measured using surface Profilometer (Surf Test SJ 201, Japan). Five tracings at different locations on each specimen were made.
Surface roughness (Ra) was determined in µm using a tracing length of 2 mm and a cutoff value of 0.25 mm to maximize filtration of surface waviness.

Measurement of Fluoride Release:
A total number of fifty disc-shaped specimens, twenty five specimens for each glass ionomer, were divided into five equal groups (n = 5/group), according to the storage medium as mentioned before. The specimens were fabricated in the split Teflon mold that was used for preparing specimens for surface roughness testing.

Statistical Analysis:
Means and standard deviations of fracture toughness, surface roughness and fluoride release were calculated for each group. The data were analyzed using three-way    Table 4. After 1 day, for conventional glass ionomer, the lowest mean fluoride release value was for specimens immersed in distilled water and the highest mean was for specimens immersed in Power Horse. For resin-modified glass ionomer, the lowest mean fluoride release value was for specimens immersed in distilled water and the highest mean was for specimens immersed in Power Horse. After 3 days, for conventional glass ionomer, the lowest mean fluoride release value was for specimens immersed in distilled water and the highest mean was for specimens immersed in Red Bull.
For resin-modified glass ionomer, the lowest mean Pocari Sweat, there was no significant difference between third, fifth and seventh days of immersion.

Discussion
It is well known that glass-ionomer cements (GICs) are clinically attractive dental restorative materials.
These cements possess certain unique properties that make them useful as restorative and adhesive materials, including adhesion to tooth structure and base metals, anticariogenic properties due to release of fluoride, thermal compatibility with tooth enamel because of low coefficients of thermal expansion similar to those of tooth structure, biocompatibility and low cytotoxicity. 15 Sport and energy drinks are popular worldwide. Sport drinks are typically formulated to prevent dehydration, In the oral cavity, restorative materials are exposed to varying environments. These include changes in temperature and acidic-base conditions from food and drinks. Therefore, the restorative materials used in the mouth should resist or show minimal change in these situations. Therefore, a long immersion time was used as an alternative for presenting the extensive effect of acidic beverages on conventional and resin-modified glass ionomer restorative materials. 18 Fracture toughness is a measurement of a material's ability to resist catastrophic failure. 19 Fracture toughness is independent of the size and geometry of the specimen and is a more reliable parameter to predict clinical performance. 20 The results showed that there was no significant difference in fracture toughness between sport and energy drinks and distilled water after 1 day for both conventional and resin-modified glass ionomer.
Whereas, after 7 days, there was a significant difference between conventional glass ionomer specimens Roughness refers to the surface texture of a material.
There are two types: the smoothness resulting from a finishing process, referred to as applied or acquired smoothness, and the smoothness of an unpolished material, referred toas inherent smoothness. Inherent smoothness depends on the filler particle size of the material. 22 Surface roughness assessment is important because it is well documented that surface micromorphology can play a role in bacterial colonization and maturation of plaque on restorative materials. 23 These interactions may predispose a restoration to the development of secondary caries and may lead to periodontal inflammation. 24,25 The results of the current study showed that conventional glass ionomer was rougher in sport and energy drinks than resin-modified glass ionomer. After 1 day of immersion, conventional glass ionomer specimens immersed in sport and energy drinks were more rough than those immersed in distilled water. Whereas, resinmodified glass ionomer specimens immersed in sport and energy drinks were not significantly different from those immersed in distilled water. This indicates that RMGI resist acid better than conventional glass ionomer  28 The results of the current study showed that both conventional and resin-modified glass ionomers release more fluoride in acidic beverages than distilled water. This high fluoride release suggests an increase in dissolution of the material and this was observed on the surface roughness. 29 However, conventional glass ionomer released fluoride more than resin-modified glass ionomer. This indicates that RMGI resist acid better than conventional glass ionomer cement. In addition, for both GICs the F released in the first day's immersion is greater than in the following storage days.
The high level of F release on the first day may be caused by the initial superficial rinsing effect

Conclusions
Based on the results and within the limitation of this study, the following conclusions can be made: 1. The fracture toughness was not significantly changed after 1 day for both conventional and resin-