The Effect of Microwave Disinfection on Tensile Strength of Dental Gypsum 1217 – 1812 :

Aims: To evaluate the effect of microwave disinfection on diametral tensile strength (DTS) of type III and type IV dental stones. Materials and Methods: A total of 80 cylindrical specimens was prepared with dimensions 40 mm in height and 20 mm in diameter. These specimens were divided into 4 main groups: Geastone type III, Zeta type III, Micromod type IV and Silky Rock type IV. Twenty specimens for each main group were divided into 10 specimens as control subgroup and 10 specimens representing microwave irradiated subgroup. The microwave irradiation was performed in a household microwave oven at 900 watts for 5 minutes. The DTS was conducted on a mechanical testing machine at a cross head speed of 1.0 mm/minute. The data were statistically analyzed using Student's t–test, analysis of variance and Duncan's Multiple Range test. Results: The microwave disinfection caused significant increase in DTS (p < 0.05) for all types of dental stone, and dental stones type IV (Micromod and Silky Rock) had significantly higher DTS (p < 0.05) than dental stones type III (Geastone and Zeta). Conclusion: Microwave disinfection of gypsum cast cause significantly increase in diametral tensile strength of type III and type IV dental stone.


INTRODUCTION
Studies demonstrated that the usual operating procedures of the prosthodontic laboratory are a possible source of crosscontamination between patients, technicians and dental personnel (1)(2)(3)(4) , and the potential sources of transmission of infectious diseases from patients to dental technicians include impressions, impression trays and gypsum casts (5) .
As impression disinfection is difficult and associated with several problems, attention was directed towards cast disinfection (6) .
Studies showed that these methods may affect the physical and mechanical properties of dental stone, such as setting time, compressive strength and dimen-sional accuracy (12)(13)(14)(15)(16) .Microwave energy has been suggested for disinfection of gypsum casts, but the effect of microwave energy on gypsum material has not been thoroughly examined.
Therefore, the purposes of this study was to investigate the diametral tensile strength (DTS) of types IV and III dental stones disinfected by microwave technique.

MATERIALS AND METHODS
In this study, 4 types of dental stones were evaluated.These stones and the water/powder ratios recommended by their manufacturers were listed in Table (1).
Split metal mold was used to construct stone cylindrical specimens with dimensions 40 mm in height and 20 mm in diameter according to ADA Specification No. 25 (17) .
The recommended powder was added to the water in a rubber bowl and mixed by hand to a smooth consistency.Mixed dental stones were poured down the side of

The Effect of Microwave Disinfection on Tensile Strength of Dental Gypsum
the inclined mold retained on a glass plate, the mold was vibrated using vibrator (BEGO, Germany) during filling to draw out air bubbles from the mixture and reduce porosity.The overfilled mold was then covered with another glass plate which was rocked into place and pressed firmly into contact to ensure flat and parallel ends.After 20 minutes, the glass plates were removed and the metal mold was dissembled carefully and the stone cylinders were easily separated from the mold.By this method, 80 specimens were prepared and all specimens were coded.Then, all specimens were stored in air at room temperature range of 23 + 2 °C for 1 hour (18) .
The total 80 specimens were divided into 4 main groups, each group represented on type of stone.For each type of dental stone (each group), 20 specimens were prepared, 10 specimens were left untreated as a control, while the other 10 specimens were microwave irradiated.
The microwave irradiation was performed in a household microwave oven (Samsung, Type CK, 99s, 2, 450 frequency) at 900 watts for 5 minutes (19) .To ensure that the specimens were adequately irradiated on all surfaces, they were first exposed for 2.5 minutes and subsequently turned upside down and irradiated again for the same amount of time.A cup with 200 ml water was placed in the microwave oven to protect the magnetron (20) .
The DTS was conducted on a mechanical testing machine (Inc., Model CN 472, EVANSTON Ill, USA) at a cross head speed of 1.0 mm/minute (18) .
An apparatus made of metal was designed and constructed especially for the test (Figure 1).The horizontal metal fracturing section was 6 cm length, 1 cm thickness and 4 cm width.It was mounted at the end of a vertical metal rod (1 cm in diameter) which is fixed to the mechanical testing machine (21) .

Hasan HR
The cylindrical stone specimen was placed in horizontal position between the table of the testing machine at the bottom and the metal fracturing section and compressed until fracture occurred (Figure 1).The compressive force recorded on the broving ring (scale) at the moment of fracture.
The DTS was then calculated in Megapascal (MPa) using the equation: (18) DTS= Where: f= breaking load; b= specimen height; d= cross-sectional diameter.
The data were statistically analyzed using Student's t-test to compare between the two subgroups: Control and microwave irradiated for each main group then analysis of variance (ANOVA) followed by Duncan's Multiple Range Test to compare between the four types of dental stone.

RESULTS
The mean and standard deviation of the DTS for each tested group and Student's t-test were calculated and listed in Table (2).In order to evaluate the effect of microwave irradiation (disinfection) on the DTS for each stone type (main group), Student's t-test was performed as shown in Table (2), which revealed that there were significant differences between microwave irradiated group and control group for all types of dental stone, and this revealed that microwave irradiated Silky Rock type IV, microwave irradiated Micromod type IV, microwave irradiated Geastone type III and microwave irradiated Zeta type III were significantly higher than the corresponding control groups, respectively.
For evaluation of the effect of type of dental stone on the DTS, ANOVA (Table 3) showed that there were significant differences between the four types of dental stone.

DISCUSSION
The most important results in this study was microwave disinfection of dental stone by household microwave oven cause significantly increase in DTS.
The clinical importance of the cycle that is used in present study (900 watts at 2.450 MHz) for 5 minutes cause striking reduction of bacteria on the cast as shown in previous study (19) .
The most critical factor in using microwave technique for disinfection of dental stones are the power and time of microwave irradiation, although previous investigation (22) showed that high power of microwave irradiation (1,400 watts) and for different time duration has bactericidal effect on different microorganisms.There was some concern that cracks or porosities in the surface might result when gypsum casts were irradiated with high wattage (1,450 watts), (23) as rapid water escape from dental stone may be harmful to the material, causing holes and cracks on the outer surface which may lead to fracture during handling.
There are little researches on the strength of gypsum materials disinfected by microwave energy (24,25) .So, in this study, an evaluation of one of the most important properties of gypsum products that's disinfected by microwave energy was performed which is tensile strength.
With the materials that exhibit limited plastic deformation, the DTS has been used successfully, (26,27) which is the test in which the compressive load is placed by a flat plate against the side of a short cylin-drical specimen.The vertical compressive force along the side of the specimen produces a tensile stress that is perpendicular to the vertical plane that pass through the center of the specimen.Fracture occurs along this vertical plane, in such situation, the tensile stress is directly proportional to the compressive load applied (27) .
For all types of dental stone (Silky Rock type IV, Micromod type IV, Geastone type III and Zeta type III) microwave disinfection cause significant increase in DTS as shown in (Figure 2).Possible scientific explanation for such increase may be related to the fact that the strength of gypsum products is conversely related to free water content in set product (28) .The one hour after mixing compressive strength is a measure of wet strength while gypsum may take as long as 7 days to dry, (29) and the dry strength may be two or more time the wet strength (27,28) .
Anusavice (27) has recommended that type III dental stone even one hour after mixing, approximately 7% excess water remains in air-dried gypsum materials, and because type IV dental stone has more gauging water, a longer drying period may be necessary to expel the excess water (21) .
So, in addition to kill pathogenic microorganisms, microwave irradiation of gypsum specimens at 900 watts for 5 minutes will remove this free water content and lead to significant increase in DTS in relation to control groups.
Other findings of this study revealed that DTS of type IV dental stone (Micromod 14.815 MPa, Silky Rock 15.25 MPa) were significantly higher than the DTS of Hasan HR type III dental stone (Geastone 8.775 MPa, Zeta 9.525 MPa) and these differences in DTS may be due to the fact that dental stone type IV contains an α-hemihydrate of the Densite type.Cuboidal shape particles and the reduced surface area produce dental stone with high mechanical properties (28) .

Figure (2):
The effect of microwave irradiation on the diametral tensile strength for the four types of dental stone

CONCLUSION
Microwave disinfection of gypsum cast at 900 watts for 5 minutes cause significantly increase in diametral tensile strength of gypsum products, and this technique could be done safely, quickly with household microwave oven without need of special skills, chemicals or additives.
Microwave disinfected dental stone type IV exhibited more DTS than that of dental stone type III.

Table ( 1
): The tested dental stones and their powder/liquid ratio

Table ( 2
): Mean, standard deviation and Student's t-test for the diametral tensile strength of the tested groups

Table 4 )
showed that Silky Rock type IV (15.250MPa) and Micromod type IV (14.815MPa) were significantly higher than those of Zeta type III (9.525 MPa) and Geastone type III (8.775 MPa).

Table ( 4
): Duncan's Multiple Range test for the effect of type of dental stone on diametral tensile strength