A Comparative Evaluation of Abrasion Resistance of Three Commercially Available Type IV Dental Stone, Dried using Three Different Drying Techniques - An In vitro Study.

BACKGROUND: The microwave technique of drying dental stone to achieve improved hardness and strength has been suggested in recent years. However, its effect on the wear properties of dental stone has not been thoroughly examined. AIM: The present study was conducted in vitro to determine the abrasion resistance of three commercially available Type IV dental stones using three different drying techniques.
MATERIALS AND METHODS: A total of 180 samples were prepared from 3 Type IV dental stones; namely Eliterock, Zhermack (Italy), Kalrock, Kalabhai industries (India), and Gyprock (Rajkot, India). Samples were subjected to drying in open air for 2 h, hot air oven at a temp of 230°C for 1 h and microwave oven for 5 min at 900Watts. Abrasion resistance was determined using a customized metallic abrasion resistance tester.
RESULTS: Microwave oven drying produced samples with increased abrasion resistance values when compared to samples dried in hot air oven and air-dried specimens. Eliterock showed significantly higher mean abrasion resistance values followed by Kalrock and Gyprock.
CONCLUSION: Drying type IV dental stone in a microwave oven at 900W for 5 min increased the abrasion resistance within short time. Copyright:

INTRODUCTION

Gypsum materials are commonly used as die material as they are easier to use, cost-effective, have the appropriate setting expansion, compatible with the majority of impression materials and familiarity.[1] Type IV and Type V dental stones are used routinely for the fabrication of definitive casts that are used in the fabrication of inlays, crowns, and bridges.[2] During the carving of the wax pattern, there are chances of accidental wearing of the gypsum die. A die material is considered ideal if they possess properties of superior abrasion resistance and high strength.[3]

When Type IV dental stone is mixed according to manufacturer's instructions, a certain amount of un-reacted water is obtained at the end of the chemical reaction, which tends to weaken the stone product and results in “wet casts.” Although they are advised to wait for 24–48 h before working on the gypsum casts, dentists and laboratory technicians often find it necessary to work on these casts soon after they are poured. These “wet casts” usually have inadequate strength, surface hardness and are easily susceptible to wear and fracture.[4]

In earlier studies, casts made of different types of gypsum products were dried in open air, hot air oven and microwave ovens, and their compressive strength and surface hardness values were compared. Furthermore, the aforementioned studies have proven that considerable time can be saved by drying the casts in a microwave oven with resultant increase in compressive strength and surface hardness. However, studies regarding effect of microwave drying on the wear properties of Type IV dental stone is scarce in literature.[45678]

Hence, this study is intended to compare and evaluate the differences in the abrasive properties of three commercially available Type IV dental stones when tested under different drying conditions (open air, hot air oven and microwave oven-dried specimens). Two null hypotheses were tested in the current study. First; that microwave oven drying would not improve the abrasion resistance of Type IV stone when compared to open-air and hot air oven drying, and second that the three types of stones used would not significantly differ in their abrasion resistance values.

MATERIALS AND METHODS

Three commercially available Type IV dental stones; Eliterock from Zhermack, Italy; Kalrock from Kalabhai industries, India and Gyprock from Rajkot, India [Figure 1] were used for preparing the test samples. A cylindrical silicone mold with dimensions of 40 mm height and 20 mm diameter was fabricated in accordance with ADA specification number 25 for preparing the test samples [Figure 2].

Figure 1

Three commercially available Type IV dental stone

Figure 2

Silicone mold fabricated according to ADA specification number 25

A total of 180 samples were prepared for testing the abrasion resistance. The stone powder and distilled water were mixed according to the manufacturer's instructions; the mixture was hand-mixed initially using a rubber bowl and spatula for 45 s and then in a mechanical vacuum mixer for 30 s to ensure a smooth, homogeneous, and bubble-free mix. After mixing all the 180 test samples were vibrated into the silicone mold and left undisturbed for 30 min until initial setting of samples occurred. Only samples with a homogeneous and smooth surface, free of surface porosities were included in the study. Then, 60 samples (20 samples each for 3 types of die stones) were dried in the open air within a temperature range of 30°C ± 50°C for 2 h [Figure 3], 60 samples were dried in a hot air oven at 2300°C for 1 h [Figure 4] and remaining 60 samples were dried in a microwave oven at 900 watts for 5 min. A microwave compatible bowl was used to keep 200 ml of water in the microwave oven during the heating process to prevent damage to the magnetron of the microwave.

Figure 3

Cylindrical samples dried in open air for 2 h

Figure 4

Samples dried in hot air oven at 230°C for 1 h

Abrasion resistance test was conducted after an interval of 2 h according to ANSI/ADA Specification number 25.[2] For evaluating the abrasion resistance, a customized metallic abrasion resistance tester/device was fabricated. This device was similar to the abrasion testing device as described by Lindquist and Stanford.[9] It consists of a rigid metallic base which supports a mounting jig. A vertical arm is attached to the metallic base, which supports a movable horizontal assembly. The horizontal arm moves freely in a back and forth direction within a range of 10 mm. A vertical Stylus with sharp pointed cutting edge is suspended at the end of the horizontal arm. The stylus is suspended by a round metal plate weighing 100 g. The apparatus also consists of a mounting jig to which the samples were attached [Figure 5].

Figure 5

Samples dried in microwave oven at 900 Watts

All the prepared samples were preweighed before the test using a digital weighing machine (Essae, DS-852J). A line of 10 mm length (5 mm on either side from a point at the center of the sample) was marked using an indelible marker along the length of the sample which corresponds to the area of the sample that was to be tested. The samples were loaded on to the mounting jig. Following the securing of the samples on the mounting jig, the stylus was adjusted such that the position of the stylus was perpendicular to the surface of sample [Figure 6]. A weight of 100 g was applied to the sample along the long axis of the stylus. Then, the reciprocating horizontal arm was driven manually 80 times over the 10 mm length of the sample. As the horizontal arm moved, the sharp cutting end of the stylus abraded the surface of the sample producing a scratch along its marked length of 10 mm. Following the abrasion test, the samples were weighed again. Before each weight measurement, the specimen was sprayed with air to remove wear debris. The difference in weights was measured as the abrasion resistance.

Figure 6

Customized device for testing abrasion resistance

The software SPSS Version 15 (IBM Corp. Released 2011. IBM SPSS Statistics for Windows, Version 20.0. Armonk, NY: USA, IBM Corp). was used for obtaining the statistical analysis of the data. The data obtained were statistically analyzed applying two-way ANOVA followed by Tukey's post hoc comparison test.

RESULTS

Two-way analysis of variance indicated significant decrease in mean values (decrease in weight) among the three drying techniques, (F (2, 171) = 163.782, P < 0.001.) as well as among the 3 type IV dental stones [F (2,171) = 8.514, P < 0.001; Table 1 and Graph 1].

Table 1

Descriptive statistics of abrasion resistance under 3 different drying techniques

Drying technique	Gypsum type	n	Preweight		Postweight		Difference=Pre-post
			Mean	SD	Mean	SD	Mean	SD
Open air drying after 2 h	Kalrock	20	23.5300	0.24199	22.3370	0.28446	1.1930	0.26668
	Gyprock	20	22.5275	0.30350	21.2080	0.40627	1.3195	0.24003
	Eliterock	20	24.0315	0.41346	22.8150	0.42197	1.2165	0.28797
	Total	60	23.3630	0.70792	22.1200	0.77343	1.2430	0.26689
Hot air oven drying at 230° for 1 h	Kalrock	20	23.5630	0.16812	22.8090	0.15708	0.7540	0.18520
	Gyprock	20	22.6845	0.19859	21.7660	0.29728	0.9185	0.32640
	Eliterock	20	24.0675	0.52383	23.2600	0.53629	0.8075	0.20292
	Total	60	23.4383	0.66502	22.6117	0.72604	0.8267	0.25177
Microwave oven drying at 900 W power for 5 min	Kalrock	20	23.5380	0.24625	23.0525	0.26447	0.4855	0.16472
	Gyprock	20	22.7495	0.17825	22.1570	0.33833	0.5925	0.27821
	Eliterock	20	24.0990	0.52125	23.8045	0.54648	0.2945	0.10480
	Total	60	23.4622	0.65487	23.0047	0.78534	0.4575	0.22945
Total	Kalrock	60	23.5437	0.21838	22.7328	0.38242	0.8108	0.35940
	Gyprock	60	22.6538	0.24785	21.7103	0.52220	0.9435	0.40950
	Eliterock	60	24.0660	0.48134	23.2932	0.64237	0.7728	0.43382

SD: Standard deviation

Graph 1

Profile plot showing the mean decrease in abrasion resistance for 3, Type IV dental stones under three different drying techniques

Tukey's post hoc test indicated a decrease in mean values (decrease in weight) for all the 3 drying techniques. These mean values were significantly different from one another. Decrease in weight (g) was maximum in open-air drying indicating least resistance to abrasion (mean decrease = 1.243, standard deviation [SD] = 0.267), followed by hot air oven (mean decrease = 0.8267, SD = 0.252). Mean decrease in weight for microwave oven drying was the least (mean decrease = 0.4575, SD = 0.229) indicating highest resistance to abrasion [Table 2 and Graph 2].

Table 2

Tukey’s post hoc test for the comparison of abrasion resistance among three different drying techniques

Drying technique (I)	Drying technique (J)	Mean difference (I-J)	P	95% CI
				Lower bound	Upper bound
Open air drying after 2 h	Hot air oven drying at 230° for 1 h	0.4163*	<0.001	0.3137*	0.5190
Open air drying after 2 h	Microwave oven drying at 900 W power for 5 min	0.7855*	<0.001	0.6828*	0.8882
Hot air oven drying at 230° for 1 h	Microwave oven drying at 900 W power for 5 min	0.3692*	<0.001	0.2665*	0.4718

P<0.001, *=significant Difference in weight among type IV stone samples (g). CI: Confidence interval

Graph 2

Profile plot showing estimated marginal means of weight of sample (g) before test

Mean decrease in weight among 3, Type IV dental stones were also significantly different from one another. Decrease was maximum for Gyprock (mean = 0.9435, SD = 0.4095), followed by Kalrock (mean = 0.8108, SD = 0.3594). Decrease in weight was the least for Eliterock [mean = 0.7728, SD = 0.4338; Table 3 and Graph 3].

Table 3

Tukey’s post hoc test for the comparison of abrasion resistance among 3, type IV dental stones

(I) Gypsum_type	(J) Gypsum_type	Mean difference (I-J)	P	95% CI
				Lower bound	Upper bound
Gyprock	Kalrock	0.1327*	0.007	0.0300	0.2353
Kalrock	Eliterock	0.0380	0.657	−0.0647	0.1407
Gyprock	Eliterock	0.1707*	<0.001	0.0680	0.2733*

P<0.001, *=significant Difference in weight among type IV stone samples (g). CI: Confidence interval

Graph 3

Profile plot showing estimated marginal means of weight of sample (g) after test

DISCUSSION

Abrasion resistance of the die material is important during waxing and in the enhancement of margins for receiving restorations.[9] Abrasion of the die at the finish line results in poor marginal adaptation and finishing of the wax patterns and the subsequent restorations.[10] It is assumed that the presence of free water radicals within the cast weakens its crystalline structure. These “wet casts” usually have inadequate strength and surface hardness. Conventional open-air drying requires drying of casts at room temperature for 30–60 min. However, it has been reported that, approximately 7% of excess water remained in air-dried gypsum cast even after an hour of mixing.[4] The presence of more gauging water in Type IV dental stone requisites a longer drying period to expel the excess water. Although manufacturers recommend a waiting period for 24–48 h, dentists and laboratory technicians often find it necessary to work with these casts soon after they are poured. Previously, different types of gypsum products were dried in open air, hot air oven and microwave ovens, and their strength and surface hardness values were compared. However, data reported in the literature regarding the effects of drying procedure, temperature, and time on the wear and abrasive properties of dental stone are scarce.[678]

Although no direct studies have been previously conducted to study the effect of microwave oven drying on abrasion resistance, Mahler indicated that surface hardness indicates abrasion resistance and flow properties of the stone.[11]

Currently, there is no standardized test to evaluate abrasion resistance of dental stone. This is because abrasion is the result of a wear process rather than the property of the material itself. Therefore, a custom-fabricated reciprocal device was used for evaluating the wear resistance of Type IV gypsum. The advantages of the abrasion testing device developed for this study are as follows:[9101112]

The device is simple and ergonomically designed

It uses a steel stylus that creates a perpendicular force across the surface of type IV die stone; the force simulates the typical abrasive action on stone dies

Adjustable number of cycles and stylus load

Accommodation of large test samples

Minimal wear on the stylus and it can be re-sharpened or replaced.

The amount of weight loss (mean weight decrease = 0.4575 g), was least for microwave dried samples indicating highest resistance to abrasion among all the samples tested. Heat production occurs during microwave drying process, excess water starts to evaporate causing fine crystals of gypsum to precipitate on the surface after the last traces of water disappear. The heat production results in more prismatic, regular shaped and densely packed crystals with little porosities, which in turn increases the abrasion resistance.[111213]

The mean weight loss (mean weight decrease = 0.8267 g) of samples dried in a hot air oven was significantly higher than microwave oven-dried samples. The temperature inside the hot air oven elevated rapidly and not gradually, resulting in production of greater heat on the outer surface than the core of the samples. This leads to rapid boiling of free water and its rapid escapement from the outer surface of the Type IV dental stone. This results in increased holes and cracks on the outer surface of the stone. Also such heat may attack the water required for crystallization, thereby reducing the strength and abrasion resistance of the Type IV gypsum. However since hot air oven results in better removal of excess water than open-air drying, hot air oven-dried samples offered better abrasion resistance when compared to air-dried samples.[1415]

Mean weight loss was highest (mean weight decrease = 1.243) in open air-dried samples indicating the least resistance to abrasion. The difference may be due, in part, to the weaker strength (green state) of the gypsum in conventional air-dried samples and it also may be due to higher water content per unit volume of material removed during the impact of the stylus. Thus, the greater volume loss of material in open air-dried samples reflected a greater sized defect (suggestive of a brittle fracture), in which a porous dry material underwent a progressive interpore fracture and consequent non uniform burst fracture across the surface of the exposed calcium sulfate dihydrate crystal structure.[101112]

Furthermore, mean weight decrease among the 3 diestones was minimum for Eliterock (mean weight decrease = 0.7728 g), followed by Kalrock (mean weight decrease = 0.08108 g) and highest for Gyprock (mean weight decrease = 0.9435 g), indicating that abrasion resistance was highest for Eliterock, followed by Kalrock and least for Gyprock dental stone.

Limitations of the study

The comparison and explanation for the differences in physical properties of the three commercially available brands of Type IV dental stones used in this study cannot be deemed complete as the manufacturers did not provide the complete chemical composition and the manufacturing process of their products. A follow-up Scanning Electron Microscope (SEM) study is required to observe the quality of crystalline structure (particle size and form) of Type IV dental stone when dried in microwave oven, hot air oven and in open air.

One main disadvantage of abrasion testing device is that the reciprocal stage has to be moved manually, causing an inconsistent force on the gypsum surface. Hence, there is a need for a mechanical motor so that a uniform load cycle can be applied on to the sample.

CONCLUSION

The microwave drying method at 900W for 5 min can be used to improve the abrasion resistance of Type IV dental stones.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.