The dimensional accuracy specimens cured by three Sherman Salim, Taizo Hamada, Hiroshima
DDS, MS,* DDS, PhDc
of rectangular acrylic resin denture base processing methods Sadamori,
The dimensional accuracy of rectangular acrylic resin specimens was examined when they were processed by three methods: a conventional method, the SR-Ivocap system, and a microwave curing methzd. The dimensional accuracy was evaluated by the change of the distance vector V, which is calculated by means of measurements of the distances between fixed points on specimens. The specimen cured by the SR-Ivocap system exhibited less dimensional change @ < 0.05) than those cured by the conventional and the microwave curing methods. The SR-Ivocap system might produce a more accurate denture base than the conventional and the microwave curing methods.(J PROSTHETDENT 1992;67:879-81.)
eat cured acrylic resin is the most popular denture base material in clinical use. There has always been a problem with shrinkage of the acrylic resin because of the curing process. Several denture base materials and processing methods have been introduced to the profession, and each of them has claimed to produce a more accurate denture base. Wolfaard et al1 stated there are many factors that can influence the dimensional change of acrylic resin dentures. The dimensional change of the denture base material has been evaluated by making specimens of various shapes. However, many factors such as the size and shape? the thickness of the denture,3 the presence of teeth,? and so on, affect the dimensional changes that might occur during processing. Therefore it is convenient and practical to use a specimen with a simple shape, rather than dentures or denture-shaped specimens, for the examination of the dimensional change of the acrylic resin itself. Thus the results of the dimensional changes could be directly attributed to the acrylic resin and to the processing method. Recently, a new injection technique and microwave curing method were introduced to the profession. This new injection technique (SR Ivocap, Ivoclar AG, Schaan, Liechtenstein), is a continuous-pressure injection technique and claims to totally compensate for the polymerization and thermal shrinkage that occurs in a heat-cured acrylic resin denture. The microwave curing method has the advantage of saving time when microwave heating is
Supported by Japan Dental Association International Scientific Exchange Fund (Dr. Salim). “Honorary research fellow, Department of Prosthetic Dentistry; Lecturer, Airlangga University, Faculty of Dentistry, Surabaya Indonesia. bAssociate Professor, Department of Prosthetic Dentistry. cProfessor and Chairman, Department of Prosthetic Dentistry. 10/l/36507
used to polymerize denture base materials. The reflection of microwaves by metallic flasks was a problem. However, Kimura and Teraka5 overcame this by using special dental flasks that are made of glass fiber-reinforced polyester resin and are held together with polycarbonate bolts. The dimensional accuracy of acrylic resin denture base material cured by processing with both the microwave technique and the SR-Ivocap method has not been fully studied. The purpose of this study was to compare the dimensional accuracy of rectangular resin specimens cured by three denture base processing methods, using the conventional processing method, the SR-Ivocap system, and the microwave curing processing method.
MATERIAL AND Sample preparation
Rectangular acrylic resin plates were made of two layers of paraffin wax (GC Dental Industrial Corp., Tokyo, Japan) measuring 60 X 25 X 3 mm on a glass plate measuring of 70 x 35 x 5 mm. The wax specimens were put on a rectangular plate whose corners were shaped so that there were four holes in the wax specimens, each with a depth of about 1.5 mm. For the embedding, a mixture of two different kinds of gypsum was used, namely Dental Plaster (Maruisi Gypsum Industrial Corp., Tokyo, Japan) and New Plastone (GC Dental Industrial, Corp.) in a ratio of 3:1, while the liquidpowder ratio was 40 ml of water to 100 gm of powder. There were 18 samples in each curing process.
In this research, the acrylic resins Bio Resin (No. 3 Dark pink, Shofu Inc., Kyoto, Japan), SR-Ivocap (Pink, Ivoclar AG), and Acron MC (No. 8 Live pink, GC Dental Industrial Corp) were used (Table I). The liquid-powder ratio of Bio Resin was 4.5 ml of liq-
I. Materials used in this investigation
Hot water bath
90 min at 70’ C 30 min at 100’ C 35 min at 100’ C
Hot water bath
SR-Ivocap system Microwave curing
II. Dimensional changes of acrylic resin specimens by three denture base processing methods (means and standard deviations)
Conventional SR-Ivocap system Microwave curing
18 18 18
0.411 0.242 0.400
0.082 0.062 0.101
uid to 10 gm of powder. After 20 minutes, the acrylic resin was packed in the flask, as recommended by the manufacturer. The curing process was an initial 90 minutes at 70” C (158O F), followed by 30 minutes at 100° C (212’ F), as recommended by Japan Industrial Standards (JIS). SR-Ivocap was packed in a capsule whose liquid-powder ratio was 30 ml of liquid to 20 gm of powder. The mixing was done using a Cap vibrator (Ivoclar AG) for 5 minutes. The curing process of the SR-Ivocap system was 35 minutes with a pressure of 6 atm, at 100” C (212O F), as recommended by the manufacturer. The curing was followed by a lo-minute cooling process using flowing water with a pressure of 6 atm. Finally, there was a further lo-minute cooling period, but without any extra pressure. The liquid-powder ratio of Acron MC was 4.3 ml of liquid to 10 gm of powder. After 20 minutes, this mixture was packed in a microwavable flask. The very short curing processwas 3 minutes using a 500 W microwave oven (EM-M 535 T, Sanyo Electric Co. Ltd., Osaka, Japan), as recommended by the manufacturer.
Measurements In-general, the norm of a six-dimensional numerical vector V = (x1, x2, x3, x4, x5, xs) is defined by
We denote the four holes in each specimen by A, B, C and D, and consider the six-dimensional vector a = (AB, BC, CD, AD, AC, BD), where AB means the distance between A and B, and so on. The distances between the holes were measured by using a Measurescope (Nikon, Nippon Kogaku K.K., Tokyo, Japan). We call v the distance vector of the specimen. O’Toole et a1.6 used the change of II~H to examine the linear distortion of a rectangular acrylic resin
Shofu Inc., Kyoto, Japan Ivoclar AG, Schaan, Liechtenstein GC Dental Industrial Tokyo, Japan
specimen. In this rnzport,we employ the chzge ofv instead of the change of IIV II. More precisely, let V 1 and 72 be the distance vectors of the specimen at-time21 and t2, respectively. While O’Toole et al. used IIIV 111-IIV 2111,we use IIT1 - %2il. Since I& -&II 2 (11~~11 - IIT our analytical method is preferable to theirs. Thus IIV 1 - 7211 can be regarded as the dimensional change between the specimen at tl and at t2. In this research, tl was the time when the holes were made in the corners of the wax specimens and t2 was the time when the observation of the specimen was made (that is, as soon as the flask was opened).
A one-way analysis of variance (ANOVA) on the data from this investigation was followed by Bonferroni’s correction to determination whether statistically significant differences existed among the samples produced by the various curing processes. Statistical analysis was conducted at the 95% level of confidence ((Y= 0.05).
RESULTS The means and standard deviations for the three denture base processing methods are presented in Table II. There was a significant difference between specimens cured by the conventional method and those cured with the SRIvocap system. The specimens cured by the microwave curing method exhibited more dimensional change than those cured with the SR-Ivocap system. No statistically significant differences were detected between the conventional and the microwave curing methods. Based on these data, the rectangular plates produced by the SR-Ivocap system are more accurate dimensionally than the samples produced by the conventional and the microwave curing methods.
DISCUSSION The dimensional changes of dentures have been examined using a variety of shapes and methods. In this research,
trolled thickness. This is because dimensional changes of specimens made with acrylic resins are affected by the shape of the specimen.2 Therefore, it is important to examine the dimensional changes of acrylic resins using a
specimen with a simple shape to permit examination of the dimensional change of the acrylic resin itself. The measurement of the dimensional change occurri%g between rectangular plates is difficult, so the change of V was used to examine the dimensional change of specimens. The four holes in each rectangular acrylic resin specimen were marked A, B, C, and D, and the six-dimensional vector 7 = (AB, BC, CD, AD, AC, BD) was analyzed. The dimensional changes of specimens were determined as the change of 7, which is a convenient and suitable method of comparing the dimensional changes of rectangular specimen. The SR-Ivocap system has less inherent processing shrinkage. 8,g However, some investigatorslot l1 reported no significant difference in dimensional change between the SR-Ivocap system and a trial-packed acrylic resin denture base resin. Acrylic resin denture base materials cured by microwave processing were compared with materials processed by conventional water bath heating for some of their properties-for instance, porosity,12-l4 residual monomer,14 molecular weight, l4 flexural strength,15 hardness,12 and transverse strength.12 However, the dimensional accuracy of acrylic resin denture material cured by the microwave technique has not been fully studied. The data in this investigation showed that the dimensional accuracy of acrylic resin itself produced by the SR-Ivocap system is more accurate than that of resins processed by the conventional and the microwave curing methods. The specimens cured by microwave processing exhibited similar dimensional change to those cured by the conventional method. The microwave curing method has the advantage of saving time and of cleaner processing equipment handling compared with the conventional processing method. Further studies will be needed to produce a more accurate denture base. In this research, only rectangular acrylic resin specimens were used. However, for the clinical situation, it is necessary to consider the influences on the dimensional accuracy of dentures from many other factors-the size and shape, the thickness of the denture, and the presence of teeth. However, this method for determining the dimensional change of the rectangular acrylic resin specimens and assessing the processing method has proved to be convenient and suitable. CONCLUSION The dimensional accuracy of rectangular acrylic resin specimens cured by the three denture base processing methods were examined for change of the distance vector
V. The specimens cured by the SR-Ivocap system exhibited less dimensional change 0, < 0.05) than those cured by either the conventional or the microwave curing method. There was not a significant (p > 0.05) difference between the conventional and the microwave curing methods. The SR-Ivocap system produced a more accurate denture base itself than the conventional and the microwave curing methods. We thank Dr. M. Masumoto of the Department of Mathematics, Hiroshima University, for helpful professionalsuggestions. REFERENCES 1. Wolfaard J, Cleaton-Jones P, Fatti P. The influence of processing variables on dimensional changes of heat-cured poly (methyl methacrylate). J PROSTHET DENT
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