The Effect of Microwave Sterilization on Abrasion Resistance of Commercially Available Acrylic Resin Teeth- An In-vitro Study

INTRODUCTION

Dentures made of acrylic resin have been identified as a source of cross-contamination between patient and dental personnel. Sterilization and disinfection are the popular and widely used methods for eradicating microorganisms from the surface of denture base materials and controlling cross-contamination^1,2 . However, disinfectant solutions can alter the physical and mechanical properties of denture base resins, as they penetrate into the material and cannot be completely eliminated by rinsing¹ . Microwave radiation is used to polymerize acrylic resins³ and is suggested as a useful alternative to disinfectant solutions⁴ .More over microwave sterilization is advantageous, as it does not require special storage, has no expiry date and does not induce resistance to Candida albicans⁵ .While a number of studies have examined the effects of microwave sterilization on the mechanical properties of denture base resins⁶ , information regarding the effects of it on commercially available acrylic resin denture 1 teeth and its effect on wear resistance is scanty .Wear of denture teeth in removable dentures may cause not only diminished chewing performance, but also a change in occlusion, later parafuctional activities and derangements of TMJ. To maintain proper function and occlusion, more attention should be given to the selection of the denture teeth associated with the opposing materials. In case of antagonistic enamel, the denture teeth as a tooth substitute should have a wear resistance similar to that of enamel and low abrasiveness against enamel⁷ . With the above background the present study is aimed at evaluating the effect of microwave sterilization on abrasion resistance of acrylic resin denture teeth.

MATERIALS AND METHODS

Thirty acrylic teeth from each brand i.e Premadent, Acrypan and Truton were taken. In each brand fifteen teeth served as control and remaining fifteen were subjected to microwave sterilization.

Preparation of Acrylic resin teeth specimens: (Fig 1, 2 and 3)

The master specimen of each brand of commercially available acrylic resin tooth was prepared by mounting the acrylic resin tooth (mandibular left first premolar) on a wax base of 10x10 mm. This was used to make a mold with silicone duplicating material. This mold was prepared for standardizing the alignment of denture teeth with base of the test specimens. Each brand of acrylic resin tooth was placed in index made in the respective mold, into which molten wax was flown. A total of 90 blocks for 3 brands (30 each) of acrylic teeth were prepared. They were numbered according to their groups for identification. These wax specimens were invested, dewaxed and acrylized by conventional method, according to the manufacturer's instructions.

Preparation of human enamel specimen preparation: (Fig 4 and 5)

Ten freshly extracted maxillary central incisors, extracted due to periodontal diseases, showing no signs of abrasion, erosion, attrition, fracture and of approximately same size were taken and embedded in a wax block of 10x10 x 5 mm. These specimens were invested; dewaxed and self-cure resin was mixed and packed following manufacturer's instructions. The flask was closed and placed in a pressure pot for 10-15 min. The specimens were recovered and surface of the specimens were finished and polished.

Wear testing:

For the purpose of the study, a custom made specialized wear testing machine (Fig 6) was designed. The machine consisted of two compartments (lower and upper), AC stepper motor, gear box and a water tank. The lower compartment of the machine was fixed on the platform on which human enamel specimens were mounted. The acrylic resin specimens were mounted in the upper compartment, which was designed such that it had a to and fro sliding movement of 4 mm per stroke under a constant load of 1 Kg for 60 cycles /min. The specimens were mounted on the wear testing machine such that buccal cusp tip of acrylic resin specimen came in contact with human enamel specimen. The water which was circulated on the specimens from the water tank served as a lubricant.

Wear testing of a control specimen: The height of a test specimen was measured before wear testing by marking two points on the specimen (Fig 3) i.e., one at the tip of the buccal cusp and second at the base of the specimen and two points were connected by a line, measured using digital vernier caliper (Fig 7) by three examiners and the mean was taken to avoid errors. The acrylic specimens were mounted on the wear-testing machine by using silicon putty in the upper compartment and human enamel specimens were fixed in the lower compartment. The wear testing machine was operated for 26.25 hrs. After wear testing, the height was measured. The difference in measurements noted before and after testing determined the total loss / total wear; this in turn determines the abrasion resistance.

Wear testing of a microwaved specimen: The height of acrylic resin tooth specimen was measured using digital vernier caliper as performed earlier. The acrylic resin tooth specimens were subjected to microwave sterilization, using microwave oven (Fig 8). Microwave sterilization is carried out by placing acrylic resin specimens in water filled glass container at 650W for 6 min. The teeth were subjected to abrasion resistance test as performed earlier. The height of the specimen was measured. The difference in measurement recorded before and after the wear testing determined the total wear; this in turn determines the abrasion resistance.

The data obtained was statistically analyzed and conclusion was drawn.

RESULTS

The descriptive Statistics of wear of control and microwave sterilized acrylic resin teeth are summarized in table 1 and in graph 1 .The ANOVA Statistics conducted in the compared groups and their significance are summarized in table 2 . Group Statistics of control and microwave sterilized groups (T-test) and their significance levels are summarized in table 3.

DISCUSSION

Prosthesis has been identified as a source of cross contamination between patient and dental personnel¹ . The increased prevalence of some infectious diseases, i.e., Hepatitis B, AIDS etc. altered the public opinion to infection control during dental service. A common practice for avoiding cross infection can be easily carried out in other disciplines of dentistry, but is sometimes difficult in Prosthodontics. Impressions, trays, casts and prostheses are all potential sources of cross contamination to and from patients, clinical personnel and dental technicians⁶ .Sterilization and disinfection have become popular and widely used method for eradicating microorganisms from the surface of denture base materials and controlling cross-contamination² .However, disinfectant solutions can alter the physical and mechanical properties of denture base resins, as they penetrate into the material and cannot be completely eliminated by rinsing¹ ,which is time consuming and considered as an inappropriate method² .So microwave sterilization was suggested as a useful alternative to disinfectant solutions. More it is advantageous as it does not require special storage, has no expiry date and does not induce resistance to Candida albicans⁵ .

Many studies were conducted to establish the effect of microwaving on the dimensional stability of acrylic dentures. They concluded that microwaving acrylic dentures at high power setting for 10 min causes unacceptable dimensional changes⁸ . However, a medium power setting for 4-6 min causes minimum changes, which is probably sustainable in long term^9,10 . Therefore microwave sterilization has been done at 650 W for 6 min in this study.

Dentures with acrylic resin teeth are most commonly used in removable prosthodontics⁷ . The advantages claimed for resin teeth are less breakage, reduction of clicking sound, a better bond between the teeth and the resin base, ease of grinding, recontouring and repolishing¹¹ .

To minimize the disadvantages of acrylic resin and to enhance certain qualities, modification was done to the polymers. The material consists of an unfilled, highly cross linked, interpenetrating polymer network. Cross-linking agents can be incorporated into linear polymers to improve properties such as stiffness, surface hardness, resistant temperature and resistance to solvent attack¹² .Therefore, 3 brands of cross linked acrylic resin teeth were chosen in this study, namely Premadent, Acrypan, &Truton. These were easily available and are most widely used.

In this study, mandibular first premolar (buccal cusp) has been used, because lingual cusp is rudimentary and has no functional contact with opposing teeth¹³ .

The major disadvantage of acrylic resin teeth is rapid wear on posterior occlusal surfaces. Rapid wear tremendously affects the vertical dimension of occlusion, tooth relationships and results in increased horizontal stresses¹⁴ , later Para functional activities and derangements of TMJ. Wear of denture teeth may permit the supraeruption of opposing natural teeth. Finally, force imbalance due to wear of denture teeth may contribute to loss of retention, encourage bone resorption and contribute to the fracture of the denture base¹⁵ .

One of the most important physical properties of artificial teeth used in the fabrication of the edentulous patient is wear resistance and the ability of these teeth to maintain a stable occlusal relationship over time¹⁶ . Therefore wear resistance of acrylic resin teeth has been checked in this study.

Prostheses like single complete dentures and partial dentures opposing all or some of the natural teeth are not an uncommon occurrence¹⁷ . Many difficulties confront the dentist rehabilitating patient with this clinical pattern. Therefore, in the present study enamel specimen has been used as abrader for the acrylic resin specimen.

While the final tests of a dental material are carefully controlled clinical investigations, these methods are time consuming, expensive, and require the use of human subjects. In addition, factors such as variable neuromuscular forces and movement, salivary pH, flow, diet exposure to abrasive or corrosive atmospheres, and oral hygiene practices are difficult to control and have marked effects on wear. For this reason the study was conducted invitro¹⁵ .

In this study custom made abrading machine was used. The acrylic resin teeth and enamel specimens were mounted on a wear testing machine, which was designed such that it had to and fro sliding movement of 4 mm per stroke under a constant load of 1 Kg for 60 cycles /min⁷ . The water which was circulated on the specimens from the water tank served as a lubricant which simulated human saliva. Motion of the machine was applied for 26.25 hours; 26.25 hours simulates the three months total occlusal contact of an individual¹³ .

The results of this study can be useful to clinician, when selecting artificial teeth for patient with removable denture against natural teeth, immediate dentures etc. as there would be greater need of teeth with improved abrasion resistance in these conditions. With recent advances in dentistry, improved sterilization procedures such as microwave sterilization have proved to be an effective method in not only eradication of micro-organisms but also in improving physical properties such as abrasion resistance of acrylic resin teeth. But, increase in abrasion resistance by microwave sterilization of acrylic resin teeth also has an advantage over time consuming and expensive procedures like incorporation of Hardy's teeth, amalgam stops in posterior teeth etc. in order to maintain occlusal vertical dimension.

It was found that abrasion resistance of PC and TC groups was significantly higher than AC group. The higher cross linking network of the resin in group PC and group TC might have contributed for higher abrasion resistance of these teeth. But, the abrasion resistance of microwaved group such as PM ,TM and AM showed no significant difference among each other. Abrasion resistance of AM group was higher than AC group. Hence, it can be stated that abrasion resistance of was increased after microwave sterilization. Thus microwave sterilization can be considered as a useful method in not only removing microorganisms but also in improving the abrasion resistance of acrylic teeth AM.

Currently there are many varieties of acrylic resin denture teeth produced by manufacturers with improved physical properties. Thus it would be important for the clinician to know the brand of teeth with better physical properties. In this study, the specimen teeth were subjected to conventional heat curing procedures similar to the processing of dentures.

The limitations in this study were that only one physical property was examined, while many other properties were not considered and out of many brands of teeth available, only three brands of teeth were selected. Moreover increase in duration and number of times of microwave sterilization after occlusal refinement might affect these physical properties. Hence, further investigation is needed in these aspects.

CONCLUSION

Under the limitations of experimental conditions, the following conclusions can be drawn:

• Abrasion resistance of PC group was higher than AC group. 
• Abrasion resistance of TC group was higher than AC group. 
• Abrasion resistance of TC group was nearly similar to PC group.
• Abrasion resistance of microwave sterilized groups such as PM, TM and AM was almost similar, but was found to be higher than the control groups (PC, AC, TC). 
• Abrasion resistance of AM group was higher than AC group. Hence, it can be stated that abrasion resistance of Acrypan increased after microwave sterilization.

Thus microwave sterilization can be considered as a useful method in not only removing microorganisms but also in improving the abrasion resistance of acrylic teeth (Acrypan).