Dentures in all stages of their construction are potential sources of cross contamination to and from patients, clinical personnel and dental technicians. Thus, dental clinic personnel and technicians should protect both themselves and patients applying relatively simple sterilization-disinfection procedures in everyday practice (1-6).
Many disinfection-sterilization methods have been suggested such as: immersion in 2% alkaline glutaraldehyde for 10 minutes, immersion in 1% sodium hypochlorite for 10 to 30 minutes, and immersion in 3% aqueous formaldehyde for 30 minutes (3). Also, immersion in a solution of 5.25% sodium hypochlorite for 5 minutes can sterilize a denture (7). Nevertheless, these chemical disinfection methods hold some disadvantages e.g. time-consuming, denture bleaching and staining, expiration date and possible oral tissue reaction (8-10). Furthermore, the chemical solutions may alter some physical-mechanical properties of denture base materials (11-13). Microwave energy is claimed to be an alternative to these traditional sterilization-disinfection methods and has been reported to be a reliable, cost effective and time saving method for complete denture disinfection (8, 14-22).
When using the microwave disinfection method, a denture is placed either into water (wet disinfection) or not (dry disinfection) and the microwave oven irradiation is mainly between 450 and 650 W. Furthermore, the time periods of irradiation range from 2 to 10 min.
A study conducted by Rohrer et al. (8) has shown that microwave irradiation at 720 W for 8 minutes, in dry conditions, can sterilize acrylic dentures contaminated with aerobic bacteria and Candida albicans. Webb et al. (14) reported that microwave irradiation at 604 W (±92) or 331 W (±34) for 6 minutes produced sterilization of dry complete upper dentures contaminated by Candida albicans and S. gordonii.
Silva et al. (16) reported that complete dentures contaminated with Candida albicans and S. aureus were sterilized after microwave irradiation at 650 W for 6 minutes. Mima et al. (17) revealed that microwave energy at 650 W for 3 minutes can be used for hard chairside reline resin sterilization. Recently, Senna et al. (22) showed that microwave irradiation at 450 W for 3 minutes or 450 W for 2 or 3 minutes combined with alkaline peroxide containing enzyme denture cleanser is an effective method for disinfection of dentures contaminated with Candida albicans. In all the above mentioned studies (16, 17, 22) for the disinfection/sterilization procedure at 650 or 450W all dentures were placed into water. A review of the literature revealed that although many studies have been conducted to evaluate the effect of microwave disinfection on the physical-mechanical properties of denture base resins (23-31), little information is available on the effect of repeated microwave disinfections on the dimensional changes of denture base materials (30-32), particularly in dry conditions (13).
The aim of this study was to evaluate the effect of repeated microwave disinfections on the dimensional stability of acrylic denture bases.
Materials and Methods
A permanent silicone elastomer mold was made (Panadent Ltd, London, England) from a standard brass die simulating an upper edentulous ridge where index marks had been incorporated by drilling holes in specific locations (33) (in the right and left posterior area (A, B) respectively and the central incisor region (C)). Eighteen casts were obtained by pouring dental stone (Vel-Mix Stone, Kerr Mfg Co, MIIn). Each stone cast was waxed up with a 3 mm thick wax sheet (Tenatex red Associated Dental Products Ltd, Purton, Swindon, Wiltshire SNS 4HT, UK). Eighteen denture bases were constructed using a heat polymerized resin (Paladon 65, Kulzer, GmbH, Germany) following a curing cycle at 74oC for 7 hours and 1 hour at 100oC in water bath (Figure 1).
The denture bases were randomly divided into 3 groups according to the disinfection procedure:
Group I: the denture bases were kept in distilled water (37°C) during the experiment period (control group).
Group II: the denture bases were individually microwaved for 6 minutes at 650 W daily for one week. During disinfection the dentures were immersed in 150 ml of distilled water (wet disinfection).
Group III: the dentures were individually microwaved following the same procedure as group II without immersing them in water (dry disinfection).
Denture bases of group II and III were microwaved every day at the same time using a domestic microwave oven (Model HF 1210, Siemens GmbH, Germany). A plastic cup, filled with 150 ml of water, was placed into the microwave oven during dry disinfection (group III), to protect the device from damage. All denture bases were kept in distilled water at 37°C between the disinfection procedures.
Method of measurement
Measurements were taken across the dimensions AB, AC, and BC, with a digital micrometer accurate to 0.01 mm (Mitutoyo, Tokyo, Japan). For each dimension, five readings were made and the mean value was calculated. The coefficient of variation for the repeated measurements did not exceed 0.9%. In order to take the measurements, the tip of the micrometer was placed on the centre of the index marks (Figure 1).
The measurements were taken on two occasions: after curing and immersion in water for 24 hours (considered to be the starting point), and one week later.
After seven dry disinfections, the denture bases showed smaller dimensional changes compared to those submitted to wet disinfections for all measured distances (Figures 2, 3, 4). This finding was statistically significant only for the distance AB (p<0.05) (Figure 2).
The main finding of this in vitro study was that all denture bases submitted to microwaving disinfection, either wet or dry, exhibited shrinkage after seven disinfections. This may be due to the fact that when acrylic denture bases are heated, as occurs during microwaving, internal stresses incorporated during the processing are released (25).
Our results revealed that denture bases which had undergone wet disinfections showed greater linear changes, compared to those occurring during dry disinfections at the posterior region of the denture bases. This can be explained by the fact that the boiling water (water starts to boil after 90 sec of irradiation) raises the temperature of the denture base, which in turn causes the residual monomer (34) conversion into polymer (25).
Denture bases which had undergone wet microwave disinfection presented linear changes up to -1.16%. This finding is in agreement with the study by Sartori et al. (24) claiming that repeated wet microwave irradiation negatively affects the internal adaptation of denture bases to their casts. Fleck et al. (26) showed that repeated wet disinfection had harmful effects on denture bases’ adaptation to the casts. Furthermore, our results are comparable with those of Seo et al. (32) who reported shrinkage of 2.36% after daily disinfections for seven days. Wagner et al. (31) revealed a decrease of 1.92% at the distance between buccal flanges after two disinfections at 700 W for 3 minutes. On the other hand, Consani et al. (35) reported that repeated wet microwave disinfections did not adversely affect the denture base adaptation.
Our finding that dry disinfection caused minor shrinkage is in agreement with the report of Polyzois et al. (13) stating that acrylic rectangular specimens which underwent dry disinfection showed negligible dimensional changes (-0.005 to 0.009%). Also, it confirms the report of Burns et al. (36) who revealed a minor shrinkage up to 0.03% in cylindrical specimens after a 15-minute at 650 W microwave disinfection. Furthermore, the finding corroborates the study of Pavan et al. (10) that microwave irradiation at 500 W for 3 min did not affect the adaptation of maxillary acrylic denture bases. However, microwaving of denture bases at 604 W for 10 min adversely affects their fit to the stone casts.
In everyday practice, a denture base material is considered to be acceptable when the linear changes across first molars occurring during processing and use are under the limit of 1%, as it was suggested by Woelfer et al. (37) about 50 years ago.
Concerning the above mentioned reports and our findings that dry disinfection caused shrinkage less than 1%, we may conclude that when there is a need for dentures to be repeatedly disinfected, a dry microwaving disinfection is preferable since it does not affect the dimensional stability of denture bases to a degree which would cause clinical problems.
Under the limitations of this laboratory study, the following conclusions can be drawn:
Disinfection using microwave energy may cause dimensional changes (shrinkage) of complete dentures.
Denture bases immersed in water during microwave irradiation exhibited greater dimensional changes compared to those which were not immersed in water.
Denture bases submitted to repeating disinfection, without being immersed in water, showed dimensional stability comparable to the untreated denture bases.
Microwaving disinfection is preferable to be carried out in dry environment because the dimensional changes occurred seem to be of no clinical significance.