Introduction
Cross-infection control in dentistry has become a major issue since the recent increase in some infectious diseases such as hepatitis B, C and AIDS (1) occurred. By dealing with removable prosthodontics the dentist, the dental personnel and the patients are constantly facing the risk of self-inoculation and transmission of microorganisms since the oral surgery environment usually includes a mixture of blood and saliva which contaminates a great number of instruments and items that are not easily disinfected nor sterilized such as prostheses, impressions, stone casts. Although ADA has published guidelines in order to prevent the transmission of infectious diseases in dental offices, many have failed (2). A number of studies have presented numerous methods to avoid cross infection in the dental practice.
Chemical disinfection was the recommended method to be used after the removal and before the insertion of the prosthesis in the patient’s mouth. The disinfectant solutions used in the past were chlorine dioxide, sodium hypochlorite, 4% chlorhexidine and glutaraldehyde (3-5). However, studies on the effects of chemical disinfection on the physical-mechanical properties of acrylic resins came to the conclusion that these were altered (6, 7). For example, dentures were presented with staining, brownish discoloration of the teeth and the acrylic denture base and patients reported a number of oral tissue reactions (1, 8). The major disadvantage of chlorine dioxide is that it was rejected by the patients because of its offensive odor (9), its bleaching action over the denture resin base and its corrosive effect on the framework (6, 10). In addition, glutaraldehyde and sodium hypochlorite presented high or medium cytotoxicity risk (11). Furthermore, studies have demonstrated that such solutions can provide a suitable environment for growth of certain bacteria (12). In conclusion, denture soaking chemical methods are considered to be time consuming or inappropriate for chairside procedures (13).
Therefore, in order to overcome the problems occurring with chemical disinfection, microwaves were suggested as a low-cost, quick, efficient, and chemical free alternative. Microwaves are a type of electromagnetic energy with wavelengths close in frequency to television transmissions and aircraft radar. It is very important to note that microwave heating is an energy conversion and differs greatly from heating in a conventional oven (1). The exact mechanism of destruction of the microwaves has yet to be fully understood. Some studies claim that the irradiation of the microorganisms is directly of thermal character, (14) while others claim that non thermal effects may play a significant role (15, 16).
It is shown that microwaving dentures is equal to or more effective than sterilizing them in a 0.02% solution of sodium hypochlorite for 8h (17). Since the main goal of microwave disinfection is to deactivate potentially pathogenic microorganisms, some studies reported that dentures microwaved in water were more effectively disinfected than dentures irradiated in dry conditions (18). Some researchers recommend microwave disinfection in order to prevent or even treat denture stomatitis, since it was shown that microwave irradiation successfully eradicates Candida species from the infected dentures (19-21).
However, microwaving may negatively affect denture resins, liners or teeth due to the material heating after irradiation. The water starts to boil after approximately 1 minute and 30 seconds and remains at this temperature until the end of the pre-set disinfection time, (14) a phenomenon that may have negative effects on denture related materials’ physical-mechanical properties. In order to moderate the cumulative distortion produced by the excessive heat, some researchers even suggested the combination of tablets containing alkaline peroxide and enzymes with microwave irradiation, thus achieving faster disinfection and less heat compared to the use of microwaves alone (22).
The aim of this paper was to overview the current scientific knowledge concerning the effect of microwave disinfection on denture related material properties.
Denture Bases
A denture base is a part of a denture which rests on the foundation area. The artificial teeth are fixed on its upper surface (23).
It is used to replace the missing alveolar ridge both in bulk and appearance. Furthermore, it is the part of a denture where the prosthetic teeth are attached and provide bracing and retention for the denture. It facilitates the transfer of occlusal forces to the abutment teeth and when the denture is tissue supported to the foundation area (23).
Dimensional stability
The dimensional stability of a denture is the degree to which the material used for its fabrication retains its primary dimensions. The clinical significance of this parameter is very crucial, as it affects the fit of a denture, making it intolerable to the tissue and as a result to the patient. Great dimensional changes can also add an extra pressure to the mucosa which might traumatize the underlying bone and increase its resorption.
Microwave disinfection’s effects on the dimensional stability of acrylic resins is widely examined in the literature since it could influence the fit, retention and stability of complete dentures (24).
Seo et al. (24) submitted denture bases to microwave disinfection at 650 W for 6 min (once per day for 7 days) and concluded that this procedure led to denture bases’ shrinkage, but the clinical relevance of these findings was not assessed. Webb et al. (17) and Pavan et al. (3) also demonstrated that denture bases are likely to undergo damaging dimensional alternations when disinfected by microwave irradiation. The irradiation parameters were 604 W for 10 minutes.
Sartori et al., (25) submitted denture bases to microwave disinfection (690W for 6 min in 500 mL of distilled water) and compared them to bases that had previously been chemically disinfected with a 100 ppm active chloride solution. They stated that bases disinfected with microwaves after a period of time did not fit to their casts, unlike bases immersed in chloride solutions. In addition, Goncalves et al., reported that microwave irradiation (650 W for 6 min in 200 mL of water for 7 times) revealed significant differences in the linear dimension of denture bases (26). Senna et al. observed significant linear distortion when the dentures were submitted to 36 cycles of microwave irradiation at 900 W for 3 min in wet environment. Nevertheless, they reported that 6 cycles of microwave irradiation at 450 or 630 W for 3 min caused no significant alternations (27). Wagner and Pikpo also observed important dimensional changes of approximately 3% when dentures were exposed twice to either 420 or 700 W of microwave irradiation in wet environment for 3 min (28). Sartori et al., (29) also investigated the effect of microwave disinfection (two cycles at 690 W for 6 min) on the tridimensional stability of denture bases and showed that this procedure generated a progressive increase in distortion after 14 days as a result.
On the contrary to the aforementioned, Consani et al. found no significant dimensional changes when they submitted dentures to microwave disinfection (650 W / 3 min) (30). Polyzois et al. (7) reported that 3 and 15 min of microwave exposure at 500 W could cause dimensional changes of no clinical importance (<0.03%) on the dentures. Rohler and Bulard (31) asserted that both wet and dry dentures’ exposal to microwave disinfection at 720 W for 16 minutes, showed no dimensional changes. Webb et al. (17) came to the same conclusion but only when dentures were irradiated with reduced exposures (331 W for 6 min).
Burns et al. (32) showed that specimens made from three different acrylic resins had no alternations on their dimensional stability after 15 min at 650 W of microwave irradiation. Fleck et al. investigated the effect of two microwave disinfection protocols (690 W / 6 min) and (345 W / 6 min) on the dimensional stability of acrylic resin denture bases. The procedure for both disinfection protocol groups was performed 3 times with a 7-day interval. They found that the first protocol had detrimental effects on the dimensional stability of denture bases, but they also suggested that 345 W for 6 min had no significant effects on them (33). Polychronakis et al. irradiated dentures daily for a week in both dry and wet conditions for 6 minutes at 650 W and concluded that the dimensional changes observed when the disinfection took place in dry conditions were of no clinical significance (34).
The controversy in the aforementioned results may be attributed to the difference in the disinfection time, power of the microwave oven, type of the materials and methods used to measure distortion (25). For example, denture resin materials have different behavior if irradiation takes place in dry or wet conditions. The immersion in water was adopted because it was believed that it increases the effectiveness of the disinfection (19). However, the water starts to boil after 90 seconds of irradiation (19), which could lead to an increase in temperature beyond the acrylic resin’s glass transition temperature and thus it would become more flexible (35). This may facilitate the warping of denture bases, caused by the release of stresses stored within the material during fabrication (36, 37). Furthermore, it is highly possible that the higher temperature of the water may enhance the dispersion of the remaining residual monomer molecules (38, 39) to the active sites of the polymer chain (40). Consequently, further polymerization may lead to shrinkage of the denture bases.
Basso et al. investigated the effects of microwave disinfection on the occlusal vertical dimension changes of complete dentures. The researchers submitted maxillary compete dentures to microwave irradiation at 650 W for 3 min. When the specimen was irradiated once a week for 4 weeks, no significant differences were observed in the occlusal vertical dimension. Nevertheless, the specimens submitted to microwave disinfection 4 times a week for 4 weeks underwent unacceptable distortion. They suggested that when dentures are exposed to many cycles of microwave irradiation, the differences in occlusal vertical dimension are caused by the release of the inherent stress established during fabrication (41).
Flexural strength
Flexural strength is the simultaneous measurement of tensile, shear bond and compressive strengths (42). It describes the loading that occurs on the denture during mastication. Clinically, diminished flexural strength could lead to dentures more prone to fracturing.
Polyzois et al. (7) suggested that the changes in flexural strength after microwave disinfection (500 W / 3 or 15 min) of denture base resins in the dry state were of no clinical importance. Consani et al. (43, 44) indicated that the method of microwave disinfection (5 cycles at 650 W for 3 min in water) had no important discrepancies in the flexural strength between disinfected and non-disinfected specimens. Ribeiro et al. in their research (650 W / 1-5 min / 2 cycles / immersed in water) came to the same conclusion (45). Konchada et al. also found no alternations in flexural strength when they irradiated dentures at 650 W for 5 min, while immersed in water (42). Senna et al. observed no significant changes in flexural strength after 6 cycles of microwave irradiation at 450 or 630 W for 3 min, even though the elastic modulus decreased after 36 irradiation cycles (27).
On the other hand, Hamouda and Ahmed demonstrated that both 5 and 15 minutes of microwave disinfection at 750 W, either in dry conditions, or immersed in water produced a reduction of the flexural properties of denture base acrylic resins and concluded that this method is not acceptable for dentures (1).
Impact strength
Impact strength is a measure of energy received by the material when it experiences sudden fracture (42). A denture with high impact strength is more resilient to fracturing. Thus it is highly favored for a disinfection procedure not to diminish dentures’ impact strength values.
Seo et al. demonstrated that the impact strength of denture bases disinfected with microwaves (650 W / 6 min) daily for a period of seven days showed no differences compared to the control group (46). Konchada et al. also concluded that microwave disinfection at 650 W for 5 min did not affect dentures’ impact strength (42). Senna et al. reported that 36 cycles of microwave irradiation at 630 or 900 W for 3 min caused no significant changes to dentures’ impact strength (27).
Conversely, Hamouda and Ahmed claimed that either immersed or non-immersed in water, microwave disinfection (750 W / 5 or 15 min) is detrimental for their impact strength - along with other properties (1).
Hardness
Hardness is directly connected with the integrity of a material and its resilience to decay by thermal, chemical or mechanical action (27). In clinical situations, hardness represents dentures’ resilience to abrasion over time. A disinfection procedure which decreases dentures’ hardness is unacceptable for clinical use because it makes them prone to wear.
Ribeiro et al. showed that the disinfection control protocol (650 W / 1-5 min / 2 cycles / immersed in water) had no detrimental effects on dentures’ hardness (45). Dixon et al. noticed an increase in hardness, but they considered this increment of no clinical significance (47). Polyzois et al. demonstrated that the hardness of a denture base resin remained unaffected during microwave disinfection (500 W / 3 and 15 min) in the dry state (7). Sartori et al. also found that microwave disinfection had no effect of clinical significance on their hardness compared to the traditional disinfection procedure, but decreased after 14 days (29). Still, they concluded that the traditional immersion in chloride solution is a safer procedure than the conventional microwave protocol (690 W for 6 min) with regard to surface hardness of dentures (29). Konchada et al. reported that microwave irradiation at 650 W for 5 min in wet conditions did not change the hardness of the dentures disinfected (42).
Consani et al. reported that microwave disinfection (650 W / 3 min / 1 cycle / in wet conditions) (43) and (650 W / 3 min / 5 cycles / in wet conditions) (44) decreased the hardness of the denture resin tested.
On the other hand, Senna et al. demonstrated that in fact microwave disinfection increases the dentures hardness after 36 irradiation cycles at 630 or 900 W / 3 min (27).
Hard and Resilient Denture Liners
Bond strength
Loss of adhesion between denture bases and liners is a common clinical failure examined in previous studies (48-51). A strong bond is necessary in order to prevent abruption of the liner, staining and retention of bacteria (52). In addition, the quality of the bonding increases the flexural strength of the liner (50). Braden has shown that the temperature of the water has an important effect on the diffusion of water into acrylic resins (51). As a result, the heat generated by microwave disinfection might cause an increase in the water sorption rate which will decrease the bond between the denture base and the liner (48) since the absorbed water acts as a plasticizer (53).
Machado et al. reported that seven cycles of microwave disinfection at 650 W for 6 min while immersed in water had no detrimental effects on the bond strength values of two hard chair-side (54) and two resilient liners (55).
Flexural Strength
The reduction of liners’ flexural strength during microwave disinfection could make the material more brittle and thus more prone to crack propagation and fracture. The residual monomer of denture liners’ resin might adversely affect their flexural strength by a plasticizing effect which diminishes interchain forces (56). In this context, microwave irradiation may escalate the degree of conversion of auto-polymerized resins, thus enhancing their flexural strength. On the other hand, microwave disinfection in wet conditions could lead to the reduction of liners’ flexural strength which might be related to the plasticizing effect of absorbed water molecules (53, 56). According to that, the residual monomer that might leach out while immersed in water, has a lesser plasticizing effect than the received water molecules, a phenomenon that leads to diminished flexural strength.
Ribeiro et al. suggested that two cycles of microwave disinfection (650 W) on four different hard liners after different exposure times (1-5 min) had no detrimental effects on their flexural strength (45). Vergani et al. (57) examined the flexural strength of four hard denture liners that were irradiated for 3, 4 and 5 minutes at 500, 550 and 650W respectively and concluded that the flexural strength of certain liners could possibly be optimized by the proper power/time combination with microwave post-polymerization irradiation, thus resulting in the improvement of the longevity of the relined dentures.
Pavarina et al. (6) demonstrated that microwave disinfection at 650 W for 6 min (2-7 cycles) increased the flexural strength of five hard chair-side reline materials. Patil et al. also investigated the effects of microwave disinfection on two resilient liners and concluded that 650 W for 5 min increased their flexural strength (58).
Porosity
The ideal surface of the liners should be smooth. However, porosity is a common clinical flaw that often occurs and leads to staining, calculus deposition and accumulation of microorganisms (59, 60). Even after the relined denture has been cleaned, there is a great chance that numerous microorganisms remain (61). Porosity through microwave disinfection may occur from the vaporization of the unreacted monomer due to the high temperatures that these materials are exposed (62).
Novais et al. examined the occurrence of porosity after microwave disinfection (650 W / 6 min / 7 cycles) in five hard liners and came to mixed results, since some liners experienced an increase in their porosity, others presented a decrease and one material remained stable (62).
Dimensional stability
Denture liners are commonly used in order to improve the clinical fit of complete dentures (18). However, during microwave disinfection liners are exposed to very high temperatures even beyond the glass transition of the polymers, resulting in their distortion (18). In addition, the high temperature affects the free monomer and promotes further polymerization and thereby extra dimensional changes (63).
Seo et al. examined the dimensional stability of four hard liners after microwave disinfection (650 W / 6 min / 7 cycles) and suggested that the procedure produced increased shrinkage on the specimens tested (24).
Basso et al. (18) evaluated the effect of microwave disinfection (650 W / 3 min / 1-5 cycles) on hard chair-side reline resins and came to mixed observations. Some of the liners presented shrinkage, but in others no wrapping was observed. Goncalves et al. also examined the dimensional stability of four hard chair-side liners after microwave disinfection (650 W / 6 min / 2 or 7 cycles) and concluded that two of them shrank and the other two remained stable (26).
Hardness
Hardness is considered a very important physical property of the liners used in removable prosthodontics. When hardness increases by time, liners lose their elasticity, an implication that leads to excessive occlusal forces to the underline mucosa, pain, soft tissue irritation and subsequently to bone resorption and denture misfit.
Dixon et al. showed that microwave disinfection produced marginal and not clinically significant effects on the hardness of two resilient liners when the irradiation took place in wet conditions (47). Ribeiro et al. also investigated the effect of microwave irradiation (650 W / 1-5 min / 2 cycles) on the hardness of four hard liners after different exposure times, and found no significant discrepancies compared to the control group (45). Machado et al. submitted two resilient liners to microwave disinfection (650 W / 6 min / 7 cycles) while the specimens were immersed in 200 mL of water and reported that the hardness of the liners was not adversely affected by microwave irradiation (55). Pavan et al. also suggested that two cycles of microwave disinfection (500 W / 3 min) did not affect the hardness values on the long-term resilient denture liners that were examined (64).
Denture teeth
Denture teeth are made of either porcelain or more prevalently by polymers. Polymer teeth are classified based on their composition and system of polymerization to conventional acrylic resin teeth and IPN (Interpenetrating Polymer Network) teeth. Conventional acrylic resin teeth have a homogeneity in their configuration and a polymer network of one resin type. On the other hand, IPN teeth chemically consist of two polymer networks mechanically tangled together. These networks are highly cross-linked and their polymerization creates strong chemical bonds (23).
The polymer teeth retention to the denture base is mainly achieved by micromechanical interlocking between the denture’s polymer and the polymer network of the prosthetic teeth (23).
Hardness
Hardness is one of the most essential physical properties of resin denture teeth examined in the literature. It is used for the assessment of prosthetic teeth wear resistance as they operate to maintain the originally established vertical dimension and masticatory activity (65-68).
Campanha et al. showed that the hardness of acrylic resin denture teeth presented no significant changes if microwave disinfection (3 cycles) is used at 650 W for 6 min in a wet environment for 90 days (67).
However, Vasconelos et al. reported that 3 cycles of microwave disinfection at 1300 W for 3 min significantly decreased denture teeth hardness, compared to the traditional glutaraldehyde and hypochloride disinfecting solutions (69). Campanha et al. (70) also suggested that seven cycles of microwave irradiation (650 W / 6 min) decreased the hardness of all acrylic resin teeth tested.
Bond strength
The unexpected detachment of teeth from dentures is a very important issue that compromises integrity and clinical service ability of complete dentures. Crack propagation, residual wax contamination of teeth and excessive stress were to blame for teeth debonding in the past (71, 72). In order to consider microwave irradiation as a reliable alternative for denture disinfection, it is crucial that it does not adversely affect the teeth/denture bond strength, consequently leading to teeth debonding after several disinfection cycles. Thus, the possible weakening effects of microwave disinfection on the teeth/denture bond strength is examined thoroughly in the literature as it is an issue of great clinical significance.
Campanha et al. investigated the effect of microwave irradiation (650 W / 3 min) on the bond between teeth and denture bases and concluded that this procedure might lead to important decrease of the bond strength values, regardless of the ridge laps treatments which may have been used (73). They also submitted acrylic specimens treated with different techniques to microwave irradiation (650 W / 3 min) while immersed in water and compared to the control group; and the irradiated specimens presented decreased teeth/resin bond strength values (74). However, in their latest study, Campanha et al. disputed the negative effects of microwave disinfection (650 W / 3 min) on the teeth/base bond strength, suggesting that 5 irradiation cycles not only do not decrease, but also increase the bond values (75).
From this overview it could be concluded that there are still reservations and lasting questions remaining unanswered. For instance, there is no established protocol for the use of microwave irradiation that ensures dentures’ disinfection by causing no implications of clinical significance on their physical-mechanical properties. Also, the exposure time and power used, seems to have been randomly selected. There is a wide range of parameters used, from 350 to 1400 W and from 1 to 20 min (76). Furthermore, the optimal irradiation frequency and the mid- and long-term effects of periodical microwave disinfection on dentures remain unknown. Thus, more studies should be conducted investigating the effects caused by microwave irradiation on denture related materials in the mid- and long-term, providing a reliable disinfection protocol and examining more physical-mechanical properties, e.g. color stability, viscoelastic properties, residual monomer, etc.
Discussion
Chemical disinfectants were the main method for preventing cross-contamination in the past. Although, their usage produced significant disadvantages such as denture staining and intra-oral tissue reactions (8). Other problems presented in the literature were a characteristic offensive odor after the use of sodium chloride, brownish discoloration of dentures and tongue (1), alterations of acrylic resins properties (7) and penetration of certain chemical disinfectants’ components in the material after the disinfection procedure (67).
On the other hand, microwave disinfection is a simple, quick and easy to use alternative. It does not produce brownish discoloration on dentures and oral tissues, nor has an unpleasant odor. It also disinfects dentures effectively and does not have an expiration date. Nevertheless, it cannot disinfect dentures with metal parts and its daily use is still debatable (76).
Considering the data from the aforementioned literature, it can be concluded that microwave irradiation is a recommended alternative for the disinfection of dentures and liners if the procedure is carried out in dry conditions for a maximum of 3 disinfection cycles. Furthermore, the optimum disinfection parameters in order to achieve best disinfection results with minimum adverse influence onto dental materials’ physical-mechanical properties are 650 W for 3 minutes.
Conclusions
Microwave disinfection (650 W / 3 min / 3 cycles) is a safe alternative for the disinfection of denture bases and liners compared to the chemical one when the procedure is carried out in dry conditions, but could possibly cause dimensional changes of clinical significance when the irradiation takes place in wet environment. More than 3 cycles of microwave disinfection in these settings could adversely affect the physical-mechanical properties of denture base resins, liners or teeth.
Microwave irradiation (650 W / 3 min) seems to have no detrimental effects of clinical importance on the flexural properties, impact strength and hardness of denture resins and the bond, flexural strength, porosity and hardness of denture liners.
The effects of microwave disinfection on the hardness of denture teeth and teeth/denture bond strength are still controversial and no safe conclusions can be drawn.
