Opportunistic and pathogenic microorganisms use the oral cavity as a reservoir [1] . While routine dental practice, there could be a high risk of contamination and infection that may even cause systemic infections [2] . Pathogens transfer from patients to members of the dental team and vice versa. Transfer of pathogens may also take place from patient to patient without intervention of the dental team via the transmission routes of contaminated surfaces, instruments or dentures. In general, the major vectors of cross-contamination of pathogens are the patient’s saliva and blood [2] [3] [4] .

As a result of the demographic change in the industrialized countries, prosthodontic treatment will increase [5] . With advancing age, the general health of patients will decline and thus older patients usually become more prone to infections. Overall, there might be a high risk of infection for all participants of dental treatment, including patients, providers, dental assistants and laboratory technicians. The omission of a proper disinfection of dental prostheses might increase the risk for transmission of communicable diseases (e.g. Hepatitis B virus) and facultative pathogenic microorganisms (e.g. Pseudomonas aeruginosa). A higher rate of Hepatitis-B-infestation compared to the average population is shown on dental technicians [6] [7] . Therefore, there is a need to reduce the possible transmission routes of pathogens in the dental setting.

The disinfection of dentures before and after contact with patients is theoretically an obligation [8] [9] . Dentures contact mucous membranes and should be cleaned of all microorganisms before handling them. These items require high-level disinfection using chemical disinfectants like aldehydes. The inactivation of pathogens without affecting the material structure of dentures is the intention of the chemical disinfecting process [9] . The definition of a high-level disinfectant is a sterilant that is used for a short contact time (e.g., 20 minutes for 2% glutaraldehyde), reaching a 6-log₁₀ kill of an appropriate Mycobacterium species. The aim of the disinfection process of dental restorations should be the elimination of pathogens to prevent transmission of infection [7] [10] [11] .

There are no agents produced exclusively for disinfection of acrylic dentures. Thus, disinfecting agents originally produced for disinfection of dental impressions are used generally. Aldehydes are the utilized agents for disinfecting acrylic dentures, which are mostly used. However, the material safety data sheets give no direct information regarding disinfection of acrylic dentures [12] .

Therefore, the purpose of this in vitro study was to determine the effect of disinfection agents on the elastic modulus, flexural strength and color stability of different denture base resins. Four different Polymethyl methacrylate-based (PMMA)-based resins and three disinfecting agents were used during this investigation.

The alternative hypothesis of this study is that the investigated chemical disinfecting agents have a significant influence on the elastic modulus, flexural strength and color change of different denture base materials.

In this present in vitro-study the influence of common dental disinfection agents on elastic modulus, flexural strength and color stability of PMMA-based resins was established. Two cold-curing (PalaXpress and Futura Gen) and two heat-curing (Paladon 65 and FuturAcryl 2000) PMMA-based resins were used [13] . The test specimens were covered by three disinfecting agents containing different primary active ingredients. Impresept and Stammopur DR were included in the aldehydes group of disinfecting agents. The disinfecting mode of action of D 050 InstruGen were oxidizing connections. Reference test series were performed using distilled water to compare the influence of water immersion compared to the possible effect of the disinfecting agents.

The investigated PMMA-based resins performed no significant effect according to the E-modulus after disinfection compared to the distilled water reference test group (Figure 3). PalaXpress test specimens showed after disinfection with Stammopur DR (2957.00 ± 93.57 MPa) minor changes and after disinfection with Impresept (3004.20 ± 26.40 MPa) the largest deviation according to the reference value of distilled water (2851.00 ± 95.23 MPa) (Figure 3). The results of Paladon 65 after disinfection with Stammopur DR (2811.40 ± 199.65 MPa) were close to the reference value of distilled water. The E-modulus values of Paladon 65 after disinfection with Impresept (2647.60 ± 157.10 MPa) showed the most deviation to the reference values of distilled water (Figure 3). However, the test results of Futura Gen after disinfection with Impresept (2853.80 ± 195.80 MPa) changed very little compared to the distilled water data, the largest deviation performed D 050 Instru-Gen (2773.80 ± 154.77 MPa). Disinfection of FuturAcryl 2000 with Impresept (2780.00 ± 75.77 MPa) performed the smallest variation to the reference values. After disinfection with D 050 Instru-Gen (2881.80 ± 58.30 MPa) the largest deviation was detectable (Figure 3). Taken together, disinfection of the investigated PMMA-based resins had no significant influence on elastic modulus (Figure 3) [13] . The minimum requirements for the E-modulus of the investigated PMMA-based resins of ≥2000 MPa (ISO 1576) were fulfilled. However, the high level of variance of the standard deviation values regarding to volume determined elasticity indicates a relatively inferior status of homogenization of the test specimens (Figure 3) [14] .

Compared to the distilled water reference test group, the investigated PMMA-based resins showed no significant effect to the flexural strength after disinfection (Figure 4). PalaXpress performed after storage in Stammopur DR (30.64 ± 1.01 N/mm²) minor changes and after disinfection with Impresept (30.88 ± 0.25 N/mm²) the largest deviation according to reference value of distilled water. Paldon 65 showed results after disinfection with Stammopur DR (28.86 ± 1.81 N/mm²), that were close to the reference value of distilled water. Flexural strength of Paladon 65 differed most after storage in Impresept (27.28 ± 1.30 N/mm²) compared to the reference values of distilled water. Test results of Futura Gen changed very little after disinfection with Impresept (29.58 ± 1.53 N/mm²) in terms of the data of the distilled water control group. The largest deviation was detected after disinfection with D 050 Instru-Gen (28.82 ± 1.59 N/mm²). The smallest variation to the reference values performed FuturAcryl 2000 after disinfection with Impresept (28.82 ± 0.52 N/mm²). The largest variation was detectable after disinfection with D 050 Instru-Gen (29.32 ± 0.49 N/mm²) (Figure 4). However, the results of this investigation are hardly comparable to other findings in literature. Thus, the proof stress limit (ε_x) was set individually at 1%.

Single used isinfection of PMMA-based denture base resins had no significant effect on elastic modulus and flexural strength (Figure 3, Figure 4) [15] . These findings are in contrast to the initial hypothesis. However, the elastic modulus and flexural strength of the PalaXpress-specimens decreased significantly after storage in distilled water compared to dry storage (Figure 3, Figure 4). The wet storage leads to a diffusion of water molecules into the material, which in turn leads to swelling and a plasticizing effect. That results in a decrease of the elastic modulus of these materials [16] [17] [18] . However, the disinfection of the tested PMMA-based denture base resins did not influence the investigated material properties relating to elastic modulus and flexural strength significantly (Figure 3, Figure 4) [15] . For general disinfecting instructions regarding to E-modulus and flexural strength, the three tested disinfecting agents can be applied to everyday use [19] [20] .

The storage in distilled water lead to color change of the test specimens from ΔE 0.66 ± 0.32 to ΔE 1.00 ± 1.14 (Figure 5). The data analysis of the color measurement shows that the investigated disinfection agents did not produce any significant color change on PalaXpress and Futura Gen (Figure 5). On the other hand, the disinfection of Paladon 65 with Impresept (ΔE 1.78 ± 1.51) and FuturAcryl 2000 with Stammopur DR (ΔE 1.25 ± 1.18) caused significant color changes compared to the control group (Figure 5). The clinical relevance of these results may be marginal, due to the fact that the measured ΔE-values were in a range of 1 to 2 points. These ΔE-values are usually not perceived visually [21] [22] . Only ΔE-values higher than 2 are visually perceptible. So only the disinfection agent D 050 Instru-Gen had a color changing effect on FuturAcryl 2000 (ΔE 2.50 ± 1.80) (Figure 5). The data shows that the disinfection process was causal for this specific color change on FuturAcryl 2000. The use of Impresept for disinfection of PalaXpress (ΔE 0.91 ± 1.13) and Paladon 65 (ΔE 1.78 ± 1.51) caused visually undetectable color alterations (Figure 5) [22] .

In summary the three investigated disinfection agents, two of them containing glutaraldehyde and one of them contain oxidizing connections, did not produce any significant changes in the tested material parameters of E-modulus and flexural strength of the PMMA-based resins [12] [15] . Processing of acrylic dentures may subject them to numerous possibilities for defects, which result in porosities, shape deviations, and failures of surface structures. Void producing defects impair the structure and downgrade the physical and biological quality of denture base resins. Additionally, these defects have a negative influence on the hygienic characteristics and they compromise the esthetics of the PMMA-based resins [17] [23] . Therefore, high pressure is typically used to reduce the described defects during the polymerization process [17] [23] .

In principle it is difficult to estimate how often a denture is disinfected during its clinical service time. Dental prostheses have to be disinfected for the first time before delivery [7] [24] . Thus, in this study the influence of single use disinfection was tested. In general, disinfecting agents should carry a wide application range. The disinfecting agent Impresept is recommended by the manufacturer for the disinfection of dental impression materials and is certified as surface disinfectant [25] . The results of this study imply that disinfection with Impresept does not impair the investigated material properties of PMMA-based resins. The universal applicability of disinfecting agents for both dental impressions and denture prosthesis is time saving and cost effective. Denture base resins are one of the most commonly used materials in the dental practice. The utilization of removable dentures will increase in the future with the expected change in demographics [5] . Thus, it is essential to understand the compatibility of utilized materials and to establish safe as well as standardized hygiene measures in the dental practice [7] [26] . The objective of all infection control procedures is to prevent transmission of infections between treated patients, dental staff and dental technicians.

A limitation of this study is the investigation of the single use disinfection. This describes the newly manufactured dental prostheses. For simulating the daily clinical practice, further studies should investigate the influence of repeated applications of disinfection agents on PMMA-based dentures. Repairs of dental prostheses could be prepared for simulating the averaged wearing period of dental prostheses. In addition, different material properties of PMMA-based resins have to be examined.

In this study, it was demonstrated that the investigated disinfecting agents did not influence elastic modulus and flexural strength of the tested denture base resins negatively. Further studies should verify the impact of long term disinfection intervals on the material properties of acrylic denture base resins.

The authors thank Dr. Seth Bozarth, DMD, Tufts University, Boston and Dr. Jamie N. Amir, DDS, Ocala, Florida, USA for their technical support and for their comments of the manuscript.

The authors declare that they have no conflict of interest.

Bensel, T., Bock, J.J., Zumpe, L., Mansour, S., Blümel, N., Seeliger, J. and Boeckler, A.F. (2018) Effect of Disinfectants on Elastic Modulus, Flexural Strength and Color Stability of Denture Base Resins. Open Journal of Stomatology, 8, 135-148. https://doi.org/10.4236/ojst.2018.84013