Antimicrobial and mechanical properties of acrylic resins with ...

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2 L.A. Casemiro et al.incorporation of antimicrobials into denture basematerials 8,10–16 .The addition of antimicrobial agents to dentalmaterials is part of a current trend 8,10–20 . Amongthe advantages of this process is the possibility ofeluting the agents from the base materials, thuspreventing or reducing bacterial, fungal and yeastcontamination.Antimicrobial zeolites have been incorporated toserve such a purpose 10,11,13–15,21,22 . Zeolites arealuminum silicate crystalline structures that presentvoid spaces measuring 3–10 angstroms in theirstructure. Antimicrobial cations, such as silver andzinc, may be lodged within the void spaces of thezeolites and be exchanged over time with othercations from their environment 14,20–22 . As this ionavailability occurs, the free cations come into contactwith the environmental microorganisms, suppressingtheir development by inactivating vitalmicrobial enzymes, interrupting RNA replicationand blocking their respiration by an oxidativeprocess 23–31 . The efficacy of antimicrobial zeolitesagainst aerobic and anaerobic bacteria and fungihas been demonstrated 20,24,32,33 .Because of their characteristics, including prolongedantimicrobial activity, low toxicity and lackof odour or flavour, zeolites have a large range ofapplications. They have been incorporated, amongothers, into polymers for packaging of food, paint,tissue, catheter and orthopedic prostheses, dentalrestorative materials and oral hygiene products13,14,20,22,34–45 . The dental materials most frequentlyassociated with antimicrobial zeolites arethe tissue conditioners. The favourable outcome ofthis association from a microbiological standpoint14,20 as well as their good viscoelastic properties15 and biocompatibility 11 , indicate that zeolitesmight also be successfully associated with othermaterials used for denture fabrication. Althoughthis procedure might potentially bring direct benefitsfor denture wearers, there have only beenlimited investigations into the addition of antimicrobialagents to acrylic resin denture bases 16,46 .The purposes of this in vitro study were to evaluatethe antimicrobial activity of acrylic resinscontaining different percentages of silver–zincantimicrobial zeolites, and to assess whether theaddition of zeolites alters the flexural and impactstrength of the denture bases.Material and methodsThe study was conducted under climate-controlledconditions (23 ± 2°C; 50 ± 10% relative airhumidity).Specimen fabricationThe tested materials with their compositions,specifications and manufacturers are listed inTable 1. Stainless steel rectangular-shaped patterns(8 · 10 · 4mm) were invested in flasks with dentalstone. After the setting of the stone, the flasks wereopened and the patterns removed, leaving rectangular-shapedcavities that were used as matrixes forthe fabrication of heat-polymerised acrylic resinspecimens.For each assay, 50 specimens were fabricatedfrom each material [a microwave-polymerised(Onda-Cryl) and two heat-polymerised (QC20 andLucitone 550) acrylic resins], and assigned to fivegroups (n = 10) according to their percentage ofsilver–zinc zeolite (Irgaguard B5000; Ciba SpecialtyChemicals Corporation, Tarrytown NY, USA): 0%(control); 2.5%; 5.0%, 7.5% and 10.0% by drywet). For this, the materials were weighted in ananalytical balance (Fisher Scientific, Pittsburgh PA,USA) and Irgaguard B5000 (zeolite) was incorporated,in pre-defined percentages, to the polymerpowder of the acrylic resins. These were mixedmanually for 3 min, by the same operator, toobtain an adequate particle distribution. TheTable 1 Tested materials.Material Manufacturer Handling dose Type of polymerisationQC20High impactLucitone 550Onda-CrylDentsply Indústria e Comércio Ltda,Petrópolis, RJ, BrazilDentsply Indústria e Comércio Ltda,Petrópolis, RJ, BrazilClássico Artigos Odontológicos Ltda.,São Paulo, SP, Brazil23g:10 ml Heat-polymerised (immersion inboiling water for 20 min)21g:10 ml Heat-polymerised (immersion inwater at 73°C for 90 min andboiling water for 30 min)7g:21 ml Microwave-polymerised with 500Wpower (3 min at 40% power, 4 min rest,then 3 min at 90% power)Ó 2008 The Gerodontology Association and Blackwell Munksgaard Ltd, Gerodontology 2008; doi:10.1111/j.1741-2358.2007.00198.x
Antimicrobial acrylic resins 3materials were then proportioned, mixed, packedand polymerised following the manufacturers’instructions. The specimens were ground withwater-cooled silicon carbide papers of decreasingabrasiveness (400-, 500- and 600-grit) and measuredto obtain rectangles of standard dimensions.Assessment of antimicrobial activityThe specimens were autoclaved and then immersedin sterile artificial saliva (School of PharmaceuticalSciences of Ribeirão Preto, University of São Paulo,SP, Brazil) at 37°C for 7 days. Thereafter, the salivawas discarded and the specimens subjected to theantimicrobial activity assays.The agar plate diffusion method was performedin triplicate using the well technique 47 . Forobtaining the inoculum, the reference bacteriastrains (C. albicans ATCC 28366 and Streptococcusmutans ATCC 25175) and the clinical isolates (C.albicans and S. mutans) (Laboratory of Microbiology,University of Franca, Franca, SP, Brazil) were replicatedin Mueller Hinton Broth (C. albicans) orBrain Heart Infusion (for S. mutans). After incubationat 37°C for 24 hours, the turbidity of C. albicansand S. mutans inoculums was adjusted to tubes 1and 0.5 McFarland standard respectively. To formthe base layer, 25 ml of Mueller Hinton Medium orBrain Heart Infusion-Agar at 50°C were depositedon sterile Petri plates (25 · 125mm). After solidification,12.5ml of the culture media inoculatedwith the test organisms (2.5ml) were added to formthe seeding layer. Four-millimetre-diameter wellswere then perforated to receive the cylindricalspecimens (4.0 · 4.0mm). The plates were kept atroom temperature for 120 min for diffusion of theantimicrobial agents 48 and then inoculated inaerobiosis or microaerophilia at 37°C for 24 hours.The zone of inhibition diameters were measuredwith sliding callipers (mm) and this measurementindicated the microbial susceptibility to the material.Data were analysed statistically by one-wayANOVA and Tukey’s post hoc test at 5% significancelevel.Flexural strengthThe specimens were submitted to a three-pointbending test using a universal testing machine(Instron 5569, Norwood, MA, USA) calibrated toprovide a crosshead speed of 5mm/min with a500kgf load cell. Testing was performed in compliancewith the specifications of ISO 178:2003standard (Plastics. Determination of flexural properties).Means (in MPa) were obtained and the datawere analyzed statistically by one-way ANOVA andTukey’s post hoc test at 5% significance level.Izod impact strengthThe specimens were taken to an impact testingmachine (Resil 25, Charlotte, NC, USA) and weresubjected to a 5.5J impact delivered by a swingingpendulum positioned at height of approximately30cm. From the energy absorbed by the specimenduring fracture the attrition value (0.022J) wassubtracted and the impact strength was determined.Testing was performed in compliance withthe specifications of ISO 180:2001 standard (Plastics.Determination of Izod impact strength) Means(in MPa) were obtained and the data were analysedstatistically by one-way ANOVA and Tukey’s post hoctest at 5% significance level.ResultsAssessment of antimicrobial activityThe means of zone of inhibition diameters (in mm)of the heat-polymerised acrylic resins containingdifferent percentages of silver–zinc zeolite areshown in Table 2.None of the control groups showed antimicrobialactivity. For all acrylic resins, the addition of 2.5%of zeolite was enough to assign antimicrobialactivity against the test strains. For QC20, thisactivity was significantly increased when the percentageof zeolite incorporated into the resin was5% (against S. mutans clinical isolate), 7.5%(against S. mutans ATCC 25175) and 10% (againstC. albicans ATCC 28366 and clinical isolate). Lucitonehad a significant increase of its antimicrobialactivity with the addition of 5% of zeolite (againstS. mutans clinical isolate) and 7.5% (againstC. albicans ATCC 28366 and clinical isolate andS. mutans ATCC 25175). Onda-Cryl had its antimicrobialactivity significantly increased with theincorporation of 7.5% (against C. albicans ATCC28366) and S. mutans ATCC 25175 and clinicalisolate) and 10% (against C. albicans clinical isolate)of silver–zinc zeolite.Flexural strengthFlexural strength results are shown in Fig. 1.Compared to the control groups, no statisticallysignificant differences (p > 0.05) were observedamong the tested acrylic resins [QC20(91.9 ± 7.5 MPa); Lucitone 550 (99.4 ± 6.7 MPa)and Onda-Cryl (0.31 ± 0.1 MPa)].Ó 2008 The Gerodontology Association and Blackwell Munksgaard Ltd, Gerodontology 2008; doi:10.1111/j.1741-2358.2007.00198.x
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