In vitro performance of self-adhesive resin cements for post-and-core build-ups: influence of chewing simulation or 1-year storage in 0.5% chloramine solution

The aim of this investigation was to test the in vitro performance of a self-adhesive resin composite core build-up in comparison with two typical conventional etch-and-rinse composite core build-up materials, before and after 1year of storage in 0.5% chloramine solution (LTS). Sixty human maxillary central incisors were divided into three groups. Teeth were root filled and decoronated. Specimens were restored using glass fiber posts cemented with a self-adhesive resin cement. Core build-ups were made with a self-adhesive (U) and two core build-up materials (C and L) applied with their corresponding bonding systems. All specimens received adhesively luted lithium disilicate crowns. Ten specimens of each group were exposed to LTS and examined monthly for cracks or other alterations. All specimens were thermocycled, mechanically loaded (TCML) and finally loaded until failure occurred. There was no statistical significant difference in regard to the number of failures during TCML without and with LTS (log rank: p = 0.225 and 0.609, respectively). The median fracture load values after static loading without LTS and with LTS did not differ significantly (Kruskal-Wallis test: p = 0.057 and 0.106, respectively), though the fracture patterns between the groups without (p = 0.024) and with LTS (p = 0.027) did. Self-adhesive cements used for core build-up have no significantly higher risk of failure compared to conventional core build-up materials in both LTS and TCML test scenarios.

Investigation of mechanical properties of modern dental composites after artificial aging for one year

This in vitro study investigated the aging behavior of dental composites with regard to surface roughness (SR), Vickers hardness (VH) and flexural strength (FS), and the study elucidated the impact of artificial aging parameters. One hundred and sixty-five rectangular specimens were prepared from five composites (Filtek Supreme XT, Filtek Silorane, CeramX, Quixfil, experimental ormocer) and subjected to various artificial aging protocols (storage in distilled water/ethanol/artificial saliva for 7, 90 and 365 days; thermal cycling, 2 x 3000 cycles 5/55 degrees C). SR, VH and FS were determined at baseline and after each aging treatment. Means and standard deviations were calculated; statistical analysis was performed using three-way ANOVA and the Tukey-Kramer multiple comparison test (alpha=.05). The results showed a significant influence in the composite and aging duration on mechanical parameters; the aging medium did not have a significant influence on VH and FS, but there was a significant influence on SR. The highest overall VH was found for theexperimental ormocer; Filtek Silorane yielded the lowest values. For FS, the significantly highest values were found for Filtek Silorane, and the lowest values were found for the experimental ormocer. Prolonged aging periods (90 or 365 days) or thermal cycling led to significant decreases in both VH and FS and significant increases in SR. The findings of the current study indicate that composites differ significantly for SR and its mechanical properties with regard to FS and VH, as well as in aging behavior. Generally, artificial aging leads to a significant decrease in mechanical properties, which underlines the relevance of continuous improvement of dental composites.

The effects of erroneous mixing of zinc carboxylate cements

The mechanical properties of luting agents are determined by the mixing ratio of powder and liquid. The purpose of this in vitro study was to evaluate the therapeutic range of zinc carboxylate cements by investigating the mechanical properties of such cements when formulated using erroneous powder/liquid ratios. Using the recommended powder/liquid ratio as a standard, four different mixing ratios (25% and 50% more or less powder) were used to prepare each carboxylate cement (Aqualox, Carboco, Durelon). A veneering composite (Sinfony) was used as control. Vickers hardness and three-body abrasion were evaluated. In each carboxylate cement, both a 25% and 50% increase in liquid content resulted in significantly lower Vickers hardness, whereas a higher powder content altered the saturation concentration but did not result in increased hardness; this effect was particularly obvious in Carboco. Durelon showed a linear relationship between wear and powder/liquid ratio, whereas Carboco and Aqualox displayed a more exponential increase in wear in mixings with an increased liquid content. The mechanical properties of carboxylate cements were altered in mixtures with an increased liquid content, but not in those with a high powder content. Mixing ratio errors greater than 25% may significantly affect the clinical performance of carboxylate cements.

Influence of artificial ageing on surface properties and Streptococcus mutans adhesion to dental composite materials

The aim of this in vitro study was to investigate the influence of artificial ageing on the surface properties and early Streptococcus mutans adhesion to current dental composites for the direct restoration of class II defects. Three hundred and thirty specimens each were prepared from five dental composites, and were randomly allotted to various artificial ageing protocols (storage in distilled water/ethanol/artificial saliva for 7/90/365 days; thermal cycling, 6,000 cycles 5/55 degrees C). Prior and after each treatment, surface roughness (R(a)) and hydrophobicity were determined, and S. mutans adhesion (ATCC 25175; 2.5 h, 37 degrees C) was simulated with and without prior exposition to human whole saliva (2 h, 37 degrees C). Adherence of S. mutans was determined fluorometrically. Means and standard deviations were calculated, and analyzed using three-way ANOVA and post-hoc analysis (alpha = 0.05). For both R(a) and S. mutans adherence to uncoated and saliva-coated specimens, significant influences of the composite material, the ageing medium and the ageing duration have been observed; for surface hydrophobicity, significant influences of the composite material and the ageing duration were found. For uncoated specimens, significant increases in S. mutans adhesion were observed with prolonged artificial ageing, whereas significant decreases in S. mutans adhesion were found for the saliva-coated specimens. The data indicate influences of the artificial ageing method on surface parameters such as R(a) and hydrophobicity as well as microbial adhesion. The results underline the relevance of saliva coating on the outcome of studies simulating microbial adhesion, and highlight differences in the susceptibility of dental composites for the adhesion of oral bacteria.

Mechanical properties and three-body wear of dental restoratives and their comparative flowable materials

OBJECTIVE

To compare wear performance and resistance to crack propagation (K1C) of commercial restorative materials and their flowable variations. A potential correlation between three-body wear and fracture toughness, modulus of elasticity, fracture work, Vickers hardness, and filler content was investigated.

METHOD AND MATERIALS

Seven restoratives (five composites, one ormocer, and one compomer) and their corresponding flowable materials were used to determine and compare the three-body wear with a bolus of millet-seed shells and rice food (Willytec). The wear characteristics were measured by profilometry after 50,000, 100,000, 150,000, and 200,000 loading cycles. The fracture toughness value, K1C (MPam1/2), for each single-edged notched specimen was measured in a three-point bending test (universal testing machine 1446, Zwick). Fracture work and modulus of elasticity were calculated from the load curves. Vickers hardness was measured (HV hardness tester, Zwick) according to DIN 50133. The veneering composite Sinfony (3M ESPE) was used as a reference material.

RESULTS

Heavily filled composites experienced less wear than their flowable variations. The nanofiller composites revealed better wear results than hybrid composites, compomers, and ormocers. After 200,000 load cycles, the lowest wear rates were detected for Grandio (14 microm; Voco), and the highest mean values were found for Dyract AP (104 microm; Dentsply DeTrey). The values for fracture toughness (K1C) ranged from 0.82 to 3.64 MPam1/2. Highest K1C data was exhibited by the nanocomposite Nanopaq (Schutz Dental). All tested restorative materials exhibited higher fracture toughness than their low-viscosity variations.

CONCLUSIONS

The wear resistance of the newer generation composites with incorporated nanofiller or microfiller particles increased to a high extent. Flowables show less resistance against wear and crack propagation because of their lower filler content. The reduced mechanical properties limit their use as a restorative to small noncontact, low-stress clinical situations.

Casting ability of selected impression materials tested in different conditions in an in vitro sulcus model

OBJECTIVE

To test casting capacities of impression materials under dry and wet sulcular conditions in vitro.

METHOD AND MATERIALS

An incisor with a circular shoulder preparation (1 mm) was inserted in a primary mold. A shiftable secondary mold allowed adaptation of sulcular depth (1 to 4 mm). An outer circular chamfer assured reproducible positioning of an impression material carrier. Tested materials were PVS of differing viscosities (extra low, Panasil Contact Plus [ELV]; low, Affinis Light Body [LV]; and medium, Virtual Monophase [MV]) and one polyether material of low viscosity (Permadyne Garant [PE]). Impressions were made with sulcular depths of 1 to 4 mm in wet and 1 and 4 mm in dry conditions, cut in half, and digitized with a light microscope (Stemi SV8). Surface area of the region of interest (ROI, at inner angle of preparation) was determined with Optimas 6.2. Medians were calculated, and statistical analysis was performed using the Mann-Whitney U test (P # .05).

RESULTS

Median values of the measurements under wet condition demonstrated the smallest ROI areas for the ELV (297-330[pixel]) and the MV (253-421[pixel]) materials followed by the LV (582-745[pixel]) and the PELV (544-823[pixel]). All materials showed significantly higher values for the wet compared to dry sulcular conditions. Repeated measurements showed no significant differences to the corresponding first determined series.

CONCLUSION

The sulcus model is applicable to assess casting abilities of impression materials in clinically approximated sulcular conditions. The PVS materials with extra low and medium viscosities showed the best properties in dry and wet conditions.

Fracture performance of computer-aided manufactured zirconia and alloy crowns

OBJECTIVE

To compare the fracture resistance and fracture performance of CAD/CAM zirconia and alloy crowns.

METHOD AND MATERIALS

One electrophoretic deposition alumina ceramic (Wolceram, Wolceram) and 4 zirconia-based systems (ce.novation, ce.novation; Cercon, DeguDent; Digizon, Amann Girrbach; and Lava, 3M ESPE) were investigated. A porcelain-fused-to-metal method (Academy, Bego Medical) was used in either conventional casting technique or laser sintering. Sixteen crowns of each material were fabricated and veneered with glass-ceramic as recommended by the manufacturers. Crown and root dimensions were measured, and 8 crowns of each system were adhesively bonded or conventionally cemented. After the crowns were artificially aged in a simulated oral environment (1,200,000 mechanical loads with 50 N; 3,000 thermal cycles with distilled water between 5 degrees C and 55 degrees C; 2 minutes per cycle), fracture resistance and fracture patterns were determined and defect sizes investigated.

RESULTS

The fracture force varied between 1,111 N and 2,038 N for conventional cementation and between 1,181 N and 2,295 N for adhesive bonding. No significant differences were found between adhesive and conventional cementations. Fracture patterns presented mostly as a chipping of the veneering, in single cases as a fracture of the core, and in 1 case as a fracture of the tooth.

CONCLUSIONS

Crown material and cementation do not have any significant influence on the fracture force and fracture performance of all-ceramic and metal-based crowns. Therefore, it may be concluded that adhesive bonding is not necessary for the application of high-strength ceramics.

Two-body wear of artificial acrylic and composite resin teeth in relation to antagonist material

STATEMENT OF PROBLEM

Wear resistance is one of the most important physical properties of artificial resin teeth, and its dependence on the antagonist material has not been clearly established.

PURPOSE

The purpose of this in vitro study was to investigate the wear resistance of representative artificial resin teeth in relation to antagonist material.

MATERIAL AND METHODS

Twenty-four standardized specimens were prepared for each of 8 tooth types representative of anterior and posterior artificial acrylic and composite resin teeth (Gnathostar, SR Orthosit PE, Condyloform II NFC, SR Postaris DCL, SR Vivodent PE, VITA Physiodens, SR Antaris DCL, Bioplus), for a total of 192 specimens. Each specimen was prepared to a diameter of 5 mm and a thickness of 2 mm on the buccal/facial tooth surfaces. The specimens were then polished and subjected to simulated mastication (50 N, 1.2 x 10(5) cycles, 1.2 Hz) using a pin-on-block design and additional thermocycling (600 cycles, 5/55 degrees C, 2 min/cycle). Three antagonists (artificial resin teeth, steel, steatite) were prepared, and 8 specimens per tooth type were tested for each antagonist. Vertical substance and volume loss were analyzed using cast replicas and an optical 3-dimensional (3-D) surface profilometer, as well as scanning electron microscopy. Data were analyzed using 2- and 1-way ANOVA and the Games-Howell test (alpha=.05).

RESULTS

For both vertical substance and volume loss, significant differences were found for the various antagonists. Lowest overall mean values (SD) for vertical substance and volume loss were measured for artificial tooth antagonists (26 (10) microm to 95 (46) microm, 1.5 (1.5) microm(3) to 10.5 (9) microm(3)), and the highest values were measured for steatite antagonists (95 (23) microm to 723 (168) microm, 8 (4) microm(3) to 245 (109) microm(3)). Intermediate mean values (SD) for vertical substance and volume loss were measured for steel antagonists (118 (88) microm to 205 (120) microm, 9 (7) microm(3) to 28 (24) microm(3)). Using artificial teeth and steel antagonists, few significant differences in wear resistance were found between the various resin teeth. In contrast, significant differences were observed with steatite antagonists.

CONCLUSIONS

The use of steatite antagonists allowed for significantly better differentiation of wear behavior between various artificial teeth than the use of artificial resin teeth or steel antagonists.

Surface properties and in vitro Streptococcus mutans adhesion to self-etching adhesives

PURPOSE

The aim of this in vitro study was to investigate the surface properties and in vitro adhesion of Streptococcus mutans to dental self-etching two-step adhesives using an artificial mouth system.

MATERIALS AND METHODS

Carriers of a denture base resin supplemented with hydroxyapatite were manufactured, polished, and surface treated with various two-step self-etching dental adhesives (Clearfil Protect Bond, Silorane System Adhesive, Adper Scotchbond SE) according to the manufacturers' instructions. For each adhesive, 14 samples were inserted in an artificial mouth system and successively rinsed with mucin solutions (2 h) and Streptococcus mutans NCTC 10449 suspension (4 h). Surface free energy of the specimens was determined before and after mucin coating. The amount of adhered bacteria was quantified using a fluorescent dye (Resazurin/Alamar Blue) for viable cell quantification.

RESULTS

Before mucin coating, significant differences in surface free energies were found between the various adhesives, with total surface free energies ranging from 34.26 mJ/m(-2) to 42.30 mJ/m(-2). Mucin coating clearly levelled surface free energy differences. Significantly lower fluorescence intensities, indicating lower adhesion of Streptococcus mutans, were found for Silorane System Adhesive than for Clearfil Protect Bond and Adper Scotchbond SE, regardless of mucin coating.

DISCUSSION

The findings of this in vitro study indicate that the initial adhesion of Streptococcus mutans is lower to Silorane System Adhesive than to Clearfil Protect Bond or Adper Scotchbond SE. Our data correlated with the common thermodynamic approaches explaining streptococcal adherence to solid surfaces.

Flexural strength of experimentally filled resins made of electron beam irradiated silica fillers

This study investigated the influence of different silica fillers on the flexural strength of experimentally filled resins. Hydrophilic (non-silanated) silica, hydrophobic silica modified by organofunctional silane, and silica modified by organofunctional silane that additionally contains polymerizable carbon double bonds were assigned into further subgroups: the first subgroup was electron beam irradiated with 10 kGy (dose rate) and the second with 30 kGy, whereas the third constituted the non-irradiated control group. In total, nine experimentally filled resin blends were mixed. Rectangular specimens were constructed, and a flexural strength test was performed. Regardless of the type of silica, specimens constructed of blends containing non-irradiated fillers showed the lowest flexural strength in comparison to their corresponding irradiated groups. With increasing dose rates from 10 to 30 kGy, filler irradiation prior to blend mixing resulted in slightly increased flexural strength values for hydrophilic as well as for organofunctional silanated silica. Specimens constructed of blends with fillers that were not only modified by silane containing polymerizable carbon double bonds but were additionally irradiated showed the highest flexural strength. The results of this study indicate that the flexural strength of filled resins could be enhanced by advance preparation of silica fillers with silane coupling agents followed by electron beam irradiation.

Streptococcal adhesion to various luting systems and the role of mixing errors

OBJECTIVE

This study aims at ranking various luting systems according to their susceptibility to adhering Streptococcus mutans and at evaluating the influence of incongruent mixing ratios on adhesion quantities.

MATERIAL AND METHODS

Circular specimens measuring 8 mm in diameter were made of nine widely used dental cements -- three of them mixed in different ratios -- and then incubated with S. mutans. Adhering streptococci were quantified using a biofluorometric assay in combination with an automated plate reader for cell quantification. Surface roughness (R(a)) was determined by perthometer measurements.

RESULTS

Meron plus revealed the highest R(a) (0.90 microm) and glass the lowest R(a) (<0.01 microm). In regular cement mixtures, the highest mean fluorescence intensities indicated the presence of many viable bacteria [Meron Plus (35,533 relative fluorescence units (rfu)), Maxcem (13,374 rfu), and Panavia F 2.0 (11,701 rfu)]. Moderate fluorescence intensities were found in Harvard (4,171 rfu), Ketac cem (3,766 rfu), Durelon (3,276 rfu), Calibra (3,259), Rely X Unicem (4,358 rfu), and Bifix SE (3,102 rfu). A medium correlation between R(a) and S. mutans adhesion was found. Changes in regular cement proportions (powder/liquid and base/catalyst, respectively) had a significant influence on relative fluorescence intensities, which linearly increased with a higher proportion of liquid in Harvard and with a higher proportion of catalyst in Calibra and Maxcem.

CONCLUSIONS

Various luting systems revealed considerable differences in their potential to adhere S. mutans. Variations from recommended cement proportions led to significant changes in the amount of adhering streptococci.

In vitro evaluation of artificial ageing on surface properties and early Candida albicans adhesion to prosthetic resins

OBJECTIVE

The aim of this in vitro study was to examine potential changes and influences of prosthetic resin surface properties on Candida albicans adhesion after surface treatment or artificial ageing.

MATERIALS AND METHODS

Standardized specimens of a denture base resin and a veneering composite were prepared, polished, and randomly subjected to different surface roughness treatments or artificial ageing protocols (storage in ethanol or artificial saliva for 7/90 d, thermocycling). Surface roughness (Ra) and surface free energy were determined prior and after each treatment. Specimens were incubated with phosphate buffered saline or whole saliva for 2 h at 37 degrees C, and later with Candida albicans suspension (2.5 h, 37 degrees C). Adherent viable fungi were quantified using a bioluminescence assay.

RESULTS

Artifical ageing did not affect substratum surface roughness, yet slight increases in substratum surface free energy and significant increases in Candida albicans adhesion were observed. Saliva coating marginally influenced Candida albicans adherence to reference and surface treated specimens, yet more pronounced differences in Candida albicans adhesion between the various artificially aged specimens were found.

CONCLUSION

No correlation between substratum surface roughness or surface free energy and Candida albicans adhesion could be established.

Strength investigation of artificial substitutes for human teeth in in vitro studies

This study investigated whether metals or resins can replace human teeth in in vitro fracture tests of endodontically treated teeth (ETT), as ETT show high heterogeneity and small availability. Eight incisor-shaped roots per group were adhesively restored with fiber-reinforced composite posts, composite core build-ups and Co-Cr-Mo crowns. Specimens were thermally cycled and mechanically loaded (TCML) and fracture strength was determined. The results varied between 0 N and 348 N. Extracted ETT may show comparable strength and survival during TCML to teeth in situ and therefore are the first choice for in vitro testing. Substitutes show comparable fracture patterns but different fracture values.

Surface properties and in vitro Streptococcus mutans adhesion to dental resin polymers

OBJECTIVES

This study aimed to characterize the surface properties of experimental resin polymers consisting of monomers differing in functionality and chain length, and to evaluate differences in Streptococcus mutans adhesion.

MATERIAL AND METHODS

Six resins were prepared (70/30 ratio UDMA/monomer); camphorquinone and ethyl-4-dimethylaminebenzoate were added for light activation. A conventional composite was used as a control. Surface free energy was determined prior and after saliva exposition (2 h, 37 degrees C). After saliva incubation (2 h, 37 degrees C), specimens were incubated with Streptococcus mutans NCTC 10449 for 2.5 h at 37 degrees C. Adherent bacteria were quantified by determining the relative substratum area covered by bacteria using SEM analysis, and by using a fluorometric assay for viable cell quantification.

RESULTS

No statistically significant differences in total surface free energies were found for uncoated specimens (mean total surface free energies ranging from 39.79 to 49.73 mJ/m(-2)); after saliva coating, statistically significant differences were observed for some of the polymers (mean total surface free energies ranging from 44.13 to 65.81 mJ/m(-2)). Few differences were observed between SEM and fluorescence quantification, finding statistically significant differences in streptococcal adhesion to the experimental polymers. Median bacteria surface coverage ranged from 1.4% for UDMA mixed with 1,10-decandiol dimethacrylate to 16.2% for the control composite material; lowest fluorescence intensities indicating lowest adhesion of bacteria were found for UDMA mixed with 1,10-decandiol dimethacrylate (median 712), and highest values indicating highest adhesion of bacteria were found for UDMA mixed with polyethyleneglycol (600) dimethacrylate (median 11974).

CONCLUSION

Streptococcus mutans adhesion appears to be different on polymers differing in monomer mixtures, yet correlations between substratum surface free energy and streptococcal adhesion were poor. Further studies are necessary to evaluate additional substratum surface properties and pellicle distribution and composition more thoroughly.

Efficacy of denture disinfection methods in controlling Candida albicans colonization in vitro

OBJECTIVE

The aim of this study was to rank 10 denture disinfection methods according to their efficacy in reducing Candida albicans (C. albicans) colonization on soft denture relining material.

MATERIAL AND METHODS

Circular specimens (diameter 8 mm) were made of soft denture relining material (Mucopren E, Kettenbach) and thermally aged. Specimens were incubated with C. albicans (strain 1386, DSMZ) followed by 1 of 10 disinfection procedures (6 soaks, 2 microwave irradiation regimes, 1 effervescent commercial cleansing product, and denture left dry overnight). Incubation with phosphate buffered saline (PBS) served as a control. Adhering fungi were quantified using a bioluminometric assay in combination with an automated plate reader for cell quantification. Scanning electron micrographs (SEMs) were made for validation.

RESULTS

Low median luminescence intensities indicated the presence of a few viable fungi after the soaking of specimens in sodium hypochlorite (10 relative luminescence units (rlu)), microwave irradiation immersed in water (8 rlu), and application of effervescent cleansing tabs (22 rlu). No statistically significant difference (p>0.05) to control PBS (200 rlu) was found after immersion in hydrogen peroxide (172 rlu), glutaraldehyde (103 rlu), household vinegar (196 rlu), Listerine coolmint (194 rlu), Plax (222 rlu), dry microwave irradiation (221 rlu) and specimens left dry overnight (165 rlu). SEM displayed C. albicans monolayers with different morphologic forms on each surface investigated.

CONCLUSIONS

Only soaking in sodium hypochlorite (1%; 10 min), microwave irradiation immersed in water (800 W; 6 min), and application of effervescent cleansing tabs (Blend-a-dent tabs; 10 min) proved to be effective against C. albicans colonization on soft denture relining material.

Experimental composites made of electron beam irradiated reinforced fillers

This study investigated the influence of electron beam irradiated reinforced fillers on the three body wear and flexural strength of experimental composite blends. Three formulations of reinforced fillers were investigated: (A) high loaded inorganic filler composite with 60 wt.% SiO2, (B) low loaded inorganic filler composite with 40 wt.% SiO2, (C) organic filler composite (precipitated Bis-phenol-A-di-methacrylate). The fillers were assigned to two subgroups of unswollen (A, B, C) and monomer swollen (As, Bs, Cs) fillers. The experimental blends (matrix: Urethane-dimethacrylate) were mixed using un-treated, annealed (90 degrees C), or electron beam irradiated fillers with 30 and 90 kGy, respectively. All specimens were heat-cured for 20 min at 140 degrees C. Three-body abrasion and flexural strength tests were performed. The highest flexural strength was evaluated for composites made of the 30 kGy irradiated type Bs filler. The comparison with annealed fillers showed that the effect was independent of increasing temperatures during the radiation process. Blends with a SiO2 content of 60 wt.% (type A, As) had significantly less wear than blends with 40 wt.% (type B, Bs) or blends with organic fillers (type C, Cs). The flexural strength of the composite could be improved by using pre-irradiated reinforced fillers. However, wear was not affected using this procedure.

Zirconia resin-bonded fixed partial dentures in the anterior maxilla

OBJECTIVE

To determine the fracture resistance of resin-bonded fixed partial dentures (RBFPDs) by examining the influence of framework design and abutment mobility.

METHOD AND MATERIALS

RBFPD frameworks were made of zirconia (Cercon Base, Degudent) or a nonprecious alloy (reference; Dentitan, Elephant Dental) and veneered with ceramic (Cercon Ceram S, Degudent). The zirconia framework design varied between a 2-retainer RBFPD with 3 different levels of tooth mobility (groups 1 to 3) and a 1-retainer cantilever version with 2 different grades of tooth mobility (groups 4 and 5). To achieve different mobility (rigid, medium, movable), the roots of the teeth were covered with a polyether material of different thicknesses. All RBFPDs were adhesively luted on prepared human teeth (Panavia 21 Ex, Kuraray). The specimens were mechanically (1.2 x 10(6); 25 N) and thermally (6,000 x 5 degrees C/55 degrees C; 2 minutes per cycle) cycled and finally loaded to failure (universal testing machine 1445, Zwick) at a speed of 1 mm/min.

RESULTS

The fracture force of the reference RBFPD (541 N) was significantly higher than that of both cantilever RBFPDs (group 4 = 271 N, group 5 = 104 N) and one 2-retainer group with rigid abutments (group 3 = 150 N). With 2 movable abutments, the fracture force increased to 261 N (group 1) and with mixed movable/rigid teeth to 324 N (group 2). Zirconia RBFPDs showed improved survival with increased tooth mobility, but the framework design showed only a minor influence on loss rate and fracture resistance.

CONCLUSIONS

Assuming chewing forces in anterior areas between 200 and 300 N, 1- and 2-retainer zirconia RBFPDs may be suitable as minimally invasive provisional alternatives to metal-supported RBFPDs.

Discolouration of orthodontic adhesives caused by food dyes and ultraviolet light

Enamel discolouration after debonding of orthodontic attachments could occur because of irreversible penetration of resin tags into the enamel structure. Adhesives could discolour because of food dyes or ultraviolet irradiation. The aim of this study was to investigate the colour stability of adhesives during ultraviolet irradiation and exposure to food colourants. Four different adhesives were exposed in a Suntest CPS+ ageing device to a xenon lamp to simulate natural daylight (Transbond XT, Enlight, RelyX Unicem, and Meron Plus AC). Tomato ketchup, Coca Cola, and tea were chosen as the food colourants. After 72 hours of exposure, colour measurements were performed by means of a spectrophotometer according to the Commission Internationale de l'Eclairage L*a*b* system and colour changes (DeltaE*) were computed. Statistical differences were investigated using two-way analysis of variance (ANOVA) and Friedman test. Unsatisfactory colour stability after in vitro exposure to food colourants and ultraviolet light was observed for the conventional adhesive systems, Transbond XT and Enlight. RelyX Unicem showed the least colour change and the resin-reinforced glass-ionomer cement (GIC), Meron Plus AC, the greatest colour change. The investigated adhesives seem to be susceptible to both internal and external discolouration. These in vitro findings indicate that the tested conventional adhesive systems reveal unsatisfactory colour stability which should be improved to avoid enamel discolouration.

An in vitro comparative assessment of different enamel contaminants during bracket bonding

In orthodontics, adhesive failures can occur because of saliva contamination during bonding. However, most in vitro studies concerning bond strength of saliva-contaminated enamel disregard the influence of temperature changes in a wet environment. The aim of the present study was to compare the influence of saliva, blood and etching gel remnant contamination on shear bond strength (SBS) after thermocycling. After etching of extracted human third molars (n = 80), a conventional primer (Transbond XT) and a moisture-insensitive primer (Transbond MIP) were evaluated using the adhesive, Transbond XT, under dry conditions and after contamination with saliva, blood and etching gel remnants. To simulate temperature changes and the moisture of saliva in the oral cavity, all samples were thermocycled (6,000 x 5 degrees C/55 degrees C) in a mastication device before SBS testing. A Mann-Whitney U test was used to determine statistical differences. Under dry conditions Transbond XT and Transbond MIP showed no significant difference in SBS. However, clinically unacceptable (P = 0.005) bond strength was observed using Transbond XT after saliva and blood contamination. In wet conditions only Transbond MIP showed sufficient bond strength. If contamination during bonding is expected, a hydrophilic primer should be used. Under dry conditions hydrophilic or hydrophobic primers could be applied. Blood contamination seems to be a more serious problem for bond strength than saliva or etching gel contamination.

Electron-beam irradiation of experimental denture base polymers

OBJECTIVE

Since the properties of polymers can be influenced using electron-beam irradiation, the aim of this study was to investigate whether electron-beam post-curing can improve the mechanical properties of experimental denture base polymers.

MATERIAL AND METHODS

Rectangular specimens of different experimental polymeric blends were electron-beam irradiated (post-cured) with 25 kGy and 200 kGy using an electron accelerator of 4.5 MeV. Fracture toughness, work of fracture, Vickers hardness and colour changes were measured and compared in non-irradiated specimens.

RESULTS

The mechanical properties of all the investigated polymers seemed to benefit from low-energy electron-beam irradiation (25 kGy). Using an energy dose of 200 kGy, all blends showed deteriorated mechanical properties resulting from chain breakage. Nevertheless, all investigated polymers had undesirable colour changes after electron-beam irradiation.

CONCLUSIONS

Mechanical properties of experimental polymethyl-methacrylate could be changed using electron-beam irradiation. Because of discolorations caused by the irradiation levels investigated, these levels cannot be recommended for practical applications.

Influence of fibre and filler reinforcement of plastic brackets: an in vitro study

In spite of their popularity in fulfilling aesthetic requirements, plastic brackets still present some disadvantages because of their low elastic modulus, decreased fracture toughness, and reduced wear resistance. Fibre-reinforced composites are well established in dentistry and consist of a polymer matrix in which reinforcing fibres are embedded. Stress is transferred from the polymer matrix to the fibres which present a high tensile strength. Hence, the mechanical properties of polymers could be improved. The purpose of this study was to compare fracture strength, fracture toughness and flexural strength of an experimental fibre-reinforced bracket material, an SiO(2) filler-reinforced bracket and an unfilled plastic bracket material (control group). Experimental brackets and specialized bars were manufactured. Tests were performed after thermal cycling (5 degrees C/55 degrees C) the samples in an artificial oral environment of a device to simulate mastication. Statistical evaluation was undertaken. The median, 25th and 75th percentiles were calculated and a Mann-Whitney U-test was performed. In this study two findings were obvious. (1) Filler reinforcement of plastic brackets improved fracture strength and fracture toughness in comparison with the unfilled bracket material. (2) Glass fibre reinforcement of orthodontic bracket materials resulted in the greatest enhancement of the mechanical properties in comparison with the other test groups. Therefore, the application of glass fibres in plastic brackets is a successful method to enhance fracture strength.

Influence of Filler Level on the Bond Strength of Orthodontic Adhesives

OBJECTIVE

To evaluate the effect of different filler contents of orthodontic adhesives on shear bond strength and Adhesive Remnant Index (ARI).

MATERIALS AND METHODS

Four experimental adhesive groups were created: group 1 was an unfilled urethane-dimethacrylate (UDMA) adhesive, group 2 consisted of UDMA and a filler content of 30 vol%, group 3 consisted of UDMA and a filler content of 50 vol%, and group 4 was manufactured with a filler level of 70 vol% in a UDMA matrix. The embedded filler was silicon dioxide. After etching and priming, stainless steel brackets were bonded to extracted human third molars (n = 60) with the experimental adhesives (15 teeth per group). After storage in distilled water for 72 hours, shear bond strength and ARI scores were evaluated. Kruskal-Wallis and post hoc test were performed for statistical analysis.

RESULTS

Shear bond strength of UDMA-based adhesives depends on filler content. Higher adhesive filler levels reveal greater bond strength between enamel and stainless steel brackets. ARI scores showed no significant difference among the groups. However, higher filled adhesives (filler content 70 vol%) seem to present higher ARI scores.

CONCLUSION

Highly filled UDMA adhesives offer greater bond strength than do lower filled or unfilled resins.

Fracture strength and bond capacities of electron irradiated fiber reinforced composites

OBJECTIVES

The aim of this in vitro study was to determine the effect of electron irradiation on the fracture strength, elongation and bond capacities on different fiber reinforced composites (FRC).

METHODS

Thirty-two bending bars (25mmx2mmx2mm) were fabricated per material (Construct: Kerr, Rastatt, Germany; everStick: StickTech, Turku, Finland; FibreKor: JenericPentron, Wallingford/CT, USA), divided into four irradiation groups (0kGy, 15kGy, 33kGy and 100kGy) and fracture loaded after irradiation. Forty-eight plates (20mmx10mmx2mm) of the FRC materials were made and divided into different pretreatment (Clearfil SE Bond Bond: Kuraray Europe, Düsseldorf, Germany; SRLink: Ivoclar-Vivadent, Schaan, Liechtenstein; MetalPrimerII: GC, Tokio, Japan; Rocatec: 3M Espe, Seefeld, Germany) and irradiation (0kGy, 15kGy and 33kGy) groups. After irradiation of the pretreated plates the veneering composite Sinfony (3M Espe, Seefeld, Germany) was applied (area Ø=5mm), water stored for 120 days and then the shear bond strength (SBS) was evaluated. Medians and 25%/75%-percentiles were calculated, statistical analysis was performed (Mann-Whitney U-test: p</=0.05).

RESULTS

Fracture strength increased significantly for all materials when irradiated, elongation decreased for everStick and Construct and increased for FibreKor. The non-irradiated various pretreatments did not influence SBS. Irradiation led to significant changes for FibreKor pretreated with SR-Link, MetalPrimerII and Clearfil SE Bond Bond.

SIGNIFICANCE

Electron irradiation can probably increase the fracture resistance of FRC fixed partial dentures if the right combinations of FRC material, pretreatment and electron irradiation doses are chosen. The FRC materials could be industrially advanced if irradiation was integrated into material development.

Acrylic removable appliances: Comparative evaluation of different postpolymerization methods

INTRODUCTION

In orthodontics, the polymeric part of removable orthodontic appliances could be a hazard for patients because high residual monomer contents can lead to allergic reactions. In addition, water uptake, color stability, and mechanical properties are influenced by the degree of conversion. The aim of this investigation was to evaluate the effectiveness of different postpolymerization methods on the properties of removable orthodontic appliance resins.

METHODS

A total of 300 rectangular specimens (5 postpolymerization groups: 20 per group) were manufactured of Orthocryl (n = 100, Dentaurum, Ispringen, Germany), Palapress (n = 100, Heraeus-Kulzer, Wehrheim, Germany), and Pro Base Cold (n = 100, Ivoclar-Vivadent, Schaan, Liechtenstein). The first 2 postpolymerization groups were postcured in a stove (10 minutes at 40 degrees C or 80 degrees C). The third resin group was postcured in a microwave at 600W for 5 minutes, and the fourth polymer group was postcured in a microwave at 600W for 10 minutes. Twenty conventionally cured samples (cold-curing under pressure: water bath 40 degrees C, 25 minutes, 2.2 bar) were the control group. Water sorption, fracture toughness, and Vickers hardness were measured.

RESULTS

Fracture toughness and Vickers hardness of the resins are slightly increased after curing. Heat as a secondary curing method decreases water uptake of resins used for removable orthodontic appliances.

CONCLUSIONS

Heat postpolymerization appears to improve the properties of removable orthodontic appliance resins.

Experimental composite brackets: influence of filler level on the mechanical properties

INTRODUCTION

Despite their popularity in satisfying esthetic demands, plastic brackets still have some disadvantages because of decreased wear resistance and hardness. Our aim in this study was to investigate the mechanical properties of an alternative bracket-polymer urethane-dimethacrylate (UDMA) reinforced by silicon dioxide fillers with several filler levels.

METHODS

Three bracket groups were created: (1) unfilled UDMA, (2) UDMA with a filler content of 40% by volume, and (3) UDMA with a filler content of 70% by volume. Flexural strength, fracture force, and Vickers hardness of the bracket groups were tested. In addition, a 3-medium wear test was performed.

RESULTS

The filler-reinforced brackets showed increased hardness, greater stiffness, and improved wear resistance compared with unfilled brackets. Higher filler concentrations in a polymer matrix result in greater stiffness of the polymer. Despite this, the filled composite brackets were not statistically more susceptible to fractures than the unfilled plastic brackets.

CONCLUSIONS

Experimental UDMA brackets reinforced with silicon dioxide fillers showed an obvious trend for improved mechanical properties compared with unfilled UDMA brackets.