Polymerization shrinkage-strain kinetics of temporary crown and bridge materials

Esthetic Integration Area concept to improve the emergence profile of fixed restorations: A dental technique

A technique for obtaining the esthetic integration and optimal emergence profile of tooth-supported and implant-supported restorations is described. Using a computer-aided design software program, data captured with an intraoral scanner were used to establish the anatomic landmarks for determining the maximum buccal volume to which a restoration can be extended. This technique could be applicable to different types of fixed-dental prostheses treatments. Advantages of this technique include the establishment of periodontal-prosthetic criteria and the improvement of clinical and laboratory communication since the same guidelines for evaluating restorative space in the buccolingual direction of tooth preparations are used to avoid overcontoured restorations.

The Technique and Material Used to Join Transfers Affect the Accuracy and Final Fit of Implant-Supported Prostheses-In Vitro Study

OBJECTIVES

 This study evaluated the linear dimensional change of polymerization of three materials and two techniques of the union of molding transfers for implant-supported prostheses used in the open-tray technique.

MATERIALS AND METHODS

 A nylon maxilla-shaped matrix was made, two osseous integrated implants were installed, and, over these two, straight conical mini-pillars were installed. Open-tray impression transfers were attached to the mini-pillars, and a silicone guide was made to standardize the connections between the transfers. The samples were divided into six groups (n = 20): PA (Pattern Resin LS, chemically activated acrylic resin in the single step technique); DU (Durallay, chemically activated acrylic resin in the single step technique); BI (Protemp4, bisacrylic resin in the single step technique); PAC (Pattern Resin LS in sectioning and joining of segments technique); DUC (Durallay, in sectioning and joining of segments technique); and BIC (Protemp4, in sectioning and joining of segments technique). The linear dimensional change values that occurred among these transfers were measured in a profile projector (VB300; Starret) coupled to the Quadra Check device, with a resolution of 0.001 mm, performed by a single calibrated operator.

STATISTICAL ANALYSIS

 Data were submitted to a two-way analysis of variance and Tukey's test (p < 0.01).

RESULTS

 Statistically significant mean values were found in all comparisons. The PA showed the lowest mean values (µm) of linear dimensional change, both in the single-step technique and in the sectioning and joining technique, in the following order: BI 255.73 (3.81), DU 173.75 (2.30), PA 95.97 (3.20), BIC 23.82 (1.71), DUC 20.85 (2.53), and PAC 13.27 (2.09). The single-step technique showed the worst results, regardless of the material.

CONCLUSION

 The sectioning and joining technique reduced the dimensional change in all materials, and the Pattern Resin LS showed the lowest shrinkage mean values, followed by Durallay and Protemp4.

In vitro fatigue and fracture testing of temporary materials from different manufacturing processes in implant-supported anterior crowns

OBJECTIVES

The aim of this study was to investigate the in vitro fatigue and fracture force of temporary implant-supported anterior crowns made of different materials with different abutment total occlusal convergence (TOC), with/without a screw channel, and with different types of fabrication.

MATERIALS AND METHODS

One hundred ninety-two implant-supported crowns were manufactured (4° or 8° TOC; with/without screw channel) form 6 materials (n = 8; 2 × additive, 3 × subtractive, 1 × automix; reference). Crowns were temporarily cemented, screw channels were closed (polytetrafluoroethylene, resin composite), and crowns were stored in water (37 °C; 10 days) before thermal cycling and mechanical loading (TCML). Fracture force was determined.

STATISTICS

Kolmogorov-Smirnov, ANOVA; Bonferroni; Kaplan-Meier; log-rank; α = 0.05.

RESULTS

Failure during TCML varied between 0 failures and total failure. Mean survival was between 1.8 × 105 and 4.8 × 105 cycles. The highest impact on survival presented the material (η2 = 0.072, p < .001). Fracture forces varied between 265.7 and 628.6 N. The highest impact on force was found for the material (η2 = 0.084, p < .001).

CONCLUSION

Additively and subtractively manufactured crowns provided similar or higher survival rates and fracture forces compared to automix crowns. The choice of material is decisive for the survival and fracture force. The fabrication is not crucial. A smaller TOC led to higher fracture force. Manually inserted screw channels had negative effects on fatigue testing.

CLINICAL RELEVANCE

The highest stability has been shown for crowns with a low TOC, which are manufactured additively and subtractively. In automix-fabricated crowns, manually inserted screw channels have negative effects.

Micro-CT analysis of 3D printed provisional crowns fitting

BACKGROUND

The provisional crowns (PR) play an important role during the course of fixed prosthesis treatment. The fitting of PR varies and depends on various factors.

OBJECTIVE

The aim of the study was to evaluate the marginal and internal fit of PR produced by three-dimensional (3D) printing technique and to compare those with PR made by the milling technique by using micro computed tomography (μCT) on three commonly used finish line designs.

METHOD

Ninety study models were made using metal die of #14 tooth (i.e. maxillary 1st premolar molar) prepared for full veneer crown with three different finish line chamfer (C), rounded shoulder (RS) and rounded shoulder with bevel (RSB). PR was fabricated on each study model, using milling (MiL group, n= 45), and 3D printing technique (3D-P group, n= 45). Marginal and internal fit of each study model was measured by micro computed tomography, at 7 Zones Pr-1- Pr-7 on each finish line. Recorded data were statistically analysed by one-way analysis of variance (ANOVA) and using Dunnett t-Test (p> 0.05).

RESULTS

The mean gap at margins was minimum for provisional crowns in 3D-P group in any finish line with minimum in rounded shoulder with bevel at zone Pr-1 30.9 ± 5.3 and at zone Pr-7 32.7 ± 5.3. In the axial region, i.e. zone Pr-2, the smallest gap was recorded in the 3D-P group and in the occlusal region, for zone Pr-3, 4 and 5, the maximum gap was recorded in the MiL group.

CONCLUSION

The provisional crowns fabricated by 3D printed technique have better marginal and internal fit than the provisional crowns made by milling technique.

Monomer Release from Dental Resins: The Current Status on Study Setup, Detection and Quantification for In Vitro Testing

Improvements in mechanical properties and a shift of focus towards esthetic dentistry led to the application of dental resins in various areas of dentistry. However, dental resins are not inert in the oral environment and may release monomers and other substances such as Bisphenol-A (BPA) due to incomplete polymerization and intraoral degradation. Current research shows that various monomers present cytotoxic, genotoxic, proinflammatory, and even mutagenic effects. Of these eluting substances, the elution of BPA in the oral environment is of particular interest due to its role as an endocrine disruptor. For this reason, the release of residual monomers and especially BPA from dental resins has been a cause for public concern. The assessment of patient exposure and potential health risks of dental monomers require a reliable experimental and analytical setup. However, the heterogeneous study design applied in current research hinders biocompatibility testing by impeding comparative analysis of different studies and transfer to the clinical situation. Therefore, this review aims to provide information on each step of a robust experimental and analytical in vitro setup that allows the collection of clinically relevant data and future meta-analytical evaluations.

Marginal and internal fit of provisional crowns fabricated using 3D printing technology

BACKGROUND

Clinicians routinely provide provisional crowns following teeth preparation. Three-dimensional (3D) printing technology could be used over conventional methods for better fit as lack of adequate fit would result in plaque accumulation, micro-leakage, teeth sensitivity, caries and periodontal diseases.

OBJECTIVE

The aim of the study was to evaluate the marginal and internal fit of provisional crowns fabricated using 3D printing technology and to compare it with that of compression molding and milling methods.

METHODS

Ninety study models were fabricated by duplicating metal master models of the maxillary first premolar molar with three different finish line chamfer, rounded shoulder and rounded shoulder with bevel. On each study model, provisional crowns were fabricated using compression molding (Mo. group, n= 30 - by over impression technique), milling (Mi. group, n= 30 - by 5-axis dental milling machine), and 3D printing method (3D-P. group, n= 30 - by 3D printer). Marginal and internal fit of the samples were evaluated by measuring gap using a scanning electron microscope with a magnification of 27 ×, at 7 zones A-G on different finish line models. The data were statistically analysed using one-way analysis of variance (ANOVA) at the 0.05 significance level. The p-values were calculated using Dunnett's test.

RESULTS

The marginal gap was minimal for the 3D-P. group for each finish line with lowest for rounded shoulder with bevel at zone A 30.6 ± 5.3 and at zone G 32.8 ± 5.4. In axial area, i.e. zones B and F, the minimum gap was noticed for the Mo. group and in Occlusal area (cusp and fossa), for zones C-E maximum gap was determined in Mi. group followed by Mo. and 3D-P. groups.

CONCLUSIONS

3D printed provisional crowns have better marginal and internal fit compared to milled and molded provisional crowns.

Marginal adaptation of three-unit interim restorations fabricated by the CAD-CA systems and the direct method before and after thermocycling

Background

Limited information is available regarding the marginal adaptation of three-unit interim restorations fabricated by different techniques from different materials. Also, the efficacy of computer-aided design/computer-aided manufacturing (CAD-CAM) systems for their fabrication is still questionable. This study aimed to assess the marginal adaptation of three-unit interim restorations fabricated by the CAD-CAM systems and the direct method from different materials before and after thermocycling.

Material and Methods

A sound mandibular second premolar and a second molar were prepared for a three-unit all-ceramic restoration. Metal dies were fabricated to simulate a case of mandibular first molar missing, and were mounted in an acrylic block. Seventy-two three-unit interim restorations were fabricated by different techniques from different materials in six groups (n=12). In the first four groups, restorations were fabricated from Tempron, Visalys Temp, Unifast III, and Acropars by the direct technique, while the Amann Girrbach and Arum CAD-CAM systems and Ceramill Temp blocks were used in the last two groups. Marginal gap in each group was measured under a stereomicroscope at ×100 magnification. The specimens then underwent 5000 thermal cycles (5-55°C), and the marginal gap was measured again afterwards. ANCOVA and Bonferroni test (for pairwise comparisons) were applied for statistical analyses (α=0.05).

Results

Amann Girrbach and Arum CAD-CAM systems were not significantly different regarding the marginal gap of restorations (P=0.999). Among the traditional interim materials, Acropars showed significantly higher marginal gap than others (P<0.001). No significant difference was noted between other traditional materials. CAD-CAM interim materials showed significantly smaller marginal gap than traditional materials (P<0.001).

Conclusions

The CAD-CAM interim materials yielded superior marginal adaptation in three-unit interim restorations compared with traditional interim materials. The type of CAD-CAM system had no significant effect on the final marginal adaptation of restorations.

Key words:Dental marginal adaptation, interim dental prosthesis, CAD-CAM.

In vitro comparison of the surface roughness of polymethyl methacrylate and bis-acrylic resins for interim restorations before and after polishing

STATEMENT OF PROBLEM

Polymethyl methacrylate and bis-acrylic based resins are widely used for interim restorations. Their initial surface roughness is important because it determines their aesthetic properties and the potential for biofilm adhesion.

PURPOSE

The purpose of this in vitro study was to assess the surface roughness and morphology of 6 bis-acrylic and 2 polymethyl methacrylate resins widely used for interim dental restorations, both before and after polishing.

MATERIAL AND METHODS

Specimens made of different bis-acrylic resins (Protemp 4, Luxatemp Star, Systemp, Telio, Structur Premium, Structur 3) or of polymethyl methacrylate (Unifast Trad, Unifast 3) were polished using a 2-step polishing system (Diatech). The average surface roughness before and after polishing (10 seconds at low speed in dry conditions) was measured by optical profilometry. Atomic force microscopy and scanning electron microscopy were used to analyze surface morphology. The Mann-Whitney and Kruskal-Wallis tests were used to evaluate the differences in roughness among specimens (α=.05), and the Pearson r correlation was computed to assess the relationship between fillers and average surface roughness.

RESULTS

In the 8 groups evaluated, the roughness significantly increased (P<.001) for Protemp 4 (from 0.12 to 0.50 μm), Luxatemp Star (0.17 to 1.19 μm), Unifast 3 (0.40 to 1.00 μm), Systemp (0.46 to 1.51 μm), Structur 3 (0.85 to 1.06 μm), Structur Premium (1.00 to 1.74 μm), and Telio (1.13 to 1.21 μm), except for Unifast Trad (9.20 to 2.59 μm). Pairwise multiple comparisons identified Protemp 4 as having the smoothest surface before and after polishing, while Unifast Trad was the roughest in both. Atomic force microscopy and scanning electron microscopy observations showed that the surface roughness of bis-acrylic resins was related to their surface morphology and average filler sizes. A positive relation between fillers and roughness was assessed (r=0.345, P<.001).

CONCLUSIONS

For the bis-acrylic interim resins, the surface roughness after polishing was correlated to the material used and its filler sizes. Nanofiller-based resins showed the smoothest surfaces. For the polymethyl methacrylate-based resins, the recently marketed Unifast 3 had the lowest overall roughness values.

Evaluation of Internal Fit and Marginal Adaptation of Provisional Crowns Fabricated with Three Different Techniques

Different techniques have been used to construct provisional crowns to protect prepared teeth. The purpose of this in vitro study was to assess the internal fit and marginal discrepancy of provisional crowns made by different methods. A total of 48 provisional crowns were constructed and divided into three groups (n = 16) according to the fabrication methods: fabricated manually-group MAN; computer-aided design/computer aided manufacturing technology-group CAM; and 3-dimensional (3D)-printed technology-group 3DP. The same standard tessellation language (STL) file was used for both CAD/CAM and 3D-printed group. The silicone-checked method was used to measure the internal gap distance. The marginal discrepancy was measured by using the polyvinyl siloxane (PVS) replica method and swept-source optical coherence tomography (OCT) scanning technique. Data were analyzed with one-way analysis of variance (ANOVA) nonparametric Kruskal-Wallis and Tukey tests at α = 0.05. At the central pit and axial walls, the gap distance mean values of group CAM were higher than those from group MAN and 3DP. The group 3DP was statistically significantly higher in gap distance at the location of occlusion than group MAN and group CAM (p < 0.05). The total gap distances assessed by silicone-checked method revealed there were no statistically significant differences between the tested groups (p > 0.05). The total mean values of absolute and horizontal marginal discrepancy of the group 3DP obtained by using the PVS-replica method and OCT scanning technique were significantly higher than the group MAN and CAM (p < 0.05). Regression correlation results of marginal discrepancy indicated a positive correlation (r = 0.902) between PVS-replica method and OCT scanning technique. The manually fabricated provisional crowns presented better internal fit and a smaller marginal discrepancy. Between different assessment techniques for marginal adaptation, PVS-replica method and OCT scanning technique have a positive correlation.

In vitro performance and fracture resistance of interim conventional or CAD-CAM implant-supported screw- or cement-retained anterior fixed partial dentures

STATEMENT OF PROBLEM

Interim restorations represent an essential clinical treatment step; however, limited information is available concerning the performance of computer-aided design and computer-aided manufacturing (CAD-CAM) interim materials.

PURPOSE

The purpose of this in vitro study was to evaluate the performance and fracture load of resin anterior implant-supported interim fixed partial dentures (IFPDs).

MATERIAL AND METHODS

Identical anterior resin IFPDs (maxillary central incisor to canine; n=16 per material) were milled from polymethylmethacrylate (PMMA) or di-methacrylate (DMA) systems with different filler content. The IFPD groups were split to simulate a chairside (cemented implant-supported prosthesis) or laboratory procedure (screw-retained implant-supported prosthesis). A cartridge DMA material served as a control. After interim cementation, combined thermocycling and mechanical loading (TCML) was performed on all restorations to approximate a maximum of 2.5 years of clinical function. Behavior during TCML and fracture force was determined, and failures were analyzed. The data were statistically investigated (Kolmogorov-Smirnov test, 1-way-ANOVA; post hoc Bonferroni, Kaplan-Meier survival, α=.05).

RESULTS

Drop out during TCML varied between no failures and complete failure during loading. For most systems, failure occurred between 120 000 and 600 000 mechanical loading cycles. For IFPDs without a screw channel fracture, values varied between 644 ±263 N and 987 ±101 N. Those with a screw channel fracture failed between 493 ±89 N and 951 ±248 N. Individual IFPDs had significantly higher mean fracture loads (P<.002), but the mean fracture values between IFPDs with and without a screw channel were not significantly different (P>.137). Failures were characterized by fracture of the connector (n=53) followed by mixed failures (n=22) or fractures at the abutment (n=21).

CONCLUSIONS

These interim materials are sufficiently fracture resistant for the fabrication of implant-supported anterior IFPDs and are expected to survive between 6 months and 2 years before failure. The stability of IFPDs depended on the type of material but not on the restoration design (with or without a screw channel).

Microcomputed tomography evaluation of cement film thickness of veneers and crowns made with conventional and 3D printed provisional materials

OBJECTIVE

To evaluate, through microcomputed tomography (μCT), the cement film thickness of veneers and crowns made with different provisional materials.

MATERIAL AND METHODS

A veneer and a crown preparation were performed on a central incisor and a second molar of a dental model, respectively, scanned with an intraoral scanner, and the .stl files were exported to an LCD-based SLA three-dimensional (3D)-Printer. Twenty-four preparations were 3D-printed for each veneer and crown and divided into four groups (n = 6/group): (a) Acrylic resin (Acrílico Marche); (b) Bisacrylic resin (Protemp 4); (c) PMMA computer-aided design and computer-aided manufacturing (CAD-CAM) (Vipiblock); and (d) 3D-printed resin for provisional restorations (Raydent C&B for temporary crown and bridge). Veneers and crowns restorations were performed and cemented with a flowable composite. Each specimen was scanned with a μCT apparatus, files were imported for data analysis, and cement film thickness was quantitatively measured. Data were analyzed by 2-way ANOVA and Tukey post-hoc tests (α = .05).

RESULTS

Crowns presented a thicker cementation film than veneers (P < .05).The bisacrylic resin showed the smallest veneer film thickness, similar to the acrylic resin (P = .151), which was not significantly different than the PMMA CAD/CAM material (P = .153). The 3D printed provisional material showed the thicker film, different than all other materials (P < .05). The bisacrylic resin showed a cement film thickness with a high number of voids in its surface. For crowns cementation, the 3D printed provisional material showed the thicker cementation film, different than all other materials (P < .05).

CONCLUSIONS

Different provisional materials present different film thicknesses. The 3D printed provisional material showed the highest veneer and crown film thicknesses. Veneers film thicknesses were smaller than crowns for all provisional materials.

CLINICAL SIGNIFICANCE

The 3D printed provisional material studied can be satisfactorily used, presenting appropriate adaptation with the tooth preparation, however, it shows the highest cement film thickness for both veneers and crowns cementations when compared with other provisional materials. A better internal fit, or smaller cement film thickness is obtained by CAD/CAM materials, acrylic and bisacrylic resins. Veneer cementation showed a smaller cement film thickness compared with crown cementation for all provisional materials.

Color stability and surface roughness of chitosan- and nanodiamond-modified bisacrylic resin

The aim of this study is to evaluate the effect of chitosan or nanodiamond incorporation on the color stability and surface roughness of a bisacrylic resin subjected to artificial aging. Four bisacrylic resins were evaluated, namely, control, chitosan-modified material, nanodiamond-modified material, and chitosan-nanodiamond-modified material. Twenty-four specimens were prepared for each material. The surface roughness was determined using a profilometer with a cut-off of 0.25 mm. The baseline color was measured according to the CIE L*a*b* system using a reflectance spectrophotometer. After these tests, the specimens were individually immersed in cola soft drink, red wine, or distilled water (n = 8) for 28 days. After the aging, the surface roughness and final color were re-evaluated. The color stability was determined using the difference between the coordinates obtained before and after the aging process. The data on roughness and color change were evaluated using ANOVA and the Tukey test (α = 0.05). The results show that the incorporation of nanodiamonds and chitosan into a bisacrylic resin provided a better color stability to the materials (p = 0.007). The storage in red wine resulted in a higher variation in the surface roughness values, especially when only the nanodiamond was incorporated to the material (p < 0.05). The incorporation of both chitosan and nanodiamonds are promising in providing an improvement in the properties of the bisacrylic resin when they are simultaneously incorporated in the product.

Laboratory workflow to obtain long-term injected resin composite interim restorations from an additive manufactured esthetic diagnostic template

OBJECTIVE

An analogue and digital workflow for the fabrication of a diagnostic 3D printed polymer template and its duplication for long-term injected composite resin interim restorations is described, because of the lack of scientific evidence in 3-dimensional (3D) printing applied to dentistry in terms of printer technology, printer parameters, postpolymerization processes, and material characteristics. In addition, in the case of 3D printed temporary resins, they cannot be relined successfully and its mechanical properties in the mouth have not been tested yet.

CONCLUSIONS

The main benefits of this approach relate to the improvement of clinical and laboratory procedures, as conventional waxing is eliminated, conventional master casts are not needed and the process is entirely automatized, improving the workflow, with minimal intervention of the laboratory technician.

CLINICAL SIGNIFICANCE

The additive manufactured diagnostic template represents the materialization of the digital diagnostic waxing and provides a powerful tool to visualize the digital diagnostic waxing in the patient's mouth and face. Furthermore, the diagnostic 3D printed template can be used for multiple applications including interim restorations, radiographical, or surgical guide fabrication. The duplication technique described provides a predictable workflow to obtain long-term injected resin composite restorations from an additive manufactured esthetic diagnostic template, improving the laboratory and chairside procedures.

Fit of interim crowns fabricated using photopolymer-jetting 3D printing

STATEMENT OF PROBLEM

The fit of interim crowns fabricated using 3-dimensional (3D) printing is unknown.

PURPOSE

The purpose of this in vitro study was to evaluate the fit of interim crowns fabricated using photopolymer-jetting 3D printing and to compare it with that of milling and compression molding methods.

MATERIAL AND METHODS

Twelve study models were fabricated by making an impression of a metal master model of the mandibular first molar. On each study model, interim crowns (N=36) were fabricated using compression molding (molding group, n=12), milling (milling group, n=12), and 3D polymer-jetting methods. The crowns were prepared as follows: molding group, overimpression technique; milling group, a 5-axis dental milling machine; and polymer-jetting group using a 3D printer. The fit of interim crowns was evaluated in the proximal, marginal, internal axial, and internal occlusal regions by using the image-superimposition and silicone-replica techniques. The Mann-Whitney U test and Kruskal-Wallis tests were used to compare the results among groups (α=.05).

RESULTS

Compared with the molding group, the milling and polymer-jetting groups showed more accurate results in the proximal and marginal regions (P<.001). In the axial regions, even though the mean discrepancy was smallest in the molding group, the data showed large deviations. In the occlusal region, the polymer-jetting group was the most accurate, and compared with the other groups, the milling group showed larger internal discrepancies (P<.001).

CONCLUSIONS

Polymer-jet 3D printing significantly enhanced the fit of interim crowns, particularly in the occlusal region.

Reducing the effect of polymerization shrinkage of temporary fixed dental prostheses by using different materials and fabrication techniques

OBJECTIVE

The aim of this laboratory study was to evaluate the horizontal and vertical effects of the polymerization shrinkage of three-unit temporary fixed dental prostheses (FDPs) on the position of the prepared teeth. In addition, the reduction of these effects by using different fabrication techniques was evaluated.

METHODS

A total of 192 temporary FDPs were fabricated using one methacrylate (MA) and two dimethacrylate (DMA) materials. Each material group (n=64) was divided into two groups according to the fabrication methods (M1: curing on the prepared teeth, M2: curing in a silicone mold). Each fabrication group was divided into four subgroups (n=8) according to the relining method used (B: no relining, S: spacer foil 300μm, DG: grinding-out with 500μm cutting depth, and FG: free grinding). The experimental apparatus consisted of two abutment teeth lowered at right angles into a silicone mold. One prepared tooth was embedded in silicone to simulate the periodontium and permit slight horizontal tooth movement. The dimensional changes were recorded with an optical microscope. The test images were superimposed and measured using image analysis software.

RESULTS

The statistical analysis showed that there were significantly higher horizontal changes for the MA than the DMA resins in M1, while there was none in M2. Regarding the vertical changes, there were significant differences between the baseline group and all relining and fabrication groups in all materials.

SIGNIFICANCE

Relining of directly fabricated temporary FDPs significantly reduces the effect of polymerization shrinkage and thus secures the position of the prepared teeth.

Comparison of the Accuracy of Different Transfer Impression Techniques for Osseointegrated Implants

The aim of this study was to evaluate 3 transfer techniques used to obtain working casts of implant-supported prostheses through the marginal misfit and strain induced to metallic framework. Thirty working casts were obtained from a metallic master cast, each one containing 2 implant analogs simulating a clinical situation of 3-unit implant-supported fixed prostheses according to the following transfer impression techniques: group A, squared transfers splinted with dental floss and acrylic resin, sectioned, and re-splinted; group B, squared transfers splinted with dental floss and bis-acrylic resin; and group N, squared transfers not splinted. A metallic framework was made for marginal misfit and strain measurements from the metallic master cast. The misfit between the metallic framework and working casts was evaluated with an optical microscope following the single-screw test protocol. In the same conditions, the strain was evaluated using strain gauges placed on the metallic framework. The data were submitted to one-way analysis of variance followed by the Tukey test (α = 5%). For both marginal misfit and strain, there were statistically significant differences between groups A and N (P &lt; .01) and groups B and N (P &lt; .01), with greater values for group N. According to the Pearson test, there was a positive correlation between the misfit and strain variables (r = 0.5642). The results of this study showed that the impression techniques with splinted transfers promoted better accuracy than the nonsplinted technique, regardless of the splinting material used.

In Vitro Fit and Cementation Resistance of Provisional Crowns for Single Implant-Supported Restorations

Abstract: This study aimed to verify marginal fit and the effect of cement film thickness standardization on retention of provisional crowns made with prefabricated acrylic cylinders on abutments, using two temporary luting agents subjected or not to mechanical cycling. Provisional crowns were made from bis-acryl (Luxatemp Fluorescence) or methyl methacrylate (Duralay) resins on acrylic cylinders and marginal fit and cement film thickness were evaluated. For retention evaluation, crowns were cemented with two temporary luting agents: non-eugenol zinc oxide (Tempbond NE) or calcium hydroxide-based (Hydcal) cements and subjected to tensile strength in a universal testing machine. After cleaning, debonded crowns were cemented again, subjected to mechanical cycling and retention was reassessed. The results of marginal fit and cement film thickness were analyzed by Student's t-test while retention of cements before and after mechanical cycling was analyzed using a mixed linear model. Methyl methacrylate crowns presented greater marginal misfit (p=0.001) and occlusal cement film thickness (p=0.003) than the bis-acryl ones. No difference was observed at axial cement film thickness (p=0.606). Resins (p=0.281) did not affect crown retention, but luting agents (p=0.029) and mechanical cycling (p=0.027) showed significant effects. The only significant interaction was mechanical cycling*luting agents, which means that luting agents were differently affected by mechanical cycling (p=0.002). In conclusion, the results showed that bis-acryl resin associated to calcium-hydroxide luting agent provided the best retention and lower cement thickness.

Effect of the processing cycle on dimensional changes of heat-polymerized denture base resins

Background:

The second processing cycle for adding the artificial teeth to heat-polymerized acrylic resin denture bases may result in dimensional changes of the denture bases. The aim of this study was to evaluate the dimensional changes of the heat-polymerized acrylic resin denture bases with one and two-cycle processing methods.

Materials and Methods:

A metal edentulous maxillary arch was used for making 40 stone casts. Maxillary complete dentures were made with heat-polymerized acrylic resins (Meliodent and Acropars) with one and two stage processing methods (n = 10 for each group). Linear dimensional changes in anteroposterior and mediolateral distances and vertical changes in the first molar region were measured following each processing cycle, using a digital caliper. Mean percentage of the dimensional changes were subjected to two-way analysis of variance and Tukey honest significant difference tests (α = 0.05).

Results:

Postpolymerization contraction occurred in both anteroposterior and mediolateral directions in all studied groups; however, the vertical dimension was increased. Acropars acrylic resin showed the highest dimensional changes and the second processing cycle significantly affected the measured distances (P < 0.05). Meliodent acrylic resin was not significantly influenced by the processing method.

Conclusion:

Reheating of the acrylic resin denture bases for the addition of denture teeth result in linear dimensional changes, which can be clinically significant based on the acrylic resin used.

A comparative evaluation of the marginal accuracy of crowns fabricated from four commercially available provisional materials: An in vitro study

Purpose:

The purpose of this in vitro study was to evaluate and compare the primary marginal accuracy of four commercially available provisional materials (Protemp 4, Luxatemp Star, Visalys Temp and DPI tooth moulding powder and liquid) at 2 time intervals (10 and 30 min).

Materials and Methods:

A customized stainless steel master model containing two interchangeable dies was used for fabrication of provisional crowns. Forty crowns (n = 10) were fabricated, and each crown was evaluated under a stereomicroscope. Vertical marginal discrepancies were noted and compared at 10 min since the start of mixing and then at 30 min.

Observations and Results:

Protemp 4 showed the least vertical marginal discrepancy (71.59 μ), followed by Luxatemp Star (91.93 μ) at 10 min. DPI showed a marginal discrepancy of 95.94 μ while Visalys Temp crowns had vertical marginal discrepancy of 106.81 μ. There was a significant difference in the marginal discrepancy values of Protemp 4 and Visalys Temp. At 30 min, there was a significant difference between the marginal discrepancy of Protemp 4 crowns (83.11 μ) and Visalys Temp crowns (128.97 μ) and between Protemp 4 and DPI (118.88 μ). No significant differences were observed between Protemp 4 and Luxatemp Star.

Conclusion:

The vertical marginal discrepancy of temporary crowns fabricated from the four commercially available provisional materials ranged from 71 to 106 μ immediately after fabrication (at 10 min from the start of mix) to 83–128 μ (30 min from the start of mix). The time elapsed after mixing had a significant influence on the marginal accuracy of the crowns.

Effects of prepolymerized particle size and polymerization kinetics on volumetric shrinkage of dental modeling resins

Dental modeling resins have been developed for use in areas where highly precise resin structures are needed. The manufacturers claim that these polymethyl methacrylate/methyl methacrylate (PMMA/MMA) resins show little or no shrinkage after polymerization. This study examined the polymerization shrinkage of five dental modeling resins as well as one temporary PMMA/MMA resin (control). The morphology and the particle size of the prepolymerized PMMA powders were investigated by scanning electron microscopy and laser diffraction particle size analysis, respectively. Linear polymerization shrinkage strains of the resins were monitored for 20 minutes using a custom-made linometer, and the final values (at 20 minutes) were converted into volumetric shrinkages. The final volumetric shrinkage values for the modeling resins were statistically similar (P > 0.05) or significantly larger (P < 0.05) than that of the control resin and were related to the polymerization kinetics (P < 0.05) rather than the PMMA bead size (P = 0.335). Therefore, the optimal control of the polymerization kinetics seems to be more important for producing high-precision resin structures rather than the use of dental modeling resins.

Estabilidade dimensional de uma resina acrílica para coroas provisórias em função de diferentes técnicas de processamento ao longo do tempo

INTRODUÇÃO: A estabilidade dimensional das resinas acrílicas influencia na adaptação das coroas provisórias e, consequentemente, no resultado do tratamento reabilitador. OBJETIVO: Avaliar a estabilidade dimensional, ao longo do tempo, de uma resina acrílica processada de diferentes maneiras. MATERIAL E MÉTODO: Cinco técnicas de polimerização da resina acrílica foram testadas: I - polimerização térmica utilizando líquido de polimerização rápida em mufla; II - autopolimerização sob pressão; III - autopolimerização utilizando pincel; IV - autopolimerização após mistura (pó + líquido) em pote Dappen e inserção em matriz de aço na fase arenosa; V - autopolimerização após mistura em pote Dappen com inserção na matriz metálica na fase plástica. Foram confeccionados dez corpos de prova para cada tipo de processamento. Avaliou-se a estabilidade dimensional por meio de matriz de aço inox confeccionada segundo a especificação nº19 da ADA. Os corpos de prova foram armazenados em água destilada a 37ºC e examinados nos períodos de 1 hora, 1, 7, 15, 10, 90 e 180 dias após a polimerização, com o auxílio de um microscópio óptico com aumento de 20×. Os resultados foram submetidos à análise de dupla variância (Teste de Tukey para Tempo e Técnica de processamento) em nível de 5% de significância. RESULTADO: Todos os grupos experimentais apresentaram alterações dimensionais entre o sétimo e o 15º dia; após esse período de tempo, houve alterações dimensionais estatisticamente significantes, para as diferentes técnicas estudadas. CONCLUSÃO: A estabilidade dimensional da resina acrílica não sofreu influência das diferentes técnicas de processamento avaliadas; entretanto, foi influenciada pelo tempo de armazenamento.

Measurement of composite shrinkage using a fibre optic Bragg grating sensor

Fibre Bragg grating is used to determine resin-based composite shrinkage. Two composite resins (Freedom from SDI and Z100 from 3M) were tested to determine the polymerization contraction behaviour. Each sample of resin was prepared with an embedded fibre Bragg grating. A LED activation unit with wavelength from 430 nm to 470 nm (Dabi Atlante) was used for resin polymerization. The wavelength position of the peak in the optical reflection spectra of the sensor was measured. The wavelength shift was related to the shrinkage deformation of the samples. Temperature and strain evolution during the curing phase of the material was monitored. The shrinkage in the longitudinal direction was 0.15 +/- 0.02% for resin Z100 (3M) and 0.06+/-0.01% for Freedom (SDI); two-thirds of shrinkage occurred after the first 50 s of illumination.

The effect of fiber orientation on the polymerization shrinkage strain of fiber-reinforced composites

OBJECTIVE

The aim of this study was to characterize the linear polymerization shrinkage strain of glass fiber-reinforced composite (FRC) according to the fiber orientation.

METHODS

Test specimens (n=5) (10.0 x 10.0 x 1.5mm) were prepared from different brands of photopolymerizable resin-preimpregnated FRC; unidirectional continuous FRC, experimental random-oriented FRC, and bidirectional continuous FRC. As control materials, particulate filler composite resin and unfilled dimethacrylate monomer resin were used. Two uniaxial strain gages (gage length 2mm) were used to measure shrinkage strains in two directions: longitudinally and transversally to the fiber direction. The uncured composite or resin was placed on top of the strain gages, covered with a separating sheet and a glass plate, and irradiated for 40s with a light-curing unit. The shrinkage strain was monitored for 300 s. ANOVA and Tukey's posthoc test were used at a significance level of 0.05.

RESULTS

ANOVA revealed that orientation of fiber and brand of material had a significant effect (P<0.05) on shrinkage strain. The unidirectional FRC revealed no shrinkage longitudinally to the fiber direction, whereas the shrinkage occurred transversally to the fiber direction. Particulate filler composite resin and unfilled resin revealed equal shrinkage strain in both of the measured directions.

SIGNIFICANCE

Anisotropic nature of FRC exists with regard to polymerization shrinkage strain. The variation of polymerization shrinkage strains of FRC compared to those of particulate filler composites and unfilled resin might be important for future clinical applications.

Marginal accuracy of interim restorations fabricated from four interim autopolymerizing resins

STATEMENT OF PROBLEM

One of the most important requirements for interim restorations is good marginal adaptation. Polymerization shrinkage of interim restorative materials can jeopardize the marginal integrity of interim restorations.

PURPOSE

The purpose of this study was to compare the marginal accuracy of interim restorations made with 4 interim materials.

MATERIAL AND METHODS

A direct technique was used to fabricate 44 interim restorations with 4 materials: Protemp 3 Garant, Trim II, Tempron, and Acropars (n=11). The interim restorations were made on a prepared molar-shaped metal die with a vinyl polysiloxane impression as a matrix. Marginal discrepancy of interim restorations was measured at the midpoint of buccal, palatal, mesial, and distal surfaces of metal die finish line with a microscope at x100 magnification. Comparisons were made with 1-way analysis of variance and the Duncan multiple range test (alpha=.05).

RESULTS

The mean marginal discrepancies of Protemp 3 Garant, Trim II, Tempron, and Acropars were 0.059, 0.063, 0.068, and 0.102 mm, respectively. Acropars exhibited the most marginal discrepancies and was significantly different from the other materials tested (P<.001). However, there were no statistical differences between the other 3 materials tested.

CONCLUSIONS

Interim restorations made from the Bis-GMA and conventional acrylic resins tested produced comparable marginal fit. Acropars demonstrated significant increases in marginal gap size.