The effects of surface machining on heat cured acrylic resin and two soft denture base materials: a scanning electron microscope and confocal microscope evaluation

The science of printing and polishing 3D printed dentures

Objective

To analyze the effectiveness of various techniques available for printing, finishing and polishing of 3D printed prosthesis.

Methods

The articles were selected from electronic databases including PubMed and Scopus. Recently, lot of advancements have been observed in the field of 3D printing in dentistry.

Results

Numerous studies were found explaining the factors affecting the surface roughness such as printing speed, direction, layer thickness, post curing, etc., and the significance in achieving a smooth surface finish of a 3D printed prosthesis. The methods employed to achieve this range, similar to conventional and chairside polishing, are to use advanced coating materials such as light cured glazes to nanoparticles.

Conclusion

3D printing is being used in day-to-day practice and the prosthesis must be aesthetic looking to satisfy the patients' expectations. There is a lack of data supporting any one polishing method for the prosthesis. There is a need for further research on the existing techniques and newer advancements yielding aesthetic prostheses with an optimal surface finish.

Effect of Orange Juice on the Properties of Heat-Polymerized and 3D-Printed Denture Materials

This study evaluated the color stability, surface roughness, and hardness of 3D-printed and heat-polymerized denture materials. A total of 90 samples were prepared, with equal numbers of 3D-printed and heat-polymerized disks. The initial hardness, surface roughness, and color values of the samples were measured. After 14 days of immersion in distilled water, natural orange juice, or commercial orange juice, the measurements were repeated. Based on the findings, 3D-printed samples exhibited a greater reduction in Vickers hardness (56.24 ± 15.81%) compared to heat-polymerized samples (18.93 ± 11.41%). Materials immersed in commercial orange juice exhibited a greater reduction in hardness compared to those in other solutions (43.13 ± 23.66). Surface roughness increased by 46.66 ± 26.8% in heat-polymerized samples and by 26.16 ± 20.78% in 3D-printed samples, with the highest increase observed in commercial orange juice (50.73 ± 28.8%) (p < 0.001). The color change (ΔE) was significantly higher in heat-polymerized samples (ΔE = 5.05 ± 0.28) than in 3D-printed samples (ΔE = 3.9 ± 0.26) (p < 0.001). This study demonstrates that the material type and immersion solutions play a critical role in determining the mechanical and optical properties of denture materials, with commercial orange juice having the most pronounced effect on surface roughness and hardness.

Benzyldimethyldodecyl Ammonium Chloride-Doped Denture-Based Resin: Impact on Strength, Surface Properties, Antifungal Activities, and In Silico Molecular Docking Analysis

Candida albicans (C. albicans) adhering to denture-based resins (DBRs) is a known cause of denture stomatitis. A new approach to prevent denture stomatitis is to include antimicrobial substances within DBRs. Here, we examined the mechanical performance and antifungal properties of DBRs containing benzyldimethyldodecyl ammonium chloride (C12BDMA-Cl) as an antimicrobial compound. C12BDMA-Cl is a quaternary ammonium compound, and its antifungal properties have never been investigated when combined with dental acrylic resin. Therefore, we modified a commercially available heat-polymerized acrylic DBR to contain 3 and 5 wt.% of C12BDMA-Cl. Unmodified DBR was used as a control group. Specimens were prepared using the conventional heat processing method. The specimen's flexural strength, elastic modulus, microhardness, and surface roughness were evaluated. C. albicans biofilm was grown on the specimens and assessed via colony-forming units (CFUs) and scanning electron microscopy (SEM). In silico molecular docking was applied to predict the potential C12BDMA-Cl inhibition activity as an antifungal drug. The 3% C12BDMA-Cl DBR demonstrated antifungal activities without a deterioration effect on the mechanical performance. SEM images indicated fewer colonies in DBR containing C12BDMA-Cl, which can be a potential approach to managing denture stomatitis. In conclusion, C12BDMA-Cl is a promising antifungal agent for preventing and treating denture stomatitis.

Impact of printing layer thickness on the flexural strength of nanocomposite 3D printed resins: An in vitro comparative study

Background

This study evaluated the influence of various printing layer thicknesses with silicon dioxide nanoparticles (SiO2NPs) incorporated as a reinforcement material on the flexural strength of 3D-printed denture base resins.

Material and Methods

Asiga (DentaBASE, Asiga, Erfurt, Germany) and NextDent (Denture 3D+, NextDent B.V., Soesterberg, The Netherlands) 3D-printed resins were modified with different concentrations of SiO2NPs (0.25 % and 0.5 wt%). A total of 180 specimens (bar-shaped, 64 × 10 × 3.3 mm) were fabricated (N = 90/resin). Each resin was subdivided into three groups (n = 30) according to the SiO2NP concentration (0 %, 0.25 %, and 0.5 wt%) Each concentration was divided into three groups (n = 10) according to the printing layer thickness (50 µm, 75 µm, and 100 µm). Specimens were printed according to the manufacturer's instructions and then subjected to 10,000 thermal cycles. A three-point bending test was used to measure the flexural strength (MPa). One-way analysis of variance (ANOVA) and Tukey's post hoc tests were used to analyze the data (α = 0.05).

Results

For both resins, printing layer thicknesses of 50 µm and 75 µm exhibited significantly higher flexural strength than 100 µm (P < 0.001). The 50 µm thickness showed the greatest flexural strength values (81.65 ± 4.77 MPa and 84.59 ± 6.21 MPa for Asiga and NextDent, respectively). The 100 µm thickness showed the lowest flexural strength values (74.35 ± 5.37 and 73.66 ± 5.55 MPa) for Asiga and NextDent, respectively. The flexural strength significantly increased with the addition of SiO2NPs with printing layer thicknesses of 50 µm and 75 µm (P < 0.001), whereas the modified and unmodified groups printed with a 100 µm layer thickness did not differ significantly. Asiga 0.25 %/50 µm and NextDent 0.5 %/50 µm showed the highest flexural strength values (97.32 ± 6.82 MPa and 97.54 ± 7.04 MPa, respectively). Scanning electron microscopy fractured surfaces analysis revealed more lamellae and irregularities with lower printing layer thicknesses and SiO2NP concentrations.

Conclusion

The flexural strength increased with printing layer thicknesses of 50 µm or 75 µm combined with SiO2NP reinforcement.

Engineering Toughness in a Brittle Vinyl Ester Resin Using Urethane Acrylate for Additive Manufacturing

Thermosetting polymers tend to have a stiffness-toughness trade-off due to the opposing relationship of stiffness and toughness on crosslink density. We hypothesize that engineering the polymer network, e.g., by incorporating urethane oligomers, we can improve the toughness by introducing variations in crosslink density. In this work, we show that a brittle methacrylated Bis-GMA resin (known as DA2) is toughened by adding a commercial urethane acrylate resin (known as Tenacious) in different proportions. The formulations are 3D printed using a vat photopolymerization technique, and their mechanical, thermal, and fracture properties are investigated. Our results show that a significant amount of Tenacious 60% w/w is required to produce parts with improved toughness. However, mechanical properties drop when the Tenacious amount is higher than 60% w/w. Overall, our results show that optimizing the amount of urethane acrylate can improve toughness without significantly sacrificing mechanical properties. In fact, the results show that synergistic effects in modulus and strength exist at specific blend concentrations.

Evaluation of the Effects of Different Polishing Protocols on the Surface Characterizations of 3D-Printed Acrylic Denture Base Resins: An In Vitro Study

Chairside polishing kits are an alternative to laboratory polishing techniques. The effects of using a chairside polishing kit on a three-dimensional (3D)-printed acrylic denture base (ADB) have not been reported previously. Thus, this study aimed to evaluate the effects of different chairside polishing techniques on the surface characterizations of ABD, including surface roughness average (Ra), average maximum profile height (Rz), and scanning electron microscopy (SEM) representations. One hundred and twenty disc-shaped specimens were fabricated from one conventional heat-polymerized (HP) ADB resin and two 3D-printed (Asiga (AS) and NextDent (ND)) ADB resins (n = 40 per material). Each group was further divided based on the polishing protocol (n = 10) as follows: conventional polishing protocol (C), microdont chairside polishing kit (M), shofu chairside polishing kit (S), and an unpolished group (U). The Ra and Rz values were measured using an optical profilometer. Two-way ANOVA and post hoc tests were used for data analysis (α = 0.05) at significant levels. In unpolished groups, there was a statistically significant difference between HP-U vs. AS-U and ND-U groups (p < 0.0001). For Ra, the lowest values were observed in HP-C, AS-S, and ND-C. While the highest values were shown in all unpolished groups. Within the material, there were statistically significant differences between the three polishing protocols (C, M, and S) vs. unpolished (p < 0.0001), while there was no significant between C, M, and S groups (p = 0.05). The Rz values had the same pattern as the Ra values. The two chairside polishing kits were comparable to conventional polishing techniques, and they can be recommended for clinical application.

3D-Printable Denture Base Resin Containing SiO2 Nanoparticles: An In Vitro Analysis of Mechanical and Surface Properties

PURPOSE

To evaluate the flexural strength (FS), impact strength (IS), surface roughness (Ra), and hardness of 3D-printed resin incorporating silicon dioxide nanoparticles (SNPs).

MATERIALS AND METHODS

A total of 320 acrylic specimens were fabricated with different dimensions according to test specifications and divided into a control group of heat denture base resin, and 3 test groups (80/test (n = 10) of unmodified, 0.25 wt%, and 0.5 wt% SNPs modified 3D-printed resin. 10,000 thermal cycles were performed to half of the fabricated specimens. FS, IS (Charpy impact), Ra, and hardness were evaluated and the collected data was analyzed with ANOVA followed by Tukey's post hoc test (α = 0.05).

RESULTS

Incorporating SNPs into 3D-printed resin significantly increased the FS, IS (at 0.5%) and hardness compared to unmodified 3D-printed resin (p < 0.001). However, the FS of pure 3D-printed and 3D/SNP-0.50% resin and IS of all 3D-printed resin groups were significantly lower than the control group (p < 0.0001). Hardness of 3D/SNP-0.25% and 3D/SNP-0.50% was significantly higher than control and unmodified 3D-printed resin (p < 0.0001), with insignificant differences between them. The Ra of all 3D-printed resin groups were significantly higher than control group (p < 0.001), while insignificant difference was found between 3D-printed groups. Thermal cycling significantly reduced FS and hardness for all tested groups, while for IS the reduction was significant only in the control and 3D/SNP-0.50% groups. Thermal cycling significantly increased Ra of the control group and unmodified 3D-printed resin (p < 0.001).

CONCLUSION

The addition of SNPs to 3D-printed denture base resin improved its mechanical properties while Ra was not significantly altered. Thermal cycling adversely affected tested properties, except IS of unmodified 3D-printed resin and 3D/SNP-0.25%, and Ra of modified 3D-printed resin.

Denture Acrylic Resin Material with Antibacterial and Protein-Repelling Properties for the Prevention of Denture Stomatitis

Denture stomatitis is a multifactorial pathological condition of the oral mucosa that affects up to 72% of denture wearers. It is commonly seen on the palatal mucosa and characterized by erythema on the oral mucosa that are in contact with the denture surface. The aim of this study was to incorporate 2-methacryloyloxyethyl phosphorylcholine (MPC) and dimethylaminohexadecyl methacrylate (DMAHDM) into a high impact polymethylmethacrylate heat-cured denture base acrylic resin as a potential treatment for denture stomatitis. We used a comparative study design to examine the effect of incorporating MPC as a protein repellent agent and DMAHDM as an antifungal agent to prevent the adherence of Candida albicans to the denture base material. The dual incorporation of MPC and DMAHDM reduced C. albicans biofilm colony-forming unit by two orders of magnitude when compared to the control group devoid of the bioactive agents. Although the addition of MPC and DMAHDM alone or in combination significantly reduced the flexural strength of the material, they showed reduced roughness values when compared to control groups. This new denture acrylic resin provides the benefit of enhancing C. albicans biofilm elimination through dual mechanisms of action, which could potentially reduce the prevalence of denture stomatitis.

Strength and Surface Properties of a 3D-Printed Denture Base Polymer

PURPOSE

This in vitro study evaluated the flexural strength, impact strength, hardness, and surface roughness of 3D-printed denture base resin subjected to thermal cycling treatment.

MATERIALS AND METHODS

According to ISO 20795-1:2013 standards, 120 acrylic resin specimens (40/flexural strength test, 40/impact strength, and 40/surface roughness and hardness test, n = 10) were fabricated and distributed into two groups: heat-polymerized; (Major.Base.20) as control and 3D-printed (NextDent) as experimental group. Half of the specimens of each group were subjected to 10,000 thermal cycles of 5 to 55°C simulating 1 year of clinical use. Flexural strength (MPa), impact strength (KJ/m² ), hardness (VHN), and surface roughness (μm) were measured using universal testing machine, Charpy's impact tester, Vickers hardness tester, and profilometer, respectively. Data were analyzed by ANOVA and Tukey honestly significant difference (HSD) test (α = 0.05).

RESULTS

The values of flexural strength (MPa) were 86.63 ± 1.0 and 69.15 ± 0.88; impact strength (KJ/m² )-6.32 ± 0.50 and 2.44 ± 0.31; hardness (VHN)-41.63 ± 2.03 and 34.62 ± 2.1; and surface roughness (μm)-0.18 ± 0.01 and 0.12 ± 0.02 for heat-polymerized and 3D-printed denture base materials, respectively. Significant differences in all tested properties were recorded between heat-polymerized and 3D-printed denture base materials (P < 0.001). Thermal cycling significantly lowered the flexural strength (63.93 ± 1.54 MPa), impact strength (2.40 ± 0.35 KJ/m² ), and hardness (30.17 ± 1.38 VHN) of 3D-printed resin in comparison to thermal cycled heat-polymerized resin, but surface roughness showed non-significant difference (p = 0.262).

CONCLUSION

3D-printed resin had inferior flexural strength, impact strength, and hardness values than heat-polymerized resin, but showed superior surface roughness. Temperature changes (thermal cycling) significantly reduced the hardness and flexural strength and increased surface roughness, but did not affect the impact strength.

Data-Driven Intelligent 3D Surface Measurement in Smart Manufacturing: Review and Outlook

High-fidelity characterization and effective monitoring of spatial and spatiotemporal processes are crucial for high-performance quality control of many manufacturing processes and systems in the era of smart manufacturing. Although the recent development in measurement technologies has made it possible to acquire high-resolution three-dimensional (3D) surface measurement data, it is generally expensive and time-consuming to use such technologies in real-world production settings. Data-driven approaches that stem from statistics and machine learning can potentially enable intelligent, cost-effective surface measurement and thus allow manufacturers to use high-resolution surface data for better decision-making without introducing substantial production cost induced by data acquisition. Among these methods, spatial and spatiotemporal interpolation techniques can draw inferences about unmeasured locations on a surface using the measurement of other locations, thus decreasing the measurement cost and time. However, interpolation methods are very sensitive to the availability of measurement data, and their performances largely depend on the measurement scheme or the sampling design, i.e., how to allocate measurement efforts. As such, sampling design is considered to be another important field that enables intelligent surface measurement. This paper reviews and summarizes the state-of-the-art research in interpolation and sampling design for surface measurement in varied manufacturing applications. Research gaps and future research directions are also identified and can serve as a fundamental guideline to industrial practitioners and researchers for future studies in these areas.

Research dissertation to published paper: the journey to a successful publication

Background Many dental professionals are now completing higher degrees that involve a research project. However, many of those research projects, although worthwhile, are not written up for publication in a peer-reviewed journal.Aim To encourage and assist novice authors in transferring their project report into a paper to submit for publication.Discussion The relationship between the authors and contributors is considered, and advice is given on how to focus on the specific research question and produce a succinct paper within the target journal's word limit. Emphasis is placed on choosing the right journal for submission and the need to follow the 'instructions to authors', as well as what happens post-submission, post-acceptance and post-publication. Furthermore, some of the difficulties that the authors have encountered on their own publication journeys are highlighted.Conclusion Great satisfaction will be derived if the novice researcher makes the most of their opportunity of undertaking a research project and subsequently getting it published in a peer-reviewed journal. Publishing gives the author recognition within their professional community, a feeling of personal achievement, can create better career perspectives and allows others to build on the work.

Flexural and Surface Properties of PMMA Denture Base Material Modified with Thymoquinone as an Antifungal Agent

PURPOSE

To evaluate the effect of addition of different concentrations of thymoquinone (TQ) on the flexural strength, elastic modulus, surface roughness, and hardness of PMMA denture base material.

MATERIALS AND METHODS

A total of 160 rectangular specimens were prepared from heat-polymerized acrylic resin, with dimensions of 65 × 10 × 2.5 mm³ for flexural strength testing and 10 × 20 × 3 mm³ for surface property testing. The specimens were divided into eight groups of 20 specimens: one control group without addition of TQ and seven test groups prepared by adding TQ to acrylic powder in concentrations of 0.5, 1, 1.5, 2, 2.5, 3, and 5 wt%. The polymer was added to the monomer before being mixed, packed, and processed using the conventional water bath method. A universal testing machine was used to measure flexural strength and elastic modulus. A profilometer and a Vickers hardness tester were used to measure surface roughness and hardness, respectively. One-way ANOVA and the Tukey-Kramer multiple-comparison test were used for statistical analysis, with statistical significance at p ≤ 0.05.

RESULTS

Addition of TQ to PMMA denture base material significantly decreased flexural strength and elastic modulus at high concentrations (p < 0.01), while no significant differences were observed at low concentrations (0.5%, 1% TQ) in comparison with the control group. At high TQ concentrations, surface roughness increased while hardness decreased (p < 0.0001), and no significant differences were observed at low concentrations (0.5%, 1% TQ) in comparison with the control group. The most favorable addition values were 0.5% and 1% TQ in all TQ groups.

CONCLUSIONS

Addition of TQ did not affect the flexural and surface properties of PMMA denture base material at low concentrations (0.5%, 1% TQ) and could be incorporated into PMMA denture base material as an antifungal agent.

A qualitative analysis to compare the effects of surface machining of conventional denture base resin and two soft liners: a scanning electron microscopic study

INTRODUCTION

The denture base acrylic resins require adjustments for various reasons. During this process there is an alteration in the surface characteristics of the denture base. Rough surfaces promote the bacterial adhesion and plaque accumulation; therefore it is important to know the character of the surface left by instrumentation on denture base materials. This study evaluated the surface characteristics of the machined surfaces of heat-cured acrylic denture base resin, GC supersoft and Permasoft softliners.

MATERIALS AND METHODS

Thirty 15×15×1.5mm acrylic resin specimens were fabricated with each of three acrylic resins: Lucitone 199 denture base resin (Group I), GC supersoft (Group II) and Permasoft (Group III) softliners. They were further divided into three sub Groups A, B and C, in which Sub Group A was control group that is smooth produced against the glass. Sub Group B was produced by machining with the tungsten carbide bur and Sub group C is machined with the stone bur. Each surface was evaluated by a Scanning electron microscope and data were analyzed by analysis of variance followed by Tukey's HSD test.

RESULTS

Stone bur produced smoother surface (Ra 3.6681μm± 0.254) on Lucitone199 than the tungsten carbide bur (Ra 5.3881μm ± 0.3373). Carbide bur produced a smoother surface on the GC super soft (Ra 1.617097μm ± 0.191767) and Permasoft softliners (Ra 2.237419μm ± 0.354259). Whereas stone bur produced rougher surface on GC supersoft(Ra 2.6μm) and Permasoft (Ra 4.184839μm ± 0.409869) softliners.

CONCLUSION

The present study shows each type of rotary instrument produces its own characteristic surface on each type of denture base materials and that care is needed when selecting the most appropriate instrument to adjust denture base materials. These results can have a significant clinical implication. While using Lucitone 199 stone bur can be used for chair side adjustments. Tungsten carbide bur can be used for GC supersoft and Permasoft softliners to achieve smoother surface.

The effect of mechanical and chemical polishing techniques on the surface roughness of heat-polymerized and visible light-polymerized acrylic denture base resins

OBJECTIVE

The purpose of this study was to compare the effects of mechanical polishing (MP) and chemical polishing (CP) on the average surface roughness (Ra) of heat-cured (HC) and light-cured (LC) denture base acrylic resins.

METHODS

A total of 120 specimens (30 × 15 × 3 mm) were prepared from one HC and one LC acrylic resin. To remove nodules and gross surface irregularities, all specimens were finished with a lathe-mounted small acrylic bur and 360-grit sandpaper. Ten finished specimens of each acrylic resin were randomly assigned to each of six polishing techniques: Resilit High-luster Polishing Liquid (RHPL), Universal Polishing Paste, Abraso-star K50, pumice, Jet Seal Liquid, or Acrypoint. MP was performed with an automatic polishing machine for 2 min, under 50 rpm and 500 g of load. CP was performed by immersing the HC and LC specimens in preheated methyl methacrylate at 75 ± 1 °C for 10 s. The surface roughness of the acrylic resin specimens was measured with a contact profilometer. The Ra values were analyzed by two-way analysis of variance, post hoc Scheffe's test, and paired t-test (p ⩽ 0.05). Polished and tested acrylic resin surfaces were evaluated by scanning electron microscopy.

RESULTS

MP was more effective than CP. The smoothest surface was obtained with the use of the RHPL on the LC (0.05 ± 0.01 μm) or HC (0.07 ± 0.01 μm) acrylic resin. Two-way ANOVA showed a statistically significant difference between MP and CP.

CONCLUSIONS

MP produced the smoothest surface of denture base acrylic resin. The mean surface roughness values after MP and CP were not influenced by the type of acrylic resin.

Roughness parameter selection for novel manufacturing processes

This work proposes a method of roughness parameter (RP) selection for novel manufacturing processes or processes where little knowledge exists about which RPs are important. The method selects a single parameter to represent a group of highly correlated parameters. Single point incremental forming (SPIF) is used as the case study for the manufacturing process. This methodology was successful in reducing the number of RPs investigated from 18 to 8 in the case study.

Surface roughness of acrylic and silicone-based soft liners: in vivo study in a rat model

PURPOSE

The aim of this in vivo animal study was to investigate changes in the surface roughness of soft liners over time.

MATERIALS AND METHODS

Forty adult Wistar rats (Rattus norvergicus albinus) were fitted with acrylic custom-made palatal plates relined by dynamic impressions and tested with the following soft liners: Dentuflex (DF), Trusoft (TS), Dentusoft (DS), and Ufi Gel P (UG). Half of the animals for each tested material had the plates fitted during the material reline procedure. Their surface roughness was read immediately (IRa group, n = 5). The other half used the palatal plates for 14 days before roughness readings were performed (FRa group, n = 5). The surface roughness (Ra) of the inner surface from the relined dentures was recorded using a Surftest SJ-401 with eight readings per specimen, and mean values were obtained. Data (μm) were analyzed by two-way ANOVA and Tukey's test (α = 0.05).

RESULTS

IRa means (2.92 ± 0.87 μm) and FRa means (3.35 ± 0.65 μm) were significantly different (p = 0.016). UG showed a lower (p = 0.01) Ra mean (2.1 ± 0.52 μm) than DF (3.94 ± 0.81 μm), TS (4.12 ± 0.64 μm), and DS (3.27 ± 0.64 μm).

CONCLUSIONS

Ufi Gel P showed the smoothest surface among the materials evaluated. The period of use resulted in changes in the surface roughness of the materials tested.

Integrity of the interface between denture base and soft liner: a scanning electron microscopic study

Aims and objectives of the study was to study the integrity of the interface between the denture base and the soft liner when the thickness of the soft liner was 0.5, 1.0 and 2.0 mm, and to study the integrity of the soft liner and denture base interface as influenced by aging process. 80 rectangular based specimens were fabricated using heat cured acrylic resin. The heat cured component of the specimen was fabricated from stainless steel template form by compression molding technique. Different thickness of silicone soft layer component was added to heat cured acrylic resin component of the specimen following the manufacturer's instructions to fabricate the group A, group B, group C and group D specimens. All the specimens were subjected to the same finishing and polishing procedures. The group A specimens was immediately scanned in scanning electron microscope after processing. A thermo statically controlled artificial saliva bath designed to maintain the temperature between 38 ± 4°C was used to simulate the oral condition and to age the group B, group C and group D specimens for 3 months after which they were subjected to scanning under a scanning electron microscope. All the aged specimens demonstrated two types of failures namely adhesive which occurred along the bond interface between the soft liner and the acrylic resin and adhesive and cohesive type of failure which occurred not only at the interface but also within the soft liner material itself. When the data was subjected to ANOVA, the group A specimens showed statistical significance with group B (P = 0.006), group C (P = 0.007) and group D specimens (P = 0.004), the level of significance being (P < 0.05). However, there was no statistical significance between group B and C (P = 0.98), group C and D specimens (P = 0.52), group B and D specimens (P = 0.70), the level of significance being (P < 0.05). Based on the results, statistical analysis of the results and within the limitations of this in vitro study the following conclusions can be drawn: The thickness of the liner did not show a statistically significant rate of failure on the integrity of the interface between the denture base resin and the soft liner and aging conditions simulating the clinical environment showed a statistically significant rate of failure on the integrity of the interface between the denture base resin and the soft liner.

Effect of Sealant Application and Thermal Cycling on Bond Strength of Tissue Conditioners to Acrylic Resin

The aim of this study was to evaluate the effect of sealer application and thermal cycling on the bond strength between tissue conditioners and acrylic resin, and to observe the type of bond failure. Two hundred eighty-eight specimens (10x16x3 mm) were made of an acrylic resin (Lucitone 500, Dentsply) using a metal muffle. Specimens were divided into four groups according to the tissue conditioner (Coe-Comfort, GC or Dentusoft, Densell) used and whether or not a sealer (Eversoft Soft Liner Sealer, Myerson) was applied. Each of the four groups was subdivided into other six subgroups (n=12) to undergo thermocycling for 45, 90, 135, 180 or 210 cycles with a dwell time of 60 s, or to be left non thermocycled (control). Tensile bond strength was measured in a universal testing machine at a crosshead speed of 5 mm/min. Sealant application had no effect on the tensile bond strength of the relined acrylic resin, regardless of the tissue conditioner used (Coe-Comfort: p=0.306 and Dentusoft: p=0.1501). The number of thermal cycles had a significant effect on the tensile bond strength of the relined acrylic resin (Coe-Comfort: p=0.002 and Dentusoft: p<0.001). Both tissue conditioners presented similar bond strength to acrylic resin. For both tissue conditioners, sealer coatings had no influence on bond strength, while different numbers of thermal cycles affected that mechanical property.

A study to evaluate and compare the shear bond strength of resilient liners with heat cure denture base resins, with and without the effect of saliva: an in vitro study

AIM

To evaluate and compare the shear bond strength of resilient liners with heat cure denture base resins in the presence or absence of saliva.

MATERIALS AND METHODS

Two commercially available heat polymerized acrylics and three commercially available denture liners were immersed in artificial saliva for 7 days and 14 days, respectively. A total of 180 (Acralyn-H, No.90 and Lucitone - 199, No.90) specimens were prepared. Total of 90 overlapping joint specimens were prepared, 45 of them using Acralyn H (AGroup) and rest 45 using Lucitone-199 (L-Group). The specimens were tested for flexural strength with a 3-point bending test on an Instron universal testing machine. The results were analyzed with a one-way analysis of variance (ANOVA).

RESULTS

The mean difference in shear bond strength (SBS) at different time intervals was found to be statistically significant (p < 0.001). Lucitone-199 recorded a significantly higher mean SBS compared to Acralyn H (p < 0.001). Further, significant differences between GC and Densply, GC and Aswin liners, and between Dentsply and Aswin were noted (p < 0.001). Difference between baseline and 7 days time interval, as well as, between baseline and 14 days time interval with respect to the mean SBS of these materials were significant (p < 0.001). Also, the mean difference in SBS between 7 days time interval and 14 days time interval was statistically significant (p < 0.001). Among the three different liners, GC yielded a higher mean SBS compared to Aswin and Dentsply at all the three time intervals. The mean SBS recorded in Dentsply and Aswin was almost same at 14 days time interval, but at baseline and 7 days, it was higher in Aswin compared to Dentsply.

CONCLUSION

Lucitone-199 recorded a higher mean SBS compared to Acralyn H. As the time interval increases, the mean SBS recorded in both the denture base materials decrease. Among the three different liners, GC yields a higher mean SBS compared to Aswin and Dentsply at all the three time intervals.

CLINICAL SIGNIFICANCE

The most common reason for failures of resilient linings in removable dentures is the separation of these linings from the denture base. Therefore, poor adhesive bond properties are one of the serious defects of the material in clinical practice.

Analysis of the performance of a standardized method for the polishing of methacrylic resins

Adhesion of micro-organisms to resin surface may be caused by inadequate polishing. Most of the studies published in literature are relative to manually prepared samples and do not take into account that test repeatability is not guaranteed a priori since skills may change from one operator to another and the quality of the work done by the same expert operator may depend on “human” factors such as the level of attention, wrist trembling, etc.

This paper aims to investigate on the efficiency and reliability of a standardized protocol for polishing methacrylic resins. For that purpose, five different methacrylic resins are considered. For each resin, 20 specimens are realized: 10 are polished by the same expert operator and 10 are polished by means of a mechanical system comprised of a milling tool, a mobile support for samples and a micrometric advance isoparallelometer. Roughness measurements are carried out with a ±0.01 µm resolution profilometer.

An extensive statistical analysis is conducted on a population of 100 specimens. Two-way Analysis of Variance (ANOVA) is carried out taking the type of resin (i) and the polishing technique (ii) as predictors in order to evaluate how those variables will finally affect the roughness of the polished surface. The significance of the variable interaction term is assessed. The null hypothesis Ho where response is independent from individual factors as well as from their interaction is assumed to hold true for p-values>0.05 (interval of confidence of the 95%).

Experimental data confirm that mechanical polishing leads to obtaining surfaces of much more uniform quality. In fact, statistical dispersion of roughness parameters can decrease significantly. This behavior is observed for all of the tested resins. Therefore, the new approach can eliminate the influence of “human” factors thus making it possible to assess the inherent features of each resin and compare different dental materials submitted to polishing.

Effects of polishing techniques on the surface roughness of acrylic denture base resins

STATEMENT OF PROBLEM

Rough surfaces of denture bases promote adhesion of microorganisms and plaque formation. It is therefore important to know how different polishing systems affect surface roughness of denture base acrylic resins.

PURPOSE

The objective of this study was to compare the effects of 4 chairside polishing kits and 2 conventional laboratory techniques used for polishing 3 different acrylic denture base resins.

MATERIAL AND METHODS

Using contact profilometric measurement, the surface texture of 54 specimens (15x30x3 mm) per acrylic material (autopolymerized ProBase Cold, heat-polymerized ProBase Hot, and injection heat-polymerized SR Ivocap plus) was studied before and after cutting with a tungsten carbide bur, and during and after chairside polishing with 4 polishing kits (Exa Technique, Acrylic Polisher HP blue, AcryPoint, Becht Polishing Cream), and after conventional polishing with 2 polishing systems (Universal Polishing Paste for Resins and Metals, Lesk Polishing Liquid). There were 9 specimens for each acrylic resin material and polishing method combination. Conventional lathe polishing with polishing paste served as the control. Mean average surface roughness (Ra) values of each specimen group were analyzed using a 2-way analysis of variance, the Scheffé post-hoc test, and paired t test (alpha=.05) with the Bonferroni adjustment. After testing the polished acrylic resin surfaces were evaluated under a scanning electron microscope.

RESULTS

The highest mean average surface roughness (Ra=2.86+/-0.8 microm to 3.99+/-1.31 microm) was measured for surfaces finished with a tungsten carbide bur. The lowest surface roughness values (Ra=0.02+/-0.01 microm) were determined for acrylic resin specimens polished with a lathe and polishing paste. The Ra values of resin specimens after polishing with chairside silicone polishing kits ranged from 0.05+/-0.0 microm to 0.35+/-0.05 microm. Mean average Ra values of specimens polished with a polishing cream alone (Ra=1.01+/-0.17 microm to 1.68+/-0.47 microm) were significantly higher (P<.05) than those obtained with other polishing systems tested, which was confirmed by scanning electron microscope images of acrylic resin surfaces. Significant differences in mean average surface roughness were found between autopolymerizing and injected heat-polymerizing resin specimens. In addition, scanning electron microscopy revealed increased porosity of autopolymerizing resin specimens.

CONCLUSIONS

Conventional laboratory polishing was found to produce the smoothest surface of denture base acrylic resin. Chairside silicone polishing kits produced a significantly smoother surface of acrylic resin than specimens polished with a tungsten carbide bur. The presence of large pores was characteristic for the autopolymerizing resin material.

Effect of finishing and polishing procedures on the gap width between a denture base resin and two long-term, resilient denture liners

STATEMENT OF PROBLEM

The junction between a long-term, resilient denture liner and the denture base is difficult to finish and polish due to differences in the hardness of the materials. Gaps tend to form during finishing and polishing procedures.

PURPOSE

This study measured the junctional gap between 2 long-term, resilient denture liners and a denture base material after different finishing and polishing procedures were performed. The surface smoothness of the 2 liner materials also was evaluated.

MATERIAL AND METHODS

Molloplast-B and an experimental, heat-polymerized methyl siloxane-resin-based denture liner were processed (according to the manufacturer's instructions) against Lucitone 199 acrylic resin. Control specimens (n = 20) were 2 mm thick and flat. Experimental specimens (n = 64) were fabricated with a raised center section: a 3- x 5- x 15-mm half-cylindrical ridge with a junction at the top of the cylinder between the liner material and acrylic resin. The specimens were finished with 1 of 6 types of burs and polished with different combinations of rubber-impregnated acrylic polishers, pumice, and tin oxide. The finished specimens were examined and photographed with a scanning electron microscope, and the largest gap along the liner/denture base junction on each specimen was measured. Three-way analysis of variance without replication (P<.05) and post-hoc t tests were used to analyze the data and compare groups. RESULTS; Averaged across finishing and polishing techniques, a larger gap was recorded for the experimental liner material (22 microm) than for Molloplast-B (14 microm) (P<.00005). Qualitative evaluation suggested that the experimental liner material polished better than Molloplast-B. The smoothest surfaces were obtained when specimens were finished with fine-tooth cross-cut carbide burs and polished with both pumice and tin oxide. A comparison between polished and unpolished specimens, averaged over materials and finishing techniques, revealed that polishing reduced gap size (P=.015).

CONCLUSION

Within the limitations of this study, the 2 denture liners tested behaved similarly. The results suggest that the size of the gap at the liner/denture base junction may be affected by finishing and polishing techniques and vary among materials.

In vitro colonisation of acrylic resin denture base materials by Streptococcus oralis and Actinomyces viscosus

AIM

The aim of the study was to compare the attachment of two typical strains of oral bacteria to four denture base materials.

DESIGN

In vitro study.

METHOD

Discs of acrylic resin denture base materials (Paladon 65, polished and unpolished; Palapress; Microbase, polished and unpolished, and Triad VLC) were placed into Petri dishes with Schaedler's medium, inoculated with Streptococcus oralis 34 or Actinomyces viscosus T14V.

MAIN OUTCOME MEASURES

After 24 h or 48 h the numbers of adhering bacteria were measured.

RESULTS

The bacteria adhered to all discs in similar numbers: 3-9 x 10(6)/ml (viable cell count) and 9-22 x 10(8)/ml (total cell count) for T14V, and 2-6 x 10(6)/ml (viable cell count) and 1.5-3 x 10(8)/ml (total cell count) for 34.

CONCLUSIONS

Polishing had little effect on adherence. Denture base materials are not resistant against adherence and possible surface damage by oral bacteria. Therefore, thorough oral hygiene is important for denture wearers.

Dental materials: 1997 literature review

This review of the published literature on dental materials for the year 1997 has been compiled by the Dental Materials Panel of UK. It continues a series of annual reviews started in 1973. Emphasis has been placed upon publications, which report upon the materials science or clinical performance of the materials. The review has been divided by accepted materials classifications (fissure sealants, glass polyalkenoate cements, dentine bonding, dental amalgam, endodontic materials, casting alloys, ceramometallic restorations and resin-bonded bridges, ceramics, denture base resins and soft lining materials, impression materials, dental implant materials, orthodontic materials, biomechanics and image processing, resin composites, and casting investment materials and waxes). Three hundred and thirty three articles have been reviewed.

Adherence of Candida albicans to denture-base materials with different surface finishes

OBJECTIVES

To assess the in vitro adherence of Candida albicans to heat-cured hard and soft denture-base materials with varying surface roughness, and to observe the effect of a mixed salivary pellicle on candidal adhesion to these surfaces.

METHODS

In vitro adhesion assays on heat-cured acrylic resin (Trevalon), Molloplast B and Novus using the type strain of C. albicans (NCPF 3153A). Surfaces for the assays were prepared using clinically appropriate rotary instruments. Unstimulated, pooled and clarified whole saliva was used to assess its effect on adhesion.

RESULTS

Significantly greater adhesion of C. albicans to rough rather than smooth surfaces was found (P < 0.001), as well as increased adhesion to the machined soft lining materials compared with acrylic. Pre-coating denture-base materials with saliva reduced candidal adhesion on all materials.

CONCLUSIONS

Rough surfaces on denture-base materials promote the adhesion of C. albicans in vitro. However, saliva reduces adhesion of C. albicans and thus diminishes the effect of surface roughness and free surface energy differences between materials.

Fact and artefact in confocal microscopy

High-resolution confocal microscopic images may be made of either the surface of a sample or beneath the surface. These images can be likened to optical tomograms, giving thin (> 0.35 micron) slices up to 200 microns below the surface of a transparent tissue: With microscopes running under normal conditions, the optical section thickness will be > 1 micron and the effective penetration into enamel and dentin a maximum of 100 microns. For maximum resolution, high-quality, high-numerical-aperture objectives should be used. Refractive index matching of the lens immersion media and the substrate will avoid distortions of images in the optical axis. Such errors could occur when imaging a considerable distance (> 40 microns) into a cell containing water, with an oil immersion objective above the cover slip. Care should be taken in the interpretation of computerized z axis reconstructions made from serial optical sections: Their validity should be checked with equivalent views made with the sample oriented in the same direction as the reconstruction. The use of fluorescent dyes in microscopy is a very powerful investigative technique. It is important that the dyes used not be labile and that they be well-fixed to the materials being examined, or the images may indicate the dye distribution rather than the material to which it is "attached". Multiple labeling experiments must have crossover control experiments to verify the distribution of the individual dyes. Valuable information can often be gained by combining information from both reflection and fluorescence images. Two-photon laser excitation of dyes gives the potential for greater depth penetration and improved resolution.