Influence of Different Repair Acrylic Resin and Thermocycling on the Flexural Strength of Denture Base Resin.

Background and Objectives: Fractured acrylic denture base is a common occurrence in clinical practice. The effective denture repair procedure is cost-effective, time conserving, and results in lesser time without denture for the patient. Along with various reinforcements and surface modifications; different acrylic resins are investigated in improving the flexural strength of the fractured site. The aim of this study was to evaluate the flexural strength of a polymethyl methacrylate (PMMA) denture base repaired with heat-polymerized (HPA), auto-polymerized (APA) and light-polymerized acrylic (LPA) resins after thermocycling. Materials and Methods: Forty rectangular shaped (50 mm × 25 mm × 3mm) PMMA specimens were fabricated. Group 1 specimens (n = 10) were kept as controls and the remaining 30 samples were sectioned at the center with a repair site dimension of 3 mm. The samples from three groups (n = 10) were repaired with HPA, APA, and LPA resins, respectively. The specimens were thermocycled for 5000 cycles and subjected to a three-point flexural test. The maximum load required to fracture the specimens was recorded, and further analyzed with ANOVA and the Games-Howell Post hoc test at the significance level p = 0.05. Results: The average maximum load and flexural strength of the control group was 173.60 N and 13.02 Mpa and corresponding values for denture repaired with HPA was 87.36 N and 6.55 Mpa. The corresponding values for APA resin and LPA resins were 62.94 N, 57.51 N, and 4.72 Mpa, 4.06 Mpa, respectively. Conclusions: The PMMA specimens repaired with HPA resins resulted in a significantly higher load to fracture compared to APA resin and LPA resin.

An audit to assess the quality and efficiency of complete and partial dentures delivered by junior hospital staff.

The average number of visits for the construction of metal-based and acrylic dentures by junior hospital staff was 10 visits. Our hypothesis was that supervision would optimise the number of visits and reduce any need for remakes. The first audit cycle was retrospective and included all patients treated by SHOs in the Prosthodontics Department. The standard of care was compared to the British Society for the Study of Prosthetic Dentistry. The re-audit showed that the time taken to completion was reduced by 2 visits for both denture types and the average length of time was reduced from 31 weeks to 22 weeks. These improvements were directly related to improved supervision by senior staff.

Dimensional accuracy of upper complete denture bases: the effect of metallic flask closure methods.

OBJECTIVES: To verify the dimensional accuracy of upper complete denture bases under the effect of different methods of metallic flask closure. MATERIALS AND METHODS: Wax record bases were assigned to six groups: 1-2: traditional clamp; 3-4: RS system; and 5-6: flask with screws. Flasks were immediately polymerised or bench stored for 6 h prior to polymerisation. Resin base-cast sets were sectioned at regions corresponding to the canines, first molars and posterior palatal zone. Gap discrepancies were measured at five points: right and left ridge crests, palatal midline, and right and left marginal limits of the flanges. An optical micrometer was used for measurement purposes. RESULTS: Data were submitted to anova, and the means compared by Tukey's test (alpha = 0.05). Results revealed significant differences in the flask closure technique, polymerisation time, section, and their interactions. Discrepancy values for the RS system and flask with screws were significantly lower than those related to the traditional clamp, regardless of whether resin polymerisation was immediate or delayed for 6 h. CONCLUSIONS: Flask closure methods should be considered when the denture base stability and comfort of the patient are being assessed during clinical use of the dentures.

Evaluation of the flexural mechanical properties of various thermoplastic denture base polymers.

This study evaluated the flexural mechanical properties of various thermoplastic denture base polymers (six polyamides, four acrylic resins, polyester, polypropylene, and polycarbonate) by three different testing conditions; specimens were tested in water bath at 37°C (Wet/Water, by ISO 20795-1), or in ambient air (Wet/Air) after being immersed in distilled water for 50 h, or after desiccation for 7 days (Dry/Air). The mean ultimate flexural strength (UFS) and flexural modulus (FM) for most products ranged from 27 to 61 MPa and from 611 to 1,783 MPa respectively, which failed to meet the minimum requirements of the international standard, except for polycarbonate (89 and 2,245 MPa). The mean UFS and FM values were ranked Dry/Air>Wet/Air>Wet/Water (p<0.05). In conclusion, the flexural mechanical properties of denture base polymers varied with the products and were significantly affected by the testing medium (air or water) and specimen conditions (wet or dry).

[Analysis of the movement of long axis and the distribution of principal stress in abutment tooth retained by conical telescope].

OBJECTIVE: To study the movement of long axis and the distribution of principal stress in the abutment teeth in removable partial denture which is retained by use of conical telescope. METHODS: An ideal three dimensional finite element model was constructed by using SCT image reconstruction technique, self-programming and ANSYS software. The static loads were applied. The displacement of the long axis and the distribution of the principal stress in the abutment teeth was analyzed. RESULTS: There is no statistic difference of displacenat and stress distribution among different three-dimensional finite element models. Generally, the abutment teeth move along the long axis itself. Similar stress distribution was observed in each three-dimensional finite element model. The maximal principal compressive stress was observed at the distal cervix of the second premolar. CONCLUSION: The abutment teeth can be well protected by use of conical telescope.

Measurement of the accuracy of dental working casts using a coordinate measuring machine.

Background/Aim: Dental impressions present a negative imprint of intraoral tissues of a patient which is, by pouring in gypsum, transferred extraorally on the working cast. Casting an accurate and precise working cast presents the first and very important step, since each of the following stages contributes to the overall error of the production process, which can lead to inadequately fitting dental restorations. The aim of this study was to promote and test a new model and technique for in vitro evaluation of the dental impression accuracy, as well as to asses the dimensional stability of impression material depending on the material bulk, and its effect on the accuracy of working casts. Methods: Impressions were made by the monophasic technique using the experimental master model. Custom trays with spacing of 1, 2 and 3 mm were constructed by rapid prototyping. The overall of 10 impressions were made with each custom tray. Working casts were made with gypsum type IV. Measurement of working casts was done 24 h later using a co-ordinate measuring machine. Results: The obtained results show that the working casts of all the three custom trays were in most cases significantly different in the transversal and sagittal planes in relation to the master model. The height of abutments was mainly unaffected. The degree of convergence showed certain significance in all the three custom trays, most pronounced in the tray with 3 mm spacing. Conclusion: The impression material bulk of 1­3 mm could provide accurate working casts when using the monophasic impression technique. The increase of the distance between abutment teeth influences the accuracy of working casts depending on the material bulk. [Projekat Ministarstva nauke Republike Srbije, br. TR 35020: Research and development of modelling methods and approaches in manufacturing of dental recoveries with the application of modern technologies and computer aided systems]

Color difference threshold determination for acrylic denture base resins.

This study aimed to set evaluation indicators, i.e., perceptibility and acceptability color difference thresholds, of color stability for acrylic denture base resins for a spectrophotometric assessing method, which offered an alternative to the visual method described in ISO 20795-1:2013. A total of 291 disk specimens 50±1 mm in diameter and 0.5±0.1 mm thick were prepared (ISO 20795-1:2013) and processed through radiation tests in an accelerated aging chamber (ISO 7491:2000) for increasing times of 0 to 42 hours. Color alterations were measured with a spectrophotometer and evaluated using the CIE L*a*b* colorimetric system. Color differences were calculated through the CIEDE2000 color difference formula. Thirty-two dental professionals without color vision deficiencies completed perceptibility and acceptability assessments under controlled conditions in vitro. An S-curve fitting procedure was used to analyze the 50:50% perceptibility and acceptability thresholds. Furthermore, perceptibility and acceptability against the differences of the three color attributes, lightness, chroma, and hue, were also investigated. According to the S-curve fitting procedure, the 50:50% perceptibility threshold was 1.71ΔE00 (r(2)=0.88) and the 50:50% acceptability threshold was 4.00 ΔE00 (r(2)=0.89). Within the limitations of this study, 1.71/4.00 ΔE00 could be used as perceptibility/acceptability thresholds for acrylic denture base resins.

Biological properties of denture base resins.

The biocompatibility and clinical efficacy of dental materials have been and continue to be a major concern of the dental profession. There are some cases of dental materials that have demonstrated problems with biocompatibility, for example, silicone soft liners. Although rare, allergic stomatitis caused by denture base resin is another example. High purity, very large polymer size, and nonoccurrence in nature significantly contribute to the acceptance of polymers used as biomaterials. New Federal Food and Drug Administration law and requirements of the specification and testing program of the American Dental Association will control the biocompatibility and clinical efficacy of dental biomaterials presently available to the dental profession.

Mechanical and thermal properties of hydroxyaptite filled poly (methyl methacrylate) heat processed denture base material.

The aim of this study was to evaluate the effect of powder-to-liquid ratio on the glass transition temperature (Tg) and the tensile properties of denture base material prepared from poly (methyl methacrylate) (PMMA) and hydroxyapatite (HA) previously treated with 3-trimethoxysilylpropyl methacrylate (gamma-MPS). Specimens for mechanical testing were prepared by adding composites powder (PMMA, BPO and HA) to the monomer (MMA and EGDMA) followed by hand mixing as in dental laboratory description usage. The glass transition temperature was studied by using differential scanning calorimetry (DSC). It was observed that the tensile properties and the Tg were affected by the powder-to-liquid ratio. The mechanical characterization of the materials were performed by using single edge notch-tension (SEN-T) specimens; the fracture toughness was slightly higher in formulation which contained HA filler compared to commercial denture base material.

Accuracy and porosity of denture bases submitted to two polymerization cycles.

In this study was verified accuracy and porosity of denture bases submitted to double cycles of polymerization. Thirty-Six master casts were made in dental stone from the impression of the metal die. They were divided in three groups. Denture bases of conventional acrylic resin were made on the casts of the groups I (GI-control) and II (GII), polymerized in water bath during 9 hours at 73 degrees C, and specific resin for microwave polymerization were made on the casts of the group III (GIII) and cured at 500 W during 3 minutes. Accuracy was verified by the weight of a impression material put between the denture base and cast die under load of 2 Kg. Afterwards, all the denture bases were relined by addition method and those of the group I and II were relined with the same material and polymerization technic. The denture bases of the group II were filled with the same resin, however cured in microwave oven with 500 W during 3 minutes. Accuracy was reevaluated. The porosity was verified by the immersion of the specimens in a solution of permanent black ink and the pores were counted through a stereo light microscope under magnification of 6.3 x. The average and standard deviation obtained for accuracy before and after the relining were respectively: GI: 0.63 +/- 0.02 - 0.66 +/- 0.03; GII: 0.63 +/- 0.03 - 0.68 +/- 0.03: GIII: 0.58 +/- 0.02 - 0.80 +/- 0.05. There was a significant difference (p < 0.05 as tested by Tukey) only for the group III. The average and standard deviation for porosity before and after the relining were respectively: GI: 8.33 +/- 0.63 - 9.25 +/- 0.85; GII: 8.17 +/- 0.98 - 10.08 +/- 1.63: GIII: 5.41 +/- 0.78 - 6.58 +/- 1.02. There was a significant difference (p < 0.05) for the group III before relining. The conclusion is that after relining, the conventional resin cured by water bath or microwaves energy showed a better adaptation, and the highest number of pores.

[Water absorption and water solubility of prosthetic materials]. / Protézis-alapanyagok vízfelvétele és vízoldékonysága.

Denture base materials investigated with using surface coating polymers fulfilled requirements of ISO Standards 1567 for water sorption and water solubility. The heat-cured polymer gave up water in a greater extent comparing to the cold-cured one. Further amount of water was retained by the surface treatment of coating. A five times increase of water solubility in samples of cold-cured resin was found comparing to the heat-cured ones. A significant decrease of water sorption was measured by using the surface coating.

Application of optical coherence tomography (OCT) to nondestructive inspection of dentures.

The porosity of denture base resins continues to be one of the undesirable characteristics of acrylic resins. It is commonly accepted that porosity of the denture not only often leads to denture fractures, but also may function as a reservoir of potential pathogens. The purpose of this study was to present the first OCT images of finished dentures using a new advanced-type OCT scanner we have developed, and to discuss the application of our new OCT system for nondestructive inspection of dentures. Ten newly fabricated full dentures of outpatient of the National Center for Geriatrics and Gerontology in Japan were selected for this study. Two types of denture base resins were used for inspection by OCT, which provided clear images of all the dentures examined. Internal structures, not visually detectable, inspection, can be observed using this OCT system. It is concluded that OCT can detect nonvisible internal structures in dentures, a finding not reported to date. OCT may, therefore, be an appropriate method for detecting interior defects in dentures nondestructively.