The properties of polymerizable luting cements

The properties of a polyacid-modified composite resin and two resin-modified luting cements have been studied. The polyacid-modified composite resin had the slowest setting reaction and, in this respect, it did not conform to the current international standard for luting cements. The compressive strength of all of the materials was studied after varying periods of storage from 24 h to 1 year. The polyacid-modified composite resin showed a distinct dip in strength at 1 month in all of the storage media, but otherwise it showed no significant variation with either age or storage medium. The resin-modified glass-ionomers showed variation at 24 h with storage medium (deionized water, 0.9% NaCl or 20 mmol dm(-3) lactic acid), but thereafter they showed little variation, until 1 year, when Vitremer luting showed a significant decline in strength in pure water. However, at 24 h and when stored in water, all of the materials had strengths that easily exceeded the minimum requirement of the current standard (70 MPa). They all took up water on storage, with diffusion coefficients ranging from 1.32 to 17. 19x10(-7) cm2 s(-1). These values were found to depend on whether the specimens were stored in pure water or in physiological saline. However, equilibrium water contents varied only slightly between water and saline. The polyacid-modified composite resin, Dyract-Cem, took up the least water, as well as showing the smallest variation in strength with age. By contrast, it was more difficult to mix than the other materials and the high viscosity of the paste led to the formation of voids and other imperfections in the specimens.

Three-body wear associated with three ceramics and enamel

STATEMENT OF PROBLEM

Wear of human enamel is a clinical concern whenever opposing teeth need to be restored using ceramic restorations.

PURPOSE

This in vitro study investigated wear of human enamel and 3 dental ceramics: a conventional porcelain (Vitadur Alpha), a low-fusing hydrothermal ceramic (Duceram-LFC), and a machinable ceramic (Vita Mark II) in a 3-body wear test.

MATERIAL AND METHODS

Thirty pairs of tooth-ceramic specimens were tested in a dental wear machine, under a standard load (40 N), rate (80 cycles/minute), and for 25,000 cycles in a simulated food slurry medium. Amount of wear was determined by measuring the height loss of the tooth and depth of wear track of the ceramic materials. Analysis of variance (ANOVA) was used to analyze the data, followed by Bonferroni multiple comparisons method to produce sets of simultaneous 95% confidence intervals.

RESULTS

ANOVA revealed significant differences between the groups for both enamel wear (P =. 002) and ceramic wear (P <.001). Further comparisons (95% CI significance level) revealed that the difference in enamel wear produced by Alpha porcelain and Duceram-LFC ceramic material was not statistically significant, whereas that produced by Vita Mark II ceramic was significantly less. Vita Mark II ceramic was significantly more resistant to wear than Alpha porcelain and Duceram-LFC ceramic. Furthermore, Alpha porcelain was significantly more resistant to wear than Duceram-LFC ceramic.

CONCLUSION

The abrasiveness of Alpha porcelain and Duceram-LFC ceramic was similar, yet both were significantly more abrasive than Vita Mark II ceramic. In addition, Vita Mark II was the most wear-resistant ceramic and Duceram-LFC ceramic the least resistant.

Size effect in resin/glass composite flexure strengths

The objective of this study was to test the hypothesis that composite restorative materials possess an elastic-brittle nature and therefore will exhibit a size effect for flexure strength data. The experimental material consisted of 20 wt% 60:40 BISGMA:TEGDMA, 10 wt% colloidal silica, and 70 wt% Sr glass and was cured by light irradiation. Two sizes of flexure specimens were fabricated: 3.2x1.6x35 mm, and 6.25x3.1x35 mm. Half of the specimens made were soaked to equilibrium weight gain in 50:50 ethanol:water. The fracture strengths were measured in four-point bending tests. The beams under load were modelled by the finite element package ABAQUS. A statistical fracture mechanics methodology embodied in a public domain computer program called CARES/LIFE, developed by NASA, utilized the ABAQUS input and the fracture strengths of the smaller specimens to predict the fracture strengths of the larger specimens. In making the computation it used an approach that combines a Weibull distribution of flaw size with Batdorf's fracture mechanical model for failure at a material flaw. Both the soaked and unsoaked specimens exhibited Weibull behaviour, with shape parameters ranging from 4.04 to 8.15. Soaking had a clearly detrimental effect on the strengths of specimens of both sizes, and produced a comparable percentage reduction in the estimated scale parameter of the fracture strength distribution. Both the soaked and unsoaked specimens also exhibited a clear and comparable size effect, i.e. the larger specimens had a fracture strength that was lower than that of the smaller specimens by roughly the same percentage. Moreover, the magnitude of the size effect was well predicted by the CARES/LIFE methodology for both the soaked and the dry specimens. The elastic-brittle character of both soaked and unsoaked composite specimens was validated by load-deflection data, the magnitude of the Weibull shape parameters of the observed fracture strength data (<10), and the observed effect of specimen size. The accuracy of CARES/LIFE in predicting the magnitude of the observed size effect in beams of two different sizes strongly suggests that CARES/LIFE will be useful for computation of failure probabilities for clinically relevant structures.

Influence of hex geometry and prosthetic table width on static and fatigue strength of dental implants

STATEMENT OF PROBLEM

Component fracture and screw loosening are prevalent concerns of contemporary dental implants.

PURPOSE

This laboratory investigation examined the influence of design factors such as the platform diameter and the hex height on the mechanical strength and quality of fit of the implant-abutment interface.

MATERIAL AND METHODS

Static and cyclic compressive bending tests were conducted on 4 and 5 mm diameter bone density-based implants. SEM evaluation of the implant-abutment interface was also conducted to assess quality of fit between the mating components.Results. The 5 mm diameter implant was stronger in both static and fatigue conditions than the 4 mm diameter implants. A comparison of the results to published literature indicated that both implants were equal to or superior to alternative prosthetic connections in an identical testing configuration.

CONCLUSION

Test results demonstrated the validity of wide diameter implants to reduce the likelihood of component fracture in contemporary dental implant systems.

Fluoride release, weight loss and erosive wear of modern aesthetic restoratives

OBJECTIVE

In this investigation, the in vitro sustained fluoride release, weight loss and erosive wear of three conventional glass ionomer cements (Fuji IX, ChemFil Superior, Ketac-Silver), three resin-modified glass ionomer cements (Fuji II LC, Vitremer, Photac-Fil), a polyacid-modified resin composite (Dyract), and a resin composite control material (Z100) were compared.

METHODS

The amounts of fluoride released and weight changes were measured for 12 weeks using a fluoride electrode with TISAB III buffer. After 12 weeks, the specimens were recharged with fluoride using 2 mL of 1.23% APF gel. The recharged specimens were assessed for the amounts of fluoride released and weight changes over another 12 weeks. At the end of the experiment, the specimens were examined with SEM and surface profilometry.

RESULTS

All materials, with the exception of Z100, showed the highest initial fluoride release rates during the first 2 days, dropping quickly over 2 weeks and becoming largely stabilised after 5 weeks, in an exponential mode. The recharging of the specimens with APF gel caused a large increase in the amounts of fluoride released during the first 2 days only. Analyses for all cements showed strong correlations between mean weight loss and cumulative fluoride release over a 5-week period following the application of the APF gel. SEM and surface profilometry found that roughness increased from the polyacid-modified resin composite to the conventional glass ionomer cements.

CONCLUSIONS

APF gel caused erosive wear of the glass ionomer cements especially, and the wear correlated well with the weight losses. To minimise surface erosion, APF gel should not be used on these cements, especially as the recharging effects are transitory.

An ex-vivo investigation into the effect of bracket displacement on the resistance to sliding

This ex-vivo study investigated the effect that repeated bracket displacement has on sliding friction and the magnitude of bracket displacement, and hence tooth movement, required to release bracket/archwire binding. The design consisted of an ex-vivo laboratory study. A jig was designed that allowed repeated displacement of a bracket to occur, while the resistance to sliding (friction) was measured using an Instron universal testing machine. One type of stainless steel bracket was used in conjunction with four archwire types (0.016-inch stainless steel, 0.019 x 0.025-inch stainless steel, 0.021 x 0.025-inch stainless steel, 0.019 x 0.025-inch beta-titanium) and four magnitudes of displacement. Repeated bracket displacement has a significant effect on the sliding resistance at the bracket/archwire interface (P < 0.001). The reduction in sliding resistance noted with displacement depended on the archwire. Over the range of displacements tested, there was an 85 and 80 per cent reduction associated with 0.021 x 0.025-inch and 0.019 x 0.025-inch stainless steel, respectively. For 0.019 x 0.025-inch beta-titanium and 0.016-inch stainless steel, these reductions were 27 and 19 per cent, respectively. The importance of true friction, given the likelihood of bracket and/or archwire displacements in vivo, may be lessened.

The effect of delayed light exposure on bond strength: light-cured resin-reinforced glass ionomer cement vs light-cured resin

Under clinical situations, the intervals between material mixing and light exposure during bracket bonding using light-cured resin-reinforced glass ionomer cement may vary for each individual bracket. This study evaluates the bond strength of light-cured resin-reinforced glass ionomer cement subjected to various time intervals between material mixing and subsequent light exposure. This investigation was conducted in two parts. The first part consisted of measuring the enamel surface temperature to define the conditions under which the second part of the study was carried out. One hundred fifteen subjects, 63 males and 52 females, participated in this study. The over-all mean temperature as measured with a noncontact infrared thermometer was 31.9 degrees C. The second part of this study assessed tensile and shear bond strengths of light-cured resin-reinforced glass ionomer cement subjected to immediate light exposure (time interval, 5 minutes) and bond strengths subjected to light exposure at 10, 20, and 40 minutes after material mixing. Light-cured resin-reinforced glass ionomer cement was then compared with light-cured composite resin. Mean tensile and shear bond strengths of light-cured resin-reinforced glass ionomer cement exposed after 40 minutes were 4.5 MPa and 20.5 MPa, respectively. This represented a reduction of approximately 20% when compared with the 5-minute group. Scheffé test showed no statistically significant differences between any two time intervals. Mean bond strengths of the light-cured resin decreased with time. Tensile and shear bond strengths of light-cured resin indicated high statistical significance within groups across time. It could therefore be concluded that the bond strength of light-cured resin-reinforced glass ionomer cement was not affected by the timing of visible light exposure; whereas, the bond strength of light-cured resin decreased as time intervals increased. Light-cured resin-reinforced glass ionomer cement may thus serve as an advantageous alternative to composite resin for bracket bonding.

Effect of three-liquid bonding agents on bond strength to a machine-milled ceramic material

This study determined the bond strengths of porcelain bonding systems joined to a ceramic material, with the aim of evaluating the effect of a catalyst for silane coupling as well as that of initiators for bonding agents. Two sizes of specimen were cut from ceramic blocks (Cerec 2 Vitablocs Mark II) and then primed with either a three-liquid self-curing bonding agent (Clearfil Porcelain Bond, CPB), a three-liquid dual-activated bonding agent (Clapearl Bonding Agent, CBA), or their components. Unprimed specimens were also used as controls. After conditioning, the two different sized plates were bonded together with a dual-cured luting agent (Clapearl DC). The specimens were stored in 37 degrees C water for 24 h and shear bond strengths were determined. Shear testing revealed that the greatest bond strength was generated in the group consisting of ceramic material primed with CBA and exposed for 20 s prior to luting. Bond strengths of the CPB-primed group and the CBA-primed group without pre-exposure were comparable, whereas the remaining groups resulted in significantly lower bond strengths than the other three groups. The priming effect was greatly enhanced when a catalyst was used with the silane coupler as compared with the application of silane alone. The results suggest that incorporation of an initiator and a catalyst for silane coupling into the bonding agent reciprocally enhances bonding between the luting agent and the ceramic material.

Evaluation of the marginal accuracy of different all-ceramic crown systems after simulation in the artificial mouth

The in vitro marginal fit of five different all-ceramic crown systems (In-Ceram, Empress staining technique, Empress veneering technique, Celay feldspathic system, Celay In-Ceram system) was evaluated before and after cyclic preloading in an artificial mouth. The crowns were adhesively luted to extracted natural maxillary incisors prepared with a 90 degrees shoulder. The results were compared to those for porcelain-fused-to-metal (PFM) crowns with circular porcelain-butt margins which were cemented with zinc phosphate cement. The analysis of the marginal discrepancies showed significant (P < 0.001) differences among the groups. Crown cementation increased the marginal gaps significantly (P < 0.01). Empress staining technique crowns showed the smallest marginal gaps (median 47 microm), followed by conventional In-Ceram crowns (median 60 microm) and Empress veneer technique crowns (median 62 microm). Celay In-Ceram crowns displayed marginal openings with a median of 78 microm, followed by Celay feldspathic crowns with a median of 99 microm. The marginal gap of the PFM control crowns showed a median of 64 microm. Ageing in the chewing simulator had no significant influence on the marginal fit of all specimens. The study indicates that all the tested all-ceramic crowns have clinically acceptable margins. However, in vivo investigations of all all-ceramic crown systems should be made before clinical routine.

Investment strength as a function of time and temperature

OBJECTIVES

The purpose of this investigation was to evaluate and compare the compressive strength characteristics of selected investments and to determine if these change as a function of time and temperature after mixing.

METHODS

Two phosphate-bonded investments and one gypsum-bonded investment were selected and 288 cylindrical specimens were fabricated (n = 9). Baseline compressive strength values were determined for each material at room temperature and compressive strength measurements were made for all materials at 700 degrees C, and at 872 degrees C only for phosphate-bonded investments. Each material was tested at 2, 6, 12, and 24 h after mixing. Specimens were heated at 15 degrees C/min and heat soaked for 5 min in a burnout oven at the applicable testing temperature. They were transferred to a preheated chamber on an Instron testing machine and subjected to a compressive load at a crosshead speed of 2 mm/min until failure. The resulting strength data are presented in MN/m2.

RESULTS

The gypsum-bonded investment did not exhibit marked differences in strength as a function of either time or temperature. The phosphate-bonded investments, on the other hand, demonstrated considerable changes in compressive strength and differed in compressive strength characteristics at 700 degrees C and 872 degrees C respectively.

CONCLUSIONS

At elevated temperatures, all materials approximated peak strength 2 h after initial mixing. At room temperature, the phosphate-bonded investments in this study were not significantly stronger than the gypsum-bonded investment material. However, they exhibited increased compressive strength as a function of time and temperature that was considerably higher than that exhibited by the gypsum-bonded investments.

Shear bond strengths of two resin-modified glass ionomer cements to porcelain

Shear bond strength of a composite resin adhesive (Concise) and two resin-modified glass ionomer cements (Fuji Ortho LC and Geristore) bonded to porcelain surface was tested. Orthodontic brackets were bonded to 120 porcelain disks (Finesse) etched with 9% HF. Samples were divided into six groups: (1) Concise, (2) Concise/silane, (3) Geristore, (4) Geristore/silane, (5) Fuji, (6) Fuji/silane. No statistical difference in mean shear bond strength was found between silanated Concise (15.8 MPa), Geristore (19.4 MPa), and Fuji (18.5 MPa) groups, which were significantly higher than nonsilanated groups. Porcelain fracture was observed in all silanated groups and nonsilanated Geristore group. We conclude that (1) silane increases bond strength to porcelain significantly for composite resin and resin-modified glass ionomer cement, (2) Concise, Geristore, and Fuji Ortho LC provide comparable shear bond strength to porcelain.

Force characteristics of nickel-titanium tension coil springs

Nickel-titanium closed coil springs are commonly used for space closure. The springs possess a high resistance to permanent deformation and the potential for relatively constant force delivery. A study was designed to determine whether relatively constant forces can be delivered and whether the force magnitudes approach the manufacturer's targeted force values. Heavy, medium, and light springs were activated 15 mm at temperatures that ranged from 15 degrees C to 60 degrees C. The forces were measured during deactivation with a specially constructed force transducer temperature chamber. Relatively constant forces can be achieved with an over-activation procedure that allows relaxation to the desired activation. The light springs delivered forces that were near the targeted force; no difference was found between the heavy and medium springs in the constant force range. The force magnitudes varied markedly depending on mouth temperature.

Stiffness, elastic limit, and strength of newer types of endodontic posts

OBJECTIVES

To determine the stiffness, elastic limit, and strength of a selection of endodontic posts recently introduced onto the market.

METHODS

Endodontic posts of zirconia (Biopost, Cerapost), titanium (PCR), and carbon fiber (Composipost) were cemented in a brass block and loaded at an angle of 45 degrees in an Instron Testing Machine. From the recorded relationship between force and deflection the three mechanical properties were determined (n = 10 in each group).

RESULTS

The ceramic posts were very stiff and strong, with no plastic behavior. The PCR post was as strong as, but less stiff than, the ceramic posts. Composipost had the lowest values for stiffness, elastic limit, and strength of the posts investigated.

CONCLUSION

The posts under investigation differed significantly with respect to mechanical properties.

Color stability and colorant effect on maxillofacial elastomers. Part III: weathering effect on color

STATEMENT OF PROBLEM

Maxillofacial prostheses are serviceable for approximately 6 months, after which they need to be refabricated because of the deterioration of color and physical properties.

PURPOSE

This third article in a 3-part series evaluated the color stability of commonly used colorant-elastomer combinations as a result of exposure to weathering.

METHODS AND MATERIAL

Fifteen specimens were fabricated for each of the 3 elastomers (Silastic medical adhesive type A, Silastic 4-4210, and Silicone A-2186) and 6 colorants (dry earth pigments, rayon fiber flocking, artist's oil paints, kaolin, liquid cosmetics, and no colorant) for a total of 270 specimens (18 groups of 15 specimens). The 15 specimens of each elastomer-colorant combination were separated into 3 test condition groups (control, time passage, and natural weathering) of 5 specimens per test condition group. Control specimens were evaluated within 1 month of fabrication. The time passage group was sealed in glass containers and kept in the dark for 6 months before testing. The natural weathering groups were placed on the roof of the dental school for 6 months and exposed to sunlight and weathering. Color and optical density data for each elastomer-colorant combination were subjected to a 1-way analysis of variance to examine effects among test conditions (control, time passage, and weathering). When significant differences were observed, the Student-Newman-Keuls multiple range test was performed to identify differences in elastomer-colorant combinations among each test condition at a significance level of .05.

RESULTS

Changes in color, as a result of weathering, were noted in many of the colorant-elastomer combinations. Also, color change occurred not only to the colored, but also to uncolored materials over time without exposure to weathering.

CONCLUSION

Clinically, the addition of colorants could have a stabilizing effect on the elastomer color when it is exposed to weathering.

Storage stability of dental luting agents

STATEMENT OF PROBLEM

Mechanical properties are considered to be important to the clinical success of dental cements. Storage time of dental luting agents (dental cements) before use and at the manufacturer may have substantial effects on their properties.

PURPOSE

This study observed and investigated the changes in the properties of water-based dental luting agents over time and on exposure to various environmental conditions.

METHODS AND MATERIAL

Properties of zinc phosphate, zinc polycarboxylate, and glass-ionomer luting agents were recorded at baseline and at intervals past 84 months, whereas the materials were exposed to controlled storage conditions as well as various conditions typical of clinical situations. Tests included viscosity, diametral tensile strength, ultimate compressive strength, rigidity, hardness, and working/setting times.

RESULTS

Viscosity increased over 24 to 60 months for the acidic polymer liquids (zinc polycarboxylate and glass ionomer). Tensile and compressive strengths began to decrease after 36 months for all cements, most acutely for glass ionomer.

CONCLUSION

Changes in critical properties on aging may not be noticed by the clinician and yet may impact clinical success. These data emphasize the importance of storage stability for perishable dental materials and portend the need for standardized tests.

Color stability of heat-polymerized and autopolymerized soft denture liners

STATEMENT OF PROBLEM

The physical and mechanical properties of soft denture liners are influenced by a number of factors. Aging is one factor that has an effect on the characteristics of polymers. The resistance of denture liners to aging can potentially influence the color stability of these liners.

PURPOSE

This study attempted to determine the color stability of soft liners by subjecting them to an in vitro accelerated aging test.

MATERIAL AND METHODS

Two autopolymerizing and 3 heat polymerizing commercially available soft denture lining materials were tested. Seven sample disks were made of each material according to manufacturers' instructions. Color measurements were recorded before and after the accelerated aging process. Color changes (DeltaE) were calculated by measuring tristimulus values at several wavelengths in the visual spectrum with the use of Commission International de l'Eclairge Lab (CIE-LAB) uniform color scale.

RESULTS

Kruskall-Wallis variance analysis and Mann-Whitney U-Wilcoxon rank sum W tests were performed for statistical comparison of the DeltaE values. Ufigel P and Simpa liner materials showed significantly greater color changes than the other tested materials. There were no significant differences in color change among Ufigel L-Molloplast B, Ufigel L-Flexor, and Molloplast B-Flexor liner materials. However, a significant difference in color change was found between Ufigel P and Simpa liners.

CONCLUSION

Heat-polymerized soft liners were more color stable than autopolymerized soft liners. No significant differences were found among heat polymerized materials; however, significant differences in color change were found between the 2 autopolymerized liners.

Wettability of silicone rubber maxillofacial prosthetic materials

STATEMENT OF PROBLEM

Maxillofacial prosthetic materials that contact skin or mucosa should have good wettability. A material that is easily wetted will form a superior lubricating layer between the supporting tissues and, thus, reduce friction and patient discomfort. The surface energy of a maxillofacial prosthetic material will give an indication of the amount of energy available for adhesion and of the susceptibility of the material to bacterial adhesion.

PURPOSE

This study evaluated the wettability and surface energies of a range of commercially available silicone rubber maxillofacial prosthetic materials.

MATERIAL AND METHODS

Contact angles and surface energies were measured by using a dynamic contact angle measuring technique. Four commonly used silicone maxillofacial materials were tested and their properties compared with those of an acrylic resin denture base material and a widely used denture soft lining material.

RESULTS

There were no significant differences in the wettability of the silicone rubber materials. All materials were significantly less wetted than the denture acrylic resin material. There were no significant differences in the surface energies of the silicone rubber materials, but all were significantly lower than denture acrylic resin material.

CONCLUSIONS

The Cahn dynamic contact angle analyzer was a quick and reproducible method for determining the contact angles and surface energies of maxillofacial materials. Further work is needed to improve the wettability of silicone rubber materials used for maxillofacial prostheses, thus, reducing their potential to produce friction with tissues.

Wear of low-fusing dental porcelains

STATEMENT OF PROBLEM

The resistance of wear of dental porcelains is of concern when selecting a material for metal-ceramic and all-ceramic restorations.

PURPOSE

An in vitro investigation was performed to rank a number of dental porcelains with respect to their wear-resistance properties. Material and methods. The wear test of 9 dentin porcelains was carried out on a wear test machine and the amount of wear was measured as the reduction of height of the specimens. The surface hardness and surface roughness were also considered.

RESULTS

Resistance of wear was lowest for Finesse porcelain, and highest for Creation porcelain. Surface hardness values of the porcelain were quite similar for all porcelains but could be classified into 3 groups with Finesses and Vita Alpha porcelains as the softest and Creation porcelain as the hardest material.

CONCLUSIONS

The low-fusing porcelain Finesse showed less abrasion resistance in comparison with Ducera Gold and Ti-Ceram porcelains. Among the high-fusing types, Vita Alpha porcelain was more wear-disposed than the others.

Damage modes in dental layer structures

Natural teeth (enamel/dentin) and most restorations are essentially layered structures. This study examines the hypothesis that coating thickness and coating/substrate mismatch are key factors in the determination of contact-induced damage in clinically relevant bilayer composites. Accordingly, we study crack patterns in two model "coating/substrate" bilayer systems conceived to simulate crown and tooth structures, at opposite extremes of elastic/plastic mismatch: porcelain on glass-infiltrated alumina ("soft/hard"); and glass-ceramic on resin composite ("hard/soft"). Hertzian contacts are used to investigate the evolution of fracture damage in the coating layers, as functions of contact load and coating thickness. The crack patterns differ radically in the two bilayer systems: In the porcelain coatings, cone cracks initiate at the coating top surface; in the glass-ceramic coatings, cone cracks again initiate at the top surface, but additional, upward-extending transverse cracks initiate at the internal coating/substrate interface, with the latter dominant. The substrate is thereby shown to have a profound influence on the damage evolution to ultimate failure in the bilayer systems. However, the cracks are highly stabilized in both systems, with wide ranges between the loads to initiate first cracking and to cause final failure, implying damage-tolerant structures. Finite element modeling is used to evaluate the tensile stresses responsible for the different crack types. The clinical relevance of these observations is considered.

Evaluation of wear of selected prosthetic materials in contact with enamel and dentin

STATEMENT OF PROBLEM

The proper choice of prosthetic materials for restoration of occlusal surfaces is important for the protection of the hard tissue of the opposing teeth and for regular functioning of the stomatognathic system.

PURPOSE

This study evaluated the resistance to wear of gold, Palliag M, Dentacolor, Chromasit, and Stellon materials and the enamel and dentin of natural teeth during functional contacts in an artificial oral environment with and without an abrasion material (Al2 O3 ).

MATERIAL AND METHODS

The 5 nonceramic prosthetic materials used in the restoration of occlusal surfaces of fixed partial dentures were examined in contact with enamel and dentin during a simulated masticatory process in 2 environments (with and without abrasion). Wear was evaluated by means of a special device designed for wear examination.

RESULTS

The findings highlighted that the wear of nonceramic prosthetic materials in contact with enamel or dentin was similar. Dentin showed greater wear when in contact with these materials than with the enamel surface.

CONCLUSION

Artificial environmental factors significantly influenced the wear of the hard tissue of teeth, especially when in contact with materials having low microhardness.

Color stability of denture base acrylic resins in three food colorants

STATEMENT OF PROBLEM

Three commonly used artificial dyes in food technology are erythrosine, tartrazine, and sunset yellow.

PURPOSE

The color stability of 5 commercially available denture base acrylic resins (QC-20, Meliodent, Trevalon, Trevalon High, and Lucitone) was studied in vitro.

METHODS

The specimens were exposed to 3% erythrosine, tartrazine, and sunset yellow solutions at 23 degrees C +/- 1 degrees C. Color changes were determined with a computer-controlled spectrophotometer. Five specimens from each material were processed, and initial color measurements were made after 1, 3, and 6 months of exposure to the staining solutions. Finally, the color stability was quantitatively measured again, and color differences (DeltaE) were calculated.

RESULTS

For the observations made in 1, 3, and 6 months intervals, the specimens that exhibited the least color change were in the sunset yellow solution. The greatest color changes observed according to the National Bureau of Standards unit system were Lucitone (2.71) in erythrosine solution, Lucitone (2.54), QC-20 (1.71) in tartrazine solution, and QC-20 (1.66) in sunset yellow solution. The changes in the other acrylic resins in the 3 solutions were slight and at trace level.

CONCLUSIONS

All materials tested were acceptable from the standpoint of color stability for long-term exposure to these food colorants.

Long-term durability of dentin bonds made with a self-etching primer, in vivo

The long-term durability of bonds between adhesive resins and dentin is of significant importance for the longevity of bonded restorations. We carried out an in vivo one-year study to evaluate the durability of resin-dentin bonds in the oral cavity, as well as to test the hypothesis that the adhesive interface would show morphological changes in vivo over time. Very shallow saucer-shaped dentin cavities were prepared in 12 intact teeth of one Japanese monkey (Macaca fuscata) under general anesthesia. The cavities were restored with Clearfil Liner Bond II and Clearfil Photo Posterior resin composite. The teeth were extracted at three different times: immediately, and 180 and 360 days after placement of the restorations. One day after the monkey was killed, specimens of the three time periods were subjected to the micro-tensile bond test at a crosshead speed of 1 mm/min. The surfaces of the failed bonds were observed under a field emission scanning electron microscope (FE-SEM). Bond strength measurements in this study were successfully performed and were stable at approximately 19 MPa during the one-year testing. Scanning electron microscopic observations of the failed surfaces revealed, at the top of the hybrid layer and within the adhesive resin, porosity which increased over time. Long-term bonds can be assessed in vivo by the combined evaluation of the microtensile bond strength and SEM morphological examination of the adhesive interface.

[A reliability growth assessment method and its application in the development of equipment in space cabin]

Objective. To assess and predict reliability of an equipment dynamically by making full use of various test informations in the development of products. Method. A new reliability growth assessment method based on army material system analysis activity (AMSAA) model was developed. The method is composed of the AMSAA model and test data conversion technology. Result. The assessment and prediction results of a space-borne equipment conform to its expectations. Conclusion. It is suggested that this method should be further researched and popularized.

Effects of dentin depth and cavity configuration on bond strength

During polymerization of resin composites, shrinkage stresses compete with resin-dentin bonds in a manner that can cause failure of the bond, depending upon the configuration of the cavity, its depth, and the restorative technique. The hypothesis tested in this study was that the effect of cavity configuration (C) and remaining dentin thickness (RDT) influence resin bond strength to the dentin of Class I cavity floors. The occlusal enamel was ground to expose a flat superficial dentin surface as a control (superficial dentin, C-factor = 1) in human extracted third molars. Cavities 3 mm long x 4 mm wide were prepared to a depth 2 mm below the ground dentin surfaces (deep dentin within cavity floor, C-factor = 3). To assess the relationship between C-factor and RDT, we removed the walls of cavities, making a deep flat surface for bonding (deep dentin, C-factor = 1). The teeth were restored with either Clearfil Liner Bond II (LB II), One-Step (OS), or Super-Bond D Liner (DL), followed by Clearfil Photo Posterior resin composite. After 24 hrs' storage in water, the teeth were sectioned vertically into 3 or 4 slabs (0.7 mm thick) and trimmed for the micro-tensile bond test so that we could determine the strength of the resin bonds to the pulpal floor. All groups gave high bond strengths to superficial dentin, but OS and DL gave significantly lower bond strengths to flat deep dentin when the C-factor was 1. When the C-factor was increased to 3 by the creation of a three-dimensional cavity preparation, the bond strengths of all materials fell (range, 21 to 35%), but the difference was significant (p < 0.05) only with DL. SEM observations of failure patterns showed that specimens with high bond strengths tended to exhibit cohesive failures within the hybrid layer, while specimens exhibiting low bond strengths showed failures at the top of the hybrid layer. Some adhesives do not bond well to deep dentin, making them more susceptible to polymerization shrinkage stress that develops in cavities with high C-factors.

Color stability and colorant effect on maxillofacial elastomers. Part I: colorant effect on physical properties

STATEMENT OF PROBLEM

The average clinical life span of a maxillofacial prosthesis is approximately 6 months, at which point it needs to be refabricated, mainly because of degradation of the color and physical properties of the prosthesis.

PURPOSE

This first part of a 3-part study evaluated the effect of coloring agents on the physical properties of maxillofacial elastomers.

METHODS AND MATERIAL

Five dumbbell-shaped and 5 trouser-shaped specimens were fabricated for each of the combinations of the 3 elastomers (Silastic medical adhesive type A, Silastic 4-4210, and Silicone A-2186) and 6 colorants (dry earth pigments, rayon fiber flocking, artist's oil paints, kaolin, liquid cosmetics, and no-colorants), for a total of 180 specimens. Evaluations of hardness and tear strength were made with the trouser-shaped specimens. Evaluations of the ultimate tensile strength and the percentage elongation were made with the dumbbell-shaped specimens. A within elastomer analysis compared the 6 colorants using a 1-way analysis of variance for each of the 4 physical properties. When significant differences were observed, the Student-Newman-Keuls multiple range test was used to identify differences between groups at a significance level of.05.

RESULTS

Physical properties of maxillofacial elastomers were changed by the incorporation of coloring agents. Dry earth pigments, kaolin, and rayon flocking acted as a solid filler without bonding to the Silicone, and artists' oils and liquid cosmetics acted as a liquid phase without bonding to the silicone matrix.

CONCLUSION

No clearly superior colorant-elastomer combination was demonstrated in all the tests in this study.