Clinical and SEM evaluations of three compomer systems in Class V carious lesions.

The aim of this study was to evaluate the clinical performance of three compomer systems: Compoglass F, Dyract AP and F2000 in Class V carious lesions. In addition, some restorations were placed in teeth scheduled for extractions in order to evaluate the compomer/dentin interface with the scanning electron microscope (SEM). A total of 175 Class V carious lesions having the gingival margins extending into cementum were restored with the tested materials. Each restoration was clinically evaluated immediately after placement, after 1 year and after 2 years using the USPHS criteria. After 1 year no loss of restoration was recorded. No significant difference between the materials was observed for any evaluation category. After 2 years, one Compoglass and two Dyract restorations were lost. No loss of F2000 was reported. No significant difference was reported between the tested materials and between the 2-year data and baseline ones. A well-defined hybrid layer as well as resin tags was seen at the interface between the tested materials and vital dentin.

3D-finite element analyses of cusp movements in a human upper premolar, restored with adhesive resin-based composites.

The combination of diverse materials and complex geometry makes stress distribution analysis in teeth very complicated. Simulation in a computerized model might enable a study of the simultaneous interaction of the many variables. A 3D solid model of a human maxillary premolar was prepared and exported into a 3D-finite element model (FEM). Additionally, a generic class II MOD cavity preparation and restoration was simulated in the FEM model by a proper choice of the mesh volumes. A validation procedure of the FEM model was executed based on a comparison of theoretical calculations and experimental data. Different rigidities were assigned to the adhesive system and restorative materials. Two different stress conditions were simulated: (a) stresses arising from the polymerization shrinkage and (b) stresses resulting from shrinkage stress in combination with vertical occlusal loading. Three different cases were analyzed: a sound tooth, a tooth with a class II MOD cavity, adhesively restored with a high (25 GPa) and one with a low (12.5GPa) elastic modulus composite. The cusp movements induced by polymerization stress and (over)-functional occlusal loading were evaluated. While cusp displacement was higher for the more rigid composites due to the pre-stressing from polymerization shrinkage, cusp movements turned out to be lower for the more flexible composites in case the restored tooth which was stressed by the occlusal loading. This preliminary study by 3D FEA on adhesively restored teeth with a class II MOD cavity indicated that Young's modulus values of the restorative materials play an essential role in the success of the restoration. Premature failure due to stresses arising from polymerization shrinkage and occlusal loading can be prevented by proper selection and combination of materials.

Effect of enamel pretreatments on bond strength of compomer.

OBJECTIVES: The objective of this study was to evaluate the effect of various enamel surface treatments on the bond strength of a compomer to enamel. METHODS: Ground bovine enamel specimens were divided into four groups. A compomer (F2000, 3M) was bonded to the specimens using different enamel surface treatments. Two groups examined the effect of application of the F2000 self-etching primer/adhesive (3M) with respect to static or dynamic priming. A third examined use of the primer/adhesive after phosphoric acid etching, and the fourth (control) group provided bond strength of the compomer to phosphoric acid etched enamel with a resin bonding system (Single Bond, 3M). Shear bond strengths for the specimens were measured after 24h storage in water at 37 degrees C. Effects of the various surface treatments on enamel were examined by SEM. RESULTS: Significant differences in bond strength of compomer to enamel were found that were related to the various surface pretreatments. Dynamic priming resulted in higher enamel bond strengths than static priming, and the best bond strengths were obtained when the enamel was etched with phosphoric acid. SEM analysis showed that depth of etch and resin penetration was also directly related to the bond strengths measured. SIGNIFICANCE: Bond strength of compomer to enamel is significantly affected by the method of pretreatment of the enamel.

A fluoride-releasing composite for dental applications.

OBJECTIVES: The aim of this work was to attempt formulation of a dental composite, for investigational purposes, which will release fluoride at a rate comparable to glass ionomer materials, while retaining physical properties adequate for potential use in a number of dental applications. METHODS: An organic fluoride material, tetrabutylammonium tetrafluoroborate (TBATFB), was incorporated into a hydrophilic monomer system made of 2,2-bis[4-(2-hydroxy-3-methacroyloxypropoxy)phenyl]-propane (BisGMA) and 2-hydroxyethyl methacrylate (HEMA). This fluoride-containing monomer system was filled with silane treated quartz to 81 wt%. The resultant material was tested for fluoride release, physical strength properties and bond strength to enamel, each in comparison to control materials. RESULTS: Cumulative fluoride release from the experimental composite was approximately linear over time and comparable to glass ionomer materials. The long term rate of release exceeded several of the glass ionomer materials tested. Diametral tensile strength was comparable to resin-modified glass ionomer (RMGI) materials. Bond strength to etched enamel was statistically equivalent to a commercially available hybrid composite control. SIGNIFICANCE: The experimental resin-based composite material could potentially be used as an alternative to glass ionomer materials in non-stress bearing restorative applications and for orthodontic bracket bonding, where high fluoride release is desirable.

Fracture strength of different core build-up designs.

PURPOSE: To investigate the resistance to fracture of three crown and core combinations, made with different core build-up systems on human teeth, using a standardized test method. MATERIALS AND METHODS: The anatomic crowns were removed from six groups (n = 8) of endodontically treated, human premolars in conformity with a brass master dye, using the Celay duplicating device. Three of these groups were provided with a core build-up with-out an endodontic post (Ti-Core, Photo-Core and Ketac-Molar); two other groups were provided with a core build-up with an endodontic post (a cast post and core, and a composite with a silica post), and one group was not provided with a core. All groups were then prepared in conformity with a master dye. This preparation ended in axial dentin, 2 mm apical from the core build-up. Thus in the sixth group (premolars without a core build-up), this preparation only affected the 2 mm axial dentin (ferrule), which resulted in a small retention area (2 mm axial dentine) for the crown. Finally, a standard crown was cemented and 24 hrs afterwards loaded until fracture in an Instron testing machine at an angle of 45 degrees. For comparison with the standardized procedures, a seventh group (Photo-core without post) was prepared manually, with dimensions similar to the standardized groups. RESULTS: Within the standardized test set-up no significant difference in strength between the 6 groups could be demonstrated. Only in the case of the hand-made Photo-Core and the cast core was the fracture load significantly different (P= 0.01). In addition, this hand-made group displayed a larger standard deviation than the standardized groups. Taking into consideration the fact that the maximum bite force in the bicuspid region is about 580 N, these tests showed that within the limitations of the standardized test set-up, where the samples had no history of fatiguing, a core build-up without a post is an alternative for the conventional cast core. In this study, the fabrication of the standardized samples proved to be accurate, fast and economical. This method also enables the evaluation of the performance of core and crown design combinations in relationship to the amount of remaining tooth structure by using different master dies and, moreover, to reduce the high standard deviation in this kind of research.

Demineralization of enamel in relation to the fluoride release of materials.

PURPOSE: To examine the reduction in enamel demineralization provided by fluoride release from a conventional glass-ionomer, a resin-modified glass-ionomer and an experimental fluoride-releasing resin-based composite compared to a conventional resin-based composite control, and to correlate the level of fluoride release with demineralization. MATERIALS AND METHODS: Enamel surfaces of extracted human incisors had a 0.4 mm thick layer of the specified test material carefully placed in a band across the mid-facial enamel to simulate a cement layer beneath an orthodontic bracket. The top surface of the test material was covered with nail varnish, leaving only the edges of the material exposed to release fluoride. The teeth were additionally covered with nail varnish to within 1 mm of the test material. Each group of teeth was placed into separate volumes of unstirred demineralizing solution at a pH of 4.7 for 4 days. The specimens were sectioned and examined by polarized light microscopy. Lesion areas were measured at distances from 100-800 microm away from the test material. Fluoride release for the test materials was measured for periods up to 5 months. RESULTS: All of the fluoride-releasing materials demonstrated a statistically significant (P< 0.05) degree of protection of enamel from demineralization compared to the non-fluoride control material. The degree of protection was greatest near the material, but lesion areas increased with distance in an inverse relationship to the amount of fluoride release. Lesions were displaced from the region near the materials and the mean displacement was directly related to amount of fluoride release. The mean lesion areas for each distance decreased with the logarithm of the cumulative fluoride release.

The influence of surface roughness on porcelain strength.

OBJECTIVES: In order to adjust occlusion, the functional surfaces of porcelain restorations are often ground and mechanical machining is even an essential part of the CAD-CAM process for these restorations. The aim of this study was to investigate the influence of the finishing procedures on the biaxial flexure strength of four commercial porcelains. METHODS: Four commercial porcelains of which two are used for metal-ceramic restorations (Flexo Ceram Dentine and Vita VM K68) and two for veneers and inlays (Duceram LFC Dentine and Cerinate BODY) are used in this study. For each porcelain, sixty discs (Ø = 22 mm, h = +/- 2.0 mm) were produced using twelve different finishing procedures. Twenty discs were left untreated, twenty discs were milled, using a high-speed diamond disc, and twenty discs were machined in a high-speed grinding/polishing device. Half of the samples were glazed. In each of these six groups, half of the samples were stored for 16 h at 80 degrees C in a 4% acetic acid solution. The biaxial flexure strength was determined using the ball-on-ring method. In each group the roughness of the surface was determined and examined via SEM. RESULTS: With the exception of Flexo Ceram Dentine, a significant correlation was found between the roughness of the surface and the biaxial strength: the smoother the surface, the stronger the sample. The differences in biaxial strength may be attributed to the stress concentration of an applied load due to the roughness of the surface caused by mechanical finishing or chemical action. The fact that the strength of Flexo Ceram Dentine was not affected by the different surface treatments is probably due to the size of the leucite particles, which apparently induce more stress concentration than the surface flaws and the roughness of the surface. SIGNIFICANCE: It was concluded that surface roughness determines the strength of a porcelain material, except where the inner structure of the material causes greater stress concentration than that caused by the combination of surface roughness and surface flaws.

Influence of ceramic and cement thickness on the masking of various types of opaque posts.

STATEMENT OF PROBLEM: Dark-colored posts may negatively affect the esthetics of all-ceramic single unit crowns as the thin layers of luting cement may not be sufficiently opaque. PURPOSE: This in vitro study evaluated the influence of the color of 2 commercially available nonmetallic opaque posts (carbon fiber and zirconia) and an experimental esthetic post, and the shade and thickness of luting cements on the esthetics of all-ceramic restorations. MATERIAL AND METHODS: Sample disks at several thickness values were made in glass-ceramic (IPS-Empress), an experimental ceramic, a zirconia, a carbon fiber post material, a resin composite material (Z100) as reference, and a luting cement (Variolink II). A laboratory procedure, with 3 possible combinations of stapling the disks, was used. This was performed for 4 substrates, 3 cement colors at 2 thickness values, and 3 heights of ceramic disks. For each combination, the shift in color was measured with a spectrophotometer. Readings were performed for 3 conditions: (1) ability of ceramic to mask the aspect of the abutment in relation to its thickness (1.0, 1.5, or 2.0 mm); (2) effect of a change in cement color (W, Y, or B) on the final color of the ceramic; and (3) influence of cement film thickness (0.1 or 0.2 mm) on the final color of the ceramic. RESULTS: When ceramic thickness was 1 mm, all other variables were visually appreciable. For ceramic thickness of 1.5 mm, color differences decreased and most differences were appreciable only with laboratory instruments. For ceramic thickness of 2.0 mm, there were no detectable, clinically relevant differences. CONCLUSION: The final esthetic result of the all-ceramic IPS-Empress glass-ceramic restoration was not affected by the presence of different substrates with different colors when the thickness was more than 2.0 mm. When ceramic thickness decreases to 1.5 mm, it is advised to take the substrate aspects into consideration. If the ceramic thickness is less than 1.0 mm, the use of a full ceramic crown is contraindicated because color matching of the abutment is required to ensure an acceptable esthetic result. Differences in cement thickness (0.1 or 0.2 mm) may slightly affect the final result. As this parameter can be controlled by the operator only to a certain extent, it cannot be considered as a procedure to correct color. Availability of different cement shades allows only minor esthetic corrections, which might be instrumentally detectable but are clinically not relevant.

Visco-elastic parameters of dental restorative materials during setting.

Contraction stresses generated in restoratives during setting are among the major problems in adhesive dentistry, since they often result in loss of adhesion from the cavity walls or in post-operative pain. The rate of stress development and the ultimate magnitude of the stress, which determine the seriousness of these problems, depend on the relatively unknown visco-elastic behavior of the restoratives during setting. The aim of this study was to determine the visco-elastic parameters during setting, to aid our understanding of the process of contraction stress development. A dynamic mechanical method was used in which the materials were subjected to periodic strain cycles in a universal testing machine during the first 60 min of setting. The visco-elastic parameters (viscosity eta and Young's modulus E) were calculated by analysis of the experimental stress-strain data with a simple mechanical model according to Maxwell. Two restorative materials from different classes were investigated: a two-paste resin composite and a conventional glass-ionomer cement. A comparison of the results showed significant differences in the development of viscosity and stiffness in the early stage of setting. The resultant relaxation time (eta/E) of the glass ionomer remained at a low level during the first 15 min, whereas that of the resin composite increased markedly. This is of clinical importance, since it implies that, during the early setting stage, glass ionomers are better capable of reducing the contraction stresses than resin composites, thus increasing the likelihood that the bond with the cavity walls will form and survive during setting.

Light-curing units, polymerization, and clinical implications.

A comprehensive review of the techniques of light initiation of dental composites with special attention to polymerization efficacy and shrinkage stress control is presented. Optimal setting of an average-sized composite restoration requires a certain quantity of light energy. The amount of energy depends on the characteristics of the light source used and the time of irradiation. Swift conversion with high-energy lamps is proportionally accompanied by rapid hardening and could negatively affect the marginal integrity of the adhesive restoration. However, preliminary stress measurements did not show an increase in the rate of stress development. Full conversion in the deeper areas of the restoration may not be possible if defects occur in one or more of the various lamp components. Prolonging the irradiation time could reduce this risk. A regular check on the energy output of the light source is recommended. Although the high-energy light sources with their extremely short irradiation times should be used in a more critical way than the conventional light sources, they save a considerable amount of time and facilitate bond application, leading to better restorations where isolation control need not be optimal.

Marginal integrity after fatigue loading of ceramic inlay restorations luted with three different cements.

PURPOSE: To evaluate the effect of fatigue loading on the quality of the cement around the margins of adhesive ceramic MOD inlays. MATERIALS AND METHODS: Non-metal IPS Corum porcelain inlays were made and cemented either with the resin-based composite Variolink, the compomer Dyract-Cem or the glass-ionomer cement Ketac-Cem. For each system, half of the specimens were subjected to thermal and mechanical load cycling to simulate fatigue loading. All teeth were then sectioned and evaluated by SEM on topography and by optical microscopy for dye penetration. RESULTS: Before loading, all systems showed good marginal integrity. After loading, inlays cemented with Variolink and Dyract-Cem showed no significant (P > 0.05) increase in marginal break down, however restorations cemented with Ketac-Cem, showed a significant (P < 0.05) deterioration at the margins.

Role of hybridization on marginal leakage and bond strength.

PURPOSE: To evaluate whether the hybrid layer significantly contributes to the seal of adhesive Class V restorations placed in vivo and to the bond strength obtained in vitro. MATERIALS AND METHODS: 20 periodontally severely compromised anterior teeth were selected for the leakage study. In each tooth, one round 3 mm wide V-shaped cavity with a maximum depth of 2 mm was prepared on the labial surface crossing the cementum-enamel junction. The sample teeth were randomly divided into two groups of ten samples each: in Group 1, Scotchbond Multi-Purpose Plus was applied in combination with Z-100 following the manufacturer's instructions. In Group 2 the bonding procedure was the same as in Group 1, except that after the phosphoric acid treatment, the cavities were treated with 5% NaOCl for 2 min. Following a period of clinical service of 60-90 days, the teeth were extracted and processed for the leakage test. RESULTS: At the enamel site, 30% of the samples of Group 1 and 70% of the samples of Group 2 showed dye penetration. At the dentin site, 50% of the samples of Group 1 showed leakage, while all the samples of Group 2 were severely infiltrated. Statistically significant differences between the groups both for the enamel sites (P < 0.05) and for dentin-cementum sites (P < 0.001) were found. Bond strength tests were performed on 2 mm thick dentin slides of tapered Class I cavities (n = 10 in each group). The mean bond strength value obtained from cavities treated without NaOCl was 15.23 (+/-1.40) MPa and with NaOCl treatment 20.52 (+/-1.70) MPa. Statistical analysis showed significant differences (P < 0.05) between the two groups. No hybrid layer was detected on the NaOCl-treated samples by scanning electron microscopy.

Debonding of adhesively restored deep Class II MOD restorations after functional loading.

PURPOSE: To assess debonding due to functional loading at the cervical dentin margins of deep Class II MOD restorations prepared with a variety of adhesive restoratives in endodontically-treated teeth. MATERIALS AND METHODS: MOD preparations and endodontic treatment were carried out on extracted sound maxillary premolars. The cavities were restored with seven material combinations: Three different resin-based composites (RBC) (Z100, Herculite XRV or Clearfil RP with their respective bonding systems); two bonded amalgam combinations (Valiant with Superbond D or with Panavia 21) and two sandwich combinations [the RBC Tetric with the polyacid-modified RBC (compomer) Compoglass or Z100 with the resin-modified glass ionomer Fuji II LC]. For each group, half of the samples were exposed to mechanical functional loading and the other half served as control. Imperfect bond formation and debonding due to loading was determined by dye penetration. In addition, the dye penetration scores were correlated with the cusp fracture strengths of similarly prepared restorations of a previous study. RESULTS: After functional loading, the RBC in combination with hybridizing dentin bonding agents showed better preservation of marginal integrity than the sandwich restorations, which in turn performed better than the amalgam restorations (P < 0.05). Debonding correlated with reduction in fracture resistance. Under the conditions of this study, debonding of adhesive MOD restorations by functional loading can best be prevented by using RBC in combination with hybridizing dentin-bonding systems.

Protective effects of resin impregnation on demineralization of enamel.

PURPOSE: To determine if resin penetrated into the enamel etch pattern, in the absence of a polymerized outer surface film, could reduce the degree of demineralization of enamel subjected to a simulated caries challenge, and to evaluate whether the addition of fluoride to the resin would enhance reductions in demineralization. MATERIALS AND METHODS: Enamel surfaces of extracted human incisors were acid-etched. One-half of the etched area was treated with an unfilled bonding resin, while the other one-half was left untreated as a control. In another group, this same procedure was followed except the unfilled bonding resin contained fluoride. The applied resin was aggressively air thinned to ensure oxygen inhibition throughout the external surface film thickness. The thinned film was visible light-cured and the area was wiped with an ethanol swab to remove the inhibited layer. The specimens were exposed to a buffer solution of pH = 4.7 for 4 days, and were sectioned and examined by polarized light microscopy and microradiography. RESULTS: In each of the two test groups, the demineralization of the resin-treated side was significantly lower than the control side (P < 0.015). Under the conditions of this study, the experimental fluoride resin did not produce statistically significant reductions in demineralization compared with the non-fluoride resin.

Clinical evaluation of a one-bottle bonding system for desensitizing exposed roots.

PURPOSE: To evaluate the effectiveness of a one-bottle adhesive system as a desensitizing treatment, with or without a prior phosphoric acid-etch step. MATERIALS AND METHODS: Two different clinical procedures using a one-bottle adhesive bonding system were tested for its efficacy on desensitizing exposed roots. Thirty subjects with a history of dentin hypersensitivity were selected and 60 exposed sensitive teeth were divided into two groups (n = 30). In Group 1 samples, Scotchbond 1 bonding system (Single Bond in USA) was applied without a prior acid-etching, while in those of Group 2, the exposed sensitive roots were acid-etched with 35% phosphoric acid for 15 seconds, prior to application of bonding agents. The sensitivity was evaluated at 0, 1, 2, 4 weeks and 3 months using cold air blast stimuli, and recorded using a visual-analogue scale. The scores were analyzed as non-parametric data by means of the Mann-Whitney and Sign tests. Five samples from each group were randomly selected in order to make scanning electron microscope observations of the sensitive dentin surfaces at baseline and at each recall using the replica technique. RESULTS: At baseline, after both desensitizing procedures all the patients reported distinct reductions in dentin sensitivity. At 1- and 2-week recalls, 20 teeth of Group 1 and 17 teeth in Group 2 exhibited a return of sensitivity, and in five teeth in each group, the sensitivity score was between 5-8. During the first recall (1-2 weeks), the sensitive teeth were retreated and showed an immediate reduction of sensitivity. At the 1-month recall, eight teeth in Group 1 and four teeth in Group 2 showed a return of some sensitivity, and in six teeth in Group 1 and in three teeth in Group 2, old amalgam restorations were replaced and this procedure almost completely resolved the sensitivity. At the 3-month recall, only five teeth from each group showed residual mild sensitivity (score 1-2). For both Groups 1 and 2 there was a statistically significant (P < 0.0005) reduction in sensitivity levels between the baseline pain scores and immediately after treatment, and between the first recall pre-treatment pain scores and the last recall pain scores. Overall, the two methods of treatment demonstrated a general decrease in sensitivity in both groups over the 3-month study period, adequately solving the clinical and patients' perception of the discomfort of sensitivity. The SEM observations of the replicas showed a progressive loss of the adhesive resin from the treated surfaces. This fact was more evident in the non-etched samples (Group 2) than in those in Group 1. The results of this study showed that exposed root dentin sealed with Scotchbond 1, with or without prior acid-etching, did not show spontaneous sensitivity after 3 months.

Influence of tissue characteristics at margins on leakage of Class II indirect porcelain restorations.

PURPOSE: To evaluate the sealing ability of Class II porcelain inlays with margins placed in cementum-dentin and enamel, luted by two new different cementing materials. MATERIALS AND METHODS: Thirty extracted molars, free from caries and restorations, were selected. The sample cavities were prepared in a standardized manner and then were divided in three groups (n = 10) at random. Group 1: The cervical margin was located 1 mm below the CEJ and the inlays were luted with EBS Multi (Espe) bonding system in combination with Compolute 1.0 Aplicap (CLA 1.0, Espe) experimental resin cement. Group 2: The cervical margin was placed 1 mm below the CEJ and Syntac bonding system (Vivadent) and Variolink II resin cement (Vivadent) were used. Group 3: The cervical margin was placed 0.5 mm above the CEJ and the same materials selected for Group 1 were used. IPS-Empress (Ivoclar) inlays were made following manufacturer's instructions. After luting procedures and 2500 thermal cycles (5 degrees and 55 degrees C), the samples were processed and evaluated for marginal leakage at cervical and occlusal site. Samples of Group 3 were first sectioned in the center of the restorations as in Group 1 and 2 and scored (Group 3A) and then two other cuts were made along the buccal and lingual areas (Group 3B) and also scored. After scoring dye penetration of Group 1 and 2 samples, half of the sections were randomly selected to evaluate: (1) resin cement thickness, (2) Hybrid layer formation at the interface between adhesive material and dental substrate and (3) to observe the morphology of the Class II cavity margins. The leakage data were statistically evaluated with the Mann-Whitney U test (P < 0.5). RESULTS: Statistically significant differences were found between Groups 3A and 3B at the occlusal site, and not at the cervical site. Samples of Group 1 showed a cement thickness of 92 microns, and 68 microns in Group 2. Hybrid layer and resin tag formation was evident in Group 1. In Group 2 samples, the hybrid layer formation was less evident and thinner than in Group 1. At cervical margins, an outer layer, not identifiable as sound dentin, of 200-300 microns thick, was noted. This layer was present in all three groups between outer margin and sound dentin. The margins located both cervically and axially in enamel mainly showed prisms cut along their long axis and a low level of structured etch pattern.

The effect of the photopolymerization method on the quality of composite resin samples.

An optimal degree of conversion and minimal polymerization shrinkage are generally antagonistic goals, as increased monomer conversion invariably leads to elevated polymerization shrinkage values. However, both parameters are indispensable for an optimal resin composite restoration. A number of approaches have been used to reduce the stress on the restoration cavity wall interface, such as dentine bonding agents to counteract polymerization shrinkage, stress-absorbing lining materials and low-intensity curing lights to control the flow capacity of the material during polymerization. However, the configuration of the cavity and cohesive fractures of the material and surrounding tooth tissues are still a problem in day-to-day clinical practice. A new photopolymerization light source, pulsed laser, ensures a higher degree of conversion and lower polymerization shrinkage, and differentiates this technique from standard polymerization methods and continuous-wave argon laser polymerization. The coherence and monochromacity of pulsed laser light set at 468 nm and the far greater intensity of laser nanopulses produce a saturation effect in the depths of the composite, thus resulting in higher monomer conversion. The total amount of energy illuminating the sample surface, which is only one-fifth of that of conventional methods, and the cooling and relaxation of the material between nanopulses may be responsible for the reduced net polymerization shrinkage.

Structural integrity of resin-modified glass ionomers as affected by the delay or omission of light activation.

Since light activation of resin-modified glass ionomers as a means of polymerizing the HEMA is usually done shortly after mixing occurs, the acid-base reaction will proceed mainly within a formed HEMA-polymer matrix. Delaying or omitting light activation may alter the structure and consequently its integrity. The aim of this study was to investigate the effect on the structural integrity of Fuji II LC, Photac-Fil, and Vitremer by delaying or omitting light initiation as compared with the integrity when light activation is performed 2 min after mixing occurs. We evaluated integrity by three-body wear experiments, conducted 8 hrs after sample preparation, to establish the integrity in the early phase of hardening, as well as after 1 wk and after 4 mos, to follow the materials throughout the process of maturation. When light activation was delayed for 1 hr, the structural integrity of Fuji II LC and Photac-Fil improved significantly in the early stages of hardening. In the case of Vitremer, an hour's delay of light activation significantly decreased integrity, which declined further when light activation was omitted. Fuji II LC was not affected by the omission of light activation, while Photac-Fil was markedly weakened. After 4 mos of aging, most of the samples of each product which had been cured by the different methods attained equal integrity, with the exception of the non-light-activated Vitremer samples, which remained weaker. We concluded that the structural integrity of resin-modified glass-ionomer cements benefits from a chemical integration of the polyalkenoate and poly-HEMA networks, as in Vitremer. Improvement in the structural integrity in the early phase for cements with a mechanical entanglement of the matrices, as in Fuji II LC and Photac-Fil, requires an acid-base reaction, a considerable portion of which may take place before activation of the HEMA polymerization.

Glass ionomer cement, an intelligent material.

The main task of a dental filling material is to permanently seal the dentinal wound and to restore the anatomy. In particular with respect to the former requirement, glass-ionomer cements exhibit, thanks to it's ingenious composition, unique properties, which allows it to react on external information in such a way that the cement automatically and adequately fulfills it's function as a proper restorative material. Because of it's setting mechanism, glass-ionomer cement restorations have a better perspective to seal the tissue than amalgam or resin composites do. As resistance to wear of glass ionomers is inferior to resin composites it proves to be a better dentin replacement, than a substitute for tooth enamel. Restorations made in a combination of glass-ionomer cement and resin composite, the so called sandwich restorations, have shown to function excellently, both at the tooth interface as well as the outer surface. As any leakage still cannot be excluded, the fluoride release at the margins of the restoration forms a wellcome contribution to the prevention of secondary caries. Once more, glass-ionomer cement shows to be intelligent by automatically activating an advantageous process when the environment requires it.