Residual stresses in glass crowns generated by polymerization and water sorption of resin cements.

This study investigated residual stresses in glass crowns cemented with resin cements. Glass caps were cemented to cylindrical cores using a conventional resin composite cement, a self-adhesive resin cement, or a methyl methacrylate (MMA)-based cement in dual-cure or self-cure mode. The cemented caps were stored in 37°C water for 28 days, and stresses on the cap surface were repeatedly measured. The water sorption, water solubility, and elastic modulus of the cements were also measured. Polymerization of the cements initially generated compressive stresses on the surfaces. Dual-curing or a greater modulus yielded greater initial stress. The stresses gradually decreased over time and lingered on the surfaces at 28 days with all the cements. Greater sorption tended to lead to greater stress reduction; however, the MMA-based cement exhibited less stress reduction despite exhibiting the greatest sorption. The use of a resin composite cement or dual-curing is recommended to reinforce crown restorations.

Comparison of polymerization stresses of dental resin composites evaluated by two indentation fracture methods.

BACKGROUND: Polymerization stress is a major problem in dental resin composite restorations. Two indentation fracture methods can be applied to evaluate the stress, however, they often calculate different values. OBJECTIVE: To compare polymerization stresses of dental composites determined by the two methods. METHODS: Glass disks with a central hole were used. Two indentation fracture methods (Methods 1 and 2) were employed to determine the polymerization stresses of low-shrinkage and bulk-fill composites. Method 1: Cracks were made in the glass surface at 300 µm from the hole. The hole was filled with the composite. Polymerization stresses at 30 min after filling were calculated from the lengths of crack extension. Method 2: The hole was filled with the composite. Cracks were introduced in the glass at 1,000 µm from the hole at 30 min after the polymerization and the stresses were calculated from the crack lengths. Stresses at composite-glass bonded interface were calculated from the stress values obtained by the two methods. RESULTS: The bulk-fill composite generated the smallest interfacial stress, and Method 1 revealed lower values than Method 2. CONCLUSIONS: The composites yielded relatively small stresses. Method 1 calculated smaller stress values, possibly affected by the lower threshold stress intensity factor.

Changes of residual stresses on the surface of leucite-reinforced ceramic restoration luted with resin composite cements during aging in water.

The objective of this study was to compare the changes in the residual stresses present on the surface of leucite-reinforced dental ceramic restorations luted with a self-adhesive and a conventional resin composite cement during aging in water. Ring specimens made of a leucite-reinforced ceramics were luted to ceramic cylinders using a self-adhesive (Panavia SA Luting Plus) or a conventional resin composite cement (Panavia V5) in dual-cure or self-cure mode. Residual stresses on the ring surface were measured using indentation fracture method at 1 h, 1, 3, 7, 14 and 28 days of the 37 °C water immersion. Water sorption, water solubility and elastic modulus of the cements were also measured. Compressive stress was generated on the surface of the ceramic rings by the polymerization of the resin composite cements, and the stresses appeared to decrease over time by water sorption of the cements. The dual-cured conventional resin composite cement remained compressive stresses on the ceramic surface, while only the self-cured self-adhesive cement, which demonstrated the greatest water sorption, generated tensile stresses during the four weeks of aging in water. The elastic moduli of cements did not significantly change through the immersion, suggesting that the stresses were less affected by the modulus. To prevent the generation of tensile stresses on the leucite-reinforced ceramic restoration, self-adhesive cements exhibiting small water sorption should be clinically selected.

Clinical effectiveness of direct resin composite restorations bonded using one-step or two-step self-etch adhesive systems: A three-year multicenter study.

The purpose of this multicenter clinical study was to compare the mid-term clinical effectiveness of direct resin composite restorations using one-step or two-step self-etch adhesives (1-SEAs or 2-SEAs). In total, 352 restorations of class I-V cavities and non-carious cervical lesions in vital teeth (1-SEAs; 52 cases, 2-SEAs; 300 cases) were placed at nine university hospitals and evaluated according to the modified USPHS criteria at baseline, and after 1, 2, and 3 years. The recall rates were 86.6% (1 year), 80.1% (2 years), and 62.2% (3 years). Two restorations failed due to fracture during the follow-up, and there was no significant difference in survival rates between 1-SEAs (97.6%) and 2-SEAs (99.4%). However, 2-SEAs exhibited significantly lower occurrences of discoloration, marginal discoloration, fracture, and plaque retention. Moreover, the subjects reported a significantly lower postoperative hypersensitivity and higher overall satisfaction at all evaluation periods if 2-SEAs were used.

Residual polymerization stresses in human premolars generated with Class II composite restorations.

The objective of this study was to assess the influence of filling techniques on residual polymerization stresses in resin composite restorations of the tooth. Flat planes were ground in buccal enamel surfaces of extracted human premolars, followed by preparing Class II cavities. Indentation cracks were introduced in the planes and crack lengths were measured mesio-distally (x-direction) and cervico-incisally (y-direction). Cavities were filled with a resin composite and an adhesive using three methods; one with bulk filling and two with differing incremental filling techniques. The x- and y-tensile stresses were calculated from crack lengths measured repeatedly over 360 min after filling. Elastic modulus and polymerization shrinkage of the composite were also measured. Filling technique and time after fillings were statistically significant only for the y-stress. The incremental techniques generated smaller stresses than the bulk filling. The stresses developed for 60 min after filling, while the modulus and the shrinkage stopped developing within 10 min and 2 min after irradiation, respectively. The incremental technique, in which the proximal portion of the cavity was filled first, was effective in decreasing the residual tensile stress generated by the polymerization of resin composite.

Changes in polymerization stress and elastic modulus of bulk-fill resin composites for 24 hours after irradiation.

The objective of this study was to investigate changes in polymerization stress and elastic modulus for light-cured bulk-fill resin composites following irradiation. Crack analysis was applied to obtain the stresses for 24 h after irradiation, which were calculated from the lengths of cracks in a glass mold bonded to the composites. The elastic modulus was repeatedly measured by nano-indentation tests performed over 24 h. The lengths of the cracks, the interfacial stress, and the elastic modulus were significantly affected by the material and time after irradiation (p<0.01). The stress and elastic modulus continued to increase for more than 6 and 1 h, respectively (p<0.05). Strong relationships (r>0.85) were revealed between the stress and elastic modulus. The bulk-fill resin composites generated smaller stresses than a flowable resin composite reported to generate relatively low stress. Post-irradiation polymerization during the first 1 h is a major determinant for the magnitude of stress.

Addition of Na2CO3 Improves the Shelf Life of Experimental 4-META Self-etch Primer.

PURPOSE: The objective of this study was to assess the influence of the addition of Na2CO3 on the shelf life and bond strength of 4-META self-etch primer. MATERIALS AND METHODS: Two 4-META self-etch primers were experimentally formulated with and without the addition of Na2CO3 (primer A and primer B, respectively). The primers were stored at 37°C for several periods of time after formulation. Clearfil SE Bond Primer (Kuraray Noritake Dental) was used as a control. A composite was bonded to bovine dentin using a combination of the primer and a bonding agent (Clearfil SE Bond), and the shear bond strengths were measured. Changes in the pH of the experimental primers were monitored and macroscopic observations were made as a function of the storage periods. RESULTS: With 0-day storage, the bond strength of primer A was equivalent to that of primer B and significantly lower than that of Clearfil SE Primer. The bond strengths of primer A were stable during 90-day storage; by contrast, the bond strengths of primer B significantly decreased during 7-day storage. primer A exhibited stable pH values during 180-day storage; primer B exhibited significantly lower pH than primer A with 0-day storage and discolored gelation during 7-day storage. CONCLUSION: By adding Na2CO3, the shelf life of the 4-META self-etch primer was obviously improved without significant deterioration in dentin bond strength. Adjusting the pH to around 4.5 was effective for obtaining sufficient bond strength and hydrolytic stability of the primer.

Interference of functional monomers with polymerization efficiency of adhesives.

The degree of conversion (DC) of camphorquinone/amine-based adhesives is affected by acidic functional monomers as a result of inactivation of the amine co-initiator through an acid-base reaction. During bonding, functional monomers of self-etch adhesives chemically interact with hydroxyapatite (HAp). Here, we tested in how far the latter interaction of functional monomers with HAp counteracts the expected reduction in DC of camphorquinone/amine-based adhesives. The DC of three experimental adhesive formulations, containing either of the two functional monomers [10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) or 4-methacryloxyethyl trimellitic acid anhydride (4-META)] or no functional monomer (no-FM; control), was measured with and without HAp powder added to the adhesive formulations. Both the variables 'functional monomer' and 'HAp' were found to be significant, with the functional monomer reducing the DC and HAp counteracting this effect. It is concluded that the functional monomers 10-MDP and 4-META interfere with the polymerization efficiency of adhesives. This interference is less prominent in the presence of HAp, which would clinically correspond to when these two functional monomers of the adhesive simultaneously interact with HAp in tooth tissue.

Polymerization stress of dental resin composite continues to develop 12 hours after irradiation.

OBJECTIVE: This study assessed the development of stress from polymerization of light-cured dental resin composites for 24 hours after irradiation. MATERIALS AND METHODS: Initial cracks were made near the edge of a cylindrical hole in glass, and crack lengths were measured. Revolution Formula 2, Kalore and Venus Diamond were polymerized in the hole. Crack lengths were measured at several intervals after irradiation up to 24 hours. Stresses at the composite-glass interface were calculated using the crack lengths. Elastic moduli of the composites were measured at the same time intervals. RESULTS: Interfacial stress and elastic modulus were significantly related to material and time. Stress continued to increase up to 12 hours after irradiation. Significantly lower stresses were measured in Kalore and Venus Diamond than Revolution Formula 2 throughout 24 hours. Stress at 24 hours was two times greater than the stress at 30 minutes. The increase in elastic modulus from 30 minutes to 24 hours ranged from 8 to 24%, which was less than the increase in stress over the same time period. CONCLUSIONS: Interfacial stress and elastic modulus of the composites demonstrated the average increases of 155% and 14%, respectively, from 30 minutes to 24 hours after irradiation. CLINICAL SIGNIFICANCE: Since the elastic modulus is a measure of stiffness and resistance to load, clinicians might consider advising their patients to avoid heavy occlusal function until the elastic modulus has reached a plateau, which would be 60 minutes for Venus Diamond, 12 hours for Kalore, and 24 hours for Revolution Formula 2.

Enamel surface changes caused by hydrogen sulfide.

BACKGROUND: Volatile sulfur compounds (VSCs) produced inside the mouth are a well-known cause of halitosis. Recent studies have suggested that VSCs modify the pathology of periodontitis by encouraging the migration of bacterial toxins associated with increased permeability of gingival epithelia, and enhancing the production of matrix metalloproteinases in gingival connective tissue. Nonetheless, the effects on the enamel of direct exposure to VSCs within the oral cavity remain unclear. In the present study, we observed the effects of VSCs in the form of hydrogen sulfide (H2S) on enamel surfaces and determined their effects on restorations. MATERIALS AND METHODS: Extracted human tooth and bovine tooth samples were divided into the H2S experimental side and the control side. We observed the effects of H2S on enamel surfaces using electron microscopy and conducted a shear test. RESULTS: We found that exposure to H2S obscured the enamel surface's crystal structure. The surface also exhibited coarseness and reticular changes. Shear testing did not reveal any differences in bond strength. CONCLUSIONS: Our findings suggested that H2S occurring inside the mouth causes changes to the crystal structure of the enamel surface that can lead to tooth wear, but that it does not diminish the effects of dental bonding in adhesive restorations.

Hydrolytic stability of three-step etch-and-rinse adhesives in occlusal class-I cavities.

OBJECTIVES: A dental adhesive without small and hydrophilic monomers such as 2-hydroxyethyl methacrylate (HEMA) and triethylene glycol dimethacrylate (TEGDMA) would be beneficial in order to avoid contact allergies. However, these monomers are important to increase infiltration and polymerization of the adhesive. Therefore, the purpose of this study was to evaluate the bonding effectiveness and bond durability of a more hydrophobic and biocompatible adhesive as compared to a conventional three-step etch-and-rinse adhesive. METHODS: Sixteen non-carious human third molars were used to determine the micro-tensile bond strength testing (µTBS) and interfacial ultrastructure by transmission electron microscopy (TEM) of the more hydrophobic cmf adhesive system (Saremco) adhesive as compared to the control OptiBond FL (Kerr). RESULTS: The more hydrophobic and biocompatible three-step etch-and-rinse adhesive was able to produce a reasonable short-time bonding effectiveness. In the long term, the collagen fibrils in the hybrid layer were not effectively protected and were prone to hydrolytic degradation. As a result, long-term bonding effectiveness of this novel adhesive was very low. CONCLUSIONS: Application of a more hydrophobic adhesive without altering the application procedure considerably results in a reduced durability of the created bond CLINICAL RELEVANCE: Omitting small and hydrophilic components from the adhesive formulation may impair the durability of your composite restoration.

X-ray diffraction analysis of three-dimensional self-reinforcing monomer and its chemical interaction with tooth and hydroxyapatite.

According to the 'Adhesion-Decalcification' concept, specific functional monomers within dental adhesives can ionically interact with hydroxyapatite (HAp). Some specific functional monomers form monomer-Ca salts due to chemical interactions. The chemical stability of the monomer-Ca salts was thought to contribute to bond durability. In the present study, we analyzed the chemical interaction between an acidic three-dimensional self-reinforcing monomer (3D-SR) of Bond Force and enamel, dentin and HAp, and assessed its chemical stability by thin-fi lm X-ray diffraction (TF-XRD). 3D-SR forms a hydrolysis-resistant Ca-salt on the dentin in a clinical application time period and on enamel and HAp in a longer time period. This suggests that the functional monomer 3D-SR is able to contribute to bond durability.

Bonding effectiveness of a new 'multi-mode' adhesive to enamel and dentine.

OBJECTIVES: Self-etch adhesives are well adopted in general practice, obviously primarily thanks to their ease of use and fast application time. Nevertheless, phosphoric acid is still often recommended to beforehand etch enamel following a so-called 'selective' enamel-etch technique, this in particular when most cavity margins end in enamel. The purpose of this study was to test if a new one-step adhesive can be applied in a multi-mode manner, this following different, either 'full' or 'selective', self-etch and etch-and-rinse approaches. Specific research hypotheses tested were that prior phosphoric-acid etching did not affect the bonding effectiveness of the one-step adhesive to enamel and dentine, and that the bonding effectiveness to dentine was also not affected when the adhesive was applied either following a 'dry-bonding' or 'wet-bonding' etch-and-rinse technique. METHODS: The micro-tensile bond strength (µTBS) of the one-step self-etch adhesive G-Bond Plus (GC, Tokyo, Japan; 1-SEA) was measured when it was bonded to bur-cut enamel following either a 'self-etch' or an 'etch-and-rinse' adhesive protocol, and to bur-cut dentine when applied following either a 'self-etch', a 'dry-bonding' or a 'wet-bonding' etch-and-rinse adhesive protocol. Bond-strength testing was corroborated by ultra-structural analysis of the interfacial interaction at enamel and dentine using transmission electron microscopy (TEM). RESULTS: Prior phosphoric-acid etching significantly increased the bonding effectiveness of the 1-SEA to enamel. A clearly enhanced micro-retentive surface was revealed by TEM. To dentine, no statistically significant difference in bonding effectiveness was recorded when the 1-SEA was either applied following a self-etch or both etch-and-rinse approaches. The 'dry-bonding' etch-and-rinse protocol was significantly more effective than its 'wet-bonding' version. TEM however revealed indications of low-quality hybridisation following both etch-and-rinse approaches, in particular in the form of a porous and poorly resin-infiltrated collagen mesh. CONCLUSIONS: While phosphoric-acid etching definitely improved bonding of the one-step self-etch adhesive to enamel, one should be more careful with additional phosphoric-acid etching of dentine. Although the bond strength was not reduced, the resultant adhesive interface appeared ultra-structurally more vulnerable to biodegradation.

Effects of functional monomers and photo-initiators on the degree of conversion of a dental adhesive.

Besides functional and cross-linking monomers, dental adhesives contain a photo-initiator system for polymerization, thereby providing physico-mechanical strength to the adhesive-tooth interface. Few studies have investigated the effect of the functional monomer and polymerization-initiation system on the polymerization efficiency of the adhesive. Here, we tested the effect of two different functional monomers (MAC-10 vs. SR) and two photo-initiator systems, camphorquinone-amine (CQ) vs. borate (BO), on the degree of conversion (DC) of different adhesive formulations. The DC of the CQ-cured adhesive formulations was significantly affected by the MAC-10 monomer. This should be ascribed to the known inactivation of the amine co-initiator through acid-base reaction. However, the SR monomer did not decrease the DC, which could be attributed to a "gel effect" or the so-called "Trommsdorff-Norrish" phenomenon of enhanced DC with more viscous resins, and to the more favorable availability of CC double bonds. In contrast, the DC of the BO-cured adhesive formulations was not affected by any acidic monomer. It is concluded that the degree of conversion of an adhesive can be affected by the functional monomer, but this depends on the kind of photo-initiator system used. As bond durability depends, among other factors, on the strength and thus degree of conversion of the adhesive, potential interaction between adhesive ingredients and the photo-initiator system definitely needs to be studied further.

TEM interfacial characterization of an experimental self-adhesive filling material bonded to enamel/dentin.

UNLABELLED: A great challenge regarding the ease-of-use of composites involves the development of 'self-adhesive' composites that no longer require a separate adhesive to bond to tooth enamel/dentin. OBJECTIVES: To characterize the interfacial ultra-structure of an experimental self-adhesive filling material bonded to enamel and dentin using transmission electron microscopy (TEM). METHODS: The experimental self-adhesive material was bonded to bur-cut human enamel and dentin, and to fractured (smear-free) dentin, strictly according to the manufacturers' instructions. The specimens were stored for 1 day in distilled water (37 °C) prior to further common specimen processing for TEM. RESULTS: The experimental self-adhesive filling material revealed a typical micro-hybrid filler distribution. At bur-cut enamel, a tight interface was formed, mostly exhibiting only tiny micro-tags without distinct surface demineralization. At bur-cut dentin, the experimental self-adhesive filling material interacted superficially, with the surface structure being more irregular because of the bur preparation. No clear resin tags were formed due to the obstruction of dentin tubules with smear plugs. At fractured dentin, the formation of a relatively thin hybrid layer of maximum a few hundreds of nanometer was disclosed without clear surface demineralization. Distinct resin tags were formed due to the absence of smear plugs. Silver-nitrate infiltration showed a pattern of spot-like appearance of nano-leakage. Ag deposition was observed more along the dentin-adhesive interface of bur-cut dentin, as compared to that of fractured dentin. SIGNIFICANCE: The obtained tight interface at both enamel and dentin demonstrates the self-adhesive capacity of the experimental self-adhesive filling material.

An eighteen-month clinical evaluation of posterior restorations with fluoride releasing adhesive and composite systems.

This study evaluated the clinical performance of a fluoride releasing restorative system (FL-Bond II & Beautifil II) in posterior restorations after 18-month placement. IRB forms were submitted with each patient approving by signing their acceptance. Seven Class I and 46 Class II restorations were placed by three clinicians in 38-patients. Each FL-Bond II & Beautifil II restoration was placed under rubber dam isolation according to manufacture's instruction. Clinical evaluation was assessed at baseline, 6 and 18-months using modified USPHS criteria. No post-operative sensitivity was reported in any restored tooth at each patient assessment. Only slight color change with some surface staining was noted. Slight marginal changes were observed in 12 of 53-restorations -seen as step irregularities when a sharp explorer was drawn across the tooth from the enamel toward the restoration interface. Utilizing USPHS evaluation criteria, the clinical performance of each (FL-Bond II & Beautifil II) posterior fluoride releasing system was clinically acceptable at 18-month.