Sabine H. Dickens

Determination of double bond conversion in dental resins by near infrared spectroscopy

OBJECTIVES This study determined the validity and practicality of near infrared (NIR) spectroscopic techniques for measurement of conversion in dental resins. METHODS Conversion measurements by NIR and mid-IR were compared using two techniques: (1) The conversion of 3mm thick photopolymerized Bis-GMA/TEGDMA resin specimens was determined by transmission NIR. Specimens were then ground and reanalyzed in KBr pellet form by mid-IR. (2) As further verification, thin resin films were photocured and analyzed by mid-IR. Multiple thin films were then compressed into a thick pellet for examination by NIR. RESULTS Conversion values obtained by NIR and mid-IR techniques did not differ significantly. A correction for changing specimen thickness due to polymerization shrinkage was applied to NIR conversion measurements since an internal standard reference peak was not employed. Sensitivity of the NIR technique was superior to those based on the mid-IR. SIGNIFICANCE The nondestructive analysis of conversion in dental resins by NIR offers advantages of convenience, practical specimen dimensions and precision compared with standard mid-IR analytical procedures. Because glass is virtually transparent in the NIR spectrum, this technique has excellent potential for use with filled dental resins as well.

Effects of the acetone content of single solution dentin bonding agents on the adhesive layer thickness and the microtensile bond strength

OBJECTIVES This study investigated the hypothesis that varying the acetone content of single solution dentin bonding agents may affect the adhesive layer thickness and microtensile bond strength (MTBS) of the bonded complex, and explored whether the adhesive layer thickness is a valid predictor for MTBS. METHODS Experimental dentin bonding agents containing (27, 37, 47, 57, or 67) mass fraction% acetone were used to bond composite resin onto occlusal dentin surfaces of extracted human molars. The adhesive layer thickness was determined by digitized image analysis. MTBS was measured after 48 h. The fracture surfaces were observed using SEM. RESULTS With increasing acetone content, MTBS varied from 38 MPa (67% acetone) to the highest MTBS of 64 MPa (37% acetone), while the adhesive layer thickness decreased linearly. Both dependent variables demonstrated moderate inverse correlation with the acetone content (p<0.0001), but were not correlated with each other (p>0.05). Ninety-four percent of the specimens showed fractures within the adhesive layer extending toward the interfaces with the hybrid layer or the composite resin. In the groups containing 57 and 67% acetone, cracks were observed at these interfaces. SIGNIFICANCE Rather than the adhesive layer thickness, interfacial cracks in specimens with acetone-rich bonding agents may have caused lower MTBS. Within the scope of this investigation, lower acetone concentrations, as could be anticipated from solvent evaporation during clinical use of the bonding agent, did not seem to lower MTBS, but rather improved the integrity of the dentin/adhesive bond.

Mechanical properties and biochemical activity of remineralizing resin-based Ca–PO4 cements

OBJECTIVE This study examined strength and bioactive (remineralizing) properties of a powder/liquid formulation (Cement I) and a more practical two-paste formulation (Cement II) of a fluoride-releasing resin-based Ca-PO(4) cement. METHODS For the remineralization potential, the dissolution of calcium, total ionic phosphate and fluoride from set cement specimens were determined in buffered saline and saliva-like solution (SLS). Artificial caries lesions were produced on coronal dentin of extracted human molars. On each tooth, two defined areas were coated, one with a composite resin (calcium-free control), the other with either Cement I or II. After incubation in SLS, the mineral loss under the cement was analyzed by digitized microradiography and compared to that under the control. RESULTS The diametral tensile strength of Cement II of approximately 30 MPa after 24 h and 23 MPa after 4 weeks was 2 to 3 times higher than that of Cement I (ANOVA, t-test, p<0.05). Calculations of the ion-activity products and Gibbs free energy from solution ion concentrations indicated a significant potential for the formation of fluor- and/or hydroxyapatite. Concurrently, both cements caused increases of 47% (Cement I) and 38% (Cement II) in the lesion mineral content over that underneath the corresponding controls. SIGNIFICANCE These in vitro results suggest that the stronger Cement II could serve as a restoration-supporting lining material and could remineralize dentin in areas where complete removal of carious tissue is contra-indicated.

Network formation and compositional drift during photo-initiated copolymerization of dimethacrylate monomers

The copolymerization of viscous monomers with a nonviscous diluent comonomer was investigated. Overall, photopolymerization kinetics was compared for equimolar mixtures of triethylene glycol dimethacrylate (TEGDMA), a low viscosity monomer, with either a rigid aromatic dimethacrylate (Bis-GMA) or a flexible aliphatic urethane dimethacrylate (UDMA), as examples of viscous comonomers. Sol fraction analysis from partial cure polymerizations provided individual monomer reactivities as a function of conversion. The resin containing UDMA reached greater maximum polymerization rate and final conversion compared with the resin based on Bis-GMA. The viscous monomers (Bis-GMA or UDMA) were found to be less reactive than the diluent comonomer (TEGDMA) at nearly all stages of the photopolymerization process. The reactivity differential was especially dramatic at the latter stages of the polymerization.

In vivo Dentin Remineralization by Calcium-phosphate Cement

Minimally invasive caries-removal procedures remove only caries-infected dentin and preserve caries-affected dentin that becomes remineralized. Dental cements containing calcium phosphate promote remineralization. This study evaluated the in vivo remineralization capacity of resin-based calcium-phosphate cement (Ca-P) used for indirect pulp-capping. Carious and sound teeth indicated for extraction were randomly restored with the Ca-P base or without base (control), followed by adhesive restoration. Study teeth were extracted after three months, followed by elemental analysis of the cavity floor. Mineral content of affected or sound dentin at the cavity floor was quantified by electron probe micro-analysis to 100-μm depth. After three months, caries-affected dentin underneath the Ca-P base showed significantly increased calcium and phosphorus content to a depth of 30 μm. Mineral content of treated caries-affected dentin was in the range of healthy dentin, revealing the capacity of Ca-P base to promote remineralization of caries-affected dentin.

A chemical phosphorylation-inspired design for Type I collagen biomimetic remineralization.

OBJECTIVES Type I collagen alone cannot initiate tissue mineralization. Sodium trimetaphosphate (STMP) is frequently employed as a chemical phosphorylating reagent in the food industry. This study examined the feasibility of using STMP as a functional analog of matrix phosphoproteins for biomimetic remineralization of resin-bonded dentin. METHODS Equilibrium adsorption and desorption studies of STMP were performed using demineralized dentin powder (DDP). Interaction between STMP and DDP was examined using Fourier transform-infrared spectroscopy. Based on those results, a bio-inspired mineralization scheme was developed for chemical phosphorylation of acid-etched dentin with STMP, followed by infiltration of the STMP-treated collagen matrix with two etch-and-rinse adhesives. Resin-dentin interfaces were remineralized in a Portland cement-simulated body fluid system, with or without the use of polyacrylic acid (PAA) as a dual biomimetic analog. Remineralized resin-dentin interfaces were examined unstained using transmission electron microscopy. RESULTS Analysis of saturation binding curves revealed the presence of irreversible phosphate group binding sites on the surface of the DDP. FT-IR provided additional evidence of chemical interaction between STMP and DDP, with increased in the peak intensities of the PO and P-O-C stretching modes. Those peaks returned to their original intensities after alkaline phosphatase treatment. Evidence of intrafibrillar apatite formation could be seen in incompletely resin-infiltrated, STMP-phosphorylated collagen matrices only when PAA was present in the SBF. SIGNIFICANCE These results reinforce the importance of PAA for sequestration of amorphous calcium phosphate nanoprecursors in the biomimetic remineralization scheme. They also highlight the role of STMP as a templating analog of dentin matrix phosphoproteins for inducing intrafibrillar remineralization of apatite nanocrystals within the collagen matrix of incompletely resin-infiltrated dentin.

Effect of a bonding agent on in vitro biochemical activities of remineralizing resin-based calcium phosphate cements

OBJECTIVES To test whether fluoride in a resin-based Ca-PO4 ion releasing cement or coating with an acidic bonding agent for improved adhesion compromised the cement remineralization potential. METHODS Cements were formulated without fluoride (Cement A) or with fluoride (Cement B). The treatment groups were A=Cement A; A2=Cement A+bonding agent; B=Cement B; B2=Cement B+bonding agent. The calcium, phosphate, and fluoride ion release in saliva-like solution (SLS) was determined from hardened cement disks without or with a coating of bonding agent. For the remineralization, two cavities were prepared in dentin of extracted human molars and demineralized. One cavity received composite resin (control); the other received treatment A, A2, B or B2. After 6 week incubation in SLS, 180 microm cross-sections were cut. The percentage remineralization was determined by transverse microradiography comparing the dentin mineral density under the cement to that under the control. RESULTS The percentage of remineralization (mean+/-S.D.) was A (39+/-14)=B (37+/-18), A2 (23+/-13), B2 (14+/-7). Two-way analysis of variance (ANOVA) and Holm-Sidak test showed a significant effect from the presence of bonding agent (p<0.05), but not from fluoride (p>0.05). The ion solution concentrations of all groups showed undersaturation with respect to dicalcium phosphate dihydrate and calcium fluoride and supersaturation for fluorapatite and hydroxyapatite suggesting a positive remineralization potential. SIGNIFICANCE Compared to the control all treatments resulted in mineral increase. The remineralization was negatively affected by the presence of the bonding agent.

In vivo Dentin Microhardness beneath a Calcium-Phosphate Cement

A minimally invasive caries-removal technique preserves potentially repairable, caries-affected dentin. Mineral-releasing cements may promote remineralization of soft residual dentin. This study evaluated the in vivo remineralization capacity of resin-based calcium-phosphate cement (Ca-PO4) used for indirect pulp-capping. Permanent carious and sound teeth indicated for extraction were excavated and restored either with or without the Ca-PO4 base (control), followed by adhesive restoration. Study teeth were extracted after 3 months, followed by sectioning and in vitro microhardness analysis of the cavity floor to 115-µm depth. Caries-affected dentin that received acid conditioning prior to Ca-PO4 basing showed significantly increased Knoop hardness near the cavity floor. The non-etched group presented results similar to those of the non-treated group. Acid etching prior to cement application increased microhardness of residual dentin near the interface after 3 months in situ.

Remineralization of human natural caries and artificial caries-like lesions with an experimental whisker-reinforced ART-composite

This paper compares the remineralization of human natural caries and artificial caries-like dentin lesions treated with a novel whisker-reinforced experimental composite resin (ART composite) with a resin-modified glass ionomer cement (RM-GIC) as control. Ten molars with moderate natural dentin caries were prepared (N). Artificial caries-like dentin lesions were prepared in occlusal dentin of 10 caries-free molars and demineralized at pH 4.3 for 48 h (A). The cavities were restored with ART composite or RM-GIC. All restored teeth were sliced into 120 μm sections. Transverse microradiography combined with digital image analysis was performed to analyze the change in mineral density at the same position in the specimens before and after 4 and 8 weeks remineralization/demineralization treatment. The mean percent remineralization ± standard deviation after 4 and 8 weeks are: N with ART composite, 27 ± 9 and 46 ± 14, respectively; N with RM-GIC, 18 ± 6 and 36 ± 11, respectively; A with ART composite, 48 ± 9 and 66 ± 11, respectively; A with RM-GIC, 50 ± 13 and 62 ± 11, respectively. There was a significant difference between the ART composite and RM-GIC for the remineralization of natural caries (P<0.05). For both restoratives there were significant differences between the remineralization of natural and artificial caries (P<0.001). The ART composite and RM-GIC remineralized natural and artificial caries differently, most likely due to differences in the microstructure and composition of the caries dentin.

Effects of adhesive, base and diluent monomers on water sorption and conversion of experimental resins ☆

OBJECTIVES To establish the relationship of resin composition and resin hydrophilicity (indicated by solubility parameters and logP) to water sorption (WS), solubility, and degree of double bond conversion (DC) of resin mixtures designed for adhesive restoratives by varying the concentration of pyromellitic glycerol dimethacrylate (PMGDM) and various co-monomers. METHODS Sixteen resin mixtures were prepared with (30-70) mass fraction % PMGDM. At given PMGDM concentrations there were up to five compositions with increasing logP. Polymer disks (13 mm x 0.7 mm) were exposed to 96% relative humidity (RH) to determine water sorption in humid atmosphere (WSH) and subsequently immersed in water for immersion water sorption (WSI) and solubility. DC was assessed by near infrared spectroscopy. RESULTS WSI was somewhat higher than WSH, which ranged from (2.1 to 5.3) mass fraction %. Both data were positively correlated to PMGDM concentrations [Pearson correlation, p<0.02; R(2)=0.74, 0.73 (WSI)] and solubility (R(2)=0.64), but not to logP. When grouped by structural similarities, i.e., base resins with bisphenol A core (Group B), Group O containing diluent monomers, or Group U containing urethane dimethacrylate, WS within each group was inversely correlated to logP with R(2)=0.98, 0.81, 0.95, and WS/solubility correlation improved with R(2)=0.88, 0.92 and 0.75, respectively. Solubility ranging from 0.3% to 2.3% was inversely related to DC (r=-0.872). Conversion ranging from 41% to 81% was lower for resins with high base monomer concentrations and highest in mixtures with UDMA. SIGNIFICANCE LogP was a good predictor of WS after grouping the resins according to functional, compositional and structural similarities. WS and conversion were reasonably well predicted from Hoys solubility parameters and other physical resin properties.