Determination of the optimal photoinitiator concentration in dental composites based on essential material properties.

OBJECTIVES: The aim of this study was to determine the concentrations of the photosensitizer (camphoroquinone, CQ) and coinitiator (ethyl-4-dimethylaminobenzoate, EDMAB) that resulted in maximum conversion but generated minimum contraction stress in experimental composites. METHODS: Experimental composites were prepared with an identical resin formulation [TEGDMA:UDMA:bis-GMA of 30.25:33.65:33.65]. Five groups of resin were prepared at varied CQ concentrations (0.1, 0.2, 0.4, 0.8 and 1.6wt% of the resin). Five subgroups of resin were prepared at each level of CQ concentration, by adding EDMAB at 0.05, 0.1, 0.2, 0.4 and 0.8wt% of the resin, resulting in 25 experimental resins. Finally, strontium glass ( approximately 3microm) and silica (0.04microm) were added at 71.5 and 12.6wt% of the composite, respectively. Samples (n=3) were then evaluated for Knoop hardness (KHN), degree of conversion (DC), depth of cure (DoC) and contraction stress (CS). RESULTS: There was an optimal CQ and EDMAB concentration that resulted in maximum DC and KHN, beyond which increased concentration resulted in a decline in those properties. KHN testing identified two regions of maxima with best overlaps occurring at CQ:EDMAB ratio of 1.44:0.42 and 1.05:1.65mol%. DC evaluation showed one region of maximum, the best overlap occurring at CQ:EDMAB ratio of 2.40:0.83mol%. DoC was 4mm. Overall, maximum CS was attained before the system reached the maximum possible conversion and hardness. SIGNIFICANCE: (1) Selection of optimal photoinitiator/amine concentration is critical to materials' formulation, for excessive amounts can compromise materials' properties. (2) There was no sufficient evidence to suggest that contraction stress can be reduced by lowering CQ/EDMAB concentration without compromising DC and KHN.

Effects of resin formulation and nanofiller surface treatment on in vitro wear of experimental hybrid resin composite.

While adding nonbonded nanofillers and lowering the viscosity of the resin matrix have shown success in reducing deleterious polymerization stresses in dental composites, their effects on wear resistance is unknown. This study evaluated abrasion and attrition wear of experimental composites with varied resin viscosities [inherent to varied ratios of TEGDMA:UDMA:bis-GMA (47:33:16 wt%; 30:33:33 wt%; 12:33:51 wt%)] and nanofiller surface treatment (12.6 wt% silanated or unsilanated silica: OX-50; 0.04 microm). Specimens (n = 6) were light cured, aged in water at 37 degrees C for 7 days, and evaluated in the new OHSU oral wear simulator (100,000 cycles). Nonbonded nanofiller increased abrasion and attrition in the low and medium viscosity composites. Increase in resin viscosity increased abrasion and attrition in composites containing silanated nanofiller, with equivocal effects in composites containing unsilanated nanofiller. Nonbonded nanofiller can lower the overall wear resistance of some composite formulations. Increasing resin viscosity generally lowers the wear resistance, but had minimal effect on composites containing nonbonded nanofiller.

Curing-light attenuation in filled-resin restorative materials.

OBJECTIVE: To characterize the attenuation of the curing light in filled resin restorative materials (FRRMs) to aid understanding of curing depth. MATERIALS AND METHODS: One hundred and eighty materials of various shades from several manufacturers were tested in various ways. One set (66 materials) was used to determine the applicability of Lambert's Law using a quartz-tungsten-halogen curing light (Optilux 400, Demetron Research) by measuring the transmitted light with a dental radiometer (Cure Rite, EFOS) for successive thicknesses of ground 10mm diameter specimens from 3 to 0.5 mm in 0.5 mm steps. A second set (17 materials) were similarly tested with separate specimens from 1 to 5mm in thickness using a transmission densitometer (DT1405, RY Parry) fitted with a curing-light dichroic filter. For a third (overlapping) set (165 materials), the 1 mm pure (reflectance-free) optical density (D1 value) was determined from two specimens, approximately 1 and approximately 2 mm thick using the densitometer as above. From D1 the critical thickness (x(CRIT)), identified as depth of cure (DoC) for an excess surface exposure factor of 2, was calculated. RESULTS: Lambert's Law was found to hold with no evidence of appreciable differential absorption effects. Attenuation coefficient and D1 were significantly correlated (P < 1 x 10(-13)). D1 varied between about 0.23 and 0.72, for corresponding x(CRIT) values of 1.3 and 0.4 mm. There was no correlation between D1 and reflectance (P > 0.09), and no systematic effect due to shade letter, but a highly significant (P < 7.5 x 10(-8)), but weak (-0.066 mm/unit), correlation between shade number and D1. SIGNIFICANCE: Depth of cure can be calculated directly from the D1 value determined via simple optical density measurements on two specimens providing that (a) an irradiation time can be determined for the surface of a specimen to be "sufficiently" cured (i.e. for DoC = 0 precisely), and (b) an excess internal surface exposure ratio can be chosen such that the corresponding DoC is attained in a practicable irradiation time.

Effects of strain rate and temperature on the mechanical properties of resin composites.

OBJECTIVE: To evaluate the effects of strain rate and temperature on the mechanical properties of resin composite restorative materials (RCs) and to investigate the construction of temperature--strain rate equivalence 'master curves'. METHODS: Four visible light-cured resin composite RCs, all of shade A3, were used: Heliomolar Radiopaque (HR) and Tetric Ceram (TC) (Ivoclar, Schaan, Liechtenstein), Filtek Z250 (FZ) (3M, St Paul, MN, USA) and Prodigy Condensable (PR) (Kerr, Orange, CA, USA). Bar specimens (10 x 1.5x 16.0 mm3) were cured for 50 s at an irradiance of 500 mW cm(-2) and were randomly distributed into groups of six for each type of material. All specimens were stored in artificial saliva at pH 6, for 7d. The specimens tested at 12, 24 and 37 degrees C were stored at the corresponding temperature but those tested at 0 degrees C were stored at 24 degrees C. Three-point bend tests for flexural strength (F), flexural modulus of elasticity (E) and total energy to failure (W) were performed at cross-head speeds (XHS) of 0.1, 1.0, 10, 50 and 100 mm min(-1) for all materials as well as at 0.01, 0.03, 0.2 and 0.5 mm min(-1) for some materials. RESULTS: There was a common pattern of behavior across materials. At constant temperature, F showed a slight variation with cross-head speed, with a broad peak in the region of 1-10 mm min(-1). E, on the other hand, showed a more marked and steady increase with XHS at all temperatures except at 0 degrees C, where it tended to level off above about 10 mm min(-1). In contrast, the values of W showed a decline with increasing XHS, except at 37 degrees C where an initial rise followed by a decline was observed. At constant XHS, increase in temperature caused a small, but highly significant (P < 10(-3)) decline in F but a marked decline in E. W, again in contrast to Fand E, showed a general increase with temperature. A master curve model for the temperature-strain-rate equivalence was fitted to the E and W data (all P < 10(-5)) and the fitted parameters interpreted in terms of strain rate and temperature sensitivity. SIGNIFICANCE: The mechanical properties of RCs are very sensitive to the test conditions of strain rate and temperature. This implies that properties determined at any temperature other than 37 degrees C, or at only one cross-head speed (or only one strain rate) are inadequate to describe their behavior in service. The master curve principle is applicable to RCs and can be used, inter alia, to determine property values under other than tested conditions. Conditions of testing in regard to XHS and temperature, as well as other factors, should clearly be stated to enable proper comparisons between studies, but more importantly the use of standardized test conditions is overdue.

Polymerization of resin composite restorative materials: exposure reciprocity.

OBJECTIVE: To examine whether there is reciprocity between irradiation time and irradiance with regard to the mechanical properties of filled, resin composite restorative materials (RCs). METHODS: Four visible light-cured RCs, all of shade A3, were used: Heliomolar Radiopaque (HR) and Tetric Ceram (TC) (Ivoclar, Schaan, Liechtenstein), Filtek Z250 (FZ) (3M, St Paul, MN, USA) and Prodigy condensable (PR) (Kerr, Orange, CA, USA). Bar specimens (1.0 x 1.5 x 16.0 mm(3)) were cured at irradiances (I) ranging from 25-1500 mW/cm(2) and irradiation times (t) of 1-3000 s. Six specimens at 250 combinations of t and I were prepared and stored in artificial saliva of pH 6, at 37 degrees C for 7d before performing three-point bend tests for flexural strength (F), flexural modulus (E) and total energy to failure (W). Contour plots of property value vs. t and I on log scales were prepared. RESULTS: The contour plots showed three regions: unset at low I.t, a plateau corresponding to more or less full property development, and connecting ramp. The boundary between the plateau and the ramp suggests the minimum acceptable exposure. No practical lower limit to irradiance was detected, but there may be no benefit from increasing I beyond about 1,000 mW/cm(2). The slopes of the contours in the log-log plots provided a test of the hypothesis of reciprocity. These slopes were approximately -1.5 for HR, TC and PR; and approximately -1 for FZ, compared with the expected value of -1. The general hypothesis therefore fails. The existence of localized maxima in property values is further evidence of that failure, even for FZ. SIGNIFICANCE: Dentists may use any lamp, including LED sources, and attain satisfactory results providing irradiation time is long enough. Manufacturers ought to supply a graph indicating the minimum acceptable exposure for each product for specified curing lamps. Calculations based on total energy delivered to guide irradiation protocols are invalid and do not recognize product behavior.

Aspects of water sorption from the air, water and artificial saliva in resin composite restorative materials.

OBJECTIVE: Primarily to establish whether artificial saliva (AS) at 37 degrees C is essential as a clinically relevant environment for testing filled, resin composite restorative materials. The effect of other storage conditions was also investigated for comparison and controls: desiccation, exposure to the laboratory atmosphere, high humidity cabinet, saturated water vapor, and deionized water. METHODS: Two visible light-cured products were used: Heliomolar Radiopaque (HR) and Tetric Ceram (TC) (Ivoclar, Schaan, Liechtenstein). Bar specimens (26 x 1.5 x 1.0 mm(3)) were cured at five overlapping spots for 60s per spot and randomly distributed into groups of six. Trial 1: one group of each material was exposed first to atmospheric air at 24 degrees C, approximately 50% RH (24WV(50)), then to water vapor at 37 degrees C, approximately 97% RH (37WV(97)), and then immersed in deionized water at 37 degrees C (37DW). Trial 2 used three groups of each material, one first exposed to 37WV(97) followed by 37DW, the other two were immediately immersed in 37DW or artificial saliva (37AS). Trial 3: two groups of each material were vacuum desiccated at 37 degrees C, then exposed to 37 degrees C, approximately 100% RH (37WV(100)), then immersed in 37DW or 37AS. Trial 4: four groups of HR were treated similarly to Trial 3; one was left under desiccation, and another in 37WV(100) for the remaining period. Three-point bend tests for flexural strength (F), flexural modulus (E), and total energy to failure (W) were performed at the end of Trials 2-4. RESULTS: Environmental moisture absorption was substantial at 24WV(50)(c. 0.2%), at least 40% of that in 37DW (HR: c. 0.7%, TC: c. 0.5%). Saturation was achievable in 37WV(100). Mass loss on desiccation (HR: c. 0.4-0.5%, TC: c. 0.25%) was reversible in 37WV(100). There were some significant effects of exposure conditions on mechanical properties (e.g. F for HR: after desiccation, 85.7+/-1.4MPa; after 37WV(100), 73.2+/-3.6MPa; difference: p<0.0002), but overall the results were unclear. After a rapid gain in mass, there was a gradual loss in both 37DW and 37AS for both materials, slightly more in 37AS than 37DW. SIGNIFICANCE: Water vapor absorption is substantial, hence attention must be paid to the laboratory working environment and conditions of storage and testing, i.e. temperature and RH must be stated to assist interpretation of data and comparisons between studies. Test conditions need to be standardized and with reference to normal oral conditions, immediate immersion in artificial saliva at 37 degrees C is the preferred treatment for these materials, whatever time of testing is chosen.

Water sorption and mechanical behaviour of cosmetic direct restorative materials in artificial saliva.

OBJECTIVES: To evaluate the water sorption and mechanical behaviour of a compomer in comparison with those of its nominal forerunners, a filled resin restorative material and a conventional glass ionomer cement. METHOD: Compomer (Dyract AP) (D-AP), filled resin (SureFil) (SF), and glass ionomer (ChemFlex) (CF) (all Dentsply, Addlestone, UK) restorative materials were tested. Forty bar specimens (26x1.5x1.0mm(3)) of each material were prepared according to the manufacturer's instructions and randomly distributed into eight groups: dry air (22% RH), saturated water vapour (WV) (100% RH), and five in artificial saliva (AS) at pH6, all at 37 degrees C, as well as untreated control (UC) (23 degrees C, 50% RH). Water sorption was assessed gravimetrically; flexural strength and elastic modulus were determined in three-point bend. The control group was tested at 24h; AS groups were separately tested after 0.5, 1, 3, 6 and 9 months; the other two at 9 months. RESULTS: Mass gain for SF, D-AP and CF in AS was up to 0.17%, 1.2% and 7.0%, respectively. CF showed a marked decrease of strength in AS compared with other groups, followed by a gradual slight rise to a peak at 3 months. Unlike SF and CF, whose flexural strength remained relatively stable, that of D-AP showed a sharp decline from the 1 month peak (P=6x10(-7)) after 6 months in AS. D-AP also showed a slight decline in flexural modulus from a peak, that of SF was quite stable, while CF showed no peak. The values of flexural strength for both CF and D-AP at 9 months were significantly lower in AS than WV, but SF showed no such difference. SIGNIFICANCE: Materials intended for service in the mouth must be stored in a realistic medium if the results of testing are to be interpretable. Dyract AP, a compomer, does not seem suitable for application in stress bearing areas as is currently recommended by its manufacturer. The rapid decline in flexural strength after 1 month of exposure to AS and its progressive fall in flexural modulus suggest a progressive deterioration of the material and this necessitates re-examination of the chemistry of compomers, if the behaviour is typical of the class.