C.H. Lloyd

Temperature rises produced by light sources and composites during curing.

Abstract A differential thermal analyser has been modified and a technique developed to enable the temperature rises which occur during the setting of visible light-cured composites to be measured. Two factors are responsible for this rise: the exothermic nature of the reaction in the material, and light used to initiate the cure which is transmitted through the composite. Both have been measured for a particular combination of one light source and one composite. The increase produced by the material is comparable to that in two paste autopolymerising systems as they set, however this is all but eclipsed by heat from the light source. The rise produced by the light increases with increasing exposure time and decreasing composite thickness, to reach up to 21.4°C for the selected combination of composite and light unit (though this rise may be reduced in vivo by other factors). The modified DTA and technique are suitable for the measurement of temperature rises (of which that from the light is of prime importance) together with corresponding times. However, whilst it is possible to obtain the heat of reaction for the composite the dominance of heat from the light source reduces the accuracy of this measurement and an alternative approach should be considered for heats of reaction.

The development of fracture toughness and fracture strength in posterior restorative materials

Abstract The changes which take place in the fracture toughness (K IC ), the fracture stress and the inherent flaw size with storage time (in water at 37°C) have been determined for three commercial products, all which have been suggested as alternatives to amalgam. Their performances have been compared with each other, and with other relevant restorative materials. The fracture toughness of the composite Occlusin rises rapidly to a high value which is maintained. Though the fracture toughness of the composite P30 peaks at a value greater than that for the selected reference amalgam, it falls considerably and progressively after this peak. The fracture toughness of the silvercontaining glass ionomer (Ketac Silver) is no greater than a conventional glass ionomer and inferior to amalgam. The inherent flaw sizes are similar for all composites, rising to become constant in time. By contrast the sizes of the inherent flaws in glass ionomers and amalgam fall in time towards constancy. Relative values and time dependencies for fracture stresses and fracture toughnesses show apparent similarities but differences do exist, which may be important to clinical performance. The main conclusion is that the fracture toughness of posterior composite varies with time and product, in one case comparable only to the lower end of the range for amalgams but in the other case substantially greater than any amalgam. Thus, a posterior composite can possess a fracture toughness in excess of that for amalgam and, on this criterion, is an acceptable substitute.

Effect of a commercial bonding agent upon the fracture toughness (K'Ic) of repaired heavily filled composite

Abstract The “repair strength” of a very heavily filled composite has been determined by the measurement of the fracture toughness at the interface. The adherend surface (obtained by fracture at 24 h) was repaired immediately, or after 1 h storage in an artificial saliva. Composite paste was applied either directly to this surface or after the surface had been coated with a bonding agent (Scotchbond). The fracture toughness of each repair was determined 24 h later. This was high (but still significantly less than that for the material itself) when the composite paste was added directly to the “clean” surface. Contamination of the surface before repair produced a further significant reduction. The presence of the bonding agent overcame this inhibition to raise the value to that obtained against an uncontaminated surface.

Application of Magnetic Resonance Microimaging to the Study of Dental Caries

Magnetic resonance microimaging (MRM) of teeth has continued to be developed. Two ways in which data can be presented have been investigated, 112–μm–thick image slices and pseudo–three–dimensional surface rendered images. Limitations of the latter have been demonstrated; the possible absence of structures having low intensity or incompleteness of the image at regions from which the signal intensity is low. This has implications for the MRM investigation of dental caries. However, all intensities are recorded and are available. Structures which give a low–intensity signal can be seen in image slices. MRM appears well suited to studying the development of dental caries, ideally in combination with other techniques. As MRM is non–destructive and non–invasive, it can be used in experiments which follow the progress of the disease, yet leaves the tissue intact for other investigations.

SOLID STATE SPATIALLY RESOLVED 1H AND 19F NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY OF DENTAL MATERIALS BY STRAY-FIELD IMAGING

As part of a program to evaluate the use of stray-field magnetic resonance microimaging (STRAFI) in dental materials research spatially resolved nuclear magnetic resonance (NMR) for solid dental cements has been investigated. By applying a quadrature echo pulse sequence to a specimen positioned in the stray-field of a NMR spectrometer superconducting magnet the magnetic resonance within a thin slice was obtained. The specimen was stepped through the field in 500 μm increments to record 1 and 19F profiles and T2 values at each point. The specimens were fully cured cylinders made from four types of restorative material (glass ionomer, resin modified glass ionomer, compomer, composite). The values for F19T2 varied with material type and reflected the nature of the matrix structure. For all materials containing 19F in the glass two values were calculated for 19F T2, one short and one long. These were relatively invariant. Solid state magic angle spinning (MAS)-NMR showed that they came from the glass. This suggests that a proportion of the element is relatively mobile (in a glass phase) and the remainder is more tightly bound (in a compound dispersed in the glass). This demonstration, that NMR microimaging of both 1H and 19F in solid dental cements is possible, opens up exciting new possibilities for investigating the distribution of these elements (in particular fluorine) in solid dental materials. ©©1999©Kluwer Academic Publishers

The application of magnetic resonance microimaging to the visible light curing of dental resins: 3. Stray-field nuclear magnetic resonance imaging (STRAFI)

OBJECTIVE To investigate the application of stray-field nuclear magnetic resonance imaging (STRAFI) to the visible light curing of dental restorative materials. STRAFI can overcome peak broadening associated with the conventional magnetic resonance microimaging (MRM) of glassy polymers, and has the potential to image dental restorative resins at both low and high degrees of conversion. METHODS Cylindrical composite specimens were light-cured from one end to produce some that were fully cured throughout their length and others that were fully cured at one end and uncured at the other. A one-dimensional probe was used to measure the magnetisation in 40 microm thick slices at 100 microm intervals along the length of the specimen. A quadrature pulse sequence was applied and the magnetisation decay recorded in a train of eight echoes. RESULTS A value for T(2) could be obtained only for the polymer (59+/-16 microms), therefore the echoes were summed to give an approximate indication of the degree of conversion. The echo sum for the monomer was significantly higher than that for the polymer. Differences in composite shade and cure time produced changes in the cure profiles. SIGNIFICANCE STRAFI produced measurements for both monomer and polymer in all stages of conversion that allowed cure profiles to be produced. Summing the decay echoes produced a qualitative measure of the condition of the material in the selected slice. The same data can be used to calculate T(2), a quantitative parameter. This first investigation has demonstrated that STRAFI is well suited to polymerisation studies.

The retention of self-threading pins embedded in various restorative materials

Abstract The retention of restorative materials by dentine pins has been investigated using specimens in which pins were embedded to a clinically realistic depth in a volume of material which corresponded to that for a posterior restoration. One week after preparation a tensile force was applied to the pin, and the failure value and failure mode recorded. Two commercial pins of identical size but contrasting thread geometries and a range of amalgam and composite products were selected. Specimens containing pins with a broad pitch thread failed primarily by fracture through the material at force levels above those for specimens containing pins with a conventional close-pitch tread. These failed by the pin pulling out. A strong correlation was found between the failure force for the former specimens and the fracture toughness (K IC ) of the embedding material. For one product the fracture toughness was sufficient to resist the applied force and tensile failure of the pin took place as an alternative failure mechanism.

Measurement of the diffusion of liquids into dental restorative resins by stray-field nuclear magnetic resonance imaging (STRAFI)

OBJECTIVES The purpose of this investigation was to determine the diffusion mechanism for water/ethanol mixtures in a diacrylate dental resin by direct observation of the absorbed liquid profiles using NMR microimaging. METHODS Frequency-swept stray-field magnetic resonance imaging (STRAFI) was used. Solutions containing 25-65% by volume ethanol remained in contact with visible light cured 54% TEGDMA: 46% modified TUDMA sheets while measurements were made. The diffusion profiles were recorded periodically for diffusion times up to 10 h, to a depth of 360 microm and with readings taken at 24 microm intervals. RESULTS For all liquid mixtures, diffusion was found to be Fickian with coefficients that increased progressively and smoothly with alcohol content in the diffusing mixture, from 2.4 x 10(-13) to 150 x 10(-13) m2 s(-1). A rule of mixtures approach, as suggested by Kwei and Zupko, gave a satisfactory description of the ethanol fraction dependence of the diffusivity. SIGNIFICANCE Frequency-swept STRAFI offers a new and unique opportunity to produce spatially resolved measurements of the liquids in dental resins to high resolution. In this study, absorption was investigated since an understanding of its mechanism is fundamental to limiting consequent environmental degradation. STRAFI has great potential for other applications, for example drying, liquid exchange, etc. Since STRAFI can discriminate 1H in the liquid from those in softened polymer additional applications are envisaged.

Fracture toughness testing: A discriminatory mechanical testing performance indicator for glass-ionomer restoratives?

OBJECTIVES To investigate the single-edge notched (SEN) bend fracture toughness (KIC) testing methodology as a reproducible and discriminatory mechanical testing protocol for encapsulated and hand-mixed glass-ionomers (GI). METHODS SEN bend test-pieces (35.0±0.1mm length, 6.0±0.1mm width, 3.0±0.1mm thickness with a sharp notch formed at mid-length by embedding a scalpel blade) were prepared for KIC testing using three encapsulated GI products (Chemfil Rock, Fuji IXGP Fast Capsule and Ionofil Molar AC). In addition, test-pieces were prepared from a hand-mixed GI product (Ionofil Molar) which contained between 100% and 20% of the manufacturers recommended powder content (in 10% decrements) for a constant weight of liquid. Groups of 20 test-pieces were prepared for each encapsulated GI product (n=3) and hand-mixed GI powder:liquid mixing ratio (n=9). Data were statistically analyzed and the coefficients of variation (CoV) determined for each encapsulated GI product and hand-mixed GI powder:liquid mixing ratio. RESULTS The KIC testing methodology failed to discriminate between the encapsulated GI products that were investigated (p=0.225). For the hand-mixed GI, the KIC testing methodology also failed to discriminate between the powder:liquid mixing ratios investigated (R(2)=0.576). The pooled CoV (10%) for the encapsulated GI products and for the powder:liquid mixing ratio groups (12%) identified the reproducibility of the test for this experiment. For the hand-mixed GI mixing ratio groups with between 100% to 50% of the recommended powder content, no trend could be discerned. SIGNIFICANCE The KIC testing methodology failed to discriminate between different encapsulated GI products and hand-mixed GI powder:liquid mixing ratio groups investigated, despite KIC being an intrinsic material property and the coefficient of variation being acceptable.