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The isothermal enthalpy changes with time of a dental composite were examined by microcalorimetry to isolate the effects of different filler concentrations and curing times on chemical aging of these composites. Urethane dimethacrylate (UDMA) monomer, zirconia-silica (ZS) powder, and 3-methacryloxypropyltrimethoxysilane (MAPM) were used as organic and inorganic matrices, and a coupling agent, respectively. The composite was mixed in different ratios and cured by visible light. The enthalpy changes with time for 0, 15, 45, 75% ZS-filled UDMA and 75% MAPM-silanated ZS-filled UDMA cured for 13, 30, 90, 150, and 300 s were measured at 37.0°, 57.0°, and 65.5°C until equilibrium. Increased curing time and filler concentration caused the excesss enthalpy changes (dH) and their rate of change (dH/dt) to increase with annealing time and apparent equilibrium was reached faster. In addition, dH showed nonlinear dependence with the increase in filler concentration by showing a maxima for samples containing 25 wt % filler. Further, filler silanation caused dH/dt to increase and required shorter times to reach apparent equilibrium. dH also reached a minimum when samples contained silanated filler, compared to composites containing unsilanated filler. It was concluded that the shorter curing time caused the occurrence of spontaneous densification, which facilitated continual resin curing; and longer curing time caused higher crosslinking of the organic phase. Moderate concentration of inorganic phase restricts the molecular motion of the surface layer of polymer onto filler particles, and the polymer is regarded as highly crosslinked, while a higher filler concentration forms aggregates that are covered by the polymer which causes a decrease in the molecular packing of the resin, and is reflected as low enthalpy values. Finally, silanation of the filler showed a highly endothermic reaction that is probably due to breaking and forming of bonds at the interface between the organic and the inorganic phases in the composites. © 1998 John Wiley & Sons, Inc. J Biomed Mater Res, 40, 224–232, 1998

The in vitro cytotoxicity of urethane dimethacrylate composites cured at different times by visible light and after different aging times and extraction treatments was evaluated using succinic dehydrogenase activity in the mitochondria of a fibroblastic cell line to reflect cell viability. In addition, extractable chemicals associated with cell response were identified. The composite samples were tested untreated, polished, or extracted with water or 75% ethanol-water. Balb/c 3T3 fibroblasts were used as the cell culture system while MTT-formazan production was used as the toxicity parameter. Cell viability was calculated as a percentage of Teflon controls. Identification of the chemicals was measured by extracting the composites with 75% ethanol-water, separating the extract by HPLC, and identifying the fractions with mass spectroscopy. In general, cell viability increased continuously with curing time for differently treated samples at high aging times (288 h) while it decreased when the composites were not aged (0 h). In addition, for 75% ethanol or water-extracted composites, cell viability increased within the first 24 h of aging and reached a plateau after 72 h. Lowest cytotoxicity occurred when the samples were extracted with the 75% ethanol solution. The highest cytotoxic effects were found when the samples were untreated. Slightly reduced cytotoxic effects were seen with polished composites. The results suggest that curing the light-activated composites for a minimum of 150 s and post-curing for 24 h is required to attain comparable biocompatibility with the Teflon control. Removing the oxygen-inhibited layer from these composites decreased the cytotoxicity by 33% while extracting the composites with 75% ethanol-water decreased it by 77%. Chemicals released from the surface accounted for approximately 40% of cellular response while about 60% of the response was due to chemical components released from the bulk. The primary leachable component from the composites was UDMA monomer. Small quantities of 1,6 hexane diol methacrylate, camphoroquinone, and 2,4,6-tritertiarybutyl phenol also were found.

The preparation and characterization of high-surface-area polymeric substrates suitable for the microcalorimetry of protein adsorption are described. High-surface-area polystyrene, poly(styrene-co-butyl methacrylate) and poly(styrene-co-allyl alcohol) were prepared by adsorbing polymer from solution onto fumed silica. Verification of adsorption of polystyrene by silica was determined by noting peak shifts of the surface silanol group in the infrared. The amount of polymer adsorbed was determined from adsorption isotherms. The minimum thickness of polystyrene required to mask silicon oxide properties was found to be that thickness at which contact angles became constant, about 35 A. Polymer densities were measured. Water contact angles on each polymer surface indicate that poly(styrene-co-allyl alcohol) has the surface most wettable by water. Polymer-water interfacial energies were estimated from pendant drop results and a harmonic mean equation along with contact angles. Two methods were used to estimate the polar and dispersion components of the three polymers. Both methods predicted polystyrene to have the highest interfacial energy against water, and one method predicted poly(styrene-co-allyl alcohol) to have the lowest. A Wilhelmy plate study verified the change in interfacial properties as a function of contact time with water. A study of the heats of adsorption of lysozyme by each substrate using a modified Tien-Calvet microcalorimeter demonstrated the suitability of the substrates for microcalorimetry.

SUMMARY The effect of the method of silanation of zirconia-silica hy 3-methacrylox ypropyItrimethoxysilane (MAPM) and 3-acryloxypropyltrimethoxysilane (APM) on the diametral tensile and transverse strength of composites made from these silanated fillers and a urethane dimethacrylate was determined after 0-lOSh of hoiling in water. The water sorption of these composites was also measur-?d at times up to 108 h. Silanation with either silane significantly increased the tensile and transverse strengths and decreased water sorption relative to comparable unsilanated controls. Silanation with MAPM from ethaaol solution at three times the minimum uniform coverage gave the best overall results, and the tensile strength tests appeared to be most predictive of effectiveness of the silane treatments.

The effects of an oligomer, urethane dimethacrylate (UDMA), on two human cell lines were studied using flow cytometry (FCM). Untreated and treated cultures of propidium iodine-stained KB (epidermal oral carcinoma cells) and human foreskin fibroblas (HFF) cells were analyzed for cellular DNA content. Concentrations of 10 and 25 μM of UDMA slightly perturbed the KB cell cycle progression at 24 and 48 h of incubation. However, the effect of 50 μ M was more pronounced at the latter incubation time period. In cell growth experiments, the sublethal concentrations (10 and 25 μM) produced inhibition of KB cell growth rate at a moderate level, which resulted in the prolongation of cell population doubling time. Significant inhibition of cell growth occurred when 50 μM (lethal concentration) was used. Data obtained from the cell cycle perturbation analysis, evidenced by FCM, correlated with the extent of inhibition in KB cell growth rates. The effects of sublethal concentrations were reversible during a 24 h period of oligomer withdrawal from culture medium. In contrast, the effects of 50 μM were not reversible. In HFF cells the depletion of S phase in the cell cycle was the major effect of 50 μM of UDMA. It was concluded that FCM technology is an ideal and practical approach for studying the cytotoxicity of components of dental composites. © 1994 John Wiley & Sons, Inc.

Effects of materials and techniques on the accuracy of models used to make indirect restorations was measured using a 4-unit posterior model containing a MOD and full crown preparation. Improved stone and fast setting epoxy dies backed with either improved stone or a thermoplastic hot melt stone were made from single-viscosity addition silicone impressions. Technique variations included heating or not heating the impression, cooling and pouring dies and placement of the hot melt stone on set or unset epoxy. The dimensions of the MOD (L, W, H) and of the crown (W, H) dies were measured at 1 and 24 h. No clinically significant changes occurred between 1 and 24 h. The stone control reproduced the dimensions of the master die best, and models made by pouring epoxy into the impression followed by immediate pouring of the hot melt stone gave the poorest reproduction. Other variations in technique using epoxy for the anatomical portion gave no practical differences in accuracy. Of the epoxy dies, those prepared from a previously heated impression with hot melt poured after the epoxy set had the best values; however, epoxy dies were smaller than stone dies. The fast set epoxy was noteworthy for rapid processing and sharp detail, however, negative changes for W and H of the crown and H and L of the MOD showed that a die spacer would be essential in the preparation of indirect restorations.

Cell culture techniques were used to determine the source of cytotoxic agents in a commercial BIS-GMA composite. The material was polymerized according to the manufacturer's directions and leachable components were removed by room temperature extraction in ethanol, chloroform, or toluene. The leachable components in the extracts were identified using infrared spectrographic analysis. Thin layer chromatographic analysis was used to determine the number of constituents. These constituents were separated by gas chromatography and then identified by mass spectrographic analysis. Succinic dehydrogenase activity and radioactive labeling with tritiated leucine were used to evaluate cell metabolism and protein synthesis, respectively. The infrared analysis of the extracts showed that the primary component was unreacted BIS-GMA. Trace amounts of 2-hydroxy-4-methoxy-benzophenone, a light stabilizer, as well as a phenyl ester of benzoic acid which was probably degraded from BIS-GMA, were detected by the mass spectrographic method. The removal of leachable components caused a 90% decrease in toxicity compared to the nonextracted BIS-GMA samples. The extracted BIS-GMA samples showed no cellular response compared to the Teflon negative control.

Dielectric characterization evaluated the influence of sorbed moisture on urethane dimethacrylate (UDMA) polymer matrix and unsilanated and silanated zirconia-silica (ZS)-filled UDMA composites. The moisture content (MC) of samples tested ranged from 0 to 7.32% with maximum values for UDMA, silanated and unsilanated ZS-filled UDMA were 3.72, 6.0 and 7.32%, respectively. Dielectric spectra were measured from 0.053 to 100 kHz and from -180 to 180;C. Small MC stiffened the polymers, which was shown by a shift of beta-peaks to higher temperatures, a decrease in the half width of bands and an increase in the change of activation energies. Higher MC resulted in water plasticizing the polymer by shifting the beta-peaks to lower temperatures. Maximum water sorption restricted the motion of the polymer chains and the b-peaks shifted to higher temperatures. Further, increasing MC shifted the a-relaxation to lower temperatures in both UDMA and composites. However, in composites with unsilanated ZS, a new relaxation occurred at high temperatures and low frequencies which was attributed to Maxwell-Wagner-Sillars (MWS) polarization. Thus, in UDMA, water occurs as fragments that form non-freezing clusters. However, in unsilanated ZS-UDMA composites, bulk water can exist at the polymer-filler interface and form disordered structures of ice at low temperatures which can evaporate at high temperatures. Silanated ZS-UDMA composites function similarly to UDMA at low MC but are more plasticized at high temperatures.

Summary The effects of molecular weight of the base silicone, filler and copolymer concentrations on compression set, strain, creep and dynamic elastic modulus were determined. The molecular weight of the base silicone had the least effect on these properties although low molecular weights showed better properties. Chain extenders were detrimental to physical properties of creep, compression set and dynamic elastic modulus. Increased copolymer concentrations decreased compression set, strain and creep and increased the dynamic elastic modulus. The useful copolymer/base functional group ratio was from 1:1 to 3:1. Filler affected properties to a lesser extent than copolymer ratio. Compression set, strain, and creep decreased and the dynamic elastic modulus increased with increasing filler with the properties being directly proportional to the weight fraction. All properties were interrelated with In strain linearly related to In compression set, creep proportional to set, and the dynamic elastic modulus was inversely correlated with strain and creep.

A two-dimensional finite element model of a mandibular quadrant was used to examine the stresses and displacements resulting from a 100 N load placed as follows: (i) distributed on the second molar, (ii) concentrated at 30 degrees to the vertical on the second molar, and (iii) distributed on the second premolar and second molar. Each tooth was supported by periodontal ligaments, cortical and cancellous bone. The modulus and Poisson's ratios for each material were selected from accepted values. The principal stresses were determined throughout the model, with special emphasis being placed for elements in the immediate vicinity of the teeth mentioned above. It was found that for the case where the load was uniformly distributed on the second molar the stresses sigma min were mostly compressive (-) in nature while sigma max were mostly tensile (+). In the case where the load was concentrated and at 30 degrees to the vertical, the magnitude of the stresses were three to five times greater and the pattern less uniform than those resulting from the distributed load. The concentrated load at 30 degrees to the vertical resulted in higher bending stresses than the distributed load and hence higher sigma max stresses.

Synthetic HAP made in the laboratory as 'synthetic bone' may be a possible replacement or repair material, since the mineral phase of dental hard tissue and bone is essentially hydroxyapatite in form. Hydroxyapatite or modified compositions of HAP appear to offer possibilities as a substitute for the original material provided by nature. A high surface area (70.4 m2/g) commercially available hydroxyapatite (HAP) of poor crystallinity was converted to a highly crystalline HAP through sintering at 1200 degrees C in an atmosphere of steam and nitrogen. Compacts were made of the powdered HAP from 500 to 25,000 psi and sintered under the same conditions. The changes in the resulting pore volumes, pore sizes, pore size distributions, densities, and surface areas of the sintered compacts were determined by mercury porosimetry. A 20-fold reduction in open pore volume for sintered compacts was observed. For compacts made at pressures above 5000 psi, the reduction in open pore volume was compensated for by the formation of closed pore volume. The sintering mechanism was attributed mainly to neck formation, growth between particles, diffusion, and plastic deformation.

The cytotoxicity of a series of dental casting alloys in the as-cast and polished condition was determined with cell culture techniques involving phase contrast microscopy to examine cell morphology and the succinic dehydrogenase histochemical reaction to measure any ring of inhibition of Balb/c 3T3 cellular respiration around alloys. Crown and bridge casting alloys and a nickel- and a cobalt-base alloy were biocompatible in the polished condition, but less so in the as-cast condition. The only two exceptions were casting alloys containing 50-60 wt% Cu. Porcelain-fused-to-metal alloys were biocompatible in either the as-cast or polished condition. This direct contact method appeared satisfactory for evaluating biocompatibility of dental casting alloys, especially since these materials are in contact with gingival tissues.

Summary The wear of dental amalgam was studied by single- and double-pass sliding and by a silicon carbide abrasion test. A dispersed amalgam showed significantly better resistance to two-body abrasion than the spherical amalgams tested. The wear of amalgam was determined by resistance to penetration and by a ductile mode of surface failure over the load range studied. Differences in the wear of amalgam and restorative resins and composites are discussed.

Summary Creep compliance measurements were shown to be effective in characterizing the elastic, retarded elastic, and viscous properties of polysulphide, silicone, and polyether impression materials. The test is particularly valuable in that the creep compliance was independent of load, allowing the mechanical properties to be represented by a single total creep compliance curve which can be used to determine the various creep components. The total recovered compliance was shown to be independent of time provided the polymerization had progressed sufficiently before testing was done. The retarded elastic and viscous compliances contributed proportionally to the total creep compliance and thus did not affect the rankings of materials. The viscous compliance of polysulphide A decreased enough after 1 h so that the creep compliance of A and D were not different. The creep compliance measurements showed that the polysulphide materials were the most viscoelastic followed by silicone and polyether impression materials. The polyether was the most nearly ideal elastic material but it had a flexibility comparable to heavy body polysulphide. Of particular interest was the observation that polyether with thinner in ratios up to 1 : 1 by length had no practical effect on the viscoelastic properties.

Summary A comparison of the accuracy of denture bases made from a conventional acrylic and four modified acrylic resins was made with the use of a non-parametric method. Twenty-five identical maxillary stone casts were made and five dentures for each resin were processed by conventional compression moulding techniques. The fit of each denture at the molar-to-molar region on its cast was evaluated by five evaluators who independently ranked the twenty-five dentures from best to worst fit after processing, after polishing, and after storage in water at 37°C for 42 days. The ranking data were analysed by Kendall's coefficient of concordance, the Kruskal-Wallis statistic, and the Mann-Whitney test. Excellent agreement among observers was obtained. After storage in water, two modified acrylic resins, Hydrocryl and Vitalon II, had a better fit on their casts than the conventional acrylic, Lucitone Hypro. Lucitone 199 had the same and Hircoe had a poorer fit than Lucitone Hypro.

Summary Three-dimensional photo-elasticity was used to study the stress distribution in complete dentures in functional positions. A new loading device was developed utilizing a linear motion bearing as its main component and mandibular dentures were evaluated for the first time. The stress concentrations were highest (2.5 fringes) on the working side of the mandibular denture. This was also demonstrated in the maxillary denture; however, the magnitude was less (1.8 fringes). The results indicate that this method could be used to quantitatively evaluate the effect of various occlusal schemes upon the denture bases.

An analysis of stress distributions within circumferential clasp arms with various tapers, was undertaken in an effort to (1) determine the area of maximum stress concentration in both tapered and non-tapered clasps, and (2) to compare this point of maximum stress concentration with the area of frequent clasp breakage. The non-tapered clasp showed better distribution of stresses than the tapered clasps. The less favourable stress concentrations could result in eventual failure of the clasp when coupled with other factors such as porosities. The maximum shear stresses in the non-tapered clasp also were lower than those in the tapered clasps. As the taper of the clasp became more accentuated, the magnitude of the stresses increased and shifted towards the tip of the clasp. The maximum tensile stress calculated in a replica of a chromium-cobalt-nickel clasp was in the order of 34 MN/m2 which is well below the ultimate tensile strength of the alloy. This observation leads to the proposal that failure of clasps is a function not only of design but also of other variables, such as porosities, surface irregularities, and fatigue.

Summary A dimethyl siloxane-triacetoxy terminated silane, Type A adhesive, can be modified with the base component of a medium grade polydimethyl siloxane with vinyl groups, MDX 4-4210, to produce more pliable maxillofacial prostheses. The mechanical properties of samples with various Type A adhesive/MDX 4-4210 ratios were determined. Addition of the MDX 4-4210 base decreased the hardness, modulus, and ultimate tensile strength, while the percentage elongation increased. The initial tear strength was relatively constant for the various ratios except for the 50/50 mix, where a 50% decrease was observed. The ability to obtain different mechanical properties by using various ratios of Type A adhesive and MDX 4-4210 could result in the production of maxillofacial prostheses which will more closely stimulate the properties of facial tissues.

Summary In this study the finite element method was used in conjunction with an axisymmetric model of an idealized first molar to determine the stresses, as well as deflections in the floor of a cylindrical cavity preparation as caused by condensers of various sizes. It was found that for the 0·25 mm thickness of dentine, the 2 mm condenser and a 2 kg load, a tensile stress of approximately 70 MN/m2 was obtained near the centre of the preparation. Under the same load and a 4 mm condenser a tensile stress of only 42 MN/m2 was obtained. Bowen & Rodriguez (1962) and Lehman (1967) reported that the tensile strength of dentine was in the order of 51.7 MN/m2 and 41.5 MN/m2 respectively. Thus it was demonstrated that for certain combinations of load, condenser size, and cavity dimensions, the tensile strength of the dentine in the cavity floor could be exceeded and failure of the cavity floor could ensue.

Four dental cements and one bis-GMA composite were tested in cultures of human periodontal ligament fibroblasts and mouse 3T3 fibroblasts. It was found that these cell types reacted to the cements with different intensities, and that there were differences in evaluation by phase contrast microscopy and succinic dehydrogenase histochemistry.