Curing light-activated composite cement through porcelain.

The purpose of the study was to determine the exposure times needed to harden a light-activated composite cement through various thicknesses of porcelain. A negative exponential model was developed with which the relative hardness of a composite could be predicted from the product of the exposure time and the square root of the light intensity at 470 nm. Hardness data obtained by varying the exposure time and porcelain thickness were found to fit this model. A relationship was derived to predict the required exposure time, t, of a composite under porcelain with a transmission coefficient, Tc, and a thickness, l:t = toTc-1/2, where to is the exposure time of the composite alone.

The effect of rotary instrumentation on the permeability of dentin.

The filtration and diffusion of tritiated water through dentin disks were measured ina split-chamber diffusion cell. The dentin had been cut with a diamond disk and the surfaces modified with a carbide fissure bur or diamond bur. Disks were given a secondary burnishing treatment with a blank bur or a modified blank bur. Burnishing reduced the permeability of dentin cut with a fissure bur.

Creep of amalgam at low stresses.

The creep and recovery of dental amalgam were studied with a torsional creep apparatus. The purpose was to investigate viscoelastic behavior in a low stress range that might result from normal chewing forces. The creep of specimens up to six months old was also studied. Four types of alloys for amalgam were used: two single-composition sphericals (SCS), an admixture-lathe-cut eutectic (ALE), and a conventional (CON). Constant torque was applied to dumbbell-shaped specimens for three hr and, after the stress was released, recovery was followed for 50 hr. All measurements were made in water at 37 degrees C after storage of the specimens for one week in 37 degrees C water. Creep was also measured at nine intervals on specimens aged from three hr to six months. The SCS amalgams exhibited the least amount of creep, followed by the ALE amalgam. The CON amalgam had the highest creep. The SCS amalgams approached linear viscoelastic behavior in the shear stress range of 0.78-4.10 MPa at 37 degrees C. They did not exhibit steady-state (viscous) creep at stresses less than 4.10 MPa. Recovery was complete but proceeded more slowly than creep. The ALE and CON amalgams exhibited viscous creep at 1.23 MPa and higher. The steady-state creep rate depended on stress, and the stress exponent ranged from 2.3-3.5. All except the ALE amalgam showed essentially constant creep compliance when aged up to six months, once the amalgamation reaction was completed after 24 to 48 hr. The compliance of the ALE amalgam gradually decreased over the six-month period.

Effect of surface preparation on phase distribution of amalgam surfaces.

Several types of amalgams were prepared with carved, burnished, or polished surfaces. Elemental analyses were made of these surfaces with the electron microprobe and the phase fractions were calculated from these data. The phase distributions were found to vary with the technique of finishing.

Estimation of the phase distribution in dental amalgam with the electron microprobe.

The phase distributions of several dental amalgams have been determined by two methods: calculation from elemental analysis data obtained with the microprobe, and measurement of phase areas on backscattered electron photographs with the image analyzer. The results of these methods agree, in general, with the distributions predicted by the stoichiometric equations.

Interferometric measurements of cusp deformation of teeth restored with composites.

A Michelson interferometry apparatus was used for measurement of the displacement of the buccal cusps of premolars after restoration of MOD preparations with composites. The effects of composite type, cavity size, and hydration conditions were examined. Interferometry permitted real-time measurement of cusp movement as it occurred. Contraction occurred very rapidly, about 1/3 of the 60-minute amount within the two-minute period of exposure to the curing light. Cusp movement was smooth rather than interrupted, indicating lack of microfracturing at deformations of 11-46 microns. Contraction, 0.94% for Heliomolar and 1.2% for P-50, was similar to the linear polymerization shrinkage of the resins. Less cusp movement occurred in small cavities than in large cavities. Hydrated teeth had less cusp movement than dehydrated teeth.

Composite resin compatibility and bond longevity of a dentin bonding agent.

The shear strength of a commercial dentin bonding agent was studied using human dentin in vitro. The agent was tested for compatibility with three proprietary composite resins. In addition, some samples were subjected to prolonged water immersion and thermocycling to determine effects on bond strength. There was no statistically significant difference in shear strengths when the three composite resins were used. In addition, there was no difference in shear strengths among those samples thermocycled or immersed for up to one year. The implications of this study are that the dentin bonding agent may be used with several composite resins with no significant difference in bond strengths, and that the bond seems stable when subjected to immersion or simulated intra-oral thermal stresses.

Build-up and repair of light-cured composites: bond strength.

Interfacial bond strengths of light-activated composites were measured as a function of age using a transverse strength test. Bond strength between layers decreased with the age of the initial layer and reflected the setting curves of the composites. The highly-filled composites exhibited the greatest bond strengths. Uncut surfaces provided a better substrate for bonding than did ground surfaces. Use of a bonding agent on both uncut and ground surfaces improved bond strengths. Mean repair strengths of light-activated composites were similar to those of self-curing composites. Composites with ground surfaces aged for one wk had mean repair strengths 27% of the cohesive strength without bonding agent and 48% with bonding agent.

Effect of clinical finishing procedures on amalgam microstructure.

Spherical amalgam specimens were burnished, burnished-polished, polished (wet), or polished (dry). The amalgams were sectioned normal to the prepared surface, elemental analyses were made at a series of depths from the surface with the electron microprobe and the phase fractions were calculated. Significant variations in phase distributions exist within 100 micrometers of the surface.

Photo-activation of resin cements through porcelain veneers.

The purpose of this study was to determine the effects of porcelain opacity, chemical catalyst, and exposure time on polymerization of light-activated resin-composite cements. Samples of microfill and hybrid composites, with and without catalyst (i.e., dual-cure and visible-light-activated), were polymerized by exposure to visible light through porcelain discs of different opacities. Microhardness testing (KHN) was used to compare degree of cure for each material at various exposure times. Porcelain opacity did not significantly affect hardness. However, the results indicated that a chemical catalyst and prolonged curing times might be essential for clinical success.

The contribution of dental amalgam to mercury in blood.

We determined the exposure to mercury from dental amalgam by comparison of blood levels of mercury before and after removal of all amalgams from ten subjects. Baseline concentrations of mercury in whole blood were measured weekly for four to 18 weeks (median = 6.6 weeks) prior to removal. All amalgams were removed in a single appointment. The subjects had an average of 14 surfaces of amalgam, seven of which were occlusal surfaces. Weekly blood sampling was continued for five to 18 weeks (median = 7.6 weeks) after the amalgams were removed. The mean baseline concentration of total mercury in whole blood of the ten subjects was 2.18 (SD = 0.90) ng Hg/mL before the amalgams were removed. The baseline mercury levels were related to the number of amalgam surfaces. The linear correlation coefficient was 0.724 with number of occlusal surfaces, and 0.433 with total number of surfaces. After removal of the amalgams, nine of the ten subjects exhibited a statistically significant decrease in blood mercury at the 95% level of confidence. The mean decrease in mercury was 1.13 (SD = 0.60) ng Hg/mL. The half-time for elimination of mercury from blood after amalgam removal was 30.2 (SD = 5.8) days. Removal of the amalgams provided an additional exposure of 1.46 (SD = 1.17) ng Hg/mL that was rapidly cleared from the blood with a half-time of 2.9 days. The daily intake of mercury from amalgam in the subjects was estimated to be at least 1.3 micrograms.

Analysis of debond rates of resin-bonded prostheses.

Debonding rates of 164 resin-bonded prostheses (RBP) placed from 1974 to 1985 were examined at recall. The yearly debond rates were calculated, the cumulative probability of failure was determined for the debonded RBPs, and Weibull distribution parameters were determined. For perforated prostheses, there was a high initial debond rate, followed by slowly decreasing rates for about 5 years. Increasing rates after 5 years indicated the beginning of wear-out. The Weibull parameters for the wear-out period of the perforated prostheses were: characteristic life = 128 (SE = 2) months, modulus = 1.27 (0.06), and threshold parameter = 0. Data were available for etched frameworks for 6 years and showed decreasing debond rates during this time. The characteristic life of the etched bridges was 318 (84) months. The cumulative failure data were also modeled with a cubic polynomial distribution that resulted in a U-shaped hazard function.

The effect of dental amalgams on mercury levels in expired air.

The expired air of a group of 48 persons, 40 with and eight without dental amalgam restorations, was analyzed for its mercury content before and after chewing. Expired air samples were collected in polyethylene bags, and a known quantity of each was pumped into the mercury detector for measurement. The results showed that examined subjects with dental amalgams had higher pre-chewing mercury levels in their expired air than those without amalgams. After chewing, these levels were increased an average of 15.6-fold in the former and remained unchanged in the latter group. It was concluded that in situ dental amalgams can increase the level of mercury in expired air.

Effect of ultrasonic vibration on post removal in extracted human premolar teeth.

The purpose of this study was to determine the effectiveness of ultrasonic vibration in removing Paraposts from extracted teeth. Paraposts were cemented in mandibular premolars to a depth of 9 mm with zinc phosphate cement and the teeth placed in four groups. Group 1 received no vibration. Group 2 received vibration for 4 min, group 3 received vibration for 12 min, and group 4 received vibration for 16 min. Tensile forces were applied to the posts and mean dislodgment forces compared. The mean force (kg) required to dislodge the Parapost in group 1 was 24.92 +/- 1.64 SEM; in group 2, 25.01 +/- 1.80; in group 3, 24.08 +/- 2.29; and in group 4, 12.41 +/- 2.60. There was a significant difference between group 4 and groups 1 to 3 (p = 0.0003). Results of this study indicate that 16 min ultrasonic vibration is an effective method for removing Paraposts from human premolar teeth.

Composition of clinically aged amalgam restorations.

Twenty-five amalgam restorations ranging in age from two to 25 years were obtained from five subjects. An electron microprobe was used to analyze the specimens for bulk elemental composition and phase composition, and the volume fractions of phases were determined by point-counting on back-scattered electron micrographs. Twenty-one of the specimens were conventional, low-copper amalgams, and the remaining four were high-copper amalgams. The bulk elemental compositions showed little variation from newly prepared amalgams except for the presence of a small amount of chloride and other contaminants. The compositions of the phases were essentially the same as is found in new amalgams, except that there was considerable internal amalgamation of gamma particles. The distribution of phases in the clinically aged amalgams was quite different from that of new amalgams. The low-copper amalgams had decreased amounts of gamma, gamma-1, and gamma-2 phases and increased beta-1 and tin-chloride. The high-copper admixed amalgams had decreased gamma, increased beta-1, and enlarged reaction rings (gamma-1 and eta').

Evaluation of new methods for composite repair.

This study evaluated the effectiveness of air abrasion (sandblasting), hydrofluoric acid (HF) etching, and acidulated phosphate fluoride (APF) for composite repair. Five different composites (three hybrids, one small-particle, and one microfill) were used to test the effects of different compositions and particle sizes. A phosphonate resin bonding agent was used for all repair procedures. In general, air abrasion provided the strongest repairs, approximately 60% of cohesive strength. The effect of HF varied with different composites, either improving or reducing the repair strength. APF gave the lowest interfacial bond strengths for all composites except the microfill.

Microleakage of indirect composite inlays.

The purpose of this in vitro study was to evaluate the microleakage of indirect composite inlays and to compare leakage of inlays with directly placed composite restorations. Standardized MOD preparations were cut in 50 extracted human molars. One gingival margin was placed in enamel above the CEJ, and the other was placed in dentin below the CEJ. Two groups of teeth were directly filled with composites (P-30 and Heliomolar) after being etched, and dentin bonding agents were applied. Two groups of teeth were restored with composite inlays that were fabricated on stone dies. The inlays were made and luted with the same two composites. The last group of teeth was restored with Heliomolar inlays luted with Dual cement. The specimens were thermocycled 300 times between 5 and 50 degrees C. Microleakage was evaluated by use of the silver-nitrate staining technique. The depth of leakage was measured microscopically after the teeth were sectioned. Both direct restorations and inlays showed substantial leakage at gingival-dentin margins; however, there was only superficial leakage at enamel margins. P-30 inlays and Heliomolar inlays cemented with Dual leaked less than direct restorations at the gingival-dentin margins. There was no difference in leakage of enamel margins of inlays and direct restorations, except that direct Heliomolar restorations leaked more than the others. There was no difference in leakage between Heliomolar restorations luted with light-cured or dual-cured cement.

Mode of failure in the dentin-adhesive resin-resin composite bonded joint as determined by strength-based (muTBS) and fracture-based (CNSB) mechanical testing.

OBJECTIVE: To determine the failure mode between dentin-adhesive resin-resin composite bonded joint produced with a chevron-notch short-bar (CNSB) and microtensile test methods. METHODS: Forty teeth were randomly selected for microtensile and forty for CNSB specimen fabrication and stored in 0.5% chloramine T at 37 degrees C until respective static load to failure testing at 30 and 180days. Failure modes were categorized by SEM and tested with Fisher's exact test. Within respective mechanical testing methods the probability of failure curve distributions being significantly different were analyzed by the Wald chi-square statistic. RESULTS: The characteristic fracture toughness at 30- and 180-day storage was 0.82 and 0.87MPam(1/2), while the Weibull Modulus (m) for the failure distributions, was 4.60 and 4.56, respectively. No significant difference was demonstrated in the failure distributions between these groups (p=0.45). The characteristic tensile strength (muTBS(o)) at 30- and 180-day storage was 52.53 and 14.71MPa with an m of 3.04 and 1.56, respectively. Failure distributions for muTBS groups were significantly different (p<0.001). K(IvM) failure modes, regardless of storage time, were within the adhesive joint with 30-day debonds primarily through the top region of the hybrid layer (THL) and after 180-days involving the bottom of the hybrid layer (BHL). The 30-day muTBS group demonstrated a propensity to debond in dentin or resin composite substrates but after 180-days storage debonds again involved the BHL. SIGNIFICANCE: The weak links in the dentin-adhesive resin-resin composite bonded joint may be the interphase regions between the THL and the adhesive resin and the BHL and dentin.

The influence of water storage and C-factor on the dentin-resin composite microtensile bond strength and debond pathway utilizing a filled and unfilled adhesive resin.

OBJECTIVE: To test the elastic wall concept utilizing adhesive resins of varying stiffness in a low- and high-C-factor cavity design after short- and long-term water storage. METHODS: A flat and box-shaped cavity was restored on occlusal dentin with a resin composite using a filled and unfilled adhesive resin from which microtensile specimens with a 0.5mm(2) cross-sectional area were formed. After storage for 30- and 150-days the microtensile bond strength (muTBS) was determined in a Zwick materials testing machine and the subsequent debond pathway was examined under scanning electron microscopy. Fisher's exact test was used to determine differences in joint and substrate failure modes and a Weibull regression model with gamma frailties was used to test for differences between failure distributions. Tests for three-way and two-way interactions were also completed for storage time, C-factor and adhesive. All tests were at 95% confidence levels. RESULTS: The characteristic strength (TBS degrees ) for the Optibond FL adhesive applied on a flat cavity was 47.57 and 20.90MPa and a box-shaped cavity was 49.26 and 17.49MPa for short- and long-term storage, respectively, while the corresponding TBS degrees for the unfilled Optibond adhesive on the flat cavity design was 36.93 and 32.68MPa and in a box-shaped cavity was 32.84 and 15.46MPa. Combining all groups according to storage time revealed a three-fold increase in the debond pathway including the bottom of the hybrid layer. SIGNIFICANCE: Evidence suggests that the durability of the bonded joint is threatened by hydrolysis and the most susceptible region is the bottom half of the hybrid layer and in low C-factor cavity designs a more flexible adhesive resin liner was more durable.

Cusp movement in premolars resulting from composite polymerization shrinkage.

This study examined the effect of cavity size, hydration conditions and type of composite on cusp movement following placement of MOD composite restorations in vitro. A microscope with a micrometer stage was used to measure deflection of cusps for 14 days after initiation of curing. The intercuspal width decreased for the first few minutes after the composites were placed, then increased toward the original dimensions. Teeth with small cavities contracted less than those with large cavities, and recovery was complete or nearly complete only in teeth with small cavities. Hydrated teeth and restorations recovered more rapidly and more completely than those that were dry. Differences in deflection of the cusps were observed with two different restorative materials.