Effect of Fluoride Release on Enamel Demineralization Adjacent to Orthodontic Brackets.

INTRODUCTION AND AIM: This study aimed to evaluate the ability of fluoride-releasing adhesives to inhibit enamel demineralization surrounding orthodontic brackets. METHODS: Two groups of 40 sound human premolars were sectioned mesio-distally. The halves were varnished, and orthodontic brackets were bonded with different adhesive materials. An area 1 mm wide surrounding the brackets was left exposed. Each specimen was immersed daily in a pH cycle for 28 days. In the second group, the specimens were exposed daily to a fluoride solution (250 ppm F-) at 37°C. The fluoride release from different groups was measured. Quantitative light-induced fluorescence (QLF) was used to quantify fluorescence loss of enamel surfaces adjacent to the brackets. Results were statistically analyzed using ANOVA at (p<0.05). RESULTS: Fluoride released from the three fluoride-releasing adhesives was significantly higher (p<0.001) in the group with daily fluoride exposures than in the group without fluoride exposures. Enamel adjacent to brackets bonded with Fuji Ortho LC, Ketac Cem, and Dyract Cem showed significantly less (p<0.001) changes in (ΔQ) value (less demineralization) than enamel bonded with Transbond, the control adhesive material. CONCLUSIONS: Using fluoride-releasing adhesives significantly reduced the level of demineralization adjacent to orthodontic brackets.

Evaluation of the coefficient of thermal expansion of human and bovine dentin by thermomechanical analysis.

The mismatch of thermal expansion and contraction between restorative materials and tooth may cause stresses at their interface, which may lead to microleakage. The present work compared the coefficient of thermal expansion (CTE) with the thermomechanical behavior of human and bovine teeth and determined if the CTE is a suitable parameter to describe tooth behavior. Fifteen human third molar and 15 bovine incisor tooth slices (6×5×2 mm) were allocated to 3 groups according to the test environment: G1 - room condition, G2 - 100% humidity, G3 - desiccated and tested in dry condition. Each specimen was weighed, heated from 20 to 70ºC at 10ºC min-1 and reweighed. The CTE was measured between 20 and 50ºC. Fresh dentin (human -0.49% ± 0.27, bovine -0.22% ± 0.16) contracted on heating under dry condition. Under wet conditions, only human teeth (-0.05% ± 0.04) showed contraction (bovine 0.00% ± 0.03) accompanied by a significantly lower (p<0.05) weight loss than in dry specimens (human 0.35% ± 0.15, bovine 0.45% ± 0.20). The desiccated dentin expanded on heating without obvious weight changes (0.00% ± 0.00). The CTE found was, respectively, in dry, wet and dissected conditions in ºC(-1): human (-66.03×10(-6), -6.82×10(-6), 5.52×10(-6)) and bovine (-33.71×10(-6), 5.47×10(-6), 4.31×10(-6)). According to its wet condition, the dentin showed different CTEs. The thermal expansion behavior of human and bovine dentin was similar. A simple evaluation of the thermal expansion behavior of tooth structure by its CTE value may not be appropriate as a meaningful consideration of the effects on the tooth-material interface.

Properties of indirect composites reinforced with monomer-impregnated glass fiber.

Sufficient flexural strength is required for long-term clinical use of fixed partial dentures made with fiber-reinforced composite. The flexural strengths of indirect composite materials reinforced with a monomer-preimpregnated glass fiber material were determined to evaluate the compatibility of the composites to glass fiber material. Four types (microhybrid, nanohybrid, microfilled, and minifilled) of indirect composites and a unidirectional long glass fiber material were selected for investigation. The composites were placed on a fiber plate and polymerized in accordance with the respective manufacturer's instructions. Rectangular bar fiber-composite specimens were machined and the flexural strength was calculated. The flexural strength of each indirect composite was also measured. The microfilled composite with the lowest filler content (70 wt%) exhibited the highest increase ratio using the fiber, although its strength without fiber reinforcement was the lowest (62.1 MPa). The fiber-microhybrid specimen demonstrated the highest mean strength (355.9 MPa), although the filler content of the microhybrid composite was comparatively low (73 wt%). The type of composite material should be considered for the selection of an optimal fiber-composite combination.

Evaluation of the coefficient of thermal expansion of human and bovine dentin by thermomechanical analysis

The mismatch of thermal expansion and contraction between restorative materials and tooth may cause stresses at their interface, which may lead to microleakage. The present work compared the coefficient of thermal expansion (CTE) with the thermomechanical behavior of human and bovine teeth and determined if the CTE is a suitable parameter to describe tooth behavior. Fifteen human third molar and 15 bovine incisor tooth slices (6×5×2 mm) were allocated to 3 groups according to the test environment: G1 - room condition, G2 - 100 percent humidity, G3 - desiccated and tested in dry condition. Each specimen was weighed, heated from 20 to 70ºC at 10ºC min−1 and reweighed. The CTE was measured between 20 and 50ºC. Fresh dentin (human -0.49 percent ± 0.27, bovine -0.22 percent ± 0.16) contracted on heating under dry condition. Under wet conditions, only human teeth (-0.05 percent ± 0.04) showed contraction (bovine 0.00 percent ± 0.03) accompanied by a significantly lower (p<0.05) weight loss than in dry specimens (human 0.35 percent ± 0.15, bovine 0.45 percent ± 0.20). The desiccated dentin expanded on heating without obvious weight changes (0.00 percent ± 0.00). The CTE found was, respectively, in dry, wet and dissected conditions in ºC-1: human (-66.03×10-6, -6.82×10-6, 5.52×10-6) and bovine (-33.71×10-6, 5.47×10-6, 4.31×10-6). According to its wet condition, the dentin showed different CTEs. The thermal expansion behavior of human and bovine dentin was similar. A simple evaluation of the thermal expansion behavior of tooth structure by its CTE value may not be appropriate as a meaningful consideration of the effects on the tooth-material interface.

A discrepância entre a expansão e a contração térmica dos materiais restauradores e o dente podem causar estresse na sua interface, podendo levar a uma microinfiltração. O presente trabalho determinou e comparou o coeficiente de expansão térmica (CET) com o comportamento termo mecânico dos dentes humanos e bovinos e determinou se o CET é um parâmetro adequado para descrever o comportamento dental. 15 fatias (6×5×2 mm) de terceiros molares humanos e 15 de incisivos bovinos foram divididos em 3 grupos de acordo com o ambiente testado: G1 - condição ambiente, G2 - 100 por cento de umidade, G3 - dissecado e testado em condição seca. Cada espécime foi pesado, aquecido de 20 a 70C a 10ºC min-1 e pesados novamente. O CET foi mensurado entre 20 e 50ºC. Dentina fresca (humana -0,49 por cento ± 0,27, bovina -0,22 por cento ± 0,16) contrai no aquecimento sobre condição seca. Em condição úmida, somente dente humano (-0,05 por cento ± 0,04) mostrou contração (bovina 0,00 por cento ± 0,03) acompanhado por uma significante (p<0.05) perda de massa que os espécimes secos. A dentina dissecada expande no aquecimento sem mudanças óbvias de peso (0,00 por cento ± 0,00). O CET encontrado foi, respectivamente, em condições seca, úmida e dissecada em ºC-1: humana (-66,03×10-6, -6,82×10-6, 5,52×10-6) e bovina (-33,71×10-6, 5,47×10-6, 4,31×10-6). De acordo com sua condição de umidade, a dentina mostrou diferentes CETs. O comportamento de expansão térmica de dentes humanos e bovinos é similar. Uma simples avaliação do coeficiente de expansão térmica da estrutura dental pelo seu valor de CET pode não ser apropriada para uma consideração significativa dos efeitos na interface dente-material restaurador.

Critical excitation spectrum of a quantum chain with a local three-spin coupling.

Using the phenomenological renormalization group (PRG), we evaluate the low-energy excitation spectrum along the critical line of a quantum spin chain having a local interaction between three Ising spins and longitudinal and transverse magnetic fields, i.e., a Turban model. The low-energy excitation spectrum found with the PRG agrees with the spectrum predicted for the (D(4),A(4)) conformal minimal model under a nontrivial correspondence between translations at the critical line and discrete lattice translations. Under this correspondence, the measurements confirm a prediction that the critical line of this quantum spin chain and the critical point of the two-dimensional three-state Potts model are in the same universality class.

Complications associated with intravenous midazolam sedation in anxious dental patients.

AIM: This prospective study was designed to establish the nature, frequency and sequelae of complications arising in patients receiving dental treatment under intravenous midazolam sedation. METHODS: All patients attending the Sedation Department at New-castle Dental Hospital for intravenous sedation over a six-month period were audited. A standardised data-collection pro forma was designed in order to collect data relating to the patient, the sedation episode, the dental treatment and any complications arising. The published standard used in this study states that the incidence of complications should be no more than 8%. RESULTS: Four hundred and one patients were included. The mean dose of midazolam administered was 7.6 mg with a mean titration rate of 0.9 mg/min. Complications were reported in 12 patients (3%), 11 of which were minor and one moderate. All complications were managed successfully within the department with no lasting sequelae. Treatment was completed in 382 (95.7%) patients, with failure to complete treatment in 17 (4.3%) patients due to disinhibition (1), poor cooperation (10), and the sedation wearing off (6). CONCLUSION: The standard was met because complications arising were infrequent (3%) and predominantly minor in nature. Complications were managed conservatively and effectively, with all patients being discharged home the same day with no lasting sequelae. The study demonstrates that intravenous midazolam provides a safe sedation technique, suitable for adult dental patients in primary care, when administered by trained personnel on carefully selected patients and in accordance with nationally agreed protocols and guidelines.

Effect of hydrofluoric acid etching duration on the roughness and flexural strength of a lithium disilicate-based glass ceramic.

The aim of this study was to examine the effect of different acid etching times on the surface roughness and flexural strength of a lithium disilicate-based glass ceramic. Ceramic bar-shaped specimens (16 mm x 2 mm x 2 mm) were produced from ceramic blocks. All specimens were polished and sonically cleaned in distilled water. Specimens were randomly divided into 5 groups (n=15). Group A (control) no treatment. Groups B-E were etched with 4.9% hydrofluoric acid (HF) for 4 different etching periods: 20 s, 60 s, 90 s and 180 s, respectively. Etched surfaces were observed under scanning electron microscopy. Surface profilometry was used to examine the roughness of the etched ceramic surfaces, and the specimens were loaded to failure using a 3-point bending test to determine the flexural strength. Data were analyzed using one-way ANOVA and Tukey's test (?=0.05). All etching periods produced significantly rougher surfaces than the control group (p<0.05). Roughness values increased with the increase of the etching time. The mean flexural strength values were (MPa): A=417 ± 55; B=367 ± 68; C=363 ± 84; D=329 ± 70; and E=314 ± 62. HF etching significantly reduced the mean flexural strength as the etching time increased (p=0.003). In conclusion, the findings of this study showed that the increase of HF etching time affected the surface roughness and the flexural strength of a lithium disilicate-based glass ceramic, confirming the study hypothesis.

Effect of hydrofluoric acid etching duration on the roughness and flexural strength of a lithium disilicate-based glass ceramic

The aim of this study was to examine the effect of different acid etching times on the surface roughness and flexural strength of a lithium disilicate-based glass ceramic. Ceramic bar-shaped specimens (16 mm x 2 mm x 2 mm) were produced from ceramic blocks. All specimens were polished and sonically cleaned in distilled water. Specimens were randomly divided into 5 groups (n=15). Group A (control) no treatment. Groups B-E were etched with 4.9 percent hydrofluoric acid (HF) for 4 different etching periods: 20 s, 60 s, 90 s and 180 s, respectively. Etched surfaces were observed under scanning electron microscopy. Surface profilometry was used to examine the roughness of the etched ceramic surfaces, and the specimens were loaded to failure using a 3-point bending test to determine the flexural strength. Data were analyzed using one-way ANOVA and Tukey's test (?=0.05). All etching periods produced significantly rougher surfaces than the control group (p<0.05). Roughness values increased with the increase of the etching time. The mean flexural strength values were (MPa): A=417 ± 55; B=367 ± 68; C=363 ± 84; D=329 ± 70; and E=314 ± 62. HF etching significantly reduced the mean flexural strength as the etching time increased (p=0.003). In conclusion, the findings of this study showed that the increase of HF etching time affected the surface roughness and the flexural strength of a lithium disilicate-based glass ceramic, confirming the study hypothesis.

O objetivo deste estudo foi avaliar o efeito de diferentes tempos de condicionamento ácido na rugosidade de superfície e resistência flexural de uma cerâmica à base de disilicato de lítio. Espécimes cerâmicos em forma de barra (16 mm x 2 mm x 2 mm) foram produzidos a partir de blocos cerâmicos. Todos os espécimes foram polidos e limpos em banho de ultrasom em água destilada. Os espécimes foram aleatoriamente divididos em 5 grupos (n=15). Grupo A (controle) sem tratamento. Grupos B-E condicionamento com ácido fluorídrico 4,9 por cento (HF) por 4 diferentes períodos de condicionamento: 20 s, 60 s, 90 s e 180 s, respectivamente. As superfícies condicionadas foram observadas sob microscopia eletrônica de varredura. Perfilometria de superfície foi utilizada para examinar a rugosidade das superfícies condicionadas, e os espécimes foram carregados até a falha pelo teste de flexão três pontos. Os valores foram analisados usando ANOVA um fator e teste de Tukey (?=0,05). Todos os períodos de condicionamento produziram superfícies significantemente mais rugosas do que o grupo controle (p<0,05). Os valores de rugosidade aumentaram com o tempo de condicionamento. Os valores médios de resistência à flexão foram (MPa): A=417 ± 55; B=367 ± 68; C=363 ± 84; D=329 ± 70; e E=314 ± 62. O condicionamento com HF reduziu significativamente os valores médios de resistência à flexão conforme o tempo de condicionamento aumentou (p=0,003). Os achados deste estudo mostraram que o aumento do tempo de condicionamento ácido influenciou a rugosidade de superfície e resistência flexural de uma cerâmica à base de disilicato de lítio confirmando a hipótese do mesmo.

Effect of an Er,Cr:YSGG laser on water perfusion in human dentine.

Changes in fluid perfusion through tubules may affect the sensitivity of exposed and restored dentine. The rate of perfusion is dependent on the structure and composition of dentine, particularly at the surface. This work analyzed the effect of treatment with an Erbium, Chromium-doped: Yttrium Scandium Gallium Garnet (Er,Cr:YSGG) laser on dentine perfusion. Extracted molars were sectioned above the mid-coronal portion, and below the cemento-enamel junction, to create crown segments. The pulp was extirpated and the dentine treated with the Er,Cr:YSGG laser, a diamond bur or sandpaper. Each specimen was mounted, with the pulp chamber oriented upwards, on a petri-dish cover in order to permit a tube filled with water to be connected to the pulp. Movement of the water meniscus over 24 h provided a measurement of the volume of water that filtered across the dentine. The dishes contained water to provide a moist environment in the relevant specimens, or were left dry to provide dry conditions. Specimens were perfused for 24 h with water pressures ranging from 20 to 60 cm. The results (in µl mm(-2) d(-1) ) showed a significant difference in the perfusion rate between treatments. The difference between perfusion in wet and dry conditions was highly significant. Laser and bur treatment of dentine significantly affected perfusion, which was higher in the former than in the latter. Therefore, laser-treated dentine may be more sensitive than bur-cut dentine.

Use of micro-Raman spectroscopy to investigate hybrid layer quality in demineralized root dentine.

Treatment of early root caries using resin adhesives to stabilize demineralized dentine and deprive bacteria of nutrients is a recognized and conservative treatment option. Essential to its success is a stable resin-dentine interface, with effective infiltration of demineralized dentine and adequate resin curing. The objective of this study was therefore to examine degree of cure (DoC) and depth of penetration of dentine adhesives in demineralized dentine using micro-Raman spectroscopy. Three commercially available adhesives were applied essentially according to manufacturers' instructions to visibly wet demineralized dentine, with two coats of primer to reduce dilution. Specimens were sectioned perpendicular to the bonded interface and micro-Raman spectroscopy performed on the cross section. Molecules associated with the adhesive were detected to depths greater than 100 µm indicating a thicker hybrid layer than seen in sound dentine. The hybrid layer showed an increase in proportion of aliphatic C=C double bonds at the base of the hybrid layer, which may be a consequence of reduced DoC or phase separation. Micro-Raman spectra suggest deep zones contain an increased concentration of HEMA and a lower concentration of BisGMA, which is likely to be less stable and may contribute to early breakdown of the dentine adhesive interface.

Neutralizing effect by resin-based materials containing silane-coated glass fillers.

The aim of this study was to evaluate the fluoride release, neutralizing ability and inhibitory effect on secondary caries of resin-based materials containing a silane-coated glass filler. Resin-based materials containing fluoro-boro-alumino-silicate glass coated by 3-methacryloxypropyltrimethoxysilane or 3-aminopropyltrimethoxysilane were used and resin material containing glass filler without coating was used as a control. The fluoride release and pH value after immersion were measured for 10 weeks. The inhibitory effect was also evaluated. During the initial period, the material with 3-aminopropyltrimethoxysilane gave the greater amount of fluoride release and produced a higher pH value compared with the other materials. However, the neutralizing ability of the material with 3-aminopropyltrimethoxysilane filler became weaker with ageing of the specimens. The mean depth of outer lesions was similar among the three materials. Within the limitations of this study, secondary caries around restorations could not be inhibited even for products showing high fluoride releasing and neutralizing ability.

A new acrylic-based fluoride-releasing cement as a potential orthodontic bonding agent.

OBJECTIVE: To develop a fluoride-releasing, acrylic-based 'easy on, easy off' bracket cement as a potential orthodontic bonding agent. MATERIAL AND METHODS: Three experimental cements were prepared in powder/liquid forms by mixing different ratios of methylmethacrylate (MMA) and 2-hydroxyethyl methacrylate (HEMA) to form the liquid (L) and sodium fluoride (NaF) and polymethylmethacrylate (PMMA) to form the powder (P). The resultant materials were tested for setting characteristics, fluoride release, hardness, strength, shear bond strength (SBS) and adhesive remnant index in comparison with resin composite and glass ionomer, which were used as control materials. The data were analyzed using ANOVA and the Kruskal-Wallis and Mann-Whitney tests. RESULTS: The experimental groups had satisfactory setting characteristics. Fluoride release of the group containing P (10% NaF, 90% PMMA) and L (60% MMA and 40% HEMA) was similar to that of glass ionomer. When experimental materials were stored in water for 7 days, their hardness was reduced and stabilized at a value lower than those for composite and PMMA. Strength was only slightly affected by water storage. The SBSs of the experimental groups were considered clinically acceptable at both 30 min and 1 month. The group containing P (10% NaF, 90% PMMA) and L (90% MMA and 10% HEMA) had a higher mean SBS than the other two experimental groups. At 1 month, there were significantly less adhesive remnants observed on the surface of enamel after debonding for the experimental groups compared with the composite. CONCLUSION: The new cement could potentially be useful as an orthodontic bonding agent.

Crown retention for non-retentive preparations using adhesive resin cements.

This study examines the effect of preparing teeth with six varying convergence angles (ranging between 12 and 120 degrees) and height (1 and 2 mm) on the retention of cast gold crowns. Six groups of 4 human premolar teeth were prepared to give a flat occlusal surface in dentine with very short axial wall heights (1 or 2 mm) and variation in axial wall convergence (between 12 and 120 degrees). Impressions were recorded of the prepared teeth and custom castings made using a high copper content precious metal alloy. The castings were luted with an adhesive resin and stored in water at 37 degrees C for 24 hours prior to determining the pull-off force in an Instron Universal testing Machine at a cross head speed of 1mm min(-1). Twenty two of the twenty four tested specimens failed within the dentine of the tooth, irrespective of preparation orientation. Statistical testing using ANOVA demonstrated that there were no differences between the bond strength values for any of the preparation convergence angles or between preparations with 1 and 2 mm axial wall heights. This suggests that attachment strength of adhesively bonded castings with minimal axial wall height preparations is not influenced by the convergence angle of the preparation. The attachment strength exceeded that cohesive strength of the underlying dentine in nearly all of the adhesively luted restorations.

Relative efficiency of radiation sources for photopolymerization.

The aim of this study was to evaluate the characteristics of new-generation light-emitting diode (LED) units in comparison with the conventional tungsten-halogen, plasma arc, and first-generation LED units reported in our previous study. The irradiance of light from new-generation LED units, the temperature rise of the bovine enamel surface, and the depth of cure of composites exposed to each unit were investigated. The irradiances in the range 400-515 nm emitted from the new-generation LED units were greater than those from the first-generation LED units. The temperature increase was 15-25 degrees C for new-generation LED units compared with a typical value of 5 degrees C for the first-generation LED units at 10 s of irradiation. The relationship between the depth of cure and the logarithm of total exposure energy suggested that LED units can cure light-cured composite resins more efficiently than tungsten-halogen or plasma arc units.

Comparative study of tubular diameter and quantity for human and bovine dentin at different depths.

This study compared the tubular dimensions and distribution of human and bovine dentin. Ten human molars and 10 bovine incisors were ground with a high-speed handpiece to obtain 3 sections at different dentin depths (superficial, middle and deep). The specimens were sputter-coated with gold to be examined under scanning electron microscopy (SEM). Three SEM micrographs were recorded randomly for each dentin depth. The number of tubules was counted and the diameter of 5 tubules selected at random was measured in each SEM micrograph. Data were analyzed statistically by ANOVA and Tukey's test (alpha=0.05). In bovine teeth, superficial dentin (4.21 microm) and middle dentin (3.98 microm) had a significantly greater (p<0.05) diameter than deep dentin (3.14 microm) tubules. In human teeth, superficial dentin tubule diameter (2.42 microm) was significantly smaller (p<0.05) than deep dentin (2.99 microm) and middle dentin (2.94 microm) tubule diameters, which did not differ significantly from each other (p>0.05). The number of tubules per square millimeter, regardless of the region, was significantly greater in human dentin (22,329) than in bovine dentin (15,964). There was a clear difference in tubule structure and morphology between human and bovine dentin.

Comparative study of tubular diameter and quantity for human and bovine dentin at different depths

This study compared the tubular dimensions and distribution of human and bovine dentin. Ten human molars and 10 bovine incisors were ground with a high-speed handpiece to obtain 3 sections at different dentin depths (superficial, middle and deep). The specimens were sputter-coated with gold to be examined under scanning electron microscopy (SEM). Three SEM micrographs were recorded randomly for each dentin depth. The number of tubules was counted and the diameter of 5 tubules selected at random was measured in each SEM micrograph. Data were analyzed statistically by ANOVA and Tukey's test (a=0.05). In bovine teeth, superficial dentin (4.21 μm) and middle dentin (3.98 μm) had a significantly greater (p<0.05) diameter than deep dentin (3.14 μm) tubules. In human teeth, superficial dentin tubule diameter (2.42 μm) was significantly smaller (p<0.05) than deep dentin (2.99 μm) and middle dentin (2.94 μm) tubule diameters, which did not differ significantly from each other (p>0.05). The number of tubules per square millimiter, regardless of the region, was significantly greater in human dentin (22,329) than in bovine dentin (15,964). There was a clear difference in tubule structure and morphology between human and bovine dentin.

Este estudo comparou a dimensão e a distribuição tubular da dentina humana e bovina. Dez molares humanos e 10 incisivos bovinos foram desgastados com alta rotação para se obter 3 secções de diferentes profundidades de dentina (superficial, média e profunda). As amostras foram cobertas com ouro em um metalizador para a observavação em MEV. Três micrografias foram tiradas aleatoriamente para cada profundidade de dentina. O número de túbulos foi contado e o diâmetro de 5 túbulos selecionados aleatoriamente foi medido para cada micrografia. Os resultados foram submetidos à análise de variância e ao teste de Tukey (a=0.05). Em dente bovino, a dentina superficial (4,21 μm) e a dentina média (3,98 μm) apresentaram diâmetro estatisticamente superior (p<0,05) à dentina profunda (3,14 μm). Em dente humano, o diâmetro dos túbulos da dentina superficial (2,42 μm) mostrou-se significativamente menor que a dentina profunda (2,99 μm) e média, (2,94 μm) as quais não diferiram entre si. O numero de túbulos por mm2, independente da região, foi significativamente maior para a dentina humana (22,329) que para dentina bovina (15,964). Houve uma clara diferença na estrutura tubular e morfológica entre dentina humana e bovina.

Polymerisation shrinkage of luting agents for crown and bridge cementation.

The volumetric contraction of a variety of luting agents, including Panavia 21, All-Bond C&B Cement, Superbond., Variolink and zinc phosphate cement, was assessed and compared using a minimal transducer The contraction among the materials tested was determined from the post-gel linear displacement of a deflecting coverslip resting on 4 silicone rubber spacers between which a mixed material was centrally located. Finally, the rate and duration of shrinkage over a period of 60 minutes from the moment of rigid contraction for all materials were investigated, and the final shrinkage values among the materials tested were compared. The analysis of variance showed that there were highly significant differences (P< 0.001) between the materials and the mean of each group was significantly different from that of any of the other groups (Tukey's test). Superbond produced the highest values of the final polymerisation shrinkage, followed by All bond C&B, Variolink, Panavia 21 and zinc phosphate cement respectively. Also, there was a marked variation in the overall magnitude of shrinkage (from 1.34% to 4.62%) among the materials tested. The method used to measure the polymerisation shrinkage in the present study was shown to be a precise measure in that it produced consistent and reproducible results. It can also offer the ability to observe the development of polymerisation shrinkage against time, during the post-gelation phase, for a range of chemically-cured resin materials.

Determination of bactericidal activity of antibacterial monomer MDPB by a viability staining method.

In this study, the bactericidal activity of antibacterial monomer MDPB (12-methacryloyloxydodecylpyridinium bromide) against Streptococcus mutans was tested by a rapid method for monitoring viability. To S. mutans culture containing fluorescence staining solution that distinguishes live from dead cells, MDPB was added at a concentration of 250, 100, 50, or 10 microg/ml. Bacterial cells were observed by fluorescence microscopy and the percentage of dead cells was calculated. After 10, 20, or 30 minutes' contact with MDPB, the live/dead ratio was measured by fluorometry and viable counts (CFU) determined by the conventional plating method. Viability staining revealed that MDPB exhibited significant bactericidal effects at 50 microg/ml or greater (ANOVA, Fisher's PLSD test), and complete killing of the cells at 250 microg/ml of MDPB was demonstrated in conjunction with a plating method. The staining method thus provided a sensitive means to determine loss of viability, and indicated the strong killing effects of MDPB on S. mutans.