| Fixed orthodontic treatment is the most widely used orthodontic treatment.The long-lasting firm adhesive force of the bracket on the enamel surface is a necessary condition to ensure that the fixed orthodontic treatment can be performed smoothly.However,long-term fixed brackets,wires and other devices interfere with the cleaning of oral hygiene.These devices also provide a favorable condition for bacteria,and the bacterial acid production leads to partial enamel demineralization and chalky pigmentation,known as White spot lesions(WSL).WSL is the most common and serious complication of fixed orthodontic treatment.Consequently,the research and development of novel functional orthodontic adhesive that can not only obtain firm adhesion with enamel and orthodontic brackets,but also exert antibacterial effect in the oral environment and promote remineralization of enamel,thus effectively preventing the occurrence of WSL is a very significant research topic.In this project,the quaternary ammonium monomer 2-methacryloxylethyl dodecyl methyl ammonium bromide(MAE-DB)was synthesized by using the heat reflux reaction,and the amorphous calcium phosphate nanoparticles(NACP)with the diameter of 110 nm were synthesized by the spray drying technique.Then,these two functional components were added to the PEHB resin matrix which has the function of bonding and ion recharging.Two novel orthodontic adhesives were produced: PEHB + 5% MAE-DB(referred to as PD),PEHB + 40% NACP + 5% MAE-DB(referred to as PND);and commonly used commercial orthodontic adhesive Transbond XT paste(3M,USA)(referred to as TB)was selected as control.By comparing the adhesive properties of three orthodontic adhesives after aging,antibacterial properties and promotion of demineralization resistance of enamel,the properties of novel antibacterial and remineralizing orthodontic adhesives were explored in order to provide a new solution for prevention and treatment of WSL and early prevention of caries.This project was divided into 4 sections.Section one: Test specimens were prepared using the three adhesives for the three-point flexural test,so as to investigate the mechanical properties of adhesives(flexural strength and elasticity modulus).In addition,the three adhesives were tested for water absorbing ability and bonding performance with enamel.Section two: Adhesive property study.The aging bond strength of three groups of adhesives was detected using shear bond strength test after 20,000 thermal cycles and 14,21 and 28 days of acid demineralization,respectively,so as to compare the adhesive properties of the three adhesives.Section three: Antibacterial property study.Discal specimens were prepared using three adhesives,respectively,followed by anaerobic culture of Streptococcus mutans.Afterwards,the adhesion and cell membrane integrity of streptococcus mutans on test specimen surface prepared using the three groups of adhesives were observed with a field emission scanning electron microscope(FESEM)and a confocal laser scanning microscope(CLSM),respectively.Besides,the bacterial survival percentages in biofilm were calculated to compare the antibacterial property of different adhesives.Section four: Remineralization capacity study.Test specimens were prepared using three adhesives,respectively,followed by immersion into the soak solution(p H 4).0.5m L solution was extracted at each detection time point to detect the calcium and phosphorus ion concentrations using an inductive coupled plasma optical emission spectrometer(ICP-OES).The enamel surface micro-topography was compared under FESEM,the 3-dimentional shape and surface roughness of enamel surface were compared using an atomic force microscope(AFM),and the microhardness of different adhesives at different locations and depths of enamel were compared using a microhardness meter,with healthy enamel was the control,so as to explore the demineralization-preventing and remineralization-promoting capacities of the three adhesives.The following results were discovered in our project:1.The novel functional orthodontic adhesive PND has similar flexural strength and elastic modulus as the commercial control adhesive TB(P > 0.05),both significantly higher than that of PD(P < 0.05).The water absorption of three orthodontic adhesives was PND > PD > TB(P < 0.05).2.The bond strength between TB and PND showed no statistically significant differences in immediate as well as at all aging time points of two aging patterns(P > 0.05).In addition,the aged bond strength in PD at the other time points was significantly lower than that in the other two groups(P < 0.05).After 20,000 thermal cycles and after acid demineralization respectively,the shear bond strength of three orthodontic adhesives decreased significantly(P < 0.05).3.The inhibitory biofilm and bactericidal effects of PD and PND with antibacterial monomer were significantly better than those of commercial control TB against Streptococcus mutans.There was compact and thick biofilm on TB surface,accompanying with bacterial expansion and connection.In comparison,sparse bacteria could be observed on PD and PND surface,and lysed or shrunken bacteria cells could also be observed.The results of CLSM showed that the bacterial survival percentages in three groups were decreased with the extension in time.For each culture time point,the bacterial survival percentages in PD and PND were significantly lower than that in TB group(P < 0.05).4.Within the 42 days of detection,orthodontic adhesive PND with NACP filler showed high levels of calcium and phosphate ion release at p H 4.The calcium and phosphate ion concentrations released in TB and PD groups were close to zero.The enamel demineralization resistance of PND group was also significantly better than that of PD and TB.After 28 days of immersion in acidic demineralization solution,obvious demineralization in enamel adjacent to the bonding region of bracket could be observed in TB group and PD group,and the exposed enamel prisms structure could also be observed.In contrast,mild enamel demineralization could be seen in PND group.AFM observed that the surface roughness of enamel in the PND group was significantly lower than that in the TB group and PD group(P < 0.05),and was no significant difference with the healthy enamel control group(P = 0.180).Meanwhile,the microhardness in PND group was higher than that in TB and PD groups at the detection depth of 30 ?m.In summary,current project has successfully synthesized a novel orthodontic adhesive containing the antibacterial quaternary ammonium monomer MAE-DB and the nano-bioactive filler NACP.Our research results indicate that,the novel antibacterial remineralizing adhesive PND containing 5% MAE-DB and 40% NACP has comparable mechanical and adhesive properties with those of the commercial orthodontic adhesive,along with favorable antibacterial and remineralization properties.Consequently,it has broad application prospect in preventing WSL occurrence and development during orthodontic treatment.Importantly,it is of great significance to further explore the methods for early caries prevention and enamel remineralization. |
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