the most common type of acrylic denture failure is debonding or fracture of the denture teeth , accounting for approximately 33% of failures.1 this failure usually occurs in the anterior region of the denture .
dentists and patients both prefer chair - side repair using auto - polymerized acrylic resin , because it is a straightforward procedure and takes less chair time than repair in the laboratory .
a likely explanation for teeth debonding is weak bond due to contamination of the tooth surfaces with substances such as wax during laboratory processing .
another cause is a chemical difference between the tooth and denture base because of different processing methods.2 weakness of the repaired interface leads to recurrent debonding .
therefore , the surface treatment method , mechanical or chemical , is important for denture repair .
the placement of a diatoric recess ( mechanical ) and the use of a bonding agent ( chemical ) on denture teeth resulted in higher bond strengths compared with no treatment.34 methyl methacrylate ( mma ) is the most frequently used chemical surface treatment agent .
mma is effective as an adhesion promoter because of its chemical similarities to denture base .
however , surface treatment with mma requires 3 minutes to effectively prime the surface for ultimately reducing adhesive failure,5 thus requiring an excessive time .
other solutions have been used as chemical surface treatments ( e.g. , chloroform , methylene chloride ( dichloromethane ) , and 4-methacryloxyethyl trimellitate anhydride ( 4-meta ) ) .
although chloroform and methylene chloride have been used , they have been identified as being carcinogenic.6 researchers6 found that two solutions , methyl formate and methyl acetate , were non - toxic and resulted in similar bond strengths to poly(methyl methacrylate ) to those obtained with methylene chloride .
another study found that methyl formate , methyl acetate , and their mixture significantly enhanced the flexural strength of heat - cured acrylic denture base resin that had been repaired with self - cured acrylic resin .
the scanning electron micrographs in this study demonstrated that the application of these solutions to heat - cured acrylic resin resulted in a 3d honeycomb appearance , whereas specimens treated with methyl methacrylate developed shallow pits.7 in addition , other studies have been conducted for investigating the surface treatment of acrylic denture base and reline resin with methyl formate - methyl acetate ( mf - ma ) .
these studies have shown that mf - ma significantly enhanced the bond strength between acrylic denture base and reline resin.89 a study of mf - ma application on acrylic denture teeth revealed that a 15-second application before packing the teeth with heat - cured acrylic denture base resulted in significantly higher micro - tensile bond strength between the teeth and the denture base , compared with the no surface treatment group .
moreover , there was no significant difference between the mf - ma and mma groups using a 15-second application on micro - tensile bond strength.10 therefore , mf - ma may be an acceptable alternative for mma because it is less toxic .
the acrylic denture tooth structure can also influence the bond strength to the denture.311 the conventional tooth has low wear resistance .
therefore , increased wear resistance is useful,12 and cross - linking the polymer matrix can increase wear resistance.13 because of this complex structure , the acrylic has less polymer penetrability than into conventional denture teeth , resulting in lower bond strength .
in contrast , another study stated that there was no significant difference in bond strength between conventional and cross - linked denture teeth to auto - polymerized acrylic resin.14 thus , crosslinking is not a major factor in reducing the strength of the joint between teeth and denture base.15 there are various methods for determining the bond strength between denture teeth and denture base , such as the american dental association specification number ( ada 15),16 international organization for standardization for synthetic resin teeth ( iso 3336),17 or the finite element stress analysis technique.16 however , these methods have been criticized concerning their accuracy in determining bond strength .
moreover , the lack of uniformity in the tooth - denture base testing methods does not allow bond strength to be investigated in a standardized manner and/or the results to be directly compared.2 past studies using mf - ma solutions have indicated that it is a non - toxic surface treatment agent and improves the bond strength of acrylic resin materials . however , there are no studies of the bond strength when using mf - ma for the repair of denture teeth with auto - polymerized acrylic resin .
the objective of this study was to evaluate the effect of surface treatment with mf - ma solution , using various application times and comparing to that of mma , on the tensile bond strength of different acrylic denture teeth repaired with auto - polymerized acrylic resin .
three brands of denture teeth were used : yamahachi new ace ( ya ) , major dent ( md ) , and cosmo hxl ( cm ) , ( table 1 ) .
seventy maxillary central incisor acrylic denture teeth of each brand were embedded with incisal surface down in auto - polymerized poly(methyl methacrylate ) ( pmma ) and were packed in 20-mm diameter polyethylene pipes .
their ridge - lap surfaces were polished with 500- and 1200-grit silicon carbide paper in a polishing machine ( nano2000 , pace technologies , st .
chemical solutions were applied to the groups as follows ( table 2 ) : no treatment ; methyl formatemethyl acetate ( mf - ma ) solution at a ratio of 25:75 ( by volume ) ( cu acrylic bond , faculty of dentistry , chulalongkorn university , bangkok , thailand ) for 15 seconds , 30 seconds , 60 seconds , 120 seconds , 180 seconds , and mma for 180 seconds .
a polyethylene sheet with a 3-mm diameter hole was placed over the treated surface and a 10-mm diameter acrylic resin ring was placed centrally over the hole in the polyethylene sheet .
auto - polymerized acrylic resin ( unifast trad , gc dental products co. , aichi , japan ) was loaded into the ring and compressed with a 1-kg weight .
the specimen was placed in a pressure cooker at 2 mpa at 60. after the acrylic resin had set , an acrylic resin rod was attached on the top with cyanoacrylate glue ( super glue , alteco chemical pte ltd . , japan ) to connect the specimen to the tensile testing machine ( fig .
the polymerized specimens were stored in deionized water at 37 for 48  2 hours and tested using a universal testing machine ( shimadzu , ez - s , bara scientific co. , ltd .
, phra nakhon si , ayutthaya , thailand ) with a 500 n load cell at a crosshead speed of 10 mm / min .
the tensile bond strength ( in mpa ) was calculated by dividing the failure force by the bond surface area .
the data were statistically analyzed using spss for windows 22 ( ibm corporation , new york , ny , usa ) . the mean and standard deviation ( sd ) for the tensile bond strength of each group was calculated and statistically analyzed using two - way analysis of variance ( anova ) and the post hoc dunnett t3 test at the 95% confidence level .
the results ( table 2 ) demonstrated that the tensile bond strengths of the negative control groups ( yac ; mdc ; cmc ) were not significantly different from each other ( p > .05 ) . for each brand ,
the surface treatment groups had significantly higher tensile bond strengths compared with the negative control group ( p < .05 ) .
the tensile bond strengths of the mma 180-second group of each brand were not significantly different from the mf - ma 15-second , 30-second , 60-second , 120-second , and 180-second groups ( p > .05 )
. however , the ya mf - ma 180-second group had a significantly higher tensile bond strength compared with the mma 180-second group ( p < .05 ) .
the bond strength of acrylic denture teeth to auto - polymerized acrylic resin can be improved by chemical surface treatment .
this improvement begins when the solvents in the surface treatment agent contact the denture teeth , dissolving their surfaces , and causing swelling of the surface layers .
subsequently , the monomer of the auto - polymerized acrylic resin material diffuses and penetrates into the interpenetration polymer network ( ipn ) matrix of the denture teeth during polymerization , which is known as the swelling phenomenon.17 this mechanism is affected by application time , polymerization temperature , type of solvent , denture teeth structure , and glass transitional temperature of the denture teeth.18 when used on acrylic denture teeth as a surface treatment , a methyl formate - methyl acetate solution acts by swelling and dissolving their surfaces , and then evaporating .
in addition , there are no carbon  carbon double bonds ( c = c ) in methyl formate or methyl acetate molecules to polymerize with the monomer in the auto - polymerized acrylic material .
thus , it would not obstruct the interlocking of the auto - polymerized resin polymer chains and the denture teeth , and the tensile bond strength would be increased .
polymer dissolving and swelling occurs when the polymer 's and solvent 's solubility parameters and polarities are close to each other .
the solubility parameter of pmma ( acrylic denture teeth ) is 18.3 mpa , whereas those of mma , mf , and ma are 18.0 , 20.9 , and 19.6 mpa , respectively.19 in addition , mma , mf , and ma molecules have the methyl ester group , which increases their ability to soften pmma.6 the mf , ma , and mf - ma mixture increased the bond strength of repaired acrylic denture base and acrylic denture base relined with rebasing material.78 thus , it is hypothesized that mf - ma mixture would result in higher tensile bond strength between acrylic denture teeth and auto - polymerized acrylic resin compared with using mma liquid .
although the results indicated that surface treatments with mf - ma and mma improved the bond strength compared to no treatment , mf - ma was superior to mma for the three brands of denture teeth for chairside repairs due to mf - ma 's reduced application time and lack of tissue irritation .
the bond strengths when using mma for 180 s for all brands was not significantly different compared with the mf - ma 15 second , 30 second , 60 second , 120 second , and 180 second groups ( p > .05 ) , and mf - ma treatment resulted in bond strengths similar to mma treatment .
the shortest application time of mf - ma treatment that had a higher bond strength compared with the control was 15 seconds , thus is the best time for chairside use for both conventional and cross - linked acrylic denture teeth . according to denture teeth types
, the results showed that the bond strengths between cross - linked ( cosmo hxl ) and conventional type ( yamahachi new ace ; major dent ) were not significantly different ( p > .05 ) , the result which agrees with a previous study.15 the reason may be the type of acrylic denture base because the bond repaired with heat - cured acrylic resin was greater than that with auto - polymerized resin.20 the stronger bond is the result of processing temperature , time , and pressure .
the polymer chain network size in conventional acrylic denture teeth is larger compared with that in the cross - linked type.21 therefore , there is more space between the polymer chains in conventional acrylic denture teeth compared with the cross - linked type ( crosslink density ) .
thus , monomer from the auto - polymerized acrylic resin can diffuse more into the conventional acrylic denture teeth , resulting in increased polymerization .
therefore , denture tooth type influences the bond strength for heat - cured acrylic resin , but not for auto - polymerized acrylic .
another reason may be that the cross - linked structure of cosmo hxl , an interpenetrating polymer network ( ipn ) consisting of only 10% highly crosslinked
if the denture tooth has a true ipn structure , the tensile bond strength between the tooth and auto - polymerized resin will be reduced because ipn denture tooth swells less after surface treatment with chemical agents ( mf - ma , mma ) .
a limitation of this study was that only conventional and cross - linked denture teeth were investigated .
the ipn denture teeth have an advantage of high wear resistance.13 this type of tooth is appropriate for dentures that occlude natural teeth . however , its bond strength to the denture base is low .
the ipn consists of two or more polymer networks , resulting in a more complex structure compared with the cross - linked type .
therefore , this type of tooth should also be studied because it will generate useful data for determining the most suitable surface treatment method in each case .
within the limitations of the present study , cu acrylic bond ( mf - ma solution ) and mma increased the bond strength of either conventional or crosslinked acrylic denture teeth to auto - polymerized acrylic resin compared to no treatment .
the application of mf - ma for 15 second can be an alternative chemical surface treatment for repairing a denture base and rebonding both conventional and cross - linked acrylic denture teeth with auto - polymerized acrylic resin .
 purposethis study evaluated the effect of chemical surface treatment using methyl formate - methyl acetate ( mf - ma ) solution on the tensile bond strength between acrylic denture teeth and auto - polymerized acrylic resin.materials and methodsseventy maxillary central incisor acrylic denture teeth for each of three different brands ( yamahachi new ace ; major dent ; cosmo hxl ) were embedded with incisal edge downwards in auto - polymerized resin in polyethylene pipes and ground with silicone carbide paper on their ridge lap surfaces . 
 the teeth of each brand were divided into seven groups ( n=10 ) : no surface treatment ( control group ) , mf - ma solution at a ratio of 25:75 ( v / v ) for 15 seconds , 30 seconds , 60 seconds , 120 seconds , 180 seconds , and mma for 180 seconds . 
 auto - polymerized acrylic resin ( unifast trad ) was applied to the ground surface and polymerized in a pressure cooker . 
 a tensile strength test was performed with a universal testing machine . 
 statistical analysis of the results was performed using two - way analysis of variance ( anova ) and post - hoc dunnett t3 test ( =.05).resultsthe surface treatment groups had significantly higher mean tensile bond strengths compared with the control group ( p<.05 ) when compared within the same brand . among the surface treatment groups of each brand , there were no significantly different tensile bond strengths between the mf - ma groups and the mma 180 second group ( p>.05 ) , except for the yamahachi new ace mf - ma 180-second group ( p<.05).conclusion15-second mf - ma solution can be an alternative chemical surface treatment for repairing a denture base and rebonding acrylic denture teeth with auto - polymerized acrylic resin , for both conventional and cross - linked teeth . 
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
CONCLUSION