heat - polymerized acrylic resin has been the most commonly used denture base material for over 60 years . however , the mechanical strength of acrylic resin is not adequate for long - term clinical performance of dentures . therefore , fracture is a common clinical occurrence , which is often seen in the midline of denture base . these fractures are often related to the poor fit of denture base , poorly balanced occlusion , problems in the design and manufacturing of the denture , low strength of the repair material , as well as the inherent stress on the denture base , which happens over time . in earlier studies , these fractures may occur inside or outside the mouth due to expelling the denture from the mouth while coughing , or simply dropping it . other reasons could be excessive bite force , improper occlusal plane , high frenal attachment , lack of balanced occlusion , poor fit and poor quality of the denture base material . since fabrication of a new denture is time - consuming and costly for patients , denture repair is considered an alternative . repaired dentures should have adequate strength , dimensional stability , and color match [ 4,5,811 ] ; moreover , the repair should be easily and quickly performed , and must be affordable . amongst various methods proposed for repairing fractured denture bases , use of auto - polymerized acrylic resins , which generally allows a simple and quick repair , heat - polymerized materials have been proven to have superior mechanical properties , compared to auto - polymerized materials . however , laboratory packing and flasking procedures are time - consuming and are associated with the risk of denture distortion by heat . consequently , autopolymerizing resin has gained more popularity due to its easy handling , saving chairside time , and not requiring laboratory processing ; moreover , the patient spends less time without denture during the repair process . in addition to the use of auto - polymerized acrylic resin , effects of reinforcement materials and surface treatment on the flexural strength of repaired dentures have been investigated in different studies [ 1517 ] . the purpose of this study was to investigate the efficacy of two types of acrylic resins and three reinforcement materials for denture base repair . materials used in this article are listed in table 1 . in the current study , we aimed to evaluate the transverse strength and modulus of elasticity of repaired acrylic denture bases using a 3-point bending test ; the results were compared with those of a heat - polymerized control group . for this purpose , a stainless steel mold with internal dimensions of 10.270.33.1 the materials used in this study heat cure acrylic resin ( meliodent , heraeus kulzer , germany ) was mixed in accordance with the manufacturer s instructions , and placed in the prepared mold and packed in the flask . acrylic resin specimens were processed for 9 hours in water bath , and kept at a constant temperature of 165f ( 73.5c ) . afterwards , 90 acrylic samples were fabricated and the superior surfaces of acrylic samples were polished . all samples were stored in distilled water at 37c for 48 hours before the test , and the prepared samples were randomly distributed into 9 groups ( c , hn , hg , hm , hp , cn , cg , cm , and cp ) . the prepared intact specimens were cut vertically in half along their long axis ( except for 10 specimens , which were allocated to the control group , that is group hg ) by a high - speed diamond disk cutter under copious irrigation , until 3 mm space was created between the two pieces . after the treatment of fractured surfaces , the heat - polymerized strip haves were fixed to a metal mold to provide space for placing the repairing resin . a groove with 36 mm length , 3 mm width , and 2.1 mm depth was prepared on all samples for placing the reinforcing materials . after placing the reinforcing materials , samples were repaired with cold / heat cure resins and prepared for 3-point bending test . the specimens were repaired as follows : 
 group c : control group with no fracture , group hn : repaired with heat cure acrylic resin with no reinforcement , group hg : repaired with heat cure acrylic resin , and reinforced with glass fiber , group hm : repaired with heat cure acrylic resin , and reinforced with metal wire ( 1 mm in diameter),group hp : repaired with heat cure acrylic resin , and reinforced with polyethylene , group cn : repaired with cold cure acrylic resin with no reinforcement , group cg : repaired with cold cure acrylic resin , and reinforced with glass fiber , group cm : repaired with cold cure acrylic resin and reinforced with metal wire ( 1 mm in diameter ) , andgroup cp : repaired with cold cure acrylic resin and reinforced with polyethylene . group c : control group with no fracture , group hn : repaired with heat cure acrylic resin with no reinforcement , group hg : repaired with heat cure acrylic resin , and reinforced with glass fiber , group hm : repaired with heat cure acrylic resin , and reinforced with metal wire ( 1 mm in diameter ) , group hp : repaired with heat cure acrylic resin , and reinforced with polyethylene , group cn : repaired with cold cure acrylic resin with no reinforcement , group cg : repaired with cold cure acrylic resin , and reinforced with glass fiber , group cm : repaired with cold cure acrylic resin and reinforced with metal wire ( 1 mm in diameter ) , and group cp : repaired with cold cure acrylic resin and reinforced with polyethylene . the repaired specimens were prepared and subjected to thermal cycling in water baths between 5c and 55c with a 30-secound dwell time for 500 cycles . each specimen was subjected to 3-point bending test , using the universal testing machine ( zwick roell , germany ) at a crosshead speed of 8 mm / min at 50 mm distance ( fig . the load was applied to the center of 2 mm repaired area , and to the center in the control group . the schematic view of 3-point bending test the materials used in this study included a heat - polymerized acrylic resin ( meliodent heat cure , heraeus kulzer , germany ) used as the base , an auto - polymerized acrylic resin ( meliodent , heraeus kulzer , germany ) used as the repair material , and three reinforcement materials including woven stainless steel wire ( 1 mm in diameter ) , glass fibers ( ribbon , angelus , brazil ) and woven polyethylene ribbon fibers ( ribbon , angelus , brazil ) . all data were statistically analyzed with oneway and two - way anova , and the differences among the groups were assessed using the tukey s test and dunnett s post - hoc test . all data were statistically analyzed with oneway and two - way anova , and the differences among the groups were assessed using the tukey s test and dunnett s post - hoc test . the mean value of the flexural strength in the control group ( no fracture ) was 1.80.202 mpa . group hg ( repaired with heat - cured acrylic resin reinforced with glass fiber ) and group cp ( repaired with cold - cured acrylic resin reinforced with polyethylene ) showed maximum ( 2.170.32 mpa ) and minimum ( 0.550.15 mpa ) flexural strength values , respectively . the mean flexural strength and standard deviation values were 1.70.474 mpa for group cm , 1.510.303 mpa for group hn , 1.160.383 mpa for group hp , 1.110.244 mpa for group cg and 1.050.331mpa for group cn . table 2 shows the comparison of flexural strength of different groups based on the reinforcing materials used in each group . analysis of one - way anova followed by dunnett s test showed a statistically significant difference between the control group and all the experimental groups except for the control group and group hn . the mean value of flexural strength in different groups two - way anova followed by tukey s test revealed significant differences between groups reinforced with different materials except between the polyethylene and metal wire groups . the effect of reinforcement with various materials on the flexural strength is shown in table 3 . the effect of reinforcement with various materials based on the flexural strength of the samples in comparison with the control group in this study , the values of flexural strength of cold cure acrylic resin with no reinforcement ( group cn ) and cold cure acrylic resin with glass fiber reinforcement ( group cg ) were lower than that of the control group ( with no fracture ) . polyzois et al reported higher fracture strength in specimens repaired with cold cure acrylic resin without reinforcement and those reinforced with glass fiber ( in comparison with the control group with no fracture ) . in our study , there was a difference between the control group and the group repaired with cold cure acrylic resin , reinforced with metal wire ; however , according to polyzois s study , the strength of the specimens repaired with cold cure acryl resin reinforced with metal wire was more than that of the control group with no fracture . in the current study , the flexural strength of the control group was greater than that of the groups repaired with cold cure acrylic resin , cold cure acrylic resin reinforced with fiber glass , and cold cure acrylic resin reinforced with metal wires , which is similar to the findings of nagai s study . in their study , glass fibers showed greater strength in comparison with metal fibers , although both were stronger than the specimens repaired with cold cure acrylic resin only . however , in contrast to the mentioned study , in the current study , the strength created with glass fibers was greater than that of metal wires . in both studies , the strength of repaired specimens with glass fibers and metal wire was greater than that of cold cure acrylic resin without reinforcement ; however , nagai also showed the greater strength of intact denture in comparison with the repaired denture . in another study , keyf et al revealed that the strength of repaired specimens was lower than that of the intact specimens . in the current study , lower strength was recorded for cold cure acrylic resin in comparison with heat cure acrylic resin . studies by leong and grant , berge , and rached et al showed that dentures repaired with cold cure acrylic resin broke at the repaired site , which may be due to the lower strength of cold cure acrylic resin ; these results confirm our findings [ 2123 ] . lower strength of cold cure acrylic resin seems to be due to the insufficient polymerization process . in the current study , the mean value of the flexural strength of group hp was 56.8% of that of control specimens ; this is generally lower than the results of previous studies , which used cold cure acrylic resin for repair of specimens with heat - polymerized resin . on the other hand , for instance , in berge s study , the reduction of the strength of dentures repaired with cold cure acrylic resin compared to the controlled prototypes was reported to be 60% . similarly , this value in a study conducted by leong and grant was reported to be 65% . in the current study , the flexural strength of group hn was 84% of the control group and 75% of the value reported by leong and grant ; in stipho s study , the recorded value was 80% . as previously mentioned , there are other studies investigating the effect of surface treatment on flexural strength . for instance , pereira et al   evaluated the effect of abrasion of fracture surfaces with silicon carbide abrasive papers and/or wetting them with methyl methacrylate . according to their study , the flexural strength of samples wetted with methyl methacrylate was greater than other groups , except for the control group . in another study , thunyakitpisal et al   evaluated the effect of methyl methacrylate , methyl formate , methyl acetate , a mixture of methyl formate - methyl acetate and re - base ii adhesive on repaired specimens . they concluded that treating surfaces with methyl formate , methyl acetate , and a mixture of methyl formate - methyl acetate solutions significantly enhanced the flexural strength of heat - polymerized acrylic denture base resin repaired with auto - polymerized acrylic resin . although several studies including the present one have reported higher strength of denture base repaired with heat - polymerized acrylic resin , use of heat cure acrylic resin is less common due to different factors such as the necessity to use a mold ( custom split cast gypsum mold ) , longer polymerization time , higher laboratory costs and patients requiring dentures at a sooner time . on the other hand , repairing denture base with cold cure acrylic resin is faster , easier and more practical . in order to enhance the mechanical properties and flexibility of denture base it has been indicated that glass fibers significantly increase the strength of dentures . in this study , glass fibers , metal wires and polyethylene fibers compared with the control group , the highest flexural strength was observed in the group repaired with heat cure acrylic resin , reinforced with glass fibers ; the difference was statistically significant . stipho and talic also showed that incorporation of glass fibers into polymethyl methacrylate increased the strength of acrylic resin . polyzois et al showed that incorporation of metal wire into acrylic resin increased the fracture and flexural strengths . he reported that metal wire incorporation plays an important role in improving the mechanical properties of acrylic resin . in a different study , polyzois et al suggested the use of metal wire to strengthen the repaired bases ; while an investigation by minami et al showed increased strength of denture base with the use of metal wires ( stainless steel or co - cr - ni wire ) . the strength of restorative materials for repairing denture base can be measured using transverse , shear and twisting strength tests . today , transverse strength test is more common , and can be performed with 3- or 4-point loading . four - point loading technique is more favorable due to its better quality control and correspondence with standards ; however , its validity for repaired denture base is doubtful , since the stress distribution pattern at the interface of base and repairing material is not known . the present study showed that there was no significant difference between the groups repaired with heat cure and cold cure acrylic resins without reinforcement . moreover , our findings revealed that repairing denture base with heat cure acrylic resin , reinforced with glass fibers increased the flexural strength of denture base . finally , among materials used in this study , repairing the denture base with heat cure acrylic resin reinforced with glass fibers showed the greatest flexural strength .
objectives : heat - polymerized acrylic resin has been the most commonly used denture base material for over 60 years . 
 however , the mechanical strength of acrylic resin is not adequate for long - term clinical performance of dentures . 
 consequently , fracture is a common clinical occurrence , which often develops in the midline of the denture base . 
 this study aimed to evaluate the efficacy of cold - cure and heat - cure acrylic resins , reinforced with glass fibers , polyethylene fibers , and metal wire for denture base repair.materials and methods : ninety specimens were prepared and allocated to nine groups . 
 ten specimens were considered as controls , and 80 were divided into 8 experimental groups . in the experimental groups 
 , the specimens were sectioned into two halves from the middle , and were then divided into two main groups : one group was repaired with heat cure acrylic resin , and the other with cold cure acrylic resin . 
 each group was divided into 4 subgroups : unreinforced , reinforced with glass fibers , polyethylene fibers , and metal wire . 
 all specimens were subjected to a 3-point bending test , and the flexural strength was calculated.results:the group repaired with heat cure acrylic resin and reinforced with glass fiber showed the highest flexural strength ; however , the group repaired with cold cure acrylic resin and reinforced with polyethylene fibers had the lowest flexural strength . 
 there was no significant difference between the groups repaired with heat cure and cold cure acrylic resins without reinforcement.conclusion:repairing denture base with heat cure acrylic resin , reinforced with glass fibers increases the flexural strength of denture base .