the use of commercially pure titanium in dentistry particularly for implant and fixed and removable partial dentures has increased dramatically in the past few years . the use of titanium and its alloys has increased because of their favorable mechanical properties , excellent biocompatibility and corrosion resistance , low density , relatively low thermal conductivity and high strength - to - weight ratio . titanium is well known for its excellent stability in oral and body fluids and for this reason , it is the metal of choice for dental implants . the electrochemical oxidation potential of titanium is 1.63v ; making titanium a very active metal . because of its high chemical reactivity , it forms a thin stable oxide ( tio2 ) layer within one minute of exposure to air , which prevents further oxidation . this phenomenon is called passivation and also occurs in aluminum ; it is commercially utilized as a protective coating in a process called anodization . access to the metal is possible only through the pores of the oxide layer . with this protective layer showed that this layer did not break down even in aggressive conditions i.e. exposure to concentrated solutions containing strong acids such as hci or h2so4 , and thus the titanium corrosion remains extremely low . similarly , cast nickel - chromium has been used extensively in dentistry since 1960 for crowns , bridges and intracanal posts . it consists of 80% nickel and 15% chromium with traces of aluminum , molybdenum and manganese . excellent corrosion resistance of these alloys has led to the usage of this alloy for dental crowns , bridge repair and cast partial denture frameworks . it is also known to be resistant to further decay and is famous for being durable , non - corrosive and minimally invasive . hayes et al . showed that the corrosion resistance of nickel - chromium especially in acidic solutions including hcl , h2so4 , hno3 , hf and h3po4 was attributed to chromium , which is the primary element influencing the general corrosion resistance . this indicates that chromium plays a strong role in maintaining the passivity of the alloy , while molybdenum acts to stabilize the passive film after a localized breakdown event . on the other hand , a rapid development has been noticed in utilization of fluoridated prophylactic gels . during the past 40 years , prophylactic application of gels and solutions containing high concentrations of fluoride at regular time intervals has become more frequent and has a noticeable impact on dental caries . topical fluoride for prevention of tooth decay has become the most widely used prophylactic measure at home and in dental offices . topical fluoride may be applied by the dental health personnel in dental offices and clinics or may be self - applied . although some exceptions exist , generally high fluoride - concentration products are applied professionally while lowfluoride concentration products are used by individuals at home . in conditions such as xerostomia and dentinal hypersensitivity in adults , repeated application of topical fluoride is required . topical fluoride is available in various forms such as solutions , gels , prophylactic pastes and varnishes . fluoride gels were developed because they were much easier to use due to their consistency and could be easily applied to trays , and thus the entire dentition could be treated simultaneously . sodium fluoride ( naf ) and acidulated phosphate fluoride ( apf ) gels have the same concentrations of fluoride and ph as their respective aqueous solutions . some gels are formulated to be thixotropic , that is , they tend to flow under pressure but remain viscous otherwise . thixotropic gels are considered to do better in interproximal penetration and do not drip from the tray as readily as nonthixotropic gels . fluoride gels are considered to be as effective as their solution counterparts with desirable working characteristics and hence fluoride gel products have dominated the market for several years . owing to the large number of favorable results , concentrated fluoridated gels have obtained the approval of the us food and drug administration . these approved dental gels have a ph range of 3.2 to 7.7 ; although acidic formulations are believed to result in fast formation of hydrofluoric acid . moreover , the efficacy of topical fluoride can be enhanced by further decreasing its ph and accelerating the formation of calcium fluoride on the tooth surface . in this context , many manufacturers have commercialized highly fluoridated gels with the fluoride ion content reaching 10,000 ppm and a ph of 5.5 . however , such concentration may have some adverse effects on the corrosion resistance of metals like titanium and nickel - chromium due to the high fluoride ion content and low ph level . there are a few studies comparing the potential risks and effects of concentrated and diluted topical fluorides on cast titanium and nickel - chromium . this study aimed at evaluating the effect of topical fluoride on the surface roughness of cast titanium and nickel - chromium . the surface defects caused by these fluoride gels and solutions may lead to increased plaque accumulation , calculus and microbial retention and may also affect esthetics . this study was conducted on 39 titanium ( orotig , titec , verona , italy ) and 39 nickel - chromium ( mealloy , dentsply , rome , italy ) test specimens . seventy - eight wax patterns were fabricated by flowing molten casting inlay wax ( schuler - dental gmbh & co. , ulm , germany ) into the rectangular shaped mold space of 15 mm  10 mm and a depth of 2 mm with the help of a thermostat ( thermo mat , dentaurum gmbh & co. , ulm , germany ) . thirty - nine wax patterns were used for preparing cast titanium samples and 39 wax patterns were used for preparing cast nickel - chromium samples . all the 78 samples were inspected for internal porosity by x - ray ( green , platino international co. , shanghai , china ) . samples with internal porosities were discarded and equal number of samples was fabricated . to ensure complete removal of the alpha - case layer in the titanium samples , surface finishing of the specimens was done using titanium finishing bur ( dentaurum , ispringen , germany ) with a hand piece ( kavo dental gmbh , bismarckring , germany ) operating at 15,000 to 20,000 rpm . for the nickel - chromium samples , tungsten carbide burs ( dentsply , rome , italy ) were used for removing surface irregularities and sequential rubber points ( dentaurum , ispringen , germany ) were used for finishing and polishing . the samples were standardized by holding them to a fixed horizontal grinding unit ( aixin medical , tianjin , china ) . for the purpose of standardization , the finishing was done by a single operator ( to prevent inter - operator error ) , for a fixed period of time and used light pressure ( to minimize intra - operator error ) and only three samples were finished and polished per day ( to minimize variation due to fatigue ) . all the samples were then inspected for their uniformly polished surfaces under a 189  152 mm magnifying lens ( best-8609e , jiada tools co. , suzhom , china ) . the samples were cleaned in an ultrasonic bath for 10 minutes in distilled water and then air - dried . the titanium and nickel - chromium specimens were divided into three groups of 13 samples each . they were placed in the respective topical fluoride solutions based on their usage in one year . group one : 2% naf was prepared by dissolving 20 g of naf powder ( sigmaultra , missouri , usa ) in one liter of distilled water in a plastic container . thirteen titanium and 13 nickel - chromium samples of group one were treated with 2% naf solution using cotton applicators for 16 minutes based on its usage in one year . group two : 1.23% apf gel ( professional gel , pascal , usa ) was dispensed from the bottle onto a plastic spatula . a uniform layer of the apf gel was applied to 13 titanium and 13 nickel - chromium samples in group two for eight minutes based on its usage in one year . group three ( control ) : distilled water was poured into a plastic container and 13 titanium and 13 nickel - chromium samples in group three were immersed in it for 16 minutes . all the samples in each group were subjected to analysis by an optical profilometer ( wyko nt 1100 , tucson , az , usa ) to estimate their roughness after their respective surface treatments . the results obtained were statistically analyzed using two - way anova and post - hoc comparison test . one sample of titanium and one sample of nickel - chromium from each group were gold - sputtered by a 150-a thin gold layer under vacuum ( 10 - 3 mbr ) and then their surface topography was evaluated under scanning electron microscope sem at 500 magnification with a resolution of 10m and the accelerating voltage of 20 kv . the elemental compositions of these surfaces were further evaluated by energy dispersive spectrometer ( eds ) . the titanium and nickel - chromium specimens were divided into three groups of 13 samples each . they were placed in the respective topical fluoride solutions based on their usage in one year . group one : 2% naf was prepared by dissolving 20 g of naf powder ( sigmaultra , missouri , usa ) in one liter of distilled water in a plastic container . thirteen titanium and 13 nickel - chromium samples of group one were treated with 2% naf solution using cotton applicators for 16 minutes based on its usage in one year . group two : 1.23% apf gel ( professional gel , pascal , usa ) was dispensed from the bottle onto a plastic spatula . a uniform layer of the apf gel was applied to 13 titanium and 13 nickel - chromium samples in group two for eight minutes based on its usage in one year . group three ( control ) : distilled water was poured into a plastic container and 13 titanium and 13 nickel - chromium samples in group three were immersed in it for 16 minutes . all the samples in each group were subjected to analysis by an optical profilometer ( wyko nt 1100 , tucson , az , usa ) to estimate their roughness after their respective surface treatments . the results obtained were statistically analyzed using two - way anova and post - hoc comparison test . one sample of titanium and one sample of nickel - chromium from each group were gold - sputtered by a 150-a thin gold layer under vacuum ( 10 - 3 mbr ) and then their surface topography was evaluated under scanning electron microscope sem at 500 magnification with a resolution of 10m and the accelerating voltage of 20 kv . the elemental compositions of these surfaces were further evaluated by energy dispersive spectrometer ( eds ) . the titanium specimens in group two showed higher increase in the mean surface roughness from 4.87770.5200 m to 6.71850.4067 m with a difference of 1.84080.4732 ( table 1 ) . the nickel - chromium specimens in group two showed higher increase in the mean surface roughness from 3.64920.6954 m to 3.70000.6832 m with a difference of 0.0508 0.1295 ( table 1 ) . the mean and standard deviation ( sd ) of surface roughness of titanium and nickel - chromium specimens in groups one , two and three before and after surface treatment . two - way anova revealed a significant difference in the mean surface roughness of the three groups of titanium and nickel - chromium ( p<0.05 ) . comparison among the three solutions used in the respective groups of titanium and nickel - chromium also showed a significant difference ( p<0.05 ) . similarly , comparison by two - way anova between groups one , two and three of titanium and nickel - chromium , respectively also showed significant differences ( p<0.05 ) ( table 2 ) . comparison of the three groups of titanium and nickel - chromium by two - way anova post - hoc test comparison showed that group two titanium specimens differed significantly from groups one and three titanium specimens ( table 3 ) and also from group two nickel - chromium specimens ( p<0.05 ) ( table 4 ) . comparison among the three groups of titanium and the three groups of nickel - chromium by post - hoc test comparison among the three respective groups of titanium and nickel - chromium by post - hoc test analysis . however , there was no significant difference among the three groups of nickel - chromium ( p>0.05 ) ( table 3 ) . the nickel - chromium specimens in group two showed higher increase in the mean surface roughness from 3.64920.6954 m to 3.70000.6832 m with a difference of 0.0508 0.1295 ( table 1 ) . two - way anova revealed a significant difference in the mean surface roughness of the three groups of titanium and nickel - chromium ( p<0.05 ) . topically applied fluorides are likely to be effective in preventing caries in susceptible patients of any age . professional fluoride treatments are especially recommended for adults with dentinal hypersensitivity and with known risk factors , such as xerostomia that may be caused by medications , metabolic disturbances like sjogren s syndrome or radiation therapy . similarly , patients undergoing orthodontic treatment exhibit a decrease in normal self - cleansing activity of the mouth . such patients have higher risk of caries beneath and adjacent to orthodontic bands and are also susceptible to developing white opaque lesions on the enamel surface due to subsurface demineralization . the aim of this study was to quantitatively and qualitatively analyze and compare the effect of topical fluorides on titanium and nickel - chromium . the comparison between the surface roughness values ( rz ) of the groups showed a statistically significant increase after apf gel treatment in titanium specimens in group two as compared to 2% naf solution treatment for specimens in group one and distilled water for specimens in group three . this can be justified by the fact that titanium is easily degraded by hydrofluoric acid ( hf ) ; and furthermore , it is rapidly dissolved by strong , and even weak acids such as acetic , formic , and phosphoric acids if the solution contains soluble fluoride [ 14,7 ] . this phenomenon is interpreted to be the result of incorporation of fluoride ions in the oxide layer , whose protective properties are considerably reduced . a low quantity of fluoride , in presence of acidic medium , induces the following chemical reaction : 
 naf  +  ch3-cooh hf+  ch3-coona 
  the hydrofluoric acid molecules can react with titanium oxide to give rise to titanium oxyfluoride in solid state : 
 ti02 + 2hfh2o+tiof2 
  many structural defects may be induced in the oxide coating due to the lattice parameters of these compounds , thereby loosening the protection from the oxide layer . boere   demonstrated that the corrosion of titanium was enhanced in an acidic environment , because f in the solution reacts with h+ to form hf , even if the naf concentration is low . the titanium specimens in group one did not show a statistically significant increase in surface roughness after treatment . this result can be attributed to the fact that corrosion resistance of titanium is maintained when the ph of the solution is neutral . nakagawa et al . reported that anodic polarization and immersion tests indicated that the passivation film on titanium was maintained in use of 2% naf ( 9048.8 ppm f ) solution at a ph higher than 6.2 . the comparison between the rz of the three groups of nickel - chromium specimens revealed that nickel - chromium displayed excellent corrosion resistance in fluoridated media . this can be justified by the fact that chromium in nickel - chromium alloys hinders active dissolution of the nickel matrix , by formation of an adherent passivating surface film . comparison between titanium and nickel - chromium specimens indicated that titanium was more susceptible to corrosion in acidic as well as neutral sodium fluoride preparations as compared to nickel - chromium . 2a ) showed dark stains that were responsible for macroscopic color changes of the titanium surface , which might also be interpreted as imperfections in the oxide layer and areas where pitting corrosion started . sem photomicrograph ( a : titanium and b : nickel chromium at 500 magnification ) of titanium and nickel - chromium specimens in group two this corrosion is characterized by the formation of gaps , inside which there was no re - passivation of the oxide layer by oxygen inhibition . this may be explained by the fact that chromium - passivized metals can be susceptible to pitting corrosion in fluoride solutions with low ph . sem photomicrograph ( a : titanium and b : nickel chromium at  500 magnification ) of titanium and nickel - chromium specimens in group one group three specimens of titanium and nickel - chromium ( fig . sem photomicrograph ( a : titanium and b : nickel chromium at 500 magnification ) of titanium and nickel - chromium specimens in group three energy dispersive spectroscopy is an analytical technique used for elemental analysis or chemical characterization of a sample . it is based on the principle that due to the unique atomic structure of each element , x rays that are characteristic of each element s atomic structure help in identification of these elements from each other . in our study , eds analysis of titanium specimens ( fig . 4 ) in group two revealed the presence of fluoride , suggesting the presence of deposits on the surface of specimens after treatment with 1.23% apf gel . identified these deposits by eds to be composed of sodium fluorotitanate ( na2tif ) that covered the whole surface of titanium disks immersed in saliva enriched with naf at concentrations of 2.5% ( i.e. , 25 g / l or 25,000 ppm ) . energy dispersive spectroscopy analysis of nickel - chromium specimens in group two and titanium specimens in group one indicated the presence of negligible amounts of fluoride . it should be noted that in our study , during the preparation of test specimens , routine procedures like removal of alpha layer , sandblasting , finishing and polishing might have caused little variation in surface roughness . in clinical situations , the actual exposure time would be longer as the patients are instructed not to rinse for 30 minutes after topical fluoride application . additional research may be carried out using different topical fluorides with variable concentrations and exposure times . further analysis with a color meter is recommended to study the macroscopic color changes occurring on the titanium surface following treatment with 1.23% apf gel . the surface corrosion caused on these restorations may lead to increased adherence of microbial flora , which can be further evaluated . based on the findings of this in vitro study , neutral naf solutions cause no significant alterations in the corrosion resistance of titanium or nickel - chromium ; hence their usage is recommended . in patients with xerostomia wherein acidic fluoridated preparations similarly , in cases of dentinal hypersensitivity , desensitizing agents like potassium nitrate may be recommended . 2a ) showed dark stains that were responsible for macroscopic color changes of the titanium surface , which might also be interpreted as imperfections in the oxide layer and areas where pitting corrosion started . sem photomicrograph ( a : titanium and b : nickel chromium at 500 magnification ) of titanium and nickel - chromium specimens in group two this corrosion is characterized by the formation of gaps , inside which there was no re - passivation of the oxide layer by oxygen inhibition . this may be explained by the fact that chromium - passivized metals can be susceptible to pitting corrosion in fluoride solutions with low ph . sem photomicrograph ( a : titanium and b : nickel chromium at  500 magnification ) of titanium and nickel - chromium specimens in group one group three specimens of titanium and nickel - chromium ( fig . sem photomicrograph ( a : titanium and b : nickel chromium at 500 magnification ) of titanium and nickel - chromium specimens in group three energy dispersive spectroscopy is an analytical technique used for elemental analysis or chemical characterization of a sample . it is based on the principle that due to the unique atomic structure of each element , x rays that are characteristic of each element s atomic structure help in identification of these elements from each other . in our study , eds analysis of titanium specimens ( fig . 4 ) in group two revealed the presence of fluoride , suggesting the presence of deposits on the surface of specimens after treatment with 1.23% apf gel . identified these deposits by eds to be composed of sodium fluorotitanate ( na2tif ) that covered the whole surface of titanium disks immersed in saliva enriched with naf at concentrations of 2.5% ( i.e. , 25 g / l or 25,000 ppm ) . energy dispersive spectroscopy analysis of nickel - chromium specimens in group two and titanium specimens in group one indicated the presence of negligible amounts of fluoride . it should be noted that in our study , during the preparation of test specimens , routine procedures like removal of alpha layer , sandblasting , finishing and polishing might have caused little variation in surface roughness . in clinical situations , the actual exposure time would be longer as the patients are instructed not to rinse for 30 minutes after topical fluoride application . additional research may be carried out using different topical fluorides with variable concentrations and exposure times . further analysis with a color meter is recommended to study the macroscopic color changes occurring on the titanium surface following treatment with 1.23% apf gel . the surface corrosion caused on these restorations may lead to increased adherence of microbial flora , which can be further evaluated . based on the findings of this in vitro study , neutral naf solutions cause no significant alterations in the corrosion resistance of titanium or nickel - chromium ; hence their usage is recommended . in patients with xerostomia wherein acidic fluoridated preparations similarly , in cases of dentinal hypersensitivity , desensitizing agents like potassium nitrate may be recommended . energy dispersive spectroscopy is an analytical technique used for elemental analysis or chemical characterization of a sample . it is based on the principle that due to the unique atomic structure of each element , x rays that are characteristic of each element s atomic structure help in identification of these elements from each other . in our study , eds analysis of titanium specimens ( fig . 4 ) in group two revealed the presence of fluoride , suggesting the presence of deposits on the surface of specimens after treatment with 1.23% apf gel . identified these deposits by eds to be composed of sodium fluorotitanate ( na2tif ) that covered the whole surface of titanium disks immersed in saliva enriched with naf at concentrations of 2.5% ( i.e. , 25 g / l or 25,000 ppm ) . energy dispersive spectroscopy analysis of nickel - chromium specimens in group two and titanium specimens in group one indicated the presence of negligible amounts of fluoride . it should be noted that in our study , during the preparation of test specimens , routine procedures like removal of alpha layer , sandblasting , finishing and polishing might have caused little variation in surface roughness . in clinical situations , the actual exposure time would be longer as the patients are instructed not to rinse for 30 minutes after topical fluoride application . additional research may be carried out using different topical fluorides with variable concentrations and exposure times . further analysis with a color meter is recommended to study the macroscopic color changes occurring on the titanium surface following treatment with 1.23% apf gel . the surface corrosion caused on these restorations may lead to increased adherence of microbial flora , which can be further evaluated . based on the findings of this in vitro study , neutral naf solutions cause no significant alterations in the corrosion resistance of titanium or nickel - chromium ; hence their usage is recommended . in patients with xerostomia wherein acidic fluoridated preparations similarly , in cases of dentinal hypersensitivity , desensitizing agents like potassium nitrate may be recommended . within the limitations of this study , it is concluded that topical fluorides with acidic ph have an effect on the surface roughness of titanium and also to a certain extent , on nickel - chromium . therefore , careful consideration must be given to the usage of these acidic fluoridated preparations in patients with these restorations . the surface defects caused by these topical fluorides may lead to increased plaque and calculus retention and may compromise aesthetics .
objectives : the aim of this in - vitro study was to evaluate the effect of topical fluoride on surface of cast titanium and nickel-chromium.materials and methods : thirty - nine rectangular specimens of titanium ( grade 2 ) and 39 rectangular specimens of nickel - chromium were cast in equal dimensions and divided into three groups of 13 samples each . 
 group one specimens of titanium and nickel - chromium were placed in 2% neutral sodium fluoride ( naf ) solution for 16 minutes . 
 group two specimens of titanium and nickel - chromium were immersed in 1.23% acidulated phosphate fluoride ( apf ) gel for eight minutes . 
 group three specimens of titanium and nickel - chromium were immersed in distilled water for 16 minutes . 
 the surface roughness of the specimens was evaluated and the data were analyzed using two - way anova and post - hoc comparison test with the level of significance set at 5% ( p < 0.05 ) . 
 the surface of the specimens was further analyzed using a scanning electron microscope ( sem ) and energy dispersive spectrometer ( eds).results : group two titanium specimens showed a statistically significant increase in surface roughness ( p<0.05 ) ; but no statistically significant increase was noted in the surface roughness of nickel - chromium specimens in groups one , two and three ( p>0.05 ) . 
 qualitative sem and eds analyses further revealed the surface corrosion of titanium ( group two ) and localized mild corrosive pitting of nickel - chromium specimens ( group two).conclusion : topical fluoride with acidic ph affects the surface roughness of titanium and to a certain extent , nickel - chromium . 
 neutral naf solutions cause no significant change in corrosion resistance of titanium or nickel - chromium .