one of the most important phenomena to be considered for an implant to be successful is resistance to corrosion , as the material to be used will be constantly submitted to mechanical and thermal forces and to the aggressiveness of the host medium13 . corrosion is the result of electrochemical reactions that occur between a metal material and the surrounding environment . titanium is known to present a high degree of immunity against the attack by the majority of mineral acids , such as chlorides , and its biocompatibility associated with good mechanical resistance make it very useful in applications as a biomaterial4 . considered to be one of the few means capable of attacking the titanium surface with corrosive action , fluoride ions are effective components in this process . the use of fluoridated gels and solutions containing high levels of fluorides , as a prophylactic measure in dentistry , has been increasingly more widespread over the last 40 years , due to the great impact on caries prevention21 . the use of these substances on titanium surfaces , including their damaging action on implant surfaces , is a subject that has been approached in few studies . which repercussion this damage to the implant surface will have on the mechanical behavior of implants over time is still unknown . the study of dental metal alloys , especially of titanium , is crucial in fluoridated mediums due to a large number of high fluoride content preparations . there are few studies on the behavior of dental implants in the face of the corrosive action of fluoride containing chemical agents , and it is necessary to characterize the material and interpret its behavior after submission of the implant - abutment sets to the action of fluoride ions . the aim of this study was to assess the action of fluoride ions on the mechanical properties of implant - abutment set by means of compression tests , mechanical cycling and metallographic characterization by scanning electronic microscopy . in this study , commercially pure titanium dental implants with internal hexagon implant - connect ar treated surface ( conexo sistema de prtese , so paulo , sp , brazil ; lot 4900 ) were used . the implants and their components ( titanium screw for internal hexagon preparation post ( conexo sistema de prtese , so paulo , sp , brazil ) were removed from their original packaging and mounted according to the manufacturer 's instructions . a manual torquemeter ( conexo sistema de prtese , so paulo , sp , brazil ) was used at 20 n for the internal hexagon implant - abutment set to attain the recommended torque . once the recommended torque was obtained , the sets were divided into two groups of mechanical test . group 1 , characterized for the non - axial compression test , with 5 samples that were not submitted to ions fluoride f(- ) ; and group 2 , characterized for the mechanical cycling test , with 5 samples that were not submitted to ions fluoride f(- ) and 5 samples that were submitted to ions fluoride f(+ ) . after that , the tests began to be carried out on the f(- ) sets that have not been submitted to the action of fluorides . the f(+ ) sets were placed into a 1,500 ppm sodium fluoride solution ph 6.8 ( school of pharmaceutical science , unesp , so paulo state university , araraquara , sp , brazil ) until the time the respective tests were carried out . according to the protocol proposed by siiril and knnen19 ( 1991 ) , a model that simulates the clinical conditions of fluoride use was designed . in this model , the test specimens were statically submerged in the sodium fluoride medium ( 1,500 ppm ph 6.8 ) for 184 hours ( 7.5 days ) , simulating contact with fluoride for 21 times a week during 5 years , with a mean of 2 minutes at a time ( mean estimate of brushing with a fluoride dentifrice 3 times a day ) . the solution was changed every 12 hours , and each implant was washed in water for a 30-second interval before being submerged again in the replaced solution . additionally , the sets were completely submerged in the solution although it is more common to find abutments exposed in the oral cavity and not the whole set . the rationale is that this is an in vitro study and can extrapolate the usual cases and simulate a screw exposition , for instance . two types of mechanical tests were carried out on the implant - abutment sets : static and dynamic . the static test was the non - axial compression test and the dynamic test was the mechanical cycling , also called fatigue test . the static test was carried out with the goal of characterizing the strength of the implant - abutment sets without fluoride contact . data obtained were the basis for the dynamic test to which f(- ) and f(+ ) sets were submitted . the tests were carried out in a servohydraulic machine ( material test system - mts 810 ; mts system corporation , minneapolis , usa ) , equipped with test star ii software . the mechanical test machine was fitted with a 10 kn load cell at a constant speed of 1 mm / min . the force was applied by means of a straight pin fixed to the load cell with a flat end with a circular cross - section . to perform mechanical cycling , a load with a value of 60% of the compression strength of the test specimens was used , and 100,000 cycles of the mechanical test machine were set at a frequency of 15 hz1,7 . surface analysis was done with a scanning electronic microscope ( model t-330 a jeol - jsm , tokyo , japan ) coupled to an energy dispersive analyzer and a photographic camera . results for fracture location and screw loosening were analyzed at a 95% confidence level with fisher 's exact test . during the compression test in the f(- ) sets from group 1 , which were not submitted to fluoride ions , and the mechanical cycling test in the f ( - ) sets from group 2 , the maximum strength limit showed no dependence to the fatigue resistance ( table 1 and 2 ) . note : > 100.000 : implant - abutment set that bears the total number of fatigue fracture location - fisher 's exact test : p=0.024 ( significant , p<0.05 ) screw loosening - fisher 's exact test : p=0.500 ( non significant , p>0.05 ) regarding the mechanical cycling test ( group 2 ) , the sets submitted to fluoride treatment , f(+)fractured before attaining the 100,000 cycles , differing from the f(- ) sets . fracture location occurred on the screw for all sets , as demonstrated in table 2 . with regard to the screw loosening variable , table 2 shows no frequency difference between f(- ) sets and f(+ ) sets . figure 1 shows the surface of a f(- ) set and figure 2 shows the surface of a f(+ ) set , from group 2 . it is possible to observe differences between them , as the first one presents some irregularities from fabrication and the second one presents signs of corrosion . figures 3 and 4 show the fracture surface in the screw characterized as mixed fracture and some areas of corrosion . figure 1 shows the surface of a f(- ) set and figure 2 shows the surface of a f(+ ) set , from group 2 . it is possible to observe differences between them , as the first one presents some irregularities from fabrication and the second one presents signs of corrosion . figures 3 and 4 show the fracture surface in the screw characterized as mixed fracture and some areas of corrosion . in this study , the implant - abutment sets were exposed to a 1,500 ppm fluoridated medium for 184 hours , taking an exposure to fluoride ions of 2 minutes daily , for 5 years , with a frequency of 3 times a day , as a reference , without taking into account the residual fluoride that remains in the mouth immediately after brushing . it is known that fluoride concentrations in saliva fall drastically after brushing with a 1,250 ppm dentifrice for 90 seconds . furthermore , fluoride concentrations even higher than 200 ppm are found in saliva , which fall during the next 30 minutes , stabilize below 29 ppm and reach 7 ppm in the following 24 hours3 . however , the presence of fluorides is not restricted to the time of duration of oral hygiene procedures ( use of dentifrice or fluoridated mouthwash ) or prophylactic therapy . then , the use of low ph fluorides may be more prejudicial to titanium22 . to evaluate the implants and its joints with their components , as well as their stability and strength , models are widely used in research in recent times , in which eccentric loads are applied to the long axis of implants , whether at angles greater than 15 degrees or by means of devices that take the moment of flexion outside the axial shaft8,11,12,14 . in this research , the experimental model adopted presents the previously mentioned characteristics , accepted as effective for reproducing the less favorable clinical conditions ( within the limitations of the case ) in the mouth . the highest maximum non - axial compression load limit values for the group of internal hexagon implants that were not submitted to the action of fluoride ions f(- ) are basically due to the stability of this system , in which the forces are distributed with greater uniformity among the implant , screw and abutment , although the maximum strength limit was shown to be practically independent of the resistance to fatigue ( table 1 and table 2 ) . implants 4 mm wide have 30% more fatigue resistance than implants of 3.75 mm , however , a more resistant implant does not solve the problem , it only takes it to another part of the system , which may be the adjacent bone15 . due to the ductility of titanium , it is not possible for a c.p.ti implant to fracture with one cycle only . with the application of cyclic load , the material starts to develop internal micro - cracks that may increase in number and size according to the number of cycles . in this study , low cycle fatigue was performed ( up to 105 cycles)4 , as is noted in table 2 , and in which it is found that the implant - abutment sets that were submitted to the action of fluoride ions f(+ ) presented failure by fracture before attaining the cycles pre - determined in the non - axial compression test . under the load conditions to which the implants in this study were submitted , the fracture location , as presented in table 2 shows that it was the screw that presented the greatest tendency to fracture , as was expected on the basis of the weak link concept for components proposed by binon2 ( 1994 ) . in this proposal , the weak link was the gold screw that connected the gold cylinder to the titanium transgingival intermediate in the nobel biocare system . this widely accepted concept was extrapolated to all the screws in the different implant systems . in previous studies , it was reported that in some set systems , the first component to present failure or fracture when submitted to fatigue was not the one designed for such purpose . this occurred in the study of basten , et al.1 ( 1996 ) , in which the titanium screw of the estheticone system component presented fracture and not the gold screw of the cylinder , as expected . the internal and external implant connections have been compared , and a better stability has been found in internal , taper - shaped connections , in the face of the action of cyclic forces11 . in this experiment , the internal connection model in implants was the internal hexagon , whose mechanical behavior is comparable with that of the tapered connection , as has already been referred to in a study in the year 200014 . this system , as was observed in this study , presented the weak link in the screw , which may reduce the possibility of an implant fracture even further . a fracture that is rare , according to the literature but , in case it does occur , may be caused due to overload by flexion of the implants . overload by flexion may be defined as a situation in which , the occlusal forces in an implant - supported prosthesis , exercise a moment of flexion by means of the cross section of the implant body , leading to the loss of marginal bone or eventually , to fracture by implant fatigue16 . in the present experimental model , this overload by flexion occurred due to the 30-degree angle in which the implant was placed . this axial force is considered a contributing factor to increase the risk of overloading by flexion when it acts in implants that present a slope greater than 15 degrees . in other circumstances , such as in the placement of a cantilever or increase of the height of a restoration ( distance from the supporting bone crest to the occlusal surface of the restoration)17 . comparing the frequency of screw loosening before and after fluoride application , no pattern was found that would suggest some type of positive or negative influence . screw loosening has been reported as part of the problems that may occur in an implant - based rehabilitating treatment . for the internal hexagon systems , a very low or non - existent screw loosening index has been resported5 , a condition that did not occur in this study , as no differences were found with respect to the screw loosening variable . it has been reported that after a time of use in the mouth , the components may lose their shine10 or undergo color changes19 . this may be attributed to the changes in the oxide layer resulting from corrosion reactions . these surface reactions are more intense when the implants are submitted to the actions of fluoride ions . macroscopically , titanium surface presented loss of shine and dark coloring in the groups submitted to the action of fluoride ions . the scanning electronic microscopic analysis showed that the titanium surface not submitted to the action of fluoride ions exhibited the expected characteristics , with irregularities resulting from the manufacturers surface treatment ( figure 1 ) . when the f(+ ) sets were analyzed , dark stains that are responsible for the macroscopic change on titanium surface color were found , which may also be interpreted as imperfections in the oxide layer and the areas were pitting corrosion started , which is a type of corrosion mainly observed in passive metals like titanium , aluminum and stainless steel4 . this corrosion is characterized by formation of gaps , inside of which there is no re - passivation of the oxide layer by oxygen inhibition , and the establishment of an anodic process23 ( figure 2 ) . the fractures by titanium compression are characterized by mixed ruptures , as they present regions with macroscopic plastic deformation . the difference is that cracking in the fragile fracture needs minimal plastic deformation to propagate itself , whereas in the ductile fracture this deformation is necessarily greater 20 . the mechanical behavior of the test specimens changed after being submitted to the action of fluoride ions . scanning electron microscopic analysis showed characteristics of mixed fracture , with predominance of fragile fracture , and the screw fracture surfaces showed regions that indicated corrosion ( figure 3 ) , by which crack propagation may have occurred . therefore , there was influence of the medium , with predominance of fragile fractures ( figure 4 ) . there is a great deal of controversy about fracture by fatigue due to the variety of mechanisms that determine the rupture of the metal . rupture by fatigue , according to souza20 ( 1974 ) , may occur in three stages : 1 ) crack nucleation , 2 ) crack propagation and 3 ) test specimen rupture . the first two stages take up practically all of the test time and , when the crack length reaches a size that leaves the tensioned section relatively small , the remaining portion can no longer resist the load and the rupture occurs suddenly . figure 5 shows a characteristic fracture by fatigue that presents three zones , zone one is produced by the gradual and progressive crack development , zone two is produced by the sudden fracture , this interface being better observed in zone 3 . a point of concentration of tensions and/or of less resistance in the implant is the level at which the internal threads end , giving way to the beginning of the solid body of the implant ( where the screw that holds the pillar is placed ) , due to the thin implant walls , is the place where fracture most likely occurs17 . siiril and knnen19 ( 1991 ) showed that the main deterioration factor for titanium , which may be present in the mouth , would be the abrasive action of brushing , but recommended avoiding prolonged exposure to the action of fluoride ions . the results of this study differ from those of these authors , as evidence was found of the modification of the titanium surface after contact with the fluoride ions , as well as of this modification acting on the mechanical performance of the materials . with regard to this aspect , the results of this investigation are in agreement with the findings of toumelim - chemla , et al22 ( 1996 ) , who pointed to high concentrations of fluoride ions and prolonged contact , as possible harmful agents to the durability of metal prostheses and rehabilitations with implants , which are present in the oral environment . additionally , reclaru and meyer18 ( 1998 ) reported that titanium and other dental alloys showed a corrosive process only when the ph dropped below 3.5 . this findings differ from those of our study and the study by nakagawa , et al.9 ( 1999 ) which revealed that titanium could be corroded by the existence of a small amount of naf if ph was considerably low or , in conditions of high ph if naf concentration was also considerably high . it has been reported that after a time of use in the mouth , the components may lose their shine10 or undergo color changes19 . this may be attributed to the changes in the oxide layer resulting from corrosion reactions . these surface reactions are more intense when the implants are submitted to the actions of fluoride ions . macroscopically , titanium surface presented loss of shine and dark coloring in the groups submitted to the action of fluoride ions . the scanning electronic microscopic analysis showed that the titanium surface not submitted to the action of fluoride ions exhibited the expected characteristics , with irregularities resulting from the manufacturers surface treatment ( figure 1 ) . when the f(+ ) sets were analyzed , dark stains that are responsible for the macroscopic change on titanium surface color were found , which may also be interpreted as imperfections in the oxide layer and the areas were pitting corrosion started , which is a type of corrosion mainly observed in passive metals like titanium , aluminum and stainless steel4 . this corrosion is characterized by formation of gaps , inside of which there is no re - passivation of the oxide layer by oxygen inhibition , and the establishment of an anodic process23 ( figure 2 ) . the fractures by titanium compression are characterized by mixed ruptures , as they present regions with macroscopic plastic deformation . the difference is that cracking in the fragile fracture needs minimal plastic deformation to propagate itself , whereas in the ductile fracture this deformation is necessarily greater 20 . the mechanical behavior of the test specimens changed after being submitted to the action of fluoride ions . scanning electron microscopic analysis showed characteristics of mixed fracture , with predominance of fragile fracture , and the screw fracture surfaces showed regions that indicated corrosion ( figure 3 ) , by which crack propagation may have occurred . therefore , there was influence of the medium , with predominance of fragile fractures ( figure 4 ) . there is a great deal of controversy about fracture by fatigue due to the variety of mechanisms that determine the rupture of the metal . rupture by fatigue , according to souza20 ( 1974 ) , may occur in three stages : 1 ) crack nucleation , 2 ) crack propagation and 3 ) test specimen rupture . the first two stages take up practically all of the test time and , when the crack length reaches a size that leaves the tensioned section relatively small , the remaining portion can no longer resist the load and the rupture occurs suddenly . figure 5 shows a characteristic fracture by fatigue that presents three zones , zone one is produced by the gradual and progressive crack development , zone two is produced by the sudden fracture , this interface being better observed in zone 3 . a point of concentration of tensions and/or of less resistance in the implant is the level at which the internal threads end , giving way to the beginning of the solid body of the implant ( where the screw that holds the pillar is placed ) , due to the thin implant walls , is the place where fracture most likely occurs17 . siiril and knnen19 ( 1991 ) showed that the main deterioration factor for titanium , which may be present in the mouth , would be the abrasive action of brushing , but recommended avoiding prolonged exposure to the action of fluoride ions . the results of this study differ from those of these authors , as evidence was found of the modification of the titanium surface after contact with the fluoride ions , as well as of this modification acting on the mechanical performance of the materials . with regard to this aspect , the results of this investigation are in agreement with the findings of toumelim - chemla , et al22 ( 1996 ) , who pointed to high concentrations of fluoride ions and prolonged contact , as possible harmful agents to the durability of metal prostheses and rehabilitations with implants , which are present in the oral environment . additionally , reclaru and meyer18 ( 1998 ) reported that titanium and other dental alloys showed a corrosive process only when the ph dropped below 3.5 . this findings differ from those of our study and the study by nakagawa , et al.9 ( 1999 ) which revealed that titanium could be corroded by the existence of a small amount of naf if ph was considerably low or , in conditions of high ph if naf concentration was also considerably high . it may be concluded that fluoride ions negatively influenced the resistance to fatigue of implant - abutment sets , and that fracture occurred before reaching the minimum number of cycles established in this study . the scanning electron microscopic analysis showed evidence of pitting corrosion , which confirms the action of fluorides on the surfaces of the studied biomaterial .
the aim of this study was to assess the influence of a fluoridated medium on the mechanical properties of an internal hexagon implant - abutment set , by means of compression , mechanical cycling and metallographic characterization by scanning electronic microscopy . 
 five years of regular use of oral hygiene with a sodium fluoride solution content of 1500 ppm were simulated , immersing the samples in this medium for 184 hours , with the solutions being changed every 12 hours . 
 data were analyzed at a 95% confidence level with fisher 's exact test . 
 after the action of fluoride ions , a negative influence occurred in the mechanical cycling test performed in a servohydraulic machine ( material test system-810 ) set to a frequency of 15 hz with 100,000 cycles and programmed to 60% of the maximum resistance of static compression test . 
 the sets tended to fracture by compression on the screw , characterized by mixed ruptures with predominance of fragile fracture , as observed by microscopy . 
 an evidence of corrosion by pitting on sample surfaces was found after the fluoride ions action . 
 it may be concluded that prolonged contact with fluoride ions is harmful to the mechanical properties of commercially pure titanium structures .