the development of titanium fixtures has brought several benefits for the rehabilitation of edentulous patients . when biological and mechanical principles are respected
, this treatment modality may successfully restore the functional and esthetic impairments caused by tooth loss3 . in spite of the significant evolution of a number of implant systems , implant design and features , such as those related to the mechanical behavior of implant - supported prostheses ,
dental prostheses do fail during function mainly due to abutment and prosthesis screw loosening and/or fracture .
in addition , it has been reported that abutment screw loosening is only surpassed by loss of osseointegration as the main cause of failure on implant - supported restorations , as shown in longitudinal follow - up studies5 . when two metal surfaces are in contact , adhesion and friction forces do limit the movement between them .
an applied method aimed to reduce this friction and improve adhesion consists of interposing a lubricating film between these surfaces .
a metal with low shear strength , such as pure gold , may act as a dry lubricant .
when compared to screws without gold coating , it has been found that gold - coated abutment screws subjected to torques of 12 , 20 , and 32 ncm aiming a 0.0064 mm opening of the implant - abutment interface presented 26 , 24 , and 24% of preload increase , respectively7 . in another study , it was found that when a torque force is applied to an abutment screw , a significant part of this force is lost due to friction between the contact points of metal surfaces , inhibiting the rotation of the screw .
thus , decreasing of friction between metallic surfaces may increase the screw rotation and , consequently , the preload .
the rotation of gold - coated abutment screws placed with torque forces of 12 , 20 , and 32 ncm increased 73 , 76 , and 62% , respectively , when compared to titanium abutment screws2 .
the purpose of this study was to evaluate by strain gauges the preload and torque removal values on three abutment screws ( gold , titanium , and titanium with surface treatment screws ) applying an equivalent torque force ( 30.070.28 ncm ) .
three machined self - tapping external hex titanium screws with 4 mm in diameter and 15 mm in length were used in this investigation ( master screw , conexo sistemas de prtese ltda .
three transmucosal cera one type abutments ( 2.0 mm height , cera one , conexo sistemas de prtese ltda . ,
reference 045022 , batch # 5080915121 ) , designed for cemented single standing implant - supported restorations were also used .
the transmucosal abutments were attached to the fixtures using 10 gold screws ( conexo , sistemas de prtese ltda .
, reference 121022 , batch # 5073147 ) ( figure 1 a a ) , 10 titanium screws ( conexo sistemas de prtese ltda .
, reference 121024 , batch # 5063223 ) ( figure 1a b ) , and 10 surface treated titanium screws ( ti - tite , conexo sistemas de prtese ltda . , reference 121026 , batch # 5063035 ) ( figure 1a c ) . a load cell adapted from a model described in the literature
this load cell presented a central void for fixture fixation in its upper portion and a horizontal plate for fixation of four strain gauges on its lower portion ( figure 1 b ) .
the abutments were placed on the upper plate of the cell ( figure 1 c ) in such way that the contact between abutments and fixtures was free of any interference .
therefore , when the abutment was fixed to the fixture by the abutment screw , tensions generated by this fixation pulled the unit against the abutment , producing a deformation in the lower plate connected to the strain gauges .
next , the force ( preload ) generated on the abutment screws was captured by the strain gauges in volts ( v ) and later converted to newton ( n ) .
the aforementioned sample was arranged into 3 different groups according to the characteristics of each abutment screw : group a was formed by gold screws , group b was formed by titanium screws , and group c was formed by surface - treated titanium screws . to determine fixation and removal torques ,
a digital torquemeter was used ( torqueleader , model tsd150 , type i , class e , part # 117317 ) , previously calibrated according with iso 6789:2003 ( e ) standard . a square hand wrench ( 1.27 mm diameter , conexo ,
, reference 062300 , batch # 5091632 ) was attached to the torquemeter to allow proper connection between torquemeter and screws ( figure 1 d ) . following the proper assembly of fixture and abutment on group a
, the gold screw was then attached to the fixture with an applied torque force of 30.070.28 ncm , being repeated afterwards for groups b and c. then , the initial preload value for the abutment screws was determined ( in volts ) and all screws were kept in their positions for 5 minutes for preload stabilization , according to the literature9 . during this the preload stabilization period ,
preload values were obtained after 1 , 2 , 3 , 4 , and 5 minutes after fixation , and thereafter a mean value was calculated .
after this evaluation , the abutment screw was removed using a digital torquemeter and the maximum value of reverse torque force required for screw removal was recorded in ncm .
next , the same screw was fixed again another 4 times , obtaining the preload and torque removal mean values .
nine other screws from group a as well as 10 other screws from b and c groups were subjected to the same procedure described above .
the values of fixation torque forces of the 3 groups were analyzed statistically by to one - way analysis of variance ( anova ) .
comparison of preload values within groups was possible using the mean of 6 measurements ( 0 to 5 minutes ) obtained at the first abutment fixation , using two - way anova .
one - way anova was used to compare the mean preload values among the groups .
tukey 's test was used for multiple comparisons among the groups . for all statistical tests ,
all fixations for each screw ( n=5 ) were considered for evaluation of the torque removal to obtain an average value , and the statistical analysis was performed similarly to the preload data .
two - way anova was used to evaluate the mean torque removal among the groups , and one - way anova to evaluate the differences between screws .
tukey 's multiple comparison test was used for individual comparisons among the groups . for all statistical tests ,
the mean fixation torque force when all screws were considered was 30.070.28 ncm ( c.i .
there were no statistically significant differences ( p=0.1244 ) among the groups regarding the fixation torque .
the gold screws presented the highest preload values and the titanium screws the lowest values ( figure 2 ) .
the mean preload values of the gold screws was 131.728.98 n , and presented statistically significant differences ( p<0.01 ) compared to groups
b and c. screw # 3 and screw # 1 presented the highest ( 140.48 n ) and the lowest ( 117.73 n ) preload values , respectively . the titanium screws presented statistically significant differences ( p<0.01 ) ( mean 37.035.69 n ) .
the highest preload value was obtained for screw # 7 ( 49.68 n ) , while screw # 3 presented the lowest value ( 25.30 n ) . in the group of surface - treated titanium screws ,
the mean preload value was 97.784.68 n. statistically significant differences ( p<0.01 ) were also observed in this group , ( maximum : 104.09 n for screw # 4 ; minimum : 90.28 n for screw # 2 ) . the removal torque presented statistically significant differences among the analyzed materials ( p<0.001 ) ( figure 3 ) .
the gold screws presented a mean removal torque of 17.641.12 ncm , i.e. they did not shown statistically significant differences ( p=0.3713 ) .
titanium screws presented a mean removal torque of 18.751.89 ncm , and showed statistically significant differences ( p<0.001 ) .
the group of surface - treated titanium screws present statistically significant difference ( p=0.004 ) ( mean : 16.431.33 ncm ) .
the results of this study are similar to those of previous investigations4,8 , which found statistically higher preload values for gold fixation screws in comparison to other tested materials , in spite of using different methodologies and materials .
a maximum preload value of 666.4 n for gold screws and 458.2 n for titanium screws was reported8 .
in addition , the finds of the present study suggest that the evaluated surface treatment of titanium screws was effective , since these screws presented higher preload values than conventional titanium screws .
most previously published reports in the dental literature do not specify the removal torque of abutment screws .
the effect of different cyclic loads has been evaluated using a 32 ncm torque for fixation of the screws , which was repeated after 10 minutes to avoid contact relaxation6 .
several manufacturers have suggested this clinical approach to decrease screw loosening . in the present study , to allow a direct comparison between results of preload and torque removal for the studied groups , all screws received a similar fixation torque of30.070.28 ncm , despite the manufacturer 's recommendations of using 35 ncm torque for the ti - tite screws .
this was performed to eliminate a possible bias caused by elastic deformation that might occur in titanium screws during fixation , which might not only quantitatively but qualitatively influence the study outcomes and impair comparison among groups .
this approach allowed that , in this investigation , only the screw was left as a study variable .
another relevant issue is that up to 10% of the initial preload may be lost to smooth contact surfaces ( embedment relaxation ) , rather than elongation stresses .
however , it has been previously observed that when the same screw is fixed several times , its preload values increased4 .
comparison to different results obtained in other studies might be a difficult task due to variations of tested products and variables that may influence the produced preload value , such as the elastic modulus of the screws , opposing joint surfaces , abutment design , friction coefficient , lubrication , rate of the applied torque and the adaptation between the fixture hexagon and the abutment2 .
the findings of the present study suggest a trend for greater preload values verified for gold screws , followed by surface - treated titanium screws and titanium screws , confirming the outcomes of previous investigations4,8 . finite element analysis has been used to evaluate preloading through elongation of the abutment screw .
a study evaluating the preload of two different systems of the same implant manufacturer in function , found that the optimal preload produced should be 75% of the screw yield stress , and also that the preload value increases dramatically as the friction coefficient between implant and screw decreases7 .
as loosening of implant / abutment joint causes clinical problems , another question that may be raised is whether the torque values recommended by the manufacturers should be increased in order to obtain greater longevity of screw tightening .
the preload produced by gold or titanium fixation screws can be evaluated through the measurement of elongation stresses .
it has been found that the stresses were 60% lower than the strengths against the torques applied according to the manufacturer 's instructions4 . on the other hand
, it has been reported that gold or titanium screws could support higher torques than those indicated by the manufacturers without presenting plastic deformations , although it has been recommended elsewhere that the stresses should not exceed 65% of the screw 's fracture strength1 .
although the results of this study showed that gold screws have a clear superiority of the produced preload , application of cyclic loads would be required for a closer simulation of the masticatory function on implanted - supported restorations .
it may be concluded : 1 . gold should be the material of choice for abutment fixation screws , since it produced the highest preload values , followed by surface - treated titanium screws and conventional titanium screws ; 2 .
titanium screws presented the highest torque removal values , followed by gold screws , and surface - treated titanium screws .
 several authors still consider the mechanical problems of fracture and component loosening as the main causes of failure of implant - supported restorations . 
 the purpose of this in vitro study was to compare the preload of three types of screw for transmucosal abutment attachment used in single implant - supported prosthesis through strain gauge and removal torque measurements . 
 three external hex fixtures were used , and each received a transmucosal abutment ( cera one ) , which was fixed to the implant with its respective screw : group a- gold screw , group b- titanium screw and group c- surface - treated titanium screw ( ti - tite ) . 
 ten screws of each type were attached applying a 30.070.28 ncm torque force and maintained in position for 5 minutes . 
 after this , the preload values were measured using strain gauges and a measurement cell . 
 gold screws presented higher preload values ( 131.728.98 n ) , followed by surface - treated titanium screws ( 97.784.68 n ) and titanium screws ( 37.035.69 n ) . 
 anova ( p<0.05 ) and tukey 's test ( p<0.05 ) were applied . 
 statistically significant differences were found among the groups for both preload and removal torque values . in conclusion 
 , gold screws may be indicated to achieve superior longevity of the abutment - implant connection and , consequently , prosthetic restoration due to greater preload values yielded . 
INTRODUCTION
MATERIAL AND METHODS
RESULTS
DISCUSSION
CONCLUSIONS