since osseointegration became a safe treatment modality in dentistry , several designs of 
 implant - abutment systems have been available for clinical use , with a plethora of data 
 being produced on titanium as the main abutment material . historically , the external 
 hexagon connection was designed to provide an engagement method for implant placement 
 and anti - rotational feature for single - unit prosthesis , and is likely the functioning 
 system with the longest clinical follow - up . despite the high survival rates of titanium abutments , 
 the esthetic outcome should also be considered as a success factor , especially in the 
 anterior esthetic zone . key elements to be considered include : height of the smile line , 
 gingival biotype , color of the neighboring teeth and the esthetic expectations of the 
 patient . in essence , implant abutments can be made of ceramic ( yttrium tetragona zirconia 
 polycristals , i.e. y - tzp ) or titanium ( commercially pure or alloyed ) and can also be 
 prefabricated or customized . the standardization of the prefabricated abutments may 
 present a limitation to the establishment of an appropriate emergence profile , 
 especially in cases where a discrepancy exists between the implant and the crown 
 diameter . in such cases , the compensation will be made by the final contour of the 
 crown , which could result in unfavorable core / porcelain thickness ratios . a way to 
 overcome this issue is the use of a customized abutment that will allow for an 
 individual emergence profile forthrightly by the abutment instead of the crown . customized - zirconia abutments have shown comparable survival rates relative to titanium 
 abutments . however , the internal connection between a zirconia customized abutment and the implant 
 continues to be a mechanical challenge . thus , to prevent the zirconia from fracturing at 
 the connection , the use of a titanium connection in bi - component aesthetic abutments 
 have been suggested . among the concerns regarding the use of cad / cam customized - zirconia abutments is their 
 unknown mechanical performance in clinical situations where the implant positioning and 
 occlusal restraints result in thin abutment walls that may negatively influence the 
 long - term prosthetic functional outcome . although the buccolingual positioning and angulation of the implant 
 is crucial for the final prosthetic outcome , oftentimes there is a need to compensate 
 non - ideal implant positioning with angled customized abutments . whereas , the mechanical 
 property of customized titanium abutments may be less affected by the resulting wall 
 thinning , due to its ductile nature , the same assumption may not hold true for brittle 
 ceramic abutments . therefore , this study evaluated the mechanical performance of thinned 
 zirconia abutments at the connection , in an attempt to simulate 2 implantation scenarios 
 in terms of inclination . in fact , in the anterior aesthetic zone , according to the 
 different available bone conditions , implant insertion may require straight ( mostly in 
 canine and bicuspid zones ) or inclined ( incisor zones ) abutments . the tested null hypothesis was that a straight bi - component abutment does not present a 
 different fatigue life compared to an angled one . twenty custom - made zirconia abutments ( sweden & martina , padua , italy ) were tested 
 according to norm uni en iso 14801:2007 ( dynamic fatigue test of endosseous dental 
 implants ) either with a straight inclination ( n=10 ) or a 25 angle relative to the 
 implant long axis ( n=10 ) ( figures 1a and b ) . in 
 order to evaluate the abutments under a worst case scenario , an industrially imposed 
 minimum thickness of 0.3 mm at the connection area with the implant testing configuration for the a ) straight zirconia ( zr ) abutment showing the 
 distances between the abutment assembly and the potting surface and b ) the 30 
 loading orientation . the area circled in b is magnified in c ) where the component 
 parts are described and the circumferential 0.3 mm thicknesses of the zirconia 
 abutment at the connection is depicted . the same patterns regarding the d ) 
 distance of the abutment assembly and e ) loading orientation are observed in the 
 25 angled abutment groups . components and thicknesses circled in e are described 
 in magnified figure f the implant fixture was inserted in a prefabricated rigid plexiglas clamping 
 device . individualized zirconia abutments were cemented on a titanium connection and 
 fixed to the implant using a titanium screw according to the manufacturer 's 
 instructions : after cleaning with alcohol , the zirconia abutment was cemented onto the 
 titanium connection ( all stone cement , sweden & martina , padua , italy ) . the remaining cement was removed 
 immediately with steam and after polymerization , the excess cement was removed . a peek ( polyetherketone ) ring was used for stress distribution at the zirconia / titanium 
 interface interposed between the zirconia abutment and retaining screw ( figure 2 ) . components of the custom - made abutment composed by an individualized zirconia 
 abutment cemented on a titanium connection linked to the implant using a titanium 
 screw . a peek ring was used for stress distribution at the zirconia / titanium 
 interface interposed between the zirconia abutment and retaining screw for mechanical testing , single load to fracture ( slf ) was performed ( 6027 , instron , usa ) 
 ( n=10 each ) , and the mean values were used to calculate the maximum load 
 ( fmax ) . the loading geometry was fixed by uni en iso 14801:2007 both for 
 the slf and dynamic loading tests . the implant / abutment system was positioned with its 
 long axis inclined at a 301 angle relative to the loading direction . the dimensions 
 of the indenter resulted in a distance of l=11.0 mm0.1 mm from the 
 center of the hemisphere to the clamping plane ( figure 
 3a ) . ( a ) single load to fracture ( slf ) and ( b ) dynamic loading testing machines a hemispherical indenter was used for the load application . eighty percent of the 
 fmax was used as a starting load value for the dynamic testing , which used 
 an uni - axial testing equipment ( fpf , italsigma , italy ) , with a 2.000 n load cell ( tstm , 
 aep , italy ) and an extensometer ( 2620 - 601 , instrom , usa ) ( figure 3b ) . fatigue was conducted at 15 hz for 5 million cycles in dry 
 conditions at 20c5c . the bending moment was calculated as follows : where y is the moment arm ( defined by l x sin 30 ) , and f is the force expressed in 
 newtons . failed samples were inspected in a polarized - light microscope ( mz - apo , carl zeiss micro 
 imaging , thornwood , ny , usa ) and then in a scanning electron microscope at an 
 acceleration voltage of 10 kv ( xvp , evo 50 , carls zeiss ag , oberkochen , 
 germany ) . mean values and standard deviations were calculated for each outcome variable ( static 
 and dynamic tests ) as a function of the straight and inclined abutment groups . all comparisons were performed by t - test assuming unequal variances [ sigmastat 3.0 
 ( spss inc . , mean values and standard deviations were calculated for each outcome variable ( static 
 and dynamic tests ) as a function of the straight and inclined abutment groups . all comparisons were performed by t - test assuming unequal variances [ sigmastat 3.0 
 ( spss inc . , the mean fmax value for slf , dynamic loading are presented in table 1 , as well as the bending moments required 
 for fracture . mean values for mechanical testing of the straight and 25 angled zirconia abutment 
 groups sd : standard deviation no significant differences in load to failure were observed between the straight and 
 angled bi - component abutments in both the static ( p=0.253 ) and dynamic tests ( p=0.135 ) . a significant difference in the bending moment required for fracture was detected 
 between the groups ( p=0.01 ) . fractures in the angled group occurred chiefly at the point of load application , whereas 
 in the straight abutments fractures were located coronally and close to the thinly 
 designed areas at the cervical region ( figure 
 4 ) . representative polarized light and scanning electron microscope micrographs of 
 abutment samples failed during fatigue . a ) straight abutment group in a frontal 
 view depicting that fractures occurred coronally at b ) thinly designed areas 
 ( pointer ) mostly close to the base of the abutment , as observed in this occlusal 
 view . c ) in the angled group , fracture occurred at the point of load application 
 involving d ) thinly designed areas ( pointer ) , as observed in this occlusal 
 view the present study simulated clinical scenarios where customized zirconia abutments 
 presented the thinnest possible section at the connection area , resulting in significant 
 implications in its mechanical response , according to our slf testing . however , when 
 evaluated under a more clinically realistic loading scenario , ( fatigue ) fmax 
 were not significantly different between the groups , but bending moments for fracture 
 were higher for the straight relative to the angled abutments . such a result was not 
 expected considering the differences in the lever scenario created for both groups , but 
 may be accounted by the difference in the moment of inertia between the two designs . the resulting failure modes where the straight abutments showed fractures at the 
 cervical region compared to fractures at the point of load application in the angled 
 abutments likely reflected the resulting bending moments for both groups . from a 
 clinical standpoint , although both scenarios may fail at similar time points , it can be 
 speculated that in a restoration supported by a thinned angled zirconia abutment , the 
 crown is more likely to shield the functional area receiving the load compared to a 
 thinned straight abutment . the 
 bending moments of both groups were significantly higher compared to values observed in 
 the literature . this could be 
 related to the double zirconia / titanium anchorage system . as to the testing methods available for the evaluation of different implant - abutment 
 system configurations , several have been described , such as the single load to 
 fracture , the use of fatigue 
 followed by the application of a static load until fracture , the staircase 
 method , fatigue limit ( iso 
 14801:2007 ) , step - stress accelerated life testing , and others . while the iso 14801 was created in 2003 and revised 
 in 2007 , with the aim of standardizing the testing procedures and data presentation in 
 the fatigue of dental implants , it has been shown that the results produced by such a 
 method should be interpreted with caution . the wide range of testing parameters allowed 
 in the iso 14801:2007 regarding the testing frequency ( 2 to 15 hz ) , environment ( water 
 or dry when testing above 15 hz ) and amount of cycles ( 2 or 5 million , depending on the 
 chosen frequency ) have shown that a very different failure probability distribution may 
 result , as well as failure 
 modes . whereas , the present 
 study utilized the highest end of the speed scale allowed by the norm , especially to 
 optimize the machine testing time , potential differences in the fatigue limit and 
 failure modes when testing in 2 hz in water may exist and warrant future investigation . it may be suggested that , while attending industry requirements for implants quality 
 assurance and control , the iso 14801:2007 testing methodology may likely continue to be 
 developed as observed from 2003 to the currently advocated version . as generally reported and also in agreement with our testing results , the discrepancy in 
 values arising from the static relative to the dynamic testing and the clinical 
 relevance of each , should be acknowledged . if the selection of a system was based on the 
 static test results , which do not simulate physiological loading scenarios where 
 repetitive , lower load cycling is the chief mechanism leading to failure , most systems 
 would be suitable irrespective of the area of application ( molar or incisor ) . with 
 static loading , materials will commonly fail at loads higher than those in fatigue . the 
 mechanisms of subcritical crack growth that should be simulated in testing include those 
 that operate in use - related failures which are stress corrosion and cyclic 
 fatigue . in this study , static 
 testing was only performed to provide load values for subsequent fatigue testing . fatigue can provide more effective ways of simulating failures observed 
 clinically . considering that the reported maximal bite force in the incisor area may vary from 108 
 n to 190 n , 
 the fmax fatigue loads observed for both thin - walled straight or angled 
 zirconia abutments are above the physiologic range . therefore , in esthetically demanding 
 areas , zirconia abutments may be indicated especially when thin peri - implant soft 
 tissues are present . although no 
 direct comparison was performed with titanium abutments , short - term clinical studies and 
 systematic reviews point toward similar success rates for both materials and no 
 influence in the failure behavior of the overlying restorative system . the postulated null hypothesis that a straight bi - component abutment would not present 
 different fmax than an angled abutment under fatigue was accepted .
although the use of zirconia abutments for implant - supported restorations has gained 
 momentum with the increasing demand for esthetics , little informed design rationale 
 has been developed to characterize their fatigue behavior under different clinical 
 scenarios . 
 however , to prevent the zirconia from fracturing , the use of a titanium 
 connection in bicomponent aesthetic abutments has been suggested.objective:mechanical testing of customized thin - walled titanium - zirconia abutments at the 
 connection with the implant was performed in order to characterize the fatigue 
 behavior and the failure modes for straight and angled abutments.material and methods : twenty custom - made bi - component abutments were tested according to iso 14801:2007 
 either at a straight or a 25 angle inclination ( n=10 each group ) . 
 fatigue was 
 conducted at 15 hz for 5 million cycles in dry conditions at 20c5c . 
 mean values 
 and standard deviations were calculated for each group . 
 all comparisons were 
 performed by t - tests assuming unequal variances . 
 the level of statistical 
 significance was set at p0.05 . 
 failed samples were inspected in a 
 polarized - light and then in a scanning electron microscope.results:straight and angled abutments mean maximum load was 296.7 n and 1,145 n , the 
 dynamic loading mean fmax was 237.4 n and 240.7 n , respectively . no 
 significant differences resulted between the straight and angled bi - component 
 abutments in both static ( p=0.253 ) and dynamic testing ( p=0.135 ) . 
 a significant 
 difference in the bending moment required for fracture was detected between the 
 groups ( p=0.01 ) . 
 fractures in the angled group occurred mainly at the point of 
 load application , whereas in the straight abutments , fractures were located 
 coronally and close to the thinly designed areas at the cervical region.conclusion:angled or straight thin - walled zirconia abutments presented similar 
 fmax under fatigue testing despite the different bending moments 
 required for fracture . 
 the main implication is that although zirconia angled or 
 straight abutments presented similar mechanical behavior , the failure mode tended 
 to be more catastrophic in straight ( fracture at the cervical region ) compared to 
 angled abutments .