since the long - term success rates of osseointegrated dental implants may be as high as 
 99% , this treatment option has 
 become increasingly important in the field of oral rehabilitation . besides single tooth 
 replacement , oral implants 
 offer the possibility of rehabilitating partial and total edentulous jaws with fixed 
 ( fdps ) or removable dental prostheses ( rdps ) . however , meta - analyses have shown that there is insufficient 
 evidence to establish clinical guidelines for either fdps or rdps in partially 
 edentulous jaws . notwithstanding , most patients prefer implant - supported 
 fdps , since this kind of prosthesis replaces the tooth under as natural conditions as 
 possible . the first 
 option is to place a screw - retained abutment onto the endosseal implant and to fix the 
 fdp by conventional cementation ; with the second option , the superstructure is directly 
 connected with the implant by a screw . there have been no consistent conclusions about 
 the long - term success of the two connection types : nissan , et al . ( 2011 ) reported that the long - term 
 outcome of cemented implant - supported fdps was superior to that of screw - retained 
 fdps . ( 2011 ) indicate 
 that screw and cement - retained restorations are equivalent with respect to most success 
 parameters as assessed by the clinician or patient . a major problem with all implant - supported fdps was identified 
 in a systematic review : technical complications related to implant components and 
 suprastructures were reported in 60 - 80% of the studies included , whereas the fixture 
 failed in less than 1% of the cases in vivo . implant overload was thought to be responsible for 
 cracks developing in the material , leading to catastrophic failure even after short 
 periods of function . cemented fdps are aesthetically superior , since they have no screw channel and 
 angulations of the implant can be compensated by the abutment . furthermore , fabrication 
 tolerances are adjusted by the cement layer and bacterial microleakage is less , 
 especially in combination with a conical implant - abutment connection . however , removal of the superstructure 
 for maintenance or hygienic reasons is very demanding or even impossible . in contrast , 
 with screw - retained fdps , these procedures can be handled easily , for example if a 
 fixation screw has become loose or has failed , or another technical or biological 
 maintenance is needed . a further advantage of these fdps is that they are less expensive 
 due to minor complexity of the manufacturing process if cad / cam technology is applied . nevertheless , screw - retained fdps require a passive fit and some studies have reported 
 that cad / cam produced frameworks may exhibit misfits and deformation stresses . one example of a screw - retained fdp is the i - bridge2 , introduced in 2005 by 
 biomain ( biomain ab , helsingborg , sweden ) . this kind of restoration is a cad / cam - milled 
 implant bridge of either titanium or cobalt chromium alloy with the possibility of 
 angling the screw channels by up to 20. due to this angulation , the screw channels can 
 be placed at the oral side of the fdp , especially in the anterior region , thus making it 
 possible to build fdps with larger spans with satisfactory aesthetics . furthermore , this 
 system is compatible with most established implant systems , since the fdp can be 
 directly connected to the fixture or with a special abutment between the implant and 
 framework , e.g. with astra tech ( see figure 
 1 ) . schematic cross - section of the i - bridge2 system there is a lack of information about the mechanical characteristics of screw - retained 
 fdps , especially when these are connected directly with the implant fixture . the authors 
 expect major stresses and distortions within the connection area and the screw which may 
 affect the mechanical characteristics of these restorations . the aim of the present 
 study was therefore to evaluate the load - bearing capacity of a five - unit milled titanium 
 implant framework ( i - bridge2 , biomain ab , helsingborg , sweden ) supported by 
 three implants and to test the influence of artificial aging from cyclic mechanical 
 loading on the load - bearing capacity . additionally , failed specimens were micro- and 
 macroscopically analyzed to identify the failure modes . the hypotheses to be tested within the present study were : 1 ) load - bearing capacity of 
 screw - retained , five - unit milled titanium implant frameworks supported by considerably 
 angulated implants is higher than static functional forces occurring in the posterior 
 region . 2 ) even after extended cyclic mechanical loading specimens show a stable 
 implant - framework connection and a sufficient load - bearing capacity for use in the 
 posterior area . for each test specimen , three implants had to be reproducibly placed into a 
 bone - simulating socket . a master model was prepared for this purpose : a silicone 
 negative ( optosil , heraeus kulzer , hanau , germany ) of a block - 55 mm in 
 length , 25 mm in height and 25 mm in depth - served as the parent for all sockets . the silicone form was cast once with polyurethane ( alphadie top , schtz 
 dental gmbh , rosbach , germany ) to generate a master socket for the implants in order 
 to simulate placement in the right mandibular canine ( 43 ) , the right mandibular 
 second premolar ( 45 ) and the right mandibular second molar ( 47 ) region . to mimic a 
 realistic clinical worst - case scenario with respect to the shape of the mandibular 
 jaw , the implants were angulated as follows : 43 : 30 buccal angulation , 45 : no 
 angulation , 47 : 30 lingual angulation . drilling holes for implant analogues were 
 prepared with a device for the manufacturing of surgical templates 
 ( gonyx , straumann gmbh , freiburg , germany ) , thus guaranteeing the 
 predefined angulation and drilling hole depth . implant analogues were placed into the 
 drilling holes and fixed with acrylic resin ( palavit g , heraeus kulzer , 
 hanau , germany ) in such a manner that a simulated bone loss of 3 mm from the implant 
 shoulder was considered in accordance with iso 14801 . the distances between the center points of the 
 implants were 14 mm ( 43 - 45 ) and 19 mm ( 45 - 47 ) . in the next step , the implant 
 analogues were prepared for modelling an i - bridge2 master framework by 
 adding viscous acrylic resin ( pattern resin ls , gc international corp . , special 
 components of the i - bridge2 system , called the i - flex ( see 
 figure 3 ) , were fixed at the implant 
 analogues . the i - flex is a screw with a spherical head that serves as a 
 substructure for the modelling and is used to define the angling of the screws 
 connecting the implant and the framework . modelling caps were then placed onto the 
 substructure and the fdp was modelled in such a manner that the occlusal surfaces 
 were planar , except for both pontics , where small cavities were included for the 
 exact load application . the distance between the shoulder of the middle implant and 
 the occlusal surface was 12 mm ( figure 2 ) . finally , the whole model was sent to the manufacturer ( biomain ) , for 
 scanning of the implant situation and the master framework and for milling 10 
 identical titanium frameworks according to the i - bridge2 system . i - bridge2 master framework made of acrylic resin special components of the i - bridge2 system , called the i - flex , placed on the 
 implants prior to master framework modelling using the special abutment ( biomain ) as interconnecting components ( see 
 also figure 1 ) , original implants 
 ( osseospeed 4.0 s , 13 mm length , astra tech , mlndal , sweden ) were 
 fixed to the frameworks with the corresponding screws . the framework - implant 
 assemblies were then consecutively placed in the above mentioned silicone negative 
 which was afterwards poured out with polyurethane ( alphadie top , schtz 
 dental gmbh , rosbach , germany ) . after the curing process was finished , all 
 implant - framework connections were removed . to assure reproducible assemblies , the 
 abutment and the frameworks were reconnected to the implants with the corresponding 
 screws and the torque given by the manufacturer ( implant - abutment 15 ncm , 
 abutment - framework 20 ncm ) . five specimens were randomly selected for cyclic 
 mechanical loading and prepared with a resilient silicone bearing at the socket 
 ( mollosil plus , detax , ettlingen , germany ) , in order to prevent socket fracture due 
 to non - planar contact during cyclic loading . specimens of the test group underwent five million cycles of mechanical loading in a 
 chewing simulator ( machine shop , hannover medical school , hannover , germany ) , with 
 100 n as the upper load limit at a frequency of 2.5 hz prior to final testing . after 
 every 250,000 load cycles , the specimens were macroscopically checked to see whether 
 the screws had loosened or failed . for this purpose , the mechanical loading was 
 stopped and the specimens were macroscopically evaluated by visual inspection 
 regarding the potential changes in the construction . furthermore , the stiffness of 
 the screw connection was tested by use of the recommended screw driver without 
 applying an additional force to the complex . as figure 
 4 shows , the load was applied onto the pontics at two points 16.5 mm apart 
 via two tungsten carbide balls ( diameter 6.0 mm ) on interposed tin foils ( thickness 
 0.2 mm ) to ensure an equally distributed load application . the loading piston was 
 mounted using an intermediate silicone layer ( mollosil plus , detax , ettlingen , 
 germany ) to prevent point - wise overload and to guarantee a homogeneous load 
 application ( see figure 4 ) . since a survey has 
 revealed that the average number of chewing cycles is about 800,000 
 per year , the 
 five million cycles applied in this study corresponded to an in - vivo 
 service period of approximately 75 months ( 6 years , 3 months ) . the force was transferred to the pontics via two tungsten 
 carbide balls after cyclic mechanical loading , the resilient silicone socket bearing and the tin 
 foils were removed and the test and control specimens were loaded in a universal 
 testing machine ( type 20k , uts testsysteme , ulm - einsingen , germany ) . load - displacement curves were recorded until failure ( defined as a drop in load of 
 more than 500 the load piston 
 was the same as that used for the cyclic mechanical loading ; the crosshead speed was 
 1 mm / min . the statistical analysis was performed using the t - test for independent 
 groups , with the level of significance set at p=0.05 . exemplarily chosen load - displacement curve of a test specimen before and after testing , all specimens were macro- and microscopically analyzed at 
 the interface of the implant and superstructure , using a reflected light microscope 
 ( m3z , wild , heerbrugg , switzerland ) . failure modes were documented via a digital 
 camera ( progres c12 plus , jenoptik , jena , germany ) with all pictures including a 
 scale bar . changes in the frameworks ' geometry due to load testing were evaluated by 
 comparing pictures of the specimens before and after the testing procedure . additionally , one specimen from each test group was selected for cross - sectional 
 analysis . for this purpose , the specimens were embedded in clear methylmethacrylate 
 ( acryfix , struers gmbh , willich , germany ) and mid - sectioned along the longitudinal 
 axis of each implant in the bucco - lingual direction using a diamond saw ( isomet 4000 , 
 buehler , illinois , usa ) . after polishing the cross - sectional surface to a roughness 
 depth of less than 9 m , the internal configuration was visually inspected and 
 photographed under a reflected - light microscope ( m3z , wild , heerbrugg , switzerland ) 
 at tenfold magnification to evaluate the failure mode . for each test specimen , three implants had to be reproducibly placed into a 
 bone - simulating socket . a master model was prepared for this purpose : a silicone 
 negative ( optosil , heraeus kulzer , hanau , germany ) of a block - 55 mm in 
 length , 25 mm in height and 25 mm in depth - served as the parent for all sockets . the silicone form was cast once with polyurethane ( alphadie top , schtz 
 dental gmbh , rosbach , germany ) to generate a master socket for the implants in order 
 to simulate placement in the right mandibular canine ( 43 ) , the right mandibular 
 second premolar ( 45 ) and the right mandibular second molar ( 47 ) region . to mimic a 
 realistic clinical worst - case scenario with respect to the shape of the mandibular 
 jaw , the implants were angulated as follows : 43 : 30 buccal angulation , 45 : no 
 angulation , 47 : 30 lingual angulation . drilling holes for implant analogues were 
 prepared with a device for the manufacturing of surgical templates 
 ( gonyx , straumann gmbh , freiburg , germany ) , thus guaranteeing the 
 predefined angulation and drilling hole depth . implant analogues were placed into the 
 drilling holes and fixed with acrylic resin ( palavit g , heraeus kulzer , 
 hanau , germany ) in such a manner that a simulated bone loss of 3 mm from the implant 
 shoulder was considered in accordance with iso 14801 . the distances between the center points of the 
 implants were 14 mm ( 43 - 45 ) and 19 mm ( 45 - 47 ) . in the next step , the implant 
 analogues were prepared for modelling an i - bridge2 master framework by 
 adding viscous acrylic resin ( pattern resin ls , gc international corp . , special 
 components of the i - bridge2 system , called the i - flex ( see 
 figure 3 ) , were fixed at the implant 
 analogues . the i - flex is a screw with a spherical head that serves as a 
 substructure for the modelling and is used to define the angling of the screws 
 connecting the implant and the framework . modelling caps were then placed onto the 
 substructure and the fdp was modelled in such a manner that the occlusal surfaces 
 were planar , except for both pontics , where small cavities were included for the 
 exact load application . the distance between the shoulder of the middle implant and 
 the occlusal surface was 12 mm ( figure 2 ) . finally , the whole model was sent to the manufacturer ( biomain ) , for 
 scanning of the implant situation and the master framework and for milling 10 
 identical titanium frameworks according to the i - bridge2 system . i - bridge2 master framework made of acrylic resin special components of the i - bridge2 system , called the i - flex , placed on the 
 implants prior to master framework modelling using the special abutment ( biomain ) as interconnecting components ( see 
 also figure 1 ) , original implants 
 ( osseospeed 4.0 s , 13 mm length , astra tech , mlndal , sweden ) were 
 fixed to the frameworks with the corresponding screws . the framework - implant 
 assemblies were then consecutively placed in the above mentioned silicone negative 
 which was afterwards poured out with polyurethane ( alphadie top , schtz 
 dental gmbh , rosbach , germany ) . after the curing process was finished , all 
 implant - framework connections were removed . to assure reproducible assemblies , the 
 abutment and the frameworks were reconnected to the implants with the corresponding 
 screws and the torque given by the manufacturer ( implant - abutment 15 ncm , 
 abutment - framework 20 ncm ) . five specimens were randomly selected for cyclic 
 mechanical loading and prepared with a resilient silicone bearing at the socket 
 ( mollosil plus , detax , ettlingen , germany ) , in order to prevent socket fracture due 
 to non - planar contact during cyclic loading . specimens of the test group underwent five million cycles of mechanical loading in a 
 chewing simulator ( machine shop , hannover medical school , hannover , germany ) , with 
 100 n as the upper load limit at a frequency of 2.5 hz prior to final testing . after 
 every 250,000 load cycles , the specimens were macroscopically checked to see whether 
 the screws had loosened or failed . for this purpose , the mechanical loading was 
 stopped and the specimens were macroscopically evaluated by visual inspection 
 regarding the potential changes in the construction . furthermore , the stiffness of 
 the screw connection was tested by use of the recommended screw driver without 
 applying an additional force to the complex . as figure 
 4 shows , the load was applied onto the pontics at two points 16.5 mm apart 
 via two tungsten carbide balls ( diameter 6.0 mm ) on interposed tin foils ( thickness 
 0.2 mm ) to ensure an equally distributed load application . the loading piston was 
 mounted using an intermediate silicone layer ( mollosil plus , detax , ettlingen , 
 germany ) to prevent point - wise overload and to guarantee a homogeneous load 
 application ( see figure 4 ) . since a survey has 
 revealed that the average number of chewing cycles is about 800,000 
 per year , the 
 five million cycles applied in this study corresponded to an in - vivo 
 service period of approximately 75 months ( 6 years , 3 months ) . after cyclic mechanical loading , the resilient silicone socket bearing and the tin 
 foils were removed and the test and control specimens were loaded in a universal 
 testing machine ( type 20k , uts testsysteme , ulm - einsingen , germany ) . load - displacement curves were recorded until failure ( defined as a drop in load of 
 more than 500 n , see figure 5 ) . the load piston 
 was the same as that used for the cyclic mechanical loading ; the crosshead speed was 
 1 mm / min . the statistical analysis was performed using the t - test for independent 
 groups , with the level of significance set at p=0.05 . before and after testing , all specimens were macro- and microscopically analyzed at 
 the interface of the implant and superstructure , using a reflected light microscope 
 ( m3z , wild , heerbrugg , switzerland ) . failure modes were documented via a digital 
 camera ( progres c12 plus , jenoptik , jena , germany ) with all pictures including a 
 scale bar . changes in the frameworks ' geometry due to load testing were evaluated by 
 comparing pictures of the specimens before and after the testing procedure . additionally , one specimen from each test group was selected for cross - sectional 
 analysis . for this purpose , the specimens were embedded in clear methylmethacrylate 
 ( acryfix , struers gmbh , willich , germany ) and mid - sectioned along the longitudinal 
 axis of each implant in the bucco - lingual direction using a diamond saw ( isomet 4000 , 
 buehler , illinois , usa ) . after polishing the cross - sectional surface to a roughness 
 depth of less than 9 m , the internal configuration was visually inspected and 
 photographed under a reflected - light microscope ( m3z , wild , heerbrugg , switzerland ) 
 at tenfold magnification to evaluate the failure mode . all specimens survived cyclic mechanical loading and no obvious failure or screw 
 loosening could be observed . load - displacement curves showed a more or less steep 
 increase until a maximum force was reached , followed by a gradually decreasing force 
 and , finally , failure . table 1 and figure 6 show the results of the load - bearing capacity testing . in comparison 
 to the control group with a load - bearing capacity of 8,496 n196 n , the aged specimens 
 exhibited a broad decrease in load - bearing capacity to 7,592 n901 n. however , the 
 cyclic mechanical loading did not significantly influence the load - bearing capacity 
 ( p=0.060 ) . mean values ( mv ) , standard deviations ( sd ) , medians ( md ) , maximum ( max ) and 
 minimum ( min ) are given box chart representing load - bearing capacity for both test groups . medians and 
 quartiles are given external inspection of the specimens revealed an identical failure mode for all 
 specimens . large deformations of the titanium framework in the abutment area accompanied 
 by a loss of vertical dimension were obvious . nevertheless , all fdps were still fixed on 
 the implants and no screw fracture could be detected . analyses of cross - sections showed framework fractures near the abutment in both the 
 control and test group ( see figure 7a - c ) . furthermore , the screw threads of the abutment and the implant were deformed . in one 
 case , polished cross - sections of embedded failed specimens of the differently angulated 
 implants ( a : + 30 , b : 0 , c : -30 ) . dental implants are subjected to functional loading during their period of wear 
 in vivo . hence , it is of crucial importance to consider cyclic 
 mechanical loading when evaluating the long - term behaviour of implant - supported 
 restorations in vitro . fatigue testing until failure is accepted as a 
 method to generate data on the fracture strength and implant longevity . a standardized guideline ( iso 14801 ) for the dynamic fatigue 
 testing of single implants has been established by the international organization for 
 standardization . in contrast 
 to single implant testing , testing of multi - implant supported fdps is not yet 
 standardized , but the experimental setup of the present study was carefully chosen to be 
 in accordance with iso 14801 . furthermore , an unfavourable clinical situation was 
 imitated as best as possible : the distance between the implant shoulder and crestal bone 
 level was adjusted to 3 mm in order to represent a typical reduction in the bone 
 support , as recommended in iso 14801 . to mimic natural bone , the implants were embedded in reinforced 
 polyurethane with an elastic modulus similar to that of bone . moreover , since in numerous clinical situations 
 implants are angulated to the restoration 's axis , in particular in the vestibulo - oral 
 direction , in the current test 
 scenario the anterior ( 43 region ) and the posterior implant ( 47 region ) were angulated 
 30 off - axis in the buccal and lingual directions , respectively . cyclic mechanical 
 loading was performed with a chewing simulator and an upper load limit of 100 n , which 
 is in accordance with the average bite forces of between 20 n and 120 n , depending on 
 the nutrition 's hardness . however , a 
 fixed number of mechanical cycles ( five million ) was applied , representing an in 
 vivo service period of approximately 75 months ( 6 years , 3 months ) . this period of wear makes it possible 
 to draw conclusions on the long - term behaviour of the implant components . even though tests were performed under 
 highly realistic conditions , the significance of the present study may be limited due to 
 the sample size of only five specimens per group . notwithstanding this , 
 the number of test samples seems to be adequate , since several other authors have 
 conducted studies on implant connection stability with the same sample size . in a systematic review , ( 2002 ) showed that technical complications related to implant 
 components and superstructures were reported in 60 - 80% of the studies included , in 
 contrast to biological complications in only 40 - 60% of the studies . the locking of multiple implants seems to 
 stabilize the whole implant - framework assembly . furthermore , this may be due to the passive fit of the 
 cad / cam - milled i - bridge2 . ( 2011 ) considered that the cad / cam is the most consistent method 
 for screw - retained implant frameworks , potentially giving an excellent fit . the load - bearing capacity of the i - bridge2 even after cyclic mechanical 
 loading was 7,592 n , which is much higher than maximum bite forces . these range 
 approximately between 150 n and 880 n in the posterior region , depending on experimental 
 conditions and the individual . nevertheless , large deformations of the framework were obvious in 
 the connection area of the implant . the onset of plastic deformation typically appears 
 earlier than the load drop which defined failure . hence , it is possible that some of the 
 veneering layer may delaminate in clinical practice before failure sets in . as the 
 load - bearing capacity of the i - bridge2 achieves approximately tenfold the 
 maximum bite forces , it can be assumed that this phenomenon may be quite rare . as a 
 limitation of the present study , it has to be mentioned that the frameworks fabricated 
 were a little bulkier than many actual clinical frameworks , thus resulting in a higher 
 load - bearing capacity . the present results suggest that screw - retained implant bridges are sufficient to 
 rehabilitate partial and total edentulous jaws . a recently published long - term 
 evaluation of full - arch implant bridges is in accordance with these findings . however , it has to be emphasized that 
 just one specific implant system was included in this study , so that conclusions for 
 other systems are hard to draw . furthermore , long - term success depends on additional 
 aspects , e. g. peri - implant soft tissue complications . the load - bearing capacity of the i - bridge2 frameworks is much higher than 
 the clinical relevant occlusal forces , even with non - optimally placed implants , so that 
 there is a huge safety margin . the cyclic mechanical loading did not significantly 
 influence the load - bearing capacity , but in vivo long - term stability 
 depends on additional aspects , e. g. bacterial microleakage . this study was supported by biomain , sweden , whose support is gratefully 
 acknowledged .
implant - supported screw - retained fixed dental prostheses ( fdps ) produced by cad/ cam 
 have been introduced in recent years for the rehabilitation of partial or total 
 endentulous jaws . 
 however , there is a lack of data about the long - term mechanical 
 characteristics.objectivethe aim of this study was to investigate the failure mode and the influence of 
 extended cyclic mechanical loading on the load - bearing capacity of these 
 frameworks.material and methodsten five - unit fdp frameworks simulating a free - end situation in the mandibular jaw 
 were manufactured according to the i - bridge2-concept 
 ( i - bridge2 , biomain ab , helsingborg , sweden ) and each was 
 screw - retained on three differently angulated astra tech implants ( 30 buccal 
 angulation/0 angulation/30 lingual angulation ) . 
 one half of the specimens was 
 tested for static load - bearing capacity without any further treatment ( control ) , 
 whereas the other half underwent five million cycles of mechanical loading with 
 100 n as the upper load limit ( test ) . 
 all specimens were loaded until failure in a 
 universal testing machine with an occlusal force applied at the pontics . 
 load - displacement curves were recorded and the failure mode was macro- and 
 microscopically analyzed . 
 the statistical analysis was performed using a t - test 
 ( p=0.05 ) . 
 resultsall the specimens survived cyclic mechanical loading and no obvious failure could 
 be observed . due to the cyclic mechanical loading , 
 the load - bearing capacity 
 decreased from 8,496 n196 n ( control ) to 7,592 n901 n ( test ) . 
 the cyclic 
 mechanical loading did not significantly influence the load - bearing capacity 
 ( p=0.060 ) . 
 the failure mode was almost identical in all specimens : large 
 deformations of the framework at the implant connection area were obvious . 
 conclusionthe load - bearing capacity of the i - bridge2 frameworks is much higher 
 than the clinically relevant occlusal forces , even with considerably angulated 
 implants . 
 however , the performance under functional loading in 
 vivo depends on additional aspects . 
 further studies are needed to 
 address these aspects .