with technological advances in the field of implant dentistry novel treatment modalities and more efficient options became available .
the custom 3d printed root analogue implant ( rai ) as defined by anssari moin et al .
[ 1 , 2 ] and figliuzzi et al .   is a futuristic treatment option for immediate implantation and immediate loading cases for a soon to be removed tooth .
advantages of the rai technique when compared to conventional screw shaped multipiece implants may encompass more cost efficiency , one - piece implant , and minimal traumatic surgical intervention [ 16 ] .
an essential factor for realization of all implant - based prosthetic reconstructions is successful osseointegration of the implant . in particular
, primary stability plays a fundamental role in one - stage implant surgery with or without immediate loading .
conventional screw type implants achieve primary stability through mechanical fixation by implant threads in bone .
numerous studies on the factors influencing primary stability ( implant shape specifications , surface modifications , bone quality , and surgical technique ) and the effect on the process of osseointegration have been performed [ 811 ] . however , primary stability for the rai technique is based on the ( targeted ) press - fit phenomenon for achieving successful osseointegration [ 13 , 6 ] . since the custom rai is based on cone beam computed tomography ( cbct ) , computer aided
design ( cad ) , and 3d printing technology an unlimited array of designs for this custom implant approach is available . every rai design option aimed at increasing initial mechanical stability for
the root part of the rai will have a different biomechanical effect on the surrounding bone and influence on the relative microdisplacement at bone - to - implant interface consequently leading to diverse osseointegration results , bone resorption , or failures .
finite element analysis ( fea ) has become an effective method in investigating bone stress / strain around implants and relative microdisplacement between bone - to - implant interfaces .
however , as with all fea studies the analysis is confined to a limited amount of factors and designs and can not be generalised , specifically not for individual cases .
thus , the aim of this study is to analyse , with the means of fea , the influence of 5 custom rai designs on stress distribution of peri - implant bone and to evaluate the impact on microdisplacement for a specific patient case .
a patient ( male , 64 years of age ) presenting a profoundly decayed upper right canine was selected and informed consent was obtained from the patient . based on the method previously described by anssari moin et al . [ 1 , 2 ] a 3d surface model of rai was constructed . in brief
the procedure was as follows : the patient was scanned with the 3d accuitomo 170 cbct system ( accuitomo 170 , 90  kvp , 5  ma , 30.8  s , 4  4  cm field of view [ fov ] , voxel 0.08  mm , morita inc . , kyoto , japan ) using the recommended scan protocol .
amira software ( v4.1 , visage imaging , carlsbad , ca ) was used for image analysis .
a region of interest limited to the tooth and its surrounding was initially selected and a threshold segmentation algorithm based on histogram analysis of grey values was used to separate the tooth ( root and crown ) from surrounding bone and periodontium . further semiautomated segmentation based on slice - by - slice analysis was implemented to enhance the segmentation by removing any residual artifacts ( figure 1 ) .
the segmented dataset was converted to 3d surface model using the marching cube algorithm and saved in the standardized triangulation language ( stl ) file format .
based on the stl model five different ( targeted ) press - fit design rai fe models have been constructed using 3d cad software ( inventor , autodesk gmbh , munich , germany ) . for the five rai models a standard identical abutment , based on morphological expectation of the original tooth crown and measurements on neighboring teeth ,
was designed at 2  mm distance coronal from the expected bone level after implantation .
subsequently , the following ( targeted ) press - fit design modifications were constructed : ( 1 ) nonmodified standard , ( 2 ) targeted press - fit prism , ( 3 ) targeted press - fit fins , ( 4 ) targeted press - fit plug , and ( 5 ) targeted press - fit bulbs , referred to as  standard ,   prism ,   fins ,  
five bone models surrounding 3  mm congruent to the respective rai models were built using femap software ( v. 11.0.1 , siemens plm software , plano , tx , usa ) .
finally , preprocessor software ( femap v. 11.0.1 , siemens plm software , plano , tx , usa ) was applied to mesh the models with quadratic tetrahedral solid elements ( figure 3 ) .
mesh refinement based on convergence analysis resulted in a mesh size of 0.5  mm .  
the following assumptions were made for the rai fe models : composition of a titanium alloy ti6al4v , young 's modulus e = 110  gpa , and poisson 's ratio  = 0.35 with the material being homogeneous , isotropic , and linearly elastic [ 13 , 14 ] .
the bone models were constructed using a homogenous isotropic linearly elastic material of 1  mm inner cortical layer ( young 's modulus e = 12.6  gpa , poisson 's ratio  = 0.3 , and shear modulus g = 5.7  gpa ) and a 2  mm outer trabecular layer ( young 's modulus e = 1.1  gpa , poisson 's ratio  = 0.3 , and shear modulus g = 0.07  gpa ) as proposed in the reviewed literature [ 13 , 14 ] .
bone - to - implant interfaces were assumed to be frictional surfaces to represent a nonosseointegrated contact situation .
a coulomb frictional method ( coefficient of friction = 0.3 ) was adopted to define linear contact behavior [ 14 , 15 ] .
two loads were applied to simulate anterior bite force : an oblique buccoapical force with a magnitude of 300  n set on 135 to the long axis of the implant and a vertical force in apical direction to the long axis of the implant with a magnitude of 150 
n , as shown in figure 3 [ 16 , 17 ] . the nodes in the outer surrounding layer of trabecular bone were constrained in all directions ( zero nodal displacement ) . numerical solving ( nastran v. 8.0 , siemens plm software , plano , tx ) and postprocessor analysis ( femap v. 11.0.1 , siemens plm software , plano , tx , usa ) was performed on the meshed bone - implant models to evaluate stress distributions on cortical and trabecular bone and deformed contact separation ( micromotion ) at the peri - implant region . based on previous research
the following measurements were recorded : the von mises equivalent stress ( vm ) at the bone peri - implant interface as a quantity of stress level for the load transfer mechanism [ 12 , 1821 ] , the tensile / maximum ( t ) and compressive / minimum ( c ) principal stresses as a criterion to evaluate the bone overloading [ 19 , 20 ] , and finally deformed contact separation ( micromotion in m ) as an indicator for initial implant stability [ 22 , 23 ] .

 figure 4 displays the average measured stress values ( in mpa ) of the principal and von mises stresses at the supporting tissues for all groups .
notably , on average the stress levels caused by oblique loading were higher when compared to vertical loading .
the standard design rai exhibited the highest von mises stress and highest minimum principal stress values ( highest compressive stress ) under both loading conditions ( vm = 252 and c = 50 ) .
the lowest von mises stress levels were numerically observed for the plug design under the different loading conditions ( figure 4(a ) , vm = 82 ; figure 4(b ) , vm = 168 ) , indicating a reduction of 67.4% and 33.3% , respectively , when compared to the standard design .
furthermore , the highest measured tensile stress in cancellous bone was 4  mpa for the standard design ( data not shown ) .
comparing behavior of von mises stress distribution caused by vertical ( figure 5(a ) ) and oblique loading ( figure 5(b ) ) , it can be observed that the cortical peri - implant bone exhibited greater stress concentration than trabecular bone . in tension stress ,
concentrations can be noted at the loaded side for the standard and prism under the oblique loading component ( figure 6 ) . however , under the same conditions the plug , fins , and bulb designs showed tensile stress intensities on the lingual side and in the buccal area of the protrusive extensions of the design ( figure 6 ) .
the apical peri - implant area indicated high von mises stress concentrations in all designs ( figure 7 ) and tensile stress peaks under both loading conditions for the standard , plug , and fin designs .
comparison of the minimum principal stress illustrated in all models the highest compressive stress concentrations on the lingual side ( figure 8) . 

table 2 shows the microdisplacement of the various rai designs from the peri - implant bone with respect to the loading conditions .
the highest magnitude of micromotion level was measured in the prism design , 32.10  m and 32.51  m under vertical and oblique loading , respectively .
remarkably , the lowest levels of contact separation were measured in the fins model followed by the bulbs design under vertical and oblique forces : 5.45  m , 6.25  m and 6.35  m , 6.42  m , respectively .
microdisplacement patterns were located at neck area in direction of the forces and in contra lateral direction in the apical area in all designs ( images not shown ) .
in this study five different designs of rai were analyzed for stress - based biomechanical behavior for a specific patient by means of finite element simulations . in the primary phase of endosseous healing multiple biomechanical mechanical factors
the von mises stress was used as an indicator for the load transfer mechanism , principal stresses as indicator to bone overloading and micromotion as indicator for initial stability .
numerical results from the current study suggest that adding targeted press - fit design characteristics to the standard rai design will decrease the amount of maximum von mises stress in the surrounding peri - implant bone , subsequently leading to more favorable load behavior for this patient .
previous studies have assumed maximum bone strength as biological limit to bone failure and activation of bone resorption [ 15 , 19 , 21 ] .
correspondingly , it has been proposed that overloading of cortical bone occurs when the maximum compressive principal stress exceeds 190  mpa and maximum tensile principal stress exceeds 130  mpa [ 15 , 19 , 21 ] .
likewise , trabecular bone overloading will occur when the compressive and tensile principal stresses exceed 5  mpa and 5  mpa , respectively [ 15 , 19 , 21 ] . according to the result of this study
it has been found that solitary prism design exceeded the maximum compressive stress criterion for cortical bone .
the standard , fins , plug , and bulbs designs exceeded the tensile stress threshold in cortical bone .
the threshold for trabecular bone overloading in tension was not reached . however , when observing the compressive stresses under oblique loading in trabecular bone , it can be noted that in the regions of the implant neck all implant designs exceeded the biological limit , inducing a risk to bone loss ( figure 8) .
the fins and bulbs designs showed the lowest levels of micromotion , indicating the most favorable primary stability .
nonetheless , it must be noted that the influence of micromotion on osseointegration is of scientific debate as some studies have suggested a more positive effect on the tissue differentiation and bone formation around implants under controlled micromotion up to 50  m .
additionally , in our study it has been found that the higher oblique loading component causes more stress concentrations on cortical and trabecular bone when compared to vertical loading .
therefore , oblique loading in the primary stage after implantation will have a more negative effect on bone healing and should be minimized . in this current study
was modeled and assumed as a homogeneous , isotropic , linearly elastic material . however , it is known that the biomechanical behavior of this living tissue is heterogeneous , anisotropic , and nonlinear [ 14 , 19 , 20 ] . moreover , a 100% osseous contact between implant and bone was assumed .
contact relationship between implant and bone was defined as linear contact behavior by using a coulomb frictional model .
although contact behavior should be defined in a nonlinear method , several studies are in agreement about adopting a linear frictional model since non - linear contact analysis is highly complex [ 14 , 25 ] . in clinical situations the actual bone - to - implant contact directly after insertion of the rai will be dependent on many factors , that is , accuracy of the rai technique on multiple levels , ( periapical ) bone defects , and surgical handling .
the quantity of in situ osseous contact after implantation of the rai will have profound effect on primary stability and stress behavior .
furthermore , the herein applied loads were static one directional loads of amplitude of 150  n ( vertical ) and 300  n ( oblique ) whereas in clinical situations considerably variable loads can be observed depending on the location of the rai in the oral cavity and patient characteristics . despite the fact that simulation methods and fe modeling were beyond the scope of this investigation ,
the current limitations can be considered as acceptable in a numerical sense and are in agreement with multiple studies [ 13 , 14 , 16 , 17 , 19 , 25 ] . especially with the rise of custom 3d printed implants questions concerning biomechanical behavior in each specific patient surface .
ideally for future implementation of custom 3d designed and printed implants easy accessible individual patients specific fea should be performed to get a better understanding of the biomechanical behavior of different implant designs for a specific case .
based on the results of this study and within the limitations of the applied methodology , it has been found that adding targeted press - fit geometry to the rai standard design , preferably fins or bulbs , will have a positive effect on stress distribution and lower concentration of bone stress and will provide a better primary stability for this patient case .
 
 objectives . the aim of this study was to analyse by means of fea the influence of 5 custom rai designs on stress distribution of peri - implant bone and to evaluate the impact on microdisplacement for a specific patient case . materials and methods . 
 a 3d surface model of a rai for the upper right canine was constructed from the cone beam computed tomography data of one patient . 
 subsequently , five ( targeted ) press - fit design modification fe models with five congruent bone models were designed :  standard ,   prism ,   fins ,   
 plug ,  and  bulbs ,  respectively . 
 preprocessor software was applied to mesh the models . 
 two loads were applied : an oblique force ( 300  n ) and a vertical force ( 150  n ) . 
 analysis was performed to evaluate stress distributions and deformed contact separation at the peri - implant region . 
 results . 
 the lowest von mises stress levels were numerically observed for the plug design . 
 the lowest levels of contact separation were measured in the fins model followed by the bulbs design . conclusions . within the limitations of the applied methodology , 
 adding targeted press - fit geometry to the rai standard design will have a positive effect on stress distribution , lower concentration of bone stress , and will provide a better primary stability for this patient specific case . 