contemporary restorations of dental defects with implants are widely applicable , clinically prospective and comfortable treatment method .
although it is important to realize that this method , as all other restorative procedures , is aimed not to change the tooth but exactly restore what has been lost in biological and mechanical aspects . an implant success rate is a numerical quantitative expression , which values the success of the implant as a matter of persistence until its fatal loss .
this rate reaches 95% and is based on the osseointegration of the implant . if the osseointegration does not occur , the implant will be rejected and will not have a successful outcome because of early complications .
there are two types of failures : early complication , which occur before the prosthetics and late complications after osseointegration and restoration [ 2,4 - 8 ] .
it means that all 95% of successful implants still have risks of late failures , which are related with longevity and quality of treatment . under functional loading condition
physiologically we can expect 1 - 1.5 mm bone loss throughout the first year and < 0.2 mm every following year
. this process could be accelerated by mechanical , chemical and biological factors . with lack of attention and control
, this physiological process may turn into a pathological inflammation of peri - implant soft tissues as peri - implant mucositis or its later form : peri - implantitis . despite high implant survival rate ,
epidemiological studies and clinicians insufficiently paying attention to the quality factor and sustainability of our final restoration .
however , there should be understood that implants can not sufficiently replace natural teeth despite their survival rates .
some of the main reasons for late implant failures can be bacterial factors , host health conditions ( diabetes mellitus and bisphosphonates ) , smoking , overloading and iatrogenic factors [ 9 - 11 ] .
major debates and the absence of consensus prevail especially due to overloading effect on implant complications
. the intact root of the tooth is covered by periodontium and has 25 to 100 m micro - movements on axial direction . in comparison
, osseointegrated dental implants can move just 3 to 5 m at the same axis .
tooth mobility is determined by deformation of periodontal ligaments while implant mobility is determined by limited deformation of the bone . in a micro - movement like this ,
the periodontal ligament works as a damper and can reduce the effect of the stress on surrounding structures .
moreover , periodontium could adjust the load because of proprioceptive sensors . according to mechanostat theory by frost ,
however , there are no clear guidelines : how much functional and para - functional loading can be injurious for dental implant , what mechanical factors can decrease load effect for such kind of restorations and what should be avoided .
the aim of this review is to find out risks of mechanical impacts of peri - implant bone loss and prosthetic influence on bone stability .
protocol and registration the review was registered on the international prospective register of systematic reviews prospero .
this protocol could be found in prospero register : http://www.crd.york.ac.uk/prospero/display_record.asp?id=crd42016037224 this review was performed following prisma statements ( preferred reporting item for systematic review and meta - analyses ) . types of publications the review included publications in english , which were published from january 2011 till april 2016 .
the selection of studies consists of randomized control trials , cohort studies , case - control studies , case reports , animal and in vitro studies .
information sources and search a literature search was performed of two databases - medline ( pubmed ) and embase websites .
the specific keywords were selected on purpose to establish a maximal informative and an accurate search .
keywords were as follows :  absorbing  ,  bone  ,  damping  ,  dental implant  ,  dis - integration 
,  occlusal  ,  overloading  ,  peri - implant  ,  shock  ,  strain  ,  stress  . according to these ,
the search was performed on pubmed and embase search systems :  dental  and  implant  and (  overloading  or  stress  or  shock  or  strain  ) and ( (  bone  and  loss  ) or  disintegration  or  damping  or  absorbing  ) .
two reviewers independently screened the title and abstract of articles derived from this broad search .
after primary screening decisions of both reviewers were compared and discussed . the third experienced reviewer accomplished a secondary screening for those cases where the disagreement was .
the inclusion of articles was done according to : the description of the usage of dental implants , the estimation of implant loading , the load transmission and distribution over peri - implant bone or bone level changes . fixed prosthetic treatment and functional loading or over - loading
should be applied in studies . in order to review the latest data , the studies over 5 years old
also , studies that analyse the effect of masticatory forces in restoration level only ( crown , abutment , screw ) without changes in the bone level were excluded .
the selected studies were divided into groups according to the type of the study : experimental in vitro , experimental in vivo and clinical .
data of experimental finite element analysis ( fea ) studies were systematized in assessing load vector and value , stress and strain in peri - implant bone , implant length , diameter , bone layers , type of bone , bone level , osseointegration level , implant design , type of implant - abutment connection , restoration ( table 1 ) . 

c = cortical bone ; t = trabecular bone ; d1 , d2 , d3 , d4 = bone density rate ; mpa = megapascals ; n = newtons ; na = not appropriate ; nd = no data ; ps = platform switch ; nps = non - platform switch ; strain = a common engineering unit measuring strain . 
 
data were collected from the included fea studies and arranged in the following fields :    bone layers  - shows types of applied bone layers ;  type of bone  - describes the density of bone ;  osseointegration level 
 characterizes the conditions of implant osseointegration ;  bone level  - describes the depth of implant position ;  design of implant  - describes the shape of implant model ;  diameter of implant  - describes diameter dimension ;  length of implant  - describes length dimension ;  implant - abutment connection  - shows type of connection ( platform switch or non - platform switch ) ;  type of supra - construction  - description of prosthesis ;  axial load  - strength value in an axial direction ;  oblique load  - strength value in an oblique direction ;  lateral load  - strength value in a lateral direction ;  stress / strain units  - measuring units
. assessment of methodological quality the intrinsic methodological and lacks of design were independently screened by two reviewers .
the qualitative assessment was accomplished for each of elected study and their risk of bias was evaluated according to the cochrane library .
they were grouped according to the type of the study : 20 - in vitro fea ; 8 - in vitro ( tests ) ; 5 - in vivo ; 3 - prospective , 6 - retrospective . the distribution of publications shows the prevalence of virtual fea mechanistic studies . with the intention of achieving methodical homogeneity ,
all fea studies ( n = 20 ) were involved in data analysis ( figure 1 ) .
study characteristics the analysis of articles , which describes the fea study , illustrates that both conditions and the obtained data are heterogeneous ( table 1 ) .
according to methods , studies differ on the different simulated conditions : a different bone base .
mainly , double layered ( cortical + trabecular ) bone model was applied and only a few models were single layered .
the authors programmed various bone types ( d1 - d4 ) and some were evaluating stress distribution on bone following density .
for example , it could be programmed as a peri - implant cylinder , as a section of the jaw or a full dental arch .
some authors provided a real scanned situation . a different osseointegration . in reality , implant is not fused with bone absolutely .
some referred to it as an ideal for simplicity ( 100% integration ) and others as a partial osseointegration for more realistic model .
there were different selection of implant length , diameter , thread and other properties in the studies .
some of the investigations were carried out without a reference of the implant - abutment connection or completely without it .
also , there were studies in which the investigation was performed only at implant - abutment model .  
the angle of oblique load was a variable too . some studies analysed stress distribution depending on load direction but mostly dominated free of choice .
the loading conditions are simulated not only at different vectors but also at different loads , which were used from 50 to 500 n.   different outcome units .
the dominant measuring of stress was expressed in von mises mpa , but studies with tangential stress were measured in mpa or strain in strain .  
the impact of loading resulted in stress or strain at the implant - bone interface was described in all 20 publications .
mainly there were stress dependence on the force acting axis [ 20 - 28 ] and all authors assured that non - axial loading increased peri - implant stress . also , implant design characteristics were described as factors causing the stress .
the alteration of stress was related with implant length , width [ 21,25,26,28 - 31 ] , macro - relief as thread , and micro - relief as porosity .
some of the articles emphasized that stress changes were caused by platform switching connection [ 20 - 22,33 ] .
the influence of abutment - implant connection was also described by another two articles , who revealed impact of inaccuracy at this interface .
prosthetic related factors as crown / implant ( c / i ) ratio   and restorative materials   were also analysed .
all the selected studies were assessed for their risk of bias according to the cochrane library .
the included studies did not report it . the sequence generation could be debatable for in vitro or perhaps impossible , especially for fea .
evaluation of allocation concealment and blinding of participants and outcomes of in vitro studies was confirmed severe and not appropriate .
the incomplete outcome data was not explained in most cases and assessment  unclear  means that no missing data was reported ( table 2 ) . 

na = not appropriate . 
  due to the study design and conditions , heterogeneity of outcomes may not achieve a comparison of results and statistical analysis .
this review was performed with an aim to systematically review risks of mechanical impact on peri - implant strain and prosthetic influence for stability across fea studies . despite the heterogeneity between the fea studies , the established trends
correlate with other in vitro , in vivo , prospective and retrospective studies from the same search . according to newton
s third law it is undisputed that chewing forces are transmitted to restoration and these forces do not decrease but transform into energy , which is distributed in certain amounts through the restoration - implant complex .
energy could be distributed into restorative materials , cement layer , abutment , screw , implants and peri - implant bone . according to reviewed studies , an overloading of peri - implant bone
fea studies have shown dependence between stress distribution and loading direction . assessing the effect of the axial force , was detected the distribution of uniform stress in peri - implant bone without concentrations in a specific surface area of the implant .
the non - axial loading of the implant increased the stress concentrations in peri - implant bone because of bending .
it is important to understand , that the axis of rotation locates at the top of the bone . in fea models under load of 100 n without any changes of other conditions except of loading direction ( axial vs. oblique ) , the stress increased and its trend of concentration was revealed under oblique loading .
angled abutments created the same effect because they shifted the load from the implant axis . without changes of load direction following the increased abutment angulation from 0 to 15 , stress concentrations increased in cortical peri - implant bone .
consequently , the oblique load creates a bending effect , which affects the bone , especially around the rotation area but the different mechanical properties of bone layers determine different displacement of the implant body .
other parameters of restorations such as cantilevers and c / i ratio could increase bending of the implant and stress or strain in the bone .
the behaviour of different sized restorations was compared : single - unit , fixed partial , fixed full .
another retrospective study did not find significant bone loss around implant when distal cantilevers were constructed with full arch prosthesis .
so the number of implants could create a counterpoise and reduce the bending effect of cantilevers .
the similar bending effect could be caused by the extended width of the implant crown .
however , the study did not find an influence between the width of the crown and peri - implant bone loss .
performed an fea study , in which the influence of different c / i ratio to stress distribution was investigated .
the dependence between peri - implant bone loss and c / i ratio was confirmed in prospective clinical studies .
otherwise , c / i ratio depends not only on crown length , but also on implant length .
this supports the fea study with the conclusion that short implants create higher stress at peri - implant cortical bone .
the shape of the implant could affect the stress distribution around the implant surface . according to fea results , as the diameter of the implant decreased ,
all studies comparing the effectiveness of conventional ( non - platform switch ) and platform switch abutments revealed that platform switch type abutments decreased stress concentrations in peri - implant bone [ 20 - 22,33 ] and its positive effect was higher for cortical bone than trabecular . on the other hand , platform switch
reduced stress from the bone , but accelerated on the interface of connection surfaces   and that could provoke strain for mechanical parts such as implant , screw and abutment .
the concept of passive fit implies that there was no gap or strain induced by misfit of the framework prior to functional loading .
this fea investigated peri - implant stress when theoretical misfit was recorded on implant - abutment level and on abutment - crown level .
approved similar findings in their study , where misfit of abutment was generated with changing of conical angle of abutment .
the passive fit of implant and related prosthetic components were considered to be very important and its absence was thought to cause complications in biological tissues and mechanical failures .
consequently , imperfections at the connection regions could generate overloading of peri - implant bone as well as in prosthetic parts .
it may be worth considering , that analog abutments or bases could cause the similar stress because of unmanaged imperfections .
study   detected that shock - absorbing capacity of implant restorations depends on damping behaviour of different dental materials under the cyclic loading conditions .
the usage of composite materials damping behaviour was growing due to visco - elasticity of such material .
changes could exist in restorative layers but that could not be detected by static linear fea .
if the damping effect of platform switch was detected , elastic prosthetic parts could accelerate the same phenomenon .
described damping behaviour of periodontal ligament and its shock - absorbance . in clinical cases ,
when an implant restoration had a natural opposite tooth , the periodontal ligament of antagonistic could absorb significant part of the loading energy .
authors established 0.2 mm peri - implant bone loss in cases with natural antagonist and 0.6 mm bone loss in cases with opposite implant restoration at the same time , and concluded , that higher bone level is in cases with opposite natural tooth .
implant loss because of direct overloading reasons was described in in vivo   study with rats and in retrospective study . in vivo based older studies detected that in case of bacterial peri - implant inflammation and overloading could accelerate bone loss .
however , we have to agree with pellegrini et al .   review , which stated that the detrimental effect of occlusal overload on bone - implant interface is still a controversial issue .
peri - implant strain could be generated by non - axial loading , cantilever prosthetic elements , crown / implant ratio , type of implant - abutment connection , misfits , properties of restoration materials and antagonistic tooth .
finite element analysis studies are not erroneous methodically and the results correlate with other experimental and clinical findings . due to the heterogeneity of finite element analysis studies and expression of their results , it is impossible to perform meta - analysis or systematic reviews .
 abstractobjectivesto systematically review risks of mechanical impact on peri - implant strain and prosthetic influence on stability across finite element studies.material and methodsan online literature search was performed on medline and embase databases published between 2011 and 2016 . following keywords tiered screening and selection of the title , abstract and full - text were performed . 
 studies of finite element analysis ( fea ) were considered for inclusion that were written in english and revealed stress concentrations or strain at peri - implant bone level.resultsthere were included 20 fea studies in total . 
 data were organized according to the following topics : bone layers , type of bone , osseointegration level , bone level , design of implant , diameter and length of implant , implant - abutment connection , type of supra - construction , loading axis , measurement units . 
 the stress or strain at implant - bone contact was measured over all studies and numerical values estimated . 
 risks of overloading were accented as non - axial loading , misfits , cantilevers and the stability of peri - implant bone was related with the usage of platform switch connection of abutment.conclusionsperi-implant area could be affected by non - axial loading , cantilever prosthetic elements , crown / implant ratio , type of implant - abutment connection , misfits , properties of restoration materials and antagonistic tooth . 
 the heterogeneity of finite element analysis studies limits systematization of data . 
 results of these studies are comparable with other findings of in vitro , in vivo , prospective and retrospective studies . 
INTRODUCTION
MATERIAL AND METHODS
RESULTS
DISCUSSION
CONCLUSIONS
ACKNOWLEDGMENTS AND DISCLOSURE STATEMENTS