Article Prosthetic and Mechanical Parameters of the Facial under the Load of Different Dental Implant Shapes: A Parametric Study

Marco Cicciù 1 , Gabriele Cervino 1 , Antonella Terranova 1, Giacomo Risitano 2 , Marcello Raffaele 2 , Filippo Cucinotta 2 , Dario Santonocito 2 and Luca Fiorillo 1,3,* 1 Department of Biomedical and Dental Sciences, Morphological and Functional Images, University of Messina, Policlinico G. Martino, Via Consolare Valeria, 98100 Messina, Italy; [email protected] (M.C.); [email protected] (G.C.); [email protected] (A.T.) 2 Department of Engineering, Messina University, 98100 Messina, Italy; [email protected] (G.R.); marcello.raff[email protected] (M.R.); fi[email protected] (F.C.); [email protected] (D.S.) 3 Multidisciplinary Department of Medical-Surgical and Dental Specialties, Second University of Naples, 80141 Naples, Italy * Correspondence: lucafi[email protected]; Tel.: +39-090-2216920

 Received: 26 October 2019; Accepted: 12 November 2019; Published: 14 November 2019 

Abstract: In recent years the science of dental materials and implantology have taken many steps forward. In particular, it has tended to optimize the implant design, the implant surface, or the connection between implant and abutment. All these features have been improved or modified to obtain a better response from the body, better biomechanics, increased bone implant contact surface, and better immunological response. The purpose of this article, carried out by a multidisciplinary team, is to evaluate and understand, through the use also of bioengineering tests, the biomechanical aspects, and those induced on the patient’s tissues, by dental implants. A comparative analysis on different dental implants of the same manufacturer was carried out to evaluate biomechanical and molecular features. Von Mises analysis has given results regarding the biomechanical behavior of these implants and above all the repercussions on the patient’s tissues. Knowing and understanding the biomechanical characteristics with studies of this type could help improve their characteristics in order to have more predictable oral rehabilitations.

Keywords: dental implants; osseointegrated implants; dental prosthesis design; biomechanical phenomena; dental occlusion; ; wound healing; immunological; bone tissue; finite element analysis

1. Introduction The dental implant (also known as endosseous implant) is a surgical-type medical device used to functionally and aesthetically rehabilitate the loss or congenital deficiency of one or more teeth, allowing the support of a prosthetic substitute through direct bone support thanks to a biological process known as osseointegration. The long-term prognosis of dental implants can be considered reliable and predictable, as it can now be based on more than forty years of worldwide clinical experience. The data reported in the literature report variable failure rates, depending on the operating techniques and types used. The type of surface treatment also appears to involve significant differences in the implant survival data. The failures are divided according to the causes, biological, biomechanical, and aesthetic. Biological failures are divided into early and late, depending on the period in which they occur. Early failure is typically linked to a deficient initial osseointegration process following the surgical procedure, more rarely to operational errors in the procedure itself, while late failures are due

Prosthesis 2019, 1, 41–53; doi:10.3390/prosthesis1010006 www.mdpi.com/journal/prosthesis Prosthesis 2019, 1 42

Prosthesis. 2019, 1, x FOR PEER REVIEW 2 of 13 to progressive infectious processes affecting the peri-implant tissues and therefore the supporting bone thatprocess surrounds following the implant the surgical (peri-implantitis) procedure, more [1–6 ].rarely Biomechanical to operational failures errors derive in the fromprocedure problems itself, due to overloadwhile late and failures functional are due trauma, to progressive which caninfectious occur processes with structural affecting failure the peri-implant at both the tissues implants and and the supportedtherefore the prosthetic supporting structures. bone that The surrounds direct implant-bone the implant connection(peri-implantitis) linked [1–6]. to the Biomechanical osseointegration processfailures leads derive to a from greater problems functional due loadto overload both on and the prostheticfunctional elementstrauma, which of the can implants occur with and on thestructural antagonist failure elements at both that the come implants into and contact the su withpported the implantprosthetic prosthetic structures. elements. The direct Theimplant- lack of bone connection linked to the osseointegration process leads to a greater functional load both on the the physiological periodontal ligament also implies the absence of the proprioceptive structures that prosthetic elements of the implants and on the antagonist elements that come into contact with the contribute to limiting trauma, through some opportune reflex mechanisms. This explains the tendency implant prosthetic elements. The lack of the physiological periodontal ligament also implies the to increase mechanical problems over time. Some systems have been proposed to limit these problems absence of the proprioceptive structures that contribute to limiting trauma, through some opportune by insertingreflex mechanisms. elastic elements This explains in the structurethe tendency of the to plants.increase Theremechanical is talk pr ofoblems aesthetic over failure time. Some when in areassystems of high have aesthetic been proposed relevance to therelimit these are exposures problems ofby metalinserting parts, elastic bone elements dehiscence in the and structure gums of with retractionthe plants. of the There interdental is talk papillaeof aesthetic and failure the creation when ofin darkareas trianglesof high aesthetic below the relevance contact pointsthere are of the teeth.exposures The success of metal or failure parts, ofbone the dehiscence implants depends and gums both with on retraction the health of status the interdental of the person papillae receiving and it, on anythe medicationscreation of dark taken triangles which havebelow a possiblethe contact impact points with of osseointegration,the teeth. The success and theor failure condition of the of the tissuesimplants of the depends mouth [both7,8]. on The the mechanical health status stress of the that person the implantreceiving would it, on an encountery medications during taken its which life must be carefullyhave a possible evaluated. impact The with correct osseointegration, planning of theand position the condition and number of the tissues of implants of the mouth is fundamental [7,8]. The for the long-termmechanical preservation stress that the of implant the prosthesis, would encounter as the biomechanical during its life forces must actingbe carefully during evaluated. The can be significant.correct planning The position of the of theposition implant and is determinednumber of byimplants the position is fundamental and angle for of thethe adjacent long-term teeth, by laboratorypreservation simulations, of the prosthesis, or by as the the use biomechanical of computerized forces tomographyacting during withchewing CAD can/CAM be significant. simulations The position of the implant is determined by the position and angle of the adjacent teeth, by and surgical guides [9]. laboratory simulations, or by the use of computerized tomography with CAD / CAM simulations and The objective of this study is to evaluate how the geometric and therefore biomechanical surgical guides [9]. characteristics of a dental implant, in addition to having repercussions on the components of the The objective of this study is to evaluate how the geometric and therefore biomechanical implant,characteristics can influence of a dental the response implant, of in the addition patient’s to oralhaving tissues repercussions [10–13]. on the components of the implant, can influence the response of the patient's oral tissues [10–13]. 2. Results 2.The Results equivalent Von Mises stress distribution at the maximum load during the masticatory cycle has beenThe evaluated. equivalent A Von vertical Mises compressive stress distribution load wasat the applied maximum on load the threeduring di thefferent masticatory prostheses cycle and afterhas the been simulation evaluated. was A performedvertical compressive the obtained load data was wereapplied post-processed. on the three different The results prostheses are presented and adoptingafter the a unifiedsimulation color was scale performed for each the kind obtained of prosthesis data were component, post-processed. ranging The fromresults a minimumare presented value in megapascaladopting a unified (MPa), color represented scale for each by bluekind of color, prosthesis and a component, maximum ranging value, representedfrom a minimum by red value color. In orderin megapascal to show the (MPa), internal represented stress arising by blue in color, the di andfferent a maximum parts of value, the finite represented element by models, red color. the stressIn hasorder been evaluatedto show the in internal section stress views. arising in the different parts of the finite element models, the stress hasAs been a first evaluated result, it in is section possible views. to see how all of the prosthesis components reach a maximum stress value lowerAs a thanfirst result, the yielding it is possible stress to of see the how titanium all of (1020the prosthesis MPa), therefore components plasticization reach a maximum and static rupturestress of value the prosthesis lower than are the avoided yielding (Figure stress of1). the titanium (1020 MPa), therefore plasticization and static rupture of the prosthesis are avoided (Figure 1).

(a) (b) (c)

Figure 1. Equivalent VonMises stress results for the complex bone fixture and prosthodontic attachments: (a) AnyOne® External; (b) AnyOne® Internal; (c) AnyOne® OneStage. Prosthesis. 2019, 1, x FOR PEER REVIEW 3 of 13 Prosthesis. 2019, 1, x FOR PEER REVIEW 3 of 13 Prosthesis 2019, 1 43 Figure 1. Equivalent Von Mises stress results for the complex bone fixture and prosthodontic attachments:Figure 1. Equivalent (a) AnyOne Von® External; Mises stress (b) AnyOne results® forInternal; the complex (c) AnyOne bone® OneStage. fixture and prosthodontic attachments: (a) AnyOne® External; (b) AnyOne® Internal; (c) AnyOne® OneStage. The AnyOne® Internal presents the most stressed area in the abutment, in the abutment-implant The AnyOne® Internal presents the most stressed area in the abutment, in the abutment-implant ® connection interface® and in the thread of the internal screw (Figure2b). On the other , AnyOne The AnyOne Internal presents the most stressed area in the abutment, in the abutment-implant® connection interface® and in the thread of the internal screw (Figure 2b). On the other hand, AnyOne External and AnyOne OneStage present the area of maximum stress in the internal screw thread and® connectionExternal and interface AnyOne and® OneStage in the thread present of the the inte arearnal of screw maximum (Figure stress 2b). Onin thethe internalother hand, screw AnyOne thread in the internal contact area® between the screw and the abutment (Figure2a–c). andExternal in the and internal AnyOne contact OneStage area between present the the screw area and of maximum the abutment stress (Figure in the 2 internal–c). screw thread and in the internal contact area between the screw and the abutment (Figure 2 –c).

(a) (b) (c) (a) (b) (c) FigureFigure 2. 2.Equivalent Equivalent Von Von Mises Mises Stress Stress results results onon thethe threethree prosthesis:prosthesis: ( (aa)) AnyOne AnyOne®® External;External; (b) ( b) ® Figure 2.® Equivalent Von Mises® Stress results on the three prosthesis: (a) AnyOne External; (b) AnyOneAnyOne® Internal; Internal; (c ()c AnyOne) AnyOne® OneStage. AnyOne® Internal; (c) AnyOne® OneStage. TheThe internal internal screw, screw, thanks thanks to to the the applied applied preload,preload, guarantees the the correct correct mechanical mechanical joining joining The internal screw, thanks to the applied preload, guarantees the correct mechanical joining betweenbetween the the implant implant and and the the abutment. abutment. ForFor allall threethree internal screws, screws, regardless regardless of of the the prosthesis prosthesis between the implant and the abutment. For all three internal screws, regardless of the prosthesis geometry,geometry, the the most most stressed stressed areas areas are are located located atat thethe contactcontact interface between between the the head head of of the the screw screw geometry, the most stressed areas are located at the contact interface between the head of the screw itselfitself and and the the abutment abutment hole hole and and also also in in the the first first threads threads ofof thethe threading (Figure (Figure 3).3). itself and the abutment hole and also in the first threads of the threading (Figure 3).

(a) (b) (c) (a) (b) (c) Figure 3. Equivalent Von Mises stress results on the internal screws: (a) AnyOne® External; (b) ®® FigureAnyOneFigure 3. 3.Equivalent® Internal;Equivalent (c Von) AnyOneVon Mises Mises® OneStage. stress stress resultsresults onon thethe internalinternal screws:screws: ( (aa)) AnyOne AnyOne External;External; (b) ( b) AnyOneAnyOne® Internal;® Internal; (c ()c AnyOne) AnyOne®® OneStage. The implant is the joining part responsible for the load transfer between the prosthesis and the boneTheThe and implant implant it has is to is the bethe joiningperfectly joining part part osseointegrated responsible responsible for forin thethordere load to transfer allow this between between load transfer. the the prosthesis prosthesis The AnyOne and and the the® bonebone and and it hasit has to to be be perfectly perfectly osseointegrated osseointegrated inin orderorder toto allowallow this load transfer. The The AnyOne AnyOne® ®

External implant presents higher stress in the proximity of the first thread of the internal nut and on the first thread of the external threading (Figure4a). The AnyOne ® Internal implant shows the most Prosthesis. 2019, 1, x FOR PEER REVIEW 4 of 13 Prosthesis 2019, 1 44 Prosthesis. 2019, 1, x FOR PEER REVIEW 4 of 13 External implant presents higher stress in the proximity of the first thread of the internal nut and on the first thread of the external threading (Figure 4a). The AnyOne® Internal implant shows the most stressedExternal area implant in the presents internal higher contact stress interface in the proxim with theity abutment of the first and thread in the of the thread internal of the nut internal and on nut stressed area in the internal contact interface with the abutment and® in the thread of the internal nut (Figurethe first4b), thread while of the the AnyOne external ®threadingOneStage (Figure implant 4a). has The lower AnyOne stress Internal values implant compared shows to the the other mosttwo (Figure 4b), while the AnyOne® OneStage implant has lower stress values compared to the other two implants,stressed area but locatedin the internal in the samecontact areas interface (Figure with4c). the abutment and in the thread of the internal nut (Figureimplants, 4b), but while located the AnyOne in the same® OneStage areas (Figure implant 4c). has lower stress values compared to the other two implants, but located in the same areas (Figure 4c).

(a) (b) (c) FigureFigure 4. Equivalent4. Equivalent(a) Von Von Mises Mises stress stress results results on on(b the the) implants: implants: ((aa)) AnyOneAnyOne® External;External;(c) ( b (b) )AnyOne AnyOne® ® ® ® Internal;FigureInternal; 4. (c Equivalent) ( AnyOnec) AnyOne VonOneStage. OneStage. Mises stress results on the implants: (a) AnyOne® External; (b) AnyOne® Internal; (c) AnyOne® OneStage. TheThe abutment abutment must must withstand withstand the the time-varying time-varying forcesforces coming from from the the masticatory masticatory cycle. cycle. The The ® AnyOneAnyOneThe abutmentExternal® External abutmentmust abutment withstand presents presents thethe thetime-varying highesthighest stress forces in the thecoming internal internal from contact contact the masticatory area area with with the cycle. the retaining retaining The screwAnyOnescrew (Figure (Figure® External5). 5). The abutmentThe AnyOne AnyOne presents®® InternalInternal the highest abutmentabutment stress has has in the the the highesthighest internal stress stresscontact in in thearea the same with same contact the contact retaining area area of of thescrewthe previous previous (Figure one, 5).one, The but but AnyOne it it exhibits exhibits® Internal also also a a large abutmentlarge stressstress has areaarea the in highest its upper upper stress part, part, in wherethe where same the the contact load load is area isapplied applied of (Figurethe(Figure previous5b). 5b). The one,The AnyOne AnyOnebut it exhibits®®OneStage, OneStage, also a inin large adjunctionadjunction stress area to to the the in internal internalits upper screw screw part, contact where contact area,the area, load presents presents is applied higher higher stress(Figurestress also also5b). in Thein the the lowerAnyOne lower contacting contacting® OneStage, part part in withadjunction with the the relativerelative to the internal implantimplant screw (Figure contact 55c).c). area, presents higher stress also in the lower contacting part with the relative implant (Figure 5c).

(a) (b) (c)

Figure 5.(a Equivalent) Von Mises stress results(b) on the abutments: (a) AnyOne® External;(c) (b) AnyOne® ® Internal; (c) AnyOne OneStage. ® ® FigureFigure 5. 5.Equivalent Equivalent Von MisesMises stress results on on the the abutments: abutments: (a ()a AnyOne) AnyOne External;® External; (b) (AnyOneb) AnyOne ® Internal;Internal; ( c(c))AnyOne AnyOne®® OneStage.OneStage. From the analysis of the stress distribution on the bone tissues (Figures 6 and 7), it is possible to ® seeFrom Fromhow the thethe analysis AnyOneanalysis of External the stress implant distribution distribution stresses on on athe thelarger bone bone part tissues tissues of the (Figures (Figures cancellous 6 and6 and bone, 7),7 ),it itisespecially ispossible possible tothe to ® seesee how how the the AnyOne AnyOne® External External implantimplant stressesstresses aalarger largerpart partof of the the cancellous cancellous bone, bone, especially especially thethe first

Prosthesis. 2019, 1, x FOR PEER REVIEW 5 of 13

first threads. On the other hand, the AnyOne® Internal presents a well-defined stressed area around Prosthesis 2019, 1 45 the thread of the implant. The AnyOne® OneStage presents, as the External device, a wide stress area Prosthesis.around 2019the , threading1, x FOR PEER of REVIEWthe implant with lower stress values for the first threads compared to5 of the 13 threads.other two On devices. the other hand, the AnyOne® Internal presents a well-defined stressed area around the ® threadfirst threads. of the implant. On the other The hand, AnyOne the ®AnyOneOneStage Internal presents, presents as the a well-defined External device, stressed a wide area stressaround area ® aroundthe thread the threading of the implant. of the The implant AnyOne with OneStage lower stress presents, values as for the the External first threads device, compared a wide stress to the area other around the threading of the implant with lower stress values for the first threads compared to the two devices. other two devices.

(a) (b) (c)

Figure 6. Stress distribution on the cancellous bone for: (a) AnyOne® External; (b) AnyOne® Internal; (c) AnyOne® OneStage. (a) (b) (c) As regards cortical tissues, a more homogeneous distribution of the stress for the AnyOne® ExternalFigureFigure and 6. 6.Stress StressAnyOne distribution distribution® OneStage onon thethe devices cancellouscancellous is registered, bone for: for: ( (whileaa)) AnyOne AnyOne greater® ®External;External; peaks (ofb) ( bstressAnyOne) AnyOne are® Internal;exhibited® Internal; by ® the( cAnyOne)(c AnyOne) AnyOne® ®Internal OneStage.OneStage. prosthesis (Figure 7).

As regards cortical tissues, a more homogeneous distribution of the stress for the AnyOne® External and AnyOne® OneStage devices is registered, while greater peaks of stress are exhibited by the AnyOne® Internal prosthesis (Figure 7).

(a) (b) (c)

FigureFigure 7. 7.Stress Stress distribution distribution on on the the bonebone tissuestissues (cancellous(cancellous and and cortical) cortical) for: for: (a ()a )AnyOne AnyOne® External;® External; (b)(b AnyOne) AnyOne® ®Internal; Internal;( (cc)) AnyOneAnyOne® OneStage.OneStage.

(a) (b) (c) 3. DiscussionAs regards cortical tissues, a more homogeneous distribution of the stress for the AnyOne® ExternalFigure and 7. AnyOne Stress distribution® OneStage on the devices bone tissues is registered, (cancellous while and greatercortical) peaksfor: (a) ofAnyOne stress® areExternal; exhibited by The finite element analysis is a useful aid for the assessment of stress rising in the bone due to (b) AnyOne® ® Internal; (c) AnyOne® OneStage. thethe AnyOne presence Internalof prosthetic prosthesis devices. (Figure It represents7). an easy way to investigate complex biomechanical systems instead of experimental techniques that are difficult to apply [14,15]. To perform a reliable 3.3. Discussion Discussion simulation, several fundamental parameters have to be taken into account, such as the bone tissues materialTheThe finite finitemodel, element element the state analysisanalysis of osseoi is antegration useful aid aid of for for the th the eimplant, assessment assessment and ofthe of stress preload stress rising rising of inthe inthe internal the bone bone duescrew. due to to thetheAlso, presence presence the reverse of of prosthetic prosthetic engineering devices.devices. procedure It represents assumes an ana fueasy easyndamental way way to to investigateimportance investigate complex in complex order biomechanicalto biomechanicalestablish the systemssystems instead instead of of experimental experimental techniques that that are are difficult difficult to to apply apply [14,15]. [14,15 To]. Toperform perform a reliable a reliable simulation,simulation, several several fundamentalfundamental parameters have have to to be be taken taken into into account, account, such such as asthe the bone bone tissues tissues materialmaterial model, model, the the state state of of osseointegration osseointegration of of the the implant, implant, and and the the preload preload of of the the internal internal screw. screw. Also, Also, the reverse engineering procedure assumes a fundamental importance in order to establish the the reverse engineering procedure assumes a fundamental importance in order to establish the correct geometry and properly models the interaction between the different prosthesis components [16].

Prosthesis 2019, 1 46

The mechanical behavior of the bone tissues is not easy to model due to the marked anisotropy and peculiar aspects depending on the individual biotype. Several authors [17–19] adopted, as a simplification, the linear elastic isotropic model in which the bone exhibits the same mechanical behavior regardless of the direction in which the load is applied. In that case, only a value of Young’s modulus and Poisson’s ratio is needed, and they can be easily retrieved from the literature or by simple mechanical tests. Other authors [20–22], in order to better represent the real mechanical behavior of the bone, adopted a linear elastic orthotropic model. In the present study, for the cancellous bone a linear elastic orthotropic model has been adopted while a transverse orthotropic model has been adopted for the cortical bone, i.e., in-plane properties (x and y direction) are the same while the third direction differs from the other two. The perfect osseointegration has been considered as a reasonable hypothesis [19,23,24]. Different biological parameters can affect the osseointegration, leading to a failure of the prosthesis [10]: Medical status of the patient, smoking, bone quality, bone age, operator experience, degree of surgical trauma, and bacterial contamination. Several authors [16,25,26] have highlighted how the internal screw preload is of fundamental importance in order to prevent the loosening of the functional contact between the abutment and the implant, especially under repeated loads. Therefore, the fatigue behavior of the internal screw connection is still an open issue [27,28]. In this study, only the maximum static load acting during the chewing cycle has been considered, hence further investigation under repeated and inclined load should be performed. The geometry of the prosthesis is of fundamental importance and it can affect the way in which the prosthesis transfers the load to the bone [29]. The neck area and the first threads of the internal screw have the maximum stress, but the AnyOne® Internal retaining screw presents smaller and well confined stress areas compared to the other two geometries. This is due to the fact that the screw has a longer shank with a reduced threaded surface respect to the AnyOne® External and the AnyOne® OneStage internal screws, which conversely present a much-extended threaded surface. The three dental implants shape adopt different configurations for the connection area with the abutment. The AnyOne® External has an external hexagonal head, while the AnyOne® Internal and OneStage present respectively a hybrid conical-hexa and octa-conical internal connection. The external configuration has the highest stress value compared to the other two internal configurations. As reported by Ceruso et al. [30], the external hexagonal connection presents micro-movement, especially under lateral load, with consequent micro-gap at the abutment-implant interface that can lead to micro-leakage and bacterial infiltration. In addition, the external solution is not able to allow a good redistribution of the stress on the implant. On the other hand, the internal connections are able to withstand in a better way to the load, redistributing the stress homogeneously on the implant and reducing the micro-gap, especially under inclined load [31,32]. The abutments are the most stressed components of the prosthetic device [33]. In this kind of component, the geometry and the shape have a great influence on the stress distribution. The AnyOne® Internal abutment presents the highest stress values followed by the AnyOne® External and AnyOne® OneStage. This behavior could be addressed to the presence of the screw seat near the loaded area of the abutment, that acts as a stress raiser due to the geometrical discontinuity. The AnyOne® External and AnyOne® OneStage are able to better distribute the load due to their unnotched shape. Several studies [34–36] have focused on the influence of the implant-abutment connection on the stress distribution in the peri-implant bone. In the present study, as observed by different authors [17,37–39], the most stressed bone tissue is the cortical one. A possible reason could be the difference in the Young’s modulus for the cortical and cancellous . The first has a value of about two order of magnitude greater than the latter, hence it is able to bear a greater stress. The implant design, the thread profile and the pitch distance have a remarkable effect on the contact area, hence on the stress distribution in peri-implant bone [38]. These properties are fundamental in order to guarantee the perfect osseointegration, transferring the correct amount of stress to avoid the bone Prosthesis 2019, 1 47 reabsorption. The AnyOne® Internal prosthesis stresses in a minor way the bone given the fact that the most stressed region is confined near the threading. As noted by Lee et al. [40], the finer pitch allow an increase of the contact area and a reduction of the stress peak in the cancellous bone. The highest peak stress has been registered in the first thread of the cortical and cancellous bone [17,18,39,41]. On the other hand, the AnyOne® External and AnyOne® OneStage prosthesis tend to stress a greater portion of the cancellous tissues. It is difficult to predict how forces are transmitted to the bone-implant interface, what happens to the implant and how the bone reacts by reshaping. First, the transmission of masticatory loads to osseointegrated implants is characterized by significant biomechanical differences with respect to natural teeth. The natural tooth is connected to the bone by the collagen fibers of the periodontal ligament which allow its intrusion up to 50–100 µm; instead the dental implant is in direct contact with the bone and the elasticity of the system depends on the elasticity of the bone. Secondly, we need to consider the biomechanical properties of bone tissue. The bone tissue is characterized by:

Anisotropy: The properties vary with the direction of the stress; • Inhomogeneity: The properties vary from point to point within the fabric; • Subjective specificity: Property values is different from one subject to another; • Viscoelasticity: Mechanical properties depend on time; the deformation is increasing over time • even at constant load; Functional adaptation: The biomechanical properties change in response to stresses. The functional • adaptation of bone is characterized by the ability of bone cells to produce or reabsorb the mineral component of the bone matrix.

According to this theory of Frost [42], four levels of increasing bone tissue are distinguished:

1. Pathologic unload zone: If no force is applied to the bone, its mineralization is gradually lost and consequently its resistance. 2. Adaptation zone: If the bone is correctly stimulated, the right physiological remodeling is created which allows the maintenance of the bone itself. 3. Overload zones: If the applied force exceeds the area of adaptation, the bone tissue reacts by opposing the external stimulus with osteoblast activation and bone apposition. 4. Pathologic overload zone: If the load exceeds the physiological range the function of the osteoblasts can be inhibited, and therefore the osteoclastic function prevails. Consequently, the bone becomes weaker and in the case of dental implants the osseointegration is lost. Finally, when the elastic limit and the resistance of the tissue are exceeded, there is a bone fracture.

4. Materials and Methods Finite elements analysis is a valid and important aid to assess the mechanical behavior of the prosthodontic devices. In particular, it is easy to predict possible bone overloads and failures that can occur on the prosthesis due to fractures. Despite finite elements analysis being a powerful tool, some fundamental parameters have to be taken into account to properly model the implants and deduce the correct results. Parameters such as model geometry, material properties, the loads, and the constrains, can severely affect the accuracy of the results. Not least, the model discretization operated by means of finite elements assumes a fundamental role in the precision of the results, therefore convergence test must always be performed. In this comparative study, three commercial prosthesis devices from the same manufacturer (MEGAGEN) with different geometric characteristic were adopted: AnyOne® External, AnyOne® Internal and AnyOne® OneStage. The simulation process undergoes through two main phases: The former involves the reverse engineering of the prosthesis, in which the stereolithography scan (STL file format) was converted into a three-dimensional CAD model and the jaw bone tissue conditions were modelled; the latter the Prosthesis 2019, 1 48

Prosthesis.definitionProsthesis. 2019 of 2019, 1 the, x, 1FOR, material x FOR PEER PEER REVIEW properties, REVIEW the discretization of the model, i.e., the creation of the mesh,8 of and8 13 of 13 the application of the loads and of the constraints on the prosthesis and bone. AfterAfterAfter the the the finite finite finite elements elements analysis analysis analysis has has has been been been perfor performed, performed,med, it it is isit possible possibleis possible to to post-processto post-process the the the data data data obtainingobtainingobtaining thethe the equivalentequivalent equivalent Von Von MisesVon Mises Mises stress stress distributionstress distri distribution onbution the on entire onthe the modelentire entire composedmodel model composed bycomposed the prosthesis by bythe the prosthesisandprosthesis the surrounding and and the the surrounding bone.surrounding bone. bone.

4.1.4.1.4.1. Reverse Reverse Reverse Engineering Engineering Engineering TheTheThe reverse reverse reverse engineering engineering engineering of of theof the the three three three prostheses prostheses prostheses was was was performed performed performed in in orderin order order to to obtainto obtain obtain a a CAD CADa CAD file file file startingstartingstarting fromfrom from thethe STLthe STL sourceSTL source source file file provided file provided provided by theby manufacturer.bythe the manufacturer. manufacturer. The STL The fileThe STL format STL file file isformat able format to is represent ableis able to to representonlyrepresent the surfaces only only the of the surfaces the surfaces model, of the noof the informationmodel, model, no noinformation about information the volume about about canthe the bevolume retrievedvolume can can frombe beretrieved it. retrieved As is possiblefrom from it. it. Asto noteAsis possible is inpossible Figure to note1toa, note some in Figurein importantFigure 1a, 1a,some detailssome important important are missing, details details like are are themissing, missing, external like like and the the internalexternal external threads,and and internal internal and threads,somethreads, components and and some some arecomponents components not completely are are not represented.not completely completely re Therefore,presented. represented. it Therefore, is necessaryTherefore, it tois it necessarymodelis necessary from to scratchmodelto model fromthefrom missing scratch scratch parts,the the missing such missing as parts, the parts, internal such such as screws, theas the intern intern andal retrievescrews,al screws, theand and missing retrieve retrieve measurements the the missing missing measurements from measurements the real fromprosthesis.from the the real realFirst, prosthesis. prosthesis. the STL First, filesFirst, the wereth STLe STL automaticallyfiles files were were automatically automatically converted converted into converted a solid into into model a solid a solidadopting model model adopting theadopting 3D ® ® thesoftwarethe 3D 3Dsoftware SpaceClaimsoftware SpaceClaim SpaceClaim, then® the, then, obtained then the the obtained modelobtained wasmodel model modified was was modified inmodified order in to orderin add order the to missingaddto add the the features.missing missing features.Thefeatures. measurements The The measurements measurements were acquired were were from acquired theacquired real from prostheses from the the real adopting real prostheses prostheses an electronic adopting adopting microscope an anelectronic electronic with a microscoperesolutionmicroscope of with 640 with a resolution480 a resolution pixel andof 64of 50 64 ×zoom 0480 × 480 pixel (Figure pixel and and8 ).5 × 5zoom × zoom (Figure (Figure 8). 8). × ×

(a) (a) (b) (b)

FigureFigureFigure 8. (8.a(a) )(STLa STL) STL to to 3Dto 3D 3DCAD CAD CAD reverse reverse reverse engineering engineering engineering process; process; process; ( (bb) ) (inb in) red,in red, red, the the thedeviation deviation deviation between between between the the the reconstructedreconstructedreconstructed geometry geometry geometry and and and the theoriginal original STL STL file file fileequals equals to 0.03to 0.03 mm. mm.

AfterAfterAfter the the the reverse reverse reverse engineering engineering engineering had had had been been been performed, performed, performed, it it was wasit was necessary necessary necessary to to verifyto verify verify the the the deviation deviation deviation of of of thethethe reconstructed reconstructed reconstructed geometry geometry geometry compared compared compared to to theto the orig original originalinal STL STL file.file. file. The The The reverse reverse reverse engineering engineering procedure procedure procedure maintainedmaintainedmaintained a maximum maximum a maximum deviation deviation respect respect to the to the STL STL STL geom geometry geometryetry file file file in in the thein the order order order of of 1/100 1of/100 1/100 of of a aof millimeter millimeter a millimeter (Figure(Figure(Figure 88b).b). 8b). The The reconstructed reconstructed geometry geometry and and a sagittalsagi a sagittalttal section section of thetheof the threethree three prostheticprosthetic prosthetic devicesdevices devices areare are reportedreportedreported in in Figure Figurein Figure 99,, in in9, whichwhichin which itit isis it possiblepossible is possible toto appreciateappreciateto appreciate thethe the didifferent ffdifferenterent components components of of the of the prosthesis prosthesis (implant,(implant,(implant, abutment abutment abutment and and and internal internal screw) screw) and and how how they they are are paired. paired.

Figure 9. Cont.

Prosthesis 2019, 1 49

Prosthesis. 2019, 1, x FOR PEER REVIEW 9 of 13

(a) (b) (c)

Figure 9. CAD reconstruction of:of: ((aa)) AnyOneAnyOne®® External; (b)) AnyOneAnyOne®® Internal; (c) AnyOneAnyOne®® OneStage.

4.2. FEM Analysis In order order to to assess assess the the stresses stresses on on the the three three differen differentt prosthetic prosthetic devices, devices, a series a series of 3D of elastic 3D elastic finite ® elementsfinite elements analysis analysis was carried was carried out using out usingSiemens Siemens NX Nastran NX Nastran® 1859. 1859.All the All prosthetic the prosthetic devices devices were weremodelled modelled with withtitanium titanium alloy alloy (Ti6Al4V), (Ti6Al4V), considered considered as asa ahomogeneous homogeneous isotropic isotropic material material whose properties areare reportedreported in in Table Table1. 1. The The interaction interaction between between the the implants implants and and the the bone bone tissues tissues of the of jawthe werejaw were taken taken into accountinto account and modelled and modelled considering considering a small a hexahedralsmall hexahedral volume volume of bone of with bone cortical with corticaland cancellous and cancellous bone tissues bone (Figuretissues 10(Figure). 10).

Table 1. Material properties and E module sources accordingly to the literature data (1,16,27,43).

Properties Cortical Bone Cancellous Bone Ti6Al4V Density [g/cm3] 1.8 1.2 4.51 Exx [GPa] 9.6 0.144 Eyy [GPa] 9.6 0.099 105 Ezz [GPa] 17.8 0.344 νxx 0.55 0.23 νyy 0.30 0.11 0.37 νzz 0.30 0.13 Gxx [GPa] 3.10 0.053 Gyy [GPa] 3.51 0.063 38.32 Gzz [GPa] 3.51 0.045

The cortical and cancellous bones exhibit a linear elastic orthotropic behavior, with the necessity of defining the Young’s modulus, Poisson’s ratio and Elastic tangential modulus in the three orthogonal directions (Table1). The solid( geometriesa) were meshed with solid(b) 4-node CTETRA4 tetrahedral( elementsc) while the contactFigure zones 10. were Reconstructed modelled withgeometry BSURFS and elementbone tissu type.es (cortical This kind and ofcancellous) finite element for: ( definesa) AnyOne a contact® regionExternal; which (b may) AnyOne act as® Internal; a source (c) or AnyOne target.® OneStage. In order to obtain reliable stress values maintaining concurrently a reasonable calculation time, a convergence test was performed and an element size of 0.2 mmThe was cortical chosen and with cancellous an acceptable bones exhibit error below a linear 5% elastic compared orthotropic to the 0.1 behavior, mm element with sizethe necessity (Table2). Theof defining final mesh the configurationYoung’s modulus, (Figure Poisson’s 11), in terms ratio of and number Elastic of nodestangential and elements,modulus forin the the three orthogonaldifferent adopted directions geometries (Table 1). after the convergence test, are reported in Table3.

TableTable 1. Material 2. Results properties of the convergence and E module test. sources Element acco sizerdingly of 0.1 mmto the as literature taken as thedata reference. (1,16,27,43).

ElementProperties Size [mm] Cortical Maximum Bone Cancellous Stress [MPa] Bone Error Ti6Al4V [%] Density [g/cm0.13] 1.8 276.69 1.2 4.51 − Exx [GPa]0.2 9.6 268.56 0.144 2.94 0.3 194.06 29.86 Eyy [GPa] 9.6 0.099 105 0.4 181.93 34.25 Ezz [GPa] 17.8 0.344 0.5 176.84 36.09 νxx 0.55 0.23

νyy 0.30 0.11 0.37

νzz 0.30 0.13

Prosthesis. 2019, 1, x FOR PEER REVIEW 9 of 13

(a) (b) (c)

Figure 9. CAD reconstruction of: (a) AnyOne® External; (b) AnyOne® Internal; (c) AnyOne® OneStage.

Prosthesis. 2019, 1, x FOR PEER REVIEW 10 of 13 4.2. FEM Analysis

In order to assess theG xxstresses [GPa] on the three 3.10 different prosthetic 0.053 devices, a series of 3D elastic finite elements analysis was carriedGyy [GPa] out using Siemens 3.51 NX Nastran 0.063® 1859. All 38.32the prosthetic devices were modelled with titaniumG alloyzz [GPa] (Ti6Al4V), 3.51considered as a 0.045 homogeneous isotropic material whose properties are reported in Table 1. The interaction between the implants and the bone tissues of the Prosthesisjaw wereThe2019 solid taken, 1 geometries into account were and meshed modelled with consideringsolid 4-node a CTETRA4 small hexahedral tetrahedral volume elements of bone while with the50 corticalcontact zonesand cancellous were modelled bone tissues with BSURFS (Figure element 10). type. This kind of finite element defines a contact region which may act as a source or target. In order to obtain reliable stress values maintaining concurrently a reasonable calculation time, a convergence test was performed and an element size of 0.2 mm was chosen with an acceptable error below 5% compared to the 0.1 mm element size (Table 2). The final mesh configuration (Figure 11), in terms of number of nodes and elements, for the three different adopted geometries after the convergence test, are reported in Table 3.

Table 2. Results of the convergence test. Element size of 0.1 mm as taken as the reference.

Element Size [mm] Maximum Stress [MPa] Error [%] 0.1 276.69 − 0.2 268.56 2.94 0.3 194.06 29.86 0.4 181.93 34.25 0.5 176.84 36.09

Table 3. Dimension of the models with 0.2 mm element size.

(a) AnyOne® External AnyOne(b) ® Internal AnyOne® OneStage(c) Figure 10.10.Elements Reconstructed geometry 498,819 and bone tissuestissu 443,946es (cortical(cortical andand cancellous)cancellous) 508,105 for:for: ((aa)) AnyOneAnyOne®® External; (bNodes)) AnyOneAnyOne ® Internal;Internal;105,804 ( (cc)) AnyOne AnyOne ®® OneStage.OneStage. 96,583 108,369

The cortical and cancellous bones exhibit a linear elastic orthotropic behavior, with the necessity of defining the Young’s modulus, Poisson’s ratio and Elastic tangential modulus in the three orthogonal directions (Table 1).

Table 1. Material properties and E module sources accordingly to the literature data (1,16,27,43).

Properties Cortical Bone Cancellous Bone Ti6Al4V Density [g/cm3] 1.8 1.2 4.51

Exx [GPa] 9.6 0.144

Eyy [GPa] 9.6 0.099 105

Ezz [GPa] 17.8 0.344

νxx 0.55 0.23

νyy 0.30 0.11 0.37

νzz 0.30 0.13

(a) (b) (c)

Figure 11. MeshMesh for for the the three three prosthetic prosthetic devices: devices: (a ()a )AnyOne AnyOne® ®External;External; (b) ( bAnyOne) AnyOne® Internal;® Internal; (c) (AnyOnec) AnyOne® OneStage.® OneStage.

Table 3. Dimension of the models with 0.2 mm element size. The hexahedral volumes of bone tissues were fixed at their lateral and lower faces and the bone- implant and bone-bone interfacesAnyOne ®weExternalre modelled AnyOne as bonded® Internal contacts in AnyOne order to® simulateOneStage the perfect osseointegrationElements of the implant. 498,819 The contact between 443,946 the metal surfaces 508,105of the prostheses was modelled as Nodesfrictional contact with 105,804 a value of the frictional 96,583 coefficient equals 108,369to 0.3. The prosthodontic component surfaces were loaded with a distributed compressive axial force of 800 N along the Y direction in order to simulate the effects of the maximum masticatory load [43] The hexahedral volumes of bone tissues were fixed at their lateral and lower faces and the (Figure 12). bone-implant and bone-bone interfaces were modelled as bonded contacts in order to simulate the perfect osseointegration of the implant. The contact between the metal surfaces of the prostheses was modelled as frictional contact with a value of the frictional coefficient equals to 0.3. The prosthodontic component surfaces were loaded with a distributed compressive axial force of 800 N along the Y direction in order to simulate the effects of the maximum masticatory load [43] (Figure 12). Prosthesis. 2019, 1, x FOR PEER REVIEW 11 of 13

The internal screw was preloaded with a force of 875 N in order to simulate the tightening torque of 35 Ncm as suggested by the manufacturer. The value of the preload was estimated with the formula: M = K D P where M is the tightening torque (expressed in Nmm), K is a global coefficient that takes into account Prosthesisthe friction2019, 1 coefficient on the thread (in this case equal to 0.2), D is the diameter of the screw 51 (expressed in mm) and P is the axial preload to apply to the screw (expressed in N).

(a) (b) (c)

FigureFigure 12. 12.Loads Loads and and boundary boundary condition condition onon thethe prosthesis.prosthesis. The The green green arrows arrows indicate indicate the the masticatory masticatory load,load, the the red arrowsred arrows indicate indicate the internal the internal screw preload,screw preload, while thewhile blue the triangles blue triangles indicate theindicate constraints: the ® ® ® (a)constraints: AnyOne® External;(a) AnyOne (b) AnyOneExternal;® (bInternal;) AnyOne (c) Internal; AnyOne (®c) OneStage.AnyOne OneStage.

5. TheConclusions internal screw was preloaded with a force of 875 N in order to simulate the tightening torque of 35 NcmHaving as suggested clarified bya thewhole manufacturer. series of biomechanical The value of thenotions preload related was estimatedto dental withimplants, the formula: and especially the influence of these on the peri-implant tissues, can lead to an improvement of the M = K D P implant surfaces and even of their shape. Knowing the· · behavior of dental implants under masticatory load, and evaluating their effects with different angles, or prosthetic components, can lead to the whererealizationM is the of tightening personalized torque rehabilitations (expressed fo inr each Nmm), patientK is and a global for every coeffi clinicalcient that need. takes into account the friction coefficient on the thread (in this case equal to 0.2), D is the diameter of the screw (expressed in mm)Author and Contributions:P is the axial For preload research toarticles apply with to theseveral screw authors, (expressed a short paragraph in N). specifying their individual contributions must be provided. The following statements should be used “conceptualization, G.R. and G.C.; 5. Conclusionsmethodology, G.R.; software, M.R., D.S. and F.C.; formal analysis, M.R., D.S. and F.C.; investigation, M.R., D.S. andHaving F.C.; data clarified curation, a whole D.S.; serieswriting—original of biomechanical draft preparation, notions related D.S.; writing—review to dental implants, and andediting, especially L.F.; visualization, M.C. and G.R.; supervision, L.F.; project administration, A.T., M.C. and G.R. the influence of these on the peri-implant tissues, can lead to an improvement of the implant surfaces andFunding: even of This their research shape. received Knowing no external the funding. behavior of dental implants under masticatory load, and evaluatingConflicts theirof Interest: effects The with authors diff erentdeclare angles, no conflict or prostheticof interest. components, can lead to the realization of personalized rehabilitations for each patient and for every clinical need. References Author Contributions: For research articles with several authors, a short paragraph specifying their individual contributions1. Pihlstrom, must B.L.; be provided.Michalowicz, The B.S.; following Johnson, statements N.W. Periodontal should diseases. be used Lancet “conceptualization, 2005, 366, 1809–1820. G.R. and G.C.; methodology,2. Bjertness, G.R.; E.; software, Hansen, M.R.,B.F.; D.S.Berseth, and G.; F.C.; Gronnesby, formal analysis, J.K. Oral M.R., hygiene D.S. and and F.C.; peri investigation,odontitis in young M.R., adults. D.S. and F.C.; dataLancet curation, 1993, D.S.; 342, writing—original1170–1171. draft preparation, D.S.; writing—review and editing, L.F.; visualization, M.C. and G.R.; supervision, L.F.; project administration, A.T., M.C. and G.R. 3. Cervino, G.; Terranova, A.; Briguglio, F.; De Stefano, R.; Famà, F.; D'Amico, C.; Amoroso, G.; Marino, S.; Funding:Gorassini,This research F.; Mastroieni, received R.; no et external al. Diabetes: funding. Oral health related quality of life and oral alterations. BioMed ConflictsRes. of Interest:Int. 2019, 2019The, authors 5907195. declare no conflict of interest.

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