Mechanical strength and fracture point of a dental implant

$Page 1: Mechanical strength and fracture point of a dental implant$
RESEARCH AND EDUCATION

aDoctoral stuUniversity, MbDoctoral stuUniversity, McResearcher,University, MdGeneral DireePrivate pracfResearcher,University, M

THE JOURNA

Mechanical strength and fracture point of a dental implantunder certification conditions: A numerical approach by finite

element analysis
Guillermo de la Rosa Castolo, MS,a Sonia V. Guevara Perez, DDS,b Pierre-Jean Arnoux, PhD,c
Laurent Badih, MS,d Franck Bonnet, DDS,e and Michel Behr, PhDf

ABSTRACTStatement of problem. Implant prosthodontics provides high-quality outcomes thanks to recenttechnological developments and certification procedures such as International Organization forStandardization (ISO) 14801. However, these certification tests are costly, and the result is highlyuncertain as the influence of design variables (materials and structure) is still unknown. Thedesign process could be significantly improved if the influence of design parameters wereidentified.

Purpose. The purpose of this in vitro study was to use finite element analysis (FEA) to assess theinfluence of design parameters on the mechanical performance of an implant in regard to testingconditions of ISO 14801 standard.

Material and methods. An endosseous dental implant was loaded under ISO 14801 testingconditions by numerical simulation, with 4 parameters evaluated under the following conditions:conditions of the contact surface area between the implant and the loading tool, length of thefixation screw, implant embedding depth, and material used for implant stiffness. FEA was used tocompare the force that needed to reach the implant’s yield and fracture strength.

Results. A dental implant’s fracture point can be increased by 41% by improving the contactsurface area, by 20% depending on the type of material, by 4% depending on the length of thefixation screw, and by 1.4% by changing the implant embedding depth.

Conclusions. FEA made it possible to evaluate 4 performance parameters of a dental implantunder ISO 14801 conditions. Under these conditions, the contact surface area was found to be themajor parameter influencing implant performance. This observation was validated experimentallyin a fatigue test under ISO 14801 conditions. (J Prosthet Dent 2018;119:611-9)

According to the World HealthOrganization, caries and peri-odontal diseases are themain causes of tooth loss.1

Worldwide, between 15% and20% of the population be-tween 35 and 44 years of agehave lost at least 1 tooth,and 30% of elderly people be-tween 65 and 74 years of agehave no natural teeth.1 Implantdentistry improves the lives ofedentulous patients thanks torecent progress in the areas ofimplant protocols, implantperformance, and dental pros-theses, as well as product ergo-nomics.2 However, implantdentistry is not always suited tomany of the situations that arise,notably when insufficient bonequality exists to ensure the im-plant’s good performance over
the long term.3,4 The implant design phase, therefore, iscritical for dealing with increasingly complex situations.3,4
dent, The French Institute of Science and Technology for Transport, Develoarseille, France; and Research engineer, Glad Medical SAS, Salon-de-Prodent, The French Institute of Science and Technology for Transport, Develoarseille, France; and Associate Professor, Department of Oral Health, NatiThe French Institute of Science and Technology for Transport, Developmarseille, France.ctor, Glad Medical SAS, Salon-de-Provence, France.tice, Le Cannet, France.The French Institute of Science and Technology for Transport, Developmearseille, France.

L OF PROSTHETIC DENTISTRY

Downloaded for scmh lib ([email protected]) at Show Chwan MemorFor personal use only. No other uses without permission.

The prognosis for an implant’s mechanical strength isstrongly influenced by load conditions during mastication5

pment and Networks (IFSTTAR) Laboratory of Applied Biomechanics, Aix-Marseillevence, France.pment and Networks (IFSTTAR) Laboratory of Applied Biomechanics, Aix-Marseilleonal University of Colombia, Bogota, Colombia.ent and Networks (IFSTTAR) Laboratory of Applied Biomechanics, Aix-Marseille

nt and Networks (IFSTTAR) Laboratory of Applied Biomechanics, Aix-Marseille

611

ial Hospital JC from ClinicalKey.com by Elsevier on May 08, 2018. Copyright ©2018. Elsevier Inc. All rights reserved.

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Clinical ImplicationsTest conditions were found to significantly influenceimplant resistance to rupture in the context ofstandard fatigue test conditions. Finite elementanalysis should be considered in the implant designprocess as a valuable tool for improving theimplant’s overall mechanical performance.

612 Volume 119 Issue 4

and also by certain physiological mechanisms such asosseointegration and bone resorption,6 certain pathologies,and patient habits.7,8 The average mastication duration isbetween 400 and 450 ms for closing the jaw and between450 and 500 ms for opening the jaw.9,10 Mastication timeduring eachmeal is estimated at between 8 and 16minuteson average, giving a result of 500 000 to 1 million cycles peryear.11Mastication forces changewith age, sex, and locationof the tooth.12 The forces applied on molars are between390 and 800 N, between 208 and 288 N for premolars andcanines andbetween 155 and200N for incisors.13-16 Lateralforces between 0 and 150 N have been reported.14,17,18

Osseointegration is an adaptation of the bone aroundthe dental implant, which was defined by Brånemarket al19 in 1952. This process, during which bone prop-erties change, is manifested in 2 stages: primaryosseointegration, which occurs in the short term after theimplant is placed, and secondary osseointegration, whichis characterized by bone growth around the implant.5,20

Factors influencing a dental implant’s osseointegrationinclude type of material,21 surface of the implant,22,23

surgical technique used for implantation,24,25 and bonequality and quantity.3,4 A bone resorption process occurs

1. Loading device2. Nominal bone level3. Connecting part4. Hemispherical loading member5. Dental implant body6. Implant holding element

z

xY

1

3

4

2

5

Figure 1. A, ISO 14801 loading system. B, Conditions for numerical simulatio

THE JOURNAL OF PROSTHETIC DENTISTRY


alongside osseointegration and is characterized by a lossof bone tissue resulting in the adaptation and evolutionof the cells involved in the osteoconduction, osteoin-duction, and immunogenetic properties of the patient.26

This process can be aggravated with age.27

All the aforementioned factors complicate the esti-mation of a reliable prognosis, although the success rateover the past 10 years has been estimated at more than94%.6 Indeed, the development of dental implant tech-nology has been accompanied by developments in theprocesses for evaluating devices which now make itpossible to guarantee a quality standard.11,28,29 Interna-tional Organization for Standardization (ISO) 14801addresses dental implants of the endosseous or trans-mucosal type30 and is used in 162 member countriesaround the world.30 This standard is used to assess theperformance of a dental implants, their related prostheticcomponents, and their fatigue strength under lessfavorable conditions than those found under clinicalconditions.6

The design and assessment phase for an implant iscostly and uncertain, hence the interest in usingnumerical modeling to improve the cost-to-performanceratio.31 The numerical approach by finite element anal-ysis (FEA) has been used for dental implant design, withstudies so far focusing on the analysis of stress distribu-tion in the implant and bone tissues in simulations withquasistatic loads.32-36 In 2 previous publications,33,37 theanalysis was rounded out with an assessment of fatigueperformances, using the approach after Wöhler, as dis-cussed by Timoshenko.38

The purpose of this study was to optimize the prob-able performances of a generic dental implant design in

Force

Load center

8 mm

3 mm

Screw

±0.5 mm

Embedding

6

30°

A Bn by finite element analysis.

de la Rosa Castolo et al


Figure 2. Dental implant and test bench components, meshingconditions.

Table 1.Meshing characteristic

System Component

Surface Meshing Volume Meshing

ElementNo.

MinimumSize (mm)

ElementNo.

MinimumSize (mm)

Dental implant 29 914 0.060 167 319 0.021

Connecting screw 3176 0.067 7250 0.037

Hemispherical member/connecting part

12 281 0.088 62139 0.052

Table 2.Mechanical properties of Ti grades 4 and 5 (Ti 6AL 4V ELI)

Material

YoungModulus(MPa)

PoissonRatio

syield

(MPa)

Max. ofDeformation

(%)smax

(MPa) Reference

Titanium grade 4 104 500 0.37 650 16 798 42

Titanium grade 5(Ti 6AL 4V ELI)

114 000 0.33 940 17 1054 43

Figure 3. A, Contact surface defined by small area. B, Congruencebetween surface of test part and dental implant increased.

April 2018 613

the context of ISO 14801 by investigating the possiblerole of 4 parameters on the implant’s performance, usingfinite element analysis.

MATERIAL AND METHODS

The present investigation was based on the identificationof parameters that may influence the performance ofendosseous dental implants under the conditions of ISO14801. Independently of the implant model, this studywas based on a single dental implant model that shouldbe considered generic.

For the study, a monobloc implant (IDP-IMP-M3.75-13; Saddle Implants) was chosen and placed under theload conditions of ISO 14801 (Fig. 1A). The implant andthe various components were connected on the testbench by a standard screw (IDP-VPMU; Saddle Im-plants) as illustrated in Figure 1B. The test bench com-ponents, called the hemispherical loading member andconnecting part, were produced specifically for the pre-sent investigation.



The first step of the study was to develop a finiteelement model of the whole system (implant and testbench components). In a second step, the model wasused in a sensitivity analysis of the implant’s perfor-mance, focusing on specific parameters related to thetest conditions: the contact surface area between theimplant and other components (a quality of interactioncriterion), the length of the fixation screw, the implantembedding depth, and the material used for implantstiffness.

Using FEA, the following measurements to evaluatethe influence of each input parameter for the dentalimplant’s performance were recorded: the location ofthe peak stress (considering the von Mises stress, as itis widely accepted as a yield criterion in materialsscience39); the applied force, Fy, needed to reach theimplant’s yield strength; the fracture force, Ffractk; andthe bending moment, M. Bending moments, M, weretaken as defined in equation 1 according to ISO1480130:

M=0:5×ð8 mm + resorptionboneÞ×F; (1)

where resorptionbone is 3 ±0.5 mm, and F is the forceapplied.

Computer-aided design (CAD) files containing thegeometry of the system elements were meshed usingsoftware (HypermeshV11; Altair) with triangularelement shapes for surface meshing and tetrahedralelements for volume meshing. Element sizes were set



Von Mises stress (MPa)7.980E+02

6.500E+02

5.688E+02

4.875E+02

4.063E+02

3.250E+02

2.438E+02

1.625E+02

8.125E+01

0.000E+00No result


6.500E+02

5.688E+02

4.875E+02

4.063E+02

3.250E+02

2.438E+02

1.625E+02

8.125E+01

0.000E+00No result

Z

Y X

Z

Y X

A

BFigure 4. Distribution of von Mises stress (MPa) on body of dentalimplant at force of 559 N for quality of congruence. A, Small area. B,Large area significantly increased.

Table 3. Forces for achieving yield strength and fracture strengthdepending on quality of part congruence and their bending moments

ParameterCongruence

Yield Strength Fracture Strength

Fy (N)Bending Moment

(Nmm) FfractkBending Moment

(Nmm)

Small area 103.0 566.5 559.0 N 3074.5

Large area 170.0 935.0 790.0 N 4325.0


at 60 mm and adjusted gradually in the contact areas(Fig. 2). The model of the hemispherical member andthe connecting part were fused and united in a singlebody. Details of volume and surface meshes are givenin Table 1.

The materials used were considered homogeneousand isotropic. The material defined for the dental implantwas titanium grade 4, the connecting screw was madeof Ti 6Al 4V ELI (titanium grade 5), and the hemi-spherical member and connecting part were made of40CrMnMoS8-6. The connecting screw, the hemispher-ical member, and the connecting part were set as rigid(kinematic condition). The behavior applied to theimplant materials was elastoplastic, after Johnson-Cook,



as discussed by Banerjee et al.40 Details of the mechan-ical properties are given in Table 2. The materials’ yieldstrength was defined by syield and the maximum stressby smax, the latter being used as the fracture point understress for the model elements.

According to ISO 14801 specifications,30 the part of thedental implant anchored in the bone must be attachedwith a stiff clamping device, with a yield strength greaterthan 3 GPa. For the simulation conditions, the dentalimplant’s anchoring was modeled by embedding nodes(kinematic condition) as illustrated in Figure 1B.

The load was applied at 30 degrees from the implant’smain axis as defined by ISO 14801.30 In the Radiosssoftware environment (Altair), a master node was used asthe loading node at the geometrical center of the hemi-spherical member, indicated as the load center inFigure 1B. The force increased linearly, reaching amaximum within 50 ms. In order to remain within qua-sistatic conditions and to limit dynamic effects, kineticenergy was relaxed during the simulation. The conditionsof contact between the parts were defined by an interfaceof the node-to-surface penalty-based contact type, with acoefficient of friction of 0.3 and a 0.005-mm gap.33 Thefinite element sensitivity analysis of the implant’s per-formance relative to test conditions concerned thefollowing input variables discussed below.

Contact surface areaSome dental implant systems include prefabricatedbeveled connecting parts, but in other designs, this part isproposed for the certification body because requirementsmust be met for the test configuration.30 In both situa-tions, a physical interface occurs between this part, thedental implant, and the clamping device. The contactsurface area was considered the quality of congruencebetween the various test components, where goodcongruence meant coinciding and perfectly fitting(Fig. 3). Two different configurations were tested todetermine the possible influence of this parameter: smallarea, where the physical contact extends over a relativelysmall surface area (Fig. 3A), and large area, wherecongruence between the test part and the dental implantwas significantly increased (Fig. 3B).

Fixation screw lengthDifferent clamping technologies have been used forattaching dental implants and prosthetic parts. In the



Von Mises stress (MPa)7.980E+026.500E+025.688E+024.875E+024.063E+023.250E+022.438E+021.625E+028.125E+010.000E+00No result

Z

Y X

A


Z

Y X

BFigure 5. Distribution of von Mises stress (MPa) at a force of 559 N forscrew length conditions. A, 3.3 mm standard size. B, 4.3 mm.

Table 4. Forces for achieving yield strength and fracture strengthdepending on size of connecting screws and their bending moments

Parameter ScrewSize (mm)



(Nmm) Ffractk (N)Bending Moment

(Nmm)

3.4 (standard size) 103.0 566.5 559.0 3074.5

4.4 117.4 645.7 581.3 3197.1

April 2018 615

present investigation, the clamping device was a screw(Fig. 1B). To determine the possible influence of thescrew’s length (and therefore the fixation method), 2configurations were tested: a standard-sized (3.4-mmlength) screw (Figure 2, note that this screw modeldoes not fill the entire space.) and with the size of thescrew increased to a length of 4.4 mm to fill the emptyspace.

Implant embedding depthIn the ISO standard, the implant embedding value of 3±0.5 mm was defined as the most representative case inview of expected bone loss. Three different configurationswere tested to determine any possible influence of thisparameter (Fig. 1B): embedding at 2.5 mm, embedding at3.0 mm, and embedding at 3.5 mm.



Type of materialsMaterials for dental implants were selected for theirmechanical properties, their chemical properties, andtheir biocompatibility.41 Two different configurationswere tested to determine the influence of the materialused for the implant: Implant using titanium grade 4materials42 and implant using titanium grade 5 materials(Ti 6AL 4V ELI).43 The properties of the tested materialsare reported in Table 2.

RESULTS

The dental implant’s force before rupture under the smallcontact surface area condition was 559 N (Fig. 4A); forthis level of loading, the implant under the large contactsurface area condition would not fracture (Fig. 4B). Forcesfor achieving yield strength, fracture strength, and theirbending moments under both of the contact conditionstested are given in Table 3.

With the standard size screw (length, 3.4 mm),implant rupture was observed at a loading force of 559N. Stress distribution on the implant is given inFigure 5A. For the longer screw (4.4 mm) and for thesame loading force of 559 N, stress distribution is givenin Figure 5B. Forces for achieving yield strength, fracturestrength, and their bending moments are detailed inTable 4.

Under the 3.5-mm embedding length condition, theimplant would fracture at a force of 555 N. At the sameiteration, stress distribution in the implants for the other2 embedding depths tested is given in Figure 6. Table 5gives details of the forces for achieving yield strength,fracture strength, and their bending moments.

The force before rupture of the implant made oftitanium grade 4 was 559 N (Fig. 7A); stress distributionon the dental implant in titanium grade 5 (Ti 6AL 4V ELI)at the same force is presented in Figure 7B. The forces forachieving yield strength and fracture of the dentalimplant, along with their bending moments, are given inTable 6.

DISCUSSION

Numerical simulation by FEA was used to evaluate theperformance of a generic dental implant under the con-ditions of ISO 14801. To various degrees, the 4 param-eters studied were found to affect the dental implant’s



Table 5. Forces for achieving yield strength and fracture strengthdepending on implant embedding depth and their bending moments

Parameter EmbeddingDepth (mm)


Fy (N)Bending

Moment (Nmm) Ffractk (N)Bending

Moment (Nmm)

2.5 103.0 540.7 563.0 2955.7

3.0 103.0 566.5 559.0 3074.5

3.5 103.0 592.5 555.0 3191.2


Z

Y X

A

2.5

mm


Z

Y X

B

3.0

mm


6.500E+02

5.688E+02

4.875E+02

4.063E+02

3.250E+02

2.438E+02

1.625E+02

8.125E+01

0.000E+00No result

Z

Y X

C

3.5

mm

Figure 6. Distribution of von Mises stress (MPa) at a force of 555 N. A,Embedding at 2.5 mm. B, Embedding at 3.0 mm. C, Embedding at 3.5 mm.




mechanical performance (fracture strength). The contactsurface area was found to play the most important role inthe dental implant’s mechanical strength.

The implant’s fracture strength was found to be inaccordance with previously reported values.37,44,45 Theimplant’s fracture strength was increased by 41% underconditions of increased contact surface area (Fig. 3B).This result was expected and can be explained by abetter load transmission and a wider stress distribution,with the effect of decreasing the maximum stress underthe same loading conditions. Likewise, the forcenecessary to reach the yield strength was increased bymore than 65%, which means that the implant’s safetyregion can be sharply increased by optimizing thecontact surface between the different test components.Concerning stress distribution, Figure 4B shows thatoptimizing the contact surface area provides relief to aregion that is naturally exposed to a high risk of fracture:the fixation screw and the threaded hole. Therefore, agood way of avoiding early fracture of the screw or theupper region of the implant (threaded hole) would be tooptimize the contact surface area of the test componentsto shift high-stress concentrations toward structurallystronger regions.

The fixation conditions themselves appear to haveless of an influence, because the force needed to reachfracture only increases by 4% under the longer screwconditions (Fig. 5) and 14% more to achieve yieldstrength. This increase in strength can be explained byoptimized load transmission. Furthermore, a shortscrew creates a particularly adverse lever arm on theupper portion of the implant (Fig. 5). This result poses aproblem. In reality, clinicians often choose to attach theimplant with a standard size screw so that, in the caseof screw fracture, the residual portion can beremoved.46 Therefore, a short screw provides a back-upsolution if it fractures but, at the same time, has moreof a chance of fracture. Further work could make itpossible to define the best compromise using numericalsimulation.

In order to validate the influence of the contact surfacearea that was recorded under the conditions of ISO14801,30 a physical test of 2 identical implants was carriedout but under different conditions of interaction (Fig. 3).These tests were carried out by an independent certifi-cation agency (Technical Centre for Mechanical Industry




6.500E+02

5.688E+02

4.875E+02

4.063E+02

3.250E+02

2.438E+02

1.625E+02

8.125E+01

0.000E+00No result

Z

Y X


9.400E+02

8.225E+02

7.050E+02

5.875E+02

4.700E+02

3.525E+02

2.350E+02

1.175E+02

0.000E+00No result

Z

Y X

A

BFigure 7. Distribution of von Mises stress (MPa) at a force of 559 N. A,Dental implant made of titanium grade 4. B, Dental implant made oftitanium grade 5.

Table 6. Forces for achieving yield strength and fracture strengthdepending on materials and their bending moments

Parameter Typeof Material



(Nmm) Ffractk (N)Bending Moment

(Nmm)

Titanium grade 4 103.0 566.5 559.0 3074.5

Titanium grade 5(Ti 6AL 4V ELI)

141.0 775.5 669.0 3679.5

April 2018 617

CETIM). The first implant was initially subjected tocyclical loading with an amplitude of 150 N for a total of 1million cycles. No fracture was observed. It was thensubjected to cyclical loading again with an increasedamplitude of 175 N under the same conditions. Theimplant fractured in the upper region of the fixation screwat cycle 758 075 (Fig. 8A). The second implant wasidentical to the previous one, but its contact surface areawith the loading tool was improved by a better definitionof tolerances and was then subjected to the same cyclicalloading procedure. This second implant, with improvedcontact conditions between components, held up underloads up to 1 million cycles at 150 N, 175 N, 200 N, and225 N. The implant finally fractured, always in the samefixation area on the screw, after 524 000 cycles with a load



of 250 N. These experimental results appear to confirmthe effect of the implant’s conditions of interaction withthe loading tool observed in the simulations (Fig. 8),encouraging a careful definition of this interaction in theearly stages of the implant design process.

As to the type of material, Table 2 shows that titaniumgrade 5 had greater resistance characteristics than tita-nium grade 4, and this was confirmed by the results ofthe simulations. The fracture force on the dental implantwas increased by nearly 20%, whereas the force forachieving yield strength increased by 37%. If the per-formances of titanium grade 5 is clearly better in terms ofmechanical strength, there will be a trend toward itsbeing progressively replaced in the production of dentalimplants by grade 4 titanium, which is less resistant butalso less toxic.32 Considerations for the materials used inthe present investigation are that, between titaniumgrades 4 and 5 (Ti 6AL 4V ELI), the mechanical charac-teristics will change if a surface treatment or materialtreatment is used.32,41

The implant embedding depth parameter was shownto have less influence on the dental implant’s strengththan any of the other parameters tested. The fractureforce on the dental implant remained practicallyunchanged (the increase was 1.4%), with a minimumembedding of 2.5 mm, whereas the force for achievingyield strength was the same for the 3 conditions tested.Although a margin of error of ±0.5 mm is tolerated withthe 3-mm level of implant embedding recommended inISO 14801, the effective final embedding value is not acritical parameter to be monitored during a certificationtest.

The present investigation made it possible to identifysignificant parameters for the mechanical strength of animplant design, in particular with regard to the resistanceto rupture. However, the observations must only beinterpreted qualitatively. Indeed, the maximum forcelevels before rupture indicated in this article depend onfactors linked to the modeling choices: the boundaryconditions, in particular the conditions of embedding ofthe base of the implant and the loading conditions (pointof application of effort, angle, amplitude); the choice ofthe implant itself, the structure of which is specific to thismodel and whose response may not be strictly identicalto any other implant model as discussed in previousstudies37,44,45; the geometry and mechanical properties,




Z

Y X

A

BFigure 8. A, Dental implant fracture in ISO 14801 fatigue test. B, Distri-bution of von Mises stress (MPa), numerical approach by finiteelement analysis.


which are fixed in the model and therefore do not takeinto account the variability that can be expected for anactual implant, both from a mechanical and a geometricalpoint of view (a sensitivity analysis of an arbitrary vari-ation of properties and the main geometric characteristicscould help to quantify this question.); and the behaviorlaw chosen for the implant, which does not take intoaccount the progressive degradation of the mechanicalproperties related to fatigue. Implementation of abehavior law that takes into account fatigue, as proposedelsewhere,33,37,38 could be the subject of further study.



CONCLUSIONS

Within the limitations of this in vitro study, the followingconclusions were drawn:

1. To various degrees, the 4 parameters under the testconditions (contact surface area, implant embeddingdepth, screw length, and material stiffness) affectedthe dental implant’s resistance to rupture.

2. The conditions of interaction between the test com-ponents, characterized here by the contact surfacearea between the implant and the surrounding testcomponents, played the most important role.

3. To validate the performance improvements of adental implant, the quality of the contact surfacearea parameter was experimentally tested under theconditions of ISO 14801. Results showed that, byimproving the conditions of this parameter, theforce at rupture increases significantly.

4. Numerical methods should be considered in theprocess of implants design, as they can improve theperformance of dental implants and their prostheticparts under the conditions of ISO 14801.

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Corresponding author:Dr Guillermo de la Rosa CastoloAix-Marseille University, IFSTTAR, LBA UMR T24, F-13016, MarseilleBd. P. Dramard, Faculty of Medicine, Sector North13916 Marseille cedex 20FRANCEEmail: [email protected]

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Mechanical strength and fracture point of a dental implant