Effe

abu

in s

rest

fini

Marco Aurélio C

aDoctoral student, Department of PbProfessor, Department of RestorativcProsthodontics Professor, DepartmedProsthodontics Professor, Departm

Carvalho et al

ct of platform connection and

tment material on stress distribution

ingle anterior implant-supported

orations: A nonlinear 3-dimensional

te element analysis

arvalho, DDS, MSc,a

Bruno Salles Sotto-Maior, DDS, MSc, PhD,b

Altair Antoninha Del Bel Cury, DDS, MSc, PhD,c andGuilherme Elias Pessanha Henriques, DDS, MSc, PhDd

Piracicaba Dental School, University of Campinas, Piracicaba,São Paulo, Brazil; Federal University of Juiz de Fora, Juiz de Fora,Minas Gerais, Brazil

Statement of problem. Although various abutment connections and materials have recently been introduced, insufficientdata exist regarding the effect of stress distribution on their mechanical performance.

Purpose. The purpose of this study was to investigate the effect of different abutment materials and platform connections onstress distribution in single anterior implant-supported restorations with the finite element method.

Material and methods. Nine experimental groups were modeled from the combination of 3 platform connections (externalhexagon, internal hexagon, and Morse tapered) and 3 abutment materials (titanium, zirconia, and hybrid) as follows: externalhexagon-titanium, external hexagon-zirconia, external hexagon-hybrid, internal hexagon-titanium, internal hexagon-zirconia,internal hexagon-hybrid, Morse tapered-titanium, Morse tapered-zirconia, and Morse tapered-hybrid. Finite element modelsconsisted of a 4 � 13-mm implant, anatomic abutment, and lithium disilicate central incisor crown cemented over theabutment. The 49 N occlusal loading was applied in 6 steps to simulate the incisal guidance. Equivalent von Mises stress(svM) was used for both the qualitative and quantitative evaluation of the implant and abutment in all the groups and themaximum (smax) and minimum (smin) principal stresses for the numerical comparison of the zirconia parts.

Results. The highest abutment svM occurred in the Morse-tapered groups and the lowest in the external hexagonehybrid,internal hexagonetitanium, and internal hexagonehybrid groups. The smax and smin values were lower in the hybrid groupsthan in the zirconia groups. The stress distribution concentrated in the abutment-implant interface in all the groups,regardless of the platform connection or abutment material.

Conclusions. The platform connection influenced the stress on abutments more than the abutment material. The stress valuesfor implants were similar among different platform connections, but greater stress concentrations were observed in internalconnections. (J Prosthet Dent 2014;-:---)

Clinical Implications

For anterior implant-supported restorations, regardless of the platformconnection, a zirconia abutment attached to a titanium base or atitanium abutment provides better mechanical behavior than purezirconia abutments.

Because of its esthetic and func-tional requirements, the maxillary an-terior single crown is still one of the

rosthodone Dentistrnt of Proent of Pro

most challenging restorations. The re-placement of missing teeth with dentalimplants is well documented as a

tics and Periodontology, Piracicaba Dental Schy, Federal University of Juiz de Fora.sthodontics and Periodontology, Piracicaba Desthodontics and Periodontology, Piracicaba De

feasible treatment with high successrates.1,2 Nevertheless, esthetic enhance-ments are still needed, mainly in

ool, University of Campinas.

ntal School, University of Campinas.ntal School, University of Campinas.

Table I. Experimental design groups

Group/ AbutmentConnection

AbutmentMaterial

External hexagon

EHTi Titanium

EHZr Zirconia

EHH Hybrid

Internal hexagon

IHTi Titanium

IHZr Zirconia

IHH Hybrid

Morse tapered

MTTi Titanium

MTZr Zirconia

MTH Hybrid

EH, external hexagon; Ti, titanium; Zr, zirconia;H, hybrid; IH, internal hexagon; MT, Morsetapered.

2 Volume - Issue -

patients with a high smile line.3,4 Theoutcome of an esthetic implant can beimproved by overcoming the opticalproblem of the gray discoloration of themarginal periimplant mucosa causedby titanium (Ti) abutments.5 Therefore,a ceramic abutment (CerAdapt; NobelBiocare) was introduced.6-8 The es-thetic benefits of ceramic abutmentsover metal abutments have been welldocumented in clinical and in vitrostudies, although, their mechanicalperformance is still of concern.3,9-14

Recent analysis of the mechanical per-formance of zirconia (Zr) abutmentswith external or internal connectionshas revealed lower fracture resistancethan Ti abutments, especially with in-ternal connections.15-21

Internal connection provides jointstability and better resistance againstrotational and lateral movementsthan external connections.22-25 How-ever, the abutment’s projection into theimplant may lead to greater stressconcentration due to the thinner abut-ment and implant walls at the inter-face.26 Moreover, fractures of ceramicinternal connection abutments havebeen reported both clinically andin vitro.12,17,18,20,26 To overcome thefragile properties of Zr at the implant-abutment interface, the 2-piece hybrid(H) Ti-Zr abutment was developed witha Ti base attached to a Zr abutmentbody.17,27 Zr is milled onto the Ti basethat is screwed onto the implant.28

Joint stability is improved by the in-ternal design made of Ti alloy, andthe ceramic body above the implantplatform enhances the esthetics. Incontrast to ceramic abutments, the Habutment presented better fatiguestrength, which suggests improved me-chanical performance.15,17,18,27

Despite recent in vitro results withH Ti-Zr abutments, the biomechanicalbehavior of these components and thedifferent platform connection interfacesis still not well explored. In addition, theuse of virtual biomechanical analysessuch as the finite element analysis(FEA) can be applied to understandthe behavior of structure stress, whichcannot be obtained from mechanical

The Journal of Prosthetic Dentis

testing.29,30 The aim of this study,therefore, was to evaluate the effectof different combinations of platformconnections and abutment materialson the distribution of stresses in theabutment and implants of singleimplant-supported anterior restora-tions by using 3-dimensional (3D) FEA.

MATERIAL AND METHODS

Experimental design

Nine 3D anterior maxilla segmentswere created based on a cone-beamcomputed tomographic image (i-CATCone Beam 3D Dental Imaging System;Imaging Sciences International). Eachmodel consisted of a dental implantdeveloped from the geometry of a 4 �13-mm implant (Titamax Ex; Neodent)with different platform connections:external hexagon (EH) or internal hexa-gon (IH), and Morse tapered (MT) withthe same macrogeometry (EH, IH, andMT) and a central incisor lithium dis-ilicate crown cemented over Ti, Zr, or Habutments (Table I). With the aid of 3Dcomputer-aided design software (Solid-Works 2013; Dassault Systèmes Solid-Works Corp), the 9 experimental models(Fig. 1) were obtained by combiningplatform connections and abutmentmaterials (EHTi, EHZr, EHH, IHTi, IHZr,IHH, MTTi, MTZr, MTH). FEA was usedto determine the maximum (tensile) andminimum (compressive) principal stressvalues for Zr and H Ti-Zr abutments andthe von Mises stress for all abutmentsand implants.

Numerical analysis

All the models were then exportedto FEA software (ANSYS Workbench 14FEA; Swanson Analysis Inc) for biome-chanical analysis. The crowns, abut-ments, screws, implants, and compactand cancellous bone were considered tobe isotropic, homogenous, and linearlyelastic (Table II). Discrete finite elementmeshes were generated by using 10-node quadratic tetrahedral elementswith 3 degrees of freedom per node.After convergence analysis31 (6%), the

try

value of the mesh size was 0.7 mm. Themodels presented a number of elementsthat ranged from 91085 to 93819 anda number of nodes that ranged from159 965 to 164975. The boundaryconditions were defined by fixing themesial and distal exterior surfaces of thebony segment in all directions. Imme-diate loading was simulated by usingnonlinear frictional contact elementswith a friction coefficient (m) of 0.3between the bone and the implant.32

Occlusal loading was applied to thepalatal surface of the lithium disilicatecrown at 6 different aligned contactareas obtained for the purpose ofsimulating the excursive movement ofthe incisal guide. The 49 N loading wasapplied at 45 degrees relative to theimplant long axis.33 The maximum(smax) and minimum (smin) principalstresses values were obtained for abut-ment comparison among Zr and H Ti-ZrH groups. Equivalent von Mises (svM)stress criteria for abutment and implantnumerical and color-coded comparisonwere obtained for all models.

RESULTS

The highest svM stress values forabutments and implants in all groups

Carvalho et al

1 Three-dimensional modeled geometries: A, Assembly parts of all groups.B, Example of internal hexagonehybrid restoration slice visualization. C and D,Complete assembly example of external hexagonetitanium with dimensions ofmaxilla segment (20 mm), cortical bone (1.5 mm), and crown (9 � 11 mm)used for all groups.

- 2014 3

are presented in Figure 2. To qualita-tively compare all the groups, the svM

stress distribution is displayed forabutments in Figure 3 and for implantin Figure 4. The highest smax and smin

for Zr and H abutments are presentedin Figure 5.

Carvalho et al

Abutments

When considering the equivalentvon Mises stress criterion and connec-tion type, the highest values were foundin the MT groups (315.61 MPa for theMTZr, followed by a decrease of 7% for

MTH and 8% for MTTi). The stresseswere concentrated on both buccal andlingual implanteabutment interfaceareas at the platform level (Fig. 3). Thelower value (91.7 MPa) was found inthe EHH group, followed by an increaseof 6.4% for the IHTi group and 9.7% forthe IHH group. The stress distributionfor the EH groups concentrated on thecervical area of the abutment, whereas,for the IH groups, it was concentratedin the IH projection and abutmentplatform rest area (Fig. 3).

With regard to the abutment mate-rial, for Ti groups, the highest svM

occurred in the MTTi group, followedby the EHTi and IHTi groups. For the Zrgroups, the highest smax and smin

values in MPa were found in the MTZrgroup (332.3 MPa, e380.37 MPa),followed by the EHZr (107.7 MPa,e250.7 MPa) and IHZr (178.32 MPa,e162.27 MPa) groups. Among the Ti-ZrH groups, when considering only the Zrabutment body, the order was similar:MTH (194.61 MPa, e235.84 MPa),followed by EHH (85.23 MPa, e87.63MPa) and IHH (67.31 MPa, e69.73MPa). When considering the Ti base,the MTH group displayed the highestsvM values (293.91 MPa), followed bya reduction of 13.9% in the EHH groupand 65.7% in the IHH group.

Implant

When considering the connectiontype, the stress patterns were almost thesame and also uniformly distributed inthe implants. They were concentratedin the palatal and buccal region of theimplant neck and decreased closer tothe implant apex, regardless the abut-ment material (Fig. 4). The mean(standard deviation) svM values rangedfrom 204.64 �12.13 MPa in the MTgroups to 220.83 �3.48 MPa in the EHgroup and 261.2 �15.23 MPa in the IHgroup. For the EH groups, the greatersvM stress concentration was found inthe external face of the first thread ofthe implant near the compact andcancellous bone interface and reached237.98 MPa in the EHH group, with a

Table II. Materials properties adopted in the study

Material Young Modulus (GPa) Poisson Ratio

Resin cement38 18.3 0.30

Cortical bone39 13.6 0.26

Cancellous bone39 1.36 0.31

Titanium39 110 0.25

Zirconia40 205 0.22

Lithium disilicate41 96 0.23

330

300

270

240

210

180

150

120

90EHTi EHZr EHH IHTi IHZr IHH MTTi MTZr MTH

Max

imum

vo

n M

ises

Stre

ss (

MP

a)

Abutment Implant

2 Comparison of maximum equivalent von Mises stresses (MPa) forabutments and implants for all groups.

3 Distribution of equivalent von Mises stresses (MPa) in abutmentsfor all groups.

4 Volume - Issue -

reduction of 11% for the EHTi and alsothe EHZr groups. For the IH groups, themaximum svM stress was concentrated

The Journal of Prosthetic Dentis

on the thinnest wall area of the implantcaused by the abutment IH projection,and reached 266.11 MPa in IHTi,

try

followed by a reduction of less than3% for both IHZr and IHH. In the MTgroups, the maximum svM stressoccurred on the internal upperabutment-implant interface at the bonelevel, which reached 226.08 MPa in theMTH group, followed by a reduction of15% in the MTZr group and also in theMTTi group. Differences in the stressdistribution in the implants weregreater regarding the connection typethan the abutment material (Fig. 4).

DISCUSSION

In addition to esthetic factors,the mechanical performance of ce-ramic abutments is a concern.2,22 Inthe present study, the combinationof 3 different abutment materialsand 3 implant platform connectionswithin implant-supported anterior sin-gle crowns was evaluated with FEA.Although mechanical tests have beenused to analyze the fracture resistance ofdifferent abutment materials and con-nections on implant single crown re-constructions, they may not helpunderstand the behavior of structurestress.29,30

In this study, the equivalent vonMises criterion was chosen for its abilityto summarize the maximum deforma-tion energy of a given body, evenamong different material properties ofsuch rigid structures as Ti and Zrabutments. The stress concentration forsvM occurs in compressive areas, onbuccal regions, on tensile areas, and onpalatal regions. The stress plots werecolor coded according to a single stresslevel scale for all implants and anotherfor all abutments, which provided astandard comparison among thegroups. The maximum and minimumprincipal stress values were used in thisstudy to compare the Zr and H groupsamong themselves because maximumand minimum principal stresses arebetter criteria for brittle materials suchas ceramics.

The platform connection influencedthe stress values and distribution moreamong abutments than among theimplants. Thinner abutment walls are

Carvalho et al

4 Distribution of equivalent von Mises stresses (MPa) in implants forall groups.

EHZr EHH IHZr IHH MTZr MTH

Co

mp

ress

ive

and

Ten

sile

Str

esse

s(M

Pa)

330290250210170130

905010

–30–70

–110–150–190–230–270–310–350–390

Abutment Zr (Tensile)Abutment Zr (Compressive)

Abutment Zr body (Tensile)Abutment Zr body (Compressive)

5 Comparison of maximum (tensile) and minimum (compressive) principalstresses among zirconia and hybrid groups.

- 2014 5

present in the MT connection designdue to the horizontal mismatch in theplatform switching condition. Com-bined with the thinner design, the brit-tleness of the ceramic material providedthe highest smax, smin, and svM valuesin the MTZr group. The IH connectionprovided the highest svM values in

Carvalho et al

implants. The implant walls of the IHgroup were thinner than those ofthe MT group, which may explain theresults for svM in implants. Whenconsidering all the groups, the loweststress values in abutments were foundin the EHH group, followed by the IHTiand IHH groups. The EH associated

with the cementation line between theTi base and Zr abutment body in the Hgroup culminated in the lowest valuesfor the EHH when compared with allthe groups. Among the Ti groups, thethicker Ti abutment wall in the IHTigroup provided the lowest values.

Although the abutment svM value forthe MTH group was similar to that ofthe MTZr group, the high svM was dueto the Ti base (293.61 MPa) and not tothe Zr abutment body (194.61 MPa),which was even lower than that of theMTTi group (286.36 MPa). The pres-ence of a Ti base in the H group signif-icantly decreased the smax and smin

stresses on the Zr abutment body incomparison with the Zr group. This factcan be attributed to the lower elasticmodulus and better elastic deformationof Ti in comparison with Zr. No FEAstudy compares the H abutment values,although, the results obtained in thepresent study corroborate those of me-chanical tests, which suggests betteroutcomes in the H groups.15,17,18

With regard to the evaluation of asingle material abutment, previousstudies reported the better mechanicalperformance of Ti over Zr abutmentsfor external and internal connec-tions.19,34 In contrast, Firidino�gluel al20 reported similar results for bothmaterials. Their findings were obtainedby comparing the results of mechanicaltests and FEA among internal connec-tion Zr and Ti abutment groups. How-ever, these results may not becompared with the present studybecause their FEA values were obtainedonly for the restoration coping and notthe abutment.

The present study is the first to useFEA to compare internal and externalconnectioon designs associated toTi, Zr, and H abutment materials.Recently, Çaglar et al35 evaluated thestress distribution patterns of IHconnection ceramic abutments and Tiabutments. The FEA results showed nodifference among them. This might bedue to the occlusal single-point loadingused. The 6-step occlusal loading usedin the present study generates morebending moments than a single-point

6 Volume - Issue -

loading. The excursive loading from thecingulum to incisal edge leads to amore-challenging scenario of abutmentresistance, which may have increasedthe discrepancy between the Ti and Zrgroups.

The findings of the present studyshould be carefully considered becauseof the interface conditions. To simulatethe immediate implant placement andprosthetic loading in the anterior zone,the “contact” type, a nonlinear fric-tional contact element, was used with a0.3 coefficient between bone and theimplant.32 The “bonded” type was usedon the interfaces of the crown, abut-ment, screw, and implant, which wereassumed to be perfectly bondedtogether. This condition interferes withthe behavior of stress dissipationthrough the structures because it gen-erates tensile stress in the palatalinterface area of the abutment andimplant once they are bonded together.Future studies with nonlinear contact atthe Ti-Zr interface should be conductedto complement the present results.

The internal distribution of stressesacquired with FEA provides importantdata that, gathered with numericalvalues of maximum stresses (smax,smin, svM) and fracture resistancevalues of mechanical tests may lead torestoration design improvements.However, the number of virtual, me-chanical, and clinical studies ofimplant-supported ceramic restorationsis still limited.4,7,12,17,27,35-37 Therefore,further clinical, virtual, and mechanicaltrials with multiple combinations ofabutment connections and materialsare required for better abutment selec-tion guidelines.

CONCLUSIONS

Within the limitations of this FEA, thefollowing conclusions can be drawn:

1. The platform connection influ-enced the stress values and distributionon abutments more than the abutmentmaterial.

2. The stress values for implantswere similar among different platform

The Journal of Prosthetic Dentis

connections, but greater stress concen-trations were observed in internalconnections.

3. TheH abutments presented similarmechanical behavior to Ti abutmentsand better mechanical behavior thanZr abutments.

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Corresponding author:Dr Marco Aurélio CarvalhoDepartment of Prosthodontics andPeriodontologyPiracicaba Dental SchoolUniversity of CampinasAvenida Limeira, 901, 13414-903Piracicaba, São PauloBRAZILE-mail: m110058@dac.unicamp.br

Copyright ª 2014 by the Editorial Council forThe Journal of Prosthetic Dentistry.