high torque

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Braz Dent J 21(6) 2010

. S. Sotto-Maior et al.

INTRODUCTION

Over the last 30 years, clinical studies withosseointegrated implants have shown excellent long-term results, with over 90% success rate (1,2). However,early failures may occur during the healing processaffecting osseointegration (3). These failures mayhave biological causes, such as periimplantitis andsystemic diseases. In addition, biochemical factors cannegatively influence implant success; for instance, boneover heating during the surgical procedure, occlusaloverload, besides the effects of tensile strength, shear andcompressive stresses in the peri-implant bone tissue (4).

The osseointegration process requires ideal stress

Influence of High Insertion Torque on ImplantPlacement - An Anisotropic Bone Stress Analysis

Bruno Salles SOTTO-MAIOREduardo Passos ROCHA2

r a O ve ra e ALMEIDAAmilcar Chagas FREITAS-JNIOR

Rodolfo Bruniera ANCHIETA2

A ta r Anton n a DEL BEL CURY

Department of Prosthodontics and Periodontology, Piracicaba Dental School,

niversity o ampinas, iracica a, , razi

epartment o enta ateria s an rost o ontics, raatu a enta c oo ,

So Paulo State University, Araatuba, SP, Brazil

e a m o t s stu y was to eva uate t e n uence o t e g va ues o nsert on torques on t e stress an stra n str ut on n cort caan cance ous ones. ase on tomograp y mag ng, a representat ve mat emat ca mo e o a part a max a was u t us ng m cs

. an o or s so twares. x mo e s were u t an e ac o t em rece ve a n mp ant w t o ne o t e o ow ng nsert ontorques: , , , , or cmon t e externa exagon. e cort ca an cance ous ones were cons ere an sotrop c. eone mp ant nter ace was cons ere per ect y on e . e numer ca ana ys s was carr e out us ng nsys or enc . . e

convergence of analysis (6%) drove the mesh refinement. Maximum principal stress ( and maximum principal strain ( ) wereobtained for cortical and cancellous bones around to implant. Pearsons correlation test was used to determine the correlation betweeninsertion torque and stress concentration in the periimplant bone tissue, considering the significance level at 5%. The increase inthe insertion torque generated an increase in the nd values for cortical and cancellous bone. The was smaller for thecancellous bone, with greater stress variation among the insertion torques. The was higher in the cancellous bone in comparisonto t e cort ca one. ccor ng to t e met o o ogy use an t e m ts o t s stu y, t can e conc u e t at g er nsert on torquesncrease tens e an compress ve stress concentrat ons n t e per mp ant one t ssue.

ey or s: enta mp ants, nsert on torque, n te e ement met o .

levels to maintain normal bone repair (5). Excessivetension may cause irreversible damage to the periimplantbone tissue (6). Conversely, too low tension mayunsatisfactorily stimulate the bone repair process (4).Recent computerized simulations have suggested thatcompressive stresses and hydrostatic tensions of theinterstitial liquid may modulate tissue differentiationand bone remodeling (4,7,8). Byrne et al. (7), usingmathematical models about cellular differentiation inbone repair, verified that the stress increase changesthe bone repair process, reducing the amount of newlyformed bone tissue by 23% and increasing the amount ofcartilage by 21%. Similarly, Checa and Prendergast (8)verified less newly formed bone and greater connective

orrespon ence: ro a. ra. ta r nton n a e e ury, epartamento e r tese e er o ont a, acu a e e onto og a e rac ca a, n vers a ee amp nas, , - rac ca a, , ras . e : + - - - . ax: + - - - . e-ma : a tcury op.un camp. r

-ent : -raz


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raz Dent J 21(6) 2010

n uence o ig insertion torque

tissue formation under elevated compressive stresses.During placement of osseointegrated implants,

the insertion torque may result in varied levels of

compressive stresses transmitted to the adjacent bone,

given that the implant bed is slightly narrower than

the diameter of the implant to be placed in order tooptimize primary stabilization (9,10). Clinical studies

have demonstrated a close relationship between initial

stabilization and the success of an osseointegrated

implant (11-13), which can be measured by the

insertion torque during implant placement (12). The

insertion torque must exceed 30 Ncm to obtain

predictable success rates (12,14), aiming at avoiding

implant micromovement and consequent connective

tissue formation (6,13). However, an excessively high

insertion torque, above 50 Ncm (15), can occur during

dense bone implant placement (12,16,17), resultingin the transmission of high compressive stresses

to the adjacent bone, in addition to compromising

osseointegration success (3).

This way, the understanding of the high valuesof insertion torque that can be used during implantplacement, without causing excessive stress in thebone tissue, would be helpful for the success of implantosseointegration.

Considering the lack of studies associatingcompressive stress and strain with the high values ofthe insertion torques during implant placement, the

aim of the present study was to evaluate the influenceof different insertion torques on stress and straindistribution in cortical and cancellous bones, using thethree-dimensional finite element method.

MATERIAL AND METHODS

fter approval by the local Research EthicsCommittee and signature of an informed consent form,a computed tomography (CT) scan was taken from apatient to obtain dicomformat images. A representativemathematical model of the anterior segment of themaxilla was built using Mimics 11.11 software(Interactive Medical Image Control System, MaterialiseInc., Leuven, Belgium) and Solid Works 2010 software(Dassault Systmes SolidWorks Corporation, Concord,

MA, USA).he geometry of an external hexagonal implant of4.5 x 11.5 mm (Strong SW model; Sistema de Implantes-SIN, So Paulo, SP, Brazil) was used to build the implantdesign (Fig. 1A) with the aided of the Solid Works 2010software. This implant was adapted to the bone segmentcorresponding to the region of the maxillary right centralincisor (Fig. 1B).

fter building, the initial model was importedto the finite element program Ansys Workbench 10.0(Swanson Analysis Systems Inc., Houston, PA, USA)to determine the regions and generate the finite element

gure . mp ant mens ons ) an mo e mens ons w t cort ca an cance ous ones ) expresse n m meters.


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presented in Figure 4. The insertion torques showedsignificant correlation with and (r=1.0,p=0.001). The models with insertion torques of 30 Ncmand 80 Ncm showed the lowest ax(4.15 MPa) and ax(3.64 x 10 4), and the highest ax (11.1 MPa) and ax

(9.72 x 10-4 values, respectively.

DISCUSSION

An adequate stability of the dental implant in

the surrounding bone plays an important role in thebone healing processes, avoiding micromovement anddamage to the bone healing process (17,19). Clinicalassessment of primary stability can be performed by theimplant insertion torque at the moment of placement (19).

Considering the importance of the insertion torque forthe osseointegration process, this study was performedby mimicking a dental implant placement using theanisotropic finite element technique. It was hypothesizedthat high values of the insertion torque can generate

Figure 3. Stress profile within cancellous bone at each torque insertion. A= 30 Ncm; B= 40 Ncm; C= 50 Ncm; D= 60 Ncm; E= 70cm; = cm.


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overcompressive stress to the peri-implant tissues andcompromising osseointegration process.

According to Ottoni et al. (14) and Irinakis andWiebe (12), the insertion torque must exceed 30 Ncm,especially for immediate load implants. However,

insertion torques above 50 Ncm are considered high(15) and generally induce excessive compressivestresses to the bone periimplant. Some studies havereported that the increase of compressive stress in thebone tissue may lead to failure in the bone healing (5)

and osseointegration process (3).he present study showed similar results to those

of a previous study (4), in which the increase of theinsertion torque generates higher compressive stressconcentrations to the peri-implant bone tissue. In the

present study, for the cancellous bone tissue, the increaseof insertion torque (from 30 to 80 Ncm) showed anincrease of 1175% for the maximum principal stress and266% for the maximum principal strain. In the corticalbone tissue both stresses were higher (267%).

Figure 4. Stress profile within cortical bone at each torque insertion. A= 30 Ncm; B= 40 Ncm; C= 50 Ncm; D= 60 Ncm; E= 70 Ncm;= cm.


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Studies based on mathematical analyses of thebone healing process presented a correlation betweenthe compressive stress and the type of tissue formedduring bone remodeling n vivo (8). It is importantto draw attention to the fact that under high stresses,

significant alterations occurred in the angiogenesisdynamics impairing the formation of new blood vessels,causing hypoxia in the periimplant tissues, thus inhibitingbone formation and favoring the formation of cartilageand connective tissue (8). It has also been emphasizedthe model of bone tissue formed by a complex three-dimensional tubule network filled with an interstitialfluid that supplies bone cells (20). This fluid would beable to transmit external stresses to bone cells through amechanism known as mechanotransduction, which refersto the conversion of mechanical energy from external

stresses into bioelectrical and biochemical signals thatmodulate the bone cell metabolism (20). Therefore,when this mechanical energy is too high, osteocytes areinduced to death, followed by recruitment of osteoclastsand bone destruction (8,20).

The results of the present study demonstrated that

the maximum principal stress increased by 648%, from

0.168 to 1.09 MPa, between torques of 50 and 60 Ncm

in the cancellous bone. High insertion torques above 50

Ncm (15) can generate high compressive stresses to the

peri-implant tissues causing blood supply deficiency and

bone necrosis during the osseointegration phase and early

implant failure (3) usually within the first month after

placement (3). A high insertion torque may occur during

implant placement in high density bone tissue (12,16,17).

This observation has been demonstrated in a study that

evaluated the relationship between bone density and the

maximum insertion torque supported by the bone tissue,

using computer tomography images and Hounsfield

scale, and found a significant correlation between bone

density, insertion torque and primary stability (17).

The cortical bone tissue had lower capacity todissipate stress as well as a more uniform increase of

the insertion torque, showing higher principal maximumstress in comparison with the cancellous tissue. Theseresults are similar to those of previous studies (9,10),which assessed the influence of the osteotomy diameterfor implant placement and the stress concentration onimplant threads. These results are explained by thedifferent mechanical properties between cancellous andcortical bones.

The computational analysis by finite elements

shows great versatility in the analysis of complex

models. This analytical method allowed identifying

the homogeneity between different models with varied

insertion torques that are difficult to obtain in an

experimental study with physical models, as well as

the same mechanical properties and dimensions for

cancellous and cortical bones. The anisotropy found inthe bone tissue was reproduced in the present study for

the cortical and cancellous bones, being characterized by

different stress responses under forces applied in varied

directions (9). Although the results of the present study

can add data to the implant/bone behavior influenced by

the high values of the insertion torque, further animal

and clinical investigation studies are needed to confirm

these findings.

ccording to the methodology used and withinthe limitations of the present study, it may be concluded

that high insertion torques can generate higher tensileand compressive stresses to the periimplant bone tissue.

RESUMO

O objetivo deste estudo foi avaliar a influncia dos altos valoresde torque de insero na distribuio de tenses e deformaesno osso cort ca e me u ar. om ase em magens e tomogra acomputa or za a, um mo e o matem t co representat vo eum segmento a max a o constru o ut zan o os programas

m cs . e o or s . e s mo e os oram constru ose ca a um rece eu um mp ante com os segu ntes torques enser o no ex gono externo: , , , , ou cm. osso

cortical e medular foi considerado anisotrpico. A interface osso/implante foi considerada perfeitamente unida. A anlise numricafoi realizada atravs do Ansys Workbench 10.0. A convergnciade anlise (6%) determinou o refinamento da malha. A tensomxima principal ( ) e a deformao mxima principal ( )oram o t os para o osso cort ca e me u ar ao re or o mp ante.

teste e corre a o e earson o ut za o para eterm nar acorre a o entre torque e nser o e e concentra o e tens ese e orma es no tec o sseo per - mp antar, cons eran o on ve e s gn c nc a e . aumento no torque e nser ogerou um aumento nos va ores para o osso cort cae medular. O foi menor para o osso medular, com maiorvariao de tenso entre os torques de insero. O foi maiorno osso medular em relao ao osso cortical. De acordo com ametodologia utilizada e com os limites do estudo, pode-se concluir

que torques alto de insero aumentou as concentraes de tensese tra o e compress o no tec o sseo per - mp antar.

ACKNOWLEDGEMENTS

s researc was supporte y t e o au o tate esearcoun at on - rant - ).

REFERENCES

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Accepted November 4, 2010

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