Fatigue Resistance of ITI Implant Abutment Connectorsaa Comparison of the Standard Cone With a Novel Internally Keyed Design

8/12/2019 Fatigue Resistance of ITI Implant Abutment Connectorsaa Comparison of the Standard Cone With a Novel Internally Keyed Design

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Jean PerriardW. Anselm WiskottAissa MellalSusanne S. Scherrer

John BotsisUrs C. Belser

Authors affiliations:

Jean Perriard, H. W. Anselm Wiskott,

Susanne S. Scherrer, Urs C. Belser, Division ofFixed Prosthodontics, School of DentalMedicine, University of Geneva, SwitzerlandAissa Mellal, John Botsis,Laboratory for AppliedMechanics and Reliability Analysis, Swiss FederalInstitute of Technology, Lausanne, Switzerland

Correspondence to:

Urs C. BelserDepartment of ProsthodonticsUniversity of GenevaSchool of Dental Medicine19, rue Barthelemy-Menn1205 GenevaSwitzerlandTel:41 22382 91 28e-mail: Urs. Belser/medecine.unige.ch

Date:

Accepted 12 November 2001

To cite this article:

Perriard J, Wiskott WA, Mellal A, Scherrer SS, BotsisJ, Belser UC. Fatigue resistance of ITI implant-abutment connectorsa comparison of the standardcone with a novel internally keyed designClin. Oral Impl. Res, 13,2002; 542549

Copyright C Blackwell Munksgaard 2002

ISSN 0905-7161

542

Fatigue resistance of ITI implant-abutment connectorsa comparisonof the standard cone with a novelinternally keyed design

Key words:dental implants; finite element analysis; mechanical stress

Abstract:The Straumann Company has recently supplemented its standard morse-taper

configuration with an octagonal internal key. During the restorative phase of implant

treatment, this additional feature was designed to ensure positional duplicability between

the laboratory and the clinical environments. It was, however, unclear whether this keying

mechanism would decrease the mechanical strength of the connection between the implant

and the abutment. This applies to keyed male and female parts but also to combinations

of the new and the standard designs. Specially constructed specimens analogs representing

all three combinations were fitted with a T-shaped bar, preangled to 15and subjected to

vertical force applications provided by a servohydrolic fatigue tester. The loading frequency

was 2 Hz and the maximum cycle number was 106. The data were evaluated using the

staircase technique. The specimens were also modeled and analyzed numerically using finite

element procedures. The samples failure locations were recorded and the displacement vs.

cycle number plots were patterned in four groups. The fatigue tests and staircase analysis

showed no difference in mechanical resistance between the standard and the internally keyed

connectors. The finite element models revealed a stress concentration located at the apical

edges of the octagonal connector. However, it appeared that this phenomenon was based on

computational rather than mechanical grounds. The locations of the failure sites were

distributed randomly across the structures, thereby indicating the absence of a locus of minor

resistance. The patterns of the displacement vs. cycle number could not be attributed to

specific combinations between the standard and the internally keyed designs. It was

concluded that both connectors are equal in their mechanical resistance to bending and

torquing forces.

Prosthetic components are subjected to a

complex pattern of horizontal and vertical

force combinations (Graf & Geering 1977).

Yet all force components do not have the

same impact with respect to material re-

sistance and incidence of failure. Force vec-

tors that are directed along the main axis

of the implant are compressive in nature

and remain well below the materials re-

sistance in compression (Glantz et al.

1993). By contrast, the bucco-lingual force

components will result in bending of the

material and it is the tension and shear

stresses thus developed that may cause

failure of the structure. Further, in contrast

to axial loading (Richter1995), the bending

effect is dependent on the height of the res-

toration and augments linearly as the

length of the lever increases (Richter1998).

It thus follows that implant connector de-

signs should be designed to ensure optimal

load transfer of bending forces.

One of the essential features of the ITIA

implant system (Straumann, Waldenburg,


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Perriard . Standard vs. internally keyed implant connectors

Fig.1. Principle of internal keying. a) Standard bico-

nal design. b) New, internally keyed, Octa design.

Switzerland) is the conical interlock that

connects the endosseous implant to the

various types of prosthodontic attach-

ments and which the company refers to as

morse taper (Sutter et al. 1993). Morse

taperis a term that stems from the tooling

industry and which designates a keying

mechanism in which a cone is fitted

within a cone (Schlosser 2001). The grip-

ping action is due to the intimate contact

and friction that develops in both elements

when the male cone is gently tapped into

the female element. This type of attach-

ment is widely used to securely fasten drill

bits or chucks to the rotating arbors of

lathes or drill presses. The taper of the cone

is indicated in degrees or in percent (d ra-

dius vs. d unit length). Percentages of 47%

are typical. Also, such morse taper designs

are characterized by their long shank, re-

sulting in length to diameter ratios of5

:1

.The ITIs cone total convergence angle is

16, its height is 2.3mm and its diameter

is 2.25mm. Thus it is not a true morse

taper as utilized in industrial applications

but rather a biconal type of keying mech-

anism whose effectiveness is significantly

increased by the preload generated on the

fraying surfaces of the cones by torque con-

trolled bolting of the abutment into the en-

dosseous implant. Such a configuration has

proven highly suited to the load transfer of

bucco-lingual bending forces both in lab-

543 | Clin. Oral Impl. Res.13, 2002/ 542549

oratory experiments (Norton 1997) and in

clinical environments (Felton et al. 1999;

Levine et al. 1999).

By design, a morse taper is rotation sym-

metric and thus lacks an antirotational

keying mechanism. This precludes accu-

rate laboratory transfers whenever the path

of draw determined by the implant needsto be altered to better accommodate an

abutment. To provide the ITI implant with

such a keying device, the Straumann Com-

pany added an internal octagon mid-level

of the cone of the implant body. Both the

standard and the new cone designs are

shown in Fig. 1(a, b). It was unclear, how-

ever, whether this additional feature would

decrease the load transferring capacity of

the joint. Therefore, the present study was

initiated to test the hypothesis that no dif-

ference existed between the standard cone

and the new internally keyed design.

Material and methods

Principle

The mechanical principle of the experi-

ment was to cyclically load combinations

of the standard (S) and the new, octagon

keyed (O) designs of implant and abutment

analogs and determine their resistance to

fatigue failure.

Rejecting or accepting the null hypoth-

esis (i.e. no difference between both con-

nector designs) consisted in comparing the

mean resistance to failure (i.e. breakage) at

Fig.2.Principle of testing setup. The specimen was embedded into a resin-filled cylinder which was angled15

off the vertical. The T-bar allowed a force application at5 mm off-center. Both features combined allowed a

torquing moment to be applied to the specimen.

106 cycles of the three groups tested; stated

differently, for each connector design in de-

termining the load level at which 50% of

specimens failed and 50% survived 106

load cycles. Conclusions were drawn after

statistical comparison of the three means.

The test specimens were divided into

three groups:O implant (S) and abutment (S),

O implant (O) and abutment (S),

O implant (O) and abutment (O).

Nine specimens were used in preruns to

adjust the machine settings, 20specimens

were used for the OO combination and 10

for both the SS and OS pairs.

In addition, the three experimental con-

ditions were modeled using finite element

procedures, the locations of the fracture

sites were recorded and the displacement

vs. number of cycles plots were analyzed.

Mechanical testing

Specimen setup

The setup of the specimens is diagram-

matically shown in Fig. 2. The specimens

(i.e. implant and abutment analogs) were

inclined by15off the vertical (Merz et al.

2000) and the abutments were fitted with

20-mm horizontal bars yielding a T-shaped

arrangement. Loading was applied in a ver-

tical direction at 5 mm off-center onto one

end of the horizontal bar. This arrange-

ment thus generated both a bending and a

torquing moment on the conical joints.

The T-bar arrangement was deemed


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necessary to determine how the specimens

would react when loaded counterclock-

wise (untightening). Therefore, in the pres-

ent setup the worst case situation (i.e. un-

screwing) was used. For testing, the speci-

mens were embedded in a cylindrical

container filled with polymethyl meth-

acrylate (PMMA) resin (Technovit 4071

,Heraeus Kulzer, Wehrheim, Germany). In

analogy with clinical implantbone re-

lationships, the implant body was posi-

tioned so that the resin was level with the

border between the polished and the rough

portion of the implant. The implant carried

a standard6taper7-mm height abutment

which was preloaded to40Ncm. The abut-

ment was fitted with a gold coping whose

purpose was to interface the softer ti-

tanium abutment with the harder steel T-

bar. The T-bar was machined with a half-

round top. Since the definitive inclinationof the specimen was to be experimentally

determined first, the half-round configur-

ation provided a resting surface normal to

the force vector applied but independent of

specimen angulation. The steel bar was

used for all test runs. A new gold coping

was used after two runs were completed.

New implant and abutment analogs were

utilized for each test.

Testing machine

The testing machine employed was a

servohydraulic fatigue tester (Hydropuls,

Schenk, Darmstadt, Germany). This ma-

chine was designed to generate various

types of loading modes (sinus, square,

ramp, preprogrammed)viaan actuator bar

that was applied to the specimens. It could

be operated in load or displacement con-

trol. The machines settings as well as the

parameters pertaining to the experiment

(time, number of cycles, loads and displace-

ments) were preset using an ancillary PC.

To ensure proper function, the machine re-

quired that the specimens always be pre-loaded to at least 20N, that is, that they

would not be fully unloaded during cyclic

force application. For the present experi-

ments, the machine was set to generate si-

Table 1. Material parameters used in the numeri-cal analyses

E (GPa) Poissons ratio

Titanium 110 0.3

Steel 206.8 0.29

PMMA 2.38 0.41

544 | Clin. Oral Impl. Res.13, 2002 / 542549

nus loadings at 2.05Hz (2Hz was chosen

initially but at that frequency the feed-

back loop that controls the oil circuitry

was unstable, hence the 2.05Hz). The

maximum load force was set according to

the staircase procedure as described below.

Staircase procedure

The procedure consists in determining the

load level (Lm) at which 50% of the

samples survive106 stress cycles and50%

fail.106 cycles is an arbitrarily set number

whose theoretical and practical basis has

been explained (Wiskott et al. 1994). The

staircase procedure is a straightforward

technique that applies to quantal (i.e. fail

or not-fail) data. The method requires the

samples to be tested consecutively in that

the outcome (fail or not-fail) of a given

samples test run determines the load level

applied to the next sample tested. If theprevious sample survived 106 cycles, the

next sample is run at the previous load

augmented by a predetermined amount. If

the previous sample failed, the next sample

is run at the previous level minus the pre-

determined amount. This generates an up-

and-down pattern of fail and not-fail loads,

hence the name staircase. After suitable

arrangement of the data, the mean (50%

failures and 50% run-outs) and the stan-

dard deviation are calculated. The compu-

tational aspects of the technique have been

described elsewhere (Wiskott et al. 1994;

Dieter 1961; Draughn 1979).

At the onset, both an entry force level

and an increment/decrement must be de-

termined before the test sequence is

Fig.3. Fatigue resistance of octagonoctagon (OO), smoothsmooth (SS) and octagonsmooth (OS) combi-

nations. Both the OO and the SS combination had overlapping confidence intervals. The SO combination

was superior to both OO and SS.

started. The entry force level was deter-

mined using the equation

F M/dsinawhereFis force applied,M

is torque applied during abutment

tightening, d is lever length and a is the

inclination of the specimen. For M

40Ncm, d5mm and a15, the esti-

mated force would be308

.8

N. Using thisvalue, it was decided to set the entry force

level to 2/3 of the maximum, that is,

205.9N. The increment/decrement was set

to20 N.

Data analysis

The results of the staircase analyses in

terms of Lm and standard deviation were

computed. Means were compared by fit-

ting with95% confidence intervals. Means

whose intervals did not overlap were con-

sidered statistically equal.

Numerical analysis

To identify zones of stress concentration,

the SS, OS and OO combinations of

connectors were modeled using a finite ele-

ment software (I-DEAS Master MS8,

SDRC, Cincinnati, OH, USA) (Curnier

1994) that was installed on a HP735work-

station in a Unix environment. For

modeling, the components were assumed

homogeneous, linearly elastic and iso-

tropic. Material characteristics are listed in

Table1. The models were a tridimensional

mesh of tetrahedra incorporating approxi-

mately 16000 elements and 3600 nodes

(combination including the Octa design in-

creased the number of cells required). A

layer of friction elements was inserted be-


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tween the implant and the abutment. Such

elements model the transfer of pressures

with respect to the relative motion of both

Fig.4.FE analysis of SS combinations. Peak stress was499Mpa.

Fig.5.FE analysis of SO combinations. Peak stress was562Mpa.

Fig.6.FE analysis of OO combinations. Peak stress was26?900Mpa.


surfaces in contact. The coefficient of fric-

tion was set to 0.5(Abkowitz et al. 1960).

The components were first subdivided into

substructures which were subsequently

meshed automatically by the software.

There was no need to specifically mesh the


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T-bar since the torquing force generated by

the lever system of the bar was computed

and integrated into the numerical simula-

tion. Calculations were performed using an

applied force of 300N.

Location of fracture sites

The location of the fracture sites was re-corded. This information was of signifi-

cance to determine whether there was a

definite locus of minor resistance inside

the connector or whether fracture occurred

at random within the structure.

Displacement recording vs. number of cycles

For each load level applied during staircase

analysis, every 50 cycles, the system re-

corded the cycle number and the displace-

ment of the machines actuator bar. This

was deemed necessary to identify possibledeficiencies in the specimen setup. It was

also meant to determine whether a sys-

tematic difference existed between speci-

mens that fractured and those that did not.

It was hypothesized that specimens that

eventually failed might present a growing

fissure which would translate into an in-

creasing displacement of the actuator bar.

Results

Fatigue resistance

The fatigue resistance of the three combi-

nations is presented in Fig.3. The confi-

dence intervals at 95% are also shown.

While the internally keyed and the stan-

dard designs clearly had overlapping con-

fidence intervals, the SO combination

presented a superior resistance to force ap-

plication.

Numerical analysis (Figs46)

Numerical analysis depicts the stress vari-ations (MPa) inside the structures under an

applied load of 300N. The stress intensity

bar on the right indicates the highest and

lowest stresses appearing in the structure

while the 2- and 3D mappings specify the

locations of the stresses. While the SS and

the SO configurations are essentially

similar in terms of the stresses induced,

the software identified extreme stress con-

centrations in the OO combination.

These appear in the apical portion of the

line angles of the Octa structure (Fig. 6).


Location of fracture sites

The location of the failure sites is summar-

ized in Fig. 7. Both fissures (dotted lines)

and overt fractures (solid lines) were ob-

served. They occurred in the implant at the

level of the tip of the screw; in the screw

threads, close to the junction with the

cone; at three levels of the SynOcta malepart and in the solid cone. For all combi-

nations there were at least three sites

where failure occurred. No preferential

location was detected.

Displacement vs. cycle number plots

Plots depicting actuator bar displacement

vs. number of cycles were generated for all

specimens. There was no systematic be-

havior that characterized either failed vs.

not-failed specimens or which differen-

tiated the three combinations. Neverthe-

less, four patterns were observed (Fig. 8):

stability over the whole run;

slow increase of actuator bar displacement,

no failure;

rapid increase of actuator bar displacement

followed by fracture;

stability followed by fracture.

Fig.7.Locations of failure sites. Encircled numbers denote the number of occurrences. Dotted lines represent

fissures. Solid lines represent fractures.

Discussion

Stress concentrations

The data presented above indicate no de-

finite trend with respect to the null hy-

pothesis (no difference between SS and O

O) in any of the tests performed, with one

exception, the inordinately high stress con-

centration on the mating surface of the

Octa connectors apical angles. When con-

sidering the numerical values obtained

(2.69104MPa peak stress) any OO combi-

nation should fail under a load in excess of

6N (600g,that is) given a tensile resistance

of titanium of 500MPa (Ashby & Jones

1986). While the magnitude of the values

computed can largely be attributed to

deficiencies in meshing of the model, the

phenomenon observed should not be re-

jected off-hand since it is known that

angles generate stress concentrations(Broek 1988). (Incidentally, these do also

appear on the cones mating surfaces in the

OS combination and may be at the origin

of the fissures shown in Fig. 7). So much so

that the machining process developed by

the manufacturer includes substantial

rounding of the angle of the SynOcta male

part as shown in Fig.9. Due to the ge-


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Fig.8.Displacement vs. number of cycle curves. Four

patterns were observed. Pattern a: stability over the

entire run. Pattern b: slow increase of maximum ac-

tuator bar displacement. Pattern c: rapid increase of

actuator bar displacement. Pattern d: slight increase

followed by fracture.

ometry of the Octa connector, the FEmodel constructed for the present study

could not be simplified to 2D or axisym-

metric meshings but included the intric-

acies of the keying mechanism. Neverthe-

less, the FE analysis conducted here was

static in that it ignored the plastic deforma-

tion of the metal. Yet this applies particu-

larly to commercially pure (i.e. unalloyed)

titanium where rapid deformation in zones

of pressure contact is expected. Hence it is

highly likely that the stress phenomenon

in the FE software is mathematically and


Fig.9.SynOcta element from the bottom. Note rounding of the external edges of the screw head.

not mechanically driven. This consider-ation holds especially in light of the fact

that no component fractured in that zone

during our tests.

Failure modes and fracture sites

The four patterns observed in the displace-

ment vs. number of cycle plots (Fig. 8) pre-

sumably represent different types of alter-

ations inside the structures. Patterns b

and c most likely indicate the progression

of a fissure at a fairly slow rate (b) and at a

faster rate (c). It is probable that specimen

b would have failed at a higher cycle

number. The specimens following pattern

a were stable and no fissuring occurred.

Pattern d, in which displacement actually

decreased before fracture, can be explained

on the basis of work-hardening, which

tends to stiffen the metal (and also to in-

crease its brittleness).

The heterogeneity observed in the frac-

ture sites basically eliminates the possi-

bility of a locus of minor resistance. This

finding positively valuates the designchosen.

Stresses in conical joints

In a previous study, we had modeled the

behavior of cemented conical joints under

lateral loading (Wiskott et al. 1999). It was

shown that crowns rotated around an axis

located mid-level of the cone and that the

stress levels in that zone decreased to a

minimum. It follows that the internal key-

ing mechanism designed by the manufac-

turer is located in a section of the cone thatbears the smallest magnitudes of stresses

and thus that the risk of failure is greatly

reduced.

Mechanics of bolted surfaces

The various keying mechanisms proposed

in dental implantology require some

understanding of the basic mechanics of

bolted joints. Consider two flat plates that

are bolted together and onto which a ten-

sile force is applied parallel to the long axis

of the bolt, in effect pulling the flat plates

apart. Initially, when the bolt is tightened,

the fraying surfaces (i.e. the portions of the

surfaces that come in contact) are drawn

together and develop a compressive force

onto the mating parts. The joint is now in

equilibrium with the compressive force

across the fraying surfaces equal to the ten-

sile stress inside the bolt. This internal

stress is called pretension and the force on

the mating surface is referred to as preload.

If an external tensile force is applied onto

the plates, the assembly responds in a

somewhat unexpected way in that the ap-plied force is not concentrated onto the

bolt but distributed along the entire fraying

surface. To some extent, the net effect of

the preload is to alter the behavior of the

two plates as if they were a single compo-

nent. This effectively shields the bolt from

large variations in tension and therefore

substantially enhances its resistance to fa-

tigue failure. The magnitude of shielding

depends on the joints material and ge-

ometry but it is not unusual that as much

as90% of the applied load is dissipatedvia


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the mating surfaces, leaving only 10% to

be borne by the screw. From the above, it

follows that, in order to ensure an optimal

mechanical continuum, the pretension in

the screw must be as high as possible,

often in the order of 6070% of the ulti-

mate tensile strength of the screw (Haack

et al.1995

). This effect has been put to usein the cone in cone design of the ITI con-

nector (Sutter et al.1993). However, due to

the machining tolerances (Binon1995) and

their near-parallel design, the vertical sides

of the Octa connector do not actually carry

load and thus the role of the internal key-

ing mechanism is to ensure positional

duplicability between laboratory and clin-

ical phases of treatment but not to provide

any noteworthy contribution as an antiro-

tational device during function.

Conclusion

The data gathered in the present study do

not provide a basis for rejecting the null

hypothesis of no difference between the

standard cone and the newly designed in-

ternally keyed (Octa) design. Therefore

both connectors are considered equal in

their mechanical resistance to bending and

torquing forces.

Resume

La compagnie Straumann a recemment remplace sa

configuration standard par une clef interne octogonale.

Durant la phase de restauration, cette addition a eteeffec-

tuee pour assurer la possibilite de reproduire la position

entre laboratoire et clinique. Il netait pas certain que ce

mecanisme de clef pouvait diminuer la force mecanique

de la connexion entre limplant et le pilier. Ceci sappli-

que aux parties males et femelles de la clef mais aussi

aux combinaisons entre les modeles nouveaux et stan-

dards. Les specimens construits specialement represen-

tant les trois combinaisons ont ete fabriques avec une

barre en forme de T preangulee a 15 degres et sujette a

des forces verticales apportees par un systeme de fatigue

servohydraulique. La frequence de charge etait de 2Hz et

le nombre de cycles maximum de 106. Les donnees ont

ete evaluees en utilisant la technique par echelons. Les

echantillons etaient aussi modeles et analyses numeri-

quement en utilisant des processus delements finis. Les

localisations des echecs ont ete notees et le deplacement

vs cycle a ete analyse par plots en quatre groupes. Les

tests de fatigue et lanalyse par echelons nont mis en

evidence aucune difference entre les connecteurs stan-

dards et les internes au niveau de la resistance mecani-

que. Les modeles delements finis ont mis en evidence

une concentration du stress localisee dans les parties api-

cales de la connexion octogonale. Cependant, il apparat

que ce phenomene etait bien plus base sur des problemes


de calcul que mecanique. Les localisations des sites avec

echec etaient distribuees au hasard le long des structures

indiquant ainsi labsence dun endroit de moindre resis-

tance. Les modeles de deplacement vs cycle ne pouvaient

pas etre attribues a des associations specifiques entre les

modeles standards et aclef interne. Les deux connexions

sont donc semblables dans leur resistance mecanique aux

forces de pliage et de torsion.

Zusammenfassung

Die Firma Straumann hat krzlich die Standard-Konus-

form mit einer achteckigen internen Kantenbahn erwei-

tert. Diese zustzliche Eigenschaft wurde entwickelt, da-

mit whrend der restaurativen Phase der Implantatthera-

pie die Uebertragung der Implantatposition zwischen

Labor und der klinischen Umgebung und umgekehrt ge-

sichert werden kann. Es ist jedoch unklar, ob dieser Ver-

schlsselungsmechanismus die mechanische Strke der

Verbindung zwischen dem Implantat und dem Sekundr-

teil beeinflusst. Dies betrifft sowohl die mnnlichen und

weiblichen Teile mit Verschlsselungsmechanismus als

auch die Kombinationen von neuen und Standardteilen.Speziell konstruierte Testanaloge, welche alle drei Kom-

binationsmglichkeiten wiederspiegelten, wurden an ei-

nem T-frmigen Balken befestigt oder um 15 abgewin-

kelt und vertikalen Krften, welche von einem servohy-

draulischen Belastungstester generiert wurden,

ausgesetzt. Die Belastungsfrequenz betrug 2 Hz und die

maximale Anzahl Belastungszyklen betrug 106. Die Da-

ten wurden mittels Treppenstufen-Technik ausgewertet.

Die Testkrper wurden ebenfalls als Computermodell

numerisch mittels der Finite-Element-Verfahren analy-

siert. Die Stellen der Misserfolge bei den Testkrpern

wurden aufgezeichnet und die Graphiken der Verschie-

bung gegenber der Anzahl Belastungszyklen wurden ge-

mss Muster in 4 Gruppen aufgeteilt.

Der Ermdungstest und die Treppenstufenanalyse zeig-

ten keinen Unterschied in der mechanischen Wider-

standsfhigkeit zwischenden Standdardteilen und den in-

tern gesicherten Verbindungen. Die Finite-Element-Mo-

delle ergaben eine Stresskonzentration, welche im

Bereich der apikalen Kanten des achteckigen Verbinders

lokalisiert war. Es schien jedoch, dass dieses Phnomen

mehr auf computertechnischen als auf mechanischen

Grnden beruhte. Die Regionen der Misserfolge waren

entlang der Strukturen zufllig verteilt. Dies zeigt, dass

keine schchste Stelle existiert. Die Muster der Displazie-

rung gegenber den Belastungszyklen konnten nicht spe-

zifischen Kombinationen zwischen den Standardteilen

und den intern gesicherten Teilen zugeordnet werden.

Es wird die Schlussfolgerung gezogen, dass beide Verbin-

dungen in Bezug auf mechanischen Widerstang gegen

Biege- und Drehrfte gleichwertig sind.

Resumen

La compan a Straumann ha suplementado recientemente

su configuracion estandar de morse-taper con una llave

octogonal interna. Durante la fase restaurativa del trata-

miento de implantes, se disen oesta caracterstica adicio-

nal para asegurar la duplicidad posicional entre las condi-

ciones de laboratorio y clnicas. De todos modos no esta-

ba claro si este mecanismo de llave disminuira la

resistencia mecanica de la conexion entre el implante y

el pilar. Esto se aplica no solo a las partes macho y hem-

bra con llave sinoa las combinaciones de los disen os nue-

vos y estandar.

Se ajustaron unos especimenes analogos especialmente

construidos representando las tres combinaciones posi-

bles a una barra con forma de T, preangulada a 15 grados

y sometida a una fuerza vertical suministrada por un pro-

bador de fatiga servohidraulico. La frecuencia de carga fue

de 2Hz y el numero maximo de ciclos fue de 106. Los

datos se evaluaron usando la tecnico e la escalera. Los

especimenes se modelaron y analizaron numericamenteusando procedimientos de elementos finitos. Se recogie-

ron las localizaciones de los fracasos de las muestras y el

desplazamiento frente al numero de ciclos se agruparon

en cuatro patrones.

Laspruebasde fatiga y el analisis de la escalera no eviden-

ciaron diferencias en la resistencia mecanica entre los co-

nectores estandar y los de llave interna. Los modelos ele-

mentos finitos evidenciaron una concentracion de estres

localizada en los bordes apicales del conector octogonal.

De todos modos, parece que este fenomeno se baso en

datos computacionales mas que en datos mecanicos. Las

localizaciones de los lugares de fracaso se distribuyeron

aleatoriamente a lo largo de las estructuras indicando por

ello la ausencia de un lugar de menor resistencia. Los

patrones de desplazamiento frente a los ciclos no se pu-

dieron atribuir a combinaciones especficas entre los dise-

n os estandar y los de llave interna.

Se concluyoque ambos conectores son iguales en su re-

sistencia mecanica a las fuerzas de doblaje y de torque.


8/8


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Documents

Fatigue Resistance of ITI Implant Abutment Connectorsaa Comparison of the Standard Cone With a Novel Internally Keyed Design