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<record><TEI><teiHeader><fileDesc><titleStmt><title xml:lang="en">Resistance of three implant-abutment interfaces to fatigue
testing</title><author><name sortKey="Ribeiro, Cleide Gisele" sort="Ribeiro, Cleide Gisele" uniqKey="Ribeiro C" first="Cleide Gisele" last="Ribeiro">Cleide Gisele Ribeiro</name><affiliation><nlm:aff id="aff01"> DDS, MSc, PhD, Graduate student, Department of Dental Implantology, University of Santa Catarina, Florianópolis, SC, Brazil.</nlm:aff></affiliation></author><author><name sortKey="Maia, Maria Luiza Cabral" sort="Maia, Maria Luiza Cabral" uniqKey="Maia M" first="Maria Luiza Cabral" last="Maia">Maria Luiza Cabral Maia</name><affiliation><nlm:aff id="aff02"> DDS, Graduate student, Department of Prosthodontics-Biomaterials, University of Geneva, Geneva, Switzerland.</nlm:aff></affiliation></author><author><name sortKey="Scherrer, Susanne S" sort="Scherrer, Susanne S" uniqKey="Scherrer S" first="Susanne S." last="Scherrer">Susanne S. Scherrer</name><affiliation><nlm:aff id="aff03"> DDS, PhD, Senior lecturer, Department of Prosthodontics-Biomaterials, University of Geneva, Geneva, Switzerland.</nlm:aff></affiliation></author><author><name sortKey="Cardoso, Antonio Carlos" sort="Cardoso, Antonio Carlos" uniqKey="Cardoso A" first="Antonio Carlos" last="Cardoso">Antonio Carlos Cardoso</name><affiliation><nlm:aff id="aff04"> DDS, MSc, PhD, Head Professor, Department of Dental Implantology, University of Santa Catarina, Florianópolis, SC, Brazil.</nlm:aff></affiliation></author><author><name sortKey="Wiskott, H W Anselm" sort="Wiskott, H W Anselm" uniqKey="Wiskott H" first="H. W. Anselm" last="Wiskott">H. W. Anselm Wiskott</name><affiliation><nlm:aff id="aff05"> DDS, MSc, PhD, Senior lecturer, Department of Prosthodontics-Biomaterials, University of Geneva, Geneva, Switzerland.</nlm:aff></affiliation></author></titleStmt><publicationStmt><idno type="wicri:source">PMC</idno><idno type="pmid">21710094</idno><idno type="pmc">4223795</idno><idno type="url">http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4223795</idno><idno type="RBID">PMC:4223795</idno><idno type="doi">10.1590/S1678-77572011005000018</idno><date when="2011">2011</date><idno type="wicri:Area/Pmc/Corpus">003023</idno><idno type="wicri:explorRef" wicri:stream="Pmc" wicri:step="Corpus" wicri:corpus="PMC">003023</idno></publicationStmt><sourceDesc><biblStruct><analytic><title xml:lang="en" level="a" type="main">Resistance of three implant-abutment interfaces to fatigue
testing</title><author><name sortKey="Ribeiro, Cleide Gisele" sort="Ribeiro, Cleide Gisele" uniqKey="Ribeiro C" first="Cleide Gisele" last="Ribeiro">Cleide Gisele Ribeiro</name><affiliation><nlm:aff id="aff01"> DDS, MSc, PhD, Graduate student, Department of Dental Implantology, University of Santa Catarina, Florianópolis, SC, Brazil.</nlm:aff></affiliation></author><author><name sortKey="Maia, Maria Luiza Cabral" sort="Maia, Maria Luiza Cabral" uniqKey="Maia M" first="Maria Luiza Cabral" last="Maia">Maria Luiza Cabral Maia</name><affiliation><nlm:aff id="aff02"> DDS, Graduate student, Department of Prosthodontics-Biomaterials, University of Geneva, Geneva, Switzerland.</nlm:aff></affiliation></author><author><name sortKey="Scherrer, Susanne S" sort="Scherrer, Susanne S" uniqKey="Scherrer S" first="Susanne S." last="Scherrer">Susanne S. Scherrer</name><affiliation><nlm:aff id="aff03"> DDS, PhD, Senior lecturer, Department of Prosthodontics-Biomaterials, University of Geneva, Geneva, Switzerland.</nlm:aff></affiliation></author><author><name sortKey="Cardoso, Antonio Carlos" sort="Cardoso, Antonio Carlos" uniqKey="Cardoso A" first="Antonio Carlos" last="Cardoso">Antonio Carlos Cardoso</name><affiliation><nlm:aff id="aff04"> DDS, MSc, PhD, Head Professor, Department of Dental Implantology, University of Santa Catarina, Florianópolis, SC, Brazil.</nlm:aff></affiliation></author><author><name sortKey="Wiskott, H W Anselm" sort="Wiskott, H W Anselm" uniqKey="Wiskott H" first="H. W. Anselm" last="Wiskott">H. W. Anselm Wiskott</name><affiliation><nlm:aff id="aff05"> DDS, MSc, PhD, Senior lecturer, Department of Prosthodontics-Biomaterials, University of Geneva, Geneva, Switzerland.</nlm:aff></affiliation></author></analytic><series><title level="j">Journal of Applied Oral Science</title><idno type="ISSN">1678-7757</idno><idno type="eISSN">1678-7765</idno><imprint><date when="2011">2011</date></imprint></series></biblStruct></sourceDesc></fileDesc><profileDesc><textClass></textClass></profileDesc></teiHeader><front><div type="abstract" xml:lang="en"><p>The design and retentive properties of implant-abutment connectors affect the
mechanical resistance of implants. A number of studies have been carried out to
compare the efficacy of connecting mechanisms between abutment and fixture.</p><sec><title>Objectives</title><p>The aims of this study were: 1) to compare 3 implant-abutment interfaces
(external hexagon, internal hexagon and cone-in-cone) regarding the fatigue
resistance of the prosthetic screw, 2) to evaluate the corresponding mode of
failure, and 3) to compare the results of this study with data obtained in
previous studies on Nobel Biocare and Straumann connectors. </p></sec><sec><title>Materials and Methods</title><p>In order to duplicate the alternating and multivectorial intraoral loading
pattern, the specimens were submitted to the rotating cantilever beam test.
The implants, abutments and restoration analogs were spun around their
longitudinal axes while a perpendicular force was applied to the external
end. The objective was to determine the force level at which 50% of the
specimens survived 10<sup>6</sup> load cycles. The mean force levels at
which 50% failed and the corresponding 95% confidence intervals were
determined using the staircase procedure. </p></sec><sec><title>Results</title><p>The external hexagon interface presented better than the cone-in-cone and
internal hexagon interfaces. There was no significant difference between the
cone-in-cone and internal hex interfaces. </p></sec><sec><title>Conclusion</title><p>Although internal connections present a more favorable design, this study did
not show any advantage in terms of strength. The external hexagon connector
used in this study yielded similar results to those obtained in a previous
study with Nobel Biocare and Straumann systems. However, the internal
connections (cone-in-cone and internal hexagon) were mechanically inferior
compared to previous results.</p></sec></div></front><back><div1 type="bibliography"><listBibl><biblStruct><analytic><author><name sortKey="Al Jabbari, Ys" uniqKey="Al Jabbari Y">YS Al Jabbari</name></author><author><name sortKey="Fournelle, R" uniqKey="Fournelle R">R Fournelle</name></author><author><name sortKey="Ziebert, G" uniqKey="Ziebert G">G Ziebert</name></author><author><name sortKey="Toth, J" uniqKey="Toth J">J Toth</name></author><author><name sortKey="Iacopino, Am" uniqKey="Iacopino A">AM Iacopino</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Al Turki, Le" uniqKey="Al Turki L">LE Al-Turki</name></author><author><name sortKey="Chai, J" uniqKey="Chai J">J Chai</name></author><author><name sortKey="Lautenschlager, Ep" uniqKey="Lautenschlager E">EP Lautenschlager</name></author><author><name sortKey="Hutten, Mc" uniqKey="Hutten M">MC Hutten</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Binon, Pp" 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sortKey="Scherrer, Ss" uniqKey="Scherrer S">SS Scherrer</name></author><author><name sortKey="Renevey, Rr" uniqKey="Renevey R">RR Renevey</name></author><author><name sortKey="Belser, Uc" uniqKey="Belser U">UC Belser</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Yousef, H" uniqKey="Yousef H">H Yousef</name></author><author><name sortKey="Luke, A" uniqKey="Luke A">A Luke</name></author><author><name sortKey="Ricci, J" uniqKey="Ricci J">J Ricci</name></author><author><name sortKey="Weiner, S" uniqKey="Weiner S">S Weiner</name></author></analytic></biblStruct></listBibl></div1></back></TEI><pmc article-type="research-article"><pmc-dir>properties open_access</pmc-dir>
<front><journal-meta><journal-id journal-id-type="nlm-ta">J Appl Oral Sci</journal-id><journal-id journal-id-type="iso-abbrev">J Appl Oral Sci</journal-id><journal-title-group><journal-title>Journal of Applied Oral Science</journal-title></journal-title-group><issn pub-type="ppub">1678-7757</issn><issn pub-type="epub">1678-7765</issn><publisher><publisher-name>Faculdade de Odontologia de Bauru da Universidade de São
Paulo</publisher-name></publisher></journal-meta><article-meta><article-id pub-id-type="pmid">21710094</article-id><article-id pub-id-type="pmc">4223795</article-id><article-id pub-id-type="doi">10.1590/S1678-77572011005000018</article-id><article-categories><subj-group subj-group-type="heading"><subject>Original Articles</subject></subj-group></article-categories><title-group><article-title>Resistance of three implant-abutment interfaces to fatigue
testing</article-title></title-group><contrib-group><contrib contrib-type="author"><name><surname>RIBEIRO</surname><given-names>Cleide Gisele</given-names></name><xref ref-type="aff" rid="aff01">1</xref><xref ref-type="corresp" rid="c01"></xref></contrib><contrib contrib-type="author"><name><surname>MAIA</surname><given-names>Maria Luiza Cabral</given-names></name><xref ref-type="aff" rid="aff02">2</xref></contrib><contrib contrib-type="author"><name><surname>SCHERRER</surname><given-names>Susanne S.</given-names></name><xref ref-type="aff" rid="aff03">3</xref></contrib><contrib contrib-type="author"><name><surname>CARDOSO</surname><given-names>Antonio Carlos</given-names></name><xref ref-type="aff" rid="aff04">4</xref></contrib><contrib contrib-type="author"><name><surname>WISKOTT</surname><given-names>H. W. Anselm</given-names></name><xref ref-type="aff" rid="aff05">5</xref></contrib></contrib-group><aff id="aff01"><label>1</label> DDS, MSc, PhD, Graduate student, Department of Dental Implantology, University of Santa Catarina, Florianópolis, SC, Brazil.</aff><aff id="aff02"><label>2</label> DDS, Graduate student, Department of Prosthodontics-Biomaterials, University of Geneva, Geneva, Switzerland.</aff><aff id="aff03"><label>3</label> DDS, PhD, Senior lecturer, Department of Prosthodontics-Biomaterials, University of Geneva, Geneva, Switzerland.</aff><aff id="aff04"><label>4</label> DDS, MSc, PhD, Head Professor, Department of Dental Implantology, University of Santa Catarina, Florianópolis, SC, Brazil.</aff><aff id="aff05"><label>5</label> DDS, MSc, PhD, Senior lecturer, Department of Prosthodontics-Biomaterials, University of Geneva, Geneva, Switzerland.</aff><author-notes><corresp id="c01"><bold>Corresponding address:</bold> Cleide Gisele Ribeiro - Avenida Barão do Rio
Branco, 2406/506 e 507 - Centro - 36010-011 - Juiz de Fora - Minas Gerais -
Brazil - Phone: +55-32-3216-6246 - Fax: +55-32-9109-2678 - e-mail:
<email>cleidegr@yahoo.com.br</email></corresp></author-notes><pub-date pub-type="epub-ppub"><season>Jul-Aug</season><year>2011</year></pub-date><pmc-comment>Fake ppub date generated by PMC from publisher
pub-date/@pub-type='epub-ppub' </pmc-comment>
<pub-date pub-type="ppub"><season>Jul-Aug</season><year>2011</year></pub-date><volume>19</volume><issue>4</issue><fpage>413</fpage><lpage>420</lpage><history><date date-type="received"><day>25</day><month>5</month><year>2009</year></date><date date-type="rev-recd"><day>27</day><month>4</month><year>2010</year></date><date date-type="accepted"><day>26</day><month>10</month><year>2010</year></date></history><permissions><license license-type="open-access" xlink:href="http://creativecommons.org/licenses/by-nc/3.0/"><license-p>This is an Open Access article distributed under the terms of the
Creative Commons Attribution Non-Commercial License which permits
unrestricted non-commercial use, distribution, and reproduction in any
medium, provided the original work is properly cited. </license-p></license></permissions><abstract><p>The design and retentive properties of implant-abutment connectors affect the
mechanical resistance of implants. A number of studies have been carried out to
compare the efficacy of connecting mechanisms between abutment and fixture.</p><sec><title>Objectives</title><p>The aims of this study were: 1) to compare 3 implant-abutment interfaces
(external hexagon, internal hexagon and cone-in-cone) regarding the fatigue
resistance of the prosthetic screw, 2) to evaluate the corresponding mode of
failure, and 3) to compare the results of this study with data obtained in
previous studies on Nobel Biocare and Straumann connectors. </p></sec><sec><title>Materials and Methods</title><p>In order to duplicate the alternating and multivectorial intraoral loading
pattern, the specimens were submitted to the rotating cantilever beam test.
The implants, abutments and restoration analogs were spun around their
longitudinal axes while a perpendicular force was applied to the external
end. The objective was to determine the force level at which 50% of the
specimens survived 10<sup>6</sup> load cycles. The mean force levels at
which 50% failed and the corresponding 95% confidence intervals were
determined using the staircase procedure. </p></sec><sec><title>Results</title><p>The external hexagon interface presented better than the cone-in-cone and
internal hexagon interfaces. There was no significant difference between the
cone-in-cone and internal hex interfaces. </p></sec><sec><title>Conclusion</title><p>Although internal connections present a more favorable design, this study did
not show any advantage in terms of strength. The external hexagon connector
used in this study yielded similar results to those obtained in a previous
study with Nobel Biocare and Straumann systems. However, the internal
connections (cone-in-cone and internal hexagon) were mechanically inferior
compared to previous results.</p></sec></abstract><kwd-group><kwd>Dental implants</kwd><kwd>Fatigue</kwd><kwd>Prosthesis failure</kwd></kwd-group></article-meta></front><body><sec sec-type="intro"><title>INTRODUCTION</title><p>Dental implantology has revolutionized the treatment for edentulous and partially
edentulous patients, and successful implant integration has been well documented for
patients with those clinical conditions. With the high rate of implant success for
edentulous individuals, the concept of osseointegration and implant therapy has
flourished as a predictable treatment modality<sup><xref rid="r10" ref-type="bibr">10</xref></sup>.</p><p>Clinical observations have indicated that the major causes of implant failure are (a)
deficient osseointegration, (b) complications of the neighboring soft tissues
(peri-mucositis and periimplantitis) and (c) mechanical complications<sup><xref rid="r13" ref-type="bibr">13</xref></sup>. Among the biomechanical problems,
screw loosening, abutment rotation, and abutment fracture are the major
issues<sup><xref rid="r15" ref-type="bibr">15</xref>,<xref rid="r28" ref-type="bibr">28</xref></sup>. In a prospective multicenter investigation,
Henry, et al.<sup><xref rid="r12" ref-type="bibr">12</xref></sup> (1996) evaluated
92 patients with 107 implants and found that the problems most frequently
experienced during the first year were related to lose screws. The two mechanisms
involved in screw loosening are: excessive bending (plastic deformation that takes
place when a load larger than the yield strength of the screw is applied) and
settling (when external loads applied to the screw interface create micromotion
between both surfaces). As the mating surfaces wear, they "settle" closer
together<sup><xref rid="r15" ref-type="bibr">15</xref></sup>. The factors
that contribute to screw instability are: misfit of the prosthesis, insufficient
tightening force, screw settling, mechanical overload, and mismatch in screw
material and design<sup><xref rid="r02" ref-type="bibr">2</xref></sup>.</p><p>A number of studies have been conducted to compare the efficacy of different
connecting mechanisms securing the abutment to the implant head<sup><xref rid="r16" ref-type="bibr">16</xref></sup>. The design of the
implant-to-abutment mating surface and the retentive properties of the screw joints
affect the mechanical resistance of the implant-abutment complex<sup><xref rid="r03" ref-type="bibr">3</xref>,<xref rid="r20" ref-type="bibr">20</xref></sup>. The implant-abutment connection is also influenced by factors
such as component fit, machining accuracy, saliva contamination and screw
preload<sup><xref rid="r04" ref-type="bibr">4</xref></sup>.</p><p>Current designs are derived from two basic designs: the "butt-joint", consisting of 2
parallel flat contacting surfaces<sup><xref rid="r03" ref-type="bibr">3</xref></sup>, and the internal "cone-in-cone" design. The latter has been
introduced in the ITI implant system (Institute Straumann AG, Waldenburg, Basel,
Switzerland) and offered a sound, stable, and self-locking interface<sup><xref rid="r29" ref-type="bibr">29</xref></sup>. Recent studies have indicated a
potential mechanical advantage of conical connectors over butt-joint
designs<sup><xref rid="r20" ref-type="bibr">20</xref></sup>. Indeed, the
mechanics of the ITI cone-in-cone<sup><xref rid="r20" ref-type="bibr">20</xref></sup> resulted in lower incidences of mechanical complications,
specifically abutment screw loosening and fracture, in comparison with those
reported for butt-joint implants<sup><xref rid="r23" ref-type="bibr">23</xref>,<xref rid="r25" ref-type="bibr">25</xref></sup>. With few exceptions, most of the
long-term clinical data on implant performance involve external hexagons. This is
primarily the result of their extensive use, the broad number of prescribed clinical
applications, the level of complications reported, and the resulting efforts to find
solutions (specific torque application to abutment screws)<sup><xref rid="r03" ref-type="bibr">3</xref></sup>. Industry surveys have shown that external hex
implants still dominate the European market<sup><xref rid="r18" ref-type="bibr">18</xref></sup>.</p><p>Fatigue is a progressive, localized and permanent structural damage that occurs in a
material subjected to repeated or fluctuating strains. Experimentally, three modes
of loading may be used to duplicate fatigue failures: direct axial loading (the
specimen is submitted to a uniform stress through its cross-section), plane-bending
(the majority of the specimen is subjected to a uniform bending stress) and
rotating-beam loading (the specimen is rotation-symmetric and is subjected to
dead-weight loading while swivel bearings permit rotation)<sup><xref rid="r24" ref-type="bibr">24</xref></sup>. In order to duplicate the multivectorial force
pattern of the mouth, a laboratory test has been developed by Wiskott, et
al.<sup><xref rid="r34" ref-type="bibr">34</xref></sup> (2004) using the
rotating beam principle. The test consists in spinning a specimen while holding it
at one end and loading it at the protruding end. The samples are thus subjected to a
360-degree field of transverse tensile and compressive force vectors.
Actuator-driven fatigue testing systems are unable to reproduce the complex force
patterns that are active clinically. Hence data obtained using rotational fatigue
testing have a superior pertinence relative to single-axis testing designs<sup><xref rid="r32" ref-type="bibr">32</xref>-<xref rid="r33" ref-type="bibr">33</xref></sup>.</p><p>To overcome some of the inherent design limitations of the external hex connector, a
variety of alternative connections have been developed. The goals of this study
were: (1) to evaluate the fatigue resistance of 3 implant-abutment connectors
(external hexagon, internal hexagon and cone-in-cone) analyzing the prosthetic
screw; 2) to determine their failure modes; and (3) to compare the obtained results
with previous data generated from Nobel Biocare-Replace and Straumann-ITI
connectors.</p></sec><sec sec-type="materials|methods"><title>MATERIAL AND METHODS</title><p>Three geometries of implant-abutment interfaces were evaluated. Thirty implants (4.0
mm diameter and 13 mm long) of each connector type were connected to Micro-unit
abutments (Conexão Sistemas de Prótese, Arujá, SP, Brazil) and torqued to 30 Ncm
using a calibrated torque controller. The Micro-unit abutments are industrially
machined prosthetic components that are intended for use in fixed partial and
complete implant-supported dentures at all sulcus depths and for all platforms. They
were designed to provide versatility while optimizing the esthetics of multiple
unit, screw retained, restorations. Therefore, the sole differences between groups
were the variations in the interface geometry between the implant head and the
Micro-unit abutment. The cone-in-cone Micro-unit abutment used in this study
presents an internal modification (internal hexagon), which is located at the bottom
of the cone to allow the angular repositioning of the abutments. The groups were set
up as follows: group A (external hex implant+micro-unit abutment+restoration
analog); group B (cone-in-cone implant+micro-unit abutment+restoration analog);
group C (internal hex implant+micro-unit abutment+restoration analog).</p><p>In order to duplicate the mouth's multivectorial force pattern, the specimens
(implant, abutment and restoration analog) were configured as rotating cantilever
beams (<xref ref-type="fig" rid="f01">Figure 1</xref>). The rotating beam principle
demands that a concentric arrangement be established between all the components. One
end of the test specimen is clamped into a collet and rotated, while a perpendicular
force is applied to the other end via a ball bearing. This perpendicular force
submits the specimens to alternating sinusoidal tension-compression stresses which,
depending on the magnitude of the load applied, cause breakage of the components
within a predetermined number of cycles. The fatigue resistance of the connectors is
expressed as the force level at which 50% of the specimens survive 10<sup>6</sup>load cycles without breakage and 50% fail.</p><fig id="f01" orientation="portrait" position="float"><label>Figure 1</label><caption><p>Schematic drawing of the sample (internal hexagon interface- group C)</p></caption><graphic xlink:href="jaos-19-04-0413-g01"></graphic></fig><sec><title>Restoration analog</title><p>The Micro-unit abutments used were 1-mm-high collar platforms. To allow valid
comparisons with previous data, the restoration analog was 20 mm in length. This
provided a 11.3 mm distance between the midplane of the ball bearing and the
emergence of the implant from the collet. The torque recommended by the
manufacturer was 20 Ncm for the abutment screw and 20 Ncm for the prosthetic
screw. At this torque, however, both screws loosened during the course of the
experiment. It was therefore decided to torque the abutment screw to 30 Ncm and
the prosthetic screw to 25 Ncm to induce failure by screw breakage and not by
screw loosening.</p></sec><sec><title>Experimental procedure and data analysis</title><p>The three implant-abutment interfaces were evaluated regarding their fatigue
resistance at 10<sup>6</sup> cycles (an arbitrarily set number whose theoretical
and practical basis has been previously explained)<sup><xref rid="r33" ref-type="bibr">33</xref></sup>. The experimental procedure required that a
number of specimens be tested in sequence. To this effect, the specimens were
loaded via the ball bearing and spun at 1.000 rpm (16.7 Hz). After
10<sup>6</sup> cycles, the experimenter checked whether the specimen was
intact or whether it had broken. If it was intact, the next specimen was loaded
at the previous magnitude plus 5 N. The same force (5 N) was subtracted from the
former load magnitude if the previous specimen had failed. This leads to the
characteristic up-and-down pattern of run-outs and failures that characterizes
the staircase procedure. After suitable arrangement of the data, the mean
F<sub>50</sub> (at which 50% of the samples failed and 50% ran out) and the
standard deviation were calculated (<xref ref-type="table" rid="t01">Table
1</xref>). When applying the staircase procedure, the examiner must set an
appropriate force increment (or decrement) (F<sub><italic>incr.</italic></sub>)
- 5 N in the present test series. If it is too large, the test looses its
discriminating potential. In this experiment, F<sub><italic>incr</italic></sub>was taken from previous studies<sup><xref rid="r32" ref-type="bibr">32</xref>,<xref rid="r34" ref-type="bibr">34</xref></sup>.</p><table-wrap id="t01" orientation="portrait" position="float"><label>Table 1</label><caption><p>Example of data arrangement for staircase analyses (external hexagon
interface)</p></caption><table frame="hsides" rules="groups"><thead><tr align="center" style="background-color:#dcddde"><th rowspan="1" colspan="1"><bold>Applied forces in newtons</bold></th><th rowspan="1" colspan="1"><bold>Force level <italic>(i)</italic></bold></th><th rowspan="1" colspan="1"><bold># of failures (n<sup>I</sup>)</bold></th><th rowspan="1" colspan="1"><bold>i n<sup>I</sup></bold></th><th rowspan="1" colspan="1"><bold>i<sup>2</sup> n<sup>I</sup></bold></th></tr></thead><tbody><tr align="center"><td rowspan="1" colspan="1">65</td><td rowspan="1" colspan="1">5</td><td rowspan="1" colspan="1">1</td><td rowspan="1" colspan="1">5</td><td rowspan="1" colspan="1">25</td></tr><tr align="center" style="background-color:#dcddde"><td rowspan="1" colspan="1">60</td><td rowspan="1" colspan="1">4</td><td rowspan="1" colspan="1">4</td><td rowspan="1" colspan="1">16</td><td rowspan="1" colspan="1">64</td></tr><tr align="center"><td rowspan="1" colspan="1">55</td><td rowspan="1" colspan="1">3</td><td rowspan="1" colspan="1">6</td><td rowspan="1" colspan="1">18</td><td rowspan="1" colspan="1">54</td></tr><tr align="center" style="background-color:#dcddde"><td rowspan="1" colspan="1">50</td><td rowspan="1" colspan="1">2</td><td rowspan="1" colspan="1">1</td><td rowspan="1" colspan="1">2</td><td rowspan="1" colspan="1">4</td></tr><tr align="center"><td rowspan="1" colspan="1">45</td><td rowspan="1" colspan="1">1</td><td rowspan="1" colspan="1">1</td><td rowspan="1" colspan="1">1</td><td rowspan="1" colspan="1">1</td></tr><tr align="center" style="background-color:#dcddde"><td rowspan="1" colspan="1">40</td><td rowspan="1" colspan="1">0</td><td rowspan="1" colspan="1">0</td><td rowspan="1" colspan="1">0</td><td rowspan="1" colspan="1">0</td></tr><tr align="center"><td rowspan="1" colspan="1"> </td><td rowspan="1" colspan="1"> </td><td rowspan="1" colspan="1">n=13</td><td rowspan="1" colspan="1">A=42</td><td rowspan="1" colspan="1">B=148</td></tr></tbody></table><table-wrap-foot><fn><p>n=Σ n<sup>I</sup> , A=Σ in<sup>I</sup>, B=Σ
i<sup>2</sup>n<sup>I</sup></p></fn></table-wrap-foot></table-wrap><p>During testing, the results were graphically charted as in <xref ref-type="fig" rid="f02">Figure 2</xref>. After all tests had been completed, they were
arranged as shown in <xref ref-type="table" rid="t01">Table 1</xref>. Taking A
and B from <xref ref-type="table" rid="t01">Table 1</xref>, F50 was calculated
as</p><mml:math id="eq01"><mml:mstyle mathcolor="#000000" mathsize="12.0pt" mathvariant="normal"><mml:msub><mml:mi mathvariant="normal">F</mml:mi><mml:mn mathsize="50.0%" mathvariant="normal">50</mml:mn></mml:msub><mml:mo mathvariant="normal">=</mml:mo><mml:msub><mml:mi mathvariant="normal">F</mml:mi><mml:mn mathsize="50.0%" mathvariant="normal">0</mml:mn></mml:msub><mml:msub><mml:mo mathvariant="normal">+</mml:mo><mml:mi mathsize="50.0%" mathvariant="normal">Fincr</mml:mi></mml:msub><mml:mspace width="0.33em"></mml:mspace><mml:mspace width="0.33em"></mml:mspace><mml:mspace width="0.33em"></mml:mspace><mml:mspace width="0.33em"></mml:mspace><mml:mspace width="0.33em"></mml:mspace><mml:mfenced close="]" open="["><mml:mrow><mml:mfrac><mml:mi mathvariant="normal">A</mml:mi><mml:mi mathvariant="italic">n</mml:mi></mml:mfrac><mml:mo mathvariant="normal">±</mml:mo><mml:mfrac><mml:mn mathvariant="normal">1</mml:mn><mml:mn mathvariant="normal">2</mml:mn></mml:mfrac></mml:mrow></mml:mfenced></mml:mstyle></mml:math><fig id="f02" orientation="portrait" position="float"><label>Figure 2</label><caption><p>Staircase data for the implant abutment interfaces analyzed in the
study</p></caption><graphic xlink:href="jaos-19-04-0413-g02"></graphic></fig><p>with: + if the test is based on run-outs, - if the test is based on failures</p><p>Whenever the number of run-outs and failures differed, data analysis was based on
the least frequent event.</p><p>The corresponding standard deviation was taken as:</p><mml:math id="eq02"><mml:mstyle mathcolor="#000000" mathsize="12.0pt" mathvariant="normal"><mml:mn mathvariant="normal">1</mml:mn><mml:mi mathvariant="normal">.</mml:mi><mml:msub><mml:mn mathvariant="normal">62</mml:mn><mml:mi mathsize="50.0%" mathvariant="normal">Fincr</mml:mi></mml:msub><mml:mspace width="0.33em"></mml:mspace><mml:mfenced close="]" open="["><mml:mrow><mml:mfrac><mml:mrow><mml:mi mathvariant="italic">nB</mml:mi><mml:mo mathvariant="italic">−</mml:mo><mml:mi mathvariant="italic">A</mml:mi><mml:mn mathvariant="italic">2</mml:mn></mml:mrow><mml:mrow><mml:mi mathvariant="italic">n</mml:mi><mml:mn mathvariant="italic">2</mml:mn></mml:mrow></mml:mfrac><mml:mo>+</mml:mo><mml:mn>0</mml:mn><mml:mi mathvariant="normal">.</mml:mi><mml:mn>029</mml:mn></mml:mrow></mml:mfenced><mml:mspace width="0.33em"></mml:mspace><mml:mi mathvariant="normal">if</mml:mi><mml:mspace width="0.33em"></mml:mspace><mml:mfrac><mml:mrow><mml:mi mathvariant="italic">nB</mml:mi><mml:mo mathvariant="italic">−</mml:mo><mml:mi mathvariant="italic">A</mml:mi><mml:mn mathvariant="italic">2</mml:mn></mml:mrow><mml:mrow><mml:mi mathvariant="italic">n</mml:mi><mml:mn mathvariant="italic">2</mml:mn></mml:mrow></mml:mfrac><mml:mo mathvariant="normal">≥</mml:mo><mml:mn mathvariant="normal">0</mml:mn><mml:mi mathvariant="normal">.</mml:mi><mml:mn mathvariant="normal">3</mml:mn></mml:mstyle></mml:math><p>and</p><mml:math id="eq03"><mml:mstyle mathcolor="#000000" mathsize="12.0pt" mathvariant="normal"><mml:mn>0</mml:mn><mml:mi mathvariant="normal">.</mml:mi><mml:mn>5</mml:mn><mml:msub><mml:mn>3</mml:mn><mml:mi mathsize="50.0%" mathvariant="normal">Fincr</mml:mi></mml:msub><mml:mspace width="0.33em"></mml:mspace><mml:mfenced close="]" open="["><mml:mrow><mml:mfrac><mml:mrow><mml:mi mathvariant="italic">nB</mml:mi><mml:mo mathvariant="italic">−</mml:mo><mml:mi mathvariant="italic">A</mml:mi><mml:mn mathvariant="italic">2</mml:mn></mml:mrow><mml:mrow><mml:mi mathvariant="italic">n</mml:mi><mml:mn mathvariant="italic">2</mml:mn></mml:mrow></mml:mfrac><mml:mo>+</mml:mo><mml:mn>0</mml:mn><mml:mi mathvariant="normal">.</mml:mi><mml:mn>029</mml:mn></mml:mrow></mml:mfenced><mml:mspace width="0.33em"></mml:mspace><mml:mi mathvariant="normal">if</mml:mi><mml:mspace width="0.33em"></mml:mspace><mml:mfrac><mml:mrow><mml:mi mathvariant="italic">nB</mml:mi><mml:mo mathvariant="italic">−</mml:mo><mml:mi mathvariant="italic">A</mml:mi><mml:mn mathvariant="italic">2</mml:mn></mml:mrow><mml:mrow><mml:mi mathvariant="italic">n</mml:mi><mml:mn mathvariant="italic">2</mml:mn></mml:mrow></mml:mfrac><mml:mo>≤</mml:mo><mml:mn mathvariant="normal">0</mml:mn><mml:mi mathvariant="normal">.</mml:mi><mml:mn mathvariant="normal">3</mml:mn></mml:mstyle></mml:math><p>Where F<sub>50</sub> was the mean force level at which 50% of specimens ran-out
and 50% failed; F<sub>0</sub> was the lowest load level at which failure
occurred; F<sub>incr</sub> was the chosen force increments or decrement, that
is, 5 N; n=Σ n<sup>i</sup> (n<sup>i</sup> : the number of failures of each load
level) (see <xref ref-type="table" rid="t01">table 1</xref>); A=Σ in<sup>i</sup>(i being the load level) and B=Σ i<sup>2</sup>n<sup>i</sup>.</p><p>To assess whether the F<sub>50</sub>'s of each group were significantly
different, the means were fitted with 95% confidence intervals using a method
described by Collins<sup><xref rid="r09" ref-type="bibr">9</xref></sup> (1993).
Means with overlapping intervals were considered equivalent.</p></sec><sec><title>Stereomicroscope examination and scanning electron microscopy (SEM)</title><p>Ten prosthetic screws of each interface were randomly selected and evaluated
using a stereomicroscope (Wild M3Z, Heerbrugg, Switzerland) to inspect thread
wear, defects and the fractured surfaces at low magnification. Stereomicroscopy
is often used to conduct preliminary observations of fractured components. After
this evaluation, 3 samples of fractured screws of each interface were
gold-sputtered and examined with a scanning electron microscope (Philips XL
Series - XL 20; Philips, Eindhoven, The Netherlands).</p></sec></sec><sec sec-type="results"><title>RESULTS</title><p>The fatigue resistance for each connector expressed as the mean force level at which
50% of the samples survived 10<sup>6</sup> cycles and 50% failed (F<sub>50</sub>) is
shown in <xref ref-type="table" rid="t02">Table 2</xref>, which also details each
mean's 95% CI. Statistically, the external hexagon interface presented superior
result compared to the conical and internal hexagon interfaces. There was no
significant difference between the conical and internal hex interfaces.</p><table-wrap id="t02" orientation="portrait" position="float"><label>Table 2</label><caption><p>Fatigue resistance of the connectors subjected to the rotating-bending
test</p></caption><table frame="hsides" rules="groups"><thead><tr align="center" style="background-color:#dcddde"><th rowspan="1" colspan="1"> </th><th rowspan="1" colspan="1"><bold>F50</bold></th><th rowspan="1" colspan="1"><bold>SD</bold></th><th align="left" rowspan="1" colspan="1"><bold>Upper</bold></th><th rowspan="1" colspan="1"><bold>Lower</bold></th></tr></thead><tbody><tr align="center"><td align="left" rowspan="1" colspan="1">External hex interface</td><td rowspan="1" colspan="1">53.5</td><td rowspan="1" colspan="1">7.80</td><td rowspan="1" colspan="1">49.5</td><td rowspan="1" colspan="1">57.5</td></tr><tr align="center" style="background-color:#dcddde"><td align="left" rowspan="1" colspan="1">Cone-in-cone interface</td><td rowspan="1" colspan="1">44.0</td><td rowspan="1" colspan="1">2.49</td><td rowspan="1" colspan="1">42.3</td><td rowspan="1" colspan="1">45.7</td></tr><tr align="center"><td align="left" rowspan="1" colspan="1">Internal hex interface</td><td rowspan="1" colspan="1">45.0</td><td rowspan="1" colspan="1">3.40</td><td rowspan="1" colspan="1">43.1</td><td rowspan="1" colspan="1">46.9</td></tr></tbody></table><table-wrap-foot><fn><p>F<sub>50</sub>=mean failure level (force level at which 50% of the
samples survive and 50% fail before 10<sup>6</sup> cycles). When the
interfaces with overlapping CIs were combined, 1 group was identified:
group A - external hex interface. SD= standard deviation</p></fn></table-wrap-foot></table-wrap><p>Analysis of the fractured screws stereomicroscopy and SEM revealed that the mode and
the region of fracture were the same for 24 of the 30 screws evaluated (fracture of
the threaded part - <xref ref-type="fig" rid="f03">Figures 3</xref>, <xref ref-type="fig" rid="f04">4</xref>). Six screws presented damages in the last two
threads but no fracture (<xref ref-type="fig" rid="f05">Figure 5</xref>). Fatigue
striations were seen on the SEM micrographs (<xref ref-type="fig" rid="f06">Figure
6</xref>). Such striations are an absolute indication of fatigue failure.
"Overload" or fast fracture zone, that is, the portions of the components where
final catastrophic failure occurred were also seen. The surface structure of this
zone was similar for all groups (<xref ref-type="fig" rid="f07">Figures
7A</xref>-<xref ref-type="fig" rid="f07">C</xref>).</p><fig id="f03" orientation="portrait" position="float"><label>Figure 3</label><caption><p>Stereomicroscope image of the prosthetic screw at the fractured surface</p></caption><graphic xlink:href="jaos-19-04-0413-g03"></graphic></fig><fig id="f04" orientation="portrait" position="float"><label>Figure 4</label><caption><p>Scanning electron microscopy (SEM) micrograph of the screw abutment at the
fractured surface</p></caption><graphic xlink:href="jaos-19-04-0413-g04"></graphic></fig><fig id="f05" orientation="portrait" position="float"><label>Figure 5</label><caption><p>Scanning electron microscopy (SEM) micrograph demonstrating damages in the
screw threads without fracture</p></caption><graphic xlink:href="jaos-19-04-0413-g05"></graphic></fig><fig id="f06" orientation="portrait" position="float"><label>Figure 6</label><caption><p>Scanning electron microscopy (SEM) micrograph showing fatigue striations</p></caption><graphic xlink:href="jaos-19-04-0413-g06"></graphic></fig><fig id="f07" orientation="portrait" position="float"><label>Figure 7</label><caption><p>Scanning electron microscopy (SEM) micrograph images of prosthetic screws
showing the same mode of fracture to all the types of implant-abutment
interfaces (acone- in-cone; b- external hexagon and c- internal hexagon)</p></caption><graphic xlink:href="jaos-19-04-0413-g07"></graphic></fig></sec><sec sec-type="discussion"><title>DISCUSSION</title><p>The effect of connector design on the mechanical resistance of a dental implant screw
joint is still fraught with uncertainties. This is demonstrated by the numerous
configurations available in today's market. Several systems are in clinical use,
most notably the external hexagon, the internal hexagon and the tapered joints.
According to Binon<sup><xref rid="r03" ref-type="bibr">3</xref></sup> (2000),
contemporary implant systems are configured with about 20 different implant/abutment
interface geometries.</p><p>Each implant-abutment interface has its pros and cons. According to Maeda, et
al.<sup><xref rid="r16" ref-type="bibr">16</xref></sup> (2006), the external
hex interface has advantages such as suitability for the two stage method, provision
of an anti-rotation mechanism, retrievability and compatibility among different
systems<sup><xref rid="r06" ref-type="bibr">6</xref></sup>. The external hex
interface provides more versatility for the laboratory technician in solving
problems related to emergence profile and esthetics, since the technician is able to
bring the porcelain of a porcelain- fused-to-gold crown closer to the implant
interface<sup><xref rid="r06" ref-type="bibr">6</xref></sup>. However,
increased screw loosening, component fracture, and difficulty in seating abutments
in deep subgingival tissues are problems commonly experienced with external hexagon
connectors<sup><xref rid="r29" ref-type="bibr">29</xref></sup>.</p><p>Regarding the internal hex system, according to Maeda, et al.<sup><xref rid="r16" ref-type="bibr">16</xref></sup> (2006), its advantages are: ease
in abutment connection, suitability for one stage implant installation, higher
stability and suitability for single-tooth restoration, higher resistance to lateral
loads due to the lower centre of rotation and better force distribution<sup><xref rid="r06" ref-type="bibr">6</xref></sup>. A systematic review conducted by
Theoharidou, et al.<sup><xref rid="r31" ref-type="bibr">31</xref></sup> (2008)
demonstrated stable abutment screw connections for internal-connection implants as
well for external-connection implants with improved screw materials (altering the
screw alloys and their surfaces) and preload. Tapered joint connections with a
conical interface have advantages of better sealing capacity in closing the
micro-gap on top of those in an internal hex system. Most <italic>in vitro</italic>studies have demonstrated that internal connections are more stable mechanically
than external flat connections<sup><xref rid="r16" ref-type="bibr">16</xref>,<xref rid="r20" ref-type="bibr">20</xref></sup>. The general focus is clearly on
deep internal connections, in which the screw takes little or no load and provides
intimate contact with the implants walls to resist micromovement<sup><xref rid="r03" ref-type="bibr">3</xref></sup>.</p><p>The present data are in agreement with those of Piermatti, et al.<sup><xref rid="r22" ref-type="bibr">22</xref></sup> (2006), who reported inferior
results for the internal hex connections when compared to external connections.
Steinebrunner, et al.<sup><xref rid="r26" ref-type="bibr">26</xref></sup> (2008)
evaluated the influence of long-term dynamic loading on the fracture strength of
different implant-abutment connectors. External hex connections yielded better
results compared to internal hex connections. Also, the internal tube-and-tube
connections with a cam indexing system obtained the superior results with regard to
longevity and fracture strength. Former research was able to show that there is a
direct correlation between the amount of misfit of the components and screw
loosening<sup><xref rid="r04" ref-type="bibr">4</xref></sup>. Binon and
McHugh<sup><xref rid="r04" ref-type="bibr">4</xref></sup> (1996) pointed
towards manufacturing tolerances as a reason for the screw loosening of the
prefabricated parts and requested manufacturers to improve the fit of their implant
components.</p><p>Preload protects the screw from breakage during cyclic loading. If the joint is
compressed, preload will be lost, the screw and the interface are subjected to
plastic deformation and the joint may separate<sup><xref rid="r35" ref-type="bibr">35</xref></sup>. The optimum preload force recommended for an implant screw
is 60-80% of the yield strength of the material from which the screw is machined. At
stresses at or beyond yield, the screw will function in its plastic deformation zone
with resulting loss of preload and inefficient function. Conversely, stresses within
the elastic region of the material are the most appropriate to resist the separation
forces induced during occlusal loading<sup><xref rid="r21" ref-type="bibr">21</xref></sup>. Thus, the greater the clamping force (preload), the tighter
the clamped joint. However, preload values should not be too high and should be
within the elastic domain, else retaining screws may yield or break under repeated
functional bite forces<sup><xref rid="r01" ref-type="bibr">1</xref></sup>. The
torque used in this study was 30 Ncm in the abutment screw and 25 Ncm in the
prosthetic screw. SEM analysis of screws tightened to 25 Ncm and to 40 Ncm
demonstrated no damages in the screw morphology thereby indicating that the torque
applied was below the elastic limit of the material. The yield strength and the
breakage strength of screws are not commonly reported by manufacturers.</p><p>The literature provides an abundance of studies that analyzed the fatigue resistance
of dental implants and prosthetic components. However, there was no standardization
of the applied forces 300 N<sup><xref rid="r01" ref-type="bibr">1</xref>,<xref rid="r35" ref-type="bibr">35</xref></sup>; 100-150 N<sup><xref rid="r07" ref-type="bibr">7</xref></sup>; 10-250 N<sup><xref rid="r17" ref-type="bibr">17</xref></sup>; 20-200 N<sup><xref rid="r08" ref-type="bibr">8</xref></sup>; 100-450 N<sup><xref rid="r11" ref-type="bibr">11</xref></sup>; 50 N<sup><xref rid="r27" ref-type="bibr">27</xref></sup>; 120
N<sup><xref rid="r26" ref-type="bibr">26</xref></sup> and in the mode of
loading (angle of load application) and simple (fatigue only) or combined (fatigue
plus monotonic load). The loading frequencies were different also. The present study
was carried out using comparable implant diameters, identical abutments and levers
therefore rendering intergroup comparisons possible. The results demonstrated that
the fatigue strength of the external hex interface used in this study was of the
same magnitude as the Nobel Biocare Replace Select, multi-unit abutment and the
Straumann ITI, standard abutment. The internal connections though (conical and
internal hexagon) revealed inferior results compared to the previous data<sup><xref rid="r32" ref-type="bibr">32</xref>,<xref rid="r34" ref-type="bibr">34</xref></sup>.</p><p>The prosthetic screw that connects the fixed dental prosthesis to the abutment is
intended as the weak link, that is, in case of occlusal overload, it is designed to
break first and thus protect the implant and the bone from damage due to excessive
stresses<sup><xref rid="r05" ref-type="bibr">5</xref>,<xref rid="r23" ref-type="bibr">23</xref></sup>. This is supported by the finding that the
incidence of abutment screw and implant fracture is much lower than that of
prosthetic screw loosening or fracturing<sup><xref rid="r05" ref-type="bibr">5</xref>,<xref rid="r19" ref-type="bibr">19</xref></sup>. Conversely,
according to Sutter, et al.<sup><xref rid="r29" ref-type="bibr">29</xref></sup>(1993), in the two-stage system it is the abutment screw that most frequently
fractures. This apparent incrongruity of the more massive abutment screw failing
before the smaller occlusal screw might be explained by simple mechanics. In a
two-stage system, the abutment screw secures the abutment to the implant. This
interface is subjected to a higher level of stress because it is located near the
alveolar crest, that is, where the applied lever is greatest. The abutment screw
therefore, is subjected to much greater forces than the occlusal screw when the
force vectors are nonaxial in nature. It is thus more susceptible to fatigue
failure, although it is a more massive structure<sup><xref rid="r30" ref-type="bibr">30</xref></sup>. The present study confirms that the prosthetic screw fails
more frequently than the abutment screw and failure varies according to the type of
interface analyzed.</p></sec><sec sec-type="conclusions"><title>CONCLUSIONS</title><p>Within the limitation of this study, the following conclusions can be drawn: 1. This
study demonstrated the superior fatigue resistance of external hex interface. There
was no significant difference between the conical and internal hex interfaces.
Probably, the quality of the surface machining of the flat-to-flat mating surfaces
(mainly, the machining accuracy of the screw and thread) determined the superior
resistance of the connector; 2; The mode and region of fracture in prosthetic screws
observed in this study suggested that failure of these screws occurred by fatigue
(presence of fatigue striations) and involved the threaded part; 3. The present
tests demonstrated that the fatigue strength of the external hex interface used in
this study was of comparable strength as that determined in a previous study on
Nobel Biocare and Straumann implants when similar abutments and level torque were
used. It is important to emphasize that the prosthetic screw used in this study was
designed specifically to support a higher torque (25 Ncm) than the conventional
torque used for this screw (10 Ncm). In addition, the micro-unit abutment received a
higher torque than the one recommended by the manufacturer; 4. The internal
connections (cone-in-cone and internal hexagon) had inferior results compared to
those found in previous results. Internal connections require accurate machining and
tolerances and the reason for the present results may be a lack of precision of the
components that allowed micromovement at the connector interface.</p></sec></body><back><ack><title>ACKNOWLEDGMENTS</title><p>Our gratitude is expressed to Conexão Sistemas de Prótese (Arujá, SP, Brazil) for
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