Mechanical Testing of Implant-Supported Anterior Crowns

Mechanical Testing of Implant-Supported Anterior Crowns
with Different Implant/Abutment Connections
Erika O. Almeida, DDS, MSc1/Amilcar C. Freitas Jr, DDS, MSc1/
Estevam A. Bonfante, DDS, MS, PhD2/Leonard Marotta, CDT, MDT, PhD 3/
Nelson R.F.A. Silva, DDS, MS, PhD4/Paulo G. Coelho, DDS, BS, MS, MSMtE, PhD3,5
Purpose: This study evaluated the reliability and failure modes of anterior implants with internal-hexagon (IH),
external-hexagon (EH), or Morse taper (MT) implant-abutment interface designs. The postulated hypothesis
was that the different implant-abutment connections would result in different reliability and failure modes
when subjected to step-stress accelerated life testing (SSALT) in water. Materials and Methods: Sixty-three
dental implants (4 × 10 mm) were divided into three groups (n = 21 each) according to connection type: EH,
IH, or MT. Commercially pure titanium abutments were screwed to the implants, and standardized maxillary
central incisor metallic crowns were cemented and subjected to SSALT in water. The probability of failure
versus number of cycles (95% two-sided confidence intervals) was calculated and plotted using a power-law
relationship for damage accumulation. Reliability for a mission of 50,000 cycles at 150 N (90% two-sided
confidence intervals) was calculated. Polarized-light and scanning electron microscopes were used for failure
analyses. Results: The beta values (confidence intervals) derived from use-level probability Weibull calculation
were 3.34 (2.22 to 5.00), 1.72 (1.14 to 2.58), and 1.05 (0.60 to 1.83) for groups EH, IH, and MT, respectively,
indicating that fatigue was an accelerating factor for all groups. Reliability was significantly different between
groups: 99% for MT, 96% for IH, and 31% for EH. Failure modes differed; EH presented abutment screw
fracture, IH showed abutment screw and implant fractures, and MT displayed abutment and abutment screw
bending or fracture. Conclusions: The postulated hypothesis that different implant-abutment connections to
support anterior single-unit replacements would result in different reliability and failure modes when subjected
to SSALT was accepted. Int J Oral Maxillofac Implants 2013;28:103–108. doi: 10.11607/jomi.2443
Key words: cementation, dental implants, fractography, implant-supported prostheses, reliability, step-stress
accelerated life testing
S
ingle-tooth replacement involving osseointegrated
implants is among the most popular and successful
treatment options, presenting 89.4% estimated survival
1Research
Scholar, Department of Biomaterials and
Biomimetics, New York University College of Dentistry, New
York, New York, USA.
2 Assistant Professor, Postgraduate Program in Dentistry,
UNIGRANRIO University, Duque de Caxias, RJ, Brazil.
3Assistant Professor, Department of Biomaterials and
Biomimetics, New York University College of Dentistry, New
York, New York, USA.
4 Assistant Professor, Department of Prosthodontics,
Department of Restorative Dentistry, Federal University of
Minas Gerais, UFMG, Belo Horizonte, MG, Brazil.
5Director for Research, Department of Periodontology and
Implant Dentistry, New York University College of Dentistry,
New York, NY, USA
Correspondence to: Dr Erika O. Almeida, 345 E. 24th Street,
Room 804S, New York, NY 10010, USA. Fax: +212-995-4244.
Email: [email protected]
©2013 by Quintessence Publishing Co Inc.
rates after 10 years in function.1 However, despite the
high success rates reported for dental implant treatment,2 mechanical complications in the prosthetic
components, such as loosening and/or fracture of the
abutment screws, have been reported and remain under investigation.3–5 Once osseointegration has been
achieved and maintained, the stability of the implantabutment connection becomes a key factor for the
success of the restoration, especially in single-tooth
replacements, where the incidence of mechanical complications is highest.6 Therefore, failures should be explored to gain insight into the mechanical behavior of
different implant-abutment connection configurations,
since they may compromise function and quality of
life.7,8
From a biomechanical point of view, one major concern among different implant-abutment connection
designs is the external force exerted on components
via oral function, which is concentrated mainly on the
abutment screw, especially in external-hexagon (EH)
connections.9,10 In contrast, internal-hexagon (IH) connections have been claimed to be more mechanically
The International Journal of Oral & Maxillofacial Implants 103
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Almeida et al
a
b
c
Fig 1 Implant connection configurations tested. (a) EH, (b) IH, (c) MT.
stable, since the load is distributed deep within the
implant, where engagement with a long internal wall
shields the abutment screw.10–12 High rates of mechanical complications (6% to 48%) have been reported for
EH implants,13–15 whereas fewer episodes of abutment
loosening (3.6% to 5.3%) seem to occur in Morse taper
(MT) connections.16
The maintenance and stability of screw-type connections are challenged by forces exceeding that of
the torqued abutment/crown system.17 However,
lower physiologic forces, applied repeatedly, although
they do not necessarily surpass the failure threshold of
the assembly, may potentially lead to gradual loosening of the implant-abutment connection and/or failure as a result of fatigue.5,18,19 The ensuing misfit of a
loose abutment connected to an implant may not only
impose taxing stresses in the crestal bone, potentially
leading to resorption,20,21 but it may also serve as a
bacterial reservoir, contributing to peri-implant tissue
inflammation.22
Laboratory fatigue studies have shown improved
mechanical behavior for prostheses supported by
implants with an internal connection versus implants
with external connections.5,23 A specific evaluation
comparing EH and IH connections to MT connections
was performed in a finite element investigation; the
results showed the lowest stress concentration in the
abutment screw of the MT connection.10 Whereas that
study provided insight about the stress concentrations
of different implant/abutment configurations, the
clinical significance of the presence of stresses per se
requires further validation by mechanical testing, such
as fatigue. Because comparisons have frequently been
made between different implant systems comprising
different implant geometry, an evaluation of the mechanical behavior and failure patterns of EH, IH, and
MT connections within the same implant system is
warranted.
The aim of this study was to evaluate the reliability and failure modes of implant-supported anterior
crowns restored with different implant connections
(IH, EH, and MT). The postulated hypothesis was that
different implant connections used as anterior singleunit replacements would show different rates of reliability and different failure modes when subjected to
step-stress accelerated life testing (SSALT).
MATERIALS AND METHODS
Sample Preparation
Sixty-three commercially pure titanium dental implants (4 mm in diameter, 10 mm in length, Emfils, Colosso Evolution System) were divided into three groups
(n = 21) according to the implant/abutment connection design: EH (HEE Evolution; Fig 1a), IH (ECI Evolution;
Fig 1b), and MT (Fig 1c). All implants were vertically embedded in acrylic resin (Orthoresin, Degudent) that had
been poured into a 25-mm-diameter plastic tube. The
top platform was left at the level of the resin surface.
A maxillary central incisor crown was waxed into an
anatomical shape and cast in cobalt-chromium alloy
(BEGO). To reproduce the anatomy of the crowns, an
impression was taken from the first waxed pattern and
used by the technician as a guide during waxing of the
remaining crowns. The respective prefabricated abutments (commercially pure titanium; Emfils, Colosso
Evolution System) were tightened with a torque gauge
according to the manufacturer’s instructions (30 Ncm)
using titanium alloy abutment screws (titanium-aluminum-vanadium). Following connection of the corresponding abutments to the implants, the cementation
surface of the crowns was blasted with aluminum oxide
(particle size ≤ 40 µm; 276 KPa of compressed air pressure), cleaned with ethanol, dried with air free of water
and oil, and then cemented (RelyX Unicem, 3M ESPE).
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Almeida et al
Table 1 Calculated Reliability for a Protocol of 50,000 Cycles at a Load of 150 N
Output
EH
IH
MT
Upper
0.4650
0.9912
1.0000
Reliability
0.3187*
0.9687**
0.9994***
Lower
0.1812
0.8917
0.9922
Beta
3.34 (2.22–5.00)
1.72 (1.14–2.58)
1.05 (0.60–1.83)
Different numbers of asterisks denote statistically significant differences.
Mechanical Testing and Reliability Analysis
For mechanical testing, the specimens were subjected
to 30-degree off-axis loading. Three specimens of each
group underwent single-load-to-failure (SLF) testing at a
crosshead speed of 1 mm/min in a universal testing machine (INSTRON 5666); a flat tungsten carbide indenter
was used to apply the load at the incisal edge of the
crown. Based upon the mean values from SLF testing,
three SSALT profiles were determined for the remaining 18 specimens of each group, which were assigned
to mild (n = 9), moderate (n = 6), or aggressive (n = 3)
fatigue profiles (ratio 3:2:1, respectively).24–26 These profiles were named based on the stepwise load increase by
which they were fatigued along the cycles. Hence, in the
mild profile, a specimen was cycled longer to reach the
same load level as that allocated in the moderate profile and even longer in the aggressive profile. The data
collected during testing at different stress profile levels
were used to estimate the parameter that best fit the
data by means of commonly used life distributions, such
as log normal, exponential, or Weibull.
The prescribed fatigue method was SSALT under
water at 9 Hz with a servo–all-electric system (Test­
Resources 800L); the indenter contacted the incisal
edge, applied the prescribed load within the step
profile, and lifted off the incisal edge. Fatigue testing
was performed until failure (bending or fracture of the
fixation screw and/or bending, partial fracture, or total
fracture of the abutment) or survival (no failure at the
end of step-stress profiles, where maximum loads were
up to 600 N).27–29
Use-level probability Weibull curves (probability of
failure versus number of cycles) with a power law relationship for damage accumulation were calculated
(Alta Pro 7, Reliasoft).30 Reliability (the probability of an
item functioning for a given amount of time without
failure) for completion of a mission of 50,000 cycles at a
load of 150 N (90% two-sided confidence bounds) was
calculated for group comparisons.
Failure Analysis
The failed samples were visually inspected and classified according to the proposed failure criteria (bending, fracture) for comparisons between groups. To
identify fractographic markings and characterize the
origin of failure and direction of propagation, the
most representative failed samples of each group were
inspected first under a polarized light microscope
(MZ-APO stereomicroscope, Carl Zeiss MicroImaging)
and then by scanning electron microscopy (SEM)
(Model S-3500N, Hitachi).26
RESULTS
SLF and Reliability
The SLF values (means ± standard deviations) were
468 ± 97 N for IH, 486 ± 51 N for EH, and 759 ± 52 N
for MT.
The beta value means (confidence intervals) and
associated upper and lower bounds derived from uselevel probability Weibull calculation (probability of failure vs number of cycles) were 3.34 (2.22 to 5.00), 1.72
(1.14 to 2.58), and 1.05 (0.60 to 1.83) for groups EH, IH,
and MT, respectively. These indicate that fatigue was
an accelerating factor in all groups (Table 1).
The step-stress–derived probability Weibull plots
and summary statistics at a 150-N load are presented
in Fig 2 and Table 1, respectively. The SSALT permitted
estimates of reliability at a given load level (Table 1).
The calculated reliability with 90% confidence intervals
for a mission of 50,000 cycles at 150 N showed that the
cumulative damage from loads reaching 150 N would
lead to implant-supported restoration survival of only
31% of EH assemblies, whereas 96% of the IH group
specimens and 99% of the MT specimens would survive. The lack of overlap between the upper and lower
limits of reliability values emphasizes the statistically
significant differences between groups; the MT group
presented the highest reliability, the EH the lowest, and
IH was positioned intermediate to the other groups.
Failure Modes
All specimens failed after SSALT. When component
failures were evaluated together, failures comprised a
combination of abutment screw bending or fracture,
abutment bending or fracture, and implant fracture.
Observed failure modes are described in Table 2.
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Almeida et al
99
EH*
Data points
Use level line
Top CB-I
Bottom CB-I
Probability of failure
90
IH*
Data points
Use level line
Top CB-I
Bottom CB-I
50
MT*
Data points
Use level line
Top CB-I
Bottom CB-I
10
5
1
100
Fig 2 Use-level probability Weibull
for tested groups showing the probability of failure as a function of number of cycles (time) given a mission of
50,000 cycles at 150 N. Use level CB
at 90% two-sided. *Cumulative damage Weibull = 175; F = 18, S = 0.
1,000
1.000E+10
10,000
1.000E+6
1.000E+8
100,000
1.000E+7
1.000E+9
No. of cycles
Table 2 Modes of Failure of Screws, Abutments, and Implants in Each Group
Test type
EH
IH
SLF
(n = 3)
Abutment screw: 1 bent, 2 fractured
Abutment: 3 intact
Implant: 3 intact
Abutment screw: 1 bent, 2 fractured
Abutment: 3 intact
Implant: 3 intact
Abutment screw: 3 bent
Abutment: 3 bent
Implant: 3 intact
SSALT
(n = 18)
Abutment screw: 18 fractured
Abutment: 18 intact
Implant: 18 intact
Abutment screw: 18 fractured
Abutment: 18 intact
Implant: 15 fractured, 3 intact
Abutment screw: 9 fractured, 9 bent
Abutment: 9 fractured, 9 bent
Implant: 18 intact
a
b
MT
c
Fig 3 SEM images of the fractured surfaces of the abutments and abutment screws together (a) EH, (b) IH, (c) MT. The white
arrows indicate compression curls, which indicate the direction of crack propagation (black arrow) and evidence of fracture origin at
the opposing tensile side (yellow arrows).
For the EH group, most failures involved abutment
screw fractures, whereas the abutments and implants remained intact after mechanical testing. For the IH group,
abutment screw and implant fractures were the chief
failure modes. This group was the only one that showed
longitudinal fractures in the implant neck area. Similarly
to the EH group, the abutments were also intact. In the
MT group, abutment and abutment screw bending or
fracture were the main failure modes. All abutments and
their screws fractured or bent, whereas the implants remained intact in this group (Fig 3, Table 2).
Observation of the polarized-light and SEM images
of the fractured surfaces of the abutment screws allowed the consistent identification of fractographic
markings (eg, compression curl), fracture origins, and
the direction of crack propagation (Fig 3).
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DISCUSSION
Because of the relevance of the mechanical characterization and failure patterns of different implant-abutment configurations in clinical practice, the present
study used SSALT in single-unit anterior crowns cemented to implants with EH, IH, and MT connections
to gain insight into the reliability of each interface. The
use of accelerated life testing in dental research has
been reported for several prosthetic restorative systems, where close similarity with the failures observed
clinically could be simulated with this methodology
in the laboratory.31–34 Its use in testing a variety of
implant-abutment systems and scenarios has also been
reported.35–38 The rationale is to reduce testing time by
causing the samples to fail more quickly, yet with relevant failure mechanisms compared to SLF tests. The
MT group presented the highest reliability, followed
by IH and EH. These findings are in agreement with
previous fatigue studies, which observed improved fatigue behavior for IH relative to EH connections.5,23 The
stress magnitude was the highest in abutment screws
of EH, followed by IH and MT connections10; therefore
the present findings of fracture in all EH and IH abutment screws but only half of the MT abutment screws
are in agreement with Pessoa et al10 and suggest the
relevance of the potential outcome (fracture) of such
stresses when translated to laboratory fatigue testing.
The reported cumulative incidence of screw or
abutment loosening in single-unit implant-supported
crowns with internal (whether IH or MT) connections
is 12.7% after 5 years of clinical service.39 Specific to
the MT connection, a recent prospective evaluation
of more than 2,500 implant-supported prostheses in
function for up to 6 years reported a very different incidence of abutment loosening of 0.37% for single-unit
crowns.40 It has been suggested that, in the MT connection, lateral loading is resisted mainly by the tapered
design, which hinders abutment micromovement. In
addition, a high pressure is normally maintained in the
matching area because of the tapered design, allowing maintenance of implant position through frictional
forces.18
Implant fracture was only observed in the IH group.
Despite this clinically negative failure scenario, significantly higher loads for failure initiation were observed
for this group compared to EH, which explains the significantly higher reliability for IH. One potential reason
for such failure behavior occurring under a worst-case
loading scenario, as simulated in the present study, is
the minimum wall thickness demanded for this system, which resulted in enlargement of the implant’s
coronal border during fatigue.41 On the other hand,
in the MT connection, locking and friction have been
reported as stabilization mechanisms, referred to as
positive or geometric locking, and are assumed to be
responsible for protecting the abutment threads from
excessive functional load.18
The analysis of the failed implant components allowed the identification of fractographic markings
and the tracing of the fracture origin and the direction
of crack propagation. The ductile fractures, which occurred as a result of stresses surpassing the material’s
yield or flow stress,42 showed a consistent crack pathway from lingual to buccal, where forces naturally occur and as was simulated in the present study.
The restoration of single-unit anterior spaces with
implant-supported crowns is a challenging scenario in
terms of longevity and esthetics. With regard to mechanical testing, the MT implant-abutment connection
presented the highest reliability in maxillary central
incisor replacements, followed by IH and EH connections. Although EH implants have been used successfully for splinted full-arch rehabilitations, it seems that
they should be avoided in the anterior maxilla because
of potential long-term complications.
CONCLUSIONS
The postulated hypothesis that implants with different
abutment connections used as anterior single-unit replacements would show different reliability and failure
modes when subjected to step-stress accelerated life
testing was accepted. The Morse taper implant connection showed the highest reliability, followed by the
internal hexagon and the external hexagon.
ACKNOWLEDGMENTS
The authors are thankful to Emfils, Colosso Evolution System
(SP, Brazil), and Marotta Dental Studio (Farmingdale, New York)
for their support. The authors reported no conflicts of interest
related to this study.
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