Keywords finite element analysis - abutment design - implants - digital image correlation
Introduction
A lost tooth in the esthetic zone can be restored by different approaches such as
implant placement, resin-bonded fixed dental prosthesis, or removable partial dentures.
Dental implants became the primary treatment option to replace a missing tooth since
it is a conservative treatment with a high survival rate.[1 ]
[2 ] However, the long-standing success of implants in the esthetic zone is influenced
by many factors, including implant position, fixture diameter, surgical procedure,
soft tissue stability, abutment design, and restorative materials.[3 ]
[4 ]
[5 ]
Among the implant abutment materials, the titanium (Ti) abutment has shown long-term
stability and biocompatibility.[6 ] However, the esthetic needs of the anterior area dictated the use of metal-free
instead of the Ti one, as the grayish shade of metallic abutment has been noticeable
through the peri-implant soft tissue.[1 ]
[5 ] As an alternative, the zirconia abutment is regularly used because it has strong
mechanical properties,[7 ] long-term stability, and biocompatibility.[8 ]
[9 ] The three mostly common types of zirconia abutments are (a) one-piece prefabricated
abutments manufactured by the implant company, (b) one-piece customized abutment made
by computer-aided design and computer-aided manufacturing (CAD/CAM), and (c) two-piece
hybrid abutment made with a customized CAD/CAM ceramic mesostructure bonded on a prefabricated
Ti base.[10 ]
[11 ] The hybrid abutments were introduced to combine the benefits of having metal abutment/implant
connection and providing an esthetic suprastructure.[12 ]
[13 ] There is a consensus in the literature that the difference in the hardness of the
materials at the implant–abutment connection zone in one-piece zirconia abutment (titanium-zirconia)
leads to excessive wear at implant interface resulting in increase in the misfit and
prosthetic complications.[3 ]
[14 ]
[15 ]
In addition to the abutment material and design, the correct implant position guided
by the upcoming prosthetic restoration location has been emphasized to achieve proper
esthetic and biological outcomes.[16 ] However, the bone morphology in the anterior region of the maxilla may guide the
implant placement in an inclined position. This inclined position can be corrected
prosthetically using angled abutments.[17 ] The consequences of the angled abutment on stress distribution in implant and bone
have been investigated,[17 ]
[18 ]
[19 ]
[20 ]
[21 ]
[22 ] where previous studies stated that angled abutments increased the stress around
the bone and implant.[19 ]
[20 ]
[21 ]
[22 ] However, there is a lack of data concerning angled hybrid abutment in prosthetic
rehabilitation.
Therefore, the aim of this study was to investigate the effect of the two-piece hybrid
zirconia abutment with different angles (axial and 15 degrees) on the stress distribution
of a unitary maxillary implant using digital image correlation (DIC) and finite element
analysis (FEA).
Materials and Methods
Digital Image Correlation
Resin Model Fabrication and Implant Placement
In the present study, the peri-implant tissue for the experimental model was standardized
using polyurethane resin (Polyurethane F160 Axson, Cercy, France) with the following
dimensions: 78 × 45 × 9.13 mm (length, height, and depth), respectively. Two blocks
were designed according to the manufacturer recommendations. After the resin curing,
the polyurethane surfaces were polished with sandpapers (#220–#600) (3M ESPE, St.
Paul, Minnesota, United States) under constant water cooling. The upper surface of
each block was drilled under constant water cooling using the surgical drills according
to the manufacturer recommendations (Conexão Sistemas de Prótese, Arujá, Brazil).
Two different inclinations have been considered. The first block received a straight
implant, maintaining the block and the implant platform parallel to each other. The
second block received a 15-degree inclined implant, in which the drilling was performed
in 15 degrees. To standardize the implant placement angulation, an adjustable surveying
table associated with handpiece was used. The proper drilling burs were connected
vertically to the surveyor arm ([Fig. 1 ]) with fixtures and squares that allowed the angle modification according to the
groups design.
Fig. 1 Drilling of the resin block while the handpiece is mounted vertically to the surveyor
table.
Abutment Fabrication and Cementation
After implant placement, the Ti bases (Conexão Sistemas de Prótese, Arujá, Brazil)
were sandblasted with 50 μm aluminum oxide (Al2 O3 ) and cleaned in an ultrasonic bath (5 minute with isopropyl alcohol). Then the prepared
Ti surface received a layer of universal primer (Monobond N, Ivoclar Vivadent ACT,
Benderstr, Liechtenstein) for 60 seconds. All Ti bases surfaces were gently dried
with air, and the screw access holes were protected with a Teflon tape. Next, according
to the manufacturer's recommendation, two zirconia mesostructures (VITA In-Ceram YZ,
Vita Zhanfabrik, Bad Säckingen, Germany) were milled and sintered. For the inclined
implant, the zirconia mesostructure was designed with 15 degrees of angulation to
correct the implant positioning ([Fig. 2 ]). To cement the zirconia mesostructure to the Ti base and create a hybrid abutment,
the intaglio and external surface of the zirconia mesostructure were sandblasted with
50 μm aluminum oxide (Al2 O3 ) and cleaned in an ultrasonic bath (5 minutes with isopropyl alcohol). Then, a universal
primer (Monobond N, Ivoclar Vivadent ACT, Benderstr, Liechtenstein) was applied for
60 seconds and gently dried with air to remove any excess. After surface treatment,
a dual cure resin cement (Multilink N system pack; Ivoclar Vivadent ACT, Benderstr,
Liechtenstein) was manipulated and applied on the Ti base and mesostructure intaglio
surface, which was seated in position under a constant load of 0.8 kg. After that,
the cement excess was removed and light cured for 20 second using a LED light curing
device (Bluephase N, Ivoclar Vivadent AG, Schaan, Liechtenstein). Next, the lithium
disilicate all-ceramic crown (IPS e.max CAD, Ivoclar Vivadent AG, Schaan, Liechtenstein)
were fabricated crystallized and polished. The crown's intaglio surface was etched
by hydrofluoric acid gel 5% (IPS Ceramic Etching Gel, Ivoclar Vivadent, Schaan, Lichtenstein)
for 20 seconds, washed with water, air dried and received the universal primer (Monobond
N, Ivoclar Vivadent ACT, Benderstr, Liechtenstein) for 60 seconds. Finally, the crown
was cemented with a resinous cement (Multilink N system pack; Ivoclar Vivadent ACT,
Benderstr, Liechtenstein) following the manufacturer recommendations. The crown fabrication
was similar for both conditions.
Fig. 2 (A ) Lateral view: angled zirconia mesostructure after the cementation on titanium base.
(B ) Buccal view: lithium disilicate crown during cementation and photopolymerization.
(C ) Crown cemented on the angled hybrid abutment after excess cement removal.
Compressive Load Application and Image Correlation
The DIC technique (in vitro) was performed to measure the deformation generated on
the surface of the polyurethane block under compressive load application.[23 ] A professional camera (Canon EOS Rebel T5 with Tamron 90 mm f/2.8 SP VC AF Macro-Lens)
was used for capturing the image sequence. The camera had a resolution of 18.00 megapixels.
Before the loading, the surface of the resin model facing the camera lens was individualized
applying a fine layer of white spray. Then, a black spray was used to produce irregular-shaped
speckles to track the image correlation analysis during the surface displacement.
A compressive non-impact progressive (0.1 mm/min) load of 150 N was applied using
a universal testing machine (DL-1000, EMIC, São José dos Campos, Brazil). When the
specimen was subjected to the loading, a special software package (Gom correlate,
Vtech Consulting Ltda, São Paulo, Brazil) was used for image analysis.
To measure the generated deformation (mm) on the block surface, sequential images
were taken at a frequency of 1 Hz until the maximum load was reached. The first image
was taken without loading, and the other photos were compared with the first image
to calculate the displacements on the surface. The deformation was calculated from
the displacement using image correlation software (GOM Correlate, Braunschweig, Germany).
Before the image processing, the surface quality has been verified ([Fig. 3 ]).[22 ]
[23 ]
[24 ]
[25 ]
Fig. 3 (A ) Isotropic substrate with prepared surface to perform the digital image correlation.
(B ) Measurement of the surface component quality prior to the calculation of the deformation
results.
Finite Element Analysis
Three-dimensional numerical models based on the in vitro setup used in the DIC were
modeled using CAD software (Rhinoceros version 4.0; McNeel North America, Seattle,
Washington, United States). The models were then exported to computer-aided engineering
software (Ansys Workbench 19.0 Ansys Inc., Canonsburg, Pennsylvania, United States)
for the finite element simulation (in silico). The mechanical properties (Young's
modulus and Poisson's ratio) of the simulated materials are summarized in [Table 1 ].[24 ]
[25 ]
Table 1
Mechanical properties of the materials used in the computational analysis
Material/Structure
Elastic modulus (MPa)
Poisson's ratio
Titanium
110,000
0.30
Polyurethane
3,600
0.30
Resin cement
7,000
0.45
Zirconia
210,000
0.33
Lithium disilicate
89,000
0.31
All materials were considered elastic, isotropic, and homogeneous. The loading configuration
followed the same for the in vitro analysis with a compressive load of 150 N. The
contacts were set as bonded. A static linear structural analysis was performed. Both
FEA models had similar mesh densities (0.3 mm each element) to increase the consistency
and accuracy, with higher node density in the cervical region of the implant (mesh
convergence test of 10%). The three-dimensional models contained a total of 434,228
nodes with 211,236 quadratic elements and 448,334 nodes with 218,102 quadratic elements
for the straight and angled model, respectively. The buccal surface of the polyurethane
block has been measured according to the direction of deformation (vertical displacement),
and the results were compared with the in vitro measurement.[23 ]
[26 ] After comparison of coherence between the experimental (DIC) and virtual results
(FEA), the von-Mises stress of the implants, Ti base, and fixation screw were calculated
to predict failure region from these ductile solids.[23 ] In addition, the Maximum Principal Stress in the mesostructure and crown were calculated
for both models.[12 ]
Results
The overall surface deformation distributions determined by both techniques can be
considered similar, as illustrated in [Fig. 4 ]. As expected, higher deformation was found in the cervical region of the peri-implant
tissue, in the central region with a higher magnitude for the angled abutment model.
Both methods showed similar trends for the deformation; however, the FEA models predicted
higher deformation magnitude (±14.6%) in comparison to DIC ([Table 2 ]).
Fig. 4 Planar deformation measured in the vertical direction. (A ) Axial implant with digital image correlation (DIC) results, (B ) axial implant with finite element analysis (FEA) results, (C ) nonaxial implant with DIC results, (D ) nonaxial implant with FEA results.
Table 2
Total deformation (mm) between both methods
DIC
FEA
Straight
Angled
Straight
Angled
0.01
0.152
0.0116
0.173
Abbreviations: DIC, digital image correlation; FEA, finite element analysis.
The magnitude over the entire region of interest as determined by both methods (DIC
and FEA) for the simulated conditions is presented in [Table 3 ]. In this sense, the FEA results are assumed as validated. Considering the stress
fields in the structure of the implant, von-Mises stress revealed higher stress concentration
in the implant with a nonaxial position ([Fig. 5 ]). This same behavior can be observed in the Ti base ([Fig. 6 ]), screw ([Fig. 7 ]), and crown ([Fig. 8 ]).
Fig. 5 von-Mises stress concentration in the implants. (A and B ) Axial implant and (C and D ) nonaxial implant.
Fig. 6 von-Mises stress concentration in the titanium bases. (A and B ) Axial implant and (C and D ) nonaxial implant.
Fig. 7 von-Mises stress concentration in the screw. (A and B ) Axial implant and (C and D ) nonaxial implant.
Fig. 8 Maximum principal stress concentration in the ceramic structures. (A and B ) Axial implant and (C and D ) nonaxial implant.
Table 3
von-Mises stress peak (MPa) for the analyzed structures for both designs
Structure
Straight
Angled
Implant
38.7
62.9
Titanium base
26.6
71.4
Fixation screw
27.8
69.8
Discussion
Several bioengineering tools have been applied to evaluate the biomechanical behavior
of implant supported prosthesis and surrounding tissue such as FEA, DIC, strain gauge
measurement, and photoelastic analysis.[20 ]
[23 ]
[27 ]
[28 ] However, every method presents limitations and there is no unique method that can
achieve all the requirements for completely displaying the biomechanical behavior
of an object subjected to a load.[29 ] For that reason, the present study applied FEA and DIC to evaluate the influence
of straight and angled hybrid abutments on the stress distribution of implant-supported
restoration.
FEA is a numerical method applied to calculate the stress concentration within a simulated
model when subjected to load.[29 ] The advantages of FEA compared with other methods are the low cost, specimens standardization,
ability to simulate complex scenarios, and predict the areas that might undergo failure.[30 ]
DIC is an optical method used to analyze the strain distribution on the surface of
an object during load application.[23 ] Unlike strain gauge tests, which are restricted to detect strains only at the contact
area, DIC is a contactless test that provides full field for the strain analysis.[26 ] Therefore, it is a preferable method to verify and validate FEA models.[29 ]
Both methodologies showed that the nonaxial implant restored with angled hybrid abutments
presented higher deformation compared with axial implant restored with a straight
abutment. Although the FEA models predicted higher deformation magnitude (±14.6%)
in comparison to DIC, this difference is considered acceptable and might be attributed
to the inaccuracy in the mechanical material properties provided by the manufacturer[23 ]
[26 ]
[31 ] and the numerical models simplifications. Thus, the similarity of the results between
the in vitro test (DIC) and the in silico test (FE) guarantees a mutual validation
of the technique and favors the recognition of internal stress results obtained by
the FEA that cannot be observed with DIC.[23 ]
[26 ]
Implant failure are related to biological and mechanical complications. Nowadays,
implant failures are predominantly related to mechanical complications rather than
biological. Implant failure related to mechanical complications includes screw loosening,
fracture of the screw, micromovements, abutment fracture, and fixture fracture.[32 ]
[33 ] Considering the abutment material, prefabricated zirconia abutment led to plastic
deformation at the Ti implant/abutment connection area[3 ]
[34 ]
[35 ]
[36 ]
[37 ] due to zirconia higher modulus of elasticity than Ti. Thus, the deformation will
be concentrated in the implant surface.[37 ] Lower fracture strength of one-piece prefabricated zirconia abutment when compared
with hybrid abutment is another problem that has been reported.[38 ]
[39 ]
[40 ] Therefore, hybrid abutment has gained popularity as they maintain stability at the
implant/abutment connection area and provide highly esthetic superstructure.[12 ]
[13 ]
[41 ]
The function of the abutment screw is to be tightened to keep a stable union between
the implant and abutment. However, screw loosening is a critical mechanical complication
that leads to micromovement and might end up with fracture.[32 ] The higher amount of stress due to the off-axis load on the inclined implant suggests
a higher chance for prosthetic complications such as screw loosening.[42 ]
[43 ]
[44 ] Screw loosening can be explained by the present results which showed that the stress
peak was located at the screw threads.
Regarding the results obtained in both tests, the high magnitude of deformation in
the cervical area of the bone around the inclined abutment model is in accordance
with the previous finite element studies.[18 ]
[19 ]
[45 ] On the other hand, Saab et al[14 ] found that strain and stresses on the bone around implant restored with angled abutment
were similar in comparison with the straight one. Tian et al[15 ] stated that inclined implants not placed in the ideal position and restored with
angled abutment can show reduced strain on the surrounding bone. Although FEA is a
beneficial bioengineering tool that aids in analyzing stress on the implant body,
prosthetic parts, and surrounding bone, it was recommended to use more than one bioengineering
tool or methodology to allow validated results.[42 ]
[46 ]
The present study analyzed only two-piece hybrid zirconia abutment with different
angulation. Other abutment designs such as one-piece abutment with different angulations
may present different results under loading. In this study, the load was applied on
the incisal edge to keep the piston in a stable position during the in vitro load
application. This load scenario represents the edge-to-edge occlusal relation that
is not the common clinical scenario and must be considered as a study limitation.
In addition, the present study considered only one specimen mechanical behavior for
the model validation, similar to other engineering procedures to validate biomedical,[47 ] airplanes[48 ] and even previous dental implants finite element models.[23 ]
[26 ]
[42 ] Therefore, the present method is not considering the standard deviation from the
different samples variation as conventional in vitro test for mean comparison between
groups, but performing the results interpretation and conclusion in a valid numerical
model.[49 ] Moreover, the simulated bone was designed as a resinous isotropic structure rather
than cortical and cancellous, which does not represent the clinical situation. Therefore,
further studies are warranted, taking into account the described limitations.
Conclusion
Based on this study, using angled hybrid abutment to correct the implant inclination
generated higher stress in the implant fixture, surrounding tissue, Ti base, screw,
and crown. Therefore, the implant should be positioned axially, whenever possible,
to reduce the mechanical complications.
