Keyword All-on-Three - implant - prosthesis - FEM analysis - OT-Bridge - MUA
Introduction
Nowadays, one of the main indications for the rehabilitation of edentulous patients,
even with extensive bone atrophy, is represented by fixed implant prosthesis.[1 ]
[2 ]
[3 ] Depending on the quality and quantity of residual bone, the level of atrophy, and
nerves positions, several implant rehabilitations can be performed.[4 ]
[5 ]
[6 ]
[7 ] Due to anatomical risk, the presence of the tongue, and the low quantity of keratinized
tissue, the full-arch rehabilitation of the mandible represents a challenging procedure
for dental surgeons. However, the arch described by the mandible is shorter than that
of the upper arch, allowing the rehabilitation of a fixed implant prosthesis with
four or also three implants. The All-On-Four technique, initially proposed by Maló
et al[8 ] is nowadays used by several clinicians.[9 ]
[10 ]
[11 ] Different connection systems between prosthetic framework and implant fixtures in
screw-retained rehabilitation are today available. For full-arch rehabilitations,
the most used is the Multi-Unit-Abutment (MUA) system, available in a variety of size
and angulations to achieve a passive prosthetic fit even in case of implant disparallelism.[7 ]
[12 ]
[13 ]
[14 ]
A valid alternative to MUA is represented by the OT-Bridge system (Rhein 83 S.R.L.,
Bologna, Italy), recently introduced in the market and also investigated in scientific
studies.[15 ]
[16 ]
[17 ] It consists of a low-profile attachment (OT-Equator), a peculiar cylindrical abutment
with an “Extragrade” region that houses an interchangeable acetal ring (Seeger ring),
allowing a retention with the OT-Equator even in the absence of the tightening screw.[16 ] The OT-Equator attachment has been initially ideated and used only for overdenture,
demonstrating a long-term surveillance on these types of rehabilitation.[18 ]
[19 ]
[20 ]
[21 ] Its use in combination with the Extragrade abutment allows the realization of fixed
prosthesis also on tilted implants with several degree of divergence. Some studies
comparing MUA and OT-Bridge systems has been conducted.[15 ]
[16 ] In all of these, an All-On-Four model was used to evaluate differences between the
two systems in terms of preload loss. The OT-Bridge system was tested also without
one or two screws to understand the possible clinical implication of avoiding the
screw insertion. No significant loss of screw tightening force was detected in comparison
to the MUA system after approximately 1-year of cyclic loadings and the OT-Bridge
system performed well even in absence of one or two prosthetic screws. In a multicenter
study with a 1-year evaluation period, the OT-Bridge system showed successful results
when used to support maxillary fixed dental prosthesis delivered on four to six implants.[21 ]
In the past few years, another prosthetic alternative to All-On-Four has emerged:
the All-On-Three technique.[22 ]
[23 ]
[24 ]
[25 ] This type of rehabilitation, specifically of the mandible, is gaining ground because
of its reduced invasiveness, safety, and predictable results in the short-medium term.[8 ]
[25 ]
[26 ] However, the connection system, the loading conditions, and the composition of the
framework could play an important role in preventing or favoring mechanical complications
such as fractures, deformation of the frameworks, chipping, and wear of the ceramic
coating.[27 ] Then, testing different framework materials and abutment system under different
loading conditions is necessary to understand the behaviour of this possible implant-prosthetic
rehabilitation.
To the best of our knowledge, there are no studies in literature comparing different
framework materials and loading conditions with MUA and OT-Bridge system in an All-On-Three.
Therefore, the aim of this study was to compare the behavior of dimensionally equal
All-On-Three frameworks by finite element method (FEM) analysis in two loading conditions
using two different abutment systems.
Materials and Methods
An epoxy resin model of the mandible was initially scanned by a high precision lab
scanner (Optical RevEng, Open technologies S.R.L., Brescia, Italy) to obtain an STL
file of the model. The mandibular geometry was simplified as a rectangular circular
object by progressively reducing the number of meshes, using a three-dimensional (3D)
mesh processing open-source software program (MeshLab, ISTI, Pisa, Italy).
Three implants (Nobel Parallel, Nobel Biocare, Kloten, Switzerland) were virtually
inserted in the mandible, parallel to each other and perpendicular to the occlusal
plane: one implant (3.75 × 10 mm) was placed near the mandibular symphysis (4.1 position)
and two (4.3 × 10 mm) were positioned at the first left and right molars (3.6 and
4.6). Soft tissues were not considered.
Then, using an appropriate software program (SolidWorks 2018, Dassault Systèmes SolidWorks
Corporation, Vélizy-Villacoublay, France), a prosthetic framework connecting the three
implants was designed. The framework was provided of a rectangular geometry bar with
a constant section of 4.8 × 5.5 mm. Three frameworks with the same design and geometry
but different materials were obtained using ([Table 1 ]):
Table 1
Mechanical characteristics of the materials used
Materials' type
Materials
Young's modulus
Poisson's ratio
E
v
[GPa]
[-]
Cortical bone without cancellous
Linear isotropic elastic
Bone
12.5
0.30
MUA 618 42
Ti Gr5 ELI
105
0.34
Seeger 618 08
POM Kepital F30-03 03
3
0.44
OT-E Profile 113 35
Ti Gr5 ELI
105
0.34
Reinforced resin framework
Trilor Arch Bioloren
26
0.4
Co-Cr framework
Magnum Lucens
194
0.3
TiAl6V4 framework
TiAl6V4
105
0.34
Abbreviations: Co-Cr, cobalt-chromium; MUA, Multi-Unit-Abutment.
Glass fiber-reinforced resin (Trilor Arch, Bioloren S.R.L., Saronno, Italy).
Cobalt-chromium (Co-Cr) (Magnum Lucens, Mesa Italia S.R.L., Travagliato, Italy).
Titanium alloy (Ti6Al4V, Arcam AB, Mölndal, Sweden).
For each material, two frameworks were created: one with the MUA system and the other
with the OT-Bridge system. As a result, the following six models were virtually generated
and tested:
Model with glass fiber-reinforced resin framework bar and the MUA system (M1).
Model with Co-Cr framework bar and the MUA system (M2).
Model with titanium alloy framework bar and the MUA system (M3).
Model with glass fiber-reinforced resin framework bar and the OT-Bridge system (M4).
Model with Co-Cr framework bar and the OT-Bridge system (M5).
Model with titanium alloy framework bar and the OT-Bridge system (M6).
A FEM analysis for all the frameworks, using the ANSYS software (Ansys, Inc., Canonsburg,
Pennsylvania, United States), was carried out. A 3D linear static parametric simulation
was developed, considering the ratio (stress and strain) between bone and prosthetic
components, and implants and OT-Equator attachments. Two different loading conditions
were applied for each FEM:
Perpendicular load of 500 N applied on the framework area between implants 4.1 and
3.6 (first loading condition) ([Fig.1 ]).
Perpendicular load of 400 N applied on a 7-mm distal cantilever to 4.6, to generate
a bending moment of the prosthetic framework (second loading condition) ([Fig.2 ]).
Fig. 1 First loading condition: application of a 500 N perpendicular load on the framework
area between implants in 4.1 and 3.6 positions.
Fig. 2 Second loading condition: application of a 400 N perpendicular load on a 7-mm distal
cantilever to 4.6 position.
Both in the first and second loading conditions, the maximum values were recorded
in terms of tension (MPa) and deformation (mm) in the axial direction at framework,
screw, and abutment level. The mechanical behavior of the different frameworks was
compared as the connection/abutment system changes.
The study was carried out with the hypotheses that linear elastic and isotropic materials,
subjected to a tension, undergo an elastic deformation proportional to the tension
itself according to a proportional factor, the Young's modulus. In the program fixed
conditions were established in terms of linear elastic and isotropic material deformation.
For the elasticity, the materials were subjected to a proportional deformation according
to the Young's modulus of the material. It was also fixed that the material had the
same mechanical and thermal properties in all directions (isotropy).
Results
Results are summarized in [Tables 2 ]
[3 ]
[4 ]
[5 ]. The incidence of each variable on tension and deformation stresses at framework,
screws, and abutment level was analyzed by matching the different results in several
ways. Bar graphs of each comparison are attached as [Supplementary File ].
Table 2
Tensions values ([Mpa] σeq,vm) expressed in the first loading condition for each abutment
system and framework material at framework, screw, and abutment level
Abt system
Framework
Screws
Abt
Value (max)
Area
Value (max)
Area
Value (max)
Area
Resin
MUA (M1)
800
3.6, 4.1, 4.6
500
3.6 thread
650
3.6, 4.6 Cone bone seat
OTP (M4)
200
3.6, 4.6
300
3.6, 4.6 thread
650
3.6, 4.6 Ot_eq and abt
Cr-Co
MUA (M2)
800
3.6, 4.1, 4.6
650
3.6 thread
680
3.6, 4.6 Cone bone seat
OTP (M5)
650
3.6, 4.6
400
3.6, 4.6 thread
500
3.6, 4.6 Ot_eq and abt
TiAl6V4
MUA (M3)
800
3.6, 4.1, 4.6
650
3.6 thread
680
3.6, 4.6 Cone bone seat
OTP (M6)
500
(3.6, 4.6
400
3.6, 4.6 thread and head
600
3.6, 4.6 Ot_eq and abt
Abbreviations: Abt, abutment; Cr-Co, chromium-cobalt; MUA, Multi-Unit-Abutment.
Table 3
Deformation values (mm) in the axial direction (z -axis) expressed in the first loading condition for each abutment system and framework
material at framework, screw, and abutment level
Abt system
Framework
Screws
Abt
Value (max)
Area
Value (max)
Area
Value (max)
Area
Resin
MUA (M1)
1.05
4.1
0.98
4.1
1.04
4.1
OTP (M4)
0.9
4.1
0.88
4.1
0.92
4.1
Cr-Co
MUA (M2)
0.97
4.1
0.91
4.1
0.97
4.1
OTP (M5)
0.88
4.1
0.83
4.1
0.87
4.1
TiAl6V4
MUA (M3)
0.99
4.1
0.93
4.1
0.99
4.1
OTP (M6)
0.89
4.1
0.85
4.1
0.88
4.1
Abbreviations: Abt, abutment; Cr-Co, chromium-cobalt; MUA, Multi-Unit-Abutment.
Table 4
Tensions values ([Mpa] σeq,vm) expressed in the second loading condition for each
abutment system and framework material at framework, screw, and abutment level
Abt system
Framework
Screws
Abt
Value (max)
Area
Value (max)
Area
Value (max)
Area
Resin
MUA (M1)
300
3.6
400
3.6 thread
650
3.6 Cone bone seat
OTP (M4)
200
3.6
200
3.6 thread
650
3.6 Ot_eq
Cr-Co
MUA (M2)
800
3.6
400
3.6 start thread
650
3.6, 4.6 Cone bone seat
OTP (M5)
300
3.6
170
3.6, 4.6 thread
600
3.6 Ot_eq
TiAl6V4
MUA (M3)
800
3.6
400
3.6 start thread
450
3.6 Cone bone seat
OTP (M6)
300
3.6
170
3.6 thread
3.6 Ot_eq
Abbreviations: Abt, abutment; Cr-Co, chromium-cobalt; MUA, Multi-Unit-Abutment.
Table 5
Deformation values (mm) in the axial direction (z -axis) expressed in the first loading condition for each abutment system and framework
material at framework, screw, and abutment level
Abt system
Framework
Screws
Abt
Value (max)
Area
Value (max)
Area
Value (max)
Area
Resin
MUA (M1)
0.21
4.1
0.19
4.1
0.2
4.1
OTP (M4)
0.18
4.1
0.16
4.1
0.17
4.1
Cr-Co
MUA (M2)
0.19
4.1
0.18
4.1
0.19
4.1
OTP (M5)
0.17
4.1
0.16
4.1
0.17
4.1
TiAl6V4
MUA (M3)
0.2
4.1
0.18
4.1
0.2
4.1
OTP (M6)
0.17
4.1
0.16
4.1
0.17
4.1
Abbreviations: Abt, abutment; Cr-Co, chromium-cobalt; MUA, Multi-Unit-Abutment.
Materials
Regarding materials in the first loading condition, glass fiber-reinforced resin with
Co-Cr and titanium decreased, respectively, of 70 and 40% the maximum stress tension
of the framework using OT-Bridge (M4, M5, M6). When the distal cantilever is loaded
(second loading condition), the same material had up to 63% less of maximum tension
values compared to Co-Cr and titanium material in the MUA system (M1, M2, M3). Screws
and abutment tension was not so affected by framework material.
Instead, deformation occurred for all the prosthetic components in all models in the
range of 0.16 to 0.21 mm and resulted always higher for glass fiber-reinforced resin
than metal frameworks.
Abutment System
In most of the cases, changing the connection type from MUA to OT-Bridge reduced tension
values. Notable differences were observed in M1 versus M4 (800 vs. 200 MPa) and with
Co-Cr and titanium at framework level applying the second loading condition (decrease
of 37.5%). Screw tensions comparing the models showed a decrease from 40% in the first
loading condition to 67.5% in the second loading condition. At abutment level, changing
MUA with OT-Bridge slightly decreased the tension values only with Co-Cr (M2 vs. M5)
especially in the first loading condition; a raise of 25% was observed with titanium
(M3 vs. M6) in the second loading condition and was the only case in which tension
values increased when passing from MUA to OT-Bridge. OT-Bridge showed a larger distribution
of tension stresses across the framework, reducing their intensification in small
areas for both the loading conditions.
Changing abutment system slightly influenced the deformation level of the different
prosthetic components. A mild decrease of deformation values was observed when using
OT-Bridge instead of MUA; the maximum value of difference (0.15 mm) was reached at
framework level with M1 versus M4 in the first loading condition.
Loading Conditions
In all the models tested, the first loading condition reached higher tension stress
and deformation values than the second loading condition. By passing from the first
to the second loading condition, tension values decreased, in some cases, up to 62,5%
at abutment (M1) and screw (M5, M6) level. Also, the deformation of the framework
in axial direction (z -axis) decreased by about 80% in the second loading condition for all the materials
and in all the components. However, the deformation of the framework bar was less
uniformly distributed when the load is applied to the distal cantilever, resulting
in higher deformation near the closest abutment similarly to tension distribution.
In addition, in the first loading condition the anterior area of the framework reached
maximum deformation while the distal portion had the lowest; changing loading set
inversed the deformed location.
Discussion
Full-arch implant prosthesis must be planned and fabricated considering the distribution
of forces on implant components. Materials and abutment system affected the survival
rate of implants, prosthesis, and the onset of mechanical complications. Despite all
its inherent limitations,[28 ] FEM analysis represents a valid tool for studying the behaviour of implant-prosthetic
components with different configurations, giving initial evaluation on the feasibility
of a rehabilitation.[29 ]
The purpose of this FEM was to evaluate the mechanical behavior of a fixed prosthesis
anchored on three implants using different materials for the framework bar, different
connection system, and applying two different loading conditions to simulate occlusion.
A mandibular model with all cortical bone and without cancellous was projected to
maximize stresses at implant-prosthetic components, simulating the worst case scenario.[30 ] Perfect passivity between the components was assumed to avoid the appearance of
internal tensions that may confound the analysis.
Materials
Co-Cr and titanium alloy frameworks generate greater tensions that were largely distributed,
involving less framework deformation. Oppositely, despite the lower tensions, the
glass fiber-reinforced resin material tends to concentrate them in a smaller surface,
thus exposing the bar to greater deformations. This behaviour could be explained because
of the material properties and to the rigidity provided by metal alloy. On the contrary,
the lower Young's modulus of resin base framework may lead to a greater absorption
of the load in the area of application without any or little tension distribution.
It is important to also consider the mechanical resistance of these materials, when
planning a framework, to prevent mechanical complications such as chipping, fractures,
and screw loosening under masticatory loads. Because of the lower rigidity and higher
shock absorbance concentration of glass fiber-reinforced resin compared to metal alloys
frameworks, it is important to plan a proper framework dimension when choosing this
material. In addition, it is important to correctly select the material in relation
to the clinical case. A higher framework resistance could be preferable in patients
with higher masticatory forces and smaller loading distribution. Furthermore, for
definitive restorations with ceramic coatings, it may be recommended to select a framework
material that does not expose the bar to deformations in order to reduce chipping
and fracture risks. Proper framework design is necessary in relation to the specific
clinical case, number and position of implants, and to the material used.
Abutment
More concentrated local tensions were recorded and observed with the MUA system. Results
proved that OT-Bridge distributes the acting tensions more uniformly, reducing their
intensification for both the loading conditions. This could be due to the OT-Equator
structural configuration which seems to collect the strength over the head of the
retainer and not only in a single point[31 ] ([Figs. 3 ] and [4 ]).
Fig. 3 Tension distribution at framework area of M2 (cobalt-chromium framework bar and Multi-Unit-Abutment
[MUA] system) in the second loading condition. Tension values are expressed by a colorimetric
scale. The location of minimum and maximum values are indicated.
Fig. 4 Tension distribution at framework area of M4 (glass fiber-reinforced resin framework
bar and OT-Bridge system) in the second loading condition. Tension values are expressed
by a colorimetric scale. The location of minimum and maximum values are indicated.
Another important result is that higher tensions and deformation were reached by MUA
at all levels. This phenomenon could be described by the low profile and overall smaller
size of the OT-Equator attachment which provides the framework of a greater dimensions
and consequently greater mechanical properties. In addition, it is possible that in
the OT-Bridge system, a portion of stress is transmitted to the Seeger ring which
provides the interlocking connection between the implant and the prosthesis. Then,
if an amount of energy is dissipated in this way, especially in case of distal cantilever
loading, a reduction of the transmitted energy happens, decreasing stresses.
Loading Conditions
In the first loading condition, the distribution of 500 N load was extended over a
large area inside the framework arch determining an overall greater stress on both
the connection types (MUA or OT-Bridge). Passing from the first to the second loading
condition (400 N applied on a 7-mm distal cantilever), tension and deformation decreased
for all the tested models. However, the deformation was much more concentrated in
the region where the load is applied near the distal cantilever. This is in relation
to loading application, involving a bending moment of the prosthetic framework with
related tension in all the prosthetic components, especially in the bar region between
the anterior and the distal implant after the first implant prosthetic connection.
Other FEM analysis in literature evaluating framework materials, abutment systems,
and loading conditions can be found. Rubo et al[30 ] found that stress increase proportionally to the increase in cantilever length and
inversely to the increase in the elastic modulus of cancellous bone. Moreover, they
concluded that a stiffer framework may allow better stress distribution, which is
in accordance with our study results. This relation about rigidity and stress distribution
was also highlighted by other studies testing polymeric materials (polyether ether
ketone and polyetherketoneketone) at FEM.[32 ]
Regarding abutment type, rigid abutment design showed to decrease the peak stresses
in the screw and the deflection of the superstructure.[33 ]
[34 ] In addition, in another study, the conical implant connected to a solid, internal,
conical abutment furnishes lower stresses on the alveolar bone and prosthesis and
greater stresses on the abutment compared to a stepped cylinder implant connected
to a screw-retained, internal hexagonal abutment.[35 ] In our study, we focused only on the prosthetic components, but it is important
to underline that the design of implant-abutment interface could affect the surrounding
bone tissue. Conical implant-abutment interface decreased the stress at bone-implant
interface, resulting also in a more apical shear stress transmission compared to the
flat top interface.[36 ]
About the OT-Bridge system, only one FEM was published by Cervino et al[17 ]; however, they focused on the stability of OT Bridge prosthesis in an All-On-Four
mandibular model, concluding that at maximum one abutment can be unscrewed to ensure
an adequate stability of the system. In another FEM evaluating different overdenture
attachments,[31 ] the locator and OT-Equator system was found to offer better stress distribution
compared to the traditional universal abutment. Furthermore, OT-Equator favored a
higher stress on the retainer gum with minor stress located around the peri-implant
bone tissue and fixture. This favorable stress distribution of OT-Equator attachment
was also recognized in our study, where it was able to dissipate stress tensions over
an extended area, differently from MUA.
Concerning forces application, the literature demonstrated that cantilever loading
increased stress proportionally to its length.[30 ] In addition, stresses clustered at the elements closest to the loading point distribution.
This is in accordance with our study where tension stresses in the cantilever loading
condition was spread especially in the portion of the framework near the closest abutment.
Literature provided also evidence that axial and nonaxial occlusal loads influence
both stress distribution on prosthetic components and on the bone remodeling phenomena.[37 ]
[38 ]
Despite the advantages of finite element analysis in the biomedical area,[29 ] a virtual simulation of a clinical condition using a computer software is limited.[39 ] In addition, in this study, only two loading conditions in two different moments
were tested with a unidirectional loading while the occlusion normally produces multidirectional
forces also in simultaneous moments. The framework design and the section of the bar
are also related to the clinical case, and this could affect the mechanical properties
and the distribution of the loading forces. Furthermore, ceramization of the prosthetic
framework may change the pattern and the magnitude of occlusion forces distribution
on framework bar, screws, and abutment. Then, further investigation and studies with
more samples, and possibly in
vivo conditions are required to evaluate the effectiveness of the All-On-Three technique
and of the OT-Bridge system.
Conclusion
Metal alloy materials reduce the framework deformation during loadings because of
the high mechanical properties. The OT-Bridge system raised the mechanical properties
of the framework because of its smaller size and spread tension stresses more uniformly
than MUA. Cantilever loading concentrated stress tensions and deformation in smaller
areas for all the connection systems.
