Shock Absorption Capacity of Restorative Materials for Dental Implant Prostheses: An In Vitro Study

Abstract

Purpose:
To measure the vertical occlusal forces transmitted through crowns made of different restorative materials onto simulated peri-implant bone.

Materials and methods:
The study was conducted using a masticatory robot that is able to reproduce the mandibular movements and forces exerted during mastication. During robot mastication, the forces transmitted onto the simulated peri-implant bone were recorded using nine different restorative materials for the simulated single crown: zirconia, two glass-ceramics, a gold alloy, three composite resins, and two acrylic resins. Three identical sample crowns for each material were used. Each crown was placed under 100 masticatory cycles, occluding with the flat upper surface of the robot to evaluate the vertical forces transmitted. Two-way analysis of variance was used. Alpha was set at .05.

Results:
The statistical evaluation of the force peaks recorded on the vertical z-axis showed mean values of 641.8 N for zirconia; 484.5 N and 344.5 N, respectively, for the two glass-ceramics; 344.8 N for gold alloy; 293.6 N, 236 N, and 187.4 N, respectively, for the three composite resins; and 39.3 N and 28.3 N, respectively, for the two acrylic resins. Significant differences were found between materials (P < .0001), except for the comparison between gold alloy and one of the glass-ceramics.

Conclusion:
Composite and above all acrylic resin crowns were more able to absorb shock from occlusal forces than crowns made of zirconia, ceramic material, or gold alloy.

Full text

Volume 26, Number 6, 2013 549
Implant dentistry has become an increasingly effective
method for correcting edentulism, either partially or
completely. Implant treatments exhibit an overall excel-
lent clinical success rate in the long term.1–4 Despite its
rare occurrence, the reasons for peri-implant bone loss
and implant failure in some patients are not completely
understood. Multifactorial aspects (general health,
bone quality and quantity, surgical procedure, implant
characteristics, parafunctional habits, occlusal over-
loading, medications, bacterial insult, etc) potentially
induce peri-implant bone damage. However, the role
of some of these aspects in reaching and maintaining
osseointegration is controversial.5 Several authors con-
sider occlusal load a crucial factor affecting the dental
implant healing phase and the long-term survival and
success of dental implants.6 –12
In teeth, a semi-elastic connection between the
tooth and bone exists (periodontal tissue), whereas
in implants, a direct and relatively rigid connection
between the bone and implant is achieved if healing
without complications has taken place.13,14 Therefore,
a direct transmission of forces on the peri-implant
bone without any shock-absorbing element is conse-
quent to implant loading.14 It can usually be achieved
by the adaptation capacity of peri-implant bone ar-
chitecture toward changing load conditions.15,16
According to Frost,15,16 within the range of a physi-
ologic loading, bone undergoes its physiologic turn-
over. In mild overloading, below bone’s microdamage
threshold, modeling drifts can begin adding to and/
or reshaping bone. But in the case of a pathologic
overload, bone fractures and bone resorption may oc-
cur.15,16 For these reasons, it appears to be impor tant
to control the forces transmitted on the bone-implant
interface. However, the amount of load defined as
overload has not been quantified because the range
of host physiologic adaptability varies. Overload can
be considered the amount of force that overextends
the host sites adaptation potential.
a
Assistant Professor, Department of Fixed and Implant
Prosthodontics, University of Genoa, Genoa, Italy.
b
Lecturer, Department of Prosthodontics, University of Turin,
Turi n , I ta ly.
c
Lecturer, Department of Health Sciences, Section of Biostatistics,
University of Genoa, Genoa, Italy.
d
Chief and Professor, Department of Fixed and Implant
Prosthodontics, University of Genoa, Genoa, Italy.
Correspondence to: Dr Maria Menini, Department of
Prosthodontics (Pad. 4), Ospedale S. Martino,
L. Rosanna Benzi 10, 16132 Genova, Italy.
Fax: + 39 0103537402. Email: maria.menini@unige.it
©2013 by Quintessence Publishing Co Inc.
Shock Absorption Capacity of Restorative Materials for
Dental Implant Prostheses: An In Vitro Study
Maria Menini, DDS, PhDa/Enrico Conserva, DDSa/ Tiziano Tealdo, DDSa/Marco Bevilacqua, DDSa/
Francesco Pera, DDS, PhDb/Alessio Signori, MScc/Paolo Pera, MD, DDS, PhDd
Purpose: To measure the vertical occlusal forces transmitted through crowns made of
different restorative materials onto simulated peri-implant bone. Materials and Methods:
The study was conducted using a masticatory robot that is able to reproduce the
mandibular movements and forces exerted during mastication. During robot mastication,
the forces transmitted onto the simulated peri-implant bone were recorded using nine
different restorative materials for the simulated single crown: zirconia, two glass-ceramics, a
gold alloy, three composite resins, and two acrylic resins. Three identical sample crowns for
each material were used. Each crown was placed under 100 masticatory cycles, occluding
with the flat upper surface of the robot to evaluate the vertical forces transmitted. Two-way
analysis of variance was used. Alpha was set at .05. Results: The statistical evaluation of
the force peaks recorded on the vertical z-axis showed mean values of 641.8 N for zirconia;
484.5 N and 344.5 N, respectively, for the two glass-ceramics; 344.8 N for gold alloy;
293.6 N, 236 N, and 187.4 N, respectively, for the three composite resins; and 39.3 N and
28.3 N, respectively, for the two acrylic resins. Significant differences were found between
materials (P < .0001), except for the comparison between gold alloy and one of the
glass-ceramics. Conclusion: Composite and above all acrylic resin crowns were more
able to absorb shock from occlusal forces than crowns made of zirconia, ceramic material,
or gold alloy. Int J Prosthodont 2013;26:549–556. doi: 10.11607/ijp.3241
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550 The International Journal of Prosthodontics
Shock Absorption of Restorative Materials
Clinical evidence on the impact of overloading on
peri-implant bone is not available. Only some case re-
ports17–19 and animal studies9,12 , 20 are present. In fact,
clinical trials evaluating overloading are difficult to
design due to ethical reasons. Moreover, it is gener-
ally impossible to identify the reason for peri-implant
bone loss in clinical cases, distinguishing overload-
ing from other potential sources of bone loss. It is the
authors’ opinion that a prudent approach to implant
prosthodontics should be aimed at avoiding the risk
of overloading the implants. In vitro studies21–25 also
demonstrate that off-axial loads increase stress on
the bone-implant interface with respect to axial loads
and may also be responsible for increased resorption
of crestal bone.20
Some authors maintain that the type of material
used for the prosthesis supported by the titanium im-
plant could affect occlusal load.14,26–32 In particular, in
the 1980s, some investigators recommended resilient
occlusal materials such as acrylic resin to reduce the
forces exerted on implants.14,33,34
However, contrasting results on this topic35–38 sug-
gest the need for further investigation. The role of
dental materials in occlusal stress transmission onto
peri-implant bone seems to be especially relevant
over the past few years because of the increasing use
of esthetic but rigid materials, such as glass-ceramic
and zirconia. These materials are reported to have
excellent mechanical and biologic properties,39,40 but
their impact on peri-implant bone and on the whole
masticatory system has not yet been investigated.
The aim of this study was to investigate in vitro the
shock absorption capacity of nine different restor-
ative materials, including both traditional and modern
esthetic materials, using a masticatory robot.
Materials and Methods
A masticatory robot able to simulate human chewing
in vitro was used (Fig 1), reproducing three-dimen-
sionally the masticatory movements and loads ex-
erted during mastication, as described in a previous
paper.26
The movable part of the robot is composed of a
Stewart platform and simulates the mandible. The
fixed upper part of the robot simulates the maxilla.
A sensor-equipped base is placed on the moving
platform and records the degree of force being trans-
mitted through the three axes (x, y, and z).
The sensor-equipped base supports a pin that
simulates the implant-abutment system (Fig 2a).
The samples to be tested are placed on the pin and
stressed in the various directions during the robot’s
mastication.
b
Fig 1 (left) Sensor-equipped masticatory robot.
Fig 2 (below) Pin simulating the implant-abutment system
with a ceramic sample crown. (a) A groove was made on the
pin to match a ridge inside the sample crown, so that the crown
would sit precisely on the pin without any possibility of rotation
or other movement during testing. (b) The sample crown has
been inserted onto the pin.
a
Groove
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Volume 26, Number 6, 2013 551
Menini et al
The materials tested were yttrium cation-doped
tetragonal zirconia polycrystals (Procera Zirconia,
Nobel Biocare), a lithium disilicate pressable ceramic
(Empress 2, Ivoclar Vivadent), a low-fusing leucite-
based pressable ceramic (Finesse, Dentsply), a gold
alloy (Ney-Oro CB, Dentsply), a microfilled hybrid
composite resin (Experience, DEI Italia), a microfilled
composite resin (Adoro, Ivoclar Vivadent), a nano-
hybrid composite resin (Signum, Heraeus Kulzer), and
two acrylic resins (Easytemp 2, DEI Italia and Acry
Plus V, Ruthinium) (Table 1).
In total, 27 identical sample crowns were made
(three for each material). The occlusal surfaces were
semispherical in shape (6.5-mm diameter) (Fig 2b).
The main axis of the sample was 11-mm long. The
sample crowns presented a single contact point at the
center of the occlusal surface when occluding with
the flat maxilla of the robot. At this point, the thickness
of the material tested was 5 mm. Each sample was
measured on its main and smaller axes. The material
thickness at the contact point was also measured with
calipers to verify that all crowns were identical.
The specimens tested were chosen at random and
not in a pre-established sequence. Each crown was
placed under 100 chewing cycles with the sample
crown occluding with the flat fixed maxilla of the ro-
bot. The masticatory robot was programmed to follow
a trajectory reproducing human chewing, as described
in the previous paper.26 The masticator traced this
trajectory in all tests described and the movements
were executed independently from generated force.
Vertical loads (kg) transmitted at the simulated
peri-implant bone were recorded using strain gauges
stuck on the sensorized base supporting the simu-
lated implant-abutment system.
With MATLAB 6.1 (MathWorks), the maximum val-
ues of the forces recorded for each masticatory cycle
were highlighted. These values underwent statistical
analysis using SPSS software (version 18.0, IBM). Two-
way analysis of variance (ANOVA) was used to compare
transmitted stresses between the nine materials tested
and across the three sample crowns of each material.
All tests were two-tailed. Alpha was set at .05.
Post hoc comparisons were assessed by means
of the Scheffe test or, alternatively, by means of the
Tamhane test when homogeneity of variances among
materials was not satisfied.
Vertical loads were converted and are found
throughout the paper in Newtons.
Results
The ANOVA found a significant difference between
the forces transmitted using different materials, and
the Scheffe post hoc test was applied. Within the ma-
terials, an internal comparison showed a significant
difference with P < .0001. Only the difference in mean
maximum force between Ney-Oro and Finesse was
not statistically significant (P > .999).
Comparisons within sample crowns made for each
material did not show significant differences, and one
unique mean was reported for each material.
The force transmitted through the simulated im-
plant onto the simulated peri-implant bone by zirco-
nia (mean 641.8 N) was the greatest ( Table 2).
The slope of the curve, representing the force
transmitted onto the peri-implant level, showed that
materials with greater elastic moduli have steeper
peaks compared with other materials, that is, the
maximum force is reached more rapidly.
Table 1 Elastic Moduli of Tested Materials
Material Manufacturer Type of material Elastic modulus (MPa)
Procera Zirconia Nobel Biocare Zirconia 210,000
Empress 2 Ivoclar Vivadent Glass-ceramic 96,000
Ney-Oro CB Dentsply Gold alloy 77,000
Finesse Dentsply Glass-ceramic 70,000
Experience DEI Italia Composite resin 13,000
Adoro Ivoclar Vivadent Composite resin 7,000 ± 500
Signum Heraeus Kulzer Composite resin 3,500
Easytemp 2 DEI Italia Acrylic resin 2,300
AcryPlus V Ruthinium Acrylic resin N/A
N/A = not available.
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552 The International Journal of Prosthodontics
Shock Absorption of Restorative Materials
Discussion
In this investigation, the use of different restorative
materials significantly affected stress transmission on
the simulated peri-implant bone. In fact, more elastic
materials reduced the stress recorded.
The difference in stress transmission between the
gold alloy and one of the two glass-ceramics was the
only difference not statistically significant, presum-
ably because of their similar Young’s moduli (Table 1).
Zirconia and ceramic crowns also showed steeper
peaks of force compared with other materials. These
were considered effects of the different elastic moduli
of the materials tested.
According to Skalak,14 the viscoelastic behavior of
an acrylic resin as occlusal material would be enough
to delay the transmission of force and reduce its peak
compared with materials with greater elastic moduli.
An in vitro study by Gracis et al32 concluded that
the harder and stiffer the material, the higher the
force transmitted onto the implant and the shorter
the rise time. In fact, according to Hooke’s law, the
higher the modulus of elasticity of a material, the less
the material will deform under pressure and the more
likely the force will be transferred through the mate-
rial.41 Conversely, the more resilient the material, the
more easily it will deform under pressure, the longer
the rise time, and the smaller the stress.
However, a review of the literature over the last
20 years demonstrated that many articles refute the
existence of a shock absorption capacity of resilient
dental materials.42–49
Some of these studies have used Instron ma-
chines48 and some have used finite element analysis
(FEA).44,46,47 These studies have several limitations.
They do not accurately reproduce the mandibular
kinematics. Instron machines perform intermittent
movements in only a single plane. They do not repli-
cate the same masticatory cycle that occurs clinically
with mastication.
With regard to FEA, which included a virtual simu-
lation, the validity of the mathematical model is dif-
ficult to estimate objectively, and the assumptions
made in the use of FEA in implant dentistry must be
taken into account when interpreting the results. In
fact, during the modeling process, several simplifica-
tions are necessary (model geometry, material prop-
erties, applied boundary conditions, etc) and greatly
affect the predictive accuracy of FEA.50
An experiment conducted on beagle dogs51 did
not show any clinical, radiographic, or histologic dif-
ferences between peri-implant tissues surrounding
prosthetic restorations made with composite resin
versus those made with ceramic materials. However,
this study did not control the amount of force exerted
onto the implants, and dogs do not replicate human
mastication.
In vivo studies41,43,49 have measured masticatory
forces transmitted through various restorative mate-
rials in patients without finding significant differences
in the results.
This type of test requires that sensors and connect-
ing wires be applied intraorally, which raises several
concerns. For instance, this type of testing may al-
ter the masticatory cycles of the study participants
and therefore may distort the results. Moreover, the
technique is not conducive to studying humans over
long experimental periods, and the masticatory cycles
are not identical. In addition, it is not possible to di-
rectly measure the forces transmitted onto the bone-
implant interface.
Using the masticatory robot, an attempt was made
to overcome the limitations associated with previ-
ous studies, approximating the three-dimensional
nature of masticatory function by an in vitro model.
The forces were measured by strain gauges attached
to the sensorized base to which the simulated den-
tal implant was screwed; therefore, it was considered
that the forces were recorded at the simulated peri-
implant bone.
Even though non-axial forces seem to be a more
relevant factor for bone maintenance compared with
axial forces, in the present paper, only data regarding
vertical forces have been reported. In fact, previous
papers26,27 showed that the percentage difference
of force using different materials was superimpos-
able on the three axes; data for the three axes were
redundant. For this reason, in the present research,
the sample crowns were left to occlude with a flat
surface and not with the reproduction of the maxilla.
Table 2 Comparison of the Maximum Forces (N)
Transmitted onto the Simulated Peri-implant Bone
Material Mean force (SD)
Difference of force
vs zirconia (%) P
Procera Zirconia 641.8 (6.8)
Empress 2 484.5 (5.5) –24.51
Ney-Oro CB 344.8 (5.7) –46.28
Finesse 344.5 (3.5) –46.32
Experience 293.6 (16.3) –54.25 < .0001
Adoro 236 (4.2) –62.23
Signum 187.4 (6.7) –70.80
Easytemp 2 39.3 (2.3) –93.88
AcryPlus V 28.3 (4.2) –95.59
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Volume 26, Number 6, 2013 553
Menini et al
Occluding with a flat surface, forces on the horizontal
plane were near zero and only data recorded on the
vertical axis were considered for statistical analysis.
The present in vitro setup presents several limita-
tions in simulating the clinical situation. Namely, the
moving platform and the upper part of the robot, sim-
ulating the maxilla, are rigid systems that cannot re-
produce the inherent elasticity of human tissues. The
elastic properties of implant, abutment, and screws
were not properly simulated.
Moreover, no attempt was made to simulate the oral
environment in terms of humidity and temperature.
Comparability of the in vitro and in vivo loading
conditions is limited. Therefore, the absolute values of
force recorded at the peri-implant bone in the present
study cannot be directly correlated to the forces that
would be present in vivo.
It should also be noted that the masticatory system
is provided with protective and self-regulatory mecha-
nisms not simulated in the present in vitro setup. In
fact, natural teeth are equipped with periodontal
mechanoreceptors that signal information about tooth
loads and are involved in the control of human jaw
actions aiming at preventing accidental excessive oc-
clusal loads.52 On the other hand, dental implants lack
periodontal receptors. However, a tactile sensibility at
the level of dental implants (so-called osseopercep-
tion) has been demonstrated and could be responsible
for an implant-mediated sensory-motor control.53
Despite the limits of the present in vitro setup in
simulating the oral implant situation, the attempt was
made to eliminate all possible variables involved. The
standardized in vitro system allowed for fabrication
of identical sample crowns that were all submitted to
identical loading conditions.
A previous paper26 demonstrated that the mas-
ticatory robot is able to reproduce, several times
over, identical masticatory cycles. The paper also
confirmed the precision of the machine during data
collection, therefore validating the reliability of the
method. In fact, the small variations found showed
that the tests are also repeatable and effective under
lengthy testing.
The only variable in the system described was the
material from which the crowns were made, which is
mandatory for a reliable comparison of different mate-
rials. The system was designed to make a comparison
between different materials effective and repeatable.
In the present study, a single crown was tested,
demonstrating a shock absorption potential for acryl-
ic resin. However, contrasting results could be found
using multiunit prostheses.41,44,54,55 In fact, stiff pros-
thetic materials are supposed to distribute the stress
more evenly to the abutments and implants. It is the
authors’ opinion that, in multiunit prostheses, a stiff
substructure (ie, gold alloy) rigidly splinting the im-
plants would be the best option to evenly distribute
loads. The shock absorption capacity of more resil-
ient restorative materials could be used at the lev-
el of the occlusal surface in association with a stiff
substructure.14
The present paper evaluates the shock absorption
capacity of nine restorative materials, including gold
alloy and zirconia, which were not tested in previ-
ous studies.26,27 To the authors’ knowledge, there are
no published studies evaluating the shock absorp-
tion capacity of zirconia. In the last few decades, the
growing patient demand for highly esthetic restora-
tions has led to the development of new all-ceramic
materials such as zirconia.
Zirconia minimizes the dark color transmit-
ted through peri-implant tissues associated with
metal components. Moreover, zirconia restorations
yield higher fracture loads than alumina or lithium
disilicate.56,57
Both the increasing industrial pressure and grow-
ing enthusiasm for attractive esthetic outcomes have
led to the widespread use of all-ceramic restora-
tions and zirconia, even though their impact on the
masticatory system has not been sufficiently tested.
The esthetic characteristics, as well as the biocom-
patibility, and the most common shortcomings of all-
ceramic restorations (brittleness, chipping of the ve-
neering ceramic, fracture strength) have been thor-
oughly investigated for zirconia.40,58 Zirconia is also
considered to have excellent mechanical properties,59
but, so far, the biomechanical consequences of such
a rigid and stiff material in the masticatory system
have not been investigated by the scientific literature.
In fact, zirconia’s elastic modulus and coefficient of
abrasion are much higher than those of natural teeth.
Only a few studies60–62 report assessments of
periodontal or peri-implant tissues around teeth or
implants supporting zirconia restorations after func-
tional loading. To the authors’ knowledge, no clinical
studies report possible consequences at the level of
the antagonist arch or any gnathological consider-
ation. Moreover, to date, the observational period for
the majority of trials on zirconia restorations is quite
short.57
Two systematic reviews on all-ceramic dental
materials and zirconia also underlined the fact that
none of the cited clinical trials took bruxism into ac-
count. More often, such a parafunction figured into
the exclusion criteria. Consequently, the authors
suggested that, since parafunctions were not con-
sidered in any clinical investigation, they should be
regarded as a potential limitation for zirconia-based
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554 The International Journal of Prosthodontics
Shock Absorption of Restorative Materials
restorations.39,6 3 One reason for this suggestion could
be the increased risk of chipping and fracture of
zirconia-based restorations in parafunctional pa-
tients, but evidence is lacking on possible harmful
effects on the masticatory system using zirconia res-
torations when a parafunction is present.
Larsson et al64 noticed that significantly more
porcelain veneer fractures are reported for implant-
supported zirconia fixed dental prostheses when
compared with tooth-supported restorations. One
explanation for this finding could be the role played
by the periodontal ligament, which allows for shock
absorption, sensory function, and tooth movement.
This hypothesis also suggests that the possible harm-
ful effects of zirconia restorations on the masticatory
system would be made worse when dealing with im-
plant-supported restorations in comparison to tooth-
supported restorations. In fact, a shock-absorbing
element is lacking in implant restorations and higher
loads can occur with implant-associated propriocep-
tion loss.52
The choice of the restorative material to be used
in implant restorations should be made in light of
newly introduced concepts of osseosufficiency and
osseoseparation5: as long as the host, the implant,
and the clinical procedures induce and allow for
maintaining osseointegration, an osseosufficiency
state is present. But some patient-related or nonpa-
tient-related factors could induce osseoseparation,
compromising the obtainment or maintenance of
osseointegration. As reported earlier, evidence is
lacking on the role of overloading in peri-implant
bone loss. However, bone has been demonstrated
to be sensitive to loading conditions.65 This suggests
that to control the occlusal loads in implant prosth-
odontics as much as possible, clinicians should aim
to reduce load entity and extra-axial loads. Based on
the present in vitro results, if the aim is reducing load
entity, zirconia is not the proper restorative material
to be used. These findings need to be supported by
clinical trials to investigate their clinical relevance
Conclusion
Within the limitations of this in vitro study, several
conclusions can be drawn. Zirconia, glass-ceramic,
and gold alloy transmitted higher stresses to the
simulated peri-implant bone. In contrast, composite
resin materials were able to significantly reduce the
values of force recorded compared to stiffer materi-
als. In fact, the use of composite resins and acrylic
resins reduced occlusal stress by up to –70.80% and
–95.59%, respectively, compared with zirconia.
Acknowledgments
The construction of the masticatory robot was financed by the
Ministr y of Instruction, Universit y and Research (MIUR), Italy,
under the auspices of the Research of National Interest Project s
(PRIN, 2002). The authors wish to thank Prof Giambattista Ravera
(Depar tment of Health Sciences, University of Genoa) for the sta-
tistical analysis, dental technician Paolo Pagliari for the labora-
tory support, and engineers Giuseppe Casalino, PhD, Fabio Giorgi,
Tommaso Bozzo, and Enrico Simetti (Depar tment of Informatics
of Systems Theory and Telematics, University of Genoa, Italy). The
authors repor ted no conflicts of interest related to this study.
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Literature Abstract
Identification of risk factors for fracture of veneering materials and screw loosening of implant-supported fixed partial
dentures in partially edentulous cases
The purpose of this retrospective study was to determine the risk factors for fracture of veneering materials and screw loosening of
implant-supported fixed partial dentures. A total of 182 patients had 219 suprastructures inserted. One hundred twenty patients (149
facing suprastructures) were included in a subgroup to investigate the risk factors of fracture of veneering materials, and 81 patients
(92 suprastructures) were included in a subgroup to analyze the risk factors for abutment screw loosening. A Cox proportional haz-
ards regression model was performed to identify the risk factors related to technical complications, and eight factors were regarded
as candidate risk factors. It was suggested that a screw-retained suprastructure was a significant risk factor for fracture of veneering
materials, and connection of suprastructures with natural teeth was a significant risk factor for screw loosening. Further investigations
involving dynamic factors, such as occlusal force and bruxism, should be considered as predictors that may be helpful in studying the
risk factors of fracture of veneering materials and screw loosening.
Noda K, Arakawa H , Maekawa K, Hara ES, Yamaz aki S, Kimura- Ono A , Sonoyama W, Minakuchi H, Matsuka Y, Kuboki T. J Oral Rehabil
2013;40:214–220 . Reprints: Takuo Kuboki, Depar tment of Oral Rehabilitation and Re generative Medi cine, Okayama University, Graduate School
of Medicine, Dentistry and Pharmac eutical Scien ces, Okayama, 700 -8525, Japan. Email: kuboki@md.okayama-u.ac.jp—Arthur S. Sham,
Hong Kong
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