Journal of Oral Implantology Photoelastic analysis on different retention methods of implant-supported prosthesis. --Manuscript Draft-Manuscript Number:
AAID-JOI-D-12-00200R3
Full Title:
Photoelastic analysis on different retention methods of implant-supported prosthesis.
Short Title:
Photoelastic analysis of stresses on implants.
Article Type:
Research
Keywords:
dental implants; suprastructure; stress; prosthesis
Corresponding Author:
Angelica Castro Pimentel, Ph.D. University of Santo Amaro Sao Paulo, Sao Paulo BRAZIL
Corresponding Author Secondary Information: Corresponding Author's Institution:
University of Santo Amaro
Corresponding Author's Secondary Institution: First Author:
Angelica Castro Pimentel, Ph.D.
First Author Secondary Information: Order of Authors:
Angelica Castro Pimentel, Ph.D. Marcello Roberto Manzi, School of Dentistry Cristiane Ibanhês Polo, School of Dentistry Claudio Luiz Sendyk, Doctor in Prosthodontics Maria da Graça Naclério- Homem, Full Professor in Implant Dentistry Wilson Roberto Sendyk, Full Professor in Implant Dentistry
Order of Authors Secondary Information: Abstract:
The aim of this study was to evaluate the stress distribution of different retention systems (screwed, cemented and mixed) in 5-unit implant-supported fixed partial dentures, through photoelasticity method. Twenty standardized titanium suprastructures were manufactured, of which five were screw retained, five were cement retained and 10 were mixed (with an alternating sequence of abutments), each supported by five external hexagon (4.0 × 11.5 mm) implants. A circular polariscope was used, and an axial compressive load of 100 N was applied on a universal testing machine. The results were photographed and qualitatively analyzed. We observed the formation of isochromatic fringes as a result of the stresses generated around the implant after installation of the different suprastructures and after the application of a compressive axial load of 100 N. We conclude that a lack of passive adaptation was observed in all suprastructures with the formation of low-magnitude stress in some implants. When cemented and mixed suprastructures were subjected to a compressive load, they displayed lower levels of stress distribution and lower intensity fringes compared to the screwed prosthesis.
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Photoelastic analysis on different retention methods of implant-supported prosthesis Angélica Castro Pimentel* Marcello Roberto Manzi** Cristiane Ibanhês Polo*** Claudio Luiz Sendyk **** Maria da Graça Naclério-Homem***** Wilson Roberto Sendyk ****** Corresponding author Angélica Castro Pimentel Endereço: Rua Waldemar Carlos Pereira, 1798 Vila Matilde –São Paulo-SP Cep 03533-002 Brazil E-mail: [email protected] Tel.: (0xx11) 27850766 or (0xx11) 99494-6035 Contributing authors Angélica Castro Pimentel* Professor at the Department of Implantology at the University of Santo Amaro, São Paulo, Brazil. Marcello Roberto Manzi** School of Dentistry, University of São Paulo (USP), São Paulo, SP, Brazil , Department of Oral and Maxillofacial Surgery and Traumatology. Cristiane Ibanhes Polo*** School of Dentistry, University of São Paulo (USP), São Paulo, SP, Brazil , Department of Oral and Maxillofacial Surgery and Traumatology. Cláudio Luiz Sendyk**** Doctor in Prosthodontics from the University of São Paulo (USP) – São Paulo, Brazil Maria da Graça Naclério-Homem***** Full Professor at the University of São Paulo (USP) - São Paulo, Brazil. Wilson Roberto Sendyk****** Full Professor at the State University of Campinas (UNICAMP). Professor and Coordinator of the Graduate Program in Implant Dentistry at the University of Santo Amaro (UNISA) São Paulo, Brazil.
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Photoelastic analysis on different retention methods of implant-supported prosthesis
45 46
ABSTRACT
47
The aim of this study was to evaluate the stress distribution of different retention systems
48
(screwed, cemented and mixed) in 5-unit implant-supported fixed partial dentures, through
49
photoelasticity method. Twenty standardized titanium suprastructures were manufactured,
50
of which five were screw retained, five were cement retained and 10 were mixed (with an
51
alternating sequence of abutments), each supported by five external hexagon (4.0 × 11.5
52
mm) implants. A circular polariscope was used, and an axial compressive load of 100 N
53
was applied on a universal testing machine. The results were photographed and
54
qualitatively analyzed. We observed the formation of isochromatic fringes as a result of the
55
stresses generated around the implant after installation of the different suprastructures and
56
after the application of a compressive axial load of 100 N. We conclude that a lack of
57
passive adaptation was observed in all suprastructures with the formation of low-magnitude
58
stress in some implants. When cemented and mixed suprastructures were subjected to a
59
compressive load, they displayed lower levels of stress distribution and lower intensity
60
fringes compared to the screwed prosthesis.
61 62 63 64 65
Key Words - dental implants; suprastructure; stress; prosthesis.
66
INTRODUCTION
67 68
The biomechanical behavior of prosthetic dental implants, especially related to the
69
passive adaptation and the stress generation on the implant and surrounding tissues, has
70
received attention from several researchers.1,2 The choice between screwed or cemented
71
prosthesis retention has strongly influenced the final treatment planning because it directly
72
affects the force transmitted to the components and the bone-implant interface.3,4 The
73
dental literature is plenty of papers that state the superiority of one technique over the
74
other.5,6 The main advantage of screwed prostheses is related to their reversibility.
75
Aesthetics and occlusion are highlighted as advantages of cemented retention, in addition to
76
the simplicity of manufacturing and reduced component costs.7,8
77
Photoelastic analysis has been widely used in dentistry to study the distribution of stresses
78
around partially removable9 or fixed10 prosthesis and osseointegrated implants.11,12 This
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technique enables a two-dimensional view, providing information on the magnitude and
80
concentration of stress.13 Experimental papers evaluated different implant designs14,15 and
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others studies compared the screwed and cemented retention systems through
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photoelasticity. However, despite the great number of researches focusing on stress
83
distribution with different prosthetic retention systems, there is no study mixing different
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types of prosthetic retention systems16,17. Programmed stress tests were used in this study
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to determine the transfer of stress by screwed, cemented and mixed suprastructures to
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implants with the goal of evaluating the passivity and stress distribution of fixed partial
87
prostheses, on each type of suprastructure.
88
Hypothesis: Do the mixed prosthesis have different stress distribution compared the
89
screwed and cemented prosthetic retention systems? The aim of this study was to evaluate
90
the stress distribution of different retention systems (screwed, cemented and mixed) in 5-
91
unit implant-supported fixed partial dentures through photoelasticity.
92 93
MATERIALS AND METHODS
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A preliminary plaster model was made that measured 25 mm high, 35 mm long and
95
12 mm thick. A motor coupled to a parallelometer was used to drill five holes that were 15
96
mm deep with a 4.35-mm drill (Conexão Sistema de Prótese Ltda, São Paulo, Brazil). Five
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external hex implant analogs (Conexão Sistema de Prótese Ltda, São Paulo, Brazil) were
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inserted into these holes with cyanoacrylate and type IV plaster. The analogs were parallel
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and equidistant, with a 7-mm center-to-center separation distance. A layer of colorless
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enamel (Colorama, Cosbra Cosmetics, São Paulo, Brazil) was applied to the plaster, which
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provided a smooth and polished surface. This construction was then used as a final working
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model and served as the standard for preparing the photoelastic model.
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To obtain the photoelastic model, the plaster model was duplicated using photoelastic resin
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PL-2 (Measurement Group Inc., Raleigh, NC, USA) in a beaker by mixing resin and
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hardener at a 1:1 ratio (50 ml each) and manipulated using a plastic tube. A lid was made
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with pink number 07 wax (Epoxiglas, Diadema, Brazil). Then, the plaster model with
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analogs was connected to the molding components and fastened to the wax lid inside of a
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plastic tube, such that the model was fixed (i.e., did not move) inside the tube. Laboratory
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silicone for model duplication (Silicone Master, Talladium Inc., Valencia, CA, USA) was
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poured inside each tube until the silicone completely filled the plaster model and the
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components connected to the analog moldings. After 2 h (the silicone setting time), the wax
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lid was removed from the plastic tube, and the plaster model was removed from the silicone
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mold.
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After obtaining the mold, five external hex implants, each with a diameter of 3.75
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mm and a height of 11.5 mm (Conexão Sistema de Prótese Ltda, São Paulo, Brazil), were
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set. Then, the PL-2 photoelastic resin (Measurement Group Inc.) was stirred with a glass
117
rod in a circular motion for 5 min to obtain a mixture with a homogenous color. The
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container was then placed in a vacuum chamber (Fast Vac, JB, Brazil) for 30 min to
119
eliminate air from the resin. At this point, the resin appeared translucent with a yellowish
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color, and it was poured into the mold, thereby covering all of the threads of the implants
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and mimicking the biological condition of osseointegration, to obtain a photoelastic model
122
with implants. This assembly was placed in a vacuum chamber to remove air bubbles for
123
30 min and allowed to rest for 48 h in a cool, ventilated area for complete polymerization
124
of the photoelastic resin.
125
The plaster model was used to make 20 suprastructures, i.e., five in each of the
126
following groups: 1) screwed prosthesis, 2) cemented prosthesis, 3) mixed prosthesis 1, and
127
4) mixed prosthesis 2.
128 129
1) Screwed prosthesis:
130
To prepare the screwed prostheses, five estheticone abutments (Conexão Sistema de
131
Prótese Ltda, São Paulo, Brazil) with a brace height of 2 mm were positioned on the
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analogs in the plaster model. The abutments were fastened by applying 20 Ncm of torque to
133
each with a manual torque wrench (Conexão Sistema de Prótese Ltda, São Paulo, Brazil).
134
Five titanium copings without an anti-rotational system were placed on the abutments with
135
a torque of 10N using a manual torque wrench. To form a titanium suprastructure, the
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copings were joined by laser welding using a Nd:YAG (neodymium-doped:yttrium
137
aluminum garnet) laser (Deka, Calenzano, Italy) with an average power of 30 W and a
138
continuous pulse of 0.5 Hz lasting 0.5 ms.
139 140
2) Cemented prosthesis
141
To prepare the cemented prostheses, five straight abutments (Conexão Sistema de
142
Prótese Ltda, São Paulo, Brazil) were positioned on the analogs attached to the plaster
143
model. These abutments had a brace height of 1 mm and were fastened by applying a
144
torque of 20 Ncm using a manual torque wrench. The abutment suprastructure was waxed,
145
thereby maintaining the insertion axis of the screws.
146
The suprastructure was then cast and polished with grade 1 titanium (Tritan, Dentaurum,
147
Ispringen, Germany) according to the standard methodology. This suprastructure was
148
separated into four sections using a carborundum disc and rejoined with laser welding.
149 150
3) Mixed prosthesis 1
151
To prepare the type 1 mixed prostheses, alternating straight abutments (Conexão
152
Sistema de Prótese Ltda, São Paulo, Brazil) and estheticone abutments (Conexão Sistema
153
de Prótese Ltda, São Paulo, Brazil) were positioned on the analogs, which were attached to
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the plaster model with screw fittings. Three straight abutments with a brace height of 1 mm
155
and two estheticone abutments with a brace height of 2 mm were applied with a torque of
156
20 Ncm using a manual torque wrench. The straight abutments were waxed, thereby
157
preserving the insertion axis of the screws.
158
The suprastructure was then cast and polished with grade 1 titanium according to the
159
standard methodology. Then, the castings were welded to the titanium copings of the
160
estheticone abutments, which formed a suprastructure that began and ended with straight
161
abutments.
162 163
4) Mixed prosthesis 2
164
To prepare the type 2 mixed prostheses, the same methodology was applied as for
165
the type 1 mixed prostheses, but the number and sequence of the abutments was changed.
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Three estheticone abutments with a brace height of 1 mm and two straight abutments with a
167
brace height of 2 mm were fitted with a torque of 20 Ncm using a manual torque wrench.
168
The straight abutments were waxed, thereby preserving the insertion axis of the screws,
169
which formed an suprastructure that began and ended with estheticone abutments. The
170
alternative of changing the type and amount of abutment would be to check the influence of
171
the stress distribution. The manufactured mixed suprastructure began and ended with
172
estheticone abutments. Internal adjustments were made to the cemented and mixed (type 1
173
and 2) suprastructure using silicone fluid (Oranwash L, Zermack, Rovigo, Italy) and a high-
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speed carbide bur (Extra Torque 605 C, Kavo, Biberach, Germany) to facilitate the flow of
175
cement and obtain a better prosthesis fitting.
176 177
Before installation of the abutments, an initial photoelastic assessment of the models
178
was made by placing the models in a container (Campestre, São B. Campo, Brazil) filled
179
with mineral oil to improve the sharpness of the images obtained with a circular
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polariscope. This device is capable of providing a dark field using polarized lens with
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parallel axis. Thus, it is possible to observe the isochromatic fringes, which have colorful
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patterns and show the stress intensity. Isocline fringes are also visible, which are dark lines
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superimposed on the colored fringes and represent the direction of the stress. After this
184
analysis, the model was considered free of residual stresses and ready to begin testing. To
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tighten the abutment screws, a torque wrench (Conexão Sistema de Prótese Ltda, São
186
Paulo, Brazil) was used by following the manufacturer's recommendations. A torque of 10
187
Ncm was applied to the screws copings, as recommended by the manufacturer. In the
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cemented and mixed (type 1 and 2) suprastructure, the temporary cement Temp Bond NE
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(Kerr Corporation, Orange, CA, USA) was used on the straight abutments because it allows
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the removal of the structures when necessary without changing the position of the
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abutments.
192
Through the photoelastic analysis, it was possible to visualize the stresses at the resin-
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implant interface at two moments: 1) during the installation of the suprastructures and 2)
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after applying a vertical compressive load of 100 N on a universal testing machine
195
(Mecmesin, Slinford, England) at a central point of the suprastructures. Images were
196
recorded with a digital camcorder, FD-717 (Sony, Foster City, CA, USA). The selected
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images were qualitatively assessed in terms of the direction of propagation and the intensity
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of the stress. A greater number of fringes indicate a greater magnitude of stress, and fringes
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that are closer to each other indicate a greater concentration of stress on each region of the
200
test block.18
201 202
After each test, the models underwent a heat treatment to eliminate any residual stress, which consisted of immersion in water at 55ºC for 5 min.19
203 204
RESULTS
205
In the present study, we observed the formation of isochromatic fringes resulting
206
from stresses generated around the implants after installation of different suprastructure and
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after applying an axial load of 100 N. Among the images obtained, one from each structure
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group was selected based on image quality criteria. All images were evaluated by the same
209
operator.
210
Figure 1a to 1d show the lower number fringes (black colour) generated in the
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model after the installation of the suprastructure. In all of the suprastructure, generalized
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stresses were observed that dissipated from the apex to the region of the middle third of
213
some implants, which characterized them as lacking passivity.
214
After applying a compressive load of 100 N at the midpoint of the suprastructure
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(Figure 2a to 2d), there was an increase in the number of fringes that showed the presence
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of tensions in the photoelastic model (red and green colours). Considering the number of
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fringes of high intensity, the largest area of stress concentration between the implants
218
occurred in the apical and cervical regions, with the greatest intensity in the screwed
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suprastructure, followed by the mixed 2, mixed 1 and cemented suprastructure in
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descending order.
221 222 223
224
DISCUSSION
225
Most studies of the biomechanical behavior of implants under occlusal loads are
226
performed using finite element analysis20,21 or occlusal testing,22,23 where it is possible to
227
quantify the generation of stresses transferred to the implants. Adding to these works,
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photoelastic analysis12,13 is proposed by the present study as a methodology to qualify how
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the transmission of stress occurs in partially fixed prosthetic implants. Photoelastic analysis
230
permitted a comparison of the distribution of stresses induced by different partially fixed
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prosthetics (screwed, cemented and mixed) with five elements in regions adjacent to the
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implants during installation and after applying a vertical compressive load of 100 N. The
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photoelastic method demonstrated the advantage of providing direct visual information
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about the pattern of stresses that occurred throughout the model after applying loads, which
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allowed the magnitude of the stress to be located and qualified.18 Although photoelastic
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models are used in dentistry to assess stress, they are limited with regard to predicting
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biological responses to specific loads because this technique does not quantify the stress
238
thresholds of the various bone types.24
239
The methods of retention, such as cement, screws and combinations thereof, provide
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discrete stress levels at the time of suprastructure installation, even after being internally
241
adjusted with silicone fluid. The four types of implanted fixed partial prosthetics did not
242
show a passive fit and produced stresses of low magnitude in some implants which was
243
likely related to laboratory inaccuracies, the fabrication model and the molds. 25,26 The lack
244
of a passive fit of the prostheses generates significant stresses on the implants.27 According
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to some authors, a complete passive suprastructure is not possible through laboratory,
246
clinical and conventional procedures. The methods for assessing fit often do not show
247
inaccuracies, and a more sensitive technique for measuring stresses should be used through
248
the application of technological resources to obtain a clinically acceptable level of passivity
249
that does not cause any aesthetic, biological or functional problems for the patient.28,29,30
250
In this study, the greatest intensity and concentration of tension after the application
251
of compressive load of 100 N.cm was observed in the screwed suprastructure, followed by
252
the mixed 2, mixed 1 and cemented respectively. We believe that the presence of cement
253
may have been a favorable factor in lower dissipation of tension between the cemented
254
prostheses and mixed 2, since the presence of cement absorbs an amount of load and stress
255
that would be transferred to the implants.
256
The splinting of fixed partial prosthetics is another interesting point in this study
257
because the stress distribution occurred in all implants but not equally, probably due to the
258
different retention systems. These findings corroborates another literature reports about
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stress distribution in prostheses subjected to occlusal forces, where tensions are higher in
260
screwed prostheses and smaller in the cemented ones22, probably owing to the hypotheses
261
mentioned above.
262
Comparing the mixed supraestruturas it was observed a higher concentration of
263
tension in the cervical and apical region in suprastructure mixed 2, probably related to the
264
amount of screwed abutments.
265
Although we do not know precisely the levels of loads that may be harmful to
266
osseointegration,31,32 it is recommended careful control of axial or extra-axial loads through
267
the prosthetic planning. However, clinically, the bone is strongest under compressive
268
forces, weaker under tensile loads, and even weaker yet in shear loads.
269
The combined retention between the cement and screw systems generated less stress
270
than the screwed prosthesis, making it a good rehabilitative choice. The use of cement also
271
provided a degree of tolerance to poor adjustment, retrievability and the ease of repair with
272
the screw.33,34
273
The mixed and cemented prostheses displayed lower levels of stress distribution and
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a lower fringe intensity than the screwed implants when subjected to compressive loads in
275
both the cervical and apical regions of the implants, but it was not clear whether these
276
differences were related to the retention system.
277 278
ACKNOWLEDGEMENTS
279
We thank all who have contributed directly and indirectly to this work. My sincere thanks.
280 281
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FIGURE CAPTIONS
379 380 381
Figure 1 – Demonstration of the results after installation of the suprastructures (passive
382
fit). A. Screwed suprastructures; B. cemented suprastructures; C. mixed suprastructures 1;
383
and D. mixed suprastructures 2.
384 385
Figure 2 - Demonstration of the results observed after application of a 100-N compressive
386
load. A. Load application on screwed suprastructures; B. load application on cemented
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suprastructures; C. application of mixed load 1; and D. application of mixed load 2.
388 389 390 391 392 393
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Figure 2 Click here to download high resolution image
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AAID-JOI-D-12-00200R3
Full Title:
Photoelastic analysis on different retention methods of implant-supported prosthesis.
Short Title:
Photoelastic analysis of stresses on implants.
Article Type:
Research
Keywords:
dental implants; suprastructure; stress; prosthesis
Corresponding Author:
Angelica Castro Pimentel, Ph.D. University of Santo Amaro Sao Paulo, Sao Paulo BRAZIL
Corresponding Author Secondary Information: Corresponding Author's Institution:
University of Santo Amaro
Corresponding Author's Secondary Institution: First Author:
Angelica Castro Pimentel, Ph.D.
First Author Secondary Information: Order of Authors:
Angelica Castro Pimentel, Ph.D. Marcello Roberto Manzi, School of Dentistry Cristiane Ibanhês Polo, School of Dentistry Claudio Luiz Sendyk, Doctor in Prosthodontics Maria da Graça Naclério- Homem, Full Professor in Implant Dentistry Wilson Roberto Sendyk, Full Professor in Implant Dentistry
Order of Authors Secondary Information: Abstract:
The aim of this study was to evaluate the stress distribution of different retention systems (screwed, cemented and mixed) in 5-unit implant-supported fixed partial dentures, through photoelasticity method. Twenty standardized titanium suprastructures were manufactured, of which five were screw retained, five were cement retained and 10 were mixed (with an alternating sequence of abutments), each supported by five external hexagon (4.0 × 11.5 mm) implants. A circular polariscope was used, and an axial compressive load of 100 N was applied on a universal testing machine. The results were photographed and qualitatively analyzed. We observed the formation of isochromatic fringes as a result of the stresses generated around the implant after installation of the different suprastructures and after the application of a compressive axial load of 100 N. We conclude that a lack of passive adaptation was observed in all suprastructures with the formation of low-magnitude stress in some implants. When cemented and mixed suprastructures were subjected to a compressive load, they displayed lower levels of stress distribution and lower intensity fringes compared to the screwed prosthesis.
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Photoelastic analysis on different retention methods of implant-supported prosthesis Angélica Castro Pimentel* Marcello Roberto Manzi** Cristiane Ibanhês Polo*** Claudio Luiz Sendyk **** Maria da Graça Naclério-Homem***** Wilson Roberto Sendyk ****** Corresponding author Angélica Castro Pimentel Endereço: Rua Waldemar Carlos Pereira, 1798 Vila Matilde –São Paulo-SP Cep 03533-002 Brazil E-mail: [email protected] Tel.: (0xx11) 27850766 or (0xx11) 99494-6035 Contributing authors Angélica Castro Pimentel* Professor at the Department of Implantology at the University of Santo Amaro, São Paulo, Brazil. Marcello Roberto Manzi** School of Dentistry, University of São Paulo (USP), São Paulo, SP, Brazil , Department of Oral and Maxillofacial Surgery and Traumatology. Cristiane Ibanhes Polo*** School of Dentistry, University of São Paulo (USP), São Paulo, SP, Brazil , Department of Oral and Maxillofacial Surgery and Traumatology. Cláudio Luiz Sendyk**** Doctor in Prosthodontics from the University of São Paulo (USP) – São Paulo, Brazil Maria da Graça Naclério-Homem***** Full Professor at the University of São Paulo (USP) - São Paulo, Brazil. Wilson Roberto Sendyk****** Full Professor at the State University of Campinas (UNICAMP). Professor and Coordinator of the Graduate Program in Implant Dentistry at the University of Santo Amaro (UNISA) São Paulo, Brazil.
44
Photoelastic analysis on different retention methods of implant-supported prosthesis
45 46
ABSTRACT
47
The aim of this study was to evaluate the stress distribution of different retention systems
48
(screwed, cemented and mixed) in 5-unit implant-supported fixed partial dentures, through
49
photoelasticity method. Twenty standardized titanium suprastructures were manufactured,
50
of which five were screw retained, five were cement retained and 10 were mixed (with an
51
alternating sequence of abutments), each supported by five external hexagon (4.0 × 11.5
52
mm) implants. A circular polariscope was used, and an axial compressive load of 100 N
53
was applied on a universal testing machine. The results were photographed and
54
qualitatively analyzed. We observed the formation of isochromatic fringes as a result of the
55
stresses generated around the implant after installation of the different suprastructures and
56
after the application of a compressive axial load of 100 N. We conclude that a lack of
57
passive adaptation was observed in all suprastructures with the formation of low-magnitude
58
stress in some implants. When cemented and mixed suprastructures were subjected to a
59
compressive load, they displayed lower levels of stress distribution and lower intensity
60
fringes compared to the screwed prosthesis.
61 62 63 64 65
Key Words - dental implants; suprastructure; stress; prosthesis.
66
INTRODUCTION
67 68
The biomechanical behavior of prosthetic dental implants, especially related to the
69
passive adaptation and the stress generation on the implant and surrounding tissues, has
70
received attention from several researchers.1,2 The choice between screwed or cemented
71
prosthesis retention has strongly influenced the final treatment planning because it directly
72
affects the force transmitted to the components and the bone-implant interface.3,4 The
73
dental literature is plenty of papers that state the superiority of one technique over the
74
other.5,6 The main advantage of screwed prostheses is related to their reversibility.
75
Aesthetics and occlusion are highlighted as advantages of cemented retention, in addition to
76
the simplicity of manufacturing and reduced component costs.7,8
77
Photoelastic analysis has been widely used in dentistry to study the distribution of stresses
78
around partially removable9 or fixed10 prosthesis and osseointegrated implants.11,12 This
79
technique enables a two-dimensional view, providing information on the magnitude and
80
concentration of stress.13 Experimental papers evaluated different implant designs14,15 and
81
others studies compared the screwed and cemented retention systems through
82
photoelasticity. However, despite the great number of researches focusing on stress
83
distribution with different prosthetic retention systems, there is no study mixing different
84
types of prosthetic retention systems16,17. Programmed stress tests were used in this study
85
to determine the transfer of stress by screwed, cemented and mixed suprastructures to
86
implants with the goal of evaluating the passivity and stress distribution of fixed partial
87
prostheses, on each type of suprastructure.
88
Hypothesis: Do the mixed prosthesis have different stress distribution compared the
89
screwed and cemented prosthetic retention systems? The aim of this study was to evaluate
90
the stress distribution of different retention systems (screwed, cemented and mixed) in 5-
91
unit implant-supported fixed partial dentures through photoelasticity.
92 93
MATERIALS AND METHODS
94
A preliminary plaster model was made that measured 25 mm high, 35 mm long and
95
12 mm thick. A motor coupled to a parallelometer was used to drill five holes that were 15
96
mm deep with a 4.35-mm drill (Conexão Sistema de Prótese Ltda, São Paulo, Brazil). Five
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external hex implant analogs (Conexão Sistema de Prótese Ltda, São Paulo, Brazil) were
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inserted into these holes with cyanoacrylate and type IV plaster. The analogs were parallel
99
and equidistant, with a 7-mm center-to-center separation distance. A layer of colorless
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enamel (Colorama, Cosbra Cosmetics, São Paulo, Brazil) was applied to the plaster, which
101
provided a smooth and polished surface. This construction was then used as a final working
102
model and served as the standard for preparing the photoelastic model.
103
To obtain the photoelastic model, the plaster model was duplicated using photoelastic resin
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PL-2 (Measurement Group Inc., Raleigh, NC, USA) in a beaker by mixing resin and
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hardener at a 1:1 ratio (50 ml each) and manipulated using a plastic tube. A lid was made
106
with pink number 07 wax (Epoxiglas, Diadema, Brazil). Then, the plaster model with
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analogs was connected to the molding components and fastened to the wax lid inside of a
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plastic tube, such that the model was fixed (i.e., did not move) inside the tube. Laboratory
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silicone for model duplication (Silicone Master, Talladium Inc., Valencia, CA, USA) was
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poured inside each tube until the silicone completely filled the plaster model and the
111
components connected to the analog moldings. After 2 h (the silicone setting time), the wax
112
lid was removed from the plastic tube, and the plaster model was removed from the silicone
113
mold.
114
After obtaining the mold, five external hex implants, each with a diameter of 3.75
115
mm and a height of 11.5 mm (Conexão Sistema de Prótese Ltda, São Paulo, Brazil), were
116
set. Then, the PL-2 photoelastic resin (Measurement Group Inc.) was stirred with a glass
117
rod in a circular motion for 5 min to obtain a mixture with a homogenous color. The
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container was then placed in a vacuum chamber (Fast Vac, JB, Brazil) for 30 min to
119
eliminate air from the resin. At this point, the resin appeared translucent with a yellowish
120
color, and it was poured into the mold, thereby covering all of the threads of the implants
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and mimicking the biological condition of osseointegration, to obtain a photoelastic model
122
with implants. This assembly was placed in a vacuum chamber to remove air bubbles for
123
30 min and allowed to rest for 48 h in a cool, ventilated area for complete polymerization
124
of the photoelastic resin.
125
The plaster model was used to make 20 suprastructures, i.e., five in each of the
126
following groups: 1) screwed prosthesis, 2) cemented prosthesis, 3) mixed prosthesis 1, and
127
4) mixed prosthesis 2.
128 129
1) Screwed prosthesis:
130
To prepare the screwed prostheses, five estheticone abutments (Conexão Sistema de
131
Prótese Ltda, São Paulo, Brazil) with a brace height of 2 mm were positioned on the
132
analogs in the plaster model. The abutments were fastened by applying 20 Ncm of torque to
133
each with a manual torque wrench (Conexão Sistema de Prótese Ltda, São Paulo, Brazil).
134
Five titanium copings without an anti-rotational system were placed on the abutments with
135
a torque of 10N using a manual torque wrench. To form a titanium suprastructure, the
136
copings were joined by laser welding using a Nd:YAG (neodymium-doped:yttrium
137
aluminum garnet) laser (Deka, Calenzano, Italy) with an average power of 30 W and a
138
continuous pulse of 0.5 Hz lasting 0.5 ms.
139 140
2) Cemented prosthesis
141
To prepare the cemented prostheses, five straight abutments (Conexão Sistema de
142
Prótese Ltda, São Paulo, Brazil) were positioned on the analogs attached to the plaster
143
model. These abutments had a brace height of 1 mm and were fastened by applying a
144
torque of 20 Ncm using a manual torque wrench. The abutment suprastructure was waxed,
145
thereby maintaining the insertion axis of the screws.
146
The suprastructure was then cast and polished with grade 1 titanium (Tritan, Dentaurum,
147
Ispringen, Germany) according to the standard methodology. This suprastructure was
148
separated into four sections using a carborundum disc and rejoined with laser welding.
149 150
3) Mixed prosthesis 1
151
To prepare the type 1 mixed prostheses, alternating straight abutments (Conexão
152
Sistema de Prótese Ltda, São Paulo, Brazil) and estheticone abutments (Conexão Sistema
153
de Prótese Ltda, São Paulo, Brazil) were positioned on the analogs, which were attached to
154
the plaster model with screw fittings. Three straight abutments with a brace height of 1 mm
155
and two estheticone abutments with a brace height of 2 mm were applied with a torque of
156
20 Ncm using a manual torque wrench. The straight abutments were waxed, thereby
157
preserving the insertion axis of the screws.
158
The suprastructure was then cast and polished with grade 1 titanium according to the
159
standard methodology. Then, the castings were welded to the titanium copings of the
160
estheticone abutments, which formed a suprastructure that began and ended with straight
161
abutments.
162 163
4) Mixed prosthesis 2
164
To prepare the type 2 mixed prostheses, the same methodology was applied as for
165
the type 1 mixed prostheses, but the number and sequence of the abutments was changed.
166
Three estheticone abutments with a brace height of 1 mm and two straight abutments with a
167
brace height of 2 mm were fitted with a torque of 20 Ncm using a manual torque wrench.
168
The straight abutments were waxed, thereby preserving the insertion axis of the screws,
169
which formed an suprastructure that began and ended with estheticone abutments. The
170
alternative of changing the type and amount of abutment would be to check the influence of
171
the stress distribution. The manufactured mixed suprastructure began and ended with
172
estheticone abutments. Internal adjustments were made to the cemented and mixed (type 1
173
and 2) suprastructure using silicone fluid (Oranwash L, Zermack, Rovigo, Italy) and a high-
174
speed carbide bur (Extra Torque 605 C, Kavo, Biberach, Germany) to facilitate the flow of
175
cement and obtain a better prosthesis fitting.
176 177
Before installation of the abutments, an initial photoelastic assessment of the models
178
was made by placing the models in a container (Campestre, São B. Campo, Brazil) filled
179
with mineral oil to improve the sharpness of the images obtained with a circular
180
polariscope. This device is capable of providing a dark field using polarized lens with
181
parallel axis. Thus, it is possible to observe the isochromatic fringes, which have colorful
182
patterns and show the stress intensity. Isocline fringes are also visible, which are dark lines
183
superimposed on the colored fringes and represent the direction of the stress. After this
184
analysis, the model was considered free of residual stresses and ready to begin testing. To
185
tighten the abutment screws, a torque wrench (Conexão Sistema de Prótese Ltda, São
186
Paulo, Brazil) was used by following the manufacturer's recommendations. A torque of 10
187
Ncm was applied to the screws copings, as recommended by the manufacturer. In the
188
cemented and mixed (type 1 and 2) suprastructure, the temporary cement Temp Bond NE
189
(Kerr Corporation, Orange, CA, USA) was used on the straight abutments because it allows
190
the removal of the structures when necessary without changing the position of the
191
abutments.
192
Through the photoelastic analysis, it was possible to visualize the stresses at the resin-
193
implant interface at two moments: 1) during the installation of the suprastructures and 2)
194
after applying a vertical compressive load of 100 N on a universal testing machine
195
(Mecmesin, Slinford, England) at a central point of the suprastructures. Images were
196
recorded with a digital camcorder, FD-717 (Sony, Foster City, CA, USA). The selected
197
images were qualitatively assessed in terms of the direction of propagation and the intensity
198
of the stress. A greater number of fringes indicate a greater magnitude of stress, and fringes
199
that are closer to each other indicate a greater concentration of stress on each region of the
200
test block.18
201 202
After each test, the models underwent a heat treatment to eliminate any residual stress, which consisted of immersion in water at 55ºC for 5 min.19
203 204
RESULTS
205
In the present study, we observed the formation of isochromatic fringes resulting
206
from stresses generated around the implants after installation of different suprastructure and
207
after applying an axial load of 100 N. Among the images obtained, one from each structure
208
group was selected based on image quality criteria. All images were evaluated by the same
209
operator.
210
Figure 1a to 1d show the lower number fringes (black colour) generated in the
211
model after the installation of the suprastructure. In all of the suprastructure, generalized
212
stresses were observed that dissipated from the apex to the region of the middle third of
213
some implants, which characterized them as lacking passivity.
214
After applying a compressive load of 100 N at the midpoint of the suprastructure
215
(Figure 2a to 2d), there was an increase in the number of fringes that showed the presence
216
of tensions in the photoelastic model (red and green colours). Considering the number of
217
fringes of high intensity, the largest area of stress concentration between the implants
218
occurred in the apical and cervical regions, with the greatest intensity in the screwed
219
suprastructure, followed by the mixed 2, mixed 1 and cemented suprastructure in
220
descending order.
221 222 223
224
DISCUSSION
225
Most studies of the biomechanical behavior of implants under occlusal loads are
226
performed using finite element analysis20,21 or occlusal testing,22,23 where it is possible to
227
quantify the generation of stresses transferred to the implants. Adding to these works,
228
photoelastic analysis12,13 is proposed by the present study as a methodology to qualify how
229
the transmission of stress occurs in partially fixed prosthetic implants. Photoelastic analysis
230
permitted a comparison of the distribution of stresses induced by different partially fixed
231
prosthetics (screwed, cemented and mixed) with five elements in regions adjacent to the
232
implants during installation and after applying a vertical compressive load of 100 N. The
233
photoelastic method demonstrated the advantage of providing direct visual information
234
about the pattern of stresses that occurred throughout the model after applying loads, which
235
allowed the magnitude of the stress to be located and qualified.18 Although photoelastic
236
models are used in dentistry to assess stress, they are limited with regard to predicting
237
biological responses to specific loads because this technique does not quantify the stress
238
thresholds of the various bone types.24
239
The methods of retention, such as cement, screws and combinations thereof, provide
240
discrete stress levels at the time of suprastructure installation, even after being internally
241
adjusted with silicone fluid. The four types of implanted fixed partial prosthetics did not
242
show a passive fit and produced stresses of low magnitude in some implants which was
243
likely related to laboratory inaccuracies, the fabrication model and the molds. 25,26 The lack
244
of a passive fit of the prostheses generates significant stresses on the implants.27 According
245
to some authors, a complete passive suprastructure is not possible through laboratory,
246
clinical and conventional procedures. The methods for assessing fit often do not show
247
inaccuracies, and a more sensitive technique for measuring stresses should be used through
248
the application of technological resources to obtain a clinically acceptable level of passivity
249
that does not cause any aesthetic, biological or functional problems for the patient.28,29,30
250
In this study, the greatest intensity and concentration of tension after the application
251
of compressive load of 100 N.cm was observed in the screwed suprastructure, followed by
252
the mixed 2, mixed 1 and cemented respectively. We believe that the presence of cement
253
may have been a favorable factor in lower dissipation of tension between the cemented
254
prostheses and mixed 2, since the presence of cement absorbs an amount of load and stress
255
that would be transferred to the implants.
256
The splinting of fixed partial prosthetics is another interesting point in this study
257
because the stress distribution occurred in all implants but not equally, probably due to the
258
different retention systems. These findings corroborates another literature reports about
259
stress distribution in prostheses subjected to occlusal forces, where tensions are higher in
260
screwed prostheses and smaller in the cemented ones22, probably owing to the hypotheses
261
mentioned above.
262
Comparing the mixed supraestruturas it was observed a higher concentration of
263
tension in the cervical and apical region in suprastructure mixed 2, probably related to the
264
amount of screwed abutments.
265
Although we do not know precisely the levels of loads that may be harmful to
266
osseointegration,31,32 it is recommended careful control of axial or extra-axial loads through
267
the prosthetic planning. However, clinically, the bone is strongest under compressive
268
forces, weaker under tensile loads, and even weaker yet in shear loads.
269
The combined retention between the cement and screw systems generated less stress
270
than the screwed prosthesis, making it a good rehabilitative choice. The use of cement also
271
provided a degree of tolerance to poor adjustment, retrievability and the ease of repair with
272
the screw.33,34
273
The mixed and cemented prostheses displayed lower levels of stress distribution and
274
a lower fringe intensity than the screwed implants when subjected to compressive loads in
275
both the cervical and apical regions of the implants, but it was not clear whether these
276
differences were related to the retention system.
277 278
ACKNOWLEDGEMENTS
279
We thank all who have contributed directly and indirectly to this work. My sincere thanks.
280 281
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FIGURE CAPTIONS
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Figure 1 – Demonstration of the results after installation of the suprastructures (passive
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fit). A. Screwed suprastructures; B. cemented suprastructures; C. mixed suprastructures 1;
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and D. mixed suprastructures 2.
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Figure 2 - Demonstration of the results observed after application of a 100-N compressive
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load. A. Load application on screwed suprastructures; B. load application on cemented
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suprastructures; C. application of mixed load 1; and D. application of mixed load 2.
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