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Load-Application Devices: A Comparative Strain Gauge Analysis

Article in Brazilian Dental Journal · May 2015

DOI: 10.1590/0103-6440201300321 · Source: PubMed

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Renato Nishioka Luis Gustavo Vasconcellos

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Brazilian Dental Journal (2015) 26(3): 258-262 ISSN 0103-6440
http://dx.doi.org/10.1590/0103-6440201300321

Load-Application Devices:
1Department of Dental Materials
and Prosthodontics, São José dos
Campos Dental School, UNESP
A Comparative Strain Gauge Analysis - Universidade Estadual Paulista,
São José dos Campos, SP, Brazil
2Department of Bioscience and

Oral Diagnosis, São José dos

Campos Dental School, UNESP
Renato Sussumu Nishioka1, Luis Gustavo Oliveira de Vasconcellos1, Renata
- Universidade Estadual Paulista,
Pilli Jóias2, Sigmar de Mello Rode1 São José dos Campos, SP, Brazil

Correspondence: Prof. Dr. Sigmar

de Mello Rode, Avenida Francisco
In view of the low loading values commonly employed in dentistry, a load-application José Longo, 777, Jd São Dimas,
device (LAD) was developed as option to the universal testing machine (UTM), using 12245-000 São José dos Campos,
strain gauge analysis. The aim of this study was to develop a load-application device SP, Brasil. Tel: +55-12-3947-9056.
(LAD) and compare the LAD with the UTM apparatus under axial and non-axial loads. An e-mail:sigmarrode@uol.com.br
external hexagonal implant was inserted into a polyurethane block and one EsthetiCone
abutment was connected to the implant. A plastic prosthetic cylinder was screwed onto the
abutment and a conical pattern crown was fabricated using acrylic resin. An impression
was made and ten identical standard acrylic resin patterns were obtained from the crown
impression, which were cast in nickel-chromium alloy (n=10). Four strain gauges were
bonded diametrically around the implant. The specimens were subjected to central (C)
and lateral (L) axial loads of 30 kgf, on both devices: G1: LAD/C; G2: LAD/L; G3: UTM/C;
G4: UTM/L. The data (με) were statistically analyzed by repeated measures ANOVA and
Tukey’s test (p<0.05). No statistically significant difference was found between the UTM Key Words: dental implants,
and LAD devices, regardless of the type of load. It was concluded that the LAD is a reliable dental prosthesis, implant-
alternative, which induces microstrains to implants similar to those obtained with the UTM. supported dental prosthesis.

Introduction that certain materials undergo changes in their electrical

Occlusal overload has been indicated as the primary resistivity when subjected to a force. Materials have
factor for peri-implant bone resorption, implant failure different resistivities, which can be measured accurately
and implant-supported prostheses failure (1,2). The at the site where the strain gage is attached, using a
maintenance of the bone/implant interface is particularly Wheatstone’s bridge circuit (9,19). This technique has
dependent on the control of biomechanical loads, since been proposed to evaluate strains in implant-supported
according to current bone physiology theories, occlusal prostheses in vitro (9,11,13,16-18,20), in vivo (15) and
forces affect the bone around the implant (3-5). The under static (13,16) and/or dynamic loads (21).
response to an increased mechanical stress below a certain Strain gauge studies in implantology use low loading
threshold will be a strengthening of the bone by increasing values varying from 20 to 300 N (8,10,11,13,18,19,22). Some
the bone density or apposition of bone. On the other hand, works used custom-built load-application devices (8,10,11)
fatigue micro-damage resulting in bone resorption may be while others used universal testing machines (13,19,22).
the result of mechanical stress beyond this threshold (3-5). However, the force of the universal testing machine is too
Moreover, during the functional load application on the great for testing small values employed in dentistry, since
implant, the direction of the forces not always coincides it is an adaptation from the engineering that requires
with its long axis. Conversely, when the occlusal force is high force. Thus, the aim of this study was to develop a
applied on different locations and in a direction that creates load-application device (LAD) and compare the LAD with
leverage, it may cause stresses on the bone adjacent to the the universal testing machine (UTM) apparatus under axial
implant (4). In vitro (6) and in vivo (7) studies have revealed and non-axial loads. The work hypotheses were: 1- both
the negative effect of the application of non-axial loads devices would produce similar magnitude of microstrain
when compared with axial loads. for both loading conditions; 2- the lateral load would
Several techniques have been employed to evaluate generate greater magnitude of microstrain.
the biomechanical loads on implants, such as photoelastic
stress analysis (8-10), finite elements stress analysis (6,11), Material and Methods
mathematical calculations (12) and strain gauge analysis A load-application device (LAD) was developed with
(9,11,13-18). different magnitudes of static vertical loading (Fig. 1). The
Strain gauge analysis is a technique for measuring loads are applied according to the amount of weight and
microstrains, which involves the use of electrical resistance position of the compressor pin, varying from 5 to 40 kgf
or strain gauges. Strain gauges are based on the principle at 5 kgf intervals.
 Braz Dent J 26(3) 2015

Preparation of the Samples casts were ultrasonically cleaned, finished and polished. The
An external implant 3.75 mm in diameter and 13 fit and passivity of the superstructures were checked by
mm deep (Master screw implants; Conexão Sistemas de direct visual examination associated with a clinical probe
Prótese, São Paulo, SP, Brazil) was arranged in the middle (23). Superstructures showing instability were excluded.
of a polyurethane block (Polyurethane F16; Axson, Cergy,
France) measuring 95 x 45 x 30 mm. EsthetiCone abutment Strain Gauge Analysis
(Conexão) was screwed onto the implant with 20 Ncm using Four strain gauges (L2A-06-062LW-120; Vishay, Raleigh,
a manual torque wrench (Conexão). NC, USA) were diametrically bonded around the implant
Plastic prosthetic cylinder (Conexão) was screwed onto the surface of the polyurethane block (Fig. 2), using
onto the abutment and a conical pattern was built using methyl-2-cyanoacrylate adhesive (M-Bond 200; Vishay
acrylic resin (Duralay; Reliance Dental, Worth, IL, USA) with Measurements Group, Raleigh, NC, USA). Each strain gauge
a 4.1 mm base, 8 mm upper platform and 8 mm high. A was connected separately, and the four strain gauges were
referential mark was made on the outermost portion of arranged in series to form a one-fourth Wheatstone’s
the larger base of the cone for the subsequent application bridge. The wires from the strain gauges were connected
of the non-axial loads. to a multichannel bridge amplifier to form one leg of the
A polyvinyl siloxane impression (Elite; Zhermack, Rovigo, bridge. A computer (Intel 775P Pentium 4 Q6600; Acer,
Italy) was made and ten acrylic resin patterns (GC Pattern Miami, FL, USA) was interfaced with the bridge amplifier
Resin; GC Europe NV, Leuven, Belgium) were obtained from to record the output signal of polyurethane surface. Data
the impression. The patterns were sprued, invested and acquisition system software (System 5000 Model 5100B;
cast in Co-Cr alloy (Wirobond SG; Bremen, Germany). The Vishay) was used to record the data.

Strain gauge: load-application devices

Application of the Static Vertical Load
Each specimen was screwed to the abutment using a
torque of 10 Ncm. All of the strain gauges were zeroed
and calibrated prior to each loading. Static vertical (axial)
loads of 30 kgf were applied for 10 s on the entry hole of
the retention screw (Fig. 3) and on the referential mark
situated 4 mm from the center of the specimen (Fig. 3),
which were applied using a load-application device (LAD)
or the universal testing machine (DL-1000; Emic, São José
dos Pinhais, PR, Brazil). The magnitude of microstrain was
recorded in units of microstrain (με). This procedure was
made two more times, completing three readings per
loading location.
Figure 1. Load-application device (LAD) with positioned weights; 1: This experiment followed a factorial scheme 2x2 type.
lower base; 2: steel rods; 3: upper base; 4: load bar; 5: load bar cradle; The experimental variables were device (LAD and UTM)
6: pressure pin; 7: hole no. 1; 8: hole no. 2; 9: eccentric cradle; 10:
two 1 kg weights, 11: one 2 kg weight.

Figure 3. Application of axial load on the test specimen, using the

Figure 2. Experimental model and strain gauge locations. load-application device (LAD).
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 Braz Dent J 26(3) 2015

and load location (center and lateral). The specimens were the load location promoted statistically significant values
randomly assigned to the load location conditions. (p=0.0001). The interaction effect was not statistically
significant, demonstrating that the device effect was similar
Statistical Analysis to the one for load location.
Data obtained were submitted to statistical analysis
using the following softwares: GraphPadPrism (GraphPad Discussion
Software, Inc, version 4.00, 2003, La Jolla, CA, USA), MINITAB The cervical region of the implant is the site where the
(Minitab, version 14.12, 2004, State College, PA, USA) and highest stresses occur (20), regardless of the type of bone
STATISTIX (Analytical Software Inc., version 8.0, 2003, and the design of the implant (24). In this study, the strain
Tallahassee, FL, USA). The statistics consisted of analysis gauges were bonded tangentially to the implant platform
of variance of repeated measurements for two factors on the polyurethane block. This positioning of the strain
(device and loading location), in which the variable loading gauges method has been used in previous studies (13,15-
location was considered as a repeated factor. The study of 18,20). In addition, the flat surface of the polyurethane
the interaction effect was conducted by graphs. Multiple block facilitates the positioning and bonding of the strain
comparisons among the means for the four experimental gauges when compared with other studies, which are
conditions were made by the Tukey’s test. Significance bonded to the implants (11), to the abutment (22) and to
level was set at 5%. the metallic structures of the prosthesis (15,20).
This study compared the microstrains generated around
Results of the implant after the application of axial (center and
Table 1 shows the descriptive statistical data, analyzing lateral) static loads applied by devices (LAD and UTM).
the mean values of microstrain obtained with each strain According to Frost (3) and Wiskott and Belser (5), bone
gauge (SG), for the devices (LAD and UTM) at each load homeostasis occurs when the level of microstrain remains
location (center and lateral). within the range from 100 to 2000 με and 50 to 1500 με,
R.S. Nishioka et.al.

The mean values of microstrain (με) of all the groups respectively. The null hypotheses were accepted, as Table 1
were calculated and are shown in Figure 4. shows that the values of microstrain obtained after applying
The statistical repeated measures ANOVA indicated that the 30 kgf load (center and lateral) in both load-application
devices remained within the level of bone homeostasis or
normal load (3,5).
Table 1. Values of microstrain (µε) obtained at each point where load In this study it was found that when the load was axial
was applied with the load-application device (LAD) and the universal
testing machine (UTM) on each strain gauge the microstrain values were smaller and equally distributed
among the four strain gages. In contrast, when non axial
Device Loading point Strain gauge Mean ± Sd loads were applied on the specimen, the highest microstrain
values were found by the SG 4 (Fig 2) placed closest to
01 294.2 ± 138.5
the tip of load application, indicating that the amount of
02 275.2 ± 177.5
Axial load transmitted to the bone/implant interface depends
03 281.4 ± 139.6
on the site where the load was applied (6-8,12). Therefore,
04 379.6 ± 246.6
the first work hypothesis was accepted.
LAD
01 320.5 ± 113.7 Table 1 shows that the highest microstrain was obtained
02 735.4 ± 163.4
by the lateral load applied by both devices occurred in
Non-axial
03 287.4 ± 129.9
04 1421.0 ± 328

01 486.0 ± 78.4
02 131.1 ± 86.0
Axial
03 497.8 ± 86.1
04 123.2 ± 76.7
UTM
01 478.0 ± 55.3
02 762.2 ± 87.4
Non-axial
03 507.0 ± 61.5
Figure 4. Mean and standard deviation of microstrain (µε) for the two
04 1152.9 ± 112.6
devices at each loading point.
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 Braz Dent J 26(3) 2015

the SG 4, followed by the microstrains generated in the to be promising for the photoelasticity analysis, since it
SG 2. In other words, the lateral load caused stretching of does not prevent the transmission of the polarized light
the SG 4 and shortening of the SG 2. Similar result was in photoelastic models.
reported by Hekimoglu et al. (21) on implants in occlusion It may be concluded that the LAD can be considered a
with natural teeth, on implants under axial and non-axial reliable alternative, which induces microstrains similar to
loads and also on the cervical region of the implant during those obtained with the UTM regardless of the load location.
non-axial loading. The lateral load significantly increased the microstrains
Table 2 indicates that there was no statistically around the implant.
significant difference between the values obtained with
LAD and UTM, regardless of the type of load. This fact Resumo
validates the use of the LAD for strain gauge studies. Considerando os valores relativamente pequenos utilizados em odontologia
However, Table 1 showed that standard deviation of the para os ensaios de carregamento verticais axiais, foi desenvolvido um
dispositivo de aplicação de carga (DAC) para substituir a máquina de
values obtained by LAD was greater than that showed ensaios universal (EMIC). O objetivo deste estudo foi desenvolver um
by UTM. The explanation for this fact may be due to the DAC e compará-lo com a EMIC por meio da utilização de carregamentos
velocity of load application, i.e., in UTM the load is applied axiais e não-axiais. Num bloco de poliuretano foi inserido um implante
hexágono externo, o qual foi conectado a um pilar protético esteticone.
gradually, beginning at the moment of the tip application Sobre o pilar protético foi parafusada uma coifa plástica e um pilar cônico
at 0.1 kgf until the load reaches 30 kgf. In contrast, LAD foi modelado em resina acrílica, que foi moldada para a obtenção de
load application is quicker. dez enceramentos iguais que foram fundidos em níquel cromo. Quatro
extensômetros foram diametralmente colados ao redor do implante.
With regards to the load location (Table 2) the lateral Cada corpo de prova foi submetido a cargas axiais central (C) e lateral
load produced significantly higher microstrain values than (L) de 30 kgf, em ambos os dispositivos: G1) DAC/C; G2) DAC/L; G3)
those produced by central load. Based on this finding, it can EMIC/C; G4) EMIC/L. Os dados (µε) foram analisados estatisticamente

Strain gauge: load-application devices

pelos testes de ANOVA para medidas repetidas e de Tukey (p<0,05). Não
be inferred that occlusal contacts positioned laterally along houve diferença estatisticamente significante entre os dispositivos DAC e
the axis of the implant produce higher stresses around the EMIC, independente do tipo de carga. A aplicação de carga não-axial (NA)
implant, and contribute to periimplant bone resorption. determinou um aumento significante de tensões ao redor do implante. Foi
concluído que o DAC é uma opção confiável, a qual induz microtensão
Babier and Schepers (7) analyzed the in vivo influence of em implantes de forma semelhante à EMIC.
axial and non-axial loading in bone remodeling showing
that non-axial loading induces greater cell response, with
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