Investigating prospectives of using
fiber optics based sensors in
dentistry
Tarek Elsarnagawy1, Mohamed-Tarek
El-Wakad2
other hand the sensitivity of the FOS
was much better than ESG. Results
also showed that using optical sources
of lower wavelength improved the
sensitivity as well as the accuracy of
the optical system.
1. Overview
Kurzfassung: Ziel dieser Studie ist
einen
Vergleich
zwischen
der
Anwendung von Glasfasertechnologie
und
elecktrische
Dehnungsmessstreifen (DMS) darzustellen um es
auf dem gbiet der Zahnmedizin
einzusetzen. Es repraesentiert
die
Empfindlichkeit des faseroptischen
Sensors (FOS) gegeueber die vom
DMS wenn Kraft einwirkt. Unter
anderem zeigte sich dass der
vorhandene Linearitaetsfehler beim
FOS von systematischer Art ist
(wiederholbarer Muster), d.h., durch
einfache Klibrierungsprozeduren kann
dieser Fehler behoben werden.
Weiterhin verbesserte sich die FOSGenauigkeit
als
auch
die
Empfindlichkeit durch das Einsetzen
von
Laserquellen
kuerzerer
Wellenlaenge. Alle Ergebnisse sind im
Text dargestellt und diskutiert.
Abstract: This study aims at comparing
the technology of fiber optics to that of
electrical strain gauges (ESG) when
both used in the field of dentistry. It
compared the accuracy and sensitivity
of the strain gauges versus that of
optical fibers under the same loading
conditions. Results showed that
accuracy of strain gages is a little
better than fiber optics sensor (FOS).
However, the errors in the fiber
readings showed a pattern that was
repeatable in every measurement series
that could be corrected through a
simple calibration procedure. On the
1
Tarek99@web.de
2
mtwakad@yahoo.com
The bite force is an important
parameter, which need to be
determined to be used in many areas of
dentistry. It is used to compare
chewing forces in patients with
disorders to those who are normal [1].
It is, also, of great importance to
measure bite forces on dentures to
assess the performance of the treatment
in restoring the normal function.
Recent studies measured bite forces
under fixed partial dentures supported
by implants [2], in complete over
denture supported with implants [3,4]
and in full denture wearer [5]. In
another study the bite force was
measured in all types of dentures as
well as natural dentition [6]. Due to the
importance of bite and chewing force
measurement, studies can be found in
the literature dated back to early 1900.
Many techniques are used in designing
bite force transducer. Some transducers
use piezoelectric sensor [7], strain gages
[2,4,5,6], force sensing resistors [8],
and pressure sensor [1].
Reviewing the bite force literature it was
found that the most commonly used
technique was strain gages. They offer
many advantages over other techniques
such as simple installation that can be
carried out with little training, available
circuitry to measure its linear output over a
large range of forces, and because it is
easy to analytically anticipate the
magnitude of the gage output with little
well defined calculations. However, the
resistance strain gages have their
limitations. One of the strain gage
limitations is the limited sensitivity of
strain gages appearing in their low
gage factor magnitude (about 2).
Another and the most important
limitation is the relatively large size of
the gauge as applies to bite force
transducers. The large size of the
gauges requires the height of the bite
force transducer to be in the range of
about 10 millimeters which causes a
bite opening. Such bite opening has
been proved to reduce the magnitude
of the bite force a patient can exert.
Consequently, it does not give a real
measure of such force.
Thus, new advances in the area of bite
force transducers and measurements
should offer a miniature transducer of
height less than 5mm. It would, also,
be advantageous if the accuracy and
sensitivity of the new transducer can
be improved compared to those using
the strain gage. Since it would be used
in the oral cavity, it would be
advantageous
if
no
electrical
connections are involved in the cavity
for safety measures.
Fiber optics provide advantages over
their electrical counterparts, namely,
immunity
to
electromagnetic
interference, lightweight, small size,
high sensitivity, large bandwidth, and
ease in implementing multiplexed or
distributed sensors. Strain, temperature
and pressure are the most widely
studied measurands and the fiber
grating sensor represents the most
widely studied technology for optical
fiber sensors. Today, some success has
been found in the commercialization of
optical fiber sensors. However, in
various fields they still suffer from
competition with other mature sensor
technologies. However, new ideas are
being continuously developed and
tested not only for the traditional
measurands but also for new
applications such as the development
of fiber optics in the field of nanosensors, nano-biosensors, PH sensors,
humidity sensors, and fluorescent
biosensors. In addition, optical fiber
sensors possess extreme sensitivity as
well as electrical passivity. The later
advantage is important for the safety in
some applications such as in medical
sensors [9]. Developed fiber optic
sensors along with some light
processing techniques have been
known to be the most sensitive
technologies to measure displacements
as small as 10–10 mm. Hence, the idea
to use optical fibers for means of
sensing is not a new one. Several
publications have shown its variety of
application. A load cell for loads up
10000 N with a resolution in the range
of 10-6 and stability was presented.
Furthermore, Zhao presented an
electromagnetic force sensor based on
fiber Bragg grating [10]. The bend loss
in optic fibers was used in the
measurement of distributed force over
a surface [11].
Referring to the listed applications and
advantages,
this
article
will
demonstrate a comparison study
between electrical strain gauges and
optic fiber strain gauges for high
accuracy measurement of strain. The
results will be used as a base for future
works in the field of single- & multipoint force measurement.
2. Material and Method
This is a preliminary study to develop
an optical technique to be used to
measure forces in dentistry. Optical
interferometry is one of the most
sensitive technologies to measure
small displacements which can later be
related to forces. Displacements in the
range of nanometers can be resolved
by applying interferometric techniques.
The common fiber interferometric
sensor configurations that are usually
applied in fiber optical sensors are
Michelson, Mach-Zehnder and Sagnac
interferometers.
In this work the Michelson fiber
optical sensor configuration is used to
sense strain. This interferometric
principle was chosen because it is
optimum with the use of the 3x3-fibercoupler. In accordance a digital
readout is simply obtained with a
photo detector and a phase-based-updown counter. Hence, a lot of
digitization electronics are saved.
Therefore, this technique was used in
the optical system of the experimental
setup as described later in this section.
To validate the optical system results
strain gauges were used since it is the
most common sensors applied in
dentistry transducers.
2.1. Experimental setup
The experimental setup consists of 4
components as follows:
1. Specimen:
A
cylindrical
specimen was used to resemble
a dental implant. On this
specimen both the stain gauge
and fiber optic were mounted
using proper adhesive (EPOTEK 353ND).
2. Optical system: This system
has 3 subcomponents as
described below:
 Optical
Sources:
Two
different sources were used.
The first source used was a
pigtail single-mode diode
Laser
(Seastar,
Diode
Mitsubishi ML5415,  =
826,3 nm). To ensure a
constant output wavelength,
the Laser diode module is
electronically temperature and
current controlled which is
important
for
the
interferometric configuration.
The second source used was a
HeNe-Laser with a shorter
wavelength (632nm). The
purpose of this source was to
study the effect of shorter
wavelength on the accuracy of
the output
 Single-mode fiber (SM800FIBERCORE LTD) of an
outer diameter of 125μm,
240μm
coating
outer
diameter, numerical aperture
of 0.11 and a launch spot size
of 6μm is used to act as the
sensor and reference of the
interferometer.
 The fiber coupler: the function
of this coupler is to condition
the light paths within the
sensor and the reference fiber.
In addition, it transfers the
light from the laser source
into the fiber and back to be
processed. This type of
coupler provides a constant
phase shift between the
signals in the coupler arms
which is 120 degree. This
leads to the operation at semi
quadrature for means of
detection of the induced phase
variation
due
to
the
measurand.
3. Strain gauge system: Strain
gauge (Electrical strain gauges
ESG:Typ 3/120XY11, HBM, 
= 11 ppm/K) was monted on
the same specimen on which
the fiber is mounted. The strain
gauge was connected to stain
meter (Vishay, System 6000,
model 6200 scanner, NC,
USA).
4. Temperature
Sensor:
A
temperature sensor (AD590)
was used in the setup. The
purpose of the sensor was to
monitor the end of the transient
heating stage of the strain
gauge related to the initiation of
current flow into the gauge. At
the end of such stage
measurements can start without
errors due to the heating effect.
Figure 1 below is a block diagram
showing the previously mentioned
components as it appears in the
experimental setup and
Figure 1: Basic schematic diagram
of experimental setup
2.2. Calibration procedure
The ESG and FOS were mounted on
the same specimen, so that both
sensors experience the same strain. A
static load was applied on the
specimen at an increment of 10 kN
starting from zero load up to 100 kN.
The output data from the FOS and the
ESG are of different units. Since the
main function of these gauges are
supposed to measure strain, therefore it
is more reasonable to change data from
each sensor into strain for comparison
purposes. The output of the FOS was
converted into strain using the
following
Phase-Strain-Sensitivity
equation
G
 4  1

  n 1  n²(1  ) p12 p11
L   2

…(1)
This equation takes into consideration
mechanical, optical and geometrical
properties of the fiber. By substituting
the used values of the FOS in above
equation it leads to G (for single mode
standard fiber) = 1.89107 radm-1.
Similarly, the output of the ESG was
converted to strain, using equation
relating Wheatstone bridge output to
strain and take into consideration the
strain gage factor as in the following
equation (2):
GaugeFactor 
R

R …….(2)
3. Result and Discussion
Figure2 shows the results of the
calibration process involving ESG and
both FOS with laser diode and with
helium. As shown in the figure, the
calibration curves are both linear
within the range of the force. This
gives the advantage of being able to
predict the applied force using simple
calibration equations obtained through
linear curve fitting.
Figure 2: The output strain from
all strain during calibration process
The figure, also, shows that the
accuracy of both FOS curves as
indicated by the slope of their lines is
less than that of ESG. However,
among the FOS sensors the heliumbased sensor is more accurate than the
laser diode-based sensor. It can be
clearly seen that the results of the
helium-based sensor is very close to
that of (ESG). Thus, the shorter the
wavelength used in the optical sensor
the closer the result is going to be to
the strain gauge results. Consequently,
using an even shorter wavelength than
the helium (e.g. Excimer lasers with 
in the range of 150 to 350 nm) is
expected to give results even closer to
those of the strain gauge.
To explore the accuracy of all used
sensors in more details the results of all
sensors were plotted against the
theoretical strain that should have been
measured (Fig 3). The theoretical
strain was calculated using axial strain
equation
which
takes
into
consideration
the
geometrical
properties of the structure on which the
sensors are mounted as well as the
mechanical properties of the structure
material. Figure 3 shows that the
results of ESG besides being linear, it
has a slope of about 43 degrees (ideal:
45deg.). This means that the
relationship between strain measured
by ESG and the theoretical one is
almost one-to-one relationship. The
deviation is about 7.5% from the
theoretical reading. This is acceptable
considering the factors that may cause
systematic errors and may lower the
accuracy that occurs during the
mounting process of the gauge such as
the gage alignment and the thickness
of adhesive layer.
Also, in figure 3 the slope of the fitted
line into the results of laser diodebased FOS is about 38 degrees. It
shows a deviation of 22 % from the
expected theoretical value. This is a
larger deviation compared to that of
ESG. This larger deviation is on one
hand due to the adhesive that acts with
lateral forces on the embedded fiber
which causes an increase of the
photoelastic effect (sandwich effect).
Since the photoelastic effect opposes
the longitudinal measuring effect, this
causes a reduction of sensitivity as
well as accuracy. This comes in
addition to the much thicker glue layer
used to sandwich the optical fibers, as
well as the slippage that may occur
between the fiber and its sheath.
On the other hand, the slope of the
fitted line into the results of heliumbased FOS is about 41.5 degrees. It
shows a deviation of 11.5 % from the
expected theoretical value. This
deviation is almost half of the laser
diode deviation and is a little higher
than that of ESG. The deviation can be
explained due to the same factors
mentioned in the laser diode section.
However, it is not as high as the laser
diode due to the shorter wavelength of
helium (632nm) compared to laser
diode wave length (835 nm).
Figure 3: Comparing the strain
output of all sensors to theoretical
strain
Generally, the accuracy of the fiber
sensor is lower than that of the
electrical gauge. However, the error is
merely systematic. Thus, the value of
the error can be either deduced and
algebraically added to the measured
value or incurred into the equation of
the calibration line in order to
compensate for this error. On the other
hand, such error limitation is counter
acted by the advantages of safely using
it in the oral cavity since it requires no
electrical connections.
In terms of sensitivity, it can be easily
shown that it is higher in the case of
FOS compared to ESG. Since the
measurement in the FOS is counts,
thus, the smallest detectable signal is
one count. According to equation (1)
the smallest detectable strain value by
FOS can be determined to be 0.017 µstrain. On the other hand, a sensitive
commercial strain meter can detect
down to one µ-strain. This shows how
high the sensitivity of the FOS
compared to the ESG. Hence, the FOS
can be applied to measure very small
as well as high acting forces at the
same time which need to be measured
in dental occlusal analysis systems to
reduce risks of implant failure,
traumatized teeth, unstable dentures,
ineffective splints and porcelain
features or anywhere occlusion plays a
role.
4. Conclusions
The purpose of this study was to
compare fiber optic sensor against
electrical strain gauge sensor when
used in dentistry and/or other
biological environment. Within the
limits of this study the following can
be concluded:
1. Optical sensors provide a safer
and a more sensitive application
in the biological environment as
compared to electrical sensors.
2. The accuracy of optical fiber
sensor is a little less than that of
the stain gauge.
3. The inaccuracy is repeatable and
can be corrected by calibration
means.
4. The accuracy and sensitivity and
hence resolution of the fiber optic
sensor can be improved by using
optical light sources of shorter
wavelengths.
5. In future articles, results of
applying FOS without adhesive
will be discussed as a way to
improve its functionality.
6. References
[1] LR Bonjardim, MB Gaviao, LJ
Pereira, PM Castelo, "Bite force
determination in adolescents with
and without temporomandibular
dysfunction", J Oral Rehabil. 2005
Aug; 32(8):577-83.
[2] MC Cehreli, K Akca, "Narrowdiameter implants as terminal
support for occlusal three-unit
FPDs: a biomechanical analysis",
Int J Periodontics Restorative
Dent. 2004 Dec;24(6):513 – 519.
[3] R Mericske-Stern, GA Zarb, "In
vivo measurements of some
functional aspects with mandibular
fixed prostheses supported by
implants" Clin Oral Implants Res.
1996A Jun;7(2):153 – 161.
[4] FM van Kampen, A van der Bilt,
MS Cune, F Bosman, "The
influence of various attachment
types in mandibular implantretained overdentures on maximum
bite force and EMG", J Dent Res.
2002 Mar;81(3):170-173.
[5] MT
El-Wakd,
JB
Brunski,
"Development of a multidirectional
transducer to measure loading on
dental implants", 3rd Inter Conf.
Faculty of Engineering, Cairo,
Egypt, Faculty of Eng. Al-Azhar
Univ., Dec. 1993.
[6] F Muller, MR Heath, R Ott,
"Maximum bite force after the
replacement of complete dentures",
Gerodontology. 2001 Jul;18(1):5862.
[7] FA Fontijn_Tekamp, , AP Slagter,
A Van Der Bilt, MA Van T Hol, DJ
Witter, W Kalk, JA Jansen, "Biting
and chewing in overdentures, full
dentures, and natural dentition", J.
Dental Research, 2000 Jul, 79(9):
1519 – 1524.
[8] CP Fernandes, PO Glantz, SA
Svensson, A Bergmark, " A novel
sensor
for
bite
force
determinations", Dent Mater. 2003
Mar;19(2):118-26.
[9] T Elsarnagawy, "A fiber-optic
based strain sensor for medical
applications",
First
annual
conference
on
Biomedical
Engineering ACBME, Cairo, 2002.
[10] Y
Zhao,
Y
Liao,
"Discrimination methods
and
demodulation techniques for fiber
Bragg grating sensors", Optics and
lasers in Engineering. 2004 Jan;
41(1): p 1-18.
[11] WC Wang et. al., "A shear and
plantar pressure sensor based on
fiber-optic bend loss", Journal of
Rehabilitation
Research
&
Development. 2005 Jun; 42(3): p
315-326.