Design and evaluation of a Force Sensing Resistor based bite force measuring device. V. Chongsuvivatwong and A.F. Geater

Transcription

1 78 Int. J. Biomedical Engineering and Technology, Vol. 4, No. 1, 2010 Design and evaluation of a Force Sensing Resistor based bite force measuring device U. Thongudomporn* Department of Preventive Dentistry, Prince of Songkla University, Hatyai, Songkhla 90110, Thailand Fax: udom.t@psu.ac.th *Corresponding author P. Smithmaitrie Department of Mechanical Engineering, Prince of Songkla University, Hatyai, Songkhla 90110, Thailand Fax: spruitti@me.psu.ac.th V. Chongsuvivatwong and A.F. Geater Epidemiology Unit, Prince of Songkla University, Hatyai, Songkhla 90110, Thailand Fax: cvirasak@medicine.psu.ac.th Alan.g@psu.ac.th Abstract: A Force Sensing Resistor (FSR) based bite force measuring device has been developed to determine human Maximum Bite Force (MBF) generated from a group of posterior teeth, using inexpensive parts and common laboratory electronic instruments. The relationship between applied force and sensor output fitted well with a polynomial regression equation. The mean ± SD of the error was 3.9 ± 3.4% and that of the coefficient of variation was 2.2 ± 0.5%. The mean maximum hysteresis was ± 2.72 mv. Unilateral MBF measured by this instrument on 80 adolescents was 692 ± N in males and 613 ± N in females. Clinical test-retest repeatability was favourable (r = ). This FSR sensor could be useful for the determination of MBF in clinical and field research work. Keywords: bite force; FSR; force sensing resistors; calibration; accuracy; reliability. Reference to this paper should be made as follows: Thongudomporn, U., Smithmaitrie, P., Chongsuvivatwong, V. and Geater, A.F. (2010) Design and evaluation of a Force Sensing Resistor based bite force measuring device, Int. J. Biomedical Engineering and Technology, Vol. 4, No. 1, pp Copyright 2010 Inderscience Enterprises Ltd.

2 Design and evaluation of a Force Sensing Resistor based bite force 79 Biographical notes: Udom Thongudomporn is an Assistant Professor in Orthodontics at the Faculty of Dentisty, Prince of Songkla University with 15 years of teaching and research experience. He is currently a PhD candidate in Epidemiology. His main interests and research activities are mainly on the effect of masticatory function on craniofacial growth and development. Pruittikorn Smithmaitrie received the BEng (ME) Degree from the Prince of Songkla University, Thailand, in 1996, and the MS (ME) Degree from Vanderbilt University, USA, in 2000, and the PhD (ME) Degree from University of Kentucky, USA, in He is currently an Assistant Professor at the Department of Mechanical Engineering, Prince of Songkla University. He is the Coordinator of the Mechatronics Curriculum of the Mechanical Engineering Department. His research interests focus on ultrasonic motors, piezoelectric devices and mechatronics for rehabilitation. He has published many international journal papers in these topics. Virasakdi Chongsuvivatwong is a Full Professor in Epidemiology with more than 25 years of teaching and research experience in international health. He wrote a package epicalc to enable R statistical software working for epidemiological analysis smoothly. He is also a co-author of more than 120 papers published in international peer-reviewed journals and received over 600 citations based on Scorpus search. Alan F. Geater is currently a Lecturer in Epidemiology at Prince of Songkla University, Thailand. His research career has spanned almost 40 years, including such diverse areas as mammalian reproductive physiology, coral reef ecology and environmental epidemiology. His current interest is in the epidemiology and spatial modelling of environmental contaminants. He is a contributing author of over 100 publications in international peer-reviewed journals. 1 Introduction Bite force is the force exerted between the maxillary and mandibular teeth resulting from the action of jaw elevator muscles when the mandible is elevated. Researchers have used Maximum Bite Force (MBF) as an indicator of the masticatory function assessment (Ahlberg et al., 2003; Cosme et al., 2005; Hagberg, 1987). Various designs of clinic-based bite force measuring devices have been developed ranging from simple mechanical equipment to complex electronic devices (Kazuo et al., 1998; Lyons et al., 1992; Shiau and Wang, 1993; Waltimo and Kononen, 1993, 1995). They have been generally claimed to be acceptably accurate and reliable. However, recorded forces have varied with the design of force measuring device and the recording technique (Bakke et al., 1989). A simple device based on thin and compact, but inexpensive, sensors would be useful for a large-scale epidemiological survey. Force Sensing Resistor (FSR), has been used for pressure and force measurement in several applications, including muscle force (Smith and Hudson, 1994), functional spinal load (Jensen et al., 1991), finger and handgrip force (Hedman, 1992; Kargov et al., 2004; Nikonovas et al., 2004), and heel force (Pataky et al., 2005). It is flexible, thin, light, inexpensive, robust under repetitive use, and resistant to temperature, chemical and moisture (Cleary et al., 2001; Coronado et al., 1999). This type of sensor was also used to

3 80 U. Thongudomporn et al. determine bite force produced from single-pair teeth with limited calibration force range (Fernandes et al., 2003). Our aim was to develop and evaluate a FSR-based force measuring device to determine bite forces produced from a group of posterior teeth, which is closer to normal biting physiology. 2 Materials and methods 2.1 Design specification In Figures 1 and 2, the sensor was a 0.46 mm thick circular-shaped FSR (FSR Part No 402, Interlink Electronics, Camarillo, CA, USA), with an active area of 12.7 mm diameter. FSR s are polymer thick-film devices which exhibit a decrease in resistance with an increase in the force applied to the active surface (Interlink Electronics, 2006). They consist of conducting interdigitated electrodes deposited on a thermoplastic sheet which is faced with a second sheet coated with semiconductive polyetherimide ink. Applying a force to the surface of the FSR causes semiconductive particles to be compressed against the conducting electrodes, thus reducing the resistance of the sheet (Interlink Electronics, 2006). A stainless steel disk of 12 mm diameter and 0.5 mm thickness was attached on the active area of the sensor to direct the bite force to the sensor s active area. These two elements were sandwiched between a pair of rectangular shaped ( mm) stainless steel plates. The remaining sandwiched space was filled with viscoelastic material (Silagum Light, DMG, Hamburg, Germany) to keep the plates from rocking. Two rubber sheets of 3 mm thickness were attached to the outer surface of the stainless steel plates to produce the dental protection. All components were bonded with cyanoacrylate adhesive. The total device thickness was 11 mm, which was within the 9 20 mm range recommended by earlier studies (Lindauer et al., 1993; van Steenberghe and De Vries, 1978). To prevent cross-contamination, the device was covered with the thumb tip part of a disposable latex glove which was changed after each use and the biting surfaces were sterilised with disinfecting solution. Figure 1 (A) A circular-shaped Force Sensing Resistor (FSR); (B) top view and (C) side view of the FSR after being attached with the stainless steel plates and the dental protection (A) (B) (C)

4 Design and evaluation of a Force Sensing Resistor based bite force 81 Figure 2 (A) dimensions of the Force Sensing Resistor and (B) design diagram of the bite force measuring device (A) (B) The sensor was connected to a current-to-voltage converter circuit (Figure 3). The output voltage was transferred to a digital storage oscilloscope (Model GDS-800, Good Will Instrument Co., Ltd., Taiwan). The data were transferred to a microcomputer and analysed with Free Capture software, version Figure 3 Schematic diagram of the electronic interface (see online version for colours) The output of the circuit is described by the following equation: V out = R R o FSR V ref where V out is the voltage output; V ref is the reference voltage; R o is the reference resistance and R FSR is the FSR resistance. The reference voltage was 5 V and the reference resistance was 25Ω. The sampling rate of the oscilloscope was 25 samples/sec. 2.2 Calibration The sensor composition at the calibration stage was in the same condition as in clinical use, i.e., the sensor was calibrated with attached dental protection. The friction of the

5 82 U. Thongudomporn et al. rubber surface prevented slipping of teeth when clenching, hence forces expressed through the sensor were mainly in the vertical direction resembling the automated condition of the validating instrument, a Lloyd Universal Testing Machine (LRS Plus model, AMETEK Lloyd Instruments Ltd, Hampshire, UK). Force from the load cell was determined by the Nexygen material testing software (AMETEK Lloyd Instruments Ltd., Hampshire, UK). At calibration, the device was placed on a flat stainless steel platform and was compressed with a cylindrical-shaped stainless steel loading fixture of 3.5 cm diameter to the pre-determined force levels for 2 s. Three stepwise calibrations were carried out at 100 N increments and decrements alternately within the range of N, starting in the up-scale direction. A polynomial regression model under least-mean-squares technique was used to finalise the calibration curve. 2.3 Accuracy, reliability and hysteresis error In the real situation, the bite force measuring device should remain accurate and reliable in its used condition, which may be different from the unused baseline condition. Hence, the device was subjected to 180 natural bites prior to the test of accuracy, reliability, and hysteresis error. Forces interpolated from the calibration curve were compared against applied forces read from the universal testing machine with five series of seven levels of force (270, 430, 620, 840, 1050, 1220 and 1360 N) alternately run in step-up and step-down directions. Error of measurement (E force ) was calculated using the following equation: E force = [(F sensor F loadcell )/F loadcell ] 100% where F sensor is the bite force sensor measurement of force and F loadcell is the applied load cell measurement of force. The E force s of all replications at the same applied force level were averaged to represent the measurement error of that particular force level. The 35 values of E force s were pooled and the grand average value of E force ± SD was calculated. Next, coefficient of variation (%CV), the standard deviation as a percentage of the mean of F sensor s (SD 100/mean) at each level of force read from the device, was computed. Then, the values of %CV of all force levels were used to compute the grand average %CV ± SD of the device. Hysteresis error, defined as the difference in the sensor output response during step-up loading and step-down loading at the same force, was determined from two cycles of seven levels of force corresponding to those in the test of accuracy, starting with step-up direction. Maximum hysteresis of each cycle was identified and the mean of these values was calculated. 2.4 Clinical assessment The clinical component of this study was approved by the ethics committee of the Faculty of Medicine, Prince of Songkla University, and the informed consent of all subjects participating in the investigation was obtained from their parents. Eighty adolescents (39 males and 41 females) were randomly selected from a secondary school in Hatyai city, Songkhla province, Thailand. The age range was from 12.9 to 14.7 years, with a mean ± SD of 13.8 ± 0.4 years. The age difference of males and females was not statistically significant (p = 0.52). All subjects must have had full eruption of permanent

6 Design and evaluation of a Force Sensing Resistor based bite force 83 dentition from right first molar to left first molar and been free from any craniofacial deformity, dental anomalies, temporomandibular joint disorders, major tooth destruction or reconstruction, and history of orthodontic treatment. After getting acquainted with the device and procedure, the subject was seated upright with relaxed head posture on a stool without a backrest. The device was placed between the upper and lower posterior teeth from upper first premolar onwards posteriorly. The subject then was instructed and encouraged to bite as hard as possible for a period of 2 s, three times in succession, with at least 20 s of resting between each bite. The procedure was then repeated on the contralateral side after one minute interval. The greatest values of the right and left MBF s were averaged to represent the subject s unilateral MBF. The recording process for all subjects was performed exclusively by the first author to minimise inter-observer error. MBF s of the first 20 consecutive subjects who participated in the initial bite force testing were re-measured after a 3-week interval following the same procedure. The correlation coefficient of the MBF between the test and re-test measurement was computed. The level of statistical significance was set at p < 0.05 for all statistical analyses. Figure 4 Representative calibration curve of the bite-force measuring device. The curve was linearised with a least squares fifth-order polynomial regression equation 3 Results The final fitted calibration curve was a fifth-order polynomial regression equation with high level of coefficient of determination (R 2 = ) (Figure 4). The grand average E force ± SD was 3.9 ± 3.4% with a maximum error of 7.2% at 1050 N. Average %CV was 2.2 ± 0.5 (Table 1). The mean ± SD of maximum hysteresis was ± 2.72 mv.

7 84 U. Thongudomporn et al. Table 1 Summary of accuracy and repeatability of the bite-force sensor Force (N) Accuracy error (%) Repeatability CV (%) Average 3.9 ± ± 0.5 The mean ± SD MBF s of males and females were 692 ± N and 613 ± N respectively (independent t-test, p < 0.005). The correlation coefficient (r) of the right and left MBF between the test and re-test measurement was 0.92 and 0.88 respectively (p < 0.001). 4 Discussion This study describes the design of a device for measuring bite force generated from a group of posterior teeth using the FSR as a sensor. Comparing the read forces with known loads under automated conditions, the device was accurate and reliable, although the calibration curve was not absolutely linear. The amount of hysteresis was small. The device was clinically reliable. With our calibrating range of N, and that of Fernandes and co-workers (2003) of N, polynomial relationships of fifth order between applied force and sensor output were consistently found. The reason for such repeated phenomena has yet to be researched. The high R 2 confirmed that interpolation of the result within this force range can be considered acceptable. Table 2 compares the size, calibration and clinical test results of three earlier bite force measuring devices, and the present one. When dental protection is taken into account, the thickness of the device varies between mm. A positive relationship tendency between the thickness of the device and the maximum calibration force may suggest a need for more robust materials to withstand larger loads. Test parameters recorded in this study are within the range of those reported earlier. The varying MBF values could be due to several factors, such as anatomical and physiologic characteristics of the subjects, position of the mandible, vertical separation of the jaws and number of teeth involved (Bakke, 2006; Braun et al., 1995) In clinical use for a single patient, the application of individual occlusal guide has been used to warrant the identical point of force application for repeated test (Khan et al., 2007; Tortopidis et al., 1998). However, this method is not practical for most epidemiological studies when a large number of subjects is to be tested. In the present study, the consistency of test and re-test right-left maximum bite force indicates that the protocol of placing the sensor between upper and lower posterior teeth from upper first premolar onwards posteriorly is sufficient to produce within-subject reliability.

8 Design and evaluation of a Force Sensing Resistor based bite force 85 Table 2 Comparison of sensor type, size, calibration outcomes, and clinical test results of four bite force measuring devices Floystrand et al. (1982) Waltimo and Kononen (1993) Fernandes et al. (2003) Present study Sensor type Semiconductor Kistler quartz Force Sensing Resistor Force Sensing Resistor Dimension (W L H) (mm) In Vitro calibration Calibration force range (N) Accuracy Na Correction coefficients a * 12 Na 22** Ø12, H = 7.5** ** N = N = 0.89 Correlation coefficient b = % error Reliability 4%CV SD range N Na 2.2%CV In Vivo study Test against other device Na Na Non-significant difference from strain gauged bite force meter (p = 0.21) Na Reliability Intra-individual variation < 15 N Na 93.8% reliability c Test-retest correlation coefficient = Clinical test Age range (years) Sample size 16 (Male = 8, Female = 8) Maximum bite force (N) 30 (Male = 15, Female = 15) Male = 847 ± 131 Female = 597 ± 94 6 (Male = 3, Female = 3) 80 (Male = 39, Female = 41) Male = 692 ± Female = 613 ± *Height of metal housing only. Dental protection component and metal lever not included. **Height measured with dental protection component. Na = Data not available. a Calculated after 500 clinical measurements. b Correlation between bite force signal from the sensor vs. force input from the universal testing machine. c Tested at submaximum bite force level using visual feedback method. While a flat testing probe could be easily standardised making assessment results comparable between studies, the force was solely vertical which may not represent human dental anatomy and actual masticatory function. Further in vivo assessment of this device against available bite force meters should be performed. Despite these limitations, the good accuracy and reliability of this inexpensive and easy to set up device imply that it can be a potentially good instrument for epidemiological study of bite force. Regular monitoring of the device is recommended to detect errors which may arise from unexpected accidents or from slow deterioration.

9 86 U. Thongudomporn et al. Acknowledgements The project was a part of the thesis of the first author to fulfill the requirement of the PhD Degree on Epidemiology at Prince of Songkla University. The study was supported by funding from the Epidemiology Unit, Faculty of Medicine, Prince of Songkla University. The authors thank Mrs. Chanya Chuenarrom for technical support. References Ahlberg, J.P., Koveo, O.A., Hurmerinta, K.A., Zepa, I., Nissinen, M.J. and Kononen, M.H. (2003) Maximal bite force and its association with signs and symptoms of TMD, occlusion, and body mass index in a cohort of young adults, Cranio, Vol. 21, pp Bakke, M. (2006) Bite force and occlusion, Semin. Orthod., Vol. 12, pp Bakke, M., Michler, L., Han, K. and Moller, E. (1989) Clinical significance of isometric bite force versus electrical activity in temporal and masseter muscles, Scand. J. Dent. Res., Vol. 97, pp Braun, S., Bantleon, H.P., Hnat, W.P., Freudenthaler, J.W., Marcotte, M.R. and Johnson, B.E. (1995) A study of bite force, Part 1: relationship to various physical characteristics, Angle. Orthod., Vol. 65, pp Cleary, J., Morrison, P. and Brook, P. (2001) Push technology the computer meets the FSR, Paper presented at the 14th Annual Conference of the National Advisory Committee on Computing Qualifications, 2 5 July, Napier, New Zealand. Coronado, J.L., Ulloa-Perez, A., Garcia-Cordova, F. and Villalba-Fernandez, J.J. (1999) Experimental development for thumb-index pinch force measurements during object manipulation with application to robotic hands, IEEE on System, Man and Cybernetic, SMC-99, Vol. 6, pp Cosme, D.C., Baldisserotto, S.M., Canabarro, S.A. and Shinkai, R.S. (2005) Bruxism and voluntary maximal bite force in young dentate adults, Int. J. Prosthodont., Vol. 18, pp Fernandes, C.P., Glantz, P.O., Svensson, S.A. and Bergmark, A. (2003) A novel sensor for bite force determinations, Dent. Mater., Vol. 19, pp Floystrand, F., Kleven, E. and Oilo, G. (1982) A novel miniature bite force recorder and its clinical application, Acta. Odontol. Scand., Vol. 40, pp Hagberg, C. (1987) Assessments of bite force: a review, J. Craniomandib. Disord. Facial. Oral. Pain., Vol. 1, pp Hedman, T. (1992) A new transducer for facet force measurement in the lumbar spine, J. Biomech., Vol. 25, pp Interlink Electronics (2006) Force Sensing Resistors. Integration Guide and Evaluation Parts Catalog with Selected Electrical Interfaces, Camarillo, CA, USA. Jensen, T.R., Radwin, R.G. and Webster, J.G. (1991) A conductive polymer sensor for measuring external finger forces, J. Biomech., Vol. 24, pp Kargov, A., Pylatiuk, C., Martin, J., Schulz, S. and Doderlein, L. (2004) A comparison of the grip force distribution in natural hands and in prosthetic hands, Disabil. Rehabil., Vol. 26, pp Kazuo, E., Shigetaka, Y., Masatsugu, S. and Akira, H. (1998) Development and clinical application of a new occlusal force measurement apparatus wave forms and FFT power spectrum analysis, Pathophysiology, Vol. 4, pp Khan, A.A., McCreary, B., Owatz, C.B., Schindler, W.G., Schwartz, S.A., Keiser, K. and Hargreaves, K.M. (2007) The development of a diagnostic instrument for the measurement of mechanical allodynia, J. Endod., Vol. 33, pp

10 Design and evaluation of a Force Sensing Resistor based bite force 87 Lindauer, S., Gay, J. and Rendell, J. (1993) Effect of jaw opening on masticatory muscle EMG-force characteristics, J. Dent. Res., Vol. 72, pp Lyons, M.F., Sharkey, S.W. and Lamey, P.J. (1992) An evaluation of the T-scan computerized occlusal analysis system, Int J Prosthodont, Vol. 5, pp Nikonovas, A., Harrison, A.J., Hoult, S. and Sammut, D. (2004) The application of force-sensing resistor sensors for measuring forces developed by the human hand, Proc. Inst. Mech. Eng., Vol. 218, pp Pataky, Z., Assal, J.P., Conne, P., Vuagnat, H. and Golay, A. (2005) Plantar pressure distribution in Type 2 diabetic patients without peripheral neuropathy and peripheral vascular disease, Diabet. Med., Vol. 22, pp Shiau, W. and Wang, J.S. (1993) The effects of dental condition on hand-strength and maximum bite force, Cranio., Vol. 11, pp Smith, B.P. and Hudson, W.B. (1994) The use of force sensing resistors for muscle force measurements, Biomed. Sci. Instrum., Vol. 30, pp Tortopidis, D., Lyons, M.F., Baxendale, R.H. and Gilmour, W.H. (1998) The variability of bite force measurement between sessions, in different positions within the dental arch, J. Oral. Rehabil., Vol. 25, pp van Steenberghe, D. and De Vries, J.H. (1978) The development of a maximum clenching force between two antagonistic teeth, J. Periodontal. Res., Vol. 13, pp Waltimo, A. and Kononen, M. (1993) A novel bite force recorder and maximal isometric bite force values for healthy young adults, Scand. J. Dent. Res., Vol. 101, pp Waltimo, A. and Kononen, M. (1995) Maximal bite force and its association with signs and symptoms of craniomandibular disorders in young Finnish non-patients, Acta. Odontol. Scand., Vol. 53, pp