- Email: [email protected]
Telemetry system to study functional occlusal forces
Telemetry
system to study functional
W. D. McCall, Jr., Ph.D,* J. A. De Boever, L.D.S., D.M.D., M. M. Ash, Jr., D.D.S., M.S.‘** Univemity
of ?v4ichigan, School of Dentistry.
tatic bite (closing) force has been investigated for centuries by many investigators using different methods, equipment, and conditions.‘-“’ Occlusal forces exerted by the masticatory muscles have also been investigated during chewing and swallowing, using strain gauges,“.” pressure transducers,‘“-‘” and piezoelectric devices.“‘-Z! Methods of simultaneously recording functional occlusal forces and mandibular movements have also been reported.‘!‘-” In very few reports, however, has the electrical activity of the masticatory muscles been related to the occlusal forces.“-” These investigations have contributed significantly to our understanding of the neuromuscular basis of the masticatory system. However, factors such as the disturbing influence of transoral linkages have caused a continued search for more precise and independent methods that do not disturb the system. Therefore, telemetric devices have been developed to continously monitor occlusal forces during function 1I-L’i. The purpose of the present research was to develop an intraoral force telemetry system that would register occlusal forces during function without the disturbing influence of transoral linkages.
METHODS placement.
Strain gauges vary their resistance in response to deformation and. in conjunction with beams, measure forces. The development of semiconductor strain gauges has made available gauge factors of ‘Awxiate Dentist?.
Professor, Department of Oral .Medicinc, State University of New York at Buffalo.
School of Buffalo.
N. Y.
l *Associate University,
Profcuor, C;hent.
Department Belgium.
of Prosthodontics,
Ghent
***Professor IIcntistry, Supported
and ChaIrman, Department of Occlusion. School University of Michigan, Ann Arbor, STich. by USPHS Grant DE 02i31.
98
1978
JULY
VOLUME
40
NUMBER
I
and
Ann Plrhor. %lich,
S
Occlusal surface and transducer
Ph.D,**
occlusal forces
State of
100 to 200, whereas foil or wire gauges have gauge factors of 2. Prior to gauge installation, stone casts of the patient were mounted on an articulator and the occlusal surface of the pontic was waxed up according to a wax-added technique. The occlusal surface was then cast in gold. A negative of this surface and the abutment teeth was made in plaster and also cast. The gold occlusal surface was reduced 1.5 mm. providing an occlusal table for mounting the sensors. The occlusal table was soldered to a rigid frame and to the two partial denture attachments for the abutment teeth. The eight strain gauges* were attached to the occlusal table; four sensors were in the mesial, central, and distal fossae; two sensors under the supporting cusps; and two under the nonsupporting cusps (Fig. 1). The sensors were covered with a uniform layer of cold-cure acrylic resint using the negative mold previously obtained from the occlusal surface. Description of the transmitter. Since the electronic characteristics of the present telemetric system for measuring occlusal forces have been described elsewhere.:’ only the essential features will be summarized here. The transmittert is composed of three main elements: the ring counter, analog switches, and radio frequency (RF) module. The transmitter components are shown in Fig. 2. The ring counter is used as a commutator to operate a series of field effect transitor (FET) switches. There is one switch for each sensor and one additional switch for a synchronization (SYNC) pulse. These FET switches are operated sequentially by the ring counter to form a nine-pulse commutated signal. The solid-state strain sensors have an output of 20 mV maximum. This is amplified by a factor of five in a DC ‘Mcdel tKcsin $Film
DOI-04..5OCO. Perm. Dental Microclcctronics,
Celesko Electronics, (:anoga Park. Calit Manufacturing Company. Akron. Ohm Burlington. Mass.
TELEMETRIC
STUDY
OF OCCLUSAL
FORCES
BUCCAL
LINGUAL Fig. 1. Placement of strain gauges. The placement of the semiconductor strain gauges under the occlusal surface is shown schematically. The view is from the occlusal surface. The force information from each strain gauge was multiplexed for transmission and demultiplexed for analog presentation. amplifier, and the resulting signal is applied to the RF module. The RF module frequency modulates the nominally 120 MHz carrier proportionally to the pulse amplitude (PAM) commutated from the sensors. Full pulse amplitude causes 0.25 MHz of deviation from the carrier. The eight data pulses are 0.30 to 0.45 of the sync pulse (Fig. 3). The sampling rate is approximately 300 Hz for each channel. A small antenna in the buccal fold provides radiation of the signals for several feet. The power for the transmitter is provided by four small 1.5 V DC batteries* in a waterproof battery case with a screw cap. The battery drain is 3.5 mA. By meticulous selection of batteries, a continuous experimental session of up to 2 hours could be obtained. Construction of the transmitter. All electronic components of the transmitter were mounted on a board and tested on the bench. After all the components were soldered underneath the pontic, the transmitter was embedded with epoxy resin.t Testing and assembly time for each transmitter was approximately 250 hours. The configuration of the assembled transmitter is shown in Figs. 4 and 5. Receiver and decoder. A loop antenna around the patient’s neck picks up the RF signal and sends it to a conventional FM superheterodyne receiverx for amplification and demodulation into a video signal. The sync pulse is extracted from the video signal and used both to reset a counter and to delete the sync pulse from the nine pulses, leaving the data pulses. Clock pulses are generated from the data pulses and *Mallory MS 212. tResin 5005. Amrez Co., Sterling Heights, Mich. fModel GPR-20, Defense Electronics Inc., Ann Arbor,
THE JOURNAL
OF PROSTHETK
DENTISTRY
Mich
Fig. 2. Electronic components of the eight-channel occluRing sal force transmitter. Clockwisr fronl :efi [L) 7ighi: counter, FET-switches, DC amplifier, RF-module, coil, resistor of the sync pulse, capacitor, fr>zlr I .S V batteries.
Fig. 3. The signal emitted by the transmitter and demodulated by the receiver consists of a sync pulse and eight pulses varying linearly in amplitude with the resistance of the strain gauges under the occlusal surface. Calibrations: 1 V and 0.5 msec. used to drive a sample and hold circuit for each channel. Thus the output of the decoder is eight analog signals corresponding to the volta,qes from the eight sensors. Tests and calibration. The transmitter underwent a series of tests prior to use in the patient. The embedded transmitter was checked for both elecrrical and water leakage by operating it in water. Afterward, the stability of the signal was verified by connecting the transmitter to a power supply and recording for 24 hours. Each channel of each transmitter was calibrated by applying knowrl forces on the occlusal surface and recording the response from the decoder. A semidynamic calibration instrumentation system was developed to move the precision XY (horizontal) table on which the transmitter was mounted in the XY plane automatically in either a continuous or intermittent mode.” In this way the force could be applied anywhere on the occlusal
99
MCCALL,
DeBClEVER,
AND
ASH
Fig, 4. Configuration
of the assembled transmitter. I, Strain gauges. 2, Gold occlusal table. 3, Ring counter. 4, FET-switches. 5, DC amplifier. 6, RF-module. 7, Coil. 8, Antenna. 9, Battery case. A, Acrylic resin occlusal surface. B, Waterproof epoxy resin. surface. The table could be repositioned to the starting point to within +0.04 mm. Force was applied to the occlusal surface in the Z (vertical) direction by means of a small ball connected to an air piston, spring loaded in a retracted position. Precise force values were obtained with this system. All calibration curves were recorded on polygraph paper and measured twice with Boley gauges to 0.1 mm. The average of the two measurements was recorded. The velocity of the movement of the calibration table had a minimal influence on the output, since increasing the velocity of the table by 300 times changed the output by less than 5%. Calibrations at 22” C and 37” C did not show any variation in amplitude. The effect of the acrylic resin covering the strain gauges on the transient response or the frequency response was not investigated. The immediate reproducibility was checked by three calibrations during and after each recording session involving the patient. The reproducibility of the telemetry system was verified and found to be within 2% of the means. Variations due to temperature, water leakage, the speed of the calibration procedure, and movement were found to be minimal. The eight channels of force data, transmitted from the patient’s partial denture and received and
100
Fig. 5. A, Occlusal view of the radiotransmltter partial denture under construction before putting the resin on the surface. Arrows, Micro semiconductor strain gauges; Bu. Partial denture attachment. B. View of the finished transmitter bridge before insertion into the mouth. 6rc. Partial denture attachment. decoded as described above, were available along with electromyographic (EMG) data, described in a companion paper.“’ These data were recorded on the same polygraph by using the pen driver amplifier only. A block diagram of the data recording and analysis equipment is shown in Fig. 6. Three force channels and the EMG from the masseter muscle on the partial denture side were also recorded on FM tape for later replay in time expansion. Data analysis. The experimental protocol was planned around functional events, chewing various foods under various occlusal conditions, and a few voluntary but nonchewing movements such as tapping the teeth together. Each event consisted ot 15 strokes, occasionally fewer if swallowing occurred. The raw data, then. consisted of a polygraph record-
IULY
1971)
VOLUME
40
NUMBER
1
TELEMETRIC
STUDY
OF OCCLUSAL
FORCES
_-
_.-
.-
-~ - ..--
~..
E ~~ _
~. .~-
Fig. 6. Block diagram of the recording and analyzing system. All equipment, except the path from digitizer to cards to computer, were operational during the experiment. The patient /ZI with the bridge (I) is sitting in a radiofrequency-shielded room (3). The received signal (4) is decoded (61 and recorded on a polygraph (10) together with the electrical muscle activity (7j. The signals are stored on a tape recorder (13). The recordings are digitized at a later date (I J) and analyzed by computer (Ji). ing of the eight channels of force data and four channels of EMG data for each of the 15 strokes. The force data on the polygraph paper were measured by an electronic digitizer accurate to 0.001 inch which punched the data and an identifying code number on cards for later computer analysis. The data recorded on FM tape were replayed with the time scale expanded by four and recorded on the polygraph with a paper speed of 100 mm/set. From each event, 15 chewing strokes were replayed, and for each stroke up to 25 parameters were measured and punched onto cards by the digitizer. A number of special computer programs were written to convert the measurements from inches to pounds by incorporating the calibration data. The results were plotted with a Calcomp plotter” and analyzed statistically.
DISCUSSION The forces exerted on the occlusal surface by patients vary in time, duration, and position. Since the calibration system is only semidynamic, it is not ideal. In some reports however, the recording devices were calibrated by applying a static force to a part or the whole occlusal surface by means of a ballzo, 23 or a loading platform.‘4 In other reports the pressure transducers were calibrated before incorporation into prostheses,6. lb or the calibration remained unknown.” The output of the sensors is a function of three variables: the magnitude of the force, the area of contact between the force and the occlusal surface, and the distance from the force to the sensor. The calibration showed the linearity of the strain gauges
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
with the magnitude of the applied force. The calibrations were performed with a standardized contact surface between calibrator and transmitter. Occlusal contact telemetry studies showed that in almost 75% of the chewing strokes occlusal contact occ~rs.‘~ In these cases the zone and the extent of contact are known. In other instances, with food between the teeth, the contact surface varied and depended on the kind of food. Therefore, comparison of data obtained by chewing different kinds of food even in the same session must be made very carefully. On the other hand, the comparison of the values obtained in chewing the same food in different sessions can be made with more confidence. Other authors have discussed methods and devices to measure oral muscle forces.“’ They have stressed that artifacts can arise from several sources and that consideration must be given to each. From the description and calibration of the present transmitter, no artifacts exist from muscle displacement by the intraoral device, temperature fluctuations in the mouth, inadequate waterproofing of the electrical components, extraneous force due to movement of wires, nonlinear response of the device, internal factors affecting reproducibility, or movement of the transducer relative to the tooth surface. The system bandwidth is a possible limitation to accuracy. The overall bandwidth of each channel was limited to 10 Hz by the final DC amplifier in the decoder. This bandwidth would preclude analysis of pulse shape and transient phenomena if this information had been desired. Comparisons of taperecorded data indicated that the peak was increased only about 10% by increasing the bandwidth from 10 Hz to 100 Hz. Thus this bandwidth limitation
MCCALL,
does not unduly affect the peak data. In the analysis of the direct tracings, the bandwidth was not taken into consideration. In the replayed tracings, this factor is a possible limitation. In measuring lateral muscle forces, e.g., from the tongue, the zone of contact between transducer and muscle varies considerably. In our experiments the contact occurs between two small stable surfaces, and the muscles do not contact the surface direct-
17.
‘Y.
19.
CONCLUSION
20.
Considering the complexity of the problem in measuring functional occlusal forces of the masticatory muscles,an accurate and manageable telemetric system has been developed. Many tests in vitro and hundreds of recordings in several patients prove its value in the study of the physiologic mechanism of the stomatognathic system.
15. 16.
18.
21.
22.
23.
24.
REFERENCES I 2.
3. 4. 5. 6. 7. 8. 9.
10.
11. 12. 13. 14.
102
Bore& G. A.: De motu animalium (1680). Tram Royal Sot Med 26:71, 1933. Brekhus, P. J., Armstrong, W. D., and Simon, W. J.: Stimulation of the muscles of mastication. J. Dent Res 20:87. 1941. Worrier, H. K., and Andenon, M. N.: Biting force measurement on children. Aust Dent J 4&l, 1944. O’Rourke, J. ‘I‘.: Oral Physiology. St. Louis, 1951, The C. V. Mosby Company, pp 66-103. Cehlig, H.: Ueber die Kaukraft. Deutsch Zahnaerztl 2 8:30, 1953. Kraft, E.: Ueber die Bedtutung der Kaukraft fiir das Kaugeschehen. Zahnaentl Praxis 13:129, 1969. Martinko, V.: Messungen von Kriifte bei KauvorgCngen. Deutsch Zahnaentl Z ‘2Oz910, 1965. Jenkins, G. N.: The Physiology of the Mouth, ed 3. Philadelphia, 1966, F. A. Davis Company, pp 422-428. Linderholm, H, and WennstrGm, A.: Isometric bite force and its relation to general muscle force and body build. Acta Odgntol Stand 8:679, 1970. Marklund,‘G., and Wennstrtim, A.: A pilot study concerning the relation between manifest anxiety and bite force. Svensk ‘I‘andlaek T 65:107, 1971. Brudevold, F.: A basic study of the chewing forces of the denture wearer. J Am Dent Assoc 43:45, 1951. Yurkstas, A., and Curby, W. A.: Force analysis of prosthetic appliances during function. J PROSMET DENT 3:82, 1953. Anderson, D. J.: A method of recording masticatory loads. J Dent Res 32:785, 1953. Anderson, D. J.: Measurement of stress in mastication. J Dent Res 35:644, 1956.
25.
26. 27.
28.
29.
30.
31.
DcBOEVER,
AND
ASH
Anderson, D. J., and P.icton, I). C.: Masticatory ctress in normal and modified occlusion. J Dent Res 37:3 12, 1958. Eichner, K.: Aufschliisse iiber den Kauvorgang durch elektronisxhe Kaukraftmessungen. Deutsch Zahnaerzr Z 19:415. 1964. Windecker, D.: Messung des durchschnittlichen Kaukraftaufnahmevermogens van Prothesen mittels einer cigenstahiler Kaukraftmessdose. Deutsch Zahnaerztl % 19~808, 1964. Beam, E. M.: Some masticatory force pattern produced by full denture wearers. Dent Pratt Dent Ret 22:342. 1972 Briner, M.: Kaubewegung und Kaudruck in ihren wcchselseitigen Buiehungen. Med Dis Ziirich 1952. Nyquist, G.. and Owall, B.: Masticatory load registration during function. Odontol Rev 19:45, 1968. Graf, H., Grassl, H., and Aeberhard, H. J.: A method for measurement of occlusal forces in three directions. Hclvet Odontol Acta 18:7, 1974. Atkinson, H. F., and Shephard, R. W.: Masticatory movements and the resulting force. Arch Oral Biol 12:195, 1967. Ahlgren, J., and &all, B.: Muscular activity and chewing forces. A polygraphic study of human mandibular movement. Arch Oral Biol 15:271, 1970. Pameijer, C. H., and Stallard, R. E.: Intraoral forces during function in relation lo tooth contact and muscle physiology. Proc 49th Meeting Intern Assoc Dent Res, 1971 (Abstr No. 402) Scott, I.. and Ash M. M., Jr.,: A six channel intraoral transmitter for measuring occlusal forces. J PROSWET DEW l&56, 1966. Kydd, W. L., and Mullins, G.: A telemetry system for intraoral pressures. Arch Oral Biol 8:235, 1963. Scott, I., and Ash M. M., Jr.,: Micro-miniature intraoral telemetry system. Proc 6th Intern Telemetq Conference, LOS Angeles, 1970, pp 122-127. Ash. M. M., Jr., and Scott, I. S.: Calibrarlon of six channel intraoral occlusal forces transmitters. Report 07975. Office of Research Administration, University of Michigan. Ann Arbor, Michigan, 1967. De Bcevcr, J., McCall, W. D., Holden, S., and Ash, M. M Functional occlusal forces: An investigation by telemetry. J PROSTHET DEN.r (Submitted for publication). Pameijer, J. H., Glickman, I., and Roeber, F. W.: Intraoral occlusal telemetry. III. Tooth contact in chewing, swallowing, and bruxism. J Periodontol 40:253, 1969. Lear, C. S. C., Cats, J., Grossman, R. C., Glanagan. J. B.. and Morrees, C. F. A.: Measurement of lateral muscle forces on dental arches. Arch Oral Biol 10:669. 1965.
Rcprtnt rcque~~slo. DR. J. DF. B~EVER JhPAR’IMENT
OF PROsTHoUoN.rIcS
STOMATOLOClE AKADEMISCH
ZIEKENHUIS
DE PINTELAAK.
135
B 9000 BELGIUM
GENT
JULY
1978
VOLUME
40
NUMBER
1
system to study functional
W. D. McCall, Jr., Ph.D,* J. A. De Boever, L.D.S., D.M.D., M. M. Ash, Jr., D.D.S., M.S.‘** Univemity
of ?v4ichigan, School of Dentistry.
tatic bite (closing) force has been investigated for centuries by many investigators using different methods, equipment, and conditions.‘-“’ Occlusal forces exerted by the masticatory muscles have also been investigated during chewing and swallowing, using strain gauges,“.” pressure transducers,‘“-‘” and piezoelectric devices.“‘-Z! Methods of simultaneously recording functional occlusal forces and mandibular movements have also been reported.‘!‘-” In very few reports, however, has the electrical activity of the masticatory muscles been related to the occlusal forces.“-” These investigations have contributed significantly to our understanding of the neuromuscular basis of the masticatory system. However, factors such as the disturbing influence of transoral linkages have caused a continued search for more precise and independent methods that do not disturb the system. Therefore, telemetric devices have been developed to continously monitor occlusal forces during function 1I-L’i. The purpose of the present research was to develop an intraoral force telemetry system that would register occlusal forces during function without the disturbing influence of transoral linkages.
METHODS placement.
Strain gauges vary their resistance in response to deformation and. in conjunction with beams, measure forces. The development of semiconductor strain gauges has made available gauge factors of ‘Awxiate Dentist?.
Professor, Department of Oral .Medicinc, State University of New York at Buffalo.
School of Buffalo.
N. Y.
l *Associate University,
Profcuor, C;hent.
Department Belgium.
of Prosthodontics,
Ghent
***Professor IIcntistry, Supported
and ChaIrman, Department of Occlusion. School University of Michigan, Ann Arbor, STich. by USPHS Grant DE 02i31.
98
1978
JULY
VOLUME
40
NUMBER
I
and
Ann Plrhor. %lich,
S
Occlusal surface and transducer
Ph.D,**
occlusal forces
State of
100 to 200, whereas foil or wire gauges have gauge factors of 2. Prior to gauge installation, stone casts of the patient were mounted on an articulator and the occlusal surface of the pontic was waxed up according to a wax-added technique. The occlusal surface was then cast in gold. A negative of this surface and the abutment teeth was made in plaster and also cast. The gold occlusal surface was reduced 1.5 mm. providing an occlusal table for mounting the sensors. The occlusal table was soldered to a rigid frame and to the two partial denture attachments for the abutment teeth. The eight strain gauges* were attached to the occlusal table; four sensors were in the mesial, central, and distal fossae; two sensors under the supporting cusps; and two under the nonsupporting cusps (Fig. 1). The sensors were covered with a uniform layer of cold-cure acrylic resint using the negative mold previously obtained from the occlusal surface. Description of the transmitter. Since the electronic characteristics of the present telemetric system for measuring occlusal forces have been described elsewhere.:’ only the essential features will be summarized here. The transmittert is composed of three main elements: the ring counter, analog switches, and radio frequency (RF) module. The transmitter components are shown in Fig. 2. The ring counter is used as a commutator to operate a series of field effect transitor (FET) switches. There is one switch for each sensor and one additional switch for a synchronization (SYNC) pulse. These FET switches are operated sequentially by the ring counter to form a nine-pulse commutated signal. The solid-state strain sensors have an output of 20 mV maximum. This is amplified by a factor of five in a DC ‘Mcdel tKcsin $Film
DOI-04..5OCO. Perm. Dental Microclcctronics,
Celesko Electronics, (:anoga Park. Calit Manufacturing Company. Akron. Ohm Burlington. Mass.
TELEMETRIC
STUDY
OF OCCLUSAL
FORCES
BUCCAL
LINGUAL Fig. 1. Placement of strain gauges. The placement of the semiconductor strain gauges under the occlusal surface is shown schematically. The view is from the occlusal surface. The force information from each strain gauge was multiplexed for transmission and demultiplexed for analog presentation. amplifier, and the resulting signal is applied to the RF module. The RF module frequency modulates the nominally 120 MHz carrier proportionally to the pulse amplitude (PAM) commutated from the sensors. Full pulse amplitude causes 0.25 MHz of deviation from the carrier. The eight data pulses are 0.30 to 0.45 of the sync pulse (Fig. 3). The sampling rate is approximately 300 Hz for each channel. A small antenna in the buccal fold provides radiation of the signals for several feet. The power for the transmitter is provided by four small 1.5 V DC batteries* in a waterproof battery case with a screw cap. The battery drain is 3.5 mA. By meticulous selection of batteries, a continuous experimental session of up to 2 hours could be obtained. Construction of the transmitter. All electronic components of the transmitter were mounted on a board and tested on the bench. After all the components were soldered underneath the pontic, the transmitter was embedded with epoxy resin.t Testing and assembly time for each transmitter was approximately 250 hours. The configuration of the assembled transmitter is shown in Figs. 4 and 5. Receiver and decoder. A loop antenna around the patient’s neck picks up the RF signal and sends it to a conventional FM superheterodyne receiverx for amplification and demodulation into a video signal. The sync pulse is extracted from the video signal and used both to reset a counter and to delete the sync pulse from the nine pulses, leaving the data pulses. Clock pulses are generated from the data pulses and *Mallory MS 212. tResin 5005. Amrez Co., Sterling Heights, Mich. fModel GPR-20, Defense Electronics Inc., Ann Arbor,
THE JOURNAL
OF PROSTHETK
DENTISTRY
Mich
Fig. 2. Electronic components of the eight-channel occluRing sal force transmitter. Clockwisr fronl :efi [L) 7ighi: counter, FET-switches, DC amplifier, RF-module, coil, resistor of the sync pulse, capacitor, fr>zlr I .S V batteries.
Fig. 3. The signal emitted by the transmitter and demodulated by the receiver consists of a sync pulse and eight pulses varying linearly in amplitude with the resistance of the strain gauges under the occlusal surface. Calibrations: 1 V and 0.5 msec. used to drive a sample and hold circuit for each channel. Thus the output of the decoder is eight analog signals corresponding to the volta,qes from the eight sensors. Tests and calibration. The transmitter underwent a series of tests prior to use in the patient. The embedded transmitter was checked for both elecrrical and water leakage by operating it in water. Afterward, the stability of the signal was verified by connecting the transmitter to a power supply and recording for 24 hours. Each channel of each transmitter was calibrated by applying knowrl forces on the occlusal surface and recording the response from the decoder. A semidynamic calibration instrumentation system was developed to move the precision XY (horizontal) table on which the transmitter was mounted in the XY plane automatically in either a continuous or intermittent mode.” In this way the force could be applied anywhere on the occlusal
99
MCCALL,
DeBClEVER,
AND
ASH
Fig, 4. Configuration
of the assembled transmitter. I, Strain gauges. 2, Gold occlusal table. 3, Ring counter. 4, FET-switches. 5, DC amplifier. 6, RF-module. 7, Coil. 8, Antenna. 9, Battery case. A, Acrylic resin occlusal surface. B, Waterproof epoxy resin. surface. The table could be repositioned to the starting point to within +0.04 mm. Force was applied to the occlusal surface in the Z (vertical) direction by means of a small ball connected to an air piston, spring loaded in a retracted position. Precise force values were obtained with this system. All calibration curves were recorded on polygraph paper and measured twice with Boley gauges to 0.1 mm. The average of the two measurements was recorded. The velocity of the movement of the calibration table had a minimal influence on the output, since increasing the velocity of the table by 300 times changed the output by less than 5%. Calibrations at 22” C and 37” C did not show any variation in amplitude. The effect of the acrylic resin covering the strain gauges on the transient response or the frequency response was not investigated. The immediate reproducibility was checked by three calibrations during and after each recording session involving the patient. The reproducibility of the telemetry system was verified and found to be within 2% of the means. Variations due to temperature, water leakage, the speed of the calibration procedure, and movement were found to be minimal. The eight channels of force data, transmitted from the patient’s partial denture and received and
100
Fig. 5. A, Occlusal view of the radiotransmltter partial denture under construction before putting the resin on the surface. Arrows, Micro semiconductor strain gauges; Bu. Partial denture attachment. B. View of the finished transmitter bridge before insertion into the mouth. 6rc. Partial denture attachment. decoded as described above, were available along with electromyographic (EMG) data, described in a companion paper.“’ These data were recorded on the same polygraph by using the pen driver amplifier only. A block diagram of the data recording and analysis equipment is shown in Fig. 6. Three force channels and the EMG from the masseter muscle on the partial denture side were also recorded on FM tape for later replay in time expansion. Data analysis. The experimental protocol was planned around functional events, chewing various foods under various occlusal conditions, and a few voluntary but nonchewing movements such as tapping the teeth together. Each event consisted ot 15 strokes, occasionally fewer if swallowing occurred. The raw data, then. consisted of a polygraph record-
IULY
1971)
VOLUME
40
NUMBER
1
TELEMETRIC
STUDY
OF OCCLUSAL
FORCES
_-
_.-
.-
-~ - ..--
~..
E ~~ _
~. .~-
Fig. 6. Block diagram of the recording and analyzing system. All equipment, except the path from digitizer to cards to computer, were operational during the experiment. The patient /ZI with the bridge (I) is sitting in a radiofrequency-shielded room (3). The received signal (4) is decoded (61 and recorded on a polygraph (10) together with the electrical muscle activity (7j. The signals are stored on a tape recorder (13). The recordings are digitized at a later date (I J) and analyzed by computer (Ji). ing of the eight channels of force data and four channels of EMG data for each of the 15 strokes. The force data on the polygraph paper were measured by an electronic digitizer accurate to 0.001 inch which punched the data and an identifying code number on cards for later computer analysis. The data recorded on FM tape were replayed with the time scale expanded by four and recorded on the polygraph with a paper speed of 100 mm/set. From each event, 15 chewing strokes were replayed, and for each stroke up to 25 parameters were measured and punched onto cards by the digitizer. A number of special computer programs were written to convert the measurements from inches to pounds by incorporating the calibration data. The results were plotted with a Calcomp plotter” and analyzed statistically.
DISCUSSION The forces exerted on the occlusal surface by patients vary in time, duration, and position. Since the calibration system is only semidynamic, it is not ideal. In some reports however, the recording devices were calibrated by applying a static force to a part or the whole occlusal surface by means of a ballzo, 23 or a loading platform.‘4 In other reports the pressure transducers were calibrated before incorporation into prostheses,6. lb or the calibration remained unknown.” The output of the sensors is a function of three variables: the magnitude of the force, the area of contact between the force and the occlusal surface, and the distance from the force to the sensor. The calibration showed the linearity of the strain gauges
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
with the magnitude of the applied force. The calibrations were performed with a standardized contact surface between calibrator and transmitter. Occlusal contact telemetry studies showed that in almost 75% of the chewing strokes occlusal contact occ~rs.‘~ In these cases the zone and the extent of contact are known. In other instances, with food between the teeth, the contact surface varied and depended on the kind of food. Therefore, comparison of data obtained by chewing different kinds of food even in the same session must be made very carefully. On the other hand, the comparison of the values obtained in chewing the same food in different sessions can be made with more confidence. Other authors have discussed methods and devices to measure oral muscle forces.“’ They have stressed that artifacts can arise from several sources and that consideration must be given to each. From the description and calibration of the present transmitter, no artifacts exist from muscle displacement by the intraoral device, temperature fluctuations in the mouth, inadequate waterproofing of the electrical components, extraneous force due to movement of wires, nonlinear response of the device, internal factors affecting reproducibility, or movement of the transducer relative to the tooth surface. The system bandwidth is a possible limitation to accuracy. The overall bandwidth of each channel was limited to 10 Hz by the final DC amplifier in the decoder. This bandwidth would preclude analysis of pulse shape and transient phenomena if this information had been desired. Comparisons of taperecorded data indicated that the peak was increased only about 10% by increasing the bandwidth from 10 Hz to 100 Hz. Thus this bandwidth limitation
MCCALL,
does not unduly affect the peak data. In the analysis of the direct tracings, the bandwidth was not taken into consideration. In the replayed tracings, this factor is a possible limitation. In measuring lateral muscle forces, e.g., from the tongue, the zone of contact between transducer and muscle varies considerably. In our experiments the contact occurs between two small stable surfaces, and the muscles do not contact the surface direct-
17.
‘Y.
19.
CONCLUSION
20.
Considering the complexity of the problem in measuring functional occlusal forces of the masticatory muscles,an accurate and manageable telemetric system has been developed. Many tests in vitro and hundreds of recordings in several patients prove its value in the study of the physiologic mechanism of the stomatognathic system.
15. 16.
18.
21.
22.
23.
24.
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Rcprtnt rcque~~slo. DR. J. DF. B~EVER JhPAR’IMENT
OF PROsTHoUoN.rIcS
STOMATOLOClE AKADEMISCH
ZIEKENHUIS
DE PINTELAAK.
135
B 9000 BELGIUM
GENT
JULY
1978
VOLUME
40
NUMBER
1