Measurement of intraoral muscle forces during functional exercises David E. Lindeman, DDS, MS,* and Robert N. Moore, DDS, PhD, EdD** Littleton, Colo., and Lhwohz, Neb.

Measurements of intraoral muscle force with foil strain gauges, load cells, and pressure transducers bonded to a Tru-Tain stent and to a lip bumper appliance were tested by means of seven functional exercises in five adult subjects over a 5-day interval. The measuring devices and the functional exercises were tested for replicability and validity. Results showed that the pressure transducer was the superior measuring device with respect to size, sensitivity, thermal compensation, factory uniformity, replicability, and validity. The device most susceptible to error, on the basis of these factors, was the foil strain gauge. Of the seven functional exercises used, the pronunciation of the words "phone," "mom," and "church" and the exercise of swallowing were replicable over time. The other three exercises--chewing gum, sucking, and blowing on a straw--were determined to be unreliable in terms of rep/icability over time. Overall pressure values recorded were significantly higher than in previous reports. Pressure values were higher for the Tru-Tain stent than for the lip bumper. (AMJ ORTHOD DENTOFACORTHOP 1990;97:289-300.)

T h e basis of orthodontic correction with functional appliances and the concept of muscle modification have been controversial since the inception of these appliances. Most appliances incorporate some type of tissue or muscular shield on the theory that if opposing muscular forces on one side are temporarily removed or modified by such a shield, permanent bony changes can be achieved in conjunction with readapration of both oral and facial musculature in this region. Many studies have recorded resting intraoral pressures of both the buccal and the lingual musculature. To date, few have examined functional pressures, partly because of the extreme difficulty in measuring muscle forces in the oral environment. The purpose of this study was twofold: 1. To develop a technique to quantify muscle forces intraoraUy with the use of a noninvasive intraoral appliance. Three systems were tested to ascertain the best combination or single system for use. 2. To quantitate intraoral muscular forces in a pilot study of five adult volunteers. This study provided oral testing of the appliance and gave numerical values for repeated intraoral measurements. From this test, the accuracy of repeated measures was determined. McNamara et al. ~3 have carded out extensive studies on nonhuman primates with respect to muscle ad-

From the University of Nebraska Medical Center, College of Dentistry. *Private practice, Littleton, Colo. **Professor and Chairman, Department of Orthodontics. 811112525

aptation in the facial region. Their studies showed that skeletal adaptation proceeds until muscle activity is restored to normal levels, suggesting that a correlation existed between occurrence and disappearance of altered muscle function in regard to reestablishment of skeletal balance. As skeletal balance was restored through structural adaptations, the need for compensatory muscle function was reduced. FrtinkeF6 developed a new functional appliance to bring about these structural adaptations. Frankel attributes the orthodontic correction achieved with his functional appliance to the presence of shields that eliminate inhibitive cheek pressures and establish a new equilibrium between the forces of the tongue and the cheeks. Comparison of opposing forces between the cheeks, lips, and tongue has been examined by several authors. Winders, 7 Proffit,s Kydd, 9 and Lear et al. '° found substantially higher readings on the lingual side of the teeth. The concept that the teeth sit in a balanced envelope of opposing muscle forces appears more complex than originally thought, and this led Weinstein et al." to theorize that a tooth may have more than one stable position within an arch. This theory, when applied to functional appliances, might then support the idea that a new state of equilibrium can be established if muscle adaptation can be achieved. In an attempt to change the lip equilibrium, Fr/inkeW has incorporated lip bumper-style shields in his functional appliances. He believes that decrowding the labial movement of incisors results not only from the release of tissue pressures but also from tissue ten289

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Am. J. Orthod. Dentofae. Orthop. April 1990

Lindeman and Moore

r

1

"i I

,,

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II

I

Fig. 1. Foil strain gauge. Fig. 3. Calibration of Tru-Tain stent-strain gauge appliance.

~ o n

straingauge diaphragm~ ~ , ~ -"ActiveSurface'-

force

pressure

Fig. 2. Entran load cell (left) and pressure transducer (right).

sion placed on labial bone by the shields. It is believed that this tractionlike force modifies the buccal alveolar wall, thus returning the muscle forces to equilibrium. The measurement of intraoral muscular force or pressure, as opposed to electromyographic muscular activity, is a more representative measure of true muscle influence on the underlying dental structure. Several techniques have been used to specifically measure muscle force, 13~g the most common being the foil strain gauge. 7"1°'t9"23In 1984 Otaki 2~ measured lingual tongue pressures with electronic pressure transducers mounted on removable splints. His results showed that lateral tongue pressures are greater than anterior tongue igressures. The results of the studies vary considerably, partially because of the rapid technologic advances that are being made. A major variable is the effect of head posture on intraoral pressures. Hellsings and L'Estrange z5 reported a high degree of change in pressure readings as a result of changes in flexing and extension of the head. They also reported that the mode of breathing significantly changes the oral pressures. These findings, however, have not been substantiated

by other investigators. Wood, 26 Hensel, 27 and Archer and Vig 28 could not find any significant change in resting pressures due to changes in head position. In a recent study, Ingervall and Thuer 29 found no significant change in pressures on the tooth surface during the functional exercises of chewing and swallowing, in either the natural or the extended head positions. During extension of the head, with the teeth at rest, a small but significant increase in pressure was noted on both the teeth and the alveolar process. The study also showed that there was an increase in pressure on the alveolar process during head extension while the chewing exercise was being performed. Head posture may well have some influence on muscular pressures. However, the methods of detection and recording of head position require further research with well-defined parameters. In most of the previous studies, the methods used to quantify pressures have varied widely and there still remains no clear method or technique for measurement of these intraoral forces.

Pressure-sensindevi g ces Three types of pressure-sensing device were used: foil strain gauges, load cells, and pressure transducers. All three devices operate on the principle of the Wheatstone bridge. The strain gauge is made up of a foil wire bonded to a flexible backing (Fig. 1). Once the gauge is firmly attached to a structure, stressing or bending the gauge up to its elastic limit causes a temporary elongation of the wire. This results in a change in resistance for the gauge that is measurable with instruments. Both the pressure transducer and the load cell pressure transducer incorporate the same basic principle as the foilstrain gauge, but they are more complex in their

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Measurement of hztraoral muscle forces during functional exercises 291

design and mechanism of action (Fig. 2). Both are sealed, self-contained units that contain four semiconductive strain gauges. These gauges are bonded to a thin circular diaphragm on which the forces of pressures act. The pressure transducer is designed to measure pressures exerted by a gas or a fluid. Since gases and fluids exert equal forces in all directions from any one point, a pressure can be determined by division of a known force by the surface area of the pressure transducer. The load cell and the strain gauge, however, are designed to measure force rather than pressure. If the force is acting evenly over the entire measuring surface, then a pressure can also be calculated.

Appliances Two different appliances were chosen to act as carriers for the three types of pressure-sensing device. A Tru-Tain stent (Tru-Tain, Inc., Rochester, Minn.) was used for measurements made over the labial surfaces of the teeth, and a lip bumper was used for readings made in the labial vestibule. Three appliances of each type were made for each subject. The Tru-Tain stent, which is used as a thin, removable retainer after the completion of orthodontic treatment, is made of a thermal polyvinyl plastic. A Biostar machine (Great Lakes Orthodontics, Tonowanda, N.Y.) was used to form a 0.5 mm sheet of the plastic material over stone models of the subject's teeth. The sheet was then removed and trimmed to cover only the mandibular incisors and canines. The stent had excellent retention to the teeth and allowed the pressuresensing devices to be removed and repeatedly replaced in exactly the same position. All lip bumpers were made by the same investigator, who used the subject's arch forms for construction. Bonded molar tubes 0.036 inch in diameter were attached to the subject's first molars. Alginate impressions of the mandibular arch were taken and poured in dental stone. The lip bumper frameworks were then contoured to fit each subject's arch, and a soldered stop was placed at the tube. This was done to fix and standardize the distance of the lip bumper 7 mm anterior to the labial surfaces of the incisor teeth. Once three frameworks were made for each subject, the acrylic was processed onto the framework. The pressuresensing devices were then attached to the appliances.

Strain gauges A full-bridge micro strain gauge arrangement* was fabricated with CEA-1 I-UN-350 preformed foil strain gauges obtained from Micromeasurements Group. The *Measurements Group, Inc., Raleigh, N.C.

gauge had an overall size of 11.2 × 9.8 mm. When bonded to the Tru-Tain stent and the lip bumper, the strain gauge was a standardized distance of 3.5 mm and 7 mm, respectively, from the tooth surface. The 350-ohm resistance of these gauges allowed for greater sensitivity. They were attached to the intraoral appliances with an AE-10 adhesive, which is coordinated for CEA-type gauges and supplied in a GAK-AE10/15 application kit. Shielded 30-gauge wire was then soldered to the gauges with a Mark V soldering station and 361-A20R-25 rosin solder. Once lead wires are attached, all rosin flux must be removed with M-line flux solvent. The strain gauges were then insulated with a protective coating, which consisted of a layer of microcrystalline wax, followed by a layer of 3145 RTV coating. The change in resistance generated by distortion of the gauges was measured with a P-3500 strain indicator, which amplified the output readings to a thermal strip chart recorder. A change in resistance was measured as a maximum of + 2 volts of direct current passed through the gauge from the P-3500 strain indicator. Each gauge was bonded onto either the Tru-Tain stent or the lip bumper appliance and calibrated. To accomplish this, two similar techniques were used. The Tru-Tain appliance with bonded strain gauges was fitted on the subject's model and placed under the calibration instrument, with the labial surface of the strain gauges parallel to the floor (Fig. 3). A 5 mm thickness of synthetic tissue medium, which simulated the lip and evenly distributed the forces over the surface area of the end of the load arm, was then placed between the surface of the appliance gauge and the perpendicular load arm of the calibration instrument. Gram weights were added to the platform, and mean changes in resistance versus pressure were graphed for the compression vectors of force (Fig. 4). Each appliance was tested five times and the results were averaged. The lip bumper appliance with bonded strain gauges was fitted to a dental stone model with molar tubes and calibrated in a similar fashion.

Load cells The ELF-500-2X ultraminiature load cell had an overall diameter of 12.7 mm, with an active surface diameter of 6.4 mm and a thickness of 2.8 mm. When bonded to the Tru-Tain stent and the lip bumper, the load cell's active surface was a standardized distance of 3.3 mm and 7 nun, respectively, from the tooth surface. The load cell has an operating pressure range of 0 to 18 psi, a useful frequency range of 0 to 1200 Hz, and a temperature range of 0 to 100 ° C. The unit

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Lindeman and Moore

STRAIN GAUGE CALIBRATION SubJect: S.A.

1000

o Lip B u m p e r Trutain

9OO 80O 7OO 600 o

.>_. 5 0 0 :~ 4 0 0 300200 1000 0

10

20

30

40

50

60

70

80

90 100 110 120

Newton per Meter Squared ( 1 0 3 ) Fig. 4. Strain gauge calibrationof one test subject.

Fig. 5. Calibration of load cell.

has a temperature-compensation module that limits any thermal shift due to temperature fluctuations to 2% per 100 ° F. An excitation of 5 volts was selected to help minimize thermal shock deviations by allowing for faster thermal compensation. The hysteresis, or the failure of two related events to keep pace with each other, for the load cell is 2% per full-scale output. The piston design of the load cell allowed it to be calibrated before mounting (Fig. 5). Each load cell was attached to the surface plate with sticky wax and powered, zeroed, and balanced with a ---5 volts direct current from an Entran MM-35 series digital transducer meter. The diameter of the perpendicular load arm for the calibration instrument exactly matched the diameter

of the active surface of the load cell. When the surface area over which the force was acting was known, pressures rather than forces could be determined. Gram weights were then added on the platform and changes in millivolt resistance versus pressure were recorded. Each load cell was calibrated five times, and the mean values were graphed for the compression vectors of force. After all load cells were calibrated, a comparison of factory calibration values versus laboratory calibration values was graphed (Fig. 6). The physical limitations of our testing apparatus precluded verification of factory calibrations at low force levels. However, the results obtained at higher levels suggested that t h e factory calibrations with microstrain gauges were very accurate, and the factory calibrations were used for the testing of subjects. On completion of calibrations, the load cells were mounted to either the Tru-Tain or the lip bumper appliances by means of Dow Coming RTV 3145 silicone. The devices were centered over the labial surface of the right lower lateral incisor, and the silicone was allowed to set for 48 hours before testing. Pressure transducer

The EPL 6-125-50X flatline pressure transducer has an overall size of 3.2 × 7.6 mm, a center diaphragm diameter of 3.05 mm, and a thickness of 1.0 mm. When bonded to the Tru-Tain stent and the lip bumper, the active surface of the pressure transducer was a stan-

Volume 97 Number 4

Measurement of intraoral muscle forces during functional exercises 293

LOAD CELL CALIBRATION Serial Numbers: Q - 0 9 , Q - l 0, Q-11

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Fig. 6. Comparisonof experimentaland factoryloadcell calibrations.

dardized distance of 1.5 mm and 7 mm, respectively, from the tooth surface. The pressure transducer has an operating pressure range of 0 to 50 psi, a useful frequency range of 0 to 12,000 Hz, and a temperature range of 0 to 100 ° C. Like the load cell, the pressure transducer has a temperature-compensation module that limits any thermal shift due to temperature fluctuations to 2% per 100 ° F. Again, a low-excitation voltage of 5 volts was selected to reduce thermal shock and enhance faster thermal compensation. The hysteresis for the load cells is also 2% per full-scale output. The pressure transducers were calibrated with a calibration pressure chamber (Fig. 7). Each transducer was attached with sticky wax to a plastic 2 × 2 inch plate, placed inside the chamber, and powered, zeroed, and balanced with a --+5-volt direct current from an Entran PS-30-A power supply. Air pressure was increased in the chamber and measured with a mercury manometer. Each calibration sequence was repeated five times, and the mean changes in millivolt resistance versus pressure were compared with factory calibrations (Fig. 8). On completion of calibrations, the pressure transducers were mounted in exactly the same way as the load cells. Experimental

design

Five adult volunteers with Class I occlusions, normal overbites and overjets, and no neuromuscular or speech defects were tested on five consecutive days.

Fig. 7. Calibration of

pressure transducer in a pressure

chamber.

Muscle forces were recorded during seven functional exercises. 7 When a pressure-sensing device was placed in the mouth, it was allowed to reach ambient oral temperature as detected by the thermal 0 shift. The load cells and pressure transducers accomplished this almost immediately, while the strain gauges required approximately 5 minutes. Once the thermal shift was 0, the devices were zeroed under the resting pressure of the lip. To standardize the time intervals between exercises and to produce repeatable instructions to the subject,

294

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Lindeman and Moore

PRESSURE TRANSDUCER CALIBRATION Serial N u m b e r s : D - 0 1 , D - 1 5 , D - 1 7 ~ . . -

400 35O 300

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N e w t o n s per Meter S q u a r e d ( 1 0 3 ) Fig. 8. Comparison of experimental and factory pressure transducer calibrations.

Fig. 9. Strain gauge mounted on Tru-Tain stent.

Fig. 10. Load cell mounted on Tru-Tain stent.

an audiotape was made. The subjects were asked to repeat the words "phone," "morn," and "church." They were then asked to suck and blow on a closed-end straw, swallow water, and chew gum. The subjects listened to the audiotape through headphones, and the investigators monitored the tape with additional sets of headphones. All six appliances and devices were tested each day (Figs. 9 to 14). All exercises were repeated twice for each appliance and pressure-sensing device, and the results were averaged.

All data were compiled and converted into newtons per square meter. Pearson correlation coefficients were used to test device reliability and to establish Cronbach's alpha values, 3° which provide a numerical way of assessing both device and functional exercise replicability. Factor analyses were used to give numerical representation of validity.31 Each factor analysis generated a factor that, theoretically, represented all of the measured variance for each functional exercise. This functional exercise factor score was then correlated with

Volume 97 Number 4

Measurement of hztraoral muscle forces during functional exercises 295

Fig. 13. Load cell mounted on lip bumper. Fig. 11. Pressure transducer mounted on Tru-Tain stent.

Fig. i4. Pressure transducer mounted on lip bumper. Fig. 12. Strain gauge mounted on tip bumper.

the corresponding average scores that were previously calculated for each subject by measurement device, appliance, and functional exercise. RESULTS

Analysis of Cronbach's alpha values for device replicability showed that the Tru-Tain-load cell combination was most replicable (Table I). The lip bumperload cell combination had the lowest replicability. Swallowing had the highest replicability of the functional exercises, while sucking on a straw had the lowest replicability. According to the number of statistically significant Pearson correlation coefficients, all devices except the lip bumper-strain gauge combination had significant

validity (Table II). The number of statistically significant correlations for the exercises "phone," "mom," "church," and Swallowing with either device was high. For chewing, sucking, and blowing, the number of statistically significant correlations was low. To assess the nature of the data distribution in the sample, confidence intervals were calculated around skewness and kurtosis values at p < 0.05 for the phm netic measurements. Of the 37 measurements, two showed significant platykurtie distributions, four were positively skewed, and one was negatively skewed. Because only a small portion of the data showed significant deviance and because skewed distributions were not consistent, these significant findings are probably both spurious in view of the large number of analyses

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Am. J. Orthod. Dentofac. Orthop. April 1990

Table I. Cronbach's alpha values for reliability coefficients showing replicability of functional exercises and

measuring devices Tru-Tain

Lip bumper

Exercise

Strain gauge

Load cell

Pressure transducer

Strain gauge

"Phone" "Mom" "Church" Sucking Blowing Swallowing Chewing

0.97 0.97 0.95 0.00 0.88 0.97 0.86

0.99 0.99 0.98 0.90 0.94 0.98 0.96

0.85 0.96 0.74 0.94 0.97 0.99 0.97

4

1

2

Rank

Load cell

Pressure transducer

0.95 0.81 0.96 0.72 0.96 0.97 0.75

0.88 0.88 0.87 0.94 0.96 0.77 0.83

0.96 0.94 0.96 0.87 0.93 0.97 0.85

5

6

3

I

Rank

p --< 0.05 per alpha :>0.8.

and incidental to the same number of cases (n = 5) on which the analyses were performed. The data distribution therefore approximates a normal distribution. To address the issue of validity, factor analyses were performed for the functional exercises (Table III). A factor analysis for sucking on a straw could not be performed because of large amplitude shifts during a few recording sessions. A percentage score that describes the amount of variance accounted for by each functional exercise was also calculated from the factor analyses. The exercises of chewing and blowing accounted for 46% and 61%, respectively, of the variance within that exercise. The remaining four exercises accounted for 70% to 74% of the variance. When the factors for blowing and chewing are discounted for all devices, the lip bumper-pressure transducer combination has the highest degree of validity and the lip bumper-strain gauge device again has the lowest. The mean pressure readings for each exercise and device are shown in Table IV. For the Tru-Tain appliance, the pressure transducers had the highest values, while the load cells and the strain gauges had similar, but far lesser, values. For the lip bumper appliance, the strain gauges had the highest values, followed by the pressure transducers and the load cells. DISCUSSION

This study focused on different methods of measuring intraoral lip pressures and found that the pressure transducer was the best overall measuring device. The worst measuring device was the foil strain gauge. Of the seven functional exercises tested, swallowing and pronunciation of "phone," "morn," and "church" had high values for replicability. The exercises of chewing, sucking, and blowing did not show good replicability. The factors of size, thermal compensation, calibration, errant loading, accuracy, and validity also contribute to

the results for both the measuring device and the functional exercises. The thickness and the overall size factor of intraoral measuring devices are extremely important. Investigators have studied the pressure response created by artificially increasing the thickness of the sensing device and found that as the effective thickness of the sensing device was increased beyond 2 mm in thickness, proportional increases in mean pressures were obtained.~°'2° Thus, the effective size of an oral pressure-sensing device should be less than 2 mm in thickness. Both in thickness (1.0 mm) and in diameter (3.2 mm), the pressure transducer was superior to the load cell and the strain gauge. The load cell size was roughly equivalent to the area required by the strain gauge and was 2.8 mm thick. The ease of placement of both the load ceil and the strain gauge devices was hampered by the large surface area required for mounting. Although both were functional in the oral environment, they were not as ideally suited as was the pressure transducer. Thermal compensation and temperature drift can play a key role in the accuracy of pressure-sensing devices. Having been only mildlY addressed by others, x°'~SaT'3~the importance of these factors cannot be overlooked. The load cell and the pressure transducer are sealed, self-contained units that are thermally compensated in the factory during manufacture. The foil strain gauge is an uncompensated system and is susceptible to wide fluctuations in readings a s a result of varying ambient temperatures. The foil strain gauge requires intraoral zeroing and balancing of temperatures to accurately record strain. This is a critical problem when one is measuring functional exercises, such as swallowing water or speaking, which might cause changes in the ambient oral temperature. Calibration of the measuring device is extremely

Voh~me97 Number 4

Measurement o f intraoral muscle forces during functional exercises

29"I

Table II. Pearson's correlation coefficients to test validity of devices Tru-Tah~ Exercise "Phone" Tru-Tain

Lip bumper

Lip bumper

Device

Strain gauge

Load cell

Strain gauge Load cell Pressure transducer Strain guage Load cell Pressure transducer

0.65 0.83* 0.62 0.49 0.89*

-0.20 0.97* 0.86* 0.93*

0.22 0.23 0.53

Strain gauge Load cell Pressure transducer Strain gauge Load cell Pressure transducer

0.72* 0.52 0.91" 0.87* 0.92*

-0.18 0.66 0.68 0.45

0.50 0.42 0.79*

Strain gauge Load cell Pressure transducer Strain gauge Load cell Pressure transducer

0.81" 0.76* 0.79* 0.67 0.65

-0.38 0.96* 0.3 I 0.22

Strain gauge Load cell Pressure transducer Strain gauge Load cell Pressure transducer

------

Strain gauge Load cell Pressure transducer Strain gauge Load cell Pressure transducer

Pressure transducer

Strain gauge

Load cell

0.94* 0.88*

-0.75*

0.61 0.89*

-0.77*

0.34 0.99* 0.98*

0.28 0.16

-0.98*

-0.71 * 0.66 0.88* 0.03

-0.46 0.75* 0.25

-0.25 0.54

-0.24

0.57 0.59 0.01 0.40 0.61

-0.73* 0.68 0.85 0.10

0.09 0.85* 0.23

0.52 0.43

-0.22

Strain gauge Load cell Pressure transducer Strain gauge Load cell Pressure transducer

0.80* 0.55 0.85* 0.53 0.85*

-0.93* 0.45 0.91" 0.89*

0.11 0.91" 0.84*

0.17 0.49

-0.65

Strain gauge Load cell Pressure transducer Strain gauge Load cell Pressure transducer

0.40 0.05 0.24 0.81" 0.12

-0.91 * 0.45 0.43 0.51

0.59 0.06 0.30

0.I1 0.53

-0.53

Pressure transducer

" M o r n "

Tru-Tain

Lip bumper

"Church" Tru-Tain

Lip bumper

Sucking Tru-Tain

Lip bumper

Blowing Tru-Tain

Lip bumper

Swallowing Tru-Tain

Lip bumper

Chewing Tru-Tain

Lip bumper

m

m

m

*Significant (p --< 0.05) between exercises and devices. --Uncollectable data.

important if quantitative values are to be obtained. The load cell and the pressure transducer are factory calibrated for a specific temperature range. For experimental purposes, the factory calibrations were como

pared with our laboratory calibrations (Figs. 6 and 8). Although the methods used in the laboratory were less sophisticated than those of the factory, the results were very similar. The pressure transducer calibration values

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Am. J. Orthod.Dentofac.Orthop. April 1990

Table III, The correlation of each set of measurements to one of six corresponding functional factor scores

Tru-Tain Exercise "Phone" "Mom" "Church" Blowing Swallowing Chewing

Str°in I L°a I gauge cell 0.85 0.99 0.93 0.60 0.88 0.65

Lip bumper Pressure transducer

Strain gauge

0.53 0.59 0.82 0.96 0.88 0.74

0.94 0.91 0.67 0.73 0.55 0.37

0.93 0.68 0.70 0.98 0.99 0.95

I

Load cell 0.86 0.86 0.88 0.93 0.84 0.69

I

Pressure transducer 0.98 0.96 0.84 0.14 0.93 0.54

Table IV. Means and standard deviations for all devices and functional exercises (in N / m 2)

Tru-Tain Exercise

Strain gauge

] [

Lip bumper

Load cell

Pressure transducer

Strain gauge

10,471 7,959

9,003 5,792

60,075 17,218

12,165 9,761

6,767 4,649

6,531 4,748

[ [

Load cell

Pressure transducer

50,688 40,444

9,692 3,034

29,432 15,435

82,169 23,364

29,806 12,598

6,127 1,280

27,275 13,199

5,871 4,905

59,395 18,380

12,854 9,988

6,176 2,009

19,168 13,820

34,534 1,517

21,640 8,077

76,505 24,330

182,708 53, I 11

23,157 8,589

37,174 12,046

26,014 9,456

24,802 9,771

78,820 24,566

94,757 40,680

26,093 8,441

35,893 13,08t

16,952 14,342

14,342 8,638

54,185 36,041

83,232 75,480

10,589 2,807

20,163 14,460

20,478 7,831

9,249 6,353

100,953 18,035

53,515 27,245

8,284 2,138

37,371 10,037

"Phone" SD "Morn" SD "Church" SD Sucking SD Blowing SD Swallowing SD Chewing SD

had the best fit overall and were extremely linear. The load cell values were somewhat less ideal, and yet they showed good linear qualities. The variation between experimental and factory values for the load cell was most likely due to the inability of the experimental device to completely control off-center loading of the load-sensing diaphragm. Since each foil strain gauge was permanently bonded to the appliance, the two together acted as the pressure-sensing device and had to be calibrated individually. No two strain gauges had the same degree of sensitivity relative to pressure. The lip bumper-strain

gauge combination was less sensitive than the Tru-Tain stent-strain gauge combination and may account for the noncorrelation of the pressure reading to those of the other devices (Table III). The graphs for both the Tru-Tain stent and the lip bumper were relatively linear, suggesting that the strain gauge and the appliances work well together and operate within their moduli of elasticity (Fig. 5). Distortion of recorded pressures among the devices used, other than those explained by statistical analyses, can be accounted for by vector addition or errant loading of the measuring device. An inherent assumption made

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l~teasurement of hztraoral muscle forces during functional exercises 299

by all researchers trying to quantify intraoral pressures is that all forces are acting perpendicularly to the active surface of the measuring device. It is also assumed that all off-angle vectors of force are insignificant or have a resultant vector that cancels them out. In the case of the load cell and the strain gauge, off-center loading affects measurements. The load cell and the strain gauge, by design, are made to operate perpendicularly to the vector of force. The piston of the load cell would limit the magnitude of error somewhat in comparison to the foil strain gauge. The pressure transducer, on the other hand, incorporates a diaphragm that reacts to pressure rather than force. The assumption made in using the pressure transducer is that the lip is exerting a fluid pressure than than a solid force and that this pressure is transmitted throughout the fluid in all directions. The validity tests show that although all devices had good replicability, the lip bumper-strain gauge device was invalid because it did not vary in relation to the other devices (Table II). This may be linked to the poor sensitivity shown in the calibration charts (Fig. 5) or to the permanent deformation of the lip bumper appliance caused by repeated placement during removal from the subject's mouth. To date no authors have reported on the issue of validity of their devices; they have discussed only the issue of repeatability. The four exercises of pronouncing "phone," "mom," and "church" and swallowing all rank high and exhibit high levels of device correlation (Table I). There was a low ranking for replicability and for device validity for the exercises of chewing, sucking, and blowing. This is of particular interest because Thur and Ingervall zT,~srecently have used a chewing sequence as part of their data-collection process. The reason these exercises had a low replicability and validity is open to speculation. Possible explanations may be muscle fatigue or lack of muscle pattern coordination during different test intervals. Another explanation may be the lack of muscle effort necessary to perform a habitual task, such as making a phonetic sound or swallowing a fluid, in comparison with a sustained muscle action, such as chewing or blowing through a straw. 33 The direct purpose of this study was not to report lip pressures for the functional exercises but to establish a reproductible method that would allow the quantification of changes in lip pressure over time. It should be noted, however, that the mean pressures reported in Table IV were greater than pressures previously reported by others, 7-1°'16-23"2529"32but meaningful comparison is hampered by the fact that the projection of the measuring device from the tooth surface is unknown.

The highest values were recorded with the Tru-Tain pressure transducer and the lip bumper-strain gauge devices. It is interesting to note that lip bumper pressure increased with strain gauges and decreased with pressure transducers when compared with the same devices mounted on the Tru-Tain appliance. Load cell values were similar for both appliances. Gould and Picton 2° and Lear et al., ~° using strain gauges, noted a pressure increase as the recording device was advanced outward from the tooth. In our study this was observed with strain gauges but not with load cells or pressure transducers. It is reasonable to postulate that placement of the lip bumper appliance 7 mm anteriorly into the vestibule caused the sarcomere lengths to be extended beyond their optimal limits of contraction. It could be concluded that there is a limit to which lip tonicity can be taxed and still result in an increased contraction. Once that limit is crossed, a decrease in the pressure of contraction would result. This theory has been recently supported by Proffit and Phillips, 2~ but it does not explain the differences in the recording devices. SUMMARY AND CONCLUSIONS

The strain gauge, load cell, and pressure transducer measuring devices were evaluated. Repeated tests were run with the seven established functional exercises of pronouncing "phone," "morn," and "church," sucking, blowing, swallowing, and chewing. The devices and functional exercises were evaluated for replicability and validity of measurement in comparison to each other. The intent was to better define a method that can be applied to future research. From the data examined and statistically analyzed by this study, the following conclusions may be drawn: 1. When the factors of size, sensitivity, thermal compensation, factory uniformity, accuracy, and valicity are taken into account, the best overall measuring device is the pressure transducer and the measuring device most susceptible to experimental error was the foil strain gauge. 2. All devices, regardless of the appliance used, acted in a replicable fashion. A rank ordering was established that showed the Tru-Tain-load cell device to be the most replicable and the lip bumper-strain gauge to be the least replicable. 3. Some functional exercises were more replicable for all devices than others. A rank ordering was established that showed the exercises of swallowing and pronouncing "phone" to be most repticable and the chewing and sucking exercises to be least replicable.

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4. The validity tests for the lip bumper-strain gauge device was poor, and it was therefore classified as an invalid measuring device. 5. The functional exercises of chewing, sucking, and blowing exhibit poor device correlation coefficients and low factor analyses. 6. Pressures recorded 7 mm from the tooth surface tended to be decreased or equal in comparison with those recorded against the tooth surface. 7. Mean pressures recorded by the devices were higher than previously reported pressures in the literature. The pressure transducers' values were much higher than those obtained with the load cell or the strain gauge. 8. The fluid environment of the oral cavity serves to distribute muscular pressures evenly over the surface area of the pressure transducer, whereas the load cell and the strain gauge require that the surface contact by evenly distributed over the entire pressure-sensitive area. The statistical consultation by Dr. Linda DuBois is gratefully acknowledged. This research was supported by the University o f Nebraska Orthodontic Development Fund and the College of Dentistry.

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Am. J. Orthod. Dentofac. Orthop. April 1990 14. Fcldstein L. An instrument for measuring muscular forces acting on the teeth. AM J ORTI1OD 1950;36:856-9. 15. Margolis HI, Prakash P. A new instrument for recording oral muscle forces: the photoelectric myodynagraph. J Dent Res 1954;33:425-34. 16. Sims FW. The pressure exerted on the maxillary and mandibular central incisors by the perioral and lingual musculature in acceptable occlusion. AM J ORTHOD 1958;44:64-5. 17. Thuer U, Janson T, Ingervall B. Application in children of a new method for the measurement of forces from the lips on the teeth. Eur J Orthod 1985;7:63-78. 18. Thuer U, Ingervall B. Pressures from the lips on the teeth and malocclusion. AM J ORTtlOD DENTOFACORTHOP 1986;90:23442. 19. Winders RV. Forces exerted on the dentition by the perioral and lingual musculature during swallowing. Angle Orthod 1958; 28:226-35. 20. Gould MSE, Picton DCA. A method of measuring forces acting on the teeth from the lips, cheeks and tongue. Br Dent J 1962;112:235-42. 21. Gould MSE, Picton DCA. A study of pressures exerted by the lips and cheeks on the teeth of subjects with normal occlusion. Arch Oral Biol 1964;9:469-78. 22. Gould MSE, Picton DCA. A study of pressures exerted by the lips and cheeks on the teeth of subjects with Angle's Class II Division l, Class II Division 2 and Class III maloeclusions compared with those of subjects with normal occlusions. Arch Oral Biol 1968;13:527-41. 23. Proffit WR, Phillips C. Adaptations in lip posture and pressure following orthognathic surgery. AM J ORTHODDENTOFACORTIIOP 1988;93:294-302. 24. Otaki K. Studies of tongue size and tongue pressure. Odontol 1984;72:163-86. 25. Hellsing E, L'Estrange P. Changes in lip pressure following extension and flexion of the head and at changed mode of breathing. AM J ORTIIODDENTOFACORTHOP 1987;91:286-93. 26. Wood LW. The relationship of variations in head position to tongue pressures [Thesis]. Chapel Hill, North Carolina: University of North Carolina, 1981. 27. Hensel S. Kopfhaltung und Wiechteilfunktion--experimentelle Untersuchungen. Stomatol DDR 1983;33:249-59. 28. Archer SY, Vig PS. Effects of head position on intraoral pressures in Class I and Class II adults. AM J OR'nXOD 1985;87: 311-8. 29. Ingervall B, Thuer U. Cheek pressure and head posture. Angle Orthod 1988;58:47-57. 30. Cronback LJ. Coefficient alpha and the internal structure of tests. Psychometrika 1951;16:297-334. 31. SPSSX User's Guide. A complete guide to SPSSX language and operations. Chicago: McGraw-Hill, 1983:647-62. 32. Alderisio JP, Lahr R. An electronic technic for recording the myodynamic forces of the lip, cheek and tongue. J Dent Res 1953;32:548-53. 33. Hixon TJ, Hardy JC. Restricted mobility of the speech articulators in cerebral palsy. J Speech Hear Disord 1964;29:293-306. Reprint requests to: Dr. Robert N. Moore Department of Orthodontics University of Nebraska Medical Center College of Dentistry 40th and Moldrege Lincoln, NE 68588

Measurement of intraoral muscle forces during functional exercises.

Measurements of intraoral muscle force with foil strain gauges, load cells, and pressure transducers bonded to a Tru-Tain stent and to a lip bumper ap...
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