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Preliminary tests of a new device to monitor orthodontic headgear use
Preliminary Tests of a N e w Device to Monitor Orthodontic Headgear Use Elizabeth K. Lyons and Douglas S. Ramsay The orthodontic community has had a long-standing interest in measuring headgear use characteristics. Progress in microelectronics has made it possible to incorporate the electronics for a headgear-monitoring device entirely within the body of an orthodontic headgear force module. A novel headgear-monitoring device was developed recently and subjected to laboratory and clinical testing. Results of these initial tests are described and suggest that the headgear monitor should be able to measure the temporal characteristics of headgear wear, estimate how much force is delivered, detect patient attempts to falsify headgear use, and provide readily accessible feedback to patients, parents, and orthodontists about headgear use. (Semin Orthod 2002;8:29-34.) Copyright 2002, Elsevier Science (USA). All
rights reserved.
linical research has shown that o r t h o d o n t i c h e a d g e a r t r e a t m e n t is effective for the correction o f Angle Class II malocclusions. ~-3 T h e removable n a t u r e o f a h e a d g e a r places the p r o p e r i m p l e m e n t a t i o n o f the o r t h o d o n t i s t ' s prescription for its use u n d e r the c o n t r o l o f the patient. Patients typically c o n t r o l w h e n a n d for how l o n g h e a d g e a r is worn, a n d d e p e n d i n g o n the appliance, patients can m a n i p u l a t e the a m o u n t o f force the h e a d g e a r delivers. A patient's willingness to wear a h e a d g e a r is t h o u g h t to be i m p o r t a n t if t r e a t m e n t is to succeed. Tulloch a n d colleagues 4 m a k e this p o i n t clearly, "The c h a n c e o f i m p r o v e m e n t in an u n c o o p e r a tive child is the same as the c h a n c e o f improvem e n t in an u n t r e a t e d o n e - small, b u t n o t zero." In contrast, there is little k n o w n a b o u t h o w m u c h p a t i e n t a d h e r e n c e to the p r e s c r i b e d head-
C
From the Departments of Orthodontics, Pediatric Dentistry, and t~sychology, Unive~:;ity of Washington, Seattle, WA. Supported by NIH grant; KO4-DFO0379 and R41 DP:12430, the University c~ Washington Orthodontic Alumni Association, the Wa.~hington 7}ehnology (;enter, and the University of Washington Royalty Research Fund. Address correspondence to Douglas S. Ramsczv, DMD, Phi), MSD, Depa~Onent of Pediatric Dentistry, Univey:;ity of Washington, Box #357136, Seattle, WA 98195-7136. Copyright 2002, Elsevier Science (USA). All r~gtzts *~served. 1073 8746/02/0801-0006535.00/0 doi: l O.1053/sodo. 2002.28170
gear r e g i m e n is n e e d e d to p r o d u c e the desired t r e a t m e n t o u t c o m e . 5,6 I n d e e d , there is little research in any health care field that describes h o w partial p a t i e n t a d h e r e n c e relates to clinical o u t c o m e . 7 A s s u m i n g that p a t i e n t a d h e r e n c e to a h e a d g e a r r e g i m e n is a m e d i a t o r o f clinical outc o m e t h e n p o o r a d h e r e n c e may lead to increased t r e a t m e n t time, increased cost, a n d failure to c o r r e c t the malocclusion, which in turn, may necessitate a less p r e f e r r e d alternative treatment. Being able to identify p o o r l y c o m p l i a n t patients w o u l d allow timely t r e a t m e n t plan m o d ifications or the i m p l e m e n t a t i o n o f strategies to e n h a n c e p a t i e n t a d h e r e n c e . 6,7 In addition, the ability to m e a s u r e i m p o r t a n t characteristics o f h e a d g e a r use would m a k e it possible to evaluate the effect o f partial a d h e r e n c e o n clinical outc o m e . U n f o r t u n a t e l y , it is usually difficult to know how well an individual is a d h e r i n g to a t r e a t m e n t r e g i m e n . ~' O r t h o d o n t i s t s have l o n g a p p r e c i a t e d the potential value o f m e a s u r i n g the time a h e a d g e a r has b e e n w o r n a n d there have b e e n n u m e r o u s attempts to m a k e a device that c o u l d provide this i n f o r m a t i o n . N o r t h c u t t i n t r o d u c e d the first c o m m e r c i a l l y available timing h e a d g e a r in 1974. 8 Unfortunately, this timing h e a d g e a r was b o t h inaccurate -~ a n d susceptible to patient attempts to falsify h e a d g e a r wear. C o n t i n u e d interest in m e a s u r i n g h e a d g e a r use lead to the
Seminars in Orthodontics, Vol 8, No 1 (March), 2002: pp 29-34
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Lyons and Ramsay
publication of m e t h o d s for fabricating h e a d g e a r timers? °q~ These timers were bulky and often necessitated placing the electronics in the neck strap, which could diminish patient comfort. Some designs r e m o v e d the safety disconnect feature that should be a part of all h e a d g e a r force modules. Most designs offered little protection against patient attempts to falsit}~ h e a d g e a r use data and provided only a cumulative measure of wear time. Finally, these devices provided the orthodontist with intermittent h e a d g e a r wear information, and n o n e gave the patients easily accessible feedback a b o u t their degree of adherence to the h e a d g e a r wear r e g i m e n ) ,6 Newer devices to measure h e a d g e a r use have b e e n developed. T h e O r t h o Kinetics Corporation (Vista, CA) has m a d e a connnercially available h e a d g e a r timer. An optical sensor, which can detect when the spring in the module is compressed, activates the timer in this h e a d g e a r force module. This device can record each period of h e a d g e a r use, and the record of headgear wear can be reviewed during visits at the orthodontic office. Unfortunately, it is possible for patients to activate the timer without actually wearing the headgear. To create a long-term record of falsified wear data, however, a patient would n e e d to a d h e r e to this pattern of deceptive behavior over a long period (eg, r e m e m b e r to compress the spring before b e d and allow the spring to relax on awakening). Scientists at the University of Florida have developed a microprocessor-controlled headgear-timing device that can record the a m o u n t of spring compression that occurs during h e a d g e a r use. 13 This design has the advantage of measuring the dynamic spring compression that occurs during h e a d g e a r wear and so can provide information a b o u t h e a d g e a r use and the a m o u n t of force p r o d u c e d by the spring. The device has limited clinical utility because it is somewhat fragile and its design requires connecting the bulky electronics, located in the neckstrap, to the h e a d g e a r force module. A novel h e a d g e a r m o n i t o r has b e e n develo p e d at the University of Washington (US Patent #5,980,026); the rationale underlying its design is described elsewhere. 5,~; In brief, the guiding design principles were to m a k e the device small, durable, conffortable, and to maintain the safety breakaway feature so that it could be used clinically. T h e device was designed to 1) measure
the temporal characteristics of h e a d g e a r wear; 2) estimate how m u c h force is delivered by the headgear; 3) detect patient attempts to falsify h e a d g e a r use; and 4) provide readily accessible feedback to the patients, parents, and orthodontists about h e a d g e a r use. Initial prototypes were fabricated and evaluated in laboratory and clinical tests. T h e electronics and battery of the prototype m o n i t o r are contained entirely within the headgear force module. T h e m o n i t o r uses a microprocessor with nonvolatile m e m o r y , m a g n e t sensors, an infrared light-emitting diode (LED) and phototransistor for bi-directional communication and a 4-digit liquid crystal display (LCD) screen to display output. A lithium battm y powers the monitor. A small cylindrical rare-earth m a g n e t is e m b e d d e d in the end of the adjustm e n t strap. T h e basic unit of m e a s u r e m e n t is the position of the m a g n e t within the body of the force module, which changes as the strap is e x t e n d e d and the internal spring is compressed. By measuring the position of the adjustment strap within the force module, it is possible to d e t e r m i n e when the h e a d g e a r is in use. In addition, the force resulting fi-om a known a m o u n t of spring compression can be calculated from calibration data stored in a look-up table in the m o n i t o r ' s memory.
Laboratory Testing T h r e e laboratory tests are described. The first test m e a s u r e d the accuracy of the clock crystal that determines when h e a d g e a r wear occurred. The second test d e t e r m i n e d whether the monitor could accurately measure the a m o u n t of spring compression. T h e third test evaluated whether the a m o u n t of force g e n e r a t e d by the spring during its compression could be estim a t e d by knowing the a m o u n t of spring compression.
Clock Accuracy T h e microprocessor in the h e a d g e a r m o n i t o r is e q u i p p e d with a standard 32,768-Hz crystal oscillator internal clock. This clock is used to register the h e a d g e a r use data in real time. To test the accuracy of the timing unit, the clock was synchronized with the National Bureau of Standards' clock t h r o u g h internet access. The head-
31
Device to Monitor O,¢hodontic Headgear (2~e
gear monitors were activated and left r u n n i n g for 72 hours. After 72 hours, the internal c l o c k OI1 the microprocessor was c o m p a r e d to the National Bureau of Standards' clock. There was approximately a 2-second delay in the internal clock out of the r u n n i n g time of 259,200 seconds, indicating 99.999% accuracy for the internal clock timer.
2s Estimated 20 Magnet
15
Position 10
(mm)
s 0
A/
"100
80 Differential 60 Hall Output 40 (A-D Values) 2O
0 5 10 15 20 25
0 5 10 15 20 25
Known Magnet Position (mm)
Measuring Adjustment Strap Position and Spring Compression Pulling the headgear adjustment strap t h r o u g h the force module causes the spring to compress. The a m o u n t of spring compression can be determined by knowing the position of the adjustm e n t strap within the force module. This was calibrated with the output from the magnet sensors in the headgear m o n i t o r by using a MTS machine (Material Testing System model Sintech 2 / S , MTS Systems Corporation, Triangle Park, NC). The MTS machine allowed the body of the force module to be m o u n t e d to a fixed lower unit while the adjustment strap was connected to a moving unit attached to a load cell. The MTS was p r o g r a m m e d to provide various known amounts of adjustment strap extension. The extension values can be p r o g r a m m e d to run in different increments to a specified extension distance and to have varying holding times at any position. The MTS was p r o g r a m m e d to move the adjustment strap at l-ram increments through the range of movement. At each l-ram increment, the adjustment strap was held in position, and the output from both magnet sensors was recorded. The location of the adjustment strap and the output from both sensors were entered into a look-up table in the m e m o r y of the headgear monitor. An algorithm was used to calculate the position of the adjustment strap based on the output from the two m a g n e t sensors. The accuracy of this m e t h o d for determining the position of the adjustment strap was evaluated. The MTS system was p r o g r a m m e d to move the adjustment strap through a series of known positions, and the estimated position of the strap was calculated by using the output from the sensors. Figure 1 illustrates that there was excellent agreement between the known position of the strap as determined by the MTS machine and the estimated position of the strap
Figure 1. A magnet embedded in the end of the headgear adjustment strap moves through the body of the headgear torce module as the MTS machine pulls the strap. By using Hall sensors to measure magnetic field strength, it was possible to obtain excellent resolution in estimating magnet position relative to the known magnet position. as d e t e r m i n e d by comparing the sensor output with the calibration data in the look-up table.
Estimating Force from Knowing the Adjustment Strap Position Orthodontists are interested in knowing the a m o u n t of force generated from the spring in the headgear force module. This force is often assessed clinically by using a fbrce gauge when the headgear is being adjusted to a patient. Therefore, it was important to ascertain whether the a m o u n t of force from the spring could be estimated by knowing the a m o u n t of strap extension. For this m e t h o d to work there must be a direct relationship between headgear strap position and force. The load cell of the MTS machine was also able to measure the tensile force at any given strap extension. Therefore, it was possible to describe the relationship between adjustment strap position and force. The MTS extension program was run with each headgear module extended to a distance of 20 m m and returned to its initial position at both slow and rapid speeds. The force n e e d e d to extend the headgear strap was recorded over the extension and return range. As shown in Figure 2, the relationship was, for the most part, linear, although there was some hysteresis. Whereas it is possible to obtain a reasonable approximation of the force delivered at a given a m o u n t of strap extension, a goal of future headgear monitor development is to reduce this hysteresis to improve the accuracy of force estimates. Interest-
Lyons and Ramsay
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1400, 1200. 1000,
MTS pullingthe
a d j u s t ~
Force 800, (gin) 600, 400,
/2
J . ~
~ ~ s h i n g the P" adjustmentstrap
f
200, 0
5 10 15 20 25 Known Magnet Position (ram)
Figure 2. Using the MTS machine, representative data collected flcom a single module demonstrate that there is a good, but not perfect, relationship between the position of the adjustment strap and force. ingly, several c o m m e r c i a l l y available h e a d g e a r force m o d u l e s were also evaluated, a n d a large a m o u n t o f hysteresis was o b s e r v e d in s o m e models.
Clinical Testing Two clinical tests are described. T h e first test m e a s u r e d the effect o f h e a d posture o n spring compression. T h e s e c o n d test described h o w spring c o m p r e s s i o n c h a n g e s d u r i n g h e a d g e a r wear.
collection o f 14 data points at each h e a d position. Spring c o m p r e s s i o n in the left a n d right m o n i t o r s was inversely related d u r i n g lateral h e a d m o v e m e n t s . As the h e a d t u r n e d to the right, the right m o n i t o r ' s spring relaxed while the left m o n i t o r ' s spring was compressed. T h e opposite was true w h e n t u r n i n g the h e a d to the left. In contrast, a positive relationship in spring c o m p r e s s i o n was observed for the right a n d left m o n i t o r s w h e n the h e a d moves u p or down. Both right a n d left springs are c o m p r e s s e d w h e n l o o k i n g u p w a r d a n d b o t h are relaxed w h e n looking downward. This i n d e p e n d e n t replication o f the relationship b e t w e e n h e a d posture a n d force suggests that these two different h e a d g e a r m o n itor designs are m e a s u r i n g the same p h e n o m e non.
Headgear Wear Data Collected During Patient Use For clinical data collection, two h e a d g e a r m o n itors were w o r n with a cervical h e a d g e a r appliance, a n d data were collected f r o m b o t h the subject's left a n d right sides simultaneously. T h e subject wore the m o n i t o r i n g h e a d g e a r for f o u r episodes d u r i n g a 2 4 - h o u r p e r i o d a n d carefully r e c o r d e d the times the h e a d g e a r was w o r n a n d
Maximum Spring 25 Compresson
F
2O
Estimated
Magnet is
Effect of Head Posture on Spring Compression Research f r o m the University o f Florida f o u n d that h e a d g e a r force varies as a f u n c t i o n o f h e a d position. 13 Because the h e a d g e a r m o n i t o r s at the University Florida a n d the University o f W a s h i n g t o n b o t h m e a s u r e an i n d e x o f spring compression, it s h o u l d be possible to replicate these findings using o u r different h e a d g e a r design. To evaluate the effect o f h e a d position o n spring c o m p r e s s i o n , a subject wore a m o n i t o r o n each side o f the head. T h e subject t h e n t u r n e d h e r h e a d t h r o u g h a range o f m o t i o n , l o o k i n g left, right, upward, a n d downward. Each h e a d position was held for 30 seconds. T h e results are shown in Figure 3. This g r a p h represents an average o f two cycles o f h e a d m o v e m e n t s with a
Position (ram)
10
Passive Spring 0 Position
Left
~R i g h t
Up
= i i = Down Left Right Up
=
=
Down
Head Position Left Side Monitor
Figure 3.
Right Side Monitor
Two headgear monitors were used with a cervical headgear appliance to collect data from both the subject's left and right sides sinmhaneously. This subject was asked to move her head into different positions and then to hold still in that positkm for approximately 30 seconds while magnet sensor data were collected. A look-up table algorithm was used to convert the magnet sensor output into an estimate of magnet position and thus a measure of spring compression.
Device to Monitor Orthodontic Headgear Use
removed. O u t p u t from both magnet sensors were collected at a rate of four samples per minute, and these data were converted to an estimate of adjustment strap position by using the look-up table. The four episodes of headgear use were detected accurately by the monitor. Figure 4 illustrates the position of the adjustment strap for both monitors during an eighth o u r period when the subject was asleep. Alt h o u g h the subject was sleeping, there was considerable variation in the degree of spring compression over time, which is indicative of head movement. These data also support the observation that the spring compression of the right and left sides are frequently related (eg, as one side compresses, the other relaxes). As could be expected, data collected while the subject was awake exhibited greater variability than data collected during sleep. W h e n the headgear is not being worn, there is very little variability in the position of the adjustment strap over time. Similarly, when the monitors were placed on a m a n n e q u i n head to simulate (falsify) headgear use, the data showed an absence of variability in the m o v e m e n t of the adjustment strap. Thus, actual wear can be distinguished from static attempts to falsify headgear use by measuring the variability in the position of the adjustment strap over time. As shown in Figure 4, there is considerable variability in the data, although there was a 45-minute period between hours 6 and 7 when there was little variability in the position of the adjustment strap even t h o u g h it was being worn. Research
Estimated Magnet Position for Left Monitor
from the University of Florida has suggested that an absence of m o v e m e n t over an h o u r would be a good criterion to indicate no wear. ~4 O n e of our design goals 5 was for the m o n i t o r "to provide continuous, accurate, and easily interpretable visual feedback to patients, parents, and clinicians about headgear use." This is another unique aspect of our design because no headgear timers have provided feedback directly to patients in an effort to enhance adherence via improved self-regulation. 6 The LCD on the monitor can be p r o g r a m m e d to show the average daily wear time a n d / o r the cumulative n u m b e r of hours of wear since the last visit. This feature is convenient for the orthodontist to assess the patient's progress, and it provides the patient with o n g o i n g feedback to regulate his or her own behavior. Self-monitoring has been suggested as a useful strategy to enhance patient adherence. 5-7
Conclusion These initial laboratory and clinical tests were conducted to validate the functionality of this novel approach to monitoring headgear wear. Based on the encouraging findings of these tests, an improved version of the headgear m o n i t o r is being developed, and these monitors will undergo extensive evaluation in private orthodontic offices. The development of a sophisticated headgear m o n i t o r will be valuable for both scientific and clinical orthodontic applications. Sufficient flexibility has been designed into this headgear m o n i t o r so that it can meet the needs of scientists as well as clinicians.
15 105
Acknowledgment
{mm)
We would like to acknowledge the contributions of Chris Prall, Mani Soma, and Susan Herring to this article.
o Estimated Magnet Position for
33
15 10
Right Monitor (mm)
References
st 0-- , 0
,
i
2
4
Time
Figure
,
6
,
8
(hours)
4. T w o h e a d g e a r m o n i t o r s w e r e w o r n w i t h a
cervical headgear appliance to collect data from both the subject's left and right sides simultaneously. Although the subject was asleep, there was considerable variation in the degree of spring compression over time which is indicative of head movement.
1. Ghatari J, King G..l, Tulloch JF. Early t r e a t m e n t of Class II, division 1 m a l o c c l u s i o n - - C o m p a r i s o n of alternative t r e a t m e n t modalities. Clin O r t h o d Res 1998;1:107-117. 2. Keeling SD, W h e e l e r TT, King GJ, et al. Ameroposterior skeletal a n d dental changes after early Class II t r e a t m e n t with bionators a n d headgear. Am.J O r t h o d Dentofacial O r t h o p 1998;113:40-50. 3. Tulloch JF, Phillips C, Koch G, et al. T h e effect of early intervention on skeletal pattern in Class II malocclusion:
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A randomized clinical trial. Am J Orthod Dentothcial Orthop 1997;111:391-400. Tulloch JF, Proffit WR, Phillips C. Influences on the outcome of early treatment for Class II m a l o c c l u s i o n . Am J Orthod Dentofi~cial Orthop 1997;111:533-542. Ramsay DS, Soma M, Sarason IG. Enhancing patient adherence: The role of technology and its application to orthodontics, in McNamara JA Jr, Trotman CA (eds): Creating the Compliant Patient. Craniotacial Growth Series, Center for Human Growth and Development, University of Michigan, Ann Arbor, MI, 1997;33:141-165. Lyons EK, Ramsay DS. A self-regulation model of patient compliance in orthodontics: Implications for the design of a headgear monitor. Semin Orthod 2000;6:224-230. Dunbar-Jacob J, Schlenk E. Patient adherence to treatment regimen, in Baum A, Revenson TA, Singer JE (eds): Handbook of Health Psychology. Mahwah, NJ: Lawrence Erlbaum Associates, 2001, pp 571-580.
8. Northcutt ME. The timing headgear. J Clin Orthod 1974;8:321-324. 9. Banks PA, Read MJF. An investigation into the reliability of the timing headgear, gr J Orthod 1987;14:263-267. 10. Cureton SL, Regennitter FJ, Orhell MG. An accurate, inexpensive headgear timer..J Clin Orthod 1991;25:749754. 11. Guray E, Orhan M. Selcuk lype headgem~timer (STHT). Am J Orthod Dentofacial Orthop 1997;111:87-92. 12. Kyriacou PA, Jones DP. Compliance monitor t6r use with removable orthodontic headgear appliances. Med Biol Eng Cnmput 1997;35:57-60. 13. Johnson PD, Bar-Zion Y, Taylor M, et al. Effects of head posture on headgear force application..J Clin Orthod 1999;33:94-97. 14. McGorray SP, Dean GS, Keeling SD, et al. Monitoring orthodontic headgear wear with a micro-electronic data acquisition system. J Dent Res 1997;76:127 (abstr 909).
rights reserved.
linical research has shown that o r t h o d o n t i c h e a d g e a r t r e a t m e n t is effective for the correction o f Angle Class II malocclusions. ~-3 T h e removable n a t u r e o f a h e a d g e a r places the p r o p e r i m p l e m e n t a t i o n o f the o r t h o d o n t i s t ' s prescription for its use u n d e r the c o n t r o l o f the patient. Patients typically c o n t r o l w h e n a n d for how l o n g h e a d g e a r is worn, a n d d e p e n d i n g o n the appliance, patients can m a n i p u l a t e the a m o u n t o f force the h e a d g e a r delivers. A patient's willingness to wear a h e a d g e a r is t h o u g h t to be i m p o r t a n t if t r e a t m e n t is to succeed. Tulloch a n d colleagues 4 m a k e this p o i n t clearly, "The c h a n c e o f i m p r o v e m e n t in an u n c o o p e r a tive child is the same as the c h a n c e o f improvem e n t in an u n t r e a t e d o n e - small, b u t n o t zero." In contrast, there is little k n o w n a b o u t h o w m u c h p a t i e n t a d h e r e n c e to the p r e s c r i b e d head-
C
From the Departments of Orthodontics, Pediatric Dentistry, and t~sychology, Unive~:;ity of Washington, Seattle, WA. Supported by NIH grant; KO4-DFO0379 and R41 DP:12430, the University c~ Washington Orthodontic Alumni Association, the Wa.~hington 7}ehnology (;enter, and the University of Washington Royalty Research Fund. Address correspondence to Douglas S. Ramsczv, DMD, Phi), MSD, Depa~Onent of Pediatric Dentistry, Univey:;ity of Washington, Box #357136, Seattle, WA 98195-7136. Copyright 2002, Elsevier Science (USA). All r~gtzts *~served. 1073 8746/02/0801-0006535.00/0 doi: l O.1053/sodo. 2002.28170
gear r e g i m e n is n e e d e d to p r o d u c e the desired t r e a t m e n t o u t c o m e . 5,6 I n d e e d , there is little research in any health care field that describes h o w partial p a t i e n t a d h e r e n c e relates to clinical o u t c o m e . 7 A s s u m i n g that p a t i e n t a d h e r e n c e to a h e a d g e a r r e g i m e n is a m e d i a t o r o f clinical outc o m e t h e n p o o r a d h e r e n c e may lead to increased t r e a t m e n t time, increased cost, a n d failure to c o r r e c t the malocclusion, which in turn, may necessitate a less p r e f e r r e d alternative treatment. Being able to identify p o o r l y c o m p l i a n t patients w o u l d allow timely t r e a t m e n t plan m o d ifications or the i m p l e m e n t a t i o n o f strategies to e n h a n c e p a t i e n t a d h e r e n c e . 6,7 In addition, the ability to m e a s u r e i m p o r t a n t characteristics o f h e a d g e a r use would m a k e it possible to evaluate the effect o f partial a d h e r e n c e o n clinical outc o m e . U n f o r t u n a t e l y , it is usually difficult to know how well an individual is a d h e r i n g to a t r e a t m e n t r e g i m e n . ~' O r t h o d o n t i s t s have l o n g a p p r e c i a t e d the potential value o f m e a s u r i n g the time a h e a d g e a r has b e e n w o r n a n d there have b e e n n u m e r o u s attempts to m a k e a device that c o u l d provide this i n f o r m a t i o n . N o r t h c u t t i n t r o d u c e d the first c o m m e r c i a l l y available timing h e a d g e a r in 1974. 8 Unfortunately, this timing h e a d g e a r was b o t h inaccurate -~ a n d susceptible to patient attempts to falsify h e a d g e a r wear. C o n t i n u e d interest in m e a s u r i n g h e a d g e a r use lead to the
Seminars in Orthodontics, Vol 8, No 1 (March), 2002: pp 29-34
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Lyons and Ramsay
publication of m e t h o d s for fabricating h e a d g e a r timers? °q~ These timers were bulky and often necessitated placing the electronics in the neck strap, which could diminish patient comfort. Some designs r e m o v e d the safety disconnect feature that should be a part of all h e a d g e a r force modules. Most designs offered little protection against patient attempts to falsit}~ h e a d g e a r use data and provided only a cumulative measure of wear time. Finally, these devices provided the orthodontist with intermittent h e a d g e a r wear information, and n o n e gave the patients easily accessible feedback a b o u t their degree of adherence to the h e a d g e a r wear r e g i m e n ) ,6 Newer devices to measure h e a d g e a r use have b e e n developed. T h e O r t h o Kinetics Corporation (Vista, CA) has m a d e a connnercially available h e a d g e a r timer. An optical sensor, which can detect when the spring in the module is compressed, activates the timer in this h e a d g e a r force module. This device can record each period of h e a d g e a r use, and the record of headgear wear can be reviewed during visits at the orthodontic office. Unfortunately, it is possible for patients to activate the timer without actually wearing the headgear. To create a long-term record of falsified wear data, however, a patient would n e e d to a d h e r e to this pattern of deceptive behavior over a long period (eg, r e m e m b e r to compress the spring before b e d and allow the spring to relax on awakening). Scientists at the University of Florida have developed a microprocessor-controlled headgear-timing device that can record the a m o u n t of spring compression that occurs during h e a d g e a r use. 13 This design has the advantage of measuring the dynamic spring compression that occurs during h e a d g e a r wear and so can provide information a b o u t h e a d g e a r use and the a m o u n t of force p r o d u c e d by the spring. The device has limited clinical utility because it is somewhat fragile and its design requires connecting the bulky electronics, located in the neckstrap, to the h e a d g e a r force module. A novel h e a d g e a r m o n i t o r has b e e n develo p e d at the University of Washington (US Patent #5,980,026); the rationale underlying its design is described elsewhere. 5,~; In brief, the guiding design principles were to m a k e the device small, durable, conffortable, and to maintain the safety breakaway feature so that it could be used clinically. T h e device was designed to 1) measure
the temporal characteristics of h e a d g e a r wear; 2) estimate how m u c h force is delivered by the headgear; 3) detect patient attempts to falsify h e a d g e a r use; and 4) provide readily accessible feedback to the patients, parents, and orthodontists about h e a d g e a r use. Initial prototypes were fabricated and evaluated in laboratory and clinical tests. T h e electronics and battery of the prototype m o n i t o r are contained entirely within the headgear force module. T h e m o n i t o r uses a microprocessor with nonvolatile m e m o r y , m a g n e t sensors, an infrared light-emitting diode (LED) and phototransistor for bi-directional communication and a 4-digit liquid crystal display (LCD) screen to display output. A lithium battm y powers the monitor. A small cylindrical rare-earth m a g n e t is e m b e d d e d in the end of the adjustm e n t strap. T h e basic unit of m e a s u r e m e n t is the position of the m a g n e t within the body of the force module, which changes as the strap is e x t e n d e d and the internal spring is compressed. By measuring the position of the adjustment strap within the force module, it is possible to d e t e r m i n e when the h e a d g e a r is in use. In addition, the force resulting fi-om a known a m o u n t of spring compression can be calculated from calibration data stored in a look-up table in the m o n i t o r ' s memory.
Laboratory Testing T h r e e laboratory tests are described. The first test m e a s u r e d the accuracy of the clock crystal that determines when h e a d g e a r wear occurred. The second test d e t e r m i n e d whether the monitor could accurately measure the a m o u n t of spring compression. T h e third test evaluated whether the a m o u n t of force g e n e r a t e d by the spring during its compression could be estim a t e d by knowing the a m o u n t of spring compression.
Clock Accuracy T h e microprocessor in the h e a d g e a r m o n i t o r is e q u i p p e d with a standard 32,768-Hz crystal oscillator internal clock. This clock is used to register the h e a d g e a r use data in real time. To test the accuracy of the timing unit, the clock was synchronized with the National Bureau of Standards' clock t h r o u g h internet access. The head-
31
Device to Monitor O,¢hodontic Headgear (2~e
gear monitors were activated and left r u n n i n g for 72 hours. After 72 hours, the internal c l o c k OI1 the microprocessor was c o m p a r e d to the National Bureau of Standards' clock. There was approximately a 2-second delay in the internal clock out of the r u n n i n g time of 259,200 seconds, indicating 99.999% accuracy for the internal clock timer.
2s Estimated 20 Magnet
15
Position 10
(mm)
s 0
A/
"100
80 Differential 60 Hall Output 40 (A-D Values) 2O
0 5 10 15 20 25
0 5 10 15 20 25
Known Magnet Position (mm)
Measuring Adjustment Strap Position and Spring Compression Pulling the headgear adjustment strap t h r o u g h the force module causes the spring to compress. The a m o u n t of spring compression can be determined by knowing the position of the adjustm e n t strap within the force module. This was calibrated with the output from the magnet sensors in the headgear m o n i t o r by using a MTS machine (Material Testing System model Sintech 2 / S , MTS Systems Corporation, Triangle Park, NC). The MTS machine allowed the body of the force module to be m o u n t e d to a fixed lower unit while the adjustment strap was connected to a moving unit attached to a load cell. The MTS was p r o g r a m m e d to provide various known amounts of adjustment strap extension. The extension values can be p r o g r a m m e d to run in different increments to a specified extension distance and to have varying holding times at any position. The MTS was p r o g r a m m e d to move the adjustment strap at l-ram increments through the range of movement. At each l-ram increment, the adjustment strap was held in position, and the output from both magnet sensors was recorded. The location of the adjustment strap and the output from both sensors were entered into a look-up table in the m e m o r y of the headgear monitor. An algorithm was used to calculate the position of the adjustment strap based on the output from the two m a g n e t sensors. The accuracy of this m e t h o d for determining the position of the adjustment strap was evaluated. The MTS system was p r o g r a m m e d to move the adjustment strap through a series of known positions, and the estimated position of the strap was calculated by using the output from the sensors. Figure 1 illustrates that there was excellent agreement between the known position of the strap as determined by the MTS machine and the estimated position of the strap
Figure 1. A magnet embedded in the end of the headgear adjustment strap moves through the body of the headgear torce module as the MTS machine pulls the strap. By using Hall sensors to measure magnetic field strength, it was possible to obtain excellent resolution in estimating magnet position relative to the known magnet position. as d e t e r m i n e d by comparing the sensor output with the calibration data in the look-up table.
Estimating Force from Knowing the Adjustment Strap Position Orthodontists are interested in knowing the a m o u n t of force generated from the spring in the headgear force module. This force is often assessed clinically by using a fbrce gauge when the headgear is being adjusted to a patient. Therefore, it was important to ascertain whether the a m o u n t of force from the spring could be estimated by knowing the a m o u n t of strap extension. For this m e t h o d to work there must be a direct relationship between headgear strap position and force. The load cell of the MTS machine was also able to measure the tensile force at any given strap extension. Therefore, it was possible to describe the relationship between adjustment strap position and force. The MTS extension program was run with each headgear module extended to a distance of 20 m m and returned to its initial position at both slow and rapid speeds. The force n e e d e d to extend the headgear strap was recorded over the extension and return range. As shown in Figure 2, the relationship was, for the most part, linear, although there was some hysteresis. Whereas it is possible to obtain a reasonable approximation of the force delivered at a given a m o u n t of strap extension, a goal of future headgear monitor development is to reduce this hysteresis to improve the accuracy of force estimates. Interest-
Lyons and Ramsay
32
1400, 1200. 1000,
MTS pullingthe
a d j u s t ~
Force 800, (gin) 600, 400,
/2
J . ~
~ ~ s h i n g the P" adjustmentstrap
f
200, 0
5 10 15 20 25 Known Magnet Position (ram)
Figure 2. Using the MTS machine, representative data collected flcom a single module demonstrate that there is a good, but not perfect, relationship between the position of the adjustment strap and force. ingly, several c o m m e r c i a l l y available h e a d g e a r force m o d u l e s were also evaluated, a n d a large a m o u n t o f hysteresis was o b s e r v e d in s o m e models.
Clinical Testing Two clinical tests are described. T h e first test m e a s u r e d the effect o f h e a d posture o n spring compression. T h e s e c o n d test described h o w spring c o m p r e s s i o n c h a n g e s d u r i n g h e a d g e a r wear.
collection o f 14 data points at each h e a d position. Spring c o m p r e s s i o n in the left a n d right m o n i t o r s was inversely related d u r i n g lateral h e a d m o v e m e n t s . As the h e a d t u r n e d to the right, the right m o n i t o r ' s spring relaxed while the left m o n i t o r ' s spring was compressed. T h e opposite was true w h e n t u r n i n g the h e a d to the left. In contrast, a positive relationship in spring c o m p r e s s i o n was observed for the right a n d left m o n i t o r s w h e n the h e a d moves u p or down. Both right a n d left springs are c o m p r e s s e d w h e n l o o k i n g u p w a r d a n d b o t h are relaxed w h e n looking downward. This i n d e p e n d e n t replication o f the relationship b e t w e e n h e a d posture a n d force suggests that these two different h e a d g e a r m o n itor designs are m e a s u r i n g the same p h e n o m e non.
Headgear Wear Data Collected During Patient Use For clinical data collection, two h e a d g e a r m o n itors were w o r n with a cervical h e a d g e a r appliance, a n d data were collected f r o m b o t h the subject's left a n d right sides simultaneously. T h e subject wore the m o n i t o r i n g h e a d g e a r for f o u r episodes d u r i n g a 2 4 - h o u r p e r i o d a n d carefully r e c o r d e d the times the h e a d g e a r was w o r n a n d
Maximum Spring 25 Compresson
F
2O
Estimated
Magnet is
Effect of Head Posture on Spring Compression Research f r o m the University o f Florida f o u n d that h e a d g e a r force varies as a f u n c t i o n o f h e a d position. 13 Because the h e a d g e a r m o n i t o r s at the University Florida a n d the University o f W a s h i n g t o n b o t h m e a s u r e an i n d e x o f spring compression, it s h o u l d be possible to replicate these findings using o u r different h e a d g e a r design. To evaluate the effect o f h e a d position o n spring c o m p r e s s i o n , a subject wore a m o n i t o r o n each side o f the head. T h e subject t h e n t u r n e d h e r h e a d t h r o u g h a range o f m o t i o n , l o o k i n g left, right, upward, a n d downward. Each h e a d position was held for 30 seconds. T h e results are shown in Figure 3. This g r a p h represents an average o f two cycles o f h e a d m o v e m e n t s with a
Position (ram)
10
Passive Spring 0 Position
Left
~R i g h t
Up
= i i = Down Left Right Up
=
=
Down
Head Position Left Side Monitor
Figure 3.
Right Side Monitor
Two headgear monitors were used with a cervical headgear appliance to collect data from both the subject's left and right sides sinmhaneously. This subject was asked to move her head into different positions and then to hold still in that positkm for approximately 30 seconds while magnet sensor data were collected. A look-up table algorithm was used to convert the magnet sensor output into an estimate of magnet position and thus a measure of spring compression.
Device to Monitor Orthodontic Headgear Use
removed. O u t p u t from both magnet sensors were collected at a rate of four samples per minute, and these data were converted to an estimate of adjustment strap position by using the look-up table. The four episodes of headgear use were detected accurately by the monitor. Figure 4 illustrates the position of the adjustment strap for both monitors during an eighth o u r period when the subject was asleep. Alt h o u g h the subject was sleeping, there was considerable variation in the degree of spring compression over time, which is indicative of head movement. These data also support the observation that the spring compression of the right and left sides are frequently related (eg, as one side compresses, the other relaxes). As could be expected, data collected while the subject was awake exhibited greater variability than data collected during sleep. W h e n the headgear is not being worn, there is very little variability in the position of the adjustment strap over time. Similarly, when the monitors were placed on a m a n n e q u i n head to simulate (falsify) headgear use, the data showed an absence of variability in the m o v e m e n t of the adjustment strap. Thus, actual wear can be distinguished from static attempts to falsify headgear use by measuring the variability in the position of the adjustment strap over time. As shown in Figure 4, there is considerable variability in the data, although there was a 45-minute period between hours 6 and 7 when there was little variability in the position of the adjustment strap even t h o u g h it was being worn. Research
Estimated Magnet Position for Left Monitor
from the University of Florida has suggested that an absence of m o v e m e n t over an h o u r would be a good criterion to indicate no wear. ~4 O n e of our design goals 5 was for the m o n i t o r "to provide continuous, accurate, and easily interpretable visual feedback to patients, parents, and clinicians about headgear use." This is another unique aspect of our design because no headgear timers have provided feedback directly to patients in an effort to enhance adherence via improved self-regulation. 6 The LCD on the monitor can be p r o g r a m m e d to show the average daily wear time a n d / o r the cumulative n u m b e r of hours of wear since the last visit. This feature is convenient for the orthodontist to assess the patient's progress, and it provides the patient with o n g o i n g feedback to regulate his or her own behavior. Self-monitoring has been suggested as a useful strategy to enhance patient adherence. 5-7
Conclusion These initial laboratory and clinical tests were conducted to validate the functionality of this novel approach to monitoring headgear wear. Based on the encouraging findings of these tests, an improved version of the headgear m o n i t o r is being developed, and these monitors will undergo extensive evaluation in private orthodontic offices. The development of a sophisticated headgear m o n i t o r will be valuable for both scientific and clinical orthodontic applications. Sufficient flexibility has been designed into this headgear m o n i t o r so that it can meet the needs of scientists as well as clinicians.
15 105
Acknowledgment
{mm)
We would like to acknowledge the contributions of Chris Prall, Mani Soma, and Susan Herring to this article.
o Estimated Magnet Position for
33
15 10
Right Monitor (mm)
References
st 0-- , 0
,
i
2
4
Time
Figure
,
6
,
8
(hours)
4. T w o h e a d g e a r m o n i t o r s w e r e w o r n w i t h a
cervical headgear appliance to collect data from both the subject's left and right sides simultaneously. Although the subject was asleep, there was considerable variation in the degree of spring compression over time which is indicative of head movement.
1. Ghatari J, King G..l, Tulloch JF. Early t r e a t m e n t of Class II, division 1 m a l o c c l u s i o n - - C o m p a r i s o n of alternative t r e a t m e n t modalities. Clin O r t h o d Res 1998;1:107-117. 2. Keeling SD, W h e e l e r TT, King GJ, et al. Ameroposterior skeletal a n d dental changes after early Class II t r e a t m e n t with bionators a n d headgear. Am.J O r t h o d Dentofacial O r t h o p 1998;113:40-50. 3. Tulloch JF, Phillips C, Koch G, et al. T h e effect of early intervention on skeletal pattern in Class II malocclusion:
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8. Northcutt ME. The timing headgear. J Clin Orthod 1974;8:321-324. 9. Banks PA, Read MJF. An investigation into the reliability of the timing headgear, gr J Orthod 1987;14:263-267. 10. Cureton SL, Regennitter FJ, Orhell MG. An accurate, inexpensive headgear timer..J Clin Orthod 1991;25:749754. 11. Guray E, Orhan M. Selcuk lype headgem~timer (STHT). Am J Orthod Dentofacial Orthop 1997;111:87-92. 12. Kyriacou PA, Jones DP. Compliance monitor t6r use with removable orthodontic headgear appliances. Med Biol Eng Cnmput 1997;35:57-60. 13. Johnson PD, Bar-Zion Y, Taylor M, et al. Effects of head posture on headgear force application..J Clin Orthod 1999;33:94-97. 14. McGorray SP, Dean GS, Keeling SD, et al. Monitoring orthodontic headgear wear with a micro-electronic data acquisition system. J Dent Res 1997;76:127 (abstr 909).