American Journal of ORTHODONTICS and DENTOFACIAL ORTHOPEDICS Founded ill 1915
Vohune 100 Number 5
November 1991
Copyright © 1991 by the American Association of Orthodontists
ORIGINAL ARTICLES
Intrusion of posterior teeth with magnets: An experiment in nongrowing baboons Michael G. Woods, MDSc, FRACDS, D. Orth., RCS,* and Ram S. Nanda, DDS, MS, PhD** Oklahoma Cit~, Okla.
T h e use of occlusal bite blocks containing repelling magnets has been proposed as the treatment procedure for patients who have anterior open bites. ~,2 The repelling force of the opposing magnets is reported to cause intrusion of the posterior teeth, allowing the mandible to rotate upward and forward in much the same way that it would if the maxillae were surgically impacted. 3 Despite the fact that these magnet appliances have been marketed to the profession for many years, it is still unclear whether the clinical changes seen are due to the magnets themselves, the muscular response to the artificially increased vertical dimension, or some other factors associated with the facial development of particular orthodontic patients. In an earlier study using repelling-magnet bite blocks in growing animals, 4 it was found that these appliances altered the amount and direction of maxillary displacement occurring during growth, caused changes in mandibular shape, and caused depression of underlying buccal teeth. However, in control animals wearing passive appliances of the same dimensions, similar responses were also noted. It was therefore suggested that attempts should be made to develop magnet appliances that do not cause such an increase in the vertical dimension and that these appliances should perhaps be used at a later age, when the facial skeleton is less plastic. It was the purpose of this study to assess the dental and skeletal effects of an updated repelling-magnet appliance in nongrowing anFrom the University of Oklahoma. This study was supported by grants from the Foundation for Orthodontic Research, Pacific Palisades, Calif., and the Presbyterian Health Foundation of Oklahoma. *Visiting assistant professor, Department of Orthodontics. **Professor and chairman, Department of Orthodontics. 811120300
Fig. 1. Surgical placement of metallic markers.
imals over a period similar to that suggested for use in human orthodontic patients.
MATERIALS AND METHODS Four female adult baboons (Papio cynocephah~s) were chosen for this study. All were approximately 9 years old, with complete permanent dentitions. All animals had ceased active growth approximately 5 years previously. During all experimental procedures, the animals were heavily sedated with ketamine hydrochloride and Rompun. Before the experimental period, metallic markers were placed directly in the left maxillary and mandibular cortices of each animal (Fig. 1). Amalgam markers we_realso placed in the upper and lower left first and second molars and left second premolars. No skeletal or dental markers were placed on the right side. Initial photographs, cephalometric radiographs, and impressions for the study casts were then taken. A radiographic machine (General Electric Co., Milwaukee, Wise.) and a cephalostat (B. F. Wehmer, Franklin Park, I11.) had already been adjusted and calibrated ~ so that good393
394
Woods and Nanda
Am. ] . Orthod. Dentofac. Orthop. November 1991
Fig. 2. Active magnet appliances following insertion.
quality cephalometric radiographs could be taken of the animals. A steel high chair had also been modified to support the baboons in the cephalostat while the radiographs were taken. The consistency of the midsagittal plane was maintained with the use of a plastic snoutholder, attached to the cephalostat2 The use of this device allowed the snout of each heavily sedated animal to be held perpendicular to the x-ray beam and at the same horizontal level relative to the earrods of the cephalostat. The distance between the midsagittal plane
and the medial surface of the film cassette was also constant for all radiographs of all animals. Acrylic bite blocks were constructed for the four animals. All were designed to cover the first and second molars and the second premolars so that there was an interocclusal opening of 5 mm between the most posterior teeth. The right and left blocks were joined by temporary cast palatal and lingual bars for initial seating. The blocks for the two animals wearing active appliances contained repelling cobalt-samarium mag-
Magnet intrusion 395
Voh.,e 100
Number 5
ANIMAL 1. WITH MAGNETS CRANIAL SUPERIMPOSITION ~ *
ANIMAL 1. WITH MAGNETS MAXILLARY AND MANDIBULAR TOOTH MOVEMENTS
Inserlion of A p p l i ~
Fig. 3. Animal 1. Magnets, left side. A, Before treatment. B, After 4 months. C, Cephalometric superimposition on cranial structures. D, Superimposition on maxillary and mandibular implant lines.
nets (Sm Co5) (Active Vertical Correctors, Inc., Fort Wayne, Ind.)Y '8 Cobalt-samarium is a highly reactive rare earth material. Therefore, to isolate the cobalt-samarium from the oral environment, the magnets were hermetically sealed in stainless steel capsules that were, in turn, embedded in the acrylic blocks. Two magnets were placed in each quadrant. Each of the four magnet pairs in each animal delivered a maximum of approximately 500 gm repelling force applied to the upper and lower buccal teeth. This was less than the maximum 700 gm delivered by each unit in the previous study." The blocks for the two control animals contained only empty magnet cases. The acrylic blocks were bonded to the teeth with composite resin. After the appliances had been inserted, the cast palatal and lingual bars were removed so that the animals could not use them as levers for the removal of the appliances. The appliances were equilibrated in the mouth as necessary to ensure maximum occlusal contact (Fig. 2). Throughout the experimental period, the animals were individually caged and fed a normal diet of laboratory chow, apples, bananas, oranges, and water. At the end of the 4month experimental period, the appliances were removed and further records were taken. All experimental animals adapted well to the presence of the appliances within 1 to 2 weeks
after the insertion. The weight of each animal was stable during the experimental period. CEPHALOMETRIC ANALYSIS
Each cephalometric radiograph was enlarged to twice its original size for more accurate tracing of the craniofacial structures. The left buccal teeth were traced on each radiograph with individual plastic templates o f these teeth made from the initial radiograph of each animal. To assess the displacement of the maxillary complex, we superimposed serial radiographs on the inferior border o f the sphenoid portion o f the cranial base and on the intercranial contours, such as the orbital roof. To assess dental changes within the mandible and the maxilla, we made serial superimpositions over the metallic bone markers and the internal structures. The movements o f the dental markers during the experimental period were then measured and recorded from the enlarged tracings. The radiographs were retraced randomly and further sets o f measurements were taken. Paired t tests were performed to determine the investigator's tracing and measurement errors. The differ-
396
Woods and Nanda
Am. J. Orthod. Dentofac. Orthop. November 1991
ANIMAL 2. WITH MAGNETS CRANIAL SUPERIMPOSmON ~ e
ANIMAL 2. WITH MAGNETS MAXILLARY AND MANDIBULAR TOOTH MOVEMENTS
Insertion of A 1 ~ i ~
•
g
I
og
• Insertionof App4iances o Removalof Appliances
Fig. 4. Animal 2. Magnets, left side. A, Before treatment. B, After 4 months. C, Cephalometric superimposition on cranial structures. D, Superimposition on maxillary and mandibular implant lines.
ences between the two measurements taken on separate occasions for each dental marker were used to calculate p values. No differences were, in fact, found to be significant at the p < 0.05 level. As a precautionary measure, however, the averages of the two measurements were used for further analysis. These were halved, corrected to the nearest 0.5 ram, and then tabulated. The absolute value for each measurement, however, was not considered to be as important as the demonstration of a difference, or otherwise, in the intrusive effects of active and control appliances. RESULTS The movements of all dental markers occurring during the 4-month experimental period are presented in Table I. They are also illustrated in Figs. 3 to 6.
Active magnets In both animals wearing the active magnet appliances, there were significant movements of the upper and lower dental markers toward their respective skeletal markers. The buccal teeth had therefore been
pushed back into their sockets. The amount of anterior compensatory eruption and consequent backward mandibular rotation was minimal. No changes were evident in the positions of the maxillary bone markers in relation to the cranium.
Passive acrylic blocks In both control animals, there was little, if any, movement of the dental markers during the experimental period. The amount of anterior eruption and backward mandibular rotation was also minimal. Once again, no changes were evident in the positions of the maxillary bone markers. DISCUSSION After a previous study of the effects of magnet bite blocks in growing animals,4 it was still unclear whether the observed depression of the buccal teeth was due to the presence of the magnets themselves or merely to the intrinsic muscular response to the artificially increased vertical dimension. It was suggested, therefore, that thinner magnet appliances should be developed and
Volume 100
Magnet intrusion
Number 5
~,~ :
•
",
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/
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. -
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.
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• Ins~'tionof Appliance~ ...... o Removalof Appliances •
•
!
Insertion of Appliances
o Removal of Ap~iances
B
qb
Fig. 5. Animal 3. Control, left side. A, Before treatment. B, After 4 months. C, Cephalometric superimposition on cranial structures. D, Superimposition on maxillary and mandibular implant lines.
that they should perhaps be used at a later age, when the facial skeleton would be less plastic. It was hypothesized that one might then see a difference between the effects of control and active magnet appliances. Indeed, that has been the ease in this study. The fact that the magnet appliance did cause depression of the posterior teeth in these adult animals supports previous findings 4 and claims.~ However, in contrast to the previous study in growing baboons, the effects of the active appliances were reduced, and no effects at all were seen in the control animals. This might be explained, at least in part, by the fact that additional work was required to keep the repelling magnets together. The magnet animals were therefore forced to adopt a lower mandibular resting position than that dictated by the acrylic blocks alone. For that reason, the potential muscular intrusion force is likely to have been greater in the animals that wore the magnet appliances. Unfortunately, this does not explain the lack of any real effects in the control animals. It would appear, therefore, that there must be certain factors associated with the magnets themselves that promote
the pathophysiologic reactions necessary for the buccal teeth to be depressed in their sockets. If, however, the major factor is the increased force delivered by the magnets, then it would appear to contradict traditional orthodontic thinking, since the potential intrusive force delivered to each tooth by these magnet appliances is already outside the range currently recommended for safe and efficient intrusion in human beings. 9''° On the other hand, one must realize that, since the repelling-force levels to drop off markedly as the magnets are separated (in a fashion similar to an inverse-square relationship), the actual forces delivered by the appliance may, in fact, have been much lower. Obviously, it is likely that some factor is involved. For instance, the possibility that pulsating electromagnetic fields might be set-up by movements of the magnets within the mouth has been canvassedJ These fields are of low frequency and are of a random nature in the mouth. However, electromagnetic fields have been shown experimentally to increase vascularity and to affect bony metabolism in localized areas." In the absence of a better explanation, therefore, it would be
398
Woods a n d N a n d a
Am. J. Orthod. Dentofac. Orthop. November 1991
A N I M A L 4. C O N T R O L C R A N I A L SUPERIMPOSITION
\
ANIMAL 4. CONTROL MAXILLARY A N D MANDIBULAR TOOTH MOVEMENTS
• Inlertio~ of Appliance~ . . . . . . . o Removalof Ap~im~s
,. I
I
dP
~
J
•
• Inl,e~'tton of Appliances o Removal
of Appl~nces
D Fig. 6. Animal 4. Control, left side. A, Before treatment. B, After 4 months. C, Cephalometric superimposition on cranial structures. D, Superimposition on maxillary and mandibular implant lines.
Table I. Movements of dental markers in all animals, during the 4-month experimental period !
Su~fiorve~icalmovementsofm~illarydentalm~kers 6 5 4 In~fiorveaicalmovementsofmandibul~dentalmxkers 6 5 4
reasonable to suggest at this time that electromagnetic fields might be involved in increasing the response within bone to potential intrusive forces delivered by the repelling magnets. In any case, it is essential that further work be directed toward this area. The fact that there was no apparent maxillary skeletal displacement and certainly little, if any, mandibular remodeling in any of these experimental animals was
Animal 1 magnets
Animal 2 magnets
Animal 3 control
[ [
Animal 4 control
3.0 3.0 2.5
2.0 2.0 2.0
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l.O 0.5 0.5
3.0 3.0 3.0
2.0 2.0 2.5
1.0 1.0 0.5
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to be expected. Craniofacial growth had, after all, been complete in all animals for approximately 5 years. While one might expect there to be mild remodeling changes if the study were to be continued for a much longer period, it is hardly surprising that the skeletal changes previously reported to occur in growing animals in response to increased vertical dimension n~6 were not observed here. Similarly, the amount of an-
Volume 100 Number 5
terior eruption was far less in all these animals than had been observed in the growing animals of the previous study. 4 However, it should be remembered that in the previous study significant vertical facial and dentoalveolar growth was occurring during the experimental period. Once again, no attempt was made in this study to limit the amount of anterior dentoalveolar compensation, since the aim of the experiment was merely to demonstrate the ability or inability of magnetcontaining bite blocks to depress buccal teeih. In extrapolating the results of a study performed in baboons to the treatment of human subjects, one should be aware that there are obvious differences between the configuration of the craniofacial region in human beings and that in baboons. While a human patient with open bite often demonstrates a predominantly vertical growth pattern, the baboon demonstrates a more anterior direction of facial growth. Any effects seen in baboons may therefore have been influenced by the different orientation of the mandibular muscles. While that should be kept in mind, the aim of this study was to examine the possibility.that teeth could be depressed in nongrowing animals. Since it has been shown that the repelling magnets used in conjunction with an artificially increased vertical dimension can depress buccal teeth, it would be reasonable to suggest that consideration could be given to their use in the treatment of an orthodontic patient in whom a decrease in posterior dentoalveolar height is required. SUGGESTIONS FOR CLINICAL TRIAL
The results of this and the previous study show that it is possible to use occlusal bite blocks containing repelling magnets to push molar teeth back into their sockets. However, several problems related to human treatment remain. Such questions must be addressed before an unqualified recommendation could be given for the use of these repelling-magnet bite blocks in all patients with anterior open bites. 1. Stability. The fact that it is possible to depress teeth, no matter how this is achieved, does not necessarily mean that any resulting decrease in facial height would be stable in the long tenn. For that reason, it is necessary that a well-controlled prospective clinical trial be undertaken in human patients, with long-term follow-up of the stability of any vertical changes. 2. Patient cooperation. The two animal studies have in no way addressed the problem of patient cooperation when these appliances are used. Without adequate cooperation, the appliances simply may not be worn long enough for the desired results to be achieved. This, however, is no different from the problems related to other fixed and removable orthodontic appliances. For that reason, the clinical trial should also include an
Magnet intrusion
399
investigation of the differences between the effects of these magnet bite blocks when worn either as removable or as fixed appliances. 3. Age and nature of the deformity. The clinical trial would also include a quantitative investigation of the amount of posterior intrusion and the actual range of effects which might be expected in the correction of human open bites. Only then will it be possible for clinicians to make sound decisions regarding the use of these appliances. For instance, their use may be limited to patients with only dental open bites. On the other hand, it may be possible to achieve significant and stable correction in patients with mild skeletal open bites. It may even be possible to achieve stable correction of some open bites in adults. In any case, it would be reasonable to suggest at this time that there will always be patients requiring orthognathic procedures to correct major dental and skeletal vertical discrepancies. 4. Cause. Finally, the clinical trial should also include an investigation of the effects of these magnet appliances on open bites with different causes. For instance, the likely effects in a patient with a thumbsucking habit might be very different from those in a patient with a history of long-standing chronic airway disturbance. These might be different again from the effects in a patient with a familial long-face tendency. We thank Mr. Willy Clagg and Mrs. Judy Amico for their help in the preparation of this manuscript. We would also like to extend our appreciation to Dr. E. L. Dellinger and the staff of Active Vertical Correctors, Inc. for their help and advice during this project, and for providing all experimental appliances. REFERENCES
1. DellingerEL. A clinical assessment of the ActiveVertical Corrector: a nonsurglcal alternative for skeletal open-bite. AMJ ORmoo 1986;89:428-36. 2. Kalra V, Burstone CJ, Nanda R. Effects of a fixed magnetic appliance on the dentofacial complex. AMJ ORmooDENTOFAC ORTHOP1989;95:467-78. 3. Bell WH, Creekmore TD, Alexander RG. Surgical correction of the long-facedsyndrome. AMJ OR'mOO1977;71:40-67. 4. WoodsgiG, NandaRS. Intrusionof posteriorteeth withmagnets: an experiment in growing baboons. Angle Orthod 1988; April:136-50. 5. WoodsMG. The mechanics of lower incisor intrusion: experiments in nongrowing baboons. AM J OR]HODDENTOFACORTHOP 1988;93:186-95. 6. Tsutsui H, Konouchi Y, Sasaki H, Shiota M, Ushita T. Studies on the SmCo magnet as a dental material. J Dent Res 1977;58:1597-606. 7. Blechman AM, Smilcy M. Magnetic forces in orthodontics. AM J ORmOD 1977;72:1-22. 8. Blechman AM. Magnetic force systems in orthodontics. AM J OR~tOD 1985;87:201-10. 9. Dellinger EL. A histologic and cephalometric investigation of premolar intrusion in the Macaca speciosa monkey. AM J OR"rHOD 1967;53:325-55.
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Woods and Nanda
10. Reitan K. Biomechanical principles and reactions. In: Graber TM, Swain BF, eds. Orthodontics, current principles and techniques. St. Louis: CV Mosby, 1985:101-92. 11. Gerling JA, Sinclair PM, Roa RL. The effect of pulsating electromagnetic fields on condylar growth in guinea pigs. AMJ ORTHOD1985;87:211-23. 12. Altuna G, Woodside DG. Response of the midface to treatment with increased vertical occlusal forces: treatment and posttreatment effects in monkeys. Angle Orthod 1985;55:251-63. 13. Breitner C. Alteration of occlusal relations induced by experimental procedure. AMJ ORTHOD1943;29:277o89.
Am.
J. Orthod. Dentofac. Orthop. 1Vovember1991
14. McNamara JA. An experimental study of increased.vertical dimension in the growing face. A.,,IJ OR~OD 1977;71:382-95. 15. Sergl HG, Farmand M. Experiments xbithunilateral bile plates in rabbits. Angle Orthod 1975;45:108-14. 16..Thompson JR, Brodie AG. Factors affecting the position of the mandible. J Am Dent Assoc 1942;29:925-41. Reprint requests to: Dr. Michael Woods 599 Dandenong Rd. Malvem, Victoria, 3143 Ausu'alia
BOUND VOLUMES AVAILABLE TO SUBSCRIBERS Bound volumes of the AMERICAN JOURNAL OF ORTHODONTICS AND DENTOFACIAL ORTHOPEDICS are available to subscribers (only) for the 1991 issues from the Pub!isher, at a cost o f $48.00 ($62.36 Canada and $59.00 international) for Vol. 99 (January-June) and Vol. 100 (July-December). Shipping charges are included. Each bound volume contains a subject and author index and all advertising is removed. Copies are shipped within 60 days after publication o f the last issue in the volume. The binding is durable buckram with the journal name, Volume number, and year stamped in gold on the spine. Payment must accompany all orders. Contact M o s b y - Y e a r Book, Inc., Circulation Department, 11830 Westline Industrial Drive, St. Louis, M O 63146-3318, USA, telephone (800)325-4177, ext. 4351. S u b s c r i p t i o n s m u s t b e in force to qualify, B o u n d v o l u m e s a r e not a v a i l a b l e in p l a c e o f a r e g u l a r J o u r n a l s u b s c r i p t i o n .
Vohune 100 Number 5
November 1991
Copyright © 1991 by the American Association of Orthodontists
ORIGINAL ARTICLES
Intrusion of posterior teeth with magnets: An experiment in nongrowing baboons Michael G. Woods, MDSc, FRACDS, D. Orth., RCS,* and Ram S. Nanda, DDS, MS, PhD** Oklahoma Cit~, Okla.
T h e use of occlusal bite blocks containing repelling magnets has been proposed as the treatment procedure for patients who have anterior open bites. ~,2 The repelling force of the opposing magnets is reported to cause intrusion of the posterior teeth, allowing the mandible to rotate upward and forward in much the same way that it would if the maxillae were surgically impacted. 3 Despite the fact that these magnet appliances have been marketed to the profession for many years, it is still unclear whether the clinical changes seen are due to the magnets themselves, the muscular response to the artificially increased vertical dimension, or some other factors associated with the facial development of particular orthodontic patients. In an earlier study using repelling-magnet bite blocks in growing animals, 4 it was found that these appliances altered the amount and direction of maxillary displacement occurring during growth, caused changes in mandibular shape, and caused depression of underlying buccal teeth. However, in control animals wearing passive appliances of the same dimensions, similar responses were also noted. It was therefore suggested that attempts should be made to develop magnet appliances that do not cause such an increase in the vertical dimension and that these appliances should perhaps be used at a later age, when the facial skeleton is less plastic. It was the purpose of this study to assess the dental and skeletal effects of an updated repelling-magnet appliance in nongrowing anFrom the University of Oklahoma. This study was supported by grants from the Foundation for Orthodontic Research, Pacific Palisades, Calif., and the Presbyterian Health Foundation of Oklahoma. *Visiting assistant professor, Department of Orthodontics. **Professor and chairman, Department of Orthodontics. 811120300
Fig. 1. Surgical placement of metallic markers.
imals over a period similar to that suggested for use in human orthodontic patients.
MATERIALS AND METHODS Four female adult baboons (Papio cynocephah~s) were chosen for this study. All were approximately 9 years old, with complete permanent dentitions. All animals had ceased active growth approximately 5 years previously. During all experimental procedures, the animals were heavily sedated with ketamine hydrochloride and Rompun. Before the experimental period, metallic markers were placed directly in the left maxillary and mandibular cortices of each animal (Fig. 1). Amalgam markers we_realso placed in the upper and lower left first and second molars and left second premolars. No skeletal or dental markers were placed on the right side. Initial photographs, cephalometric radiographs, and impressions for the study casts were then taken. A radiographic machine (General Electric Co., Milwaukee, Wise.) and a cephalostat (B. F. Wehmer, Franklin Park, I11.) had already been adjusted and calibrated ~ so that good393
394
Woods and Nanda
Am. ] . Orthod. Dentofac. Orthop. November 1991
Fig. 2. Active magnet appliances following insertion.
quality cephalometric radiographs could be taken of the animals. A steel high chair had also been modified to support the baboons in the cephalostat while the radiographs were taken. The consistency of the midsagittal plane was maintained with the use of a plastic snoutholder, attached to the cephalostat2 The use of this device allowed the snout of each heavily sedated animal to be held perpendicular to the x-ray beam and at the same horizontal level relative to the earrods of the cephalostat. The distance between the midsagittal plane
and the medial surface of the film cassette was also constant for all radiographs of all animals. Acrylic bite blocks were constructed for the four animals. All were designed to cover the first and second molars and the second premolars so that there was an interocclusal opening of 5 mm between the most posterior teeth. The right and left blocks were joined by temporary cast palatal and lingual bars for initial seating. The blocks for the two animals wearing active appliances contained repelling cobalt-samarium mag-
Magnet intrusion 395
Voh.,e 100
Number 5
ANIMAL 1. WITH MAGNETS CRANIAL SUPERIMPOSITION ~ *
ANIMAL 1. WITH MAGNETS MAXILLARY AND MANDIBULAR TOOTH MOVEMENTS
Inserlion of A p p l i ~
Fig. 3. Animal 1. Magnets, left side. A, Before treatment. B, After 4 months. C, Cephalometric superimposition on cranial structures. D, Superimposition on maxillary and mandibular implant lines.
nets (Sm Co5) (Active Vertical Correctors, Inc., Fort Wayne, Ind.)Y '8 Cobalt-samarium is a highly reactive rare earth material. Therefore, to isolate the cobalt-samarium from the oral environment, the magnets were hermetically sealed in stainless steel capsules that were, in turn, embedded in the acrylic blocks. Two magnets were placed in each quadrant. Each of the four magnet pairs in each animal delivered a maximum of approximately 500 gm repelling force applied to the upper and lower buccal teeth. This was less than the maximum 700 gm delivered by each unit in the previous study." The blocks for the two control animals contained only empty magnet cases. The acrylic blocks were bonded to the teeth with composite resin. After the appliances had been inserted, the cast palatal and lingual bars were removed so that the animals could not use them as levers for the removal of the appliances. The appliances were equilibrated in the mouth as necessary to ensure maximum occlusal contact (Fig. 2). Throughout the experimental period, the animals were individually caged and fed a normal diet of laboratory chow, apples, bananas, oranges, and water. At the end of the 4month experimental period, the appliances were removed and further records were taken. All experimental animals adapted well to the presence of the appliances within 1 to 2 weeks
after the insertion. The weight of each animal was stable during the experimental period. CEPHALOMETRIC ANALYSIS
Each cephalometric radiograph was enlarged to twice its original size for more accurate tracing of the craniofacial structures. The left buccal teeth were traced on each radiograph with individual plastic templates o f these teeth made from the initial radiograph of each animal. To assess the displacement of the maxillary complex, we superimposed serial radiographs on the inferior border o f the sphenoid portion o f the cranial base and on the intercranial contours, such as the orbital roof. To assess dental changes within the mandible and the maxilla, we made serial superimpositions over the metallic bone markers and the internal structures. The movements o f the dental markers during the experimental period were then measured and recorded from the enlarged tracings. The radiographs were retraced randomly and further sets o f measurements were taken. Paired t tests were performed to determine the investigator's tracing and measurement errors. The differ-
396
Woods and Nanda
Am. J. Orthod. Dentofac. Orthop. November 1991
ANIMAL 2. WITH MAGNETS CRANIAL SUPERIMPOSmON ~ e
ANIMAL 2. WITH MAGNETS MAXILLARY AND MANDIBULAR TOOTH MOVEMENTS
Insertion of A 1 ~ i ~
•
g
I
og
• Insertionof App4iances o Removalof Appliances
Fig. 4. Animal 2. Magnets, left side. A, Before treatment. B, After 4 months. C, Cephalometric superimposition on cranial structures. D, Superimposition on maxillary and mandibular implant lines.
ences between the two measurements taken on separate occasions for each dental marker were used to calculate p values. No differences were, in fact, found to be significant at the p < 0.05 level. As a precautionary measure, however, the averages of the two measurements were used for further analysis. These were halved, corrected to the nearest 0.5 ram, and then tabulated. The absolute value for each measurement, however, was not considered to be as important as the demonstration of a difference, or otherwise, in the intrusive effects of active and control appliances. RESULTS The movements of all dental markers occurring during the 4-month experimental period are presented in Table I. They are also illustrated in Figs. 3 to 6.
Active magnets In both animals wearing the active magnet appliances, there were significant movements of the upper and lower dental markers toward their respective skeletal markers. The buccal teeth had therefore been
pushed back into their sockets. The amount of anterior compensatory eruption and consequent backward mandibular rotation was minimal. No changes were evident in the positions of the maxillary bone markers in relation to the cranium.
Passive acrylic blocks In both control animals, there was little, if any, movement of the dental markers during the experimental period. The amount of anterior eruption and backward mandibular rotation was also minimal. Once again, no changes were evident in the positions of the maxillary bone markers. DISCUSSION After a previous study of the effects of magnet bite blocks in growing animals,4 it was still unclear whether the observed depression of the buccal teeth was due to the presence of the magnets themselves or merely to the intrinsic muscular response to the artificially increased vertical dimension. It was suggested, therefore, that thinner magnet appliances should be developed and
Volume 100
Magnet intrusion
Number 5
~,~ :
•
",
ANIMAL 3. CONTROL CRANIAL SUPERIMPOSITION ~
/
;
,,. - ' ~ . _ ~ . ".'.:...~
-->.~
. -
~=
397
,,, .:~
.
--
ANIMAL 3. CONTROL MAXILLARY AND MANDIBULAR TOOTH MOVEMENTS
• Ins~'tionof Appliance~ ...... o Removalof Appliances •
•
!
Insertion of Appliances
o Removal of Ap~iances
B
qb
Fig. 5. Animal 3. Control, left side. A, Before treatment. B, After 4 months. C, Cephalometric superimposition on cranial structures. D, Superimposition on maxillary and mandibular implant lines.
that they should perhaps be used at a later age, when the facial skeleton would be less plastic. It was hypothesized that one might then see a difference between the effects of control and active magnet appliances. Indeed, that has been the ease in this study. The fact that the magnet appliance did cause depression of the posterior teeth in these adult animals supports previous findings 4 and claims.~ However, in contrast to the previous study in growing baboons, the effects of the active appliances were reduced, and no effects at all were seen in the control animals. This might be explained, at least in part, by the fact that additional work was required to keep the repelling magnets together. The magnet animals were therefore forced to adopt a lower mandibular resting position than that dictated by the acrylic blocks alone. For that reason, the potential muscular intrusion force is likely to have been greater in the animals that wore the magnet appliances. Unfortunately, this does not explain the lack of any real effects in the control animals. It would appear, therefore, that there must be certain factors associated with the magnets themselves that promote
the pathophysiologic reactions necessary for the buccal teeth to be depressed in their sockets. If, however, the major factor is the increased force delivered by the magnets, then it would appear to contradict traditional orthodontic thinking, since the potential intrusive force delivered to each tooth by these magnet appliances is already outside the range currently recommended for safe and efficient intrusion in human beings. 9''° On the other hand, one must realize that, since the repelling-force levels to drop off markedly as the magnets are separated (in a fashion similar to an inverse-square relationship), the actual forces delivered by the appliance may, in fact, have been much lower. Obviously, it is likely that some factor is involved. For instance, the possibility that pulsating electromagnetic fields might be set-up by movements of the magnets within the mouth has been canvassedJ These fields are of low frequency and are of a random nature in the mouth. However, electromagnetic fields have been shown experimentally to increase vascularity and to affect bony metabolism in localized areas." In the absence of a better explanation, therefore, it would be
398
Woods a n d N a n d a
Am. J. Orthod. Dentofac. Orthop. November 1991
A N I M A L 4. C O N T R O L C R A N I A L SUPERIMPOSITION
\
ANIMAL 4. CONTROL MAXILLARY A N D MANDIBULAR TOOTH MOVEMENTS
• Inlertio~ of Appliance~ . . . . . . . o Removalof Ap~im~s
,. I
I
dP
~
J
•
• Inl,e~'tton of Appliances o Removal
of Appl~nces
D Fig. 6. Animal 4. Control, left side. A, Before treatment. B, After 4 months. C, Cephalometric superimposition on cranial structures. D, Superimposition on maxillary and mandibular implant lines.
Table I. Movements of dental markers in all animals, during the 4-month experimental period !
Su~fiorve~icalmovementsofm~illarydentalm~kers 6 5 4 In~fiorveaicalmovementsofmandibul~dentalmxkers 6 5 4
reasonable to suggest at this time that electromagnetic fields might be involved in increasing the response within bone to potential intrusive forces delivered by the repelling magnets. In any case, it is essential that further work be directed toward this area. The fact that there was no apparent maxillary skeletal displacement and certainly little, if any, mandibular remodeling in any of these experimental animals was
Animal 1 magnets
Animal 2 magnets
Animal 3 control
[ [
Animal 4 control
3.0 3.0 2.5
2.0 2.0 2.0
l.O l.O 0.5
l.O 0.5 0.5
3.0 3.0 3.0
2.0 2.0 2.5
1.0 1.0 0.5
i.0 1.0 0.5
to be expected. Craniofacial growth had, after all, been complete in all animals for approximately 5 years. While one might expect there to be mild remodeling changes if the study were to be continued for a much longer period, it is hardly surprising that the skeletal changes previously reported to occur in growing animals in response to increased vertical dimension n~6 were not observed here. Similarly, the amount of an-
Volume 100 Number 5
terior eruption was far less in all these animals than had been observed in the growing animals of the previous study. 4 However, it should be remembered that in the previous study significant vertical facial and dentoalveolar growth was occurring during the experimental period. Once again, no attempt was made in this study to limit the amount of anterior dentoalveolar compensation, since the aim of the experiment was merely to demonstrate the ability or inability of magnetcontaining bite blocks to depress buccal teeih. In extrapolating the results of a study performed in baboons to the treatment of human subjects, one should be aware that there are obvious differences between the configuration of the craniofacial region in human beings and that in baboons. While a human patient with open bite often demonstrates a predominantly vertical growth pattern, the baboon demonstrates a more anterior direction of facial growth. Any effects seen in baboons may therefore have been influenced by the different orientation of the mandibular muscles. While that should be kept in mind, the aim of this study was to examine the possibility.that teeth could be depressed in nongrowing animals. Since it has been shown that the repelling magnets used in conjunction with an artificially increased vertical dimension can depress buccal teeth, it would be reasonable to suggest that consideration could be given to their use in the treatment of an orthodontic patient in whom a decrease in posterior dentoalveolar height is required. SUGGESTIONS FOR CLINICAL TRIAL
The results of this and the previous study show that it is possible to use occlusal bite blocks containing repelling magnets to push molar teeth back into their sockets. However, several problems related to human treatment remain. Such questions must be addressed before an unqualified recommendation could be given for the use of these repelling-magnet bite blocks in all patients with anterior open bites. 1. Stability. The fact that it is possible to depress teeth, no matter how this is achieved, does not necessarily mean that any resulting decrease in facial height would be stable in the long tenn. For that reason, it is necessary that a well-controlled prospective clinical trial be undertaken in human patients, with long-term follow-up of the stability of any vertical changes. 2. Patient cooperation. The two animal studies have in no way addressed the problem of patient cooperation when these appliances are used. Without adequate cooperation, the appliances simply may not be worn long enough for the desired results to be achieved. This, however, is no different from the problems related to other fixed and removable orthodontic appliances. For that reason, the clinical trial should also include an
Magnet intrusion
399
investigation of the differences between the effects of these magnet bite blocks when worn either as removable or as fixed appliances. 3. Age and nature of the deformity. The clinical trial would also include a quantitative investigation of the amount of posterior intrusion and the actual range of effects which might be expected in the correction of human open bites. Only then will it be possible for clinicians to make sound decisions regarding the use of these appliances. For instance, their use may be limited to patients with only dental open bites. On the other hand, it may be possible to achieve significant and stable correction in patients with mild skeletal open bites. It may even be possible to achieve stable correction of some open bites in adults. In any case, it would be reasonable to suggest at this time that there will always be patients requiring orthognathic procedures to correct major dental and skeletal vertical discrepancies. 4. Cause. Finally, the clinical trial should also include an investigation of the effects of these magnet appliances on open bites with different causes. For instance, the likely effects in a patient with a thumbsucking habit might be very different from those in a patient with a history of long-standing chronic airway disturbance. These might be different again from the effects in a patient with a familial long-face tendency. We thank Mr. Willy Clagg and Mrs. Judy Amico for their help in the preparation of this manuscript. We would also like to extend our appreciation to Dr. E. L. Dellinger and the staff of Active Vertical Correctors, Inc. for their help and advice during this project, and for providing all experimental appliances. REFERENCES
1. DellingerEL. A clinical assessment of the ActiveVertical Corrector: a nonsurglcal alternative for skeletal open-bite. AMJ ORmoo 1986;89:428-36. 2. Kalra V, Burstone CJ, Nanda R. Effects of a fixed magnetic appliance on the dentofacial complex. AMJ ORmooDENTOFAC ORTHOP1989;95:467-78. 3. Bell WH, Creekmore TD, Alexander RG. Surgical correction of the long-facedsyndrome. AMJ OR'mOO1977;71:40-67. 4. WoodsgiG, NandaRS. Intrusionof posteriorteeth withmagnets: an experiment in growing baboons. Angle Orthod 1988; April:136-50. 5. WoodsMG. The mechanics of lower incisor intrusion: experiments in nongrowing baboons. AM J OR]HODDENTOFACORTHOP 1988;93:186-95. 6. Tsutsui H, Konouchi Y, Sasaki H, Shiota M, Ushita T. Studies on the SmCo magnet as a dental material. J Dent Res 1977;58:1597-606. 7. Blechman AM, Smilcy M. Magnetic forces in orthodontics. AM J ORmOD 1977;72:1-22. 8. Blechman AM. Magnetic force systems in orthodontics. AM J OR~tOD 1985;87:201-10. 9. Dellinger EL. A histologic and cephalometric investigation of premolar intrusion in the Macaca speciosa monkey. AM J OR"rHOD 1967;53:325-55.
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10. Reitan K. Biomechanical principles and reactions. In: Graber TM, Swain BF, eds. Orthodontics, current principles and techniques. St. Louis: CV Mosby, 1985:101-92. 11. Gerling JA, Sinclair PM, Roa RL. The effect of pulsating electromagnetic fields on condylar growth in guinea pigs. AMJ ORTHOD1985;87:211-23. 12. Altuna G, Woodside DG. Response of the midface to treatment with increased vertical occlusal forces: treatment and posttreatment effects in monkeys. Angle Orthod 1985;55:251-63. 13. Breitner C. Alteration of occlusal relations induced by experimental procedure. AMJ ORTHOD1943;29:277o89.
Am.
J. Orthod. Dentofac. Orthop. 1Vovember1991
14. McNamara JA. An experimental study of increased.vertical dimension in the growing face. A.,,IJ OR~OD 1977;71:382-95. 15. Sergl HG, Farmand M. Experiments xbithunilateral bile plates in rabbits. Angle Orthod 1975;45:108-14. 16..Thompson JR, Brodie AG. Factors affecting the position of the mandible. J Am Dent Assoc 1942;29:925-41. Reprint requests to: Dr. Michael Woods 599 Dandenong Rd. Malvem, Victoria, 3143 Ausu'alia
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