Magnetic force in orthodontics Abraham M. Blechman, B.S., D.D.S., MS.,* and Harry Smiiey, B.A., D.D.S.** New
York.
N.
Y.
E
lastics and springs have been the main source of force in orthodontic therapy. The present study shows that small permanent magnetic devices offer a new, innovative, and biologically safe approach in generating intermaxillary, intramaxillary and extraoral force. Present methods of treatment, especially with elastics, may be satisfactory, but problems do exist. For example, lack of cooperation on the part of the patient can result in either failure or compromise of treatment objectives. Polygraph studies at Loyola University emphasized this lack of cooperation by showing that adults as well as children were often not reliable in using elastics is $ in fact, those patients were carefully selected prior to treatment for good motivation and were usually thought by their orthodontists to be cooperative. Magnets can eliminate this problem of the patient’s cooperation, since they are totally operator-controlled. Continuous application of force results in decreasing treatment time. Lengthy treatment can produce periodontal disturbances, root resorption, and caries. A shorter treatment time may make orthodontic therapy available to a wider spectrum of the population. Permanent magnets can also provide better directional force control. For example, with mandibular opening, when magnets are used for intermaxillary force, the air gap increases, and the vertical component of the force vector decreases rapidly, first as the square and then as the cube of the distance. This is an advantage over the use of elastics, with which the vertical force vector increases upon mandibular opening, thus producing an undesirable increase in the cant of the occlusal plane. Magnets can also be used for continuous high-deflection, low-rate force with precise control. Proper momentto-force ratios can be achieved, and tipping, root translation, and root torquing are also Possible. It is a total force system with a favorable benefit-to-risk ratio. Intermaxillary elastics also account for the loosening of bands because of the large vertical vector of force. On the other hand, magnetic devices generate a more horizontal force vector, which does not tend to unseat bands. Obviously, loose bands require much time for re-cementation, and also increase the patient’s susceptibility to caries. Talc and starch may have been used as dusting powders on orthodontic elastics, and, as reports in the medical literature have indicated for some time, these agents, in a medical situation, may be related to granulomatosis, pulmonary talcosis, immunogenic reactions, md, possibly, carcinoma. 3~8 Some of these reactions demonstrate a clinical latency so that they may not appear for many years after the patient’s initial exposure. In addition, *Division St., New **Yonkers,
of Orthodontics, School York, N. Y. 10032. N. Y.
ooO2-9416/78/100435+09$00.90/0
0
of Dental
and Qral
1978 The C. V Mosby Co.
Surgery,
Columbia
University,
630 West
168
435
Am. J. Orrhod. October 197X
nothing is known of the possible long-term chemical leaching out of products from elastics in the oral environment. We live in an environment of magnetic fields, both natural and artificial. Evolution and normal biologic processes may very well be magnetic-field dependent (0.3 to 0.6 gauss at sea level).” Since electric and magnetic fields are interdependent, physiologically generated magnetic fields exist constantly, and we are also exposed to artificial magnetic fields of significant intensity at home, in our automobiles, in the wrist watches we wear, and when walking under high-power lines. Therefore, we thought it wise to consult with authorities on the biomagnetic effects to be expected from the devices that were to be used in this study. They evaluated our designs and concluded that no adverse bioeffects were to be expected. Physics
of magnets
In biomagnetic experiments, three parameters need to be considered: the gradient, the field intensity, and the direction of the field vector to which the biologic sample is exposed. 1. Gradient:
$ . The gradient usually characterizes the inhomogeneity of the field. ( > The physical difference between an inhomogeneous field and a homogeneous field is that an inhomogeneous field exerts an accelerating force upon particles which are more para-or more dia-magnetic than their surroundings, and this accelerating force apparently accounts for all observed biomagnetic effects. Magnetic fields used in an intermaxillary mode vary from inhomogeneous when the mandible is closed or in rest position, to less inhomogeneous when the mandible is moving. Under inhomogeneous conditions, only saliva or food will occupy the space between the poles where the peak field intensity and largest gradient exist. Saliva and food have a magnetic permeability, approximately equal to one, so that para- or dia-magnetic effects are negligible. Since B -
F,
the adjacent
exposed tissue perceives so little of the field that para- and dia-magnetic effects are nonexistent for such weak fields. Therefore, the inhomogeneity of the field and the resultant gradient should not pose a biologic hazard. 2. Field intensity: (H). To compute the field B produced by a permanent magnet in the surrounding environment, the relationship used is: B = pH, where H = field intensity, or magnetomotive force (oersteds), and B = field flux density (gauss) and or. = 1, for air and most other biologic material. Therefore, B = H for most of this program. Maximum operating conditions for AlNiCo 5 and samarium cobalt magnets can be obtained from the operating point in the second quadrant of the hysteresis curve. This operating point can be determined by differentiating the curve and taking the first derivative. The operating point also locates the maximum energy product (BH),,, as shown in Fig. 1. For the most efficient use of magnetic material (i.e., the smallest size magnet for a given output), a magnet should be designed to operate at the point of maximum energy. The external energy that each cubic centimeter of magnetic material can supply is equal to E.
The higher the (BH), the stronger the magnet, and ultimately the more force is
available. 3. Direction ofJeld vector. The direction of electron spin in the third shell of the atom nrnrl~u-~s
R mnon~tir
moment
which
nrrnr~ntc
fnr
the
msyetic
field
A limited
volume
Volume 14 Number 4
Magnetic MYAGNETIZATION CURVE
\
1400
1200
1000
DEMAGNETIZING
600 FORCE
600
400
I
I
,force
in orthodontics
EXTERNAL ENERGY CURVE
200 EXTERNAL
H (OERGTEDS)
ENERGY
(GWSS-OERSTEO
Fig.
1. The intersection of the operating maximum flux and energy are produced Permanent Magnets Manual.)
437
G*H,,X
IO’
1
load lines and demagnetization curves shows where the for AlNiCo 5 and AlNiCo 8. (Hitachi Magnetics Corporation
(about a million-billion atoms or 10m8c.c.) of these tiny electromagnets form domains of internal field energy, which can be reoriented in direction to establish external field energy. (Once magnetized, a permanent magnet requires no further energy to maintain the field.) The direction of the field vector is determined by the mode used (i.e., intramaxillary or intermaxillary). In this article, we are concerned only with an intramaxillary application; therefore, the vector is parallel to the dental arch. For intermaxillary force, the vector is different, and will be the subject of another article. The force generated in this experiment with two AlNiCo 5 magnets with a magnetic induction of 1,200 gauss was measured to be approximately 30 grams with a 3 to 4 mm. air gap. If samarium cobalt magnets were used, the peak induction would be 3,500 gauss with the same air gap, and the force generated would be approximately 100 grams. (A Hall-Effect gaussmeter was used to determine gauss.) Samarium cobalt obviously has a much larger coercive force (-H), a higher-energy product, and better recoil permeability properties than AlNiCo 5. lioeffects
with magnetic devices: Whole-body
exposure
Authorities in the field (personal communications) have overwhelmingly supported our approach as being biologically safe by extrapolation. For example, Dr. Peter Neurath of Tufts University School of Medicine exposed animals to fields with a gradient product (z
. B) , equal to 10” (e
j2, and observed no toxic effect.‘” The gradient
product generated in this program was 500 gauss per millimeter. This is at least 1,000 times smaller than Neurath’s product. (Practically, gauss and oersteds are used interchangeably, since B = H).
Am. J. Orthod. October 197X
Nahas, of Columbia University College of Physicians and Surgeons, has reported no toxic or histopathologic effects on rats exposed for a period of one month to permanent magnetic fields that ranged from 200 to 1,200 gauss. ” This experiment was repeated six times with the same negative result. Biochemical analysis and histopathologic examinations of the internal viscera were employed in this study. These rats were subjected to low-frequency fields due to random wandering in the vertical field (unpublished data). Bioeffects
with
magnetic
devices:
Localized
body
exposure
Magnets in dentistry have been limited entirely to prostheses (magnets in dentures or implanted in mandibular alveolar bone). One recent report deals with magnets used to retain an obturator. Magnets have been used in various localized medical situations. Bassett, at Columbia University College of Physicians and Surgeons, has successfully used electromagnetic fields clinically to enhance the calcification process in bone fractures’* and has not reported any deleterious effects. Hilal and Driller described clinical systems which involved the permanent implantation of magnetic materials in the vascular system. One system provided for delivery and permanent implantation of a magnetic embolus to occlude a specific cerebral artery for treatment of arteriovenous malformations in a young child. A small (0.7 by 2.5 mm.) cylinder of PtCo was the embolus. This was used clinically with no reported adverse effect.i3* I4 In addition, there is increasing usage of permanently implanted magnetic materials in other biologic systems. One German investigator recently used permanently implanted magnets to mobilize eyelids affected by facial nerve paralysis.‘” Others have used magnets for permanent retention of an artificial eye in the socket. No adverse effect has been reported in regard to the use of these devices. Induced
electric
field
effects
A conductor moving in a magnetic field will induce an electromotive force in an orthogonal direction (Faraday effect). It has been suggested that biologic tissue may react similarly if moved in a sufficiently strong magnetic field. This situation could apply to the blood vessels in the mouth near the orthodontic magnetic devices. In blood vessels, the induced electromotive force is proportional to the cross-sectional diameter of the vessel, velocity of vascular Bow (the conductor), and the field flux density (B). According to Jochim,i6 a peak signal of 3 mV. can be expected with peak blood flow velocities of 300 cm./sec., 1,000 gauss field, and vessels 1 cm. in diameter. If we assume a flow of 1 cm./sec. in a vessel I mm. in diameter in the presence of a 2,000 gauss peak field (values consistent with the placement of a small magnet near small vessels in the oral cavity), the following voltage is expected: 3 mV X 1 cm./sec. Loo0 g. 1 mm. X ~ l,ooo
300 cm./sec.
X
g.
~ 10 mm.
= 2
X
1Om.3mV.
Assuming a uniform magnetic field which induces a linear gradient within the vessel, the gradient across the individual red blood cells will be approximately: 2
X
10m3 mV.
X
llOzl’,“y
=
2 X 10e5 mV.“7’
Normal resting potential across cell membranes is 60 to 100 mV. Therefore, the expected induced electromotive forces are several orders of magnitude lower than the
Volumr Number
74 4
Magnetic ,force in orthodontics
Fig.
2. AlNiCo
5 magnets
bonded
to cat’s
439
dentition.
resting potential, and it is extremely unlikely that there could be any appreciable physiologic effect because of the field produced by the magnetic devices. The conclusion drawn from the above formulations indicates that the magnetic fields which we expect to generate in this program will be safe and will produce no adverse tissue reaction. The system proposed for orthodontic therapy will consist of two magnets which exert attractive forces on each other. However, the field produced by each of these open-circuit magnets will not be homogeneous. Simple design aids have been developed. These graphs can be used to predict the expected axial field for cylindrical permanent magnets of varying length/diameter (L/D) dimensions. However, these data apply only to a single magnet. Therefore, extrapolation with experimental verification is required in order to determine the field developed between two magnets. Computation of the expected field of each individual magnet, followed by superposition of spatial components, will not be an accurate indicator of the resultant field. Simple computational methods are not readily available for accurate determination of the developed force in the arrangement proposed in this program. For this reason, we have measured these forces under simulated conditions to determine their order of magnitude. By way of approximation, it can be stated that the generated force is proportional to B2 the gradient. The force generated in this experiment with two attracting AlNiCo 5 magnets with a magnetic induction of 1,200 gauss was measured to be approximately 30 $nms with a 3 to 4 mm. air gap. If samarium cobalt magnets were used, the peak uction would be 3,500 gauss with the same air gap, and the force generated would be +tproximately 100 grams. (A Hall-Effect gauss meter was used to determine gauss.) Samarium cobalt obviously has a much larger coercive force (-H,), a higher energy product, and better recoil permeability properties than those of AlNiCo 5. In this pilot experiment, we decided to use AlNiCo 5 instead of samarium or platinum cobalt magnets, for several reasons. The anatomy of the cat’s dentition necessitated a I-g, slender, cylindrical magnet, and the length-to-diameter (L/D) ratio for AlNiCo 5 of 4: I approached this constraint. The L/D ratio for samarium cobalt is approximately 1 : 1,
Am. J. Orthod. October 1978
Fig. 3. AlNiCo
5 magnets
mounted
in cat’s
maxilla
thereby making it less suitable for this situation. Platinum cobalt was simply too expensive. We are aware of the many other advantages of the latter two magnetic materials, and have used them. Furthermore, for testing purposes, the large flux leakage or fringing of AlNiCo 5 was used in order to intentionally expose the adjacent tissues to a larger field than would be possible with samarium cobalt. Obviously, samarium cobalt is the magnet of choice in the human application. Method
In a 6% pound, 8-month-old female cat, an AlNiCo 5 cylindrical magnet (13 mm. long and 1 mm. in diameter, magnetized axially) was attached to the upper right molars by means of Whaledent pins, and another similar magnet (9 mm. long and 1 mm. in diameter) was attached to the canine. All magnets were totally coated with fast-curing acrylic in order to eliminate corrosion products. Although the optimum L/D ratio of AlNiCo 5 is 4: 1, the anatomy of the cat’s dentition necessitated the above-mentioned dimensions. On the upper left side, Whaledent pins were inserted into the molar, and one pin was inserted into the canine. These pins were used for attachment of a %-inch light latex elastic. In the cat a natural diastema exists, distal to the canine, so that distal movement of the canine can be observed. (See Figs. 2 and 3.) Another 6 pound, 8-month-old female cat was prepared in exactly the same way as the previously mentioned animal with AlNiCo 5 magnets on the upper right side. However, on the upper left side of the second cat, sham magnets (identical to the magnets in size and weight, but non-magnetic) were installed. Therefore, both animals served as their own controls. The animals were weighed once a month for 9 months, and showed a gradually increasing growth curve up to 8 pounds, which was within normal limits. They were given physical examinations periodically by the attending veterinarian, and appeared to be normal in every way clinically. Oral examinations revealed no abnormal findings, except a slight local inflammation due to the physical presence of fast-curing acrylic which was used to coat the magnets.
Volume 74 Number 4
Magnetic force in orthodontics
Fig. 4. Lateral view radiograph demonstrating
5. I
.al view
6f&&&trating
441
closure of air gap due to distal movemc znt (3f canine.
open air gap due to lack ofmovement
of s;hann magnetic
After 9 months, all canines showed approximately equal distal movemen It, e:xcept for the canines with the sham magnets. Intraoral and lateral plate x-ray films of the head revealed no pathologic alterations, and substantiated the distal movement of the: canines. (See Figs. 4, 5, and 6.) After the animals had been killed, samples were taken from all oral tissue:s aczljacent to
Am. .I Orthod October 1978
Fig.
6. Occlusal
view
radiograph
comparing
active
and
sham
magnets.
the magnetic field, as well as from all internal viscera. All findings were normal. A veterinary pathologist examined the tissue samples and verified the biologic safety of the magnetic fields. l* The pathologist’s findings were corroborated by a research periodontist, especially the oral sections. Discussion To our knowledge, permanent magnets have apparently never been used to produce orthodontic forces. This work has demonstrated the biologic safety of magnets as used here and has made available to the orthodontist a new “space-age” modality that is totally operator-controlled, more precise and more efficient than any method now in use. Operator-controlled movement is less traumatic to adjacent tissues because of the relatively continuous and consistent application of force, resulting in a shorter duration of treatment. (As the air gap decreases, a novel situation is produced, because greater force is generated by the approaching magnets.) Lengthy treatment can result in periodontal disturbances, root resorption, and caries. Shorter periods of treatment may also make orthodontic therapy available to a wider spectrum of the population. REFERENCES I. Campisi, R. S.: A study of truthfulness in male orthodontic patients from the appraisal of certain autonomic responses to questions concerning cooperation. Unpublished M.S. thesis, Loyola University Dental School, Chicago, 1963. 2. Cavanaugh, T. P., Jr.: A study of truthfulness in female orthodontic patients. Unpublished M.S. thesis, Loyola University Dental School, Chicago, 1963. 3. Saxen, L.: Foreign material and post-operative adhesions, N. Engl. J. Med. 279: 200, 1968. 4. Sternheb, J. J., et al.: Starch peritonitis and its prevention, Arch. Surg. 112: 458-461, 77. 5. Grant, B. F., et al.: Allergic starch peritonitis in the guinea pig, Br. J. Surg. 63: 864-866, 76. 6. Grant, J. B.. et al.: The immunogenicity of starch glove powder and talc, Br. J. Surg. 63: 864-866, 76. 7. Cruthirds, T. P., Cole, F. H., and Paul, R. N.: Pulmonary talcosis as a result of massive aspiration of baby powder, South. Med. J. 70: 626-628, 1977. 8. Pfenninzer, J., et al.: Powder aspiration in children, Report of two cases, Arch. Dis. Child. 52: 157-159, 1977.
Volume 74 Number 4
Mugnatic
force
in orthodontics
443
9. Aceta, Tobias, and Silver: Some studies on the biologic effects of magnetic fields, I.E.E.E. Transactions on Magnetics, Vol. MAG-6, No. 2, 36X-373, June, 1970. IO. Neurath, P.: Personal communication, June, 1974. 11. Nahas, G.: Personal communication, June, 1977. 12. Bassett, C. A. L., Pilla, A. A.. Pawluck. R. J.: A non-operative salvage of surgically-resistant pseudoarthroses and non-unions by pulsing electro-magnetic fields, Clin. Grthop. 124: 128 142, 1977. 13. Hilal, S., Michelsen, W., Driller, J., and Leonard, E. : Magnetically guided devices for vascular exploration and treatment, Radiology 113: 529-540, 1974. 14. Driller, J.: Kinetics of magnetically guided catheters, I.E.E.E. Transactions on Magnetics, Vol. MAG-6, No. 3, 467-471, Sept. 1970. 15. Momma, W. G., et al.: A simple method of restoring lid function in facial nerve paralysis with permanent magnets, Klin. Monatsbl. Augenheilkd. 169: 529-533, 1976. 16. Jochim, K.: Transactions on Biomedical Electronics, I.E.E.E., Vol. BME-9, No, 4, 228-235, July 1962. 17. Driller, J.: Personal communication, May, 1977. 18. Davis, T.: Personal communication, July, 1976.
York.
N.
Y.
E
lastics and springs have been the main source of force in orthodontic therapy. The present study shows that small permanent magnetic devices offer a new, innovative, and biologically safe approach in generating intermaxillary, intramaxillary and extraoral force. Present methods of treatment, especially with elastics, may be satisfactory, but problems do exist. For example, lack of cooperation on the part of the patient can result in either failure or compromise of treatment objectives. Polygraph studies at Loyola University emphasized this lack of cooperation by showing that adults as well as children were often not reliable in using elastics is $ in fact, those patients were carefully selected prior to treatment for good motivation and were usually thought by their orthodontists to be cooperative. Magnets can eliminate this problem of the patient’s cooperation, since they are totally operator-controlled. Continuous application of force results in decreasing treatment time. Lengthy treatment can produce periodontal disturbances, root resorption, and caries. A shorter treatment time may make orthodontic therapy available to a wider spectrum of the population. Permanent magnets can also provide better directional force control. For example, with mandibular opening, when magnets are used for intermaxillary force, the air gap increases, and the vertical component of the force vector decreases rapidly, first as the square and then as the cube of the distance. This is an advantage over the use of elastics, with which the vertical force vector increases upon mandibular opening, thus producing an undesirable increase in the cant of the occlusal plane. Magnets can also be used for continuous high-deflection, low-rate force with precise control. Proper momentto-force ratios can be achieved, and tipping, root translation, and root torquing are also Possible. It is a total force system with a favorable benefit-to-risk ratio. Intermaxillary elastics also account for the loosening of bands because of the large vertical vector of force. On the other hand, magnetic devices generate a more horizontal force vector, which does not tend to unseat bands. Obviously, loose bands require much time for re-cementation, and also increase the patient’s susceptibility to caries. Talc and starch may have been used as dusting powders on orthodontic elastics, and, as reports in the medical literature have indicated for some time, these agents, in a medical situation, may be related to granulomatosis, pulmonary talcosis, immunogenic reactions, md, possibly, carcinoma. 3~8 Some of these reactions demonstrate a clinical latency so that they may not appear for many years after the patient’s initial exposure. In addition, *Division St., New **Yonkers,
of Orthodontics, School York, N. Y. 10032. N. Y.
ooO2-9416/78/100435+09$00.90/0
0
of Dental
and Qral
1978 The C. V Mosby Co.
Surgery,
Columbia
University,
630 West
168
435
Am. J. Orrhod. October 197X
nothing is known of the possible long-term chemical leaching out of products from elastics in the oral environment. We live in an environment of magnetic fields, both natural and artificial. Evolution and normal biologic processes may very well be magnetic-field dependent (0.3 to 0.6 gauss at sea level).” Since electric and magnetic fields are interdependent, physiologically generated magnetic fields exist constantly, and we are also exposed to artificial magnetic fields of significant intensity at home, in our automobiles, in the wrist watches we wear, and when walking under high-power lines. Therefore, we thought it wise to consult with authorities on the biomagnetic effects to be expected from the devices that were to be used in this study. They evaluated our designs and concluded that no adverse bioeffects were to be expected. Physics
of magnets
In biomagnetic experiments, three parameters need to be considered: the gradient, the field intensity, and the direction of the field vector to which the biologic sample is exposed. 1. Gradient:
$ . The gradient usually characterizes the inhomogeneity of the field. ( > The physical difference between an inhomogeneous field and a homogeneous field is that an inhomogeneous field exerts an accelerating force upon particles which are more para-or more dia-magnetic than their surroundings, and this accelerating force apparently accounts for all observed biomagnetic effects. Magnetic fields used in an intermaxillary mode vary from inhomogeneous when the mandible is closed or in rest position, to less inhomogeneous when the mandible is moving. Under inhomogeneous conditions, only saliva or food will occupy the space between the poles where the peak field intensity and largest gradient exist. Saliva and food have a magnetic permeability, approximately equal to one, so that para- or dia-magnetic effects are negligible. Since B -
F,
the adjacent
exposed tissue perceives so little of the field that para- and dia-magnetic effects are nonexistent for such weak fields. Therefore, the inhomogeneity of the field and the resultant gradient should not pose a biologic hazard. 2. Field intensity: (H). To compute the field B produced by a permanent magnet in the surrounding environment, the relationship used is: B = pH, where H = field intensity, or magnetomotive force (oersteds), and B = field flux density (gauss) and or. = 1, for air and most other biologic material. Therefore, B = H for most of this program. Maximum operating conditions for AlNiCo 5 and samarium cobalt magnets can be obtained from the operating point in the second quadrant of the hysteresis curve. This operating point can be determined by differentiating the curve and taking the first derivative. The operating point also locates the maximum energy product (BH),,, as shown in Fig. 1. For the most efficient use of magnetic material (i.e., the smallest size magnet for a given output), a magnet should be designed to operate at the point of maximum energy. The external energy that each cubic centimeter of magnetic material can supply is equal to E.
The higher the (BH), the stronger the magnet, and ultimately the more force is
available. 3. Direction ofJeld vector. The direction of electron spin in the third shell of the atom nrnrl~u-~s
R mnon~tir
moment
which
nrrnr~ntc
fnr
the
msyetic
field
A limited
volume
Volume 14 Number 4
Magnetic MYAGNETIZATION CURVE
\
1400
1200
1000
DEMAGNETIZING
600 FORCE
600
400
I
I
,force
in orthodontics
EXTERNAL ENERGY CURVE
200 EXTERNAL
H (OERGTEDS)
ENERGY
(GWSS-OERSTEO
Fig.
1. The intersection of the operating maximum flux and energy are produced Permanent Magnets Manual.)
437
G*H,,X
IO’
1
load lines and demagnetization curves shows where the for AlNiCo 5 and AlNiCo 8. (Hitachi Magnetics Corporation
(about a million-billion atoms or 10m8c.c.) of these tiny electromagnets form domains of internal field energy, which can be reoriented in direction to establish external field energy. (Once magnetized, a permanent magnet requires no further energy to maintain the field.) The direction of the field vector is determined by the mode used (i.e., intramaxillary or intermaxillary). In this article, we are concerned only with an intramaxillary application; therefore, the vector is parallel to the dental arch. For intermaxillary force, the vector is different, and will be the subject of another article. The force generated in this experiment with two AlNiCo 5 magnets with a magnetic induction of 1,200 gauss was measured to be approximately 30 grams with a 3 to 4 mm. air gap. If samarium cobalt magnets were used, the peak induction would be 3,500 gauss with the same air gap, and the force generated would be approximately 100 grams. (A Hall-Effect gaussmeter was used to determine gauss.) Samarium cobalt obviously has a much larger coercive force (-H), a higher-energy product, and better recoil permeability properties than AlNiCo 5. lioeffects
with magnetic devices: Whole-body
exposure
Authorities in the field (personal communications) have overwhelmingly supported our approach as being biologically safe by extrapolation. For example, Dr. Peter Neurath of Tufts University School of Medicine exposed animals to fields with a gradient product (z
. B) , equal to 10” (e
j2, and observed no toxic effect.‘” The gradient
product generated in this program was 500 gauss per millimeter. This is at least 1,000 times smaller than Neurath’s product. (Practically, gauss and oersteds are used interchangeably, since B = H).
Am. J. Orthod. October 197X
Nahas, of Columbia University College of Physicians and Surgeons, has reported no toxic or histopathologic effects on rats exposed for a period of one month to permanent magnetic fields that ranged from 200 to 1,200 gauss. ” This experiment was repeated six times with the same negative result. Biochemical analysis and histopathologic examinations of the internal viscera were employed in this study. These rats were subjected to low-frequency fields due to random wandering in the vertical field (unpublished data). Bioeffects
with
magnetic
devices:
Localized
body
exposure
Magnets in dentistry have been limited entirely to prostheses (magnets in dentures or implanted in mandibular alveolar bone). One recent report deals with magnets used to retain an obturator. Magnets have been used in various localized medical situations. Bassett, at Columbia University College of Physicians and Surgeons, has successfully used electromagnetic fields clinically to enhance the calcification process in bone fractures’* and has not reported any deleterious effects. Hilal and Driller described clinical systems which involved the permanent implantation of magnetic materials in the vascular system. One system provided for delivery and permanent implantation of a magnetic embolus to occlude a specific cerebral artery for treatment of arteriovenous malformations in a young child. A small (0.7 by 2.5 mm.) cylinder of PtCo was the embolus. This was used clinically with no reported adverse effect.i3* I4 In addition, there is increasing usage of permanently implanted magnetic materials in other biologic systems. One German investigator recently used permanently implanted magnets to mobilize eyelids affected by facial nerve paralysis.‘” Others have used magnets for permanent retention of an artificial eye in the socket. No adverse effect has been reported in regard to the use of these devices. Induced
electric
field
effects
A conductor moving in a magnetic field will induce an electromotive force in an orthogonal direction (Faraday effect). It has been suggested that biologic tissue may react similarly if moved in a sufficiently strong magnetic field. This situation could apply to the blood vessels in the mouth near the orthodontic magnetic devices. In blood vessels, the induced electromotive force is proportional to the cross-sectional diameter of the vessel, velocity of vascular Bow (the conductor), and the field flux density (B). According to Jochim,i6 a peak signal of 3 mV. can be expected with peak blood flow velocities of 300 cm./sec., 1,000 gauss field, and vessels 1 cm. in diameter. If we assume a flow of 1 cm./sec. in a vessel I mm. in diameter in the presence of a 2,000 gauss peak field (values consistent with the placement of a small magnet near small vessels in the oral cavity), the following voltage is expected: 3 mV X 1 cm./sec. Loo0 g. 1 mm. X ~ l,ooo
300 cm./sec.
X
g.
~ 10 mm.
= 2
X
1Om.3mV.
Assuming a uniform magnetic field which induces a linear gradient within the vessel, the gradient across the individual red blood cells will be approximately: 2
X
10m3 mV.
X
llOzl’,“y
=
2 X 10e5 mV.“7’
Normal resting potential across cell membranes is 60 to 100 mV. Therefore, the expected induced electromotive forces are several orders of magnitude lower than the
Volumr Number
74 4
Magnetic ,force in orthodontics
Fig.
2. AlNiCo
5 magnets
bonded
to cat’s
439
dentition.
resting potential, and it is extremely unlikely that there could be any appreciable physiologic effect because of the field produced by the magnetic devices. The conclusion drawn from the above formulations indicates that the magnetic fields which we expect to generate in this program will be safe and will produce no adverse tissue reaction. The system proposed for orthodontic therapy will consist of two magnets which exert attractive forces on each other. However, the field produced by each of these open-circuit magnets will not be homogeneous. Simple design aids have been developed. These graphs can be used to predict the expected axial field for cylindrical permanent magnets of varying length/diameter (L/D) dimensions. However, these data apply only to a single magnet. Therefore, extrapolation with experimental verification is required in order to determine the field developed between two magnets. Computation of the expected field of each individual magnet, followed by superposition of spatial components, will not be an accurate indicator of the resultant field. Simple computational methods are not readily available for accurate determination of the developed force in the arrangement proposed in this program. For this reason, we have measured these forces under simulated conditions to determine their order of magnitude. By way of approximation, it can be stated that the generated force is proportional to B2 the gradient. The force generated in this experiment with two attracting AlNiCo 5 magnets with a magnetic induction of 1,200 gauss was measured to be approximately 30 $nms with a 3 to 4 mm. air gap. If samarium cobalt magnets were used, the peak uction would be 3,500 gauss with the same air gap, and the force generated would be +tproximately 100 grams. (A Hall-Effect gauss meter was used to determine gauss.) Samarium cobalt obviously has a much larger coercive force (-H,), a higher energy product, and better recoil permeability properties than those of AlNiCo 5. In this pilot experiment, we decided to use AlNiCo 5 instead of samarium or platinum cobalt magnets, for several reasons. The anatomy of the cat’s dentition necessitated a I-g, slender, cylindrical magnet, and the length-to-diameter (L/D) ratio for AlNiCo 5 of 4: I approached this constraint. The L/D ratio for samarium cobalt is approximately 1 : 1,
Am. J. Orthod. October 1978
Fig. 3. AlNiCo
5 magnets
mounted
in cat’s
maxilla
thereby making it less suitable for this situation. Platinum cobalt was simply too expensive. We are aware of the many other advantages of the latter two magnetic materials, and have used them. Furthermore, for testing purposes, the large flux leakage or fringing of AlNiCo 5 was used in order to intentionally expose the adjacent tissues to a larger field than would be possible with samarium cobalt. Obviously, samarium cobalt is the magnet of choice in the human application. Method
In a 6% pound, 8-month-old female cat, an AlNiCo 5 cylindrical magnet (13 mm. long and 1 mm. in diameter, magnetized axially) was attached to the upper right molars by means of Whaledent pins, and another similar magnet (9 mm. long and 1 mm. in diameter) was attached to the canine. All magnets were totally coated with fast-curing acrylic in order to eliminate corrosion products. Although the optimum L/D ratio of AlNiCo 5 is 4: 1, the anatomy of the cat’s dentition necessitated the above-mentioned dimensions. On the upper left side, Whaledent pins were inserted into the molar, and one pin was inserted into the canine. These pins were used for attachment of a %-inch light latex elastic. In the cat a natural diastema exists, distal to the canine, so that distal movement of the canine can be observed. (See Figs. 2 and 3.) Another 6 pound, 8-month-old female cat was prepared in exactly the same way as the previously mentioned animal with AlNiCo 5 magnets on the upper right side. However, on the upper left side of the second cat, sham magnets (identical to the magnets in size and weight, but non-magnetic) were installed. Therefore, both animals served as their own controls. The animals were weighed once a month for 9 months, and showed a gradually increasing growth curve up to 8 pounds, which was within normal limits. They were given physical examinations periodically by the attending veterinarian, and appeared to be normal in every way clinically. Oral examinations revealed no abnormal findings, except a slight local inflammation due to the physical presence of fast-curing acrylic which was used to coat the magnets.
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Fig. 4. Lateral view radiograph demonstrating
5. I
.al view
6f&&&trating
441
closure of air gap due to distal movemc znt (3f canine.
open air gap due to lack ofmovement
of s;hann magnetic
After 9 months, all canines showed approximately equal distal movemen It, e:xcept for the canines with the sham magnets. Intraoral and lateral plate x-ray films of the head revealed no pathologic alterations, and substantiated the distal movement of the: canines. (See Figs. 4, 5, and 6.) After the animals had been killed, samples were taken from all oral tissue:s aczljacent to
Am. .I Orthod October 1978
Fig.
6. Occlusal
view
radiograph
comparing
active
and
sham
magnets.
the magnetic field, as well as from all internal viscera. All findings were normal. A veterinary pathologist examined the tissue samples and verified the biologic safety of the magnetic fields. l* The pathologist’s findings were corroborated by a research periodontist, especially the oral sections. Discussion To our knowledge, permanent magnets have apparently never been used to produce orthodontic forces. This work has demonstrated the biologic safety of magnets as used here and has made available to the orthodontist a new “space-age” modality that is totally operator-controlled, more precise and more efficient than any method now in use. Operator-controlled movement is less traumatic to adjacent tissues because of the relatively continuous and consistent application of force, resulting in a shorter duration of treatment. (As the air gap decreases, a novel situation is produced, because greater force is generated by the approaching magnets.) Lengthy treatment can result in periodontal disturbances, root resorption, and caries. Shorter periods of treatment may also make orthodontic therapy available to a wider spectrum of the population. REFERENCES I. Campisi, R. S.: A study of truthfulness in male orthodontic patients from the appraisal of certain autonomic responses to questions concerning cooperation. Unpublished M.S. thesis, Loyola University Dental School, Chicago, 1963. 2. Cavanaugh, T. P., Jr.: A study of truthfulness in female orthodontic patients. Unpublished M.S. thesis, Loyola University Dental School, Chicago, 1963. 3. Saxen, L.: Foreign material and post-operative adhesions, N. Engl. J. Med. 279: 200, 1968. 4. Sternheb, J. J., et al.: Starch peritonitis and its prevention, Arch. Surg. 112: 458-461, 77. 5. Grant, B. F., et al.: Allergic starch peritonitis in the guinea pig, Br. J. Surg. 63: 864-866, 76. 6. Grant, J. B.. et al.: The immunogenicity of starch glove powder and talc, Br. J. Surg. 63: 864-866, 76. 7. Cruthirds, T. P., Cole, F. H., and Paul, R. N.: Pulmonary talcosis as a result of massive aspiration of baby powder, South. Med. J. 70: 626-628, 1977. 8. Pfenninzer, J., et al.: Powder aspiration in children, Report of two cases, Arch. Dis. Child. 52: 157-159, 1977.
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9. Aceta, Tobias, and Silver: Some studies on the biologic effects of magnetic fields, I.E.E.E. Transactions on Magnetics, Vol. MAG-6, No. 2, 36X-373, June, 1970. IO. Neurath, P.: Personal communication, June, 1974. 11. Nahas, G.: Personal communication, June, 1977. 12. Bassett, C. A. L., Pilla, A. A.. Pawluck. R. J.: A non-operative salvage of surgically-resistant pseudoarthroses and non-unions by pulsing electro-magnetic fields, Clin. Grthop. 124: 128 142, 1977. 13. Hilal, S., Michelsen, W., Driller, J., and Leonard, E. : Magnetically guided devices for vascular exploration and treatment, Radiology 113: 529-540, 1974. 14. Driller, J.: Kinetics of magnetically guided catheters, I.E.E.E. Transactions on Magnetics, Vol. MAG-6, No. 3, 467-471, Sept. 1970. 15. Momma, W. G., et al.: A simple method of restoring lid function in facial nerve paralysis with permanent magnets, Klin. Monatsbl. Augenheilkd. 169: 529-533, 1976. 16. Jochim, K.: Transactions on Biomedical Electronics, I.E.E.E., Vol. BME-9, No, 4, 228-235, July 1962. 17. Driller, J.: Personal communication, May, 1977. 18. Davis, T.: Personal communication, July, 1976.
