Pi i 0889540689904101

Effects of a$xed magnetic appliance on the dentofa/ciaZ complex

Varun Kalra, BDS, MDS, D. Orth., MS,* Charles J. Burstone, DDS, MS,** and Ravindra Nanda, BDS, MDS, PhD*** Farmington, Corm.

The purpose of the study was to design and evaluate the effects of a fixed magnetic appliance that hinged the mandible open and exerted an intrusive force on the teeth. Ten patients between the ages of 8 years and 10 years 6 months, with Class II, Division 1 malocclusion associated with mandibular retrusion and increased lower facial height, were treated with this appliance. The length of treatment was 4 months, after which the appliance was removed and the patients were followed up for 4 months. Ten children with similar age, sex, and dentofacial characteristics acted as controls and did not receive any appliance therapy. Changes in morphology of the dentofacial complex were evaluated by use of lateral cephalograms and study models. In addition temporomandibular joint and muscle functions were assessed. During treatment mandibular length increased 3.2 mm, angle of facial convexity decreased 2.8, the upper and lower teeth intruded an average of 1.5 mm each, and the mandibular plane angle decreased 1.3. In the follow-up period, some rebound eruption was noted; however, all other changes were stable. (AM J ORTHOD DENTOFAC ORTHOP 1989;95:467-78.)

I t has been estimated that two thirds of the patients treated by orthodontists in the United States have mandibular retrusion characteristics. Some of these patients also have an increased lower facial height and large interlabial gap. In such patients treatment with conventional orthodontic appliances may lead to Class I occlusion. However, this often does not provide a satisfactory result in terms of stability and facial es- thetics. At present the only definitive method of im- proving dentofacial harmony in these patients is by means of surgical superior repositioning of the maxilla. This procedure increases the interocclusal space and allows upward and forward autorotation of the mandible, thereby decreasing lower facial height and facial convexity. Often concomitant with superior repositioning of the maxilla, the mandible also is surgically ad- vanced to further reduce mandibular retrusion. Recently Dellinger reported on the use of a magnetic appliance to treat patients with skeletal open bite. This appliance resulted in intrusion of posterior teeth and an upward and forward autorotation of the mandible.

There were two hypotheses for this study. (1) If all erupted teeth in the upper and lower arches could be

From the University 3f Connecticut School of Dental Medicine. Supported in part by NIH Grant DE-03953-12. *.ksistant Professor. Department of Orthodontics. **Professor and Chairman. Departmenr of Orthodontics. ***Professor. Department of Orthcdomics.

LINGUAL WIRE

Fig. 1. Schematic drawing of appliance.

intruded with an appliance, the mandible would auto- rotate upward and forward into the interocclusal space created. (2) If this appliance could displace the condyle downward and forward, away from the posterior part of the glenoid fossa, stimulation of condylar growth might occur. Both these effects, an increase in length of the mandible and an upward and forward autorotation of the mandible, would be beneficial in treating Class II malocclusions associated with increased lower facial height and a retrusive mandible. The objectives of this study therefore were to (1) design an appliance that hinges the mandible open and exerts an intrusive force

467

466 Kalra, But-stone, and Nanda Am. J. Orrhod. Denrofuc. Orthop. June 1989

Fig. 2. Occlusal view of appliance.

Table I. Force produced by the appliance

Distance between upper Repelling force and lower splints (mm) fgm)

0.5 3500 1.0 2640 1.5 2040 2.0 1630 2.5 1315 3.0 1080 3.5 890 4.0 730 4.5 600 5.0 480 6.0 330

appliances were removed and the patients observed for an additional 4 months. Subjects in the control group were studied for 8 months.

Appliance design

on the teeth and (2) evaluate the effects of this appliance on the morphology and function of the dentofacial complex.

MATERIALS AND METHODS

The subject pool in the study consisted of 20 boys and girls who were between the ages of 8 years and 10 years 6 months and who had Class II, Division 1 malocclusion associated with mandibular retrusion. The angle of facial convexity, lower facial height, and interlabial gap were greater than normal. The permanent first molars and incisors had erupted; the overjet ranged from 6 to 10 mm and the overbite varied from 0 to 3 mm. The 10 subjects in the treatment group and the 10 subjects in the control group were closely matched with regard to age, sex, and dentofacial characteristics.

The working bite was taken with the mandible in centric relation and opened 7 to 8 mm in the permanent first molar region. The appliance consisted of upper and lower acrylic splints that were bonded on the occlusal halves of the permanent first molars, deciduous molars or premolars, and canines (Figs. 1 through 3). Samar- ium cobalt magnets measuring 20 X 8 x 2 mm were encased in a stainless steel case 0.007-inch thick and embedded into the upper and lower acrylic splints in a repelling mode. In addition a 0.028-inch wire was embedded in the acrylic. This wire rested on the lingual surfaces of the four permanent incisors and was indi- vidually bonded to them; thereby intrusive forces were transmitted to the entire arch. The size and shape of the magnets were designed in collaboration with Re- coma Inc.* The steel cases were fabricated in cooperation with the Bioengineering Department, University of Connecticut Health Center. The forces produced by the appliance are listed in Table I.

Cephalometric analysis

Changes in craniofacial morphology caused by growth and treatment were determined from a set of lateral cephalometric head films. A set consisted of two cephalograms, one taken with the mandible in centric relation and the other with the mouth wide open to

The active treatment time was 4 months, after which *Recoma Inc., Fairfield, N.J.

Vo/ume 9s Numhrr 6

Fig. 3. Buccal view of appliance.

facilitate visualization of the condyles. For each patient in the treatment group, the first set of head films was taken before treatment, the second set of films after 4 months active treatment, and the third set 4 months posttreatment. In the control group, an initial set and a final set of cephalograms were taken 8 months apart. Changes between the two sets of films in the control group were divided by two to obtain changes throughout a 4-month period. Subsequent cephalograms were superimposed on the anterior cranial base of the previous cephalogram as described by Baumrind, Miller, and Molthen. Vertical and horizontal positional changes of certain landmarks were measured in relation to two Cartesian coordinate systems. In the first system, the originally constructed Frankfort horizontal (FH) plane (constructed by subtracting 7 from the sella-nasion line) served as the X axis and a line perpendicular to it through sella served as the Y axis. In the second system, the original natural occlusal plane formed the X axis and a line perpendicular to it through sella formed the Y axis. The two coordinate systems were transferred from the first tracing to the next. In addition certain other linear and angular measurements were assessed (Figs. 4 through 6). Separate maxillary and mandibular tracings were superimposed3 to determine changes in the position of the teeth within the maxilla and the mandible. Changes in the position of the teeth were measured in relation to the original occlusal plane (X axis) and a line perpendicular to it through sella (Y axis). The length of the mandible, condylion- prognathion (Co-Pgn), was measured from the open mouth cephalogram.

J cos co* COP

N FH

Fig. 4. Cephalometric landmarks and linear measurements used in the study. (1) N-ANS, Upper anterior facial height measured perpendicular to Frankfort horizontal (FH) plane. (2) ANS- Me, Lower anterior facial height measured perpendicular to FH plane. (3) A-El to occlusal plane. (4) Condylion-pogonion (Co- Pg), Length of mandible parallel to FH plane. (5) Co-Go, Pos- terior facial height measured perpendicular to FH plane.

Assessment of temporomandibular joint and muscle function

An assessment of the function of the masticatory system was conducted to ascertain the status of any signs and symptoms of dysfunction that were present or that might develop. The assessment was based on a

470 Kalra, Burstone, and Nanda Am. J. Orthod. Dentofac. Orthop. June 1989

Fig. 5. Mandibular length, condylion-prognathion (Co-Pgn), measured from wide open mouth cephalogram.

clinical dysfunction examination and on anamnestic dysfunction as described by Helkimo.4

For the treatment group, assessments were made at the following time intervals: before treatment, 3 days after insertion of appliance, 3 months after insertion of appliance, on removal of appliance, and 4 months posttreatment. For the control group, assessments were made at the start of the examination period and 8 months later.

Results of the cephalometric and study model analyses were statistically analyzed. To assess the significance of differences between the treatment and control groups, mean and standard deviation were calculated and Students t tests were performed. The null hypothesis was rejected at the 0.05% level of confidence.

The size of the combined error in locating, super- imposing, and measuring the changes in different landmarks was calculated with the following. formula:

J Zd2 SE measurement = - 2n The combined standard error did not exceed 0.6 mm in the horizontal and vertical dimensions.

RESULTS

The effects of the magnetic appliance during treatment and during the follow-up period are given in Tables II and III.

Mandible

During treatment the length of the mandible, condylion-prognathion (Co-Pgn) , increased 3.2 mm as compared with 0.8 mm in the control group (p < 0.001). In the follow-up period, no difference was seen in growth rates of the treatment and control groups (Fig. 7).

Fig. 6. Angular measurements used in the study. (6) 1 to-FH plane. (7) Y axis. (8) N-A-Pg, Angle of facial convexity. (9) 1 to mandibular plane. (10) FH-MP, Mandibular plane angle.

In the treatment group, the mandible autorotated upward and forward as judged by the mandibular plane angle and Y axis. The mandibular plane angle decreased 1.3 and 0.3; the Y axis decreased 1.1 and 0.3 in the treatment and follow-up periods, respectively. These changes were small but statistically significant.

Overall facial form

Downward and forward displacement of2 the maxilla, as represented by anterior nasal spine and point A, did not show any significant differences between the treated and control groups.

There was a significant decrease in the angle of facial convexity (p < 0.001) (Fig. 8) and improvement in the relationship of A-B to occlusal plane (p < 0.001) (Fig. 9).

Even though the mandible autorotated upward and forward, increased mandibular growth was responsible for an increase in anterior (ANS-Me) and posterior (Co- Go) facial heights (p < 0.001). Fig. 10 shows the change in overall facial form in the treatment group as compared with the control group.

Dentition

During treatment the upper and lower incisors intruded approximately 1.3 mm each and the molars 1.6 mm each. During this period, in the control group, these teeth erupted 0.3 and 0.4 mm, respectively (Fig. 11). In the follow-up period, the posterior teeth reerupted until they achieved occlusal contact with their antago- nists (Fig. 12). However, overall the lower incisor

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ESfects of fired magnetic appliance on dentofacial complex 471

Table II. Comparison of changes between treatment and control groups during treatment period

Measurements

Treatment p-oup Control group (N = IO) (N = IO)

Mean SD Mean SD Mean

difference p value

Mandible Length

Co-Pgn (mm) Co-Pg parallel to FH (mm)

Displacement in relation to FH Pg horizontal (mm) Pg vertical (mm)

Rotation MP-FH () Y axis ()

Maxilla Displacement in relation to FH

Point A horizontal (mm) Point A vertical (mm) ANS horizontal (mm) ANS ven:ical (mm)

Facial height ANS-Me perpendicular to FH (mm) Co-Go perpendicular to FH (mm)

Facial convex,@ N-A.-Pg () A-B perpendicular to OP (mm)

Dentitinn Overjet (mm) Overbite (mm) Molar relation (mm)* Upper incisor to FH () Lower incisor to MP ()

+3.2 0.5 +2.1 0.6

+0.8 0.2 2.4 1-0.5 0.2 1.6

< 0.001 < 0.001

+2.6 + 1.9

+0.5 +0.5

< 0.001 < 0.01

0.9 I.1

0.2 0.3

2.1 1.4

- 1.3 - 1.1

0.8 0.5

0.2 0.1

1.3 1.1

< 0.001 < 0.001

0 0

+0.5 +0.3 +0.5 +0.2

0.2 0.2 0.3 0.2

+0.4 +0.2 +0.4 +0.2

0.3 0.2 0.2 0.3

0.1 NS 0.1 NS 0.1 NS 0 NS

+ 1.8 1.1 +2.6 0.9

+0.4 0.2 +0.6 0.2

1.4 2.0

i 0.01 < 0.001

-2.8 0.9 +2.3 0.9

-0.1 +0.1

0.1 0.1

2.7 2.2

< 0.001 < 0.001

-2.1 0.7 -3.8 0.9 +2.0 0.8 +0.2 0.6 -0.1 0.7

-0.1 0.2 +0.1 0.2 -0.1 0.1 +0.2 0.5 +0.1 0.6

2.0 < 0.001 3.9 < 0.001 2.1 < 0.001 0 NS 0.2 NS

Changes in tooth position within the maxilla and mandible (measured from maxillary and mandibular superimposition tracings)** Vertical displacement

Upper molar (mm) - 1.6 0.3 +0.2 0.1 1.8 < 0.001 Lower molar (mm) + 1.6 0.3 -0.3 0.2 1.9 < 0.001 Upper incisor (mm) - 1.4 0.4 +0.4 0.3 1.8 < 0.001 Lower incisor (mm) + 1.2 0.2 -0.3 0.1 1.5 < 0.001

Horizontal displacement Upper molar (mm) +0.1 0.4 +0.1 0.2 0 NS Lower mlolar (mm) +0.2 0.2 0 0.3 0.2 NS Upper incisor (mm) 0 0.4 +0.1 0.1 0.1 NS Lower incisor (mm) 0 0.5 0 0.3 0 NS

+ Denotes downward or forward displacement of landmark. - Denotes upward or backward displacement of landmark. * + Denotes imprcvement of molar relationship toward Class I. **The maxilla was superimposed on the hard palate and the anterior maxillary structures with main consideration being given to the region between point A ar,d the anterior nasal spine with the images of the superior surfaces of the hard palate aligned. The mandible was superimposed on the inner tables of the symphysis and the line of distal extension of the mandible.

(p < O.OOl), upper incisor (p < O.Ol), and upper molar (p < 0.05) showed less eruption in the treated group than in the control group as seen in Table IV and Fig. 13.

In the treatment period, the overjet was reduced 2.1

mm and the molar relationship improved 2.0 mm toward Class I occlusion as a result of increased forward displacement of the mandible in relation to the maxilla; this remained unchanged during the follow-up period.

Magnets used in the repelling mode produce lateral


Tabie III. Comparison of changes between treatment and control groups during follow-up period

Measurements

Treatment group (N = 10)

Mean SD

Control group (N = 10)

Mean SD Mean

difference p value

Length Co-Pgn (mm) Co-Pg parallel to FH (mm)

Displacement in relation to FH Pg horizontal (mm) Pg vertical (mm)

Rotation MP-FH () Y axis ()

Maxilla

+0.7 +os

0.2 0.3

+0.6 0.2 +0.4 0.2

-0.4 -0.3

0.2 0.2

Displacement in relation to FH Point A horizontal (mm) Point A vertical (mm) ANS horizontal (mm) ANS vertical (mm)

Facial height ANS-Me perpendicular to FH (mm) Co-Go perpendicular to FH (mm)

Facial convexity N-A-Pg () A-B perpendicular to OP (mm)

Dentition

+0.3 +0.4 +0.4 +0.2

0.2 0.4 0.2 0.3

+0.2 +0.5

0.2 0.2

-0.3 +0.3

0.2 0.2

Overjet (mm) 0 0.3 Overbite (mm) +2.8 0.4 Molar relation (mm)* +0.1 0.2 Upper incisor to FH () +0.4 0.6 Lower incisor to MP () -0.4 0.5

i-O.8 0.2 +0.5 0.2

0.1 NS 0 NS

+0.5 +0.5

0.2 0.3

0.1 NS 0.1 NS

0 0

0.2 0.1

0.4 0.3

< 0.05 < 0.05

+0.4 +0.2 +0.4 +0.2

0.3 0.2 0.2 0.3

0.1 NS 0.2 NS 0 NS 0 NS

to.4 0.2 +0.6 0.2

0.2 NS 0.1 NS

0.1 0.1

-0.1 +0.1

0.2 0.2

< 0.05 < 0.05

-0.1 0.2 +0.1 0.2 -0.1 0.1 +0.2 0.5 to.1 0.6

0.1 NS 2.1 < 0.001 0.2 NS 0.2 NS 0.5 NS

Changes in tooth position within the maxilla and mandible (measured from maxillary and mandibular superimposition tracings)** Vertical displacement

Upper molar (mm) Lower molar (mm) Upper incisor (mm) Lower incisor (mm)

Horizontal displacement Upper molar (mm) Lower molar (mm) Upper incisor (mm) Lower incisor (mm)

+ 1.8 0.4 -2.1 0.5 +1.4 0.5 - 1.4 0.3

+0.2 0.1 1.6 -0.3 0.2 1.8 +0.4 0.3 1.0 -0.3 0.1 1.1

< 0.001 < 0.001 < O.ctOl < 0.001

+0.1 +0.2 +0.2 -0.1

0.4 0.3 0.2 0.4

+0.1 0

+0.1 0

0.2 0.3 0.1 0.3

0 NS 0.2 NS 0.1 NS 0.1 NS

+ Denotes downward or forward displacement of landmark. - Denotes upward or backward displacement of landmark. * + Denotes improvement of molar relationship to Class I. **The maxilla was superimposed on the hard palate and the anterior maxillary structures with main consideration given to the region between point A and the anterior nasal spine with the images of the superior surface of the hard palate aligned. The mandible was superimposed on the inner tables of the symphysis and the line of distal extension of the mandible.

forces as they are moved toward each other. This re- stilted in buccolingual tipping of the teeth in the pos-, terior segments. It was found that in centric relation, seven patients had a molar crossbite on one side and

an increased buccal overjet on the other side. However, during the follow-up period, the teeth uprighted to their original inclinations, thereby correcting the crossbite on one side and the increased buccal overjet on

Volume 95

Number 6 EfJects of,fixed magnetic appliance on dentofacial complex 473

Co-Pgn H Treatment Group q Control Group

(MM) 0

1 Treatment Period Follow-up Perlod

Fig. 7. Change in mandibular length (Co-Pgn).

the other side. The intermolar width remained unchanged.

Function

None of the patients in the treatment or control groups complained of difficulty in opening the mouth maximally, of stiffness of jaws, locking, luxation or pain on movement of the mandible, pain in the region of the temporomandibular joint, or pain of the masticatory musculature during any stage of the study. Sim- ilarly clinical examination of the muscles of mastication and temporomandibular joint function showed that patients in the treatment group did not experience any discomfort, pain, or temporomandibular joint dysfunction at any stage of treatment.

The appliance was accepted extremely well by the children and, apart from initial awkwardness, none complained of discomfort nor difficulty in speech and eating. Patients used a fluoride rinse daily and main- tained good oral hygiene; as a result neither caries nor decalcification was noted during treatment.

DISCUSSION

The two most important findings in the study were that the length of the mandible increased significantly in the treated group and the entire upper and lower arches intruded during treatment.

In 4 months of treatment, the length of the mandible in the treated group increased 3.2 mm as compared with 0.8 mm in the control group. The specially constructed magnetic appliance held the mouth open 7 to

8 mm in the first molar region when the upper and lower splints were in contact, and 10 to 11 mm open when the mandible was in the acquired rest position. This caused the condylar heads to rotate and translate forward, away from the posterior aspect of the glenoid fossa. A number of researchers5-7 have shown increased condylar growth in animal studies with appliances that caused protraction or hyperpropulsion of the condyles. Reports on the use of functional appliances, either a form of activator or the Frankel FR, present contradic- tory results. Some studies*-* show increase in mandibular growth; others22-28 find that mandibular growth is not increased with the use of either type of appliance. Conflicting results with the use of removable functional appliances could be attributed to the fact that they are dependent on patient cooperation and clinical expertise and at best are worn only part of the day.

It is possible that in growing persons there is stimulation of condylar growth when the condyles are pro- tracted by the functional appliance, but this ceases when the appliance is removed from the mouth. All ani- ma15- and human studies29-33 that use fixed splint-like devices to hold the mandible forward 24 hours a day show an increase in mandibular growth. It could be that the determining factor is the amount of time the appliance is worn each day. Moss and Salentijn34 claim that growth at the condyle appears to take place as a secondary phenomenon to fill the space left by the mandible as it is displaced forward by the tissues around it. Enlow also reports condylar growth to be of a compensatory nature rather than a primary process.


N-A-Pg q Treatment Group b9 Control Group

-4' Treatment Period Follow-up Period

Fig. 8. Change in the angle of facial convexity (N-A-Pg).

A-B (OP) S Treatment Group RI Control Group

MN 0

-1' Treatment Period Follow-up Period

Fig. 9. Change in the relationship of A-B to occlusal plane.

Petrovi? and McNamara, Connelly, and McBride12 attributed increased condylar growth to increased activity in the lateral pterygoid muscles. Later studies3-7 in- dicated that the effect of the lateral pterygoid muscles may be mediated to the condyles via the stretch of the posterior portion of the capsule, the meniscotemporal ligament as the condyles come forward. Whetten and JohnstonJ6 have shown that severing the lateral pterygoid msucle did not affect growth of the condyle on the affected side. Recently McNamara and Carlson noted that the adaptive changes in the temporomandib-

ular region may be caused by alteration in the biome- chanical or biophysical environment of the joint that may be produced by muscular or nonmuscular forces. With the magnetic appliance in the mouth, the condyles moved down the articular slope about 10 mm away from the posterior aspect of the glenoid fossa. This could cause articular tissue strain with increased condylar growth as a fill-in process. However, the present state of knowledge precludes a definite answer to the mechanism of increased condylar growth.

After removal of the appliances, the rate of growth

V&me 95 Number 6

Ejfects ofjked magnetic appliance on dentofacial comple?r 475

_ , , u :

q R I : ! , i ,-- I Y. I , , =._ .* I , , I ,I I -._* a. / ,,;, ;;

v

--.* ,/ . *. *..* : : -*. : : a a......, Fig. 10. Composite tracing of lateral cephalograms showing the mean differences in dentofacial form between the treatment and control groups. Differences between the two groups are interpreted as changes caused by treatment. -, Pretreatment; - - -, 4 months posttreatment. The lateral cephalogram was superimposed on the anatomic structures of the floor of the anterior cranial fossa with primary consideration given to the region between the anterior clinoid process and crista galli.

of the mandible was found to be similar to that in the control group. Petrovic3 and McNamara3 have reported a decrea.sed amount of growth for a period following removal of hyper-propulsion appliances in an- imals. However, human studies29-33 with the Herbst appliance have shown normal mandibular growth after removal of the appliance. The results of this study con- cur with those of the latter.

It was decided to use magnets to provide the intrusive force on the teeth since any other form of me- chanical device would either interfere with function or not provide such an efficient system. The size and shape of the magnets were dictated by space available in the mouth, patient comfort, and the force that they could generate. Samarium cobalt magnets were chosen since they have an excellent ratio of magnetic force to size.3g4 In addition they are far superior in resisting loss of magnetic energy with time and are safe to use in the mouth.394 However, samarium cobalt is suscep- tible to corrosion in the oral environment39; therefore

Fig. 11. Composite of maxillary and mandibular superimposition tracings showing the mean differences in the position of the teeth between the treatment and control groups during the treatment period. Differences between the two groups are interpreted as changes caused by treatment. -, Pretreatment; - - -, posttreatment. The maxilla was superimposed on the hard palate and the anterior maxillary structures with main consideration given to the region between point A and the anterior nasal spine with the images of the superior surfaces of the hard palate aligned. The mandible was superimposed on the inner tables of the symphysis and the line of distal extension of the mandible.

the magnets were encased in a stainless steel case and embedded in the acrylic splints so that they were not exposed to the oral environment.

With the appliance in the mouth, the subject tended to maintain an interocclusal space of about 3.0 mm between the upper and lower splints. At this distance the magnets produce a repelling force of 1080 gm, thereby subjecting each tooth in the arch to an intrusive force of approximately 90 gm. Burstone recommends an intrusive force of 20 gm for incisors. Dellingep3 showed that a force of 100 gm was adequate to intrude premolars in dogs. However, an optimum force value has not been established for intrusion of posterior teeth or large segments of teeth. Since some of the teeth covered by the appliance had large root surfaces, a force in the range of 90 gm per tooth was considered adequate, though perhaps excessive, for intrusion. More research is needed in this area to determine an optimum intrusive force.

In a repelling mode, when the magnets are moved toward each other, lateral forces are generated. In most patients this caused the mandible to deviate about 2 mm to one side in the acquired rest position. This shearing force generally caused both upper buccal segments to be tipped to the right side of the patient and both lower buccal segments to be tipped to the left side. This re-


Fig. 12. Composite of maxillary and mandibular superimposition tracings showing the mean difference in the position of the teeth between the treatment and control groups during the follow-up period. Differences between the two groups are interpreted as the amount of relapse. -, Posttreatment; - - -, 4 months posttreatment. The maxilla was superimposed on the hard palate and the anterior maxillary structures with main consideration given to the region between point A and the anterior nasal spine with the images of the superior surfaces of the hard palate aligned. The mandible was superimposed on the inner tables of the symphysis and the line of distal extension of the mandible.

Fig. 13. Composite of maxillary and mandibular superimposition tracings showing the mean differences in the position of the teeth between the treatment and control groups during treatment plus follow-up. Differences between the two groups are interpreted as overall additional change in the treated group. -, Pretreatment; - - -, 4 months posttreatment. The maxilla was superimposed on the hard palate and the anterior maxillary structures with main consideration given to the region between point A and the anterior nasal spine with the images of the superior surfaces of the hard palate aligned. The mandible was superimposed on the inner tables of the symphysis and the line of distal extension of the mandible.

Table IV. Comparison of changes in tooth position between treatment and control groups during treatment plus follow-up (measured from maxillary and mandibular superimposition tracings)*

Treatment group Control group (N = IO) (N = 10)

Mean Measurements Mean SD Mean SD difference p value

Dent&ion Vertical displacement

Upper molar (mm) +0.2 0.2 +0.4 0.2 0.2

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.Effects of @fixed magnetic appliance on dentqfizcial complex 477

segments in the lower tipped to the other side, there were no significant changes in the upper and lower intermolar arch widths. In an effort to minimize or prevent the occurrence of crossbites, it is proposed that the amount of repelling force produced by the magnets be reduced, thereby also reducing the lateral forces generated. If buccal crossbites still occurred, they would be mild in nattire and self-correcting once the appliance was removed. Treatment with the magnetic appliance is followed by a phase of fixed appliance treatment to correct dental relationships; therefore correction of any buccolingual tipping, were it to remain, could readily be achieved at this stage.

On removal of appliances, it was noticed that since the teeth had intruded, occlusion occurred only on the gum pads covering the unerupted second molars, leav- ing the posterior teeth about 4.0 mm out of occlusion. In retrospect examination of pretreatment study models and x-ray films showed that even though the upper and lower second molars were not close to eruption, the clearance between the soft tissues of the upper and lower pads averaged only about 1.0 mm. When the appliances were removed, upward and forward a&o- rotation of the mandible was limited by the gap between these pads.

Autorotation of the mandible occurs when the free- way space is increased after surgical superior repositioning of the maxilla.A7 In such instances the mandible acquires a new rest position, which appears sta- b1e.4s-47 [t could therefore be assumed that if the gum pads had not caused obstruction, the mandible would have autorotated further and acquired a new, potentially stable rest position. Dellinge? reported on cases in which intrusion of posterior teeth had been achieved with the use of an Active Magnetic Vertical Corrector. He found that both intrusion of teeth and the ensuing autorotation of the mandible were stable 3 years later. In this study the teeth were prevented from achieving occlusal contact, resulting in reeruption until full occlusion of the posterior teeth was obtained. Neverthe- less, throughout the entire treatment and follow-up period, the teeth in the treated group underwent statistically significant intrusion as compared with the control group. The limited autorotation of the mandible that was achieved was stable. In fact during the follow-up period, the mandible autorotated further. The appliance may have caused mild inflammation of the soft tissues distal to the first molars; once the appliances were removed, the inflammation gradually subsided and this allowed the mlandible to close a littler further. Since the slight intrusion and autorotation achieved were both stable, it can be hypothesized that if the gum pads had not prevented further autorotation, the enhanced auto-

rotation achieved would also have remained stable and the teeth would not have reerupted to the extent they did. In the future it is proposed that removal of the tissue overlying the upper and/or lower second molars about a week before removal of appliances would be beneficial. This would allow autorotation of the mandible as determined by the amount of intrusion.

CONCLUSIONS

A fixed magnetic appliance was designed that hinged the mandible open and exerted an intrusive force on the teeth. Treatment with this appliance resulted in:

1. An increase in length of the mandible 2. Intrusion of teeth 3. Upward and forward autorotation of the man-

dible 4. Reduction of A-B to occlusal plane 5. Improvement in the angle of facial convexity 6. Creation of temporary buccal crossbite caused

by the shearing force of repelling magnets During follow-up there was some rebound eruption

of teeth; however, all other changes were stable. This appliance presents a promising mode of im-

proving facial harmony in patients with Class II, Di- vision 1 malocclusion associated with mandibular retrusion, increased lower facial height, and increased interlabial gap. In addition reduction in overjet and improvement in molar relationship toward Class I occlusion make the second stage of conventional orthodontic treatment less demanding. Further research and development of the appliance are advocated.

REFERENCES I.

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