Activators: A practical approach

Activators:

A practical approach

Hans-Casper

D.M.D.,*

Baltimore,

Hirzel,

and John

M. Grewe,

D.D.S., Ph.D.**

Md.

I

n recent years orthodontists in the United States have become increasingly interested in understanding and utilizing the activator as an orthodontic appliance for the correction of specific morphologic disharmonies. For a historical account of activators and other functional appliances, the reader should refer to articles published in the AMERICAN JOURNAL OF ORTHODONTICS the 1CJ+j’()‘s.%10, 1% 21, 2%26, e purpose of the present article is to discuss and summarize the biologic effects of the activator. Two cases treated exclusively with this functional appliance will be presented as illustrations. The activator to which we refer (Fig. 1) is the type currently in use at the University of Ziirich and the University of Maryland. Although it resembles Robin’P original monobloc more than Andresen’s’ activator,l both appliances have basically the same effect and serve the same purpose. A construction bite, which ultimately determines the degree of activation and the mandibular position, must be obtained prior to fabrication of the activator. The construction bite used and cited in this article is obtained by first determining the physiologic rest position and then displacing the mandible vertically 2 to 3 mm. and horizontally 5 to 8 mm. (Fig. 2). The vertical displacement, only 2 to 3 mm. beyond the freeway space, is used to keep the activator in place by overextending the closing musculature. This method contrasts with that of Harvold and Vargervik,s who advocate an opening of 8 mm. or more. The horizontally displaced mandible activates such mandibular retractors as the posterior portion of the temporal muscle and the suprahyoid musculature which, together with the fibrous ligaments, are the source of force for the activator. The activator itself prevents the mandible from sliding backward and transfers the force to the maxilla, which is essentially the anchorage unit for the anteriorly displaced mandible. Fig. 3 illustrates the force vectors in relation to the maxilla and mandible.

during

21 “Jh

*Assistant Professor, Department of Orthodontics, University of Maryland School of Dentistry. Present address : Mellingerstr. 1, CH5400, Baden, Switzerland. **Associate Professor and Chairman, Department of Orthodontics, University of Maryland School of Dentistry.

557

55% Hirzel

Fig.

1. Modified

made Bio’logic Division

of

0.032

a”nd Gre’we

activator. inch

effects of the 1 malocclusion

Note

stainless functional

Am. J. Orthod. November 1914

the steel

spurs

mesial

to the

upper

molars.

The

labial

bow

is

wire. appliance

in correcting

Class

II,

A number of authors have reported various morphologic changes that have occurred as a result of the utilization of forces created by functional appliances. Clinical findings have been detailed by Demisch,4 Dietrich, Frlnkel,G Harvold and Vargervik,s Hotz,l” Pfeiffer and Grobety,21 Tulley,2G and Valinoti.2’ Animal findings have been reported by McNamara,l” Moyers and associates,20 and Stackli and Willert.23 The following material is a synthesis of this information and of our own observations. For clarity of presentation, we have divided the biologic effects into the following seven categories: (I) remodeling of the mandibular condyle, (2) redirection or retardation of horizontal maxillary growth, (3) anterior-downward rotation of the maxilla, (4) mandibular rotation, (5) dental arch changes anteroposteriorly, (6) altering eruption of the teeth within the buccal segments, and (7) incisor tipping. Remodehg effect on th,e mandibuh condyle. There has been considerable controversy concerning the extent to which activator treatment can influence mandibular growth. Andresen and Hlupl,l Korkhaus,l” Marschner and Harris,16 and, more recently, Demisch’ have shown clinically that activator treatment has a positive influence on mandibular growth. Bjijrk,2 Harvold and Vargervik* Meach,l” and Dietrich” did not observe such an effect. However, using cephalometric laminagraphs, May Ii observed that the horizontal growth of the condyle was more than three times as great in the activator group as in the control group in a study at the University of Minnesota. StGckli and Willert’” have demonstrated histologic changes in the condylar region following functional anterior displacement of the mandible in rhesus monkeys. They observed adaptive tissue changes in the three general layers of condylar cartilage and also noted surface changes at the anterior aspect of the postglenoid process. Joho,‘” Moyers2” and McNamara18 have observed cephalometrically similar alterations.

Volume 66 Num her 5

Activators:

A pmctical

approach

559

2-3mm

02

1 2 3 4

POSTERIORPART OF TEMPORALMUSCLE SUPRAHVOlD MuscuLnTuRE FIBROUSCOHPONENTS

CENTRIC RELATION

ANCHORAGE

0 3

Fig.

2. Displacement

of the

Fig. 3. Force system created pull the mandible backward. which is the anchorage unit

mandible by

for

the

construction

bite.

an activator. The muscular The activator stabilizes the in the whole system.

and fibrous components mandible against the

(l-3) maxilla,

Thus, current findings have failed to determine conclusively the effect of activator treatment on mandibular growth and development. There is no doubt, however, that adaptive changes occur on a cellular level in condyles of the functionally displaced mandible in animals and probably in human beings as well. The clinical significance in the latter is, however, questionable. growth. Studies by HarRedirection or retardation of h.orkontal maxillary vold and Vargervik,s Jakobsson, I2 and Meachlg indicated that the basal areas of the maxilla are retarded in their normal forward development. Conversely, Bj6rk’sz early cephalometric studies have been interpreted as indicating that reactions is limited to the teeth and alveolar bone, with no effect upon growth in the facial skeleton. Pfeiffer and C:robetyzl and Dietrich have shown that this effect upon maxillary growth can be altered efficiently by combining activator and headgear treatment. It is, however, uncertain so far whether the observed effect upon sutural growth consists of (1) decrease in cellular activity in those

560

Hirzel

and Grewe

Am. J. Orthod. November 1974

sutures, (2) retardation of growth and/or redirection of growth in a more vertical direction, or (3) all these effects combined. Further investigation on a biochemical, autoradiographic, and electron microscopic level is needed to explain this clinically well-demonstrated react,ion. Anterior-downward rofn,fim of the maxilla.. Demisch,l Harvold and Vargervik,8 and Jakobsson” have shown not only a reduction in the forward displacement of the maxilla but also an anterior-downward rotation of the entire maxilla. The explanation for this finding lies in the action of the force vector against the maxillary dental arch well below the center of resistance of the maxilla. Xandibulnr rotation. Harvold and TTargerviks have observed an increase in the height of the mandibular alveolar process and a variable degree and rate of eruption of teeth in the buccal segment. The resulting backward rotation of the mandible may be compensated for by the vertical growth of the condyle at a later age. This hinging of the mandible or the greater eruption of mandibular buccal teeth ultimately decides the degree of overbite reduction. An increase in lower facial height is inevitable, however. This hinging of the mandible is detrimental in Class II malocclusions with high mandibular plane angles. Dental arch. changes a,n’teroposteriorly. There is some additional controversy concerning the extent to which dental arch changes or distal migration of maxillary teeth and mesial migration of mandibular teeth can be induced by an activator. Bjijrkz attributed all changes to the dentoalveolar region. Distal migration of maxillary molars and mesial migration of mandibular posterior teeth have also been reported by Andresen and Haupll as well as by Korkhaus.lS The latter, however, used numerous small inclined planes within the activator to direct erupt,ion of the teeth. Harvold and Vargcrvik,” who modified the original Andrcsen appliance and cover only the upper posterior teeth with acrylic, found neither distal migration of maxillary teeth nor increased mesial migration of mandibular teeth. This is consistent with Xeach’s’!’ findings that the eruption pattern of the maxillary molars was not changed by activator treatment. Altered emptio?l of th.e teeth within the buccal segments. By selectively removing acryliP3 26 or by using the activator design proposed by Harvold and Vargervik,s the vertical eruption of the maxillary posterior teeth can be inhibited and the eruption of their antagonists can take place undisturbed and free of occlusal interferences. This difference in vertical eruptions is, according to Harvold and Vargervik,” an important source in transforming a Class II malocclusion into a neutral occlusion but, as mentioned previously, is always accompanied by an increase in lower facial height. Incisor tipping. The immediate response to the anterior displacement of the mandible occurs within the dental arch, mainly in the incisor region. The force returning the mandible to its original position is transmitted by the activator and its labial bow to the maxillary dentition and particularly to the maxillary incisors. This results in lingual tipping.S, ‘3 I2 However, this effect can be decreased by using ball clasps or single arrowhead clasps mesial to the upper

Activators:

m-e

'

SNA SNB ANB ANS PNS-MeGa &NA

i-NB Fig. occur

4. Two-stage rather

rapidly.

treatment Stage

6-63 -w-m 3-64

6-63 82' 77O

3o" 23' by the

bttage 2

activator. skeletal

Stage

approach

561

3-64 ----S-67

79O 77O 2O 28' 21° 27'

82' 76' 6' 3o" 24O 28'

5O 27’

II is the

A practical

I represents

the

dental

changes

which

reaction.

molars, as proposed by Herren and Demisch.4 The behavior of the lower incisors is just the opposite. Jakobsson, I2 Bj6rk,2 Harvold and Vargervik,* and Dietrich have shown that, in spite of the long lingual extension of the activator (which is thought to take as much force as possible from the lower teeth and transmit it to the body of the mandible), an increase in lower incisor inclination occurs. In order to decrease the amount of “dumping” during treatment, there should be no contact between the activator and the lingual aspect of the lower incisors; there should be coverage of the incisal edge and a portion of the labial surface of the incisors.11l 24 Pfeiffer and Grobety21 and Dietrich have demonstrated that this protrusive effect can be avoided and may be reversed into retrusion by the simultaneous use of an activator and a headgear. The dental changes that is, dental arch changes, altered eruption, and incisor tipping,-occur rather rapidly,-whereas ‘skeletal changes-that is, condylar remodeling and maxillary growth retardation-generally occur later. Therefore, the average activator treatment of a Class II, Division 1 malocclusion consists of a first stage in which dental changes predominate and a second stage in which an increase in SNB and a decrease in SNA can be observed. The skeletal stage, in effect, stabilizes the earlier dental corrections (Fig. 4). These biologic effects must be considered when indications and contraindications for such treatment are being formulated. Furthermore, they must be considered separately for the dental and the skeletal components.

562

Fig.

Hirzel

5. Case

Am. J. Orthod. November 1974

and Gmwe

1. A,

Facial

profile

of

71/2-year-old

boy

with

100 per cent overbite, and 12 mm. overjet. 8, Facial profile of activator treatment followed by a 3-year posttreatment Dental casts (right side) before treatment. D, Dental casts Incisor relationship prior to treatment. treatment. G to I, intraoral photographs a 3-year

posttreatment

Contraindications

period

for

activator

with

a Class

II molar

relationship,

of the same boy after 3 years period with no retention. C, (left side] before treatment. E,

F, Incisor relationship after after 3 years of activator

9 months treatment

of activator followed by

no retention.

treatment

in Class

II, Division

I malocclusion

The contraindications for activator treatment in a Class 11, Division 1 malocclusion can be divided into the following factors : Lack of growth Skeletal factors : Unfavorable growth pattern (high mandibular plane angle, backward rotating mandible, etc. ) Excessive lower facial height Excessive sagittal discrepancies Dental-skeletal factors : Sagittal discrepancies Transversal discrepancies Dental factors : Retruded maxillary incisors Protruded mandibular incisors Severe crowding Severe spacing Severe rotations Active extrusion or intrusion required These contraindications relate to a total activator treatment which includes no other appliances. Any time an activator is used for partial or preliminary treatment to achieve a certain goal, one or more of the contraindications can be disregarded. Indications

for

activator

treatment

in Class

II,

Division

1 malocclusion

Ideal skeletal characteristics of the patient are a favorable growth pattern and normal or diminished lower facial height. An acceptable dental arch-apical

VoZume Number

66 5

Activators:

Fig.

5, C-l.

For

legend,

see

opposite

A practical

page.

approach

543

544

Hirzel

Am. J. Orthod. November 1974

awl Grewe

C.M. -2-65

C.M. -

Fig.

6.

tracing and of after

Case

1. A,

after

9 months

3-year treatment. 3 years

Cephalometric

posttreatment The

tracings

of treatment

of

tracing

treatment.

at

period

without

are

superimposed

and

3-year

the

beginning

of

C, Cephalometric

posttreatment

on

treatment.

tracing

retention.

D, the

Over-all

ethmoid

period

2-65

without

after

B, Cephalometric 3 years

changes triad.

E, retention.

of after

Over-all

treatment 9

months changes

Volume Number

Fig.

7.

Case

clusion, ment. by

2.

A,

10 mm. C,

activator ing

Activators:

GG 5

Incisor

photographs

of

labial

profile and

G, no

Incisor

appliance 31/z

an

D and

face-bows after

of

therapy.

E, Dental Note

profile

casts. after

undertaken

of activator

girl

B, Facial

relationship

was years

81/z-year-old

no overbite.

relationship.

treatment.

2 months two

Facial

overjet,

the

with after

F, Incisor 2 years

partial

following treatment.

A practical

of relapse. the

a Class 31/z

approach

II,

years

relationship activator H, relapse

Division of

1 maloc-

activator after

(G).

followed

treatment I to

treat-

2 years

treatment Further

565

K,

utilizIntraoral

of

566

Hirzel

a.?d G-ewe

Am. J. Orthod. November 1974

Fig.

7, F-K.

For legend,

see p. 565.

base relationships is absolutely necessary. Dentally, the maxillary incisors should be protruded and the mandibular incisors retruded or well positioned. No severe crowding, spacing, or rotations should be present. This vigorous handling of indications is necessary to minimize the number of failures and maximize the number of successes. Patient cooperation as an indication or contraindication factor must also be considered. Herren!’ and Ikmisch” have reported how they approach the cooperation problem in observation periods prior to treatment. In summary, the ideal patient is in the middle mixed-dentition period, with good arches and an abnormal maxillomandibular relation. For girls, this is usually between the ages of 7 and 11 years; for boys, it is between the ages of 8 and 12 yea.rs. When all permanent teeth (excluding the third molars) have erupted, the success rate is minimal, Sth correction achieved by the activator limited to the dentoalveolar region.

Volume Number

66 5

Activators:

A practical

approach

567

Fig. 8. Case 2. A, Cephalometric tracing at beginning of treatment. Note the protrusion of the lower incisors. 6, Cephalometric tracing after 26 months of treatment. The lower incisors are now severely protruded. C, Cephalometric tracing at the end of treatment. D, Over-all superimposition of the first 2-year treatment period. The excessive dental reaction and the poor skeletal response are clearly visible. E, The dental relapse within 2 months. F, Overall changes after 3 years 10 months of treatment, superimposed on the ethmoid triad.

560

Hirzel

Am. J. Orthod. November 1974

and Grewe H.B. ---

Fig. Case

4-69 b-11

8, D-F.

For legend,

see

p. 567.

reports

The following two cases, both treated at the University of Ziirich Dental School, illustrate the principles of activator treatment. The first case, illustrating the ideal, is that of a 7%.year-old boy with a Class II molar relationship, 100 per cent overbite, and 12 mm. overjet (Fig. 5). The clinical result after 9 months of treatment can be seen in Fig. 5, F. The final result, at the end of 3 years of treatment and 3 years posttreatment mith no retention, is shown in Fig. 5, B and G to 1. The cephalometric tracings superimposed on the ethmoid triad demonstrate Stage I in which dental movements dominate (Fig. 6, n). Over-all changes are shown in Fig. 6, B. The differences in forward growth of maxilla and mandible are clearly visible. The second case is an example of what may occur when some indication requirements are not present (Fig. 7). An Sr$-year-old girl with a Class II, Division 1 malocclusion, 10 mm. overjet, and no overbite was treated with the same type of activator. After 2 years of treatment, the overjet had disappeared (Fig. 7, F). The discontinuation of treatment led to a partial relapse within 2 months (Fig. 7, G). Further treatment, utilizing two labial bows on the appliance, was then undertaken (Fig. 7, H). The final result after 18 months of additional treatment is shown in Fig. 7, I to K. The cephalometric tracings in Fig. 8, B and B reveal that the lower incisors were protrusive at the beginning of treatment and were even more protrusive after 2 years of treatment. The final tracing (Fig. 8, C) shows the lower incisors still in a protruded position but close to the initial value. The unsuccessful initial

Volume Number

66

Activntors:

5

phase and the almost immediate relapse are and E; the excellent amount of mandibular the final result possible (Fig. 8, F) .

demonstrated growth in

A practical

in the superimpositions the second phase of

approach. in Fig. treatment

569 8, D made

In summary, we contend that the activator should be considered as an integral appliance alternative within a modern orthodontic practice. It is obviously assumed that the problem, diagnosis, treatment objectives, and plan always dictate the appliance choice. The appliance should never be the dominant consideration In this context, we have presented material in support of judicious use of the activator. It should be noted that many of the principles, indications, and contraindications in activator therapy are similar to those in headgear therapy. The ideal patient for activator treatment is a cooperative child with normal growth in the mixed-dentition stage with good dental arch-apical base relationship, normal lower facial height, retruded or normally positioned mandibular incisors, protruded maxillary incisors, and no severe crowding, spacing, or rotations. The authors wish to express their Orthodontic Department, University of of the material presented in this article.

appreciation Zurich Dental

to

Professor Dr. R. Hotz, School, Zurich, Switzerland,

Chairman, for part

REFERENCES

1. Andresen, V., and IILupl, K.: Funktionskieferorthopiddie, die Grundlage des Norwegischen systems, Leipzig, 1935, Hermann Meusser. 2. Bjork, A.: The principle of the Andresen method of orthodontic treatment; a discussion based on cephalometric x-ray analysis of treated cases, AM. J. ORTHOD. 37: 437-458, 1951. 3. Cunat, J. J.: Activators: An orthopedic puzzle, AM. J. ORTHOD. 65: 16-27, 1974. 4. Demisch, A.: Auswirkung der Distalbisstherapie mit dem Aktivator auf das Gesichtsskelett, Schweiz. Monatsschr. Zahnheilkd. 83: 1072-1092, 1973. 5. Dietrich, U. C. : Aktivator-mandibulare Reaktion, Schweiz. Monatsschr. Zahnheilkd. 83: 1093-1104, 1973. 6. FrHnkel, R.: The treatment of Class II, Division 1 malocclusion with functional correctors, AM. J. ORTHOD. 55: 265-275, 1969. 7. Freunthaller, P.: Cephalometric observations in Class II, Division 1 malocclusions treated with the activator, Angle Orthod. 37: 18-25, 1967. 8. Harvold, E. P., and Vargervik, K.: Morphogenetic response to activator treatment, AM. J. ORTHOD. 60: 478.490, 1971. 9. Herren, P.: Die Wirkungsweise des Aktivators, Schweiz. Monatsschr. Zahnheilkd. 63: 829-878, 1953. 10. Hotz, R..: Application and appliance manipulation of functional forces, AM. J. O,~HOD. 58: 459-478, 1970. 11. Hotz, R.: Orthodontics in daily practice, Bern, 1974, Hans Huber Medical Publisher. 12. Jakobsson, S. 0.: Cephalometric evaluation of treatment effect on Class II, Division 1 malocclusions, AM. J. ORTHOD. 53: 446-457, 1967. 13. Joho, J. P.: Changes in form and size of the mandible in the orthopedically treated Macacus irus, Tr. Eur. Orthod. Sot. 44: 161-173, 1968. 14. Jorgensen, S. E.: Activators in orthodontic treatment ; indications and advantages, AM. J. ORTHOD. 65: 260-269, 1974. 15. Korkhaus, G.: Present orthodontic thought in Germany, AM. J. OR~~HOD. 46: 270-287, 1960.

57Q Hirxel

Am J. Orthod. November 1974

a,d, G-ewe

16. Marschner, J. F., and Harris, J. E.: Mandibular growth Orthod. 36: 89-93, 1966. 17. May, J. F.: A Iaminogrnphic and cephalometric evaluation occurring during activator treatment, M.8. thesis, University 18. McXamara, J. Jr.: Neuromuscular and skeletal adaptations Monograph No. 1, Crania-facial growth scrics, Ann Arbor,

rind

Class

11 treatment,

Angle

of dental and skeletal of Minnesota, 1972. to alter orofacial 1972, University of

changes function, Michigan

PIYZSS.

19. Meac11, C. L. : A cephalomctrie comparison of bony profile changes in (Ilass 11, Division I patients treated with cxtraoral forccx and functional jaw orthopedics, AX J. ORTHOU. 52: 353-370, 1966. 20. Moyers, R. E., et al. : Eixpcrimcntal production of Class III in rhesus monkeys, Tr. Eur. Orthod. Sot. 46: 67-74, 1970. 21. PfeifYcsr, J. I’., and Grobety, I).: Simultaneous USC of cervical appliance and activator: An orthodontic approach to fixed appliance therapy, Au. J. ORTHOD. 61: 353-373, 1972. --. w Posen, A. L. : The monobloc, Angle Orthod. 38: 121-128, 1968. 23. Htiickli, P. W., and Willert, H. G.: Tissue reactions in the tcml)(~ro~~andibular joint resulting from anterior displacement of the mandible in monkeys, Aal. J. ORTHOD. 60: 142-155,

1971. 24. Rix, R,. E.: Further thought on monobloe therapy, Dent. Pratt. 16: 389, 1966. 25. Robin, P.: Observation sur un nouvel appareil de redressement, Rev. Stomatol. 1902. 26. Tulley, W. J.: The scope and limitations of treatment with the activator, Anf.

9: 423-432, J.

ORTHOD.

61: 562-577, 1972. 27.

Valinoti,

1973.

J. R.:

The

European

activator:

Its

basis

and

use,

A&r.

J.

ORTHOD. 63: 561-580,