Molar Distalization With the Herbst Appliance Mesou Lai

This article reviews the dental effects of the Herbst appliance as well as its long-term effects on the dentition. The Herbst appliance exhibits a pro- nounced high-pull headgear effect on the maxillary molars. Without reten- tion, the molars tend to return to their former anteroposterior positions after the removal of the appliance. These distal movements of the maxillary molars are favorable in Class II correction. Reciprocal mesial movements of the mandibular dentition, however, may not be desirable in many instances. Even though partial recovery occurs after Herbst therapy, treatment effects can persist in the mandibular arch. The latter effect can be beneficial to those patients who have initial retroclined mandibular incisors, but unfavorable in those patients with proclined lower incisors at the beginning of treatment. (Semin Orthod 2000;6:119-128.) Copyright© 2000by W.B. Saunders Company

O ne c o m m o n p rob lem facing orthodontists is the t rea tment of Class II malocclusions.

In correct ing this type of malocclusion into a Class I relationship, one or more of a variety of changes in anteroposter ior relations must occur, including distal movemen t of maxillary teeth, mesial mo vem en t of mandibular teeth, growth, a n d / o r or thopedic changes of apical basal skel- etons. The Herbs t appliance, developed by Emil Herbs t 1,2 in 1905, has been used in an a t tempt to alter the am oun t and direction of basal bone growth.

The Herbs t appliance is a bi te- jumping device that features a bilateral telescoping mechanism. Telescopic tube-an&rod assemblies extend f rom the region of the maxillary first molars to the region of the mandibular premolars and keep the mandible in a constantly p ro t ruded position. The Herbst appliance is used mainly in the correct ion of Class II malocclusions. In addition, this appliance can be used as an anchorage appliance for space closure ~,4 dur ing protract ion

From the Division of Orthodontics, Department of Growth and Development, Harvard School of Dental Medicine, Boston, MA.

Address cor~vspondence to Mesou Lai, BDS, DDS, MS, Harvard School of Dental Medicine, Department of Growth and Development, 188 Longwood Ave, Boston, MA 02I 15.

Copyright © 2000 by W.B. Saunders Company 1073-8746/00/0602-0006510. 00/0 doi: l O. 1053/od.2000.5898

of the mandibular molars or dur ing retraction of the maxillary buccal segment after molar distal- ization. 5

Class II t rea tment with the Herbs t appliance has gained increasing populari ty since it was re in t roduced by Pancherz in 1979. 6 Many ar- ticles on appliance design as well as clinical m a n a g e m e n t of the appliance have been pub- lished since that time. 7-21 The banded design of Pancherz 6 has been modif ied in various ways in an a t tempt to improve t rea tment efficacy. For example, stainless steel crowns have been substi- tuted for bands on the anchor teeth. 7-u Cast splints 12 and acrylic splints 13-17 also have been used to carry the telescope mechanism. Flexible spring modules have been substituted for the rigid tube-and-rod design to allow greater range of mandibula r movements . 18 A cantilever design is particularly beneficial dur ing early Herbst therapy before the erupt ion of the mandibular first premolars. ~,4,19

During the past 2 decades, many studies have been p e r f o r m e d to evaluate the t rea tment ef- fects of the Herbs t appliances on the craniofacial skeleton. 22-47 Dentoalveolar as well as skeletal changes contr ibute greatly to the Class II correc- tion dur ing Herbs t t reatment. This article re- views the dentoalveolar effects of the Herbs t appliance as well as its long-term effects on the dentition. Maxillary incisors are not always incor-

Seminars in Orthodontics, Vol 6, No 2 (June), 2000: pp 119-128 119

120 Mesou Lai

p o r a t e d in to the app l i ance ; t he re fo re , move- m e n t o f the max i l l a ry incisors is n o t c o n s i d e r e d in this ar t icle . This a r t ic le does n o t i n t e n d to c o m p a r e the effects a m o n g the d i f f e ren t des igns because o t h e r factors a f fec t ing t r e a t m e n t , such as gender , age at s tar t o f t r e a tmen t , l e n g t h o f t r e a tmen t , o r t r e a t m e n t p ro toco l , vary a m o n g these studies.

Effects on the Maxillary Dentition

Changes During Therapy

T h e H e r b s t a p p l i a n c e is c o m p l e t e l y t o o t h - b o r n e a n d uses b o t h the m a x i l l a r y a n d m a n d i b u l a r d e n t i t i o n to t rans fe r the fo rce e x e r t e d f rom the te lescopic a rms o f the H e r b s t b i t ez jumping mech- an i sm to the bases o f the max i l l a a n d the m a n d i b l e . T h e te lescopic system p r o d u c e s a p o s t e r o s u p e r i o r l y d i r e c t e d force on the maxi l - la ry p o s t e r i o r t ee th a n d an an t e r io r ly d i r e c t e d force on the m a n d i b u l a r den t i t i on . As a resul t , Class II m o l a r c o r r e c t i o n gene ra l l y is a c o m b i n a - t ion o f skele ta l a n d d e n t o a l v e o l a r changes i r re- spect ive o f facial m o r p h o l o g y 22,2~ o r e thn i c back- g r o u n d . 24,25 T h e skele ta l changes are the resu l t o f d i f fe ren t i a l g rowth b e t w e e n the m a n d i b l e a n d the maxi l la . D e n t o a l v e o l a r m o v e m e n t s i n c l u d e b o t h the dis ta l m o v e m e n t o f the max i l l a ry mo- lars a n d mes ia l m o v e m e n t o f the m a n d i b u l a r molars .

T h e skele ta l a n d d e n t o a l v e o l a r c o n t r i b u t i o n s to Class II m o l a r c o r r e c t i o n d u r i n g H e r b s t

t h e r a p y de r ived f rom d i f f e ren t s tudies a re l is ted in Table 1. 22,23,25-33 In the ma jo r i ty o f studies,

dis tal t oo th m o v e m e n t s o f max i l l a ry molars , s e c o n d a r y to m a n d i b u l a r skele ta l changes , con- t r ibu te s ignif icant ly to the sagi t ta l m o l a r cor rec -

t ion, t h o u g h t h e r e is a la rge var ia t ion in the a m o u n t o f max i l l a ry m o l a r d i s ta l iza t ion a m o n g these studies. In gene ra l , m a x i l l a r y m o l a r distal- iza t ion has b e e n shown to c o m p r i s e approx i - ma te ly 25% to 40% of m o l a r c o r r e c t i o n with the b a n d e d H e r b s t a p p l i a n c e , 22,23,25-29 whereas in the

acryl ic des ign it a ccoun t s for 20% to 25% of the cor rec t ion . ~1-3~ S u p e r i m p o s e d c e p h a l o m e t r i c trac- ings showing the t r e a t m e n t changes o f 40 Class II Division 1 pa t ien t s t r e a t ed with the acryl ic-spl in t H e r b s t a p p l i a n c e f r o m a p rev ious s tudy ~2 are shown in F igure 1.

T h e distal m o v e m e n t s o f the u p p e r molars , in con t r a s t to the n o r m a l g rowth p a t t e r n in which these tee th mig ra t e mesia l ly t h r o u g h the a lveolar processes , i nd ica t e tha t the H e r b s t a p p l i a n c e has a p r o n o u n c e d d is ta l iz ing effect o n the max i l l a ry molars . 26,~1,33-35 In add i t i on , the e r u p t i o n o f the m a x i l l a r y p o s t e r i o r t ee th a re i n h i b i t e d by the a p p l i a n c e Y ,32,35 D i sp l acemen t s o f the max i l l a ry

mola r s by the H e r b s t a p p l i a n c e as well as con t ro l da t a f r o m previous ly p u b l i s h e d s tudies a re shown in Table 2. Differences be tween the m e a n changes in the two g r o u p s are c o n s i d e r e d to be the t r e a t m e n t effects. T h e d is ta l iz ing effects a re re- p o r t e d to r a n g e f rom an average o f 1.8 m m in the s tudy by F r a n c h i e t a133 to 2.8 m m in the s tudy

Table 1. Skeletal and Dental Components of Class II Molar Correction During Herbst Therapy

Skeletal Changes Dental Changes Sample Age Tx Interval Molar

Authors Size (y) (mo) Correction Maxilla Mandible Maxilla Mandible

Pancherz ~6 1982 22 12.1 6 +6.7 -0.3 +3.1 +2.6 +1.3 Pancherz et a127 1986 40 12.5 7 +6.3 -0.3 +2.5 +2.0 +2.1 Pancherz et al 2s 1989 18 10-13 6 +6.4 -0.1 + 1.9 +2.7 + 1.9 Rufet a122 1997 16 11-14 7 +6.4* -0.2 +3.0 +1.5 +2.1

15 11-14 7 +5.7t -0.5 +1.9 +2.4 +1.9 Obijou et a123 1997 14 Prepubertal/pubertal 7.5 +5.9~ -0.8 +3.5 +1.6 +1.6 Wong et a125 1997 14 13.4 6-8 +7.2§ -0.2 +3.4 +1.9 +2.1

14 13.3 6-8 +6.711 -0.3 +2.3 +2.6 +2.1 Konik et a129 1997 21 Postpubertal 7.7 +6.1 -0.1 +2.4 +2.0 +1.8 Valant et al~° 1989 32 10.2 10 +7.1 +0.7 +3.3 +1.5 +1.6 Windmiller 31 1993 46 13.1 11.6 +5.4 -1.0 +4.0 +1.0 +1.4 Lai et a132 1998 40 13.0 12 +5.7 -1.0 +4.1 +1.3 +1.3 Franchi et al 3~ 1999 55 12.8 12 +5.3 -0.6 +3.1 +1.4 +1.4

NOTE. +, Indicates favorable *Hyperdivergent. tHypodivergent. $Class II Division 2. §Chinese sample. ItCaucasian sample.

change for correction; - , indicates unfavorable change for correction.

by Panche rz . 26 T h e int rus ive effects a re app rox i - ma te ly 1 ram. 3°,3z,35 T h e a m o u n t o f dis tal a n d ver t ica l m o v e m e n t s o f max i l l a ry mola r s is f o u n d to be i n d e p e n d e n t o f the p r e s e n c e o r ab se nc e o f e r u p t e d s e c o n d mola r s 35 o r o f the somat ic ma tu - r i ty level o f pa t i en t s at the s tar t o f t r e a tme n t . 29,35

A

C

B

Molar Distalization With Herbst Appliance 121

C / . . ......... ,,,.,

i ' \ ,. ' :

] :

Posttreatment Changes

General ly , the Class II r e l a t i onsh ip is overcor- r e c t e d in to a Class I I I r e l a t i o n s h i p with an uns t ab l e i n t e r c u s p a t i o n at the e n d o f H e r b s t t r e a t m e n t (Fig 1). Fo l lowing the r emova l o f the a pp l i a nc e , the occ lus ion t h e n set t les in to a Class I r e l a t i onsh ip ma in ly as a resu l t o f d e n t a l re lapse , which i nc ludes mes ia l m o v e m e n t o f the maxi l - l a ry mola r s a n d dis ta l m o v e m e n t o f the m a n d i b u - lar molars . 26,27,39,33,35 In the maxi l la , the mola r s

t e n d to r e t u r n to the i r o r ig ina l pos i t i on af te r the t r e a t m e n t is d i s c o n t i n u e d , r e su l t ing in l i t t le con- t r i bu t i on to the Class II m o l a r co r r ec t ion . T h e c o n t r i b u t i o n o f m a x i l l a r y m o l a r d i s ta l iza t ion de- creases to 11% at the e n d o f the 12-month p o s t t r e a t m e n t p e r i o d (0.5 m m of 4.6 m m cor rec - t ion) . 26 Two s tudies eva lua t ing two-phase t reat- m e n t with the acryl ic H e r b s t a p p l i a n c e r e p o r t e d n o c o n t r i b u t i o n at all a t the e n d o f s e c o n d phase t r e a tme n t . ~2,33 T h e skele ta l a n d d e n t o a l v e o l a r c ha nge s tha t a c c o m p l i s h the Class II m o l a r c o r r e c t i o n w h e n the Class I sagi t ta l r e l a t i onsh ip is e s t ab l i shed a re p r e s e n t e d in Table 3. F igure 2 shows the t r e a t m e n t c ha nge s o f 40 pa t i en t s t r e a t e d with the acryl ic H e r b s t i m m e d i a t e l y fol- lowed u p by edgewise therapy. 32

D u r i n g a pos t -He rbs t p e r i o d o f 16 mon ths , F r a n c h i e t aP ~ r e p o r t e d tha t the a n t e r i o r move- m e n t o f the max i l l a ry mola r s was s ignif icant ly l a rge r in the t r e a t e d g r o u p t han in the u n t r e a t e d

/ /

Figure 1. Superimposition of the cephalometric trac- ings showing the treatment changes of 40 Class II, Division 1 patients (20 women and 20 men) treated with the acrylic-splint Herbst appliance. The tracing of pretreatment is shown by a solid line. The tracing of post-Herbst is shown by a dot ted line. Superimposition at the anterior cranial base (A). Class II molars were overcorrected with a posterior openbite. The maxilla moved downward while the mandible moved forward and downward as a consequence of an increase in mandibular length. There was a slight clockwise rota- tion of the palatal plane. The mandibular plane closed approximately 0.6 ° , which was not significantly differ- ent from that seen in the control. Maxillary regional superimposition showing the movements of the maxil- lary teeth within the maxilla (B). The molars were moved distally and held in position vertically while the incisors were not affected by the treatment. Mandibu- lar regional superimposition showing the movements of the mandibular teeth within the mandible (C). The mandibular molars were moved forward and their eruptions were enhanced. The incisors were t ipped forward and their eruptions were inhibited.

122 Mesou Lai

Table 2. Changes of Maxillary Molar Position During Herbst Therapy

Sample Age Interval Authors Size ( y ) (too)

Changes

Herbst Control P Value

Sagittal Changes Pancherz 26 1982 22 12.1 6 -2.6 +0.2 <.01 McNamara et al ~4 1990 45 12.0 12 - 1.4 + 1.3 <.001 Pancherz et a135 1993 45 12.4 7 -2.1 +0.3 <.001 Windmiller 31 1993 22 11.1 11.6 -0.7 +1.4 <.001 Franchi et al ~ 1999 55 12.8 12 -1.4 +0.4 Significant

Vertical Changes Valant et al 3° 1989 32 10.2 10 +0.2 + 1.0 <.001 Pancherz et al ~5 1993 45 12.4 7 - 0.7 + 0.4 <.001 Lai et a132 1998 40 13.0 12 -0.2 +1.0 <.001

NOTE. +, Indicates mesial or extrusive movement; - , indicates distal or intrusive movement.

Class II pat ients . These pos t t r e a tmen t relapses coun te r ac t ed the t r e a t m e n t effect of the Herbs t appl iance , thus, overall, the maxi l lary molars were no t affected by the app l i ance in the sagittal d i r ec t i on . 33 N o r m a l growth d e v e l o p m e n t a l

changes prevail thereaf ter with respect to the maxi l lary den ta l changes (ie, the maxi l lary mo- lars move mesially un t i l growth is completed).~5

Vertical alveolar change , however, was compa- rable with that seen in the con t ro l g roup d u r i n g a 17-month post-Herbst interval . 32 A shor te r maxi l lary pos ter ior alveolar he igh t was n o t e d at the e n d of 2-stage t rea tment .

The Influence of Retention on Posttreatment Relapse

It m igh t be expec ted that the distal m o v e m e n t of the maxil lary molars d u r i n g Herbs t therapy could be m a i n t a i n e d if r e t e n t i o n is achieved. In a shor t - te rm follow-up, there is a d i f ference be- tween the r e t e n t i o n a n d u n r e t a i n e d groups with respect to sagittal mo la r relapse after removal of the retainer. Accord ing to Panche rz a n d Han- s e n , 27 there is less relapse (1.1 mm, P < .01) in

the g roup r e t a ined with activator or maxi l lary plate t han in the u n r e t a i n e d g roup d u r i n g the 12-month pos t t r ea tmen t per iod. The difference,

however, d imin ishes with t ime. Panche rz and A ne hus - P a nc he r z 35,36 f o u n d that 1- to 2-year post-

t r e a t m e n t r e t e n t i on on a long- te rm basis d id no t have a s ignif icant effect on the pos i t ion of the maxi l lary molars.

Effects on the Mandibular D e n t i t i o n

Changes During Therapy

In add i t ion to the maxi l lary mola r distalization, mesial m o v e m e n t of the m a n d i b u l a r molars also con t r ibu tes significantly to Class II mo la r correc- t ion d u r i n g Herbs t therapyY 2,2~,25-33 In studies on

the b a n d e d Herbs t appl iances , these changes usually are less t h a n the distal m o v e m e n t of the maxi l la ry molars (Table 1) a n d con t r ibu t e ap- proximate ly 20% to 30% of the overall correc- t ion. 22'23'25-29 In the acrylic Herbs t studies, these

changes are comparab le a nd accoun t for approxi- mately 25 % of the mo l a r correc t ion . 3133

Herbs t appl iances displace the m a n d i b u l a r den t i t i on anter ior ly to a greater ex ten t w h e n the resu l t ing den toa lveo la r changes are c o m p a r e d with those of the con t ro l g roup 26,31,33,34,37 or with

data der ived f rom a growth study. 32 The a m o u n t of den toa lveo la r effects on m a n d i b u l a r molars was r epor t ed to be an average of 0.8 m m 33 to 2.2

Table 3. Skeletal and Dental Components of Class II Molar Correction as Class I Molar is Established

Molar Skeletal Change Dental Change

Authors Correction Maxilla Mandible Maxilla Mandible

Pancherz et a127 1986 +4.6* - 1.7 +4.2 +0.5 + 1.6 Lai et a137 1998 +3.8 t -1.7 +4.7 -0.2 +1.0 Franchi et al ~ 1999 +3.75 -1.1 +4.2 -0.2 +0.9

NOTE. +, Indicates favorable change for correction; - , indicates unfavorable change for correction. * 12 months post-Herbst treatment. t l 7 months post-Herbst treatment. ++16 months post-Herbst treatment.

Molar Distalization Wi th Herbst Appl iance 1 2 3

m m 31 (Table 4). A greater effect on the incisor posi t ion (Table 5), r ang ing f r o m 1.7 m m 33 to 2.4 ram, ~7 was r epo r t ed with the excep t ion o f the study by Windmil le r 31 in which no significant dif ference was n o t e d between t reated and con- trol groups. In addit ion, procl inat ion o f mandibu-

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lar incisors occurred concomitantly with the ante- r ior d i sp lacement o f the dent i t ion. 6,12,24,s0,s2,37.39 The incisors were t ipped fo rward to a significant level, with r e p o r t e d amoun t s r ang ing f r o m an average o f 2.0 °3o to 8.4 °37 (Table 6).

The Influence o f Anchorage on Dental Movement

It seems clear that an te r ior d i sp lacement o f the m a n d i b u l a r dent i t ion has b e e n a significant side effect o f Herbs t therapy. The lower ancho rage could be increased by inco rpora t ing addi t ional denta l units into the appl iance o r splint ing teeth together, thus, p rovid ing m o r e o f a skeletal co r rec t ion and restr ict ing an te r ior m o v e m e n t o f the lower dent i t ion. However, it has no t yet been shown that any o f the presen t a n c h o r a g e systems used in Herbs t t r ea tmen t are able to prevent an te r ior m o v e m e n t o f the mand ibu l a r teeth and proc l ina t ion o f incisors. The study by Panche rz and H a n s e n 4° c o m p a r i n g the effects o f five di f ferent m a n d i b u l a r a n c h o r a g e systems showed that a n c h o r a g e loss was inevitable regardless o f the a n c h o r a g e system used. Even when using a

Figure 2. Superimposition of the cephalometric trac- ings showing the treatment changes of 40 Class II, Division 1 patients (20 women and 20 men) treated with the acrylic Herbst appliance immediately fol- lowed by a second phase of edgewise therapy. The tracing of pretreatment is shown by the solid line. The tracing of post-Herbst treatment is shown by the dotted line. The tracing of postedgewise treatment is shown by the hyphenated line. Superimposition at the anterior cranial base (A). The maxilla moved down- ward and the mandible moved downward and slightly forward during the fixed appliance therapy. At the end of two-stage treatment, a Class I molar relation- ship was established. The palatal plane rotated back to pretreatment angulation. The mandibular plane closed approximately 0.8 ° , which was comparable with that seen in the control. Maxillary regional superimposi- tion showing the movements of the maxillary teeth within the maxilla (B). During the edgewise treat- ment, the molars returned to their pretreatment positions sagittally and had a similar amount of erup- tion as that of the control, resulting in shorter maxil- lary posterior alveolar height when compared with the control. The incisors were retracted. Mandibular re- gional superimposition showing the movements of the mandibular teeth within the mandible (C). There was a pardal relapse of dental movement for both molars and incisors. The mandibular dentition, however, were still positioned more anteriorly at the end of treatment.

124 Mesou Lai

Table 4. Changes of Mandibular Molar Position During Herbst Therapy

Sample Age Interval Authors Size ( y ) ( mo )

Changes

Herbst Control P Value

Pancherz z6 1982 22 12.1 6 + 1.3 +0.3 <.001 Pancherz et a137 1985 29 12.0 7 + 1.7 + 0.2 <.001 McNamara et a134 1990 45 12.0 12 + 1.4 +0.5 <.001 Windmi l le r sl 1993 22 11.1 11.6 +1.7 - 0 . 5 <.001 Lai et a132 1998 40 13.0 12 +1.5 +0.1 <.001 Franchi et a133 1999 55 12.8 12 + 1.4 +0.6 Significant

NOTE. + Indicates mesial tooth movement. - indicates distal tooth movement.

chrome cast f ramework for anchorage 12 or splint- ing teeth together as one rigid unit, 3°-33 dentoal- veolar changes in the mandibular arch could not be avoided, a l though there are less mandibula r dentoalveolar changes repor ted in the studies using acrylic Herbs t appliances. 3°-33

The Influence o f Somatic Maturation on Dental Movement

When the amoun t of mandibular tooth move- men t was related to the matura t ion level of the t reated patient, it did not seem that the growth per iod in which the pat ient was treated was an impor tan t concern provided that t rea tment did not start very late dur ing growth. 37m,42 In patients whose Herbs t therapies started 1-year postpuberta l growth, greater changes in the po- sitions of the mand ibu la r incisors were re- ported.29,37,41

The Influence o f Incisor Proclination on Gingiva

The effect of or thodont ic proclinat ion of the mandibular incisors on gingival recession has been a subject of debate. Ruf et a143 investigated the relationship between the amount of mandibu- lar incisor proclinat ion dur ing t rea tment and the deve lopment of gingival recession 6 months after t rea tment in 98 children and adolescents. A

total of 392 mandibula r incisors were examined. Only 12 of the 392 teeth developed a recession or their preexisting recessions became worse. No relationship was found between the amoun t of proclinat ion and the increase in crown length or deve lopment of recession. Therefore , they con- cluded that labial movemen t of mandibular inci- sors did not result in gingival recession.

Posttreatment Changes

Dental relapses after the removal of the Herbs t appliance also are evident in the mandibular dentition. In contrast to the maxillary molars, mandibular molars do not tend to re turn to their p re t rea tment position. Class II molar cor- rections occur as a result of mandibular skeletal changes and mesial movemen t of the mandi- bular molars at the end of the 12-month post- t rea tment per iod 27 or the end of 2-stage treat- men t 32,33 (Table 3).

On a short- term basis, a l though rebound occurs after the appliance is removed, the effects of the Herbs t appliance on the mandibular denti t ion seem to be maintained, but to a lesser extent (ie, more anteriorly posi t ioned mandibu- lar teeth and more procl ined mandibular inci- sors when compared with the controls).Sz,~ These effects on the mandibular denti t ion are main- tained th roughout the growth period. 44

Table 5. Changes of Mandibular Incisor Position During Herbst Therapy

Sample Age Interval Authors Size (y ) ( mo )

Changes

Herbst Control P Value

Pancherz 26 1982 22 12.1 6 + 1.8 +0.0 <.001 Pancherz et al s7 1985 29 12.0 7 +2.4 +0.0 <.001 McNamara et al s4 1990 45 12.0 12 + 1.6 - 0 . 4 <.001 Windmi l le r sl 1993 21 11.1 11.6 +1.1 +0.4 NS Lai et al 3z 1998 40 13.0 12 +1.6 - 0 . 5 <.001 Franchi et al 3s 1999 55 12.8 12 + 1.3 - 0 . 4 S

NOTE. +, Indicates mesial tooth movement; - , indicates distal tooth movement. Abbreviations: NS, not significant; S, significant.

Molar Distalization With Herbst Appliance 125

Table 6. Changes of Mandibular Incisor Inclination During Herbst Therapy

Sample Age Interval Authors Size ( y ) ( mo )

Changes

Herbst Control P Value

Pancherz 6 1979 10 Prepuber ta l 6 +5.4 0.0 <.001 Wies lander 12 1984 18 8.3 5 +3.6 +0.3 <.01 Pancherz et a137 1985 29 12.0 7 +8.3 -0 .1 <.001 Valant et al 3° 1989 32 10.2 10 +2.5 +0.5 <.05 Sidhu et a124 1995 8 11.8 8 +4.9 - 0 . 4 <.05 Lai et a132 1998 40 13.0 12 +5.1 - 0 . 5 <.001

NOTE. +, Indicates labial procl inat ion; - indicates l ingual retroclination.

StabUity

Long-term stability of the mandibula r dentition, the incisors in particular, has been of p r imary concern because there is a strong tendency for the teeth to re turn to their p re t rea tment posi- tion. Many orthodontists believe that this ten- dency eventually will overtake the t rea tment effect and that relapse will occur in the buccal segments or result in the deve lopment of post- t rea tment crowding.

In a long-term study on the results of Herbs t t reatment, Pancherz 36 compared two groups of Herbst- treated patients with and without relapse in the occlusion. Skeletal and dentoalveolar changes in the mandibular arch were found to be similar in both groups 5 years after t reatment. The reason for relapse was thought to be the anter ior movements of maxillary denti t ion ow- ing to muscular influence f rom the lips or tongue, or to an unstable occlusal condit ion after t reatment.

The mandibular incisors are displaced anteri- orly to a great extent dur ing t rea tment and recover partially dur ing the immediate posttreat- men t period. This overall movemen t results in a net effect of slightly more anteriorly posi t ioned teeth. In the short- term follow-up, the move- ments of the incisors do not seem to cause crowding of the lower anteriors. 45,46 In a long- te rm perspective, Hansen et a146 assessed the relationship between the changes of mandibular incisor inclination and the deve lopment of ante- rior crowding. They found that the decrease of available space and increase in irregularity index were similar between the Herbst- treated patients and the untreated normal subjects dur ing a 5- to 10-year follow-up period. Thus, the deve lopment of incisal crowding was thought to be associated with normal craniofacial growth changes ra ther than the use of the Herbs t appliance.

Discuss ion

Distalization of maxillary molars into a Class I relationship is one t rea tment modality for Class II malocclusions. This movemen t can be achieved by ei ther extraoral traction 48-5° or intraoral appli- ances such as removable plates, 51 repell ing mag- nets, 52-57 nickel-titanium coil springs, 56-59 super- elastic wires, 6° the Pendulum appliance (Ormco Corp, Orange, CA),6144 the Wilson arch (Rocky Mountain Orthodontics, Denver, CO), 65 Herbst appliances, 22-35 the Jasper J u m p e r (American Orthodontics, Sheboygan, WI),18 and sliding jigs with Class II elastics. 66

Molar distalization usually can be achieved in a relatively short per iod of t ime (3 to 4 months) with repel l ing magnets , 54-57 superelastic coil springs, 5c~5s the Pendulum appliance, 61-64 or the Wilson arch. 65 These appliances p roduced distal movemen t at the rate of 0.6 to 1.2 m m per month . In comparison, the Herbs t appliance displaced the maxillary molars at a slower rate (Table 1). This reduced rate of movemen t could be due in par t to the fact that the Herbs t appliance was originally designed to alter the growth of basal bones ra ther than to distalize the maxillary molars. Premolars a n d / o r anter ior teeth, in addition to molars, are incorpora ted into the appliance to restrict undesirable dento- alveolar movements .

In the correct ion of Class II malocclusions, moving the maxillary molars distally without intrusion usually is undesirable, especially in those patients with hyperdivergent growth pat- terns, because of the tendency to rotate the mandible downward and backward. The Herbs t telescoping bite-jumping mechanism places a distal and intrusive force on the maxillary molars and the force vector passes occlusally to the center of resistance. This force system produces

126 Mesou Lai

backward and upward movements of maxillary molars in conjunction with distal crown tip- ping. 67 Because of the intrusive effect, distal movements of maxillary molars do not tend to open the mandible. 6,12,3°'32,34 These effects are similar to those produced by high-pull head- gear. 4s-5° In contrast, most of the rapid molar distalization appliances tend to cause the man- dible to rotate downward and backward, open- ing the mandibular plane angle. Of the seven studies 55,56,5s,61"64 that evaluated the mandibular plane changes during distalization, five stud- ies 56'58'61'62'64 repor ted that the mandible rotated downward and backward approximately 1 °.

The Herbst appliance, like other interarch Class II mechanics such as Class II elastics and the Wilson arch, uses the mandibular arch as anchorage to distalize the maxillary molars. The reciprocal force displaces the mandibular denti- tion anteriorly and proclines the incisors. 65,66 Although all of these appliances are effective in correct ing the sagittal relationship of the denti- tion, the vertical changes produced by these appliances differ. The vertical force of Class II elastics extrudes the maxillary incisors and man- dibular molars, and these changes result in a clockwise rotation of the occlusal and the man- dibular planes as well as an increase in lower anterior facial height. 66 The intrusive force of the Herbst appliance on the maxillary molars and mandibular anter ior teeth results in an increase in the occlusal plane angle, whereas it has no t reatment effect on the mandibular plane.6,12,30-32,34

Summary The Herbst appliance exhibits a p ronounced high-pull headgear effect on the maxillary mo- lars. Without retention, the molars tend to re- turn to their fo rmer anteroposter ior positions after the removal of the appliance. These distal movements of the maxillary molars are favorable in Class II correction. Reciprocal mesial move- ments of the mandibular dentition, on the other hand, may not be desirable in many instances. Even though partial recovery occurs after Herbst therapy, t reatment effects can persist in the mandibular arch. The latter effect can be benefi- cial to those patients who have initial retroclined mandibular incisors, but unfavorable in those

patients with proclined lower incisors at the beginning of treatment.

Acknowledgment The author thanks Dr George Cisneros, James A. McNamara, Jr, and Leslie A. Will for their assis- tance in the preparat ion of this manuscript.

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