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1/9

uropean Journal of Orthodontics19 (1997) 93-101 9 1997 Europ ean Ortho dont ic Socie ty

C la s s I I s u b d i v i s i o n t r e a t m e n t w i t h t i p b a c k m o m e n t s

B h a v n a S h r o f f , S t e v e n J . L i n d a u e r a n d C h a r le s J . B u r s t o n e

Dep artmen ts of Ortho don t ics, Un iversi ty of Maryland School of Dent istry, Me dical Col lege of

Vi rg in ia, Vi rg in ia Com mon wea l th Univers i ty and Un ivers i ty of Connecticut , USA

SU M M AR Y U n i l a t e r a l t i p - bac k m ec han i c s a r e c ha l l eng i ng bec aus e o f a num be r o f undes i r ab l e

s i d e - e ff e c ts a s s o c ia t e d w i t h t h e i r u s e d u r i n g o r t h o d o n t i c t r e a t m e n t . T h e p u r p o s e o f th i s p a p e r

is t o r e v i e w t h e d i f f e r e n t ia l d i a g n o s i s a n d t r e a t m e n t p l a n n i n g o f C la s s II s u b d i v i s i o n

m a l o c c l u s i o n s a n d p r e s e n t a t r e a t m e n t s t ra t e g y b a s e d o n a c a r e fu l b i o m e c h a n i c a l a n a l y s is o f

t he c l i n i c a l s i t ua t i on . Em phas i s i s p l ac ed on t he c o r r ec t i on o f m o l a r ax i a l i n c l i na t i on us i ng

u n i la t e ra l t ip - b a c k m o m e n t s in t h e t re a t m e n t o f de n t a l a s y m m e t r ie s . A p p l i a n c e d e s i g n a n d

t r e a t m e n t s e q u e n c i n g a r e a ls o d i sc u s s e d .

Introdnction

D e n t a l m a l o c c l u s i o n s i n v o l v i n g a n a s y m m e t r i c

p o s t e r i o r o c c l u s i o n w h i c h r e s u l t s f r o m t h e

p r e s e n c e o f a n a l t e r e d m e s i o - d i s t a l m o l a r a x i a l

inc l ina t ion a re d i f f i c u l t to t r e a t o r thodon t ic a l ly .

O n e o f t h e c h a l l e n g e s i n t r e a t i n g s u c h m a l -

oc c lus ions i s a s soc ia te d wi th the b iome c ha n ic a l

l im i ta t ions o f a t e c hn ique tha t use s a c on t inuous

a r c h w i r e a p p r o a c h . S u c h t e c h n i q u e s m a y l i m i t

t h e f u l l c o r r e c t i o n o f a n a s y m m e t r i c o c c l u s i o n

b e c a u s e o f f r i c t i o n b e t w e e n t h e b r a c k e t a n d

t h e a r c h w i r e a n d u n p r e d i c t a b l e f o r c e s y s t e m s .

F u r t h e r m o r e , t h e c o r r e c t io n o f a s y m m e t r i c m a l -

o c c l u s i o n s u s i n g u n i l a t e r a l t i p - b a c k m o m e n t s i s

usua l ly a c c ompa nie d by unde s i ra b le s ide -e f fe c t s

w h e n f o r c e s y s t e m s u s e d d u r i n g t r e a t m e n t a r e

n o t c a r e f u l l y a n a l y s e d a n d e v a l u a t e d p r i o r t o

a p p l i a n c e d e s i g n . T h e d i f f e r e n t i a l a n d e a r l y

d ia gnose s o f de n ta l a symme tr ie s a re the ke ys to

d e f i n i n g t h e c o r r e c t t r e a t m e n t o b j e c t i v e s a n d

d e s i g n i n g a n a p p l i a n c e w h i c h w i l l d e l i v e r t h e

d e s i r e d f o r c e s y s t e m t o a c h i e v e o r t h o d o n t i c

c or re c t ion .

iagn osis and treatment planning

A s y m m e t r i c m a l o c c l u s i o n s m a y r e s u l t f r o m t h e

pre se nc e o f a t rue ske le ta l a symme try , a de n ta l

a s y m m e t r y o r a c o m b i n a t i o n o f t h es e t w o

c l i n i c a l s i t u a t i o n s . D u r i n g t h e d i f f e r e n t i a l

d i a g n o s i s o f a d e n t a l a s y m m e t r y , c a r e f u l

e v a l u a t i o n o f t h e a n t e r o - p o s t e r i o r r e l a t i o n s h i p

of the buc c a l oc c lus ion i s unde r ta ke n in c e n t r ic

re la t ion , a nd the p re se nc e o f a func t iona l sh i f t

b e t w e e n c e n t r i c r e l a t i o n a n d c e n t r i c o c c l u s i o n

pos i t ions i s r e c orde d . Whe n the c e n t r ic r e la t ion

p o s i t i o n i s d i f f i c u l t t o f i n d , a d e p r o g r a m m i n g

sp l in t ma y be p re sc r ibe d to the pa t ie n t to he lp

e s ta b l ish a c c ura te ly the c e n t r ic r e la t ion p os i t ion .

I t i s a n i m p o r t a n t s t e p f o r r u l i n g o u t a p s e u d o -

a s y m m e t r i c b u c c a l o c c l u s i o n r e s u l t i n g f r o m a

sh i ft o f the m a ndib le dur in g c losure .

D e n t a l a s y m m e t r ie s o f t h e p o s t e r i o r o c c l u si o n

m a y r e s u l t f r o m t h e p r e s e n c e o f a m o l a r

ma lo c c lus ion in the f i r st o rde r ro ta t ion) o r the

s e c o n d o r d e r a b n o r m a l m e s i o - d i s t a l a x i al

i n c l i n a t i o n ) . A l t h o u g h t h i s p a p e r p r i m a r i l y

foc use s on the d ia gnos i s a nd c or re c t io n o f de n ta l

a s y m m e t r i e s r e s u l t i n g f r o m t h e p r e s e n c e o f a n

a b n o r m a l m o l a r a x i a l i n c l i n a t i o n , t h e r o l e o f

u n i l a t e r a l m o l a r r o t a t i o n i n c r e a t i n g d e n t a l

a sym me tr ie s wi l l a l so be d i s c us se d .

T h e p r e s en c e o f a u n i l a te r a l m o l a r r o t a t i o n i n

t h e m a x i l l a r y a r c h o b s e r v e d f r o m t h e o c c l u s a l

a spe c t ma y be a n ind ic a t ion o f m e s ia l d r i f t a nd a

m o r e C l a s s I I m o l a r r e l a t i o n s h i p o n t h a t s i d e .

D u r i n g t r e a t m e n t , p u r e r o t a t i o n o f t h e m o l a r

a round the c e n t re o f r e s i s ta nc e c a n ope n spa c e

for the a n te r io r t e e th F ig . 1 ) . E a r ly d ia gnos i s

a n d c o r r e c t i o n o f t h e a s y m m e t r y a r e c r i t i c a l t o

ob ta in a b i l a te ra l C la s s I oc c lus ion a nd a n ide a l

over je t /ov erbi te re lat ionsh~p.

A d i sc re pa nc y be twe e n the me s io -d i s ta l a x ia l

inc l ina t ion se c ond o rde r ) o f the r igh t a nd le f t

p e r m a n e n t f i r s t m o l a r s i n t r o d u c e s a d e g r e e o f

a s y m m e t r y b e t w e e n t h e r i g h t a n d l e f t b u c c a l


2/9

9 4

B . S H R O F F E T A L .

_.___ ./

C l q

.._

a

F i g u r e I S c h e m a t i c r e p r e s e n t a t i o n o f a r o t a t e d ma x i ll a r y

r i g h t p e r m a n e n t f i r st mo l a r . T h e r i g h t p e r m a n e n t f ir s t mo l a r

d i s p l ay s a me s i o - l i n g u a l r o t a t i o n a n d t h e b u c c a l o c c l u s i o n o n

t h i s s id e i s mo r e C l a s s I I . R o t a t i o n o f t h e f i r s t mo l a r u s i n g a

c o u p l e mo v e s t h e me s i a l a s p e c t o f t h e mo l a r d i s t a ll y b e c a u se

t h e mo l a r r o t a t e s a r o u n d t h e c e n tr e o f r e s i s ta n c e C R ) wh i c h

is located l ingual ly .

o c c l u s i o n a n d n e e d s t o b e d i a g n o s e d a n d

c or re c te d dur ing the in i t i a l s t a ge s o f t r e a tme nt .

For e xa mple , me s ia l inc l ina t ion o f the ma xi l l a ry

f i r st m ola r c a n be obse rve d in ca se s o f e c top ic

e r u p t i o n o f t h e p e r m a n e n t m a x i l l ar y fi rs t

mola r s . Th is me s ia l pos i t ion ing o f the ma xi l l a ry

f i r s t m o l a r r e s u l t s i n a m o r e C l a s s I I b u c c a l

oc c lus ion on tha t s ide o f the de n ta l a rc h . I t i s

t h e r e f o r e n e c e s s a r y t o c o r r e c t t h e a x ia l

inc l ina t ion o f the f i r s t mola r to a c h ie ve a n ide a l

b u c c a l o c c l u s i o n b i l a t e r a l l y . D e n t a l m i d l i n e

d i s c r e p a n c y a n d d e e p o v e r b i t e a r e o f t e n

a s soc ia te d wi th a n a symme tr ic buc c a l oc c lus ion

a n d m u s t b e c a r e f u l l y e v a l u a t e d t o s e e i f i t i s

pos s ib le to a ddre s s th i s p rob le m s imul ta ne ous ly

d u r i n g t h e t r e a t m e n t o f t h e a s y m m e t r y .

T h e m e s i o - d i s t a l a x i a l i n c l i n a t i o n o f t h e

m a x i l l a r y f i r s t p e r m a n e n t m o l a r s c a n b e

e v a l u a t e d f r o m s t u d y m o d e l s a n d l a t e r a l

c e p h a l o m e t r i c r a d i o g r a p h s t a k e n a t a 4 5 d e g r e e

a n g u l a t i o n . T h e m e s i o - d i s t a l m o l a r a x i a l

F i g u r e 2 a ) T h e a s s e s s me n t o f th e me s i o - d i s t a l a x ia l

i n c l i n a t i o n o f t h e ma x i l l a r y f i r s t p e r ma n e n t mo l a r i s

u n d e r t a k e n w i t h r e s p e c t t o a n a n t e r o - p o s t e r i o r h o r i z o n t a l

l i n e p a s s i n g t h r o u g h t h e t i p o f t h e b u c c a l c u s p s o f t h e m o l a r .

b ) W h e n t h e m a x i l l a r y f i r s t p e r m a n e n t m o l a r i s t i p p e d

f o r wa r d , t h e h o r i z o n t a l l i n e o f r e f e r e n c e i n t e r s e c t s t h e

max i l l a ry inc i so r mo re g ing iva l ly tha n obse rv ed in a ) .

i n c l i n a t i o n i s e v a l u a t e d w i t h r e s p e c t t o a n

a n t e r o - p o s t e r i o r h o r i z o n t a l l in e p a s si n g t h r o u g h

the t ip o f the buc c a l c usps o f the ma xi l l a ry f i r s t

p e r m a n e n t m o l a r s o n e a c h s i d e . C l i n i c a l l y , a

ru le r o r a s t r a igh t e dge c a n be e a s i ly a l igne d to

t h e b u c c a l c u s p s o f t h e m o l a r s a n d u s e d t o

e s t a b l i s h t h e h o r i z o n t a l l i n e o f r e f e r e n c e . T h e

r i g h t a n d l e f t h o r i z o n t a l l i n e s i n t e r s e c t t h e

m a x i l l a r y i n c i s o r s a n d t h e r i g h t a n d l e f t m o l a r

a x ia l inc l ina t ions a re the n c ompa re d (F ig . 2a , b ) .

R a d i o g r a p h i c e v a l u a t i o n a s s i s t s i n d e t e r -

m i n i n g t h e m e s i o - d i s t a l m o l a r a x i a l i n c li n a t io n s

a n d t h e s p a c e a v a i l a b l e d i s t a l t o t h e f i r s t a n d

se c ond ma xi l l a ry mola r s . La te ra l c e pha lome t r ic

r a d i o g r a p h s t a k e n a t a 4 5 d e g r e e a n g u l a t i o n

a r e u s e f u l t o a s s e s s t h e m e s i o - d i s t a l a x i a l

i n c t i n a t i o n s o f m a x i l l a r y a n d m a n d i b u l a r

pos te r io r t e e th , the i r roo t p a ra l le l ism , the l e vel o f

t h e , b o n e a n d i ts d i s t r i b u t i o n . P a n o r a m i c

a n d p e r i a p i c a l r a d i o g r a p h s o f t h e m a x i l l a r y

t u b e r o s i t y m a y a l s o b e h e l p f u l t o a s s e s s t h e


3/9

T R E A T M E N T W I T H T I P B A C K M O M E N T S 9 5

a m o u n t o f s p a c e d i s t a l t o t h e m o l a r s . A n

a s y m m e t r y b e t w e e n t h e m e s i o - d i s t a l a x i a l

inc l ina t ion o f th e r igh t an d le ft f i r st m o la r s i s no t

s e l f - c o r r e c t i n g w i t h g r o w t h . T h e m e s i o - d i s t a l

a x i a l i n c l i n a t i o n s o f p e r m a n e n t m o l a r s c h a n g e

w i t h f a v o u r a b l e g r o w t h , b u t a p r e - e x i s t i n g

d i s c r e p a n c y b e t w e e n t h e m e s i o - d i s t a l a x i a l

inc l ina t ions o f the r igh t and le f t f i r s t m ola r s i s

m a i n t a i n e d a n d r e s u l t s i n a n a s y m m e t r i c b u c c a l

o c c l u s i o n . D i a g n o s t i c t o o l s s u c h a s f r o n t a l

c e p h a l o m e t r i c r a d i o g r a p h s c a n a l s o a s s i s t i n

e s t a b l i s h i n g t h e o r i g i n o f t h e a s y m m e t r y . A

f r o n t a l c e p h a l o m e t r i c f i l m o f a p a t i e n t w i t h a n

a s y m m e t r i c o c c l u s i o n o f d e n t a l o r i g i n m a y n o t

show any ske le ta l o r ap ica l base d i s c repanc ies .

T r e a t m e n t o p t i o n s f o r s e v e r e s k e l e t a l a s y m m e t -

r i e s i n c l u d i n g f r o n t a l h o r i z o n t a l d i s c r e p a n c i e s

b e t w e e n t h e m a x i l l a r y a n d m a n d i b u l a r a p i c a l

b a s e s r a n g e f r o m s u r g i c a l t r e a t m e n t t o

o r t h o d o n t i c t r e a t m e n t i n v o l v i n g a n a s y m m e t r i c

p a t t e r n o f d e n t a l e x t r a c t i o n s t o c o m p e n s a t e f o r

the ske le ta l p rob le m .

T r e a t m e n t p l a n n i n g o f a m a l o c c l u s i o n w i t h

s o m e d e g r e e o f a s y m m e t r y i s a c h a l l e n g i n g

p r o c e s s . I n n o n - e x t r a c t i o n t h e r a p y , i f t h e

d i f f e rence be tween the r igh t and le f t m e s io -d i s ta l

a x i a l i n c l i n a t i o n o f t h e m o l a r s i s n o t d i a g n o s e d

a n d a d d r e s s e d e a r l y d u r i n g t r e a t m e n t , t h e

a s y m m e t r y b e t w e e n t h e r i g h t a n d l e f t p o s t e r i o r

o c c l u s i o n i s m a i n t a i n e d u n t i l l a t e r i n t r e a t m e n t

a n d m a y t h e n b e m o r e d i f f i c u l t t o c o r r e c t .

T h e r e fo r e , t h e m e c h a n i c s p l a n m u s t b e d e s i g n e d

t o a d d r e s s i n i t i a l l y t h e a s y m m e t r y o f t h e

p o s t e r i o r o c c l u s i o n s o a s y m m e t r i c o c c l u s i o n c a n

b e a c h i e v e d e a r ly d u r i n g t r e a t m e n t .

Un i la te ra l t ip -ba ck i s d i f f icu l t to ach ieve us ing

c o n t i n u o u s a r c h w i r e t e c h n i q u e s . T h e s e t e c h -

n i q u e s d o n o t a l l o w t h e a p p l i c a t i o n o f i d e a l

d i f f e r e n t i a l f o r c e s y s t e m s o n t h e r i g h t a n d l e f t

s ides o f the a rch wi thou t s ign i f i can t undes i r ab le

s ide-e f f ec t s , such asa change in the can t o f the

o c c l u s a l p l a n e , m o l a r e x t r u s i o n , m e s i a l r o o t

d i s p l a c e m e n t o f t h e m o l a r a n d l o w e r a r c h ef fe c ts

i f C las s I I e las t i cs a re used . A m ore e f f i c ien t

a p p r o a c h w i l l i n v o l v e a n o p t i m u m d i s s o c i a t i o n

b e t w e e n t h e m e c h a n i c s u s e d t o t r e a t t h e r i g h t

and le f t s ides o f the den ta l a r ch us ing the s eg-

m e n t e d a r c h t e c h n i q u e t o b e t t e r c o r r e c t

t h e a s y m m e t r y a n d c o n t r o l t h e u n d e s i r a b l e

s i d e - e f f e c t s B u r s t o n e e t a l . 1 9 6 1 ; B u r s t o n e ,

1962 , 1966 ; 1981 , 1985 ; Rom eo and Burs tone ,

1 9 7 7 ; S m i t h a n d B u r s t o n e , 1 9 8 4 ) . F o l l o w i n g

c o r r e c t i o n o f t h e p o s t e r i o r a s y m m e t r y a n d

a t t a i n m e n t o f s y m m e t r i c r i g h t a n d l e f t b u c c a l

o c c lu s io n s , a p p r o p r i a t e m e c h a n i c s c a n b e u s e d t o

c o n t i n u e t r e a t m e n t d e p e n d i n g o n t h e r e s o l u t i o n

o f a n y a n t e r i o r p r o b l e m .

i omechani cs

C o m m o n m e c h a n i s m s u s e d t o a ch i e ve u n i l a te r a l

m o l a r t i p - b a c k a n d t h e i r s i d e - e f f e c t s

V a r i o u s t r e a t m e n t s t r a te g a r e a v a i l a b le t o

c o r r e c t d e n t a l a s y m m e t r i e s o r t h o d o n t i c a l l y , e a c h

p o s s e ss i n g n u m e r o u s a d v a n t a g e s a n d d r a w b a c k s .

T h is pap er i s l im i ted to d i s cus sing the m echa n ics

o f u n i l a t e r a l t i p - b a c k . T h e u s e o f a s y m m e t r i c

h e a d g e a r u n i l a t e ra l fa c e b o w ) h a s b e e n

a d v o c a t e d H a a c k a n d W e i n s t e in , 1 9 58 ) a n d

p r o v i d e s a m e c h a n i s m t o d e l i v e r a n a s y m m e t r i c

f o rc e s y s t e m t o t h e m a x i l l a r y m o l a r s G a r l a n d

H e r s h e y

e t a l .

1981). A l th oug h a d i s ta l fo rce can

be del ive red uni lateral ly , a net la teral force is a lso

p r e s e n t a t t h e i n n e r b o w t e r m i n a l s a n d h a s a

tendenc y to c rea te a l ingua l c ros sb i te on the s ide

which receives the greater dis tal force. However ,

g o o d p a t i e n t c o - o p e r a t i o n i s n e c e s s a r y w h e n

e x t r a o r a l a p p l i a n c e s s u c h a s h e a d g e a r a r e

presc r ibed and m ay l im i t the i r use .

Uni la te ra l C las s I I e las t i cs a re o f ten used on

f u l l a r c h e s t o c o r r e c t t h e a n t e r o - p o s t e r i o r

p o s i t i o n o f t h e f i r s t m o l a r s , p r e m o l a r s a n d

c a n i n e s . T h i s m e c h a n i s m o f C l a s s I I c o r r e c t i o n

h a s l i m i t a t i o n s b e c a u s e i t r e l i e s o n p a t i e n t

co-opera t ion . S ign i f ican t s ide -e f f ec t s a re as soc i -

a ted wi th the wear o f un i la te ra l C las s I I e las t i cs

w h i c h d e p e n d o n t h e a m o u n t a n d p o i n t o f

a p p l i c a t i o n o f t h e f o r c e a s w e l l a s t h e l e n g t h

o f t r e a t m e n t . A m o n g t h e s e s id e - e f fe c t s a re a

s k e w i n g o f t h e a r c h e s w i t h p o s s i b le d e v e l o p m e n t

o f a m a x i l l a r y - m a n d i b u l a r m i d l i n e d i s c r e p a n c y

and asym m e t r ic over je t F ig . 3 ). Ca n t in g o f the

m a x i l l a r y a n t e r i o r o c c l u s a l p l a n e c a n a l s o b e

observ ed du e to the vert ic~al co m po ne n t o f the

Clas s I I e las t i c un i la te ra lly , and s ign i f ican t lower

i n c i s o r f l a r i n g c a n b e o b s e r v e d i f t h i s i s n o t

con t ro l led p roper ly . Pos te r io r t ee th m ay no t be

f ree to s l ide d i s ta l ly so tha t th e en t i r e a rch ro ta tes


4/9

96 B . S H R O F F E T A L .

F i g u r e 3 Oc c l u s a l a s p e c t o f t h e ma x i l l a r y a n d ma n d i b u l a r

a rches . Un i l a t e ra l C las s I I e l a st i c wea r r e su l t s in a skewing o f

b o t h a r c h e s i n o p p o s i t e d i re c t i o n s w i t h r e s p e c t t o t h e i r c e n t r e

o f re s i s t an c e C R ) .

F igu re 4 a ) Ho r izon ta l and ve rt i ca l fo rces r e su l t ing f rom a

un i l a t e ra l C las s I I e l a s ti c u sed wi th a s l id ing j ig a re show n .

b ) In the f ron ta l p l ane , the un i l a t e ra l ve r t i ca l fo rce r e su l t s in

c a n t i n g o f t h e a n t e r i o r o c c l u s a l p la n e .

i n s t ead o f t he m o la rs t i pp ing d i s ta l ly . S t eepen ing

of t he pos t e r io r occ lusa l p l ane i s a l so obse rved

u n i l a t e r a l l y r e s u l t i n g f r o m t h e v e r t i c a l c o m -

pon en t o f t he C lass I I e la s t ic .

S l i d ing ji g s a sso c i a t ed wi th u n i l a t e ra l C lass I I

e l a s t i c u se have a l so been sugges t ed t o co r rec t

d e n t a l a s y m m e t r i e s T h u r o w , 19 66 ; T w e e d ,

1966). Th ese j ig s s l i de on a con t inuou s a rch wi re

a n d t r a n s m i t a d i st a l f o rc e to t h e m o l a r f r o m t h e

un i l a t e ra l C lass I I e l a st ic . A l th ou gh the e l a s ti c is

n o t d i r e c t l y a t t a c h e d t o t h e m a i n a r c h w i r e ,

s ide -e f fec t s f rom un i l a t e ra l C lass I I m echan ics

can s t i ll be observ ed F ig . 4a ,b) .

Un i l a t e ra l C lass I I e l a s t i c wea r has a l so been

used wi th a co i l sp r ing t o t i p a m ax i l l a ry f i r s t

m o l a r d i s t a ll y i n a n a t t e m p t t o c o r r e c t t h e m o l a r

m e s i o - d i s t a l a x i a l i n c l i n a t i o n . T h e c o i l s p r i n g

de l ive rs a d i s t a l fo rce t o t he m o la r and an equa l

and oppos i t e m es i a l fo rce t o t he p rem ola rs and

canines . The uni la tera l Class II e las t ic del ivers a

d i s t a l fo rce t o co un te ra c t t he m es i a l fo rce app l i ed

by the co i l sp r ing , anda ve r t i ca l fo rce wh ich i s

re spons ib l e fo r s i de -e f fec t s i nc lud ing can t ing o f

the an t e r io r occ lusa l p l ane and s t eepen ing o f t he

pos t e r io r occ lusa l p l ane F ig . 5a ,b ) . Th e d i s ta l

fo rce app l i ed by t he un i l a t e ra l C lass I I e l a s t i c

m a y a l s o c a u s e s k e w i n g o f t h e a r c h e s . T h e

l o n g - t e r m s t a b i l i t y o f c o r r e c t i o n i n v o l v i n g t h e

use o f such Class I I m echan ics i s ques t i onab le

because i t i s based on t he m ovem en t o f t he en t i re

a r c h w h e n o n l y m o l a r t i p - b a c k i s i n d i c a t e d

t o c o r r e c t a n a b n o r m a l m e s i o - d i s t a l a x i a l

i n c l i n a t i o n o f t h i s t o o t h . F l a r i n g o f t h e l o w e r

inc i so rs and changes i n t he can t o f t he an t e r io r

a n d p o s t e r i o r o c c l u s a l p l a n e s m a y a l s o b e

undesi rable .

U n i l a t e r a l t i p - b a c k m e c h a n i c s u s i n g s t r a i g h t

c o n t i n u o u s a r c h w i r e s o r u n i l a t e r a l t i p - b a c k

bends can a l so be i nco rpo ra t ed i n t he t rea tm en t

o f d e n t a l a s y m m e t r i e s r e s u l t i n g f r o m a l t e r e d

m o l a r a x i a l i n c l i n a t i o n s . W h e n u n i l a t e r a l t i p -

b a c k m e c h a n i c s a r e a t t e m p t e d w i t h s t r a i g h t


5/9

T R E A T M E N T W I T H T I P B A C K M O M E N T S 9 7

91191

igure 6 S a g i tt a l v ie w o f t h e f o rc e s y s t e m p r o d u c e d w i t h a

straight wire extended through the bracket of a mesially

tipped m axillary first molar.

t h e a r c h t h r o u g h t h e u s e o f a h e a v y s e c t i o n a l

a rchwire , th i s s ide-ef fec t i s d i f f icul t to cont rol .

D e s i re d f orc e s y s t e m t o ac h i ev e un i l a t e ra l

F i g u r e 5 a ) F o r c e s y st e m g e n e r a t e d b y a c o i l s p r i n g

assoc ia ted w i th uni la te ra l Class I I e las t ic wear . b ) Fron ta l

v i e w s h o w i n g a c a n t i n g o f t h e a n t e r i o r o c c l u s a l p l a n e a n d

s t e e p e n i n g o f t h e p o s t e r i o r o c c l u s a l p l a n e d u e t o u n i l a t e r a l

Class I I e las t ic wear .

a r c hw i r e s , c on s ide r a b l e f r i c t i on is de ve lop e d a t

t h e b r a c k e t a n d t h e m o l a r m a y e x p er ie n c e a

f o r w a r d m o v e m e n t o f i t s r o o t w i t h o u t d i s t a l

m o v e m e n t o f it s c r o w n F i g . 6 ).

O the r s i gn i f i c a n t s i de - e f f e c t s i nc lud ing c a n t i ng

o f t h e a n t e r i o r o c c l u s a l p l a n e o c c u r a s t h e

m o l a r a c h i e v e s a b e t t e r a x i a l i n c l i n a t i o n . T h e

a p p l i c a t i o n o f a u n i l a t e r a l t i p - b a c k m o m e n t

p r o d u c e s v e r t i c a l f o r c e s , w h i c h a r e i n t r u s i v e

a n t e r i o r l y a n d e x t r u s i v e p o s t e r i o r l y F i g . 7 a ).

T he i n t r us ive f o r c e ge ne r a t e d on one s i de o f t he

a n t e r i o r p a r t o f t h e a r c h c r e a t e s a m o m e n t

a r o u n d t h e c e n t r e o f r e s i s t a n c e o f t h e a n t e r i o r

un i t f our ma x i l l a r y i nc i so r s ) . T h i s r e su l t s i n a

c a n t o f t h e a n t e r i o r o c c lu s a l p l a n e w h i c h t en d s t o

in t r ude t he a n t e r i o r t e e th on t he s i de w he r e t he

m o l a r u p r i g h t i n g i s r e c o m m e n d e d F i g . 7 b ,c ).

U n l e s s t h e a n t e r i o r i n t r u s i v e f o r c e i s o f v e r y

s m a l l m a g n i t u d e a n d d i s t r i b u t e d t o th e re s t o f

t ip back

T h e f o r c e s y s t e m n e c e s s a r y t o c o r r e c t a n

a s y m m e t r i c b u c c a l o c c l u s i o n r e s u l t i n g f r o m t h e

f o r w a r d t i p p i n g o f a m a x i l l a r y p e r m a n e n t f i r s t

m o l a r w i l l b e a n a l y s e d . I n t h i s e x a m p l e , t h e

opp os i t e s i de o f t he a r c h e xh ib i t s a n i de a l C l a s s I

o c c l u s i o n w i t h a n o r m a l a x i a l i n c l i n a t i o n o f t h e

m a x i l l a r y p e r m a n e n t f i r s t m o l a r . T o c o r r e c t t h e

a x i a l i n c l i n a t i o n o f t h e m e s i a l l y t i p p e d f i rs t

p e r m a n e n t m o l a r , a t i p - b a c k m o m e n t i s u s e d o n

th i s s ide on ly F ig . 8 ) . A s ing l e d i s t a l f o r c e

a p p l i e d a t t h e m o l a r b r a c k e t c a n a l s o b e u s e d t o

p r o d u c e t i p - b a c k b u t t h e u se o f a c o u p l e i n v o lv e s

l e s s unde s i r a b l e s i de - e f f e c t s . T he goa l i s t o

m a i n t a i n a g o o d o c c l u s i o n o n t h e C l a s s I s i d e

a nd no t t o p r oduc e i nc i so r s i de - e f f e c t s .

I t i s poss ib l e by c onne c t i ng t he r i gh t a nd l e f t

f i r s t m o l a r s w i t h a t r a n s p a l a t a l a r c h w i r e o r

h o r s e s h o e a r c h t o a p p l y a t i p - b a c k m o m e n t o n

t h e s id e p r e s e n t i n g t h e t i p p e d m o l a r B u r s t o n e

a n d M a n h a r t s b e r g e r , 1 9 88 ; B u r s t o n e , 1 9 89 ,

1 9 9 4 ) . T h e m o l a r o n t h e o p p o s i t e s i d e e x p e r i -

e n ce s a t i p - f o r w a r d m o m e n t F i g . 9) . T h i s

u n d e s i r a b l e s i d e - e f f e c t c a n b e c o n t r o l l e d b y

c onso l i da t i ng t he r e s t o f t he de n t a l a r c h us ing a

h e a v y a r c h w i r e . T h i s a p p r o a c h c a n o n l y b e u s e d


6/9

98

a SHROF F ET AL

a

Figure 7 a) Sagittal view of the force system produced with a 2 x 4 appliance with tip-back bends mesial to the first molar. b)

Frontal view of the unilateral intrusive force resulting from a unilateral tip-back. c) Cant of the occlusal plane due to a

unilateral intrusive force applied at some distance from the centre of resistance of the anterior teeth.

i h t

igure 8 Desired force system to tip-back a mo la r Figure9 Forcesystemgenerated when a tip-backmoment is

unilaterally, applied unilaterally with a transpalatal arch.

in the permanent dentition and is not suitable in

the mixed dentition because of the limited

number of permanent teeth present in the arch.

Another approach is described below.

Bilateral tip-back moments to correct the

molar axial inclination may be produced by

using a three-piece base arch Fig. 10) Shrof f

e t

a l .

1995). Bilateral tip-back springs are hooked

to the distal extensions of an anterior segment of

wire which includes the four maxillary incisors.

Anterior intrusive forces and posterior extrusive

forces are produced with the molar tip-back

moments. The intrusive force is placed through

the centre of resistance of the upper incisors to

prevent flaring. Intrusion of the four maxillary

incisors is a desirable side-effect of bilateral

tip-back mechanics which can be used in the

presence of a deep overbite. As tip-back of the

molars occurs, the anterior hooks of the tip-back

springs slide distally along the distal extension of

the anterior segment of wire. The position of

these hooks needs to be modified during

treatment to maintain the point of application of

the intrusive force through the centre of

resistance of the upper incisors. In cases where

no intrusion of the maxillary anterior teeth is

indicated, the anterior seg men t of wire is

extended to the right and left canines to

minimize the amount of intrusion.

It is possible to combine an asymmetric


7/9

TREAT MENT WITH TIP BACK MOMENTS

9 9

Figure 10 Lateral view of a three piece base arch.

l i n g u a l a r c h a c t i v a t i o n w i t h b i l a t e r a l t i p - b a c k

s p r i n g s t o c o r r e c t a n a s y m m e t r i c b u c c a l

o c c l u s i o n a s s o c i a t e d w i t h a d e e p o v e r b i t e . I n

t h e s e c a s e s , a b i l a t e r a l t i p - b a c k m o m e n t i s

a p p l i e d t o t h e r i g h t a n d l e ft m o l a r s a n d i n t r u s io n

o f t h e f o u r i n c i s o r s i s o b t a i n e d u s i n g a

t h r e e - p i e c e b a s e a r c h ( S h r o f f

e t a l .

1995).

S imul ta ne ou s ly , a p a la ta l a rc h i s use d to de l iver a

u n i l a te r a l t i p - b a c k m o m e n t t o t h e s id e re q u i r i n g

t h e m o l a r a x i a l i n c l in a t i o n c o rr e c t io n . T h e m o l a r

on the oppos i te s ide i s ma in ta ine d in i t s o r ig ina l

p o s i t i o n b e c a u s e o f o p p o s i n g m o m e n t s f r o m t h e

in t rus ion sp r ing a nd the l ingua l a rc h .

T he th re e -p ie c e ba se a rc h i s c ompose d o f a n

a n t e r i o r s e g m e n t o f w i r e p l a c e d p a s s i v e l y i n t o

the b ra c ke t s o f the four inc i so r s . T h is a n te r io r

se gme n t o f w i re is fa br ic a te d f rom 0 . 021 x 0 . 025

s ta in le s s s te e l w i re to ob ta in h igh r ig id i ty .

T i p - b a c k m o m e n t s a r e a p p l i e d t o t h e m o l a r s

b i l a t e r a l l y b y u s i n g t i p - b a c k s p r i n g s e x t e n d i n g

f rom the a ux i l i a ry tube o f the mola r to the d i s ta l

e x te ns ion o f the a n te r io r s e gme nt o f wi re . T he

a n t e r i o r p o r t i o n o f t h i s s p ri n g h a s a h o o k t h a t is

f r e e t o s l i d e a l o n g t h e d i s t a l e x t e n s i o n o f t h e

a n te r io r s e gme nt o f wi re . I t i s impor ta n t tha t the

c rown of the m ola r i s f r ee to move d i s ta l ly a s i ts

a x i a l i n c l i n a t i o n i m p r o v e s . T h e s e t i p - b a c k

s p r i n g s a r e f a b r i c a t e d f r o m 0 . 0 1 7 x 0 . 0 2 5

T M A w i r e a n d 7 0 g o f i n t r u s i v e f o r c e c a n

b e a p p l i e d o n t h e r i g h t a n d l e f t s i d e s o f t h e

a n t e r i o r s e g m e n t o f w i r e . T h i s m e c h a n i s m

d e l i v e r s a m o m e n t o f a p p r o x i m a t e l y 2 1 0 0

g / m m i f t h e d i s t a n c e b e t w e e n t h e a n t e r i o r a n d

p o s t e r i o r t e e t h i s 3 0 m m . S i m u l t a n e o u s l y , a

t r a n s p a l a t a l o r h o r s e s h o e a r c h m a d e o f 0 . 0 3 2

0 . 032 T M A or 0 . 030 s ta in le s s s te e l i s use d to

d e l iv e r e q u a l a n d o p p o s i t e m o m e n t s t o e a c h s id e

of the a rc h .

T h e r e s u l t i n g f o r c e s y s t e m i n c l u d e s a n

i n c r e a s e d t i p - b a c k m o m e n t o n t h e s i d e w h i c h

ne e ds the mo la r a x ia l inc l ina t ion c or re c t ion (F ig .

1 l a ) . T h e m o l a r o n t h e o t h e r s id e o f t h e a r c h

e xpe r ie nc e s a t ip - fo rwa rd mome nt de l ive re d by

t h e p a l a t a l a r c h a n d i s c o u n t e r a c t e d b y t h e

t i p - b a c k m o m e n t d e l i v e r e d b y t h e t i p - b a c k

spr ings . T he se two mome nts shou ld c a nc e l e a c h

o t h e r , r e s u l t i n g i n m a i n t e n a n c e o f a n i d e a l

o c c l u s i o n o n t h i s s i d e o f t h e a r c h . E q u a l

in t rus ive fo rc e s a re a pp l ie d on e a c h s ide o f the

a n t e r i o r s e g m e n t o f t e e t h a n d i n t r u s i o n o f t h e

a n t e r i o r t e e t h i s o b t a i n e d , t h u s h e l p i n g t h e

ove rb i te c or re c t ion (F ig . 1 b ) . T ip -ba c k o f the

m o l a r i s o b t a i n e d r a p i d l y . T h e a m o u n t o f

e x t rus ive fo rc e e xpe r ie nc e d by the mola r s m a y be

c oun te ra c te d by the fo rc e s o f oc c lus ion . Fur the r

c o n t r o l o f t h e v e r t i c a l d i m e n s i o n d u r i n g t h i s

pha se o f t r e a tme nt c a n be a c h ie ve d us ing a c h in

c a p wi th a s t r a igh t ve r t i c a l pu l l . T he c h in c a p i s

w o r n a t n i g h t a n d w i l l h e l p c o n t r o l t h e m o l a r

e x t r u s i o n r e s u l t i n g f r o m t i p - b a c k m e c h a n i c s .

Onc e the a x ia l inc l ina t ion o f the r igh t a nd le f t

m o l a r s a r e s y m m e t r i c , c o n t r o l o f f u r t h e r t i p -

ba c k o f the se te e th a nd o f the ve r t i c a l d im e ns ion

d u r i n g i n t r u s i o n m a y b e a c h i e v e d b y u s i n g a

h igh-pu l l he a d ge a r w i th a pu ll d i r e c te d a bove the

c e n t re o f r e s i s ta nc e o f the mola r s .

l inical applications

T he f i r s t ma jor s te p o f the t r e a tm e nt c or re c t s the

a x ia l inc l ina t ion o f the t ippe d mola r to a c h ie ve

be t te r symme try o f the buc c a l oc c lus ion a nd to

crea te space pos te r ior ly . Be tbre this is a t tempted,

g o o d a l i g n m e n t o f t h e f o u r m a x i l l a r y i n ci s o rs i s

a c h ie v e d a n d a r i g id a n t e r i o r s e g m e n t o f w i re is

p l a c e d i n t o t h e b r a c k e t s o f t h e f o u r m a x i l l a r y

i n c i s o r s w i t h a b i l a t e r a l d i s t a l e x t e n s i o n .

T ip-ba c k sp r ings a re a c t iva te d e qua l ly on bo th

s i d e s a n d b i l a t e r a l t i p - b a c k m e c h a n i c s a r e

in i t i a te d . Ca re fu l ide n t i f i c a t ion o f the po s i t io n

of the c e n t re o f r e s i s ta nc e o f the four ma xi l l a ry

inc i so r s i s unde r ta ke n on a ' l a te ra l c e pha lome t r ic

he a d f ilm . T he in t rus ive fo rc e i s a pp l ie d th ro ug h

t h e c e n t r e o f r e s i s t a n c e o f t h e f o u r m a x i l l a r y

inc i so r s thus in t rud ing the m. S imul ta ne ous ly , a

pa la ta l a rc h i s p la c e d wi th a t ip -ba c k / t ip - fo rwa rd


8/9

1 0 0 B . S H R O F F E T A L .

a

I

F i g u r e 1 1 a ) F o r c e s y s t e m r e s u l t i n g f r o m t h e s i m u l t a n e o u s

a p p l i c a t i o n o f b i l a t e r a l t i p - b a c k s g r e y a r r o w s ) u s i n g a

t h r e e - p i e c e b a s e a r c h a n d t h e a p p l i c a t i o n o f a u n i l a t e r a l

t i p - b a c k b l a c k a r r o w s ) u s i n g a t r a n s p a l a t a l a r c h . b ) T h e

a n t e r i o r s e g m e n t o f t e e t h i s i n t r u d e d .

a c t i v a t i o n t o c o r r e c t t h e a s y m m e t r i c s i d e a n d

ho ld t he oppos i t e m o la r i n it s co r rec t pos i t ion . A

t ip -back bend i s p l aced a t t he j un c tu r e be tween

t h e v e r t i c a l a n d t h e h o r i z o n t a l p a r t s o f t h e

pa l a t a l a rc h , an d t he en t i re pa l a t a l a rch i s tw i s t ed

b y i n c o r p o r a t i n g a b e n d a t i t s a p e x . T h i s

p r e v e n t s a n u n d e s i r a b l e m e s i a l f o r c e o n t h e

t i pped m o la r . F o r a ho rseshoe a rch , t he t i p -back

a c t i v a t i o n i s p l a c e d b y i n t r o d u c i n g a s m o o t h

c u r v a t u r e i n t h e a r c h B u r s t o n e , 1 9 89 ). N o

bucca l segm en t s o f w i re a re p l aced t o a l l ow the

p r e m o l a r s a n d c a n i n e t o d r i ft b a c k a s t h e m o l a r

a x i a l i n c l i n a t i o n i s c o r r e c t i n g F i g s 1 2 a ,b a n d

13a ,b ) . T ranssep t a l f i b re s a re pa r t l y re spons ib le

f o r t h e d i s t a l m i g r a t i o n o f t h e p r e m o I a r s a n d

canine . A f igure-of-e ight l iga ture wire may a lso

b e e x te n d e d f r o m t h e m o l a r t o t h e p r e m o l a r s a n d

igure 1 2 P r e t r e a t me n t v ie ws o f t h e o c c l u s i o n o n mo d e l s. a

and b ) R igh t and l e f t bucca l occ lus ions . The r igh t and l e f t

b u c c a l o c c l u s i o n s a r e a s y mm e t r i c b e c a u s e o f a d i f f e r e n c e i n

the mes io - -d i s t a l ax ia l inc l ina t ion o f the max i l l a ry pe r rnanen t

f i rs t mo l a r s . T h e l e ft p e r ma n e n t f i rs t mo l a r i s t i p p e d f o r wa r d .

c a n i n e t o e n c o u r a g e t h e i r d i s t a l d r i f t . D i s t o -

p a l a t a l r o t a t i o n o f t h e p r e m o l a r s a n d c a n i n e

m a y b e p r e v e n t e d b y s i m u l t a n e o u s l y u s i n g a

f igu re -o f -e igh t l i ga tu re on t he pa l a t a l a spec t o f

these t ee th , In t ru s ion o f t he m ax i l l a ry i nc iso rs is

a s i de -e f fec t o f t i p -back m echan ics u sed advan -

tageously to correct the deep overbi te .

M o n i t o r i n g o f p r o g r e s s i s c a r r i e d o u t a t

regu l a r in t e rva l s eve ry 4 weeks) and onc e t he

des i red co r rec t i on i s ach ieved the pa l a t a l a rch i s

m ad e pass ive to t he new m ola r pos i t i on .

U n i l a t e r a l t i p - b a c k m e c h a n i c s a r e p r i m a r i l y

u s e d t o c o r r e c t m o l a r m e s i o - d i s t a l a x i a l

i n c l i n a t i o n i n c a s e s o f d e n t a l a s y m m e t r i e s a s

described in th is paper . However, in some cases

of skele ta l asymmetries involv ing smal l skele ta l

d i s c r e p a n c i e s , u n i l a t e r a l t i p - b a c k m e c h a n i c s

m a y b e s u c c e s s f u l l y u s e d t o p r o d u c e d e n t a l

com pensa t ions .


9/9

TR EA TMEN T WI TH TI P B A C K MO MEN TS 1 1

Figure13 Intra ora l views after correction of the molar axial

inclina tion on the left side. a and b) Right and left buccal

occlusions.

Conclus ion

T h e c o r r e c t i o n o f d e n t a l a s y m m e t r y is a

f u n d a m e n t a l c o m p o n e n t o f a c h i e v i n g a n

o p t i m u m t r e a t m e n t re s ul t. D e n t a l a s y m m e t r i e s

r e s u lt i n g f r o m a n a l t e r e d m e s i o - d i s t a l m o l a r

a x i a l i n c l i n a t i o n c a n b e s u c c e ss f u ll y c o r r e c t e d

o r t h o d o n t i c a l l y w i t h o u t s id e -e f fe c ts . U n d e s i r a b l e

s i d e -e f f ec t s r e s u l t i n g f r o m c o m m o n l y u s e d

m e c h a n i c s i n c l u d e c a n t i n g o f t h e o c c l u s a l p l a n e

a n d i n c i s o r f l a r i n g . T h e p r i n c i p l e s o f t h e s e g -

m e n t e d a r c h t e c h n i q u e a r e h e l p fu l fo r d e s i g n i n g

a n a p p l i a n c e t h a t w i ll d e l iv e r a c o n t r o l l e d a n d

s t a t i c a ll y d e t e r m i n a t e f o rc e s y s t e m .

A m e t h o d h a s b e e n p r e s e n t e d fo r u n i l a t e r a l l y

t i p p i n g a m o l a r d i s t a l l y t h a t r e l ie s o n t i p - b a c k

m o m e n t s a n d n o t a d i s ta l fo rc e. By c o m b i n i n g a

l i n g u a l a r c h a n d t i p - b a c k s p ri n g s, t h e m o l a r t i ps

b a c k o n t h e d e s i r e d s i d e, th e o c c l u s i o n o n t h e

o p p o s i t e s i d e i s m a i n t a i n e d a n d r e c i p r o c a l l y

t h e i n c i s o r s a r e i n t r u d e d .

ddre ss for correspondence

D r B h a v n a S h r o f f

D e p a r t m e n t o f O r t h o d o n t i c s

U n i v e r s i t y o f M a r y l a n d D e n t a l S c h o o l

6 6 6 W . B a l t i m o r e S t r e e t

B a l t i m o r e , M D 2 12 01

U S A

c kno w l e dg e m e nt s

T h a n k s a r e e x te n d e d to M s J o - A n n W a l k e r a n d

M s B a r b a r a V a l l o n g a fo r p r e p a r i n g t h e m a n u -

s c r i p t a n d t o M s C a r o l W i l k i n s f o r th e

c o m p u t e r - g e n e r a t e d g r a p h i c s .

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