3.- Fuerzas Balanceadas_2

8/20/2019 3.- Fuerzas Balanceadas_2

1/9

0099-2399/85/1105-0203/ 02.00/0

JOURNAL OF ENDODONTICS

Copyright 9 1985 by The American Association of Endodontists

Printed in U ,S,A.

VOL. 11, NO. 5, MAY 1985

Th e Ba lanced Force Co nce pt for Inst rum entation o f

Curved Cana ls

E l Con cepto de Fue rza Ba lancead a para la

Instrumentacion de Conductos Curvos

J a m e s B . R o a n e , B S , D D S , M S , C l y d e L . S a b a l a , B S , D D S , a n d M a n v i l le G . D u n c a n s o n , J r . , D D S , P hD

C a n a l c u r v a t u r e h a s a lw a y s i n t ro d u c e d c o m p l e x i ty

i n to c a n a l p r ep a r a t io n . T h e b a l a n c e d f o r c e c o n -

c e p t , d e v e l o p e d b y t ri al a n d e r r o r e x p e r i m e n t a t io n

o v e r t h e p a s t 1 2 y r, i s p r o p o s e d a s a m e a n s o f

o v e r c o m i n g t h e c u r v a t u r e i n f lu e n c e . I ts c o n c e p t s

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

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

R o t a t io n i s p r o m o t e d a s t h e m e a n s f o r m a i n t a i n in g

m a g n i t u d e a s a c o n t ro l a n d c o u n t e r c l o c k w i s e d i r e c -

t i o n o f r o t a t i o n p r o v i d e s f i n i t e o p e r a t o r c o n t r o l . D i a -

g r a m m a t i c e v a lu a t io n s , m a t h e m a t i c a l c a l c u la t io n s ,

b e n d i n g m o m e n t s , t e s t c a n a ls , s e c t i o n e d t e e th , a n d

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

e a c h s t e p o f t h e c o n c e p t . T h e c o n c e p t c o m e s t o

f r u it io n w i t h t h e i n t r o d u c t i o n o f a n e w K - t y p e f il e

d e s i g n .

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

c o m p l e j id a d e n l a p r e p a r a c i o n d e l m i s m o . E l c o n -

c e p t o d e f u e r z a b a l a n c e a d a d e s a r r o l la d o a t r a v e s

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

u l ti m o s 1 2 a ~ o s , s e p r o p o n e c o m o u n m e d i o d e

s u p e r a r l a i n f lu e n c i a d e l a c u r v a t u r a . S u s c o n c e p t o s

u s a n m a g n i t u d e s d e f u e r z a a f in d e I o g r a r el co n t r o l

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

c o n d u c t o . S e p r o m u e v e l a ro t ac ib n c o m o m e d i o d e

m a n t e n e r l a m a g n i t u d c o m o c o n t r o l , y la d ir e c c io n

d e r o t a c i 6 n e n s e n t i d o c o n t r a r i o a la s a g u j a s d e l

r e lo j p e r m i t e u n d e f i n i d o c o n t r o l a l o p e r a d o r . S e

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

m a t e m a t i c o s , m o m e n t o s d e t o r s i6 n , p r u e b a s d e

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

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

s e v e c o m p l e t a d o c o n l a i n t r o d u c c i 6 n d e u n n u e v o

d i s e ~ o d e l i m a s ti p o K .

R oo t c u r v a t u r e i s a fr eque n t oc c u r r enc e i n t he hum a n

den t i t i on and w hen a c u r v a t u r e i s p r es en t , endodon t i c

2 3

p r epa r a t i on o f t he as s oc i a t ed pu l p c ana l s y s t em be -

com es d if fi cu l t. In fac t , curva ture in t rod uce s such com -

p l ex i t y t ha t t o t a l i n s t r um en t i ng c onc ep t s hav e been

dev e l oped t o dea l w i t h t he c u r v ed c ana l ( 1 - 5 ) . Ev en

w i t h t hes e , i t i s c om m on p l ac e f o r a c ana l l o c a t ed w i t h in

a c u r v ed r oo t t o be en l a r ged t o a s m a l le r fi na l d i am e t e r

than i t wou ld be i f i t were loca ted w i th in a s t ra igh t roo t .

Jus t i f i ca t ion fo r such a l te ra t ion i s s imp ly the fac t tha t a

c u r v a t u r e i n t r oduc es f ac t o r s i n t o p r epa r a t i on w h i c h , i f

no t c on t r o l l ed du r i ng en l a r gem en t , w i l l c aus e t r ans po r -

t a t ion , l edge f o r m a t i on , an d ev en pe r f o r a ti on ( 1 - 4 , 6 ,

7 ) . The m o r e s e v e r e t he c u r v a t u r e o f t he r oo t , the m o r e

one t ends t o r educ e t he i n t ended p r epa r a t i on d i am e t e r

i n an e f f o r t t o p r ev en t i rr ev e r s ib l e dam age o f t he c ana l

w a l l. R ed uc i ng t he p r epa r a t i on s i z e unde r s uc h c i r c um -

s t anc es appea r s t o be l og i c a l f o r t w o r eas ons : ( a )

s m a l l e r d i am e t e r p r epa r a t i on m eans l es s c u t t i ng o f t he

cana l wa l l s and consequent l y a lesser l i ke l ihood fo r

ex p r es s i on o f undes i r ab l e c u t t ing e f f ec t s and ( b ) s m a l l

d iameter f i l es a re more f l ex ib le and there fo re less l i ke ly

t o c aus e t r ans po r t a t i on du r i ng en l a r gem en t .

The c u r v a t u r e p r ob l em ap pea r s t o be s o l v ed by s m a l l

p r epa r a t i on d i am e t e r s un t i l one ex am i nes w ha t t ha t

so lu t ion sacr i f i ces in cana l debr idement and re l i ab i l i t y

o f t he f ina l s ea l . Sm a l l e r p r epa r a t i on d i am e t e r s r educ e

t he am oun t o f m ec han i c a l and c hem i c a l c l eans i ng o f

t he c ana l s pac e . Se v e r a l s t ud i es appe a r t o i nd ic a t e l es s

c om p l e t e r em ov a l o f deb r i s w h en s m a l l non f la r ed p r ep -

a r a t i ons a r e us ed ( 8 - 10 ) w h i l e o t he r s r e l a t e t o t he

f lush ing e f fec t o f i r r i gan ts and ind ica te tha t i r r i gan ts a re

no t e f f ec t i v e i n w as h i ng deb r i s f r om a s m a l l d i am e t e r

cana l (11-13) . F ina l l y , A l l i son e t a l . (14) ind ica te tha t

p r epa r a t i on s i z e / des i gn has an i n fl uenc e up on t he f ina l

s ea l. They f ound t ha t t he bes t s ea l w a s ac h i ev ed w hen

a s t e p b a c k p r e p a r a t io n w a s u s e d .

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

t he c ana l s s o f t t i s s ue c on t en t s as c om p l e t e l y as pos -

s ib le , e l im ina te as comp le te ly as phys ica l l y poss ib le any

m i c r ob i a l e l em en t s , and c r ea t e a s i t ua t i on w i t h i n t he


2/9

2 0 4 R o a n e e t a l

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

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

s u p p o r t i v e s t ru c t u r e s . T o r o u t in e l y a n d d e p e n d a b l y a c -

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

s e e m s r e a s o n a b le to d e m a n d t h e s a m e c o m p l e t e n e s s

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

i s s tr a i g h t o r c u r v e d . Va r i a t io n s i n th e s i z e o f p r e p a r a -

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

r a t h e r t h a n t h e d e g r e e o f r o o t c u r v a t u r e . I f s u c h w e r e

p o s s i b l e , s p e c i f i c a l l y d e f i n e d s t a n d a r d i z e d c a n a l p r e p -

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

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

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

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

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

s i ze ra ther than curva ture was in i t i a l l y env is ioned by

R o a n e i n 1 9 7 0 a n d , a s a r e s u l t o f 1 2 y r o f tr ia l a n d e r r o r

e x p e r i m e n t a t i o n , a p r e p a r a t i o n t e c h n i q u e h a s b e e n d e -

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

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

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

i t s va l id i t y , and to make i t s concepts ava i lab le so tha t

o t h e r s m a y a l s o e x a m i n e a n d t e s t i ts v a li dit y.

R T I O N L E

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

p h y s i c a l l a w w h i c h s t a t e s : f o r e v e r y a c t i o n t h e r e i s a n

e q u a l a n d o p p o s i t e r e a c t i o n . T o d e v e l o p t h e c o n c e p t ,

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

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

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

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

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

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

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

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

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

ing cana l p repara t ion .

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

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

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

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

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

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

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

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

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

C l i n i c a l u s a g e a n d s u b s e q u e n t p h y s i c a l a n a l y s i s i n -

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

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

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

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

s o u g h t . M o s t i m p o r t a n tl y , t h e K - t y p e d e s i g n p r o v id e s

c u t t i n g e d g e s w i t h i d e n t i c a l r a k e a n d c l e a r a n c e a n g l e s

( 1 5 ) r e g ~ d l e s s o f t h e d i r e c ti o n o f m o v e m e n t . S i n c e

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

J ou r na l o f

E n d o d o n t i c s

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

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

loss o f e f f i c iency (16) (F ig . 1 ) . B id i rec t iona l cu t t i ng

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

t io n s a v a ila b le w i t h a K - t y p e i n s t r u m e n t t h a n w o u l d b e

a v a i l a b l e i f a H e d s t r o m - t y p e i n s t r u m e n t w e r e u s e d .

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

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

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

m o m e n t o r r e s t o r i n g f o rc e , a s i t w i ll b e r e f e r re d t o

here , i s decreased.

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

t h a t i s 3 7 . 5 % l e s s th a n t h a t o f a s q u a r e f il e o f t h e s a m e

standard ized s i ze . Th is po in t i s ver i f i ed by ca lcu la t ing

t h e a re a o f tw o # 4 0 file s , a s s u m i n g o n e t o b e m a n u -

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

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

i s d e t e r m i n e d u s i n g t h e e q u a t i o n s b - - 3 R / 1 7 3 2 a n d

A = b / 4 x 1 . 7 3 2 . I n t h e s e e q u a t i o n s b r e p r e s e n t s t h e

length o f one s ide o f an equ i la te ra l t r i ang le , R the

c i rcumrad ius o r one-ha l f o f the cu t t i ng d iameter , i . e . 1 /2

o f 0 . 4 0 m m , a n d A t h e a r e a o f t h e t ri a n g le . Su b s t i tu t e

t h e k n o w n v a l u e s a n d f i n d b = 3 ( 0 . 2 ) / 1 . 7 3 2 - - 0 . 3 4 6

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

e q u a t i o n m a y b e s o l v e d f o r t h e c r o s s - s e c t i o n a r e a : A =

0 . 3 4 6 / 4 x 1 . 7 3 2 = 0 . 0 4 9 9 m m 2 . F o r a s q u a r e f ile , t h e

FiG 1. W is a l ine draw n tange nt to the c i rcumference of cut c at a

po in t o f contac t w i th one cut t ing e dge o f a t r i angu lar shaped K - type

instrument. W is us ed to descr ibe the c learanc e angles U and V into

the f l u te space assoc ia ted w i th that cu t t i ng edge. T hey are equa l and

are 60 de gree s by def ini tion. P is a tangen t l ine dra wn in a s imilar

fashion to intersect one edg e of a square K- type instrument. I t is

used to def ine the c l earance ang les Q and R whi ch are equa l and 45

degre es by definit ion. The s ides of the squa re and tr iangle are let tered

a and b, respectively , and the area s are sh ded for co mpa r ison. I t i s

easy to recognize that the square has a cross-sect ional area which

is much larger than tha t of the t riangle, i .e. 37.5 greater . The rake

angle for both the t r i ang le and squa re may be measured as the ang le

betw een the radial line d and the l ines b an d a, respect ively . B oth

shapes produce neg at i ve rake ang les ; how ever , the t r iang le en joys a

15-degree more favo rable angle. Nei ther the rake n or the c learance

angle is al tered for ei ther by the di rect ion of m ovem ent.


3/9

Vol. 11 N o. 5 Ma y 1985

e q u a t i o n s a = d/1 414 and A - - a 2 a r e r equ i r ed . I n t hes e

a r ep r es e n t s t he l eng th o f one s i de o f t he s qua r e , d t he

c u t t i ng d i am e t e r o f t he fil e o r 0 . 40 r am , and A t he a r ea

o f t he s qua r e . Subs t i t u t e t he k now n v a l ues and f i nd : a

= 0 . 4 / 1 . 41 4 = 0 . 282 m m . By us i ng t ha t f o r a , t he a r ea

equa t i on i s s o l v ed t hus l y : A = ( 0 . 282 )2 - - 0 . 07 95 m m 2 .

R es u l t s o f the p r ec ed i ng c a l c u l a t i ons r ounded t o t w o

s i gn i f i c an t num be r s i nd i c a t e t ha t w hen t he c u t t i ng r a -

d ius i s 0 .4 mm ( i .e . a 4 0 f il e ), a t r i ang le has a c ross -

s ec t i ona l a r ea o f 0 . 05 m m 2 w h i l e a s q ua r e h as an a r ea

o f 0 . 08 m m 2 . W i t h t he r ec ogn i t i on t ha t c u r v a t u r e c o u l d

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

and t ha t t he am oun t o f m e t a l , i . e . t he c r os s - s ec t i ona l

a r e a , va r ie s b e t w e e n t h e t w o s h a p e s , t h e n o n e s h a p e

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

r a t i o bas ed upon t he i r c r os s - s ec t i ona l a r eas . By us i ng

t h i s m e t hod , i t bec om es appa r en t t ha t a t r i angu l a r f i l e

h a s o n l y 6 2 . 5 % a s m u c h c r o s s - s e c t i o n a l a r e a a s a

s qua r e f i l e o f equa l c u t t i ng r ad i us and i f geom e t r y had

no a f f ec t upon bend i ng , t he t r i angu l a r f i l e w ou l d be

ex pec t ed t o dev e l op on l y 62 . 5% as l a r ge a r es i s t i ng

l oad ( r es to r i ng f o r c e ) aga i ns t t he c ana l w a l l a s a s qua r e

f i l e as s um i ng bo t h w e r e p l ac ed t h r ough t he s am e c u r -

va ture . C l in i ca l ly spe ak ing , a t r i angu la r f il e i s p red ic ted

to be m ore f lex ib le and to ap p ly a li gh te r res to r ing fo rc e

aga i ns t t he w a l l o f a c u r v ed c ana l du r i ng p r epa r a t i on .

Th i s m eans , t ha t t he t r i angu l a r g r ound i ns t r um en t w i l l

be l es s l ik e ly t o t r ans p o r t a c ana l w a l l du r i ng p r epa r a -

t ion . I ts K - t y pe b i d ir ec t iona l de s i gn a l l ow s t he o pe r a t o r

t o s e l ec t any o f f ou r po t en t i a l c u t t i ng app r oac hes , i . e .

i n s e r ti on , w i thd r aw a l , c l oc k w i s e , o r c oun t e r c l oc k w i s e

r o t a t ion . F i gu r e 1 p r ov i des a g r aph i c c om pa r i s on be -

t w een t he t r i angu l a r and s qua r e , i . e . c u t t i ng edge c on -

f igura t ion , rake , and c learance ang les .

By r ec ogn i z i ng t ha t geom e t r y has an i n f l uenc e upon

s t ruc tu ra l p roper t ies o f mater ia l s , i t i s des i rab le to es -

t ab l i s h a r e l a t i ons h i p be t w een t he r es t o r i ng f o r c e and

t he c r os s - s ec t i ona l geom e t r y r a t he r t han on l y t he a r ea

as j us t p r ev i ous ly c o ns i de r ed . To do t h i s it m us t be

r ec ogn i z ed t ha t t he r es t o r i ng f o r c e m ay be m eas u r ed

t h r ough i ts oppos i ng f o r ce , t he angu l a r de f l ec ti on be nd -

i ng m om en t . Va l ues hav e been ex pe r i m en t a l l y de t e r -

I m i n e d fo r b e n d i n g m o m e n t s b e t w e e n v a r io u s g e o m e t r ic

s hap es and w e r e r ec en t l y r epo r t ed by K r upp e t a l. (17 ).

I n t he i r s t udy , t he am oun t o f f o r c e r equ i r ed t o p r oduc e

a 60 - deg r ee angu l a r de f l ec t i on o f a t r i angu l a r s i z e 30

U n i t ek f i l e w as one - t h i r d l e s s t han t ha t r equ i r ed t o

p r oduc e an equa l de f l ec ti on o f a s q ua r e s i z e 25 U n i t ek

f i l e . The i r g r aphs i nd i c a t ed t ha t a bend i ng m om er ) t

s l i gh t l y g r ea t e r t han 40 g - c m de f l ec t ed t he s i z e 30

t r i angu l a r f i l e 60 deg r ees w h i l e a bend i ng m om en t

s l i gh t l y g r ea t e r t han 60 g - c m w as r equ i r ed t o de f l ec t

t he s i z e 25 s qua r e f i l e equa l l y . A dec r eas e i n t he

r es t o r i ng f o r c e r epo r t ed as bend i ng m om en t oc c u r r ed

in sp i te o f a g rea ter cu t t i ng rad ius , i .e . la rger ins t rum ent

s ize o f the t r i angu la r f i l e . The square f i l e i n th i s case

Balanced Force Concept 205

has 83% as l a r ge a c r os s - s ec t i ona l a r ea as t ha t o f t he

c om pa r ab l e t r i angu l a r f i l e y e t i t gene r a t es abou t one

and one-ha l f t imes as la rge o f a res to r ing fo rce , i .e .

b e n d i n g m o m e n t o f 6 0 v e r s u s 4 0 g - c m . B y r e c o g n i zi n g

t ha t t he r es t o r i ng f o r c e i s a s t a t ic a l ly ap p l ied l oad w h i c h

ho l ds t he f il e s c u t t ing s u r f ac es a ga i ns t t he c u r v a t u r e i n

a s ing le d i rec t ion and tha t i t i s the load wh ich i s re -

s pons i b l e f o r a s t r a i gh t en i ng o f t he c u r v e du r i ng

p r epa r a t i on , one c an p r ed i c t t ha t a s qua r e f i l e i s m o r e

l i ke ly to uncont ro l l ab ly s t ra igh ten the cana l , to cu t ex -

cess ive ly a t i t s t i p , and there fo re i s more l i ke ly to ledge

a p r epa r a t ion . The d i f fe r enc e in r es t o r i ng f o r c e m agn i -

t ude c oup l ed w i t h ou r un r epo r t ed c l i n i c a l and ex pe r i -

m en t a l obs e r v a t i ons l ead us t o c onc l ude t ha t a s qua r e

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

pec ia ll y w hen us i ng l a r ge i ns t r um en t s i z es .

The nex t s t ep i n unde r s t and i ng how t o m a i n t a i n

p r epa r a t i on s i z e p as t a c u r v e i s l ea r n ing t o i den t if y t he

d i r ec t i on and l oc a t i on f o r ex p r es s i on o f t he r es t o r i ng

f o r c e w h i c h i s gene r a t ed b y e l as ti c d i s to r t i on w i t h i n an

i ns t r um en t w h en i t i s pas s ed t h r ough a c ana l c u r v a t u r e .

I den t i f i c a t i on a l l ow s r ec ogn i t i on o f w he r e t he l oads

c onc e n t r a t e and p r ov i des i n f o r m a t i on us e f u l in l ea rn i ng

how t o p r ev en t e f f ec t i v e ex p r es s i on o f t hos e l oads i n

undes i r eab l e a r eas . I t i s i m po r t an t t o l ea r n t o c on t r o l

t hes e f o r c es in o r de r t o p r ev en t t hem f r om bec om i ng a

m a j o r i n f l uenc e du r i ng c ana l en l a r gem en t as t he i r

e x p r e s s io n c a n p r o d u c e d i s a s t ro u s r e s u lt s . T o a c c o m -

p li sh c on t r o l i t is nec es s a r y to c o m p a r e t he m ag n i t ude

o f l oads , app l y t he r u l e t ha t f o r ev e r y ac t i on t he r e i s an

equa l and op pos i t e r eac t i on , and f i na ll y i den t if y m o t i ons

w h i c h w i l l d i r ec t t he f o r c es app l i ed by t he ope r a t o r i n

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

c u r v a t u r e . W hen s uc h a ba l anc e i s es t ab l i s hed , t he

cana l may be en la rged th rough i t s o r ig ina l ax is , a t l eas t

in the ap ica l th i rd o f the cana l . In the process t ranspor -

ta t ion i s e l im ina ted o r a t l eas t d i sp laced coronaUy whe re

i t i s l e s s l i k e l y t o hav e s e r i ous c ons equenc es . The

des i r ed ba l anc e o f f o r c es c an be gene r a t ed s i m p l y by

r o t a t ing t he i ns t r um e n t s t o p r od uc e c ana l en l a r gem en t .

Th i s i s t r ue s i nc e r o t a t i on d i r ec t s den t i na l ha r dnes s

aga i ns t t he r es t o r i ng f o r c e o f t he c u r v ed i ns t r um en t

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

c u t t ing l oads . R o t a t i on p r ev en t s ex p r es s i on o f t he c u r -

v a t u r e - gene r a t ed r es t o r i ng f o r c e v ia m a gn i t ude , a t l eas t

fo r a l im i ted range o f f i l e s i zes . F igure 2 i l l us t ra tes the

ba l anc e o f fo r c es gene r a t ed by r o t a t ion . T he r e la t iv e

m agn i tudes o f t he f o r c e gene r a t ed f r om den t i na l r es i s t-

anc e and t he r es t o r i ng f o r c e gene r a t ed i n t he i ns t r u -

m en t by a c u r v a t u r e a r e no t de fi ned ; how ev e r , pe r f o r m -

anc e t es t s dem ons t r a t e t he v a l i d i t y o f t he as s um p t i on

t ha t t he r es t o r i ng f o r c e i s o f l e s s m agn i t ude t han t he

forces c rea ted f rom dent ina l res is tance (F igs . 3 and 4) .

The m agn i t ude o f den t i na l r es i s t anc e i s a f unc t i on o f

den t i na l ha r dnes s and gene r a t es a f o r c e du r i ng p r epa -

ra t ion wh ich remains re la t i ve ly cons tan t fo r each cana l .


4/9

2 0 6 R o a n e e t a l

FIG 2. T he e n l o s i n g i r l e representsdentinas i t contacts he cutt ing

edges of a f i le. R represents he internal force appl ied by den tinal

hardnessas i t is vectored oward the center of the instrument.S is a

restoring orce appl ied against he curva ture by the f i le attempting o

return to i ts original straight condi t ion. S remains stationary as the

instrument s rotated whi le R rotates wi th the blades. As long as R

remains greater than S, the instrumentwil l not tran spo rt the cana l;

however, should he f i le be pul ledout rather han rotated his formula

fai ls and S appl ies the primary cutting load . Transportation wi l l

frequently occur under such condi t ions.

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

w i t h i n a f i le i s a fu n c t i o n o f t h e f i l e s m a s s , g e o m e t r y ,

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

i n s t r u m e n t a t i o n c u r v e . I t s e x p r e s s i o n i s in v e r s e l y r e -

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

t ip . C o n s e q u e n t l y , t h e r e s t o r i n g f o r c e i s a v a r i a b le f o r c e

a n d i t w i l l in c r e a s e : ( a ) i f t h e m e t a l m a s s i n c r e a s e s ,

e i t h e r a s a r e s u l t o f s h a p e o r i n s t r u m e n t d i a m e t e r ; ( b )

i f t h e r a d i u s o f t h e c a n a l c u r v a t u r e i s d e c r e a s e d ; ( c ) i f

t h e a r c o f t h e c a n a l c u r v a t u r e i s in c r e a s e d ; o r ( d ) if t h e

d i s t a n c e f r o m t h e c u r v e t o t h e f il e t i p is d e c r e a s e d . B y

u s i n g t h e s e r e l a t io n s h i p s , o n e c a n i d e n t if y s p e c i f ic c a n a l

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

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

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

f o r a t i o n s . I n i ti a ll y , t h e r a d i u s o f t h e c u r v e s h o u l d b e

i n c r e a s e d b y c r e a t i n g a c a n a l a c c e s s ( 1 , 4 , 5 ) , i . e .

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

( F i g . 5 ) . T h i s e f f e c t i v e l y i n c r e a s e s t h e r a d i u s a n d d e -

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

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

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

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

d u c e l e s s d e n t in r e m o v a l a n d t h e c a n a l is e n la r g e d

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

m e n t s . A s a c o n s e q u e n c e , l a r g e r d i a m e t e r i n s t r u m e n t s

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

r a t i o n a r e l i k e l y . T o a v o i d a p i c a l e x p r e s s i o n , p r i m a r i l y

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

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

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

J o u r n a l o f E n d o d o n t i c s

O v e r e x t e n s i o n i s g e n e r a l l y c o n s i d e r e d t o b e u n d e s i r a -

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

a g e d b y m o d i f ic a t i o n o f t h e i n s t r u m e n t t i p in o r d e r t o

g a i n b a l a n c e t h r o u g h t h e r e m o v a l o f th e t e r m i n a l c u t t i n g

p o i n t s .

M o d i f i c a t io n o f t h e i n s t r u m e n t s t i p i s a r e c e n t i n n o -

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

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

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

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

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

f o r c a n a l t r a n s p o r t a t i o n . P r o p e r r e m o v a l o f t h e s e c u t -

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

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

p r e p a r a t i o n ( F i g . 5 ) . T h e m o d i f i e d i n s t r u m e n t i s n o t

p r e s e n t l y a v a i la b l e o n a c o m m e r c i a l s c a le . A p h o t o -

g r a p h o f t h e m o d i f i c a t i o n i s p r e s e n t e d i n F i g . 6 a l o n g

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

p o i n t s . T h e s e p o i n t s c u t i n r e s p o n s e t o a r e s t o r i n g

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

FIG 3. Th is electron micrograph of a prepared canal n a mandibular

incisor reveals ingual movem ent of the preparation. The access n

this ooth appliedpressure o the f i le shaft which should havecaused

movem ent facial ly. The obse rvationof a tape red preparation o th e

l ingual supports the balance of force magn i tude concept. M issing

dentin o the l ingualunbalanceddentinal esistance o that side and

al lowed the f i le to be pus hed away from the F wal l m asking enti rely

th e expressionof any restoring o rce along he F wa l l.


5/9

Vol. 11 No. 5 May 1985 Balanced Force Concept 207

FiG 4. P resented are four cases that were prepared us ing the ba lanced force concept . The m in imum ap i ca l p repara t i on d iameter was 0 .45 m m.

The secon d m olar in A w as com ple ted us ing s tandard fi les and t i p overex tens ion whereas the o ther th ree cases were com ple ted us ing m odi f ied

instruments.

6 4 TE $

L I D D EN

2 h N h L

4 CCE $ $

FtG 5. Th i s graph i c representa t ion A and ac tua l molar cana l access B i ll us t ra te how an a ccess preparat i on a l te rs the curvature s e f fec t upon

enlarging instruments. B an d C are the areas of dent in rem oval which a re responsible for ang ular change. The cha nge in instrum ent entry angle

before and a f ter canal access is i l lus trated as the angle a and repres ents a reduct ion in the arc of the canal s curvature. Spac e created by the

access a l l ows the canal curvature to be expresse d in the i ns t rument as though the rad ius o f the curve had been i nc reased. The curve becom es

more genera l i zed and d i s t r i bu tes throughout th e cana l l ength , thereby de c reas ing the curvature- re l a ted orces and the i r express ion.


6/9

2 0 8 R o a n e e t a l


i s ev ident t ha t use o f t he ba lanced fo rce concepts ,

especial ly with mod if ied instruments, ena bles one to

prepare a curved acry l ic cana l much la rger than c lass i-

ca l t echn iques , w i tho ut p roduc ing ap ica l t ranspo r ta t ion .

Fi le t ip modif icat ion and u se of the balanced force

F IG 6 . P r e s e n t e d a r e t w o t r ia n g u l a r K - t y p e f il e s . File A h a s a s t a n d a r d

t i p w i t h d is t i n c t p o i n t s c r e a t e d w h e n i t s c u t ti n g e d g e s a r e t e r m i n a t e d

w i t h a s t a n d a r d i z e d t i p .

File B

i s a p r o t o t y p e m o d i f ie d i n s t r u m e n t w i t h

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

p r o d u c e s s u p p o r t i n g t r ia n g u l a r p la n e s w h i c h d i s t r i b u t e lo a d s t o k e e p

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

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

a r e r e p la c e d b y s i x n e w e d g e s .

f rom i t s pass ive o r zero fo rce s t ra igh t l ine pos i tion (F ig .

7) . The res tor ing fo rce when present is t ransmi t ted to

the f i le t ip through i ts metal shaf t which acts as a lever

arm.

A l te r ing the ins t rument t ip removes i t s capab i l i t y t o

respond to e las tic d is to r t ion in a concent ra ted area and

t he r eby m anda t es t he ex p r es s ion o f t hos e f o rc es ov e r

the length o f each cu t t ing edge ra ther than a t t he f i le

t ip. Th us, with the terminal poin ts removed, internal

d is to r t ions es tab l ished by the cana l curva ture gene ra te

a res tor ing fo rce wh ich is p ropo r t ioned over the cu t t ing

edges and .d ispersed enough to a l low the re la t ive ly

la rger ma gn i tude o f dent ina l hardne ss to de ny not icab le

express ion . Conse quent ly , t he f i le s s t ra igh t p ro fi le be-

comes unapparent in the comple ted prepara t ion . I n t ro -

duc t ion of t ip modif ication int rodu ces an abi li ty .to en-

la rge a curved can a l even a long i t s ins ide w a l l com-

p le te ly to the apex . Th is ab i li ty does not app ear to ex is t

w i th in the methods tes ted to da te us ing s tandard

ground K- type inst ruments. (Fig. 8) . I f the resul ts seen

in F ig . 8 are comp ared w i th tho se o f Weine e t a l . (2 ), i t

s, , A

I

Z

F IG 7 . T h is h y p o t h e t i c a l c a n a l c u r v a t u r e i s s e p a r a t e d i n t o i t s c o m p o -

n e n t p a r t s .

Point A

i s t h e a x i s o f t h e cu rve , r i s t h e ra d i u s , a n d a

d e f i n e s i t s a rc . W h e n a t h e o re t i ca l f i le i s p a ss e d a ro u n d t h i s cu rve , i t

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

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

s u b s e q u e n tl y e x p r e s s e d a t t h e f il e ti p o v e r t h e / e v e r

FT

C h a n g i n g

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

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

t ip T

a w a y

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

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

c h a n g e t h e c u r v a t u r e w h i l e fi le o v e r e x t e n s i o n c a n b e u s e d t o i n c r e a s e

t h e l e ve r a rm . F i l e t i p m o d i f i ca t i o n d o e s n o t a f f e c t t h e l e ve r ; i t s i m p l y

p r e v e n t s e x p r e s s i o n o f t h e g e n e r a t e d f o r c e s . F Z i n d i c a t e s t h e f i le ' s

o r i g i n a l s t ra i g h t co n d i t i o n a n d d e t e rm i n e s t h e ze ro f o rc e l in e , i .e . n o

re s t o r i n g f o rce .

F IG 8 . P r e s e n t e d a r e t w o a c r y l ic c a n a l m o d e l s w h i c h h a v e b e e n

e n l a r g e d f r o m a n o r i g i n a l d i a m e t e r o f s i z e 2 0 t h r o u g h a f i le s i z e 5 5

u s i n g t h e b a l a n c e d f o r c e c o n c e p t . Sample A w a s p r e p a r e d w i t h

s t a n d a r d i n s t r u m e n t s u s i n g 0 . 5 - m m s t e p b a c k s e v e r y o t h e r f i l e s i z e .

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

a p i ca l l y . Sample B w a s p r e p a r e d u s i n g m o d i f i e d f i le s a n d n o s t e p b a c k .

I t is t y p i c a l o f t h e s e r i e s a s n o a p i c a l t r a n s p o r t a t i o n w a s s e e n i n t h e

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

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


7/9

Vol . 11 No. 5 May 1985

c onc ep t c an enab l e one t o eas i l y en l a r ge a c ana l f r om

a s i z e 20 t o a s i z e 55 f i l e w i t hou t r ec ogn i z ab l e t r ans -

po r t a t i on i n t he p r es enc e o f r a t he r s i gn i f i c an t c ana l

c u r v a t u r e .

T E C H N IQ U E C O N S I D E R T I O N S

To u t i l i z e t he ba l anc ed f o r c e c onc ep t s , i n s t r um en t a -

t i on s hou l d be r e f i ned i n t o p l ac em en t , c u t t i ng , and

remova l o f each f i l e us ing on ly ro ta ry mot ions . P lace-

m en t i s ac c om p l i s hed us i ng c l oc k w i s e r o t a t i on ( 18 ) and

l igh t, i nw a r d p r es s u r e . C u t t i ng i s ac c om p l i s hed us i ng

c oun t e r c l oc k w i s e r o t a t i on ( 19 ) and i nw a r d p r es s u r e

ad ju s ted to ma tch the f i le s s t re ngth , i .e . very l i gh t fo r

s m a ll i n s t r um en t s and heav y f o r v e r y l a r ge ins t r um en t s .

C l ean i ng o r deb r i s r em ov a l i s ac c om p l i s hed us i ng one

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

c l oc k w i s e r o t a t i ons . C l ean i ng i s no r m a l l y c om p l e t ed

on l y a f t e r t he des i r ed l eng t h has been r eac hed and

m a i n t a i ned w i t h c oun t e r c l oc k w i s e r o t a t i on . Ax i a l r ec i p -

r oc a t i on o r f il ing m o t i on i s us ed on l y t o p r oduc e c ana l

t r ans po r t a t i on , t o f la r e t he c o r ona l a r ea , and du r ing t he

in it ia l ope n ing o f ca lc i fi ed cana ls . Such m ot ion vo ids the

ba l anc ed f o r c e f o r m u l a t i on a nd i t s us e s hou l d be l im i t ed

t o ex t r em e l y s m a l l d i am e t e r i n s t r um en t s o r t o t ha t

po r t i on o f t he c ana l w h i c h i s c o r ona l t o t he c u r v a t u r e ,

ex c ep t w he r e t r ans po r t a t i on w i l l s e r v e t o r em ov e a

p r eex i s t i ng l edge o r s hel f. W h en unm od i f i ed i ns t r u -

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

by 0 . 25 m m w i t h eac h c hange t o t he nex t l a r ge r s i z e

i ns t r um en t i n o r de r t o p r ev en t ac c um u l a t i on o f t i p c u t s

a t a s ing le po in t . I f accumula ted , t i p cu ts w i l l c rea te a

m ec han i c a l l edge . The s t epbac k p r ec au t i on i s no t nec -

es s a r y w hen t he t i p s a r e m od i fi ed a nd f il ing ac t i on m ay

be us ed m o r e f r ee l y w i t hou t d r as t i c l o s s o f c on t r o l.

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

c l oc k w i s e c u t t i ng m ay no t be appa r en t a t f i r s t . To

unde r s t and t ha t c onc ep t , one has t o aga i n ana l y z e f i l e

des i gn and pos t u l a t e t he r eac t i ons p r oduc ed by v a r i a -

t i ons o f mot ion or d i rec t ion . The cu t t i ng edges inc l ine

dow n a K - t y pe f i l e s ha f t a t app r ox i m a t e l y 45 deg r ees

f rom the ver t i ca l ax is (F ig . 6 ) . By s tandard ized spec i f i -

c a t i on , t he c i r c um r ad i us i nc r eas es f r om t he t i p t ow a r d

t he hand l e and t he f l u t es s p ir a l c l oc k w i s e . The r es u l t is

s i m p l y t ha t a l oad de li v e red by c l oc k w i s e r o t a t ion pu l ls

a w a y f r o m t h e o p e r a t o r a n d m o v e s t h e i n s tr u m e n t

ap i c a l l y , w h i l e a l oad de l i v e r ed by c oun t e r c l oc k w i s e

r o t a t ion pu s he s t ow a r d t he ope r a t o r and m ov es t he fi le

ou t o f t he c ana l (F ig . 9 ) . As a c ons equ enc e o f t h is , t he

ope r a t o r m ay s ens e t he t o t a l o f a l l f o r c es du r i ng c oun -

t e r c l oc k w i s e r o t a t i on , t o r que p l us ou t w a r d m ov em en t ,

w h i l e he o r s he s ens es on l y t he t o r que po r t i on o f a

c l oc k w i s e r o t a t i on . The unno t ed i nw a r d m ov em en t i n

t ha t c as e p l ac es t he i ns t r um en t f u r t he r i n t o t he c ana l

and ex c es s i v e l y em beds i t s c u t t i ng edges i n t o den t i n .

T h e d e s c r i b e d m o v e m e n t s a r e a c o n s e q u e n c e o f v e c -

t o r ed f o r c es c r ea t ed w hen t he app l i ed t o r que i n t e r ac t s

p o w /

a l a n c e d F o r c e C o n c e p t

209

MOVEMENT

FIG 9. This i l lustration indicates the reactions which occur along the

blade inc l ines of a K - type f i le dur ing c lockwise and cou nterc loc kwise

rotat ion. Dent in s tr ikes the inc line opp osi te the rotat ing force deb r is

is ref lected to the de nt in s ide whi le a por t ion of the torqu e is vectore d

causing the i ns t rument to move i n to the cana l when the torque i s

c l ockwise and out o f the cana l when the torque i s counterc l ockwise.

Inward movement is labeled power and is used to place the instru-

me nt into canals . Outw ard m ovem ent is labeled control . I t is used to

i nc remental ly d i sengage the cut t ing ed ges and may be opposed by

the ope rato r pushing inw ardly to pro duce a f ini te c ontrol over the

cutting force.

wi th the c ana l wa l l a long the b lade inc lines. The resu l t -

an t fo r c es m ov e t he i ns t rum en t i n to t he c ana l w hen t he

t o r que i s app l i ed c l oc k w i s e and as t he i ns t r um en t

m ov es i nw a r d l y i t s s t anda r d i z ed t ape r f o r c es t he c u t -

t ing b l ades de epe r i n t o t he c ana l w a l l s . In o t he r w o r ds ,

a t a g iven po in t w i th in a cana l , the cu t t i ng rad ius

i nc r eas es as t he f i l e m ov es i nw a r d l y . C onv e r s e l y t ha t

r ad i us dec r eas es as t he f i l e m ov es ou t w a r d l y . I nw a r d

m ov em en t i s a r es u l t o f c l oc k w i s e i ns t r um en t r o t a t i on

w h i l e ou t w a r d m ov em en t i s a r es u l t o f c oun t e r c l oc k -

w i s e i ns t r um en t r o t a t i on ( F ig . 9 ). H enc e , no t on l y does

t he ope r a t o r s ens e t he f u l l l oad w h i l e r o t a t i ng t he

i ns t r um en t c oun t e r c l oc k w i s e bu t a l oad o f den t i n t oo

g r ea t t o s hea r w i ll c aus e t he f il e t o m ov e ou t w a r d l y and

t ha t m ov em en t w i l l s i m u l t aneous l y dec r eas e t he dep t h

o f c u t t i ng edge pene t r a t i on i n t o t he den t i n . The de -

c r eas e i n pene t r a t i on dep t h c on t i nues un t i l t he ope r a -

t o r s app l ied i nw a r d p r es s u r e e x c eed s t he s hea r

s t r eng t h o f t he t o t a l engaged den t i n and a c u t r es u l t s .

Th is re la t ionsh ip f i n i te l y ad jus ts app l ied fo rce aga ins t


8/9

2 1 R o a n e e t a l

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

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

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

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

c o u n t e r c l o c k w i s e r o ta t io n p a s t 1 2 0 d e g r e e s , o n c e d e n -

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

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

b e g i n n i n g p o s i t i o n o f a n o t h e r b y t h a t p o i n t . F u r t h e r

c o u n t e r c l o c k w i s e r o t a t io n h e l p s t o e n s u r e f ul l d i a m e t e r

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

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

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

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

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

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

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

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

r o t a t io n . W h e n e n l a r g e m e n t h a s b e e n a c c o m p l i s h e d , a

f i n a l c l o c k w i s e c l e a n i n g r o t a t i o n i s u s e d t o l o a d c a n a l

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

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

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

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

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

C l o c k w i s e l o a d i n g o f d e b r i s i s a n o n e n l a r g i n g m o t i o n

in tended on ly to fac i l i t a te remova l .

D I S U S S I O N

U t i li z a ti o n o f t h e c o n c e p t o f b a l a n c e d f o r c e s e n a b l e s

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

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

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

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

p r e s e n t e d i n F ig . 4 . W i t h o u t m o d i f ie d t i p s it is n e c e s s a r y

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

o r d e r to p r e v e n t t r a n s p o r t e d w a l l s a n d l e d g e fo r m a t i o n .

W h e n o v e r e x t e n s i o n i s u s e d , t h e t i p m u s t b e r e t r a c t e d

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

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

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

t i o n o r l e d g e . I n t h o s e c a s e s , t h e l e d g e c r e a t e d b y t h e

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

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

d u r i n g t h e f il lin g p r o c e d u r e . F i g u r e 4 A p r e s e n t s a c a s e

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

p le te ly accu ra te nor c li n ica ll y des i rab le and it ma y be

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

ins t rum en ts . E x tens ive c l in i ca l t r i a l s o f the mod i f ied f i l es

h a v e d e m o n s t r a t e d t h i s c o n c l u s i o n t o b e t ru e . Se v e r a l

c a s e s a r e p r e s e n t e d i n F ig . 4 ( B t o D ) w h i c h r e v e a l th e

cl in ical value of f i le t ip modi f icat ion.

O N L U S I O N S

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

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

I t d i f fe r~ p r imar i ly i n tha t the cu t t i ng mo t ion i s i n ten t ion-


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

l ev e l w i t h o u t b l o c k a g e , e s p e c i a l l y w h e n m o d i f ie d i n s t ru -

m e n t s a re u s e d . I n s t r u m e n t p l a c e m e n t is a c c o m p l i s h e d

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

n i fi c a n t lo a d s o n a n i n s t r u m e n t t i p w i t h o u t r e q u i ri n g t h e

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

f a c t e n a b l e s s m a l l i n s t ru m e n t s , i .e . 8 a n d 1 0 , t o b e

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

ca lc i f i ed cana ls rap id ly . In add i t i on , th i s approach to

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

s e c o n d a r y b l o c k a g e f r o m l o o s e n e d p a r t i c l e s .

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

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

b e l i m i t e d t o n o m o r e t h a n 1 8 0 d e g r e e s i n o r d e r t o

prevent over inser t ion o f the ap ica l por t ion o f the ins t ru -

m e n t i n t o d e n t i n . Su c h o v e r i n s e r t i o n c a u s e s t h e t i p t o

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

t h e f ile c o r o n a l t o t h a t p o i n t a n d i n c r e a s e s t h e l ik e l ih o o d

o f i n s t r u m e n t s e p a r a t i o n ( 2 0 ) . Ea c h p l a c e m e n t l o a d i s

f o l lo w e d b y a c u t t i n g m o t io n , c o u n t e r c l o c k w i s e r o t a t io n

o f 1 2 0 d e g r e e s o r g r e a t e r . T h i s a c t i o n c o m p l e t e l y e n -

la rges the cana l to the f i l e d iameter , f rees the ins t ru -

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

w h e n th e n e x t c l o c k w i s e r o t a t io n i s s u p p l i e d . C l o c k w i s e

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

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

r e a c h e d . O n o c c a s i o n t h e f il e b e c o m e s f ill ed w i t h d e b r i s

a n d w i l l h e s i t a t e t o a c c e p t t h e n e x t p l a c e m e n t m o t i o n .

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

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

gress . Upon re inser t ion , p repara t ion i s con t inued un t i l

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

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

t io n o f a t l e a s t 1 2 0 d e g r e e s . A g r e a t e r a n g l e o f r o t a t io n

i s d e s i r e d , i. e. o n e o r t w o r e v o l u ti o n s , b u t c a n n o t b e

safe ly accom pl i shed in a l l cana ls . Th is i s espe c ia l l y t rue

w h e n a n e x t r e m e l y s h a r p c u r v a t u r e e x i s t s a s s u c h a

curva ture can eas i l y cause fa t igue fa i l u re and resu l t i n

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

throughout the i r l eng th p roduce l i t t l e l i ke l i hood o f f rac -

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

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

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

c l o c k w i s e m o v e m e n t , i . e. 1 2 0 d e g r e e s e i t h e r d i re c t io n .

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

Sh a r p c u r v a t u r e s l o c a t e d p r im a r i ly i n t h e a p i c a l o n e -

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

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

r a d i u s v ia c a n a l a c c e s s a n d t h e i r i n f lu e n c e i s e x p r e s s e d

t h r o u g h a v e r y s h o r t le v e r a rm .

T o s u p p o r t t h e b a l an c e d f o r c e c o n c e p t a n d e s t a b li s h

i ts s a f e t y , w e h a v e a c c u m u l a t e d d a t a r e la t iv e t o i n s tr u -

me nt fa il u re dur in g c l in i ca l u se (20) . In tha t s tud y ins t ru -

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

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

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

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


9/9

Vol. 11 N o. 5 May 1985

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

p o r t e d c o u n t e r c l o c k w i s e i n s tr u m e n t w e a k n e s s 2 1 , 2 2 ) .

T h e f in d in g s o f p r ep ar a t io n s p r o d u ced in p las t i c can a l

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

sen t ed in a su b seq u en t a r t i c l e .

D r . R o a n e s a s s o c i a t e p r o f e s s o r a n d c h a ir m a n , D e p a r t m e n t o f E n d o d o n t i c s ,

U n i v e r s i ty o f O k l a h o m a C o l l e g e o f D e n t i s t r y , O k l a h o m a C i ty , O K . D r . S a b a l a i s

a s s o c i a t e p r o f e s s o r , D e p a r t m e n t o f E n d o d o n t ic s 0 U n i ve r s it y o f O k l a h o m a C o l -

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

m e n t o f D e n t a l M a t e r i a l s , U n i v e r s i t y o f O k l a h o m a H e a l t h S c i e n c e s C e n t e r ,

O k l a h o m a C it y , O K .

References

1 . M u l l a n e y T P . I n s t r u m e n t a t i o n o f f i n e l y c u r v e d c a n a ls . D e n t C U n N o r t h

A m 1 9 7 9 ; 2 3 : 5 7 5 - 9 2 .

2 . W e i n e F S , K e l ly R F , L i o P J . T h e e f f e c t o f p r e p a r a t i o n p r o c e d u r e s o n

o r i g i n a l c a n a l s h a p e a n d o n a p i c a l f o r a m e n s h a p e . J E n d o d o n 1 9 7 5 ; 1 : 2 5 5 - 6 2 .

3 . A b o u - R o s s M , F r a n k A L , G l i c k D H . T h e a n t i c u r v a t u re fi li n g m e t h o d t o

p r e p a r e t h e c u r v e d r o o t c a n a l, J A m D e n t A s s o c 1 9 8 0 ; 1 0 1 : 7 9 2 - 4 .

4 . Goe r i g A C , M i che l ich R J , S c h u l t z H H . I n s t r u m e n t a t i o n o f r o o t c a n a l s in

m o l a r s u s i n g t h e s t e p - d o w n t e c h n i q u e . J E n d o d o n 1 9 8 2 ; 8 : 5 5 0 - 4 .

5 . S c h i l d e r H , F u l to n S Y , C a n a l d e b r i d e m e n t a n d d i s i n f e c t io n . I n : C o h e n S ,

B u r n s R C , e d s . P a t h w a y s o f th e p u l p . 3 r d e d . S t . L o u is : - C V M o s b y C o . ,

1 9 8 4 : 1 7 5 - 2 0 4 .

6 . O s w a l d RJ . P r o c e d u r a l a c c i d e n t s a n d t h e i r re p a ir . D e n t C l in N o r t h A m

1 9 7 9 ; 2 3 : 5 9 3 - 6 1 6 .

B a l a n c e d

Force Concept

2 1 1

7 . W e i n e F S , K e l l y R F , B r a y K E . E f f e c t o f p r e p a r a t i o n w i th e n d o d o n t i c

h a n d p i e c e s o n o r ig i n a l c a n a l s h a p e . J E n d o d o n 1 9 7 6 ; 2 : 2 9 8 - 3 0 3 .

8 . Co f f ae K P , B r il li an t JD . Th e e f f ec t o f ser i a l p rep ara t i on versu s nonser ia l

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

m o l a r s . J E n d o d o n 1 9 7 5 ; 1 : 2 1 1 - 4 .

9 . W a l t o n R E . H i s t o io g i c e v a l u a t io n o f d i f f e r e n t m e t h o d s o f e n l a r g in g t h e

p u l p c a n a l s p a c e . J E n d o d o n 1 9 7 6 ; 2 : 3 0 4 - 1 1 .

1 0 . K l a y m a n S M , B r il li an t J D . A c o m p a r i s o n o f t h e e f f ic a c y o f s e r i a l p r e p a -

r a t io n v e r s u s G i r o m a t i c p r e p a r a t i o n . J E n d o d o n 1 9 7 5 ; 1 : 3 3 4 - 7 .

1 1 , L i t tm a n S H . E v a l u a t i o n o f r o o t c a n a l d e b r i d e m e n t b y u s e o f a r a d i o -

p a q u e m e d iu m . J E n d o d o n 1 9 7 7 ; 3 : 1 3 5 - 8 .

1 2 . C h o w T W . M e c h a n i c a l e f f e c t iv e n e s s o f r o o t c a n a l ir ri ga t io n . J E n d o d o n

1 9 8 3 ; 9 : 4 7 5 - 9 .

1 3 . R a m Z . E f f e c t i v e n e s s o f r o o t c a n a l i r ri g a t io n . O r a l S u r g 1 9 7 7 ; 4 4 : 3 0 6 -

12.

1 4 . A l li s o n D A , W e b e r C R , W a l to n R E . T h e i n f l u e n c e o f t h e m e t h o d o f

c a n a l p r e p a r a t i o n o n t h e q u a l i t y o f a p i c a l a n d c o r o n a l o b t u r a t i o n . J E n d o d o n

1 9 7 9 ; 5 : 2 9 8 - 3 0 4 .

15 . P h i ll ips RW . Me chan i cs o f cu t t i ng w i t h de nta l burs . S k i nn er s sc ience

of denta l ma ter i a l s . 8 t h ed . P h il ade lph i a : WB S au nde rs , 1982:565 ,

1 6 . F e l t R A , M o s e r J B , H e u e r M A . F l u t e d e s i g n o f e n d o d o n t i c i n s t ru m e n t s ;

i t s i n fl uence on cu t t i ng e f fi c iency . J E n dod on 198 2;8 :2 53-9 .

1 7 . K r u p p J D , B r a n t l e y W A , G e r s t e in H . A n i n v e s t i g a t i o n o f t h e t o r s i o n a l

a n d b e n d i n g p r o p e r t i e s o f s e v e r a l b r a n d s o f e n d o d o n t i c f i l e s . J E n d o d o n

1 9 8 4 ; 1 0 : 3 7 2 - 8 0 .

18 . Joh nso n HB . A me thod f o r t he f il li ng o f t he pu l p cana li cu li by d i f f us ion .

J D e n t R e s 1 9 2 2 ; 4 : 1 1 7 - 2 8 .

1 9 . L u k s S . R o o t c a n a l i n s tr u m e n t s a n d c h o i c e o f i n s t ru m e n t s . P r a ct ic a l

end odon t i cs . P h i lade l ph ia : JB U pp i n co t t Co. , 1974 :92 .

2 0 . R o a n e J B , S a b a l a C L C l o c k w i s e o r c o u n t e r cl o c k w is e , J E n d o d o n

1 9 8 4 ; 1 0 : 3 4 9 - 5 3 .

2 1 . C h e r n i c k L B , J a c o b s J J , L a u t e n s c h l a g e r E P , H e u e r M A . T o r s i o n a l

f a i lu r e o f e n d o d o n t i c f il es . J E n d o d o n 1 9 7 6 ; 2 : 9 4 - 7 .

2 2 , L a u t e n s c h l a g e r E P , J a c o b s J J , M a r s h a l l G W , J r , H e u e r M A . B r i t tl e a n d

d u c t i le t o r s i o n a l fa i l u r e s o f e n d o d o n t i c in s t r u m e n t s . J E n d o d o n 1 9 7 7 ; 3 : 1 7 5 - 8 .

Documents

3.- Fuerzas Balanceadas_2