cyclic fatigue of NiTi instruments

8/10/2019 cyclic fatigue of NiTi instruments

1/9

0099-2399/97/2302-0077503.00/0

JOURNAL OF ENDODONT{CS

Copyright 1997 by The American Association of Endodontists

S C I E N T IF IC A R T IC L E S

Pr in ted in U S A

VOL. 23 No. 2 FI~BRUARY1997

Cyc l i c F a t i g u e T es t i n g o f N i cke l T i tan i u m

E n d o d o n t i c I n s t r u m e n t s

J o h n P . P r u e tt D D S D a v i d J . C l e m e n t D D S a n d D a v i d L . C a r n e s J r. P h D

Cyc l i c fa t i gue o f n i cke l - t i tan ium, eng ine -d r i ven in -

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

ca n a l cu r va tu r e a n d o p e r a t i n g sp e e d o n t h e b r e a k -

a g e o f L i g h t sp e e d i n s t r u me n ts . A n e w me th o d o f

ca n a l cu r va tu r e e va l u a t io n t h a t a d d r e ss e d b o th a n -

g l e a n d a b r u p tn e ss o f cu r va tu r e wa s i n t r o d u ce d .

Ca n a l cu r va tu r e wa s s i mu l a te d b y co n s t r u c t i n g s i x

cu r ve d s ta i n l e ss - s te e l g u i d e t u b e s w i t h a n g l e s o f

cu rva tu re o f 30 , 45 , o r 60 degrees , and rad i i o f

cu r va tu r e o f 2 o r 5 mm. S i ze 3 0 a n d 4 0 L i g h t -

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

tu b e s a n d t h e h e a d s se cu r e d i n t h e co l l e t o f a

Ma g t r o l Dyn a mo me te r . A s i mu l a te d o p e r a t i n g l o a d

o f 1 0 g - cm wa s a p p l i e d . I n s t r u me n ts we r e a b l e t o

r o ta te f r e e l y i n t h e t e s t a p p a r a tu s a t sp e e d s o f 7 5 0 ,

1 3 0 0 , o r 2 0 0 0 r p m u n t i l se p a r a t i o n o ccu r r e d . Cy -

c l e s t o f a i l u r e we r e d e te r m i n e d . Cyc l e s t o f a i l u r e

we r e n o t a f fe c te d b y r p m. I n s t r u m e n ts d i d n o t se p -

a r a te a t t h e h e a d , b u t r a th e r a t t h e p o i n t o f ma x i -

mu m f l e xu r e o f t h e sh a f t , co r r e sp o n d i n g t o t h e

m i d p o i n t o f cu r va tu r e w i t h i n t h e g u i d e t u b e . Th e

i n s t r u me n ts w i t h l a r g e r d i a me te r sh a f ts , 4 0 , f a il e d

a f t e r s i g n i fi ca n t ly f e w e r cyc l e s t h a n d i d 3 0 i n s t r u -

me n ts u n d e r i d e n t i ca l t e s t co n d i t i o n s . Mu l t i va r i -

a b l e a n a l ys i s o f va r i a n ce i n d i ca te d t h a t cyc l e s t o

fa il u re s i g n i f ica n t l y d e c r e a se d a s t h e r a d i u s o f cu r -

v a t ur e d e c r e a s e d fr o m 5 m m t o 2 m m a n d a s t h e

a n g l e o f cu r va tu r e i n c r e a se d g r e a te r t h a n 3 0 d e -

g rees (p < 0 .05 , power = 0 .9 ) . Scann ing e lec t ron

m i c r o sco p i c e va l u a t i o n r e ve a l e d d u c t i l e f r a c tu r e

a s t h e f a t i g u e f a i lu r e m o d e . Th e se r e s u l t s i n d i ca te

tha t , fo r n i cke l - t i tan ium, eng ine -d r i ven ro ta ry i n -

s t r u me n ts , t h e r a d i u s o f cu r va tu r e , a n g l e o f cu r va -

tu r e , a n d i n s t r u me n t s i ze a r e mo r e i mp o r ta n t t h a n

o p e r a t i n g sp e e d f o r p r e d i c t i n g se p a r a t i o n . Th i s

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

t i g u e f a il u re a n d su g g e s t s t h a t s t a n d a r d i ze d f a -

t i g u e t e s t s o f n i cke l - t i t a n i u m r o ta r y i n s t r u me n ts

sh o u l d i n c l u d e d yn a m i c o p e r a t i o n i n a f l e xe d s ta te .

Th e r e su l t s a l so su g g e s t th a t t h e e f f e c t o f t h e r a -

d i u s o f cu r va tu r e a s a n i n d e p e n d e n t va r i a b l e

sh o u l d b e co n s i d e r e d wh e n e va l u a t i n g s tu d i e s o f

r o o t ca n a l i n s t r u me n ta t i o n .

77

Instrument separation is a serious concern in endodontic s. Because

stainless-steel instruments usually deform before they separate,

dentists can inspect them for visible signs of instrument deforma-

tion. A deformed instrument usually shows severe bending or

unwinding of the flutes, indicating hat the elastic limit of the metal

has been exceeded and that the instrumen t should be discarded.

Nickel-titanium Ni-Ti) endodontic instruments were intro-

duced to facilitate instrumentation of curved canals. Ni-Ti instru-

ments are superelastic and will flex far more than stainless-steel

instruments before exceeding their elastic limit 1, 2). This flexi-

bility is an important property that allows preparation of curved

canals while minimizing transportation 3). Despite this increased

flexibility, separation is still a concern with Ni-Ti instruments, and

they have been reported to undergo unexpected fracture 4). Sep-

aration can occur without any visible signs of previous permanent

deformation, apparently within the elastic limit of the instrument.

Visible inspection, therefore, is not a reliable method for evalua-

tion of used Ni-Ti instruments.

The American Dental Association ADA) Specification No. 28

5) stipulates the design requirements, testing procedures, and

physical properties for endodontic instruments. Tests detailed in

ADA Specification No. 28 are conducted in a static mode. Instru-

ments are fixed at the tip and are twisted until failure to determine

instrumen t resistance to fracture under torsional loading. Angular

deflection is used to detertnine stiffness when instruments are bent

to predetermined angles of 30, 45, and 90 degrees. Tests described

in ADA Specification No. 28 are appropriate for establishing

mini mum strength requirements of Ni-Ti hand instruments placed

under a static load I, 2, 6, 7), but ADA Specification No. 28 is not

an appropriate test of the dynamic characteristics of engine -driven

rotary instruments. All Ni-Ti, engine-dr iven otary systems require

that the ins trument be activated at a predetermined rpm before

inser tion into the canal. Therefore, to test rotary inst ruments, the


2/9

7 8 P r u e t t e t a l

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

B e c a u s e A D A S p e c i f i c a t i o n N o . 2 8 d o e s n o t c o n s i d e r c a n a l g e -

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

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

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

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

separa t ion (2, 8) .

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

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

p la s t i c de form a t ion in s t a in le s s - s te e l ins t rum e nts (1 , 2 ) . Me ta l

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

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

f o r m a t i o n ) t h r o u g h k n o w n m e c h a n i s m s c a l l e d s l i p b a n d s ( 9 ) . S u c h

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

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

s t r e s s . R o t a t i o n s u b j e c t s a n e n d o d o n t i c i n s t r u m e n t t o b o t h t e n s i l e

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

i n c u r v e d c a n a l s d e f o r m , c r e a t i n g s t r e s s w i t h i n t h e i n s tr u m e n t . H a l f

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

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

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

m e n t t o u n d e r g o o n e c o m p l e t e t e n s i o n - c o m p r e s s i o n s t r e s s c y c l e .

T h i s i s t h e m o s t d e s t r u c t i v e f o r m o f c y c l i c l o a d i n g ( 9 ) .

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

a n d s t a i n l e s s - s t e e l e n d o d o n t i c i n s t r u m e n t s ( 2 , 8 , 1 0 - 1 3 ) , f a t i g u e

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

m e t h o d o l o g y ( 2 , 1 0 , 1 1 ) . S t r es s l e v e l s d u r i n g c y c l i c l o a d i n g a r e

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

a ppl ie d loa ds . Thi s s t r e s s i s gre a te s t in the a re a of c urva tu re (14) .

More se ve re be nds c re a te gre a te r s t r e s s (14) ; a nd l a rge r , s t i f f e r

i n s t r u m e n t s w i l l e x p e r i e n c e g r e a t e r s t r e s s t h a n s m a l l e r i n s t ru m e n t s

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

T h e p u r p o s e o f t h i s s t u d y w a s 2 - f o l d ; f i r s t t o i n t r o d u c e a n e w

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

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

k n o w n t o d e t e r m i n e i n s t r u m e n t s t re s s a n d t h e r e f o r e a f f e c t i n st r u -

m e n t f a t i g u e l i f e . S e c o n d , c y c l i c f a t i g u e w a s e x a m i n e d i n a c o m -

m e r c i a l l y a v a i l a b l e N i - T i i n s t r u m e n t s y s t e m . T h e s t u d y e x a m i n e d

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

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

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

ins t rum e nts .

M A T E R ~ L S A N D M E T H O D S

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

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

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

a r b i tr a r y a n g l e m e a s u r e d i n d e g r e e s a s d e s c r i b e d b y S c h n e i d e r ( 1 5 )

( F i g . 1 ) . T o d e t e r m i n e t h e d e g r e e o f r o o t c u r v a t u r e , h e s c r i b e d a

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

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

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

a c u t e a n g l e f o r m e d ( a ) w a s d e f i n e d a s t h e d e g r e e o f ro o t c u r v a t u r e .

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

s c r i b e d u s in g t w o p a r a m e t e r s : a n g l e o f c u r v a t u r e ( a ) a n d r a d i u s o f

c u r v a t u r e ( r ) ( F i g . 2 ) . T o d e t e r m i n e t h e s e p a r a m e t e r s , a s t r a i g h t l i n e

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

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

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

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

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

3

B

F IG 1 . Deg r ee o f r oo t c ana l c u r va t u r e . T he m e t hod des c r i bed by

S c hne i de r f o r de t e r m i n i ng c ana l c u r v a t u r e us es on l y one pa r am e t e r

t o de f i ne t he ang l e . T t =, t wo s i m u l a t ed t ee t h dem ons t r a t e h ow a

d i f fe rence i n abruptne~ , , o f the curve rad ius o f curvature) w i l l vary

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

o f 43 deg r ee s and B has an ang l e o f 52 deg r ees , ev en thou gh b o t h

a n g l e s m e a s u r e d a c c o r d i n g t o o u r m e t h o d e q u a l e d 6 0 d e g r e e s .

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

ang l e .

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

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

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

c a n a l s o b e d e f i n e d b y t h e a n g l e f o r m e d ( c ~ 1 a nd c ~2) by pe rp e n-

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

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

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

T h e r a d i u s o f c u r v a t u r e ( r I a n d r 2 ) i s t h e l e n g t h o f t h e r a d i u s o f t h e

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

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

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

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

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

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

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

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

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

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

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

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

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

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

m e n t s e p a r a t i o n w e r e r a d i o g r a p h e d . R a d i o g r a p h s w e r e a l s o m a d e

o f t e e th t r e a t e d c l i n i c a l l y in w h i c h s e p a r a t i o n o f N i - T i L i g h t s p e e d

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

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

i m a g e o f t h e c a n a l p r o f i l e . A d d i t i o n a l l y , s i z e 3 0 L i g h t s p e e d

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

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

5 - m m r a d iu s o f c u r v a tu r e w i t h a n a n g l e o f c u r v a t u re o f 3 0 d e g r e e s

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

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

i n o v e r 3 0 m i n a t a s p e e d o f 2 0 0 0 r p m ) w i t h i n t h e t i m e f r a m e f o r

t h e t e s t a p p a ra t u s o r d a t a c o l l e c t io n . T h u s , a n g l e s o f c u r v a t u r e o f

3 0 , 4 5 , a n d 6 0 d e g r e e s w e r e c h o s e n f o r e v a l u a t i o n a t r a d i i o f

c u r v a t u r e o f 2 a n d 5 m m .

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


3/9

V o l. 23 N o . 2 F eb ru ary 1997

o~ = 60 o~z =6 0

r = rnrn

A B

F IG 2 . Rad i us o f c u r v a t u r e and an g l e o f c u r v a t u r e . New m e t ho d o f

des c r i b i ng c ana l geom e t r y us ing t wo pa r am e t e r s . Rad i us o f c u r v a -

t u r e ( r) and ang l e o f c u r v a t u r e ( e ) a r e de t e r m i ned on t he s am e t ee t h

as i n F ig . 1 . T hes e t ee t h ac c u r a t e l y dep i c t t he gu i de t ubes us ed i n

t h i s s t udy t ha t had a 6 0 - deg r ee ang l e ( (~ 1 = ( x2 ). A ng l e o f c u r v a t u r e

(c~) i s de t e r m i ne d b y t he ang l e f o r m ed by t h e l ines t ha t i n t e r s ec t a t

t he c i r c l e s c en t e r . T hes e t wo l ines a r e pe r pe nd i c u l a r t o t he l ines

d r awn a l ong t he l ong ax es o f t he c o r ona l and ap i c a l po r t ions o f t he

r oo t c ana l s pac e . P o i n t s a and b a r e t he po i n t s whe r e i n t he c ana l

dev i a t es f rom t he s t r a i gh t l ines and e i t he r beg i n o r end t he c u r v ed

po r t i on o f t h e r oo t c ana l s pac e . A ng l e ( (~ ) is t ak en t o be t he ang l e

f o r m ed by t he a r c in deg r ees be t w een po i n t s a and b . T he

rc

l ies

on a c i r c le w hos e s i z e is s pec i f i ed b y i t s r ad i us, an d t he c i r c le s

r ad i us is t ak en t o be t he r ad i us o f c u r v a t u r e o f t he c ana l i n t ha t l oc a l

area. The c i r c l e s rad ius i s the rad ius ( r l and r2) o f the cu rved por t i on

o f t he r oo t c ana l s pac e and de f i nes how ab r up t l y t he c ana l c u r v es .

T he c an a l geom e t r y o f t hes e t wo t ee t h d i f fe r s on l y i n the r ad i us o f

curvature ( r~ and

r 2 ,

wher eas t he ang l e o f c u r v a t u r e ( ( x1 and (x2)

equa l s 60 deg r ees . A r ep r es en t s a s weep i ng c ana l c u r v a t u r e hav i ng

a 5 - m m r ad i us o f c u r v a t u r e ( r l) . B r ep r es en t s an ab r up t c ana l c u r-

vatu re hav ing a 2-m m rad ius o f curv ature ( r2) . C l i n i ca l ly , the rad ius o f

c u r v a t u re a n d a n g l e o f c u r v a tu r e o f a n y r o o t c a n a l s p a c e c o u l d b e

m eas u r ed us i ng t h is t ec hn i que w i t h t he a i d o f a c i r c le gauge .

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

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

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

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

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

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

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

p e a t a b l y r e c o r d e d . W h e n a c t i v e d e n t i n c u t t i n g o c c u r r e d , r e c o r d e d

t o r q u e v a l u e s r a n g e d f r o m 6 t o 1 4 g - c m . T h e r e f o r e , a p r e se t t o r q u e

o f 1 0 g - c m w a s a p p l i e d t o t h e t e s t i n s t ru m e n t s d u r i n g a l l o f t h e t e s t

c o n d i t i o n s .

S i z e 3 0 a n d 4 0 L i g h t s p e e d i n s t r u m e n t s ( L i g h t s p e e d T e c h n o l -

o g i e s , I n c . , S a n A n t o n i o , T X ) w e r e e v a l u a t e d . L i g h t s p e e d i n s t r u -

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

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

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

b r e a k a g e . S i z e 4 0 w a s s e l e c t e d b e c a u s e i t i s t h e m i n i m u m m a s t e r

a p i c a l s i z e a d v o c a t e d f o r th e L i g h t s p e e d t e c h n i q u e . S i z e 3 0 w a s

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

d e s i g n t h a t i s n e a r l y p a r a l l e l . I n a d d i t i o n , L i g h t s p e e d i n s t r u m e n t s

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

r a n g e o f o p e r a t i n g s p e e d s . T e n i n s t r u m e n t s o f e a c h s i z e w e r e t e s te d

i n e a c h o f t h e s i x d i f f e r e n t e x p e r i m e n t a l c a n a l s h a p e s ( 3 a n g l e s c ~

e a c h o f 2 r a d i i r ) at t h r ee d i f f e r e n t s p e e d s ( 7 5 0 , 1 3 0 0 , a n d 2 0 0 0

r p m ) . T h i s p r o d u c e d 1 8 t e s t g r o u p s f o r e a c h i n s t r u m e n t s i z e . E a c h

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

C yc l i c F a t i g u e o f N i - T i R o t a ry I n s t ru m e n t s 79

Ni-Ti L ightspeed Inst rument

Tes t Suppor t B lock

Gu ide Tube

Ho lddown

.~ , . C lamp

Dynamomete r Co l l e t

L

I npu t sha f t t o dynamomete r , da ta aqu i s i t ion compu te r

F IG 3 . T es t appa r a t us . Cu t - awa y v i ew o f a L i gh t s peed i ns t r um e n t

i ns e r ted t h r ough t he l um en o f a s t a i n l es s -s t ee l gu i de t ube c l am ped

i n t he dy n am om et e r c o l le t . T he i ns t r um en t s hank bu t t s up aga i ns t

t he end o f t he gu i de t ube , ens u r i ng t ha t eac h i ns t r um en t was p l ac ed

t o t he s am e dep t h w i t h i n t he a r t i f i c i a l c ana l . T he dy nam om et e r

a l l owed f r ee r o t a t i on o f t he i ns t r um en t du r i ng t he c y c l ic f a t i gue t es t ,

y e t was c apab l e o f app l y i ng t o r s i ona l p r e l oad t o s i m u l a t e c l i n i c a l

u s e ,

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

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

s t u d y . A r t i f i c i a l c a n a l s w e r e f a b r i c a t e d f r o m 1 8 - g a u g e , s t a i n l e s s -

s t e e l n e e d l e s h a v i n g a n i n t e r n a l d i a m e t e r o f 0 . 8 3 r a m . R e f e r e n c e

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

t u b e s . A 2 - m m r a d i u s r e p r e s e n t e d a n a b r u p t c u r v a t u r e ; a n d a 5 m m

r a d i u s r e p r e s e n t e d a s w e e p i n g c u r v a t u r e ( F i g . 2 ) , e a c h w i t h a n g l e s

o f c u r v a t u r e o f 3 0 , 4 5 , a n d 6 0 d e g r e e s . T h i s r e s u l t e d i n s i x a r t i f i c ia l

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

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

o f t h e c a n a l w a s 7 m m f r o m t h e t i p o f t h e i n s t r u m e n t ( F i g . 3 ) . T h e

h e a d o f th e i n s t r u m e n t p r o t r u d e d - 2 m m f r o m t h e a r t if i c i a l c a n a l .

A t e s t s t a n d ( F i g . 3 ) w a s c o n s t r u c t e d t h a t a l i g n e d t h e i n n e r a s p e c t

o f t h e a r ti f i c ia l c a n a ls w i t h t h e c e n t e r l i n e o f t h e d y n a m o m e t e r a n d

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

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

D y n a m o m e t e r ( M a g t r o l , I n c . , B u f f a l o , N Y ) . C l a m p e d i n s t r u m e n t s

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

h a n d p i e c e ( A s e p t i c o m o d e l A E U - 1 7 , A s e p t i c o , In c . , W o o d i n v i l l e ,

W A ) w a s a t t a c h e d t o t h e i n s t r u m e n t s h a n k . T h e e x p e r i m e n t a l

o p e r a t i n g s p e e d o f 7 5 0 , 1 3 0 0 , o r 2 0 0 0 r p m w a s a d j u s t e d u s i n g t h e

d y n a m o m e t e r r e a d o u t t h a t p r o v i d e d a m o r e a c c u r a t e r p m d e t e r m i -

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

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

e l e c t r i c h a n d p i e c e .

T h i s s t u d y u s e d i n s tr u m e n t a ti o n e q u i p m e n t s p e c i f i e d in A D A

S p e c i f i c a t i o n N o . 2 8 f o r t o r s i o n a l t e s t i n g : t h e M a g t r o l Dynamom


4/9

80 Pruett et al.

eter. The dynamometer allows the torque applied to the instrument

to be preset and measured while the instrument rotates. All instru-

ments were operated with a head load of 10 g-cm torque. A

software program (Sunset Resources, Inc., San Antonio, TX) was

written for an IBM-compatible computer to permit data collection

while the inst rument was rotating. A n 18-rain time limit for data

collection was set as limited by the data collection program, but

instrument separation always occurred within 5 min of beginn ing

the test. Data collection from the dynamometer consisted of con-

tinuous torque, and rpm readouts were comprised of 10 averaged

readings/s. The time of instrument failure was determined when

the torque values suddenly dropped, indicating instrument separa-

tion. The number of cycles to failure was calculated from the rpm

data by multiplying the rpm by the time to failure. Data analyzed

were the number of cycles to fa ilure for each inst rument tested

under the specified angle of curvature, radius of curvature, rpm,

and instrumen t size conditions. Data were analyzed by analysis of

variance (ANOVA), with the Neuman-Kuels procedure serving as

the post-hoc test using a 95% confidence level.

Fractured inst rument fragments were collected by group. Rep-

resenta tive samples of the fracture surface from the various groups,

and sizes were examined under scanning electron microscopic

magnification with a JSM-840 scanning electron microscope

(Japanese Electronic and Optical Ltd., Japan). Another group of

instruments was partially cycled (75 to 80% to failure) using mean

cycles-to-failure data from the matching experimental groups.

These instruments were then mounted in both unflexed and flexed

positions with the area of interest visible in side view, and exam-

ined under scanning electron microscopy (SEM). For comparison,

new same-sized instruments were mounted in the same manner and

examined under SEM.

RESULTS

Operating rpm

Multivariable ANOVA indicated that operating rpm was not a

significant factor affecting cycles to failure (p < 0.10 for size 30;

p < 0.06 for size 40) (Fig. 4). No difference in cycles to failure

was recorded between the 750, 1300, and 2000 rpm groups within

the two instrument sizes. This was further verifi ed by one-way

ANOVA, wherein only 2 of the possible 36 groups in the size 30

and 40 instrument groups had cycles to failure that varied when

rpm was the only independent variable evaluated.

Instrument Shaft Diameter

Shaft diameter had a signif icant effect on cycles to failure (p .

o

1000

900

800

700

600

500

400

300

200

100

0

5ram Radius 2ram

Radius

SIZE 40

30 degrees ~ 45 degreee ~ 60

degrees

FIG 6. E ffect of angle of curvature. Angle subgro ups are arranged

according to radius of curvature for size 30 (A) and 40 (B) instru-

ments. Forty-five degree or 60-degree angles in both instrument

size groups had significantly fewer cycles to failure than the 30-

degree angle groups (*). The size 40, 2-mm radius subgroup varied

from this pattern, w ith all angles having significantly different cycles

to failure (** and ***).

meri t s ize exis t , which s igni f icant ly reduce ins t rument cycl ic fa-

tigue life.

There i s a c lear need for the developmen t of a tes t protocol for

the cycl ic fa t igue of Ni-Ti , engine-dr iven rotary ins t ruments .

Brantle y et al . 10) and Luebke et al. 11), in studies on stainless-

s tee l rotary ins t ruments , have a lso advocated the dev elopmen t of a

cycl ic fa t igue tes t protocol that s imulates c l in ica l condi t ions of

ins t rument use . Serene e t a l . 2) publ ished a shor t volume on Ni-Ti

instruments, including results from a preliminary study on rotation

to breakage cycl ic fa t igue) of hand ins t ruments . A s igni f icant

di f ference was found between c onvent ional s ta inless-s tee l f i les and

all Ni-Ti fi le groups. It was also noted that in rotation-to-breakage

studies of Ni-Ti f i les that a l lowed f ree t ip rota t ion, the range of

resul ts wi thin each same-s ize group was large and unpredic table .

That pre l iminary informat ion on rota t ion- to-breakage of hand in-

struments stated that the instruments were rotated in a 90-degree

metal tube until they broke. Defining the test parameters in such a

l imi ted manner , w i thout s t r ic t ly control l ing the var iables of curv e

geometry, may expla in the var iabi li ty in rota t ion- to-breakage cy-


6/9

82 Pruett et al Journal of Endodont ics

FIG 7. Location of separation. Ran domly selected size #40 Light-

speed instruments that w ere cycled to failure. All instruments tested

showed the same pattern of separation, with fractures occurring

within the guide tube at the midpoint of the curved segment.

D

FIG 8. Partially separated instruments. Partially separated instru-

ments ap pear normal to the eye (A), and under SEM evaluation in an

unflexed state at x150 original magnification (B).

A r r o w s

indicate

the location of the fracture in-the unflexed instrument, which is

closed and not apparent. Upon mounting the instrume nt shaft in a

flexed (stressed) position (C), incomple te fractures be came appar-

ent under SEM examination at x150 original magnification (D).

A r r o w s

indicate the same site of exam ination of the instrument shaft,

which corre lated to the midpoint of the curved segment of the guide

tube.

cles to fa i lure) resul ts and the conclus ion drawn concerning N i-Ti

ins t ruments . Parameters including radius of curvature , angle of

curvature, instrument size, and the point of maximal instrument

f lexure were a l l found to have a s igni f icant ef fec t on the number of

cycles to fa i lure and locat ion of breakage in our s tudy. Al l of these

factors should be considered when developing a tes t protocol for

the examinat ion of cycl ic fa t igue .

Mathemat ics and engineer ing pr inciples 14) suggest tha t a

system is needed that describes canal geometry in terms that

consistently describe the state of stress and cyclic fatigue on a

f lexed ins t rument shaf t . The independent var iables that descr ibe

canal shape are radius of curvature and angle of curvature . The

usual method of measur ing canal curvature publ ished by Schneider

15) in 1971 provides a means of estimating canal shape using a

single parameter: an estimated arbitrary angle Fig. 1). This

method does not consider radius of curvature as a measurable

aspect of canal geometry. Using only one parameter to descr ibe

canal shape does not d i f ferentia te the canal shape by abruptness o r

radius of curvature. Two canals measured at the same angle in

FIG 9. S tages of instrument fracture. A show s the fracture surface of

a size #30 instrume nt at 300 original magnification. The b o x e d a r ea

is enlarged in B having a x190 0 original magnification. The

aster isk

indicates the area of crack init iation on the periphery. Crack p rop-

agation is seen in B as striations marked by the ar rows Each

striation represents a jump in the fracture surface that occurred

under tension in that area during rotation. Ultimate ductile fracture is

seen in the central area of A, characterized by the presence of

microvoid formations and dimpling.

degrees by the S chneider method could have ve ry di f ferent radi i or

abruptness of curvatures, thus having a very different impact on the

di f f iculty of canal ins t rumentat ion and ins t rument fa t igue .

The more exact method of determining canal curvature and

geometry descr ibed in th is s tudy considers the radius of curvature

r) and the angle of curvature a) separately Fig. 2). Thes e are

terms that mathema tically specify the shape factors of a curved

root canal sys tem. The radius of curvature in mi l l imeters) i s the

radius of a circle that coincides with the path taken by the canal in

the area of the most abrupt curvature. A more abrupt curve cor-

responds to a smal ler radius of curvature . The angle of curvature

is the degrees arc formed between the points of devia t ion on the

ci rc le, or the angle formed between the perpendicular l ines drawn

from the tangents in tersecting at the center of the circle Fig. 2).

The angle of curvature i s independent of the radius . Thus , two

canals wi th the same degree of curvature can have radical ly di f -

ferent radii . Pilot studies were required to define systematically the

range of parameters for th is s tudy on cycl ic fa t igue . The radi i of

curvature and angles of curvature chosen could be considered so me

of the more s igni f icant tha t are encountered in c l in ica l pract ice .

When the s imulated canals in th is s tudy were measured us ing the

Schneider method, the 2-mm radius , 30- , 45- , and 60-degree angle

groups measured 26, 40, and 52 degrees , respect ively . The 5-mm

radius, 30-, 45-, and 60-degree angle groups measured 24, 35, and

43 degrees , respect ively . Authors have c i ted average canal curva-


7/9

V o l . 23 N o . 2 F eb ru ary 1997

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

b e t w e e n 2 0 t o 3 0 d e g r e e s , w i t h r a n g e s b e t w e e n 6 t o 4 8 e g r e e s

( 1 6 ) . O u r 3 0 - d e g r e e a n g l e g r o u p s a r e w e l l w i t h i n t h e a v e r a g e s o f

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

l a r g e r t h a n 3 0 d e g r e e s w o u l d b e e x p e c t e d t o b e e n c o u n t e r e d l e s s

f r e q u e n t l y .

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

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

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

d r i v e n r o t a r y i n s t r u m e n t s . C o n s i s t e n t l y , a d e c r e a s e i n t h e ra d i u s o f

c u r v a t u r e f r o m 5 m m t o 2 m m r a d i u s r e s u l t e d i n a s i g n i f i c a n t

d e c r e a s e i n t h e n u m b e r o f c y c l e s t o fa i l u r e ( F ig . 6 ) . E x p l a n a t i o n o f

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

s t a ti n g t h a t s t r e ss o n t h e i n s t r u m e n t i s i n v e r s e l y p r o p o r t i o n a l t o t h e

r a d i u s o f c u r v a tu r e . T h e r e f o r e , a s t h e r a d i u s o f c u r v a t u r e d e c r e a s e s ,

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

c r e a s es . T o o u r k n o w l e d g e , r a d i u s o f c u r v a t u r e h a s n o t b e e n

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

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

C l i n i c a l l y , t h e s w e e p i n g c u r v a t u r e o f t h e 5 - m m r a d i u s g r o u p s ,

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

m a n a g e t h a n t h e a b r u p t c u r v a t u r e r e p r e s e n t e d b y t h e 2 - m m r a d i u s

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

c a n a l o r i n d i l a c e r a t e d r o o ts . B o t h c o n d i t i o n s a r e a s s o c i a t e d w i t h a n

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

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

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

a g e s , z i p p i n g , a n d p e r f o r a ti o n . H o w e v e r , r a d i u s o f c u r v a t u r e w i t h

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

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

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

t h o r o u g h l y , a n d t h e c l i n i c al e f f e c t o f r ad i u s o f c u r v a t u r e o n i n s t r u -

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

c l e a r l y d e f i n e d .

T h e 3 0 - d e g r e e a n g l e g r o u p s h a d s i g n i f i c a n t l y g r e a t e r c y c l e s t o

f a i l u r e t h an t h e 4 5 - a n d 6 0 - d e g r e e a n g l e g r o u p s , r e g a r d l e s s o f s iz e

( F i g . 6 ) . T h i s w a s a n u n e x p e c t e d r e s u l t w h e n t h e e n g i n e e r i n g

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

s e e m s

n e g l i g i b l e o r u n c e r t ai n . I t w a s s u s p e c t e d t h a t t h e 0 .8 3 m m d i a m e t e r

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

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

s t r u m e n t s h a f t s i n th e 3 0 - d e g r e e a n g l e g r o u p s . T h i s l a c k o f c o n -

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

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

s h a f t t o f o l l o w a l a r g e r r a d iu s p a t h w i t h i n t h e g u i d e t u b e . D e p e n d -

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

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

2 - a n d 5 - r a m g r o u p s . H o w e v e r , i n t h e m o s t h i g h l y s t r e s s e d s t a t e s

s t u d i e d ( s iz e 4 0 i n s t r u m e n t s i n t h e 2 - r a m r a d i u s ) , i n c r e a s i n g a n g l e

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

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

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

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

T h e L i g h t s p e e d i n s t r u m e n t i s o n e o f s e v e r a l N i - T i s y s t e m s

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

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

i s

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

h e a d . T h e s e f e a t u r e s m a k e L i g h t s p e e d i n s t r u m e n t s g o o d e x p e r i -

m e n t a l i n s t r u m e n t s i n s e v e r a l re s p e c t s . I n p a r ti c u l a r , t h e d e s i g n o f

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

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

C yc l i c F a t i g u e o f N i - T i R o t a ry I n s t ru m en t s 83

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

c a t i o n s . T h e s l e n d e r s h a f t a l lo w s q u i t e s e v e r e b e n d s w i t h o u t p e r -

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

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

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

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

s e p a r a t i o n o f L i g h t s p e e d i n s t r u m e n t s . A l l i n s t ru m e n t s t e s t e d i n th i s

s t u d y s e p a r a t e d w i t h i n a 5 - m i n t i m e l i m i t .

A n o t h e r a d v a n t a g e o f u s i n g t h e L i g h t s p e e d i n s t r u m e n t i n th i s

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

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

w a s p e r f o r m e d a t 7 5 0 , 1 3 0 0 , a n d 2 0 0 0 r p m . T h e t h r e e r p m g r o u p s

w e r e n o t s t a t i s t i c a l l y d i f f e r e n t f r o m o n e a n o t h e r ( F i g . 4 ) . U p o n

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

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

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

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

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

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

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

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

a n d t h e c r a c k r e s h a r p e n s ( 9 ) . T h i s m i n u t e p r o p a g a t i o n o c c u r s o n

t h e o r d e r o f 1 0 - 6 s o r a p p r o x i m a t e l y a t t h e s p e ed o f s o u n d i n m e t a l

( l 7 ) , a n d t h e p r o p a g a t i o n e v e n t i s c o m p l e t e d m u c h f a s t e r th a n t h e

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

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

I t i s i m p o r t a n t t o r e c o g n i z e t h a t a n i n s t r u m e n t h a s a m e a n

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

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

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

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

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

o f c y c l e s t o f a i l u r e a v a i la b l e . T h i s i s e s p e c i a l l y t r u e w h e n a N i - T i

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

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

h a s b e e n a d v o c a t e d f o r N i - T i , e n g i n e - d r i v e n s y s t e m s , t h o s e r e c -

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

a s a c au s e o f i n s t r u m e n t s e p a r a t io n . N o n e t h e l e s s , t h e e f f e c t o f r p m

w h e n s t u d y i n g t h e b r e a k a g e o f N i - T i i n s t r u m e n t s h a s n o t b e e n

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

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

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

s p a n i s i n v e r s e l y p r o p o r t i o n a l t o i n s t r u m e n t s i z e 0 2 , 1 3 ) an d t h a t

m e t a l f a t i g u e i s d e e p l y i m p l i c a t e d i n f i l e b r e a k a g e ( 2 , 8 ). 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 h a d a l o w e r f a t i g u e l i f e , r e s u l t i n g i n s i g n i f -

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

( F i g . 5 ) . T h e d i f f e r e n c e i n a v e r a g e s h a f t d i a m e t e r s b e t w e e n

t h e s e

t w o i n s t r u m e n t s i s o n l y 0 . 0 4 6 r a m . T h 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

w a s u n d e r m o r e s t r e ss w i t h i n t h e c o n f i n e s o f a d e f i n e d c u r v a t u r e .

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

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

g e r i n c l i n i c a l p r a ct i c e . I n f a ct , r o t a r y N i - T i i n s t r u m e n t s w i t h l a r g e r

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

s m a l l e r d i a m e t e r i n s t r u m e n t s .

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

m e a n n u m b e r o f c y c l e s to f a i l u r e f o r e i t h e r s i z e 3 0 o r 4 0

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

e x p l a n a t i o n a r e re q u i r e d . F i r s t , th e p o i n t o f m a x i m a l f l e x u r e i n t h e

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

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

C l i n i c a l l y , a x i a l m o t i o n ( i n a n d o u t m o v e m e n t s ) d u r i n g i n s t r u m e n -


8/9

8 4 P r u e t t e t a l

t a t io n w i t h N i - T i, e n g i n e - d r i v e n r ot a ry i n s t r u m e n t s w o u l d b e e x -

p e c t e d , a s c o n c l u d e d b y D e d e r i c h e t a t . ( 1 2 ) e x a m i n i n g s t a i n l e s s -

s t e e l i n s t r u m e n t s , to s i g n i f i c a n t l y e x t e n d i n s t r u m e n t l if e . N i - T i

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

w i t h c o n t i n u o u s a x i a l m o t i o n i n t h e c a n a l . L i n g e r i n g a t a s i n g l e

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

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

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

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

S e c o n d , e n g i n e e r i n g a p p l i c a t i o n s o f N i - T i a l l o y s h a v e b e e n

e x t e n s i v e l y i n v e s t i g a t e d ( 1 8 - 2 0 ) , a n d t h e f a t i g u e b e h a v i o r o f N i - T i

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

a l l o y . A t l o w s t a ti c s t r e ss , t h e a l l o y e x h i b i t s l i n e a r s t r ai n . O n c e t h e

s t r e ss r i s e s a b o v e t h e y i e l d s t r e s s , s m a l l i n c r e a s e s i n t h e s t re s s w i l l

g i v e r i s e t o l a r g e i n c r e a s e s i n s t r a i n t h a t , u n l i k e m o s t m e t a l s , a r e

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

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

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

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

l o w - c y c l e f a t i g u e b e h a v i o r ( 1 8 ) , w h i c h r e s u l t s i n v e r y r a p i d r e d u c -

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

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

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

c r a c k p r o p a g a t i o n p r o c e s s . T h i s s t u d y w a s c o n d u c t e d i n t h e r a n g e

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

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

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

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

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

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

y i e l d s t r e s s , o c c u r r e d b e t w e e n a n g l e s o f 2 5 t o 3 0 d e g r e e s w h e n a

s i z e 3 0 i n s t r u m e n t p a s s e d t h r o u g h a 5 - r a m r a d i u s c u r v a tu r e . A t 3 0

d e g r e e s , l o w - c y c l e f a t i g u e b e h a v i o r r e s u l t e d . A t 2 5 d e g r e e s a n d

b e l o w , t h e f a t i g u e li f e w a s i n d e f i n i t e l y l o n g , s u g g e s t i n g e l a s t ic

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

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

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

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

M o s t c l i n i c a l a p p l i c a t i o n s o f N i - T i i n s t r u m e n t s w i l l a t l e a s t p e r i -

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

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

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

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

c a n a l c u r v a t u r e d e t e r m i n e ~ t r a d i o g r a p h i c a l l y . F l e x e d c o n d i t i o n s

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

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

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

in th is s tudy (18) .

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

u r e w i t h i n th e c u r v e d s e g m e n t o f t h e tu b e , w h e r e i n t h e s t r e s s o n t h e

i n s t r u m e n t w a s g r e a te s t ( F i g . 8 ). T h e f r a c t u r e d s u r f a c e s s e e m e d

s i m i l a r t o t h o s e p r e v i o u s l y o b s e r v e d f o r f a t i g u e t e s t i n g ( 1 i ) . U n -

f l e x e d , p a r t i a ll y f r a c t u r e d N i - T i i n s t r u m e n t s s e e m e d n o r m a l w i t h -

o u t p e r m a n e n t p l a st i c d e f o r m a t i o n. U n d e r f l e x u re , o p e n i n g o f t h e

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

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

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

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

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

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

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

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

N i - T i L i g h t s p e e d i n s t r u m e n t s a l w a y s o c c u r re d - - 2 m m b e h i n d t h e

i n s t r u m e n t h e a d ( 7 ) . T h a t f a i l u r e m o d e i s l i m i t e d t o s t a t i c t e s ti n g .

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

c l i n i c a l l y , tw o e x p l a n a t i o n s a r e p o s s i b l e . F i r st , s e p a r a t i o n o c c u r r e d

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

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

f a t i g u e f a i l u r e d i r e c tl y b e h i n d t h e h e a d . T h e h e a d i s i n f l e x i b l e a n d

m u s t p a s s t h r o u g h a n y c u r v a t u r e t h a t t h e s h a f t e x p e r i e n c e s . E x -

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

s h a f t t o f le x j u s t b e h i n d t h e h e a d . B o t h c i r c u m s t a n c e s l e a d t o s t r e s s

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

c a u s e d b y b i n d i n g a n d f a t i g u e. S e p a r a t io n o f u n b o u n d i n s t r u m e n t s

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

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

F u r t h e r s t u d y i s r e q u i r e d t o i m p r o v e t h e u n d e r s t a n d i n g o f c r i -

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

t o s e p a r a ti o n . T h e f a t ig u e o f a n i n s t r u m e n t c a n b e b r o k e n d o w n

i n t o s t a g es a s e v i d e n t o n S E M e x a m i n a t i o n ( F i g . 9 ) . S t a g e I , c r a c k

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

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

v e r s i b l e t h r o u g h p r o p e r h e a t t r e a t m e n t ( 9 ) . S t e r i l iz a t i o n h a s b e e n

s u g g e s t e d t o e n h a n c e t h e f a t i g u e l i fe o f N i - T i f i l e s a n d r e v e r s e t h e

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

p h a s e ( 2 ) . F u r t h e r m o r e , s t e r i l i z a t i o n m a y r e v e r s e t h e c r a c k i n i t i a -

t i o n s t a g e o f f r a c t u r e i n N i - T i a l l o y s , b u t t h i s i s a n a r e a f o r f u t u r e

i n v e s t i g a t i o n . T o r s i o n a l l o a d i n g d u r i n g r o t a t i o n a l u s e i s a n o t h e r

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

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

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

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

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

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

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

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

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

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

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

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

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

d e s i g n o f N i - T i , e n g i n e - d r i v e n ro t ar y i n s t r u m e n t s a n d w a r r a n t

fu r the r s tudy .

S p e c i f ic r e c o m m e n d a t i o n s f o r p r e v e n t i n g b r e a k a g e o f L i g h t -

s p e e d i n s t r u m e n t s c a n n o t b e d r a w n f r o m t h i s s tu d y . H o w e v e r ,

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

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

c a n b e m a d e . T o p r e v e n t t h e b r e a k a g e o f N i - T i , e n g i n e - d r i v e n

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

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

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

w o u l d b e e x p e c t ed t o v a r y w i d e l y d e p e n d i n g o n i n s t r u m e n t d e s ig n ,

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

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

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

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

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

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

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

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

t i o n a l t o i n s t r u m e n t st r e ss . S m a l l e r r a d i u s o f c u r v a t u r e a n d i n -

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

c a u s e d b y h i g h e r i n s t r u m e n t st r es s . C y c l e s t o f a il u r e w a s n o t

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


9/9

V o l. 2 3 N o . 2 F e b r u a r y 1 9 9 7

t o f a i lu r e w a s u n c l e a r . N e w N i - T i i n s t r u m e n t s y s t e m s a r e c u r r e n t l y

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

a g r o w i n g n u m b e r o f in c i d e n t s o f in s t r u m e n t s e p a r a t io n a r e b e i n g

i n f o r m a l l y d i s c u s s e d . G i v e n t h e i n c r e a s e d n u m b e r o f N i - T i e n d -

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

l i k e l y t o b e c o m e m o r e c o m m o n . T o d a t e t h e r e is n o te s t p r o t o c o l

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

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

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

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

m e n t s i n c l u d i n g N i - T i i n s t r u m e n t s d e s i g n e d t o b e e n g i n e - d r i v e n

a n d f u l l y p r e p a r e c u r v e d c a n a ls . F u r t h e r m o r e t h e p a r a m e t e r s o f

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

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

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

t h e s a fe u s e o f N i - T i e n g i n e - d r i v e n r o t a ry i n s t r u m e n t s w i l l r e m a i n

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

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

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

Th is s t u d y w a s su p p o r t e d i n p a r t b y a n E n d o d o n t i c S t u d e n t A w a rd f r o m

the Research and Educat ion Foundat ion o f the Amer i can Associa t i on o f

Endodont i s ts .

The op in ions, asse r t i ons, mater ia l s, and m ethodo log ies here in a re p r i va te

ones o f the au thors and are no t to be c onst rued as o f f i c ia l o r re f lect ing the

v i e w s o f t h e A m e r i ca n A sso c i a t io n o f E n d o d o n t i s ts o r t h e Fo u n d a t io n .

We gra te fu l l y acknow ledge David S . P i t tman o f Sunset Resources, Inc.

San Anton io , TX) fo r mod i fy ing and wr i t i ng the da ta co l l ect i on p rog ram fo r the

Magt ro l Dynamometer . We express our g ra t i tude to Drs. James A . Gi l l es,

Wi l l iam A. Walker I l l , and Carlos E. del Rio for thei r advice during art icle

prepara t ion .

Drs. P rue t t , C lement , and Carnes are a f f i l i a ted w i th the Depar tment o f

Endodo nt i cs/Denta l Schoo l , Un ivers ity o f Texas Hea l th S cience Center a t San

Anton io , San Anton io , TX . Address requests fo r repr in ts to Dr . David L .

Carnes, Jr . , Depar tment o f Endodont i cs/Denta l Schoo l , Un ivers i ty o f Texas

Hea l th Science Center a t San Anton io , 7703 F loyd Cur l Dr ive , San Anton io , iX

78284-7898.

C y c l i c F a t i g u e o f N i - T i R o t a r y I n s t r u m e n t s 8 5

References

1. Wal ia H, Brant ley WA, Gerstein H. An ini tia l invest igat ion of the bending

and to rs iona l p roper t i es o f n i ti nol roo t cana l f il es. J Endodon 1988;14 :346 -51 .

2 . Serene TP, Adams JD , Saxena A . N icke l - t i tan ium inst ruments: app l i -

cat io ns in endodo nt ics, St. Louis: Ishiyaku EuroAmerica, Inc., 1994.

3 . GIosson CR, Ha i le r RH, Dove SB, de l R io CE. A compar i son o f roo t

canal preparat ions using Ni-Ti hand, Ni-Ti engine driven, and K-Flex end-

odont i c i nst ruments. J Endodon 1995;21 :146-51 .

4 . C ohen S , Burns RC. Pathways o f the pu lp . 6 th ed . S t . Louis : Mos by-

Year Book, Inc., 1994:206.

5 . C ounci l on Denta l Mater ia l s and Devices. New A DA Speci f i ca t ion No.

28 fo r endodont i c f i l es and reamers. J Am Dent Assoc 1976;93 :813-8 .

6 . Cam ps J, Per to t WJ. Torsiona l and st i f fness p roper t i es o f Cana l Master

U sta in less s tee l and n i t ino l i nst ruments. J Endodon 1994;20 :395 -8 .

7 . Mars i cov e tere ES, Burgess JO, C lement D J, de l R io CE. Tors iona l

test ing o f the l i gh tspeed N iT i i nst rument system. J End odon in p ress) .

8 . So to kaw a T. An ana lys is o f c l in i ca l b reakage o f roo t cana l i nst ruments.

J Endodon 1988;14 :75-82 .

9 . D ie te r GE. Mechan ica l meta l lu rgy. 3 rd ed . New York: McG raw-Hi l l ,

1986:119, 138, 185-8, 382-7, 394.

10 . B ran t ley WA, Luebk e H, Luebke FL, M i tche l l JC. P er fo rmanc e o f

eng ine-dri ven ro ta ry endodont i c i nst ruments w i th a super imp osed be nd ing

def lect ion . V . Gates-G l idden and pees o dri ll s . J Endodo n 19 94;20 :241-5 .

11. Luebke NH, Brant ley WA, Sabri ZI, Luebke FL, Lausten LL. Physical

d imensions, to rs iona l per fo rmance, bend ing proper t i es, and meta l l u rg i ca l

characte r i s t i cs o f ro ta ry endodont i c i nst ruments. V I . Cana l Master d r i l l s . J

E n d o d o n 1 9 9 5 ; 2 1 : 2 5 9 -6 3 .

12 . Deder i ch DN, Zakar iasen KL. The e f fects o f cyc l i ca l ax ia l mot ion on

ro ta ry endodo nt i c i nst rumen t fa ti gue. Ora l Surg 198 6;61 :192- 6 .

13 . Ha ikel Y , Gasser P , A l l emann C. Dynam ic f ractu re o f hybr id endo don-

t ic hand instruments co mp ared wi th tradit ional f i les. J Endodon 1991 17:217-20.

14 . Crandal l SH, Dah l NC, Lardner TJ. An in t roduct ion to the mech an ics o f

so l i ds. 2 r id ed . New York: McGraw-Hi l l , 1972:416-77 .

15 . Schne ider SW. A compar i son o f cana l p repara t ions in s t ra igh t and

curved canals. Oral Surg 1971;32:271-5.

16. Cunningham CJ, Senia ES. A three-dimensional study of canal curvatures

in the mesial roots of mandibular molars. J Endodon 1992;18:294-300.

17 . Kann inen MF, Pope la r CH. Adva nced f ractu re mechan ics. Ne w York:

Ox ford University Press, 1985:22, 1980.

18 . Me l ton KN, M erc ie r O. Fa t igue o f N iT i thermo e last i c mar fensi tes. Acta

M e t 1 9 7 9 ; 2 7 : 13 7 -4 4 .

19 . M iyazaki S , Sug aya Y , Otsuka K . E f fects o f var ious facto rs on fa t i gue

l i fe of Ti-Ni al loys. MRS In t Mtg Aclv Ma ter 1989 ;9:251 -6.

20 . Tobush i H, Iwanaga H, Tanaka K , Hod T, Sawa da T. S t ress-st ra in -

tempe ra ture re la t ionsh ips o f T iN i shape m emor y a l loy su i tab le fo r thermo me-

chanical cycl ing. JSME Int J Series I 1992;35:271-7.

You Migh t e In te res ted

A case o f f raud was recen t ly uncovered in wh ich an imposte r had , over the past 25 years , managed to ge t

a number o f essays and le t te rs to the ed i to r pub l ished in respected journa ls . The JAM A unm asked the rasca l

because h is wr i t ing s ty le had been recogn ized (BMJ 311 523 , 1995) .

Of course , there was ano ther c lue . The f ic t ic ious name s igned to a l l those pub l ished le t te rs over a l l those

ye a rs w a s - -yo u ' l l l o ve t h i s - -T . H . F id d lema n .

Zachariah eomans

Documents

cyclic fatigue of NiTi instruments