7/21/2019 1-In Vivo Cusp Fracture of Endodontically Treated Premolars Restored With MOD Amalgam or MOD Resin Fillings
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I n v i v o c u s p f r a c t u r e o f
e n d o d o n t ic a l ly t r e a t e d
p r e m o l a r s r e s t o r e d w i t h M 0 Dam algam or M 0D res i n fillin gs
E . K . H a n s e n
Depar tment o f Techno logy, Roya l Denta lCo l lege, Copenhagen, Denmark
H a n s e n E K . In vivo c u s p f ra c t u r e o f e n d o d o n t i c a l l y t r e a t e d p r e m o l a r s r e s t o r e d
w i t h M O D a m a l g a m o r M O D r e s in fi ll in g s . D e n t M a t e r 1 9 88 : 4 : 1 69 - 17 3 .
A b s t r a c t - T h e f r e q u e n c y o f c u sp f r a c tu r e o f e n d o d o n t i c a U y t re a t e d p r e m o l a r s
w a s i n v e s t ig a t e d i n a r e t r o sp e c t i v e s tu d y . A f t e r e n d o d o n t i c t h e r a p y , t h e t e e t h
w e r e e i t h e r r e s t o r e d w i th a M O D a m a l g a m f il li n g o r w i th a n e n a m e l - b o n d e d
M O D r e s in f i ll in g . A v e r y h ig h f r e q u e n c y o f c u sp f r a c t u r e w a s f o u n d i n
p r e m o l a r s r e s t o r e d w i t h a m a l g a m : n e a r l y o n e - t h i r d f r a c t u r e d w i t h in 3 y e a r s
a f t e r e n d o d o n t i c t r e a t m e n t . T h e f r e q u e n c y o f c us p f ra c t u r e d i f f e re d a m o n g t h e
4 a m a l g a m - r e s t o r e d t e e t h ; t h e l o w e r fi rs t p r e m o l a r h a d a 1 5 - y e a r s u r v iv a l r a te
o f 7 4 % , i n c o n t r a s t to a n a v e r a g e o f 32 % f o r th e o t h e r 3 p r e m o l a r s . N o r e si n -
r e s t o r e d p r e m o l a r f r a c t u r e d d u r i n g t h e f i rs t 3 y e a r s , a n d t h e d i f fe r e n c e b e t w e e n
a m a l g a m a n d r e s i n - r e s t o r e d t e e t h w a s h i g h l y s ig n i fi c a n t a t t h e 0 - 3 y e a r i n te r v a l.
H o w e v e r , f r a c t u r e o f re s i n - r e s to r e d p r e m o l a r s d i d h a p p e n i n t h e f o ll o w i n g 3 - 1 0
y e a r i n t e r v a l. F r o m a p e r i o d o n t a l p o i n t o f v i e w , a lo w f r e q u e n c y o f c u sp
f r a c t u re c a r r i e s g r e a t w e i g h t , a n d i t i s t h e r e f o r e c o n c l u d e d t h a t a n e n a m e l -
b o n d e d r e s i n f il li n g m a y b e a t r e a t m e n t o p t i o n m u c h p r e f e r r e d t o a m a l g a m i n
t e m p o r a r i l y r e s t o r in g e n d o d o n t i c a l l y t r e a t e d p r e m o l a r s w i th M O D c a v i ti e s.
Key words: endodont i c therapy, restorative
dentistry, amalgam, enamel-bonded resin, tooth
failure.
E. Kei th Hansen, Helsing6rsgade 7, DK-3400Hi l ler6d, Denmark.
Received Apri l 27; accepted July 24, 1987.
E n d o d o n t i c t h e r a p y c a u s e s a p r o -
n o u n c e d w e a k e n i n g o f t h e t o o t h a n d a
c o n s e q u e n t h i g h r i s k o f c u sp o r c r o w n
f r a c t u r e . G h e r e t a l . ( 1 ) s t u d i e d t h e
c l i n ic a l f e a t u r e s a s s o c i a t e d w i t h t o o t h
f r a c t u re a n d f o u n d t h a t 7 1 % o f t h e
f r a c t u r e d t e e t h h a d b e e n e n d o d o n t i -
c a l l y t r e a t e d .
T o o t h f r a c t u r e i s a s e v e r e p r o b l e m
b e c a u s e t h e f r a c t u r e u s u a l l y e x t e n d s
s u b g i n g i v a l l y a n d n o t i n f r e q u e n t l y r e -
s u l t s i n l o s s o f p e r i o d o n t a l a t t a c h m e n t ;
t h e f r a c t u r e m a y e v e n b e s o v e r t i c a l
t h a t t h e t o o t h h a s t o b e e x t r a c t e d . I f t h e
t o o t h c a n b e s a v e d , t h e m a i n p r o b l e m
f o r t h e d e n t i s t i s b o t h t e c h n i c a l a n d b i o -
l o g i c a l i n n a t u r e , i . e . t h e t e c h n i c a l d i f fi -
c u l ti e s r e l a t e d t o t h e i m p r e s s i o n - t a k i n g
i t se l f a n d , b y i m p r e s s i o n - t a k i n g , t h e
b i o l o g i c a l r i sk o f f u r t h e r d a m a g i n g t h e
p e r i o d o n t a l t is s u e s (2 ) . E v e n i f t h e d e n -
t i s t s u c c e e d s i n f a b r i c a t i n g a w e l l -
a d a p t e d r e s t o r a t i o n , t h e p a t i e n t m a y
f a c e a n o t h e r p r o b l e m . M a n y i n v e s t i g a -
t o r s h a v e r e p o r t e d t h a t s u b g i n g i v a l l y
p l a c e d r e s t o r a t i o n s a l w a y s r e s u l t i n a n
i n f l a m m a t o r y p e r i o d o n t a l r e s p o n s e ( 3 -
7 ) . A s a c o n s e q u e n c e o f t h i s , i t h a s b e -
c o m e a c c e p t e d p r o c e d u r e t o m a i n t a i n
r e s t o r a t i o n m a r g i n s i n t h e s u p r a g i n g i v a l
a r e a w h e n e v e r p o s s ib l e . H o w e v e r , i n
c a s e s o f c u s p f r a c t u r e , o n e n e a r l y a l -
w a y s h a s t o a c c e p t a s u b g i n g i v a l r e s t o r -
a t io n m a r g i n a n d t h e r e b y a p r o n o u n c e d
p o s s i b il i ty o f p e r m a n e n t i n j u r y t o t h e
p e r i o d o n t a l t is s u e s.
I n o r d e r t o p r e v e n t t o o t h f r a c t u r e ,
t h e o p t i m a l r e s to r a t i o n o f e n d o d o n t i -
c a l l y t r e a t e d t e e t h i s a f u ll c r o w n o r a
c a s t i n l a y t h a t p r o t e c t s t h e c u s p s ( 8 ) ,
b u t m a n y d e n t i s ts d o n o t f i n d i t a c c e p t -
a b l e t o f a b r i c a t e t h e f i n a l r e s t o r a t i o n
b e f o r e p e r i a p i c a l h e a l in g h a s b e e n c o n -
f i r m e d w i t h r a d i o g r a p h s 6 - 1 2 m o n t h s
l a t e r. H o w e v e r , o n e o f t h e c u s p s , o r t h e
c r o w n i n w h o l e , m a y a c t u a l l y f ra c t u r e
d u r i n g t h i s o b s e r v a t i o n p e r i o d . S t i ll a n -
o t h e r p r o b l e m i s t h a t c a s t r e s t o r a t i o n s
a r e e x p e n s i v e a n d q u i t e a f e w p a t i e n t s
c a n n o t a f f o r d t h e m . I n t h e s e c a s e s, t h e
d e n t i s t h a s b u t t w o c h o i c e s : a m a l g a m
o r c o m p o s i t e r e s i n .
T h i s r e t r o s p e c t i v e s t u d y w a s u n d e r -
t a k e n t o e x a m i n e w h e t h e r , a n d t o w h a t
e x t e n t , t h e u s e o f a n e n a m e l - b o n d e d
r e s t o r a t iv e r e s i n i n M O D c a v i ti e s o f e n -
d o d o n t i c a l ly t r e a t e d p r e m o l a r s h a d r e -
d u c e d t h e f r e q u e n c y o f to o t h f r a c t u r e
w h e n c o m p a r e d t o e n d o d o n ti c a ll y
t r e a t e d p r e m o l a r s r e s t o r e d w i t h a
M O D a m a l g a m f i ll in g .
M a t e r ia l a n d m e t h o d s
T h e m a t e r i a l w a s c o l le c t e d fr o m M a y
1 9 8 6 t o F e b r u a r y 1 9 8 7 f r o m 8 d e n t i s t s
w o r k i n g a s g e n e r a l p r a c t i t i o n e r s . T h e
d a t a w e r e d e r i v e d f r o m c l in i c a l e x a m -
i n a t i o n s a n d r e v i e w o f t h e p a t i e n t s '
d e n t a l r e c o r d s a n d r a d i o g r a p h s .
T h e l i m i t i n g c r i t e r i a f o r a c c e p t a n c e
o f d a t a w e r e t h e f o l lo w i n g : ( i) a n M O D
c a v i ty in a n e n d o d o n t i c a l l y tr e a t e d p r e -
m o l a r e i t h e r r e s t o r e d w i t h a m a l g a m o r
r e s t o r e d w i t h a c o m p o s i t e r e s i n a f t e r
p r e v i o u s a c i d - e t c h i n g o f t h e e n a m e l ;
( i f ) n o w e a k e n i n g o f a n y o f t h e c u s p s
c a u s e d b y a C l a s s V r e s t o r a t i o n ; ( i i i )
n o r m a l o c c l u s io n a n d a r t i c u la t i o n ; a n d
( i v ) c o m p l e t e p e r m a n e n t d e n t i t i o n i n
t h e a n t e r i o r a n d p r e - m o l a r r e g i o n p l u s
a t le a s t o n e m o l a r i n e a c h q u a d r a n t i n -
v o l v e d i n t h e o c c l u s i o n . T h e f o l l o w i n g
i n f o r m a t i o n w a s re c o r d e d : n a m e o f t h e
12 DentalMaterials4:4. 1988
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1 7 0 H a n s e n
individual tooth, date of endodont ic
therapy, restorative material, and date
of control. For restorations with a com-
posite resin: type of cavity preparation
(bevel or butt-joint), use of an i nterme-
diate layer of low-viscous resin, and ap-
plication technique (bulk or incre-
ment). In cases of fracture, the dentists
were asked to record the date of regis-
tration of fracture, whether the fracture
involved the facial cusp, the lingual
cusp, or the crown as a whole, and also
whether the fracture resulted in extrac-
tion of the tooth. Informa tion was ob-
tained on 222 endodontic ally treated
premolars restored with either a MOD
amalgam or an enamel -bonded MOD
resin filling. It should be emphasized
that the restorative resins in this study
were applied after previous acid-etch-
ing of the ena mel, but without use of a
dentin- bonding agent.
Trial time of the endodontically
treated premolars was defined as the
time elapsing between the date of en-
dodontic therapy and (a) the date of
fracture; (b) the date of last contact ; (c)
the date of withdrawal; or (d) the
date of final registration. Total trial
time was arranged into intervals of one
year and, for each interval, the effec-
tive numb er of teeth exposed to risk of
fracture was calculated. The cumula-
tive survival rate was then calculated
using life table analysis (9, 10). Ana-
lyses of differences between the cum-
ulative survival rate of each of the 4
premolars and of differences between
MOD amalgam and MOD resin restor-
ations were tested at the 5% level of
significance by means of Chi-square
tests (10).
Resu l ts
Table 1 shows the num ber of endo don-
tically treated premolars restored with
Table 1. Number of endodontically treatedpremolars restored with a MOD amalgam orwith an enamel-bonded MOD resin filling.Number of fractures are shown in paren-theses.
Tooth MOD restorationsnumber*
Amalgam Resin
14, 24 48 (27) 23 (2)15, 25 70 (45) 11 (3)34, 44 13 (3) 2 (0)35, 45 50 (27) 4 (0)
Total 181 (102) 40 (5)
* = Viohl's two-digit system.
10 0
8 O
L UI- -
r
6o
U .I
40
g(. 9
20
%
9 ".
' , . ' \
\ - . .\ \ " - %
\ \ " ' . .
\ \ " ' ., . . . . . . . . . .
I I I I I I I _ Y E A R S A F T E R
2 /. 6 8 10 121 4
TREATMENT
Fig. 1. Cumulative survival rate of endodontically treated premolars restored with MODamalgam fillings. - . . . . lower first premolars; . . . . . . lower second premolars; - - = upperfirst premolars; . . . . upper second premolars.
a MOD amalgam or with an enamel-
bonded MOD resin filling.
F r a c t u r e p a t t e r n . Less than 10% of the
fractures included the crown as a
whole, i.e. usually only one of the cusps
was lost. When the latter occurred, the
facial cusp of the upper first premolar
fractured in 52% of the cases, in con-
trast to 29% for the upper and lower
second premolar. This difference was
statistically significant at the level of p
<0.05. The lower first premolar only
fractured lingually. The cusp fracture
nearly always ended subgingivally, and
3 of the teeth had to be extracted be-
cause of vertical root fracture.
M O D a m a l g a m . Within the first year
after endodontic therapy, 13% of the
premolars fractured and nearly one-
third failed during the first 3 years. Th e
cumulative survival rates for each of
the 4 premolars are shown in Fig. 1; for
the sake of clarity, 95% confidence in-
tervals are omitted. The lower first pre-
molar had the highest 15-year survival
rate (74%), while the upper second
premolar showed the lowest survival
rate (26%). No statistically significant
difference was found between the sur-
vival rates of the 2 upper premolars an d
that of the lower second premolar (p
>0.1), while the difference between
these 3 teeth and the lower first premo-
lar was statistically significant at the
level of p < 0.05.
M O D r e s i n . The restorative resins were
all chemically activated (Table 2); the
"unknown" resin was assessed as being
a chemically activated macrofilled com-
posite because of the surface texture,
and the fact that this restoration was
made in 1974, i.e. before both micro-
filled, hybrid, and light-activated com-
posites were marketed. Tooth failure
was independen t of the type of compos-
ite resin used (Table 2); cusp fracture
was thus found with both Adaptic and
Concise (macrofilled), Silar (micro-
filled) and P-10 (hybrid). The vast ma-
jority of the resin-restored teeth were
upper premolars, among which all 5
fractures were found (Table 1).
A bulk application technique was
used in 4 of the 5 fractured teeth; as to
the fifth fracture, no inf ormation on ap-
plication technique was obtaina ble.
Bevel preparation, both occlusally
and proximally, followed by applica-
Table 2. Restorative resins used and numberof fractures.
Material Type Number Fractureof teeth
Adaptic Ma 1 lConcise Ma 20 2P-10 Hy 9 1Silar Mi 9 1Unknown Ma 1 0
Ma = macrofilled; Mi = microfilled;Hy = hybrid.
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F r e q u e n c y o f c u s p f r a c t u r e i n v i v o 171
t i o n o f a l o w - v i s c o u s r e s i n , w a s u s e d i n
3 0 o f t h e 4 0 r e s i n - r e s t o r e d p r e m o l a r s ;
n o i n f o r m a t i o n o n b e v e l / b u t t - jo i n t an d
l o w - v i s c o u s r e s i n w a s o b t a i n a b l e f o r t h e
r e m a i n i n g 1 0 fi l li n g s . F o u r o f t h e 5 f r a c -
t u r e s w e r e f o u n d i n t e e th w i t h b e v e l e d
c a v i t y m a r g i n s .
A m a l g a m v s . r e s i n . A s m e n t i o n e d e a r -
l i e r , t h e c u m u l a t i v e s u r v i v a l r a t e o f
a m a l g a m - r e s t o r e d l o w e r f ir st p r e m o l a r s
w a s s i g n i f i c a n t l y h i g h e r t h a n t h e s u r v i -
v a l ra t e s o f t h e 3 o t h e r p r e m o l a r s ( F i g .
1 ) . L o w e r f i r s t p r e m o l a r s a r e t h e r e f o r e
n o t i n c l u d e d i n t h e s t a t i st i c a l a n a l y s i s o f
d i f fe r e n c e s b e t w e e n t h e s u r v i v a l r a t e o f
M O D a m a l g a m a n d M O D r e si n -r e -
s t o r e d t e e t h . A s e p a r a t e a n a l y s i s o f
l o w e r f i r st p r e m o l a r s i s m e a n i n g l e s s b e -
c a u s e o f th e s m a l l n u m b e r o f o b s e r v a -
t i o n s ( T a b l e 1 ) .
T h e p o o l e d c u m u l a t i v e s u r v i v a l r a t eo f t h e u p p e r f i r st p r e m o l a r s , t h e u p p e r
s e c o n d p r e m o l a r s a n d t h e l o w e r se c o n d
p r e m o l a r s i s s h o w n i n F ig . 2 .
C h i - s q u a r e a n a l y s i s i n a 2 x 2 c o n -
t i n g e n c y t a b l e r e q u i r e s t h a t t h e e x -
p e c t e d f r e q u e n c y i n e a c h c e l l n o t b e t o o
s m a l l ; th e i n t e r v a ls a n a l y z e d w e r e
t h e r e f o r e r e s t r i c t e d t o 0 - 3 y e a r s , 3 - 6
y e a r s , a n d 6 -- 10 y e a r s . A n a l y s i s b e y o n d
1 0 y e a r s w a s n o t m a d e b e c a u s e o n l y 8
r e s i n r e s t o r a t i o n s w e r e o l d e r t h a n 1 0
y e a r s , a n d o n l y 4 r e s t o r a t i o n s w e r e o l -
d e r t h a n 1 1 y e a r s . T h e a n a l y s i s ( T a b l e
3 ) s h o w e d a h i g h l y s i g n i f ic a n t d i ff e r -e n c e b e t w e e n t h e s u r v i v a l r a t e o f a m a l -
g a m a n d r e s i n - r e s t o r e d p r e m o l a r s a t p
< 0 . 0 0 1 ; t h e m a i n r e a s o n w a s t h a t n o
r e s i n - r e s to r e d t o o t h f r a c t u r e d d u r i n g
t h e f i r st 3 y e a r s . T h e r e s i n - r e s t o r e d p r e -
m o l a r s h a d a s l i g h t ly b e t t e r s u r v i v a l
r a t e d u r i n g t h e 3 ~ 5 a n d 6 - 1 0 y e a r in t e r -
v a l s ( F i g . 2 ; T a b l e 3 ) , b u t t h i s d i f f e r -
e n c e w a s n o t s t a t i s t ic a l l y s i g n i f ic a n t ( p
> 0 . 0 5 ) .
Discussion
L i k e a l l r e t r o s p e c t i v e s t u d i e s , t h i s i n -
v e s t i g a ti o n h a s a n i n h e r e n t d i s a d v a n -
t a g e i n t h a t t h e d a t a o b t a i n e d f r o m r e -
v i e w o f t h e p a t i e n t s ' r e c o r d s o n l y p a r -
t ia l ly a n s w e r e d t h e q u e s t io n s a s k e d .
M a n y e s s e n t i a l d e t a i ls m a y b e l o s t i n a
r e t r o s p e c t i v e s t u d y b e c a u s e m o s t p a -
t i e n t r e c o r d s d o n o t c o n t a i n a l l t h e i n -
f o r m a t i o n n e e d e d f o r a c a r e fu l i n v e s ti -g a t i o n . T h i s p r o b l e m d o e s n o t p r e v e n t
a d i s c u s s i o n o f t h e p o s s i b l e c a u s e s o f
t h e d i f f e r e n t s u r v i v a l r a t e s o f a m a l g a m
a n d r e s i n - r e s t o r e d p r e m o l a r s , b u t t h e
r e s u l t s o f t h i s s t u d y s h o u l d b e c a u -
t i o u s l y i n t e r p r e t e d , e s p e c i a l l y s i n c e t h e
n u m b e r o f r e s i n - re s t o r e d t e e t h w a s
r a t h e r s m a l l .
L a b o r a t o r y s t u d i e s h a v e s h o w n t h a t a
n o n - b o n d e d i n t r a c o r o n a l r e s t o r a t i o n
d o e s n o t s t re n g t h e n t h e t o o t h , a n d t h a t
t h e f r a c t u r e r e s i s ta n c e o f p r e m o l a r s
w i t h M O D c a v i t ie s r e s t o r e d w i th a m a l -
g a m o r w i th a n o n - b o n d e d c o m p o s i ter e s i n is s i m i l a r t o t h a t o f p r e m o l a r s w i t h
a n u n f i l l e d M O D c a v i t y ( 8 , 1 1 , 1 2 ).
S e v e r a l a u t h o r s h a v e p u b l i s h e d p a p e r s
o n t h e r e i n f o r c i n g ef f e ct o f a n e n a m e l -
b o n d e d c o m p o s i t e r e s t o r a t i o n w i t h o u t
p r e v i o u s a p p l i c a t i o n o f a d e n t i n - b o n d -
i n g a g e n t , a s in v e s t i g a t e d in t h e p r e s e n t
s t u d y , b u t t h e r e s u l t s a r e c o n t r a d i c t o r y .
R e e l & M i t c h e l l ( 1 3 ) , E a k l e ( 1 4 ) a n d
S t a m p a l i a e t a l . ( 1 5 ) d i d n o t f i n d a s ig -
n i f i c a n t i n c r e a s e i n t h e f r a c t u r e r e s i s -
t a n c e , i n c o n t r a s t w i t h S i m o n s e n e t a l ,
( 1 1 ), D o u g l a s ( 1 2 ) a n d M a c k e n z i e ( 1 6) ,
w h o a ll f o u n d t h a t p r e m o l a r s w i th e n -a m e l - b o n d e d c o m p o s i t e r e s t o r a t i o n s
w e r e s t r o n g e r t h a n t h o s e e i t h e r re -
s t o r e d w i t h a m a l g a m o r l e f t u n r e s t o r e d .
H o w e v e r , m e t h o d s u s e d i n v i t ro t o
m e a s u r e t h e f r a c t u r e r e s i s ta n c e o f t e e t h
w i th a n M O D p r e p a r a t i o n m a y n o t r e -
p r o d u c e w h a t a c t u a l l y h a p p e n s i n v i v o .
I n t h e p r e s e n t i n v e s t ig a t i o n o n l y 3 o f
t h e 1 0 7 f r a c t u r e s w e r e s o v e r t i c a l t h a t
t h e t o o t h h a d t o b e e x t r a c t e d . T h i s l o w
f r e q u e n c y o f v e r t i c a l fr a c t u r e i s n o t i n
a c c o r d a n c e w i t h th e i n v i t ro r e s u l t s p r e -
s e n t e d b y S t a m p a l i a e t a l. ( 1 5 ) ; t h e s e
a u t h o r s f o u n d t h a t a l l e x c e p t o n e o f 22
f r a c t u r e s o c c u r r e d a s a m i d d l e s p l i t o f
t h e t o o t h t h r o u g h t h e p u l p a l f lo o r o f
t h e c a v i ty p r e p a r a t i o n . T h e d i s c r e p a n c y
b e t w e e n t h e r e s u l t s o f t h e p r e s e n t s t u d y
a n d t h e r e s u l t s p u b l i sh e d b y S t a m p a l i a
e t a l . ( 15 ) m a y v e r y w e l l b e c a u s e d b y
t h e d i f f e r e n t d i m e n s i o n s o f t h e M O D
c a v i t ie s . A f u r t h e r p o s s i b l e e x p l a n a t i o nh a s b e e n g i v e n b y C r a i g ( 1 7 ) , w h o
p o i n t e d o u t t h a t i n v i t ro t e s t i n g o f a r i -
g i d ly s u p p o r t e d t o o t h i g n o r e s t h e r e -
s il ie n c y o f t h e p e r i o d o n t a l l i g a m e n t .
A l s o , i t d o e s n o t s e e m v e r y l i k e ly , co n -
s i d e ri n g t h e c o m p l e x o c c l u s a l f u n c t io n ,
t h a t i n v i v o l o a d i n g o f t h e c u s p s c a n b e
c o m p l e t e ly r e p r o d u c e d i n v i t ro w i t h t h e
t e s t m e t h o d s u s e d ( 1 1 -1 6 ) .
T h e m o s t a s t o n i s h i n g f i n d i n g i n t h e
p r e s e n t s t u d y W a s t h e v e r y h i g h f r e -
q u e n c y o f c u s p f r a c t u r e o f t h e a m a l -
g a m - r e s t o r e d p r e m o l a r s , w h e r e n e a r l y
o n e - t h i r d f r a c t u r e d w i t h in 3 y e a r s a f t e re n d o d o n t i c t r e a t m e n t ( T a b l e 3 ) . T h e s e
f r a c t u r e s , m o s t o f t e n s u b g i n g i v a l , m a y
r e s u lt i n p e r m a n e n t i n j u r y to t h e p e r i o -
d o n t a l t i s s u e s n o t o n l y b e c a u s e o f t h e
f r a c t u r e i t s e l f , b u t a l s o b e c a u s e e v e n a
w e l l - a d a p t e d r e s t o r a t i o n w i l l h a r d l y
e v e r b e s u r r o u n d e d b y t h e sa m e
h e a l t h y p e r i o d o n t a l t is s u e s a s th e n a t u -
r a l t o o t h s u r f a c e i t s e l f (3 - 7 ) .
T h e f re q u e n c y o f fr a c t u r e d i f fe r e d
a m o n g t h e 4 a m a l g a m - r e s t o r e d p re m o -
l a r s (F i g . 1 ) . T h e h i g h 1 5 - y e a r s u rv i v a l
r a t e o f t h e l o w e r f i rs t p r e m o l a r ( 7 4 % )
c o m p a r e d t o a n a v e r a g e o f 3 2 % f o r t h eo t h e r t h r e e p r e m o l a r s ( F i g s . 1 a n d 2 ) is
p r o b a b l y r e l a t e d t o b o t h t h e a n a t o m i c a l
f o r m a n d t h e l o c a t io n o f t h e t o o t h . A s
t o t h e l a t t e r , L e i n f e l d e r ( 1 8 ) h a s p r e -
s e n t e d a w e a r f a c t o r o f c o m p o s i t e r e si n
a s a f u n c t i o n o f l o c a t i o n : t h e l o w e r f i r st
p r e m o l a r w a s g i v e n a w e a r f a c t o r o f
1 x , t h e u p p e r f ir s t p r e m o l a r 3 x , a n d
t h e tw o s e c o n d p r e m o l a r s 4 x . T h e s e
w e a r f a c t o r s , b e i n g a f u n c t i o n o f l o -
c a t i o n , a r e i n f a ir l y g o o d a g r e e m e n t
w i t h t h e d i f f e r e n t f r e q u e n c i e s o f c u s p
f r a c tu r e o f t h e 4 p r e m o l a r s f o u n d i n t h e
p r e s e n t s t u d y ( F i g . 1 ) .
Table 3 . C hi - square analys is o f surv ival r a t es ( cusp/crown f r ac ture) of endod ont i ca l ly t r ea t ed perm olar s r es tor ed wi th M OD amalgam f il li ngsor wi th enam el -bond ed MO D res in f il li ngs.
Years af tert r e a t m e n t
M O D a m a l g a m r e s to r a t io n s M O D r e si n r e s to r a t io n s
1~ dx Ux l'x l'~-d ~ lx d~ u~ 1'~ l' x -d~ ;(2
0-3 168 53 9 163.5 110.5 38 0 1 37.5 37.5 16.5093-6 106 17 11 100.5 83.5 37 3 5 34.5 31.5 1.3756-10 78 19 9 73.5 54.5 29 2 19 19.5 17.5 2.144
Total Chi-squ are 20.028
1• = intac t at beg inning of interval ; d x = fai l lures dur ing interva l ; ux = withdrawn and los t cases dur ing interval ; 1 'x = effective num bersexposed to r i sk of f r ac ture .
12"
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172 H a n s e n
100
80
IL l
t Y
<
> 6 0
W>
~ 40<
IE
20
I I I
2 4 6 8
YEARS AFTERI I t
10 12 14 TREATMENT
Fig . 2 . Pooled cumulative survival rate of endodontically treated upper first premolars, up-per second premolars and lower second premolars. 9 = MOD amalgam restorations; o =enamel-bonded MOD resin restorations. Vertical bar = 95% confidence interval.
The frequency of facial cusp fracture
of the upper first premolars (52%),
compared with 29% for the upper and
lower second premolars, may be re-
lated to the anatomical form of both
the pulp chamber and the cusps (19).
The difference between the height of
the 2 cusps is more pronounced in the
upper first than in the upper second
premolar, and the occlusal slope of the
facial cusp is more steep (19), render-
ing the facial cusp of the upp er first pre-
molar more susceptible to fracture than
the lingual cusp. As to the 2 lower pre-
molars, endodontic therapy will most
often result in a weak lingual cusp.
Another interesting fact arises from
Fig. 2, which shows the cu mulat ive sur-
vival rates of amalgam and of resin-
restored premolars. The 3-year survival
rate of resin-restored teeth was mark-
edly better that that of the amalgam-
restored teeth. The difference between
the short-term survival rate of amalgam
and of resin-restored premolars can be
illustrated by the fact that this differ-
ence would still have been statistically
significant (p = 0.02) e ven if 5 of the 38
resin-restored teeth had fractured dur-
ing the first 3 years after endodontic
treatment.
After 3 years, cusp fracture of resin-
restored premolars happened with
nearly the same frequency as amalgam-
restored teeth. The combined use of
bevel and a low-viscous resin could no t
prevent tooth failure. This confirms the
i n v i t r o results published by Reel &
Mitchell (13) and by Douglas (12), who
did not find a significant difference be-
tween the fracture resistance of MOD
cavities with beveled or with butt-joint
cavity margins.
There are 2 more variables in the
present study which should be dis-
cussed, i.e. the application technique
and the fact that the restorative resins
were used without a dentin-bonding
agent.
As to the application technique, no
information was obtainable on one of
the 5 fractures; the other 4 failures
were found in premolars where the re-
storative resin had bee n applied with a
bulk technique. In this context it is in-
teresting that the polymerization con-
traction of bulk-applied restorative re-
sins in MOD cavities causes a defo rma-
tion of the cusps (20-22). The fractures
found in the resin-restored premolars
may have been initiated by an incom-
plete cusp fracture caused by the
stresses induced in the enamel during
the polymerization contraction of the
restorative resin. A possible solution to
this problem may be to use an oblique
incremental technique, which reduces
the wall-to-wall polymerization con-
traction (23) and thereby the cuspal de-
formation (21, 22).
No den tin-bond ing agent was used in
the resin-restored premolars and it is
thus possible that the fracture resis-
tance could have bee n increased. How-
ever, the i n v i t r o results published on
the effect of these bonding agents are
contradictory: Douglas (12) a nd Eakl e
(14) found that a den tin-bon ding agent
increased the fracture resistance of
MOD-prepared premolars, while Reel
& Mitchell (13) did not find a signifi-
cant improvement. Also, no commer-
cial dent in-bondi ng agent is as yet
strong enough to totally prevent the
formation of a contraction gap (24).
Furthermore, even strong bonding
agents have only a mediocre effect on
the high-viscous composites intended
for use in posterior teeth (25). Thus, it
is possible, but not evident, that the
frequency of cusp fracture found in the
present study could have bee n reduced
had a dent in-bond ing agent been ap-
plied.
In conclusi on, the possibility of using
enamel-bonded composites as tempor-
ary restorations for endodontically
treated premolars should be Consid-
ered. There are, of course, still many un-
solved problems with regard to the use
of composite resins in posterior teeth
and especially in large MOD cavities;
wear resistance and marginal leakage,
just to mention two of the problems.
However, from a periodontal point of
view, the low frequency of cusp frac-
ture carries great weight.
A c k n o w l e d g e m e n t - This paper was sup-ported in part by the Research Foundationof the Danish Dental Association.
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