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Lopes
et al
ome recent dye penetration studies of packable
umatic extraction.
The aim of this study was to evaluate the effect of
METHOD ND M TERI LS
A box-type Class II cavity was prepared on each
In total, 12 cavities could be restored. The vital-
1:100 000 epinephrine was administered.
After the cavity preparation, a visual examination
One cavity in each tooth (n = 6) was restored with
ced distaUy. The bulk restorations were irradiated for
The other six cavities were restored with Filtek
Z250 (3M Dental), placed with an oblique Incremen-
tal technique in three layers. Each increment was irra-
diated with the XL 1500 curing unit for 40 seconds.
The finishing procedures were carried out with fine
diamond burs (Brasseler) and sandpaper strips (3M
Dental),
After 1 week, the teeth were carefully extracted
with minimal trauma to the restored area. The roots
were immediately removed by diamond bur sectioning
under water. The teeth were cross sectioned mesiodis-
tay through the center of the restoration with a dia-
mond-impregnated copper disk (South Bay Tech-
nology) in an oil-cooled low-speed cutting machine
(Isomet. Buehler),
A polyvinyl si loxan e impres sion (Pre siden t ,
Coltene/Whaledent) was taken of the resin-dentin in-
terface to control for artifacts created during the
preparation for SEM, The specimens were subse-
quently fixed in 2.5% glutaraldehyde in 0,1-M sodium
cacodylate huffer at pH 7,4 for at least 12 hours at
4''C,
After fixation, the teeth were rinsed ivith 20 mL
of 0,2-M soditim cacodylate buffer at pH 7,4 for 1
hour with three changes, followed by distilled water
for 1 min ute. They were deh ydrated in ascending
grades of ethanol (25;o for 20 minutes, 50". ) for 20
minutes, 75 for 20 minutes, 9O''/o for 30 minutes,
and lOO^b for 60 minutes). After the final ethanol
step,
the specimens were dried by immersion in hexa-
methyldisilazane for 10 minutes'^ and air dried at
room temperature.
The specimens were then embedded in self-curing
epoxy resin and stored for 12 hours. After setting, the
epoxy casts were pohshed with waterproof sificon car-
bide papers of decreasing abrasiveness (420, 600, and
1,000 grit) and polished to a high gloss with soft fissue
disks vvith increasingly fine diamond suspensions to a
particle size of 1 pm. The specimens were washed,
dried, demineralized in 6 N of hydrochloric acid for 30
seconds, and deproteinized in2 sodium hypochlorite
for 10 minutes. After drying, the specimens were sput-
ter coated with gold-palladium by means of a Polaron
E-5000 sputter coater (Polaron), and observed under a
Phihps XL 30 SEM (Philips Electric),
RESULTS
The enamel-resin interface did not show any gap for-
mation. The dentin-resin interface formed gaps above
the hybrid layer in three specimens prepared with the
bulk placement technique (Figs la and lb) and five
specimens prepared with the incremental placement
technique (Fig 2). Table 1 shows the percentage of gap
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Lopes et al
i l a Class II cavity restored with the bulK placement technique (Filtek and P60) A gap naa formed Sequence ot the resin-denti
face of a cavity restored w itii ttiebull* placement technique.
For the restorafions placed with the oblique
mental technique (Filtek and Z250), the mean ga
mation was 6,1 , For restorations placed wit
bulk technique (Filtek and P60), the mean gap f
tion was 18.7 . A
test revealed that the increm
placement technique resuUed in a significantly
percentage of gap formafion than did the hulk
densadon technique.
i 1b Gap formed at the resm-de ntii inle -ace of a
cavity restored w ith the Puik placement rechnique.
DISCUSSION
The initial polymerization shrinkage of resin co
ites,the different coefficients of therm al exp an
this material, and dental hard tissues and adh
problems of cervical areas are the essential facto
sponsible for marginal leakage at the proximal as
and gap formation at the resin-dentin interfa
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Lopes el al
2 Class II cavity restored with the incremental placement te clinique (Fiftek and Z250). There is no gap formation. Sequence of trie
tir interface ot a cavity restored by incrementat piacement techn ique.
can reach 2.6% to 7.1% on light-cured
ed by many au tho rs. ^ ^ Layering resin com posite
ty walls.^'
The reason for the lesser amount of gap formation
Class II resin composite restoration is in the range
.0 to 2.0 when the bulk technique is use d. The in-
prep aratio n. Increm ental restoration techniques
T BLE
Gap percentage by placement technique
and resin composite
Specimen
1
2
3
4
5
6
Mean
Incremental
Z250 ( )
0.0
6.7
0.0
0.0
7.4
2Z.4
6,1
Bulk
P (
0.0
17.3
23.8
35.4
18,9
26.9
18.7
The incremental technique can incorporate air bub-
bles. Alster et a P have shown that the stress relief in
thin resin increments is proportional to the amount of
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L o D e s
et l
contributes to stress reduction. - Tbe role of air incor-
poration in the stress relief of resin composite seems
to be important and should be investigated furtber.
Altbough tbe incremental layering tecbnique bas
been widely advocated for reduction of polymerization
sbrinkage, reports and recent finite element analyses
now dispute this tbeory. ' In one of tbese studies, it
was concluded that layering resin composites results
in bigher overall sbrinkage stress and deformation of
remaining bard tissue walls.- Tbe autbors pointed out,
bowever, tbat tbe incremental tecbnique still has some
advantages over bulk placement, sucb as improved
marginal adaptation and wetting,-' wbicb was, in part,
confirmed in tbe present study.
Tbe incremental placement technique is still re-
garded as tbe most practical for otber reasons as well.
This procedure provides enbanced control of over-
bangs in tbe lateral margins prior to curing^ and a
more effective and uniform cure. ^
The present results are in agreement witb recent re-
searcb conducted in vivo, in wbicb tbere was signif-
cantly more gap formation at tbe dentin-restoration
interface, and postoperative sensitivity, after bulk
placement tban after restorations were applied in lay-
ers.''
In botb studies, neither tecbnique resulted in a
perfectly sealed interface between tbe restoration and
tbe tootb structure. Tbe bigh modulus of elasticity of
tbe bybrid resin composites probably could not relieve
tbe stress produced during polymerization. Furtber in-
vestigations should be conducted in vivo with an in-
termediate layer of tlowable resin composite, because
the use of a low-modulus resin composite may in-
crease the flexibility of the bonded assembly and may
act as stress-relaxation buffers, absorbing tbe tension
stress induced by polymerization of tbe resin compos-
ite placed over tbe resin. ' ^
CONCLUSION
Within tbe limitations of tbis short-term study, it is
concluded that tbe placement tecbnique is an impor-
tant factor wben posterior teetb are restored witb
resin composites. Tbe incremental placement of poste-
rior composites stili provides better seal than does tbe
new bulk condensation technique.
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