Correlation between marginal accuracy and fracture resistance of

Braz J Oral Sci. January-March 2007 - Vol. 6 - Number 20
Correlation between marginal accuracy and
fracture resistance of indirect composite
restorations varying cavity preparation design
Rodrigo Borges Fonseca1*
Hugo Lemes Carlo1*
Alfredo Júlio Fernandes Neto2*
Carlos José Soares1*
1
Department of Operative Dentistry and Dental
Materials, Dental School
2
Department of Occlusion and Fixed
Prosthodontics, Dental School
* Federal University of Uberlândia - Uberlândia MG - Brasil
Received for publication: November 14, 2006
Accepted: February 13, 2007
Abstract
This study evaluated the correlation between marginal accuracy and
fracture resistance of laboratory-processed resin composite (LPRC)
restorations with different cavity preparation designs. Eighty human
third mandibular molars were selected and divided into 8 groups (n=10):
G1 – conservative (-c) inlay; G2 – extensive (-e) inlay; G3 – onlay-c
with mesio-buccal cusp coverage (CC); G4 – onlay-e with mesiobuccal CC; G5 – onlay-c with buccal CC; G6 – onlay-e with buccal
CC; G7 – onlay-c with total CC; G8 – onlay-e with total CC. Indirect
composite restorations (SR Adoro, Ivoclar-Vivadent) were
manufactured and positioned over each respective preparation. Marginal
accuracy was assessed using a stereomicroscope at 3 points on buccal,
lingual, mesial and distal regions (40x magnification). After adhesive
cementation (Adper Single Bond 2 and Rely-X ARC, 3M ESPE) a load
compressive test (0.5mm/minute) was performed. Data were recorded
in mm (marginal accuracy) and Kgf (fracture resistance). In general,
Pearson correlation coefficient (a=0.05) did not show significance
between factors in study (r=0.073; p=0.520) but it showed significance
only on G6 (r=0.702; p=0.024). Irrespective of the preparation design
it seems difficult to state the clear correlation between marginal accuracy
and fracture resistance of LPRC restored teeth. In this study the effect
of one variable on another was negligible.
Key Words:
fracture resistance, marginal accuracy, correlation, composite inlays/
onlays
Correspondence to:
Carlos José Soares
Faculdade de Odontologia – Universidade Federal
de Uberlândia
Departamento de Dentística e Materiais
Odontológicos
Av. Pará, nº 1720, Campus Umuarama,
Bloco 2B, Sala 2B-24, CEP 38400-902
Uberlândia - Minas Gerais – Brazil
Phone: +55-34-32182255
Fax: +55-34-32182279
E-mail: [email protected]
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Braz J Oral Sci. 6(20):1260-1264
Correlation between marginal accuracy and fracture resistance of indirect composite restorations varying cavity preparation design
Introduction
The last developments on adhesive dentistry have brought
some great advantages on the treatment of weakened tooth
structures. Materials and techniques are improved aiming to
create a restoration that can mimic tooth substance, or better
still regenerate tooth structure. Among currently available
indirect restorative systems the second generation of
laboratory-processed resin composites (LPRC) has been
available since the early 1990s1.
Several aspects of any restorative system require investigation
but marginal accuracy seems to be of great importance due to
its influence on the clinical longevity of restorations2-7. Plaque
accumulation, gingival sucular fluid flow and bone loss,
microleakage, recurrent caries, periodontal disease3,6-8, pulpal
irritation and consequent sensivity9, marginal fracture and
disintegration of the luting cement3,10-12 are some clinical
disadvantages of poor adapted restorations. Leinfelder et al.6
have suggested as a general rule that the interfacial gap should
not exceed 100µm, but some studies have shown gaps ranging
from 0 to 200µm3,13.
Furukawa et al.14 stated that within this gap range the effect of
cement thickness on fracture resistance of LPRC restorations
can be considered to be of little clinical importance, but their
conclusion was based on a comparison between their results
and the results from other studies. Cho et al.3 proved that as
the occlusal convergence angles (6, 10 and 15 degrees)
increases, the marginal accuracy and the fracture resistance
of LPRC crows are negatively influenced. In addition, different
finishing lines had an influence on marginal accuracy and
fracture resistance of LPRC full crow restorations15. Irrespective
of the cavity preparation design, it is a common finding to
state that preparations weakens the tooth16-17 but the possibility
of establishing good adhesion between teeth tissues and
adhesive materials suggests the minor necessity to extend
cavity preparations in order to prevent future tooth fracture1820
. It is hypothesized that indirect composite restorations for
teeth with different cavity preparations could have a different
influence on the correlation between marginal accuracy and
fracture resistance. Then, if poor marginal accuracy could be
correlated to low fracture resistance a careful attention should
be spent before luting a new restoration.
The purpose of this study was to evaluate the correlation
between marginal accuracy and fracture resistance of LPRC
restorations with different cavity preparation designs. The
null hypothesis to be tested was that different cavity
preparation designs have no effect on the correlation between
marginal accuracy and fracture resistance of LPRC restored teeth.
Material and Methods
Eighty freshly extracted human third molars of similar size
were selected for this study and divided into eight groups
(n=10). Teeth were collected after patients had signed an
informed consent, in accordance with the ethics committee of
1261
Federal University of Uberlândia, Brazil (protocol #029/2003).
Calculus deposits and soft tissue were removed with a hand
scaler and the teeth were stored in 0.2% thymol solution.
The teeth roots were covered with a 0.3mm layer of a polyether
impression material (Impregum F, 3M-ESPE, St Paul, MN, USA),
to simulate the function of the periodontium, and embedded
in a polystyrene resin up to 1mm below the enamel-cementumjunction to simulate the alveolar bone21. Teeth were divided
into 8 groups (N=10): G1 - conservative (c-) inlay preparation;
G2 - extensive (-e) inlay preparation; G3 – c-onlay preparation
with mesio-buccal cusp coverage; G4 – e-onlay preparation
with mesio-buccal cusp coverage; G5 – c-onlay preparation
with buccal cusps coverage; G6 – e-onlay preparation with
buccal cusps coverage; G7 – c-onlay preparation with total
cusps coverage; and G8 – e-onlay- preparation with total cusps
coverage. All teeth received standardized preparations with
#3131 diamond burs (KG Soresen, São Paulo, Brazil) on a cavity
preparation device so that all the cavities of a single group
would have uniform dimensions. The cavity preparations were
first named conservative or extensive, being that the occlusal
isthmus width was the only difference between them (2.5 mm
for conservative, and 5.0 mm for extensive ones). The teeth
were prepared with the following standardized preparation
criteria: a 6-degree axial wall taper, a 2.5-mm-deep occlusal
isthmus and a 1.5-mm–wide chamfer placed 1.5 mm cervical to
the prepared occlusal pulpal wall, at proximal boxes. A 2.5-mmocclusal reduction was defined from the top of each cusp and,
on these groups, a 1.5-mm–wide chamfer was placed 1.5 mm
cervical to occlusal reduction. The proximal boxes were 1.5mm-deep and their buccal-lingual interfacial distances (occlusal
isthmus width) were prepared according to the experimental
group: extensive (5.0mm) or conservative (2.5mm). Cavosurface angles were approximately 90o and the internal angles
were all rounded. Figure 1 shows the general types of cavity
preparations for the different groups.
One technician made all the restorations. A one stage
impression was taken of each prepared tooth in a condensation
silicon (Silon 2APS, Dentsply, Mildford, DE, USA) by use of a
stock plastic tray, and poured with type IV stone (Durone IV,
Dentsply, Mildford, DE, USA). Stone dies were isolated with
SR Adoro Model Separator (Ivoclar-Vivadent, Schaan,
Liechtenstein, Germany) before laboratory resin insertion. A
die spacer was not used because it was suspected it could
mask preparation irregularities. SR Adoro Liner 200 (IvoclarVivadent, Schaan, Liechtenstein, Germany) was applied on a
thin layer and then SR Adoro A3 were incrementally built up.
Each layer were first pre-polymerized with an halogen light
source (Targis Quick, Ivoclar – 180 mW/cm2 at 10 mm distance)
for 10 s and then, the final restoration was post-polymerized
in a multi-focal halogen light source oven (Lumamat 100,
Ivoclar-Vivadent, Schaan, Liechtenstein, Germany) with
program 1 at 95oC and high light intensity for 25 min.
After removing the restorations from stone dies it was
Braz J Oral Sci. 6(20):1260-1264
Correlation between marginal accuracy and fracture resistance of indirect composite restorations varying cavity preparation design
Fig.1 - Schematic representation of preparation designs: (a) G1/
G2; (b) G3/G4; (c) G5/G6; (d) G7/G8.
Fig. 2 - Points of measurement of marginal accuracy.
Fig. 3 - Scatter plot of marginal accuracy (mm) vs. fracture resistance
(Kgf) with regression fit line and confidence intervals (a=0.05).
necessary to perform internal adjustment for accurate seating4.
The internal adjustment was accomplished by one operator
with a low speed hand piece (Kavo, Joinville, Santa Catarina,
Brazil) and #2131 diamond burs (KG Soresen, Barueri, Brazil).
The internal surface of the restorations was 50 µm aluminum
oxide sandblasted for 5 seconds, at a pressure of 4 bars and 2
cm of source-to-sample distance (Microjato Plus, Bio-Art, São
Paulo, Brazil). After sandblasting, the interfacial distance of
the restoration was assessed with a digital stereomicroscope
(Model STN, Olympus, Tokyo, Japan) at 3 points on buccal,
lingual, mesial and distal regions (Figure 2) with 40x
magnification and 0.0005-mm accuracy resulting in 12
measurement points per tooth. The mean marginal accuracy
for each tooth was obtained considering all measurement
points together and then the mean for each group was
calculated. A linear measurement was taken between the
restoration and the cavo-surface-angle, after the restoration
was set in place and stabilized with hueless glue (Tenaz, São
Paulo, Brazil), at two points on occlusal margins7. The marginal
accuracy, observed in linear measurement, was recorded at
selected points with values expressed in microns (µm).
After marginal accuracy assessment, the restorations were
silaneted before cementation22. The preparations (enamel and
dentin) were acid etched with 37% phosphoric acid for 15s,
rinsed and blotted dried with a moist cotton pellet. The adhesive
system Adper Single Bond 2 (3M-ESPE, St Paul, MN, USA)
was applied to the preparation and left untouched for 20s,
being light polymerized for 20s with an halogen light source,
XL-3000, at an intensity of 800 mW/cm2 (3M-ESPE, St Paul,
MN, USA). A dual-resinous cement, Rely-X ARC (3M-ESPE,
St Paul, MN, USA), was mixed and applied to the internal
surfaces of the restorations. The restoration was set in place
under an occlusal load of 500grams for 5min, the excess cement
removed with a brush, and light polymerized with XL-3000 from
the mesio-buccal, mesio-lingual, disto-buccal, disto-lingual, and
occlusal facial directions for 40 seconds in each direction.
After bonding the teeth were stored in distilled water at 37oC
in a dark container for 24h prior to testing. Teeth were then
submitted to a compressive load on a testing machine (Emic
DL-2000, EMIC, São José dos Pinhais, PR, Brazil) with a 6.0mm
sphere contacting the center of the occlusal surface, at a
crosshead speed of 0.5 mm/min. The compressive load at
fracture (Kgf) was recorded for each specimen.
Statistical analysis of the correlation between fracture
resistance and marginal accuracy was performed using Minitab
14 statistical software (Minitab Corporation, State College,
PA, USA) and SPSS 12.0 for Windows (SPSS Inc., Chicago,
USA). The Pearson correlation coefficient (r) (p<0.05) was
calculated for each experimental group and for the data of all
groups in conjunction. The Coefficient of Determination (r2),
which is the ratio of the explained variation to the total variation,
was calculated for the data of all groups in conjunction. Linear
Regression was computed, in order to clarify the correlation
between factors in study, by means of a scatter plot with a
regression fit line and confidence intervals (a=0.05).
Results
Table 1 shows the mean fracture resistance (Kgf), marginal
accuracy (mm), standard deviations, Pearson correlation
coefficient (r) of the groups and P associated values. All
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Braz J Oral Sci. 6(20):1260-1264
Correlation between marginal accuracy and fracture resistance of indirect composite restorations varying cavity preparation design
recorded data were checked for test distribution with the
Anderson-Darling test (p<0.05) which revealed normal test
distribution for all groups. Pearson correlation coefficient
showed significance only on G6 (r=0.702; p=0.024). Then,
the null hypothesis had to be rejected to all groups but not
to G6. This fact means that, in general, the marginal accuracy
of LPRC restorations seems to be poorly correlated to their
fracture resistance.
Table 2 shows the results of Linear Regression: Pearson
correlation coefficient (r), Coefficient of determination (r2), F
and P values for data of all groups in conjunction. It can be
noted that the “r” value (r=0.073) is too close to 0, what means
that the correlation of marginal accuracy and fracture resistance
is weak enough to be significant (p=0.520).
Figure 3 depicts a scatter plot with a regression fit line and
confidence intervals (a=0.05). Few data points are close to the
regression fit line and inside the lines representing the
confidence intervals. The fit line represents the trend of the
data and the space between the confidence intervals lines
shows the data points which more adhere to this trend; it
could be seen a small number of collected data points
demonstrating a relationship (r=0.073) between marginal
accuracy and fracture resistance.
Discussion
Several factors are believed to affect the marginal accuracy
and fracture resistance of laboratory-processed resin
composite restorations. For the former, scientific reports list
the influence of luting agent type23-24, modulus of elasticity of
the supporting structure, surface roughness, residual stress,
restoration thickness3, properties of the restorative material24,
manufacturing conditions of the restoration18, the employment
of an adhesive technique7,14,16,18-19,23-25 and the preparation
design3,7,15-16. Affecting marginal accuracy are the location of
cavity margins, use of incremental placement techniques,
restorative material type and methods of activation, the usage
of liners and finishing methods26. In spite of the fact that some
studies have investigated the influence of these factors both
on marginal accuracy and fracture resistance of LPRC
restorations 3,13,15,19 none of them have established the
relationship of these variables. Then, this study evaluated the
correlation between marginal accuracy and fracture resistance
of LPRC restorations with different cavity preparation designs.
Cho et al.3 have demonstrated that as the occlusal convergence
angles (6, 10 and 15 degrees) increases, the marginal accuracy
and the fracture resistance of LPRC crows is negatively
influenced. Thus, it means that when a cavity preparation
results on poor marginal accuracy it is expected a negative
influence on fracture resistance as well. Although this
conclusion seems reasonable, Furukawa et al.14 showed that
when an adhesive technique is successfully employed on the
cementation of indirect composite restorations clinical gaps
that normally range from 0 to 200µm do not significantly affect
1263
Table 1 - Mean peak fracture (Kgf), marginal accuracy (mm),
standard deviations, and Pearson correlation coefficient (r)
to fracture resistance and marginal accuracy (a = 0.05).
Mean Fracture
Resistance
(Kgf) ± SD
Groups
Mean Marginal
Accuracy
r
P
-0.183
0.612
(µ
µm) ± SD
G1
193.4 ± 55.9
128.8 ± 97.34
G2
202.2 ± 26.2
73.3 ± 54.73
0.118
0.746
G3
215.8 ± 54.5
102.6 ± 81.0
0.015
0.967
G4
191.4 ± 58.8
80.6 ± 62.84
0.511
0.131
G5
208.1 ± 81.6
122.0 ± 88.2
0.402
0.249
G6
188.3 ± 61.4
77.8 ± 60.21
0.702
0.024*
G7
221.9 ± 32.2
116.5 ± 61.12
0.235
0.513
G8
222.7 ± 60.2
110.6 ± 91.6
-0.540
0.107
* Significant at a = 0.05.
SD: Standard Deviation.
Table 2 - Results of Linear Regression: Pearson correlation
coefficient (r), Coefficient of determination (r2), F and P values
for data of all groups in conjunction (a = 0.05).
Correlations: Marginal accuracy vs. Fracture Resistance
r
r2
F
0.073
0.005
0.418
P
0.520
fracture resistance. In agreement with Furukawa et al.14 this
study showed that there is no significant correlation (r=0.073
and p=0.520; Figure 3) between marginal accuracy and fracture
resistance in gaps that ranged from 73.3µm to 128.8µm (mean
values). The square of the r coefficient is called coefficient of
determination (r2) and it is known to be the ratio of the explained
variation to the total variation. Thus, considering that in this
study r2=0.005, from a statistical point of view it means that
just 0.5% of the variation of marginal accuracy can be explained
by the relationship between marginal accuracy and fracture
resistance 27 . The remaining 99.5% of the variation is
unexplained and is due to chance or other factors.
When it is considered the several types of preparation designs
used on this study, G6 was the only one where a significant
and positive correlation could be found (r=0.702; p=0.024).
In fact, this occurrence seems not usual since one would expect
that poor marginal accuracy is associated to good fracture
resistance. If the coefficient of determination of this group
was calculated it would return r2=0.499, and this mean that
half (49.9%) of the variation on marginal accuracy can be
explained by the relationship between this factor and fracture
resistance, but for sure several other factors (50.1%) can have
an influence. A mean gap of 77.8µm was observed on this
group, and according to Furukawa et al.14 it is not probable
that on a clinical situation it would have any influence on
fracture resistance of indirect composite restorations. Several
other factors may affect both variables and this makes difficult
Braz J Oral Sci. 6(20):1260-1264
Correlation between marginal accuracy and fracture resistance of indirect composite restorations varying cavity preparation design
to establish a clear relationship. Mehl et al.28 studied different
wall thicknesses for inlay preparations restored with ceramic
or composites, showing that ceramic inlays provide
significantly greater stabilization and better marginal quality
than do composite inlays, but in the case of very thin remaining
walls (about 1.3 mm), however, the marginal quality and the
cusp-stabilizing effect are reduced. Then, similar to this study,
it can be seen that preparation design can influence both
marginal discrepancy and fracture resistance but it seems difficult
to predict the exact influence of these parameters on each other.
Studying the effect of finish line variants on marginal accuracy
and fracture resistance of Targis/Vectris crowns, Cho et al.15
showed that crowns with a shoulder finish line showed a
smaller marginal discrepancy than those with a chamfer finish
line, but the fracture resistance of the crowns with a chamfer
finish line was higher than that of the crowns with a shoulder
finish line. Thus, marginal accuracy and fracture resistance
had an opposite behavior. Again, this fact stresses the
difficulties in establishing a clear relationship between these
variables. On the present study it was only employed a chamfer
finish line but the variation in preparation design between
groups did not significantly influenced the correlation of these
variables, except for G6.
The results of this study are only indicative that at least from
the perspective of an in vitro simulated clinical situation there is
no correlation between marginal accuracy and fracture resistance
for laboratory-processed resin composite restorations. For sure,
several other factors must be studied in order to address a
stronger conclusion and definite reasonable clinical guidelines.
Further studies, especially clinical observations, are necessary
to address stronger conclusions. In accordance with the
methodology developed in this study it is possible to conclude
that marginal accuracy and fracture resistance do not have a
significant correlation to each other. Only for one group where
teeth received an extensive onlay preparation with buccal cusps
coverage (G6) it was observed a positive significant correlation.
Acknowledgements
Authors are grateful to Dentsply and KG Sorensen for full
donation of the materials used in this study, and to IvoclarVivadent and 3M-ESPE for partial donation.
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