New
York University
David
B. Kriser Dental Center
College
of Dentistry
Graduate
School of Arts and Science
Nov.
7, 1998
Department
of Dental Materials Science
345
East 24th Street
New
York, NY 10010
Tel.:
212-998-9637
Fax:
212-995-4085
Dr.
Samuel Waknine
DRM
Research Laboratories Inc.
29
Business Park Drive
Branford,
CT 06045
Dear
Dr. Waknine,
Please
find enclosed a final report entitled “Performance probability - Material,
Operator and Technique contributions :Esthetic Crown and Bridge materials.”.
This study of Crown and Bridge materials was undertaken by Dr. Seung-Il
Eom as part of MS thesis requirements for the Masters degree in Dental
Materials Science under my supervision. Additional
results and analysis which do not form part of the MS thesis are also included
in this report.
The
Biaxial flexure strength (BFS) and Diametral tensile strength (DTS) of
“DiamonCrown” microhybrid composite restorative material (DRM Res. Labs.
Inc., product) were compared to a 'Compact filled resin based glass ceramic'
and a 'Pressable ceramic' materials. The
mechanical property comparisons indicate that “DiamondCrown” ranks first
among the materials evaluated, in BFS and DTS.
Additionally,
the results of the ‘Weibull regression analysis’ of test data indicate
that the failure stress at 0.1% failure probability was estimated to be the
highest for the ‘DiamondCrown’ material; indicating the capability of
enhanced clinical performance probability compared to the other materials. The
characteristic strength values are presented for various levels of failure
probability as well.
Please
feel free to contact for any clarifications or additional information.
We
thank DRM Labs for the Material and Equipment grants required for this study.
Sincerely,
T.V.V.
Raghavan
T.
V. Vijayaraghavan, PhD
Associate
Professor, Dental Materials Science
Division
of Restorative and Prosthodontics
Performance
probability - Material, Operator and Technique contributions :Esthetic
Crown and Bridge materials.
T.
V. Vijayaraghavan, PhD.
and
Seung-Il Eom, DDS, MS
Department
of Dental Materials Science, New York University Kriser Dental center, NY
10010
Objective
The
objective of this study was to evaluate the Biaxial flexural strength (BFS)
and Diametral tensile strength (DTS) of three esthetic crown and bridge
materials and evaluate the effect of sample preparation and operator
contributions.
Introduction
and background
Composite
resin dental restorative materials are blends of hard glass or glass ceramic
particulate fillers and a softer polymeric resin matrix.
Due to the technique sensitivity of these materials, operator
manipulation and handling differences can alter the homogeneity of the
material and lead to property differences in the set form.
This is often reflected in the range of test data during mechanical
testing. The spread in test data
depends to a large extent on the differences in ‘flaw population
characteristics’ from sample to sample.
Flaws such as, voids and/or micro-porosities in the volume or on the
surface, may be material inherent, and/or infiltrated or incorporated during
manipulation technique. In the
presence of an external load or force they act as local stress concentration
points and failure is initiated at lower forces[1],[2].
The
behavior of brittle (flaw dependent strength) materials is best represented by
the “Weibull distribution function” [3],[4].
‘Weibull statistical analysis’ of test data is one method to
characterize the variability of test data of brittle materials[1].
The data from ‘N’ number of trials or replicates of a sub-group is
analyzed using ‘Weibull statistics’ to estimate two parameters; 1) s0,
termed the characteristic or median (probable) strength and ‘m’ the
‘Weibull modulus’ or shape parameter, which can be used to estimate the
failure probability. A high value
of ‘m’ indicates a greater level of homogeneity in material character,
while a lower value of ‘m’ would be an indicator of heterogeneity or a
wider spread in test data about s0.
From the value of ‘s0’and
‘m’ the failure stress for a failure probability of 0.001, 0.1, 0.9 and
0.999(1, 10, 90 or 999 in 1000).labeled as s0.001,
s0.1,
s0.9,
and s0.999
can be estimated. This allows
ways to optimize the performance of the material.
The
values of s0.001,
s0.1,
indicate the possible performance of the material, for an operator, who may be
a junior operator or a first time user; while the values s0.9,
and s0.999
reflect the performance obtainable from a master technician or an experienced
user. The value of s
0,
can be
considered as a performance of the material obtained by a ‘median’
operator. The inherent limitation
or attainable performance of the material is also represented by the value of s0.999;
i.e., as good as it gets!. The conformity of the test data to the ‘Weibull
distribution function’ for a given sub group of N trials is indicated by the
value of R(correlation coefficient); the higher the value, the greater the
confidence in the test data. When
R is >0.9 the test data has greater validity.
The
estimate of the uniaxial tensile strength for brittle materials is obtained
from Diametral Tensile or Three or Four point bend flexure testing.
Flexure testing or bend tests have the advantage that a state of pure
tension can be established on one side of the specimen.
However, the stress state under In-Vivo service conditions in the oral
environs are not purely uniaxial, but biaxial or triaxial in nature[5].
A biaxial flexure test has been used frequently for the determination
of fracture characteristics of brittle dental materials (Ban and Anusavice)
and is increasingly being adopted for the evaluation of brittle dental
materials,[6].
The ease of sample preparation, the elimination of edge effects, the
similarity to clinical size scale and intra oral loading conditions are the
main advantages of biaxial testing compared to uniaxial flexure testing.
Further, the evaluation of slightly warped specimens and the
possibility of estimation of uniaxial flexure data from biaxial test data are
additional advantages of Biaxial flexure testing
[7].
Crown and bridge restorative material are used by clinicians and/or
laboratory technologists. Unlike direct composite restorative materials, crown
and bridge materials undergo extensive manipulation in the dental laboratory
in the process of creation of an anatomical tooth form.
The technique may involve layers of various shades of the material to
create the translucency and shade of the final tooth form restoration.
Materials
and Methods
Table
1. lists the materials used in this study.
Disk
samples for Biaxial Flexure(BF)
test, of 14 mm in diameter and 1.4 mm thick were prepared following two sample
preparation procedures. The first, an Incremental
packing procedure was adopted, similar to that used in the preparation of
uniaxial flexure bars[2],
where the composite dispensed from the syringe was flattened on a glass slab
and sectioned and packed into the mold incrementally.
A metallic and/or plastic plunger was used to pack it evenly in the
stainless steel split ring mold(14 mm diameter and 1.4 mm thick).
The excess was trimmed and a plastic film followed by a glass slide was
placed on either side and clamped with butterfly clamps to allow venting of
the excess through the channel in the split ring stainless steel mold.
The number of such incremental fillings was variable and depended on
operator. Approximately 0.5 gm of
resin paste was required for one biaxal test sample.
In
the second procedure, a single step Bulk
packing was adopted, in which a length of the cylindrical segment of the
composite dispensed from the syringe was placed on a flat glass slab, and
patted down using gentle pressure along the long axis with a stainless steel
placement instrument. A
transparent plastic film was placed below the mold followed by a glass slide
or slab. The cylindrical paste
form was placed inside the ring mold. A
second plastic film followed by a glass plate was overlaid and gently pressed
to distribute the resin evenly within the circumference of the ring mold.
Two butterfly clamps were used to squeeze the excess composite through
the split ring opening. Approximate
time under clamps was 4-5 minutes for both methods of sample preparation.
A
Halogen visible light curing booth (DiamondLite VL, Halogen, (DRM Res. Labs.
Inc., Branford, CT), generating 500 mW/cm2 with a wavelength range
of 450-550 nm was used for curing the composite materials.. Samples were cured
from top and bottom. The cure
time per surface for DCR was 60
seconds (total = 120 seconds) and for ART
was 90 seconds (total = 180 seconds). After
curing, specimens were separated from the ring mold and stored in distilled
water maintained at 37 °C for 24 hours prior to testing.
A total of 20 samples were prepared for each material group.
The BF test samples for the ceramic material, EMP,
were fabricated by dental Prolab of Florida (Ocala, FL), as per
manufacturer’s instructions. All
Samples were evaluated visually with a low powered magnifier for defects on
the surfaces such as cracks, surface markings, porosity, etc.
These samples were excluded from the test group.
Testing
Biaxial flexure tests were performed using an "Instron"
machine (Instron Corp, Canton, MA), with the cross head displacement rate set
at 0.51 mm/minute. Disk specimens
were placed on eight steel balls[3]
(3.2 mm in diameter) spaced along the circumference of a circle of diameter
10.mm. A flat ended piston of
diameter 1.2 mm was used to load
the sample at the center of the disk. Tests
were carried out at room temperature. The
maximum load prior to failure was obtained from a chart recorder attached to
the test machine.
Biaxial
Flexure strength
The
maximum tensile stress under biaxial loading condition (at failure) was
calculated according to the following equations developed by Marshall (1980)[8].
The failure stress, s
f , can
be expressed as:
s
f = A.
Pf /t2
(1), where, ‘A’ is
given by the expression,
A
= (3/4p)
[2.(1+n).ln(a/r0*) + (1-n).(2a2-r0*2)/2b2
+ (1+n)]
(2)
Pf,
is the maximum load at failure in Newtons, n,
the Poisson`s ratio (for composite materials a value of 0.24 was used),
‘a’, the radius of the support circle, ‘b’ is the radius of disc
specimen, ‘r0*’, the equivalent radius given by
r0* = (1.6 r02 + t2)1/2
- 0.675t,
where,
‘t’ is the thickness of the disc specimen, and ‘r0‘is the
radius of the piston2.
The Diametral Tensile Tests (DTS)
was performed as per ADA specification #27.
Stainless steel molds 6 mm in diameter and 3 mm in height were used to
prepare DTS test specimens. The
curing modality, time and storage (post curing ) test conditions were
identical to those used for the preparation of BFS test samples.
Statistical
Analysis
Mean
and standard deviation were obtained from raw data.
Student t-Test was used for evaluation of significant differences
between mean value pairs. Weibull
regression analysis was performed using a personal computer, to evaluate the
‘Characteristic strength’, s0
and ‘Weibull modulus’, m, the
from the raw data.
Results
Tables
2. presents the results of ‘Weibull analysis” of the BFS data as a
function sample preparation procedure and operator.
The
high values of correlation coefficient, R, indicate that the distribution of
raw data shows a good fit to ‘Weibull distribution function’ for all the
materials. Mean BFS values for
the ‘Bulk’ packing was significantly higher than that for the
‘Incremental’ mode of test sample preparation procedure(t-Test;
p<0.05). Operator difference
were not significant irrespective of procedure(p>0.05). The mean BFS value
for DCR was significantly higher <0.05) than that obtained either ART or
EMP materials.
Table
3. presents the results of Weibull analysis of DTS data.
There was no significant difference in mean DTS values between Operator
‘a’ and ‘b’
at constant material and test condition (t-Test, p>0.05).
Mean DTS value for DCR was significantly higher (t-Test, p<0.05) in
comparison to ART material. The ‘m’ values range from 6.6 to 16.9.
The characteristic DTS value(s0)
for DCR wase larger for DCR material compared to ART group.
Discussion
Failure
origin:
Tensile
stresses are the primary cause of failure of brittle materials and depend on
the presence of flaws. In the BF
test, tensile stresses (biaxial) are localized on the face opposite to the
plunger. The presence of surface
or subsurface flaws on this surface can lower the limiting displacement or
load for failure origination and propagation through the thickness.
A spread in test data is therefore reflective of the variation in
surface and volume defects or flaws. The
mean strength is an unreliable estimate of the In-Vivo performance, since it
assumes no contribution from material characteristics
to the spread in strength data, other than experimental errors(machine,
operator, etc). The effect of
operator manipulation to strength is not taken into account.
In
the case of composite materials, the worst case scenario for In-Vivo failure
relates to brittle failure of the anatomical structure leading to loss of
function. The conformity of the
test data to ‘Weibull distribution function’ is an indicator of the
variations in material and stress homogeneity of the material’.
Under a constant test protocol (testing mode) or loading condition, the
variation in test data is expected to reflect a sampling variation in material
homogeneity.
Material
manipulation and Technique sensitivity
A
primary requirement of dental restorative material is the ability to create
anatomical forms. The material
must be plastic for easy manipulation to accommodate shape.
The volume of material involved in relation to the inherent defect size
is an important parameter in limiting strength.
Under a uniform external displacement or load, the orientation,
location and size of the flaw determines the limiting load at failure.
Both the size and distribution affects the fracture strength data.
A larger flaw leads to a lower load at failure and vice versa.
The spread in the data reflects this variation of the limiting flaw
size and its distribution in the test volume.
The
lower characteristic strength values for incremental filling procedure
indicates, a greater chance for incorporation of strength delimiting flaws in
the bulk, larger than originally present.
The higher strength values noted for the bulk packing procedure
reflects the absence of the effect of manipulation.
The surface characteristic are expected to be similar due to the
similarity of contact surfaces for both procedures.
The significantly lower BFS value obtained in the incremental procedure
(p<0.05) indicates that the reduction in strength is primarily due to
incorporation of defects in the bulk. This
is further, reflected in the drop in the attainable strength value for the
incremental packing technique. The
bulk packing procedure limits manipulation and may be treated as providing
the characteristic or most probable strength of the material as it is
presented by the manufacturer. The
above conclusions hold irrespective of the operator.
These are also reflected in the higher
s0.999
values obtainable for the bulk packing procedure.
The values s0.001(0.1%
probability of failure at this stress) is therefore an indicator of the effect
of operator induced effects. For
Operator ‘a’ and ‘b’,
differences in s0.001
are larger compared to that for s0.999
for DCR and ART material.
In this respect, protocols suggested by the manufacturer for
manipulation and handling should be adhered and accommodated under clinical or
laboratory use of the material. In
the case of crown and bridge materials, the use of modeling liquids,
application of thin layers for the creation of shades, the use of instruments
for contouring are expected to have a greater influence and increase the
chance for incorporation of flaws. Finally,
finishing procedures for the adjustment of occlusion, such as grinding and
polishing methodologies can also incorporate surface flaws that can affect
limiting strength values.
Clinical
implications
The
DTS (mean value) requirement as per ADA Spec. #27[9]
for Type II Direct filling resin composite materials(> 50% filler content)
is about 34 MPa. This requirement
does not take in to account any of the other parameters such as manipulation,
handling and operator expertise into account and their effect on DTS.
In the present study, both the mean DTS and the characteristic DTS
value exceed 34 MPa for DCR and ART. Of
the two materials only DCR exceeds a value of 34 MPa at probaibility of
failure of 0.1%. Based
on this a higher level of clinical performance reliability can be
expected of DCR material with s0.001>34
MPa,
compared to a material with s0.001<34
MPa.
For
an average mastication bite force of 125 lbs[10]
and an occlusal load contact area of 0.04 mm2, the localized stress
is around 140 MPa. If this were
to be localized as a biaxial bending stress the material of choice would have
to have a failure stress higher than 140 MPa to maintain material integrity
and function. This value is
exceeded at the 10 % failure probability level for DCR material for both
‘incremental’ and ‘bulk’ techniques of material manipulation,
irrespective of the operator. Based
on this the performance reliability under a normal clinical loading situation,
is expected to be higher for the DCR material compared to the ART or EMP
material.
Conclusions
1.
Significant
difference in mean BFS values were observed only for differences in sample
preparation procedure (p<0.05). Operator
difference did not affect the mean BFS(p>0.05).
2.
The
characteristic BF strength, s0,
the stress at a the lowest 0.1%, s0.001
and the highest 99.9% failure probability, s.999,
were highest for DCR material compared to ART and EMP materials.
3.
The
characteristic DTS value , s0
were higher for DCR material compared to ART material.
4.
The
comparisons of the mechanical property at the lowest levels of failure
probability indicate a greater confidence in the performance reliability of
DCR material compared to ART and EMP
Table
1. List of materials
|
Material,
(ID) |
Classification |
Shade/Batch
# Exp.
Date |
Manufacturer |
|
DiamondCrown (DCR) |
Composite, Microhybrid |
DA2/225911/
053199 A3/318940/ 043000 |
DRM
Res. Labs. Inc., Branford, CT |
|
Artglass
(ART) |
Glass
filled composite Microhybrid, |
DA3/720864
/00-12-31 |
Heraeus
Kulzer, GmBH, |
|
IPS
Empress (EMP) |
Pressable
Ceramic, |
A3/9003002540
/700976 |
Ivoclar
(Jelenko, Armonk,
NY |
Table
2. Results of Weibull statistical Analysis
- BFS data
|
Mat.
ID Incremental
packing |
s0
MPa |
R |
N |
m |
SE(m) |
s0.001 MPa |
s0.1 Mpa |
s0.90 MPa |
s0.999 MPa |
|
DCR,
Oa |
174.6
|
0.90
|
10 |
11.3
|
1.94
|
95.0
|
143.2
|
187.9
|
207.1
|
|
DCR,
Ob |
173.6
|
0.97
|
7 |
12.1
|
1.27
|
98.0
|
144.1
|
186.0
|
203.7
|
|
DCR,
Oa & Ob |
173.7
|
0.94
|
17 |
13.3
|
1.29
|
103.2
|
146.6
|
185.0
|
200.9
|
|
Bulk
packing |
|
|
|
|
|
|
|
|
|
|
DCR,
Oa |
198.3
|
0.97
|
10 |
12.8
|
1.11
|
115.7
|
166.4
|
211.6
|
230.5
|
|
DCR,
Ob |
193.4
|
0.96
|
11 |
11.2
|
1.05
|
104.1
|
158.0
|
208.4
|
230.0
|
|
DCR,
Oa & Ob |
195.2
|
0.99
|
21 |
13.3
|
0.39
|
116.2
|
164.9
|
207.9
|
225.7
|
|
ART,
Oa |
134.0
|
0.95
|
8 |
10.5
|
1.33
|
69.4
|
108.1
|
145.1
|
161.1
|
|
ART,
Ob |
137.9
|
0.99
|
11 |
13.4
|
0.70
|
82.2
|
116.6
|
146.8
|
159.4
|
|
ART,
Oa & Ob |
136.1
|
0.97
|
19 |
13.1
|
0.77
|
80.2
|
114.6
|
145.1
|
157.8
|
|
EMP |
134.2
|