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Ghazal2008-Wear-DentureTeeth.pdf
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 502–507
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Two-body wear of resin and ceramic denture teeth in
comparison to human enamel
Muhamad Ghazal a,b , Bin Yang a , Klaus Ludwig a , Matthias Kern a,∗
a
Department of Prosthodontics, Propaedeutics and Dental Materials, School of Dentistry, Christian-Albrechts University at Kiel,
Arnold-Heller Strasse 16, 24105 Kiel, Germany
b Department of Fixed Prosthodontics, School of Dentistry, University of Aleppo, Syria
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. To evaluate the two-body wear resistance of different artificial denture teeth when
Received 3 September 2006
opposed to steatite ceramic balls in a dual-axis chewing simulator.
Accepted 5 April 2007
Methods. Artificial denture teeth including the ceramic tooth Bonartic CT® , the composite resin tooth Condyloform II NFC® , the acrylic resin teeth Bonartic TCR® , Orthognath® ,
Polystar Selection® , SR Orthotyp DCL® , and Vitapan Cuspiform® , and human maxillary pre-
Keywords:
molars were tested in a chewing simulator. Wear resistance was analyzed measuring vertical
Two-body wear
substance loss and volume loss using profilometry and an optical macroscope after various
Resin teeth
chewing cycles (49 N, up to 1,200,000 cycles). Data were statistically analyzed using one-way
Composite teeth
analysis of variance (ANOVA) followed by the Fisher test (LSD) at p ≤ 0.05.
Ceramic teeth
Results. After 1,200,000 chewing cycles the mean vertical substance loss and volume loss
Human enamel
for the composite resin teeth (117 ␮m and 0.144 mm3 ) were significantly lower than for all
Chewing simulator
acrylic resin teeth (149–166 ␮m and 0.220–0.292 mm3 ) (p ≤ 0.05), but higher than for ceramic
teeth (36 ␮m and 0.029 mm3 ) and for enamel (56 ␮m and 0.033 mm3 ) (p ≤ 0.05). No significant
differences were found among the acrylic resin teeth for both parameters (p > 0.05).
Significance. The composite resin showed improved in vitro two-body wear resistance compared to modern acrylic resin denture teeth; however, it showed less wear resistance than
ceramic teeth and human enamel. Ceramic teeth should be preferred over natural teeth
when occlusal stability is considered a high priority.
© 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Wear resistance is one of the most important physical properties of artificial teeth that are used in removable partial or
complete dentures. Excessive wear might cause loss of vertical dimension of occlusion, loss of masticatory efficiency,
faulty tooth relationship, and fatigue of masticatory muscles
[1,2]. Materials used for denture tooth fabrication determine
the wear resistance to a great extent [3]. Acrylic resins and
ceramics are commonly used for artificial teeth. Ceramic teeth
∗
have been considered the most wear resistant [3]. However,
due to their brittleness, the mismatch in coefficient of thermal
expansion and the high modulus of elasticity, ceramic teeth
are more likely to fracture and crack from the denture base
than resin teeth [1–3].
Convenient handling, better toughness, and better compatibility with the acrylic denture base give the acrylic resin
teeth advantages compare to ceramic teeth [1,3]. Therefore in
removable dentures, resin denture teeth are usually used more
frequently than ceramic teeth. Recently, several new types of
Corresponding author. Tel.: +49 431 597 2874; fax: +49 431 597 2860.
E-mail address: [email protected] (M. Kern).
0109-5641/$ – see front matter © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.dental.2007.04.012
503
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 502–507
Table 1 – Tested denture teeth
Products (group codes)
Manufacturer
Lot no.
Color
Size
Type of denture teeth
Feldspathic ceramic
Micro- and nano-filled
composite resin
IPN-resin
PMMA resin, networks
TCR resin
DCL resin
PMMA u. derivative from
PMA + 14 Gew. % silicon dioxide
(SiO2 )
Bonartic CT® (P-Bon)
Condyloform II NFC® (C-NFC)
Candulor, Wangen, Switzerland
Candulor
411018
ZZ4129
J2
A2
06
N6
Polystar Selection® (R-IPN)
Orthognath® (R-Ort)
Bonartic TCR® (R-TCR)
SR Orthotyp DCL® (R-DCL)
Vitapan Cuspiform® (R-Vit)
Merz Dental, Lütjenburg, Germany
Heraeus-Kulzer, Wehrheim, Germany
Candulor
Ivoclar-Vivadent, Schaan, Liechtenstein
Vita, Bad Säckingen, Germany
2094
04-55
1495
2085
W 41
A3
A3
A3
A3
A3
XL
31
06
N5
45C
DCL: double cross-linked (also named as TCR), IPN: interpenetrating polymer networks.
resin denture teeth have been developed in order to retain the
positive characteristics of acrylic resin teeth while improving
their wear resistance. These teeth are made of cross-linked
acrylics and micro-filled composite resins [4]. Cross-linked
acrylic denture teeth have been developed by utilizing various
polymer technologies including blend polymer, interpenetrating polymer networks (IPN) [5], and double cross-linking (DCL)
to increase the resistance to crazing and wear.
The purpose of this in vitro study was to evaluate the twobody wear resistance (vertical loss and volume loss) of ceramic
denture teeth, composite teeth, and five different resin teeth
and to compare them with the wear resistance of human
enamel in a dual-axis computer controlled chewing simulator. The hypothesis of this study was that there is no difference
among these artificial denture teeth and human enamel.
2.
Materials and methods
2.1.
Description of the dual-axis chewing simulator
The wear test was performed in a dual-axis chewing simulator
(Willytec, Munich, Germany) [6]. It has eight identical sample
chambers and two stepper motors which allow computercontrolled vertical and horizontal movements between two
antagonistic specimens in each specimen chamber. The masticatory cycle in this study consisted of three phases: contact
with a vertical load of (49 N), horizontal sliding of 0.3 mm,
and separating the teeth and their antagonistic material. The
masticatory load curve is programmed by the combination of
horizontal and vertical movements. The computer unit controls the mechanical motion and the water flow of cold and
warm water baths for the thermal cycling of the specimens.
2.2.
Materials and teeth preparation
For this study, eight maxillary first premolars from each of
seven different denture teeth were prepared (Table 1). In addition, eight maxillary first human premolars were also used
in this study as control group, which were stored in 0.1%
thymol solution and used within 1 month after extraction.
All teeth were embedded in auto-polymerizing acrylic resin
(Technovit 4000, Heraeus-Kulzer, Wehrheim, Germany) using
custom-made copper holders with a diameter of 15 mm. A
custom-made surveyor was used to ensure that the occlusal
surface of the buccal cusp was aligned rectangularly to the
long axis of the specimen holder. Then, the buccal cusps of
each tooth specimen were abraded with 2500 grit abrasive
paper and finished with 4000 grit abrasive paper to a depth
of 0.5 mm to achieve a flat area of about 2.5 mm × 3 mm for
loading during the wear test. The wear test was performed
on the flat surface of the buccal cusp (Fig. 1). Steatite ceramic
balls (Höchst Ceram Tec, Wunsiedel, Germany) with a diameter of 6 mm were used as antagonistic specimens to simulate
enamel abraders.
2.3.
Wear testing and measurements
The teeth and the steatite ceramic balls were mounted in
the chewing simulator and the teeth were loaded with total
of 1,200,000 chewing cycles. The parameters of the wear test
are listed in Table 2. The effective weight of each antagonistic
steatite ball was 5 kg, which corresponds to a loading force of
49 N [7]. After 120,000, 240,000, 480,000, 840,000, and 1,200,000
loading cycles, an impression of each loaded tooth surface was
taken using a polyether impression material (Permadyne, 3 M
Espe, Seefeld, Germany). Then replicas were made using Stycast 1266 (Emerson & Cuming National Starch & Chemical,
Westerlo, Belgium).
Vertical substance loss of the teeth was measured with a
custom-made mechanical profilometer. The profilometer consisted of a stepper-motor and a feeler arm (Typ 1920, Pretec,
Bienne, Switzerland) with a stylus (5 ␮m diameter) at the tip,
which was movable in a vertical direction at a resolution of
0.02 ␮m. The replica was fixed in the profilometer, and then
the vertical feeler arm with its stylus was positioned on the
Table 2 – Test parameters
Cold/hot bath temperature
Vertical movement
Rising speed
Descending speed
Weight per sample
Kinetic energy
Dwell time
Horizontal movement
Forward speed
Backward speed
Cycle frequency
5 ◦ C/55 ◦ C
6 mm
55 mm/s
30 mm/s
5 kg
2250 × 10−6 J
60 s
0.3 mm
30 mm/s
55 mm/s
1.3 Hz
504
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 502–507
Fig. 1 – Schematic drawing of the tooth preparation for the wear test.
non-abraded portion of the flat area of the buccal cusp of the
replica. The replica was moved horizontally by the steppermotor, while the vertical deflection of the stylus was recorded
(contact scanning). The profilometer was connected to an x–yrecorder (X–Y Schreiber L800, Linseis, Selb, Germany), which
reproduced the enlarged surface profiles (magnification 20×).
This process was repeated three times at a parallel line with
a distance of 0.1 mm in the mesio-distal direction from midline. The deepest point of the profile represented the vertical
substance loss. The wear area was analyzed with an optical macroscope (Makroskop M 420, Heerburgg, Switzerland),
which was connected to a digital camera (Leica DC 100, Leica
Microsystems, Bensheim, Germany). The radius of wear area
was directly measured using the software Leica IM 50 (Leica
Microsystems) at a magnification of 25×. Since the differences in radius of wear area, measured from buccal-lingual
direction and mesio-distal direction, were very small (ranging from 0 to 0.025 mm), it was assumed that the shape of
the wear area was round. So volume loss could be mathematically calculated in sufficient approximation as a spherical
segment using the following formula for a spherical segment:
V = /6h (32 + h2 ). V = volume loss in mm3 , h = vertical loss in
mm, = radius of wear in mm. One-way analysis of variance
(ANOVA) was used to analyze the data. Multiple pair-wise
comparison of means was performed by the Fisher test (LSD) at
p ≤ 0.05.
For qualitative analysis of the abraded contact surfaces the
specimens were sputter-coated with gold by an ion-sputter
instrument (SCD 030, Balzer Union, Liechtenstein), and evaluated at a magnification of 2000 using a scanning electron
microscope (SEM, Philips XL 30 CP, Philips, Germany) operating
at 10 kV.
3.
Results
The mean vertical substance loss and the mean volume loss of
the test groups after the various chewing cycles are shown in
Tables 3 and 4. All resin teeth tested showed statistically more
wear than human enamel, while the ceramic teeth exhibited
less wear than enamel although this difference was not statistically significant. After 120,000 chewing cycles, there was
no statistically significant difference in the vertical substance
loss and the volume loss between enamel and composite resin
teeth (p > 0.05). Although statistical differences were not completely identical for vertical substance loss and volume loss
during the various experimental periods, after 1,200,000 chewing cycles, all the tested resin teeth including the composite
Table 3 – Mean vertical substance loss and standard deviations of the tested teeth after various chewing cycles in ␮m
(N = 8)a
Groups
Chewing cycles
120,000
Enamel
P-Bon
C-NFC
R-IPN
R-Ort
R-TCR
R-DCL
R-Vit
a
38
16
50
68
76
74
88
62
±
±
±
±
±
±
±
±
13 B
5A
14 BC
23 CD
11 CD
11 CD
31 E
17 CD
240,000
41
20
65
87
96
89
108
80
±
±
±
±
±
±
±
±
15 B
6A
12 C
26 D
18 DE
18 DE
29 E
23 CD
480,000
46
28
84
113
121
118
129
107
±
±
±
±
±
±
±
±
16 A
7A
20 B
25 C
18 C
19 C
34 C
33 C
840,000
53
31
102
136
141
143
147
134
Means with the same capital letter within one row are not statistically different at p ≤ 0.05 (Fisher test).
±
±
±
±
±
±
±
±
16 A
10 A
27 B
21 C
23 C
26 C
27 C
39 C
1,200,000
56
36
117
149
159
160
163
166
±
±
±
±
±
±
±
±
17 A
11 A
30 B
26 C
23 C
32 C
28 C
47 C
505
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 502–507
Table 4 – Mean volume loss and standard deviations of the tested teeth after various chewing cycles in mm3 (N = 8)a
Groups
Chewing cycles
120,000
Enamel
P-Bon
C-NFC
R-IPN
R-Ort
R-TCR
R-DCL
R-Vit
a
0.014
0.004
0.026
0.050
0.057
0.066
0.079
0.036
±
±
±
±
±
±
±
±
0.01 AB
0.003 A
0.012 AB
0.029 C
0.014 CD
0.023 CD
0.048 D
0.017 BC
240,000
0.017
0.007
0.042
0.077
0.092
0.097
0.119
0.067
±
±
±
±
±
±
±
±
0.011 AB
0.004 A
0.015 BC
0.041 D
0.027 DE
0.035 DE
0.060 E
0.032 CD
480,000
0.022
0.013
0.070
0.120
0.138
0.160
0.166
0.121
±
±
±
±
±
±
±
±
0.014 A
0.008 A
0.034 AB
0.051 B
0.037 B
0.051 B
0.080 C
0.061 B
840,000
0.029
0.020
0.108
0.175
0.191
0.216
0.201
0.197
±
±
±
±
±
±
±
±
0.017 A
0.013 A
0.056 B
0.056 C
0.052 C
0.078 C
0.075 C
0.098 C
1,200,000
0.033
0.029
0.144
0.220
0.242
0.273
0.249
0.292
±
±
±
±
±
±
±
±
0.019 A
0.015 A
0.069 B
0.073 C
0.061 C
0.104 C
0.081 C
0.136 C
Means with the same capital letter within one row are not statistically different at p ≤ 0.05 (Fisher test).
resin teeth had statistically significantly higher vertical substance loss and volume loss than enamel and ceramic teeth
(p ≤ 0.05). However, the composite resin teeth showed significantly less vertical substance loss and less volume loss than
all acrylic resin teeth (p ≤ 0.05), while no statistical difference
was detected among the acrylic resin teeth (p > 0.05).
The SEM observations of the abraded surface of the resin
and composite resin teeth after 1,200,000 chewing cycles in the
chewing simulator are presented in Fig. 2. The size of the irregular inorganic filler particles in group C-NFC varied greatly but
was well distributed in the matrix (Fig. 2A). For group R-IPN, a
few PMMA beads were missing from the matrix (Fig. 2B). The
teeth exhibited the inclusion of spherical particles, presumably PMMA beads in the matrix. In group R-Vit, the spherical
PMMA beads could be clearly identified and were homogenously distributed in the matrix. Some cracks could be seen on
the wear surface between the organic filler agglomerates and
the resin matrix. In contrast to composite teeth, the organic
fillers and resin matrix were worn to the same extent, but the
resin matrix showed micro-cracks.
Fig. 2 – SEM observation of the wear surfaces of the resin teeth. Original magnification ×2000. (A) Composite teeth NFC® ; (B)
Polystar Selection® ; (C) Orthognath® ; (D) Bonartic TCR® ; (E) SR Orthotyp DCL® ; and (F) Vitapan Cuspiform® .
506
4.
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 502–507
Discussion
Wear of tooth hard tissues and restorative materials under
clinical conditions is rather a complicated phenomenon in
contrast to other mechanical and physical properties of materials [6]. The primary mechanisms are first, attritional wear by
welding under pressure of surface asperities and subsequent
shearing of such welds to give superficial damage of occlusal
contact surfaces; second, abrasive wear by abrasive particles
in the food or the wear debris loss from a surface through the
mechanical cutting action (rubbing or friction) of a secondary
material, which is in relative motion to the first surface, and
third, erosion by the mechanical removal of food flow from a
surface [6]. The discriminating characteristics controlling the
wear rate of materials include the friction coefficient, surface
roughness, the elastic modulus, and shear strength of both
antagonistic materials [3,8].
One recent study compared 10 different dental restorative materials in five wear simulators with a round robin
test [8]. The result showed that among the five wear simulators one wear simulator named as IVOCLAR (vertical loss)
was the best with the respect to the coefficient of variation. The variables relating to the same method largely
agreed with one another (volumetric and vertical wear).
This computer-controlled chewing simulator presented [9] the
simultaneous simulation of wear mechanics and temperature
change. Therefore, in this study, this kind of two-body chewing simulator was used to compare the wear resistance of
several artificial teeth. The teeth were loaded with total of
1,200,000 cycles, which is equivalent to 5 years of clinical service [6]. Thermal cycling was used as artificial aging to obtain
an increasing wear effect [10–12].
In this study, the two-body wear approach was used as a
consequence of direct contact between the test material and
its antagonist, and this can be described as mixed wear of
adhesion, attrition and fatigue [3]. Steatite ceramic balls with
a diameter of 6 mm were used as the antagonistic material
since this ceramic has been reported to be a suitable substitute for enamel in wear tests [13,14]. This ceramic material as
antagonist produced a similar wear rate on different composites as enamel antagonists [15]. The chosen diameter of 6 mm
is more comparable to the size and shape of a molar cusp [9]
than flat specimens used as antagonistic materials in other
studies [8,16].
In the present study, ceramic denture teeth (group P-Bon)
exhibited less wear than enamel although no statistical difference was found between the two groups. This may be
explained by the type and hardness of the antagonistic steatite
ceramic, which has a similar wear rate as enamel antagonists
[13,14]. In addition, ceramic is more sensitive to fatigue wear
due to flaws in its structure than to attritional wear due to its
crystalline matrix. This result is in agreement with another
in vitro study [16] in which ceramic teeth showed more wear
resistance than human enamel, while two low-fusing ceramics showed a tribological similarity to dental enamel after
glazing [17].
In an effort to retain the positive clinical characteristics of
acrylic resin teeth while achieving improved wear resistance,
composite and cross-linked acrylic resin denture teeth have
been developed. In the current study, there was no statistically significant difference in either vertical substance loss or
volume loss between groups enamel and C-NFC after 120,000
chewing cycles (p > 0.05). These results indicate that the initial
wear resistance of the composite resin teeth was not statistically different from that of enamel. However, after 1,200,000
cycles, all resin teeth including group C-NFC showed a statistically significantly higher vertical substance loss and volume
loss than enamel. In the abrasion mechanism of composite resin materials, the size, shape, volume and hardness of
fillers, the bonding between fillers and polymer matrix, and
the polymerization dynamics, all have an effect on the wear
characteristics [8]. During the wear process of composite resin,
inorganic filler particles are exposed after the softer resin
matrix is abraded, which causes a high friction coefficient and
leads to high internal shear stresses in the polymer matrix.
This process corresponds with an increased sensitivity to wear
which might have accelerated the wear of composite resin
teeth during long-term testing. Therefore, the composite resin
teeth showed more wear than enamel after long-term chewing
simulation, which is in agreement with the SEM photos after
1,200,000 chewing cycles and also an in vivo study reporting
the wear patterns of the filled composite resin after 4.5-year
of clinical service [18]. So it might be concluded that the composite resin teeth (group C-NFC) of this study showed a high
initial wear resistance. However, long-term wear resistance of
the composite teeth still needs improvement to be comparable
with that of natural tooth enamel.
Several studies indicated that micro-filled composite teeth
possess superior wear resistance compared to conventional
acrylic resin teeth with lineal polymer chain structures
although data vary depending on study designs [19,20]. The
components of the materials influence physical parameters,
such as flexural strength, fracture toughness, Vickers hardness, modulus of elasticity, etc., which may influence the
wear resistance [3,8]. Recently, various polymer technologies
including blend polymer, interpenetrating polymer networks
[5], and double cross-linking have been used to improve the
mechanical properties of modern acrylic resin teeth. However,
the wear resistance of all acrylic resin teeth was still statistically lower than that of the composite resin teeth (group
C-NFC).
In this study group R-Ort (conventional polymethyl
methacrylate denture teeth) showed vertical substance and
volume substance losses, which did not differ from those
of other modern acrylic resin teeth after 1,200,000 chewing
cycles. The result indicates that denture teeth made of polymers with a high degree of cross-linking (IPN, DCL, TCR) are not
consistently more wear-resistant than conventional acrylic
resin teeth. It also suggests that no definite relationship exists
between the chemical composition and the wear resistance of
denture teeth [21,22]. One study that compared five brands
of acrylic resin teeth with a toothbrush abrasion machine
concluded that modified resin teeth did not show superiority
over conventional acrylic resins [23] although several previous studies suggested that modern denture teeth with a high
degree of cross-linking and composite resin teeth were more
wear-resistant than conventional acrylic teeth [19,24,25]. As a
result, the hypothesis that there is no difference among these
artificial denture teeth and human enamel has to be rejected.
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 502–507
From the wear results of this in vitro study, when the
antagonistic materials are enamel or ceramic, ceramic teeth
should be preferred for removable dentures over acrylic resin
or composite resin teeth from the viewpoint of wear resistance and the intent to keep the vertical distance of occlusion.
However, when the antagonistic materials are modern acrylic
resins or composite resins, modern acrylic resin or composite resin teeth might be a good choice for removable
dentures, due to the similarity in wear properties. Further
studies are needed to evaluate the wear resistance of the resin
teeth tested when opposed to the materials of similar wear
characteristics.
5.
Conclusions
Within the limits of this study, the following conclusions are
drawn:
1. Ceramic denture teeth demonstrated a two-body wear similar to human enamel.
2. All acrylic resin and composite resin teeth tested showed
statistically higher wear than human enamel. However, the
composite resin showed improved wear resistance compared to modern acrylic resin denture teeth.
3. The acrylic resin teeth showed differences in their wear
resistance which were not statistically significant.
Acknowledgements
Based on a DMD thesis submitted to the Christian-Albrechts
University at Kiel. The authors thank Dipl.-Inf. Juergen
Hedderich, Department of Medical Informatics and Statistics, Christian-Albrechts University at Kiel, for his statistical
advice.
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