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Original Article
A comparative study of retentive strengths
of zinc phosphate, polycarboxylate and glass
ionomer cements with stainless steel crowns –
An in vitro study
Abstract
An in vitro study was conducted to compare the retentive
strengths of zinc phosphate, polycarboxylate and glass
ionomer cements using Instron universal testing machine. Thirty
preformed and pretrimmed stainless steel crowns were used for
cementation on 30 extracted human primary molars which were
divided into three groups of 10 teeth in each group. Then the
teeth were stored in artificial saliva and incubated at 37°C for
24 h. A load was applied on to the crown and was gradually
increased till the crown showed dislodgement, and then the
readings were recorded using Instron recorder and analyzed for
statistical significance. The surface area of crown was measured
by graphical method. The retentive strength was expressed in
terms of kg/cm2, which was calculated by the equation load
divided by area. Retentive strengths of zinc phosphate (ranged
from a minimum of 16.93 to amaximum of 28.13 kg/cm2 with
mean of 21.28 kg/cm2) and glass ionomer cement (minimum of
13.69 - 28.15 kg/cm2 with mean of 20.69 kg/cm2) were greater
than that of polycarboxylate cement (minimum of 13.26 - 22.69
kg/cm2 with mean of 16.79 kg/cm2). Negligible difference
(0.59 kg/cm2) of retentive strength was observed between zinc
phosphate (21.28 kg/cm2) and glass ionomer cements (20.69
kg/cm2). Glass ionomer cements can be recommended for
cementation of stainless steel crowns because of its advantages
and the retentive strength was almost similar to that of zinc
phosphate cement.
Key words
Luting cements, retentive strength, stainless steel crowns
Introduction
The restoration of primary and permanent teeth
with advanced carious lesions has been a constant and
Raghunath Reddy MH, Subba Reddy VV1,
Basappa N2
Professor & Head, Department of Pedodontics & Preventive
Dentistry, Uttar Pradesh Dental College & Hospital, Faizabad
Road, Lucknow, Utter Pradesh, 1Principal, Professor & Head,
Department of Pedodontics & Preventive Dentistry, College
of Dental Sciences, Pavilion Road, 2Professor, Department of
Pedodontics & Preventive Dentistry, Bapuji Dental College &
Hospital, Davangere – 577004, Karnataka, India.
Correspondence:
Dr. M.H. Raghunath Reddy, Department of Pedodontics &
Preventive Dentistry, Uttar Pradesh Dental College & Hospital,
Faizabad Road, Lucknow, Utter Pradesh, India.
E mail- pedoreddy@gmail.com
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DOI:
10.4103/0970-4388.76150
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difficult problem for the dentist, to prevent premature
loss of primary teeth and to maintain normal occlusion.
Studies have shown that amalgam, a commonly used
restorative material, had to be replaced with stainless
steel crowns in 70% of multisurface amalgam
restorations. Stainless steel crowns have proved to
be efficacious and are relatively easy to use, they have
become an important factor in the restoration of
hypoplastic, endodontically treated teeth, malformed
teeth and fracture teeth to perform their normal
function.[1,2]
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Raghunath Reddy, et al.: Retentive strength of cements
In 1950, Humphery introduced preformed or stainless
steel crowns, later Helm listed indications for stainless
steel crowns in pediatric dentistry.
Many studies have investigated the retention of
stainless steel crowns and clinicians have suggested that
dental cement alone was responsible for retention of
stainless steel crowns on primary molars.[3-6] Jefferey et
al, however, believe that the significant retentive feature
is the close adaptation of the metal crown margin to
the tooth surfaces in the undercut areas of the prepared
teeth.[7] Placement of well-fitted, contoured stainless
steel crown on different crown preparations, without
cement is not possible because of its insufficient
strength to fit on the tooth. Stainless steel crowns
have been cemented with zinc phosphate cement,[4,8,9]
polycarboxylate cement,[4,5,8,9] and zinc oxide eugenol
cement, but it would appear that glass ionomer cements,
due to their adhesive properties to dentin and enamel
and fluoride releasing ability, have definite advantages
over the other cements.[5,10]
So this study was planned to compare the retentive
strengths of zinc phosphate, polycarboxylate and glass
ionomer cements with the stainless steel crowns.
Materials and Methods
The study was conducted in the Department of
Pedodontics and Preventive dentistry, Bapuji Dental
College and Hospital, Davangere in collaboration
with the Department of Metallurgy, Indian Institute
of sciences, Bangalore and Department of civil
Engineering, B.D.T., Davangere.
Instron universal testing machine: (Instron 8502) Figure 1
The specially designed Instron apparatus were used.
The applied force was directed parallel to the long axis
of the tooth, at a cross head speed of Instron of 0.05
inch/min.
A total of 30 extracted human maxillary first and
second mandibular primary molars (two maxillary
primary first molars, 10 maxillary primary second
molars, six mandibular primary first molars, 12
mandibular primary second molars) were selected
for the study. These teeth had sufficient intact tooth
structure so that a good crown marginal seal could be
obtained on the basis of a good clinical examination.
Each tooth was hand scaled and cleaned to remove soft
246
tissue debris and rinsed with deionized water. Each
tooth was mounted in self-cure acrylic resin exposing
complete crown. The occlusal surfaces of all teeth were
reduced uniformly about 1 mm to 1.5 mm. The proximal
surfaces were prepared so that contact was broken and
all mesial and distal undercuts were removed. All the
sharp angles were made rounded. Then pretrimmed and
precontoured stainless steel crown (Ion-3M) was fitted
on each tooth and crimped, contoured to allow for the
best marginal fit achievable on the basis of a thorough
examination with an explorer. The crown had two
opposing attachments (Begg’s brackets) spot welded
to facilitate the attachment of the crown removal
apparatus. Then 30 teeth were randomly divided
into three groups of 10 teeth in each group, i.e., zinc
phosphate cement, polycarboxylate cement and glass
ionomer cement, respectively [Figure 2].
The cements were manipulated according to the
manufacturer’s recommendations. They were loaded
into the crown and each crown was seated with hand
pressure. All excess cement was removed from crown
margins. After 10 min, the teeth were stored in prepared
artificial saliva and incubated at 37°C for 24 h. Retentive
strength was tested using Instron universal testing
machine. The machine [Figure 3] was fitted with an
Instron recorder. A specially designed apparatus of
Instron was used to remove the cemented crowns.
Hole was made for each specimen at the base of the
mounted teeth with self cure acrylic resin in order to
place the specimen in the specially designed Instron
apparatus[Figure 4]. A screw was passed through the
Instron apparatus and the hole of the each specimen to
stabilize the specimen during crown removal and the
screw was made tight.
The load was applied from zero reading and then
gradually increased. The loading was continued until
the cemented stainless steel crowns showed first
dislodgement during crown removal. The machine
was stopped by pressing the button after cemented
stainless steel crowns showed first dislodgement and
recorded the readings. The specimen was removed and
new specimen was placed in the Instron apparatus and
the screw was made tight. The same procedure was
followed for all the specimens. The applied load was
directed parallel to the long axis of the tooth during
crown removal. Cross head speed of Instron was 0.05
inch/min. The retentive strength values were recorded,
expressed in terms of kg/cm2, which was calculated by
the formula: load / area.
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Raghunath Reddy, et al.: Retentive strength of cements
Figure 1: Instron apparatus and specimen
Figure 2: Armamentarium
Figure 3: Specimen mounted on Instron
The surface areas of crowns have been determined
by cut opening of the crowns and their surfaces have
been developed on graph sheet and the areas of these
developed surfaces have been determined by counting
the squares on these developed areas.
Results
An in vitro study of three dental cements was performed
to test the retention of stainless steel crowns on
extracted primary molars using Instron Universal
testing machine.
The Wilcoxon Mann–Whitney ‘ U ‘ test was performed
to compare the retentive strength between each group
to see whether there were any significant differences
in strengths among the groups.
Kruskal–Wallis one-way analysis of variance was
performed to know the difference in retentive strength
exhibited by different cements.
Figure 4: Instron universal testing machine
The analysis revealed that there were significant
differences in retentive strengths of different cements.
The retentive strength of the zinc phosphate,
polycarboxylate and glass ionomer cements were
ranged from 16.93 to 28.13, 13.26 to 22.69 and 13.69
to 28.15 kg/cm2 respectively. The mean retentive
strength with standard deviation for zinc phosphate,
polycarboxylate and glass ionomer cements were
21.28 ± 2.91, 16.79 ± 3.28 and 20.69 ± 4.14 kg/cm2
respectively [Tables 1 and 2].
The retentive strength of zinc phosphate ranged
from 16.93 kg/cm2 to 28.13 kg/cm2 with an average
of 21.28 ± 2.91 kg/cm2. For polycarboxylate cement,
the retentive strength ranged from 13.26 to 22.69kg/
cm2 with an average of 16.79 ± 3.28 kg/cm2. The
results showed that zinc phosphate cement strength
was greater than that of polycarboxylate cement and
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Raghunath Reddy, et al.: Retentive strength of cements
the difference was statistically significant (P<0.05)
[Table 3].
The retentive strength of zinc phosphate ranged from
16.93 to 28.13 kg/cm2 with an average of 21.28 ±
2.91kg/cm2. For glass ionomer cement, the retentive
strength ranged from 13.69 to 28.15 kg/cm2 with an
average of 20.69 ± 4.14 kg/cm2. The mean retentive
strength of the glass ionomer cement was lesser than
that of phosphate cement, but the difference was not
statistically significant [Table 4].
The comparison of the retentive strength of
polycarboxylate and glass ionomer cements was also
assessed. The retentive strength of polycarboxylate
cement ranged from 13.26 to 22.69 kg/cm2 with an
average of 16.79 ± 3.28 kg/cm2 with an average of
20.69 ± 4.14 kg/cm2. The mean retentive strength
of the glass ionomer cement was greater than that
of polycarboxylate cement and the difference was
statistically significant (P<0.05) [Table 5].
The results showed that zinc phosphate cement had
better retentive strength than glass ionomer cements
but the difference was not statistically significant. Both
the zinc phosphate and glass ionomer cements showed
better retentive strength than the polycarboxylate
cement and the difference was statistically significant
(P<0.05).
Table 1: Retentive strengths of the cements (kg/cm2)
Zinc phosphate
Polycarboxylate
Glass ionomer
15.60
13.26
22.69
20.06
15.89
17.73
20.46
13.71
13.93
14.60
16.79
03.28
23.65
16.91
13.69
18.55
24.76
21.19
18.62
28.15
19.95
21.46
20.69
04.14
21.50
18.56
20.28
21.66
21.82
16.93
20.65
28.13
21.06
22.18
Mean 21.28
SD 02.91
Table 2: Retentive strengths of zinc phosphate, polycarboxylate
and glass ionomer cements
Cements
No. of
cases
Mini - maxi
(kg/cm2)
Mean
Stand. dev.
10
10
10
16.93–28.13
13.69–28.15
13.26–22.69
21.28
20.69
16.79
2.91
3.28
3.28
Zinc phosphate
Glass ionomer
Polycarboxylate
(Wilcoxon’s Mann–Whitney ‘U ‘test)
Table 3: Retentive strengths of zinc phosphate and
polycarboxylate cements
Cements
No. of Mini - maxi Mean
cases
(kg/cm2)
Stand
dev.
Significance
P>0.05
Significant
Zinc phosphate
10
16.93–28.13
21.28
2.91
Polycarboxylate
10
13.26–22.69
16.79
3.28
(Wilcoxon’s Mann–Whitney ‘U ‘test)
Table 4: Retentive strengths of zinc phosphate and glass
ionomer cements
Discussion
An in vitro study was carried out to compare the retentive
strength of the three luting cements.
Cements
Zinc phosphate
Glass ionomer
No. of Mini - maxi Mean
cases
(kg/cm2)
10
10
16.93–28.13
13.69–28.15
21.28
20.69
Stand
dev.
Significance
2.91
4.14
Not significant
Zinc phosphate is the oldest of the luting cements used
widely for cementation of stainless steel crowns. This
cement is generally considered as adequate for clinical
crown retention, even though its retentive properties are
purely mechanical in nature. It is brittle, has a relatively
high solubility in the mouth, and it does not adhere to
tooth structure. They do have high compressive strength
and are also a potentially caustic substance, to vital pulp
tissue due to their low pH.[11,12]
(Wilcoxon’s Mann–Whitney ‘U ‘test)
The use of fluoride releasing cements with the potential
for chemical adhesion and mechanical retention helps in
retention of the crown. The polycarboxylates form an
ionic bond with enamel and dentin and have a higher
adhesive strength than the zinc phosphate.[11] Addition of
fluoride to this cement results an increase in strength and
anticariogenic properties.[13] They also have a somewhat
lower compressive strength than that of zinc phosphate
cements and are relatively non toxic to vital tissue.[14-16]
The glass ionomer cements are also used because it
forms a strong ionic bond to enamel, dentin and nickel
chrome alloys providing a high adhesive strength[16-20]
and releases fluoride from the setting cement. It gives
a high compressive strength[18,21,22] because of the
248
(Wilcoxon’s Mann–Whitney ‘U ‘test)
Table 5: Retentive strengths of polycarboxylate and glass
ionomer cements
Cements
No. of Mini - maxi Mean
cases
(kg/cm2)
Stand
dev.
Significance
P> 0.05
Significant
Polycarboxylate
10
13.26–22.69
16.79
3.28
Glass ionomer
10
13.69–28.15
20.69
4.14
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Raghunath Reddy, et al.: Retentive strength of cements
unique composition and structure of Al3+ and Ca2+ ions
and polyacrylic acid solution, which is also found to be
relatively nontoxic to vital pulpal tissue.[23]
Studies compared the retention of stainless steel crowns
with and without cementation (i.e., mechanical retention
from the crown alone and retention due to cementation)
and they found the retention due to cementation to be
far greater than that gained from mechanical retention
alone. This clearly shows that the placement of cement is
necessary for the placement of stainless steel crowns.[3-6]
Zinc phosphate, polycarboxylate and glass ionomer
cements were used in this study, because these are the
commonly used luting cements for cementation of
stainless steel crowns in routine clinical practice.
Primary molars were selected for this study because
stainless steel crowns are more widely used on primary
molars to prevent premature tooth loss and development
of future malocclusion.
Pretrimmed and precontoured stainless steel crowns
were used in this study because to standardize the surface
area of the crowns as in case of other type of crowns
trimming is necessary which gives an intra clinician
variation in surface area and the specimens were stored
in the artificial saliva because it simulates human saliva.
Instron tensometer, Hounsfield tensometer and Instron
universal testing machines were used to measure the
retentive strength of the cements. Whereas, in the
present study, Instron universal testing machine was
used because of the easy availability than Instron
tensometer and Hounsfield tensometer.
Mathewson found the highest retentive strength with
copper phosphate cement than zinc phosphate and
polycarboxylate cements, which he attributed to the low
pH of the copper cement during the setting reaction
and a possible acid etching effect on the tooth creating
a better bond between the cement and the tooth. He
also speculated that the acidity of the copper cement
potentially was harmful to pulp tissues in vital teeth. So in
this study, copper phosphate cement was not considered
for testing retentive strength.[4]
David R. Myers and Garcia Godoy reported that no
significant difference was found between zinc phosphate
and polycarboxylate cements in the retention ability of
the cements. Whereas in this study, zinc phosphate cement
showed better retentive strength than polycarboxylate
cement, which was statistically significant (P<0.05). The
difference may be due to the fact that zinc phosphate
cement lies on mechanical interlocking for its retentive
effect and on close physical adaptation for sealing
restorative margins, but it does not provide any chemical
bonding to tooth or metal surfaces.[8,9]
Noffsinger et al found that no significant difference
between the overall mean retentive forces of the two
glass ionomer cements and polycarboxylate cement using
preformed crowns fitted to extracted third molar teeth,
whereas in this study glass ionomer cement showed
better retentive strength than polycarboxylate cement,
which was statistically significant (P<0.05).[5] The exact
reason for this difference is not known, but it may due
to powder - liquid ratio, viscosity and shorter working
time. Advantage of the glass ionomer over the zinc
polycarboxylate is the fluidity or lower viscosity of the
mixed glass ionomer cement.[24] Studies have shown that
glass ionomers have higher adhesive strength and form
stronger bonds to tooth structure and stainless steel than
the polycarboxylate cements.[16-20]
Polycarboxylates form an ionic bond with enamel and
dentin and have a higher adhesive strength than zinc
phosphate.[11] The chemical reaction of polycarboxylate
cement is by which zinc ions link adjacent poly acrylic
acid molecules producing a large cross-linked chelate
structure. The poly acrylic acid molecules have the
ability to chelate to calcium ions in tooth enamel as well
as to stainless steel. This shows that polycarboxylate
cements are suitable for cementation of stainless steel
crowns.[25,26]
In the present study, zinc phosphate cement showed
better retentive strength than glass ionomer cement.
The lesser retentive strength of glass ionomer may be
due to the advantage of the cement during initial phase
of setting, moisture can adversely affect the hardness
of the surface.[27-29]
From this study, glass ionomer cements can be
recommended for cementation of stainless steel
crowns. Because these cements have the ability to bond
to enamel and base metals,[16-20] release fluoride from
setting cement [21] and are found to be nontoxic to vital
pulp tissue.[23] In the present study, the mean retentive
strength of zinc phosphate cement was 21.28kg/cm2 and
glass ionomer cement showed 20.69 kg/cm2, which was
not of much difference. So glass ionomer cements can be
recommended for cementation of stainless steel crowns.
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Raghunath Reddy, et al.: Retentive strength of cements
Summary and Conclusion
This in vitro study was conducted to compare the
retentive strengths of zinc phosphate, polycarboxylate
and glass ionomer cements using Instron universal
testing machine. Thirty preformed and pretrimmed
stainless steel crowns were used for cementation on 30
extracted human primary molars.
6.
A load was applied on to the crown and was gradually
increased till the crown showed dislodgement, and then
the readings were recorded and analyzed for statistical
significance. The surface area of crown was measured by
graphical method. The retentive strength was expressed
in terms of kg/cm2, which was calculated by the equation
load divided by area.
9.
From the present study, it can be concluded that:
• The retentive strength of zinc phosphate cement
ranged from a minimum of 16.93 to a maximum of
28.13 kg/cm2 with the mean of 21.28 kg/cm2.
• The retentive strength of polycarboxylate cement
ranged from a minimum of 13.26 to a maximum of
22.69 kg/cm2 with the mean of 16.79 kg/cm2.
• The retentive strength of glass ionomer cement
ranged from a minimum of 13.69 to a maximum of
28.15 kg/cm2 with the mean of 20.69 kg/cm2.
• The retentive strengths of zinc phosphate and glass
ionomer cements were greater than the retentive
strength of polycarboxylate cement.
• Negligible difference (0.59 kg/cm2) of retentive
strength was observed between zinc phosphate (21.28
kg/cm2) and glass ionomer cements (20.69 kg/cm2).
• Glass ionomer cements can be recommended for
cementation of stainless steel crowns because of its
advantages and the retentive strength was almost
similar to that of zinc phosphate cement.
7.
8.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
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Source of Support: Nil, Conflict of Interest: Nil
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