An in vitro comparison of retentive force and deformation of acetal resin and
cobalt-chromium clasps
Tugberk Arda, BDS,a and Ayla Arikan, PhDb
University of Marmara, Faculty of Dentistry, Department of Prosthodontics, Istanbul, Turkey
Statement of problem. The use of metal clasps on anterior teeth may cause esthetic problems. Recently,
acetal resins have been used as an alternative tooth-colored denture clasp material to improve esthetics.
However, there are few studies to support acetal resin use.
Purpose. The purpose of this in vitro study was to compare the retentive force and deformation of acetal resin
and cobalt-chromium clasps after 36 months of simulated clinical use.
Material and methods. Forty clasps each of acetal resin (1.2 or 2.0 mm thick) and cobalt-chromium
(Dentorium)(1.2 mm thick) were fabricated using half-round standard prefabricated clasp patterns. The groups
were further subdivided (n = 10) into the type of tooth (premolar or molar metal model) and undercut (0.25
mm or 0.50 mm). The retentive force of the clasps was measured in distilled water by a specially designed
insertion-removal testing apparatus with intervals corresponding to 0, 6, 12, 18, 24, 30, and 36 months of
simulated clinical use of a removable partial denture. The distance between the clasp tips (mm) was measured
with a microscope before and after the insertion-removal testing procedure. Comparison of the mean values of
the retentive force (gram force) of the clasps and the distance (mm) between the clasp tips was conducted with
3-way analysis of variance and a Least Significant Difference (LSD) multiple range test (a=.05).
Results. The mean values of tensile load required to dislodge acetal resin clasps with 1.2-mm thickness (111.6 g
or 0.11 N) and with 2.0-mm thickness (178.4 g or 1.75 N) was significantly lower than that to dislodge CoCr clasps (694.1 g or 6.81 N) (P,.001). The retentive force needed to dislodge all 3 types of clasps was
significantly lower for the molar than premolar models and also lower for the models with 0.25-mm undercuts
than for those with 0.50-mm undercuts (P,.001). After 36 months of simulated clinical use, there was evidence
of deformation in the cobalt-chromium clasps but no deformation noted for the acetal resin clasps. The retentive
force of cobalt-chromium clasps (297.4 g or 2.91 N) after deformation remained significantly higher (P,.001)
than the retentive force of acetal resin clasps that were 1.2 mm (110.7 g or 1.08 N) and 2.0 mm thick (177.5 g
or 1.74 N), respectively.
Conclusion. Within the limitations of this study, the results suggest that both thicknesses of acetal resin clasps
evaluated required less force for insertion and removal than Co-Cr clasps over a simulated 36-month
period. (J Prosthet Dent 2005;94:267-74.)
CLINICAL IMPLICATIONS
The results of this in vitro study demonstrate that the retentive force for an acetal resin clasp may
not be sufficient for removable partial dentures (RPDs) due to the significantly low retentive
force required for removal. Flexibility of acetal resin clasps would allow for the retentive clasp
arm of RPDs to be placed in deeper undercuts on abutments rather than Co-Cr, because acetal
resin is not as stiff as Co-Cr alloy. Although low retentive force is required for removal of these
clasps, the flexibility and the long-term retentive resiliency of the clasp suggest that acetal resin
clasps may be suitable for RPDs where esthetics or periodontal health is a primary concern.
R
estoration of esthetics is an important factor to
consider in the fabrication of a removable partial denture
(RPD).1 Several types of polymers and metal alloys have
Marmara University Research Foundation financially supported this
research.
a
Research Assistant.
b
Professor.
SEPTEMBER 2005
been used in RPD construction. Frequently, RPD clasps
made from the same alloy as the metal framework. The
most common alloys used for clasps are cobalt-chromium (Co-Cr) alloy and gold and titanium alloys, although these may be unesthetic.2 The methods to
overcome this esthetic dilemma have included covering
clasps with tooth-colored resin,3,4 using lingually positioned clasps,5,6 and engaging mesial rather than distal
undercuts.7 While RPDs with precision attachments
THE JOURNAL OF PROSTHETIC DENTISTRY 267
THE JOURNAL OF PROSTHETIC DENTISTRY
may be esthetically satisfying, however, they are expensive and more difficult to fabricate.8,9
Polyoxymethylene (POM), also known as acetal
resin, has been used as an alternative denture base
and denture clasp material since 1986 and was promoted, primarily, for superior esthetics.10 Acetal
resins are formed by the polymerization of formaldehyde.11 The homopolymer (POM) is a chain of
alternating methyl groups linked by an oxygen molecule.11 Because of its biocompatibility, it was considered as an RPD framework material for patients with
allergic reactions to Co-Cr alloys.10 It is reported
to have a sufficiently high resilience and modulus of
elasticity to allow its use in the manufacture of retentive clasps, connectors, and support elements for
RPDs.10,11
Several investigations have determined the properties of the materials used to fabricate RPD clasps.12-25
In view of the many variables involved with clinical
evaluation, most of the studies on clasp materials
and designs have been in vitro investigations.26
Some of the studies of clasps were performed under
a constant load12-19 or a constant displacement.19-24
Investigators have considered the long-term effectiveness of the clasps and the effects that the clasp might
have on the abutment teeth.22-24 A clasp arm design
producing less stress is important for predictable
long-term use of an RPD. Three factors—clasp material, clasp form, and the amount of undercut—affect
the design of a clasp arm.12 Clasp form involves the
elements of length, curvature, cross-sectional dimension, and taper. Among these, the first 2 elements
are determined by the abutment tooth contour, and
the latter 2 elements are under the control of the dentist or technician. Furthermore, clasp form is associated with stress distribution, which affects fatigue
and permanent deformation.25 Poor fit may cause
the decrease of retention and failure of RPD function.27 However, the mechanical properties of a
clasp material are generally determined by the alloy
used.28
Metals and metal alloys undergo permanent deformation and fatigue when exposed to repeated stress.29
The fatigue of a denture clasp is based on the repeated deflection of the clasp during insertion and
removal of the RPD over the undercuts of the teeth.2
Although extensive work has been performed to determine the properties of a variety of materials used for
RPD clasps,17,18,22-24 little is known about how acetal
resin functions in this application.
In this study, the tensile load required to dislodge acetal resin clasps with 2 different thicknesses (for a molar
or premolar) and 2 different amounts of undercut was
investigated after repeated cycles of placement and removal simulating 0, 6, 12, 18, 24, 30, and 36 months
of clinical use. Furthermore, the magnitude of tensile
268
ARDA AND ARIKAN
load required to dislodge clasps made of a conventional
dental Co-Cr alloy was also compared to that required to
dislodge acetal resin clasps. The permanent deformation
of the clasps was also determined after 36 months of simulated clinical use.
MATERIAL AND METHODS
White acetal resin (Polyoxymethylene; Pressing
Dental Srl, San Marino, Italy) and a conventional dental
Co-Cr alloy (Co 64%; Cr 28%; Mo 5%; Dentorium, New
York, NY) were evaluated in this investigation.
Impressions of a maxillary first premolar and first molar
from a Dentoform (Model 1362; Columbia Dentoform
Corp, New York, NY) were made with reversible hydrocolloid (Speedex; Coltene/Whaledent Inc, Cuyahoga
Falls, Ohio), and wax (Casting wax, Dentalwachse;
Karl Berg GmbH, Engen, Germany) models were prepared. The wax molar tooth was fixed in the center of
a wax plate with dimensions of 1.2 3 4 3 3 mm and
trimmed on a surveyor (APF400; Amann Dental
Equipment, Koblach, Austria) to provide mesial (8
mm) and lingual guide planes (6 mm) and to create a
0.25-mm undercut area on the distobuccal surface. An
occlusal rest seat 2 mm deep was prepared on the mesial
occlusal surface. Small amounts of wax were placed on
the distobuccal and distolingual line angles of the tooth
as reference points to standardize location of the tips
and lengths of both retentive arms (8 mm for premolar,
12 mm for molar) and reciprocating arms. The same procedure was used to make molar models with a 0.50-mm
undercut and premolar models with 0.25-mm and
0.50-mm undercuts. Each wax model was duplicated
(Lab putty; Omega GmbH, Bliedersdorf, Germany)
and poured with molten casting wax to make 10 identical sets of wax models for the molar and premolar, each
with different-sized undercuts. Forty wax models were
invested singly in casting rings (BEGO, Bremen,
Germany) and cast with a Co-Cr alloy (Dentorium), following manufacturer recommendations. The specimens
included 10 premolar clasps with 0.25-mm undercut,
10 premolar clasps with 0.50-mm undercut, 10 molar
clasps with 0.25-mm undercut, and 10 molar clasps
with 0.50-mm undercut. The specimens were then
trimmed, airborne-particle abraded, finished, and electropolished (Eltropol SL; BEGO). The guide planes
were evaluated for parallelism, and slight inaccuracies
were adjusted using a milling machine (APF400;
Amann Dental Equipment). Next, impressions of each
metal model were made in regular-body silicone impression material (Lab putty; Omega GmbH, Bingen,
Germany) with custom impression trays. Each impression was poured with die-investment material
(Wirovest; BEGO) to make 1 refractory cast for Co-Cr
clasps and, with Type IV dental stone (Cere Rock; GC
Germany GmbH, Munchen, Germany), to make 2 refractory casts for the acetal resin clasps.
VOLUME 94 NUMBER 3
ARDA AND ARIKAN
Fig. 1. Wax clasp pattern on model.
Fig. 3. Testing apparatus.
For Co-Cr clasps, preformed half-round standard
(1.2 mm thick) clasp patterns with occlusal rests, and
retentive and reciprocal arms (Klammern Clasp;
Dentaurum, Ispringen, Germany) were adapted on the
refractory investment cast. A wax plate (4 3 7 3 3 mm)
was attached to the minor connector parallel to the
path of insertion (Fig. 1). The plate was used later
for maintaining the clasp in the testing machine.
The assembly (cast and pattern) was invested
(Wirovest; BEGO) according to the manufacturer’s
instructions and cast in Co-Cr alloy. The clasp was
trimmed, airborne-particle abraded, finished, and electropolished using standardized procedures.29
For the acetal resin clasps, the same preformed halfround standard (1.2 mm thick) clasp patterns were
adapted on the refractory cast. For fabrication of the
2.0-mm-thick acetal resin clasps, the occlusal rest, and
retentive and reciprocal arms of the clasp were adapted
on the refractory die by using a straight semicircular
clasp pattern (Megadental Vertrieb GmbH, Budingen,
Germany). The previously described, similar-sized wax
plate was attached to the minor connector parallel to
SEPTEMBER 2005
THE JOURNAL OF PROSTHETIC DENTISTRY
Fig. 2. 1.2-mm- and 2.0-mm-thick acetal resin clasps and
Co-Cr clasps on metal models.
the path of insertion (Fig 1). First, the assembly (stone
model and pattern) was placed at a distance of approximately 2.5 cm from the injection opening of the special
flask (Muffle-Type 100; Pressing Dental Srl) with Type
IV dental stone (Marble Stone; Pressing Dental Srl).
One cylinder of the acetal resin was placed into the injection tube, and then the tube was placed in the injection
machine (J-100; Pressing Dental Srl). Once in the injection machine, the parameters of the machine were set as
follows: preinjection time with the material maintained
at 220°C (melting temperature) for 20 minutes, postinjection time with the temperature maintained at
the desired level of 220°C for 3 minutes, and injection
pressure of 4 bar. At the end of the process, the flask
was removed from the initial position, and the clasp
was deflasked. The clasp was polished with rubber points
(Pressing Dental Srl), followed by using polishing paste
for acetal and acrylic resins (Universal polish; Pressing
Dental Srl). Each of 40 acetal resin clasps, 1.2 or
2.0 mm thick, and Co-Cr clasps with 1.2-mm thickness
(Fig. 2) were divided into 4 subgroups (n = 10) according to the type of tooth (molar or premolar) and amount
of undercut (0.25 mm or 0.50 mm).
Prior to the retention testing, the clasps were placed
in acrylic resin blocks (Meliodent; Heraeus Kulzer,
Hanau, Germany) (2.5 3 4 3 5 mm) to standardize
the measurements with a microscope. The distances between the tips of the retentive and reciprocal arms of the
clasps were measured with a microscope (Toolmaker
TM-505; Mitutoyo Ltd, Singapore) and recorded to
the nearest 0.005 mm. The retention testing of the
clasps was conducted using a specially designed insertion-removal apparatus (Festo AG & Co, KG,
Istanbul, Turkey). The apparatus allowed the placement
(insertion) of the clasp to its predetermined terminal position and its subsequent removal from the metal model.
The retentive force of the clasp (g) was also measured
during removal (Fig. 3).
269
THE JOURNAL OF PROSTHETIC DENTISTRY
ARDA AND ARIKAN
Table I. Comparison between mean forces required to dislodge clasp at first period of cycles using 3-way ANOVA
Source
Type III sum of squares
df
Mean square
F
Sig.
8129100.217
107341.008
247611.675
13459.717
212570.150
4025.208
7413.117
55631.700
21690513.000
8777152.792
2
1
1
2
2
1
2
108
120
119
4064550.108
107341.008
247611.675
6729.858
106285.075
4025.208
3706.558
515.108
7890.670
208.385
480.698
13.065
206.335
7.814
7.196
,.001
,.001
,.001
,.001
,.001
.006
.001
Material
Tooth
Undercut
Material 3 Tooth
Material 3 Undercut
Tooth 3 Undercut
Material 3 Tooth 3 Undercut
Error
Total
Corrected total
Difference significant at P,.05.
Material, 1.2-mm- or 2.0-mm-thick acetyl resin clasp or Co-Cr clasp; Tooth, molar or premolar; Undercut, 0.25 mm or 0.50 mm.
Table II. Pairwise comparison of retentive force values (g) and SDs for clasps at first period of simulated clinical use*
Molar
Premolar
0.25 mm
Mean 6 SD
Acetal
Acetal
Acetal
Co-Cr
Acetal
Co-Cr
resin (1.2 mm)
resin (2.0 mm)
resin (1.2 mm)
resin (2.0 mm)
85.3
126.7
85.3
563.5
126.7
563.5
6
6
6
6
6
6
2.4
4.1
2.4
4.3
4.1
14.3
0.50 mm
P
Mean 6 SD
,.001
,.001
,.001
104.6
169.2
104.6
739.5
169.2
739.5
6
6
6
6
6
6
3.7
8.1
3.7
45.9
8.1
45.9
0.25 mm
P
,.001
,.001
,.001
Mean 6 SD
114.8
190.2
114.8
615.2
190.2
615.2
6
6
6
6
6
6
3.6
8.5
3.6
14.5
8.5
14.5
0.50 mm
P
Mean 6 SD
,.001
,.001
,.001
141.8
227.5
141.8
858.2
227.5
858.2
6
6
6
6
6
6
3.3
7.4
3.3
58.2
7.4
58.2
P
,.001
,.001
,.001
*0 insertion-removal cycle.
Table III. Pairwise comparison of retentive force values (g) and SDs for clasps after 36 months of simulated clinical use*
Molar
Premolar
0.25 mm
Mean 6 SD
Acetal
Acetal
Acetal
Co-Cr
Acetal
Co-Cr
resin (1.2 mm)
resin (2.0 mm)
resin (1.2 mm)
resin (2.0 mm)
84.7
126.4
84.7
255.6
126.4
255.6
6
6
6
6
6
6
2.6
4
2.6
33.3
4
33.3
0.50 mm
P
,.001
,.001
,.001
Mean 6 SD
103.5
168.4
103.5
314.2
168.4
314.2
6
6
6
6
6
6
4
8
4
34.1
8
34.1
0.25 mm
P
,.001
,.001
,.001
Mean 6 SD
113.3
188.2
113.3
279.0
188.2
279.0
6
6
6
6
6
6
3.4
9
3.4
28.5
9
28.5
0.50 mm
P
,.001
,.001
,.001
Mean 6 SD
141.1
226.9
141.1
340.9
226.9
340.9
6
6
6
6
6
6
3.5
7.5
3.5
29.4
7.5
29.4
P
,.001
,.001
,.001
*4380 insertion-removal cycles.
The clasp attached to the testing apparatus was placed
on the corresponding abutment metal model fixed on a
stainless steel container. The container was filled with
distilled water. Cycles (4380) of placement and removal
of the clasp, simulating 3-years of clinical use,15 were
performed along the path of insertion and removal determined by preliminary surveying procedures of the
abutment metal model. A tensile load was applied at a
crosshead speed of 10 mm per minute to the clasp until
it was dislodged. The sensor (Spider SW; MettlerToledo, Inc, Columbus, Ohio) connected to the load
cell detected the magnitude of the tensile load applied
at the moment the clasp was removed from the metal
model. The maximum loads required to remove the
270
clasp at 7 different periods of 0, 730, 1460, 2190,
2920, 3650, and 4380 continuous cycles were recorded
by the computer (Inspiron 8600; Dell Inc, Round Rock,
Tex) connected to the sensor. First, acetal resin clasps
and then Co-Cr clasps were tested to avoid any possible
surface attrition of the models. After fatigue testing, the
distance between the tips of the retentive and reciprocal
arms of each clasp, which were placed in the acrylic resin
blocks in the same position as previously described, was
measured with a microscope (Toolmaker TM-505;
Mitutoyo Ltd) and recorded.
The mean values and SDs of the retentive force magnitudes were recorded for the 7 periods for dislodgement of each clasp. The distances between the clasp
VOLUME 94 NUMBER 3
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THE JOURNAL OF PROSTHETIC DENTISTRY
Table IV. Mean values 6 SDs of average retentive force (g)
of clasps on molar and premolar with 0.25- and 0.50-mm
undercut and comparison at different test periods using LSD
multiple range test
Acetal resin
(1.2 mm)
Cycle
0
730 (6 mos)
1460 (12 mos)
2190 (18 mos)
2920 (24 mos)
3650 (30 mos)
4380 (36 mos)
111.6
111.3
110.8
110.7
110.7
110.6
110.7
6
6
6
6
6
6
6
20.9
20.8
21.1
20.8
21.0
21.0
20.9
Acetal resin
(2.0 mm)
178.4
178.4
178.2
178.1
177.8
180.2
177.5
6
6
6
6
6
6
6
37.5
37.4
37.5
37.3
37.5
44.2
37.4
Co-Cr
(1.2 mm)
694.1
538.2
433.9
393.2
355.5
322.3
297.4
6
6
6
6
6
6
6
121.5*
124.0*
87.0*
79.3*
68.6
54.0
44.8
*Differences significant at P,.05.
tips measured before and after the insertion-removal test
were calculated. Comparison of the data was conducted
with 3-way analysis of variance (ANOVA) and a least significant difference (LSD) multiple range test (a=.05).
RESULTS
Comparison of the mean retentive force with the 3way ANOVA for each of the different type of clasps at
the initial period is shown in Table I. The main effects
for the 3 factors studied, namely, material type, tooth
type, and undercut type, were significant. The main effects for the 3 factors were significant for all test periods.
Tables II and III represent the mean values of the retentive force required to cause dislodgement of each
clasp from molar and premolar teeth with 0.25-mm
and 0.50-mm undercuts and the ANOVA comparison
of mean values of each group at the initial and 36-month
periods. The mean retentive force at the first period
of testing in the test groups varied from 85.3 6 2.4
to 858.2 6 28.2 g (0.84 N to 8.41 N). The greatest retentive force required to remove the clasps was for
Co-Cr clasps, and the lowest retentive force was for
the 1.2-mm-thick acetal resin clasps. The mean retentive force for the 36-month test period varied from
84.7 6 2.6 to 340.9 6 19.9 g (0.83 N to 3.33 N). At
the initial and 36-month periods, the retentive force required to dislodge 1.2-mm-thick acetal resin clasps from
the molar and premolar teeth with both 0.25-mm and
0.50-mm undercuts was significantly lower (P,.001)
than the retentive force needed for the 2.0-mm-thick acetal resin and Co-Cr clasps. The retentive force required
for removal of the 2.0-mm-thick acetal resin clasps was
also significantly lower (P,.001) than that the retentive
force required for removal of the Co-Cr clasps from the
molar and premolar teeth with 0.25-mm and 0.50-mm
undercuts.
During repeated cycles of placement and removal, the
retentive force required to remove the Co-Cr clasps decreased significantly from the initial to the 6-, 12-, and
SEPTEMBER 2005
Table V. Mean values 6 SDs of distances (mm) between
clasp tips before and after simulated clinical use*
Before test
After test
Acetal resin
(1.2 mm)
Acetal resin
(2.0 mm)
Co-Cr
(1.2 mm)
6.3 6 1.0
6.3 6 1.0
6.3 6 1.1
6.3 6 1.1
6.1 6 1.0y
6.7 6 1.0y
*36 months, 4380 cycles.
y
Differences significant at P,.05.
18-month cycles (P,.001). Also at 30 and 36 months,
the retention for these clasps was significantly less than
the retention at 18 and 24 months. However, the retentive force required for both thicknesses of acetal resin
clasps demonstrated no significant change over the
7 periods tested (Table IV).
Table V represents the mean values of the distances
between clasp tips measured before and after testing
and comparison of the data with the ANOVA. After
36 months of simulated clinical use, there was significant
increase (P=.022) in the distances between the tips of
Co-Cr clasps, but not for both thicknesses of the acetal
resin clasps. The difference between the distances of
the tips from the models for both types of acetal resin
clasps measured before and after fatigue testing was
significantly lower (P,.001) than the distances of
Co-Cr clasp tips (Table VI).
DISCUSSION
Retentive clasp arms must be capable of flexing and
returning to their original form and should retain an
RPD satisfactorily. The tooth should not be unduly
stressed or permanently distorted during service and
should provide esthetic results.23 The clinical experience
of loss of retention of the RPD after the prosthesis is
worn for some time raises the question of whether constant deflection of the clasp during insertion and removal of the denture fatigues the clasp.2 Acetal resin is
marketed for the direct retainers attached to a Co-Cr
RPD framework, as well as the supportive components
of RPDs. As purported by the manufacturer, acetal resin
is available in 20 color shades matching the Vita shade
guide (Vitapan; VITA Zahnfabrik, Bad Sackingen,
Germany).
Acetal resin has a relatively high proportional limit
with little viscous flow, enabling it to behave elastically
over a large enough range to be used as a material for
clasp fabrication.11 Turner et al10 found that if all other
variables were equal, a 15-mm-long Co-Cr clasp of
1-mm diameter would exhibit the same stiffness as an acetal resin clasp, 5 mm in length and 1.4 mm in diameter.
For this reason, thicker acetal resin clasps were used for
comparison in the present study.
271
THE JOURNAL OF PROSTHETIC DENTISTRY
ARDA AND ARIKAN
Table VI. Pairwise comparison of difference (mm) between distances of clasp tips measured before and after
simulated clinical use*
Molar
Premolar
0.25 mm
Mean
Acetal resin (1.2
Acetal resin (2.0
Acetal resin (1.2
Co-Cr (1.2 mm)
Acetal resin (2.0
Co-Cr (1.2 mm)
mm)
mm)
mm)
mm)
0.002
0.003
0.002
0.201
0.003
0.201
0.50 mm
P
.926
,.001
,.001
Mean
0.001
0.001
0.001
0.455
0.001
0.455
0.25 mm
P
.975
,.001
,.001
Mean
0.003
0.001
0.003
0.203
0.001
0.203
0.50 mm
P
.901
,.001
,.001
Mean
0.003
0.001
0.003
0.393
0.001
0.393
P
.901
,.001
,.001
*36 months or 4380 cycles.
The study was designed to compare clasps in function
in 2 different amounts of undercuts. The 0.25-mm undercut was selected because it represents the undercut
commonly used for Co-Cr clasps. With the increased requirements of esthetics, more patients are requesting
that dentists conceal RPD clasps by placing them closer
to the gingival, where the undercuts tend to be larger.
The stiffness of Co-Cr clasps makes them unsuitable
for placement in larger undercuts due to unacceptable
stresses on the abutments.22 One property of acetal
resins that has created interest for use in RPDs is the
low modulus of elasticity, which allows for their use
in larger retentive undercuts than recommended for
Co-Cr alloys.10,11 This may be advantageous in clinical
situations in which esthetics and/or periodontal health
are priorities.24 The second undercut condition
(0.50 mm) was chosen to determine whether fabrication
of clasps from acetal resin would be a possible alternative
to the metal clasps in large undercuts. Greater than
0.50-mm-undercut conditions were not considered because Co-Cr clasps in a deeper undercut may be difficult
to repair.
Abutment condition includes the shape of the abutment and the frictional coefficient between the abutment and the clasp. The frictional coefficient between
a tooth and cast metal clasp metals has been reported
to be approximately 0.2 in the presence of saliva.27
Sato et al27 reported that frictional coefficient values in
wet conditions with water are nearly the same as values
in wet conditions with saliva. In the present study, the
insertion and removal test was performed in wet conditions with distilled water.
The results of the present study showed that acetal
resin clasps of both dimensions had significantly lower
retentive force (111.6 g or 1.09 N and 178.4 g or
1.74 N, respectively) than Co-Cr clasps (694.1 g or
6.80 N). Ahmad et al20 found that 4.77-N retention
was required to dislodge a Co-Cr clasp from a
0.25-mm undercut. Frank and Nicholls14 concluded
that 300 to 750 g (2.94 N to 7.35 N) represented an
acceptable amount of retention for a bilateral distal extension RPD. The flexibility of a clasp arm affects the re272
tention and the function of an RPD. If a clasp is too
flexible, the clasp may not provide adequate retention
for the RPD when the framework design is based on
the recommended principles for Co-Cr alloys.2 In the
present study, the results demonstrate that the retentive
force for an acetal resin clasp might not be sufficient for
RPDs because of the significantly low retentive force required for removal. It should be noted that other factors, in addition to the type of retentive clasp and
depth of undercut, influence the retention of an RPD.
For example, the presence of a guiding plane on the
proximal surface increases the retention.1 Clinical experience indicates that ineffective reciprocation may result
in denture instability and lack of retention. The number
and distribution of the abutment teeth, the amount of
wax block-out, and the fit of the framework are other
factors that influence the amount of retention obtained.14 The experimental design of this study tested
a single-clasp system. When 2 or 3 acetal resin clasps
are used in the fabrication of an RPD and all other mentioned factors are also considered, acetal resin clasps may
be sufficient for clinical use.
The mean retentive force of the 1.2-mm-thick acetal
resin clasps was lower than that of 2.0-mm-thick acetal
resin clasps. Not surprisingly, the greatest retentive force
for acetal resin clasps was found in the 2.0-mm-thick
clasps designed to engage the 0.50-mm undercut area
of the premolar. The acetal resin clasps exhibited greater
retentive forces when placed in both 0.25-mm and
0.50-mm undercuts of the premolar due to the shorter
length of the clasp arms. Turner et al10 suggested that
in clinical use, acetal resin clasps should have a shorter
length, a relatively larger cross-sectional area, and engage deeper undercuts to have adequate retention.
Fitton et al11 stated that to gain adequate retention
from acetal resin clasps, the clasp should have a greater
cross-sectional area than a metal clasp. The present
study confirms these findings. The acetal resin clasp
must be thicker and shorter than a standard clasp and
engage a deeper undercut to achieve clinically acceptable
retention. This is due to greater flexibility of the acetal
resin (Elastic modulus; 2.9 to 3.5 kN/mm2)11
VOLUME 94 NUMBER 3
ARDA AND ARIKAN
when compared to Cr- Co alloy (Elastic modulus; 22.43
kN/mm2).29 It could be argued that a larger, bulkier
clasp design would be detrimental to oral health by contributing to plaque accumulation. However, if plaque
control is established and the patient presents for regular
recall visits, there is no evidence suggesting that any
harm will result.26 Further study using deeper undercuts
and thicker retentive clasp arms is recommended to
provide additional information for using acetal resin
clasps.
The results of this study indicate that acetal resin
clasps with 1.2-mm thickness and with 2.0-mm thickness are resistant to deformation and may offer a clinical
advantage over the conventional metal clasps. The retentive forces of both types of acetal resin clasps did
not decrease over the cycling periods. Under the conditions of the present study, Co-Cr clasps lost retentive
force within 730 cycles of placement and removal and
continued to lose retentive force during the remaining
testing period. Past studies have indicated that there
was a loss of retention because of permanent deformation of the Co-Cr clasps.12,15,25 The attrition of the inner surface of the Co-Cr clasps and/or the surface of the
model may be other potential causes of the reduction of
the retention. This possibility could not be completely
eliminated in this study because there was no quantitative evaluation of the specimen surface characteristics.
However, no attrition on these surfaces was observed
visually.
Although the retentive force of the Co-Cr clasps decreased in the test cycles, it was still greater than that of
both types of acetal resin clasps at the end of the test period. The greater load generated by the Co-Cr clasp was
attributed to a higher stiffness or modulus of elasticity
than acetal resin. With this study design, in which the
shapes and dimensions of the abutments were standardized, the retention magnitude of the clasps should
be closely related to the elastic modulus of the clasp
material.
Bates12 showed that a 10-mm-long Co-Cr clasp
would display a tip deflection of 0.15 mm at the proportional limit. Morris et al13 suggested that permanent
deformation of a clasp tip by more than 0.025 mm
may be significant. The results of the present study
showed a significant deformatio of Co-Cr clasps after
36 months of simulated clinical use. However, neither
type of acetal resin clasps showed significant deformation after 36 months of simulated clinical use (Table VI).
Suggesting that acetal resin is a better material because it assists in overcoming the poor esthetics of anterior clasping and demonstrates greater flexibility
resulting in reduced loads on the abutment teeth may
not be adequate to change the choice of the material.
The lower retention provided by acetal resin clasps
should be considered in clinical use. Further research
on the clinical efficacy of the acetal resin clasps is needed
SEPTEMBER 2005
THE JOURNAL OF PROSTHETIC DENTISTRY
to determine whether these materials are suitable alternatives for RPD clasps.
CONCLUSIONS
Within the limitations of this study, the following
conclusions were drawn:
1. The mean retentive force required to remove the
1.2-mm- and 2.0-mm-thick acetal resin clasps was
found to be significantly lower (P,.001) than those
required for removal of the Co-Cr clasps.
2. The retentive force required for 1.2-mm- and
2.0-mm-thick acetal resin clasps demonstrated no significant change over the 7 periods tested.
3. Co-Cr clasps lost significant amounts of retentive
force from the initial to the 6-, 12- and 18-month
periods (P,.001) and continued to lose the retentive
force during the remainder of the test.
4. Although the mean retentive force of the Co-Cr
clasps decreased in the test cycles, it remained significantly greater (P,.001) than that of both types of acetal
resin clasps at the end of the 36 months of simulated
clinical use.
5. The 1.2-mm- and 2.0-mm-thick acetal resin
clasps tested did not show significant deformation after
36 months of simulated clinical use.
6. Co-Cr clasps showed significant deformation
(P=.022) after 36 months of simulated clinical use.
The authors thank the Festo Co and Yildiz University, Faculty of
Metallurgy, for technical support.
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ARDA AND ARIKAN
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Reprint requests to:
DR TUGBERK ARDA
UNIVERSITY OF MARMARA
FACULTY OF DENTISTRY, DEPARTMENT OF PROSTHODONTICS
GUZELBAHCE BUYUKCIFLIK SOK. NO: 6 80200
NISANTASI, ISTANBUL
TURKEY
FAX: 90-0-212-246 52 47
E-MAIL: [email protected]
0022-3913/$30.00
Copyright Ó 2005 by The Editorial Council of The Journal of Prosthetic
Dentistry.
doi:10.1016/j.prosdent.2005.06.009
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VOLUME 94 NUMBER 3