evaluation of fatigue resistance of esthetic and …
TRANSCRIPT
EVALUATION OF FATIGUE RESISTANCE OF ESTHETIC AND
METAL CLASPS FOR REMOVABLE PARTIAL DENTURE– AN IN
VITRO STUDY
A Dissertation submitted to
THE TAMILNADU DR.M.G.R. MEDICAL UNIVERSITY
CHENNAI - 600032
In partial fulfilment for the degree of
MASTER OF DENTAL SURGERY
BRANCH – I
DEPARTMENT OF PROSTHODONTICS
& CROWN AND BRIDGE
2017 – 2020
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INTRODUCTION
The Prosthetic rehabili tation should be able to recover the patient’s masticatory
function, phonetics and aesthetics because it will significantly affect the quality
of l ife (1 )
. Despite of the advocated performance and success rates of fixed
partial dentures (FPD), removable partial dentures (RPD) are sti ll being used for
oral rehabili tation. The extension of the edentulous space s, bone loss, short
clinical crowns and the financial conditions of the patient may be a reason to
seek for other forms of rehabili tation of the stomatognathic system restoration
such as removable partial dentures (RPD). The most common alloys used for
clasps were cobalt -chromium (Co-Cr) alloy, gold and ti tanium alloys, although
these may be unesthetic (5 )
.The emphasis on the physical appearance in this
contemporary society has increased the demand for esthetic dental restorations
recently (2 )
. The display of the direct retainer assemblies of RPD is the biggest
esthetical problem. Many trials have been carried out to overcome this problem
such as etching the clasp arm and coating it with a layer of tooth -color resin and
lingual retention design engagement of mesial rather than distal undercuts and
use of gingival rather than occlusally approaching clasps. Unless clasps can be
avoided by using precision attachments, which are relatively expensive and
technically demanding, some of the removable partial denture framework will
invariably be visible (3 )
. Several types of metal alloys and polymers have been
used in removable partial denture construction (6 -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. Acetal resins are fabricated from the
polymerization of formaldehyde. The homopolymer (POM) is a chain of
alternating methyl groups linked by an oxygen molecule. Because of i ts
biocompatibili ty, i t was widely considered as a removable partial denture
framework material for patients with allergic reactions to Co -Cr alloys. It has
been reported that resil ience and modulus of elasticity are high and sufficient
enough to allow its useage in the manufacture of retentive clasps, connecto rs
and other support elements for the fabrication of removable partial dentures.
Polyetheretherketon (PEEK) and polyetherketonketon (PEKK) are the polymers
from the group of polyaryletherketone (PAEK) which is a relatively new family
of high-temperature thermoplastic polymers consists of an aromatic backbone
molecular chain, interconnected by ketone with ether as its functional groups
(10 ). In medicine PEEK has been demonstrated to be excellent substitute for
t i tanium in orthopedic applications (10 , 11 )
and it has been used in dentistry as
provisional implant abutment (12 )
. PEEK is highly flexibile and elastic in nature,
because of these mechanical properties it could decrease the stress on abutment
teeth. Direct retainers are also fabricated these days in a tooth-colored material
constructed from acetal resin and polyetheretherketone (PEEK) have been used
to enhance the esthetic property of direct me tal retainer assemblies of removable
partial denture (4 )
.
Several investigations have determined the properties of the materials used to
fabricate removable partial denture clasps. Fatigue, which is the loss of
mechanical properties of material after repeated loading, affects denture
construction. In the mid-1970s stress distribution in denture clasps was studied
and reported that there is a theoretic possibility of fatigue failure of clasps. The
coarse grain structure of the cobalt – chromium ally clasp suggests the poor
fatigue strength of the clasps (12 , 13 , 14 )
.
Studies were conducted to evaluate the long-term effectiveness of the clasps and
the effects that the clasp that might have on the abutment teeth, which has
concluded that the less stress design of the clasp arm is important to predict the
long-term success of the removable partial denture(12 )
. However, the alloy which
is been used for the fabrication will determine the mechanical properties of a
clasp. Metals and metal alloys undergo permanent deformation and fatigue when
exposed to repeated stresses. The fatigue of a denture clasp is based on the
repeated deflection of the clasp during insertion and removal of the removable
partial denture over the undercuts of the teeth (22 )
. Although extensive work has
been performed to determine the properties of a variety of materials used for
removable partial denture clasps, yet the review of l i terature did not reveal
enough studies with regard to the fatigue resistance of the esthetic clasps (acetal
resin clasp, PEEK clasp) and metal alloy clasp (Co- Cr). Hence the present
study was carried out to evaluate and compare the fatigue resistance of the
esthetic clasps namely Acetal resin clasp and PEEK clasp and metal alloy clasp
namely Co- Cr clasp.
AIM AND OBJECTIVES
Aim
The present study is aimed to evaluate the fatigue resistance of esthetic and
metal clasps for removable partial denture on abutment teeth.
Objectives
This invitro study was conducted with the following objective :
To evaluate the fatigue resistance of the metal clasp – Cobalt Chromium
alloy
To evaluate the fatigue resistance of the esthetic clasp – Acetal resin
To evaluate the fatigue resistance of the esthetic clasp – PEEK polymer
To compare and evaluate the fatigue resistance of esthetic and Metal clasp
– Cobalt Chromium, Ace tal resin and PEEK
Scope of the study
The display of the direct retainer assemblies of removable partial denture is the
biggest esthetical problem. Recently thermoplastic resins have been introduced
into the market, to fabricate the direct retainers in a tooth-colored material .
Fatigue test is believed to simulate the clinical situation. Studies have shown
that some materials and removable partial denture designs possess greater
fatigue resistance than do other materials. Thus, information on the fatigue
behaviour of materials and structures would guide dentists and dental
technicians during removable partial denture design, material selection and
fabrication.
Null hypothesis
The fatigue resistance of the esthetic clasps are inadequate when compared to
the conventional Co-Cr metal clasps.
Alternative hypothesis
The fatigue resistance of the esthetic clasps are adequate when compared to the
conventional Co-Cr metal clasps.
REVIEW OF LITERATURE
Retention of a removable prosthesis is a unique concern when it is compared
with the other prostheses. In case of crown or fixed partial denture, the
combined use of preparation geometry (i .e., resistance and retention form) and a
luting agent can fix the prosthes is to the tooth in a manner which will resists a ll
the forces that are acting upon the teeth . This direction of forces can be towards
the t issue, across the t issue, or away from the tissue.
General ly, the forces that are acting to move the prostheses toward and across
the supporting teeth and/or t issue are the greatest in intensity. This is because
most often these are the forces of occlusion. Forces which are acting to displace
the prosthesis from the tissue can consist of gravity acting ag ainst a maxillary
prosthesis, the action of adherent foods acting to displace the prosthesis on
opening of the mouth during mastication , or functional forces acting across a
fulcrum to dislodge the prosthesis from its basal t issue . The first two of these
forces are seldom at the magnitude of functional forces, and the latter is
minimized through the use of adequate support in the RPD design. The
component part of the RPD which are applied to resist this movement away from
the teeth and tissues to provide retention for the prosthesis is called the direct
retainer.
A direct retainer is the part of the removable dental denture which engages an
abutment tooth or an implant to resist the displacement of the prosthesis away
from its basal seat t issues. The abili ty of the to resist this movement is greatly
influenced by the stabili ty and support of the removable partial denture . The
stability and the support of the prosthesis are provided by the major and minor
connectors, rests, and tissue bases.
This relationship of the supportive and retentive components of the RPD
highlights the relative impor tance of these component parts. T he removable
partial denture must have retention appropriate to resist the dislodging forces
acting on it .
Retention is provided by means of primary and secondary. Primary retention for
the removable partial denture is accomplished mechanically by placing the
retaining elements (direct retainers) on the abutment teeth. Secondary retention
of the removable partial denture is provided by the minor connectors contact
with the guiding planes, denture bases. The latter is similar to the retention of
complete denture prosthesis. It is proportionate to the accuracy of the
impression registration, the accuracy o f the fi t of the denture bases and the total
involved area of the contact. Retention can also be provid ed by the engagement
of an attachment mechanism on dental implants.
Bates (1965) (16)
studied the mechanical properties of co balt chromium alloys (Co-
Cr) and their relation to the RPDs. The author reported that the minimum
undercut to be tested for the Co-Cr alloys should be 0.25 mm and the clasps
should be at least 15 mm long. Where undercuts greater than 0.25 mm are
available on the teeth, a gold clasp is to be preferred, since it has adequate
flexibili ty and a safety margin which are not available with Co-Cr alloys.
Jochen (1972) (12)
conducted a study to evaluate the different path of insertion in
the RPD. He has recommended the use of planned parallel guiding planes for
RPDs to avoid different path of insertion .
Holt (1981) (13)
suggested that the most important consideration is that the guiding
plane retention has less potential for causing supporting structure damage than
does the clasp retention.
Stewart et al. (1983) (14)
said that the guide planes could be a mean of providing
additional frictional resistance and therefore i t contributes to the retention of a
RPD.
Vallittu and Kokkonen (1995) (5)
suggested that there is a significant difference
which exists in the fatigue resistance of the RPD clasps that are made from the
different commercial cast metals, which may also, causes loss of retention of the
RPD and the clasp failures.
Bridgeman et al. (1997) (7)
compared the t itanium and cobalt -chromium RPD clasps,
they have mentioned that the long-term retentive resil iency of the pure t i tanium
and ti tanium alloy clasps suggests that these materials are more suitable for the
fabrication of RPDs than cobalt-chromium alloy.
Rodrigues et al. (2002) (16)
has compared the circumferential RPD clasps (E-clasps)
made from the commercially pure t i tanium and the identical clasps are made
from the two different cobalt -chromium alloys. The testing was done to evaluate
the defects on the casts radiographically during its insertion and removal of the
clasps. The authors have suggested that the commercially pure t i tanium clasps
maintained its retention even over a simulated 5-year period, with lower
retention force than the identical cobalt -chromium clasps.
Pavarina et al. (2002) and Varjão et al. (2012) (8)
described a technique in which light
polymerized composite material is used to obtain retention for RP D retainers
when usable natural undercuts are unavailable.
Kim et al. (2004) (17)
have investigated the retentive force of various types of clasps
during repeated cycles of placement and removal to determine whether the
t i tanium alloy clasps maintain their initial retentive force under varied
conditions, including the different retentive undercut depths and clasp size s.
The authors have finally concluded that although the end -point retention for all
the clasps were similar, there was a less change in the retentive force of the cast
t i tanium alloy clasps even after repeated cycling sequences of simulated
placement and removal.
Khan (2005) and Shah (2013) (10)
stated that the design of the RPD should always
follow some basic biomechanical requirements, such as retention, stability and
support. The design has to fol low the requirements to enhance the achievement
of an aesthetic rehabilitation .
Ozkan et al (2005) (14)
compared the color stabili ty of pigments in acetal resin with
conventional polymethyl methacrylate (PMMA). The results showed there was a
slight color change seen for both the material s after 4000 thermal cycles. This
discoloration of both the materials was significant after 12,000 thermal cycles.
However, these discoloration values were clinically acceptable.
Arda and Arikan (2005) (14)
simulated a 36-month clinical use of RPD clasps made
of acetal resin and assessed their retentive force and deformation by comparison
with similar clasps cast of Co–Cr. The result showed that there was no
deformation seen for the acetal resin clasp even after 36 months of simulated
clinical use unlike the Co–Cr clasp which presented an increase in the distance
between the t ips. However, the acetal resin clasps require s less force for
insertion and removal than Co–Cr clasps even after the simulated 36 months
clinical period.
Khan (2005) (10)
suggested an alternative path of insertion for a RPD which allows
one part of the framework to be seated first followed by the remaining part . This
technique decreases the need of clasps. Among all the rotational path of
insertion is the most used technique. It is indicated most often in cases of
missing anterior teeth. It has an advantage of not being dislodged even with the
forces which are perpendicular to the plane of occlusion.
Khan (2005) (10)
advised that the extra-coronal direct retainers are not pleasing for
the patients who are concerned about their aesthetic. It is the only framework
components of a RPD that can be placed on visible surfaces of the teeth. He
also suggested that the commonly used, circumferential clasp encircles is more
than 180 degrees and therefore i t is not desirable for anterior teeth. For a distal
extension situation, the equipoise system is a good aesthetic option and a back -
action clasp, normally attached to premolars, can al so be chosen. Hidden clasp
is one of the good options for the anterior teeth rehabilitation (Kennedy Class
IV cases). When the teeth have no natural buccal undercut or short clinical
crown, bal l-clasp can be used.
Khan (2005) (12)
suggested an a lternative to RPI-system, RLS-system can be a
good solution when teeth lack of buccal undercut or when ever they are
aesthetically desirable.
Khan (2005) and Shah (2013) (13)
suggested a twin-flex clasp, which is a flexible
wire-soldered clasp and can be another option for distal extensions situations,
but galvanic corrosion can occur.
Shah (2013) (13)
said that a round-rest distal depression clasp is an aesthetic
alternative on maxillary abutment anterior teeth because the metal will not be
visible on its facial surface.
Khan (2005) (12)
said that the use of fixed crowns or shaping enamel surface and
the use of composites to improve the aesthetic demands and retention of a RPD
are viable solutions. But i t can also increase the cost of an oral rehabilitation.
He also advocated, avoiding the usage of clasps or allowing their placement on a
less visible position.
Khan (2005)(13)
proposed that the Technopolymer frameworks which are
manufactured from the thermoplastic acetal resin (polyoxymethylene), has a
highly crystall ine structure which ensures the greater flexibility, high transverse
strength and radiolucency. However, the disadvantages include bulkiness, lack
of adjustabili ty, need for speci al equipment , technic sensitive and increased
cost.
Sandu (2007), Nascimento, (2013) (19)
said that the surface of the metal structures can
be covered with the resource of ceramics, acetate re sins and nylon and it has
been reported. Commonly these alternatives failed due to i ts complete or
cohesive fractures, especially in c lasps which are more demanding in terms of
flexibili ty and are continuously subjected to fatigue and wear process. The other
method to mask the luster of dental clasps has been studied and it consists of a
coating cobalt -chromium alloy with white powder inks. These resin powder inks,
are mainly composed of polyamide resin. It represents an optimal and unique
feature in terms of i ts physical behavior and aesthetic property. In their
preliminary mechanical and biocompatibility tests which was conducted in
invitro assay shows that the powder inks have an adequate cell response with
cultured human fibroblastic cell s which supports i ts potential application in
various biomedical areas.
Cheng et al. (2010) (15)
conducted a study which shows that after a test simulating
period of five year service, cast Co-Cr alloy clasps exhibited a residual retentive
force to satisfy the requirements for the clinical usage .
Yota Takabayashi (2010) (21)
conducted a study to evaluate the characteristics of
denture thermoplastic resins for non-metal clasp dentures . The following
conclusions were drawn after conducting water absorption solubility, flexural
strength and modulus of elasticity, tensile strength and color stability tests to
reveal the mechanical and physical properties of thermoplastic and conventional
acrylic resins. It was concluded that though the flexural strength and modulus of
elasticity were relatively low in the thermoplastic resins, they demonstrated
great toughness and resistance to fracture. Hence the thermoplastic resins could
withstand stress through a considerable degree of def lection, indicating that
they have sufficient longevity for repeated insertion and removal from the oral
cavity. The water absorption values of all the tested materials met the ISO
standards for Type 3 denture base materials, which indicate that the
thermoplastic resins are stable and hygienic materials.
Souza et al. (2011) (12)
reported that frameworks fabricated in commercially pure
t i tanium tend to decrease in retentive strength over a period of t ime and also
have a potential risk of fracture when it is used in less than 0.75 mm of
undercut .
Thomas (2011) (22)
said that there are several tooth shades available for esthetic
clasps, but long-term studies must be conducted.
Tannous et al. (2012) (18)
stated that if a RPD would be removed approximately for
four t imes each day for 11 years, there would be about 16,000 insertions and
removals.
Tannous et al. (2012) (30)
conducted an in vitro research , showed that the clasps
which are made of PEEK have lower resistance forces than the ones made from
cobalt - chrome. Scientists have searched for combinations with other materials,
to improve PEEK’s properties.
Shah (2013) (13)
advocates that a fter diagnosis and treatment planning, the phase of
surveying is indispensable for the decisio ns to make regarding the design of a
RPD. In this phase the possible paths of insertion, the location and depth of the
remaining teeth undercuts, parallelism of guide planes can be evaluated . The
surveying allows us to determine the location of clasp arms and arrangement of
prosthetic artificial teeth to derive its maximum aesthetic .
Nascimento, (2013) (19)
stated that Acetal resin can a lso be used only on clasps as
well as nylon clasps and attached to a metal framework. However, studies
showed that the deformation of these metal free direct retainers shows higher
deformation than their metal alloy counterparts. This may adversely affect i ts
clinical performance which can lead to the loss of retentive characteristics .
Singh, (2013) (32)
suggested that a flexible denture has aesthetic advantages over a
conventional RPD. It is an alternative option when patients are allergic to
acrylic, but i ts l imitations do not satisfy all clinical cases. It is also difficult to
repair and to maintain .
Zoidis P et al. (2015) (28)
suggested that PEEK is a biocompatible material and can
be used as an alternative framework material for removable partial dentures .
Najeeb S et al (2016) (22)
said that the PEEK possess a tensile strength of 80 MPa,
Young’s modulus 3-4 GP, CFR-PEEK 120 MPa. It is non-allergic and also has a
low plaque affinity.
Lieberman et al. (2016) (29)
conducted an invitro research comparing PEEK, poly
methyl methacrylate (PMMA) and composite resin . The results showed that the
PEEK has the lowest solubili ty and water absorption values.
MATERIALS AND METHODOLOGY
In this research, a comprehensive study was carried out to ev aluate and
compare the fatigue resistance of Cobalt - Chromium clasps, Acetal Resin and
PEEK on four different cyclic loading.
MATERIALS AND METHODS
Materials used for this study were commercially available Cobalt -
Chromium alloy namely Wironit , Bego, Germany, Acetal resin namely
Biodentaplast, Bredent, Germany and PEEK BioHpp Bredent, Germany . These
three materials were used with single depth of undercut on the extracted
maxillary first premolar tooth for this study.
The materials used for this invitro study were tabulated below:
SNO COMMERCIAL
NAME OF THE
MATERIAL
FORM OF
THE
MATERIAL
COMPOSITION MANUFACTURER
DETAILS
1 Wiront Pellets Co-64%, Cr28.65%,
Si, Mn, Ctrace
Bego, GmbH & Co. KG,
Germany
2 Biodentaplast Catridges Acetal resin (POM)
(PolyOxymethylene)
Bredent, GmbH & Co.
KG, Germany
3
Circumferential
Wax Pat terns
(For Co-Cr
clasps).
Preformed
half-round
patterns
Paraffin, Ceresin,
Bees wax, Resins &
other waxes
Bego,Wachsschablonen,
GmbH & Co.KG
Germany
4
Circumferential
Wax Pat terns
(For Resin
clasps).
Preformed
half-round
patterns
Paraffin, Ceresin,
Bees wax, Resins &
Other waxes.
Protek Wachskleber,
Bredeent, GmbH & Co.
KG Germany
5 Expandorock Powder &
Liquid
Type III Dental
stone
Bredent, GmbH & Co.
KG, Germany
6 Wirovest Powder &
Liquid
Phosphate bonded
investment &
Colloidal sil ica
Bego, GmbH & Co. KG,
Germany
EQUIPMENTS USED IN THIS STUDY
Dental Surveyor - Ney Surveyor; Dentsply, New York, New York, USA)
Centrifugal casting machine (Modular 3S, Italy).
Injection Moulding Machine (Thermopress 400 unit; Bredent)
Electrolytic polishing unit (BEGO, Germany)
CADCAM milling machine (Roland, DWX-50, USA).
Universal testing machine ( Instron 3365: Buckinghamshire, England)
GROUPING OF SAMPLES:
There are three different groups of sampl es were used for this study. These
groups and the corresponding number of specimens used are l isted below:
Group I Control - Metal alloy - Cobalt Chromium claps -20nos
Group II Esthetic clasp- Resin - Acetal resin clasps -20nos
GroupIII Esthetic clasp - Polymer-PEEK -20nos
Extracted maxillary first premolar was selected for this study to be used as an
abutment . All patients were informed about the purpose of the study and using
of their extracted teeth according to the institutional ethical committee of
Madha Dental College and Hospital , Chennai. One laboratory custom made
Aluminum model (30 mm in length, 20 mm in width, and 25 m m in height) was
used for fabrication the testing samples .
FABRICATION OF METAL DIE STUDY MODEL
The extracted maxillary premolar tooth were embedded in each type 2 Dental
plaster model ((Orthokal, khalabai, Mumbai, India ) vertically upto the cemento-
enamel junction. The base of the plaster block and its superior surface were
made parallel to the surveyor table (Ney Surveyor; Dentsply, New York, New
York, USA) to evaluate the 0.01 inch distobuccal undercut and height of contour
by using undercut guage . (22 )
(figure.1) (Figure.2). Minor tooth preparations
were performed to provide rest seat. The rest seat preparation was triangular in
shape with the base of the triangle resting on the marginal ridge; the rounded
apex of the triangle was directed toward the center of the occlusal surface of the
tooth .The width of the rest seat preparation was one third of the distance
between the buccal and the palatal cusp tips. The prepared depth of the occlusal
rest seat was 2 mm. The floor of the rest seat preparatio n was spoon-shaped and
directed toward the center of the occlusal surface of the tooth. Impression of
the prepared maxillary premolar was made with condensation sil icone
impression material (C- sil icon impression material; Zherma ck, Milan Italy) and
a wax model representing the extracted maxillary first premolar was poured (44 )
.
A ledge was placed on the buccal surface to standardize the locations and
lengths of the retentive arms. On the palatal surface, a piece of wax, rectangular
in shape, was placed as a reference to standardize the locations and lengt hs of
the reciprocal arms (45 )
. After finishing the wax pattern was invested with
Phosphate bonded investment material (Wirovest, Bego, GmbH & Co. KG,
Germany) to fabricate one aluminium die model (Figure :9). The wax patterns
were sprued, invested and casted using Allumium alloy (Wironit -BEGO,
Germany) using high -frequency induction melting technology with a centrifugal
casting machine (Modular 3S, Italy).
CLASP DESIGN
The Co- Cr and Acetal resin clasp was designed from the most common
conventional design patterns. The PEEK clasp was designed according to the
novel dimension using DWOS (software 3D – RPD software) machine and the
clasps were milled using -3D Roland milling machine (Figure :12). However the
occlusal rest was spoon shaped 3mm width and 1mm thickness and at right
angles to the minor connector on the proximal side. A pin cylinder holder was
added to all the clasps designs to ser ve as an attachment for f ixing the clasps to
the sample j ig holders of the testing machines.
FABRICATION OF CLASP
Preformed half round standard Aker clasp patterns (1.2 mm) (readymade
Aker clasp pattern; Bego, Italy) wi th occlusal rest , retentive arm, reciprocal
arm and minor connector were adapted on the refractory casts, with one
clasp pattern for each refractory cast. A wax plate with dimensions 4×3×7
mm was prepared on the distobuccal aspect and attached to the minor
connector parallel to the path of insertion. A rounded vertical plastic sprue
(46 ), 20 mm in length and 3 mm in diameter, was attached to the prepared
wax plate, which was used latter on for maintaining the clasp in the testing
machine (47 )
Steps for fabrication of clasps as follow: The wax pattern of the
Aker clasp assembly was sprued at the thickest part of the clasp wax pattern
with 20 mm sprue length and 3 mm diameter. Five wax patterns per flask
were invested and all the small sprues were attached to a large sprue with 50
mm length and 10 mm diameter (Figure 8,9) . After the surface tension
reducing solution was applied to the wax patterns, they were invested in a
vaseline-insulated flask (a luminum flask; Bredent) . The wax patterns were
sprued, invested, and cast using Co -Cr alloy (Wironit-BEGO, Germany)
with a phosphate-bonded investment material using high -frequency
induction melting technology with a centrifugal casting machine (Modular
3S, Italy).
Similarly, for fabricating the Acetal resin clasp, six wax patterns per flask were
invested and all the small sprues were attached to a large sprue with 50 mm
length and 10 mm in diameter. After the surface tension reducing solution was
applied to the wax patterns, they were invested in a vaseline -insulated flask
(aluminum flask; Bredent). Hard stone was used as investment. Gypsum paste
was poured into one of the two halves of the flask and the duplicated casts
containing the spruing of the clasp patterns were dipped. Gypsum paste was
poured into one of the two halves of the flask and th e duplicated casts
containing the spruing of the clasp patterns were dipped. When the investment
finally set, the gypsum surface was insulated and the second half of the flask
was assembled. The same hard stone was prepared and poured into the upper
chamber of the flask, covering thoroughly the wax pattern and sprues. After the
gypsum set, the flask was submerged in warm water in a thermostatic container.
The two halves of the flask were then disassembled and the wax was boiled out
using clean hot water. The mold was then insulated using a special agent, which
was applied in a single layer on the gypsum surface. Preheating temperature and
time were checked (15 min at 220°C).
The corresponding cartridge of injecting acetal resin was selected. The cartridge
was introduced into the heating cylinder. When the programmed preheating time
elapsed, an audible signal was heard. The two halves of the flask were
assembled and fastened with screws. The flask was inserted and secured i n the
corresponding place of the injecting unit (Thermo press 400 unit; Bredent)
(Figure :10).
The injecting procedure was initiated and the flask was left to slowly cool down
for 8 h. Before investment removal, screws were loosened and the flask was
gently disassembled. The stone blocking the vents in the upper side of the flask
was removed using the hook and a mallet . The sprues were cut off using carbide
and diamond burs using low pressure to avoid overheating the material . Acetal
resin clasps were ready to be tested (Fig. 6).
For fabricating the PEEK Akers c lasp, the design was first converted to STL
file forms; figure (10 )
. This is to fabricate the 3D sample . For machining
Clasps, the Roland milling machine milled PEEK clasps from digital patterns
into 3D PEEK clasps using PEEK BioHpp- Juvora™ blank discs (Roland,
DWX-50, USA). This was accomplished using the various steps of the machine
software processing program. The entire clasp specimens were cut from their
connectors. PEEK clasps were finished and polished using the conventional
method. Meanwhile the Co-Cr clasps were l ightly cleaned using a sandblaster
with airborne-particle abrasion using 80 µm aluminum oxide particles (Renfert ,
Germany). Then, white compound was applied for dry polishing and fin ally the
clasps were electrical polished using Electrolytic polishing unit (BEGO,
Germany). A digital micrometer was used to optimise the clasp dimensions
after the finishing and polishing procedures.
The obtained wax patterns of clasps were thus prepared and casted. Each
metallic clasp was evaluated for casting defects and porosity. The porosity of all
specimens was examined radiographically by usin g an intraoral X-ray machine
(5 ). Each testing model with the tooth was attached from its base to the fixed
compartment of the testing machine with a load cell of 5 kN. The occlusal rest
of the Aker clasp was fully seated in i ts rest seat. The vertical sprue was
attached to the movable compartment of the universal testing machine (14 )
(Figure : 12, 13, 14).
Each clasp was initially activated by withdrawing the clasp over the maximum
convexity of the tooth until the complete separation of the clasp from the tooth.
To perform the retention test , an insertion/removal test set up was used. This
test allowed the placement (insertion) of the clasp to its predetermined terminal
position and its subsequent removal from this position, thus simulating the
placement and removal of a RPD (14 , 16 )
.
The clasps were subjected to a cyclic insertion removal test . The force needed to
remove the fi t ted clasp was measured in Newton. The test was performed with
41cycles/ minute at a constant spe ed of 35.7mm/second (22 )
After the
measurement of the retentive force of the fit ted clasps, the clasps were cycled
on and off the dies at 5 different periods of 0,730,1460,2190,2920 cycles,
representing the simulated insertion and removal period of 6, 12, 18, and 24
months respectively, were recorded by the computer software. After this
simulated clinical use, the force of removal was re -measured to determine the
reduction in the amount of retentiveness remaining (20 )
.
Figure 1: Testing Model on a surveyor
Figure 2:Test model with 0.01 undercut guage
Figure 3: Representation of prepared Metal die
Figure 4: Representation of prepared Metal die with wax pattern of Akers clasp
Figure 5: Group 1 Cobalt Chromium clasp
Figure 6: Group 2 Acetal resin clasp
Figure 7: Group 3 PEEK clasp
Figure 8: Clasps design in calcinable pattern
Figure 9: Clasps prepared for the investment
Figure.10 : Acetal resin clasps were injected the resin with the Thermopress 400 machine from
the Bredent
Figure.11: PEEK production of route to STL file
Figure 12: CAD CAM milling of PEEK Clasps
Figure 13: Clasps under cyclic loading test
Figure 14 : Universal testing Machine
RESULTS
STATISTICAL ANALYSIS
Based on the testing values of the raw data collection of three different
groups namely
Group I Control - Metal alloy - Cobalt Chromium claps -20nos
Group II Esthetic clasp- Resin - Acetal resin clasps -20nos
GroupIII Esthetic clasp - Polymer-PEEK -20nos
The mean and the standard deviation (SD) values of fatigue resistance and each
group were analysed statistically with one - way ANOVA analysis (Table 1 )
(Chart 1).
In group I, the retention force ranged from 9.61 to 9.2 GPa with a mean value
and standard deviation of 9.38 ± 0.10 for pretest (0 cycling).
For 730 cycles, the retention force ranged from 8.8 to 7.6 GPa with a
mean and standard deviation value of 7.87 ± 0.40.
For 1460 cycles, the retention force ranged from 6.92 to 6.0 GPa with a
mean and standard deviation value of 6.33 ± 0.31.
For 2190 cycles, the retention force ranged from 4.92 to 4.19 GPa with a
mean and standard deviation value of 4.31 ± 0.26.
For 2920 cycles, the retention force ranged from 2.68 to 2.58 GPa with a
mean and standard deviation value of 2.61 ± 0.05.
In group II, the retention force ranged from 1.72 to 1.6 GPa with a mean value
and standard deviation of 1.70 ± 0.03 for pretest (0 cycling). (Table 1) (Chart
2).
For 730 cycles, the retention force ranged from 1.52 to 1.40 GPa with a
mean and standard deviation value of 1.48 ± 0.05.
For 1460 cycles, the retention force ranged from 1.31 to 1.28 GPa with a
mean and standard deviation value of 1.29 ± 0.03.
For 2190 cycles, the retention force ranged from 1.08 to 0.97 GPa with a
mean and standard deviation value of 1.04 ± 0.04.
For 2920 cycles, the retention force ranged from 0.98 to 0.8 GPa with a
mean and standard deviation value of 0.91 ± 0.04
In group III, the retention force ranged from 2.59 to 2.32 GPa with a mean value
and standard deviation of 2.46± 0.07 for pretest (0 cycling). (Table 1) (Chart 3).
For 730 cycles, the retention force ranged from 2.5 to 2.09 GPa with a
mean and standard deviation value of 2.21 ± 0.09.
For 1460 cycles, the retention force ranged from 1.9 to 1.4 GPa with a
mean and standard deviation value of 1.86 ± 0.18.
For 2190 cycles, the retention force ranged from 1.51 to 1.40 GPa with a
mean and standard deviation value of 1.50 ± 0.03.
For 2920 cycles, the retention force ranged from 1.49 to 1.01 GPa w ith a
mean and standard deviation value of 1.28 ± 0.11.
TABLES AND CHARTS
Table 1: Means and standard deviations of retention forces of different
groups at different intervals
Group 1 Group 2 Group 3
Cycle 0 9.38 ± 0.10 1.70 ± 0.03 2.46 ± 0.07
Cycle 730 7.87 ± 0.40 1.48 ± 0.05 2.21 ± 0.09
Cycle 1460 6.33 ± 0.31 1.29 ± 0.03 1.86 ± 0.18
Cycle 2190 4.31 ± 0.26 1.04 ± 0.04 1.50 ± 0.03
Cycle 2920 2.61 ± 0.05 0.91 ± 0.04 1.28 ± 0.11
Table 2: One-way ANOVA analysis of variance for retentive force (N) for
Group I
Data for retentive force of the tested clasp materials for Group I with 0.01 inch
undercut before and after cyclic loading with 95% confidence interval showing
statistically significant with p value <0.00 5 .
ANOVA
SUM OF
SQUARES Df
MEAN
SQUARE F SIG.
BETWEEN
GROUPS
586.978
4
146.744
2205.299
.000
WITHIN
GROUPS
6.321
95
0.067
TOTAL
593.299
99
Table 3: One-way ANOVA analysis of variance for retentive force (N) for
Group II
Data for retentive force of the tested clasp materials for Group II with 0.01 inch
undercut before and after cyclic loading with 95% confidence interval showing
statistically significant with p value <0.005.
ANOVA
SUM OF
SQUARES Df
MEAN
SQUARE F SIG.
BETWEEN
GROUPS
8.200
4
2.050
1237.685
.000
WITHIN
GROUPS
0.157
95
0.002
TOTAL
8.357
99
Table 4: One-way ANOVA analysis of variance for retentive force (N) for
Group III
Data for retentive force of the tested clasp materials for Group III with 0.01
inch undercut before and after cyclic loading with 95% confidence interval
showing statistically significant with p value <0.005.
ANOVA
SUM OF
SQUARES Df
MEAN
SQUARE F SIG.
BETWEEN
GROUPS
19.110
4
4.778
417.762
.000
WITHIN
GROUPS
1.086
95
0.011
TOTAL
20.197
99
TABLE 5: ONE-WAY ANOVA ANALYSIS OF VARIANCE FOR
RETENTIVE FORCE (N) FOR ALL THE THREE GROUPS AT 0 CYCLE
Data for retentive force of tested clasp materials for all three groups with 0.01
inch undercut at 0 cycle interval (pre cyclic loading) showing statistically
significant with p value <0.005 for all the three groups.
ANOVA
SUM OF
SQUARES Df
MEAN
SQUARE F SIG.
BETWEEN
GROUPS
716.083
2
358.042
65017.596
.000
WITHIN
GROUPS
0.314 57
.006
TOTAL 716.397 59
(I)
GROUP
(J)
GROUP
MEAN
DIFFERENCE
(I-J)
STD.
ERRO
R
SIG.
95% CONFIDENCE
INTERVAL
Table 6: One-way ANOVA analysis of variance for retentive force (N) for
all the three groups at 730 cycle
LOWER
BOUND
UPPER
BOUND
1
2
7.682000*
.02346
7
.000
7.62412
7.73988
3
6.914500*
.02346
7
.000
6.85662
6.97238
2
1
-7.682000*
.02346
7
.000
-7.73988
-7.62412
3
-.767500*
.02346
7
.000
-.82538
-.70962
3
1
-6.914500*
.02346
7
.000
-6.97238
-6.85662
2
.767500*
.02346
7
.000
.70962
.82538
Data for retentive force of tested clasp materials for all three groups with 0.01
inch undercut at 730 cycle interval showing statistically significant with p
value <0.005 for all the three groups.
ANOVA
SUM OF
SQUARES Df
MEAN
SQUARE F SIG.
BETWEEN
GROUPS
490.083
2
245.041
4333.376
0.000
WITHIN
GROUPS
3.223
57
0.057
TOTAL
493.306
59
(I)
GROU
P
(J)
GROUP
MEAN
DIFFEREN
CE (I-J)
STD.
ERROR SIG.
95% CONFIDENCE
INTERVAL
LOWER
BOUND
UPPER
BOUND
1
2
6.393000*
.075198
.000
6.20751
6.57849
3
5.667000*
.075198
.000
5.48151
5.85249
2
1
-6.393000*
.075198
.000
-6.57849
-6.20751
3
-.726000*
.075198
.000
-.91149
-.54051
3
1
-5.667000*
.075198
.000
-5.85249
-5.48151
2
.726000*
.075198
.000
0.54051
.91149
Table 7: One-way ANOVA analysis of variance for retentive force (N) for
all the three groups at 730 cycle
Data for retentive force of tested clasp materials for all three groups with 0.01
inch undercut at 730 cycle interval showing statistically significant with p
value <0.005 for all the three groups.
ANOVA
SUM OF
SQUARES Df
MEAN
SQUARE F SIG.
BETWEEN
GROUPS
304.433
2
152.217 3632.229 .000
WITHIN
GROUPS
2.389 57 .042
TOTAL 306.822 59
(I)
GROUP
(J)
GROUP
MEAN
DIFFERENC
E (I-J)
STD.
ERROR SIG.
95% CONFIDENCE
INTERVAL
LOWER
BOUND
UPPER
BOUND
1
2
5.035000*
.064736
.000
4.87532
5.19468
3
4.471750*
.064736
.000
4.31207
4.63143
2
1
-5.035000*
.064736
.000
-5.19468
-4.87532
3
-.563250*
.064736
.000
-.72293
-.40357
3
1
-4.471750*
.064736
.000
-4.63143
-4.31207
2
.563250*
.064736
.000
.40357
.72293
Table 8: One-way ANOVA analysis of variance for retentive force (N) for
all the three groups at 2190 cycle
Data for retentive force of tested clasp materials for all three groups with 0.01
inch undercut at 2190 cycle interval showing statistically significant with p
value <0.005 for all the three groups.
ANOVA
SUM OF
SQUARES Df
MEAN
SQUARE F SIG.
BETWEEN
GROUPS
125.146
2
62.573
2652.564
.000
WITHIN
GROUPS
1.345
57
.024
TOTAL
126.491
59
(I)
GROUP
(J)
GROUP
MEAN
DIFFEREN
CE (I-J)
STD.
ERROR SIG.
95% CONFIDENCE
INTERVAL
LOWER
BOUND
UPPER
BOUND
1
2
3.266000*
.048569
.000
3.14619
3.38581
3
2.810250*
.048569
.000
2.69044
2.93006
2
1
-3.266000*
.048569
.000
-3.38581
-3.14619
3
-.455750*
.048569
.000
-.57556
-.33594
3
1
-2.810250*
.048569
.000
-2.93006
-2.69044
2
.455750*
.048569
.000
.33594
.57556
Table 9: One-way ANOVA analysis of variance for retentive force (N) for
all the three groups at 2920 cycle
Data for retentive force of tested clasp materials for all three groups with 0.01
inch undercut at 2920 cycle interval showing statistically signi ficant with p
value <0.005 for all the three groups.
ANOVA
SUM OF
SQUARES Df
MEAN
SQUARE F SIG.
BETWEEN
GROUPS
32.040
2
16.020
3097.284
.000
WITHIN
GROUPS
.295
57
.005
TOTAL
32.335
59
(I)
GROUP
(J)
GROUP
MEAN
DIFFEREN
CE (I-J)
STD.
ERROR SIG.
95% CONFIDENCE
INTERVAL
LOWER
BOUND
UPPER
BOUND
1
2
1.702000*
.022743
.000
1.64590
1.75810
3
1.331000*
.022743
.000
1.27490
1.38710
2
1
-1.702000*
.022743
.000
-1.75810
-1.64590
3
-.371000*
.022743
.000
-.42710
-.31490
3
1
-1.331000*
.022743
.000
-1.38710
-1.27490
2
.371000*
.022743
.000
.31490
.42710
Chart 1: This Chart represents the means and standard deviations of
retention forces of group I at different intervals
0
1
2
3
4
5
6
7
8
9
10
0 730 1460 2190 2920
CO-Cr
CO-Cr
Chart 2: This Chart represents the means and standard deviations of
retention forces of group II at different intervals
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0 730 1460 2190 2920
Acetal Resin
Acetal Resin
Chart 3: This Chart represents the means and standard deviations of
retention forces of group III at different intervals
0
0.5
1
1.5
2
2.5
3
0 730 1460 2190 2920
PEEK
PEEK
Chart 4: This Chart represents the retention forces of different groups at 0
cycle
Chart 5: This Chart represents the retention forces of different groups at
730 cycles
0
1
2
3
4
5
6
7
8
9
10
0 Cycle
Co-Cr
Acetal Resin
PEEK
0
1
2
3
4
5
6
7
8
9
730 cycle
Co - Cr
Acetal resin
PEEK
Chart 6: This Chart represents the retention forces of different groups at
1460 cycles
Chart 7: This Chart represents the retention forces of different groups at
2190 cycles
0
1
2
3
4
5
6
7
1460 cycle
Co- Cr
Acetalresin
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
2190 cycle
Co - Cr
Acetal resin
PEEK
Chart 8: This Chart represents the retention forces of different groups at
2920 cycles
0
0.5
1
1.5
2
2.5
3
2920 cycle
Co - C r
Acetal resin
PEEK
DISCUSSION
Prosthetic rehabili tation should be able to recreate patients function and
esthetics. An increased awareness of aesthetics in dentistry has resulted in the
need for removable partial dentures that reveal l i tt le or no metal supporting
structures or retentive elements. The dentist has expanded the scope of esthetic
fixed prostheses as many patients demand a removable partial denture (RPD) for
health, anatomic, psychological or financial reasons. Fabricating an
aesthetically pleasing removable partial denture while avoiding the unsightly
display associated with conventional clasp assemblies often presents a challenge
to dentists. Changing the removable partial denture system by designing with
esthetic improvement can bring a favourable outcome for the patients (4 )
. The
esthetic changes can be done in materials used to support the denture teeth and
retain the removable partial denture in the mouth (5 ) .
The material used for the fabrication of removable partial denture should have
enough flexibility for clasp and rigidity for other components of partial denture
(19 ). Retentive clasp arms must be capable of flexing and returning to i ts original
form and should retain the denture in situ satisfactorily without causing unduly
stress on the abutment teeth. It should n ot get distorted permanently during
service and should also provide good aesthetic results (22 )
.The base metal alloys
offers high value of modulus of elasticity and strength, excellent corrosion
resistance and very low metal cost compared with the alternat ive type IV gold
casting alloys (14 )
. Wear of the tooth surface may result in a reduction in the
circumference of the tooth and shortcoming of the metal clasps is their poor
esthetic appearance (16 )
. The clinical experience of loss of retention of the
removable partial denture 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 assembly (5 ,15 )
.
The study was designed to compare ‘C’clasps/ Akers clasp/ Circumferential
clasp in three different materials on premolar teeth. The 0.01 inch undercut was
selected as i t represents the undercut commonly used for Co -Cr clasps. With the
increased requirements of esthetics, more patients are requesting that dentists
conceal removable partial denture clasps by placing them closer to the gingiva,
where the undercuts tend to be larger. One property of acetal resin is that i t has
created interest for use in removable partial dentures is the low modulus of
elasticity, which allows for their use in larger retentive undercuts than Co -Cr
alloys. Compared to noble metal alloys, the flexibili ty of the Co -Cr is higher,
the only disadvantage in clinical situations is its esthetics (17 )
.
With reference to the tooth se lection done in this study the extracted first
maxillary premolar was selected to standardize the study (18 )
. Metal die of
premolar crown was prepared for testing retentive forces and fatigue resistance
of clasps by the insertion/removal. The preformed wax pattern for Aker’s clasp
was used to facili tate the standardization of the shape and thickness of clasps
and to eliminate the factors that may affect flexibility and also to eliminat e the
manual variations (18 , 19 )
.
The retentive strength values were captured at the first insertion and removal
and then after 730, 1460, 2190, and 2920 continuous cycles corresponding to 6,
12, 18, and 24 months, respectively, in service were recorded by the computer
software. Fatigue is responsible for 90% of all service failure which occurs as
cyclic bending during insertion/removal and mastication (19 )
. In this study
fatigue was observed after 24 months of cyclic loading indicating that long term
denture wear results in loss of i ts ret entive property.
Acetal resin has been used as an alternative denture base and denture clasp
material since 1986 and was promoted, primarily for i ts superior esthetics (11 )
.
Acetal resin is marketed for the direct retainers attached to a Co -Cr removable
partial denture framework, as well as the supportive components of removable
partial dentures. As purported by the manufacturer, acetal resin is available in
20 color shades matching the Vita shade guide (Vitapan; VITA Zahnfabrik, Bad
Sackingen, Germany). It has a relatively high proportional l imit with l i t t le
viscous flow, enabling it to behave elastically over a large range enough to be
used as a material for clasp fabrication (7 )
.
The results obtained in the study were similar with the other studies, which
compared deformation of acetal resin and metal alloy direct retainers of
removable partial denture after repeated dislodgments over a test die for a
simulated period (42 , 6 )
. The present study confirms these findings, because the
retentive force of Acetal resin clasps in 0.01 inch undercut show statistically
significant difference over the five periods tested (2 years of simulated use).
Acetal resin clasp in 0.01 inch undercut sho wed no significant difference in
retentive force after 730 cycles; this may be attributed to the greater flexibility
of the material .
Abutment condition includes the shape of the abutment and the frictional
coefficient between the abutment and the clasp h as been reported to be
approximately 0.2 in the presence of saliva. Sato et al (9 )
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 distil led water which mimics close to the
clinical conditions.
Ahmad et al (10 )
found that 4.77-N retention was required to dislodge a Co -Cr
clasp from a 0.01 inch undercut. Frank and Nicholls [11] conclu ded 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 flexibili ty of a clasp arm affects the
retention and the function of an RPD. If a clasp is too flexible, the clasp may
not provide adequate retention for the removable partial denture even when the
framework design is based on the recommended principles for Co -Cr alloys. In
the present study, the results demonstrate that the retentive force for an acetal
resin clasp reduces substantially on repeated cyclic loading indicating loss of
retention on prolonged clinical use (5 )
. The ineffective reciprocation, number
and distribution of the abutment teeth, the amount of wax block -out and the fi t
of the framework are other factors that inf luence the amount of retention
obtained. In the clinical situation this could have a cumulative effect.
Fitton et al (12 )
stated that to gain adequate retention from acetal resin clasps,
the clasp should have a greater cross -sectional area than a metal cl asp. 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 (Elasti c
modulus; 2.9 to 3.5 kN/mm2). 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 .
With the emergence of carbon fiber reinforced PEEK (CF/PEEK), a new
composite material was exploited for fracture fixation and femoral prosthesis in
artificial hip joints surgery (20 )
. It has good thermal stability up to 335.8° C (21 )
.
It is a good biocompatible material with the flexural modulus is 140 -170 MPa,
density – 1300 kg/m3 (22 ,27 , 28 )
. Special chemical structure of PEEK exhibits
stable chemical and physical properties: stabili ty at high temperatures (l ike
steril ization processes), resistance to most substances apart from concentrated
sulfuric acid and wear-resistance (20 )
. In this study it was found that the clinical
acceptable value of PEEK was found to be adequate t il l 2190 cycle, Liebe rman
et al . in vitro research comparing PEEK, poly methyl methacrylate (PMMA) and
composite resin showed that PEEK has the lowest solubili ty and water
absorption values (29 )
.
As PEEK is a quite new material in prosthodontics, i t makes i t more attractive
to patients with high aesthetic requirements (30 , 31 )
. However, due to i ts grayish
brown color PEEK is not suitable for monolithic aesthetic res torations of
anterior teeth (32 )
. More aesthetic material l ike composite should be used for
coating to get an aesthetic result . Previous l i teratures stated that many surface
conditioning methods of PEEK are offered to improve bonding with resin
composite crowns. Air abrasion with and wit hout sil ica coating creates wettable
surface, but etching with sulfuric acid makes it rough and so chemically
processed surface may be advocated (33 )
.
Mechanical properties of the PEEK are similar to dentin and enamel. Thus it has
superiority over metal alloys and ceramic restorations. CAD -CAM milled PEEK
fixed prostheses has resistance to fracture is 2354N. It has higher resistance
than li thium disil icate ceramic (950N), aluminium (85 1N) or zirconia (981-
1331N) (34 )
. However, there are no clinical data a bout PEEK’s abrasion with
other materials such as metal alloys, ceramics, dentin or enamel. A study which
was done by Stawarczyk et al stated that the mastication on cyclic loading of the
teeth with a 400 N force. As PEEK has high fracture resistance to lo ading which
makes it suitable for fabrication removable partial denture frames (32 )
.
Modified PEEK containing 20% ceramic fil lers known as BioHPP (Bredent
GmbH Senden, Germany) was used in this study. PEEK is relatively weak
mechanically in homogenic form. Tannous et al . (30 )
in vitro research showed
that clasps made of PEEK have lower resistance forces than the ones made from
cobalt – chrome alloy. Scientists have researched for combinations with other
materials, to improve PEEK’s properties (22 )
. Hence these informations supports
that PEEK is a good alternative material to Cr -Co frames for the patients with
high aesthetic requirements. PEEK is being used in manuf acturing fixed
restorations (35 )
, dental implants, individual abutments, removable prosth eses
and their parts and even maxillary obturator prostheses (22 , 36 ) .
This in-vitro study was carried out to compare the retentive forces of Co -Cr,
Acetal resin and PEEK clasps with an undercut of 0.01 inch at different
intervals. This experiment was conducted for 2920 cycles to show wear, if a
removable partial denture would be removed four t imes each day for two years.
The outcome shows that retention loss is l ikely to occur only after two years of
clinical usuage. This study closely mimics long term us e of removable partial
denture in relation to loss of retention (37 )
.
Loss of retention of the clasps due to fatigue resistance test was considered as a
good indicator of permanent deformation of the clasps (5 , 38 )
. Before loading, the
highest mean value of retentive force was found with a Co –Cr clasp followed by
a PEEK clasp and whereas the lowest mean value of retentive force was found
with an Acetal resin clasp. This lower retentive force of Acetal resin clasps was
due to greater flexibili ty. In the cu rrent study, the retentive force of the Co –Cr,
Acetal resin and PEEK clasps decreased in the test cycles, but that of Co –Cr and
PEEK was sti l l greater than that of Acetal resin clasps at the end of the test
period.
Acetal resin clasp had the same diamete r as metal clasps and not larger in cross -
sectional area, as reported in a study done by Turner et al . (39 )
, who stated that
to obtain stiffness similar to that of a cast Co –Cr clasp measuring 15 mm in
length and 1 mm in diameter was suitable Acetal resin clasp must be shorter (5
mm) and have a larger cross-sectional diameter (1.4 mm). Fitton et al . (6 )
, in
their study, stated that the Acetal resin clasps must have greater cross -section
area than metal clasps to provide adequate retention. The lower retenti ve force
of long term usuage of acetal resin clasps compared with other clasp types is in
agreement with the results obtained in the respective studies by Arda and Arikan
(14 ) and Sato et al .
(40 ).
Within the l imitation of the study, the fatigue resistance of Co - Cr, Acetal resin
and PEEK was evaluated only using vertical dislodging forces acting on the
clasps. There would have been variations due to intraoperative sensitivity and
variabili ty. The ul timate test for any research conclusion is i ts application in a
patient and the final evaluation in the oral cavity.
Hence the future scope of the research has to be further studied on the fatigue
resistance using the horizontal forces acting on the clas p, activation of the
clasp, other physical properties and microstructure of the clasps on functional
loading could also be done. The use of PEEK in removable partial denture
framework has to be further studied. As the outcome of the study acetal resin
and PEEK can be advocated for the removable partial denture components.
CONCLUSION
In this study, the fatigue resistance of the esthetic clasp and metal clasp was
evaluated. Experimental tests were conducted on Cobalt - Chromium C clasp,
Acetal resin C clasp and PEEK C clasp. Within the l imitation of the study, the
following conclusion could be made:
PEEK clasps 0.01 mm undercut provide sufficient retention nearly similar
that of Co-Cr clasps.
The esthetic clasp fabricated on Acetal resin and PEEK showed
significant fatigue resistance under pre and post cyclic loading of
730,1430,2190,2920 which is a simulated period corresponding to 6, 12,
18 and 24 months of simulated clinical use of a RPD.
Ac clasps had significantly lower retentive strength than Co-Cr and PEEK
clasps.
All clasps exhibited continuous significant decrease in retentive strength
from the first period of cyclic loading ti l l the end of the cycling.
The mean retentive strength required to remove Ac clasps was fo und to
be significantly lower than that required for the removal of Co –Cr clasps
and PEEK clasps
The retentive strength required for Co –Cr, Ac and PEEK clasps
demonstrated significant change over the five periods tested.
The mean retentive strength of Co–Cr and PEEK clasps showed marked
decrease after 2920 cycles and was nearly equal to that of acetal resin,
but sti l l a significant difference was found.
Co–Cr, Acetal Resin and PEEK clasps showed significant deformation
after 24 months of simulated clin ical use.
SUMMARY
Comprehensive treatment plan for partially edentulous patients is usually more
complicated than treatment plan formulated for edentulous patients or for
patients who do not require the replacement of missing teeth. The major factor
for RPD success is the retention. The traditional use of metal clasp like cobalt –
chromium (Co–Cr), gold, stainless steel, and ti tanium satisfy the principles of
RPD design, however the esthetic requirements of such clasps is hampered, as
i t’s obvious display conflicts with the patient’s prosthetic confidentiali ty. The
components of the prosthesis should be, whenever possible, discreet to improve
the RPD esthetics. The poor esthetics of CoCr clasps lead to the search for
thermoplastic resin clasps. In this study, an attempt is mad e to evaluate the
fatigue resistance of the Esthetic clasps and whether i t could be used in the
fabrication of clasps for RPD. This study would help us to know if Esthetic
clasps with 1.0 mm in cross section diameter engaging 0.01 inch would provide
sufficient retention and fatigue resistance and be nearly similar that of Co -Cr
clasps.
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