EVALUATION OF FATIGUE RESISTANCE OF ESTHETIC AND …

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.







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).








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 2920 cycles, the retention force ranged from 1.49 to 1.01 GPa w ith a


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


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


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


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


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






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






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




inch undercut at 2920 cycle interval showing statistically signi ficant with p


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


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


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


1460 cycles


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


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.

REFERENCES

1. Retentive force comparison between esthetic and metal clasps for

removable partial denture , Eron Toshio Colauto yamamoto, Braz Dent Sci

2017 Jul/Sep;20(3)

2. Flexible resins: an esthetic option for partially edentulous patients,

Simone Guimarães Farias GOMES, RGO, Rev Gaúch Odontol, Porto

Alegre, v.63, n.1, p. 81 -86, jan./mar., 2014.

3. Turner JW, Radford DR, Sherriff M. Flexural properties and surface

finishing of acetal resin denture clasps. J Prosthodont 1999;8: 188 -95

4. Esthetic designs of removable partial dentures. Chu CH, Gen Dent, 2003

Jul-Aug; 51(4):322-4.

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