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Post-fatigue Fracture Resistance of Deep Marginal Elevation in CAD-CAM

Lithium Disilicate and Monolithic Zirconia Crowns.

Thesis Research Proposal

By: Dr. Sahar Abdulrahman Mohammed Al Ateeq

Postgraduate student in Prosthodontics

To

Department of Prosthodontics

Riyadh Elm University

Riyadh, Kingdom of Saudi Arabia

Supervisor: Dr. Fawaz AL Qahtani. BDS, MDS, DABP, FACP

Associate Professor, Department of Prosthetic Dental Sciences,

School of Dentistry, Prince Sattam bin Abdulaziz University

Date of Submission: …………………………….

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Introduction

As patients and dentists desire increased to have more esthetic

restorations, all ceramic crowns became the best option to meet these

expectations (Dhima, Carr et al. 2014). Meanwhile, these restorations provide

enough fracture resistance and more natural color which led the dentists to use

them in the anterior and posterior teeth more often( Juntavee and Sirisathit

2018) (Choi, Kim et al. 2017) (Dhima, Carr et al. 2014). The most famous

ceramic materials are lithium disilicate and zirconia, as lithium disilicate having

higher translucency and lower mechanical strength (Pieger, Salman et al. 2014).

In the other hand, zirconia has higher mechanical properties but lower esthetic.

Lithium disilicate was introduced by Ivoclar Vivadent in 1998 as IPS

Impress II, which was stopped by the manufacture and renovated into IPS

E.max which is present in both perssable (IPS e.max press) and machinable

(IPS e.max CAD) (Pieger, Salman et al. 2014). Despite that both materials have

70% same crystal content, they only differ in the size of the crystal, which leads

to the difference of flexural strength (Arnason 2017). It is 360 MPa and 460

MPa for IPS e.max CAD and IPS e.max press, respectively. In term of the

marginal fit of both materials, there is no significant difference (Al Hamad, Al

Quran et al. 2019) (Dhima, Carr et al. 2014).

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Zirconia is considered as polycrystalline ceramic with no glassy phase

(Guazzato, Albakry et al. 2004) (Nakamura, Harada et al. 2015). In a room

temperature, the zirconia will present in a monoclinic state, exposing the

monoclinic zirconia to stress will lead to change it into tetragonal and then

cubic zirconia which are considered unstable zirconia and can be reversed into

monoclinic zirconia by removing the heat. Tetragonal and cubic zirconia can be

stabilized by adding small yttria (Guazzato, Albakry et al. 2004) (Zhao, Wei et

al. 2014). Due to the higher crystalline content of the zirconia, the fracture

resistance can reach up to 3377 N (Schmitz and Beani 2016) (Denry and Kelly

2008) and a more opaque color which is producing a less esthetic outcome than

lithium disilicate (Guarda, Correr et al. 2013). Therefore, zirconia can be the

material of choice as a fixed dental prosthesis in the posterior region especially

in patients who have a higher force (Pieger, Salman et al. 2014) (Guarda, Correr

et al. 2013) (Sun, Zhou et al. 2014).

In the mid-1980s, computer-aided designing technology (CAD)/computer

aided manufacturing (CAM) technology started to be used in dentistry. The

CAD/CAM has significantly changed the background of dentistry, the accuracy

of the restorations by using CAD/CAM succeeded the conventional, as well as

it is easier and faster (Arnason 2017). Moreover, the improvement of CAD

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technology, systems and programs, and CAM knowledge has helped with a

rapid increase the use it. (Arnason 2017)

In the oral environment, many factors could alter the physical and

mechanical properties of the ceramic restorations. For example, projecting the

ceramic restorations to dynamic and fluctuating stress could lead to microscopic

cracks, which are considered as fatigue fracture (Heintze, Monreal et al. 2018).

As well as the presence of continuous loading during the mastication with

thermal variation results in stress concentration that can induce cracks and

weaken the restoration. Moreover, if these stresses exceeded the load-bearing

capacity, this will result in catastrophic fracture of the weakened part of the

restoration (Guarda, Correr et al. 2013)(Güngör and Nemli 2018)

The fracture the resistance after thermo-mechanical loading of monolithic

zirconia and pressed lithium disilicate (IPS e.max press) was compared by

Johansson et al. (Hamed, Bakry et al. 2018) (Raigrodski, Hillstead et al. 2012).

This study showed that zirconia has a higher strength compared to lithium

disilicate with same occlusal thickness. Another study by Sun et al.

(Baladhandayutham, Lawson et al. 2015) reported 1 mm monolithic zirconia

crown resulted in the same fracture resistance value to 1.5 mm of a metal

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ceramic crown. In the other hand, 1.5 mm of lithium disilicate crown had a

significantly lower value. Nakamura et al. (Baladhandayutham, Lawson et al.

2015) reported that thickness .5 mm of monolithic zirconia crown had a higher

fracture resistance of 1.5 mm of lithium disilicate.

The operative obstacles of crown fabrication with subgingival margins

start with difficulties in tooth preparation, impression, adhesive cementation,

and finishing and polishing of the margin. To make the clinical less susceptible

to faults, a technique that was suggested Dietschi and Spreafico in 1998 called

cervical marginal relocation (CMR) which was renamed by Pascal Magne in

2012 to deep marginal elevations (DME). This technique is considered non-

invasive substitute to the invasive surgical crown lengthening (Juloski, Koken et

al. 2018)

The cervical marginal location might be done simultaneously with

immediate dentine sealing, which increases the bonding strength and helps in

the sealing of the margin. Furthermore, the deep marginal elevation has the

following advantages: 1) dentine sealing, 2) reinforcement of the undermine and

filling of the undercut, 3) provide the geometry needed for partial coverage

restoration, in inlay and onlay indirect restoration (Taktak, Mghirbi et al. 2018)

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Deep sub-gingival margin is commonly encountered in daily dental

practice. Where it represents a challenge for the dental practitioner. Because of

the advent in the adhesive dentistry, inventing new materials and increase the

esthetic demands from both patients and dentists, usually the treatment plan of

such teeth will end up with indirect adhesive restoration. Thus, two main

problems will resurface:1) biological nature problem and 2) technical operative

problem. (Juloski, Koken et al. 2018)

To overcome the deep sub-gingival margin, the margin can be exposed

surgically by displacing the tissue more apically losing from the tooth support

tissue leading to attachment loss. Also, it might lead to anatomical problem like

proximity of the roots and roots concavities which are considered hard for the

patient to maintain his/her oral hygiene resulting in additional new challenges

(Magne and Spreafico 2012)

In 2014, Zaruba et al. concluded in their study, that the location of the

cervical margin by the placement of resin composite before the final impression

of the inlay would result in a marginal adaptation not differ than the margin of

the inlay on dentine. The authors reported as well, such a technique will not

only help in the cementation of the inlay, but it will help in the preparation by

not causing trauma to the gingiva and the final impression taking (conventional

or digitally) (Zaruba, Kasper et al. 2014). Another study that was done by

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Sperafico et al. 2016 comparing the deep marginal elevation with a different

type of flow resin composite. As a result of this study, there is no significant

difference in the marginal adaptation or microleakage before and after the

thermo-mechanical loading among the groups (Spreafico, Marchesi et al. 2016).

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Research Proposal

Aim:

The aim of the study in vitro is to evaluate the post-fatigue fracture resistance of

the deep marginal elevation (proximal box elevation, cervical marginal

relocation) in full coverage all-ceramic crowns.

Null Hypothesis:

1) There is no difference in the fracture resistance where all-ceramic crown

margin on sound tooth structure or on composite restoration.

2) There is no difference in the fracture resistance between CAD CAM zirconia

on composite margin and IPS E.max CAD on composite margin.

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Sample size and groups:

80 extracted molar teeth.

Alpha level of significance= .05 with estimated SD of .6 and max. difference of

1.2 and power of 89.8% the sample of each group should be at least 10.

E = Enamel, D = Dentine, e = IPS. E.max CAD, z = CAD-CAM Zirconia, 1 =

one side ( Mesial ), 2 = two sides ( Mesial and Distal)

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Variable to be tested

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Fracture resistance

Group nameeE

Group nameeD

Group nameeE

Group nameeD

Group namezD

Group namee1

Group namee2

Group namez2

Group namez1

Group namee2

Group namee1

Group namez2

Group namez1

Group namee2

Group namee1

VSGroup name

zEGroup name

zD

Group nameeD

Group namez1

Group namez2

VS

VS

VS

Material and Methods:

Specimen preparation

The type of study in Vitro and the sample that will use 80 intact extracted

molar teeth with no caries, or fracture noticed with similar dimension,

endodontic treatment of all the teeth.

After that, MOD cavity preparation divides to MOD cavity preparation. eE

group and zE group the proximal box above the CEJ by 1 mm.

For the eD group and zD group the proximal box below the CEJ by 1 mm, e1c

group and z1 group the proximal box on the distal side is 1 mm above the CEJ

and on the mesial side 1 mm below the CEJ, e2 group and z2 group the

proximal box on the mesial and distal is 1 mm below the CEJ.

All the sample will restore by composite restoration; The teeth will be

etched by phosphoric acid 37% for the enamel, dentine 3 step etch rinse

adhesive system.

MOD cavity will be restored with conventional composite restoration, the

proximal box below the CEJ will be restored with hybrid flowable composite in

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two increments of 1 mm. (Except for the eD and zD). The Preparation of the

teeth for the crowns (According to the manufacturer) Occlusal clearance: 1.5

mm. Axial clearance: 1.5 mm for lithium disilicate and 1.2 mm for zirconia.

Finish line: 1 mm above the CEJ (Except for the eD and zD, the finish line will

be below the CEJ by 1 mm).

Finish line type: chamfer finish line, the width of 1 mm. Teeth will be

scanned by an extraoral scanner, lithium disilicate crowns and zirconia crowns

will be milled by CAD CAM machine. IPS E.max CAD and CAD CAM

zirconia will be cemented by relyX unicem (universal resin cement).

Teeth will be mounted in cold-cure plastic resin below the CEJ by 2 mm

simulating the bone.

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Test

1) Thermo-mechanical loading, the sample will be placed in the chewing

simulator (CS-4.2, by SD Mechatronik) in which it will be subjected to

thermo-cycling and dynamic loading simultaneously. The sample will be

subjected to 1,200,000 cycles representing 5 years of load and thermal

changes in the oral cavity, at the frequency of 1.6 Hz with a constant

vertical load of 80 N. At the same time, the samples will be thermo-cycled

in a water bath between 5 C and 55 C with dwell time of 40 seconds. If

crack or fracture happened during the fatigue test, the device will detect it

and stop itself.

2) A stainless steel sphere will be used as the antagonist. For the

standardization of the occlusal contact, the sphere will contact the occlusal

surface of the crowns at a three-point contact (tripod contact).

3) After the thermo-mechanical loading test (fatigue test) is finished, the

sample will subjected to the fracture resistant test device (CS-4.2, by SD

Mechatronik), in which the sample will be subjected to a force up 1000 N.

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The test will be stopped when either the crown fracture, any part of the

tooth, bulging out of the composite or de-bonding of the composite. The

force value at which sample fracture will be recorded in Newton (N)

through the software.

4) Force valve at which the fracture of the complex happens will be recorded.

Also, the mode of failure of each sample will be recorded to compare each

group to the other as well as the extension of the fracture in each sample.

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Clinical Implication:

Deep margin in the mesial and/or distal side of the tooth during

preparation of the crown is a common scenario yet it is a challenging one in

making the decision whether to extract or to restore the tooth by crown

lengthening which might expose the furcation area in the molar teeth or making

the crown-root ratio unfavorable which might lead to extraction of the tooth

eventually. As well as during preparation due to the blood from the lacerated

gingiva and inability of the clinician to inspect the tooth margin properly. Also,

in impression taking whether it is conventional or digital. Furthermore, during

the cementation procedure as this area can be easily contaminated with blood

and saliva, which lead decrease the bond of the adhesive.

In this study we suggest a solution that can be used by the clinician which

might help to avoid surgical procedure like crown lengthening that can lead to

poor prognosis of the tooth and during preparation, impression taking and

cementation of the crowns.

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References

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