cuspal deflection in mod molars restored with bulk-fill

Cuspal Deflection in MOD Molars Restored with Bulk-fill

Versus Layered Resin Composite Restorations (A

Comparative in-vitro Study)

Dahlia Mowafaq Mohammed

1, Sawsan Hameed Aljubori

2

1Ninavah Health Directorate, Mosul, Iraq.

2Department of Conservative Dentistry, College of Dentistry, University of Mosul, Mosul, Iraq.

Email Address: [email protected]

Received: 20 Nov 2021, Revised: 23 Nov 2021, Accepted: 1 Dec 2021, Online: 26 Dec 2021

Abstract

Objectives: To evaluate the accuracy of the bulk-fill application technique with its effect on the cuspal

deflection of molars with class II MOD cavities filled with bulk-fill and conventional composite resin

restorations. Aims of the study: The aims of this study focused on comparing and evaluating the cuspal

deflection of bulk-fill composite resin materials (high and low viscosity) with conventional composite resin

material using incremental and bulk-fill application techniques by assessing shrinkage stress and elastic

modulus(E£). Materials and Methods: Thirty human molars teeth received standardized class II mesio-occlusal-

distal cavity preparation and were restored with three types of composites: [Filtek (Z350XT), 3M-ESPE (USA)],

[SDR surfil plus (DENSPLY)], [Tetric- Evoceram bulk- fill (Ivoclar viva-dent)]. Teeth were divided into three

groups (n=10) according to the type of composite resin, then each group was subdivided into two groups (n=5)

depending on the application techniques (Bulk-fill and Incremental filling). Filtek Z350XT composite was used

as a control group in incremental and bulk-fill techniques which was tested as a conventional incremental

material according to manufacture instruction represented as positive control subgroup and also as bulk-fill

product, represented as negative control subgroup. The buccal and palatal cuspal deflection was measured using

a strain measurement system, in addition, thirty specimens were prepared for each composite resin material and

used to evaluate and measure the elastic modulus (E£) by three-point bending test using a universal testing

machine (Gester, Total Testing Solution). Results: Teeth filled with bulk-fill composite resin restoration had

significantly lower total cuspal movement values compared with conventional composite resin restoration for all

groups ( incremental and bulk-fill application techniques). Incremental and bulk-fill application techniques

groups with low-viscosity SDR surefil plus and high-viscosity Tetric- Evoceram bulk-fill composite resin

restoration were not significantly different from each other. Bulk filled and incremental filled with low-

viscosity SDR surefil plus have a significantly lower cuspal movement than all other groups. P-value less than

or equal to 0.05 was considered statistically significant. Conclusion: Incremental application technique was the

best technique to lower the polymerization shrinkage stress and it's complication. Bulk-fill application technique

of low-stress bulk-fill composite resin restorations achieved comparable results to incremental application

technique. The use of the incremental technique with low polymerization shrinkage stress has dwindled but it is

safer to continue to use the incremental application technique, as a significant number of the bulk-fill and

conventional composite resins produced higher polymerization shrinkage stress without it.

Keywords: Bulk fill restorative material, Cuspal deflection, Intercuspal distance, Elastic modulus.

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Research

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Journal of Global Scientific Research (ISSN: 2523-9376)

6 (12) 2021/ 1952-1971

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Mohammed, D.M. & Aljubori, S.H. Journal of Global Scientific Research (ISSN: 2523-9376) 2021/ 6 (12) 1953

1. Introduction

Since the development of the composite

resin as an esthetic restoration alternative

to old-dental amalgam restoration in the

late 1950s. Composite resins are mercury-

free, adhered to the dental structure

through the dental adhesive system, low

thermal conductivity. The materials have

found increasing application in modern

preventive and conservative dentistry as

the material undergo catastrophic

development; alternative photo-initiators

system, application of nano- filler particles

technology, novel- resin monomer, blend –

resin monomer, and the development of

the translucent dental composite resin to

the blue light permit curing up to 4mm

thickness of the restorations and produce a

restoration with natural tooth appearance

with long term performance (Kalliecharan

et al., 2016; Ricci et al., 2019).

The incremental technique is based on

polymerizing with resin-based composite

layers < 2.0 mm thick to ensure a complete

polymerization, secure the adhesion of the

restoration to the cavity wall, and can help

achieve a good quality of the margin. The

incremental technique increasing the

unbounded area for each increment layer

allows for material to flow, maximizes the

relaxation of polymerization shrinkage

stress, minimized the stress on the

prepared cavity walls and its complications

(Deliperi and Bardwell, 2002;

Chandrasekhar et al., 2017 ).

The main disadvantages of the incremental

filling technique are time-consuming (as

multiple increments must be inserted and

individually light cured), lack of bond

between layers, technique sensitivity,

contamination, or incorporation of voids to

the mass of the restoration. Recently, the

concept of bulk-fill technique ( the

restoration placed in a 4.0-mm bulk in one

increment ) was introduced by some

authors not only to save time but also to

reduce stress at the cavosurface margins

and to reduce the potential for errors

(Giachetti et al., 2006; Menees et al.,

2015).

Cuspal deflection occurs in teeth restored

with composite resin materials as the result

of the interaction of polymerization

shrinkage stress in the composite resin

restoration and the compliance of the

remaining teeth structure after cavity

preparation that impact the adaptation of

the restoration to the cavity walls (Lee et

al., 2007; Hamama et al., 2011; Oskoee et

al; 2012).

The deformation occurred as a

combination of stresses in restoration,

across the tooth restoration interface and

the tooth structure. Cusp compliance is an

important factor that affects the amount of

cusp deflection, compliance is defined as a

change in dimension of the object to unit

force and has the opposite meaning to

stiffness, thus the degree of compliance

can affect the result of the measurement

stress. The teeth with high compliance,

that the cusps will deflect more easily, the

teeth with cavities exhibit relatively a high

compliance. Cuspal deflection may cause a

change in the occlusal contact relation of

the teeth and may cause postoperative

teeth sensibility or with greater magnitude

of this deflection lead to the greater degree

of micro deformation in tooth cusps and

consequently increase the possibility of

fracture under the stress far below the

maximum strength of the restored tooth

may occur (Oliveira et al; 2012; Kim et

al., 2016)

Inward displacement of the cusp was much

slower and longer than polymerization

shrinkage of composites therefore cuspal

deflection was used by many authors to

study the influence of restorative

procedures and restorative material

properties and provided indirect

quantifying measurements of

polymerization stress in the composite

being restricted by bonded to the cavity


walls, as the polymerization stress cannot

be directly and simply measured in

restored teeth cavities (McCullock and

Smith, 1986; Campodonico et al., 2011;

Karaman and Ozgunalty, 2013).

The measured cuspal deflections averaged

15μm to 50μm but varied according to the

technique used, the measurement method,

the cavity size, and tooth type. An absence

of standardization of tooth size, tooth

cavity preparation, and restoration

technique renders comparison of result

difficult since the cusps contraction

depends on the amount of the remaining

tooth structure. Some studies combined

cuspal deflection with microleakage

analysis for shrinkage stress determination

(Moorthy et al., 2012 and Kweon et al.,

2012).

Different inexpensive and simple

measurement devices had been used to

measure cuspal deflection included non-

contact and contact methods. Non-contact

methods such as; Digital image correlation

(DIC), Microscopy, Laser scanning, and

3D micro-computed tomography Contact

methods such as Twin-channel deflection

measurement gauge, Linear variable

displacement transducers (LVDT), Direct

current differential transformers (DCDTs),

the Deflection of metal strip with

microscopy indicated cusps alignment,

Digital micrometer, and Strain gauge

measurement system (Palin et al., 2005,

Faisal and Nashaat in 2018).

A modification of the strain gauge method,

which was developed in cooperation

between the engineering and dental staff

(Hamama et al., 2011). they were

relatively small grid areas of only a few

millimeters. Bonded on the external

surface of buccal and lingual cusps about

2.5mm from the cusp tips. It had been

reported that cusp fracture in restored teeth

commonly occurs in this area and it was

considered that this area was likely to be

the region subjected to the highest strains

(Lee et al., 2007; Shebl et al., 2018).

The strain gauge measurement system has

many computerized advantages and it

provided accurate quantitative and

qualitative measurements, details, a large

amount of data storage, and easy recall as

the process of micro deformation

experienced by the cusp led to the

covariance of electrical resistance later this

covariance was sent to the data acquisition

panel and conveyed into digital signs,

which displayed in curve format which

was consisted of peaks and valleys which

read in specified software and could be

provided a further expression in future

studies. The accuracy of these

measurements is more reliable than with

conventional measuring systems (in which

the measurements were based only on

deflection range values or difference

between the post-curing and pre-curing

values) as the data are automatically

calculated by a specified software program

(SI Prog). In addition, the result from

measurements showed that the highest

levels of strain were produced during

exposure of the restoration to the light-

curing source for polymerization

(Hamama et al., 2011; Nashaat and

Mohammed, 2018).

The main limitation of this measurement

system is that it was needed to obtain two

symmetric channels buccally and lingually

to judge whether the contraction occurs

simultaneously in both cusps or one cusp

more than the other. In addition, a greater

variety of micro deformation results might

occur due to the influence of a greater

number of factors. Factors that could

influence or interfere with the passage of

the current strain gauge measurement

system might include: the orientation of

the strain gauge sensor during the bonding

on the external surface of the cusp, thermal

vacillations, and the number of

information that might be lost in the path

between the strain gauge sensor and the


entrance device (data acquisition) (

Hamama et al., 2011; Rocha et al., 2013;

Shebl et al., 2018).

The purpose of this study was intended to

evaluate the effect of different application

techniques ( bulk-fill ⁄ incremental ) of

bulk-fill (low and high viscosity) and

conventional composite materials and their

roles to reduce the shrinkage stress by

assessing the cuspal deflection of class II

media-occluso-distal composite resin

restorations. The cuspal deflection was

measured using a strain measurement

system. The influence of the elastic

modulus of the different types of

composite resin restoration on the

polymerization shrinkage stress is also

verified.

2. Materıals and Methods

Materials:

The main materials used in this study: [

Filtek (Z350XT), 3M-ESPE(USA)],[ SDR

surfil plus (DENSPLY)], [Tetric-

Evoceram bulk- fill (Ivoclar viva-dent)],[ OptiBond FL (Kerr)].

Methods:

Selection of Teeth

Thirty extracted sound human third molars

(n=30) have been collected within four

months for this study. The teeth collected

from patients ranged from 18-33 years.

The inclusion criteria that the selected

teeth were free from .decay, cracks,

.attrition, previous restoration, any

malformation defects, and any visible teeth

defect resulting from the extraction, the

teeth with a complete root formation. The

teeth were selected to have nearly

standardized intercuspal width and

mesiodistal length which were measured

with a digital caliper, within a maximum

mean of deviation of 10%, from their

respective specific mean (9.5-10.5mm and

10.5-11.5mm).

The teeth were debrided from blood,

saliva. rinsed with water, the teeth were

disinfected in 0.1% thymol solution ( as it

alters less the conditions of dental

structures for adhesion) and after that

stored in distal water at room temperature

to keep the specimens hydrated until the

time of the study was carried out.

Preparation of the Mold and Mounting of

Teeth

Each tooth was marked 2±0.5mm above

the (CEJ) amelo-cemental junction with an

indelible pen. Polyvinyl chloride (PVC)

retentive tube, with two centimeters in

diameter and two centimeters in length,

was used as a mold for mounting the teeth,

Figure (1a). Each tube was filled with

polyvinyl siloxane impression material

(Dursoil putty addition polyvinyl siloxane

impression material, Germany), and by

aiding of surveyor mounted the occlusal

surface of the teeth with sticky wax so the

roots of each tooth had been positioned at

the center of the retentive tube, with the

long axis of the tooth parallel to the sides

of (PVC) were extended 2± 0.5mm below

the cement-enamel junction.

For the proximal cavities, to facilitate the

material placement, improve the

restorative material adaptation and shorten

the finishing steps, an impression for the

tooth crown was made prior to cavity

preparation with polyvinyl silicone which

was placed in a (1.5cm) diameter

cylindrical plastic tube used as a tray to

create a custom matrix for restoring

proximal box, Figure (1b). After setting

the impression material the plastic tube

was removed and trimmed of all the

material overlaying the occlusal surface of

the tooth, Figure (1c).


Grouping of the Specimens

Thirty prepared teeth were selected

randomly for strain measurements and

distributed into three experimental groups

according to composite material (n=10),

then each group was subdivided into two

equal subgroups according to filling

technique (n=5): Incremental and bulk-fill

technique.

Preparation of the Teeth

Standardized large slot (MOD) cavities

were prepared without proximal boxes in

order to reduce the preparation variation.

Cavities were prepared in each tooth by

using of milling machine (Bio-art), Figure

(2), flat-ended fissure bur (HP559S31)

(diameter 1.20mm) is mounted in a

straight handpiece with coolant water, the

cavity preparation was carried within a

stainless steel frame which is referred to it

as "guiding frame".

The guiding frame was attached to the

acrylic mold fixture by an adjusting screw

and was adjusted to be as near as possible

over the occlusal surface of the tooth and

the vertical part of the "guiding frame"

over the marginal ridge to ensure

standardization of the cavity in all

preparation. The result is nearly

standardized cavity width (4±0.3) and the

cavity depth was (4±0.3), Figure(3), the

bur was replaced every five preparation to

ensure high cutting efficiency. The

accuracy of cavity measurements was

aided by using a periodontal probe, using

dental magnifier loops with (2.5x

magnification).

Figure (1): A: The teeth mounted in polyvinyl chloride (PVC) retentive tube, B: A

cylindrical plastic tube of (1.5cm) in diameter used as a tray , C: custom matrix

for restoring proximal box.

A C B

Figure (2):Bio-art milling

machine.

Figure (3): Standardized

large slot (MOD) cavity.


Restorative Application

After standard class II cavities were

prepared for each sample, samples were

acid-etched and bonded using the same

total-etch system (Kerr Gel Etchant and

OptiBond FL, Kerr) to reduce the number

of variables. The silicon putty band which

was previously prepared for each

respective tooth has been used. Cavities

were encircled with silicon putty matrix

band with a mylar strip was adapted

against the proximal aspect on the teeth

being restored. Cavities were then restored

with resin composite restoration.

Composite in all groups applied in the

prepared cavities depending on the filling

techniques. In an incremental technique

(horizontal incremental technique) two

layers ( the thickness of each increment of

resin composite is not more than 2mm).

Each increment should be fully

polymerized before the next one is inserted

into the cavity for (20) seconds. For the

bulk-fill technique, the cavity filled in one

increment, the thickness of the increment

of resin composite is 4mm thickness which

was verified by the using periodontal

probe. Then light cured for (20) seconds,

using the light emitting diode (LED)

(eighteenth) at an irradiance of

(1500mw/cm²), the intensity of light cure

measured by radiometer before application

with a wavelength range between (380-

515). The light-curing tip touching the

slopes of the cusp tips of the teeth for

standardized the position of the light cure .

After removal the impression matrix

additional curing from the facial and

lingual aspect of the proximal boxes to

ensure complete polymerization. The

intensity of (LED) light source was

monitored before and after curing

throughout the study with a radiometer.

Strain Measurement System

The strain measurement system consists of

a four-channel amplifier circuit

(SunFounder-5V-4-Channel-Relay-

Module,15-20 mA), analog to digital

converter (AD) card (SunFounder AD

analog to Digital Convertermcp3004),

personal computer (Lenovo, L420) with

strain indicator program ( SIProg, Data

loger 4 channel ver.) and strain gauge

sensor, Figure (4) which designed for this

study by the technical engineer Nazar

Ahmed.

A

Figure (4) Strain measurement system: A: Four channel amplifier circuit,

B:Analog to digital converter (AD) card, CPersonal computer, D:Strain

Indicator Program ( SIProg), E: Strain gauge sensor

B C

D E


The cuspal strain was measured with strain

gauges (PA-06-060CC-350L) with pre-

attached thin copper cables which had

internal electrical resistance of 350Ω, a

gauge factor of 2.00-2.20, gauge grid

dimensions 3.2 x 3.06mm², small size,

mass, easy attachment, high sensitive to

strain, low cost and low sensitive to

temperature and other ambient conditions.

For strain gauge installation, it is important

that the external surface of the test

specimen on which the strain gauge

bonded should be cleaned. To do so, the

strain gauge's bonding site on the external

buccal and lingual surfaces of each tooth

should be carefully sandblasted with

abrasive paper (no 0.320), Figure (5a), the

abraded area should be fin cleaned by

soaking the area with a cloth dampened in

a small amount of acetone, this preparation

aided to obtain a sleekly, shiny surface,

and somewhat large braded area ( 20mm to

30mm) larger than the instillation area,

which provides a careful adaptation of

strain gauge on the tooth contour and

provides more security bonding and

stability during measurement without

interference with the boundaries of the

restoration and impression matrix band.

Two precision strain gauges were bonded

on the decontaminated external surface of

the buccal and lingual cusps of each

unrestored tooth with adhesive (super glue,

INGCO) and polythene sheet onto the

gauge and pressed down with thumb to the

cervical area of the cavity's external walls,

Figure (5b) In addition, to compensate for

any dimensional deviations due to

temperature effects, with the same

procedure two strain gauges were bonded

on the external surface of another tooth

performed with the same cavity dimension

but free from restoration ( acts as a passive

control sample) and connected to the

analog to digital converter card.

The upper portion of buccal and lingual

surfaces of polyvinyl siloxane impression

matrix that was encircled the tooth was

customized without encircling the cervical

portion of buccal and lingual surfaces,

Figure (6a) to avoid hocking the placement

of strain gauge and wrong measurement

due to pre-stress from the impression band,

later the specimens were placed on

specialized 3D printing stage for

standardized the position of the specimen,

Figure (6b).

A B

B

Figure (5): A: Sand- blasted the external surface of

tooth, B: strain gauge were bonded on the external

tooth surface with adhesive.


The strain gauge constituted two thin lead-

terminals, one-terminal connected to the

full-bridge while the other end had been

connected to the (SI Prog) strain indicator

program. Full bridge wheat stone bridge

circuits, it may be advantageous to make

all four elements of the bridge "active" for

even greater sensitivity (provided that the

change in resistance caused by the applied

force is equal for all four strain gauges).

However, the output voltage is directly

proportional to an applied force.

SI Prog ( Data loger 4 channel ver.) was

fully written in homestead using Matlab

packages. It consists of a Title bar is the

name of this program strain indicator

program. The menu bar from which the SI

Prog is processed by clicking on the start

option also can select other options in this

bar either running or running continuously.

The options panel contains the waveform

chart box and CH1-CH4 boxes from which

the number of operating channels can

select. The measuring information panel

when the scientific name of the material is

inserted in the VISA resource name

cabinet also contains the stop pushbuttons

cabinet. The display area in which the

collecting data appeared as a curve

between the time in minutes and the strain

in the microstrain unit (με).

The strain gauge recorded the data (the

changes in voltage signals) during the

polymerization process then amplified by

recording system, the amplified signals

were transferred using an analog-to-digital

converter (AD) card to a (SI Prog) for

analysis of the measurements that recorded

up to (180s) following the initiation of

light irradiation, no further cuspal recoil of

the buccal and lingual cusp occurred

following 180s post-irradiation, then by

clicking on the stop pushbuttons in the

measuring information panel

corresponding graphs are displayed of the

buccal and lingual cusp area the strain

reading is displayed as a graph

representing the relation between the strain

measurement and corresponding voltage.

The collecting data for both buccal and

lingual cuspal deflection were recorded

and tabulated to a Microsoft Excel ( 2010)

software program.

The collecting data appeared as a curve

between the time in second (sec) and the

strain in the microstrain unit (με). These

readings for the full wheat stone bridge are

directly proportional to the internal cuspal

contraction of the buccal and lingual cusps

of tested specimens. The sum of the buccal

and lingual cuspal contraction or

deflection measurements by microstrain

(με) also was calculated for each

specimen.

Flexural Modulus (ES)

Composites resin materials were prepared

in a half-spilt stainless-steel mold with

dimension 2.0 (depth) x 2.0 (width) x 25.0

A B

Figure (6): (A) impression matrix was customized without encircle the upper portion of buccal

surface, (B) Specialized 3D printing stage for standardized the position of the specimen.


(length) mm³, with metallic base, Figure

(7a) For packing of composite into the

mold amylar strip and a glass slid

microscope was used to make a flat

surface of the specimen and prevent the

interfering of oxygen with the most

superior layer of the composite material.

The specimen were then polymerized, the

light curing procedure was performed with

3 overlapping sessions, to cure the entire

length of the specimen with 20 seconds for

each secssion, by LED (eighteenth ) with

light intensity (1500) mw/cm², wave

length of (380-515) nm. This procedure

was repeated on the opposite side after

removal the metallic base.

Ten specimens were prepared for each

group. After light curing, the specimens

were then free from the mold, Figure (7b)

then gently wet-ground (stand paper no

0.320) to remove any accessories or flash.

Specimen's dimensions were measured by

digital caliper to an accuracy of 0.01mm,

measurements were performed at 3 points

for the width, height, and their (average

values were used in the calculation of

bending strength), then transferred to distal

water kept at 37ºC for 24 hours.

The three-point bending fixture was

performed according to ISO No.4049:2009

that consisted of 2 rods (diameter 2mm)

mounted parallel with 20mm between their

center (support span 20mm) the test was

performed immediately after removing the

specimens from the distilled water and

without drying the specimens. For a three-

point flexural test, a rectangular sample of

the resin composite material is supported

at its two ends, in the middle there is no

support but the ends are sturdy a load or

force is applied to the middle section until

the material breaks, or permanently bend

.With a 2mm diameter of the loading

piston and cross-head speed of 0.5

mm/min until failure occured (Universal

testing Machine, Gester, total testing

solution), using a preloaded of 0.5N.

The flexural strength values were recorded

with computer software (ver.1.6.4.9), the

modulus of elasticity (E£) was calculated

from the slope of the linear part of the

diagram of the force-deflection using the

following equation:

E = FL³ / 4BH³d

F: is the maximum load (Newton's), L: is

the distance between the arms

(millimeters), B: is the width of the

samples (millimeters), H: is the height of

the specimen (in millimeters), D: is the

deflection corresponding to the load F.

After collecting the data, the elastic

modulus (E£) have been calculated and

measured in (GPa).

A B

Figure (7): A: stainless-steel mold, B:

specimens were free from the mold.


Statistical Analysis

The data for this study were collected,

tabulated, and statically analyzed by using

a compatible personal computer with a

statistical package for social. science IBM

SPSS( SPSS for Windows, IBM Corp.,

Version 25) for statistical analysis.

Normality of data distributions and

homogeneity of variance tests was

assessed with Kolmogorov-Smirnov and

Shapiro-Wilk at the significances level of

(P≤ 0.05). All data passed the tests and

was distributed normally, they were

subjected to the parametric statistical

analysis.

Thirty tooth specimens were tested for

cuspal deflection. Cuspal deflection of

each specimen of the same group was

statistically analyzed using paired sample-

t-test for both buccal and lingual cusps

comparison and the independent-sample t-

test for comparison of two application

techniques. One-Way ANOVA Test and

Duncan's Multiple Rane Test for

comparison of the cuspal deflection of

three types of composite materials using

incremental and bulk fill application

technique, Significant differences were

considered when (P≤ 0.05).

Thirty specimens of the testing materials

were prepared for flexural modulus. One-

way ANOVA test and Duncan's Multiple

Rane Test were performed to identify the

significance of the flexural modulus

among composite resin materials groups at

( p≤0.05).

3. Results

Cuspal Deflection Measurements

The behavior of the cuspal deflection of

each specimen of the same group consisted

of summation of both buccal and lingual

cuspal deflection were statistically

analyzed using paired-sample- t-test and

the independent-sample t-test of two

application techniques shown in Table(1),

the results indicated there was a significant

difference between buccal and lingual

cusps, with the lingual cusp has higher

cuspal deflection than the buccal cusp.

The results of the application techniques

for (FiltekZ350XT) there is significant

differences between incremental (control

positive group) and bulk fill (control

negative group) application technique at (P

≤ 0.05), while there are no significant

differences between bulk fill and

incremental application technique for

(Tetric EvoCeram Bulk fill ) and (SDR

Surefill Plus) at (P ≤ 0.05) as shown in.

Table (1) Comparison of cuspal deflection

in specimens with bulk fill and

incremental application technique for

conventional and bulk-fill composite

resins.

N: number of specimens; SD: stander of deviation.

Composite Resin

Materials

Application

Techniques

N

Mean(μs) and SD

of Buccal cusp

Mean(μs) and SD

of Lingual cusp Sig.

(2-tailed)

(FiltekZ350XT) Bulk -fill 5 519.52±11.17 564.42±20.21 0.004

Incremental 5 321.16±11.14 367.55±8.23 0.000

Tetric EvoCeram

Bulk fill

Bulk -fill 5 271.68±2.57 328.50±10.69 0.000

Incremental 5 262.34±19.53 328.18±13.20 0.001

SDR Surefill

Plus

Bulk -fill 5 167.10±2.88 272.25±6.14 0.000

Incremental 5 164.34±90 263.85±19.22 0.001


Table (2) Descriptive statistic of elastic modulus(GPa) for each group.

Composite Resin

Materials

N

Minimum

Median

Maximum

Mean

SD

FiltekZ350XT

10

6.73

8.06

9.54

8.01

0.95

Tetric Evo Ceram

Bulk fill

10

4.88

7.03

7.81

6.61

1.03

SDR Surefill

Plus

10

2.72

3.24

3.63

3.16

0.29


One-Way ANOVA test result for cuspal

deflection of three types of composite

materials using incremental and bulk fill

application technique represented in

Figure (8) indicated that there was a

significant difference between control and

bulk-fill composites groups at (P ≤ 0.05).

The behavior and values of the cuspal

deflection for three groups of composite

materials (FiltekZ350XT, Tetric

EvoCeram Bulk-fill, SDR Surefill Plus)

and application techniques ( incremental

and bulk fill) also were compared. Cuspal

deflection for (SDR Surefill) showed the

lower values while (FiltekZ350XT)

showed the higher values.

The behavior of the buccal and lingual

cusps for all groups was also compared

using One-Way(ANOVA) there was a

significant difference between the two

cusps, the lingual cusp showed a higher

strain than the buccal cusp for all groups.

(SDR Surefill Plus) in the incremental

application had lower values of lingual

cuspal deformation than that in bulk fill

application technique while

(FiltekZ350XT) had the higher values of

lingual cuspal deflection as showed in

Figures (9 and 10).


A B

C D

Figure (10): Cuspal deflection in microstrians(µS) measured with strain gauges placed on the

external surfaces of the buccal and lingual cusps. (A) Buccal cusp; (B) Lingual cusp of tooth sample

filled with incremental technique. (C) Buccal cusp; (D) Lingual cusp of tooth sample filled with

bulk-fill technique.


Elastic Modulus Test

The mean of elastic modulus and standard deviations of tested composite materials were

shown in Table (2).

Table (1) Comparison of cuspal deflection in specimens with bulk fill and incremental

application technique for conventional and bulk-fill composite resins.


Table (2) Descriptive statistic of elastic modulus(GPa) for each group.

Composite Resin

Materials

N

Minimum

Median

Maximum

Mean

SD

FiltekZ350XT

10

6.73

8.06

9.54

8.01

0.95

Tetric Evo Ceram

Bulk fill

10

4.88

7.03

7.81

6.61

1.03

SDR Surefill

Plus

10

2.72

3.24

3.63

3.16

0.29


A One-way (ANOVA) test was performed

to identify the significance of the flexural

modulus among composite resin materials

groups at ( p≤0.05). The result indicated

there were highly significant differences

between composite materials at (P≤ 0.05).

Duncan's Multiple Range Test for flexural

modulus values of different types of

composites in the following order (SDR

Surefill Plus, Tetric EvoCeram Bulk fill,

FiltekZ350XT) as shown in Figure (11 ).

There was a significant difference, among

composite groups, and the (FiltekZ350XT

has the highest value while SDR Surefill

Plus has the lowest value at the significant

level (P≤ 0.05).

Composite Resin

Materials

Application

Techniques

N

Mean(μs) and

SD of Buccal

cusp

Mean(μs) and

SD of Lingual

cusp

Sig.

(2-tailed)

(FiltekZ350XT)

Bulk -fill

5

519.52±11.17

564.42±20.21

0.004

Incremental

5

321.16±11.14

367.55±8.23

0.000

Tetric EvoCeram

Bulk fill

Bulk -fill

5

271.68±2.57

328.50±10.69

0.000

Incremental 5 262.34±19.53 328.18±13.20

0.001

SDR Surefill

Plus

Bulk -fill

5

167.10±2.88

272.25±6.14

0.000

Incremental 5 164.34±90 263.85±19.22 0.001


4. Discussion

Cuspal deflection is defined as a linear

movement or recoil of cusp tips of the

tooth which result from complex

interactions between the polymerization

shrinkage stress of the composite and the

compliance cavity's wall of the tooth

(Singhal et al., 2017).

Cuspal deflection or deformation occurred

as a biomechanical phenomenon which

referred to the fact that deflection is

proportional to the cubed power of the

cusp length and inversely to the cubic cusp

thickness of cantilever cusp thickness

(Alexandra et al., 2016).

Extracted teeth have been used in this

study to eliminate the problems associated

with the compliance of the testing system

or the supporting dental structures. The

extracted human maxillary and mandibular

third molars teeth were selected with the

buccolingual/palatal width (BL/PW) of

teeth to vary between (9.5-10.5mm and

10.5-11.5mm) to accommodate for the size

variance and to reduce the increased

morphology variances the third molars

teeth with (two buccal and two lingual/

palatal ) cusps were included. The

characteristic of the third molar that has

significantly reduced in length of the

anatomic crown and a wider surface area

for strain gauge bonding in compared to

premolar which is mostly used in other

studies ( Alexandra et al., 2016; Politi et

al., 2018).

A large slot II MOD cavity preparations,

the cusp remain after cavity preparation

under the occlusal load were performed on

third molars teeth in this study as this

preparation has a number of advantages

that: The first one the amount of cuspal

deflection is directly related to the amount

of tooth structure loss, weaken the

remaining tooth structure, high C-factor,

lost the marginal ridge integrity that results

in a significant loss in relative cuspal

stiffness; consequently, the cusp

compliance increases for best cuspal

deflection and provides aping of clinical

state (Jlekh and Abdul-Ameer, 2018).

The second one this preparation would

increase the sensitivity to strain gauge

sensor, in addition, it was found that

polymerization shrinkage of the composite

resin materials produced a significant

inward cuspal displacement in three

surfaces versus two surface restoration,

also in MOD preparation need a large

amount of filling material. The third one

with this type of preparation, the cusps that


remain after cavity preparation were

reported to act under the occlusal load as a

cantilever beam, which increases the depth

of the prepared cavity, while the prepared

cavity floor acts for cusp bending as a

fulcrum.

The factors influencing the stress

formation included: polymerization

shrinkage stress, the elastic modulus of the

composite resins, C-factors of the restored

cavity, the curing mode, and the type of

the dentin adhesive system. In this study,

the size and the shape of the cavity, curing

mode, and the dentin adhesive system was

standardized for all specimens.

The main variables causing the difference

between the cuspal deflection values of

conventional composite and bulk-fill

composites may be due to two factors of a

combination thereof. The first factor was

materials' chemical composition of the

different groups of composites

(conventional and bulk-fill composites)

and the second factor was the type of

application technique: Incremental (two

layers and two sessions of light-curing)

Bulk-fill (one layer and one session of

light-curing) application techniques

(Elsharkasi et al., 2018; Oliveria et al.,

2018).

The matrix band placement on teeth

prepared with MOD cavities may cause

cuspal bending and pre-stress the tooth

prior to polymerization of composite

restoration (Mohammed and Nashaat,

2018) therefore in this study impression

matrix band with modification on the

buccal and lingual surface has been used

during the restorative procedure to prevent

the interference with the adaptation of

strain gauge sensor.

After the tooth restoration, the initial

period of negative cuspal deflection is

critical value because during this period

occlusal contact is normally adapted,

which could cause a greater tendency to

tooth fracture till a period of relaxation

reaches (Gonzalez-Lopez et al., 2006 a).

The result of this study showed that after

completing the restorative procedure an

inward cuspal deflection for all groups this

deflection was continued for several

minutes( at time intervals of 0.5 min to 3

min) and there is no further cuspal

movement for both cusps occurred after 3

min or (180s) post-irradiation this result

can be explained: inward cuspal

deflection, might be attributed to the

amount of the remaining free radicals,

double bonds in the resin base composite

restoration which persisted to react after

the polymerization reaction process

consequence, the polymerization stress

developed and causes inward recoil of

both cusps (McHugh et al., 2017;

Elsharkasi et al., 2018; Jlekh et al., 2018).

The present study compared the influence

of application technique ( incremental and

bulk-fill) on the mean of cusp deflection

for conventional and bulk-fill ( high and

low viscosity) composites resin. The

results showed that in (Filtek ZX50XT)

groups there is a significant difference (p ≤

0.05) between incremental and bulk-fill

techniques, the incremental technique

show less cuspal deflection compared with

bulk-fill technique, and this could explain

as a recommended maximum depth of cure

is about 2mm for each incremental layered

of composites resin materials. Therefore,

in the bulk-fill technique, the depth of

polymerization might not be totally

completed causes internal stress to be

developed within the structure of the

material as well as more cuspal recoil, so

the clinical application of the conventional

composite with bulk-fill technique may not

be applicable, the bulk-fill technique with

conventional composite not recommended

for clinical use, these findings are in

agreement with (Park et al., 2008) reported

that when the conventional composite used

in both incremental and bulk-fill

application techniques, the increment


technique was shown considerably lower

cuspal deflection than that for bulk-fill

technique implying that the bulk-filling

technique led to significant more cuspal

deflection than did the incremental filling

technique with no significant differences

between oblique and horizontal layered

techniques.

On the other hand for (Tetric Evo Ceram

Bulk-fill) and (SDR surefill Plus) groups,

there was no significant difference (p

≥0.05) between the mean of cuspal

deflection for both incremental and bulk-

fill application techniques, this could be

explained as the bulk-fill composite resin

that used in this study according to the

level of polymerization shrinkage stress

can be classified into: low-viscosity bulk-

fill with low stress (SDR surefill Plus) and

high-viscosity bulk-fill with low stress

(Tetric Evo Ceram Bulk-fill).

(SDR surefill Plus) is based on "Stress

Decrease Resin" technology, which means

that in the backbone of the polymerizable

resin a substance that is chemically

embedded known as a "polymerization

modulator" has an appositive effect in

lowering the development elasticity

modulus, allowing for the reduction in the

polymerization stress without any negative

effect in the rate of the polymerization

reaction and the degree of conversion and

this effect occurred as a result of the

synergistically interacts between the

polymerization modulator and the

camphoroquinone photo-initiator (Patel et

al., 2016; Al-Bargash et al., 2017;

Tsujimoto et al., 2020).

Tetric Evo Ceram Bulk-fill composites

resin is known to have filler within the

matrix of the composite resin, which is

partially functionalized by silane known

a special stress reliever״ allowing reliable

polymerization and depth of cure at 4mm

depth for posterior restoration, in addition,

to di-benzoyl germanium which a newly

patented light initiator. Furthermore, this

bulk-fill composite contains ″Ivocerin״

which provide a high absorption

coefficient and consequence increase the

efficiency of the quantum, Ivocerin also

characterized by high reactivity to the light

provide rapid polymerization reaction with

lead an increase in the depth of cure (Patel

et al., 2016; Al-Bargash et al., 2017;

Tsujimoto et al., 2020).

Although the difference in the viscosity of

the two bulk-fill between low-viscosity

and high-viscosity bulk-fill composite

resin but both materials have low stress. In

this study, the incremental application

technique doesn't appear to offer any

advantages over the bulk fill application

technique for both (SDR surefill Plus)

(Tetric Evo Ceram Bulk-fill composites

resin). Thus in the low-stress bulk-fill

resins, the effect of the incremental

technique on the polymerization stress has

a minimal influence and the advance in the

chemical composition of the composite

resin materials are more important in

reducing the generated polymerization

shrinkage stress than the choice of the type

of the application technique this result was

supported by ( Shimatani et al., 2018)

which suggests that the cuspal deflection

of the composites resin materials is

primarily influenced by materials

chemical composition rather than the

application filling technique.

This result also came in agreement with (

Meeries et al., 2018) provided a systemic

review of polymerization shrinkage stress

of resin composites which found that

modification of composite resin matrix had

the largest redounding on minimizing

stress development and also this result

came in agreement with the study carried

by (Műnchow et al., 2018) showed in

those reviews, the technique protocols for

application of composite resin materials no

longer impacted with a great contribution

to polymerization shrinkage stress and the

consequence complication associated with

cuspal deflection and had fewer influences


than light-curing method. On the other

hand to minimize the development of the

polymerization shrinkage the amending in

the resin matrix made the largest effects.

Also, this result came in agreement with

(Tsujimoto et al., 2020) which concluded

that the cuspal deflection of some high and

low-viscosity bulk-fill resin was

significantly reduced by using incremental

techniques, however, the resultant

improvement of incremental techniques

was not applicable to all the bulk-fill

resins, so the effect of incremental

techniques on polymerization shrinkage

stress of low-and high bulk-fill resins was

material predicated. Thus, the use of the

incremental technique with low

polymerization shrinkage stress has

dwindled but it is safer to continue to use

the incremental application technique, as a

significant number of the bulk-fill and

conventional composite resins produced

higher polymerization shrinkage stress

without it.

The lingual cusps had higher cuspal

deflection than buccal cusps for all

experimental groups. This can be

explained by that the amount of cuspal

deflection being influenced by the amount

of remaining tooth structure. The cervical

areas of third molars narrower lingually,

and less amount of the remaining tooth

structure than they did buccally, and, thus,

the less stiff could be expected of the

lingual cusps than the buccal cusps this

result came in agreement with (Bicalho et

al., 2014) found that when cuspal

deflection measured with strain gauge

sensor validated the finite element analysis

on the third molars the lingual cusps had a

significant higher cuspal deflection in

compared to the buccal cusp also reported

that the negative effect of the residual

polymerization shrinkage stress can

minimize through the combination of the

low-post-gel shrinkage composite resins

and incremental application techniques

that provides appropriately polymerized

the restoration, but without increasing in

the number of increment, with the

thickness of increment layer that is large

enough but not exceeding 2mm.

Moreover, the effect of the filling

techniques ( incremental and bulk-fill) was

proved by measuring the flexural modulus

of the tested composite resin materials.

The ( SDR surefil Plus) has the lowest

elastic modulus values, in (Tetric Evo

Ceram Bulk-fill) the moderate elastic

modulus and the highest elastic modulus in

( Filtek Z350XT). During the

polymerization process, the elastic

modulus of the material plays an important

role in the generation of polymerization

stress, it was found that the viscosity of the

resin base materials increase during the

polymerization reaction with the formation

of bonded cross-linked network

accompanied by a rapid increase in the

material rigidity, while the volumetric

changes of the materials occurred it was

restricted with the perimeter of the

surrounding tooth structure lead to

increase the stress in the internal structure

of the material (Shebl et al., 2018).

According to the present study the bulk-fill

composite resin has shown less

polymerization shrinkage stress than the

conventional composite resin, with ( SDR

surefil Plus) has the lowest cuspal

deflection and (FiltekZ350XT) the highest

cuspal deflection, the possible explanation

for the positive result of SDR surefill Plus

may be despite its contain a lower filler

volume with higher polymerization

contraction values, but the resin matrix

contains high molecular weight

polymerization modulators, with greater

flexibility of the urethane modulators

groups lead to more uniform network

formation with a delay in the rate of

contraction reaction which allows to

compensate the effect of the

polymerization contraction and result in

low stress. The low elastic modulus and

the low stress generated have a synergetic


effect in reducing the amount of cuspal

deflection. Conversely, (Tetric Evo Ceram

Bulk-fill) which shows moderate values

for the elastic modulus, though having a

high filler content, the Tetric Evo Ceram

Bulk-fill contains pre-polymerized fillers

(with a modulus of elasticity 10MPa) that

are included in the total filler amount

consequently lower fraction of inorganic

filler thus their effect increases the elastic

modulus is concordantly lower, these

fillers act as a ″shrinkage stress reliever"

improved the elasticity of the material

through keeping a chemical cushion

between the coarse filler particles.

FiltekZX350 showed higher stress because

of higher elastic modulus and higher post-

gel shrinkage (Ilie et al., 2013; Kim et al.,

2015). This finding came in agreement

with (Braga et al., 2018) that indicated the

composite resin materials with high elastic

modulus values and the polymerization

shrinkage restricted by bonding to the

cavity walls, deformed less when they are

stressed, and produced more rigid

restoration will result in higher shrinkage

stress.

As ( SDR surefil Plus) produced lower t

modulus of elasticity compared to (Tetric

Evo Ceram Bulk-fill), so the cuspal

deflection was lower due to

polymerization stress that generated by (

SDR sure fil Plus) is lower than (Tetric

Evo Ceram Bulk-fill) this could be

explained by the resin composite with a

higher elastic modulus has limited flow

generate higher polymerization stress. This

result came in conformed with (Olafsson

et al., 2017 and Jlekh and Abdul-Ameer,

2018 ) reported that the incrementally

placed conventional composite resin

resulted in greater cuspal deflection when

compared with bulk-fill resin-based

material and the amount of cuspal

deflection in the bulk-fill resin-based

material, depends on the type of composite

resin material, the lower cuspal deflection

attribute to that the new bulk-fill

composite resins are produced lower

polymerization shrinkage stress than those

of conventional composite resins through

the incorporation of "stress-reliever" to

change the dynamic of the polymerization

reaction using either; novel chemistry,

increase filler loading, decrease resin

matrix.

5. Conclusion

Bulk fill composite resin of all

groups (incremental and bulk-fill

application techniques) have shown

less cuspal deflection than the

conventional composite resin, with (

SDR surefill Plus, low viscosity

bulk-fill) having the lowest cuspal

deflection and (FiltekZ350XT) the

highest cuspal that generated by

polymerization shrinkage stress.

Although the incremental application

technique decreased the

polymerization shrinkage stress and

it's a complication associated with

cuspal deflection better than the

bulk-fill application technique in

comparing between the control

groups (conventional composite resin

material) but doesn't appear to offer

any advantages over bulk-fill

technique for both low stress (SDR

surefil Plus) and (Tetric Evo Ceram

Bulk-fill composites resin) bulk-fill

composite resin.

The result of the present study cuspal

deflection is primarily influenced by

materials' chemical composition and

properties rather than the type of the

application technique.

It is preferable always to use the

incremental application technique. A

significant number of composite

resins produced higher

polymerization shrinkage stress.


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