Effect of Temperatures on Polymerization Stress and Microleakage of Class V Composite Restorations

doi:10.4236/ojcm.2013.34011

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Open Journal of Composite Materials, 2013, 3, 107-112

http://dx.doi.org/10.4236/ojcm.2013.34011 Published Online October 2013 (http://www.scirp.org/journal/ojcm)

107

Effect of Temperatures on Polymerization Stress and

Microleakage of Class V Composite Restorations

Pavinee Padipatvuthikul Didron1*, Wojciech Chrzanowski2, Ayman Ellakwa3,4

1Department of General Dentistry, Srinakharinwirot University, Bangkok, Thailand; 2Faculty of Pharmacy, University of Sydney,

Sydney, Australia; 3Faculty of Dentistry, The University of Sydney, Sydney, Australia; 4Faculty of Dentistry, Tanta University, Tanta,

Egypt.

Email: *[email protected]

Received July 22nd, 2013; revised August 22nd, 2013; accepted August 31st, 2013

tion License, which permits unrestricted use, distribution, and reproduction in any me dium, prov ided the original work is properly cited.

ABSTRACT

The loss of interfacial integrity was identified as one of the major causes for replacement of resin com posite restorations.

Preheating procedure has been prov en to enhance flowability and ad aptation of resin composites and increase their de-

gree of conversion. The purpose of this study was to investigate polymerization contraction stress produced in resin

composites after preheating to 37˚C and 60˚C, and measure microleakage of Class V restorations restored with pre-

heated composites. Three resin composites (GC Kalore, Gradia Direct X, Filtek Supreme XT) at room te mpe rature, 3 7˚C,

and 60˚C were investigated. Maximum contraction stress of the composites (n = 5) was evaluated in a modified low-

compliance device. Samples were light-cured for 40 seconds and the maximum force was recorded during 15 minutes.

Calculations were done to adjust for the system’s compliance and obtain linear shrinkage values of composites. Data

were analyzed by Multivariated Analysis of Variance (MANOVA) and Tukey’s test for multiple comparisons (α =

0.05). Seventy-two Class V cavities were prepared on the buccal surfaces of extracted premolars and divided into 9

groups. The teeth were restored with composites at 3 temperatures and were thermo-cycled between 5˚C and 55˚C with

a one-minute dwell-time for 1000 cycles. The teeth were sealed with wax and nail vanish before placed in 0.5% tolu-

idine blue dye for 24 hours. The teeth were embedded in self-curing resin and sectioned bucco-lingually with a

slow-speed diamond saw, providing 3 sections per restoration. Microleakage was rated by two evaluators using a 0 - 4

ordinal scale at the occlusal and cervical margins under light microscope. Microleakage data were analyzed with

Kruskal-Wallis ANOVA and Mann-Whitney U test (α = 0.05). Results indicate that preheating co mposites to 37˚C and

60˚C significantly increased polymerization contraction stress of composites (p < 0.05). A significantly greater amount

of leak age wa s found a t the cerv ical ma rgins (p < 0.05). For all tested materials, preh eating compos ites to 60˚C resulted

in significantly less microleakage at the cervical margin.

Keywords: Preheating; Dental Composite; Polymerization Shrinkage; Contraction Stress; Microleakage

1. Introduction

Improvements in resin composites’ mechanical proper-

ties and their reduced polymerization shrinkage during

the past decade encouraged clinicians to use resin com-

posites more frequently for posterior restorations. Ma-

jorities of improvements aimed to improve microstruc-

ture of the material including monomer composition, size,

shape, and distribution of inorganic filler particles and

targeted mainly at increasing the filler load of resin

composites. However, increasing the filler load resulted

in higher viscosity and led to concerns about handling,

packing, and adaptation of the material. Many attempts

were made to enhance composite adaptation and decrease

microleakage between composites and dental cavity, ei-

ther by using the flowable composite as a base material,

chemical and laser treatments of dentin or by preheating

the composites to lower their viscosity.

The effect of lower viscosity in improvements of adap-

tation has been proven important. Th is is the primary ba-

sis for producing flowable resin composites in which the

lower viscosity can be achieved by decreasing their filler

contents and making changes in the matrix chemistry,

which severely reduce their physical properties. Many

polymers exhibit lower viscosity when they are heated

because thermal vibrations force the composite monomer

further apart and allow them to slide by each other more

*Corresponding a uthor.

Effect of Temperatures on Polymerization Stress and Microleakage of Class V Composite Restorations

108

readily. This property leads to a concept of warming or

preheating composite resins before photopolymeriza-

tion, which will decrease the viscosity and increase flow

of resin composites. In addition, composites cured at

elevated temperatures have been proven to increase po-

lymerization rate and have a higher degree of conversion

[1-3], which could result in improved mechanical proper-

ties [4]. Also, it has been reported that increasing com-

posite temperature up to 60 ˚C might enhance the conver-

sion degree on the top and in 2 mm of the bottom sur-

faces [2].

However, it has been reported that the increased de-

gree of conversion associated with preheating would re-

sult in increased polymerization sh rinkage [5]. One study

has shown that temperature has a significant effect on

polymerization shrinkage of microf illed composites. Pre-

heating composites to relatively high temperatures (54˚C

or 68˚C) causes a significant increase in volumetric

shrinkage, but preheating composites to a body tempera-

ture causes similar shrinkage to that at room temperature

[6]. Advantages of preheating the resin are to make more

durable, highly filled, highly viscous conventional com-

posite resin, to reduce the viscosity, to provide flow val-

ues that are similar to those of less filled, flowable resin

composites, without undermining the mechanical proper-

ties. However, more investigation is necessary to deter-

mine the si de effects of preh eating procedu res such as an

increase of the polymerization shrinkage which causes

stress at the tooth/restoration interface and may cause

microleakage of the restorations. The main objective of

this study was to evaluate the effect of preheating on

polymerization contraction stress of composites and the

in vitro marginal microleakage. It was hypothesized that

increasing the preheating temp erature would in crease the

polymerization stress and increase the microleakage at

resin composites/tooth in terfaces.

2. Materials and Methods

2.1. Polymerization Contraction Stress

Measurement

The materials used in this study are indicated in Table 1.

Polymerization contraction stress was measured by a

modified low-complicance device (Figure 1), consisting

of 2 parts; the first part is a load cell, which a brass steel

piston was attached. A brass steel piston (10 mm diame-

ter; 30 mm length) was used as the bonding substrate for

the composite. The piston had one surface abraded with

#180-grit sandpaper, coated with silane coupling agent

prior to the application of a thin layer of unfilled resin

(Adper™ Single Bond 2), and light-cured for 20 seconds

(MiniLED, Satelec, France). A ringshape teflon mold

was inserted at the end of the brass piston, created a cy-

lindrical cavity (dimension 10 mm diameter × 1 mm

Table 1. Materials used in the present study, their respec-

tive batch number and manufacture r.

Material Batch number

Manufacturer

Filtek™ Supreme XTN151598 3M ESPE, Dental Products,

St. Paul MN

GC KALORE™ 910071 GC Corporation,

Tokyo, Japan

Gradia™ Direct X 1201271 GC Corporation, Tok yo,

Japan

Adper™ Single Bond 2N283944 3M ESPE, Dental Products,

St. Paul MN

Scotchbond™ EtchantN287300 3M ESPE, Dental Products,

St. Paul MN

Figure 1. The test setup for the polymerization stress meas-

urements.

thickness) for resin composite. Uncured resin composite

was preheated and inserted in the mold, the mold was

then removed. The brass piston with the composite was

heated up to specific test temperature on a thermal con-

trolled plate, a thermocouple was placed inside a hole in

the brass piston at all time to measure the composite tem-

perature.

The second part is a brass cylinder, holding a clear

perspex disc (22 mm diameter; 1 mm thickness). The

brass cylinder has a slot which allowed for the placement

of the light guide of a curing unit in contact with the

perspex disc. The top surface of perspex disc was abrad-

ed with #180-grit sandpaper, cleaned, coated with a thin

layer of unfilled resin (Adper™ Single Bond 2), and

light-cured for 20 sec. When the desire temperature was

reached, the brass piston was fixed to the load cell. The

brass piston was driven down to just touched the perspex

surface. The composite was photo-activated through the

clear perspex disc. As the composite polymerized, con-

traction force was followed for 15 min. The force values

were converted to nominal stress by dividing them by the

cross-sectional area of the specimen (78.5 mm2). Maxi-

mum contraction stress (Smax) was subjected to statisti-

cal analysis.

Effect of Temperatures on Polymerization Stress and Microleakage of Class V Composite Restorations

109

Data were analyzed by Multivariated Analysis of Va-

riance (MANOVA), entering resin composite and tem-

perature as main factors, and Tukey’s test for multiple

comparisons (α = 0.05).

2.2. Microleakage Test

2.2.1. Tooth Selection

Seventy-two extracted caries and restoration-free perma-

nent human premolars were selected, remaining soft tis-

sue removed, and stored in deionized water for a maxi-

mum duration of 4 weeks. The teeth were cleaned with

slurry of pumice and water, rinsed thoroughly with tap

water, and then examined macroscopically with mag-

nification for defects in the enamel and dentin.

2.2.2. Sample Prepa ration

The teeth were randomly assigned into nine groups of

eight. A Class V preparation was made in the buccal sur-

face of each tooth. The occlusal margin of the cavities

were in enamel and the gingival margins located 1.5 mm

apical to the cemento-enamel junction. Preparations were

made with a 329 carbide bur in a high-speed handpiece

equipped with water spray. Cavity dimensions were stan-

dardized (5.0 mm in width, 3.0 mm in height, and 2 mm

in depth). For all groups, enamel and dentin are etched

with 35% phosphoric acid gel (Scotchbond™ etchant,

3M Dental Products, MN, USA) for 20 s and 15 s re-

spectively, rinsed for 20 s, and air-dried to obtain chalky-

white appearance enamel and moist dentin. All cavities

are treated with a resin-based adhesive system. The ad-

hesive used in this study is Adper™ Single Bond 2 (3M

ESPE, St. Paul, MN, USA). Wet-bonding technique is

followed as recommended by the manufacturer, moist

dentin was clinically evidenced by a uniform shiny sur-

face on which water was not pooled. A fully saturated

brush tip for each coat is used, applying two consecutive

coats of Adper™ Single Bond 2 adhesive to prepared

enamel and dentin. Later, the surface was dried gently for

5s and light cured for 20 s (miniLED, Satelec, France).

The cavities were restored as follow:

Group 1: restored with a room temperature Filtek Z350

XT;

Group 2: restored with a preheated (37˚C) Filtek Z350

XT;

Group 3: restored with a preheated (60˚C) Filtek Z350

XT;

Group 4: re stored wi th a room temperature GC Kalore;

Group 5: restor ed wi t h a prehe a te d (3 7˚C) GC Kalore;

Group 6: restor ed wi t h a prehe a te d (6 0˚C) GC Kalore;

Group 7: restored with a room temperature Gradia di-

rect X;

Group 8: restored with a preheated (37˚C) Gradia di-

rect X;

Group 9: restored with a preheated (60˚C) Gradia di-

rect X.

All restorations were done two increments with the

first against the gingival wall, and light-cured for 20 s.

Excess materials are removed with a No.170 bur, fol-

lowed by finishing and polishing with the Softlex disk

system (3M Dental Products Division, St. Paul, MN,

USA). The restored teeth are stored in deionized water at

37˚C for 1 da y before f urther treat ment.

All samples are thermocycled for 1000 cycles between

5 and 55˚C with a dwell time of 1 min, before immersion

in dye. The apices of the teeth are sealed with blue wax

and coated with a nail polish 1 mm short of the restora-

tion margins in order to reduce other leakage elsewhere

that could lead to false positive results. The teeth were

immersed in 0.5% toluidine-blue solution for 24 hour at

room temperature. The superficial dye is removed with a

pumice slurry and rubber cup after removal of the speci-

mens from the dye solution. Teeth are then mounted in a

cold-cure epoxy resin (Leco®, Leco Corporation, MI,

USA) to facilitate handling during sectioning.

2.2.3. Mi croleakag e Test

To measure the extent of microleakage, the teeth were

sectioned longitudinally through the restorations in a

bucco-lingual direction with a low speed diamond saw

(IsoMet™, Buehler Ltd., Lake Bluff, IL, USA), provid-

ing 3 sections per restoration. The sectioned teeth were

evaluated with a stereomicroscope (Leica MZ8, Leica

Microscopy System Ltd., Heerbrugg, Switzerland) at 20

× magnification. The degree of microleakage determined

through dye penetration was scored according to stan-

d ardized criteria (0 to 4; Tab le 2, Figure 2). Double blind-

ed evaluators measured the slices and then the Kappa test

was performed. Differences in the frequency distribution

of scores between groups were assessed using the Krus-

kal-Wallis test and assessments within the groups were

assessed using the Mann-Whitney U test. The results of

testing were analyzed with statistical software (IBM®

SPSS® Statistics Base 21, IBM, USA). Significance is

considered at the 0.05 level.

3. Results

Average polymerization contraction stress of composites

at room temperature and when preheated to 37˚C and

60˚C are shown in Table 3 and Figure 3. Figure 4

shows trend lin es of the polymerization contraction force

during 0 - 60 second. Results indicate that preheating

composites to 37˚C and 60˚C significantly increased po-

lymerization contraction stress and the developmental

rate of polymerization contraction force (p < 0.05). Sta-

tistically greater amount of leakage was found at the cer-

vical margins compared to the occlusal margins (p < 0.05)

for the group restored with composites at room tempera-

ture and the group restored with composites at 37˚C. No

Effect of Temperatures on Polymerization Stress and Microleakage of Class V Composite Restorations

110

Table 2. Microleakage scoring criteria.

0 No dye penetration

1 Dye pe netration up to one-half of the cavity wall

2 Dye penetration up to total cavity wall

3 Dye penetration up to one-half of the axial wall

4 Dye penetration more than one-half of the axial wall

Table 3. Average polymerization contraction stress of com-

posites at room temperature and when preheated to 37˚C

and 60˚C.

Materials Room Temp37˚C 60˚C

GC Kalore 4.18 ± 0.02 8.10 ± 0.07 9.85 ± 0.04

Gradia Direct X 5.61 ± 0.02 8.62 ± 0.03 10.72 ± 0.05

Filtek Supreme XT 11.83 ± 0.0114.21 ± 0.05 16.39 ± 0. 05

Figure 2. Diagram of Class V cavity and microleakage scor-

ing.

statistical significant differences were observed among

materials and temperatures at the occlusal margin. In

contrast, for all materials tested, the 60˚C preheated sam-

ples showed statistically lower microleakage at the cer-

vical margin. In fact, there was no microleakage ob-

served at the cervical margin for the group restored with

composite preheated to 60˚C.

The polymerization contraction stress results were

analyzed using General Linear Model, Multivariated Ana-

lysis of Variance (MANOVA), Tukey’s and Scheffe’s

Post Hoc test. Highly significant differences were found

both between materials and between temperatures (p <

0.01).

The frequency distribution of different degrees of mi-

croleakage in the groups is shown in Table 4. There were

significant differences between the microleakage scores

for the enamel and dentin (p < 0.05). Less microleakage

was observed at the occlusal margins than at the cerv ical

margins. Only 2 restorations showed microleakage at the

occlusal margins. There were no significant differences

between materials and temperatures at the occlusal mar-

gins (p > 0.05). However, at the cervical margins, there

Figure 3. Polymerization contraction stress (MPa) of com-

posites at different temperatures.

Figure 4. Polymerization contraction force during 0 - 60

second.

Effect of Temperatures on Polymerization Stress and Microleakage of Class V Composite Restorations

111

Table 4. Frequency distribution of micr oleakage for each experimental group.

Occlusal Gingival

Group 0 1 2 3 4 0 1 2 3 4

Group 1: Filtek Supreme (control) 7 1 0 0 0 4 3 0 1 0

Group 2: Filtek Supreme (37˚C) 8 0 0 0 0 5 3 0 0 0

Group 3: Filtek Supreme (60˚C) 8 0 0 0 0 8 0 0 0 0

Group 4: GC Kalore (control) 8 0 0 0 0 5 2 0 1 0

Group 2: GC Kalore (37˚C) 8 0 0 0 0 6 2 0 0 0

Group 3: GC Kalore (60˚C) 8 0 0 0 0 8 0 0 0 0

Group 5: Gradia Direct X (control) 7 1 0 0 0 5 3 0 0 0

Group 6: Gradia Direct X (37˚C) 8 0 0 0 0 6 1 0 0 1

Group 7: Gradia Direct X (60˚C) 8 0 0 0 0 8 0 0 0 0

were significant differences among the preheated groups

(p < 0.05). Preheat composites to 60˚C significantly re-

duced the degree of cervical leakage in all materials

tested. In fact, no microleakage was observed at the cer-

vical margin in this group.

Figure 5 show representative stereomicroscopic im-

ages of the samples with and without cervical microleak-

age.

4. Discussion

The objective of this experimental study was to evaluate

the effect of preheating temperatures on polymerization

contraction stress of composites and to investigate the

extent of in vitro marginal microleakage of Class V resin

composite restoration restored with preheated composites

as compare to the cavity restored with room temperature

composites. The extent of leakage after thermal cycling

is relevant to clinical practice since microleakage of sa-

liva, oral fluids and bacteria at the tooth-restoration in-

terface has been linked causally to a range of problems,

including marginal staining, postoperative sensitivity,

and secondary caries. The integrity and durability of the

marginal seal is an important factor in the longevity of

adhesive dental restorative materials, particularly for com-

posite resins. The absence of a seal at restoration margins

permits the entry of oral bacteria and fluids, which can

result in postoperative sensitivity, adverse pulpal re-

sponses and recurrent caries [7]. In the present study,

standardized methods were used to minimize confound-

ing factors. The dye penetration test is the most widely

used laboratory meth od for assessing leakage, and fo r the

purposes of this study a simple grading system was used

[8]. The apical extent of the test cavities was intention-

ally placed into the root surface because leakage at this

site is known to be a clinical concern when Class II and

Class V cavities are restored with composite resin mate-

rials.

The results show that preheating composites signifi-

(a) (b)

Figure 5. Representative stereo-microscopic images of the

cavity with microleakage (a) and without cervical micro-

leakage (b).

cantly increased polymerization contraction stress of

composites and for all materials tested, preheating to

60˚C resulted in significant reduction of microleakage at

the cervical margin. This is agreed with Fróes-Salgado et

al. who evaluated the effect of pre-heating on marginal

adaptation, monomer conversion, flexural strength, mi-

crohardness, and polymer cross-linking of a resin com-

posite under a non-isothermal condition. They found th at

under non-isothermal conditions (similar to a clinical

situation) preheating composite to 68˚C did not improve

the degree of conversion, flexural strength or polymer

crosslinking, but yielded enhanced marginal adaptation

[9]. Wagner et al. also found that preheating composites

can improve adaptation of resin composites to tooth

structure and significantly reduced microleakage, al-

though delay of light curing after placement appears to

be counterproductive and diminishes the positive effects

from the preheating treatment [10]. They also concluded

that flowable liner was less effective than preheating the

composite in reducing microleakage. Interestingly, the

previous expectation that composite preheating could

worsen the marginal adaptatio n of composites to th e cav-

ity walls due to the increased conversion rates [1,2] and

consequently increased polymerization shrinkage [11,12]

Effect of Temperatures on Polymerization Stress and Microleakage of Class V Composite Restorations

112

was not confirmed by the results o f the present inv estiga-

tion. The results shows that preheating composite to high

temperatures lead to an increase in polymerization con-

traction stress but less microleakage at the cervical mar-

gin. This is in contrast with several studies that have

shown a direct relationship between contraction stress

and marginal leakage in resin composite restorations

[13-15]. This was also expected because resin compos-

ites exhibit a six to eight ti mes greater thermal expansion

than the surrounding tooth structures [9,16], polymeriza-

tion shrinkage along with thermal contraction might cre-

ate high interfacial stresses in preheated composites upon

thermal equilibrium, with detrimental effects on marginal

adaptation, integrity and seal [13]. Further investigation

is therefore needed regarding advantages and disadvan-

tages of dental composite preheating before we can make

a conclusion if this method is appropriate in dental prac-

tice.

5. Conclusion

The present finding suggests that in the challenging

situation of the cervical restoration which extends onto

the root surface, preheating composites to 60˚C signifi-

cantly reduced microleakage at the tooth-restoration in-

terfaces. Preheating however results in non-desirable

increase of polymerization contraction stress. More in-

vestigation should be done regarding the consequence of

the increased stress at the tooth restoration interfaces to

the strength of the restored tooth.

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