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Operative Dentistry, 2011, 36-3, 326-334

Comparison of Premolar Cuspal Deflection in Bulk or in Incremental Composite Restoration Methods ME Kim  SH Park

Clinical Relevance For class II composite restorations, choosing composites with a low shrinkage value and elastic modulus, applying an incremental filling technique of more than three layers, and polymerizing them with sufficient curing time are needed to reduce the cuspal deflection, which is induced by the polymerization shrinkage stress of composites.

SUMMARY Objectives: This study examined the cuspal deflection of maxillary premolars when either a bulk filling or incremental filling technique was employed using a range of composites with different elastic moduli. Methods: Four brands of composite materials, Heliomolar (HM, Ivoclar Vivadent, Schaan, Liechtenstein), Heliomolar HB (HH, Ivoclar Vivadent, Schaan, Liechtenstein), Filtec Supreme XT (FS, 3M Dental Product, St Paul, Myeung-Eun Kim, DDS, PhD, Department of Conservative Dentistry, Yonsei University, Seoul, Korea *Sung-Ho Park, DDS, PhD, professor, Department of Conservative Dentistry, Yonsei University, Seoul, Korea *Corresponding author: Department of Conservative Dentistry, Yonsei University, 134, Shinchon-Dong, Seodaemun-Gu, Seoul, Korea, 120–752; e-mail: sunghopark@ yuhs.ac DOI: 10.2341/10-315-L

MN, USA), and Renew (RN, Bisco Inc, Schaumburg, IL, USA), as well as three filling techniques, bulk filling, two-layer incremental filling, and three-layer incremental filling methods, were used. One hundred twenty caries-free human premolars were collected and divided into four groups according to the filling material used. Each of these four groups was then subdivided into three groups according to filling method. In group 1, a bulk filling of 0.15 g of each resin was inserted and lightcured with LED light from the occlusal, mesial, and distal surfaces for 60 seconds each. Group 2 was given two horizontal increments, 0.08 g and 0.07 g, with each increment light-cured from the occlusal, mesial, and distal surfaces for 30 seconds each. In group 3, three horizontal increments of 0.05 g were used, each of which was light-cured from the occlusal, mesial, and distal surfaces for 20 seconds each. The cuspal deflection was measured using a cus-

Kim & Park: Cuspal Deflection in Bulk or Incremental Methods

tomized cuspal deflection measuring machine for 10 minutes after initiating light polymerization. The elastic modulus of each composite resin material was measured using a threepoint bending test. Two-way analysis of variance (ANOVA) with a Dunnet test was used to examine the effect of the two variables (curing methods, materials) on the amount of cuspal deflection at the 95% confidence level. In each material, groups 1, 2 and 3 were compared using one-way ANOVA and a Dunnet test at the 95% confidence level. The elastic moduli of HM, HH, FS, and RN were compared using one-way ANOVA and a Tukey test at the 95% confidence level. The relationship between the amount of cuspal deflection in each group and the elastic modulus of the composite was analyzed using a Pearson correlation test. Results: The amount of cuspal deflection in HH was larger than in the other materials (HM, FS, and RN; p,0.05). There was no significant difference between HM, FS, and RN. The amount of cuspal deflection was greatest in group 1, followed in order by groups 2 and 3 (p,0.05). The amount of cuspal deflection was in the following order: group 123 in HM, and 1.2, 3 in HH, FS, and RN. The elastic modulus was HH.RN.FS.HM (p,0.05). There was a positive correlation between the cuspal deflection and the elastic modulus of the composite. Conclusions: The incremental filling techniques reduced the amount of cuspal deflection in all composite groups with different elastic moduli. The amount of cuspal deflection showed a positive correlation with the elastic modulus of the composite. INTRODUCTION Resin-based dental composite materials exhibit a 1%-3% decrease in volume during polymerization.1,2 Polymerization shrinkage stresses can initiate failure of the composite-tooth interface, which can cause postoperative sensitivity, microcracks, microleakage, and secondary dental caries.3 It was reported that placing a composite into class II cavities can lead to the inward deformation of the cusp, with the amount of deformation varying from 15 to 50 lm.4-15 Many factors affect the level of cuspal deflection, such as the size and shape of the cavity,16,17 Young’s modulus of the composite resin,18 amount of polymerization shrinkage,8,11 use of an intermediary flowable liner,10 type of curing light,12 and placement techniques.9,15,19-22

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Although the incremental filling technique is preferred over the bulk filling method, it is unclear if the incremental filling technique can reduce the amount of cuspal deflection compared with the bulk filling technique. Some studies reported that the amount of cuspal deflection was reduced when the cavities were restored with a composite placed in multiple but small increments,3,13,15,19 whereas others claimed that there was no evidence of incremental filling with an advantage in cuspal deflection over bulk filling.9,20,23-25 Lee and others13 reported that incremental filling and indirect restoration decreased the amount of cuspal deflection by 34% and 32.2%, respectively, compared with a bulk filling. Park and others15, in their in vitro study using an aluminum block to minimize the substrate variations inherent with the use of natural teeth, reported that the bulk filling technique produced significantly more cuspal deflection than the incremental techniques, whereas there was no significant difference between the horizontal and oblique incremental methods. This should be confirmed using a tooth substrate. On the other hand, Versluis and others23, in their finite element study, reported that incremental filling techniques increased the amount of cuspal deflection more than the bulk filling technique. This was attributed to the incremental deformation of the preparation, which effectively decreases the total amount of composite needed to fill the cavity. The finite element method can integrate the parameters for geometry and material properties but cannot simulate the transitional change in resin flow during polymerization precisely.23 Abbas and others,9 in their in vitro study using premolars, reported that cuspal deflection was increased significantly with an incremental cure compared with a bulk cure but the level of microleakage was higher in the bulk cure. They suggested that the bulk cure regimen induced an incomplete cure that resulted in low cuspal deflection and higher microleakage. It is important to determine whether the lower cuspal deflection in bulk cure is due to an incomplete cure or to another reason, as Versluis and others23 indicated. If it is due to incomplete cure, it would have different results when the curing time is extended or curing lights with a high-power density are applied. Another issue in cuspal deflection is the elastic modulus of the composites. Ausiello and others,18,26 in their finite element method study, reported that the cusp displacement was affected by the elastic modulus of the composites. They reported that a less rigid restoration can reflex the applied stress

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through greater elastic deformation, which is transferred to the lower levels of cusp deformation. However, Lee and Park,27 in their in vitro study in which the amount of polymerization shrinkage and cuspal deflection was measured using composites with a known elastic modulus and filler content, reported that even flexible composites could lead to greater cuspal deflection than rigid composites. They suggested that this was possible because both the amount of polymerization shrinkage and elastic modulus appeared to affect the cuspal deflection. The incorporation of less inorganic fillers and flexible monomers into the composites could help decrease the elastic modulus but might increase the amount of polymerization shrinkage significantly. Therefore, selecting composites with different elastic moduli but a similar amount of shrinkage would be needed to examine the influence of the elastic modulus on cuspal deflection. The cuspal deflection might be affected differently in composites with different moduli when a bulk filling or incremental filling technique is applied. Rees and others25 and Hyun and Park20 reported no difference in cuspal deflection between the bulk and incrementally filled composites. Interestingly, they used Heliomolar as a filling material, which has a low elastic modulus.28 A comparison of the cuspal deflection of a bulk filled or incrementally filled tooth in which composites with different elastic moduli are used might provide an answer to this controversy. This study compared the cuspal deflection of the maxillary premolars when a bulk filling and incremental filling technique were employed using composites with different elastic moduli. In this study, sufficient light-curing time was applied to prevent incomplete cure of the composites. In addition, composites with a similar amount of polymerization shrinkage but different elastic moduli were selected.

Table 1: Restorative Materials Used in This Study Materials

Code

Manufacturer

Volumetric shrinkage (%)

Heliomolar

HM

Ivoclar Vivadent, Schaan, Liechtenstein

3.2

Heliomolar HB

HH

Ivoclar Vivadent, Schaan, Liechtenstein

3.3

Filtec supreme XT

FS

3M Dental Products, St Paul, Minn, USA

2.8

Renew

RN

Bisco Inc, Schaumburg, IL, USA

3.1

the filling method: group 1, bulk placement; group 2, horizontal placement of two layers; and group 3, horizontal placement of three layers (Table 2). For all groups and subgroups, there was no significant difference in the dimensions of the tooth specimens. Preparation of a Modified MOD Cavity and Adhesive Application Before preparing the teeth, an outline of the cavity was drawn with a lead pencil, and a parallel-sided mesio-occlusodistal (MOD) cavity, 3.5 mm in width and 3 mm in depth, without buccal or lingual extension was prepared using diamond burs with water spray cooling (Figure 1). The dimensions were confirmed using a prefabricated hexahedral resin block, which had the same size as the cavity dimensions. The block was placed into the cavity, and the preparation was adjusted until it fit the prepared cavity.

MATERIALS AND METHODS One hundred twenty intact human maxillary premolars were collected after extraction and stored immediately in a saline solution. The dimensions of the teeth were strictly controlled: the buccolingual dimension was between 9.6 and 9.7 mm, and the mesiodistal dimension was between 7.6 and 7.7 mm. From the internal data bank, which records the volumetric shrinkage of composite resins that have been previously measured using AcuVol (Bisco Inc, Schaumburg, IL, USA), four different composites with an A2 shade and ,0.5% difference in volumetric shrinkage were selected (Table 1). Each material group was subdivided into three groups according to

Table 2:

Cavity-Filling Methods and Curing Times in Each Group Method

Composite, g

Curing time, s

Group 1

Bulk filling

0.15

60þ60þ60

Group 2

Two-layer increments

0.8þ0.7

(30þ30þ30) þ(30þ30þ30)

Group 3

Three-layer increments

0.5þ0.5þ0.5

(20þ20þ20) þ(20þ20þ20) þ(20þ20þ20)

Kim & Park: Cuspal Deflection in Bulk or Incremental Methods

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two measuring pins in contact with the cusp tips of the buccal and lingual surfaces. The point where the pin was positioned on the tooth surface was controlled through the specimens. To minimize any tooth mobility, a specimen stabilizer made from putty impression material was used to sustain the specimen (Figure 3).

Figure 1. Schematic diagram of the parallel-sided, tunnel-shaped MOD cavity.

The cavities were flushed with copious amounts of water, and the enamel was then etched with 37% phosphoric acid (Total Etch, Ivoclar Vivadent, Schaan, Liechtenstein), rinsed with water, and dried completely. AdheSE (Ivoclar Vivadent) was then applied according to the manufacturer’s instructions and lightcured for 20 seconds using a Bluephase LED curing light (Ivoclar Vivadent). Measurement of Cuspal Deflection The specimen was positioned in a custom-made cuspal deflection measuring system (Figure 2) (R&B Inc, Daejon, Korea) using the screw and pin in the system. The system was designed to detect any cusp deflection during polymerization from the

Figure 2. Schematic diagram of the custom-made cuspal deflection measuring system.

In group 1, a 0.15-g bulk filling was used. The amount of resin material was weighed on an electronic balance and placed into the cavities (Table 2). Before light-curing, the initial distance sensed by the two crossheads was set to the baseline value of 0. All specimens in group 1 were light-cured from the occlusal, mesial, and distal surfaces for 60 seconds each using the LED curing light (Bluephase, Ivoclar Vivadent) with a power density of 900 mW/cm2, when measured using a Coltolux Light Meter (Colte´ne, Altsta¨tten, Switzerland), making a total curing time of 180 seconds. The tip of the curing light was kept within 2 mm of the tooth specimen. As light-curing began, the degree of inward cuspal movement was measured and recorded using the system. The inward cuspal movement altered the position of the measuring pin and floating lever, which was detected using a linear scale sensor (Lie 5, Numeric Jena Gmbh, Jena, Germany). The data were stored in a computer simultaneously every 0.5 seconds for 10 minutes (Table 2). In group 2, two separate increments, 0.08 g and 0.07 g, were placed into the cavity. Each layer was light-cured from the occlusal, mesial, and distal surfaces for 30 seconds, resulting in a total curing time of 180 seconds (Table 2). In group 3, each increment was light-cured at the occlusal, mesial,

Figure 3. Specimen placed in the cuspal deflection measuring system.

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and distal surfaces for 20 seconds. Therefore, the total curing time was 180 seconds in all three groups (Table 2). In groups 2 and 3, the cuspal deflection was measured in the same way as reported for group 1. In groups 2 and 3, the cuspal deflection measurement was continued without stopping while the second or third fillings were placed on the top of the previous filling. Elastic Modulus Measurement A test specimen, 253232 mm in size, was prepared using a stainless-steel mold according to ISO 4049. The flexural strength test apparatus was calibrated to provide a constant crosshead speed of 0.75 mm/ min. The apparatus consisted of two rods (2 mm in diameter) mounted parallel to each other with a 20mm distance between centers. A third rod (2 mm in diameter) was centered between and parallel to the other two so that the three rods in combination could be used to give a three-point reference to the specimen. The displacement of the resin specimen at a 10N load was measured. The elastic modulus was calculated using the following formula: E¼

F1l3 ; 4bh3 d

where E is the elastic modulus, F1 is the load (in Newtons) at a convenient point (10N) in the straightline portion of the tract, d is the deflection (in millimeters) at load F1, l is the distance (in millimeters) between the supports (20 mm), b is the width (in millimeters) of the specimen measured immediately before testing, and h is the height (in millimeters) of the specimen measured immediately before testing. Statistical Analysis The amount of the cuspal deflection in the four filling materials and three curing methods were analyzed by two-way analysis of variance (ANOVA) with a Dunnet test at the 95% confidence level. The amount of cuspal deflection in the three curing methods was compared using one-way ANOVA with a Tukey test in each composite material at the 95% confidence level. The correlation between the amount of cuspal deflection and the elastic modulus of the composite was analyzed using a Pearson correlation test. RESULTS Table 3 summarizes the amount of cuspal deflection. The amount of cuspal deflection differed according to the material and curing method used (p,0.05).

Table 3: Mean Value of Cuspal Deflection (lm) (n¼10)a Group 1

Group 2

Group 3

Heliomolar

14.56 6 1.52a 12.42 6 1.82ab 10.41 6 1.97b

Heliomolar HB

19.33 6 3.48a

16.17 6 1.37b

14.33 6 1.92b

Filtec supreme 15.22 6 1.49a XT

12.45 6 1.07b

11.58 6 2.27b

Renew

12.02 6 2.37b

10.33 6 1.65b

14.43 6 0.56a

a Small letters a, b beside figures represent the results of statistical analysis of one-way analysis of variance with a Tukey test in each material. a and b are different at the p¼0.05 level.

There was no correlation between the materials and curing methods (Table 4). The Dunnet test revealed the amount of cuspal deflection in Heliomolar HB (HH, Ivoclar Vivadent, Schaan, Liechtenstein) to be larger than the other materials, Heliomolar (HM, Ivoclar Vivadent), Filtec Supreme XT (FS, 3M Dental Product, St Paul, MN, USA), and Renew (RN, Bisco Inc); p,0.05). There was no significant difference between HM, FS, and RN. The amount of cuspal deflection was in the following order: group 1.group 2.group 3 (p,0.05). The amount of cuspal deflection was group 123 in HM and 1.2, 3 in HH, FS, and RN (p,0.05; Table 3). Figure 4 (a-d) shows the amount of cuspal deflection as a function of time. The elastic moduli listed in Table 5 was HH.RN.FS.HM (p,0.05). Statistical analysis revealed a positive correlation between the cuspal deflection and elastic modulus of the composite. The Pearson correlation coefficient in groups 1, 2, and 3 was 0.619, 0.665, and 0.528, respectively. DISCUSSION In this study, the cuspal deflection of the bulk or incrementally filled cavity ranged from 10.33 to 19.33 lm, which is similar to that found in other studies.8,21,25,29 The cuspal deflection observed was in the order of group 1.group 2.group 3. There was no correlation between the materials and curing methods. These results suggest that the reported lower cuspal deflection in the bulk cure compared with the incremental cure appears to be due to incomplete cure of the composites. In a light-curing composite, most of the polymerization shrinkage occurs in a short period of time,

Kim & Park: Cuspal Deflection in Bulk or Incremental Methods

Table 4:

331

Results of Two-Way Analysis of Variance

Source Corrected model

df

Type III sum of squares

F

Significance

11

67.501

14.264

0.000

22,222.136

1

22,222.136

4695.995

0.000

Material

371.934

3

123.978

26.199

0.000

Method

362.830

2

181.415

38.337

0.000

7.742

6

1.290

.273

0.949

Error

511.072

108

4.732

Total

23,475.714

120

1253.578

119

Intercept

Material3method

Corrected total

742.506(a)

Mean square

particularly with a high–power-density curing light, where approximately 85%-90% of polymerization shrinkage occurs within 20 seconds of light curing, even though a slight increase continues.8,30,31 However, Figure 4 shows that cuspal deflection was much slower and longer than polymerization shrinkage of the composites. During the dark polymerization period, more cuspal deflection may be induced, even though the absolute amount of polymerization shrinkage was smaller because the material is stiffer in this period than in the light-curing period.8 It is also possible that the remaining tooth structure appears to resist flexure in the early phase of the polymerization process.8 Versluis and others23 assumed that the reason for the greater cuspal deflection in an incrementally filled cavity versus a bulk filled cavity was due to the incremental deformation of the preparation, which effectively decreases the total amount of composite required to fill the cavity. However, considering the speed of cuspal deflection in the present study, the amount of incremental deformation would be so limited as to have very limited influence. In this study, the cuspal deflection of the incrementally filled group was lower than that of the bulk filled group in all composite groups with different elastic moduli. However, there appeared to be some differences according to the elastic moduli. In HM, which had the lowest elastic modulus, the cuspal deflection was group 123, whereas it was group 1.2, 3 in HH, FS, and RN. Interestingly, there was

no significant difference in cuspal deflection between the two layer incremental techniques and the bulk placement technique in HM, which is consistent with the results reported by Hyun and Park. 20 A significant difference was only found between the bulk placement and three layer incremental placement technique. More increments might be needed in HM on account of its lower elastic modulus than the other materials. HH, which has the highest elastic modulus, showed the largest cuspal deflection. Heliomolar HB falls into the category of what is known as a packable or condensable resin composite. This material is less sticky due to a slight modification of the proportional composition of the monomer mixture. A comparison of the amount of cuspal deflection between groups 1, 2, and 3 revealed a larger difference between groups 1 and 2 than between groups 2 and 3 in all materials. In particular, in HH, FS, and RN, there was no significant difference in the amount of cuspal deflection between groups 2 and 3, even though the value was slightly lower in group 3 (Table 3; Figure 4). Based on this limited information, it is possible that the amount of cuspal deflection did not decrease lineally with the number of increments, and the effect appears to decrease with the increments. However, more research will be needed. The Pearson correlation test revealed a positive correlation between the elastic modulus of the

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332

Figure 4. Change in the amount of cuspal deflection as a function of time in (a) HM, (b) HH, (c) FS, and (d) RN.

composite and amount of cuspal deflection. HH, which had the highest elastic modulus, showed a greater amount of cuspal deflection than the other materials. However, the Pearson correlation coefficient was in the moderate range. More factors, such as the amount of polymerization shrinkage, should be considered in a future study to develop a study Table 5:

Result of Elastic Modulusa F1, N

D, mm

E, GPa

Heliomolar

10

0.44 6 0.03

2.81 6 0.23a

Heliomolar HB

10

0.16 6 0.01

7.76 6 1.00d

Filtec supreme XT

10

0.34 6 0.03

3.64 6 0.41b

Renew

10

0.29 6 0.02

4.32 6 0.37c

a

a, b, c, and d are different at the p¼0.05 level.

model with greater accuracy. In this study, as the four different composites showed a ,0.5% difference in volumetric shrinkage, its effects on cuspal deflection were minimized. Figure 4 shows that there was a slight increase in cuspal deflection in all groups when the light was turned off. For example, there was an increase in the graph at 180 seconds in group 1 in all composites. The light-curing process increased the temperature in the tooth by more than 108C when high-powerdensity curing light was used.32,33 Considering that the linear coefficient of thermal expansion for the tooth and composites is 8.3-11.4 PPM/8C and 26.543.4 PPM/8C, respectively,34 several additional micrometers of linear contraction might have occurred in the tooth and composites when the light was turned off after 180 seconds of light activation.8 CONCLUSION The incremental filling techniques reduced the amount of cuspal deflection in all composite groups

Kim & Park: Cuspal Deflection in Bulk or Incremental Methods

with different elastic moduli. The amount of cuspal deflection has a positive correlation with the elastic modulus of the composite. Acknowledgements

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ite using an LED light curing unit Journal of Dentistry 35(2) 97-103. 13. Lee MR, Cho BH, Son HH, Um CM & Lee IB (2007) Influence of cavity dimension and restoration methods on the cusp deflection of premolars in composite restoration Dental Materials 23(3) 288-295.

This work was supported by a faculty research grant of Yonsei University College of Dentistry for 2006 (6-2006-0018) and the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST) (No. R13-2003-01305002-0).

14. Palin WM, Fleming GJ, Nathwani H, Burke FJ & Randall RC (2005) In vitro cuspal deflection and microleakage of maxillary premolars restored with novel low-shrink dental composites Dental Materials 21(4) 324-335.

(Accepted 8 February 2011)

15. Park J, Chang J, Ferracane J & Lee IB (2008) How should composite be layered to reduce shrinkage stress: incremental or bulk filling? Dental Materials 24(11) 1501-1505.

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