INTRODUCTION

An endodontic treated tooth generally has lost tooth structure, either by caries or access preparation. Therefore, it may not be able to withstand a chewing force1). Core build-up materials provide an abutment having a normal anatomical form as well as increased resistance and shape retention for the restoration. The ideal core build-up material should provide sufficient compressive and tensile strengths in order to withstand the masticatory and para-functional forces2,3).

Composite resin is a common material used for core build-up to strengthen lost tooth structure. However, it needs to be restored using an incremental technique to ensure proper polymerization. However, multiple increments can leave voids and gaps due to the difficulty during placement in a deep cavity4). Another problem when using a resin composite is polymerization shrinkage that occurs at the resin composite-tooth interface. This phenomenon produces interfacial gaps, which can lead to microleakage, marginal discoloration and secondary caries5). Therefore, bulk fill resin composites have been introduced to overcome the problem of layering techniques leading to voids or contamination between layers6). Additionally, it has been claimed that it is possible to save the restorative procedure time by placing up to 4–5 mm thickness that can be photo-polymerized in one step7). Another material commonly used for core build-up is dual cured resin composite. It was developed in an attempt to overcome the limitations of light-cured material. It can be polymerized by both light and self-activation8). Therefore, the dual cured composite is a useful material for restoring deep cavities. However, there have been few studies comparing the physical

properties of conventional composite resin (FiltekTM

Z350), new generation (high-viscosity) of bulk fill (FiltekTM Bulk fill), and dual-cure composite resin (MultiCore®

Flow) as core build-up materials. In addition, the use of informative Weibull analysis9) of these materials is still limited.

This study aims to evaluate and compare the physical properties of three commercial core build-up materials using compressive strength, flexural strength and microhardness.

The null hypothesis tested herein was that the three core build-up materials have no difference in physical properties.

MATERIALS AND METHODS

Study materialsThree types of resin composites were used in this study (Table 1): conventional resin composite (FiltekTM Z350, 3M ESPE, St. Paul, MN, USA), high viscosity bulk-fill resin composite (FiltekTM Bulk-fill, 3M ESPE) and core build-up material (MultiCore® Flow, Ivoclar Vivadent, Schaan, Leichtenstein). Each specimen was prepared according to the manufacturer’s recommendation under fluorescence light with the room temperature 22°C.

Compressive strengthFifteen specimens of each group were prepared using a stainless steel split mold with cylindrical shape size 6 mm (height)×4 mm (diameter). Resin composite was filled into the mold that placed between two glass slides and Mylar sheets. For conventional composite group, the specimen was placed 2 mm incremental layers. Each layer was cured for 20 s using a light-emitting diode (LED) curing unit (Elipar S10, 3M ESPE). For the high viscosity bulk fill, the specimens were placed

Comparison of physical properties of three commercial composite core build-up materialsSasinisa WARANGKULKASEMKIT and Piyapanna PUMPALUK

Department of Advanced General Dentistry, Faculty of Dentistry, Mahidol University, 6 Yothi Road, Ratchathewi District, Bangkok 10400, ThailandCorresponding author, Piyapanna PUMPALUK; E-mail: piyapanna@hotmail.com

Various materials have been used for core build-up when restoring the coronal portion of the tooth. Currently, bulk-fill resin composites have been produced to restore a large posterior cavity in single increment. This study aimed to evaluate the compressive strength, flexural strength, and microhardness of three commercial composite core build-up materials. All data were analyzed by one-way ANOVA and Tukey test methods (α=0.05). Flexural strength data were subjected to Weibull statistics analysis. All three groups presented significant differences in the compressive strength, flexural strength, and Knoop hardness. FiltekTM Z350 XT had the greatest compressive strength (MPa) and Knoop hardness while FiltekTM bulk fill had the highest flexural strength. MultiCore®Flow had the lowest properties; however, it revealed the highest Weibull modulus (m) value. With regard to the properties tested in this study, bulk-fill resin composite can be used as an alternative to conventional resin composite for core build-up material.

Keywords: Flexural strength, Weibull statistics, Core build-up material, Bulk fill, Resin composite

Color figures can be viewed in the online issue, which is avail-able at J-STAGE.Received Jan 31, 2018: Accepted May 25, 2018doi:10.4012/dmj.2018-038 JOI JST.JSTAGE/dmj/2018-038

Dental Materials Journal 2019; 38(2): 177–181

Table 1 Materials used in this study

Material CompositionFillerload

(wt%/vol%)Manufacturer

Batch Number(lot number)

FiltekTM Z350

Light cure composite:Bisphenol-A-glycidyl methacrylate (Bis-GMA), Ethoxylated bisphenol-A dimethacrylate (Bis-EMA), Urethane dimethacrylate (UDMA) with small amounts of Triethyleneglycol dimethacrylate (TEGDMA) non-agglomerated/non-aggregated, 20 nm nanosilica filler, and loosely bound agglomerated zirconia/silica nanocluster

78.50/59.53M ESPE, St.Paul, MN, USA

N708953

Filtek TM Bulk fill

Light cure composite:Bis-GMA, Bis-EMA, Aromatic urethane dimethacrylate (AUDMA), UDMA and 1, 12-dodecane-DMA non-agglomerated/non-aggregated 20 nm silica filler, a non-agglomerated/non-aggregated 4 to 11 nm zirconia filler, an aggregated zirconia/silica cluster filler and a ytterbium trifluoride filler

76.5/42.5 3M ESPE N682084

MultiCore® Flow

Dual cure composite:Bis-GMA, UDMA, TEGDMAInorganic fillers (barium glass, Ba-Al-fluorosilicate glass, silicon dioxide, and ytterbium trifluoride)

54.65/46Ivoclar Vivadent, Schaan, Leichtenstein

T35741

by bulk technique up to 3 mm and each layer was cured for 20 s. Dual-cured core build-up materials was placed in one bulk and then cured for 20 s. All specimens were irradiated at the top and bottom surfaces. They were stored in distrilled water at 37°C for 24 h with 100% humidity. Test specimens were subjected to the compressive strength tests using a universal testing machine (LR10K, LLOYD Instrument, UK) at a cross head speed 0.5 mm/min. The stress at fracture (S) was calculated according to the following equation:

S=F/((d/2)2π)Where F is the load at fracture (N), d is the mean diameter of the specimen (mm)

Flexural strengthFlexural strength (σ) was determined by a three-point bending test. Thirty-five specimens in each group were fabricated using a stainless steel split mold with dimentions of 25±0.1 mm (length)×2±0.1 mm (height)×2±0.1 mm (width), as specified in ISO 4049:2009 standards10). The specimens were photo-polymerized using LED curing unit (Elipar S10, 3M ESPE). The center of the specimen was cured first and then the polymerization was performed by curing overlapping regions between right and left from the center until the entire specimen surface were polymerized. Each area was cured for 20 s and irradiation was performed at the top and bottom of the specimens. All specimens were stored in distrilled water at 37°C for 24 h with 100% humidity. Then they were submitted to the universal testing machine (EZ-S, SHIMADZU, Kyoto, Japan) at a cross head speed of 0.75 mm/min until fracture occured. All data were calculated according to the following

equation:σ=3Fl/2bh2

Where F is the maximum load (N), l is the distance between the supports (mm), b is the width of the specimen (mm), and h is the height of specimen (mm)

Knoop microhardnessThe Knoop microhardness test was performed in order to determine the microhardness of resin composite, which can predict the clinical performance of restorations. The specimens were made in a stainless steel split mold size 2 mm (height)×3 mm (diameter). All specimens were then stored in distrilled water at 37°C for 24 h prior to testing. The specimens were submitted to the Knoop hardness test (KHN) using a load of 10 g with dwell time of 20 s when using a digital microhardness tester (FM-ARS-900, Future-test, Kanagawa, Japan). The specimens were positioned beneath the indenter of a digital microhardness tester and five indentations were measured on each specimen surface.

Fracture analysisThe fracture surfaces of the specimens were coated with gold for 60 s in Quorum Q150R ES (serial no. 15112, Quorum Technologies, East Sussex, UK) at a sputter current of 25 mA. They were observed in a scanning electron microscope (SEM; JSM-IT300LV, serial no. MP1372001500150, JEOL, Tokyo, Japan)

Statistical analysisData obtained from the compressive, flexural, and knoop hardness tests were analyzed by One-way ANOVA and Tukey’s test (α=0.05). Reliability of materials and

178 Dent Mater J 2019; 38(2): 177–181

Fig. 1 Weibull analyses.

Table 2 Mean values of compressive strength (MPa, n=15), flexural strength (MPa, n=35), Knoop hardness (n=5) and Weibull statistic values

MaterialsCompressive strength (SD)

Flexural strength (SD)

Knoop hardness (SD)

Weibull statistic

m σ0 (MPa) r2

FiltekTM Z350 283.43 (19.04) 125.22 (10.84) 66.22 (1.86) 11.38 131.83 0.89

FiltekTM Bulk fill 239.75 (16.58) 142.43 (10.77) 48.99 (1.66) 15.76 147.21 0.95

MultiCore® Flow 193.25 (16.15) 114.71 (8.73) 40.69 (1.83) 16.60 118.03 0.99

Fig. 2 Images showing internal pores at the fracture surfaces of three materials at ×300; (A) FiltekTM Z350, (B) FiltekTM Bulk fill, (C) Multicore®Flow and at ×700, (D) FiltekTM Z350, (E) FiltekTM Bulk fill, (F) Multicore®Flow.

probability of failure were analyzed by Weibull analysis9) using the flexural strength data. The following equation was used to evaluate the cumulative probability of failure, Pf

Pf=1−exp(−(σ/σ0)m)Where Pf is the probability of failure, σ is the flexural strength, σ0 is the characteristic strength (Pf=63.2%), m is the Weibull modulus. Plotting ln[ln1/1−Pf)] against lnσ will provide a slope with the value of the Weibull modulus.

RESULTS

The results from the ANOVA analyses indicated that there were significant differences at the 95% level (p<0.05) in the mean compressive strength, flexural strength, and Knoop hardness of three materials (Table 2). FiltekTM Z350 had the greatest compressive strength and Knoop hardness, which were 283.43 MPa and 66.22 respectively, followed by FiltekTM Bulk fill (239.75 MPa, 48.99) and MultiCore® Flow (193.25 MPa, 40.69). On

the other hand, FiltekTM Bulk fill had the greatest mean flexural strength (142.43 MPa), followed by FiltekTM

Z350 (125.22 MPa) and MultiCore® Flow (114.71 MPa).The result from the Weibull analyses (Fig. 1)

179Dent Mater J 2019; 38(2): 177–181

showed that MultiCore® Flow had the greatest Weibull modulus (16.60), followed by FiltekTM Bulk fill (15.76) and FiltekTM Z350 (11.38). The characteristic strengths of FiltekTM Bulk fill, FiltekTM Z350 and MultiCore® Flow were 147.21, 131.83 and114.71 MPa, respectively.

The SEM images (Fig. 2) show the defects associated with the fracture sites. FiltekTM Z350 had the largest porosity size, while MultiCore®Flow had the lowest size of such defects.

DISCUSSION

Mechanical properties are important factors for the success of core build-up restorative dental materials. This is because they must withstand the forces due to mastication and para-function. This study evaluated some of the properties, including compressive strength, flexural strength, and Knoop hardness (KHN) of three resin composite materials. The null hypothesis of this study was rejected, since there were statistically significant differences in the properties of the three groups. The results of this study demonstrated that FiltekTMZ350 had the greatest compressive strength and Knoop hardness, whereas FiltekTMBulk fill had the greatest flexural strength. These results confirm an earlier conclusion11) that bulk fill resin-based composites show lower mechanical properties (except for flexural strength) than nanohybrid and microhybrid resin-based composites.

It is known that the mechanical properties of the composites are related to their filler contents12) and to the type and size of filler13). From the composition (Table 1), FiltekTMZ350 has a higher filler content (78.5 wt%, 59.5 vol%) than FiltekTMBulk fill (76.5 wt%, 42.5 vol%) and MultiCore®Flow (54.65 wt%, 46 vol%). For the filler types, silica and zirconia fillers are found in FiltekTMZ350 and FiltekTMBulk fill, whereas MultiCore®Flow contains barium glass and silicon dioxide fillers. Bulk fill composite materials have been developed to offer low polymerization shrinkage, easy use and improved depth of cure. When it is necessary to increase the depth of penetration of the light initiating the cure, the amount of filler particles has to be reduced. Therefore, FiltekTMBulk fill has lower compressive strength and hardness compared to FiltekTMZ350. MultiCore®Flow has the lowest filler content and also has no zirconia in its composition. Consequently, it has the lowest strength. Nevertheless, all three materials tested were found to have compressive strength values (>100 MPa) greater than the minimum value (50 MPa) recommended for dental amalgam, which is clinically well-proven for core build-up2).

Both filler morphology and filler loading influence the flexural strength, flexural modulus, and hardness12).

Moreover, the type of monomer in the matrix influences these properties. One study14) reported that monomer containing Bis-GMA or TEGDMA substituted by UDMA results in an increase in flexural strength. Also that substitution of Bis-GMA by TEGDMA reduces the flexural strength14). In this study, TEGDMA monomer

contained in FiltekTMZ350 and MultiCore®Flow leads to lower flexural strength compared to FiltekTMBulk fill.

Weibull statistics relate to the reliability of the material in use. It does this by providing the probability of failure. Simple measurement of fracture strength alone cannot predict structure failure. That is because it provides an insight into only the stresses that the material will withstand for a given flaw size distribution15). On the other hand, the Weibull modulus (m) is a parameter that describes the variability of the strength of brittle materials. A high Weibull modulus indicates higher reliability of materials16). In addition, materials with a high Weibull modulus are more predictable and less likely to break at a stress much lower than a mean experimental value17). The second parameter of the Weibull analysis is the characteristic strength (σ). This is a parameter that corresponds to the stress level giving a 63.2% probability of failure7,18). Thus Weibull characteristic strength values (Pf=63.2%) are slightly greater than the mean strength values (Pf=50%).

In this study, Multicore®Flow has a Weibull modulus (m) higher than other groups. This may be because of the lower viscosity of uncured Multicore®Flow, As a result surface defects within the material are reduced and crack propagation is minimized. SEM analysis confirms this result, showing the smallest defects on the fractured surfaces (Figs. 2C and F). FiltekTM Bulk fill has similar m value to Multicore®Flow while FiltekTMZ350 has the lowest m value. The result from SEM of the FiltekTMZ350 group (Figs. 2A and D) showed that there were the largest porosities on the fracture surfaces. These porosities may result from the placement technique that involves building up the material in multiple increments. This technique necessitates higher chair time and increases the risk of voids and contamination between layers. Another factor responsible for the gaps in the material is clinical skill of the operator. The operator that carefully performed and strictly followed according to the manufacturer’s instruction will give the good clinical outcome. One study revealed that operator skill and experience play a major role in the post-operative sensitivity outcome19). Therefore, materials that use a bulk technique tend to provide smaller gaps than do conventional resin composites that using an incremental technique. Furthermore, in this study, FiltekTM Bulk fill had a similar m value as an earlier study which had m=14.211). On the other hand, it was higher than that found by Vidhawan et al.7) In addition, the m value of FiltekTMZ350 in this project (m=11.38) is higher than that reported as m=8.320) These differences in m value could be due to the differences in methodology used in the earlier work.

Although the strength of core build-up material is the factor that influenced the fracture resistance of the abutment, the matching moduli between the material and the dentin is also important. If there is too mismatch of the elastic values, interfacial stress may occur from either thermal, mechanical, or shrinkage strain in the material21). Therefore, core build-up material should

180 Dent Mater J 2019; 38(2): 177–181

have high elastic modulus similar to tooth structure to withstand the forces of mastication and polymerization shrinkage stresses22). The elastic modulus values of FiltekTM Z350, FiltekTM Bulk fill and Multicore® were reported approximately 1423), 10.123) and 924) Gpa, respectively. Flexural strength of dentin ranged from 245 to 280 MPa25). and modulus of elasticity of dentin ranged from 11–20 Gpa26). Therefore, FiltekTM Z350 has strength and modulus value approaches to dentin’s more than other groups.

CONCLUSION

Three chosen core build-up materials have significant differences in compressive strength, flexural strength and Knoop hardness. FiltekTMZ350 has the highest mean compressive strength and Knoop hardness whereas FiltekTMBulk fill have the highest flexural strength. MultiCore®Flow and FiltekTMBulk fill have higher Weibull modulus than FiltekTMZ350. Based on these results, FiltekTM Z350 is the material of choice for core build-up. Another alternative for core material is bulk fill resin composite because it exhibited high strength and reliability. Importantly, it can be cured as a single placement, thereby reducing the patient chair time.

ACKNOWLEDGMENTS

The author wishes to thank the staff at the Research Unit, Chulalongkorn Univeristy for valuable advice and use of the research equipment.

REFERENCES

1) Gher ME Jr, Dunlap RM, Anderson MH, Kuhl LV. Clinical survey of fractured teeth. J Am Dent Assoc 1987; 114: 174-177.

2) Yuzugullu B, Ciftci Y, Saygili G, Canay S. Diametral tensile and compressive strengths of several types of core materials. J Prosthodont 2008; 17: 102-107.

3) Cho GC, Kaneko LM, Donovan TE, White SN. Diametral and compressive strength of dental core materials. J Prosthet Dent 1999; 82: 272-276.

4) Kapoor N, Bahuguna N, Anand S. Influence of composite insertion technique on gap formation. J Conserv Dent 2016; 19: 77-81.

5) Krejci I, Lutz F. Marginal adaptation of Class V restorations using different restorative techniques. J Dent 1991; 19: 24-32.

6) Sarrett DC. Clinical challenges and the relevance of materials testing for posterior composite restorations. Dent Mater 2005; 21: 9-20.

7) Vidhawan SA, Yap AU, Ornaghi BP, Banas K, Neo JC, Pfeifer CS, Rosa V. Fatigue stipulation of bulk-fill composites: An in

vitro appraisal. Dent Mater 2015; 31: 1068-1074.8) Kournetas N, Tzoutzas I, Eliades G. Monomer conversion in

dual-cured core buildup materials. Oper Dent 2011; 36: 92-97.

9) Weibull W. Statistical distribution function of wide applicability. J Appl Mech 1951; 18: 293-297.

10) International Standard Organization (ISO). Dentistry–Polymer-based restorative materials; 2009: ISO4049.

11) Ilie N, Bucuta S, Draenert M. Bulk-fill resin-based composites: an in vitro assessment of their mechanical performance. Oper Dent 2013; 38: 618-625.

12) Kim KH, Ong JL, Okuno O. The effect of filler loading and morphology on the mechanical properties of contemporary composites. J Prosthet Dent 2002; 87: 642-649.

13) Li Y, Swartz ML, Phillips RW, Moore BK, Roberts TA. Effect of filler content and size on properties of composites. J Dent Res 1985; 64: 1396-1401.

14) Asmussen E, Peutzfeldt A. Influence of UEDMA BisGMA and TEGDMA on selected mechanical properties of experimental resin composites. Dent Mater 1998; 14: 51-56.

15) Kelly JR. Perspectives on strength. Dent Mater 1995; 11: 103-110.

16) Thompson GA. Determining the slow crack growth parameter and Weibull two-parameter estimates of bilaminate disks by constant displacement-rate flexural testing. Dent Mater 2004; 20: 51-62.

17) Quinn JB, Quinn GD. A practical and systematic review of Weibull statistics for reporting strengths of dental materials. Dent Mater 2010; 26: 135-147.

18) Bona AD, Anusavice KJ, DeHoff PH. Weibull analysis and flexural strength of hot-pressed core and veneered ceramic structures. Dent Mater 2003; 19: 662-669.

19) Sancakli HS, Yildiz E, Bayrak I, Ozel S. Effect of different adhesive strategies on the post-operative sensitivity of class I composite restorations. Eur J Dent 2014; 8: 15-22.

20) Rodrigues SA Jr, Ferracane JL, Della Bona A. Flexural strength and weibull analysis of a microhybrid and a nanofill composite evaluated by 3- and 4-point bending test. Dent Mater 2008; 24: 426-431.

21) Watts DC. Elastic moduli and visco-elastic relaxation. J Dent 1994; 22: 154-158.

22) Combe EC, Shaglouf AM, Watts DC, Wilson NH. Mechanical properties of direct core build-up materials. Dent Mater 1999; 15: 158-165.

23) Rosatto CM, Bicalho AA, Veríssimo C, Bragança GF, Rodrigues MP, Tantbirojn D, Versluis A, Soares CJ. Mechanical properties, shrinkage stress, cuspal strain and fracture resistance of molars restored with bulk-fill composites and incremental filling technique. J Dent 2015; 43: 1519-1528.

24) Lendenmann U. Scientific documentation MultiCore®. Research and Development Scientific Services. Ivoclar Vivadent AG, Liechtenstein; 2004.

25) Waters NE. The mechanical properties of biological materials. Cambridge: Cambridge University Press; 1980.

26) Marshall GW Jr, Balooch M, Gallagher RR, Gansky SA, Marshall SJ. Mechanical properties of the dentinoenamel junction: AFM studies of nanohardness, elastic modulus, and fracture. J Biomed Mater Res 2001; 54: 87-95.

181Dent Mater J 2019; 38(2): 177–181