Bi-axial flexural strength of dual-polymerizing agents cemented to human dentin after photo-activation with different light-curing systems

Abstract

Objectives

This study aimed to assess the bi-axial flexural strength of two dual-polymerizing resin luting agents cemented to human dentin when photo-activated with different light-curing units.

Materials and methods

Two dual-cured resin cements: choice (CH) and Variolink II (VL) were tested. Hybrid composite resin (Z-250) discs (12 × 1.5 mm) were fabricated. Three types of light-curing units were used halogen-curing unit (QTH), light-emitting diode (LED) and plasma arc (PAC). Sixty dentin discs of 0.5 mm thickness were prepared from extracted human teeth. A circular mold (2.5 mm in height and 12 mm diameter) was utilized to create supporting structure for dentin, resin cement complex. The resin luting cement (0.5 mm) was placed on the previously prepared dentin discs and covered with the prefabricated composite discs. Photo-activation of cements was performed for 40 s with QTH and LED units and for 3 s with PAC. The specimens were divided into 12 groups (20 specimens for each light source). Six groups were kept in distilled water for 24 h and the rest were stored for 6 weeks. Bi-axial flexural strength was determined using Instron machine. The data was analyzed using two-way ANOVA and Tukey test for comparison.

Results

The findings indicated that the bi-axial flexural strength values for both cements CH and VL were higher for 24 h over 6 weeks but not statistically significant when cured with QTH. Meanwhile, when LED light was used for photo-activation the cements, the flexural strength values reported were statistically higher of 24 h over 6 weeks storage at P = 0.4^E−6 However, PAC light did not record any statistically significant difference between two duration for the CH cement although when used for polymerization of VL the reported value for 6 weeks were statistically significantly higher value than 24 h duration at P = 0.002.

Conclusion

When high immediate flexural strength is preferred in clinical situation photo-activation the cements with LED reported the greatest value.

1. Introduction

The traditional approaches of tooth preparation that were required by direct metallic restorations are being replaced by techniques associated esthetic materials. This has resulted in more conservative preparation and has heightened patient’s awareness of dental esthetic. In addition, when larger indirect restorations are needed, more people desire esthetic choice. Many of these choices depend on the use of resin-based luting cement for additional retention and strength within the restorative material (Duke, 2000). The clinical success of composite and ceramic indirect restorations is attributed to reliable bond between adhesive cementing systems (resin cement/bonding agents) and mineralized dental tissues (Inokoshi et al., 1993). Composite luting cements were classified according to their initiation systems, auto-polymerization, and recent development concentrated on dual-cure mechanism (Karmer et al., 2000).

Dual-cured systems consist of a mixture of monomers and catalysts and are formulated so as not to depend solely on light activation for proper cure (Arrais et al., 2007a,b). It was reported that under proper conditions, dual-cured cements would provide clinical success. A clinical evaluation checked at the 11-year performance of indirect composite inlays placed with dual-cured resin cement. A success rate of 83.3% was reported, although 45% of the margins were placed in dentin and a high caries population was included (Duke, 2000). Dental professionals have a variety of curing lights from which to choose such as quartz tungsten halogen (QTH), plasma arc (PAC), laser or light-emitting diode (LED) (Vandewalle et al., 2005). These lights have different characteristics and claimed advantages (Rueggeberg, 1999).

The commonly used unit for polymerization of composite material is a halogen-curing unit (QTH). These units have specific drawbacks, including decrease of the light output with time. This results in a reduction of curing effectiveness of the QTH curing unit over time. Also, the light intensity decreases with time even within a short irradiation period (Nomoto et al., 1998). This may result in a lower degree of monomer conversion for the material and consequent negative clinical implication (Nomoto et al., 2004; Rueggeberg and Jordan, 1993).

The innovative light-emitting diode (LED) technology based on semiconductors, has opened new and interesting views in the field of photo-polymerization. LEDs add the advantages of a soft-start polymerization safety, efficiency, economy and the longer lifetime of LED light. Despite their lower light emission, LEDs are capable of a polymerization qualitatively comparable with other light sources (La Torre et al., 2003; Mills et al., 2002).

In recent years, plasma arc light-curing unit that deliver high light intensity output for faster curing have been introduced with the claim of relatively short curing time (Nomoto et al., 2004; Pettemerides et al., 2001).

Mechanical properties including fracture toughness, flexural strength and others are important properties of dental resin-based composite materials (Miyazaki et al., 1996). Bi-axial flexural strength testing is known to be advantageous over uni-axial, diameteral tensile and compressive testing methods (Morrel et al., 1999; de With and Wagemens, 1989). The smaller disc shaped specimens utilized for the bi-axial testing result in an improved representation of the volume and dimension of clinical restorations (Plain et al., 2003). In addition, it was suggested that bi-axial flexural strength testing of dental resin-based composites provides a more reliable testing method than three points flexural (Plain et al., 2003). Therefore, the aim of the current study was to assess the bi-axial flexural strength of two dual-polymerizing resin luting agents cemented to human dentin and photo-activated with different light-curing units.

2. Materials and methods

Resin luting cements used in the present study were Choice (Bisco, USA) and Variolink II (Ivoclar Vivadent, Schaan, Liechtenstein). Filtek™ Z-250 (3 M ESPE Dental Products, St. Paul, MN, USA) was used to fabricate the composite resin discs (Table 1).

Table 1.

Characteristics of the light-curing units.

Light cure unit	Light intensity (mW/cm2)	Manufacturer	Exposure time
Halogen (QTH)	800	ESPE Elipar High Light 3 M, Germany	40 s
Light emitted diode (LED)	800	Ultra-Lume LEDS, Ultradent USA	40 s
Plasma arc (PAC)	1370	Apollo 95E USA	3 s

The three types of light-curing units used to cure the luting cements were conventional halogen light (QTH), light-emitting diodes curing unit (LED) and plasma arc (PAC) as shown in Table 2.

Table 2.

The mean and standard deviation of bi-axial flexural strength (MFlst) for the tested materials groups (MPa).

Light	Cement	Group	24 h Storage	Group	Six weeks storage	P value
			MFlst		MFlst
QTH	CH	1	84.8 ± 8.1	7	65.0 ± 12.7	0.19
	VL	2	76.5 ± 12.9	8	59.1 ± 12.8	0.064
LED	CH	3	103.6 ± 6.3	9	72.5 ± 9.4	0.00⁎
	VL	4	107.8 ± 12.6	10	78.0 ± 5.6	0.01⁎
PAC	CH	5	83.4 ± 14.4	11	73.3 ± 11.7	0.257
	VL	6	74.1 ± 4.5	12	89.6 ± 6.1	0.02⁎

^⁎Denotes significant differences at P = 0.05.

2.1. Specimen preparation

Extracted non-carious human premolars and molars human teeth were collected and stored in 0.25% thymol solution. Sixty dentin discs of 0.5 mm thickness were prepared from these teeth by cutting them with Isomat™ 2000 (Buehler Ltd., Lake Bluff, IL, USA) precision saw machine. Digital gauge (Ultra-cal Mark III, Fowler, Co. Inc., Sylvac Newton, MI, USA) was utilized to confirm the thickness of all the dentin discs. A circular Teflon® mold (12 mm in diameter and 1.5 mm height) was used to fabricate sixty composite discs from Filtek™ Z-250 composite. The mold was placed on a transparent matrix strip supported by a microscopic glass slide. The mold was overfilled with composite. The mold and material was covered with another matrix strip and a microscopic glass. Light pressure was applied until the upper matrix strip and slide came into contact with the mold to expel excess material and avoid air entrapment. Z-250 was photo-activated with LED through the top glass slide in four quadrants to insure adequate polymerization of the material. The curing time used was 20 s as recommended by the manufacturers.

Sixty specimens were prepared, 20 for each light-curing source. The resin luting cements (0.5 mm) were placed on the previously prepared dentin discs and composite specimens was placed over the cement. A circular mold (2.5 mm in heights and 12 mm diameter) was fabricated to create supporting structure for the dentin, resin cement complex. For photo-activation, the curing light tips were positioned close to the composite disc without any space between the tip and the disc. Light-curing was performed for 40 s with QTH and LED units and for three seconds with the PAC unit according to manufacturer’s instructions. The specimens were divided into 12 groups. Six groups were kept in distilled water for 24 h and the other six groups for 6 weeks (in temperature/humidity controlled room).

2.2. Bi-axial flexural strength measurement

The bi-axial flexural strength was determined by centrally loading the specimens on a 10 mm knife-edge support a across head speed of 0.2 mm/min with 1 mm diameter steel piston. The specimens were loaded to failure using a universal tensile-testing instrument Instron software (Instron Ltd., Model 8500⁺, MA, USA). The load (N) and extension (mm) at failure was recorded. The bi-axial flexural strength was calculated according to the equation of Timoshenkos and Woinowsky-Krieger (1959)

$${}^p{\sigma }_{\frac{\text{max}}{{\text{H}}_2}}=\frac{(1+v)[0.485\ln (9)+0.52]+0.48}{h}$$ (1)

σ_max = the maximum tensile strength (MPa), P = the measured load at fracture (N), a = the radius of the knife-edge support (mm), h = the specimen thickness at the fracture point (mm), v = the Poisson’s ratio (0.25). This ratio was taken according to the recommendation in the standard for all the materials.

3. Result

The mean and SD of bi-axial flexural strength of data for each group t is presented in Table 2. Group 4 had the highest value (107.84 ± 12.62 MPa). Meanwhile, Group 8 recorded the lowest value (59.06 ± 12.8 MPa) among all groups. Two-way ANOVA and Tukey’s test for comparison at 0.05 significance level on the resultant data identified that after 24 h duration when LED was used for polymerizing the cements there was statistically significant difference in bi-axial flexural strength between LED, QTH and PAC, where LED recorded the highest value at P = 0.000. Meanwhile, no significant difference was found between halogen and PAC lights as the confidence intervals were overlapped, as shown in Table 3 and Fig. 1.

Table 3.

The mean and standard deviation of bi-axial flexural strength (MPa) for the tested cements when polymerized with different light-curing units.

Duration	Light type	Mean	SD	P value
24 h Storage	QTH	80.6∗	11.0
	LED	105.7	9.7	0.01
	PAC	78∗	11.2
Six weeks storage	QTH	62.1	12.4	0.002
	LED	75.5∗∗	7.9
	PAC	81.5∗∗	12.3

^∗,∗∗ Denotes statistically significant differences within the duration groups at P = 0.05.

Figure 1.

The bi-axial flexural strength of tested cements after 24 h storage in distilled water.

In addition, after 6 weeks storage duration the 95% confidence intervals revealed no statistically significant difference in flexural strength characteristic of the tested cements when cured with LED and PAC lights. However, photo-activation with QTH light recorded statistically significantly lower value at P = 0.01 than the other lights (LED and PAC), Table 3 and Fig. 2.

Figure 2.

The bi-axial flexural strength of tested cements after 6 weeks storage in distilled water.

The flexural strength value of CH cement when photo-activated with QTH and PAC reported no statistically significant difference between 24 h and 6 weeks as shown in Table 2. Meanwhile polymerization with LED revealed statistically significant difference between both duration at P = 0.000 Table 2. On the other hand, when VL was cured with QTH revealed no statistically significant difference between both durations. However, statistically significant difference (P = 0.01) was reported when LED was used for photo-activation where flexural strength after 24 h (107.8 ± 12.6 MPa) and after 6 weeks (78.0 ± 5.6 MPa). Although with PAC the opposite was recorded where 24 h flexural strength value were (74.1 ± 4.5) which was significantly differ than the value recorded after 6 weeks (89.6 ± 6.1 MPa) at P = 0.002 as presented in Table 2.

4. Discussion

The ability to thoroughly cure resin cement has desirable effects on the physical properties of the cement. The dual-cure cements are favorable in an attempt to make a universal resin system. Esthetic results and longevity of indirect composite resin restorations depend on each step of the clinical and laboratory procedures. Cementation is the most critical step and involves the application of both adhesive system and resin luting agent (Mark et al., 2002).

The stresses involved in a resin-based cement system are associated with function, the polymerization process and dimensional changes after polymerization. It has been shown that functional stresses should be well-resisted by resin cements (Love and Purton, 1998).

In the present study, when CH and VL were polymerized with LED, they recorded the highest value among the other curing light units (QTH and PAC).

It was reported that different light intensities used with the same exposure time had some effect on flexural strength and modulus. This is the case with LED where the same exposure time was used with both QTH and LED, where the flexural strength was reduced when QTH than with LED which indicate higher light intensity of LED. Low intensity exposure that causes slow polymerization yields longer molecular chains with higher flow properties (Unterbrink and Muessner, 1995). Meanwhile, it results in improved marginal adaptation, and that higher intensity with shorter exposure time was not optimal for the polymerization of light-cured composite restorations (Uno and Asmussen, 1991). However, clinicians should be aware of the greater thermal potential when using LED units when compared with other types of light activation curing devices (Taher et al., 2008).

The resin matrix has been reported to be a critical component that influences the mechanical properties of the resin-based composite (Dietchi et al., 1994), because it is responsible for the sorption of the resin-based systems (Deepa and Krishnan, 2000). The monomer of CH is bisphenol diglycidy methacrylate (5–30% concentration range). On the other hand, the VL resin is bis-GMA, urethane dimethacrylate and triethylene glycol dimethacrylate with 26.3 wt.% (base) and 22 wt.% catalyst with a high viscosity. The difference in resin content and type changes many of the physical properties of these resins (Duke, 2000). This could be the explanation of higher flexural strength of CH when cured with three different light-curing units after 24 h storage. It was suggested that the increased bi-axial flexural strength was attributed to an increased degree of conversion and cross-linking densities (Plain et al., 2003).

Also, it was postulated that the decreased polymerization rate generated a temporary excess of free volume that enhanced the mobility of the polymerizing system. An increased mobility of the reactive species within CH allowed the system to reach higher degrees of conversion and was manifested as an increase in flexural strength following 24 h compared with limited mobility of free radical presented in the matrix of VL (Andrzejewska, 2001).

In addition, in clinical situations where bending forces are anticipated, adhesive resin cements exhibiting a high immediate flexural strength might be preferred (Pace et al., 2007). In the present study, photo-activation of VL with LED had the greatest early strength value which could be the cement of choice for adhesive bonded restorations that have minimal resistance and retention form. The flexural strength values recorded of CH and VL after 24 h storage when the cement was photo-activated with PAC was reduced than the other units LED and QTH this could be attributed to the finding that the PAC unit require longer radiation time than that recommended by its manufacturers (3 s) to create a depth of cure equal to that produced by the QTH or LED for 20 s of irradiation (Nomoto et al., 2004).

Resin cements as a group are virtually insoluble in the oral environment (White et al., 1992). However, it was concluded that long-term aging in water had little influence on flexural strength (Ferracane et al., 1998). The previous finding is in agreement with the result of the present study. The hypothesis was that water sorption causes a softening of the polymer resin component by swelling the network and reducing the frictional forces between polymer chains. Once the network is saturated with water and becomes softened, the composite structure stabilizes and there is no further reduction in properties (Ferracane et al., 1998). In the present study the data revealed that after 6 weeks storage the flexural strength was reduced with both cements with three lights except with VL when polymerized with PAC (89.6 ± 6.1 MPa) which produced higher value than 24 h storage (74.1 ± 4.5 MPa). The explanation could be that significant residual or continued polymerization occurred for these cement by PAC (Pace et al., 2007). This finding is in agreement with other (Pace et al., 2007) although different cements were used in that study.

The measurement of the strength of brittle material under bi-axial flexural conditions rather than uni-axial flexural is often more reliable, because the maximum tensile stresses occur within the central loading area, and edge failures have no effect on specimen fracture (Ban and Anusavice, 1990). Also, the bi-axial test is simpler to perform and provides a better simulation of clinically relevant sample size than that used for other strength tests (Ban and Anusavice, 1990). Evaluation of bi-axial flexural strength data of light-activated resin-based dental materials between different test centers would provide more reliable and consistent data for comparison with the clinical situation (Plain et al., 2003). However, whether measuring the tensile, flexural or compressive strength of a material the data obtained will only provide sufficient information for comparative purpose (Plain et al., 2003).

However, it should be noted that there are some limitation to this study as reported earlier (Hooshmand et al., 2008). The bi-axial flexural strengths reported will not reflect the actual fracture strength in the clinical situation because of different environmental and loading condition. Factors such as dentinal fluid movement and internal stress related to cavity configuration for indirect restorations should be evaluated in further studies (Arrais et al., 2007a,b; De Menezes et al., 2006). Future investigation on the role of chemical bond at the resin cement–dentin interface is also suggested since the adhesive role was not included in the present study. Also, further studies of veneering materials and cements (in vivo–in vitro) and testing of other mechanical properties such as elastic modulus or fracture toughness, may provide a better understanding and resemblances of clinical situation.

5. Conclusion

Within the conditions of this in vitro study, it can be concluded that, in general the bi-axial flexural strength value of CH was higher than VL cement. The flexural strength of CH cement was reduced by longer storage in water when photo-activated with the different tested light-curing systems. VL cement affection with water storage was reduced when cured with LED and QTH, but in contrast its value was increased by time when PAC was used for curing.

Clinical significance: when high immediate flexural strength is preferred in clinical situation photo-activation of cements with LED light reported greatest value.