Evaluation and Comparison of Flexural Strength and Diametral Tensile Strength of Four Commercially Available Resin Luting Cements Before and After Immersion in Artificial Saliva – An In vitro Study

Introduction

Attachment of a prosthesis meant to be fixed on to a tooth/ teeth or in certain cases on implant abutment is done so by using a luting agent.¹ The term luting infers any material used to attach or cement indirect restorations to prepared teeth. There are various types of luting agents that are commercially available.² Depending on the clinical condition and kind of prosthesis to be luted, the choice of a luting agent is done.

Resin cements were introduced in mid 1980s. It has polymers that contain fillers to reduce water sorption and coefficient of thermal expansion, which also increases the strength. ³

Luting of indirect dental restorations is a critical step in determining their success. In wet and warm oral environment, luting cements must sustain masticatory stresses for years. They must maintain their integrity when these stresses transfer from crowns to tooth structure.⁴ There are numerous choices to select and decide on the kind of luting agent to use. The newest choice includes the professed “universal” self-etching/ self-adhesive resin luting agents, which claim to be appropriate for all kinds of indirect restorations. The goal in the progress of these newer self-adhesive resin luting agents is to combine the easy handling along with improved mechanical properties of traditional resin luting agents.⁵ Due to decreased number of steps and extensive range of applications, the ease-of-use is alluring to the practitioner.⁶ Compared to traditional resin luting agents which involves an additional step of etch and rinse, the newer self-adhesive luting agents comprise the essential chemical components in one product. They do not eliminate the smear layer, but incorporate it into the cement which forms dentin/ resin hybrid layer, thus not exposing dentinal tubules decreasing patient discomfort,.^5-9 But the mechanical properties of resin luting agents are generally greater than newer self-adhesive luting agents, thus compromising on mechanical properties in order to reduce sensitivity in patients.5 Ideally, in certain clinical situations wherein mechanical properties like strength are critical, it would be worth using the self-adhesive resin luting cement.^10,11

The objective of this study was to test two mechanical properties of all four commercially available newer self-etching/ self-adhesive resin luting cements. Since the mode of adhesion for these cements is similar, comparing and evaluating the mechanical properties would provide knowledge to aid in material selection, as their mechanical properties may influence clinical performance.

Materials and Methods

Materials

In total, 160 samples were fabricated. Twenty specimens were tested for each cement with 10 specimens measured after the immersion in artificial saliva and other 10 were as it is (control group). Since there were four resin luting cements, they were categorized as:

A. SpeedCEM plus (Ivoclar)

B. Maxcem Elite (Kerr)

C. Estecem plus (Tokuyama)

D. Panavia F 2.0.

Methods

Preparation of metal dies

Stainless steel dies of two different dimensions were made for flexural strength test and indirect tensile (IDT) strength test (Figure 1). For IDT strength, a customized disc shaped mould with a circular indentation of 6 mm diameter and 2 mm depth was fabricated and for flexural strength, a rectangular mould of 25×2×2 mm was fabricated. All of these were fabricated according to ISO 4049:2009.

Specimen preparation for flexural strength and indirect tensile strength tests

Specimens with dimensions of 25.0 x 2.0 x 2.0 mm in sets of 10 were made. According to the manufacturer’s instructions, luting agents were mixed and filled into customized moulds. To lessen the voids and irregularities in the specimens, moulds were marginally overfilled and pressed with a glass slide by applying hand pressure to remove any extra material. The specimens were allowed to set according to the recommended setting time. Light curing unit was used to ensure complete photoactivation and uniform curing of the specimens. After the final set of the luting cements, samples were removed carefully from the mould and deflashed with a sandpaper for irregularities. For IDT strength, cylindrical test specimens (6 mm diameter and 2 mm depth) were produced and similar procedure was followed.

Measurement of flexural strength (Figure 2)

The measurement of flexural strength was accomplished according to ISO 4049 guidelines. The height and the width of each bar was measured in the middle of the specimen with a digital caliper. For the measurement of flexural strength, the specimens were positioned on two cylindrical supports of 2 mm diameter. The supports were at a distance of 20 mm. Universal testing machine was used which had a cylindrical loading piston with a diameter of 2 mm. This was placed in the middle of the bar and loading was performed. The crosshead speed was set at 1 mm/min. Fracture load was recorded and the flexural strength (σ FS) was calculated according to the following equation:²²

σ FS = 3Fl /2bh²

b: width of the specimen

F: load at fracture

h: height of the specimen

l: distance between the supports (20 mm)

Measurement of indirect tensile strength (Figure 3)

Diameter and height of the samples were measured and were loaded perpendicular to the cylinder axis until fracture. Load at fracture was noted and the indirect tensile strength (σ IDTS) was calculated according to the following equation:

σ IDTS = 2F/ (πdh)

F: load at fracture

d: diameter of the specimen

h: height of the specimen

Effect of storage in artificial saliva

Once the samples were stored in artificial saliva for 24 hours at room temperature, flexural strength and IDT strength were measured.

Results

The numerical values obtained in the study are given in Table 1, 2 and Graph 1, 2.

Flexural strength test

Deduction of numerical results of flexural strength of control groups in the decreasing order was as follows: Group B > Group A > Group D >Group C. Similarly, after storage in artificial saliva for 24 hours, the groups in decreasing order were Group A > Group D > Group B > Group C. Group B showed highest flexural strength without any storage media which significantly decreased once stored in artificial saliva for 24 hours. Group C and D showed similar decrease in the flexural strength after 24 hours in storage media. Whereas Group A showed a slight amount of increase in the flexural strength due to the after effects of storage media.

Indirect tensile strength

Deduction of numerical results of indirect tensile strength of control groups in the decreasing order was as follows: Group B > Group A > Group D > Group C. Similarly, after 24 hours of storage in artificial saliva, the groups in decreasing order were Group B > Group A > Group D > Group C.

Correlation of flexural strength and indirect tensile strength (Table 2, Graph 2)

There was a high positive correlation between the two mechanical properties of specimens of control group, while the correlation was moderately positive among the mechanical properties of specimens that were immersed in artificial saliva. 

Discussion

In the past few years, there has been considerable enhancement in the cements available for luting dental prostheses. As a result, an extensive variety of options for cementation of fixed restorations exists these days for the clinicians. Self- adhesive resin is comparatively a newer category of luting agents.² Self-adhesive resin cements are also known as auto-adhesive luting cements. A distinguishing advantage of auto-adhesive resin cements is reduced chair-side time due to reduction in application steps.¹² Traditional resin luting cements to attain the adhesive effect, necessitates an acid to “process” the tooth surface (etching). However, in case of self-adhesive resin cements, the phosphoric acid ester monomer is included to achieve the etching step.¹⁷ This means that self-adhesive resin cements directly etch the tooth surface without the additional step of acid treatment.^5,6

It is appropriate to test and know the comparative nature of newer self-adhesive resin luting agents in terms of their mechanical properties. This will eventually estimate their clinical performance.^9,13 Mechanical strength during function is usually evaluated by flexural strength as it closely resembles the stresses prompted during function.In a clinical scenario, the layer of cement must have acceptable flexural strength to withstand transfer of stresses from restoration to the tooth without breaking. Tensile strength is also imperative as most luting agents are very brittle and prone to tensile failure.¹⁸

Therefore, the current study was designed to test and compare the flexural strength and IDT strength of four self-adhesive luting agents and also to evaluate the effect of storage in artificial saliva for 24 hours on these properties.

Flexural strength: Determines the ability of a material to resist bending forces. It determines the extent of force necessary to break and bend the specimens of specific thickness area when loaded. This is also called as “Transverse strength.”25 Oilo G stated that the flexural properties of many materials may be more important than their tensile, shear, or compressive strengths, because resin-bonded FPDs are more likely to be subjected to bending forces than to other types of stresses.⁴⁷

Flexural strength in the current study was determined using three-point bending test. When the results were compared, Group B (Tokuyama Estecem plus) had considerable higher flexural strength followed by Group A (Speed Cem Plus) and Group D (Panavia F2), whereas Group C (Maxcem Elite) had the lowest flexural strength among the four resin luting cements tested. The credible reason for maximum strength of Group B (Tokuyama Estecem plus) could be because of the amount of filler content in the cement as Group B has 74% by volume filler content which was highest among all the cements tested.^48,49

This can be substantiated by a study conducted by Kim et al., where the filler morphology and loading on mechanical property of resin composites were tested. They concluded that the resin composite with maximum filler by volume displayed maximum flexural strength.¹⁹

Similarly, Ilie et al., conducted a study and concluded that the presence of hybrid inorganic fillers in the composition of some materials improves mechanical properties.³¹

Effect of saliva: Laboratory assessment of the mechanical properties of luting cement can be a useful guide to the clinician in determining where and when a particular luting cement to be used.²⁴ Therefore this study also focused on the effect of storage in artificial saliva on mechanical properties of the four tested luting cements.²⁸ “Aging”, which refers to the possible physical or chemical changes in the cement over time may lead to an increase or decrease in strength and retention.^29,35 The failure of luting cements is due to the formation of micro cracks and/or bacterial access or by gross failure and crown displacement.^34,37

Storing the specimens in artificial saliva for 24 hours had a substantial effect on all the materials tested. All the materials showed a significant reduction in flexural strength, except for Group A which showed slight increase in flexural strength after 24 hours in artificial saliva. This outcome can be elucidated by an increased conversion rate.^40,49

The outcome demonstrated a relative reduction in the mechanical properties of resin luting cements tested, which were aged in artificial saliva. This reduction in properties is related predominantly to the uptake of water by the polymer.^42,44 Ferracane et al., hypothesized that water (fluid) sorption results in softening of the polymer resin component by swelling the network which causes frictional forces between polymer chains.¹³After saturation of the network with water and becoming soft, the structure within the cement stabilizes, thereby causing no more decrease in properties within the time frame studied.^14,15This limited decrease in properties offers proof that further degradation, such as filler/ matrix interfacial hydrolysis or polymer matrix crazing, may be absent or may not continue significantly once the resin becomes saturated and remains wet.^20,21 This provides further evidence that the degradative but limited effect of water sorption is liable for the variation in properties seen in the current study.³⁶However, the effect of other solvents and esterases present in the oral cavity may have a more detrimental and sustained effect than artificial saliva on the mechanical properties of resin luting cements.¹⁶

Ferracane’s study showed that long-term aging of resin cement in water had slight impact on flexural strength along with fracture toughness and microhardness. There is only limited degradation of composites in water medium. Other solvents may be more aggressive and produce different results.¹³

Indirect tensile strength: IDT strength testing is a better indicator to asses aging of the dental cements. This can be elucidated by degradation mechanism.²¹ Water molecules diffusion into the material causes decomposition of chemical bonds superficially, causing decrease in mechanical strength at the surface of the samples. Compared to compressive strength test, the IDT test is more sensitive to surface defects.²⁶ Therefore, the outcome of surface degradation is more seeming in tensile test when compared to compressive test.^{23, 27}

In this study, when resin luting cements were tested and compared with each other, there was a substantial difference in the results of four materials. Group B (Tokuyama Estecem plus) had considerable maximum IDT strength, followed by Group A (Speed Cem Plus) and Group D (Panavia F2). Group C (Maxcem Elite) had the lowest IDT strength among all the four resin luting cements. Similar results were obtained once the samples were tested after 24 hours of storage in artificial saliva. There was a substantial reduction in the indirect tensile strength of each cement when compared to before and after storage of specimen in artificial saliva. This reduction in properties is related predominantly to the uptake of water by the polymer as mentioned above.^28,29

Curing mode: The dual-curing procedure was used for all the materials. Apparently, many studies revealed that the effect of dual-curing resulted in greater values than the values obtained after autopolymerizing alone.^31,38 This was more marked for flexural strength than for IDT strength. This shows that greater degree of conversion of molecules can be achieved on dual curing which in turn improves mechanical properties.³³ Oguri M et al., stated that the degree of conversion is influenced by the kind of photo-initiator and functional monomer used in a cement.^45,46

Rohr et al., stated that there was a considerable effect on flexural strength of the resin luting agents. Better polymerization was achieved for the dual-polymerized when two initiation systems were used.⁴³ The results reported by them are in agreement with our findings. Therefore, it is recommended that dual cure luting agents should be light initiated to attain improved mechanical properties like higher strength and modulus of elasticity.^39,41

Correlation of flexural strength and indirect tensile strength: Pearson’s correlation measures the association between the strength of two variables.⁴⁰ In this study, we correlated flexural strength with indirect tensile strength pre-and post-immersion in artificial saliva. The correlation between these two mechanical properties before immersion in artificial saliva was highly positively correlated and there was high statistical significance seen (r=.799; p ˂0.001). Likewise, the correlation between mechanical properties after immersion in artificial saliva was moderately positive and statistically significant (r= .608; p ˂0.001). Hence, there was a significant relationship between flexural and IDT strength (directly proportional), which means that an increase in flexural strength would lead to higher indirect tensile strength.²⁷

The drawback of the present study is that the tests do not simulate precise clinical circumstances. The durability of any luting cement can be influenced by numerous other factors such as clinical conditions, extent of moisture contamination, mixing technique, etc.³⁰ In this study, only static forces were considered but not the complete dynamic forces that act on the cements in the oral cavity. However, these in vitro results are important for screening mechanical properties like the flexural and IDT strength qualities of different resin luting cements. For further evaluation, additional studies simulating clinical situations are required.³²

Conclusion

The choice of luting agents is significantly influenced by strength parameters. Long term clinical trials can determine whether in vitro laboratory study results correlate with in vivo study results. Within the limitations of the existing study, it can be concluded that:

1. Among the cements tested, Estecem plus (Tokuyama) had superior flexural strength and indirect tensile strength compared to other three materials tested.

2. There was significant difference in flexural strength, and indirect tensile strength of the resin luting cements, before and after the immersion in artificial saliva. A marked reduction in both was noticed when immersed in artificial saliva.

3. Continuity and the clinical durability of resin luting cement may be compromised due to changes in mechanical properties when it is in contact with saliva in the oral cavity.

4. Statistically significant results were obtained in the test groups for both the parameters.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest