Effects of thermal fatigue on shear punch strength of tooth-colored restoratives.

AIMS: This study investigated the effect of thermal fatigue on the shear strength of a range of tooth-colored restorative materials including giomers, zirconia-reinforced glass ionomer cement (GIC), nano-particle resin-modified GIC, highly viscous GICs, and composite resin.
MATERIALS AND METHODS: Twenty specimens of each material were fabricated in standardized washers (17 mm outer diameter, 9 mm internal diameter, 1 mm thick). The specimens were cured, stored in 100% humidity at 37.5°C for 24 h, and randomly divided into two groups of 10. Group A specimens were nonthermocycled (NT) and stored in distilled water at 37°C for 168 h. Group B specimens were thermocycled (TC) for 10,000 cycles (168 h) with baths X, Y, and Z adjusted to 35°C, 15°C, and 45°C, respectively. Each cycle had dwell times of 28 s in X, and 2s in Y/Z in the order XYXZ. Specimens then underwent shear punch testing at a crosshead speed of 0.5 mm/min with a 2 kN load cell. Statistical analysis of shear strength was done using t-test and two-way ANOVA/Scheffe's post hoc test at significance level P < 0.05.
RESULTS: The effect of thermal fatigue on shear strength was material dependent. Except for the "sculptable" giomer (Beautifil II) and a highly viscous GIC (Fuji IX GP Fast), no significant differences in shear strength were generally observed between the NT and TC groups. For both groups, the composite resin (Filtek Z250XT) had the highest shear strength while the zirconia-reinforced (zirconomer) and a highly viscous GIC (Ketac Molar Quick) had the lowest.
CONCLUSIONS: The effect of thermocycling on shear strength was material dependent. Thermal fatigue, however, did not significantly influence the shear strength of most materials assessed. The "sculptable" composite and giomer were significantly stronger than the other materials evaluated. Shear strength of the "flowable" injectable hybrid giomer was intermediate between the composite and GICs.

RCT Entities:   Population Interventions Outcomes

INTRODUCTION

Thermal changes in the mouth may affect the clinical longevity of restorations. Intraoral temperature changes are induced during eating, drinking, and even breathing.[1] These thermal changes induce mechanical stresses and can cause gap volume changes leading to marginal leakage, staining as well as secondary caries.[234] Although the effects of thermal fatigue on physicomechanical properties have been investigated, results were equivocal. While some studies have reported that thermal fatigue significantly reduced fracture resistance[56] and impaired marginal adaptation,[78] others have found that it improves physicomechanical properties, including bond strength[9] and hardness.[1011] Reasons suggested for the disparity in results include differences in thermocycling regimen, type of restorative material, and the “C” factor.[12] When comparing these studies, it is also important to take into consideration the differences in research design and the mechanical properties investigated.

As new and novel tooth-colored restoratives are introduced to the dental profession, studies on their durability and longevity, especially when subjected to intraoral thermal and other stresses are required. The two ends of the continuum of tooth-colored restoratives are represented by composite resins and glass ionomer cements (GICs), respectively. While composites offer superior strength, excellent esthetics, and high wear resistance, GICs offer sustained fluoride release/recharge and chemical bonding to teeth. In an effort to combine the advantages of these two materials, various hybrid materials have been developed. These hybrid materials include resin-modified GICs (RMGICs), compomers (polyacid-modified composites), and giomers.

Giomer restoratives utilize prereacted glass ionomer (PRG) technology to form its glass ionomer phase. The glass ionomer fillers consist of fluorosilicate particles that are prereacted with polyacrylic acid, and these fillers are incorporated into a resin matrix. The PRG phase is assumed to promote sustained fluoride release via ligand exchanges within the ion-rich hydrogel without disrupting the integrity of the filler–matrix interface that typically occurs in compomer materials.[13]

To improve physicomechanical properties and reduce early moisture sensitivity, novel GIC materials are constantly being developed. Recent improvements in GIC restoratives had been achieved through advances in glass formulations, refinement of glass particle sizes, increasing powder: Liquid ratios, incorporation of resins, and zirconia reinforcement. While thermocycling was reported to enhance the hardness of giomers due to increased postcure maturation on contact with heat,[14] literature pertaining to the effects of thermocycling on the new glass ionomer materials are still scarce. Research is thus warranted to better comprehend the mechanical performance of these innovative restorative materials.

The aim of this study was to evaluate the effects of thermal fatigue on the shear strength of a range of tooth-colored restorative materials including giomers, zirconia-reinforced, and highly viscous GICs, nano-particle RMGICs as well as a composite resin. It was hypothesized that thermal fatigue has no significant effect on the shear strength of these materials.

MATERIALS AND METHODS

Seven restorative materials were selected from the range of currently available tooth-colored restorative materials. They included giomers (Beautifil II [BF2], Shofu; Beautifil Flow Plus [BFP], Shofu), novel zirconia-reinforced GIC (Zirconomer [ZRM], Shofu), highly viscous GIC (Fuji IX GP Fast [F9F], GC; Ketac Molar Quick [KMQ], 3M-ESPE), nano-particle resin-modified GIC (Ketac Nano [KN], 3M-ESPE), and a composite resin (Filtek Z250XT [Z250], 3M-ESPE). The detailed technical profiles of the materials evaluated are shown in Table 1. The materials were mixed according to the manufacturers' instructions where applicable and/or dispensed directly into stainless steel ring washers (17 mm outer diameter, 9 mm inner diameter, and 1 mm thickness), covered with mylar strips and sandwiched between two glass slides. Gentle pressure was applied to the glass slides to extrude excess material. Light-cured specimens were then polymerized using an LED light cure unit (DB-686-Ib, Coxo, Foshan Guangdong, China). The mean intensity of the light source was checked with a commercial radiometer (Light Meter-200, Rolence Enterprise Inc., Chungli, Taiwan) and found to be above 1200 Mw/Cm2. All light-polymerized materials were light-cured for 40 s and left alone for 15 min. Chemically-cured materials were also allowed to set for 15 min. After the initial setting period, the glass slides and mylar strips were removed, and specimens were then stored in 100% relative humidity at 37°C for 24 h. Thickness of the specimens was measured by a digital caliper to ensure that the specimens were within 1.00 ± 0.01 mm thickness.

Table 1

Technical profiles of the materials evaluated

Twenty specimens were made for each material and randomly divided into two groups of 10. Specimens in Group A formed the nonthermocycled group (NT) and were stored in distilled water at 37°C for 168 h. Specimens in Group B formed the thermocycled group (TC) and were thermal cycled for 10000 cycles. Thermal cycling was conducted using a computer thermal fatigue system (NUS-AMA, Singapore, Singapore) comprising a two-axis computer-controlled robotic gantry system, temperature-controlled baths, and a controller with a software interface. Employing gale and Darvell's advocated thermal cycling regimen,[15] the temperatures of the water in the baths X, Y, and Z were adjusted to 35°C, 15°C, and 45°C, respectively. One thermal cycle consisted of the cycle XYXZ, with dwell time in container X being 28s, and 2s each in containers Y and Z. The total time for each thermal cycle was 1 min leading to a total thermal cycling time of 7 days (168 h in total).

Shear strength testing was conducted using a custom-designed micro-punch apparatus [Figure 1] mounted on an Instron Micro-tester (Model 5848, Instron Corp, Norwood, MA, USA). The specimens were positioned in the apparatus with the cured side facing up, via means of a self-locating recess, which provided a snug fit. The specimens were restrained using a screw clamp to a torque of 2.5 Nm using a torque wrench. A tool steel punch with a flat end 1.95-mm in diameter was used to create shear force by sliding through a punch hole with a radial clearance of 0.01-mm. Prior to testing, the entire experimental setup, including the Instron machine and its 2 kN load cell, was calibrated to ensure minimal frictional force as compared to the force value required to fracture the test specimens. Testing was done at a crosshead speed of 0.5-mm/min, and the maximum load was recorded. Statistical analysis was carried out using SPSS version 20.0 (IBM SPSS Inc., Chicago, IL, USA). Shear strength data were subjected to t-test and two-way ANOVA/Scheffe's post hoc test at significance level P < 0.05.

Figure 1

Schematic representation of the shear punch test apparatus

RESULTS

The mean shear strength values of the materials after incubation (NT) and thermocycling are shown in Figure 2 and Table 2. The effect of thermal fatigue on shear strength was found to be material dependent. t-test showed no significant differences in shear strength between NT and TC specimens for all materials with the exception of F9F and BF2. Two-way ANOVA showed shear strength to be both material and thermal fatigue dependent. Results of two-way ANOVA and Scheffe's post hoc test for both TC and NT groups are shown in Table 3. Z250 and BF2 were found to be significantly stronger while ZRM and KMQ were significantly weaker than the other materials evaluated. The shear strength of the “flowable” injectable hybrid giomer BFP was intermediate between the composite and glass ionomers.

Figure 2

Mean shear strength of materials after incubation and thermocycling

Table 2

Mean shear strength of materials after incubation and thermocycling

Table 3

Results of statistical analysis between materials for both nonthermocycled and thermocycled groups

DISCUSSION

The thermal cycling protocol used in this study was based on Gale and Darvell and their suggested estimate of 10,000 cycles. The extremes in temperatures were fixed at 15°C and 45°C as they were the lowest/highest comfortable temperatures reported from in vivo studies.[15] As the effect of thermal fatigue on shear strength was found to be material dependent, the null hypothesis was rejected. No significant differences in shear strength between TC and NT specimens were found for the majority of materials evaluated. Findings corroborated the work of Mair and Vowles, who concluded that “the effects of cyclic temperature changes likely to be encountered in the mouth are not a significant factor in the reduction of the fracture strength of composite filling materials”.[16] The shear strengths of F9F and BF2 were, however, significantly increased by thermal cycling. Improvements in strength can be attributed to an increase in “bound” water in the glass ionomer components that may be enhanced by thermal fatigue.[1718] As the hydration of GICs increases, so will the strength of the GICs. Although BFP, ZRM, KN, and KMQ also contained glass ionomers; their shear strengths were not significantly affected by thermocycling. The discrepancies in findings may be attributed to differences in glass ionomer content and composition and warrants further investigation.

Shear punch test was chosen as the mode of testing in our experiment. Shear punch test has been used for the standard testing of plastics and was advocated as a standard specification test across a broad range of restorative materials.[192021] Results are clinically relevant as shear stresses are induced in teeth and restorations during mastication by occlusal or incisal forces.[2122] The advantages of the shear punch test have been reported by Nomoto et al. The main advantage, over other tests, is the ease of preparing good quality specimens. The “quality of the edges of the disc around the circumference has no direct influence on the testing outcome” for shear punch testing. On the other hand, the quality of the surfaces and edges of specimens is most critical in flexural, compressive, and diametral testing. The only requirement for shear punch testing is that the two main faces of the disc specimens are flat and parallel.[19] Ensuring parallel faces are greatly facilitated by the use of standard washers for specimen preparation, which was done in this study.

Nomoto et al.[19] reported that shear punch strength values decreased in the order of composite > giomer > amalgam > compomer > RMGIC > GIC > polycarboxylate. In comparison, ranking of shear strength in this study was as follows: Z250 (composite resin) > BF2 (giomer) > F9F (highly viscous GIC) > KN (nano-particle RMGIC) > BFP (injectable hybrid giomer) > ZRM (zirconia-reinforced GIC) = KMQ (highly viscous GIC). Z250, a nano-hybrid composite resin, had the highest shear strength. Composite resins are generally stronger than GICs. Nomoto et al.[19] reported that the higher strength associated with composite materials is consistent with their ability to resist fracture in relatively thin sections when used in adhesive restorations. In addition, the mechanical properties of composite materials are improved in direct relation to the amount of filler added. This is the same for glass ionomers with increasing powder: Liquid ratio up to a critical level. The high shear strength of Z250 can be attributed to not only its high filler content (81.8% weight) but also to the addition of surface-modified zirconia/silica with a median particle size of approximately 3 microns or less as well as nonagglomerated/non-aggregated 20 nanometer surface-modified silica particles. Furthermore, Z250 is a nano-hybrid composite resin, which contains sub-100 nm to micron-sized particles. According to its manufacturer,[23] the wide distribution of particle sizes can lead to high filler loading with resultant high strength and wear resistance.

The second strongest material was shown to be BF2. This can also be attributed to its high filler loading (83.0% weight) obtained by the surface PRG fillers and discrete nano fillers (10–20 nm). The results of this study were consistent with others including that of Yap et al.,[11] who used a microindentation technique and found BF2 to be significantly harder than highly viscous and resin-modified GICs as well as some composites. The significantly lower shear strength of the injectable hybrid composite BFP when compared to BF2 can be attributed to the lower filler loading (67.3% weight).

The GICs in this study were ZRM, F9F, KMQ, and KN. Among the GICs, F9F was the strongest, followed by KN. ZRM and KMQ were significantly weaker than the other materials evaluated. Highly viscous GIC F9F is an improved version of Fuji IX GP. Wang et al.[21] reported that “the shortened maturation time of Fuji IX GP Fast may make it less susceptible to the effect of early water exposure.” This could have contributed to the high strength of F9F. The higher strength of F9F can also be attributed to its higher powder: Liquid ratio (3.6:1). Kanachanavasita et al.[24] has showed that for restorative grade materials, resin-modified cements can absorb water up to 7% by mass. The amount of water uptake by RMGICs is dependent on its poly (HEMA) content. Hence, it is possible that water sorption could have lowered the strength of the RMGIC, KN.

The nano-particle RMGIC, KN was found to be the second strongest GIC. According to the manufacturer, it contains significantly fewer voids than Fuji II LC and Fuji IX as it is dispensed from the Quick Mix Capsule. Nomoto et al. have reported that “the porosity of a cured restorative material may lead to reduced physical properties”.[25] Hence, fewer voids lead to reduced porosity and this may in turn contribute to increased physical properties. An interesting point to note is that unlike most GICs which require a powder and liquid to be mixed, KN uses a paste-paste system to provide faster, easier, less messy and more reproducible dispensing, and mixing compared to powder-liquid systems.[26] The nanotechnology in KN, together with resin modification, could contribute to its higher strength compared to KMQ and ZRM.

KMQ is a “heavy-bodied, metal-free glass ionomer”,[27] with a powder to liquid ratio of 3.4:1. The powder contains Calcium-Lanthanum-Aluminum fluorosilicate glass while the liquid consists of mainly polycarboxylic acid as in conventional GICs. The low shear strength of KMQ may be attributed to the fact that it is not reinforced by resin or nanoparticles (unlike KN) and has a lower powder: Liquid ratio than F9F (3.4:1 vs. 3.6:1). The zirconia-reinforced GIC, ZRM, was noncapsulated and required hand-mixing. A capsulated version of this material was recently introduced into the market. When compared to the other capsulated and triturated materials, an even mix and consistent texture was difficult to achieve with ZRM. This could be a contributing factor to its lower shear strength. No significant difference in shear strength was observed between ZRM and KMQ.

CONCLUSION

The effect of thermocycling on shear strength was found to be material dependent. While the shear strengths of most materials were not affected by thermal fatigue, strength of the “sculptable” giomer (BF2), and a highly viscous GIC (F9F) were significantly increased by thermocycling. The regular “sculptable” composite (Z250) had the highest shear punch strength while another highly viscous GIC (KMQ) and zirconia-reinforced GIC (ZRM) had the lowest. The shear strength of the “flowable” injectable hybrid giomer (BFP) was intermediate between the composite and GICs.

Financial support and sponsorship

NUS UROP grant.

Conflicts of interest

There are no conflicts of interest.