INTRODUCTION

Restorative resin composites have been used in dentistry for 41 years.¹ Notwithstanding the indisputable technological advances introduced during these decades, the volumetric shrinkage of methacrylate-based resin composite remains a major concern.² The development of the stresses in tooth structure can be manifested clinically as cuspal deflection. Teeth with mesio-occluso-distal (MOD) cavities have recorded the highest cuspal deflection values when direct resin composite restorations are used.³ Thus, several approaches have been taken to diminish polymerization shrinkage.^4-6 One such approach incorporated 40 wt% of pre-cured composite particles to reduce the volume of shrinking matrix available during polymerization; however, this was found to increase the material’s stiffness.⁵ Clinically, the use of different modes of curing has been recommended, yet this did not significantly reduce the shrinkage.⁶

Silorane-based resin composite has been on the market since 2007. A silorane molecule contains a siloxane core with four attached oxirane rings that open upon polymerization and bond with other monomers.⁴ The oxirane ring opening leads to a volumetric expansion that, to a certain degree, counteracts the volumetric shrinkage resulting from monomers bonding. ⁴ Previous studies confirmed that silorane-based resin composite has a total volumetric shrinkage of 1% or less compared with the methacrylate-based composites that resulted in 2%-3.5% shrinkage.^4,7 This low shrinkage property of silorane-based resin composite might lessen the need to control the negative effects that occur when restoring cavities with a high configuration factor (C-factor) and diminish the need for using the incremental application technique. Consequently, restoration of complex cavities in posterior teeth would become easier. Meanwhile, low shrinkage does not necessarily result in low contraction stress, which may manifest itself clinically by cuspal deflection.⁸ Therefore, it is important to verify the behavior of this new composite in this term. The null hypothesis tested was that there is no difference between the two materials in regards to the incremental changes and the cumulative changes in cuspal deflection.

MATERIALS AND METHODS

Selection and Standardization of Teeth

Freshly extracted non-carious human maxillary second premolars, extracted for orthodontic purposes from patients aged from 15 to 18 years, were collected. The teeth were inspected using a magnifying lens (6×) (Bausch and Lomb, Rochester, NY, USA) to confirm the absence of visible hypoplastic defects or cracks. The maximum buccopalatal width and the maximum mesiodistal width were measured using a digital caliper (Mitutoyo Corp, Kawasaki, Japan) as shown in Figures 1A and 1B. The cementoenamel junction (CEJ) was demarcated with an indelible pen. The height of the palatal cusp was measured from the cusp tip to the demarcated CEJ (Figure 1C). For the distal proximal surface, the mid-proximal line extending from crest of the marginal ridge to the CEJ was also demarcated and measured (Figure 1D). From the collected premolars, 40 teeth that showed less than 5% variation in the measured dimensions were selected. A second line was drawn so that the height of the crown segment left above its level was standardized to 10 mm for all specimens. The level of this line varied by 2 ± 0.5 mm apical to the CEJ to accommodate the 5% difference in cusp height. The apical one third of each selected tooth was trimmed using an Isomet saw (Buehler Ltd, Lake Bluff, IL, USA) to aid in the specimen mounting (Figure 1E). Teeth were stored in a phosphate buffer solution (pH=7) with 0.02% sodium azide at 4°C. Teeth were kept moist except when the study protocol required isolation from moisture.⁹

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Grouping of Specimens and Restoration Placement

The prepared specimens were divided into two main groups of 20 specimens each, according to the resin composite material used. The materials used in the current study are presented in Table 1. A methacrylate-based microhybrid resin composite Filtek Z250 (3M ESPE, St Paul, MN, USA) was used in group 1, whereas a low shrinkage silorane-based microhybrid resin composite Filtek P90 (3M ESPE, St Paul, MN, USA) was used in group 2. The cuspal deflection of each specimen (n=20/group) was recorded using a stereomicroscope before curing, just after curing (within 30 seconds), and every five minutes over a period of 30 minutes. The end result was eight records for each specimen and a total of 40 × 8 = 320 readings for both tested groups. Clearfil SE Bond (Kuraray Medical Inc, Osaka, Japan) was used for the Filtek Z250 resin composite group, whereas the P90 System Adhesive (3M ESPE, Seefeld, Germany) was used for the Filtek P90 resin composite group. Both adhesive systems were applied according to the manufacturer's instructions as shown in Table 2. The cavities were thoroughly washed with distilled water to remove any debris found during preparation. Before any of the adhesive systems were applied, the enamel was etched for 15 seconds with Scotchbond etchant gel (3M ESPE, St Paul, MN, USA) followed by thorough washing with water for 10 seconds and drying for five seconds. Each prepared specimen was bulk filled (one increment) with the proposed resin composite.¹⁰ The resin composite increments were weighed using a sensitive digital balance (0.0875 ± 0.002 g). The resin composite was light-cured with a light-emitting diode (LED) light curing unit (Satelec Acteon Group, Mérignac Cedex, France). The intensity of the light curing unit (1250 mW/cm²) was checked using an LED radiometer (Kerr Dental Specialties, Orange, CA, USA). Light curing was done from the distal, mesial, and occlusal sides for 40 seconds each, resulting in a total curing time of 120 seconds.¹¹

Table 1:  Resin Composite Restorative Materials and Adhesive Systems Used in the Study

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Table 2:  Mode of Application of the Adhesive Systems Used in the Study

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RESULTS

Statistical analysis using t-tests revealed that there was no significant difference between the tooth dimensions (p= 0.1), cavity dimensions (p=0.1), or C-factor (p≥0.4) in both groups. The descriptive statistics and test of significance of the mean changes in the cuspal deflection (μm) for Filtek Z250 and Filtek P90 are presented in Table 3. Statistical analysis comparing the change in cuspal deflections over time using one-way ANOVA revealed significant differences for both groups as shown in Table 3. Statistical analysis comparing the cumulative change in cuspal deflection over time using one-way ANOVA revealed significant difference for both groups, as shown in Table 4. Comparing the two groups at each time interval using a t-test revealed that there were significant differences at 10, 15, 20, 25, and 30 minutes.

Table 3:  Descriptive Statistics and Test Significance of the Mean Changes in the Cuspal Movement (μm) for the Filtek Z250 and Filtek P90 Groups

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Table 4:  Descriptive Analysis and Test of Significance of Cumulative Change in Cuspal Movement (μm) of Filtek Z250 and Filtek P90 at Different Time Intervals

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DISCUSSION

The thickness of the remaining cusps plays an important role in determining the degree of cuspal deflection. Cusps of teeth with large cavity preparations displayed greater deflection than those of teeth with small cavities.¹⁴ It has been reported that one of the major constraints that influences cuspal deflection measurements is the difference in cusp thicknesses among natural teeth.^15,16 Therefore, in the current study, the problem of possessing different cusp thicknesses was eliminated by keeping the size differences between the teeth less than 5%^9,10,17-20 and by standardizing the cavity preparation and restoration methods. A stereomicroscope was used to measure the cusp deflection without contacting the teeth.¹² In contrast, other methods that come in contact with the teeth, such as transducers, may stiffen the cusps, thereby interfering with their free movement.¹²

The statistical analysis demonstrated a significant difference between the two materials; thus, the null hypothesis was rejected. The finding could be explained based on the different polymerization processes of the two materials. The cationic ring opening of oxirane moieties in the silorane-based composite yields a reduced volumetric shrinkage^21,22 in contrast to the free radical addition reaction of the double bonds of the dimethacrylate-based composite.^4,7,23 When acidic cations attack the oxirane rings, the cleavage and opening of these rings gain space, thus counteracting the loss of volume occurring in the subsequent step when covalent bonds are formed.⁷ The lower shrinkage of silorane-based material due to its alternative resin formulation was confirmed by other authors.²⁴ Nevertheless, there was a difference in the obtained values between both studies, which may be related to the difference in the testing conditions, approaches, and time intervals at which the cuspal deflections were recorded.^15,25,26

Concerning the incremental changes in cuspal deflection, the recorded measurements of each restoration were not consistent over the tested intervals. Additionally, both restorations exhibited no significant difference in cuspal deflection values at all recorded time intervals, except at 10 minutes and 30 minutes. This indicates that the amount of change in the cuspal deflection was not consistent for each restoration and the amount of change for Filtek P90 was not constantly lower than that of Filtek Z250. The high initial nonsignificant cuspal deflection shown by Filtek P90 may indicate its high polymerization stress value in spite of its low volumetric shrinkage. Previous research supports this point in that low volumetric shrinkage does not necessarily correspond to a low polymerization stress development.^2,5 Marchesi and others²² support this assumption, as they have revealed no significant difference between Filtek P90 and Filtek Z250 in the recorded shrinkage stresses after five minutes. Additionally, the microtensile results recorded by Van Ende and others²⁷ failed to confirm that silorane resin composite (Filtek P90) induces less stress at the adhesive interface compared with methacrylate resin. It seems that the polymerization degree and rate, the elastic modulus acquired during polymerization, and the volumetric changes are important contributing factors in contraction stress development.²⁸ Regarding the polymerization rate, although the cycloaliphatic oxirane sites of silorane have high reactivity and shrink less during polymerization, the extent to which the reactions will achieve better polymerization and higher cross-linking density compared with free-radical polymerization is not yet clear.²⁹ Additionally, the change in the composite viscoelastic behavior that occurs during polymerization makes polymerization stress development quite a complex event.² Therefore, any analysis of shrinkage stresses should consider the restorative material properties and all other contributing factors.

CONCLUSIONS

Incremental cuspal deflections of both materials over the tested intervals were almost comparable. However, after five minutes of curing, the silorane-based resin composite surpassed the methacrylate-based resin composite in controlling the cumulative cuspal deflection.