Flexural Strength and Depth of Cure of Single Shade Dental Composites

Aims: This work aims to assess the flexural strength and depth of cure of Optishade, Omnichroma and Z350 dental resin composites. Study Design: Experimental Laboratory Study. Methods: To assess flexural strength as per ISO standards, 15 samples of each of the three materials were made (n=5) with the dimensions 25x2x2 mm. They were then subjected to 3 point bending testing on a universal testing machine. To assess depth of cure as per ISO standard, 15 cylindrical samples 4 mm in diameter and 6 mm in height were created (n=5) and scraping test was performed. Results: There was a significant difference between the 3 materials in both flexural strength and depth of cure. Z350 had the lowest depth of cure and the highest flexural strength. Conclusion: Within the limitations of this study, all three tested materials fell within the ISO requirementsfor dental resin compositesfor both flexural strength and depth of cure.


INTRODUCTION
Dental resin composites are currently considered the material of choice for many dental practitioners [1,2]. Good esthetics andconstantly improving physical properties play a large part in the increase of their usage [3]. In the quest for a perfectly camouflaged restoration,several composite systems have a large variety of shades available to help match multiple teeth. These shades are often layered in an attempt to get the best match to the adjacent tooth structure.. Unfortunately, even with the existence of a plethora of shades in several composite systems, the creation of an imperceptible restoration is not always achieved in clinical situations. This is due toseveral material and operator-related factors [4]. In an effort to simplify the process of shade selection and improve the final esthetic results, manufacturers have recently introduced "single" and "group shade" dental composites. These materialsare meant to camouflage into the tooth structure once they have been placed [5]. Simply defined, "single shade" dental resin composites are those that use only one shade for all restorations.. Similarly, "group shade" dental resin composites have a narrower group of shades to replace the wide array frequently found in esthetic restorative materials. These materials have been shown to camouflage very well into the surrounding tooth structure [6,7].
While esthetic propertiesare undeniably important in anterior restorations,all direct restorative materials are subjected to a significant mechanical challenge during function. A composite restoration needs to be strong enough to withstand the mastication process. Flexural tests are often used to measure composite strength [8]. The International Organization for Standards (ISO)states that theflexural strength should be determined using a 3-point bending test for polymer-based restorative materials [9]. This willgive clinicians an indication of the material's ability to withstand deformation and fracture [8,10]. Another important material property is the depth of cure. A well-cured restoration will have the optimal properties possible for that material. It follows that under-curing a restoration will result in suboptimal properties. Under-cured restorations may result in secondary caries, less wear resistance and restorative failure [11,12]. Furthermore, chemicals are more likely to be leached into the oral cavity by under-polymerized resin dental composites [13]. The ISO has a standardized testing regimen for depth of cure measurement [9]. Measuring depth of cure, as per the ISO standard, aims to ensure the clinician knows the maximum increment size which will be adequately polymerized during restoration placement.
There are several papers in the currentliterature assessing the esthetic qualities of single and group shade dental resin composite materials [14][15][16][17][18][19][20]. However, the literature discussing the physical and mechanical properties of these materials is limited at this time [15,21]. This work aims to assess the flexural strength and depth of cure of a single shade and group shade dental resin composite compared to a conventional one. The tested null hypotheses are: 1. There is no difference between the flexural strength of the three materials tested 2. There is no difference between the depth of cure of the three materials tested

MATERIALS AND METHODS
The details of the materials used in this study are listed in Table 1.  [9]. For the setup, a glass slide and Mylar strip were placed under the mold. The material to be tested was then placed into the mold as a single increment. Next,a Mylar strip was placed over the test material and pressed with a glass slide, effectively sandwiching the mold between the glass slides/Mylar strips. This removed excess material prior to light curing.Each specimen was light cured on both the bottom and top surfaces using five overlapping exposures of 20 seconds each, by a LED light curing unit (LCU) (3M ESPE Elipar, St Paul, MN, USA). The irradiationwas measured with a hand held radiometer (Bluephase Meter II, Ivoclar, Amherst, NY, USA )at 1000 mW/cm 2 immediately before specimen preparation. 320 grit abrasive paper was used to finish the specimens. Samples were then stored in for 24 hours in distilled water at 37°C.

Flexural Strength Testing
Flexural Testing was done on a universal testing machine 5944 (Instron, Norwood, MA, USA) with a crosshead speed of 0.5 mm/min. As per the ISOstandard [9], the formula used to calculate flexural strength is: Where "F = maximum load exerted on the specimen in Newtons (N) l = distance between the supports (in mm) b = width at the center of the specimen (in mm), measured immediately before testing h= height at the center of the specimen (in mm), measured immediately before testing"

Depth of Cure
Specimens of Filtek Z350, Omnichromaand Optishadewere prepared, with 5 specimens per group. Thespecimens were made by using a stainless steel cylindrical split mold [New Age Research, USA]. The internal dimensions of the moldwere 6 mm in length and 4 mm in diameter.
For specimen preparation, the mold was positioned on a glass slide with a transparent Mylar strip on it. The mold was then filled with the desired dental resin composite material and covered with another Mylar strip and then a glass slide. The mold and strips were pressed between the slides in order to displace excess material. The mold was then placed on filter paper, the upper glass slide removed and the material light cured as per manufacturers' recommendations. The LCU irradiation was measured with a hand held radiometer at 1000 mW/cm 2 immediately before specimen preparation. The specimen was removed from the mold directly aftercuring was completed. A plastic spatula was used to remove any uncured material. A digital micrometer[Mitutoyo Co, Kawasaki, Japan] was used to measure cured cylinder height. The measured value was divided by two to obtain the depth of cure. The results were analyzed using one-way analysis of variance (ANOVA) followed by pairwise comparison using Bonferroni method. Tests were conducted to test the effect of the material on the flexural strength and depth of cure. Statistical software SPSS Ver. 17 [IBM, Armonk, NY, USA] was used at 0.05 significance level.

Flexural Strength
There was a statistically significant difference between the 3 materials tested (p= 0.00132). The mean flexural strength of all the materials (in MPas) along with their standard deviations is shown in Fig. 1    Filler loading 78.5% by weight (63.3% by volume) of non-agglomorated/non-aggregated silica fillers (20 nm), non-agglomorated/non-aggregated zirconia fillers (4 -11 nm) and aggregated zircona/silica cluster fillers (20 nm silica and 4 -11 nm zirconia). The average cluster particle size is 0.6 -10 microns.
Bonferroni pairwise comparison revealed the difference was between Optishade and Z350 (p= 0.00238) as well as Omnichroma and Z350 (p =0.00487). There was no statistically significant difference between Omnichroma and Optishade (p=1).

Depth of Cure
One way ANOVA revealed a statistically significant difference between all three materials (p= 0.00011). The mean depths of cure, along with their SD are shown in Fig. 2.
Bonferroni pairwise comparison revealed the difference was between Optishade and Z350 (p= 0.00018) as well as Omnichroma and Z350 (p =0.00069). There was no statistically significant difference between Omnichroma and Optishade (p = 1).

DISCUSSION
Dental restorative materials are subjected to a multitude of stress types, which must be considered when evaluating a dental filling material. Some examples of these stresses are compressive, tensile and shear. These simple stresses form the foundation of other, more complex,patterns such as flexural stress [22]. Flexural strength has been correlated to clinical wear of composites [23,24]. Currently, the ISO standard uses a3-point flexural testing for resinbased filling materials [9].As both Optishade and Omnichroma are relatively new to the dental marketplace, the current standardISOtesting procedureswere used to measuretheir flexural strength and depth of cure. The results were then compared to Z350, which is a conventional composite andhas been in the marketplace for longer.
At present, themajority of the published work on single and group shade dental resin composites focus mainly on their esthetic properties [6,7,[16][17][18]20,25,26]. There is very limited published work available in the literature regarding any other physical or mechanical properties of these materials [15,21]. This work aims to begin filling that gap.
The null hypotheses were both rejected in this work. The alternate hypotheses were accepted asthere was a significant difference between the materials tested in both flexural strength and depth of cure results.
Regarding the flexural strength of the materials tested, all 3 materials were above the ISO's standard of 80 MPa. Z350 exhibited a significantly higher flexural strength than either Optishade or Omnichroma. While all three materials are highly filled, Z350 has the lowest filler loading of any of the materials tested in .

Mean Depth of Cure Values
this work at 78.5% by weight.Omnichroma has 79% by weight and Optishade has 81% by weight. It has been found that while strength increases with an increase in filler loading, strength declines at very high filler levels [22]. Another possible reason for the increase in Z350's flexural strength may be the presence of the aggregated zircona/silica cluster fillers. Nanoclusters have been found to significantly increase the strength of nanohybrid dental resin composites [27]. The cause is believed to be that the nanoclusters absorb and dissipate crack propagation [28]. The polymer matrixcomposition ofZ350 differs to that ofthe other two materials in this work. While Omnichroma has a BisGMA free matrix, both Optishade and Z350 have BisGMA and TEGDMA in their matrixes. Both Z350 and Omnichroma have UDMA. Z350 is the only material tested which has Bis EMA in its polymer matrix. AlthoughBis-EMA's structure is similar to Bis-GMA, Bis-EMA has a lower viscosity and is more flexible, which could result in higher overall conversion [29]. Studies have found that polymers with Bis-EMA had a high flexural strength [30]. The result of this work was not in agreement with that of Takuma et al, who found that Omnichroma had a flexural strength of similar value or greater than the other dental resin composites tested [21]. However, none of the comparative materialsthey testedhad nanoclusters. The comparators were esthetic, group shade or giomer resin composite materials. Thus, the results of that work are expected to be different from the ones in this study. Schweppe et al.reported flexural strength results of 86.4 MPa for Omnichroma, which is similar to what was found in this work [15].
Adequate curing is a requirement for dental resin composite restorations to achieve their optimal properties, while a multitude of restorative failures have been reported due to under-curing [12]. Dental resin composite restorations which are inadequately cured have less than optimal physical and mechanical properties [31,32]. Several factors affect the polymerization of dental resin composites, among which are the wavelength of the dental curing light [33]. The light cure used in this workwas the same for all materials prepared The light cure unit'sirradiance was measured prior to sample preparation and found to be 1000 mW/cm 2 at each reading. This effectively eliminated the effect of variations within the LCUsuch as type of light cure unit, wavelength emitted, etc. The factors which remain are all material related. Theyinclude material shade, filler type and volume [34].
Darker material shade and larger filler size have both been shown to decrease light penetration into the dental resin composite, while smaller particles and lighter shades allowed more light penetration from the LCU [35]. The depth of cure was positively correlated with the light penetration through the dental resin composite [36].

Both
Optishade and Ominchroma had significantly higher depths of cure when compared with Z350. This may be due, in part, to the higher translucency of both these materials in comparison to shade A2 Z350. Both Optishade and Omnichroma are marketed as translucent materials which are capable of reflecting the shade of the surrounding tooth structure. The depth of cure of Z350 was in agreement with other published work [32,37,38]. While all three materials tested may be classed as nanocomposites due to the size of their fillers, both Optishade and Omnichroma have smaller particles than Z350. As mentioned earlier, Z350 also has nanoclusters which increase the filler size. Thus, it follows that Z350 had the lowest depths of cure measured. Despite the differences found, each of the materials cured above the 2 mm depth recommended by their manufacturers. This meant that each of the materials had a depth of cure which was more than 1.5 mm and thus they all conformed to the current ISO requirement [9]. Ensuring that 2 mm increments are used while placing the esthetic restorations will allow each material to polymerize satisfactorily, all else being equal. This knowledge will help guide dental clinicians during incremental placement of dental resin composites, ensuring adequate polymerization of each increment and increasing the chances of the final restoration's success and longevity.

CONCLUSION
In conclusion, within the limitations of this study, both Optishade and Omnichroma showed a higher depth of cure anda lower flexural strength than Z350. All the materials tested conformed to the ISO requirements for both flexural strength and depth of cure.

CONSENT
It is not applicable.

ETHICAL APPROVAL
As per international standard or university standard written ethical approval has been collected and preserved by the author(s).