Tests and Methods:

1. Fatigue:we found only few studies which tested the fatigue of dental filling materials . Each used a different method:

1. Cylindrical specimens, 4.0 mm in diameter and 8.0 mm long, were prepared by composite insertion into a stainless steel split mold. The materials were photo-cured on each lateral face. The specimens were then stored indistilled water at 35 ± 2°C for 7 days. Following storage, the compression strength test was performed in an MTS 810 mechanical test machine (MTS Systems Corporation – 14000 Technology Drive, Eden Prairie, MN, USA) at a 0.5 mm/min cross-head speed, determining the mean compression strength of each composite (n = 15).[1]

2. In the study human molars and premolars (type of teeth) were used. The cavities deep of 3 mm were filled with composite materials. There was a contact between the material and enamel and dentin. The composite material was applied in the layers, each of 2 mm thick, and was polymerized by halogen lamp for 40 sec. Then, teeth samples were placed in special holders and exposed to cyclic mechanical loads in contact with buffer solution with pH=6.8.

Degradation test of composite materials were estimated using special mastication simulator. The tester simulate mechanical loads that taking place during natural mastication process to the highest degree. The simulator enableto design the parameters of really operating condition and estimate the course of wear process and defects of dental fillings relate to clinical conditions.[2]

3.Four commercial dental composite filling materials that are used in load bearing restorations in posterior teeth were investigated. The composite materials were injected into glass capillary tubes resulting in finished specimens that were 0.85mm in diameter and the test length was 14mm for the material “Alert” and 12mm for the other three materials. The resins were photo polymerized with a Coltolux 4 light (Coltene Wahaladent, Dentalvertiebs GmbH, Konstanz/Germany) directed at the side of the capillary tube using 40 s exposure time foreach 5mm of length of the tube. Thorough curing was achieved because of the small diameter of the specimens. The specimens were stored in a dry beaker at 21 ◦C for 24h after fabrication and subsequently tested dry at room temperature 21 ◦C. The specimens were mounted between a rod and a permanent magnet by means of additional composite as a cement, using a jig for centering. The specimens were tested in a torsional creep apparatus that has been described in previous articles . A permanent samarium cobalt magnet fixed to the end of the specimen generated torque. The magnet produced a torque of 0.0145Nm/A at the center of a Helmholtz coil. A thin mirror 8.2mmin diameter and 1.5mmthick was cemented to the magnet to reflect a laser beam to a chart at a distance D of 944 cm. Tests of quasistatic modulus were done by placing the apparatus under DC current to apply a constanttorque. The deformation of the specimen was recorded for a period of time, and then the stress was released to zero. Recovery followed for 10 times the period under load. For fatigue measurements, AC current was used to drive the specimen at torsional resonance. The amplitude of angular displacement of the driven end was measured on the chart at the resonance frequency[3]

2. Flexural StrengthTen samples of each material were prepared by the same investigator according to International Standards Organization(ISO) 4049 specifications (25 × 2 × 2 mm) by using stainless steel split molds and light cured for 20s by using a blue light-emitting diode with a 550mW/cm2 light source (Hilux Ledmax 550, Benlioğlu Dental, Ankara, Turkey) according to the manufacturer’s directions. After polymerization, samples were stored in distilled water at 37˚C for 24 hours. The flexural tests were performed with a Universal Testing Machine (LF Plus, LLOYD, Instrument, AmetekInc, England) at a cross-head speed of 1mm/min, with 2 Kn loading force.[4]

3. Compressive StrengthTen cylindrical samples of each material were prepared by the same investigator according to American Dental Association (ADA) 27 specifications (3 mm diameter × 6 mm height) by using stainless steel split molds and

light cured for 20 s by using a blue light-emitting diode with a 550 mW/cm2 light source (Hilux Ledmax 550). The universal posterior composite samples were prepared incrementally. The other materials were prepared according to the to the manufacturer’s directions for cavities deeper than 4/5 mm height. Samples for compressive strength tests were stored in distilled water at 37˚C for 24 hours. The compressive tests were performed with aUniversal Testing Machine (LF Plus, LLOYD, Instrument, Ametek Inc, England) at a cross-head speed for 0.05mm/min.Compressive strength and flexural strength tests data’s were subjected to one-way variance analysis andKruskal-Wallis tests at a significance level of 0.05. [4]

4. Depth of Cure and Knoop Hardness TestsThese tests provide an indication of the total depth to which the composite will cure or the surface hardness you will achieve when the composite is irradiated by a curing light for the amount of time recommended by the manufacturer. Methods: We measured the depth of cure for a cylindrical sample of 11 composites according to the standard test method (ISO 4049-2009).16 We also recorded a relative bottom-to-top hardness measurement based on the Knoop hardness test of each specimen. [5]

5. Fracture ToughnessFracture toughness is an intrinsic property of a material and is a measure of the energy required to propagate a crack from an existing defect. For this study, notch-less triangular prism (NTP) specimens (6 mm x 6 mm x 6 mm x 12 mm) were made from each core material (n = 15). Each side of the sample was light-cured for 30 seconds, then all samples were polymerized in a visible light-polymerizing (VLP) chamber for 5 minutes. The samples were stored in distilled water at 37ºC for 7 days prior to testing. NTP fracture toughness was determinedin a universal testing machine (Instron Model 3345, Instron Corp., www.instron.com).

Each sample was scorched at the location of tensile forces in order to create a defect. Force was applied until failure of the specimen occurred. The relationships that were used to calculate the FT (KIC) were proposed by Barker8 and adopted by ASTM standard E1304. The equation is as follows:

where Pmax = the maximum load at fracture, D = the specimen diameter, W = the specimen length, and Y*min = the minimum of the dimensionless stress intensity factor coefficient (= 28).9 [6]

References:[1]: http://www.scielo.br/scielo.php?pid=S1806-83242005000400007&script=sci_arttext

[2]: VIBROMECHANIKA. JOURNAL OF VIBROENGINEERING. December 2009. Volume 11, Issue 4, ISSN1392-8716

[3]: Fatigue of packable dental composites. Y. Papadogiannisa, , R.S. Lakesb, G. Palaghiasa, M. Helvatjoglu-∗Antoniadesa, D. Papadogiannisc. Dental Materials 2 3 ( 2 0 0 7 ) 235–242

[4]: Published Online April 2014 in SciRes. http://www.scirp.org/journal/ojcm , http://dx.doi.org/10.4236/ojcm.2014.42013

[5]: A Laboratory Evaluation of Bulk-Fill Versus Traditional Multi-Increment-Fill Resin-Based Composites. Amer Tiba, PhD; Gregory G. Zeller, DDS, MS; Cameron Estrich, MPH; Albert Hong. Volume 8, Issue 3 - American Dental Association.

[6]: Comparison of Mechanical Properties of Five Commercial Dental Core Build-Up Materials. Compendium. January 2013, Volume 34, Issue 1Published by AEGIS Communications.