2.1

Microhardness profiles

Microhardness (Vickers hardness; HV) profiles were generated by means of a re-usable, block-shaped, and custom-made Teflon mold with a semicircular notch of 15 mm in length and 4 mm in diameter ( Fig. 1 A ). The semicircular notch was entirely filled with one of the five resin composites. Then, the mold was covered with a Mylar strip (Hawe Stopstrip Straight, KerrHawe, Bioggio, Switzerland) and the resin composite in the semicircular notch was made flush with the mold by use of a glass slide. Excessive resin was removed and the mold was covered by a Teflon shell ( Fig. 1 B). A second Mylar strip was placed on the semicircular opening ( Fig. 1 C) and the resin composite was light-cured for 20 s through the semicircular opening (top surface) keeping the tip end of the light-curing unit centered and in contact with the second Mylar strip. After light-curing, the shell and both Mylar strips were removed and the mold including the resin composite specimen ( Fig. 1 D) was placed in a black photo-resistant box at 100% humidity and the box was stored in an incubator (Memmert UM 500, Memmert GmbH & Co., Schwabach, Germany) at 37 °C for 24 h. After storage, the mold including the resin composite specimen was placed under a microhardness indentation device (Fischerscope HM2000, Helmut Fischer GmbH, Sindelfingen, Germany). Subsequently, six HV measurements were made on the resin composite specimen at defined distances (“depths”), beginning with an HV measurement at a depth of 1 mm from the irradiated top surface followed by HV measurements at a depth of 2 mm, 3 mm, 4 mm, 5 mm, and 6 mm (i.e. at increasing distances from the top surface). Programming of the hardness indentation device for defined distances and reproducible placement of the mold ensured that the HV measurements were made along the same axis on each specimen. HV measurements were performed in a force-controlled mode for 50 s with the test load increasing and decreasing between 0.4 and 500 mN at a constant speed. For each of the five resin composites, n = 8 specimens were prepared resulting in eight HV profiles per resin composite.

Specimen preparation. Microhardness profiles: A, Teflon mold with semicircular notch; B, Teflon shell; C, semicircular opening for light-curing; D, mold including the light-cured resin composite specimen. Microhardness (HV) and shear bond strength (SBS) at increasing increment thickness: E, split Teflon molds; F, dentin underlay (mid-coronal dentin of a ground, embedded human molar); G, mold filled with resin composite for future HV measurement; H, dentin specimen (mid-coronal dentin of ground, embedded human molars); I, specimen for future SBS measurement.

2.2

Microhardness at increasing increment thickness

Another 210 resin composite specimens were produced for measurement of microhardness (Vickers hardness; HV) on the bottom surface (i.e. on the surface opposing the irradiated top surface) of specimens of increasing increment thickness ( n = 14/group; 15 groups (5 resin composites, 3 increment thicknesses)). Specimens of the five resin composites were prepared in re-usable custom-made Teflon molds (split Teflon molds with an inner diameter of 3.6 mm and a height of 2 mm, 4 mm, or 6 mm mimicking three increment thicknesses ( Fig. 1 E)). The molds were the same as those later used for preparation of shear bond strength specimens. During preparation of HV specimens, the molds were placed on a dentin surface used as underlay in analogy to preparation of specimens bonded to dentin for measurement of shear bond strength. In order to obtain the dentin surface, one extracted human molar was ground from the occlusal surface until at mid-coronal dentin. Grinding was performed with a grit #220 SiC abrasive paper (Struers, Ballerup, Denmark) on a grinding machine (Struers LaboPol-21, Struers). The molar was embedded in self-curing acrylic resin (Paladur, Heraeus Kulzer GmbH, Hanau, Germany) in a cylindrical stainless steel mold. After removal of the mold, the mid-coronal dentin of the embedded molar was ground with a grit #500 SiC abrasive paper (Struers). Centrally, the dentin surface of the embedded molar exhibited a shade of A4/5M3 (VITA Easyshade Advance 4.0, VITA Zahnfabrik, Bad Säckingen, Germany). The dentin underlay ( Fig. 1 F) was re-used throughout the study and when not in use, it was stored at 100% humidity and 37 °C. During preparation of the HV specimens, the dentin underlay was covered with a first Mylar strip (Hawe Stopstrip Straight, KerrHawe) prior to placement of the respective Teflon mold. The mold was then filled in bulk with one of the five resin composites and covered with a Mylar strip. The resin composite was made flush with the mold by use of a glass slide and after light-curing for 20 s, the filled mold was removed from the dentin underlay and from both Mylar strips ( Fig. 1 G). The mold was then placed in a black photo-resistant box at 100% humidity and the box was stored in an incubator (Memmert UM 500, Memmert GmbH & Co.) at 37 °C for 24 h. After storage, HV measurements were performed on the bottom surface of each specimen (i.e. the surface formerly in contact with the first Mylar strip and opposing the irradiated top surface). Five HV measurements per specimen were performed with the same microhardness indentation device and in the same force-controlled mode as described above (one HV measurement in the center and four HV measurements toward the periphery of the specimen). Out of the five HV measurements per specimen, one mean value was calculated resulting in 14 HV values per group for statistical analyses.

2.3

Shear bond strength at increasing increment thickness

A final amount of 255 extracted human molars without restorations or caries was used for measurement of shear bond strength (SBS) at increasing increment thickness ( n = 17 molars/group; 15 groups (5 resin composites, 3 increment thicknesses)). The molars were cleaned under tap water with a scaler and a hard toothbrush to remove calculus and debris, stored in 2% chloramine solution, and kept at 4 °C until needed. For preparation of dentin specimens ( Fig. 1 H), molars were apically shortened with a water-cooled diamond saw (IsoMet Low Speed Saw, Buehler, Lake Bluff, IL, USA) and ground from the occlusal surface until at mid-coronal dentin. Grinding was performed with a grit #220 SiC abrasive paper (Struers) on a grinding machine (Struers LaboPol-21, Struers). The molars were embedded in Paladur (Heraeus Kulzer GmbH) in cylindrical stainless steel molds. After removal of the molds, the mid-coronal dentin of the embedded molars was ground with grit #500 SiC abrasive papers (Struers) for 10 s to obtain a standardized smear layer. The grit #500 SiC abrasive papers were changed after grinding of 10 dentin specimens. The dentin specimens were subsequently stored in tap water at room temperature. The ground, dentin surfaces of the specimens were air-dried and for each specimen, the bonding area on the dentin surface was defined and isolated by use of perforated self-adhesive tape (diameter of the perforation ∼4 mm). The bonding area of the dentin surface was then treated with the three-step etch-and-rinse adhesive system OptiBond FL (Kerr, Scafati, Italy) according to manufacturer’s instructions (etching (15 s; Gel Etchant; LOT-number: 4841467), water spray (>15 s), air-dry (∼3 s); OptiBond FL Prime (15 s; LOT-number: 4856727), air-dry (∼5 s); OptiBond FL Adhesive (10 s; LOT-number: 4856728), air-dry (∼3 s), light-cure (5 s)).

After adhesive treatment, a split Teflon mold as described above (inner diameter: 3.6 mm; height: 2 mm, 4 mm, or 6 mm ( Fig. 1 E)) was clamped to the dentin surface of each specimen and the respective mold was filled in bulk with one of the five resin composites and covered with a Mylar strip. The resin composite was made flush with the mold by use of a glass slide and after light-curing for 20 s, the SBS specimen was placed in a black photo-resistant box. Five minutes after completion of light-curing and at room temperature, the specimen was freed from the Teflon mold ( Fig. 1 I). All specimens were stored in the black photo-resistant boxes at 100% humidity and 37 °C for 24 h. After storage, specimens were subjected to SBS testing by use of a wire (stainless steel, diameter 0.6 mm) in a universal testing machine (Zwick Z010, Zwick GmbH & Co., Ulm, Germany) at a crosshead speed of 1 mm/min. The maximum force ( F _max (N)) was recorded (testXpert software V9.0, Zwick GmbH & Co.) and the SBS values (MPa) were calculated ( F _max (N)/bonding area (mm ² ); bonding area = r ² × π = ∼10.2 mm ² (radius ( r ) = 1.8 mm)) resulting in 17 SBS values per group for statistical analyses.

After SBS testing, the failure mode of each specimen was determined under a stereomicroscope (Leica ZOOM 2000, Leica, Buffalo, NY, USA) at 40× magnification and classified as (1) cohesive failure in dentin, (2) adhesive failure at dentin – adhesive interface, (3) adhesive failure at adhesive – resin composite interface, (4) cohesive failure in resin composite, or (5) mixed failure (combinations of failure modes (1)–(4)).

2.4

Statistical analyses

HV values obtained through measurement of HV profiles as well as failure modes after SBS testing were analysed descriptively whereas HV values obtained through measurement of HV at increasing increment thickness and SBS values were analysed with nonparametric one-way ANOVA followed by post hoc Wilcoxon rank sum tests and Bonferroni–Holm adjustment for multiple testing. All calculations were performed with R version 2.12.1 (The R Foundation for Statistical Computing, Vienna, Austria; www.R-project.org ).

For both HV and SBS, data of preliminary tests was statistically analysed with NCSS/PASS 2005 (NCSS, Kaysville, UT, USA) for sample size determination after the level of significance had been set at α = 0.05.