1

Introduction

Over recent decades, dental amalgam has been increasingly replaced as a restorative material by alternative restorative materials, such as glass ionomer cements, compomers, resin composites and derivate products. Although amalgam is still available for direct posterior fillings, resin composite is increasingly used due to its superior esthetic properties . Longevity and annual failure rates were shown for both materials . Relatively more secondary caries was related to composite and more fracture failures to amalgam restorations by another retrospective study, comparing posterior composite and amalgam restorations without differences in longevity . It was found that larger-sized restorations showed reduced longevity .

Light cured resin composites are usually placed in increment thicknesses of 2 mm or less into cavity preparations through incremental layering . Exposure to blue light from an occlusal direction follows, before repeating increments until the cavity preparation is filled . Improved physical properties, enhanced marginal adaptation and reduced cytotoxicity of the resin composite is achieved by adequate polymerization . A second rationale for the incremental technique is to reduce shrinkage occurring during polymerization. If polymerization shrinkage is not reduced, the associated shrinkage-stress may cause cuspal deformation with resulting sensitivity and micro-cracks in the resin structure , which can result in marginal gaps, micro-leakage and secondary caries.

There are several disadvantages of the incremental layering technique: the possibility of contamination between the layers, bonding failures and increased processing time. Bulk fill (BF) resin composites allow the placement of increments up to 4 mm in thickness . As opposed to placement and curing of multiple increments, BF resin composites require less chairside time .

Investigation of the depth of cure of resin-composites has been made by various methods. Apart from the simple “scrape test” used in International Standard: ISO 4049, one of the most widely adopted procedures is to consider the profile of hardness versus depth and to note the depth for which hardness just equals or exceeds 80% of the maximum hardness – at or close to the surface .

To achieve adequate physical and mechanical properties, sufficient curing of BF resin composites is essential . Improving the absorption spectrum and the photo initiator’s reactivity may increase the depth of cure of light curing resin composites, as well as optimizing the LCU and increasing the translucency of the materials . One of the most commonly used photoinitiators is a combination of camphorquinone (CQ) and different types of tertiary amine . New photoinitiators such as dibenzoyl germanium derivates (Ivocerin), which are more light reactive than CQ, are incorporated in TEC-BF as an additional photoinitiator, to increase the depth of cure . Another important aspect for sufficient curing is light energy: Insufficient light energy and duration of exposure may result in a low degree of conversion , reduced microhardness and inferior mechanical properties . The depth of cure is obviously translucency dependent and some BF resin composites are more translucent to blue light than some conventional resin composites . For conventional composites an incremental filling technique is used, as it has been shown that light energy transmitted through resin composite decreases exponentially with resin composite thickness. More precisely, this is the case after a correction has been made for surface reflection and the Apparent Absorption ( A ′) then increases linearly with depth .

The overall aim of this study was to obtain further experimental information about a possible reciprocity between light irradiance (mW/cm ² ) and irradiation time (s) for resin-composites. The product of these quantities has the units of energy density (J/cm ² ). So the general reciprocity hypothesis is that: the same photo-cure outcomes will result from applying essentially constant energy densities despite reciprocal variations in the irradiance and time-period. The specific clinical motivation is to achieve equivalent curing by using high irradiance coupled with shorter irradiation times. Previous studies suggest that such reciprocity may only hold to a very limited extent. Hence the overall aim can be re-expressed as the identification of limits to reciprocity for representative bulk-filled (BF) and conventional (non-BF) resin-composites when subjected to different irradiation regimes : i.e. polymerized by a series of light-curing units (LCUs) of increasing radiant emittance (mW/cm ² ), for systematically varied irradiation times.

The specific objectives were to investigate:

• (i)

  Hardness versus Depth profiles of representative BF and non-BF resin composites, when each material is subjected to different irradiation regimes.

• (ii)

  Whether or not curing with high energy density at short irradiation time generates acceptable hardness profiles.

• (iii)

  Whether translucent materials are cured more efficiently than high-viscosity resin composites in 4 mm depth molds.

• (iv)

  Whether maximum hardness correlates with composite filler loading.

Based on these objectives the following null hypotheses were formulated:

• (I)

  Shorter curing times with high energy density do not result in similar depth of cure to prolonged curing times with moderate energy density.

• (II)

  Translucent materials do not show more efficient curing than more opaque resin composites in 4 mm cavity preparations.

• (III)

  Maximum hardness does not correlate with composite filler loading.

2

Materials and methods

Sets of flat-cylindrical solid acetal test molds with a circular cavity were prepared with increasing thickness (1, 2 3, 4, 5 and, 6 mm), and with an internal diameter of 5.0 mm. To reduce light reflection back up the materials filling the cavities, dark covered microscope slides were used as the base. Transparent plastic film was positioned between the dark base and the acetal mold to achieve a plane surface. Six BF composite materials (four low viscosity [X-BF, V-BF, F-BF, SDR], two high viscosity materials [TEC-BF, SF-BF]) and two conventional composite materials (TEC, CXM) ( Table 1 ) were filled into the mold cavities. After filling the cavities, the upper surface of each specimen was marked and covered with another piece of transparent film. Specimens were produced in triplicate. The resin composites were cured from the marked top side with specific irradiation times ( Table 2 ), which were calculated for each composite separately as specified by the manufacturers for three different curing lights. The energy densities (J/cm ² ) received by the resin composites, are the mathematical product of irradiance (mW/cm ² ) and the exposure time (s) . E _D = I × t.

After curing the resin composites, removing the transparent film and inserting the specimens into glass bottles, they were stored dry, in the dark for 24 h at 37 °C.