1

Introduction

For several decades, many clinician groups have wished for shorter irradiation times when photocuring dental resin-based composite (RBC) restorations. For some time, manufacturers primarily addressed this request by increasing the radiant emittance of light curing units (LCUs). This included development of plasma arc LCUs for dentistry with radiant emittance near 2000 mW/cm² [ ]. These plasma LCUs were advocated for rapid, 3-s curing, as being equivalent to that of 40 or 60 s exposures from a QTH (Quartz Tungsten Halogen) light [ ]. This concept failed, given that subsequent studies clearly attested insufficient polymerization [ , ] and emphasized the need for multiple 3-s exposures to achieve a clinically adequate performance [ , ].

Associated with these approaches was the concept of “exposure reciprocity”: that one could deliver the required photon-dose over a shorter irradiation period by significantly increasing the light irradiance (photons/second). This concept has proved controversial over the past 20 years and has been dismissed as unworkable [ , ]. However, many of the earlier studies were on resin-composites incorporating Norrish Type II photoinitiators, principally camphorquinone (CQ)/Amine. More recently, the Stansbury-Bowman group has established that, for mechanistic reasons, exposure reciprocity may be achievable using Norrish Type I photoinitiators [ ]. Very recently, a NIST group has established empirically that, within appropriate ranges of irradiance and resin-composite viscosity, conditions may be met for valid exposure reciprocity [ ]. Although this evidence is empirical it has been known for some time that system viscosity has a significant influence by reducing the rate of free-radical termination reactions.

The use of LCUs with high radiant emittance raised, in addition, concerns for: (i) potential injury of the pulp and soft tissues by development of high temperatures [ ] plus (ii) the accumulation of residual stresses in the polymerized material. This second aspect has been demonstrated at both the macroscopic [ ] and molecular [ ] scales. The latter has been interpreted as due to greater chain segment extension at fast polymerization rates creating residual stress via reduced structural order and free volume [ ].

Technological progress in the development of high power LED (light emitting diode) chips [ ] has enabled progressively higher radiant emittance (up to 3000–7000 mW/cm²) in modern LED LCUs [ ]. However, an attempt to achieve adequate cure of methacrylate-based RBCs with 1–3 s irradiation was restated, but then again failed [ ].

A major reduction in curing time (to 3 s) in methacrylate-based RBCs was achieved only when conventional Norrish Type II photo-initiators (CQ/Amine) were replaced by Norrish Type I photoinitiators ( e.g. monoacylphosphine oxide) used at appropriate curing conditions (radiant emittance >500 mW/cm², wavelength range 395–415 nm) [ ]. However, this approach has not yet been implemented in commercial RBCs, since a change in the output spectrum of LED-LCUs would have been necessary.

Recently, a new approach has been proposed to reduce the curing time of RBCs. It connects curing with high radiant emittance, to a modification of the polymerization mechanism in a specific dimethacrylate-based RBC: Tetric PowerFill (Ivoclar Vivadent AG). This RBC incorporate a reversible addition-fragmentation chain transfer (RAFT) polymerization mechanism. 3 s-irradiation is proposed for a bulk-fill application. This means that the bulk-fill RBC needs to be adequately cured in increments of at least 4 mm. RAFT polymerization has been previously incorporated in a bulk-fill RBC (Filtek One Bulk Fill, 3 M). That material was designed to have improved depth of cure combined with reduced translucency to achieve improved esthetic appearance [ , ].

It was therefore the aim of the present study to assess the outcomes of fast curing (3 s), with high radiant emittance, on the polymerization process and mechanical properties of an experimental bulk-fill RBC modified to incorporate RAFT polymerization. In addition, the tolerance of this RBC to improper curing conditions was to be quantified, including increased exposure distance and/or angulation of the LCU, as may occur clinically, when access to a restoration is obstructed. A prototype LCU, designed for fast curing with high radiance emittance, was used and characterized.

The null hypotheses tested were that:

• a)

  For a RAFT polymerized bulk-fill RBC, fast 3 s curing with high irradiance (curing mode flash ) is not significantly different to curing for 10 s (mode high , 10 s) in terms of polymerisation kinetics, degree of conversion (DC), various macro and micro-mechanical properties, and with increase of exposure distance and LCU angulation during curing;

• b)

  Under identical curing conditions, the RAFT-polymerised bulk-fill RBC shows no significant differences in polymerisation kinetics, DC and micro-mechanical properties, for each analyzed specimen depth, from those exhibited by an established, regular bulk-fill RBC of comparable chemical composition cured by conventional free radical polymerization.

2

Materials and methods

An experimental bulk-fill RBC modified for RAFT polymerization (F-Composit 2, Abb. FC2 ; IV A, LOT W92823, Ivoclar Vivadent, Schaan, Liechtenstein; current commercial name Tetric PowerFill) was analyzed in relation to an already established bulk-fill RBC (Tetric EvoCeram Bulk Fill, Abb. TEVO-BF , IV A, LOT X24046, Ivoclar Vivadent, Schaan, Liechtenstein).

A Prototype LCU (Ivoclar Vivadent, Schaan, Liechtenstein, SN: 1428000006, current commercial name Bluephase Power Cure) was used for polymerization in two different curing modes ( flash for 3 s and high for 10 s).

The curing characteristics of this LCU (radiant emittance, radiant exposure and spectral distribution) were determined for both curing modes as a function of exposure distance. Degree of conversion and polymerisation kinetics parameters of the above-described RBCs were assessed in real-time for five min at two clinical relevant increment thicknesses (2-mm and 4-mm). The variation in micro-mechanical properties, measured in 100-μm steps in a line profile along 6-mm high specimens pre-stored in water at 37 °C for 24 h, was used to calculate the depth of cure (DOC). The spatial distribution of the micro-mechanical parameters (Vickers hardness and indentation modulus) was recorded in 500-μm steps, in specimens cured under ideal curing conditions (LCU in proximate contact to the specimen surface) as well as in simulated, clinical relevant curing conditions (variation of exposure distance and/or LCU angulation).