An estimated 800 million direct resin-based composite (RBC) restorations were placed in 2014 [1]. Many of these RBC restorations will be replaced due to fractures of the RBC [2], [3], [4], [5], [6]. Light exposure times that were once 60 seconds per increment [7] have now been reduced to 10–20 s [8], [9], [10], [11] for many RBCs, and at least two manufacturers claim that their light-curing unit (LCU) can polymerize RBCs in just one second [12], [13], although at least two 3 s exposures are recommended for larger (>8 mm wide) and deeper RBC restorations (> 5 mm deep) [13].

The ability of a LCU to effectively photo-polymerize a resin is governed by the first and second laws of photochemistry. According to the first law, the Grotthuss-Draper law, a photochemical reaction can only occur if the chemical substance absorbs light. The second law, known as the Stark-Einstein law, states that for each absorbed photon, only one molecule within the system can be activated [14], [15]. Therefore, the LCU must emit light within the appropriate wavelength range to polymerize RBCs effectively. Even if two LCUs deliver the same power, variations in their emitted wavelengths may lead to different curing outcomes [16], [17]. Additionally, Beer’s law describes the exponential decrease in transmitted light as RBC thickness increases. Since monomer conversion depends on the total energy delivered over a given exposure time [18], [19], it is crucial to follow the manufacturer’s instructions for the RBC, its thickness and the LCU used to achieve optimal polymerization [20].

A new LCU, Pinkwave (Apex, Racine, WI) [21] emits four distinct wavelength bands that deliver: red (625 – 750 nm) and infrared (800 – 900 nm), as well as blue (420 – 500 nm), and violet light (380 – 420 nm). The manufacturer asserts that this ‘quad-wave’ LCU differs from traditional LCUs available in the market because it delivers more energy to depths beyond the capabilities of traditional blue light emitting LCUs [22]. This is because of the Rayleigh dispersion that means that the wavelength is inversely related to the amount of light scattering [21]. In other words, as the wavelength increases, the amount of light dispersion increases. Thus, while violet and blue light are more reactive due to their shorter wavelengths, they do not penetrate as far into the RBC as red or infrared light [21]. The PinkWave manufacturer also claims that their ‘quad-wave’ LCU can reduce polymerization shrinkage stress and photo-polymerize RBCs in 3 s in the Boost mode, that is then followed by a further 20 s in the standard mode, or by using 6 s in the Boost mode at the end of the procedure [22]. However, this reduction in polymerization shrinkage may due to a lower degree of conversion when using this type of LCU. Ivoclar (Ivoclar, Schaan, Liechtenstein) recommends that its RBC, PowerFill, can be photo-polymerized in 3 s with a high irradiance of 2700–3300 mW/cm2. This recommendation is possible due to the reversible fragmentation chain transfer (RAFT) polymerization mechanism. The RAFT process is very similar to conventional radical polymerization, except that it includes a specific chain transfer agent (known as a RAFT agent), which introduces two additional steps into the polymerization mechanism [23].

A recent study reported that after 10 years, 70 % of the reasons for replacing RBC restorations were due to fractures and wear [1]. Another study reported that after 33 years, 60.3 % of the failures resulted from fractures [24]. This may occur because the RBCs used to restore teeth can undergo plastic deformation [25] followed by catastrophic crack growth under a high applied stress [26], [27], [28]. Since it appears that many RBC restorations are now replaced due to fracture [1], [2], [3], [4], [5], [6], the fracture resistance of RBCs has now become highly relevant. This property is usually determined using fracture toughness measurements, which is the ability of the RBC to resist pre-crack/flaw propagation [27], [28], [29]. The fracture happens when the stress intensity factor (K) exceeds the critical value (KIC) [28], [29] and these results have been reported to predict the clinical performance of the RBC [30].

Various fracture toughness test protocols such as the 3-point bend test, the 4-point bend test, and the single-edge notched test [29], [31] have been developed. Each test method has advantages and disadvantages, and it is important to choose the correct test method for the specific material and the specific condition being tested [32]. The single-edge notch 3-point bending test is commonly used due its simplicity and acceptance [25], [29]. The results from biaxial tests have been reported to correlate with those from 3-point flexure tests and they show less variability in the data [33]. The ISO 4049 standard [25], tests a 25 ± 2 mm × 2 ± 0.1 mm × 2 ± 0.1 mm specimen of RBC. To photo-polymerize this 25 mm long beam, the ISO standard requires multiple overlapping exposures, and the beam must also be exposed to light from both sides. While this test method will provide the best result for the RBC, this test does not represent the clinical reality that is used to photo-polymerize RBCs in the mouth. Thus, the results from using the ISO 4049 test may not be clinically relevant, and a different test method should be considered [34].

The validity and accuracy of fracture toughness measurements is influenced by several factors, including the size and shape of the test specimen, the test method used, the test environment, the preparation of the specimen, the loading rate and the duration applied during the test, and the instrumentation used for testing [29], [34], [35], [36]. When the mechanical test is performed using a low crosshead speed, this can lead to creep within the RBC, and produce inaccurate results. Therefore, it is important to carefully consider these factors when selecting the test method [30]. Theoretically, the fracture toughness should not change with the specimen geometry or measurement technique [29], but the storage and test conditions may affect the results [27], [28]. The composition, shape, content, and distribution of the fillers in the RBC have also been reported to affect the fracture toughness of RBCs [28], [29], [35], [37]. The distribution of internal flaws, air bubbles and inter-particle bonding will also affect crack propagation under stress and the resultant KIC values [29]. The size and shape of the test specimen have also been shown to be important factors when determining the accuracy of fracture toughness measurements in RBCs [35]. The specimen must be large enough to withstand the forces applied during the test, yet small enough to be easily handled and tested under clinically relevant conditions. Additionally, the size of the specimen should represent how the material that is being tested will be used clinically, otherwise, the results may not be clinically relevant [34].

Since clinicians must now decide between using a standard 10 second light exposure, or a shorter 1–3 second exposure, a quadwave LCU or a laser diode LCU, this study evaluated the physicomechanical properties of different RBCs when photo-polymerized using these new technologies. The null hypotheses were:1)

There would be no differences among the LCUs and the manufacturer recommended exposure times for these LCUs on the physicomechanical properties of the four RBCs evaluated;

2)

There would be no differences between the physicomechanical properties of the four RBCs photo-polymerized by different LCUs manufacturer recommended exposure times.