Effectiveness of Cure of Resin-Based Composites
Effectiveness of cure of RBCs is a crucial parameter that evaluates the extent of material polymerization. An RBC that is cured effectively ensures optimum clinical performance due to an acceptable DC of monomers into polymers, which results in enhanced mechanical properties of the material. The effectiveness of RBC cure may be assessed directly or indirectly. Direct methods using vibrational spectroscopy such as IR spectroscopy and laser Raman spectroscopy assess DC. However, indirect methods include the ISO 4049 method and SH testing.
These methods provide a direct approach to determine the effectiveness of cure by measuring the DC (Asmussen, 1982b; Eliades, Vougiouklakis, & Caputo, 1987; Ferracane & Greener, 1984; Rueggeberg & Craig, 1988). This is achieved by measuring both the percentage of carbon-carbon single bonds in the cured material and the percentage of unreacted C=C bonds. Vibrational spectroscopy can be classified into two techniques. The first technique is Fourier transform infrared spectroscopy (FTIR), which is sometimes documented as IR spectroscopy and is based on light absorption. The second technique, Raman spectroscopy is based on light scattering (Figure 4.7). Two devices are popularly used; Fourier transform-Raman spectroscopy (FT-Raman) and micro-Raman spectroscopy (MRS) (De Santis & Baldi, 2004). Other available techniques include differential thermal analysis (DTA), differential scanning calorimetry (DSC), and nuclear magnetic resonance (NMR) (Alshihri, Santini, & Aldossary, 2018).
FIGURE 4.7 Raman spectroscopy is based on light scattering. Raman spectroscopy is an effective method in determining the early changes in human enamel caused by artificial caries. (Adapted, with permission, from Buchwald & Buchwald, 2019.)
ISO 4049 Method
Manufacturers use the ISO 4049 method primarily to certify that their RBCs will achieve the minimal depth of cure requirement. They also provide recommendations for light curing times relative to RBC increment thickness to adequately polymerize the material. Some studies have also used this method to determine the effectiveness of cure of the tested RBC (Ruyter & Oysaed, 1982). Samples are prepared in a cylindrical stainless steel mold of dimensions 6 mm long x 4 mm diameter. If the manufacturer claims a depth of cure of 3 mm. the mold should be at least 2 mm longer than twice the claimed depth of cure. After polymerization of the specimen, it should be removed from the mold and scraping of uncured material is done with a plastic instrument. A micrometer is used to measure the height of the cured material, where the values are divided by 2 to give the depth of cure of the tested RBC (ISO_4049, 2009). According to ISO 4049-2009, the depth of cure should be no more than 0.5 mm below the value stated by the manufacturer.
The rationale behind ISO 4049 is unspecified and the correlations between the test and clinical performance are lacking. It was concluded that the ISO 4049 method overestimated the depth of cure when it was compared with Knoop hardness profiles (Moore, Platt, Borges, Chu, & Katsilieri, 2008) and Vickers hardness estimations of the DC (Flury, Hayoz, Peutzfeldt, Husler, & Lussi, 2012).
Surface Hardness (SH)
SH is a well-accepted method and has been used to indirectly probe polymer network conversion; therefore, hardness profiles can be used to measure the depth of cure. Ideally, SH of RBC should be equal or close to equal throughout the restoration and for the life of the restoration. However, the SH of bulk-fill RBC has different values at different depths (Garcia et al., 2014). Therefore, sufficient cure was defined as SH of more than 80% between the bottom and top surfaces (B/T ratio) of RBC samples (Johnston, Leung, & Fan, 1985).
As a measure of the completeness of conversion, the bottom to top Knoop hardness number (B/T-KHN) was approximately 2.5 times more sensitive than the bottom to top DC (B/T-DC) ratio (Bouschlicher, Rueggeberg, & Wilson. 2004). B/T-KHN ratios provide an accurate, simple method of assessing the efficacy of photoinitiation strategies (curing light/exposure duration) instead of using more complex FTIR methods to determine the DC (Bouschlicher et al., 2004).
The Influence of Curing Light Distance on the Effectiveness of Cure of Resin-Based Composites
Among the factors that affect the effectiveness of cure of RBCs is the distance between the surface of the restoration and the LCU guide. Manufacturers recommend positioning the curing light guide as close as possible to the RBC restoration surface. These recommendations are typically based on testing the material in ideal laboratory conditions, commonly at a curing light distance of 0 mm (Shortall, Price, MacKenzie, & Burke, 2016). However, in a clinical setting, this curing light distance is difficult to achieve especially in Class II cavities. One study measured the depths of Class II proximal cavities of 1.146 extracted teeth. They reported an average depth of 4-5 mm for mandibular premolars, 5-6 mm for maxillary premolars, and 5-7 mm for molars (Hansen & Asmussen, 1997). This was in agreement with another study that found that 6.3 mm (+ 0.7 mm) was the typical Class II cavity depth when the LCU guide was positioned directly on the tooth or the restoration surface (Price, Derand, Sedarous, Andreou, & Loney, 2000). Additionally, more than 15% of the mandibular molars were reported to have a proximal cavity depth of more than 8 mm (Hansen & Asmussen, 1997).
Several studies have investigated the impact of curing light distance on the effectiveness of cure of conventional RBCs. Most of these studies found that the SH of the RBC decreases with increasing the curing light distance (Caldas, de Almeida, Correr-Sobrinho, Sinhoreti, & Consani, 2003; Rode, Kawano, & Turbino, 2007; Thome, Steagall, Tachibana, Braga, & Turbino, 2007). Other studies found that there was no statistical difference of the top SH of RBCs at small curing light distances; however, the bottom SH was statistically significant at the same small curing light distances (Aguiar, Lazzari, Lima. Ambrosano. & Lovadino, 2005; Pires, Cvitko, Denehy, & Swift, 1993). To date, most of the studies assessing the effectiveness of cure of RBCs at different curing light distances were done on conventional RBCs; however, scarce studies were done on bulk-fill RBCs.