Effects of Preheating and Precooling on the Hardness and Shrinkage of a Composite Resin Cured with QTH and LED

Abstract

The aim of this study was to evaluate in vitro the hardness and shrinkage of a pre-cooled or preheated hybrid composite resin cured by a quartz-tungsten-halogen light (QTH) and light-emitting diode (LED) curing units. The temperature on the tip of the devices was also investigated. Specimens of Charisma resin composite were produced with a metal mold kept under 37°C. The syringes were submitted to 4°C, 23°C, and 60°C (n=20) before light-curing, which was carried out with the Optilux 501 VCL and Elipar FreeLight 2 units for 20 seconds. The specimens were kept under 37°C in a high humidity condition and darkness for 48 hours. The Knoop hardness test was carried out with a 50 gram-force (gf) load for 10 seconds, and the measurement of the shrinkage gap was carried out using an optical microscope. The data were subjected to analysis of variance and the Games-Howell test (α=0.05). The mean hardness of the groups were similar, irrespective of the temperatures (p>0.05). For 4°C and 60°C, the top surface light-cured by LED presented significantly reduced shrinkage when compared with the bottom and to both surfaces cured by QTH (p<0.05). It was concluded that the hardness was not affected by pre-cooling or preheating. However, polymerization shrinkage was slightly affected by different pre-polymerization temperatures. The QTH-curing generated greater shrinkage than LED-curing only when the composite was preheated. Different temperatures did not affect the composite hardness and shrinkage when cured by a LED curing unit.

Effect of Preheating on Microleakage and Microhardness of Composite Resin (An in Vitro Study)

“…Didron et al (38) pointed out that Preheating composites to higher temperature significantly increased the rate of polymerization and polymerization contraction stress. The increased stress at elevated temperature seems to be a consequence of the system thermal contraction rather than an increase in materials' conversion, since the composites' mechanical properties were not significantly improved at elevated temperatures (13,30).…”

“…Recent literatures recommend that chair side warming of composite resins before photo polymerization. The increase in temperature of universal composite with high filler loading may enhance its flowability which can be advantageous in placement of composites, and better adaptation to the cavity, thus increase the durability of the restoration (13,14).…”

Influence of a Repeated Preheating Procedure on Mechanical Properties of Three Resin Composites

“…25,26 There is a general consensus in the literature on the absence of harmful effects of preheating procedures on the mechanical properties of resin composites. 2,3,15 In a recent study, Osternack and others 27 concluded that composite hardness was not affected by precooling or preheating procedures. However, the majority of previous studies did not consider repeated preheating cycles.…”

Influence of storage temperature on vickers microhardness of resin composite

“…These results are in agreement with a previous study where they showed that at room temperature only 50% to 75% conversion of monomers could be achieved. 21,38 Low monomer conversion rate reduces the mechanical strength of restoration, and oxidation of unsaturated monomers may give rise to composite colour changes as well as allergic reactions. An increase in the degree of conversion improves surface hardness, flexural strength, modulus, fracture toughness, diametral tensile strength, and wear resistance.…”

“…Moreover, as a result of enhanced flowability the placement and adaptation of composites will be facilitated, thus increasing the durability of the restoration. 21,22 King Saud University…”