A Comparative Evaluation of Pre-Heat Treatment, Post-Cure Heat Treatment and Combined Heat Treatment on Degree of Conversion of a Bulk-Fill Composite: An In Vitro study

INTRODUCTION: The need for amalgam alternatives and the rising demand for aesthetic restorations have led to the increased popularity of resin composite restorations for posterior teeth. One of the recent innovations in resin composite application technique involves preheating of resin composite material before insertion into the cavity. Preheating of resin composites before placing in the cavity with a chairside thermal assist unit shows an improvement in composite properties and it also reduces the curing time. The increased flow obtained by pre-heating makes the handling easier and adapts to the walls of the prepared cavity. The most important benefit of resin composite preheating is the enhancement of the maximal polymerization rate and the overall monomer conversion. The most common reasons for composite restoration being replaced are fracture, secondary caries and discoloration. Therefore, various techniques have been introduced to promote conversion rate and mechanical properties. In order to approach this, soft-start light polymerization methods and more recently pre-heating of composites were employed. Most composite restorations achieve only 45–70% monomer conversion after the initial light curing. Post-cure heating can enhance the degree of conversion and improve the mechanical properties of the composite. Also, relief of internal stresses which are generated in the composites during the initial light curing may improve the mechanical properties of resin composites as well. Furthermore, studies have shown increased biocompatibility of the resin composite after heat treatment. Depending on the post-cure temperature, time, and resin composite formulation, researchers have reported increase in hardness, tensile and flexural strength, and fracture toughness of composite restorations. AIMS AND OBJECTIVES: AIM: To comparatively evaluate the effect on degree of conversion of Tetric NCeram Bulk Fill composite after pre-heat treatment, post-cure heat treatment and combined heat treatment. OBJECTIVE: To observe the effect of temperature on degree of conversion of Tetric NCeram Bulk Fill composite. METHODOLOGY: The specimens used in this study were prepared from commercially available light cure bulk fill composite resin Tetric N-Ceram Bulk Fill (Ivoclar Vivadent, Liechtenstein, Germany). The dimensions of the specimens were 10mm X 10mm X 4mm which were prepared using customized stainless steel molds. Light cure bulk fill composite was placed into the mold as a single increment using a teflon coated instrument, a clear mylar strip followed by another glass slab was placed over the mold and pressed to remove the excess material. After removing the glass slab, the specimens in group I were photopolymerised using Bluephase N Polywave LED (Ivoclar Vivadent, Liechtenstein, Germany), a light curing unit of intensity 1200mW/cm2 for 10secs. In group II pre-heating of composite Tetric N-Ceram Bulk Fill to a temperature of 60ºC for 3 minutes was carried out in a dental laboratory wax melter and light cured. In group III and IV post-cure heat treatment group, the light cured specimens were subjected to heat treatment with the plugger tip of Sybron Endo (Obturation System Kerr, Orange, USA) at 100ºC and 200°C for 10 secs. In group V and VI, combined heat treatment group, the composite is preheated at 60°C, light cured and post-cure heat treatment was done with the plugger tip of Sybron Endo at 100ºC and 200°C for 10secs. After 24 hours of light proof storage at 37⸰C, top surface of the samples were analyzed for degree of conversion using Raman spectrometer and values for degree of conversion of the composite resin was obtained using baseline guide technique. RESULT: Tetric N-Ceram Bulk Fill composite with combined heat treatment provided higher degree of conversion when compared to other groups. CLINICAL IMPLICATION: It is known that the degree of conversion of composite resins may directly affect their clinical performance. The greater the quantity of monomers transformed to polymers, the higher will be the degree of conversion of composite resins and the better will be the material properties. The polymeric chains with lower density of cross-links are more susceptible to the action of solvents and consequently to plastification. These cross-links usually reduce the permeability of the polymer by reducing the existing free volume. Thus secondary curing is a method employed to increase the degree of conversion of composite resins