16 P.P. Lizymol / Trends Biomater. Artif. Organs, 36(1), 16-20 (2022) *Coresponding author E-mail address: lizymol@sctimst.ac.in (Dr. P.P. Lizymol) X-ray Microcomputed Tomography (μCT) Studies of a New Low Shrinkage Visible Light Cure Dental Composite (Ormo48): Effects of Thermal Cycling on Physical Properties P.P. Lizymol Division of Dental Products, Biomedical Technology Wing, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Poojappura, Thiruvananthapuram 695012, India Received: 9 December 2021 Accepted: 25 December 2021 Published online: 22 January 2022 Keywords: dental composite, thermal cycling, microhardness, microcomputed tomography, diametral tensile strength, Ormo48 This study investigated the effects of cyclic temperature changes on internal porosity generation, diametral tensile strength (DTS) and surface hardness of an experimental organically modified ceramic composite (Ormo48) dental restorative material with superior shrinkage characteristics. In this study, a novel organically modified ceramic resin has been synthesized and used as the binding resin along with quartz filler for composite preparation. The photocured composite samples were subjected to thermal cycling up to 2000 cycles from 5 o C to 55 o C in a dental thermal cycler and the properties were evaluated as per ISO and ADA standards. MicroCT analysis was carried out to find the effect of cyclic temperature changes on the porosity and internal structure of the cured restorative. Statistical evaluation using analysis of variance (ANOVA single factor) showed that thermal cycling up to 500 cycles resulted in significant decrease in DTS whereas no significant change in surface hardness was noticed. With thermal cycling both pore size and pore volume of cured Ormo48 increased as evidenced by microCT results. © (2022) Society for Biomaterials & Artificial Organs #20056222 Original Article Introduction Polymer based composite materials used in dentistry for direct restorative purpose are mostly in situ polymerizable methacrylate based thermoset materials, which have been widely acceptable due to their better aesthetic, and concerns about dental amalgam fillings [1] and became the good choice of dentists to satisfy the demands of patient’s aesthetics. Like any other composite system, a dental composite is a material system composed of two or more constituents mainly an organic resin and inorganic filler, which are essentially insoluble in each other. The major components of restorative materials are the inorganic filler or combination of different fillers forming hybrid system. The filler particles used for the dental composite preparation were commonly quartz, nano silica or radiopaque glass alone or in combination. The binding resin generally comprised of high molecular weight dimethacrylates monomers including (bisphenol-A glycidyl methacrylate) or urethane di/tetra methacrylates. Most current research efforts are directed towards reducing polymerisation shrinkage of the existing resin system by incorporating different monomers as well as developing entirely new monomers include ring-opening polymerisation, prepare analogues of BisGMA, use of liquid crystalline monomers, thiol-ene reaction, bicycloacrylates, siloranes, spiro orthocarbonates and organically modified ceramics for dental composites [2-6]. The use of ring-opening polymerisation (ROP) could help circumvent polymerisation shrinkage, since the polymerisation reaction involves the breaking of a bond for every new bond formed [1]. Radical ring-opening polymerisation, of bicycloacrylates, is an example of monomer with low polymerization shrinkage However, the polymers displayed a greater water uptake, which limits its long-term strength [2-3]. Spiro orthocarbonates have shown reduced shrinkage because of the double ring-opening polymerisation, with two bonds broken for every new bond created, leading to volume expansion upon polymerisation [4]. However, due to their poor curing characteristics they have insufficient mechanical strength. A new www.biomaterials.org.in/tibao