Influence Of Different Power Density And Shades On The Microhardness Of Nano-composite Resins At Different Energy Level

Light-cured composite resins were introduced to stomatology in1976. Due to theiradvantages, excellent esthetics, ease of handling, and ability to bond to tooth structures, theyare widely used in restorative dentistry. Many studies have shown that only adequatepolymerization of composite resin can obtain optimal mechanical properties and clinicalperformance. The blue light activate the photoinitiator and promote the conversion ofmonomers of the dental materials. But the conversion of the monomer is never fully, part ofthem become residual monomers eventually. Inadequate polymerization will lead to lowerphysical and mechanical properties of the resin and cannot guarantee the longevity of therestoration. Residual monomers have a cytotoxicity effect on pulp tissue and will act asplasticizers and change the properties of the material. Their clinical consequences include thefracture of dentin and enamel, post-operative sensitivity, micro-infiltration, and secondarycaries, so a lot of research have been devoted to improve the conversion degree. Someoneconsidered that only total energy density influences the degree of cure and properties of aresin composite. Energy density is defined as power density multiplied by exposure time.That is, the same properties could be achieved either at high power densities applied for ashort time or at low power densities applied for a correspondingly long time. But, somestudies have questioned the principle of constant energy density recently. They believe thatwhen the energy density constant at different combination, degree of conversion, depth ofcure and hardness, etc. are differentResearches about light-cured composite resin are more interested in the influence ofdifferent irradiation distance and time. There have less attention to influence of the energydensity, and resin color either. Therefore, this experiment had energy density remained,discusses the influence of different power density and shades on the microhardness ofnano-composite resins. We chose two energy densities—12J/cm2and16J/cm2,and seewhether they can be up to standard, to prove or disprove the principle of constant energydensity.We choose three LEDs with400mW/cm2,800mW/cm2,1000mW/cm2as curing light(power density was measure by radiometer three times, and make sure the value error of plusor minus20mW/cm2). Resin sample in2mm thickness has been confirmed with good uniformity and curing. Different colors and transparency can affect the performance of resin,therefore we choose commonly used4kinds of color A1, A2, A3, A3.5nano-composite resinof two different manufacturers as experimental material.First put resin in a copper ring, the thickness is2mm, both sides are covered withpolyester film to prevent the oxidation reaction during the curing.3LED lights cured twobrands of nano-composite resin with corresponding curing time. After photocuring, thespecimens were stored in the distilled water in a light proof container at37℃for24hours.The VHNs of the specimen’s top and bottom surface were determined. We use statisticalanalysis to compare these values.The average bottom surface VHNs is lower than the top surface VHNs. But no samplesare less than80%of the maximum value, that means every group can meet the clinical needsunder these experimental conditions. The composites received energy level of16J/cm2havebetter degree of conversion than that of12J/cm2. For the VHNs of specimen’s bottomsurface, the curing light with800mW/cm2could obtain relatively ideal results and the valuesdecreased significantly along with the shades changing darker. All of the experimentalgroups could obtain the valid bottom surface hardness percent values.Through statistical analysis on the VHNs, the following conclusions could be drawn:1. Energy density is defined as power density multiplied by exposure time. The ‘totalenergy level rule’ is wrong.2. The composites received energy level of16J/cm2have better degree of conversionthan that of12J/cm2, but both resin in every group can meet the clinical needs under theseexperimental conditions.3. The curing light with800mW/cm2could obtain relatively ideal results.4. The values decreased significantly along with the shades changing darker.