Journal of the College of Physicians and Surgeons Pakistan 2011, Vol. 21 (7): 411-414 411 INTRODUCTION Composite resin has been widely used in dental resto- ration, due to low cost and conservative technique. To evaluate the elastic properties of composite resins, the methods which are mostly used are the Knoop and Vickers microhardness test. 1 Hardness tests are considered as an indirect method to evaluate the degree of polymerization of composite resins and which have already been reported to correlate with the degree of conversion of carbon double bonds. 2 Depth of cure and microhardness are considered to be essential physical properties of composite resin materials that are relevant to the clinical technique of incremental packing and curing. Hardness is a property of a material that enables it to resist plastic deformation, usually by penetration. However, the term hardness may also refer to resistance to bending, scratching, abrasion or cutting. 3 Nanocomposite combine the advantages of hybrid and micro filled composite in the same restorative material showing favourable mechanical property, higher surface quality and increased wear resistance. 4 In addition to materials characteristics, light curing units significantly influence the degree of polymerization of light-activated composite resins. The most important features associated with the effectiveness of light curing seem to be the intensity of the light emitted the spectral output of the light source and the curing mode. 5 Most LED equipments provide a power output of approximately 300 mW/cm 2 and have been reported to polymerize composite resins. LEDs are known to use less power, have a longer life and greater durability than conventional filament lamps. They have a narrow spectral range with a peak around 470 nm, which matches the optimum absorption wavelength for the activation of the camphorquinone photo initiator. 6 LED units generate minimal heat. The efficiency of conversion of electrical energy to useable curing energy is higher for blue LEDs than for conventional QTH lamps (14% vs. 1%, respectively). 7 The irradiance of light emission depends on the power (Watts) of the curing unit as well as the time (seconds) and the surface area (cm 2 ) where the light is spread over. The energy density (irradiance x irradiation time) ABSTRACT Objective: To determine the microhardness and depth of cure of nanocomposite using different irradiation times on both upper and lower surfaces of composite material. Study Design: In-vitro experimental study design. Place and Duration of Study: Dr. Ishrat-ul-Ebad Khan Institute of Oral Health Sciences, Dow University of Health Sciences and NED University of Engineering and Technology, Karachi, from March to May 2010. Methodology: Total 120 cylinder shaped specimens; 60 specimens for depth of cure test and 60 specimens for micro hardness test were fabricated using A3 shade of nanocomposite (Filtek Z350 XT, 3M ESPE). For each irradiation time four groups were made (Group 1 = 20s), (Group 2 = 30s), (Group 3 = 40s) and (Group 4 = 60s). For each group fifteen specimens were used. The resin was placed and polymerized into a cylindrical plastic mold. Depth of cure was measured by using micrometer. Micro Vickers hardness was measured on both top and bottom surfaces. SPSS-16.0 was used for statistical analysis. Results: There was statistically significant difference in the depth of cure between all groups showing the highest value in group 4 (p < 0.001). For hardness on top surface, there was a statistically significant difference in between groups 1 and 2 (p=0.001), groups 1 and 3 (p < 0.001), groups 1 and 4 (p < 0.001). There was no statistically significant difference between groups 2 and 3, groups 2 and 4 and in between groups 3 and 4. For hardness on bottom surface, there was statistically significant difference in between all groups showing the highest value in group 4 (p < 0.001). Conclusion: Depth of cure and hardness was increased by increasing irradiation time. Hardness on the top surface was higher than bottom surface values. Key words: Nanocomposite. Depth of cure. Vickers hardness test. Irradiation time. 1 Department of Science of Dental Materials/Operative Dentistry 2 , Dr. Ishrat-ul-Ebad Khan Institute of Oral Health Sciences (DIEKIOHS), DUHS, Karachi. 3 Department of Material Engineering, NED University, Karachi. Correspondence: Dr. Sidra Akram, R-955, Block 15, F.B Area, Dastagir Society, Karachi. E-mail: drsidra1@hotmail.com Received June 19, 2010; accepted May 26, 2011. Effect of Different Irradiation Times on Microhardness and Depth of Cure of a Nanocomposite Resin Sidra Akram 1 , Syed Yawar Ali Abidi 2 , Shahbaz Ahmed 2 , Ashraf Ali Meo 3 and Fazal-ur-Rehman Qazi 2 ORIGINAL ARTICLE