101 Abstract Objectives: Information about the retentive strength of luting agents for zirconium oxide–based crowns is limited. The purpose was to determine the ability of selected luting agents to retain a representative zirconium oxide ceramic crown under clinically simulated conditions. Methods: 56 sound freshly extracted frst permanent molars were selected. Teeth were divided randomly into two groups based on the type of zirconia system used. Each group was further subdivided based on the type of resin cement used and each subgroup was further subdivided into two halves based on thermocycling. Shear bond strength was measured using Universal Testing Machine and then the samples were observed under magnifcation (80X) using a Stereomicroscope to identify the nature of bond failure. Student t test was applied on the data obtained. The log transformation, if required, was applied to normalize the data and p > 0.05 was considered signifcant. Results : Mean bond strength of Panavia F2.0 with Cercon before and after thermocycling was 9.45 Mpa, 13.45 Mpa and With Ziecon was 9.59 Mpa and 12.37 Mpa respectively. Mean bond strength of Rely X U200 with Cercon before and after thermocycling was 8.10Mpa and 11.81Mpa and with Ziecon was 8.12 Mpa and 10.63Mpa respectively. Signifcance: Panavia F2.0 was found to be better presented highly signifcant results than Rely X U200 with both Zirconia Systems. Thermocycling signifcantaly affects the bond strength of both the resin cements with dentin. There was no signifcant difference was observed between shear bond strength of two zirconia systems. Highlights: • This study revealed that the type and composition of Zirconia systems does not affect the shear bond strength of Zirconia to dentin. • Composition, type of resin cement, and oral conditions affect the bond strength of Zirconia to dentin. Evaluation of Infuence of Thermocycling on Shear Bond Strength of Two Different Zirconia Systems Bonded to Dentin Using Resin Cements - An In Vitro Study Manbir Singh, Sharad Gupta, Abhishek Nagpal, Akshay Bhargava, Hari Parkash, Megha Sethi Department of Prosthodontics, ITS-CDSR, Murad Nagar, Uttar Pradesh, India Corresponding author: Dr. Megha Sethi, Department of Prosthodontics, ITS-CDSR, MuradNagar, Uttar Pradesh, India, Tel: 8958709738; e-mail: meghasethi04@gmail.com Introduction The popularity of all-ceramic dental restorations has increased in recent years. Indeed, many patients are more interested in having esthetic appearance than any other feature of dental service. Dental ceramic restorations have been of increasing interest among dentists and patients, due to their expectations for more natural looking restorations [1]. Despite their good mechanical properties, the Porcelain Fused to Metal (PFM) restorations do not always provide optimal cosmetic values [2]. Zirconia may exist in three crystallographic forms, cubic, tetragonal and monoclinic. All of these phases are variants on the cubic fuorite structure, depending on the addition of minor components. Specifc phases are said to be stabilized at room temperature by the minor components such as such as calcium, magnesia, yttrium or ceria [3]. Adhesion to dentin is obtained by infltration of resin into etched dentin, producing a micromechanical interlock with partially demineralized dentin, which underlies the hybrid layer or resin interdiffusion zone [4]. Resin cements may be classifed as total-etch, self-etch and self-adhesive, depending upon their application to dental tissues. Total-etch resin cement requires the use of phosphoric acid followed by primer and adhesive before the application of resin cement. Self-etch resin cements use an acidic primer, which is not rinsed away, to modify the dental surfaces before bonding. Self-adhesive resin cements bond to dental tissues without previous application of any bonding adhesive [5]. Bond strength of zirconia ceramic mainly depends on the type of resin cement, primers used for bonding and the intra oral conditions [6]. There are certain indications for use of resin cements such as self-adhesive resin cement is indicated with tooth preparations having adequate taper (2 ° –5 ° ), whereas self-etching resin cement is recommended for tooth with a short clinical crown (<3mm) and over-tapered preparations (>5 ° ) [6]. The bonding of zirconia substructures should be based on both micromechanical and chemical bonding since the micromechanical retention supports chemical bonding and if bonding is based only on chemical compounds, some debonding might happen in moist environments such as in the mouth [7]. Airborne-particle abrasion with Al 2 O 3 abrasive particles, tribochemical silica coating or fring glass pearls as a monolayer to the inner surface of zirconia substructure can be used as surface pre-treatments prior to bonding to achieve better bond strength between the zirconia material and resin cement [8]. Intra oral temperature change is another important factor that affects the bonding between the tooth structure and the zirconia core materials. The difference in the coeffcient of thermal expansion between tooth structure and restorative