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Objective: The interface between the adhesive resin and fiber posts has an important role in their performance. However, It was found that the most frequent cause of their failure is debonding at the adhesive resin-dentin interface. The aim of the present study was to evaluate the effect of dentinal wall laser treatment: Erbium, Diode or no treatment; on push out bond strength of Glassix fiber-reinforced post at different regions of root canal [cervical, middle, apical]. materials and methods: A total of 30 freshly extracted natural human maxillary central incisors were equally divided into three groups after root canal treatment, decoronatin and post space preparation. Teeth of group 1 were treated with Er,Cr:YSGG 2790-nm laser, group 2 were treated with 940-nm diode laser, while the teeth of group 3 [control] had no laser treatment. Glassix fiber posts were then cemented to the whole samples using Rely X Unicem 2 self adhesive resin cement. Three slices of 3mmthickness: cervical, middle and apical were prepared from each root. Then a push-out test was performed using universal testing machine at a strain rate of 0.5 mm/min. The results revealed that laser treatment generally is an important factor in attaining improvement in bond strength of fiber post to root canal dentin. The Er,Cr:YSGG -2790 nm laser afforded greater bond strength than the 940-nm diode laser. Moreover, better retention of the glass fiber-reinforced post was achievable in the coronal regions of root canals, followed by the middle, then the apical regions. KEy wORDs: Laser; Root canal dentin; Push-out test; Post retention; Fiber reinforced post; Self-adhesive. (1552) Eman A. Essam E.D.J. Vol. 62, No. 2 cores has resulted in the advent of post-and-core systems without any metal, especially transparent glass or quartz fiber-reinforced composite posts. [2] Several studies have confirmed the appropriateness of the physical properties of fiber-reinforced composite posts. The most important advantage of these posts, in addition to their esthetic results, is their modulus of elasticity which is similar to that of the tooth structure. It is believed that a modulus of elasticity similar to that of the tooth structure and the possibility of formation of an adhesive bond between various materials (root and coronal dentin, luting agents, fiber posts and composite cores) decrease the risk of root fracture.[2,3] Retention of fiber posts depends on several factors such as bond strength of post-resin cement and resin cement-root canal dentin. Some studies have indicated that there are no voids in the postcement interface and the bond strength of cementdentin is less than the post-cement interface. In the other words, cement-dentin interfaces is the weak point in bonded fiber posts.[4,5] Because of various dentinal morphology in different areas of the root canal, the bonding quality is different in coronal, middle, and apical regions. On the other hand, for achieving an optimal bond in interface, adhesive and resin cement should be polymerized well. Although, it is hard to transmit light to the apical region of the root canal, dualcure adhesives may result in a more acceptable and better bond in comparison with lightcure ones.[6] Bonding mechanism to the dentin root canal is micromechanical because of adhesive penetration to the demineralized dentinal surface and formation of hybrid layer, resin tag, and also adhesive lateral branches. [7] The immediate bonding effectiveness of methacrylate resin cements is favourable due to the formation of a resin–dentine interdiffusion zone and resin tags that provide micromechanical interlocking between resin and demineralised root dentine. However, the technique-sensitivity[8] and time-consuming bonding procedures associated with the use of multistep resin cement systems compromise their popularity for clinical use. Selfadhesive resin cement systems do not require complex dentine surface pre-treatment and appear to be more well-received by clinicians for luting of glass fibre posts. However, the high cavity configuration factor (ratio between bonded and nonbonded surface areas) associated with the bonding of fibre posts to long, narrow post-spaces[9] may result in the generation of extensive polymerisation shrinkage stresses that jeopardise the integrity of resin-dentine bonding interface. The limited ability of bond post spaces to relieve shrinkage stress by resin flow renders the retention of resin cementluted glass fibre posts unpredictable, as a substantial part of the retention is derived from friction.[1,8] In addition to the routinely used chemical substances, other technologies have been investigated for the treatment of root canal dentine, such as laser irradiation. Lasers with a wide range of characteristics (Nd:YAG, 810 nm diode, Er:YAG) have been investigated in the field of endodontics to evaluate the potential of smear layer removal, antibacterial activity and sealer adhesion. [10] Recently 980 nm diode laser has been launched to the market. [11,12] There are several studies that evaluate the penetration potential [11,12,13] and antibacterial activity of this laser. Nevertheless, the findings of these studies are not consistent enough to elucidate their mechanism of action on root canal structures. [14] Thus, Diode lasers emitting at 980 nm may have research and clinical applications because they transmit energy through thin flexible fibers that are compatible with the dimensions and curved shapes of root canals. [15,16] Diode lasers have a good penetration potential, with high absorption peaks for melanin and hemoglobin and limited interactions with water and hydroxyapatite. The power output of these lasers ranges from 0.5 to 7 W and is delivered in two operating modes: continuous wave and pulsed mode. However, the endodontic applications of this technology have largely not been addressed by the literature. Coluzzi [17] reported that EffEctivEnEss of dEntinal wall lasEr trEatmEnt on rEtEntion (1553) diode lasers have a depth of penetration per pulse that is 10,000 times greater than that of the Er:YAG laser and may act more deeply within the dentinal tubules. Gutknecht et al [15] reported that irradiation with a 980-nm diode laser can eliminate bacteria that have migrated deep into the dentine (up to 500 μm), whereas chemical solutions can only reach 100 μm. Ultrastructural alterations in dentine irradiated with a diode laser can be observed by scanning electron microscopy (SEM) [12, 16,18] and depend mainly on laser parameters such as output power, frequency, and application mode because these parameters are directly related to the increase in temperature in dental tissues.[18] The Er:YAG laser (wavelength 2,940 nm) is approved by the FDA for cleaning, shaping and enlarging the root canal. [19] Previous studies have tested the ability and the effects of this laser on root canal walls and have indicated that the Er:YAG laser is a suitable instrument for removal of the smear layer in root canals. [20] Furthermore, George et al. in an investigation of the ability of both the Er:YAG and Er,Cr:YSGG lasers equipped with conical shaped radially firing tips and plain tips to remove the smear layer from the apical third of the root canal showed a laser activation of EDTA and a better performance of conical fibers compared to plain fibers for improving the action of EDTAC in dissolving smear layer. [21] Hence, during the past decade, the effectiveness of lasers, including the Nd:YAG[22], Er:YAG [23] and Er,Cr:YSGG [24] lasers, in the removal of tooth hard tissues and in the preparation of cavities has undergone extensive evaluation. In addition, decreases in microbial counts in the root canals have been demonstrated after laser irradiation. Therefore, laser irradiation is considered as a coadjuvant treatment for conventional root canal therapy. [25] The aim of the present study was to evaluate the effect of dentinal wall laser treatment [whether Er,Cr: YSGG 2790-nm laser, 940-nm diode laser or no laser treatment( control)] on push out bond strength of glass fiber-reinforced post at different regions of root canal [cervical, middle, apical]. The null hypothesis tested was that there would be no difference between bond strength of laser-treated and un-treated groups. Also, there would be no difference in push out bond strength of fiber post at different root canal segments. maTERIals aND mEThODs A total of 30 freshly extracted sound human maxillary central incisors without any cracks or carious lesions, confirmed by evaluation with a dental explorer and under a magnifying loop were used in the present study. The teeth had been extracted due to periodontal and orthodontic reasons and were stored in saline solution of 0.9 % concentration at room temperature. The teeth were collected from an oral and maxillofacial surgery department after the patients’ informed consent had been obtained under a protocol approved by the Regional Medical Research Ethics Committee. Teeth with any canal obstruction or with working lengths less than 14 mm were excluded from the study. The teeth were decoronated 1 mm coronal to the cementoenamel junction after removing all the calculi and debris from the root surface using hand scalers. Micracut 150 precision cutter (Metkon instrument Ltd, Bursa, Turkey), under water coolant was used for decoronation. The canals were shaped and prepared 1mm short of the apex. Firstly treated with manual K-files (Dentsply Maillefer, Ballaigues, Switzerland) from size 0.6 to 25, in order to obtain an initial glide path, and then prepared by rotary NiTi Pro-taper mounted at c smart 3 motor (Forshan Coxo medical instrument )with instrument S1, S2, F1, F2, and F3 (Dentsply-Maillefer, Ballaigues, Swiss) and 2.5% sodium hypochlorite for rinsing. The root canals were rinsed with water and dried with paper points (Meta Biomed Co. LTD, Baotou, China). They were then obturated with guttapercha (Dentsply-Maillefer, Ba

EFFECTIVENESS OF DENTINAL WALL LASER TREATMENT ON RETENTION OF FIBER POST TO DIFFERENT REGIONS OF ROOT CANAL