The an unpredictable substrate for adhesion due to

 The choice of using direct composite resin as a restorative material in dentistry has become more popular because of its esthetic property. Due to polymerization shrinkage of these materials, successful adhesion to enamel and dentin is an indispensable prerequisite for clinical success, otherwise gap formation would endanger the clinical success.

On the other hand, polymerization shrinkage stress is only present when shrinking  materials are bonded. Enamel bonding is meanwhile accepted as clinically strong and durable, because acidic etchants, such as 30–40% phosphoric acid, create enamel  microporosities allowing the penetration of monomers consecutively generating  micromechanical retention. In contrast, dentin is an unpredictable substrate for adhesion due to facts like tubular structure, high organic content, and intrinsic wetness(1). However, the main problem faced by composites is polymerization shrinkage and stresses which depends on multiple factors such as the configuration factor, composition of resin composites, material properties, various incremental placement techniques and different modes of curing(2).

According to Karthick et al. 3 to overcome this problem, various methods have been employed; the incremental curing technique being one of them. The various incremental techniques used are faciolingual layering (vertical), gingiva-occlusal layering (horizontal), three site technique, wedge-shaped layering (oblique), successive cusp build-up technique, bulk technique, and centripetal build-up.

4However, this technique of  multiple increment has many disadvantages: It is difficult to place the multiple increments leading to an increase in the arduousness of the task and the time it takes to complete it. If not performed properly, placing multiple layers can result in polymerization shrinkage and marginal leakage.2If this technique not carried out effectively, areas of uncured or partially cured composite resin may remain at the base or between layers at the bottom of each increment.

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This can lead to reduction in strength, prevent adequate sealing of the restoration or cause post-operative sensitivity and early failure of the restoration(6) In light of this, a group of new products were recently introduced, known as “bulk-fill composites.” These materials are recommended for insertion in a maximum 4-mm bulk due to their high reactivity to light curing.5The rationale of the bulk-fill resins is to reduce clinical steps by filling the cavity in “single” increment, leading to a reduced porosity and a uniform consistency for the restoration, further reducing the clinical time taken and cost factor for the patient.2The Objectives of this study were1. To compare the shear bond strength of microhybrid and bulkfill composite.2.

To compare the shear bond strength of  bulkfill,  horizontal and oblique layering techniques.3. To determine the mode of failures using scanning electron microscope.Methodology60 freshly extracted human first molars were collected and their root surfaces were cleaned.

These teeth were mounted in self-cure acrylic resin blocks and rinsed with distilled water. A flat dentinal surface of these teeth was prepared by removing 1.5–2.0 mm of their occlusal surfaces with the help of single-sided diamond disc for testing. These samples were randomly divided into two groups of 30 samples each: group 1 for microhybrid composite and group 2 for nanohybrid bulkfill composite resin. The two groups were further divided into three subgroups based on the layering techniques (group A- bulkfill, group B -horizontal and group C-oblique layering techniques) These samples were stored in normo-saline at 37 °C for 24 h, thermocycled for 500 cycles.

These specimens were  loaded in Universal testing machine at a speed of 0.5 mm/min till failure for determining the shear bond strength. The mode of failures of each group were observed under a scanning electron microscope and ANOVA  and T value calculation analysis was  applied.