الفهرس 
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Abstract
In this study we aimed to evaluate the influence of specimen dimension and loading technique on shear bond strength & failure mode results using invitro testing & FEA to numerically analyze stresses generated during shear testing for better interpretation and confirmation of our results.
Ten sound disinfected human lower wisdom teeth were transversally sectioned using diamond disc and a straight hand piece at low speed under air-water coolant into two pieces at the intermediate dentin level midway between the central occlusal fossa and cementoenamel junction. Radicular tooth portion was sectioned transversally at cementoenamel junction yielding twenty tooth halves.
Polypropylene (PP) tubes with 1.8 cm internal diameter and 1 cm height were used as mold in which the sectioned teeth were placed and embedded in cold cured acrylic resin. Eighty macro and micro flowable composite specimens of 1.8 and 0.8 mm internal diameter respectively and 1.5 mm thickness were bonded to dentinal substrate. Four study groups were created (n=20); Macroshear wireloop Gp1, Microshear wireloop Gp2, Macroshear chisel Gp3, Microshear chisel Gp4. They were tested for SBS using chisel and wireloop loading techniques by universal testing machine followed by failure mode analysis using digital microscopy and SEM. Two- and one-way ANOVA were used to compare stress at failure values of different groups while Kruskal Wallis test was used to compare between failure modes of the tested groups. Marc/Mentat (Santa Ana, CA, USA) software was used for virtual model creation. Four main models were created simulating macro & micro-shear composite specimens bonded to tooth substrate (similar to the experimental model) loaded by Chisel and wireloop techniques.
Material properties values were assigned as Elastic modulus and Poisson’s ratio from literature. All enlisted materials were assumed to be isotropic, homogeneous, with linearly elastic behavior.
Twenty-four finite element models were created, those models were used to assess:
i) Dentinal and composite stress maps (contours) for macro and micro shear simulations.
j) Dentinal and composite stress maps for CH & WL loaded simulations.
k) Dentinal stress maps and dentinal maximum nodal stress values for central and peripheral bonded composite specimen simulations.
l) Dentinal and composite stress maps and dentinal maximum nodal stress values for tetragonal & hexagonal element class simulations.
m) Dentinal and composite stress maps for simulations with different Loading offsets (zero, 0.1 & 0.5 mm).
n) Vector displacement (composite specimen bending) values.
o) Rate of stress development in CH and WL loaded simulations.
p) Contact frictional forces.
In vitro shear testing showed that Gp4 had recorded the highest mean stress at failure 54.1 ± 14.1 MPa, and the highest fraction of adhesive failures in relation to the other groups.
FEA showed smaller contour maps for micro in comparison to macro simulations at the same stress scale. CH loading showed concentrated stress contours and higher stress values. No major difference was existing between central vs. peripheral positioned specimens, similarly no major difference was detected between tetragonal & hexagonal element classes. Noticeable differences were observed by various offset loadings. Two rather than one dentinal maximum stress bands were observed with 0.5 mm offset loading. Vector displacement values showed gradual increase by increasing the value of offset loading. CH loading showed rapid development of stresses through simulation increments in comparison with the WL. CFF analysis for the four main groups at flush position showed best frictional force distribution for Gp2 while at 0.5 and 0.1 mm offsets for WL & CH simulations respectively, Gp4 showed the best frictional force distribution.
Conclusively, loading technique and specimen size are prime factors influencing shear bond strength and failure mode results. Microshear bond strength test is more reliable and recommended than the macroshear one. Chisel loading is more recommended than wireloop whenever accurate adhesive interface engagement can’t be guaranteed.