الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
With advances in technology, novel composite resin materials have been introduced with modifications in fillers sizes, shapes, orientation and types that help to improve the mechanical and physical properties of the material to serve as suitable restorations for posterior teeth. The purpose of this in vitro study was to evaluate the fracture resistance of maxillary premolars with MOD cavities restored with recent different types of posterior composite materials (bulkfill posterior, nanoceramic filled, microhybrid and nanohybrid), and to assess the biaxial flexural strength of samples of prefabricated discs of those types of composites.
For fracture resistance test, a total of 60 sound extracted permanent maxillary human premolars were used in this study. The root portions of all teeth were embedded in self-cure acrylic resin blocks at 2 mm below the CEJ. 60 specimens were randomly divided into 3 main groups (A,B and C) where group A (n=10) served as a positive control with intact unprepared teeth, group C (n=10) served as negative control with prepared unrestored teeth with MOD cavities and group B was further subdivided into: Subgroup B1(n=10) (MOD cavities restored Filtek Bulkfill posterior composite/ Single bond universal adhesive), Subgroup B2(n=10) (MOD cavities restored with Ceram X Spheretec composite/Prime & Bond Universal adhesive), Subgroup B3 (n=10) (MOD cavities restored with Swisstec composite/ One coat 7 Universal adhesive) and Subgroup B4(n=10) (MOD cavities restored with Harmonize composite/ Optibond XTR Self Etch Adhesive). After water storage, thermocycling (1200 cycles, 5ºC/55ºC) with a 30 second dwell time and a transfer time of 10 seconds, then all the specimens were subjected to load cycling at 240,000 cycles corresponding 1 year of clinical service. Afterwards, all the specimens were subjected to axial compression in a universal testing machine at a speed of 0.5 mm/min till failure. Modes of failure were assessed and the results in various groups were compared and statistically analyzed.
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For biaxial flexural strength test, a total of 40 prefabricated cylindrical composite discs were made of each composite material using a custom made teflon mold with 9 mm diameter and 1.2 mm thickness. 4 main groups were tested with 10 specimens each: group I (Filtek Bulkfill Posterior composite discs), group II (Ceram X Spheretec composite discs), group III (Swisstec composite discs) and group IV (Harmonize composite discs). All the specimens were stored in distilled water at 37º C for one week prior to testing. A universal testing machine was used; a ball-on-three-ball testing method was applied where the specimens were supported by three stainless-steel ball bearings with diameter of 1.0 mm equally spaced along a support circle of diameter 6.8 mm. A ball used on the loading surface with a 1.0 mm. diameter at a cross-head speed 1 mm/min and the maximum load (P) was recorded. Fractured fragments were evaluated and biaxial flexural strength was determined with the use of a certain set of equations.
The results regarding fracture resistance test were as follows: group A (Positive control 1517.20 ± 268.68 N), group C (Negative control 559.50 ± 85.03 N), Subgroup B1 (Filtek Bulkfill 813.70 ± 86.73 N), Subgroup B2 (Ceram X Spheretec 1179.00 ± 108.75N), Subgroup B3 (Swisstec 657.90 ± 77.02N) and Subgroup B4 (Harmonize 940.30 ± 111.17N). Post-hoc multiple comparisons test demonstrated significant differences between groups. The analysis of failure modes revealed that, mixed and pure cohesive tooth fractures were the most common types of failure for all groups, with Subgroup B2 showing equal types of failures (50% mixed, 50% cohesive tooth failure).Also most of the failures were of favorable type of failures that can be repaired.
The results of biaxial flexural strength test were as follows: group I (Filtek Bulkfill 207.605 ± 189.918 MPa), group II (Ceram X Spheretc of 151.221 ± 23.653 MPa), group III (Swisstec 165.241 ± 39.450 MPa), group IV (Harmonize 164.284 ± 26.212 MPa). The post-hoc pairwise comparison using Bonferroni method revealed that the biaxial flexural strength in
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Filtek Bulkfill was statistically significantly higher than all other tested groups. Analyzing failure modes of the fractured fragments showed that: In group III (Swisstec), 40% of the specimens were fractured into three fragments while 60% were fractured into two fragments. Three fractured fragments were most frequently observed in group I (Filtek Bulkfill), group II (Ceram X Spheretec) and group IV (Harmonize).