الفهرس 
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Abstract
The restoration and reinforcement of pulpless teeth is an important step in preventing fracture of the teeth. The amount of tooth structure that remains after endodontic treatment is an important consideration. The primary objective of post and core build up is to replace the missing coronal tooth structure sufficiently to provide the required retention and resistance form for final restoration. There is a definite correlation between post material and fracture of roots. The post material should have the same modulus of elasticity as the root dentin to distribute the applied forces evenly along the length of the post.
In this in vitro study, thirty freshly extracted sound single rooted human maxillary central incisors standardized in lengths and widths were obtained. They were examined carefully and free of defect specimens were selected.
All specimens were cut 2mm above the most incisal point of cementoenamel junction perpendicular to the long axis of the tooth to obtain 2mm ferrule. They were divided randomly into three group each group consist of 10 specimens; group (A) zirconia post (CosmoPost) with composite core, group (B) glass fiber reinforced post (FRC Postec Plus) with composite core and group C Cast metal post and core (as control).
Specimens were endodontically treated and obturated with gutta percha and resin sealer (AH 26). Post spaces were prepared having (10 mm) depth from sectioned surface of root using CosmoPost 1.7 drill for group A and FRC Postec Plus size 3 drill for group Band C. All specimens had standard core length (4mm) and post length (10mm), and standard preparation with chamfer finish line (0.8mm) by aid of handpiece mounted on dental surveyor. Core and crown former obtained by vacuumed poly ethylene sheet was used to fabricated all specimens cores and crowns.
All posts and cores and their crowns were cemented with (Multicore Flow) luting cement. They were mounted in acrylic block with thin layer of light body silicon impression material in-between to simulate the periodontal ligament. The test specimens were thermocycled in a thermocycling machine for 500 cycles. They were loaded by universal testing machine at 135 degree until specimens fracture.
The result show that there was no significant difference in fracture resistance between zirconia post, glass fiber post and cast metal post and core (P value = 0.72). The mean maximum force for group A CosmoPost (951.04±280.76 N), group B FRC Postec Plus (883.94±245.50 N) and group C cast metal post and core (860.69±236.98N).
The result also show that there was a significant difference in the failure modes between zirconia post, glass fiber post and cast metal post and core (P value = 0.05). 40% of all sample represented Type I failure mode (favorable) and 60% represented Type II failure mode (catastrophic). Group A represented 10% Type I and 90%Type II. Group B represented 60% Type I and 40 % Type II. Group C represented 50% for both types.
Endodontically treated teeth restored with posts and cores of different materials tend to exhibit similar fracture resistance when abundant tooth structure remains. Anterior teeth with a 2 mm ferrule restored with a zirconia, FRC post, cast metal post and full-coverage crown would resist normal occlusal forces.