الفهرس 
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Abstract
This study aimed to assess the effect of repeated piezoelectric ultrasonic periodic scaling strokes on microleakage around high translucent zirconia and hybrid resin ceramic crowns margins.
Forty freshly extracted human maxillary first premolars were used in this study. Each premolar was mounted, in a multi-hole Teflon mold, along its long axis with 3mm of cervical portion of root above a transparent epoxy resin block level. Premolars were prepared with a standard method to receive full veneer crowns, with total convergence angle 12 degrees. The occlusal reductions for functioning cusp and non-functioning cusp were 1.5mm, 1mm respectively. The axial reduction and heavy chamfer finish-line were 1.5mm, 1mm respectively. The finish line position was 1mm above cementoenamel junction.
The sample was divided with blind randomization technique into two equal groups, based on the material used for crowns fabrication. group A (n= 20): High translucent zirconia (HTZ) crowns and group B (n=20): Hybrid resin ceramics (HRC). After crowns fabrication, they were cemented with dual cured self-adhesive resin cement according to manufacturer’s instructions under static load 5kg.
Each group was then subdivided according to number of ultrasonic scaler strokes into four subgroups 1-4 (n=5), where subgroups A1 and B1 were not exposed to ultrasonic stroke cycles, subgroups A2 and B2 exposed to 6 complete stroke cycles of ultrasonic scaling, which represent 3 years of periodic oral hygiene recalls. While subgroups A3 and B3 exposed to 10 complete ultrasonic scaling stroke cycles, which represent 5 years of periodic oral hygiene recalls. Subgroups A4 and B4 (n=5) for each: exposed to 14 complete stroke cycles of ultrasonic scaling, which represent 7 years of periodic oral hygiene recalls.
Each complete stroke cycle was 60 seconds; every sample aspect was stroked for 15 seconds/cycle. Stainless steel tips size G2 and G4 were used for instrumentation by standard pressure, at 0◦ tip angulation, with lateral side of the tip, at medium power of device under sufficient water coolant.
After samples immersion in 2% methylene blue dye for 24hours, they were be sectioned buccolingually at the center of the crown using diamond disc attached to electric precision cutting saw. Each specimen was assessed at the buccal and the lingual margins using stereomicroscope . The captured images were transferred to a computer and the dye penetration percentage was calculated. Scanning electron microscope captured images were magnified at X120 and evaluated. The obtained data was collected, tabulated and statistically analyzed.
The results of HTZ crowns microleakage percentage showed that there was significant increase in leakage after 5 years and 7 years of repeated strokes. On the other hand HRC crowns subgroups showed statistical insignificant increase in the mean. Scanning electron microscope analysis was made to confirm HTZ group results.