الفهرس 
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Abstract
Calcium hydroxide is considered an effective intracanal medicament for the eradication of many bacterial strains and its byproducts in addition to its role in the relief of inflammation. However it has a critical disadvantage through its weakening effect of the mechanical properties of root canal dentin. Many vehicles were used for mixing of calcium hydroxide and it was proofed by many authors that the mixing vehicle has a direct effect on the degree and the rate of calcium hydroxide formulations. In our study three vehicles were used; saline, chlorhexidine and iodoform.
This study consisted of three parts;
The first part was designed to detect the chemical effect of calcium hydroxide on the root canal dentin and the surrounding periapical tissues regarding the changes in pH and the calcium weight percent after immersion of the dressed samples in normal saline for one, two and three months.
PH changes were measured using digital pH meter and the changes in the internal and external dentin calcium weight percent were measured using EDAX system.
The results of this section showed an increase in the pH by time for both dentin and the surrounding media with higher readings recorded for the saline vehicle. It also showed high calcium weight percent for the internal dentin than the external dentin for the iodoform vehicle than saline than chlorhexidine. It was noted that the calcium weight percent for both internal and external dentine decreases by time for the three groups.
The second part of the study was designed to evaluate the mechanical effect of calcium hydroxide on the root canal dentin as regarding the changes in the fracture resistance and the dentin microhardness after immersion of the dressed samples in normal saline for one, two and three months.
The results of this section showed decrease in the fracture resistance in samples dressed with calcium hydroxide by time. With the use of iodoform vehicle the fracture resistance was better than saline vehicle and chlorhexidine vehicle in one and two months immersion period . However after three month there was a strong DROP in the fracture resistance in the three groups while the saline and chlorhexidine showed better fracture resistance than the iodoform vehicle. It is important to mention that this DROP reaches about 1/3 of the fracture resistance of the undressed samples. Regarding dentin microhardness the results showed better dentin microhardness values for the external dentin than the internal dentin for the three tested groups. The dentin microhardness decreases with time especially with the use of chlorhexidine vehicle rather than with the use of saline or iodoform vehicles for both internal and external dentin.
The third part of this study was designed to test the healing potentiality of different formulations of calcium hydroxide on the periapical tissues of dogs with induced periapical lesions.
Six mongrel dogs were used for this section with 16 teeth for each dog. After the induction of a periapical lesion teeth were dressed with the three different formulations while four teeth one in each quadrant for each dog was left empty (control). The animals were classified according to the time of scarification into three groups; two weeks, four weeks and six weeks. After scarification histological preparation of the samples was done and stained with H&E stain to be ready for the histopathological analysis to detect the number of the inflammatory cells using Image j. analysis software. The results revealed that the calcium hydroxide formulations accelerate the healing potentiality by time and the saline vehicle was better than iodoform and chlorhexidine.