Statement of Problem: Nowadays Mineral Trioxide aggregate (MTA) is widely used for root end fillings, pulp capping, perforation repair and other endodontic treatments. Investigations have shown similar physical and chemical properties for Portland cement and Root MTA with those described for MTA. Purpose: The aim of this in vitro study was to evaluate the tissue reaction to implanted MTA, Portland cement and Root MTA in the mandible of cats. Materials and Methods: Under asepsis condition and

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... eral anesthesia, a mucoperiosteal flap, following the application of local anesthesia, was elevated to expose mandibular symphysis. Two small holes in both sides of mandible were drilled. MTA, Portland cement and Root MTA were mixed according to the manufacturers, recommendation and placed in bony cavities. In positive control group, the test material was Zinc oxide powder plus tricresoformalin. In negative control group, the bony cavities were left untreated. After 3,6 and 12 weeks, the animals were sacrificed and the mandibular sections were prepared for histologic examination under light microscope. The presence and thickness of inflammation, presence of fibrosis capsule, the severity of fibrosis and bone formation were investigated. The data were submitted to Exact Fisher test, chi square test and Kruskal-Wallis test for statistical analysis. Results: No statistically significant differences were found in the degree of inflammation, presence of fibrotic capsule, severity of fibrosis and inflammation thickness between Root MTA, Portland cement and MTA (P>0.05). There was no statistical difference in bone formation between MTA and Portland cement (P>0.05). However, bone formation was not found in any of the Root MTA specimens and the observed tissue was exclusively of fibrosis type. Conclusion: The physical and histological results observed with MTA are similar to those of Root MTA and Portland cement. Additionally, all of these three materials are biocompatible. However, in order to replace Root MTA and Portland cement type I as less expensive and suitable substitutes for MTA, more longer-term studies with larger number of samples are suggested.