Materials and Methods: Thirty extracted human single-rooted teeth were used in this study. Standard root canal treatment was carried out. Root-ends were resected, and retrocavities were prepared. The teeth were randomly divided into the following two groups (n=15): In group 1, the internal surface of the cavities was coated with blood. Then, the cavities were filled with MTA. The roots were immersed in molds containing fresh blood. In group 2, the aforementioned procedures were performed except that synthetic tissue fluid (STF) was used instead of fresh blood. After incubation, the specimens were prepared for scanning electron microscopic analysis. To assess the marginal adaptation, “gap perimeter” and “maximum gap width” were measured, respectively. The surface microstructure was also examined at higher magnifications. Independent samples t-test and Mann-Whitney U test were used to analyze data.

Results: Maximum gap width and gap perimeter in the blood-exposed group were significantly larger than those in the STF-exposed group (p<0.01). In the blood-exposed group, the crystals tended to be more rounded and less angular compared with the STF-exposed group, and there was a general lack of needle-like crystals. 

Conclusions: Exposure to blood during setting has a negative effect on marginal adaptation of MTA, and blood-exposed MTA has a different surface microstructure compared to STF-exposed MTA.