Abstract: Objective To investigate the texture of root surface and collagen fiber exposure after etching with different modes of citric acid, and to explore the best condition for root replantation. Methods Root pieces （5 mm×5 mm） were prepared from molars of beagle dog and randomly divided into seven groups. Group A was treated as negative control, and root pieces in group B were soaking in 30% citric acid solution for 1 hour. The root surfaces of remaining five groups were rubbed with cotton pellets soaked in the 30% citric acid solution for 3 min, 5 min, 10 min, 20 rain and 30 min separately. Scanning electron microscopy was used to observe the presence of smear layer and the area of exposed collagen fibers. Inorganic loss after acid etching was detected by thermo gravimetric analyzer. BMSCs were seeded and cultured for 24 hour on the surface of citric acid-etched root. The condition of cell proliferation was tested by Cell Counting Kit-8. Results Smear layer was observed in group A. The root surface in group B was smooth, without any exposed collagen fibers. Collagen fibers exposure was observed in group C with no smear layer. The exposed amount of collagen was different according to the different processing time. The exposed amount of collagen in C-5 group was higher than in C-3 group, and the processing time of more than 10 minutes were appeared to remove not only the mineral component but also the collagenous matrix. Cell adhesion and cell proliferation in etched group were better than in control group. Conclusion The root surfacerubbed with cotton pellets soaked in the 30% citric acid solution for 5 min seems to be an ideal root surface conditioning method.

Key words: Citric acid, Root surface conditioning, Cell colonization