METHODS: Thirty root canals were prepared to apical size .04/#40 with Kerr K3 instruments (Sybron, Orange/CA/USA). Micro-CT scans (GE Locus SP, London/ON/Canada) of the prepared root canals were taken prior to obturation in order to ensure consistent taper (.04) and size (#40) as required for the particular obturation technique. Teeth not matching the required criteria were replaced. Teeth were randomly assigned to group1 (n=15) and obturated with a glass ionomer-based (ActiveGP sealer/ActiveGP cone, Brasseler USA, Savannah/GA/USA) or group2 (n=15) and obturated with a flowable, silicone-based cold obturation material (GuttaFlow/gutta-percha cone, Coltene/Whaledent, Langenau/Germany). Specimens were stored for 1 week (37°C/100% humidity) to ensure complete sealer setting. Canals were retreated step-by-step to size 40, size 45, size 50 and size 55 taper .04 with EndoSequence instruments (Brasseler USA). Micro-CT scans (8µm) were taken after every enlargement and required retreatment time recorded. Evaluation of micro-CT scans was performed by Amira 4.1 software (Mercury Computer Systems, Berlin/Germany). Residues of root-fillings (coronal, middle and apical canal level) were expressed as percentage of canal surface area. Statistics were computed with SPSS 14.0 software (SPSS Corporation, Chicago/IL/USA).
RESULTS: Residue evaluation did not show significant differences between both groups (t-test, P>0.05). The percentage of residue-coated canal surface (PRCS) in all specimens was significantly higher in the apical third (ANOVA, P<0.05). Stepwise enlargement of the root canals from .04/#40 to .04/#55 significantly decreased the amount of residues in both groups (t-test, P<0.05). Retreatment time was not significantly affected by sealer type (P>0.05).
CONCLUSION: Within the limits of this study, root fillings with ActiveGP or GuttaFlow were retreatable with nickel-titanium rotary instruments. Enlargement of root canals up to 3 sizes is recommended to obtain virtually residue-free canal walls.