Methods: canals of 30 straight extracted roots (20 incisors, 5 premolars, 2 cuspids, 3 molar palatal roots) were instrumented using ProTaper instruments (Dentsply-Maillefer,Ballaigues,Switzerland), NaOCl 5,25% and EDTA 17%. Working-length was set using Morita DentaportZX (MoritaMfg.Corp.,Japan) to the apical foramen. Roots were obturated using the lateral condensation technique and master cones (Meta Biomed Co, Korea) and auxiliary cones size A(Dentsply-Maillefer,Ballaigues,Switzerland) and sealer AH+(Dentsply-DeTrey,Zurich,Switzerland). After 2 weeks, gutta-percha was removed using the ProTaper retreatment and rotary ProTaper Universal instruments(Dentsply-Maillefer, Ballaigues, Switzerland), according to the manufacturer's indications. Last retreatment instrument was one size greater than the last instrument used for instrumentation. Control radiographs were taken. Roots were embedded in acrylate. 5 transversal sections were performed at 0.5mm, 1mm, 1.5mm, 2 and 2,5mm, and were examined under direct, polarized light microscope and stereomicroscope(OlympusBX51M, at 50X and100X magnification). Images were captured using an Olympus ColorView IIIu digital camera and analyzed with the Analysis Auto system. Data were registered for each section: total perimeter of the canal and the residual gutta-percha perimeter. The Mann-Whitney test was used to compare the residual gutta-percha perimeter between sections.
Results: the mean percentage of the residual gutta-percha perimeter area varied from 49.01±34.13 at 0.5mm of apex, to 35.77±32.55 at 1mm, to 40.44±33.76 at 1.5mm, to 52.47±26.84 at 2mm, and to 38.13±36.11 at 2.5mm. A significant difference was found between the residual gutta-percha perimeter at 1mm and 2mm, and between 2 and 2.5mm (p<.05).
Conclusions: The highest percentage of residual gutta-percha was observed at 2mm of apex.When using rotary ProTaper retreatment and Universal instruments for removal of gutta-percha, a relatively high mean percentage (between 35-52%) of the root canal perimeter harbors residual material within the last 2.5mm of the re-instrumented canal.