OBJECTIVES: To investigate the effectiveness of instrumentation using Protaper Next (PN
Dentsply Sirona, Maillefer, Ballaigues, Switzerland) and Waveone Gold (WG
Dentsply Sirona, Maillefer, Ballaigues, Switzerland) systems on the area of untouched surface (US), accumulated hard tissue debris (AHTD), and the filling ability of two obturation techniques on the volume percentage (vol%) of voids within C-shaped root canals of mandibular first premolars. METHODS: A total of 64 mandibular first premolars with C-shaped canals were scanned, matched, and subsequently assigned to two shaping groups (n = 32): PN and WG. After instrumentation, the specimens were randomly allocated into two obturation subgroups (n = 16): continuous wave compaction (CWC) and single-cone (SC) techniques. The US% and AHTD% post- instrumentation, as well as the vol% of voids after obturation, were calculated from micro-computed tomography. Data were analyzed using comparisons for two groups (PN vs. WG) or two subgroups (CWC vs. SC) at α = 0.05. RESULTS: The untouched canal wall area was 18.75% and 22.69% in the PN and WG groups (p >
0.05), respectively. The apical third had higher US% than the coronal third (p <
0.05) in the two shaping groups. Instrumentation with WG left more debris (26.48%) than PN (8.36%) in the apical 1-3 mm (p <
0.05). In PN and WG group, the vol% of voids had no significant difference between the CWC and SC subgroups (p >
0.05). The apical region had significantly more voids than the coronal region of canal space in the two obturation subgroups regardless of which system was applied (p <
0.05). CONCLUSIONS: Both PN and WG systems showed similar performance on US after instrumenting C-shaped canals of the mandibular first premolar. WG left significantly more AHTD compared with PN in the apical region. In PN or WG group, SC yielded similar obturation quality when compared with CWC. Both CWC and SC obturation techniques provided inferior filling quality in the apical region than that observed in the coronal region.