Full Working Length Negotiation in Challenging Narrow Canals Using a Single Conservative Asymmetric Reciprocating Instrument

AIM: To compare the ability of Reciproc Minima instruments (sizes 20 and 25) and Reciproc Blue R25 to reach the full working length (FWL) in the mesial canals of mandibular molars without prior scouting or glide path preparation. METHODOLOGY: A total of 330 mesial canals from 165 extracted mandibular molars were included. Teeth with curvature of 0°-20° were randomly allocated to three groups (n = 110 canals): Reciproc Blue R25, Reciproc Minima 25, or Reciproc Minima 20. No scouting or glide path preparation was performed. Canal preparation was carried out in reciprocating motion according to the manufacturers' instructions. In canals where Reciproc Blue R25 failed to reach the FWL, Reciproc Minima 25 and, if necessary, Reciproc Minima 20 were used. The primary outcome was the frequency of reaching the FWL. Secondary outcomes included instrument fracture, deformation, and preparation time. Canals in which the FWL was not reached were analysed by micro-CT to investigate anatomical factors. Data were analysed using chi-squared, Kruskal-Wallis, Bonferroni post hoc, and McNemar tests (α = 0.05). RESULTS: The frequency of reaching the FWL differed significantly among the instruments (p < 0.001). Reciproc Blue R25 reached the FWL in 70.91% of canals, Reciproc Minima 25 in 83.64%, and Reciproc Minima 20 in 96.36%. No instrument fractures occurred, and deformation rates were low and comparable among groups (p = 0.552). Among canals in which Reciproc Blue R25 failed to reach the FWL, Reciproc Minima 25 failed to reach the FWL in all cases. However, Reciproc Minima 20 subsequently achieved the FWL in 78.13% and 83.33% of Reciproc Blue R25 and Reciproc Minima 25 cases, respectively (p < 0.001). Preparation time also differed significantly among groups (p < 0.001), with Reciproc Minima 20 reaching the FWL faster than both Reciproc Blue R25 and Reciproc Minima 25. CONCLUSIONS: Reciproc Minima instruments showed greater ability than Reciproc Blue R25 to mechanically scout mesial canals of mandibular molars. Reciproc Minima 20 showed the highest frequency of reaching the FWL, including in canals where larger instruments failed, without increasing deformation or fracture and with shorter preparation times.