A clinical method to determine the optimal apical preparation size. Part I.

Objective: To establish a new approach for determination of the optimal apical preparation size (APS).

Study Design: After preflaring 212 root canals from 80 extracted molars, electronic working length (WL) was measured to establish where the apical preparation size (APS) should be determined. Subsequently, nontapered, noncutting special instruments were used to reach WL. The diameter of the largest special instrument (SI) width that had to overcome some resistance to advance to WL was defined as d(SI). After sectioning the roots apically, the diameter of a potential rotary instrument was determined for each section, allowing a complete cutting of the root canal wall. The estimation of APS was related to d(SI).

Results: Theoretically, individual APS of d(SI) + 0.60 mm yielded circumferential apical preparations in 98% of the cases. When shaping palatal/distal root canals of molars to d(SI) + 0.40 mm, and mesiobuccal/mesiolingual/distobuccal root canals of molars to d(SI) + 0.30 mm, complete preparations of root canal walls resulted in 78% (95% CI, 69%-87%) and 72% (95% CI, 65%-80%) of canals, respectively.

Conclusion: The described approach allowed evaluation of the APS for most root canals. Root canals should be shaped to larger sizes than normally recommended.