The aim of this study was to compare sizes of the first instrument with or without a taper that binds at the apical constriction of a root canal after coronal flaring.

A total of 310 canals were evaluated in patients presenting for root canal therapy. Canals with intact apices were selected. After gaining straight line endodontic access, the coronal third was flared using Gates-Glidden drills. Working length was determined using an apex locator. ISO Standard K-files (tapered) were passively introduced into the canals starting with a No. 15 file. The first K-file size to bind against the canal walls without pushing and to reach the working length was recorded as the FKFB (First K File to Bind). Next, ISO Standard Lightspeed files (non-tapered instruments) starting with No. 20 were then gently introduced by hand to each canal in ascending order to the working length. The first size of a Lightspeed instrument to bind against the canal walls and reach the working length was recorded as FLSB (First Light Speed to Bind). In all instances a larger file was introduced to ensure it could not reach the same depth (i.e., working length). Statistical analysis was carried out using a univariate analysis of variance (ANOVA).

The average size of the FLSB to bind against the canal walls first at the working length was approximately two ISO sizes larger than the FKFB (P<0.001).

The clinician should consider introducing a non-tapered instrument to working length after coronal flaring because determination of the initial narrow apical canal diameter plays a major factor in identifying the extent of final apical shaping. Because the first non-tapered instrument that binds the apical constriction is larger than the corresponding tapered instrument, it better reflects the actual narrow apical diameter of the canal.

The initiation of canal instrumentation with a K-file size three sizes beyond the mean values of the FLSB will result in greater final enlargement of the canal compared to starting with the FKFB. This increased canal enlargement facilitates improved mechanical and chemical cleansing of the root canal ensuring removal of more microorganisms and their substrates, thus, improving the outcome of the treatment.

Darda S, Manwar N, Chandak M, Shori DD. An In Vivo Evaluation of Two Types of Files used to Accurately Determine the Diameter of the Apical Constriction of a Root Canal: An In Vivo Study. J Contemp Dent Pract 2009 July; (10)4:043-050.