Canal negotiation to working length

Precurving the negotiating instrument at canal negotiation is not essential but may be needed in cases of moderate to severe curvature; in case the file stops before the estimated working length, either the file should be precurved to be able to bypass an impediment or more space needs to be created midroot or in the apical third.

These two conditions are defined by the tactile feedback the clinician gets from the file; a loose resistance indicates the presence of a ledge or acute curve, while so-called rubbery resistance suggests the presence of a tight canal space or the presence of compacted soft tissue that prevents the file from penetrating deeper into the canal.  

West differentiates four specific conditions that require technique modifications:
1. Apical blockage
2. Mismatch between canal curve and instrument
3. Too large file tip
4. Presence of restrictive coronal dentin

All these conditions may be remedied by either changing the negotiation file, its curvature, or by modifying the coronal two-thirds of the canal.  

The apical endpoint of root canal treatment should ideally be at the apical constriction, the narrowest diameter of the canal. This point is believed to coincide with the cement-dentinal junction (CDJ), based on histologic sections and ground specimens. However, the position and anatomy of the CDJ varies considerably from tooth to tooth, from root to root, and from wall to wall in each canal. Moreover, the CDJ cannot be located precisely on radiographs. For this reason, some have advocated terminating the preparation in necrotic cases at 0.5 to 1 mm short of the radiographic apex and 1–2 mm short in cases involving irreversible pulpitis, although there is no definitive validation for this strategy at present.

The apical endpoint of root canal treatment should ideally be at the apical constriction, the narrowest diameter of the canal. This point is believed to coincide with the cement-dentinal junction (CDJ), however, the position and anatomy of the CDJ varies considerably from tooth to tooth, from root to root, and from wall to wall in each canal. Moreover, the CDJ cannot be located precisely on radiographs. For this reason, some have advocated terminating the preparation in necrotic cases at 0.5 to 1 mm short of the radiographic apex and 1–2 mm short in cases involving irreversible pulpitis.

While specific manufacturer guidelines may vary, electronic apex locators work best in molars after drying the pulp chamber with a cotton pellet. A small file is advanced until the apex locator marks apical patency and then pulled back about 0.5 mm to determine the working length of the canal. It is best to start with a #8 or #10 K-file depending on the size of the canal and confirm the length with a flexible #15 file. Apex locator can produce irregular signals in large canals and so it is desirable to check the working length with larger files that come closer to binding at the foramen. The better the adaptation of the tip of the instrument to the foramen, the more accurate the length determination.

It is recommended to explore and shape a root canal at least to a #15 size instrument before the use of a rotary NiTi instrument to full working length to create a glide path for the safe advancement of the rotary instrument tip.   Typically, K-files sizes #10 and #15 are used in watch-winding or balanced-force motion to create and secure the glide path.  The presence of an appropriate glide path is indicated by the fact that a straight, not precurved size #15 K-file can passively and smoothly travel to working length. This should be tested from 1 to 2 mm off working length, as well as from 5 to 6 mm away.  For long-oval or flat canals, often found in distal roots of mandibular molars, the
preparation of two glide paths and ultimately two canal pathways is indicated. These will be located in the far buccal and far lingual aspects of the canal; such a shape promotes canal wall contact, facilitates debridement, and ultimately simplifies obturation.