Root canal preparation and obturation to the correct working length is well established as a prognostic factor in the favourable outcome of endodontic treatment. Ideally the root canal preparation would end at the junction of the root canal and the periodontium, but this junction can only be determined histologically. Clinically a number of factors aid in the establishment of the appropriate working length, one of the most important being the use of an electronic apex locator.
Apex locators work on the basis that the impedance across oral tissues is relatively constant. The value at the apical foramen is the value at which the majority of apex locators are calibrated, and values short of the apical foramen are not calibrated as accurately. Therefore it is important to establish the reading at the terminal point of the root canal. Studies have shown that the apical constriction is an average of 0.5mm short of the apical foramen.
1. Analyse the root anatomy for curvature and establish an estimated working length from the pre operative radiograph.
2. The coronal aspect of the canal should be opened/prepared to provide straight line access or a glide path to the apical aspect of the root canal.
3. Modern apex locators generally function well in the presence of fluids and irrigants in the root canal, but prior to using the apex locator excess irrigating fluids are removed from the access cavity.
4. Once the lip hook and file holder are attached, in most cases a size 15 or 20 file (see troubleshooting) is advanced into the root canal until the blue scale on the apex locator reaches the apex and red triangle on the screen of the root ZX. This indicates that the file is now at the apical foramen (see diagram).
5. A diagnostic radiograph is taken with the file at this length. If the radiograph confirms the file to be at the apex this length is effectively the canal length.
6. Since the apical constriction is on average 0.5 mm from the apical foramen, the working length is calculated by subtracting 0.5 mm from the canal length. The canal can now be prepared to the working length (see diagram).
If you are an advocate of patency filing, a size 8 file should be placed to the canal length to maintain the patency of the root canal.
Subsequent use of the apex locator.
Apex locators can be used to check that the master file is still within the root canal by placing it to the working length at the completion of the preparation. With the file at working length the blue bars should advance approximately to the blue line on the right hand side of the scale. This is short of the apex and indicates the file is within the root canal. Should the treatment be carried out over 2 visits the procedure can be repeated prior to commencing obturation.
1. The anatomy of the root canal dictates the size of the file that is used to establish the working length. In sclerosed canals a size 6, 8 or 10 may be the first file that can be negotiated to working length and will be the file used with the apex locator. Conversely in large canals it may be necessary to use a size 40, 50 or 60 to obtain an accurate apex locator reading. If the blue bars move over a range of readings,.a sequentially larger file is used until a stable and reproducible reading is obtained.
2. If the file touches a metallic restoration the apex locator will short circuit and give an inaccurate reading. To overcome this a section of the TGpex plastic applicator tip can be cut from the applicator and placed over the file. The plastic will prevent the file contacting the metal and an accurate reading then obtained
3. If there is vital tissue in a root canal it may cause a reading to appear short. To overcome this the file should be curved in the apical region to try and avoid contact with the tissue in the root canal.
4. Finally, if a perforation is suspected the apex locator can be used to confirm this, should an apex reading be obtained well short of the apex.