6. Instrumentation (Techniques, File Systems, File Types, and Techniques)

The success of non-surgical endodontic root canal treatment requires the use of files and instruments to remove necrotic and infected tissues. The cleaning and shaping are separate concepts but are always performed together. The goal of cleaning the root canal is the removal of necrotic pulp and infected tissues. The goal of shaping the canal is to maintain the apical foramen as small as possible in its original anatomic position. A good endodontic treatment outcome is dependent on the removal of necrotic pulp and infected tissues to a low level that cannot cause a flare up which will require retreatment. After a straight-line access cavity has been cut to allow direct access of the instruments into the root canals, and the orifices of the root canals have been identified. The next step is to instrument the root canals. The instrumentation process can be simplified by dividing the procedure in a series of steps. The majority of teeth are approximately 19-25 mm in length. Most roots are 9-15mm and most crowns are 10mm in length. An easy concept is to divide the root canal into three regions; coronal, middle and apical. Each of these regions is likely to be between 3-5mm in length. Dividing the root canal into three regions is a helpful strategy for instrumenting complicated calcified root canals with a challenging morphology. It is necessary to accurately measure tooth length in order to carry out and fulfill the basic tenets of root canal therapy. This measurement should be 0.5mm to 1mm short of the radiographic apical foramen, to create an apical stop within the tooth structure in order to confine instrumentation and the filling material. The radiograhic length is the length of the tooth as it appears on the radiograph. The estimated working length is the radiographic length minus 1mm. The final working length is -1mm substracted from the anatomical apex measure from the working length radiograph. NiTi rotary instruments have proved to be extremely successful for cleaning and shaping root canals, but they should not be used when the dentinal walls are extremely thin to avoid perforation of the root canal. Curved canals are the most challenging to instrument, because the distortion of the files and instruments will cut into the curve to reduce its angle, and place pressure on the cutting tips in an opposite direction, thereby increasing the risk of cutting a perforation. The risk of cutting a perforation in curved canals increases when larger file sizes are used. To avoid perforations, the concept of anti-curvature filing is to prepare a straight line access through the root canal to the apical region, by filing away the bulky root structure to create a displacement space, and by not touching the thin root walls which are in danger of being perforated. When a perforation occurs, it should be repaired immediately using restorative materials, with a thin liner of MTA being placed against the vital periodontal tissues or bone. The prognosis of a perforated tooth depends on the size, location, and the time taken to repair it.