After adequate access opening has been prepared, a sharp endodontic explorer is typically used to locate the orifices and to assess for straight-line access. Pre-flaring the coronal third should be carried out with the goal of creating a “funnel” at each canal orifice so that instruments easily “drop into” the canal without the clinician having to “search and find” canals. Gates Glidden burs and many other “orifice opener” instruments are available for creating such funnels without gouging the pulpal floor. All dentin triangles in anterior teeth and dentinal overhangs in posterior teeth are removed to reveal orifices and establish straight-line access to the middle third of each root canal. Preparation in this manner makes the subsequent steps easier and faster while eliminating any unnecessary bending of instruments which can later lead to broken instruments and ledged canals.

Schilder (1974) described the cleaning and shaping process, often called “instrumentation,” in great detail. He emphasized the importance of achieving certain objectives that result in successful endodontic treatment [1]:

A continuous funneled and tapered preparation ensures adequate delivery of irrigants close enough to the working length, and it exposes lateral canals by shortening its length. Tapered preparations increase the contact between the file and dentinal walls, whereas in parallel preparation it is less likely to have such contact. Obturation also will be enhanced with funneled preparations, and compaction forces are distributed evenly resulting in a better seal.

The diameter of the root canal should be as small as the canal preparation apically. This kind of preparation will transmit the compaction forces to the smallest area including lateral canals, fins, and isthmus. This results in denser compaction of gutta-percha. Caution must be taken not to weaken the coronal structure by creating too large of a taper. This is particularly true in teeth that have internal resorption.

Clinicians, during root canal therapy, frequently deviate from the canal’s original path thereby creating a new artificial path, commonly called “transportation” of the canal. This is most often caused by the rigidity of the instruments and their inability to accurately follow the curves of the canals. One objective of proper root canal instrumentation is to minimize transportation because it can result in unnecessarily removing of healthy dentin and incomplete debridement of the harmful contents within the main canal. However, conservative straightening of the canal’s coronal third is often desirable in order to achieve better access to the apical third. Transportation of the foramen is one of the most common reasons for wet or bleeding canals, which can result in chronic persistent inflammation and failure of root canal therapy. The rigidity of the metals creates a lever effect when the instrument goes around a curve. This exerts a lateral force, which can lead to transporting the foramen. In severe cases, a lateral perforation through the side of the root is the result. Creating straight-line access to at least the mid-root of the canal helps to reduce this level effect and reduces the lateral force of the instrument leaning against the side wall of the canal. The use of copious amounts of irrigation during instrumentation also helps prevent accumulation of dentinal shavings and canal blockage that also cause instruments to deviate away from the canal’s original path.

The following biological objectives are necessary for adequate root canal preparation: