Knowing the desired end point of endodontic therapy in a clinical context is essential. Seeing a white line to the apex of a root is very different from achieving the three-dimensional cleansing, shaping and obturation of the canal space from the orifice of the canal to the minor constriction (MC) of the apical foramen. A white line, while it may reach the apex, may have no relevance to whether the objectives of root canal therapy were met (Figures 1-2).
Every step in root canal treatment should move the clinician closer to the treatment goal above. Bearing this in mind can eliminate unnecessary steps or motions that do not take the clinician closer to this goal. For example, working without a rubber dam might save a minute or two in its routine application, but its absence puts the entire treatment process at risk from microbial contamination, lack of tactile over the tooth as well as the risk of aspiration. The two minutes saved probably cost 15-20 minutes in slowed treatment and make all treatment goals far harder to meet.
One challenge in endodontics that makes the achievement of its end point harder to conceptualise is that the procedure is performed primarily via tactile sense, especially when not working through a surgical operating microscope (SOM), (Global Surgical, ST. Louis, MO, USA). By contrast, crown preparation and impressions, etc, are procedures in which the clinician can easily define and see the desired end point (occlusal clearance is achieved, walls tapered, finish line even and smooth, etc).
We do not have this visualisation in endodontics except to the extent that the SOM allows. As a result, a mental awareness of where the clinician is at all times in the root and what is happening to the anatomy of the canal as a result of the actions of the clinician is essential.
Inherent in this recommendation is that the clinician must have a clear appreciation of how long, curved, fluted, calcified etc, a given root is before taking, for example, a rotary nickel titanium (RNT) file down the canal. The value of knowing what the taper and tip size of the particular instrument will do to the canal cannot be overstated.
In the broadest clinical terms, the desired end point of root canal therapy is the complete and total extraction of the pulp. Endodontics should mimic extraction. Extraction works and the patient heals because the entire diseased pulp is removed from the patient.
The application to endodontics is similar, the greater the endodontic procedure mimics extraction, the greater the chance that the patient will heal. Conversely, the more pulp (dead, dying or vital) left after treatment, the greater the chance of failure. Debris and bacteria left in root canal systems allows the apical migration of bacteria through the portals of exit as well as potentially allowing tissue fluids to reach bacteria in the root canal system; in either event, entities that increase the chances of failure.
Instrumentation during root canal treatment is the primary means by which bactericidal irrigants are allowed access to reach the canal system to kill bacteria, dissolve tissue and clear the smear layer. Instrumenting canals with RNT files is actually dependent at this time in large measure on a very low-tech means, i.e. the negotiation and initial enlargement of canal spaces with small hand files.
Small hand K files, #6-10, are used to assure that canals are open, negotiable and patent along their entire length to later allow RNT files to traverse the pre-enlarged canal path. One adjunct that can accentuate hand file use significantly is the M4 reciprocating handpiece attachment (SybronEndo, Orange, CA, USA). The fits onto an E type attachment of an electric motor. The hand file that negotiates the canal space to the estimated working length has an M4 then attached to it, while still in place in the canal and in the presence of a viscous EDTA gel such as File-Eze (Ultradent, South Jordan, UT, USA).
Using reciprocation at 900 RPM, a #6 hand file in an M4 can enlarge a canal to the size of a #8 or #10, and a #8 can enlarge the canal in an M4 to a #10 or even the diameter of a #15 hand K file. The file, while attached to the M4, is used gently with a vertical amplitude of 1-3mm until it spins freely, then the next size hand file is used until the desired initial enlargement has occurred.
Using the M4, the clinician might ask ‘how much hand filing will be required’? Hand filing is an essential first step in the enlargement of the canal and makes safe RNT enlargement and irrigation far more predictable. Hand filing should almost always precede RNT instrumentation and the canal must be at least to the size of a #15 hand K file before RNT files are inserted into the canal.
Initial radiographs with ideally three digital pictures (one mesial, one buccal and one distal) taken with a system like Dexis (Dexis digital radiography, Alpharetta, GA, USA) can give strong clues to how much initial hand enlargement will be required. If no canals are visualised or appear very thin radiographically, referral is often the best option.
The initial pre-enlargement of the canal with small hand files (with or without the M4) can make the enlargement of the canal predictable, safe and efficient with RNT instruments with the caveat that the RNT are used appropriately. I use K3 for its flexibility, durability, safety and tactile control (SybronEndo, Orange, CA, USA).
A fixed tapered series of instruments, they are available in a variety of tip sizes (#15-60). K3 is best-used crown down, i.e. from larger tapers and tip sizes to smaller. K3 use is gentle, passive and done with minimal engagement of dentin per insertion. One practical byproduct of this sizing is that enhanced master apical diameters can be prepared.
As a comprehensive system, K3 is in a class by itself – it can be used for virtually any canal anatomy and create the aforementioned enhanced master apical diameters. These enhanced diameters are consistent in the endodontic literature with cleaner final prepared canals. Powering RNT files of all types is predictable with two motors I can recommend, the TCM III (SybronEndo, Orange, CA, USA) and the Electrotorque TLC (Kavo, Lake Zurich, Il, USA).
As a stand-alone motor, the TCM III is very economical and dependable with a maximum RPM of 900. For a comprehensive handpiece system, the Kavo Electrotorque TLC can provide high, low and electric endodontic motor functions by changing attachments on a box system that is wired into the dental unit at the chairside.
The end point of endodontic cleansing/shaping is the prepared canal that has been enlarged in taper and tip size to a biologically relevant diameter. Once the first RNT file reaches the true working length (determined with an electronic apex locator), the MC is gauged with a hand file and the canal enlarged relative to this diameter. For example, if a #20 hand K file resists displacement through the MC, with K3 the clinician can prepare the canal to a larger diameter. For a canal that gauges to a #20, it is relatively simple to enlarge the canal to a #40 K3 RNT file at the MC. After the preparation of the final diameter, obturation can commence.