The crown-down approach is the most effective procedure for root canal debridement, especially in the molars. However, in simpler cases, a combination of crown-down preparation, rotary glide path creation and single-length preparation can be very effective too. This hybrid technique will be described in this article.
The crown-down filing technique was introduced many years ago for rotary nickel–titanium (NiTi) files. File systems such as ProFile (Dentsply Sirona), K3 (Kerr Dental) and HERO 642 (MicroMega) were used for this technique, which became popular in the 1990s. However, for the majority of clinicians, it remained quite complicated. For that reason, systems like ProTaper Universal (Dentsply Sirona) and Mtwo (VDW) were designed. During this time, there was a shift away from the crown-down technique towards the single-length technique, and for many dentists, the single-length technique was much clearer and easier. Nevertheless, the single-length technique is very sensitive for some clinical problems, for example, for working length (WL) determination, the first step that has to be performed with hand files. In more curved canals, the WL check needs to be performed after each NiTi file has been used, because it can shorten by 0.5–0.7 mm. This happens because in more curved canals reaching the WL with every NiTi file makes the curvature slightly less curved. Thus, the final length of the canal can be shorter. This WL determination issue will cause the over-preparation of the apical constriction and overfilling of the canal.
How to prepare canals in the molars easily
First of all, pre-flaring is performed with the starter file in the MG3 set of files (Shenzhen Perfect Medical Instruments), reaching approximately one-third of the root canal length—usually no more than 5 mm. This will reduce the canal curvature and allow you to reach the apical area more easily. In addition, the length of the canal should not reduce during the shaping procedure.
After this procedure, in straight canals, the WL can be established with hand files and the G1 (20/0.04) and G2 files (25/0.06) used for shaping. In larger canals, the G3 file (35/0.04) can be used. The G1, G2 and G3 files can be used for the single-length technique. In canals with curvatures smaller than 30°, this technique will allow you to work in a simple and efficient way.
For more curved canals, you can also use the hybrid technique, which I like the most for treatment of molar root canal systems. After using the starter file, the glide path and temporary WL need to be established. After this procedure, the G2 file is taken to between half and two-thirds of the WL. This step is considered deep flaring. In the next step, the canal is rinsed and the WL established one more time. This stage is mandatory in curved canals. In addition, a rotary glide path can be created with the P1 and P2 files. After this procedure, the G1, G2 and G3 files (if needed) are used for the single-length technique.
The crown-down technique is considered the most efficient for debridement of the root canal space. However, it being quite a difficult technique, many clinicians prefer to use the single-length technique. Owing to the WL determination issue in curved canals that typically occur in the molars, the single-length technique should be modified. In canals with curvatures of about 30°, pre-flaring should be performed. In more curved canals, deep flaring is very useful for preserving the apical constriction and avoiding procedural problems.