Main causes of failure in endodontic treatments
One of the main causes of failed endodontic treatments is the presence of bacterial biofilm in the root canal system, especially inside areas of complex anatomy and lateral canals.5 As a file — whether it is a hand file or NiTi rotary — is only able to reach the main canal and is unable to instrument any lateral anatomy, irrigation is key to endodontic success to clean bacteria and pupal tissue that the file cannot contact. A root canal left undiscovered or contaminated is likely to be the reason for the treatment failure.
The persistent endodontic infection may be due to the difficulty in reducing the microbial load below a certain threshold. These microorganisms (isolated or within a biofilm) may be located in the ramifications of the root canal, including isthmus areas.6 According to one micro-CT study, 85% of all molars have isthmus areas.7 Therefore, the presence of difficult access anatomic areas is not an exception, but a rule. The current micro-CT studies require an anatomic pairing before the research itself. It is remarkable that many times the isthmus area is larger than the root canal area. With that in mind, in order to reduce the microbial load below the necessary threshold, it is imperative to thoroughly clean these areas using irrigation combined with some type of agitation process (mechanic, sonic, ultrasonic, or laser agitation).
Besides the clinical diagnosis and the case planning, knowing the root canal morphology and its many potential variations is fundamental to achieving success in endodontic treatment.8 When dealing with C-shaped canals, rotary and reciprocating instruments can easily shape the canals, which can then be cleaned and disinfected with the appropriate irrigating solutions. But many research papers have shown that mechanical preparation does not contact the entire canal system.1-3,9-11 On top of that, during preparation the debris produced when cutting the dentin walls of the canal are pushed inside lateral canals and isthmus areas (Figure 3).
The problem is quite clear: The vast majority of root canals do not have a simple anatomy. There is a high occurrence of flattened and oval-shaped canals, especially in mandibular molars; and the available rotary/reciprocating systems cut dentin in a conical shape, leaving a considerable percentage of untouched areas.1-3
Presence of isthmus
With the exception of upper anterior teeth, the isthmus is a common anatomic structure in human teeth. The isthmus can be described as a small horizontal groove that unites two root canals and can run all the way to the root or be limited to the coronal or mid portion of the canal, or even be present only at the apical portion. The isthmus is an area difficult clean and decontaminate with mechanical preparation due to its perpendicular position to the file, and it can have a major impact in the success of a root canal treatment.5,8
In a clinical study,12 1,400 teeth from 618 patients were evaluated using cone beam computed tomography. The study demonstrated that 87.9% of mandibular molars have isthmus present (Figure 4).
A high prevalence of isthmus areas means the practitioner must pay special attention during the preparation of mandibular molars. The use of ultrasonics and optical magnification is a fundamental tool to locate, identify, and clean this intricate anatomy.
Besides the high percentage of isthmus areas in mandibular molars, another clinical difficulty is the presence of a third canal in the mesial root, the middle mesial canal. This canal is located between the mesiobuccal and mesiolingual canals. The entrance point for the middle mesial canal is very hard to find without the use of magnification; however, in studies using the operative microscope, the canal was found in 46% of the investigated molars.13
The instrumentation, either manual or mechanized, is restricted by the main canal diameter, having little or no effect on flattened areas and twists/curvature of the root canal. Specially designed ultrasonic instruments allied to ultrasonic cavitation can favor the treatment inside flattened regions — raising the percentage of touched wall areas by the activated irrigants and performing a thorough cleaning without promoting excessive loss of tooth structure.
Protocol to prepare the mesial root canals in mandibular molars (Figure 5)
- After preparing the mesiobuccal and mesiolingual canals, remove the dentin over the isthmus using a diamond-coated ultrasonic tip. This procedure is best accomplished with a thick cone, pear, or sphere-shaped tip.
- Once the isthmus is visible through magnification, initiate its cleaning using a thin tip such as the E18D diamond-coated ultrasonic tip (Helse Ultrasonic, Santa Rosa de Viterbo, Brazil) or the UCT-1 diamond-coated ultrasonic tip (Vista Dental, Racine, Wisconsin) or the BUC-1A (Kerr Dental, Orange, California).
- In addition to cleaning this area, we must look for the middle mesial canal, which is generally located closest to the mesiolingual than to the mesiobuccal canal.
- Once the entrance is found, prepare the canal with carbon steel hand files, since they have a proper stiffness for this situation. A No. 6 carbon steel file is ideal for initial exploration of the canal due to its greater stiffness compared to a stainless steel file of the same size.