Please activate JavaScript!
Please install Adobe Flash Player, click here for download
ePaper created 2018-04-25, 08:59:39 | version 1.38.0
12 mCME Dental Tribune Middle East & Africa Edition | 2/2018 Canal preparation and obturation: An updated view of the two pillars of nonsurgical endodontics mCME articles in Dental Tribune have been approved by: HAAD as having educational content for 1 CME Credit Hour DHA awarded this program for 1 CPD Credit Point CAPP designates this activity for 1 CE Credit By Dr Ove A. Peters, USA The ultimate goal of endodontic treatment is the long-term retention in function of teeth with pulpal or periapical pathosis. Depending on the diagnosis, this therapy typically involves the preparation and obtura- tion of all root canals. Both steps are critical to an optimal long-term out- come. This article is intended to up- date clinicians on the current under- standing of best practices in the two pillars of nonsurgical endodontics, canal preparation and obturation, and to highlight strategies for deci- sion making in both uncomplicated and more diffi cult endodontic cases. Prior to initiating therapy, a clini- cian must establish a diagnosis, take a thorough patient history and conduct clinical tests. Recently, judi- cious use of cone-beam computed tomography (CBCT) has augmented the clinically available imaging mo- dalities. Verifying the mental image of canal anatomy goes a long way to promote success in canal prepa- ration. For example, a missed canal frequently is associated with endo- dontic failures.1 As most maxillary molars have two canals in the mesiobuccal root, case referral to an endodontist for microscope-supported treatment should be considered. Endodontists are increasingly using CBCT and the operating microscope to diagnose and treat anatomically challenging teeth, such as those with unusual root anatomies, congenital variants or iatrogenic alteration. The endo- dontic specialist, using appropriate strategies, can achieve good out- comes even in cases with signifi cant challenges (Fig. 1). Preparation of the endodontic space The goal of canal preparation is to provide adequate access for disin- fecting solutions without making major preparation errors such as perforations, canal transportations, instrument fractures or unnecessary removal of tooth structure. The in- troduction of nickel-titanium (NiTi) instruments to endodontics almost two decades ago2 has resulted in dramatic improvements for success- ful canal preparation for generalists and specialists. Today there are more than 50 canal preparation systems; however, not every instrument sys- tem is suitable for every clinician and not all cases lend themselves to rotary preparation. Several key factors have added versa- tility in this regard, for example, the emergence of special designs such as orifi ce shapers and mechanized glide path fi les. Another recent de- velopment is the application of heat treatment to NiTi alloy, both before and after the fi le is manufactured. Deeper knowledge of metallurgical properties is desirable for clinicians who want to capitalize on these new alloys. Finally, more recent strategies such as minimally invasive endo- dontics have emerged.3 Basic nickel titanium metallurgy What makes NiTi so special? It is highly resistant to corrosion and, more importantly, it is highly elastic and fracture-resistant. NiTi exists re- versibly in two conformations, mar- tensite and austenite, depending on external tension and ambient tem- perature. While steel allows 3 per- cent elastic deformation, NiTi in the austenitic form can withstand defor- mations of up to 7 percent without permanent damage or plastic defor- mation.4 Knowing this is critical for rotary endodontic instruments for two reasons. First, during prepara- tion of curved canals, forces between the canal wall and abrading instru- ments are smaller with more elastic instruments, hence less preparation errors are likely to occur. Second, rotation in curved canals will bend instruments once per rotation, which ultimately will lead to work hardening and brittle fracture, also known as cyclic fatigue. Steel can withstand up to 20 complete bend- ing cycles, while NiTi can endure up to 1,000 cycles.4 Recently manufacturers have learned to produce NiTi instruments that are in the martensitic state and even more fl exible than previous fi les. Figure 2 shows how instrument conditions (austenite vs. martensite) are determined in the testing labora- tory, using prescribed heating and cooling cycles.5 Heat-treated fi les with high martensite content typi- cally do not have a silver metallic shade but are colored due to an ox- ide layer, such as gold or blue. It is important to note that CM fi les frequently deform; however, with a delicate touch, cutting is adequate and often even superior to con- ventional NiTi instruments.6 It is imperative for clinicians to retrain themselves prior to using these new instruments to avoid excessive de- formation and subsequent instru- ment fracture. in Preparation strategies Experimental and clinical evidence suggests that the use of NiTi in- struments combined with rotary movement results improved preparation quality. Specifi cally, the incidence of gross preparation errors is greatly reduced.7 Canals with wide oval or ribbon-shaped cross-sections present diffi culties for rotary instru- ments and strategies such as cir- cumferential fi ling and ultrasonics should be used in those canals. Studies found that oscillating instru- ments recommended for these canal types did not perform as well,8 par- ticularly in curved canals. Specifi c instruments developed to address these challenges include the Self- Adjusting File (SAF) System (ReDent- NOVA, Raana, Israel), TRUShape® (Dentsply Sirona, Tulsa, Okla.) and XP Endo® (Brasseler, Savannah, Ga.). However, there is no direct clinical evidence that these instruments lead to better outcomes. Canal transportation with contem- porary NiTi rotaries, measured as un- desirable changes of the canal center seen in cross-sections of natural teeth, is usually very small. This in- dicates that canal walls are not exces- sively thinned and apical canal paths are only minimally straightened (Fig. 1), even when preparing curved root canals. While preparation usually removes dentin somewhat preferen- tially toward the outside of the cur- vature,9 current NiTi instruments, including reciprocating fi les, can enlarge the canal path safely while minimizing procedural errors. Almost all current rotaries are non- landed, meaning they have sharp cutting edges, and they can be used in lateral action toward a specifi c point on the perimeter. This “brushing” ac- tion allows the clinician to actively change canal paths away from the furcation in the coronal and middle thirds of the root canal10 but may cre- ate apical canal straightening when taken beyond the apical constric- tion. Circumferential engagement of canal walls by active instruments may lead to a threading-in effect, but contemporary rotaries are designed with variable pitch and helical angle to counteract this tendency. An important design element for all contemporary rotaries is a passive, non-cutting tip that guides the cut- ting planes to allow for more evenly distributed dentin removal. Rotaries with cutting, active tips such as dedi- cated retreatment fi les should be used with caution to avoid prepara- tion errors. NiTi instrument usage As a general rule, fl exible instru- ments are not very resistant to tor- sional load but are resistant to cyclic fatigue. Conversely, more rigid fi les can withstand more torque but are susceptible to fatigue. The greater the amount and the more peripheral the distribution of metal in the cross section, the stiffer the fi le.11 Therefore, a fi le with greater taper and larger di- ameter is more susceptible to fatigue failure moreover, a canal curvature that is more coronal is more vulner- able to fi le fracture. Instrument handling has been shown to be associated with fi le frac- ture. For example, a lower rotational Fig. 1: Root canal treatment of tooth #3 diagnosed with pulp necrosis and acute apical periodontitis. The mesiobuccal root has a signifi cant curve and two canals with separate apical foramina. Case courtesy of Dr Jeffrey Kawilarang. (Photos/Provided by American Association of Endodontists) Fig. 2: Behavior of controlled memory nickel-titanium rotaries compared with standard instruments. Shown are data from Typhoon Differential scanning calorimetry, which document the transition between austenite and martensite at about 5 degrees C for standard NiTi and at about 25 degrees C for controlled-memory (CM) alloy (A). At room temperature, this results in a drastically increased fatigue lifespan (B).Image A modifi ed and reprinted with permission from Shen et al .J Endod 2011; 37:1566-1571. (Photos/Provided by American Association of Endodontists) speed (~250 rpm) results in delayed build-up of fatigue12 and reduced incidence of taper lock.13 Material im- perfections such as microfractures and milling marks are believed to act as fracture initiation sites.14 Such sur- face imperfections after manufac- turing can be removed by electropo- lishing but it is unclear if this process extends fatigue life.15 the Manufacturers’ recommendations stress that rotaries should be ad- vanced with very light pressure; however, recommendations differ with regard to the way the instruments are moved. A typical recommendation is to move the in- strument into the canal gently in an in-and-out motion for three to four cycles, directed away from the fur- cation, then withdraw to clean the fl utes. It is diffi cult to determine exactly the apically exerted force in the clinical setting; experiments have suggested that forces start at about 1 Newton (N) and range up to 5 N.16 Precise torque limits have been discussed as a means to reduce failure. Most clinicians use torque-controlled mo- tors, which are based on presetting a maximum current for a DC electric motor. To reduce friction, manufacturers often recommend the use of gel- based lubricants in dentin; how- ÿPage 13