Genius Proflex Preflaring Pack (4 Pack)

Customized Heat Treatment

The importance of the heat-treatment 

by: Carlos A. Spironelli Ramos 

Root canal instrumentation is accomplished by the use of endodontic instruments and irrigating solutions under aseptic working conditions. Root canal instrumentation may be carried out using hand-held or engine-driven instruments. Up until the last decade of the past century, endodontic instruments were manufactured out of stainless-steel. Stainless-steel files have an inherent stiffness that increases as the instrument size increases. As a result, when preparing a curved root canal, restoring forces attempt to return the instrument to its original shape, especially when the operator uses a filing motion. Therefore, in curved canals, steel instruments must be pre-curved for use, which effectively prevents them from being used in a rotary motion. An instrument that is too stiff will cut more on the convex (outer) side than on the concave (inner) side, thereby straightening the curve. 

Engine-driven files manufactured out of a nickel–titanium (NiTi) alloy have proven to be a valuable adjunct for root canal therapy. NiTi instruments are highly flexible and elastic. Advantageously, NiTi rotary shaping files have nearly eliminated the iatrogenic instrumentation complications that are often connected to endodontic steel instruments. NiTi instruments were introduced over two decades ago. Since their first appearance, instrument design has changed considerably; progress has been made in manufacturing as well as alloy heat-treatment. Traditionally files had been produced according to empiric designs, and even today many instruments are still devised by individual clinicians rather than developed through an evidence-based approach. Clinical procedures and ideal working parameters are still being refined as new instruments continue to be introduced to the market. With new versions rapidly becoming available, the clinician may find it difficult to choose the file and technique most suitable for an individual case or even during one case instrumentation protocol. NiTi instruments have undergone a revolution regarding different designs to produce an instrument that cuts effectively while exhibiting resistance to fracture even in the most challenging anatomical confines. One should always bear in mind that all file systems have benefits and weaknesses. Instrument properties are derived from the type of alloy, degree of taper, cross-sectional design and heat-treatment protocol.  

Nickel–titanium (NiTi) alloy has been used in endodontics for almost 30 years and has brought a major breakthrough to root canal therapy. NiTi endodontic instruments with super-elasticity have gained extensive popularity amongst clinicians due to their higher flexibility and greater torsional resistance than the traditional stainless-steel ones. Therefore, an increasing number of super-elastic NiTi endodontic files with various geometry designs (cross-sectional shape with or without “radial lands” or sharp cutting edges, constant or variable pitch, and progressive or constant taper) have been developed. However, the undesirable and unexpected separation of NiTi endodontic rotary files during root canal instrumentation caused by flexural (cyclic) fatigue and/or torsional overload still remains a serious concern and drawback in clinical use. Torsion overload is one of the primary mechanisms responsible for the intracanal separation of NiTi endodontic instruments, which accounts for 55.7% of the failures of NiTi rotary files. It can be generated within the rotary file when the tip or some part of the instrument is locked against the canal wall while the shank of the file (driven by the handpiece) continues to rotate or is subjected to excessive pressure by the operator. The tip fractures when the handpiece torque exceeds the ultimate strength of the metal. High torsional stiffness is desirable for the clinical performance of small sizes of rotary files owing to the enhanced cutting efficiency and reduced torsional failure risk. Cyclic fatigue occurs when a metal is subjected to repeated cycles of tension and compression that cause its structure to break down, ultimately leading to fracture. It is the main reason for the majority of broken instruments. The fracture caused by cyclic fatigue of NiTi endodontic instruments is difficult to detect during clinical practice due to the invisible signs of permanent deformation during cyclic fatigue. That is why many attempts have been made to improve the resistance to cyclic fatigue for NiTi files, predominantly novel heat-treatment protocols. 

In recent years, novel kinds of NiTi endodontic files fabricated by proprietary heat-treatment processes are being propose to mitigate file breakage during its clinical use. In this case, normally the heat-treatment gives more flexibility to the file, increasing cyclic fatigue resistance. This particular feature is adequate for larger sizes of the files since they have more metal mass. If the same method is accomplished in small files sizes, a side effect can occur leading the file to unwinding when some pressure is undergoing. 

NiTi endodontic files used in clinical practice are subjected to both flexural fatigue and torsional load simultaneously during root canal preparation procedures, probably leading to instrument separation due to hybrid forces. Bending is imposed by the root canal anatomy, in the case of blockage, it is also associated with torsion. Clinically, both cyclic fatigue and torsional failure probably occur simultaneously. The mechanical properties of NiTi endodontic instruments including flexibility, torsional resistance, and flexural fatigue are fundamental requirements of endodontic instruments for successful use. The flexibility is beneficial for maintaining the original shape of root canals, especially for the ones with severe curvatures. Adequate torsional resistance and flexural fatigue resistance favor reducing the occurrence of file breakage. Thus, flexibility and resistance to fracture constitute properties expected for an ideal root canal file.  

From a material point of view, the properties of NiTi alloys depend on their chemical composition, phase constitution, and fabrication procedures, among which the metallurgical properties including the chemical composition and phase constitution are the internal factors and the fabrication procedures such as cold working, heat-treatment before and/or after grinding the blank are the factors that can determine mechanical properties for NiTi endodontic files. Controlling mechanical properties and their association with the metallurgical properties of NiTi rotary instruments is helpful for clinicians to make decisions regarding which instruments are appropriate for each phase of the instrumentation procedures of root canal therapy. The influence of metallurgical properties on endodontic files mechanical properties is possible to be adjusted to each file size by controlling the heat-treatment for each specific file on the instrumentation sequence. In this sense, tailoring the heat-treatment for each file size can keep flexibility to larges files while maintain torsional resistance to small sizes files. 

NiTi alloy can exist in two different temperature-dependent crystal structures called martensite (lower temperature or daughter phase) and austenite (higher temperature or parent phase). The crystal lattice structure can be altered by either temperature or stress. This is important because several properties of the two forms are notably different. Near equiatomic NiTi alloys contain three microstructural phases (i.e., austenite, martensite, and R-phase), the character and relative proportions of which determine the mechanical properties of the metal. When the material is in its martensite form, it is soft and ductile and can easily be deformed, while austenitic NiTi is quite strong and hard. The conventional super-elastic NiTi file has an austenite structure at room and body temperatures. It is well known that the nature of the alloy and the manufacturing process greatly affect the mechanical behavior of the instrument. To improve the fracture resistance of NiTi files, new alloys were introduced to manufacture NiTi files or developed new manufacturing processes. A series of proprietary thermomechanical processing procedures has been developed with the objective of producing super-elastic NiTi wire blanks that contain the substantially stable martensite phase under clinical conditions. Enhancements in these areas of material management have led to the development of the new generation of endodontic NiTi instruments. NiTi files with heat-treated process contain a mixture of austenite and martensite conditions at body temperature. The martensitic phase of NiTi has some unique properties that have made it an ideal material for many applications. The martensitic phase transformation has excellent damping characteristics because of the energy absorption characteristics of its twinned phase structure. In addition, the martensitic form of NiTi has an excellent fatigue resistance. 

Therefore, endodontic files with high expression of austenitic phase will have more torsional fatigue resistance, while files with high expression of martensitic phase will have a better cyclic (flexure) fatigue resistance. If the same heat-treatment is performed for all sizes, some of them will benefit of the accomplished metallurgical changes while others will be harmed. One method doesn’t equally benefit all sizes needs and it is imperative that customize heat-treatment protocols be applied so that all files’ sizes are adequately improved in its mechanical behavior.