Rotary Ni-Ti Instrumentation Part 2-1 PDF

Rotary instrumentation Since the revelatory development of nickel titanium instruments, the strategies of root canal preparation have been changed. The constant search for automation of root canal shaping instruments was the main aim of the developers, seeking to reduce patient and clinician...

Rotary instrumentation Since the revelatory development of nickel titanium instruments, the strategies of root canal preparation have been changed. The constant search for automation of root canal shaping instruments was the main aim of the developers, seeking to reduce patient and clinician stress, as well as to preserve the essential characteristics of the canal shape. Before NiTi instruments manufacturing, many attempts in toward automation of canal instrumentation have failed, because the use of stainless-steel instruments with powered rotary device often results in loss of instrument control within the root canal with exaggeration of root end aberrations. However, with the rotary nickel-titanium files, a new phase for automation of root canal shaping has begun and thanks to its characteristics of the metallic alloy and its super elasticity. These instruments assure a quick shaping of the root canal system, with better maintained in their original anatomy than stainless steel hand techniques. Instrument fracture and separation: Engine-driven NiTi files are efficient and convenient for root canal cleaning and shaping. However, safety is a major issue that must be considered in clinical practice since separation of NiTi instruments often occurs accidentally. NiTi instruments possess a risk of separation or fracture during use because of torsional or cyclic fatigue. Distortion of rotary NiTi files is often not visible without magnification. Consequently, these instruments can fracture without any warning signs as opposed to stainless steel instruments where distortion can be perceived easily. Cyclic fatigue occurs due to the instrument rotating in a curved canal and generating repetitive tension/compression cycles that eventually lead to metal Rotary Ni-Ti Systems fatigue, whereas torsional fatigue is a result of different parts of the file rotating at different speeds. Clinically, torsional fatigue is the result of the instrument tip binding to the canal dentin, whereas the shank continues to rotate, creating a torque that results in torsional fracture. Torsional fatigue resistance is basically how much a file can twist before fracturing. This property is desirable during use in a narrow and constricted canal where torsional load would be high during use. Manufacturers are constantly trying to develop changes in NiTi rotary systems to improve the mechanical properties of these instruments, especially the fatigue resistance. One of the changes incorporated was heat treatment of these alloys, which produced instruments with superior mechanical properties. Other methods used produced instruments with different cross-sectional designs or incorporated new manufacturing processes. General design features of rotary instruments: To date, more than 160 rotary NiTi systems have been used in clinical practice. Unlike stainless steel instruments that follow a unified international standard, no unified standard has been established for the design of rotary NiTi systems, and tip diameters, tapers, and blade lengths. Alloy type: Mainly the NiTi rotary file systems are present in two types of alloys: 1. Conventional super-elastic alloy: All the NiTi rotary instruments manufactured approximately since the late 80s to 2010 is manufactured from super elastic alloy which is flexible and can bend under stress and regain the original straight shape in room temperature. Page 2 of 9 Rotary Ni-Ti Systems The bad side of this type of alloy is that the signs of permanent deformation are not evident because of its superelasticity. In addition, instrument safety and fracture resistance are major concerns. 2. Heat treated NiTi alloy: With the continuous development of metallurgical technology, designers and manufacturers have improved the physical properties of metals, increasing fatigue resistance by changing the alloy chemical and crystal phase composition. Thermal treatment, the most frequently used metal processing method, is applied to heat and cool a certain material under specific conditions several times. The most processing developments aim to provide an alloy with martensite alloy or R-phase alloy in the room or body temperature rather than austenite alloy. Some examples of heat-treated alloys are M-wire, gold alloy, blue alloy, controlled memory wire, max wire and R-phase wire. The controlled memory wire is one of the novel innovations. Compared with the nickel content of 54.5%–57% of conventional NiTi alloy, CM NiTi wire contain less nickel (52%) which improves the mechanical properties of the alloy and with post-manufacturing heat treatments on alloy, further increases the austenite phase transition temperature of the CM-wire to 55 °C such that the crystal structure at room temperature is dominated by martensite. In conclusion, CM wires have become metal alloys with controlled memory, improved fracture resistance, good flexibility, and resistance to cyclic fatigue. The CM-wire file can be prebend to conform to the curvature of the root canal. If the instrument undergoes deformation (such as uncoiling) after use, it can return to initial Page 3 of 9 Rotary Ni-Ti Systems shape once heated such as autoclaved and can be reused, i.e., the so-called controlled shape memory effect. Taper The taper is the ratio of the diameter to the length of an instrument, which refers to the increase in diameter of the file per mm increase in length. Files with large and medium tapers are used for rapid cutting of dentin, and few changing times of files during instrumentation. Files with a small taper are conducive to the shaping of a good apical morphology and maintenance of the original axial direction of the root canal. Rotary NiTi files are usually designed with a large taper, and a section from the root canal orifice to the apical stop is prepared into a uniformity tapered funnel shape using the crown-down preparation technique. 1. Constant taper instruments: Most instruments are designed with a constant taper, i.e., the taper does not change on the same instrument along the length of the file blade. Many systems adopt this type of taper design provide 2, 4, 6, 8, 10 and 12% tapered instruments. 2. Multiple taper instruments: These instruments are characterized by different tapered ration along the length of cutting blade. It could be subdivided into: - increasing taper multi-taper files: the taper increase along the distance from file tip to the shaft. Usually, the taper of these types of files starts with small taper and increases gradually towards the file shaft. Example of these instruments is the shaping files in ProTaper system. - Decreasing taper muti-taper files: the taper decrease along the distance from file tip to the shaft. Usually, the taper of these types of files starts with large taper and decreases gradually towards the file Page 4 of 9 Rotary Ni-Ti Systems shaft. Example of these instruments is the finishing files in ProTaper system. - Variable multi-taper files: in these instruments the taper usually increase initially at the apical side of the file and then decrease at more coronal levels to prevent production of large cross sectional surface area. Example of these instruments is HyFlex EDM system. Cross sectional design: Finite element (FE) analysis shows that the mechanical behavior of an instrument during the root canal shaping process is affected by the cross-sectional shape, that is why manufacturers focus on improvement and optimization of instrument cross sections. For the same shape, an instrument with a larger cross- sectional area has higher torsional stiffness in comparison with instruments with small cross-sectional area which is also more flexible. There are two main cross-sectional designs, symmetrical cross section instruments at which the instruments center of rotation is at same place of center of rotation of rotating hand piece. And asymmetrical cross sectional design instruments at which the instrument center of rotation is offset from the center of rotation of the handpiece. This is made by making a relief from one side of the instrument along the blade length and gives the instrument the ability to rotate eccentrically or asymmetrically, resulting in off-centered movement. Because of the unique design of these instruments, many benefits could be gained: 1. Reduced cross sectional area aids to increase flexibility and reduced transportation. 2. Reduced file contact with canal walls during rotation decrease screw in effect. Page 5 of 9 Rotary Ni-Ti Systems 3. Enhanced cleaning space. 4. Enhanced instrument fracture resistance. Development and evolution of rotary instruments: From the 1990s until now, NiTi rotary instruments have undergone revolutionary changes in terms of the construction and as a result physical characteristic of the NiTi instrument. Furthermore, the design, shape, and number of instruments used in each group have highly been modified. The aim of modifying and improving these instruments is to develop a NiTi rotary instrument which cuts and removes the dentin strongly and also resistant to fracture even in the most challenging narrow, curved root canals. Another purpose of modifying and improving these instruments is to simplify the cleaning and shaping stage and to reduce the number of using instruments along with preserving the original shape of the prepared root canals. First Generation: This category of NiTi rotary instruments was first introduced to the market. The most important characteristic of the first-generation NiTi rotary files is having three equally spaced U-shaped grooves ending with passive cutting radial lands along with constant 0.04 - 0.06 tapers over the full working lengths. Radial lands are the peripheral portion of a rotary instrument that is flat and smooth. It is described as a flawed area that prevents the file from locking within the dentin. radial lands allow cutting to occur through a passive planning action (scraping the surface), and its design is to center the instrument in the central space. The most important NiTi rotary instruments within this category are Page 6 of 9 Rotary Ni-Ti Systems LightSpeed (similar to Gates-Glidden but NiTi and smaller in tips sizes), Profile and profile GT rotary systems. Several research showed that all first-generation rotary instruments created centered root canal preparation and caused low procedural errors. The main deficiency of this generation of NiTi rotary instruments was requiring numerous files to achieve tapered canal shape (with lightspeed system as produce cylindrical canal preparation because of it end cutting tip design), complexity of use, and increased contact with the canal walls which increase chance for instrument separation or fracture. Second Generation The second generation of NiTi rotary files have active cutting edges with greater cutting efficiency rather than presence of radial lands, so the number of instruments required to achieve complete cleaning and shaping was almost less in comparison with the previous generation. Notable systems in this generation are ProTaper Universal, Hero Shaper system and Race rotary files. The main disadvantage of this generation is the screw in effect. Because of the active cutting edge of the file engage into the dentin, it tends to form a track that allow the “leading flute” to attract the file mechanically deeper in the canal by the power of rotation available by the endodontic motor. The screw in effect increases as the contact of file with canal wall increases at deeper levels of the canal. Many instrument design features had been developed to overcome this issue such as: 1. Varying the pitch length of the file so the track formed by the leading flute is erased by the following one, reducing the screw in effect (adapted pitch concept applied in HeroShaper instruments). Page 7 of 9 Rotary Ni-Ti Systems 2. Alternating the cutting edge by placing non twisted parts along with other twisted parts through the length od the blade (RaCe rotary system). Third Generation: It was in late 2007 that the manufacturers started to apply special heat treatment technologies on NiTi alloys to improve the safety (enhance fracture resistance) of these instruments, especially in the curved root canals (please revise former section “Alloy Type”). In making third generation of the NiTi rotary files, the manufacturers have highly focused on metallurgic properties of the NiTi alloy using special heat treatment on wires which results in reduction of the cyclic fatigue of the files and also reduction of the separation risk of the instruments which is highly demanded by the practitioners. Applying M-wire and R-phase technologies and electrical discharge methods make instruments with high memory shapes and low risk of separation. Twisted files (r-phase files), Profile GTX Series (M-wire), HyFlex CM Files (controlled memory), and Vortex Blue (blue alloy [hard titanium oxide surface layer]) are notable files in this group which have been exposed to heat treatments to increase flexibility and safety. Fourth Generation In 2008, Yared proposed the concept of reciprocating motion of engine- driven NiTi system instead of full rotation based on a balanced-force technique, as the root canal is prepared by different angles in the clockwise (CW) and counterclockwise (CCW) rotations. The use of a single file technique to achieve a thorough cleaning and shaping goals at this phase was another success which was also derived from the reciprocating philosophy in cleaning and shaping the root Page 8 of 9 Rotary Ni-Ti Systems canal systems. Many studies have shown that the Wave One and the One Shape single-file systems can efficiently reduce the bacterial number in the root canal along with preserving the original shape of it. WaveOne and Reciproc systems are featured instruments of fourth generation. Fifth Generation In this generation, the efficiency of canal shaping has been improved by offsetting the center of rotation (the center of rotation is not at the same location as the center of the file during rotation). The offset designed files produce a mechanical wave of motion that distributes along the full length of the NiTi file which improves cutting and removing the debris in comparison with a centered mass rotating instrument. Furthermore, this offset design reduces the taper lock or the screwing effect which causes instrument separation. HyFlex EDM, Revo-S, One Shape, and ProTaper Next are important files of the fifth generation. It is noteworthy to say that there are some instrument that can belong to different instrument generations, as the previous classification is based over different design, development process and motion criteria. As an example, Reciproc blue CM system is a heat treated reciprocating single file system without radial land, so it can be considered as 2nd, 3rd, and 4th generation instrument. Page 9 of 9