July 2, 2019
by Barbara Müller, BSc, MSc, PhD
Nowhere in dentistry is technical progress as rapid as in modern endodontics. The development of flexible nickel-titanium files in the late 1980s created entirely new and hitherto
unknown opportunities in the mechanical preparation of root canals.
Numerous recent innovations have made endodontics significantly easier. The introduction of nickel-titanium alloy rotary files was a major milestone. The secret behind this versatile material is Nitinol; an alloy that consists of approximately 55 per cent nickel and up to 45 per cent titanium. It is this specific combination that provides the material its pseudo-elastic deformation properties. Another outstanding feature is the option of giving the material a shape memory. In 2011 a method for modifying the “DNA” of NiTi such that the files were given a true shape-memory, allowing exceptionally precise working was developed (Coltène/Whaledent GmbH + Co. KG, Langenau, Germany). As a result, dentists could safely and confidently prepare very curved canals without the fear of unexpectedly breaking the instruments. The largely tension-free behaviour of this new generation file was well received as previously it was almost impossible to manually bend NiTi files. The varying use of descriptive terminology has been confusing; thus, it is necessary to take a more detailed look at physical and molecular relationships to understand what NiTi systems with a “controlled memory” effect can offer.
Two Types of Deformation
The use of files in the root canal unavoidably leads to deformation of the file. Rubber is the best known elastic deformable material: a stretched rubber elastic band (Fig. 1) generally returns to its original shape as soon as the stretching force is no longer applied. During this process, the applied energy escapes and can be clearly measured; a thermometer shows a slightly increased temperature of the material, during the reversal of the stretching process (Fig. 2). This is elastic deformation and is completely reversible. The molecular structure of the rubber shows no changes to the molecular bonds. A comparable metal elastic deformation is the thin spiral metal toy which, after an initial push, runs down stairs one step at a time without any visible signs of material fatigue.
Plastic deformation is obvious in a car accident that damages the car’s bodywork. The damage is usually irreversible, even if the dents are repaired and painted. Inhere, the structure of the metal is altered (Fig. 3), the bonds change and the molecules diffuse (Fig. 4). In case of a repeat collision with a previously damaged car, the damaged car door or passenger compartment will buckle much more readily, as the new molecular structure significantly reduces the overall stability of the material. Re-painting the traces of an accident adds little mechanical resistance to the car.
In dentistry, the differentiation between elastic, transient deformation and irreversible, plastic deformation is equally important. A plastically deformed NiTi file will break easily due to material fatigue, and plastic deformation can usually not be detected (even under magnification let alone with the naked eye), due to the high bounce back effect of conventional NiTi material. Minute and invisible microfractures which occur during the metal cutting manufacturing process can increase the risk of unexpected instrument failure. Unfortunately, the pseudo elasticity of conventional NiTi files often masks existing plastic deformation: visibly, the damaged file does not differ from an unused file, but the consequences during endodontic preparation can be serious. Until now, the dentist had no prospect of verifying the clinical condition of the used instrument. Even disposable files offer no guarantees, althoughthey do increase safety somewhat.
Shape Memory Increases Safety
The development of a new generation of NiTi files finally solved this problem. It is possible to differentiate between the elastic and plastic deformation of nickel-titanium alloys. To achieve this, the material used must possess a true shape memory. Ultimately, shape memory is no more than “training” the material to “memorise” a certain shape under different conditions. After deforming the material (by bending or other forces), its shape memory automatically returns it to its original shape as soon as the external conditions change. Temperature or pressure changes are examples of such changed parameters. Alternatively, magnetism or simple chemical processes can induce a return to the original shape.
For endodontic instruments, the practical advantages of this principle are evident: a NiTi file with “controlled memory” adapts to the anatomical shape of the root canal during the entire treatment procedure. In case of resistance or a block in the canal, the file bypasses this stress situation by changing their cross-sectional shape. After use, the file is subjected to a thermal change during autoclaving; the heat returns the instrument to its original shape.
If the uniform spiral structure of the file can no longer be reattained, the file is plastically deformed and should no longer be used. For the first time it is possible to visually differentiate between elastic and plastic deformation in the file with the naked eye, considerably increasing clinical safety, even when faced with short set-up times or inexperienced personnel.
Like a Phoenix from the Ashes
The reason why Nitinol can be trained so reliably lies in its inner structure. Nickel-titanium alloys display two crystallographic phases: the austenite phase at high temperatures and the martensite phase at lower temperatures. In the martensite phase, Nitinol can be bent into complicated shapes without effort. Without further external influences, the bent NiTi file with “controlled memory” remains in this position at room temperature. In the austenite phase, (higher temperatures), the material adopts its original structure and the molecules form a cubic face-centred lattice structure. Heat induces the phase transformation and the file returns to its original condition during sterilisation. This controlled bounce back effect can be demonstrated with a conventional lighter. When placed over the flame, the bent instrument changes to its original straight file shape within a few seconds (Fig. 5).
Based on this technology, COLTENE introduced its HyFlex CM file series in 2011. (“CM” is short for “controlled memory”). CM offers up to 300% higher fatigue resistance compared with conventional NiTi files. HyFlex CM adapts perfectly to prevailing canal anatomies, considerably reducing the risk of perforation. The instrument moves perfectly in the centre of the canal, preventing a shift in course (via falsa). The net result is optimized cleaning and preparation of the root canal for obturation. Like a Phoenix rising from the ashes, the NiTi file is regenerated by autoclaving, and ready for its next use (until the end of its life cycle where an uneven, bent shape can be clearly seen). The files are available pre-sterilised for dentists who prefer working with disposable instruments.
Practical Advantages of Modular NiTi Systems
The future is trending towards modular NiTi systems. The advantage of such variable instrument sets is their high degree of versatility. The considerable complexity of the human root canal anatomy always presents challenges: hidden isthmuses, side canals, and horizontal lateral canals can quickly turn cases into difficult journeys. Depending on the type and position of the tooth, radiographic findings do not always clearly identify all cannals. Modular NiTi systems enable variable approaches depending on the anatomical situation; fast instrumentation with only a few files or highly precise canal shaping with an array of NiTi files.
High Cutting Performance through Spark Discharge
In 2015 COLTENE introduced HyFlex EDM, a 5th generation NiTi file. Spark erosion generates a hardened surface which improves cutting performance. Classical NiTi files are milled on CNC machines. Electrical Discharge Machining (EDM) uses spark erosion, which hardens the surface of the NiTi file, resulting in superior fracture resistance and improved cutting efficiency. Repeated bombardment of the alloy with sparks melts the material and even leads to evaporation in some places. The result is a file with a distinctly textured surface where heat creates a new surface hardness (Fig. 6) offering good cutting properties (similar to a serrated edge knife).
The end result is an almost unbreakable file that achieves rapid results with reduced instrumentation. The HyFlex EDM facilitates rotary instrumentation and reduces the number of files without making compromises in adapting to the natural root canal anatomy.
In simple cases, two files are sufficient to clean and efficiently prepare the root canal. All that is required, is a slow speed handpiece which can be operated at 500 rpm or lower at a recommended torque of up to 2.5 Ncm. To prepare a mechanical glide path, the dentist uses a 10/.05 Glidepath file, which is introduced up to the full working length with dabbing up and down movements (at 300 rpm). As soon as resistance is felt, patency is verified with a 15/.02 hand file. Final preparation in the central and apical area is then performed using the HyFlex EDM 25/.~ OneFile (400-500 rpm), also with gentle dabbing without applying pressure and progressing to the full working length (Fig. 7). Due to the high cutting efficiency it is important to proceed for only 1-2 mm without applying pressure, to clean the file in between insertions, and to rinse the canal thoroughly.
Optionally, the root canal can be extended coronally in advance using the 25/.12 Orifice Opener. In more complex cases, and depending on the clinical situation and the extent of curvature of the canal, the sequence for preparation can be complemented with the newly introduced HyFlex EDM 15/.03 Glidepath File and 20/.05 Preparation File. Of special interest is the option of also preparing large canals safely with HyFlex EDM in the apical area using the extremely flexible Finishing Files sizes 40/.04, 50/.03 or 60/.02. This straightforward procedure has attracted dental practitioners to the important goal of tooth preservation, and made team training relatively easy.
Improved instrumentation facilitates endodontic treatment. New obturation materials may also generate new treatment options within the next few years. Novel paradigm shifts with innovative preparation or entirely chemical approaches may be just around the corner. The utilisation of natural, regenerative processes combined with the rational implementation of current technical options, assist both general practitioners and endodontic specialists to offer predictable solutions for significantly damaged teeth.
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About the Author
Dr Barbara Müller studied Agricultural Biology at the University Hohenheim and received her Master of Science at the University of Georgia, USA. She completed her PhD at the University of Ulm. Dr Müller was the Manager of research and Development at Coltène/Whaledent GmbH and was responsible for the development of numerous products. She is currently Head of the Endodontics Product Segment and lectures frequently at endodontic meetings