A short history of the NiTi file revolution
Nowhere in dentistry is technical progress as rapid as it is in modern endodontics. The development of flexible nickel-titanium (NiTi) files in the late 1980s created entirely new and hitherto unknown opportunities in the mechanical preparation of root canals. The following article gives a descriptive explanation of the decisive technical differences between a conventional file system and the latest generation of instruments. Employing a number of different scenarios, the article examines the opportunities available to ENDO specialists and beginners through the smart application of modular NiTi systems in different treatment situations.
Numerous innovations have made work significantly easier for endodontists over the past few years. There can be little doubt that the introduction of rotary files made of nickel-titanium alloys is a major milestone. But what exactly is the secret behind this versatile material? Nitinol alloys generally consist of approximately 55% nickel and up to 45% titanium. It is this combination that lends the material its pseudo-elastic deformation properties. The second outstanding feature is the option of giving the material a shape memory. Back in 2011, the Swiss dental specialist COLTENE had already developed a method for modifying the ‘DNA’ of NiTi, in which the files were given a true shape memory and thus allowed exceptionally precise working. In future, dentists were to be in a position where even strongly curved canals could be prepared safely and confidently without having to fear unexpected breaking of the instrument. The largely tension-free behaviour of this new file generation caused quite a sensation in the industry, as it had hitherto not been possible to simply bend NiTi files by hand. The widely varying use of physical terminology over the years has been confusing, so to give a clearer understanding of what NiTi systems with a ‘controlled memory’ effect can offer, it pays to take a more profound look at the physical and molecular relationships.
Two types of deformation
The use of files in the root canal unavoidably leads to deformation of the file material. If we take classical rubber, probably the best known elastic deformable material, the issue is clear: if one stretches a rubber elastic band (Fig. 1) it generally returns to its previous shape by itself as soon as force is no longer applied. During this process, the applied energy escapes and this can be clearly measured at a physical level. Using a thermometer, the reversal of the stretching process shows a slightly increased temperature of the material (Fig. 2a). This type of deformation is elastic and completely reversible at the same time. Examination of the molecular structure of the rubber shows there are no changes to the molecular bonds. A comparable example for the elastic deformation of metals is the thin metal spiral children's toy, which, after an initial shove, runs down stairs automatically and repeatedly several times without showing any visible signs of fatigue of the material (Fig. 2b).
The second type of deformation is plastic deformation. This type of plastic deformation is well known to anyone who has ever been involved in a car accident with damage to the car's bodywork. The damage is usually irreversible, even if the dents have been repaired and resprayed. The reason is the change in the molecular structure of the metal (Fig. 3) where the bonds change and the molecules diffuse (Fig. 4). In case of a repeat collision with such a previously damaged car, the damaged car door or passenger compartment will buckle much more readily as the new molecular structure has an extremely negative effect on the overall stability of the material. It goes without saying that respraying the traces of an accident adds only little mechanical resistance to the car; in dentistry, the differentiation between elastic, transient deformation and irreversible, plastic deformation is at least equally important. A plastically deformed NiTi file will break too easily due to material fatigue, and plastic deformation can usually not be detected under a magnifying glass or microscope, let alone with the naked eye, due to the high bounceback effect of conventional NiTi material. Minute and also invisible microfractures that occur during the metal cutting manufacturing process can increase the risk of unexpected breaking of the instrument. Put into plain words, this means that the pseudoelasticity of conventional NiTi files often masks existing plastic deformation: visibly, the damaged file does not differ from an unused file, but the consequences during preparation can be serious. Until now, the treating dentist had no opportunity of checking the actual condition of the used instruments by himself, even the use of disposable files offers no guarantee, although it does increase safety somewhat.
Shape memory increases safety
With the development of a new generation of NiTi files, COLTENE finally solved this quality control problem with a very simple trick. Meanwhile it has become 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 nothing else but ‘training’ the material to ‘memorise’ a certain shape under different conditions. After deforming the material by bending or similar means, a material with shape memory automatically returns to its original shape as soon as the external conditions are changed. Temperature or pressure deviations are examples of such changed parameters. Alternatively, one can induce a return to the original position through magnetism or via a simple chemical process.
Applied to endodontic instruments, the practical advantages of this principle are soon evident: a NiTi file with a ‘controlled memory’ effect adopts the anatomical shape of the root canal during the entire treatment procedure. In case of resistance or blocking in the canal, the files bypass this stress situation by the spirals changing their cross-section. After use, the file is subjected to new temperature conditions during autoclaving. The effect of heat automatically returns the instrument to its original shape. However, if the uniform spiral structure can no longer be reproduced, then the file is plastically deformed and should no longer be used. For the first time it is possible to visibly differentiate between elastic and plastic deformation with the naked eye, which considerably adds to application safety in the dental practice. Regardless of short set-up times or frequent changes in personnel, even ENDO beginners can quickly scrutinise and assess the situation without a doubt—a considerable advantage in often stressful daily treatment routines.
Like a Phoenix from the ashes
The reason why Nitinol can be trained so reliably lies in its inner structure. NiTi alloys display two crystallographic phases: the austenite phase at high temperatures, and the martensite phase at rather lower temperatures. In the martensite phase, Nitinol can be bent into complicated structures without effort. Without further external influences, the bent NiTi file with ‘controlled memory’ effect therefore remains in this position at room temperature. In the austenite phase, i.e. at higher temperatures, the material can adopt 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 even be demonstrated directly using a conventional lighter. When placed over the flame, the heated instrument visibly changes from its bent form back to the classical straight file shape in only a few seconds (Fig. 5).
Based on these insights, COLTENE presented its HyFlex CM file series for the first time in 2011. The abbreviation ‘CM’ stands for the above described ‘controlled memory’ effect. This special property results in an up to 300% higher fatigue resistance compared with conventional NiTi files, with an added highlight: the HyFlex CM excels through its extreme flexibility. Due to its special characteristics, the file adapts perfectly to prevailing canal anatomies and thus considerably reduces the risk of perforation. In addition, the instrument moves perfectly in the centre of the canal. This effectively prevents a shift in course or via falsa. The result is perfect cleaning and preparation of the root canal for subsequent obturation. Like a Phoenix from the ashes, the NiTi file is regenerated by autoclaving and ready for its next application until it reaches the end of its life cycle by clearly displaying an uneven, bent shape. The files are available pre-sterilised for dentists who prefer working with disposable instruments, especially as endo experts also benefit from the exceptional flexibility and fracture safety of HyFlex in single use.
Practical advantages of modular NiTi systems
Looking into the future, the trend is definitely moving towards modular NiTi systems. The advantage of such variable instrument sets is mainly given by their high degree of versatility. The considerable complexity of the human root canal anatomy always presents dentists with new challenges. Hidden isthmuses and side canals, but also horizontal lateral canals, can quickly turn patient cases into a journey into the unknown, especially as, depending on the type and position of the tooth, the X-ray findings do not always clearly identify all branches. Modular NiTi systems enable working with confidence in virtually every situation. Depending on the anatomical situation, the dentist can choose between fast instrumentation with only a few files or highly precise canal shaping with a skilful combination of different NiTi files.
High cutting performance through spark discharge
With the HyFlex EDM (COLTENE) the fifth generation of NiTi files has been recently presented at the IDS 2015. Spark erosion generates a unique, hardened surface, which improves cutting performance even further. Whereas classical NiTi files are traditionally milled on CNC machines, so-called Electrical Discharge Machining (EDM for short) processes the workpieces with the aid of electrical discharge. The repeated bombarding of the alloy with sparks melts the material or even leads to evaporation in some places. The result is a file with a distinctly textured surface where heat creates a new surface hardness, which – similar to a bread knife with serrated edge—displays particularly good cutting properties (Fig. 6).
The result of this innovative process is an unbreakable file, predestined for dentists who wish to realise reliable results quickly with a reduced file sequence. In particular, the HyFlex EDM facilitates working with rotary instruments for ENDO beginners: due to their enormous flexibility, the number of files used can be reduced significantly 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 as with the classical method. All that is required, is a slow speed handpiece which can be operated at up to 500rpm at a recommended torque of up to 2.5Ncm. 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 300rpm). As soon as resistance is felt, check patency using a 20/.02 hand file. Final preparation in the central and apical area is then performed using the HyFlex EDM 25/.~ One File (400–500rpm), 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 only proceed for 1–2mm without applying pressure to clean the file in between 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 in question, the sequence for preparation can be complemented with sizes from the HyFlex CM range (i.e. 15/04, 30/04). Of special interest is the option of also preparing large canals safely with HyFlex EDM in the apical area using the Finishing Files in the sizes 40/.04, 50/.03 or 60/.02, which are also extremely flexible. The novel system solutions available on the market have made the question of the ideal file sequence less of an issue. This uncomplicated handling has increasingly attracted dental practitioners to the supreme discipline of tooth preservation, and training of the entire practice team is relatively easy.
In the long term, modern instruments will increasingly support ENDO experts in their daily work. Sophisticated materials with so-far unexplored properties have the potential of defining completely new treatment standards in only a few years. We may soon be facing another paradigm shift to highly energetic or even entirely chemical-biological methods. A first taste of things to come is presently given by the 2-in-1 filling systems, where contact with fluids forms hydroxylapatite crystals, which also support regeneration in the root canal. This is yet a further milestone in materials research on the path to optimal endodontic therapy. The utilisation of natural, regenerative processes combined with the intelligent use of current technical options, will help both ENDO specialists and beginners to create sustainable solutions for a large number of indications.
Editorial note: This article was published in roots – international magazine of endodontology No. 01/2015.