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Research that matters: studies on fatigue of rotary and reciprocating NiTi root canal instruments

2019; Wiley; Volume: 52; Issue: 10 Linguagem: Inglês

10.1111/iej.13194

1365-2591

Michael Hülsmann,

Occupational health in dentistry

From time to time regular readers of scientific journals will ask him/herself, which of the papers will remain in their consciousness for at least a few days or, even more importantly, have an impact on their clinical practice. Often the answer is rather depressing as many of the studies prove to be of little value from a clinician`s perspective or even when judged using rigorous scientific criteria. Are these papers just printed paper? The endodontic literature is replete with studies on the cyclic and fatigue behavior of all kinds and sizes of existing rotary nickel-titanium instruments and with each new instrument that arrives the number of such studies seems to increase in an exponential manner. From the standpoint of an Associate Editor of the IEJ with a limited number of pages per year and a limited number of reviewers the main questions to be asked are: Do we really need such studies to improve the quality of root canal treatment? And do we have to publish the detailed data in the International Endodontic Journal?' These were the main questions being discussed in a previous Editorial in the International Endodontic Journal with a focus on 'Root canal preparation, retreatment and working length studies' (Hülsmann 2013). Five years later from an Editor's perspective, it seems timely to address these problems again. Fracture of endodontic instruments is not an uncommon occurrence during root canal preparation and has been reported for all types of instruments: stainless steel as well as nickel-titanium and hand instruments, as well as rotary or reciprocating instruments (Parashos et al. 2004, Zinelis 2018). It is not the fracture itself, but the sequelae of the incident that may compromise the results of treatment and the prognosis of the tooth, as the region of the canal apical to a fragment will likely remain insufficiently disinfected and not properly filled (Parashos & Messer 2006, McGuigang et al. 2013). So, once more, a technical problem will result in biological problems. Nickel-titanium rotary root canal instruments have been regarded as especially prone to fracture due to their metallic properties. Indeed, there have been early studies suggesting a significantly higher fracture incidence than for harder, less flexible and more rigid stainless steel instruments (Hülsmann et al. 2005). The introduction of speed- and torque-controlled motor systems and also the increasing experience in the use of such instruments seem to have drastically reduced the incidence of instrument fractures, resulting in a frequency comparable to that of stainless steel instruments (Spili et al. 2005, Lambrianidis 2017). Additionally, the development of improved nickel-titanium alloys and modified instrument designs have contributed to a substantial degree to this improved safety of use. Nevertheless, there remains a plethora of published studies and submitted manuscripts on the fracture susceptibility of new alloys and new instruments. This ever-increasing number of papers on testing of nickel-titanium systems and new alloys could fill several issues of the International Endodontic Journal each year. The most prominent and frequent are studies on fatigue: resistance to torque is one study model, investigating the risk of fracture when the instrument tip is firmly engaged and the motor continues to rotate or reciprocate, respectively. The second type of studies/publications is on cyclic fatigue, counting the number of rotations or working time till fracture. For both types of studies, more or less standardized study designs have been described with only minor details varying between the studies, thereby at first glance allowing good comparability of the results and the investigated instruments. It should be noted that such studies are easy to perform once the design is established. Cyclic fatigue can be investigated in a static or in a dynamic study design. Unfortunately, many studies still use a static approach, which clearly does not resemble the clinical reality. In the static model, compression and torsion of the file over time are located at the same point or region of the instrument. In a dynamic model with the dental handpiece performing an additional up and down movement with a defined and standardized amplitude, the position of the greatest stress is distributed along a larger part of the instrument, logically increasing the number of rotations until fracture. Nevertheless, the clinical relevance of these studies is controversial. Pure rotation inside a root canal without any torque on the instrument is unlikely to happen. So, it may be questioned whether it is of major importance to know the number of rotations for each file of each system. In a previous editorial, it was suggested that it should be the responsibility of the manufacturers to provide relevant data (Hülsmann 2013). Secondly, as demonstrated in a review paper in this issue of the IEJ (Hülsmann et al. 2019), the results from studies on cyclic fatigue – static or dynamic in nature – vary considerably. There are differences up to more than 100% between dynamic and static tests, and even the results between static tests differ significantly. The same is true for dynamic tests with the number of dynamic studies being clearly lower. Nevertheless, when looking at the details of the published studies a large variance in the study protocols becomes evident, making comparisons of results very difficult and problematic and to some degree even impossible (Plotino et al. 2009). Recent studies have demonstrated that several factors such as environmental temperature (Capar et al. 2014, Dosanjhi et al. 2017, Grande et al. 2017, Shen et al. 2018) and trajectory of the instruments when introduced into canals (Plotino et al. 2010) influence the outcome substantially. As these data are not reported in the vast majority of fatigue studies, the results from these studies cannot be regarded as valid or comparable. When considering these facts, the scientific value of such studies is debatable and at the same time they confirm that the results are useless for the general dental practitioner as well as for the endodontic specialist when they are looking for information on a specific instrument system. In short, further publication of fatigue studies at the present time is not justified.