Minimally invasive access cavities: does size really matter?

Over the last few years, minimally invasive treatments have been embraced in all fields of medicine as a consequence of advances in microsystem engineering, nanotechnology, laser therapy, and high-resolution imaging tools for diagnostics and guidance of surgical instruments. Dentistry as a matter of course is following this trend, for example, by focusing on new ways to treat dental caries by preserving as much sound tooth structure as possible. In the context of Endodontics, an influential proposal was made by Clark & Khademi (2010) who advocated maximum dentine preservation during access cavity preparation and root canal shaping based on the following core arguments: (i) the logical reasoning that claims that the overall loss of dentine mass would inevitably reduce the ability of the tooth to resist intermittent masticatory forces in the long term and (ii) no manmade material can properly replace the lost dentinal tissue (Clark & Khademi 2010). It is important to point out that, as happened in other medical fields, this proposal was based on intense technical training and expertise associated with the use of certain technologies such as cone-beam computed tomography, magnification, high illumination, ultrasonic tips, irrigation devices and heat-treated NiTi instruments.

From this starting point, the debatable concept of ‘the more dentine is preserved, the better the treatment outcome’ has been followed religiously by some proponents and influencers in the field of Endodontology, cumulating in extremely conservative approaches popularly known as ‘ninja’ and ‘truss’ access. Apart from nomenclature issues, which include more than 20 terms to designate similar cavity designs thus creating obstacles for scholarly communication (Silva et al. 2020), it is always timely to be reminded of the fact that Endodontics is or at least should be a science-based specialty, which cannot rely on pure deductive reasoning. There are three classical reasons why the clinical decision-making process should not be guided by extrapolation or ‘common sense’: the hindsight bias, the judgmental overconfidence and our habitual tendency to perceive unreal patterns in random events. On the other hand, sound scientific evidence is able to correct our flawed understanding and provide us with an unfiltered view of reality. That said, even after 10 years of the first proposal to apply minimally invasive concepts in Endodontics, two critical questions still remain: (i) how strong is the evidence supporting the preparation of minimally invasive access cavities? and (ii) are minimally invasive access cavities able to improve the long-term health and survival of teeth?

These are not easy-to-answer questions, and conclusive answers are not available at this time. However, most laboratory studies do not support the claim that minimally invasive access preparations preserve the fracture resistance of root filled teeth better than a conventional access cavity. As a matter of fact, the negative effects of minimal access cavities related to canal orifice location, and the quality of chemomechanical canal preparation and filling procedures appear to overshadow the perceived benefits (Saygili et al.2018, Rover et al.2020, Silva et al.2020, Vieira et al.2020). Moreover, iatrogenic deviations, instrument fractures and tooth discoloration have also been reported as potential consequences of extremely minimal access  preparations  (Alovisi et al. 2018, Marchesan et al.2018, Silva et al. 2020). Unfortunately, there are no clinical studies available that have followed the healing rate of teeth accessed with a minimally invasive approach, and therefore, no evidence is available to support the claim that conservative access cavities would improve the long-term survival of root filled teeth.

Despite this lack of clinical evidence, supporters of minimally invasive access cavities base their argument on the economical reasoning for saving dentine, whilst paying less attention to the possible insufficient disinfection and debridement as a result of this approach in infected root canals. This behaviour can be explained by ‘common-sense’ observation bias. Disinfection cannot be directly measured or observed from the chairside. Rather, it is inferred when specific technical parameters are achieved during canal preparation and filling, such as the ‘white lines’ on radiographs, which may give a false 2D impression of proper disinfection. It would appear that clinicians are more prone to associate a higher risk of failure with short root fillings rather than those canals prepared to a minimal size with consequent narrow or even poorly condensed filling materials, which are commonly seen in radiographs of root filled treated teeth with minimally invasive access cavities. Therefore, even though reducing the fracture rate of root filled teeth is highly desirable, the unproven rationale of ‘saving dentine’ during access preparation makes little sense if ineffective canal disinfection and debridement are the inevitable consequence. First, a technique should prove its benefit and further, its safety. An example of this reversal of scientific rationale was the recent discussions on drugs available for the treatment of patients infected with COVID-19. So far, few beneficial effects have been demonstrated scientifically, yet supporters were applauding some associated mild side effects.

Apart from the perception of ‘saving dentine’, the appearance of technically well-treated canals, depicted on radiographs and clinical images, is taken as ‘proof’ of the technical skills of the operator, which has rendered the concept of minimal access cavities a trending/hot topic on social media. The necessity for advanced technical skills to perform minimally invasive cavities fuels self-promotion, which turns out to be the major motivation for dentists worldwide to copy and follow suit. However, this behaviour has obviously driven some clinicians to ignore basic rules of disinfection, as can be ‘appreciated’ in countless cases posted on social media, in which minimally invasive cavities were prepared through flawed resin restorations, carious tissue or deficient crowns. In a specialty in which disinfection is historically overshadowed by the white lines observed on a radiograph, the minimally invasive access cavity concept stands as a new mantra for an old unresolved problem.

Although there is a strong intuitive appeal to reduce the fracture rate of root filled teeth by minimal access, in a scientific-based rationale, proper science is mandatory to prove efficacy (results in a controlled environment) and, eventually, effectiveness (good results in everyday clinics). In a properly designed experiment, be it clinical or in the laboratory, the core concept centres around a so-called null hypothesis. In the current context and because the benefit of a conservative access cavity is not evident, it should not be advocated as being better than the conventional approach unless such proof evolves. It is not ethical or even reasonable to propose and implement a new standard technique until a negative clinical proof is published, since it is not the purpose of science to demonstrate that a treatment does not work. This would be the reversal of the burden of the proof. In the framework of the best evidence-based practice, new concepts such as minimally invasive access preparations should be assessed ideally in randomized controlled clinical trials against the reference intervention. Yet, even 10 years after the first opinionated article on the topic (Clark & Khademi 2010), there is no experimental or clinical evidence on the safety of this procedure or its positive impact on treatment outcome. Thus, despite the passionate way that some clinicians defend one side versus the other, usually ignoring experts’ opinions for the sake of personal values and commercial informative reports, the current body of evidence is too weak and incomplete to guide decision-making. It is still based on laboratory studies, which rarely favour the minimally invasive approach.

Undeniably, the ability to perform root canal treatment through a minimal access cavity is highly dependent on training and extended chair time to achieve the technical goals of proper root canal preparation and filling. It therefore demands a long learning curve and that, beside the fact that most teeth that require root canal treatment are heavily restored or broken down in the first place, is where the greatest flaw of this technique may be found. From the educational standpoint, it seems unwise to make endodontic practice  even  more  technically demanding and complex to learn and master than it already is. Regardless if the targets are undergraduate or postgraduate students, this approach clearly increases the difficulty in all subsequent steps of root canal treatment, potentially raising the possibility of procedural accidents and complications, which, consequently, may impact on the long-term prognosis. Additionally, minimally invasive access preparation stands for a counter-movement towards technical simplification that has been lately accomplished in Endodontics by the mechanization of root canal preparation.

In conclusion, those that endorse and promote the minimally invasive concept still need to demonstrate its potential to straighten the survival curve of root filled teeth without jeopardizing their healing rate, as subsequent steps of root canal treatment become more complex to perform well. In other words, minimally invasive access preparation needs to be associated with unequivocally positive results to justify the extra operative effort in comparison with traditional endodontic access. Having said that, further discussions around the topic in a scientific-based environment are more than timely and welcome. Hitherto, and considering that additional research is needed to provide a minimally reliable body of evidence on this topic, the introduction of minimally invasive access cavity preparation into routine clinical practice and/ or training of undergraduate and postgraduate students seems untimely and imprudent – and even reckless. Therefore, this editorial aims to encourage the endodontic community to develop a reliable scientific and clinical knowledge base to confirm whether the operative efforts and costs of conducting a root canal treatment under a constricted access cavity really are able to improve the overall quality of care, patient satisfaction and their quality of life.

Authors: E. J. N. L. Silva, M. A. Versiani, E. M. Souza, G. De-Deus

References:

1. Alovisi M, Pasqualini D, Musso E et al. (2018) Influence of contracted endodontic access on root canal geometry: an in vitro study. Journal of Endodontics 44, 614–20.
2. Clark D, Khademi J (2010) Modern molar endodontic access and directed dentin conservation. Dental Clinics of North America 54, 249–73.
3. Marchesan MA, James CM, Lloyd A, Morrow BR, García-Godoy F (2018) Effect of access design on intracoronal bleaching of endodontically treated teeth: an ex vivo study. Journal of Esthetic and Restorative Dentistry 30, E61– 7.
4. Rover G, Lima CO, Belladonna FG et al. (2020) Influence of minimally invasive endodontic access cavities on root canal shaping and filling ability, pulp chamber cleaning and fracture resistance of extracted human mandibular incisors. International Endodontic Journal 53, 1530–9.
5. Saygili G, Uysal B, Omar B, Ertas ET, Ertas H (2018) Evaluation of relationship between endodontic access cavity types and secondary mesiobuccal canal detection. BMC Oral Health 6, 121.
6. Silva EJNL, Pinto KP, Ferreira CM et al. (2020) Current status on minimal access cavity preparations: a critical analysis and a proposal for a universal nomenclature. International Endodontic Journal 53, 1618–35.
7. Vieira GCS, Pérez AR, Alves FRF et al. (2020) Impact of contracted endodontic cavities on root canal disinfection and shaping. Journal of Endodontics 46, 655–61.