The last few decades have seen the emergence of a new buzz phrase in the medical field – minimally invasive. Dentistry in general and Endodontics in particular are no exception to this trend. Despite a lack of consensus in terminologies as well as treatment strategies, indeed, as clinicians (and as patients), we want minimal loss of biological tissues. Can the minimally invasive strategy be applied to root canal treatments, or is it just a terminology that is masked with the current trend elsewhere in the medical and dental field that we intend to adopt but cannot? What are the basic philosophies and principles that underlie this minimally invasive and minimal intervention concepts? How much do we know about their benefits, or lack of, from the endodontic context? This chapter comprehensively reviews the current evidence on minimally invasive endodontics, based on a solid foundation of the basic principles.
Intervention: ‘the act of interfering with the outcome or course, especially of a condition or process (as to prevent harm or improve functioning)’; invasive: ‘reaching or taking over surrounding tissues.’ It is not an exaggeration when one says that no terms in Dentistry have been more subjected to the over-use, misuse, and abuse. Should dental treatments be minimum interventions or should they be minimally invasive? A simple look at the meaning of intervention is likely to tell us that both these goals are possible, depending on which part of the tooth and what disease is being treated. From a holistic perspective, minimal intervention dentistry encompasses identification, diagnosis, risk assessment, prevention, control, restoration/rehabilitation, recall, patient education, and empowerment [1, 2]. Minimally invasive on the other hand should be employed in all the surgical procedures that are performed, to conserve/preserve biological tissues including the tooth structure, bone, and soft tissues. Thus, one may contend that minimum intervention is a philosophy while being minimally invasive is a technical goal.
6.2 Embracing the Concept of Minimally Invasive Endodontics
From the endodontic context, is minimum intervention and are minimally invasive procedures possible or are they just fashion trends? Technological advancements such as three-dimensional imaging techniques, the operating microscope, and the knowledge of dentine biomechanics, stem cells, biomaterials, and ‘biotherapies’  are likely to make both minimum intervention oral healthcare delivery and minimally invasive endodontics a clinical reality. The main goal of root canal treatment is the prevention or treatment of apical periodontitis for long-term retention of a functional tooth. To achieve this, a rational approach is preserving or conserving the structural integrity of the tooth. At the same time, these trends should not compromise the debridement of the root canal system to achieve reduction of the infectious load to subcritical levels to initiate physiological healing . That said, there appears to be a consensus that teeth with infected and/or necrotic pulps cannot be rendered completely microbe-free with contemporary treatment procedures .
6.2.1 The Need for Patient-focused Approaches
The art and science of minimally invasive operative dentistry has been defined as that clinical approach that uses the altered and irreversibly damaged histopathological changes that occur in diseased tissues as the biological basis to determine the extent of surgical intervention during a procedure [6–8]. The underlying tenet is the maximal preservation of sound or repairable tissues, with only minimal sacrifice where operative technique dictates. This is in stark contrast to the more conventional approaches to surgical intervention, which prioritise tissue sacrifice for operative technique/materials over natural tissue biology.
Minimally invasive operative dentistry forms part of the team-delivered, patient-focused minimum intervention oral healthcare delivery approach of managing and maintaining oral and general health in patients and populations. As well as the operative management of disease, minimum intervention healthcare delivery focuses on detecting and diagnosing conditions early and, in combination with longitudinal and progressive patient disease susceptibility assessment, instigating the appropriate patient-focused, targeted prevention and disease control regimes where necessary. An important focus is on patient engagement, their perceived value and responsibility in their personal health maintenance, with mutually agreed tailored recall consultation intervals with appropriate members of the oral healthcare team [2, 7, 8]. These care delivery principles apply equally to both general and oral health and for many specific conditions, including dental caries.
For many years, root canal treatment provided pain relief and restoration of function and aesthetics in patients around the world. The main objectives of root canal treatment were always the prevention of apical periodontitis, healing of apical periodontitis, and the lifetime retention of the root filled tooth. These objectives are met usually through cleaning, shaping, and filling of the infected root canal system (the so-called endodontic triad). It has to be accepted that clinical procedures in treatment were subjected to modifications to ensure safety and ease, which may have oftentimes disregarded the need for the tooth’s long-term survival. Removing dental tissue to facilitate the procedure of root canal treatment was considered advisable in the past, whenever the risk of procedural complications was high. However, with the current technological developments that Endodontology has experienced, removing tissue to facilitate a surgical procedure seems obsolete and cannot be considered minimally invasive and therefore beneficial for the patient.
Medicine and Dentistry, in general, have moved towards minimum intervention concepts that benefit the patients in the long term, in maintaining health and lifestyle. This patient-focused approach also affects endodontic concepts. Minimum intervention dentistry seeks to bridge the gap between prevention and dental intervention, focusing on the patient understanding, appreciating, and valuing their oral and dental health. This approach, alongside minimally invasive surgical techniques, highlights the application of a systematic respect for the original tissue. This implies that the dental profession recognises that an artefact is of less biological value than the original tissue. The common delineator is tissue preservation preferably by preventing disease from occurring and intercepting its progress, but also by removing and replacing as little tissue as possible .
It is now recognised that a frequent reason for the failure of root canal treated teeth is the pronounced loss of dental tissues due to unrestorable caries and fractures . The fracture of these teeth is suggested to be directly related to the quantity of tissue lost and to the specific cavity configuration prepared . The more tissue removed, the greater is the weakening of the remaining tooth structure and the reduction of tooth stiffness and resistance to fracture . Access cavity and root canal treatment procedures per se appear to have only a small effect on the tooth, reducing the relative stiffness by 5%, compared with 20% reduction in tooth stiffness because of restorative procedures for an occlusal restoration or 63% if both marginal ridges were lost in a mesio-occluso-distal (MOD) preparation . This suggests that the marginal ridges are important elements for the fracture resistance of the tooth. In agreement with the above findings, Lang et al.  reported that access preparation (removal of the pulp chamber roof), as well as post preparation, resulted in a significant increase in deformability of the tooth, while the removal of dentine at the wall of the canal without extensive alteration of the outline of the root canals (by manual widening) resulted in no significant increase in deformability.
In molars, cuspal deflection increases with increasing cavity size and is greatest following endodontic access cavity preparation , suggesting that a cuspal coverage protective restoration is needed in molars after access cavity preparation. Over the years, many different types of restoration were developed and recommended for root filled teeth . According to the clinical situation, the objective was always to compensate for tissue loss and design a restoration that might protect the remaining tooth from fracturing under occlusal load. The reason that root filled teeth may be easily prone to fractures is attributed to trauma or fatigue failure from repeated stress overloading. With normal functional stresses, a tooth can fracture because of the reduced mechanical properties of teeth with incomplete root formation, caries, tooth wear, highly invasive operative dentistry procedures, and because of the changes in tooth structure that occur due to ageing, loss of pulp vitality, and root canal treatment .
6.2.2 Technological Advancements in Endodontics that Has Made Minimally Invasive Strategies Possible
The current technological advancements that enabled the process of tissue preservation in endodontics can be summarised as follows:
Magnification: The dental operating microscope (DOM), although not new, provides visual feedback, depth perception, magnification, and coaxial illumination and improves ergonomics for use in all stages of dental treatment .
Small field of view, high-resolution cone-beam computed tomography (CBCT) imaging: Current advancement in CBCT imaging results in visualization of the three-dimensional root canal anatomy for most configurations . Moreover, dedicated software can be applied to measure different root canal parameters [18, 19], and computer-aided design (CAD) and computer-aided manufacturing (CAM) technologies can leverage cone CBCT data for the production of materials to be used in surgical and nonsurgical root canal treatments as well as in educational settings.
Ultrasonic (US) tips and devices: US may guarantee clinicians with an increased visibility and precision during the operating procedures, thus promoting more conservative treatments.
Martensitic-flexible and fatigue-resistant files: Current advancements in thermo-mechanical processing of NiTi alloys resulted in the manufacturing of extremely resistant and flexible instruments. These instruments can be pre-curved, and they can be used safely through confined spaces without the need for convenience form and extension for prevention . Although newly introduced small instruments with reduced taper have become available to be more conservative in root canal preparations, more innovation and research in needed in this area of instrument development and their performance.
Evolution of irrigation techniques: Recent advancements in irrigant activation techniques (US, high-power-sonic, multisonic, or laser-assisted activation) suggest that disinfection of the root canal system may be possible with minimal or even no instrumentation [21, 22]. Moreover, the development of new irrigating solutions might be able to diffuse and penetrate easier in the anatomic intricacies of the root canal system [23–25]. Furthermore, the use of nanoparticles may also provide a solution to minimally invasive preparations owing to their remarkable biofilm elimination  and collagen cross-linking effects .
Evolution of bioactive and biocompatible materials: Recent evolutions in bioactive root filling materials resulted in the development of simplified techniques to fill the root canal system .
3D fixed (static) and/or dynamic guidance : These new technologies may help the clinician to perform conservative treatments even in the most difficult cases, such as completely calcified canals or access through crown and bridges.
Adhesive dentistry procedures: Increase in the quality of the restorative procedures may help to achieve a better prognosis even with partial adhesive restorations of root canal treated teeth.
Under this framework, removing tissue to facilitate the stages of root canal treatment seems obsolete in Endodontics. Tissue preservation emerges as the new paradigm shift that might result in the lifetime retention of natural teeth.
6.2.3 What Does Minimum Intervention Root Canal Treatment Encompass?
Minimum intervention dentistry always begins with detection, diagnosis, and then prevention/control strategies. For Endodontics, preventive measures should be adopted to ensure the protection of pulp vitality, sensibility, and health. Thus, trauma prevention and caries prevention can be considered the first line of defence. Once the pulp tissue is reversibly affected, vital pulp therapy treatments follow as the minimally invasive treatment approach. Always remember that a vital tissue with possible immune competence inside the root canal system prevents apical periodontitis. Once the pulp is rendered necrotic, root canal treatment is the next step. Minimal tissue removal is always the objective but without jeopardizing the safety and efficacy of the procedure.
6.3 Rationale for Minimally Invasive Root Canal Treatment
6.3.1 Failure of Root Canal Treatment: Microbial Causes
Root filled teeth may fail either because of microbial or structural causes. It is known now that microbial causes (intra- and extraradicular biofilms) and other nonmicrobial factors are associated with unsatisfactory outcomes of root filled teeth [30–34]. Root canal disinfection is considered challenging as the microbiota are organised into biofilms. Biofilms are highly organised, surface-attached, spatially oriented communities of densely packed microbes embedded in a biopolymeric matrix (extracellular polymeric substances, or EPS) [25, 35–37]. The microorganisms in biofilms are more resistant to antimicrobial agents than planktonic cells, which may be due to the diffusion barrier created by the highly protective EPS matrix, phenotypic changes in the cells, or differences in microbial cell-to-cell communication [25, 37]. There is evidence to show that the biofilm matrix alone (in the absence of live bacterial cells) can induce a chronic inflammatory response . Studies are now beginning to finely characterise the biofilm matrix relevant to endodontic pathogens . Thus, while procedural errors may compromise treatment outcomes indirectly, the inability to accomplish intracanal microbial reduction appears to be the direct cause of failure.
In the case of teeth with vital pulps, compromised debridement results in remnant pulp tissues, which may serve as a nidus or nutritional source, resulting in secondary infection, contributing to post-treatment disease . In general, the ability to debride the root canal system (i.e., remove the pulp tissue, microbial biomass, and hard tissue debris) is linked to the access cavity and the apical preparation diameters .
6.3.2 Failure of Root Canal Treatment: Structural Causes
Vertical root fractures account for at least 5–10% of tooth loss post-treatment . However, several clinical surveys and follow-up studies show variable numbers of teeth with vertical root fractures in root filled teeth, ranging from 3.69% to 20% . The complex biomechanics of fractures in intact and root canal treated teeth have been thoroughly and elegantly reviewed elsewhere . Several reasons may contribute to the weakening of root canal treated teeth. These may be iatrogenic (loss of tooth structure from dental caries, access cavity preparation, excessive removal of radicular dentine during instrumentation, dentine defects induced during root canal preparation and post-space preparation, proteolytic and demineralizing actions of root canal irrigants, and forces exerted during root canal filling procedures) or non-iatrogenic (root canal anatomy, position of the tooth in the arch, ageing of dental tissues) [44–46].
From this ongoing discussion, one may conclude that loss of tooth structure weakens root canal treated teeth. Thus, three logical questions arise: At what stage of treatment does minimally invasive ‘endodontics’ start? If a tooth were to be root canal treated, can minimally prepared access cavities and root canal results improve the fracture resistance of teeth? Can these minimally invasive preparations satisfactorily debride root canal systems? The next sections of this chapter attempt to answer this question with the available evidence base.
6.4 Minimum Intervention in Endodontics: Prevention Is Better than Cure?
Endodontology/Endodontics can be defined as the field of Dentistry studying the biology and pathology of the dental pulp and periapical tissues and with the prevention, diagnosis, and treatment of diseases and injuries in these tissues. Root canal treatment/therapy (RCT) is often used synonymously, but erroneously, with the term endodontics. Indeed, RCT is a specific procedure to manage the already necrotic pulp tissues and space, to help maintain the functionality of the tooth in the dental arch and to prevent re-infection. However, the key term in the definition above is ‘prevention.’ Indeed, ideal endodontics should aim to preserve pulp tissue integrity and sensibility throughout the life-course of the tooth and patient. While several technological advances have introduced new strategies in applying minimal intervention in endodontics, without compromising treatment efficiency and effectiveness , it is well understood that one of the major causes for pulp disease is the development of the caries process in an individual. Spread and progress of the carious lesion, unabated, will lead to pulp pathosis, with potentially irreversible, damaging consequences. Therefore, controlling the caries process and preventing carious lesion progress is an important underpinning element of successful endodontics. Indeed, the critical role of dental education in applying minimally invasive dentistry at different levels of prevention has been reviewed in detail .
6.5 Minimally Invasive Management of the Deep Carious Lesion
A deep, cavitated carious lesion in dentine can cause pain, be a potential stagnation site for a dysbiotic plaque biofilm, and lead to structural/functional issues and ultimately poor aesthetics, all requiring restorative intervention. To prevent injury to the pulp and the sound tissues of the tooth, a minimally invasive method of selective excavation of carious tissue is essential with the aim of preserving pulp sensibility and tooth structure. Carious dentine can be considered as histopathological zones spreading throughout the depth of the lesion viz., contaminated (infected) and demineralised (affected) dentine, dependent on the stage of the caries process. To avoid the risk of unnecessary vital pulp exposure in deep lesions, demineralised (affected) dentine that is present close to the pulp should be conserved. This is based on the concept that the remaining microorganisms in the deeper dentine adjacent to a vital pulp can be sealed off and terminated after placement of an adhesive, bio-interactive restoration. Suitable adhesive materials such as glass ionomer cement (GIC), dental adhesives, or tricalcium silicate cements with antibacterial properties and having the ability to bond and seal chemically to the remaining dentine provide a potential seal to arrest the caries process and facilitate tissue rejuvenation by ionic regenerative responses of the dentine-pulp complex [48, 49].
Excavation of carious tissue from cavitated, shallower cavities more distant from the pulp is more straightforward. In shallow cavities, the focus of management is centred on maximising the physical properties of the chosen restorative material in contrast to managing the pathological or biological caries process. Restorative materials require sufficient mechanical support from the underlying and surrounding residual tooth structure and gain their intrinsic mechanical properties when an adequate bulk of material exists. Hence, a balance between tissue preservation/management of the biological processes and the mechanical requirements and properties of restorative materials needs to be achieved for successful long-term tooth restoration. In such circumstances, to ensure an adequate bulk of restorative material, a more conventional, complete carious tissue removal protocol may be followed .
Lesion-pulp proximity: The depth of the lesion adversely affects the quality and thickness of the remaining dentine and in turn the protection of the vital pulp. The extent of advancement of the lesion is linked directly to the severity of the pathological caries process. In deep carious lesions, demineralised dentine close to the pulp is conserved to minimise the risk of pulp exposure. In more superficial, cavitated lesions, a more conventional, complete carious tissue removal protocol can be followed to ensure adequate space to achieve the required bulk of the final restorative material.
Extent of viable, remaining, supragingival, and restorable tooth structure: Prior to operative intervention, evaluation of the functional and aesthetic restorability of the tooth is essential. More tooth structure is conserved when employing a minimally invasive approach that removes only contaminated (infected) dentine. This conserved tooth structure can retain and support the final sealed restoration. Natural tooth substance is the optimal restorative material, and smaller volume cavities can be managed more easily by the operator and the patient. An exposed restoration surface with a reduced surface area and cleansable margins in regions of the mouth with easy access to oral hygiene aids can increase the patient’s ability to disrupt the plaque biofilm regularly leading to a decreased risk of secondary caries development adjacent to the restoration.
Pulp sensibility: Evaluation of pulp sensibility is necessary from the time of onset of clinical signs and symptoms. Simultaneously, other relevant non-invasive investigations including radiographs, and electrical and thermal pulp testing should be performed. If the caries process is arrested with a sealed restoration and effective control of the plaque biofilm by the patient follows, then the signs and symptoms of acute reversible pulpitis can resolve by facilitating the healing process of the dentine-pulp complex and its inflammatory mediators.
Caries susceptibility of the patient: The assessment of caries susceptibility (risk) is essential as it affects the outcomes of non-operative caries prevention regimes. A motivated, cooperative patient can be more easily converted and maintained at a state of low caries susceptibility. Such individuals following non-operative prevention regimes can benefit from the long-term survival of any kind of restoration. In individuals with high caries susceptibility and less motivated towards oral hygiene maintenance, a reduced duration of survival of all restorations is observed. All kinds of restorations placed in an unsuitable high-risk oral environment will be compromised.
Clinical factors: While considering the selective removal of soft carious dentine and placement of a sealed, bio-interactive restoration, various factors need to be considered. Vital pulp therapy procedures are described in detail elsewhere in this textbook.
6.6 Minimally Invasive Root Canal Treatment Procedures
Traditionally, endodontic access cavity designs were dictated by the underlying anatomy, the pathology, the existing restoration, and the need to facilitate all the subsequent stages of root canal treatment. In this regard, the outline form of the access cavities was guided by the shape, size, and position of the pulp chamber but also by operator convenience. Although such access design offered many advantages to the operator, it took less account of the need to preserve sound tooth structure to allow the long-term function of the tooth. Traditional access cavity designs were questioned, regarded as legacy concepts, and modified to fit the current trends of tissue preservation in endodontics.
For several decades, the traditional access cavity designs were subject to various principles that were left unchanged over time. The principles of traditional endodontic access cavities include [51, 52]:
- The removal of all carious dentine and defective restorations,
- The outline form that is dictated by the occlusal extend of the prepared cavity with divergent axial walls,
- The convenience form that is dictated by the degree of dentine to be removed at specific locations so as to achieve a straight-line access to root canal orifices,
- The ‘toilet’ cleaning of the cavity, and
- The extension for prevention that is dictated by the removal of dentine obstructions to extend the straight-line access to the apical foramen or the primary curvature of the root canal.
These traditional access cavities offered several inherent advantages such as the enhanced visibility; the improved clinical exploration of the floor anatomy and canal orifices; the facilitation of cleaning, shaping, and canal filling procedures; and the alteration of root canal curvature parameters in a way that would minimise complications .
6.7 Access Cavity Terminology
When discussing new concepts of access cavity design, it is important to define the common new terminology that will help clinicians communicate .
The traditional endodontic cavity (TEC) has been defined as a cavity that aims to perform a complete unroofing of the pulp chamber, exposure of all the pulp horns, and a straight-line access to the root canals with coronally divergent walls without undercuts, to visualise the pulp chamber floor and all the root canal orifices from the same visual angulation  (Figure 6.1).
The conservative endodontic cavity (CEC) involves the partial unroofing of the pulp chamber with preservation of the pulp horns, with slightly convergent walls occlusally beveled, to visualise the pulp chamber floor and all the root canal orifices from different visual angulations , following the principles given by Clark and Khademi . It means that clinicians can visualise the chamber space and the floor even when not using the same angulation but also tilting the mirror (Figure 6.2).