Access cavity in endodontics: Balancing precision, preservation, and clinical needs

Abstract

Endodontic access cavity preparation is a critical step that directly influences subsequent endodontic procedures. Procedural errors in this initial phase, whether in position, depth, or extension, can inevitably compromise the outcome of the entire treatment. Although access cavity preparation was historically believed not to weaken the tooth, more recent studies suggest otherwise. Therefore, adhering to tissue preservation principles is essential even during access cavity preparation. The mantra “as small as practical” should guide every stage of this step, balancing tissue preservation with clinical imperatives. Access cavity is dynamic, is not dogmatically predetermined, is adaptable at any time to intraoperative requirements, and must be tailored to each specific case. Achieving this, however, necessitates appropriate equipment and competencies, which require a relatively long learning curve for the clinician.

INTRODUCTION

The success of an endodontic treatment primarily depends on five universally recognized factors:[1,2,3,4]

1. Access cavity preparation

2. Shaping

3. Disinfection

4. Three-dimensional obturation of the root canal

5. Postendodontic restoration.

These five factors are closely interconnected, making it challenging to determine which is most important. However, access cavity preparation, the first step of endodontic treatment, directly influences the subsequent phases.[5,6] Mistakes at this step, whether in position, depth, or extension, could inevitably compromise the overall treatment outcome.

Executing an access cavity requires an in-depth understanding of teeth’s internal and external anatomy. It is a strategic phase designed to facilitate the localization of all canal orifices, shaping, disinfection, canal obturation, while respecting tissue preservation, and, ultimately, postendodontic restoration.[7,8]

This work aims to present a philosophy that views the access cavity as dynamic rather than dogmatically predetermined. It must adapt to each specific case while being guided by the clinician’s technical expertise and equipment. The principle “as small as practical” should be applied throughout every step, balancing tissue preservation with clinical requirements. The access cavity must remain modifiable during the procedure to meet intraoperative needs. Routine use of adequate equipment, including an operating microscope, inherently contributes to the preparation of access cavities that respect healthy dental tissues.

SPECIFICATIONS OF THE ACCESS CAVITY

To enable other phases of endodontic treatment, the access cavity must meet the following criteria:

• Eliminate the pulp chamber content: Complete removal of the pulp tissue is essential to prevent contamination and discoloration. Sodium hypochlorite digestion ensures effective cleaning during treatment. In rare cases, ultrasonic instruments may be employed to remove residual tissue

• Visualize the pulpal floor: The pulpal floor and canal orifices must be visible. Complete visualization in a single view is not mandatory; multiple views using angled mirrors suffice

• Preserve the pulpal chamber walls: Retaining natural chamber walls helps guide instruments toward canal orifices

• Facilitate instrument access to canal orifices: Grooves or pathways can be created in the cavity walls, avoiding systematic relocation or flaring of canal orifices

• Provide support for temporary obturations: Temporary restorations should create a hermetic seal without collapsing into the chamber. Proper compaction of sealing material over sterile cotton pellets ensures this

• Maintain four walls: Teeth often lack one or more walls due to caries or defective restorations. Restoration or reinforcement of these walls is imperative to:

  1. Ensure proper isolation

  2. Maintain an irrigation reservoir

  3. Provide reference points within the cavity for maintaining working lengths.

TRADITIONAL VERSUS CONSERVATIVE ACCESS CAVITIES

Traditional endodontic access cavity

Traditionally, the silhouette of the access cavity is predetermined by the application of Ingle’s principles of “straight-line access” and “form of convenience.”[9,10] Ingle’s principles derive from Black’s dogma of “extension for prevention.”[11] The resulting type of access cavity could be defined as “static” since its shape is preestablished in advance, irrespective of the anatomy of the tooth element and the presence or absence of carious lesions or restorations. These principles oblige us to remove healthy tissue to prevent possible iatrogenic complications and take absolutely no account of the principle of tissue preservation so dear to conservators.

The main aim of the traditional approach is to achieve complete degassing of the pulp chamber, allowing access and visibility to the root canal orifices in a single view so that all root canal orifices can be located[12,13] and procedural complications such as root canal transport[13,14] or instrument fracture[15] can be avoided. The contour line of the access cavity should be, as a minimum, the projection of the pulp chamber roof at an occlusal or lingual level for anterior teeth.[9,10] The more distal the tooth in the arch, the larger and more mesiodistally inclined the access cavity, in keeping with the principle of convenience. The more curved the canals, the greater the amount of healthy dentin removed at the pericervical level, keeping with the principle of straight-line access.[9,10]

Conservative endodontic access cavity

Over the last decade, the concept of minimally invasive endodontics (MIE),[16,17,18,19,20] applicable to every phase of endodontic treatment, including preparation of the access cavity, and aimed at preserving tooth strength, has gained ground in our discipline, but not without controversy. At first, this “movement” met with immense resistance, sparking heated discussions between proponents and detractors of MIE. However, a recent survey of American Association of Endodontists members revealed that 43% claim to prepare conservative endodontic access cavities (CECs).[21]

CECs are smaller in volume than traditional endodontic access cavities (TECs). Its preparation focuses more on tissue preservation, emphasizing the importance of preserving the pericingular and pericervical dentin, located 4 mm above and 4 mm below the crestal bone. It is crucial for the distribution of functional stresses during function and parafunction.

While the concept of MIE may be justified, it is also important to note that, from a clinical point of view, this paradigm shift must be accompanied by a high-performance technical platform. This approach implies the use of the operating microscope, dedicated ultrasound instruments, martensitic shaping instruments and technologies enabling irrigant activation and three-dimensional (3D) filling, as well, in selected cases, as the use of 3D radiological examinations. Indeed, knowledge of endodontic anatomy will enable cone-beam computed tomography (CBCT) to be performed only when necessary, following the European Society of Endodontology and American Association of Endodontists guidelines.[22,23,24]

Various types of CEC have been proposed, and there is some confusion as to the terms used to designate them, which is why several researchers have attempted to categorize these different types into just 6.[25] Others then proposed classifying access cavities into CEC, TEC, and ultraconservative (UEC) based on the amount of enamel and dentin removed during access cavity preparation, generating >15% tissue removal for TEC, ≤15% for CEC, and ≤6% for UEC, respectively.[26]

Nevertheless, although standardizing nomenclature may be useful from an academic point of view for communication between researchers, clinically, this diatribe is sterile. CEC should be seen as the preparation of a dynamic access cavity that takes into account the “as small as practical” divide by considering the unique anatomy of the dental element being treated while respecting the objectives that an access cavity must satisfy.

The aim of preparing a TEC would be to simplify the subsequent stages of endodontic treatment in order to achieve high-quality therapy. Unfortunately, however, the epidemiological data are indisputable and witness the failure of our education and healthcare system. Success rates for properly performed endodontic treatment are incredibly low,[27,28,29] and rates of periapical lesions in root canal-treated teeth are statistically higher than in untreated teeth.[30] The quality of endodontic treatment is intimately linked to the presence and extent of periapical lesions.[28,31]

Simplifying a surgical procedure that requires knowledge, skill, and a long learning curve is probably not the right solution. Endodontics is a very demanding branch of dentistry, and 6 years of training is probably insufficient to produce a clinician capable of dealing with endodontics. More targeted training with an adequate technical platform would enable us to train more effective clinicians. It is unfortunate to have to reiterate this. However, an impeccable knowledge of teeth’s internal and external 3D anatomy and its variations should be an indispensable prerequisite for performing “lege et artis” endodontic therapy.

Furthermore, a recent article points out that a large percentage of students feel they lack the necessary anatomical knowledge.[32] It is extremely important to emphasize the concept of 3D anatomical knowledge. Moreover, thanks to countless scientific works based on micro-computed tomography (CT) technology, it is accessible to all practitioners today.[33,34,35,36]

The clinician’s familiarity with retroalveolar radiography has, so to speak, encouraged the loss of a true anatomical memory, leading to ignorance of the anatomy in mesiodistal (MD) projection that cannot be detected by conventional two-dimensional (2D) radiography.[37]

Visualization of the operative field is a further key point, especially in creating the access cavity. It seems incredible that some authors stress the fact that the minimally invasive approach requires direct vision of the surgical field as if this were something exceptional.[25,38] Direct viewing of the surgical field requires the use of dedicated ultrasound, long-necked burs, and indirect vision under the operating microscope.

In no branch of surgery, would the surgeon be prepared to operate blindly and for a valid reason. Visualization of the surgical field should be a prerequisite, regardless of whether CEC or TEC is being prepared. Whatever the volume of the access cavity, canal orifices are so small in diameter that they cannot be visualized without the aid of a high-performance light source and magnification system.[39,40]

Indeed, the need to work with a high-performance light source de facto disqualifies the dental loupes in favor of the operating microscope. Moreover, visualization of the surgical field is a necessary, but not sufficient, condition for the performance of our surgical act since visualization of the surgical field in indirect vision does not guarantee the 3D spatial control necessary for the smooth progress of endodontic treatment.[41]

Consequently, it is imperative that every university be able to pass on to students the anatomical and clinical skills needed to perform endodontic therapy in accordance with the latest scientific advances and that practitioners continue to benefit from ongoing training throughout their careers.

Apparently, CEC should not be an exercise in style. However, it should only be performed if it represents an advantage, particularly in terms of fracture resistance and, thus, the longevity of the dental element. This enables the biological and mechanical objectives of endodontic therapy to be achieved simultaneously.

ADVANTAGES AND CHALLENGES OF CONSERVATIVE ACCESS CAVITIES

Fracture resistance, stress localization, and quantification

Contrary to popular belief, scientific evidence evaluating the impact of endodontic access cavity type on tooth longevity is very limited. Ingle’s TEC preparation protocols were mainly supported by the work of Reeh et al.,[42] who concluded that access cavity preparation on maxillary second premolars only weakened the tooth by a negligible 5%. These results were subsequently generalized to other dental elements.

However, this study, like many others on the subject, is subject to several biases that render it clinically irrelevant:

Indeed, the first critical point of these studies is to have well-standardized samples

1. In terms of tissue diameter, density, and morphology, the volume, thickness, height, width, and morphology of dental tissues are likely to influence the fracture resistance of teeth. Tissue volumes can vary from tooth to tooth, even with the same external diameters, so it is important to perform micro-CT analysis to select anatomically similar samples. Therefore, it is obvious that when extracted teeth are used for scientific purposes, there will always be parameters that are very difficult to control and could jeopardize the study. Therefore, to minimize this problem, all standardizable factors should be harmonized, starting with weight, volume, and density, which have a more consequential effect than the MD or buccolingual dimensions of the samples[43]

2. Regarding the fatigue undergone by the dental element prior to avulsion, the age of the tooth is an important aspect of sample selection. Certainly, aging has a negative impact on tooth toughness and ductility, reducing the strength limit of dentin.[44,45,46,47,48] Clinically, most fractures of endodontically treated tooth elements are caused by cyclic fatigue with subcritical loads that are well below the load capacity required to cause a catastrophic fracture; the cumulative effect of these subcritical loads generates the initiation of a “crack” that propagates over time and becomes a true fracture[49]

3. The extraction technique should be atraumatic for all specimens and, of course, not generate microcracks, but this is very difficult to demonstrate

4. Sample storage conditions after extraction may also affect sample characteristics.

The second critical point is sample preparation prior to testing, which can affect the outcome of experimental procedures

1. Various literature studies comparing TEC and CEC perform root canal treatments, from preparation of the access cavity to obturation, without the aid of the operating microscope.[18,50,51,52] This is undoubtedly a colossal bias. CEC preparation requires a high-performance technical platform, including an operating microscope

2. The total volume of dental tissue removed or the exact dimensions of the access cavities are difficult to standardize

3. Simulation of the periodontal ligament on test specimens is essential. Conclusions drawn from studies where the periodontal ligament is not simulated may be questionable. Simulation of the periodontal ligament could result in a change in stress location and concentration[53,54]

4. The specimens tested should all be restored, considering that depending on the type of restoration, the tooth will have a different fracture resistance. In the study by Reeh et al.,[42] while storage conditions are mentioned, no mention is made of tissue volumes or even the external diameters of the specimens; neither the age of the teeth nor the extraction technique is mentioned; the total volume of tissue removed is not determined; the periodontal ligament is not simulated; the specimens tested are not restored after cavity preparations, they have not undergone thermomechanical aging and they have been subjected to a maximum force of 111 N, whereas the physiological forces to which teeth are subjected in the lateral-posterior sector range from 250 N to 290 N[55,56] and may exceed 770 N when subjected to pathological forces.[56] Consequently, conclusions on the influence of access cavity preparation on tooth strength, like all other conclusions drawn from studies such as that by Reeh et al.[42] with innumerable biases, are scientifically questionable and raise questions about the reliability of the results.

Finite element analysis (FEA) has the merit of standardizing the evaluation of a variable under test while virtually fixing all other parameters, thus providing reliable results and numerically controlled tests.[57] Zelic et al.,[58] using a combined method of fracture testing and FEA, concluded that access cavity preparation on maxillary second premolars significantly weakens the tooth. Their results indicate that access cavity preparation weakens the tooth by 29.25%. This result, obtained by attempting to minimize all the methodological drawbacks of Reeh’s study,[42] overturns the assumption that access cavity preparation has a negligible impact on the strength of the tooth element. This same approach was subsequently used to evaluate stress concentration zones in standardized models in which different types of access cavities were simulated.[59,60,61,62,63,64] The conclusion of these studies is unequivocal: in models where a TEC is simulated, stress concentration in the pericervical region is greater than in models where a CEC is simulated, and the fracture resistance of a tooth with a CEC is greater than that of a tooth with a TEC.

Influence of the conservative access cavity on endodontic procedures

The question arises, however, whether too small an access might affect the subsequent stages of root canal treatment by compromising:

1. The location of root canal orifices

2. Cleaning and disinfection of the pulp network

3. Shaping

4. Root canal filling

5. Tooth esthetic causing discoloration due to a possible failure to remove pulp tissue or filling materials from the pulp chamber.

Locating root canal openings

Root canal orifice location, an essential step in shaping, cleaning, and filling a root canal, is not a question to be linked solely to the size of the access cavity but rather, once again, to the knowledge and skills of the clinician and depends on the use of a high-performance technical platform and anatomical knowledge. The study by Mendes et al.[64] showed that, concerning the middle-mesial-canal localization on mandibular molars performed by an endodontist using a dedicated microscope and ultrasound, no statistically significant difference was established, whether the access cavity was traditional or minimally invasive. The same result was demonstrated by Saygili et al.[65] for the second mesiobuccal canal (MB2) localization on maxillary molars.

Cleaning and disinfection

The literature results regarding pulpal system cleaning and disinfection procedures are discordant. However, in various studies, irrigants are not activated,[66,67] although it is clear that in the case of minimally invasive access, irrigation activation is generally necessary. Other studies in which irrigators are activated conclude that there is no difference in cleaning and disinfection between the two types of cavities.[68,69]

Shaping

Researchers use the percentage of walls touched by root canal instruments, respect for root canal anatomy, root canal transportation, and reduction of intracanal bacterial load to evaluate shaping. However, probably due to the undemanding selection of samples, studies reach contradictory conclusions.[14,67,70,71,72,73,74]

We are convinced that focusing on the amount of wall surface touched by the instruments rather than on the action of the irrigators is probably an inappropriate way of looking at things since shaping enables irrigators to clean out and empty the pulp networks. More attention should be paid to the activation of irrigation solutions rather than a mechanical design focused on root canal shaping.

Apical transportation is an important parameter to consider, too, as it would be directly related to the success of endodontic treatment.[75] Indeed, it favors the persistence of microorganisms and organic tissue remnants in the dentin walls, compromising the disinfection and sealing of the root canal system.[76] Although studies come to contradictory conclusions, it would appear that the use of instruments with a taper of no more than 6% does not produce significant apical transportation.[71,77] Besides, using instruments with a taper >6% would make absolutely no sense with the minimally invasive philosophy.

Another element to consider is the instrument fracture of shaping instruments. Findings on the percentage of instruments fractured, depending on the type of access cavity, tell us unequivocally that there is no statistical difference between the two types of cavity.[66,67]

Root canal filling

Root canal filling ability and the presence of intracanal voids are assessed to determine the quality of root canal obturation. However, once again, scientific results are discordant.[66,71,73,78] Given the limited and conflicting evidence, the influence of access cavity size on filling quality remains an open area for further research. However, with the growing adoption of hydraulic cold compaction techniques, these challenges should no longer be a significant concern.

Tooth esthetic

A further criterion is the ability to properly remove pulp cameral tissue and any obturation material from the pulp chamber after completion of the obturation phase itself so as not to lead to tooth pigmentation. Once again, scientific results are discordant; some studies reported greater amounts of residual filling material in the pulp chamber[74] when CEC was performed, whereas others reported no difference,[76] which were difficult to remove even with a microscope and ultrasonic tips, potentially affecting esthetics and extending operative time.

Indeed, the persistence of obturation materials or pulp tissue can lead to tooth discoloration and negatively impact adhesive materials.[79,80] At present, the cold hydraulic condensation technique, which by definition uses hydrophilic sealers, can undoubtedly overcome this problem without any additional effort on the part of the clinician.

However, larger-scale clinical trials would be needed to fill the knowledge gaps regarding patient-related factors, which are more difficult to manage in the laboratory.

Influence of the access cavity on postendodontic restoration

Minimally invasive approaches in endodontic procedures, especially in access cavity preparation, aid in preserving sound tooth tissues, particularly at the level of the cervical area, where coronal fractures are most common, ultimately impacting the restorative strategy.[58] In fact, endodontics is part of a more complex treatment plan designed to restore the tooth’s original function within the oral system. Therefore, clinicians should always approach their procedures with a clear vision of the final postendodontic restoration. Moreover, to determine the most appropriate restoration, it is essential to consider recent research highlighting that tooth structure weakened by TEC preparation[57,58,59,60,61,62,63] can compromise the tooth’s integrity as significantly as the loss of a marginal ridge.[42] This finding, as discussed, challenges earlier assumptions that access cavities alone have minimal impact on tooth stiffness and underscores the importance of minimally invasive approaches in preserving structural integrity.[58,81]

Minimally invasive access cavities can preserve more critical structures like the pericervical dentin, reducing the need for extensive restorations and positively influencing the treatment plan.[61,82,83]

Additionally, advancements in restorative materials and bonding techniques allow for highly durable adhesive restorations at reduced thicknesses, enabling the clinician to preserve sound tooth structure. Both these important technological advancements could reduce the sacrifice of sound tooth structure required during restorative procedures and cavity preparation, as partial adhesive restorations with complete or partial cuspal coverage may be performed in most of the teeth endodontically treated following the minimally invasive procedures described in this paper.[81]

The guiding principle is to preserve as much tooth structure as possible by seeking the right balance in relation to clinical needs at every stage, from the access cavity preparation to the type of coronal restoration.[84,85] From a biomechanical perspective, preserving tooth structure is essential for maintaining the delicate balance between biological, mechanical, adhesive, functional, and esthetic factors. Conserving coronal tissues and applying invasive endodontic procedures help safeguard this equilibrium, ensuring restored teeth’s long-term integrity and performance. Therefore, the dogmatic correlation between endodontics and full-crown restoration, in which a complete cuspal coverage with a full crown was advocated for any endodontically treated tooth, independently of the amount of residual tooth structure, to reduce the risk for fracture and improve the prognosis of these teeth, may be no more actual, given the present advancements described above.[84]

However, restoring endodontically treated teeth is a multifaceted process that requires careful consideration of both tissue loss and clinical needs and is mainly based on:[86]

1. Residual tooth structure: The number, thickness, and quality of remaining walls, particularly in the cervical region where fractures often occur

2. Tooth position in the arch: Anterior teeth, premolars, and molars differ in functional and structural demands

3. Role in rehabilitation: Single restorations versus teeth incorporated into partial or full-mouth rehabilitations

4. Occlusion and parafunctions: The presence of parafunctional habits and occlusal forces

5. Antagonist teeth: The nature and type of opposing dentition

6. Adjacent teeth: The presence or absence of mesial and distal neighbors.

Clinically, while restorative decision-making is inherently multifactorial and difficult to generalize, a simplified guideline can be applied in cases involving a single restoration—provided the patient has natural opposing teeth, is classified as Angle Class I, and exhibits no signs of parafunctional habits. The tooth element in question has one mesial and one distal tooth; the restoration choice depends largely on the access cavity design (CEC vs. TEC) and the extent of structural loss.[87,88,89]

In the rare case of endodontic treatment on an intact tooth, if a CEC was performed, the restoration would generally be a direct restoration without cuspal coverage, regardless of the tooth type. Conversely, cusp thickness would often be reduced if a TEC was performed, so an indirect restoration with cuspal protection would likely be required.

In case of more significant tissue loss, such as the loss of one or both marginal ridges, the preservation of pericervical dentin achieved with CEC may guide the clinician toward an adhesive partial restoration with selective or full cuspal coverage, depending on the remaining cusp thickness. In contrast, if a TEC was performed, the structural compromise would more often necessitate either an indirect restoration with full cuspal coverage or a complete crown.

While these clinical recommendations can be valuable, they should be regarded as a flexible clinical guideline rather than a rigid decision tree. However, tissue preservation could be beneficial in lengthening the restorative cycle of teeth. Defective restorations are eventually replaced by larger restorations that will 1 day fail again, leading to even larger restorations, including post- and core approaches, increasing the risk of complications and, ultimately, tooth loss.[84] The preservation of tooth structure is closely linked to fracture resistance[90,91] while also minimizing catastrophic failures and extending the longevity of the restored tooth.[92,93] Moreover, there is no documented literature supporting the placement of a crown over an indirect adhesive restoration or a direct restoration in cases also of severely compromised Endodontically treated teeth (ETT).[94,95]

In this context, by preserving critical structures such as pericervical dentin, CEC designs help minimize the need for extensive restorations. Therefore, based on the preceding analysis, CEC combined with adhesive partial restorations, offering cuspal coverage to protect weakened cusps while preserving sound tooth tissues with minimal sacrifice of tooth structure, can extend the longevity of the restorative cycle of teeth while preventing the excessive volumetric reduction often required for full crowns, which are frequently necessary for ETT treated with TEC, due to the significant loss of pericervical dentin and overall structural compromise. The application of these concepts could be beneficial because the lifespan of ETT improves in direct proportion to the amount of preserved sound tooth structure, regardless of the type of restoration placed.[91,96]

The clinician’s approach should remain dynamic, integrating technological advancements and patient-specific factors to optimize outcomes.

PRACTICAL GUIDE FOR CONSERVATIVE ACCESS CAVITY PREPARATION

The preparation of a CEC increases the fracture resistance of the dental element. It, therefore, does not compromise the subsequent stages of root canal treatment, provided that a high-performance technical platform is available. This strategy includes using an operating microscope, dedicated ultrasonic instruments, martensitic shaping instruments, and activating irrigants, enabling 3D cleaning and obturation. 3D diagnostic modalities such as CBCT could be used if protocols are respected. Indeed, the systematic use of CBCT in case planning, as suggested by some authors, should be avoided.[19] The interpretation of 2D radiography coupled with the use of the operating microscope, ultrasonic instruments, and martensitic instruments is more than sufficient to treat cases of moderate difficulty. Of course, this presupposes an impeccable knowledge of anatomy, as every practitioner should.

These technological advances make it possible to create small access cavities, known as CECs while respecting biological and mechanical imperatives.

However, balancing and adapting to each case is essential, respecting the “as small as practical” principle. CEC preparation is designed to minimize the deposition of healthy dental tissue, particularly at the pericervical level, without lengthening the operating time. This type of cavity is an evolving and not a static preparation, which must vary according to the anatomy, the presence, or absence of reconstructions or carious lesions on the dental element, and which will vary according to the operator’s skillfulness. It is not an exercise in style, but it must strike a perfect balance between tissue preservation and clinical requirements.

From a clinical point of view, the process leading up to the preparation of the access cavity can be divided into two main stages:

1. Preaccess analysis

2. Access technique and location of root canal orifices.

Preaccess analysis

Once the need for endodontic treatment has been determined:

1. The first step in the preaccess analysis is to identify the enamel-cement junction (ECJ)[96] using a periodontal probe, exploring the entire tooth circumference. This strategy will provide a mental overview of the silhouette of the pulp chamber at the ECJ. As the crown’s long axis does not coincide with the pulp chamber, using occlusal landmarks to prepare the access cavity could lead to procedural errors, especially when the tooth to be treated bears a prosthetic restoration

2. The second step is to analyze the preoperative radiograph(s) to determine the MD inclination of the tooth. Moreover, to assess the buccolingual inclination, information is extrapolated from the circumferential probing or, if necessary, the CBCT examination

3. The third step is to determine the radiographic distance between a coronal reference point and the floor of the pulp chamber

4. The fourth step is to assess the distance between the roof and the floor of the pulp chamber. Teeth that appear to have calcified pulp chambers, with a reduced or nonexistent roof-floor distance, should be treated with great care

5. The fifth step consists of choosing the point of occlusal penetration and the inclination to be given to the drill. This point is not predetermined and changes according to the clinical and radiological characteristics of the target tooth. The internal anatomy of the pulp chamber, extrapolated from knowledge of anatomy, radiographic analysis, and clinical visualization, will dictate the final shape of the access cavity. The cavity is created in several stages.

Meticulous preparation of the preaccess analysis phase is the prerequisite for preparing a correct access cavity.

Like an architect or sculptor, before constructing a building or sculpting a statue, they draw sketches and then 3D models. The clinician will have to carry out the same work, either mentally or, especially at the beginning, on the digital radiograph, by tracing linear markers which will represent the MD limits of the access cavity. These marks will be reproduced on the occlusal surface for molars and premolars, the lingual surface, and the incisal edges for anterior teeth. In more complicated cases, and always respecting the protocols, a 3D radiological examination will be carried out if necessary.

Access technique

Schematically, the perioperative stages of preparing the access cavity can be divided into four phases [Video 1]:

1. Penetration phase

2. Enlargement phase

3. Root canal orifice location phase

4. Finishing phase.

Penetration phase

This first phase aims to make the pulp breakthrough and gain access to the area of the pulp chamber where it is most represented.

The penetration phase is carried out using a long-shank cylindrical diamond bur with a diameter ranging from 0.12 mm to 0.14 mm, depending on the tooth to be treated. Obviously, if access is to be gained through a prosthetic restoration, specific transmetal, ceramic, or zirconia burs are available. The bur will advance to the point in the pulp chamber predetermined during the analysis. In general, this point coincides with the area where the pulp is most represented.

In any case, the reference point-chamber floor distance calculated during the analysis phase must not be exceeded. Although the passage of the drill from the dentinal tissue to the pulp can be detected by the clinician, who will report a clear sensation of the drill falling into the pulp chamber, this sensation will only be perceived if the pulp chamber is at least 2 mm deep. In the case of a calcified or shallow pulp chamber, this sensation will be completely absent. At this stage, visibility will not be ideal, as the head of the contra-angle handpiece is in our field of vision. Nevertheless, the operating field can be visualized, provided that the burs used have an elongated neck and indirect vision.

If necessary, in more complicated cases, this phase can be split into two: a first phase during which we will use the cylindrical bur, which will be at most 2 mm from the reference point to chamber floor distance calculated during the preaccess analysis, and a second phase during which we will use an ultrasonic tip working at a dedicated point such as the Start X3 (Dentsply Sirona Dentsply, Chem. du Verger 3, 1338 Ballaigues, Switzerland) to perform the breakthrough, in order to have an optimal view of the operating field. The tip will be able to work up to the predetermined distance safely and with no risk of iatrogenic errors.

If there is any doubt, an intraoperative X-ray or, if necessary, a CBCT examination should be carried out; blind enlargement or, worse still, apical deepening of the access cavity would be taking unnecessary risks.

Enlargement phase

This phase aims to widen the access cavity, which will then take on its almost definitive shape. However, the walls of the pulp chamber will be scrupulously respected without altering them. If possible, and taking into account clinical needs, the chamber’s roof will not be completely removed, and no maneuvers will be carried out to strip the cavity.

Dedicated ultrasound tips will be used in this phase. During this phase, dedicated ultrasonic tips enable highly precise and controlled instrumentation. With optimal visualization of the operating field, these tips allow the clinician to perform detailed work, similar to sculpting, ensuring accuracy and preservation of tooth structure. The clinician will be able to visualize the action of these instruments on the dentin and enamel of the dental element at all times. Clinically, in the majority of cases, we will use ultrasonic tips that do not work at the tip, such as the Start X1 (Dentsply Sirona Dentsply, Chem. du Verger 3, 1338 Ballaigues, Switzerland), thus limiting the risk of perforation. However, in the case of intrachamber calcifications, it will be necessary to use ultrasonic tips working at the tip, considering that patent root canal orifices should be located deeper in these cases. The access cavity should be slightly wider than the coronal projection of the root canal orifices. This will often make it possible to retain part of the roof of the chamber.

Of course, the tops of the cusps should be preserved when designing the access cavity for premolars and molars. Clinically, for mandibular molars, easy access to the distal canal is possible even when the projection of the distal canal orifice(s) will be more mesial, resulting in an access cavity with a reduced MD diameter compared to a traditional access cavity. Similarly, for maxillary molars, the projection of the palatal canal orifice will be more buccal, resulting in an access cavity with a reduced biparietal diameter compared to a traditional access cavity.

For premolars, the access cavity will be centered in the proximal direction and barely oval, with a long buccopalatal axis whose extension in the buccopalatal direction will be approximately 1/3 of the inter-cuspid distance.

In the case of the incisors and canines, the access cavity will be moved more coronally toward the incisal edge while respecting the peri-cingular dentin. At the end of the widening phase, the access cavity will have reached its almost definitive shape.

Root canal orifice location phase

During the orifice location phase, the aim of which is to locate and widen the canal entrances in a controlled manner, care must be taken to geometrically respect the wall–floor passage surfaces without altering them and without increasing the wall–floor angles to obtain highly obtuse angles as was traditionally done due to the use of different burs in this phase.

The essential instruments in this phase are the straight endodontic probe and the ultrasound inserts. If the canal orifices are visible, first, the probe is used to point out the canal and confirm its presence, and then, the ultrasound insert is used to widen the canal entrance as little as possible. The insert used in this case is again the Start X1. The nonworking part of the Start X1 is 0.5 mm, so its precise use will enable canal orifices to be enlarged in a controlled, nondestructive manner. If the root canal opening is not visible because it is hidden by the reactionary dentin, as is often the case with MB2, we use point-working inserts such as Start X3 or ultrasonic K files. At this stage, if it is possible to obtain a clear visualization of the entire floor and canal orifices using several viewpoints, it is also possible not to eliminate the entire roof of the pulp chamber. If clinical needs dictate, the walls of the access cavity can be sculpted in a parallel or divergent manner from the cameral roof to the occlusal enamel. Hierarchically, however, it is once again important to respect the pericervical dentin and, therefore, the wall–floor passage surfaces without altering them.

Retroalveolar radiography can only give us limited information about the number of canals and their position. Provided that the entire pulpal floor is visualized, the clinician should be able to locate all the canal orifices using a series of anatomical landmarks on the cameral floor, which have been explained in the form of “anatomical laws.” The law of color change will guide us in determining when, at the level of the floor–wall junction, it is no longer necessary to widen. Normally, according to this law, the color of the floor is always darker than that of the pulp chamber walls. Some exceptions should be kept in mind. In the case of pulp stones, dark regions are not visible, and in some cases, with extra roots on the buccal side for mandibular molars, this dark region does not exist on the extra root side. By virtue of the laws of symmetry, which are valid for all dental elements, with the exception of the maxillary molars, we can identify a missing canal and locate it precisely.

The laws of orifice location allow us to know the number of orifices and to identify their position because they state that canal orifices are always located at the wall/floor junction at the angles of the polygon representing the pulp floor. Furthermore, this is also true in the case of intracameral calcifications since these laws allow us to know where we need to start depositing reaction dentin in order to locate the hidden canal. This knowledge helps us, of course, to identify hidden canals, but also teeth where the number of canals is less than the norm.

In the design phase, we may be more ambitious than expected and create an access cavity that is not compatible with clinical needs because it is too narrow; in this case, we should not hesitate to widen the access cavity, which is always modifiable and dynamic.

Finishing phase

The aim of the finishing phase is to create straight, direct access to the canal orifices without eliminating the first canal curve.

Real insertion grooves or braces will be necessary to guide the shaping instruments toward the canal orifices.

Tooth-by-tooth application

Visualizing the access cavity on an intact tooth rarely represents clinical reality since intact teeth rarely require endodontic treatment.

Pedagogically, this procedure can be justified by the fact that the intact tooth should be the easiest model for creating an access cavity; however, it should be avoided.

Outlines often do not reflect the anatomical reality and even less the proportions between the enamel tissue, the dentin, and the pulp. In these diagrams, the access cavity for the maxillary and mandibular molars is often too mesial. On other occasions, the pulp volume is overestimated, giving a sketched view of the contour lines of the CEC concerning the pulp volume.

The cavity design of the different access cavities we propose in this article is superimposed on micro-CT sections of real teeth to be as close as possible to anatomical reality. Nevertheless, in clinical practice, teeth requiring endodontic treatment often already have a conservative restoration or a carious lesion. In these cases, in order to respect the hierarchical principles mentioned above, it would be preferable, after careful assessment, to create an endodontic access cavity guided by the Caries-guided Access Cavity (CariesAC) or the Caries-guided Access Cavity (RestoAC), whether it is located mesially, distally, buccally, or lingually. However, clinically speaking, in the case of a distal lesion in the premolars or molars, the advantages and disadvantages of creating a cavity guided by the lesion must be carefully assessed, as must the advantages and disadvantages of creating a cavity guided by the lesion in the case of a distal or mesial lesion in the incisors or canines.

It would be impossible to cover all possible access cavities here, depending on the clinical situation and the dental element in question. However, for each group of teeth, we will discuss the preparation of the CEC as if the tooth was intact, and in the case of preexisting restorations (or caries) in the figures, we will suggest carrying out a RestoAC guided by the lesion. Once again, the access cavity must be prepared on a case-by-case basis, and the same tooth may have a different silhouette. We will need to be inventive and imaginative, depending on the anatomy and condition of the tooth.

Maxillary incisive-canine group

The access cavity is circular or slightly oval along the coronal-radicular axis and is centered in the MD direction in the vast majority of cases.

The projection of the root canal orifice, in reality, falls on the buccal surface of the crown; however, although the access cavity can be prepared at the buccal level, clinically, in the absence of the indirect prosthetic plane, the access cavity will most often be made between the incisal edge and the most coronal third of the crown on the palatal side [Figure 1a]. The practitioner starts with the cylindrical bur at the level of (or slightly below) the incisal edge, keeping the bur almost parallel to the inclination of the buccal cervical third of the crown. Depending on the anatomy, the MD extension of the access cavity should be adjusted. The more sclerotic the pulp, the smaller the MD extension. In all cases, the pericingular dentin should be respected.

Figure 1.

(a) Micro-computed tomography (micro-CT) reconstruction of a maxillary incisor showing intact tooth, traditional endodontic access cavity (TEC), conservative endodontic access cavity (CEC), and RestoAC. Access is shown from sagittal, coronal, and palatal views. The red area represents the tooth structure removed in the TEC group, the green area the tooth structure removed in the CEC group, the blue area the tooth structure removed in the RestoAC group, and the white area represents the restoration in the RestoAC group. (b) Micro-CT reconstruction of a maxillary premolar showing intact tooth, TEC, CEC, and RestoAC. Access is shown from sagittal, coronal, and axial views. The red area represents the tooth structure removed in the TEC group, the green area the tooth structure removed in the CEC group, the blue area the tooth structure removed in the RestoAC group, and the white area represents the restoration in the RestoAC group. (c) Micro-CT reconstruction of a three-rooted maxillary premolar: (A) Buccal aspect with opaque tissues; (B) Buccal aspect with transparent enamel and dentin: The location of pulp chamber in green and root canals in red is possible; (C) Coronal slice with CEC overlay in green; (D) Occlusal view with CEC overlay in green in the case of three-rooted maxillary premolar; Notice the “T” shape of the access cavity with a MD enlargement in the buccal side. (d) Micro-CT reconstruction of a maxillary first molar showing intact tooth, TEC, CEC, and RestoAC. Access is shown from sagittal, coronal, and axial views. The red area represents the tooth structure removed in the TEC group, the green area the tooth structure removed in the CEC group, the blue area the tooth structure removed in the RestoAC group, and the white area represents the restoration in the RestoAC group. TEC: Traditional endodontic access cavity, CEC: Conservative endodontic access cavity

For worn elements with an occlusal surface rather than an incisal edge, the access cavity can be made exclusively on the occlusal side. This will give us absolutely straight access to the canal and avoid removing additional dentin at the palatal and pericingular levels.

Anterior teeth statistically have the highest number of iatrogenic perforations during preparation of the access cavity. These consequences can be justified due to the very design of the TEC, which is carried out along two different axes and in two stages. In particular, when the pulp chamber is sclerotic in the case of preparation of a TEC, the drill, placed perpendicular to the cingulum, can advance in a buccal direction until a buccal perforation is made since the pulp horn is located much more apically. This case, and the sensation of “falling into a void” previously mentioned, will not occur.

Preparing the access cavity in the anterior teeth from the coronal third very close to the incisal edge means that only one axis can be used, and access can be achieved in a single step. Naturally, the risk of iatrogenic perforation in this context is virtually nil. Access to the canal will be perfectly straight, which will facilitate shaping.

Maxillary premolars

The pulp chamber of the maxillary premolars has two horns, a B and an L, with its major axis in the Buccolingual (BL) direction. The external contour of the access cavity will have an oval silhouette with a major axis BL, centered in the direction MD [Figure 1b]. Its extension in the BL direction will be approximately 1/3 of the intercuspid distance, and the external contour of the access cavity in relation to the central sulcus will be shifted more buccally.

Rarely, maxillary premolars may have two buccal canals and one palatal canal. In this case, the coronal diameters, especially at the cervical level on the buccal side, are increased and probing often reveals a depression at the buccal level, which should suggest the presence of a second buccal canal. The access cavity will have a “T” shape, with an MD extension on the buccal side [Figure 1c].

Maxillary first molar

The maxillary first molar has four functional cusps and an additional cusp on the lingual surface, the Carabelli cusp. The ML and DB cusps are linked by the oblique ridge known as the enamel bridge. This dental element usually has four canals and three distinct roots. The pulp chamber is larger in diameter in the BL direction and generally has four horns. The shape of the pulp floor is trapezoidal. The silhouette of the access cavity, which should not involve the cusp tips and the oblique ridge, will be trapezoidal [Figure 1d]. Various authors propose a mesial extension in the preparation of the access cavity to locate the MB2 canal, but this is not necessary and would compromise the stability of the mesial marginal ridge. Considering the central fossa, the palatal extension of the access cavity can be reduced in relation to the B extension, allowing part of the possible to leave part of the pulp chamber roof.

Regarding the location of the MB2, it is important to recall the morphogenesis of the MB root.

At first, it contains a single laminar canal of semi-lunar shape, with a major pole, the future orifice of MB1, and a minor pole, the future orifice of MB2, with the function and deposition of dentin. In the second phase, two canals, MB1 and MB2, are “formed,” largely interconnected by isthmuses and interconnections. For this reason, if there is only one canal in the MB root, the orifice will be oval; if there are two canals, the orifices will be round.

Under certain conditions, when there is a single oval MB canal orifice, the MB2 canal may originate from the mid-third of MB1. In such cases, the MB cusp can limit the entrance of MB2 due to a difficult access angle. In these exceptional conditions, it is important to create real insertion grooves to facilitate proper instrumentation. If present, removing secondary and tertiary dentin at floor level with dedicated ultrasonic inserts under microscopic vision will help locate the MB2 with the preparation of a CEC. The use of the law of localization and color change undoubtedly simplifies its localization.[96]

Maxillary second molar

The anatomy of the maxillary second molar may be similar to that of the first molar. Or it may have three cusps. In this case, the shape of the floor is triangular or almost in a straight line where the orifices are located along a line with the orifice of the distal canal in the middle between the MB and P canals. More rarely, it may present only two canals, a B and a P, in a single root or two separate roots. Roots may be separated or fused, and a C-shaped anatomy is often observed in the case of fusion. The external silhouette of the access cavity can vary from a trapezoidal shape, with a reduced MD diameter compared to that of a maxillary first molar, to a triangular or elliptical shape with a major diameter in the BL direction.

Maxillary third molar

In terms of external anatomy, the crown may have 4, 3, 2, or 1 cusp characteristic of conoidal elements. It may have 4, 3, 2, or 1 canal. The external shape of the access cavity for an intact third molar may be trapezoidal, triangular, oval, or circular, depending on its anatomical variability.

Mandibular central and lateral incisors

These teeth have the smallest MD diameter compared to the other dental elements. Their MD diameter gradually decreases from the incisal edge to the apical foramen. BL diameters are larger than MD diameters from the cingulum to the apical third.

For this reason, mandibular incisors are not easy teeth to treat. Moreover, they often have two canals, one B and one L. The pulp cavity is fairly wide in the BL direction and narrow in the MD direction. The access cavity is circular or slightly oval along the coronoradicular axis, centered in the MD direction, and located between the incisal edge and the most coronal third of the crown on the lingual side [Figure 2a].

Figure 2.

(a) Micro-CT reconstruction of a mandibular incisor showing intact tooth, traditional endodontic access cavity (TEC), conservative endodontic access cavity (CEC), and RestoAC. Access is shown from sagittal, coronal, and lingual views. The red area represents the tooth structure removed in the TEC group, the green area the tooth structure removed in the CEC group, the blue area the tooth structure removed in the RestoAC group, and the white area represents the restoration in the RestoAC group. (b) Micro-CT reconstruction of a mandibular premolar with one canal showing intact tooth, TEC, CEC, and RestoAC. Access is shown from sagittal, coronal, and axial views. The red area represents the tooth structure removed in the TEC group, the green area the tooth structure removed in the CEC group, the blue area the tooth structure removed in the RestoAC group, and the white area represents the restoration in the RestoAC group. (c) Micro-CT reconstruction of a mandibular premolar with two canals showing an intact tooth, TEC, CEC, and RestoAC. Access is shown from sagittal, coronal, and axial views. The red area represents the tooth structure removed in the TEC group, the green area the tooth structure removed in the CEC group, the blue area the tooth structure removed in the RestoAC group, and the white area represents the restoration in the RestoAC group. (d) Micro-CT reconstruction of a mandibular molar showing intact tooth, TEC, CEC, and RestoAC. Access is shown from sagittal, coronal, and axial views. The red area represents the tooth structure removed in the TEC group, the green area the tooth structure removed in the CEC group, the blue area the tooth structure removed in the RestoAC group, and the white area represents the restoration in the RestoAC group. TEC: Traditional endodontic access cavity, CEC: Conservative endodontic access cavity

Creating such a coronal access cavity has multiple benefits. It allows straight access to the canal, is conservative, minimizes the risk of intraoperative perforation, and, if two canals are present, allows the L canal to be located and shaped relatively easily.

Mandibular canine

The external anatomy of the lower canine is very similar to that of the upper canine. However, the MD and BL diameters are smaller, and the cusp and cingulum are less pronounced. The pulp chamber reproduces the coronal anatomy more represented in its BL than in MD diameter. Although considered a single-rooted, single-canal tooth, between 6% and 24% may present one root and two canals, and between 0% and 5%, two roots and two canals.[97,98,99]

The access cavity will be oval-shaped, like the incisors, and located coronally without necessarily involving the canine margin. The access cavity may have a slightly larger BL diameter if a second canal is suspected.

Mandibular first premolar

It has two cusps, a fairly wide B and a much smaller L. It is considered a single-rooted, single-canal tooth in most cases. However, there are innumerable anatomical variations; in over 20% of cases, it is said to have at least two canals or a C-shaped configuration, with fused roots and a groove generally located mesially. More rarely, it may present two roots with two or three canals. The access cavity is centered in the MD direction and displaced buccally. It is generally circular in shape [Figure 2b]. Naturally, since the access cavity is planned according to the anatomy of the tooth:

1. In the case of two canals, one L and one B, the access cavity should be slightly wider buccally to permit easier management of the lingual canal [Figure 2c]

2. In the case of a C-shaped anatomy, the access cavity external contour will be a little more generous than for a one- or two-canal element.

Mandibular second premolar

The access cavity is circular, very similar to the first premolar, but with less variability than the mandibular first premolar. A particular variant of this dental element is its two-channel configuration with Vertucci’s type 5, in which case the access cavity will be slightly oval.

Mandibular first molar

The mandibular first molar is generally the largest element of the mandible. It has three V-shaped cusps, two L-shaped cusps, and two roots, one mesial and one distal. The pulp chamber is larger in diameter in the MD direction and generally has five horns.

The access cavity design for the first mandibular molar is influenced by the tubercle’s angulation in relation to the root and the pulp chamber’s coronal position. Due to this anatomical variation, the pulp chamber may be more coronal, necessitating a careful approach when designing the access cavity.

In cases where the tubercle has a larger inclination, the traditional straight-line access may not be ideal, and modifications are required to improve visibility and access to all canal orifices. The inclination can also affect the extension of the access cavity, requiring an adjustment in the mesial or distal direction to accommodate the natural anatomy. Therefore, access cavity preparation should be adapted to this anatomical consideration, ensuring that all canals are adequately located while preserving structural integrity. Using magnification and ultrasonic instruments can assist in refining the cavity design, particularly in cases where the pulp chamber is positioned more coronally. The morphology of the mesial root initially contains a single semi-lunar laminar canal, which, with function, generally gives rise to two canals, the MB and ML, largely interconnected by isthmuses and interconnections. The D root most often contains a laminar canal, but the two-canal configuration is by no means an exception.

Occasionally, following the deposition of secondary dentin, a third canal is formed in the mesial root, known as the middle mesial canal. The outer contour of the access cavity of an intact tooth is often trapezoidal in shape, more offset mesially, with a larger BL diameter on the M side than on the D side [Figure 2d]. The outer contour of the access cavity of an intact tooth is often trapezoidal, more offset mesially, with a larger BL diameter on the M side than on the D side [Figure 2d]. Distal extension of the access cavity to allow a complete unfolding of the pulp chamber is unnecessary since access to the distal canal(s) is fairly straight.

Mandibular second molar

The anatomy of the lower second molar is similar to that of the lower first molar, but its dimensions are smaller. It has two buccal cusps, two lingual cusps, and four horns in the pulp chamber. Its best-known anatomical variant is the C-shaped anatomy, characterized by root fusion, a generally lingual groove and at least one CBCT cross-section corresponding to the C1–C5 configuration of Fan’s classification.[100] In most cases, the access cavity is similar to the first molar, but the diameters are reduced. It is often a three-canal tooth, but it can frequently have four or two canals and, more rarely, one canal.

Mandibular third molar

The anatomy of the third molar, like the access cavity, is similar to that of the second molar, but it may present anatomical variations with 5, 4, or 3 cusps and 4, 3, 2, and 1 canals.

CONCLUSION

CEC preparation enhances tooth fracture resistance without compromising. This phase can be carried out either with an ultrasonic insert alone, such as the Start X1, or after first drawing the grooves (pregrooves) with the ultrasonic insert and then, in order to speed up the procedure but still under microscopic control, with a cylindrical burr with an elongated neck, which will work in complete safety by resting on the pregrooves made beforehand by ultrasound.

During this phase, it is essential to avoid intentionally depositing the triangle of dentin often present in the first coronal third of the canal to gain straighter access to the canal. Certainly, this would remove healthy dentin in the pericervical zone, which is fundamental for the structural stability of the dental element.[58,59,60] Although visible on the postoperative radiograph, this action is often underestimated because much of the dentin removed is located buccolingually and is, therefore, invisible radiographically. It is worth remembering that martensitic shaping instruments are safe and that the literature on this subject is unequivocal: There is no additional risk of instrument fracture when preparing a CEC. The clinician provides root canal treatment by employing advanced equipment and adhering to modern anatomical principles. However, mastering this approach requires a steep learning curve, underscoring the need for continuous education and technical expertise in endodontics.[101,102,103,104,105]

Highlights

1. The preparation of the endodontic access cavity is crucial for successful treatment, influencing both positioning and outcome

2. Recent studies emphasize the importance of tissue preservation during access cavity preparation, challenging traditional beliefs

3. Effective access cavity design must balance precision with clinical needs, adapting dynamically to each case and requiring advanced skills.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.