Endodontic access cavities: science versus trends

Dr. Chafic Safi discusses how the design of access cavities can affect root canal treatment

Introduction

The prevention and treatment of apical periodontitis has always been paramount for any clinician performing a root canal treatment. By prevention, the clinician aims at stopping an ingress of microbes into the root canal system before affecting the periapical tissues. Once these microbes colonize the root canal system and reach a certain critical concentration, periapical tissues develop a radiolucency, a sign of endodontic disease. The removal of these microbes and lowering their count restores the health of the diseased tissues. In other words, the aim of a root canal treatment is microbial control.¹

This control is achieved through a series of steps involving the chemo-mechanical debridement and disinfection of the root canal system. Mechanically, root canal systems are instrumented with files up to a certain diameter.^2,3,4 The resulting space is chemically disinfected by the action of irrigation solutions and then filled and sealed using various available techniques.^4,5,6 Finally, a proper coronal restoration is placed to ensure a fluid tight seal as well as to consolidate the tooth.^7,8 The prognosis of such a goal has been reported to be in the range of 90%-96%. ^9,10

In the recent decade, technological advancements have simplified root canal treatments. The advent and accessibility of magnification and illumination, as well as cone beam computed tomography (CBCT), and other techniques made it possible to localize and visualize the pulpal space and canal orifices with more ease. This resulted in a shift in the design of access cavities, going from traditional cavities to less invasive ones such as conservative, ultra conservative (ninja), truss, caries-oriented, and restorative-oriented access cavities.

Endodontic access cavities: the science

The access cavity is the first clinical step in a root canal treatment. Its design and execution can dictate the healing or the persistence of apical periodontitis. The purpose of an access cavity is to offer the clinician a proper entry into the pulpal chamber, the canal orifices, and into the canals all the way to the working length. It should allow the easy way-in and way-out of all the mechanical and chemical armamentarium during a root canal treatment.¹¹

Traditionally, the endodontic access cavity was designed in a way that aimed to remove any obstruction in the way of reaching the canal orifices. This included the complete unroofing of the chamber, exposing all pulp horns, and obtaining a straight-line access to the apex — avoiding the need to alter visual angulation. This design also aimed at reducing the likelihood of iatrogenic errors.

There are various factors that influence the size of such cavities, notably the anatomical location of the orifices and visualizing these orifices without tilting the mirror.¹² This results in an access cavity that is widest occlusally and smallest at the level of the orifices.^12,13 Hence the drawback of such access cavity design is that it requires the removal of healthy tooth structure, specifically the pericervical dentin — the dentin present 4 mm above and 6 mm below the crestal bone which helps to distribute any forces placed on the tooth (Figure 1).¹⁴ This access design, known as Traditional Endodontic Cavity (TEC) is presently being questioned, giving rise to various access designs emphasizing the preservation of this pericervical dentin.^14,15

Endodontic access cavities: The trends

The increasing popularity of magnification, illumination, and CBCT technology, along with the goal to preserve pericervical dentin, gave rise to a series of new endodontic access cavities. These include conservative, ultra conservative (ninja), truss, guided, caries-oriented, and restorative access cavities.

Conservative Endodontic Cavity (Figure 2A)

The Conservative Endodontic Cavity (CEC) is a first step towards a less invasive access cavity while executing a root canal treatment. It is based on the partial deroofing of the pulpal chamber while preserving pulpal horns, resulting in a slight occlusal convergence. This elimination of straight-line access makes orifices visible one at a time using different visual angulation.¹⁶

It has been shown that CEC might increase the strength of endodontically treated teeth especially in mandibular premolars and molars.¹⁷ However, there are concerns about the ability of effective chemo-mechanical debridement and disinfection of the root canal system as well as procedural errors due to reduced direct vision.¹⁸

Ultraconservative Endodontic Cavity (Ninja) (Figure 2B)

The Ultraconservative Endodontic Cavity (UEC), also known as Ninja access, is a more minimalistic version of the CEC. It is performed by creating a cavity in the occlusal surface starting from the central fossa or incisal edge and reaching the pulpal floor with an extremely conservative deroofing of the chamber and preservation of the pulp horns. No further extension is made.15 This results in a super convergent cavity and the preservation of not only percervical dentin but also occlusal enamel. This ultraconservative shape makes it more challenging to locate, clean, disinfect, and obturate the root canal system. It also increases the risks of missed canals which can then lead to failure of the treatment. The increased mechanical stability and increased long-term survival claimed to be attributes to such cavity design are limited and debatable.17 There is also increased treatment time to be considered.

Truss Access Cavity (Figure 2C)

In the discussion of smaller access cavities with the aim of preserving pericervical dentin, we include the Truss Access Cavity. This design yields separate access cavities on the occlusal surface, aligned with the corresponding canals, to expose the orifices and leaving the truss of dentin intact in between.¹⁹

On mandibular molars, this yields two separate cavities, one mesial and one distal. On maxillary molars, the mesiobuccal and distobuccal orifices are accessed through one opening and the palatal through a separate opening. One of the major challenges of such design is inaccuracy which may lead to perforations and other errors.¹⁵

In a study comparing CEC to truss cavities, there was no difference in fracture resistance between the two. Considering the higher risk and difficulty of the truss access, it is advisable to opt for the conservative access.²⁰

There is a major lack of evidence to support the use of truss access cavities and whether they can result in effective tissue cleaning.

Guided Access Cavity (Figure 3)

The advancements in CBCT and in imaging software make it possible for a clinician to have an access cavity guide. Similar to a surgical guide in implant dentistry, this access guide/stent is placed in the patient’s mouth and allows for guided dentin removal. This allows the clinician to plan their access cavity in a way that conserves the dentin and saves time. This access cavity design is mainly used in obliterated root canal systems to reduce the risks of canal deviation and perforation. However, the lack of accuracy of the stent as well as its cumbersomeness in posterior regions tend to make this kind of access cavity less popular.^{15, 21}

Caries Oriented and Restorative-Oriented Access Cavity (Figures 2D and 2E)

Remaining in the quest of maximizing tooth structure to reduce post-endodontic fractures, comes the caries and restorative-oriented access cavities. The cavities are done through the caries or through an existing restoration, allowing this to dictate the overall design and preserve sound tooth structure.19 One main disadvantage of these designs is the increased stress and strain on instruments, and hence an increased risk of instrument separation. Due to the restricted access, there is a debate that appropriate chemo-mechanical debridement is compromised, therefore increasing the risk of post treatment disease.¹⁵

“The increasing popularity of magnification, illumination, and CBCT technology, along with the goal to preserve pericervical dentin, gave rise to a series of new endodontic access cavities.”

Science versus trends

The transformational shift into more restricted endodontic access cavities with the claim of stronger teeth was investigated by many, especially TEC versus CEC and UEC.

In a study comparing TEC versus CEC and UEC, extracted premolars and molars with TEC showed lower fracture strength than the ones prepared with CEC or UEC. The ultraconservative “ninja” endodontic cavity access did not increase the fracture strength of teeth compared with the ones prepared with CEC.¹⁵ It seems consistent in many other studies that UEC does not offer an increased resistance to fracture compared to TEC or CEC. Moreover, there is evidence that UEC makes cleaning procedures more difficult, increasing the total time required for a root canal treatment.²⁰

The effect of CEC is, however, not consistent in the literature, with systematic reviews showing no evidence that supports the use of CEC over TEC for the increase of fracture resistance in human teeth.^20,22 It should be mentioned, however, that TEC does sacrifice more percervical dentin, but it is not clear if that can impact the fracture resistance of endodontically treated teeth.

However, not only did many studies’ results show no benefits associated with CECs, but some also present evidence that this access modality resulted in less root canal detection, less preservation of the original canal anatomy particularly at the apical level, and more complications regarding the restoration.^16,23,24

As for guided access cavities and caries and restoration-oriented cavities, there are only case reports and case series pertaining to them, hence questioning the validity of their clinical implementation.

Lastly, what seems important in terms of fracture resistance is not the access cavity design but rather the number of walls remaining at time of access. It has been shown that molars with two remaining walls are significantly less resistant to fracture compared to four- and three-walled cavities, regardless of the cavity design.²⁰ This of course, raises the importance of post-endodontic cuspal coverage.

Conclusion

The wave of technological advancements that the field of endodontics has experienced in the last decade has transformed the way a root canal treatment is executed. However, the main purpose remains the same — microbial control. This control relies heavily on the action of chemo-mechanically debriding the root canal system and sealing it afterwards. This in turn relies on having an adequate access cavity opening. The traditional access opening has been redesigned into more restricted preparations with the claim of better preservation of tooth structure. However, at the present time, there is not enough supporting evidence for the incorporation of such cavities into clinical practice, and clinicians must remain cautious and base themselves on current scientific evidence.