Controversies in endodontic access cavity design: A literature review

Access cavity preparation

The access cavity preparation generally refers to the part of the cavity from the occlusal table to the canal orifices. Black's principles of cavity preparation, including outline, convenience, retention, and resistance forms, speculate the basis of TEC. The outline form of the endodontic cavity must be correctly shaped and positioned to establish complete access for instrumentation, from cavosurface margin to apical foramen. Convenience form, as conceived by Black, is a modification of the cavity outline form to establish greater convenience in the placement of intracoronal restorations. Later, removal of the remaining carious dentine and defective restorations in an endodontic cavity preparation is necessary. It must be removed for three reasons:

Another important reason to eliminate undermined and unsupported tooth structure is to evaluate whether the tooth is restorable or not, and also to minimize the possibility of tooth fracture in future. It is imprudent and unlikely that a clinician would leave the structure of the diseased tooth intact to create a textbook access cavity.⁹ In 2010, Clark and Khademi introduced the concept of contracted endodontic access cavity design, in a series of case reports.¹⁰ The basis of a CEC was kept in terms of saving the pericervical dentine (PCD) and leaving small overhangs of the pulp chamber roof behind. The most important tooth structure responsible for long-term survival is considered to be the PCD, the dentine structure located 4 mm below and 4 mm above the alveolar crest¹ (Figure 2), which serves as the neck of the tooth and is responsible for distribution of functional and mechanical stresses inside the tooth.^11,12 More of the occlusal tooth structure can be sacrificed than the cervical tooth structure, as the key pericervical tooth structure should remain as unaltered as possible.¹ A new contracted access cavity, using a different set of burs than those used for TEC, was suggested. In a CEC, measurement reference points may change; for example, in the past, the reference for the mesial canals of mandibular molars was often the corresponding MB cusp. Now it may be found as a reference more to the distal, as it would preserve PCD and overhanging dentine. The simultaneous placement of 4 or 5 gutta-percha points for a cone fit radiograph in the more constricted access may be difficult without trimming to eliminate binding. Clark and Khademi recommended not removing the pulp tissue under the overhang until the obturation was finished; that way the operator only has to clean it once.¹⁰

Another approach to the conservation of tooth structure is the orifice-directed design (TREC) (also known as the ‘truss’ access cavity) in which separate cavities are prepared.

The main objective of these access cavity designs is to preserve strategic dentine (ie to leave a dentine bridge between the two cavities thus prepared).² In TREC, the cavities are prepared over the mesial and distal canals of the mandibular tooth, respectively, guided by computed tomographic images. The pulp chamber roof is maintained beneath the ‘truss’ of the tooth structure, between the mesial and distal cavities.² However, TREC significantly impaired the debridement of the pulp chamber.⁵ Only one case report has been published so far regarding a ‘truss’ type cavity design, however, no follow-up of this case was presented.¹³

Hence, the long-term outcome of this type of treatment is unknown. Another ultra-conservative technique has recently been introduced, which is also known as the ‘ninja’ access cavity design.⁶ An NEC is actually an even smaller access cavity than the one made for a CEC on the occlusal surface, that should enable the clinician to find and access all the orifices of the canals, but there is not sufficient data or literature discussing this type. However, the NEC could jeopardize the complete removal of infected pulp tissue from the pulp chamber and make canal instrumentation more difficult and less safe.

Histological evaluation

In a study conducted on mandibular molars, one type of CEC design (Truss access) was tested, to acertain whether it was able to debride the pulp chamber completely or not, and to evaluate the remaining pulp tissue (RPT) in both experimental group the (TREC) and control group (TEC). Although both groups showed the presence of RPT in the pulp chamber, the amount in the TEC group was significantly lower than that of the TREC group. There was no substantial difference in RPT present in the isthmus (within the canals) between the TEC and TREC groups at any level.² Only the area of the pulp chamber under the truss was assessed, which could serve as a nutritional source for the remaining bacterial biomass, leading to persistent infection. The results of this study showed two important findings: the pulp chamber showed a significantly reduced amount of remaining pulp tissue in TEC compared to TREC access, and there was no difference in the amount of remaining tissue in the root canals, close to and at the isthmus area of the root thirds between the two access cavity designs. In another study, the effect of different access cavity designs (lingual cingulum, lingual conventional and incisal straight-line) (Figure 3) on the ability of endodontics files to plane the root canal walls in maxillary anterior teeth was tested.¹⁴ It was found that instrumentation did not allow the entire root canal wall to be instrumented during the preparation of canals, irrespective of the access cavity design. However, the incisal straight-line access cavity allowed a larger proportion of the root canal walls to be instrumented as compared to the conventional lingual access cavity and the cingulum access cavity.¹⁴ Nevertheless, it is still uncertain what happens to the incisal part of the pulp chamber in incisal straight-line and lingual cingulum access, as no histological evaluation was performed to identify any remaining pulp tissue under the pulp horns. Whether the pulp remnants and possible remaining bacterial biomass would later be troublesome for the patient or not, is still unknown. Further investigations are required to establish the prognosis of root canal treatment in both anterior and posterior teeth with these new designs. Furthermore, no study has been done so far to identify possible remaining pulp tissue in the pulp chamber in other CEC and NEC designs. This holds relevance as the pulp chamber is not completely unroofed and remaining infected pulp tissue under the pulp horns may lead to contamination of the rest of the root canal system and subsequent failure.

Fracture testing

Fracture tests are used primarily to compare the fracture strength or fracture resistance of teeth. Various studies that compared the fracture resistance of endodontically treated teeth with different access cavity designs are listed in Table 1.

It has been shown that conservative endodontic access cavity designs, specifically the ‘ninja’ and ‘truss’ types, are successful in maintaining the pericervical dentine, and hence increasing the fracture resistance of the tooth. However, a group of researchers concluded that it was the loss of the mesial or distal marginal ridge that affected the fracture resistance of endodontically treated teeth rather than the access cavity design itself.⁵ Furthermore, Krishan et al¹⁵ found no advantage of conservative access cavity design over traditional design in terms of fracture resistance in anterior teeth. Another study compared teeth with different access cavity designs and sound teeth. It was found that unrestorable type fractures after fracture testing were noted considerably more often than the restorable type in access teeth, irrespective of the access cavity design.¹⁶ Besides, a huge limitation to the findings of the previous studies is that all of them were performed in vitro on almost non-carious teeth, and the age of the patients, from whom the teeth were extracted, was not recorded. As with the increasing age of the patient, the brittleness and hence the fracture ability of a tooth increases,¹⁷ it is necessary that the age be considered and mentioned. Furthermore, all these studies used static loading to determine the fracture strength. Ideally, cyclic loading, as compared to static loading, would correspond better to the natural loading during chewing. Additionally, there was no simulation of periodontal ligament in most of these studies. Although this simulation is necessary, a standardized periodontal ligament simulation model has not yet been introduced.¹⁸

The challenges and changes

Bio-minimalism recognizes that the pericervical dentinal (PCD) zone is essential to support the residual coronal tooth structure during functional loading stress, and acts ostensibly to minimize cuspal flexion during mastication.⁹ However, the fracture strength of endodontically treated teeth could also be affected by insufficient dental residue, due to the caries causing the loss of one or more dentinal walls.¹⁹ Fortunately, technological advances in armamentarium has brought the objectives of minimally invasive endodontics closer. Cone beam computed tomography aids the clinician in avoiding the removal of excessive hard tissue by allowing assessment of the angulation and orientation of the root canals.⁹ Despite the limited clinical evidence for the use of contracted access cavity designs, the growing interest and technological advances in image-guided endodontics can prove to be a paradigm shift in root canal treatment.² Besides, we now have files and finishers that adjust to the original canal shape, scrape biofilm in a way similar to periodontal scalers and make it easier for irrigants to act on exposed microbes.⁹ Furthermore, with the advent and use of the dental operating microscope, root canals can be detected and cleaned optimally, even through minimally invasive endodontic access cavities. However, currently there is little evidence (mainly from in vitro studies) available to prove that conservative and ultra-conservative access cavity designs are advantageous over their traditional counterpart, especially when the need to clean the root canal adequately remains an overarching objective of non-surgical endodontic treatment.² Besides, no clinical trials have been reported so far on patients with these newly introduced contracted cavity designs. Additionally, owing to the fact that pulpal remnants were seen in the pulp chamber while examining the histological sections of the ‘truss’ type CEC,² the long-term success rate is unknown. Perhaps case selection (based on multifactorial evaluation, including the condition of the pulp, vital or necrotic), level of difficulty (presence of calcifications, curvatures, etc) and accessibility, in addition to the equipment and facilities available, would serve as a reason in which conservative access cavities could be prepared for certain cases without compromising the ability to locate all canals and the efficiency of subsequent root canal treatment procedures.²⁰

Conclusion

Currently, there is no conclusive evidence to suggest that conservative or ultra-conservative access cavity design can help retain endodontically treated teeth longer by increasing their fracture resistance. Furthermore, there is no conclusive evidence that the biological principles (complete disinfection) of endodontic treatment can be adequately achieved with these conservative access cavity designs. Hence, conservative or ultra-conservative access cavity designs should be used with extreme caution. Perhaps the objective of conservative cavity preparation should be avoided from ‘removing the smallest possible tooth structure’ to ‘removing as little as necessary’. Besides, to validate these newly introduced access cavity designs, more research needs to be conducted, as the studies remain few and fragmentary.