Introduction

Vertical root fracture of endodontically treated teeth is a prevalent issue. One study looked at the reasons of non-vital teeth extractions suggested that 28% of the extracted teeth had a vertical root fracture versus 40% had a periodontal disease. However, the VRF percentage might be under-reported in that study because VRF might lead to periodontal diseases if not detected earlier.1

Any functional tooth is constantly under a masticatory force which periodically motivates scientists to evaluate the fracture resistance of certain dental materials under specific conditions. This fracture resistance can be defined as the ability of a tooth structure to withstand the masticatory force without being cracked or fractured. There are various factors that might influence the fracture resistance of any tooth significantly. These factors include cavities, trauma, erosion, aging, malocclusion, accidents, and caries. In most cases, endodontically-treated teeth are more fragile due to non-conservative endodontic access cavity preparation, and removal of the arched roof of pulp chamber.

Generally, root canal treatment can reduce fracture resistance of the non-vital teeth. Randow and Glantz in their published study suggested that removing vital pulp leads to loss of protective feedback mechanism, which may contribute to tooth fracture.2 Furthermore, excessive use of irrigant solutions during conventional root canal treatment such, ethylenediaminetetraacetic acid (EDTA), and sodium hypochlorite might alter the microstructure of the root canal dentin which consequently makes the tooth prone to fracture.3 Also, it was reported that the loss of dentine and other significant anatomic structures such as cusps, ridges, and roof of the pulp chamber may result in tooth fracture even after the definitive restoration is placed.4 It was reported that access cavity preparation reduces tooth stiffness only by 5%, while restorative procedures are the main reason for the significant reduction in tooth stiffness.5 Ernest et al study also pointed out that the loss of marginal ridges significantly reduces tooth stiffness by 63%.5 Additionally, it reported that the structural integrity of the tooth is influenced by the extension of the carious lesion and later by the boundary of the access cavity, which is designed to eradicate all bacteria and undermined enamel.5 This loss in tooth structure certainly put the endodontically treated tooth under a high risk of fracture if not restored properly and in timely manner. Because posterior endodontic treated teeth (ETT) are usually under a constant occlusal force, these teeth are at high risk of fracture, especially if this destructive occlusal force exceeds the elastic limit of the enamel and dentin.

Cuspal coverage of posterior teeth after root canal treatment is essential to prevent cracks and fracture. One study reported that non-vital endodontically treated posterior teeth without cuspal coverage are lost at six times higher rate than teeth with complete cuspal coverage.6

It is still debatable which restoration might provide ETT with prolonged survival rate. Indirect adhesive restorations might provide better longevity for the endodontically treated teeth when compared to direct restorations. It was reported that amalgam, bonded composite, glass ionomer cement can be used to restore the integrity of the teeth, but under functional load, these teeth are more prone to fracture.5,7 In addition, polymerization shrinkage still appears to be a significant drawback of resin based direct restorations despite the improvement of their mechanical properties.8 Moreover, one study reported that restoring extensive coronally damaged tooth with direct restorations such as composite and amalgam, could only last for 5 years, which is considered a short period of time.9 On the other hand, better long-term survival rate, 10 years, was reported when endodontically treated teeth was restored by indirect restorations.9 In the same context, a comparative study published by Al-Dabbagh et al suggested a higher success rate and survival rate of indirect restorations especially conventional crowns when compared to endo-crowns.10 Additionally, a superior fracture resistance was noted when fibre post was followed by an indirect restoration when compared to direct ones. Chotvorrarak et al, found higher survival rate of endodontically treated molars restored with fibre post and crown, compared to those restored with fibre post and direct composite build up.11 Also, Full coverage indirect restoration can prolong the survival of endodontically treated teeth. A study conducted by Suksaphar et al showed a higher survival rate of endodontically treated teeth restored with full coverage crown (95.1%) compared to those restored with direct composites (77.0%).12 In contrast, Durre Sadaf et al, reported a higher extraction rate by 2.05 times for endodontically treated teeth which received crowns more likely than those in which only a composite build-up was performed.13

Despite the fact that many studies favour indirect restorations over direct ones, deciding on the best indirect restoration is still debatable. Inlay and Onlay restorations have many advantages over full-coverage crowns, which include conserving sound tooth structure, allowing cementation without hydrolytic behaviour, better marginal placement and visualization, and providing practical maintenance of oral hygiene and periodontium. One of the most common types of indirect restorations of ETT is ceramic inlays. However, still there some drawback associated with these restorations including fracture susceptibility and abrasive to the opposing natural teeth. Thus, the launch of the new computer aided design and computer-aided manufacturing (CAD/CAM) allowed clinician to fabricate various materials into indirect restorations. Full coverage indirect restoration can still prolong the survival of endodontically treated teeth. A study conducted by Suksaphar et al showed a higher survival rate of endodontically treated teeth restored with full coverage crown (95.1%) compared to those restored with direct composites (77.0%).12 Controversy exists regarding the most appropriate indirect restoration for endodontically treated teeth. A study conducted by Rayyan et al, who reported superior mechanical properties and strength of endo-crowns when compared with post and core retained conventional crowns with and without a ferrule. Nevertheless, Guo et al, found no differences in the fracture mode and survival rate of endo-crowns in comparison with glass fibre post and core retained conventional crown.14

One of the most important factors to consider when it comes to restoring ETT is the fracture resistance of the restoration. To the best knowledge of the authors of the present paper, this is the first paper that summarizes the fracture resistance of various direct and indirect restorations of ETT. This paper might provide clinician with good insight to help them on their clinical decision during ETT restoration.

Method

A survey of the literature was conducted to identify the main articles that sought to explain the fracture resistance of various restorations of endodontically treated teeth. The research was carried out in PubMed searching engine utilizing the keywords indirect restorations, direct restorations, fracture resistance, Onlay, Inlay, Endocrowns, and endodontically treated teeth. The eligibility criteria were: complete articles published in the English language, experimental studies, clinical studies and reviews that were published between the year of 2000 to the year of 2022. Exclusion criteria were given outside of English and unpublished, conference articles, and letters to the editor. Two reviewers analysed all titles and abstracts of the articles found, independently and in duplicate. Articles that did not meet the inclusion criteria were excluded. In case of disagreement between reviewers, it was resolved through debate, aiming to seek the best evidence related fracture resistance of various restorations of endodontically treated teeth The present study did not intend to be a systematic review but a narrative review, summarizing the field of restorations of endodontically treated teeth, and providing an updated overview on the subject. The narrative review was the model chosen because the theme is wide ranging, heterogeneous data not conducive to meta-analysis.

Results

Twenty-one articles were included in the study that met the inclusion criteria. Indirect restorations such as Onlay, Inlay, Overlay, and Endocrown appears to have superior fracture resistance when compared to direct restorations such as amalgam and composite. The results of the in-vitro studies are not standardized and varies depending on the type of the tooth being tested, the angulation of the tooth during the test, using lining material underneath the tooth to mimic the periodontal ligament effect, using artificial saliva, and thermocycling of the samples. Losing the marginal ridge, type of the luting cement, and involvement of the functional cusps, appear to be significant factors that influence the clinician decision on how to restore endodontically treated teeth.

Direct and indirect composite resin restorations

Losing parts of tooth structures is considered to be one of the most important factors that might influence the fracture resistance of tooth structure after receiving root canal treatment. An in-vitro study conducted by Plotino et al compared the fracture resistance of different access cavity designs.15 The results of Plotino et al study showed that losing the marginal ridge was a crucial factor that significantly reduced the fracture resistance despite the cavity design.15 However, one of the limitations presented in Plotino et al study was that the cavities were not restored prior to subjecting teeth to fracture.15 Plotino et al study confirmed the finding of Reeh et al classic study which suggested that MOD cavity preparation reduces cuspal rigidity by 63%, and losing one marginal ridge reduces tooth rigidity by 46%.5 Restoring large cavities with direct or indirect composite resin restorations has some drawbacks. There is a study showed that composite resin restoration, whether by direct or indirect approach, did not improve the fracture resistance of extensively compromised teeth.16 Plantino in his in vitro study showed that both direct and indirect composite restorations displayed a reduction in fracture resistance by 42% and 44%, respectively, compared to intact molars.16 Due to these drawbacks, many studies came after in an attempt to improve the mechanical properties of direct or indirect composite resin restorations. One study published by Daher et al showed that molars restored by direct nanohybrid composite with fiber-reinforcing rings warped in an “X” shape around the remaining buccal and lingual walls of molars before placing the restorations present comparable fracture resistance and less catastrophic failures than CAD/CAM resin composite Inlays and Onlays.17 Furthermore, endodontically treated molars restored with a composite restoration reinforced by a horizontal fiberglass post have fracture resistance similar to molars restored CAD-CAM ceramic Onlays.18 On different teeth than molars, studies have shown that premolars with conservative cavities and restored by direct composite resin exhibit resistance to fracture similar to feldspathic ceramic inlays because the quantity of residual tooth structure plays a significant role in fracture resistance.19

Inlay Ceramic restorations

There are few numbers of studies that looked into the fracture resistance of endodontically treated teeth with inlay restorations. For instance, an in-vitro study conducted by Seow et al, pointed at the limitation of inlays as their results suggested that Intra-coronal inlay restores only 75% of the strength of sound premolars.20 Furthermore, an important finding in Seow et al was that palatal cusp coverage can improve the biomechanical properties of inlay restoration in maxillary premolars after receiving root canal treatment.20 Additionally, no improvement in teeth fracture resistance was noted with bonded CAD/CAM ceramic inlays. A study published by Mergulhão et al showed that, when compared to sound premolars, bonded CAD/CAM ceramic inlays were associated with severe fractures (type 2 and type 3, which are fractures below cemento-enamel junction of palatal cusp; and fractures of palatal cusp and central portion of the tooth exposing the root canal cavity, respectively as defined by Mergulhão in his paper).21 On a good note, inlay restorations demonstrated superior properties when compared to direct restorations such as amalgam and composite, especially when the marginal ridge is lost. In an in-vitro study conducted by Cobankara et al, it was shown that in endodontically treated molars, indirect ceramic inlays have higher fracture resistance and lower unrestorable fracture incidents than conventional MOD amalgam and resin composite.22 However, molars with conservative access cavity and intact marginal ridges restored with amalgam, direct composite resin, or crown have higher fracture resistance than teeth restored with indirect inlays, which required further tooth preparation to eliminate the undesirable undercuts.23 Interestingly, using composite resin reinforced with polyethylene fibres as a base material under MOD ceramic inlays in endodontically treated teeth could result in more favourable failure modes.24

Onlay and overlay ceramic restorations

Cuspal coverage raises the fracture resistance of endodontically treated teeth as suggested by Seow et al study which reported that all-ceramic onlays or inlays with palatal cusp coverage are the best for restoring endodontically treated maxillary premolars.20 Another in vitro published by Gupta et al compared between the fracture resistance of full coverage crown and partial porcelain onlays.25 Gupta et al study suggested that partial porcelain onlays are more conservative approach for restoring endodontically treated premolars, however, if the functional cusp is involved, the full coverage crown appears to be the best treatment of choice in premolars as they are exposed to more lateral destructive force compared to molars.25 Additionally, according to in vitro study by Bitter et al on premolars, it was revealed that in MOD cavities with remaining palatal and buccal wall thickness of 2 mm should restored with onlays with cuspal coverage to improve the fracture resistance.26 Another study by YU et al suggested that lithium disilicate CAD/CAM onlays could be more reliable than crowns for the reinforcement of root canal treated premolars with MOD cavities as full coverage crowns are associated with more severe fractures.27

In another context, one study showed the possibility of using thick overlays (2.5-3.5 mm) to restore badly decayed teeth in a patient with high load requirements.28 Another study reported that fracture strength of all composite resin overlays is higher than the likely bite forces, therefore, it could be an acceptable conservative approach to restore endodontically treated molars. However, teeth were not exposed to thermocycling fatigue test in the aforementioned study, which put more stress on the adhesive bond.29

Endo-crown restoration

Many studies compared the fracture resistance of Endo-crowns to other types of indirect restorations such as inlays and onlays. Those studies are in agreement with Kassis et al study, which after thermocycling all samples showed that Endo-crowns have superior fracture resistance when compared to inlays restorations.30 On contrast, the results of Krance et al in vitro study suggests that when suitable tooth structure and sufficient interact restorative space exists, endodontically treated molars restored with lithium disilicate complete crowns based on preparations with amalgam core foundations containing 1 mm and 2 mm of dentin axial wall height could serve as a suitable restorative option that may provide more recoverable failure modes than endo-crown restorations.31

The effect of cement type on fracture resistance

The material used to make the restoration and the cement type could be considered as crucial factors that might influence the fracture resistance. One study published by Ortega et al found that using TargisTM ceromer inlays with elastic moduli closer to dentine and Enforce cement with low elastic moduli are the best combinations between cement and restorative material.32 Moreover, another study by showed that MOD resin inlays luted with conventional 3MTM RelyXTM -ARC- could maintain cuspal deflection stability and showed higher fracture resistance than inlays luted with the other self-adhesive cements 3MTM RelyXTM Unicem2Clicker, Maxcem ElitTM, Maxim SeTTM, due to the fact that the application of an adhesive system on the tooth substrate before the resin cement allows better adhesion at the tooth/restoration interface.33 Similarly, another study showed that premolars restored with cast Onlays cemented with zinc phosphate cement have the highest fracture resistance than cast inlay restorations luted by PanaviaTM Ex resin cement.34

Conclusion

This narrative review concludes that within the limitation of the published in vitro studies, losing tooth structure especially marginal ridges could be considered as the most determent factor that influence the clinician decision on how to restore endodontically treated teeth. The knowledge published in the literature is very limited as there is standardization between the in vitro studies which led to wide range of findings. These variations include type of tested tooth (majority premolars), testing fracture resistance at variable angles, using a lining material underneath the tooth structure to mimic the effect of periodontal ligaments, using artificial saliva to mimic the oral cavity, and finally thermocycling which is not used in all studies. Well controlled clinical studies are needed to reliably evaluate the fracture resistance of various direct and indirect restorations of endodontically treated teeth.