Research that matters: debunking the myth of the “fracture resistance” of root filled teeth

Abstract

There is no topic within Endodontics that causes more controversy and interest than the mechanical failure of root filled teeth. Although such failures present in scenarios ranging from cuspal fractures to vertical root fractures, all involve the development of cracks (Rivera & Walton 2007). As a result of microbial contamination, these fracture lines can trigger the development of endodontic symptoms or periodontal bone loss (Ricucci et al. 2015). In some cases, the outcome of many structural failures are catastrophic and unrestorable leading to the inevitable extraction of the root filled teeth (PradeepKumar et al. 2016). The term “fracture resistance” of root filled teeth is often used in endodontic research; however, in materials science or engineering, the term “fracture resistance” has a specific meaning, that is: the increase in fracture toughness with crack extension, which is a material property. What dental researchers do most often without really understanding the difference is to measure the “load capacity” of the restored tooth, which is a property of structures consisting of components made of different materials. As the reader can acknowledge, there are currently no standards for testing the “load capacity” of teeth that have undergone root canal treatment. Many researchers use static loading to prove or disapprove various hypotheses ranging from the effect of different intracanal antimicrobials (Andreasen et al. 2002, Dotto et al. 2020) to the effect of various access cavity designs (Rover et al. 2017, 2020, Sabeti et al. 2018) on the mechanical failure of teeth. However, it must be emphasized that these tests measure load capacity and not fracture resistance. Although some information might be obtained using this approach, many of these studies lack the fundamental biomechanical understanding to support their results. More importantly, the fact that only very limited information can be obtained from a static fracture test is rarely acknowledged, with the consequence that readers are often misled about the results and implications of a study. During the last 40 years, Dentistry has progressively moved from the use of ductile materials (e.g. gold) to the use of brittle materials that rely on adhesion to restore teeth. A similar trend occurred in high-temperature engineering when engineers switched from metals to ceramics, which needed a fundamental change in the methods for structural design against failure. Therefore, similar engineering approaches should be adopted in Dentistry during the design and testing of dental restorations. Even though the restorative sciences started to move from static loading to more clinically relevant methods more than 20 years ago (Braemet al. 1994), clearly there is a lack of a consensus amongst endodontic researchers on how the root filled tooth should be tested mechanically. The first step towards improving research and avoiding confirmatory bias in this area is to understand the fundamental nature of failure of teeth and restorations in service. With the exception of traumatic fractures, there is substantial evidence that cracks and subsequent modes of tooth failure (cracked tooth, cuspal fracture, split tooth, vertical root fracture) develop in root filled teeth as a result of cyclic loading in the form of mastication over a period of time (Mireku et al. 2010, Kishen 2015, Pradeepkumar et al. 2016, Arola 2017). By definition, fatigue failure occurs due to the application of a fluctuating stress that is much lower than that required to cause a catastrophic failure during a static test with monotonically increasing stress (Drummond 2008, Campbell 2008, Taha et al. 2014, Arola 2017). In engineering, approximately 90% of failures are caused by cyclic fatigue (Campbell 2008); the remaining 10% are early failures due to inadequate designs, manufacturing faults or accidental loads. The cyclic nature of chewing and the delayed failure of restorations after a phase of oral function indicates that a comprehension of fatigue is essential in ensuring the longevity of the restored tooth (Arola 2017). The time taken or the number of cycles (N) required to cause failure reduces exponentially with the stress amplitude (σ), i.e. , where σ0 is the static fracture strength and α is the fatigue life reduction factor. Thus, the load and the number of cycles are equally important in a cyclic fatigue experiment. This leads to the most important concept or tool in analyzing fatigue failure, the stress-life curve; see Figure 1. Published endodontic experiments using static loading in these regards can only provide the critical stress amplitude for one cycle, which is unlikely to be adequate for long-term failure prediction. A restored tooth consists of several different tissues, materials and their interfaces, each with a different stress-life curve. Further, under masticatory loading, each region is subjected to a different stress amplitude. The most vulnerable region is not necessarily the most highly stressed, but it is the one with the shortest life expectancy. The best restorative design is the one in which all components are subjected to their optimum individual stress level that will give them approximately the same time to failure. Teeth and many restorative materials are relatively brittle in nature, which means that they are stronger in compression than in tension, and they may demonstrate a large variation in their fracture strength and service life (Kinney et al. 2003, Park et al. 2008). Even highly standardized samples in terms of materials and dimensions (such as CAD/CAM restorations) do not all fail at the same load or the same time when subjected to the same mechanical test. The mechanics of materials theory states that brittle materials (such as enamel and dentine) fail because of the unstable extension of pre-existing flaws that will have a range of different sizes and orientations within different samples. Interestingly, a specimen with the largest flaw is not necessarily the weakest if that flaw is located in a region or oriented in a direction that is not highly stressed (Fok & Chew, 2020). Crack size and orientation can occur randomly, which is why results from predictions for dental materials or restored teeth need to be expressed in terms of failure probability. Thus, instead of simply designing a restoration that can last for a certain period of time, which will contain a large uncertainty, its probability of failure as a function of time should be specified, which is the most clinically significant variable and is compatible with the results from most clinical research on failure of dental restorations. For analysis or design purposes, the weakest-link theory coupled with Weibull’s power function can be used. If a restoration can be repaired or a tooth restored, a more relaxed failure probability maybe acceptable. However, because the catastrophic failure of a root filled tooth could lead to tooth loss, a stricter requirement may well be necessary, for example a failure probability of less than 5% within the first 5 years of service. Environmental effects have rarely been taken into consideration when testing for tooth and restoration failure in the past. Dental materials and interfaces can degrade over time through corrosion, wear and hydrolysis. Therefore, mechanical properties, including the stress-life curve, are not constant over time. A classic example in engineering terms is “corrosive fatigue”. By definition, it is the mechanical degradation of a material under the combined action of corrosion and cyclic loading. (See Fig. 2). A more general term is “environmental stress cracking” or “environmentally assisted fatigue”. In the endodontic field, restorative materials and tooth tissues interact within a complex environment that includes chemical (pH variations), physical (cyclic fatigue, parafunction) and biological challenges such as biofilm colonization (Zhang et al. 2020). Not only are the mechanical loads already degrading the materials and their interfaces, but additional biological and chemical challenges accelerate the degradation (Carrera et al. 2017). For example, a composite-dentine interface in a root filled tooth subjected to low pH and enzymatic attack due to plaque accumulation can fail more rapidly compared to the same interface working in an inert environment such as air (Orrego et al. 2017). Whilst these important factors that contribute to the degradation of restorative materials have been evaluated separately, they need to be considered simultaneously to assess any synergistic effects in future studies (Orrego et al. 2017). These variables can be considered relevant when the root filled tooth is restored using direct resin composite restorations because the degradation and debonding of the interface increases cuspal flexure leaving the root filled tooth vulnerable to the occurrence of fracture events (Taha et al. 2014). It is the opinion of the authors that researchers who are seeking to assess the effect of different variables on the service life of root filled teeth should take the following topics into consideration: (1) clinically, most failures are caused by cyclic fatigue with a subcritical load which is much lower than the load capacity. Thus, authors must use cyclic fatigue instead of static loading; (2) the load and the number of cycles are equally important; (3) authors should report the probability of failure as a function of time; and (4) materials should be ideally aged before or during testing. The application of these steps will help the endodontic community to properly evaluate the long-term effect of root canal irrigants and medicaments, microcracks generated by canal instrumentation, the effect of minimally invasive access cavity designs and restorative procedures on the expected service life of root filled teeth. They can therefore provide a more representative methodology for studying the influencing factors, detection and treatment of cracked teeth. In this context, recommendations for restoration designs must not be based solely on tests performed using static load. This test is no longer acceptable. Knowledge of these topics is also essential when re-evaluating and assessing the validity of previous study designs.

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