Summary

The prognosis of root filled teeth depends not only on successful root canal treatment but also on having an adequate amount of remaining tooth structure. A successful restoration of the tooth can prevent recontamination and strengthen the tooth in normal function. This chapter discusses the implications of numerous restorative factors on the prognosis of root filled teeth with special emphasis on evidence from clinical studies. The chapter also presents detailed information to enable effective clinical decision making.

21.1 Introduction

Even though developments in Dentistry over the last 50 years have allowed patients to retain severely compromised teeth for significant periods of time, the restoration of root filled teeth remains a challenge that is in constant evolution. Failure of tooth restorations are not uncommon, including loss or failure of direct restorations, post loosening and root fracture, marginal gaps, and loss of retention of single crowns [1–4]. The prognosis of root filled teeth depends not only on the quality of root canal treatment but also on the quality of the coronal restoration [5]. Adequate restorations can improve endodontic outcomes, prevent canal recontamination, and replace missing tooth structure, thereby strengthening the tooth and restoring its normal function and aesthetics [6–8]. On the other hand, substantial loss of tooth structure in root filled teeth diminishes their clinical performance [9, 10]. It is commonly accepted that the preservation of coronal tooth structure and placing high-quality restorations after root canal treatment should be the primary focus while restoring root filled teeth [11]. The objectives of a definitive restoration following root canal treatment are [12]:

• to prevent bacterial reinfection from coronal leakage or saliva into the root canal system.
• to restore function, anatomy, and aesthetics, if applicable.
• to protect the residual tooth structure and the coronal restoration against mechanical failure, including fracture.
• to prevent recurrent caries lesions around temporary restorations, which are susceptible to mechanical breakdown and dissolution in the mouth.
• to ensure healthy marginal periodontal tissues.

21.2 Examination of Root filled Teeth Before Selection of a Treatment Approach

21.2.1 Ferrule

Studies have highlighted the significance of a ferrule for improving the stress-bearing ability of restorations [13, 14]. A ferrule is defined as circumferential parallel dentinal walls extending coronally from the finish line which, after being encircled by a crown, improves stress distribution within the tooth thereby providing a protective effect (ferrule effect) [15–17]. To be effective, it is generally recommended that the ferrule should be 1.5–2.0 mm in height and 1.0 mm in thickness [13, 14, 17]. If the clinical condition does not permit a complete circumferential ferrule, studies have suggested that at least an incomplete ferrule of 2.0 mm should be present on the buccal and lingual walls [17]. In addition, a laboratory study concluded that teeth with a 2 mm-high ferrule were more resistant to fracture than teeth with a 2 mm-high ferrule and absence of the lingual wall and one proximal wall [18].

Whenever an adequate ferrule (circumferential or buccolingual) cannot be obtained, other approaches such as crown lengthening or orthodontic extrusion can be attempted. Studies have indicated that in addition to a 360-degree ferrule, preservation of coronal walls is critical for the long-term survival of tooth restorations [1, 19]. A “wall” is defined as a residual coronal structure of at least 3 mm in height above the gingival level [20]. Results of Finite Element Analysis (FEA) support the retention of 2 mm of remaining coronal dentine as an important factor that positively influences the strain, stress distribution, fracture resistance, and mode of failure of root filled incisors, regardless of the method of crown-root restoration [21]. In laboratory studies, restorations on teeth without a ferrule have been reported to have a high rate of fracture compared to other treatment options [4].

A thicker ferrule has been reported to increase the fracture resistance for cast post and cores when at least 1 mm thick, despite producing more unfavourable failures [22]. Thus, ferrule thickness should be considered when choosing different posts to reduce the occurrence of unfavourable failures. In the absence of a ferrule, the use of a cast post and core results in more favourable failures compared to glass fibre posts [22]. According to another finite element analysis study, the existence of a ferrule also results in less damaging stress distribution to the roots. High levels of tensile stress inside root canals were measured with a cast post and core, which should be avoided to restore weakened roots, mostly in the absence of a ferrule [23]. On the other hand, although the presence of a ferrule and preservation of coronal walls has been strongly advocated, a recent systematic review indicated that the ferrule effect did not appear to significantly reduce the failure rate of fibre post-and-core restorations [24].

21.2.2 Remaining Coronal Walls

Failure risk is significantly higher for root filled teeth when four coronal walls have been lost. At 3 years, a lower fracture resistance was observed for teeth missing all the coronal walls versus those having at least one remaining wall irrespective of the presence or absence of a ferrule [25]. It has been reported that the majority of restoration dislodgements and root fractures occur in teeth where the remaining coronal structure before build-up involved only one residual wall at the most [14]. Post-retained crowns were connected with the greatest favourable outcome in teeth with one to two remaining coronal wall(s), whereas post-free crowns were superior when greater tooth structure was available [4]. A meta-analysis suggested that coronal wall absence might increase the risk of fibre-post (fibre-P) and core-restoration failure [26].

21.2.3 Marginal Ridges

Endodontic access cavity preparation increases cuspal deflection by 2–3 times in teeth with loss of one or both marginal ridges [27]. The strength of teeth is influenced more by the loss of marginal ridge integrity than by the effects of root canal treatment [28]. Endodontic interventions only reduced the stiffness of a tooth by 5% whereas the effect of an occlusal cavity preparation reduced stiffness by 20%. A mesio-occlusal-distal preparation, with loss of two marginal ridges, reduced tooth stiffness by 63%. Therefore, the loss of both marginal ridges with further dentine removal during pulp chamber cavity preparation may cause weakening of teeth and results in increased cuspal deflection during function, leading to a greater incidence of tooth fracture [29, 30]. Clinical data on root filled teeth revealed that no failures occurred clinically over 2 years when both marginal ridges were intact [14]. Preserving only one intact marginal ridge in molars does not fully conserve the strength of adjacent cusps [31].

21.3 Evidence from Clinical Studies Regarding Factors Affecting the Prognosis of Root filled Teeth

21.3.1 Outcome Measures and Clinical Questions Addressed by Clinical Studies

Clinical studies assessing the prognosis of root filled teeth have evaluated two main outcomes: (i) survival of the restoration defined as the absence of catastrophic failure; a failure leading to loss of the restoration or tooth, and (ii) success—defined as the absence of both catastrophic failure (restoration or tooth loss) and noncatastrophic failures.

For this section, a total of 30 studies on this topic were reviewed: 15 randomised clinical trials, 2 nonrandomised clinical trials, 9 systematic reviews (5 of them included a meta-analysis), 2 clinical practice guidelines, and 2 cost-effectiveness analyses. Detailed characteristics of the reviewed studies are listed in Tables 21.1–21.3. The data abstracted from this evidence were organised to answer clinical key questions with different levels of confidence. The Grading of Recommendations, Assessment, Development and Evaluation (GRADE) approach was used to assess the collective quality (certainty) of the evidence pertaining to different key questions and treatment recommendations [32]. The GRADE quality (certainty) of the evidence represents confidence that the treatment effects are appropriate to inform the recommendations and is rated as high, moderate, low, or very low defined as follows:

• High: We are very confident that the true effect lies close to that of the estimate of the effect [32].
• Moderate: We are moderately confident in the effect estimate. The true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different [32].
• Low: Our confidence in the effect estimate is limited: The true effect may be substantially different from the estimate of the effect [32].
• Very low: We have very little confidence in the effect estimate: The true effect is likely to be substantially different from the estimate of effect [32].

21.3.1.1 Clinical Key Questions

Does presence of a ferrule or number of remaining coronal walls improve the survival and/or success of root filled teeth? The clinical literature reveals that the presence of a ferrule alone is not sufficient for good long-term prognosis of root filled teeth (moderate certainty). Retention of coronal walls in addition to the ferrule is important for improving the prognosis (moderate certainty). Although some studies demonstrated that the presence or absence of ferrule did not affect the prognosis of root filled teeth [1, 14, 24], most studies concur that in addition to the ferrule, retention of coronal walls has a significant influence on survival and success [1, 11, 14, 19, 33]. It has been demonstrated that retaining all four coronal walls on root filled teeth was not associated with failure regardless of the restorative procedure; in addition, failures increased as the number of coronal walls decreased [1, 14].

Do crowns improve survival and/or success of root filled teeth? In the short-term (≤5 years), root filled teeth restored with direct restorations compared to crowns exhibit similar clinical performance (low certainty). However, crowns may improve the long-term (10–20 years) prognosis while being cost-effective (very low certainty). Randomised controlled trials demonstrated similar short-term (<5 years) clinical performance when root filled teeth were restored with direct restorations compared to crowns [34, 35]. On the other hand, lower quality studies suggest that crowns may lead to better long-term (10–20 years) survival, success, and cost-effectiveness [36].

Do posts improve the survival and/or success of root filled teeth? Posts improve the survival and success of root filled teeth only when substantial tooth structure is missing (fewer than two remaining walls; lack of ferrule) (moderate certainty). Evidence also suggests that posts improve survival and success of root filled teeth when substantial tooth structure is missing (fewer than two remaining walls; lack of ferrule) [14, 33, 38, 52]. However, posts do not affect the prognosis of root filled teeth when adequate tooth structure is present (two or more walls; adequate ferrule) [19, 33, 38, 39].

If posts are indicated, which post types/materials improve the survival and/or success of root filled teeth? Post material (metal versus fibre) or method of fabrication (prefabricated versus custom) do not influence survival or success of root filled teeth (very low certainty). Prefabricated metal posts are the most cost-effective followed closely by fibre posts and then cast post and core (very low certainty). Active posts and cast post and core may be associated with higher catastrophic failures (very low certainty). Hence, passive prefabricated posts (metal or fibre posts) are recommended due to reduced treatment time and complexity along with associated cost savings (very low certainty). Individual studies reported mixed results regarding the influence of post material (metal versus fibre) on the clinical performance of root filled teeth. Most studies demonstrated that there were no significant differences in outcomes with either material [19, 40–42], while some indicated that fibre posts might result in improved outcomes [2, 43]. Systematic reviews of randomised clinical trials revealed no difference between the post types in the short- to medium-term (6–10 years) but highlighted the need for longer term studies [44, 45]. Similarly, method of post fabrication (prefabricated versus customised) did not have a significant influence on survival or success in most studies [19, 40, 41, 46]. Metal posts were the most cost-effective followed by fibre posts and then cast-posts/cores [47]. Some studies indicated that active metal posts and cast post and cores may be associated with higher incidence of catastrophic failures (root fractures) [2, 48]. Additionally, reduced treatment time and procedural complexity along with associated cost savings may favour the use of passive prefabricated posts over custom cast post and core.

Although formal risk of bias assessment of individual studies was not undertaken, most of the included randomised clinical trials were judged to be high or unclear risk of bias according to the systematic reviews that combined findings from these trials. Most randomised clinical trials lacked adequate randomisation, allocation concealment and blinding, which are considered important methods to minimise sources of bias. Additionally, clinical trials mostly utilised short- (≤5 years) to medium-term (6–10 years) follow-up periods. The limited long-term (>10 years) follow-up evidence was derived from a single randomised clinical trial, three nonrandomised trials, and two cost-effectiveness analyses. Nonrandomised clinical trials are inherently rated lower than randomised trials in the hierarchy of evidence due to the risk of selection, performance, and detection biases. On the other hand, although cost-effectiveness analyses provides practical information for decision-making, they rely completely on indirect comparisons.

Several inconsistencies of effect estimates were noted across the included studies, which made summarising the findings difficult. This could be related to extensive heterogeneity between studies. Studies were heterogeneous regarding baseline clinical characteristics such as different types of teeth (e.g. anterior, premolar, molar; maxillary or mandibular) or differing amounts of tooth structure loss (e.g. number of existing walls; presence/absence of ferrule). Additional heterogeneity was introduced due to different intervention and control groups across studies such as different post systems/materials (e.g. titanium, stainless steel, cobalt chromium, gold; or carbon fibre, glass fibre) or differences in clinical procedures (e.g. different post treatments or different luting cements for posts or crowns). Furthermore, studies assessed outcomes of prognosis differently (restoration success, restoration survival, tooth survival) while utilising different definitions for these outcomes.

21.4 Decision-making for Restoration of Root filled Teeth

Based on the summarised findings, restorative recommendations and good practice statements can be formulated to simplify clinical decision-making. Recommendations are based on the best available evidence and have an associated GRADE quality rating of the collective evidence. Whereas good practice statements are based on indirect evidence (expert opinion, clinical judgment, or patient values and preferences) and do not have an associated quality rating. Restorative recommendations and good practice statements according to various clinical situations (based on the amount of remaining coronal tooth structure) along with applicable quality ratings are presented below:

21.4.1 Root filled Teeth with Minimal Loss of Coronal Structure

Clinical situation 1: Root filled teeth with three/four remaining coronal walls

Restorative recommendation:

• Post is not needed (moderate certainty) [19, 33, 38–40, 49].
• Final restoration with direct resin composite (low certainty) [34, 35, 37, 50, 51].

Good practice statement:

Consider partial coverage if clinically indicated.

Explanation: Possible clinical situations where partial coverage might be appropriate: discoloured anterior teeth; fractured, cracked tooth; or undermined cusps, for instance.

21.4.2 Root filled Teeth with Significant Loss of Coronal Structure

Clinical situation 2: Root filled teeth with two remaining coronal walls

Restorative recommendation:

Final restoration with an indirect restoration (partial or full crown) (very low certainty) [36, 37, 52].

Explanation: Choice of partial versus full coverage restoration will depend on clinical factors such as condition of remaining tooth structure, discolouration, forces on the tooth, and occlusion.

Good practice statement:

Consider use of a prefabricated post if core retention is questionable.

Explanation: After caries excavation and removal of unsupported tooth structure or defective restoration, if it is determined that the remaining tooth structure might not be able to adequately retain the core restoration, then a post should be utilised.

Clinical situation 3: Root filled tooth with one or no remaining coronal wall. Complete or partial ferrule present or achievable through adjunctive procedures.

Restorative recommendation:

• Prefabricated post should be used for core retention (moderate certainty) [1, 14, 33, 37, 38, 40, 52].
• Final restoration with a full crown (very low certainty) [36, 37, 52].
• Extract and consider replacement if prognosis is severely compromised.

Explanation: Extraction and replacement with a removable/fixed prosthesis in the absence of walls and ferrule unless orthodontic extrusion or crown lengthening is performed (moderate certainty) [1, 11, 14, 24].

Good practice statement:

• Consider cuspal coverage resin composite if prognosis of crown is questionable.

Explanation: Cuspal coverage direct resin composite restorations (in addition to a prefabricated post) may be more suitable when it is anticipated that the ability of the tooth-post-crown complex to bear occlusal forces will be significantly compromised after crown preparation procedures (e.g. undermined cusps or marginal ridges).

• b) Maintain the tooth even if prognosis is compromised.

Explanation: Possible clinical situations where the clinician and the patient may decide to maintain the tooth even when the prognosis is compromised might include: tooth with minimal occlusal forces (e.g. mandibular first premolars or incisors), patient’s willingness to save the tooth in spite of the risk of failure, maintaining the tooth for a short amount of time (e.g. to preserve bone for implant procedures or until other definitive treatment options are finalised), and financial constraints regarding replacement options.

21.5 Clinical Considerations for the Management of Root filled Teeth Using Posts

21.5.1 Relevance of Tooth Anatomy

The role of tooth anatomy on root canal disinfection has been demonstrated clearly in clinical and laboratory studies [53, 54]. In order to avoid iatrogenic accidents, a knowledge of root canal anatomy is also relevant when the treatment includes the placement of an intraradicular post.

The first priority for preparation of the post space is the preservation of the root canal dentine [55]. Failure to understand root and root canal morphology during this procedure can lead to mishaps including gouging of the coronal and radicular dentine, stripping of the root canal walls, or root perforation [55]. Whereas some of these iatrogenic errors are not connected to immediate acute symptoms, perforations will be associated with periodontal damage, lateral bone loss, or furcation involvement if they are not managed properly (Figure 21.1). It is known from clinical data that a significant number of iatrogenic events, which include perforations, could be attributed to post space preparation [56]. In addition to the periodontal problems associated with perforations, the excessive loss of root dentine creates another undesirable effect, as it can predispose to the development of vertical root fractures [57]. For all these reasons, it is of utmost importance for the operator to fully understand the anatomy of the root and use adequate post sizes that fit inside the root canal space with minimal dentine removal.

Radiographic examination is essential to determine the anatomical characteristics of the root that will receive the intraradicular post. Because radiographic interpretation is based on two-dimensional images, more than one angulation might be needed to disclose anatomical variants of interest that may be unnoticed with conventional radiography. This includes the recognition of the root shape, root length, presence of invaginations or concavities, presence of canal splitting and root canal curvatures [55] (Figure 21.2). Even with all the advantages of using periapical radiographic views with different angulations, clinicians should be aware that radiographic images can overestimate the thickness of the dentinal walls and cannot be considered accurate when determining this critical factor [58].