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20 Jan 2017

By Maciej Zarow, Walter Devoto et al

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A clear guideline to restorative therapy for premolar and molar teeth

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Restoration of endodontically treated teeth represents a challenge for the practitioner, because it requires profound knowledge not only of restorative dentistry, but of endodontics and periodontics as well. The main reason for loss of endodontically treated teeth is their bad reconstruction. Unfortunately, the clinical concepts regarding the restoration of root canal treated teeth are not clear and often based on conjecture due to a lack of sound empirical data. The diversity of published opinions is confusing and may lead to less-than-optimal treatment selections. There is also an emerging debate about whether a post is necessary. The purpose of the present article is to organise this topic in evidence-based principles and provide dental practitioners with clear guidelines about restorative therapy for premolar and molar teeth.

 

Download the free eBook "Reconstruction of endodontically treated posterior teeth - with or without post?"

 

Current status of endodontically treated teeth

Root canal treated teeth are supposed to be structurally different from unrestored vital teeth. It has been suggested that endodontically treated teeth dry over time and dentin undergoes changes in collagen cross-linking. Therefore it has been suggested that endodontically treated teeth are more brittle and may fracture more easily than vital teeth.

Fennis et al studied more than 46,000 patients from insurance claims and reported significantly more fractures in teeth with endodontic treatment. Moreover, after root canal treatment, the teeth usually presented inadequate remaining coronal structure. It is believed that it is the loss of the tooth structure from caries, trauma, or both that makes endodontically treated teeth more susceptible to fracture. Randow and Glantz reported that teeth have a protective feedback mechanism that is lost when the pulp is removed, which may also contribute to tooth fracture.

When a large portion of the clinical crown has been lost to damage, it is often impossible to achieve sufficient anchorage of a restoration in the remaining dentin. In such situations, a root canal-retained restoration is proposed to be required. For many years the cast gold post and core has been regarded as the “gold standard” in post-and-core restorations due to its superior success rate. Alternatives to cast posts and cores have been developed. The use of prefabricated posts and custom made build-ups with composite simplifies the restorative procedure because all steps can be completed chair-side, and fair clinical success can be expected.

Prefabricated posts were initially made of gold-plated brass stainless steel, but were improved later by using titanium alloys as the basic material. The utilization of metallic posts yields a root fracture index of approximately 2 to 4%, which has been assigned to stress concentration. If two materials of different mechanical properties are united, stress is concentrated in the weaker material. This is apparent with metal and zirconia posts, which are mechanically stronger than natural tooth structure. For that reason, fiber posts were developed that presented an elasticity modulus (E) closer to that of dentin (fiber post = 20 GPa, dentin = 18 GPa) when compared to cast posts and prefabricated metallic (E = 200 GPa) and ceramic posts (E = 150 GPa). This consequently allows for the absorption and uniform distribution of stresses to the remaining root structure instead of concentrating them.

 

 The carbon fiber post was designed initially, followed by quartz fiber posts and glass fiber posts. The quartz and glass fiber posts were developed to compensate for certain esthetic limitations of the carbon fiber posts, since all of these posts present similar characteristics from a mechanical standpoint. Currently, prefabricated fiberreinforced composite (FRC) posts are being used increasingly in dental clinical practice. Recently however, several papers supported the use of a direct restoration without placing any posts for restoring endodontically treated teeth.

A recent study by Krejci et al38 discussed the need for a re-evaluation of post use, especially where adhesive techniques are used to construct the core. Fokkinga et al39 found no differences in mean failure load and failure mode between root canal premolars restored with or without posts. Earlier, several comparative in vitro studies demonstrated that the use of posts did not increase the fracture resistance significantly. Posts are used to retain the core material, so the indication for post insertion depends on missing hard tissues and the extent of either destruction or viable structure in the tooth being considered for endodontic treatment.

 

Ferrule effect

Additionally, the ferrule effect has a great influence on fracture resistance, especially in de-coronated teeth. The ferrule is a band that encircles the external dimension of the residual tooth, similar to the metal bands around barrel. A properly executed ferrule reduces the incidence of fracture in nonvital teeth by reinforcing the tooth at its external surface and dissipating force that concentrates at the narrowest circumference of the tooth. Crown preparation with as little as 1 mm coronal extension of dentin above the margin has double the fracture resistance of preparations in which the core terminates on a flat surface immediately above the margin. If deep destruction of the teeth renders a sufficient ferrule impossible, a periodontal crown lengthening or orthodontic extrusion can be performed.

Various in vitro studies have shown that fracture resistance can be significantly increased by the use of a ferrule; the post length or design are of secondary importance for fracture resistance if a sufficient ferrule can be provided. Bolhuis et al postulated that the crown ferrule is a more important issue than the choice between a post and core, or a core reconstruction with adhesive fillings only. The researchers examined de-coronated, root-treated premolars. These were rebuilt by a core build-up without an endodontic post or by a core build-up with an endodontic post (a cast post and core, and a composite with a silica post), an additional group was not provided with a core at all. No significant difference in fracture strength among the different groups could be demonstrated. However, the above-mentioned study was a pilot study; it was not controlled and randomized.

 

Restroation of endodontically treated molars vs premolars

For the dental practitioner it is important to take advantage of the anatomy of the molar teeth. Unless the destruction of coronal tooth structure is extensive, the pulp chamber and canals provide adequate retention for a core build-up. Thanks to these anatomical features, endodontically treated molars do not require posts. However, in a case of complete loss of natural hard tissue, a post can be required. Premolars have less hard tissue and smaller pulp chambers to retain a core build-up after endodontic treatment than do molars. Premolars are also more likely than molars to

 

be subjected to lateral forces during mastication. It has been well documented that the fracture resistance of a tooth depends on the angle of applied load, with oblique forces being more detrimental.

In retrospective clinical reports, premolars were found to be the most frequently fractured teeth. For these reasons, they may require posts more often than molars. In a recent study, Ferrari et al56 concluded, over a 2-year observation period, that fiber post placement resulted in a significant reduction of failure risk for endodontically treated premolars. In particular, fiber posts appeared to contribute a protective role against root fracture. Because of the delicate root morphology present in some premolars, special care must be exercised when preparing a post space. It is important to place the post for at least the length of the coronal restoration, but with a minimum of 4 to 5 mm of apical root filling to ensure an adequate seal.

 

Although recent reports described no benefits in fracture resistance with fiber post reinforcement, researchers emphasized that tooth fracture had a more favourable, restorable pattern when fiber posts were placed in premolars. In vitro studies with endodontically treated premolars demonstrated catastrophic root fractures when no fiber posts were applied.

Current consensus in restorative dentistry is that de-cementation or failure of posts is preferable to fracture of residual tooth structure. Whatever foundation will be carried out, post or composite core build-up only, it is important to perform it immediately after endodontic treatment whenever possible. In vitro studies have shown that exposure of coronal gutta-percha to bacterial contamination can lead to migration of bacteria to the apex in a matter of days.

 

Core build up

Currently, composite is the most popular build-up and has some characteristics of an ideal build-up material. It can be easily bonded to many of the current posts and to the remaining tooth structure to increase retention.63 It has high tensile strength, low solubility, and the tooth can be prepared for an indirect restoration immediately after polymerization. Some of the negative features of resin-based composite are polymerization shrinkage, hydroscopic expansion as a result of water adsorption, and incorporation of voids during build-up, especially when self-cured composites are applied.

The C factor, defined as the ratio of the bonded to un-bonded surface area of cavities, is highly unfavourable in root canals where it can range from 20 to 200. Adhesion to dentin on the pulp floor is generally not as strong or reliable as to coronal dentin. Furthermore, resin-based composite is incompatible with zinc oxide eugenol in many root canal sealers, the combination of which can result in incomplete polymerization.

In the case of molars where posts are not used it is important to increase the surface of dentin by removing the gutta-percha remnants from the orifice of the root canals. It has been shown that contamination of dentin with temporary cements, saliva, and blood may lead to reduced bond strengths. Therefore, a rubber dam is an absolute requirement during the restorative procedure68 and perfect cleaning of the dentinal surface should be performed.

 

When light-curing composite is applied as a core build-up material, special care must be taken to ensure the proper time for polymerization and adequate lamp intensity are applied.69 Concerning the final restoration, there is convincing evidence that cuspal coverage should be provided for endodontically treated posterior teeth. Sorensen et al concluded from the retrospective evaluation of 1,273 endodontically treated teeth that the presence of cuspal coverage was the only significant restorative variable to predict long-term success.

 

Conclusions and guidelines for practitioners

As a result of the present review and the authors’ clinical experience, several conclusions and guidelines for practitioners can be drawn.

  • In the case of endodontically treated molars, if adequate coronal dentin is present and the pulp chamber provides adequate retention for a core build-up, no additional fiber post reinforcement is needed.

 

A fiber post should be used during reconstruction of endodontically treated molars in the case where there is an absence of coronal dentin. During core build-up reconstruction, especially when no post is applied, special care has to be focused on achieving the best possible adhesion to the dentin through:

 

Rubber dam isolation
Removing gutta-percha remnants from the orifice of the root canals
Perfect cleaning of the dentinal surface
Adequate application of the bonding system
Adequate light-curing of the bonding system and resin composite
Incremental placement of the composite to reduce polymerization shrinkage and achieve satisfactory marginal adaptation.
  • During the restoration of endodontically treated premolars, use of a fiber post is recommended for the length at least equal to the length of the crown.
  • Fiber post placement or composite core build-up restoration should be performed immediately after endodontic treatment whenever possible.

 

  • During the restoration of endodontically treated premolars, use of a fiber post is recommended for the length at least equal to the length of the crown.

  •  Fiber post placement or composite core build-up restoration should be performed immediately after endodontic treatment whenever possible.

 

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