Modern endodontic principles part 7: the restorative interface

Direct restoration

If the coronal structures are largely intact (particularly marginal ridges) and loading is favourable, a simple plastic restoration can be placed in the access cavity. Placing a bonded restoration immediately following obturation whilst the rubber dam remains in place is good practice (Figure 1).

The core

Core is the term used to describe a restoration that is placed in order to build up a broken down tooth before receiving an indirect restoration. The role of a core is reliant upon how much coronal dentine remains. For the majority of pulpless teeth a structural core material is required as this will form the bulk of the final preparation. Consequently, it will be subjected to significant functional stresses and must therefore exhibit adequate mechanical properties to resist these. There are essentially four types of direct core material available:

The relevant properties of these are summarized in Table 1.

Endodontically-treated teeth typically lack sufficient coronal dentine in which retentive features can be prepared to retain a core without compromising the strength of the remaining tooth structure. Where a large pulp chamber exists, extension of amalgam/composite into this space alone may be sufficient. Where the pulp chamber is small or there is more significant tooth loss, the Nayyar technique should be used:⁷ 2–3 mm of the coronal root filling is removed and an amalgam or composite core placed to fill the coronal preparation, the pulp chamber and the extension into the roots (Figure 2). This technique has been widely adopted for premolars as well as molars and a core of this type will function well even when there is as little as one sound cusp remaining. Given bonded cores do not require additional retentive features, clinicians should consider resin composite cores, where possible, to preserve tooth structure.

Ferrule

A key component in the predictability of restoration of endodontically-treated teeth is the ‘ferrule’. The ferrule is that part of the crown that encircles the remaining tooth structure (Figure 3). The ferrule binds the remaining tooth together, simultaneously preventing root fracture and providing resistance form to prevent dislodgement of the crown during function⁸ (Figure 4). Four factors which directly influence the ferrule have previously been identified.⁹

4. Type of tooth and the extent of lateral load

Lateral forces have a greater potential to damage the tooth-restoration interface when compared with vertical loads.¹⁴ Hence, a differential approach needs to be adopted when it comes to the restoration of anterior and posterior teeth.

If there is no ferrule, the tooth should be viewed as compromised and the patient informed accordingly. It should not, however, be considered an absolute contraindication to treatment, rather an important prognostic feature to be included in the consent process (Figure 5). With minimal or absent ferrule, the clinician becomes dependent upon intraradicular aids (eg Nayyar cores and posts) to retain the coronal restoration. Crown lengthening and/or orthodontic extrusion may be considered in cases where the tooth is of strategic importance or planned as an abutment, therefore demanding improved mechanical characteristics.

Posts

The primary function of a post is to retain a core in a tooth with extensive loss of coronal tooth structure.¹⁵ Posts are associated with three significant drawbacks¹⁶ (Figures 6 and 7):

As such, the decision whether to use a post must be made judiciously and currently there appears to be a definite trend towards reduced post usage. Without clear guidelines from definitive research, specific factors for the individual tooth and clinical situation require careful consideration. Ultimately, the evaluation of whether a post is needed is based on how much natural tooth substance remains to retain a core build-up and support the final restoration after caries removal and endodontic treatment are completed. It is sensible to consider placement of a post only if there is such significant loss of coronal tooth tissue that a core cannot be retained. Practically, the divergent roots and pulp chamber space of molars should mean posts should not be required, but anteriorly these features are not present. Therefore, for structurally compromised anterior teeth, post-retained cores remain an essential part of restorative dentistry.

If the decision is made to place an intra-radicular post, there are several aspects to consider, including the type of post, size and shape of post, conservation of tooth structure and the creation of the post space.

Non-metal post systems

Traditionally, the principal posts of choice have been direct or cast metal posts. These have several drawbacks:

Carbon, zirconia and fibre composite posts have all been introduced as alternatives to metal posts. These systems are deemed more biologically compatible with tooth tissue.¹ Furthermore, if amalgam is phased out of dentistry, clinicians must become confident with resin-based technology.

The more aesthetic glass and quartz fibre post have now replaced carbon fibre posts. Moreover, they can be bonded to dentine. Zirconium posts cannot be etched, therefore, it is not possible to bond a composite core material to the post, making core retention a problem.²¹ Retrieval of zirconium and ceramic posts is very difficult if endodontic retreatment is necessary, or if the post fractures. Some ceramic materials can be removed by grinding away the remaining post material with a bur, but this is a tedious and dangerous procedure. It is impossible to grind away a zirconium post. For these reasons, ceramic and zirconium posts should not be used.

The main advantage of using a fibre post is that the modes of failure are generally more retrievable than those of metal post systems. The use of fibre posts reduces the risk of root fracture significantly as the modulus of elasticity of these posts is closer to that of dentine.²² Fibre posts provide greater elastic support to the core of a tooth than metal posts²³ (Figure 8).

The most common causes of failure associated with fibre post systems are post de-cementation and secondary caries.²⁴ Of these, post de-cementation has been suggested as the most frequent complication.²⁵ There are many challenges to successful bonding in the root canal that may explain this mode of failure:

The dental literature relating to the different types of posts presents too many variables to enable a true comparison between all available post types. The profession lacks long-term clinical results, with a high level of evidence pertaining to survival data for the various post systems.³⁰ The presence of a ferrule of 1.5–2 mm sound coronal tooth structure between the core and the finish line is more important in fracture resistance than the post design or type.³¹ Nonetheless, there is a growing understanding of the benefits of resin-bonded fibre-composite post systems. Given the more predictable and less catastrophic mode of failure, we would advocate these in conjunction with composite cores wherever possible.

However, no one system has universal application. Parallel-sided or tapering pre-fabricated posts are recommended for conservatively prepared root canals in teeth with roots of circular cross-section. Excessively flared canals are most effectively managed by a traditional custom cast post. In vitro studies have confirmed that parallel-sided posts are more retentive than tapered posts and that threaded posts are the most retentive.³² Threaded posts which screw into dentine are not recommended as they generate residual stress in the dentine, which may result in root fracture.

Conservation of tooth structure

In creating post space, care must be used to remove only minimal tooth structure from the canal. Excessive enlargement can perforate or weaken the root, which may then split during post cementation or subsequent function. Most roots are narrower mesio-distally than facio-lingually and often have proximal concavities that cannot be seen on standard periapical radiographs. Experimentally, most root fractures originate from these concavities, because the remaining dentine thickness is minimal.³³ Therefore, the root canal should be enlarged only enough to enable the post to fit accurately and yet passively whilst ensuring strength and retention.

Retention form

Simultaneous dislodgement of an anterior crown with its retaining post and core is a frequent occurrence and results from inadequate retention form of the prepared tooth.³⁴ Post retention is affected by the following factors.

Resistance form

The resistance to lateral displacement largely stems from the presence of a ferrule effect.

Figure 9 demonstrates six key features of post design, placement and subsequent preparation that may all contribute to long-term success of the endodontically-treated and extensively compromised tooth.

Indirect restorations

The overwhelming majority of reports in the literature supports the need for cuspal coverage restorations of endodontically-treated posterior teeth, and a strong association between the success of endodontically-treated teeth and crowned teeth has been shown.³⁸ However, no consensus exists regarding the preferred type of final restoration for endodontically-treated teeth.³⁹ More recently, partial restorations such as indirect onlays are advocated as these restorations preserve more sound tooth structure than does a full coverage crown, while at the same time providing cuspal coverage to protect weakened cusps⁴⁰ (Figure 10). If it is anticipated that after crown preparation the buccal and/or lingual walls will have less than 1 mm remaining dentine thickness, a partial coverage restoration should be considered.

Whatever the evidence suggests, the patient must be involved in the decision-making process. Though best practice indicates indirect restoration of many endodontically-treated teeth, many patients do not want or cannot afford this ideal. It is imperative in these circumstances that the core is placed to the best possible standard and consideration given to providing cuspal coverage with the chosen plastic restoration (Figure 11). Figures 12 and 13 demonstrate algorithms that may help clinicians decide upon optimum strategy for restoring the root-filled tooth.

Prosthodontic interface

Abutment tooth assessment

Endodontically-treated teeth are often required as abutments for fixed and removable prostheses. With this comes a very specific set of problems that must be recognized when planning restorative treatment and getting patients to consent to the likely success of such treatment plans.

Posterior teeth are predominantly exposed to axially directed forces. When such a tooth acts as an abutment, it is now also exposed to greater non-axial forces. This is especially true if we consider cantilever bridges and the terminal abutment for a Kennedy Classification I partial denture. In such situations, extra consideration needs to be given to the amount of remaining tooth tissue, with the resistance form of any planned restorations being particularly important. This involves careful assessment not only of clinical crown height, but also the thickness of remaining dentine. In order to do this, all existing restorations and carious tissue must be removed and the amount of tissue removal for any planned restoration also needs to be visualized. This can be particularly challenging when abutments are planned to be restored with crowns incorporating milled guide surfaces, which require extra tissue removal (Figure 14).

It is advisable to remove any remaining tooth tissue which will be less than 1 mm thick, as this will be prone to fracture. Once this has been undertaken, the clinician can now better assess the suitability of the tooth to act as an abutment and if extra retention from the root canal will be required in order to retain a restoration or core. Location and span of the prosthesis will also be important, as different forces will be exerted on anterior and posterior abutments. Anticipation of the forces which the tooth will be subjected to require assessment of any parafunctional activity and the opposing dentition. Obviously, this is only a small part of the abutment tooth assessment and the factors that need consideration are presented in Table 2.

Table 2. Factors to consider when assessing the favourability of a tooth to act as an abutment.

Clinical Examination

Tooth type, alignment, drifting, over-eruption Length and location of edentulous span Existing restoration size and quality Periodontal support Clinical crown height Inter-occlusal clearance Assessment of opposing dentition or prosthesis

Occlusal Examination

Evidence of parafunction Static contacts Dynamic contacts Presence of interferences (working and non-working side, RCP-ICP, protrusive)

Radiographic Examination (consider horizontal parallax views for multi-rooted teeth)

Root length, configuration Quality of endodontic treatment Presence of periapical disease Crown:root ratio Canal curvature, preparation taper Periodontal bone levels

Clinical Investigation

Removal of existing restorations and carious tissue Examination for the presence of cracks on the pulpal floor Assessment of remaining coronal tooth tissue

Prosthesis design

Once careful consideration of the factors outlined in Table 2 has been undertaken, the feasibility of a fixed or removable option can be considered.

Abutments for fixed prostheses

Abutment teeth requiring a post-retained restoration should not be used as the sole abutment for a fixed prosthesis (cantilever design) as there is a significant risk of root fracture and de-cementation¹⁶ (Figure 15). When fixed-fixed designs are planned, the clinician needs to consider the different support which can be offered by the two abutments if only one is root-filled. Although tempting, utilizing double abutments can result in further problems and should be avoided. Such restorations are more difficult for patients to clean and for maintenance of periodontal health. Additionally, the forces generated during function can cause the most distal retainer to de-cement, resulting in microleakage and recurrent caries.

Other options for managing such situations include the use of a fixed-moveable design (Figure 16). Fixed moveable bridges theoretically allow less transmission of force to the weaker abutment by allowing some independent movement of abutments and facilitating removal of the compromised abutment without destroying the prosthesis. Clinicians must be aware that joints only allow movement in a vertical direction, so may have limited use when lateral forces are considered. In addition, joints may require additional destruction of tooth structure and consideration should be given to the positioning of the joint.

Endodontics as an adjunct to prosthodontic treatment

In rare circumstances, endodontic treatment may be provided electively in order to facilitate prosthodontic reconstruction (Table 3). Elective devitalization of a tooth should not be undertaken lightly and a meticulous assessment of the risks and benefits must be undertaken and thoroughly discussed with the patient. The clinician must have a realistic perspective of his/her own capabilities and that the canal morphology is favourable for a positive outcome. Moreover, the clinician needs to be confident that the resultant restoration will have the best chance of success and this needs to be carefully planned before endodontic treatment is instigated (Table 4). Despite this, when used appropriately, it can significantly enhance the outcome of prosthodontic treatment.

Conclusions

Patients are not well served if the endodontic treatment is successful but the coronal restoration fails. Long-term success of endodontic treatment is highly dependent upon the restorative treatment that follows. There is not one post, core, or final restoration that can be used in all clinical situations. Clinicians must consider the functional and aesthetic endpoints for each tooth when considering the restorative protocol. Clinicians must also be aware that endodontically-treated teeth present challenges when used as abutments. An awareness of these is essential in restorative treatment planning. The restoration process is a delicate balance between preserving and sacrificing tooth structure. Whatever the clinical scenario, certain key philosophies must be remembered:

If there is no ferrule, the prognosis becomes guarded for that tooth.