The Foundation Restoration: General Considerations

Although there are many materials and techniques to fabricate a foundation restoration, no combination of materials can substitute for tooth structure. As a general rule, the more tooth structure that remains, the better the long-term prognosis of the restoration. The coronal tooth structure located above the gingival level will help to create a ferrule.^6,85,106,134 The ferrule is formed by the walls and margins of the crown, encasing at least 2 to 3 mm of sound tooth structure. A properly executed ferrule significantly reduces the incidence of fracture in endodontically treated teeth by reinforcing the tooth at its external surface and dissipating forces that concentrate at the narrowest circumference of the tooth.^101,181 A longer ferrule increases fracture resistance significantly.⁸⁶ The ferrule also resists lateral forces from posts and leverage from the crown in function and increases the retention and resistance of the restoration. To be successful, the crown and crown preparation together must meet five requirements:

Root anatomy can also have significant influence over post placement and selection. Root curvature, furcations, developmental depressions, and root concavities observed at the external surface of the root are all likely to be reproduced inside the root canal. Within the same root, the shape of the canal will vary between the cervical level and the apical foramen.⁶⁶ As a result, severe alteration of the natural shape of the canal is often necessary to adapt a circular post inside the root. This increases the risk of root perforation, especially in mesial roots of maxillary and mandibular molars which exhibit deep concavities on the furcal surface of their mesial root.^17,95 The tooth is also weakened if root dentin is sacrificed to place a larger-diameter post. A study using three-dimensional electronic speckle-pattern interferometry (ESPI) evaluated the effects of root canal preparation and post placement on the rigidity of human roots.⁹⁹ ESPI has the major advantage of being able to assess tooth deformation in real time and can be used repeatedly on the same root because of the nondestructive nature of the test. Study results indicate that root deformability is significantly increased after the preparation of a post space. Thus, preservation of root structure is also a guiding principle in the decision to use a post, the selection of the post, and the preparation of the post space. This is a reason why not every endodontically treated tooth needs a post and why more conservative approaches that do not rely on the use of a post are currently being developed.

However, a post may be used in the root of a structurally damaged tooth in which additional retention is needed for the core and coronal restoration. Posts should provide as many of the following clinical features as possible:

From a mechanical point of view, an endodontic post should not break, should not break the root, and should not distort or allow movement of the core and crown. An ideal post would have an optimal combination of resilience, stiffness, flexibility, and strength. Resilience is the ability to deflect elastically under force without permanent damage. It is a valuable quality in endodontic posts, but too much flexibility in a narrow post compromises its ability to retain the core and crown under functional forces. Stiffness describes a material’s ability to resist deformation when stressed. The stiffness of a material is an inherent physical property of that material, regardless of size. However, the actual flexibility of a post depends both on the diameter of a specific post and on the modulus of elasticity of the post’s material. Posts with a lower modulus of elasticity are more flexible than posts of the same diameter with a higher modulus of elasticity. Posts made of non-stiff materials (low modulus of elasticity) are more resilient, absorb more impact force, and transmit less force to the root than stiff posts, but low-modulus posts fail at lower levels of force than high-modulus posts.^105,126,150

Excessive flexing of the post and micromovement of the core are particular risks in teeth with minimal remaining tooth structure, because these teeth lack their own cervical stiffness as a result of the missing dentin. Post flexion can also distort and open crown margins. Open margins can result in potentially devastating caries or endodontic leakage and apical reinfection. Extensive caries extending into the root can be as irreparable as root fracture. Because rigid posts flex and bend less than nonrigid posts, they can limit movement of the core and possible disruption of the crown margins and cement seal. However, the force must go somewhere. Force from a stiff post is transmitted to the root, next to the apex of the post. An attempt to strengthen a weak root by adding a stiff post can instead make the root weaker as a result of the force concentration of a stiff rod in a more flexible material. Stress concentration in the post/root complex can lead to the self-destructive process of cracking and fracturing. Root fracture is particularly a risk in teeth with minimal remaining tooth structure to support a ferrule.

Roots also flex under force, which is a function of both the modulus of elasticity of dentin and the diameter of the root. Dentin is relatively flexible, and posts can be flexible or stiff. Although no material can behave exactly like dentin, a post with functional behavior similar to that of dentin is beneficial when the post must be placed next to dentin. Posts have been developed with a modulus of elasticity closer to dentin than that offered by traditional metal posts. But posts are significantly narrower than roots, and the actual deflection of a post within dentin is a function of both the modulus of elasticity and the diameter. The modulus of elasticity of various posts, compared with that of dentin, represents only one aspect of flexion.

In summary, an ideal post would be resilient enough to cushion an impact by stretching elastically, thereby reducing the resulting stress to the root. It would then return to normal without permanent distortion. At the same time, this ideal post would be stiff enough not to distort, permanently bend, or structurally fail under mastication forces. Finally, the perfect post would combine the ideal level of flexibility and strength in a narrow-diameter structure, which is dictated by root canal morphology. Current post systems are designed to provide the best compromise between the desired properties and inherent limitations of available materials.

Why Roots Fracture

Structures subjected to low but repeated forces can appear to fracture suddenly for no apparent reason. This phenomenon, also known as fatigue failure, occurs when a material or a tissue is subjected to cyclic loading. Fatigue may be characterized as a progressive failure phenomenon that proceeds by the initiation and propagation of cracks; many failures of teeth or materials observed in the mouth are fatigue-related. Because teeth are subjected to fluctuating cycles of loading and unloading during mastication, fatigue failure of dentin, posts, cores, crown margins, or adhesive components are all likely to occur.¹⁶⁰ Mechanical loading will favor the propagation of microcracks that will progress from the coronal to the apical region of the tooth.

Initial failure of crown margins from fatigue loading is clinically undetectable. However, when measured in vitro, early failure resulted in significant leakage of crown margins, extending between the tooth, restoration, and post space.⁴⁸ Particularly in teeth with minimal remaining tooth structure, fatigue can cause endodontic posts to bend permanently or break, or it can cause a fiber-matrix complex to disintegrate.

Fatigue failure of nonvital teeth restored with a post is more catastrophic because it may result in a complete fracture of the root. A post placed into a dentin root will function physically like any structural rod anchored in another material. This means that the forces applied on the post are transmitted to the root dentin with characteristic patterns depending on the modulus of elasticity of both the post and the dentin. If the post has a higher modulus than the dentin, the stress concentration is adjacent to the bottom of the post (Fig. 22-3). This is evident in clinical cases of root fracture originating at the apex of a rigid post.

FIG. 22-3 A, Stress distribution within a metallic post and core foundation and residual tooth structure, according to photoelastic and FEM studies. The post is cemented and usually penetrates the apical portion of the root. Functional stresses accumulate inside the foundation, slightly around the post and further inside the canal, around the apex of the post; there is less stress buildup in the cervical area compared to that with a fiber post (as shown in Fig. 22-3, B). This configuration more ideally protects the coronocervical structures but when failing, results in severe untreatable root fractures. B, Stress distribution within a fiber post/composite foundation and residual tooth structure, according to photoelastic and FEM studies. The post is bonded to the canal walls and penetrates the canal less apically. Functional stresses mainly accumulate around the post in the cervical area. This configuration protects the cervical area less efficiently but tends to prevent untreatable root fractures. The presence of a ferrule is mandatory.

(Adapted from Dietschi D, Duc O, Krejci I, Sadan A: Biomechanical considerations for the restoration of endodontically treated teeth: a systematic review of the literature—Part 1. Composition and micro- and macrostructure alterations. Quintessence Int 38:733–743, 2007.)

When the stiffness of the endodontic post is similar to that of dentin, stresses are not concentrated in the dentin adjacent to the apex of the post but rather dissipated by both the coronal and the root dentin (see Fig. 22-3). A resilient post can also prevent a sudden blow by stretching elastically, which reduces the transient forces against the tooth, but a post that is too elastic becomes too flexible for retaining a core and a crown when the tooth cannot do so on its own. A resilient post that is overloaded fails with less force than a stiffer post. This limits the amount of resilience that can be designed into a post.

Direct Foundation Restorations

In general, the evolution of foundation restorations has been to diminish invasiveness and eliminate some components in selected cases. When a sufficient amount of tissue is present at the periphery of the prepared tooth, a direct foundation restoration is indicated. In the direct technique, a prefabricated post is cemented inside the root canal, and the core is built directly on the prepared tooth. For other clinical situations, an indirect custom-cast post and core may be indicated.

Various materials can be used to fabricate a direct foundation restoration. Although there is growing interest in using resin-based materials such as resin composites or fiber-reinforced resin posts, more traditional materials such as amalgam are still used for that purpose.²⁸ For clarity, the components used for fabricating a direct foundation restoration (e.g., the endodontic post and core material) are described individually.

Indirect Foundation Restorations: Cast Post and Core

For many years, the cast metal post and core has been the traditional method for fabricating the foundation restoration of a prosthetic crown. Classically, smooth-sided, tapered posts conforming to the taper of the root canal are fabricated from high noble alloys, although noble and base-metal classes of dental alloys have also been used. Noble alloys used for post and core fabrication have high stiffness (approximately 80 to 100 GPa), strength (1500 MPa), hardness, and excellent resistance to corrosion.³²

One advantage of the cast post/core system is that the core is an integral extension of the post, and that the core does not depend on mechanical means for retention on the post. This construction prevents dislodgment of the core from the post and root when minimal tooth structure remains. However, the cast post/core system also has several disadvantages. Valuable tooth structure must be removed to create a path of insertion or withdrawal. Second, the procedure is expensive because two appointments are needed, and laboratory costs may be significant. The laboratory phase is technique sensitive. Metal casting of a pattern with a large core and a small-diameter post can result in porosity in the gold at the post/core interface. Fracture of the metal at this interface under function results in failure of the restoration. Most important, the cast post/core system has a higher clinical rate of root fracture than preformed posts.^47,166

Studies on cast post retention have shown that the post must fit the prepared root canal as closely as possible to be perfectly retained. When a ferrule is present, custom cast posts and cores exhibit a higher fracture resistance compared to composite cores built on prefabricated metallic posts or carbon posts.¹⁰⁴ Cast posts are also known to exhibit the least amount of retention and are associated with a higher failure rate compared to prefabricated parallel-sided posts. In a classic retrospective study (1 to 20 years) of 1273 endodontically treated teeth in general practice, 245 (19.2%) were restored with tapered cast posts and cores. Among these, 12.7% were deemed failures. This failure rate was higher than that for the other passive post systems used. Of particular concern was the fact that 39% of the failures led to unrestorable teeth requiring extraction. Thirty-six percent of the failures were due to loss of retention, and 58% were due to the fracture of the root. It has been suggested that tapered smooth-sided posts have a “wedging” effect under functional loading, and it is this that leads to increased risk of root fracture.¹⁶⁸

One 6-year retrospective study reported a success rate higher than 90% using a cast post and core as a foundation restoration.⁹ The lower failure rate and fewer root fractures were attributed to the presence of an adequate ferrule and careful tooth preparation. Attention has also been drawn to the fact that the higher failure rate may be due to the fact that nearly half of the posts were shorter than recommended from the literature. A venting groove for the cement along the axis of the post results in less stress on residual tissues.

Traditional Cements

Zinc phosphate cements or polycarboxylate cements are still used for cementation of posts and crowns. They are generally supplied as a powder and a liquid and their physical properties are highly influenced by the mixing ratio of the components. Their compressive strength is about 100 MPa, and elastic moduli are lower than that of dentin (5 to 12 GPa). Zinc phosphate cement is mostly used for cementing metal restorations and posts; film thickness of the zinc phosphate cement is less than 25 µm. These cements provide retention through mechanical means and have no chemical bond to the post or to dentin but provide clinically sufficient retention for posts in teeth with adequate tooth structure.

Glass Ionomer Luting Cements

Glass ionomer cements are a mixture of glass particles and polyacids, but resin monomers may also be added. Depending on the resin content, glass ionomer cements can be classified as either conventional or resin-modified glass ionomer cements. Conventional glass ionomer cements have compressive strengths ranging between 100 and 200 MPa; the Young modulus is generally about 5 GPa. They are mechanically more resistant than zinc phosphate cements, and they can bond to dentin with values ranging between 3 and 5 MPa. Some authors still recommend the use of glass ionomer cements for the cementation of metallic posts. Major advantages of conventional glass ionomer cements are their ease of manipulation, chemical setting, and ability to bond to both tooth and post.

On the contrary, resin-modified glass ionomer cements are not indicated for post cementation, because these cements exhibit hygroscopic expansion that can promote fracture of the root.

Resin-Based Luting Cements

Today there is a trend toward the use adhesive cements for bonding endodontic posts during the restoration of nonvital teeth. The rationale for using adhesive cements is based on the premise that bonding posts to root canal dentin will reinforce the tooth and help retain the post and the restoration.⁵⁰ Contemporary resin-based luting cements have been shown to exhibit compressive strengths around 200 MPa and elastic moduli between 4 and 10 GPa.²⁵ These materials may be polymerized through a chemical reaction, a photopolymerization process, or a combination of both mechanisms. Photopolymerization of these resin-based materials is often necessary to maximize strength and rigidity.

Most luting cements require a pretreatment of the root canal dentin with either etch-and-rinse or self-etching adhesives. Both types of adhesives have been shown to form hybrid layers along the walls of the post spaces.¹¹ However, bonding to root canal dentin may be compromised by the use of endodontic irrigants such as sodium hypochlorite, hydrogen peroxide, or their combination.¹²¹ Because these chemicals are strong oxidizing agents, they leave behind an oxygen-rich layer on the dentin surface that inhibits the polymerization of the resin.¹⁵⁹ Previous research has shown that the bond strength of C&B Metabond to root canal dentin was reduced by half when the dentin was previously treated with 5% sodium hypochlorite (NaOCl) or 15% EDTA/10% urea peroxide (RC Prep, Premier Dental, Plymath Meeting, PA).¹¹³ Other reports indicate that the contamination of the dentin walls by eugenol diffusing from endodontic sealers may also affect the retention of bonded posts.^70,176 Further, it is difficult to control the amount of moisture left in a root canal after acid etching, making impregnation of collagen fibers with etch-and-rinse adhesives problematic. The use of self-etching adhesives has been proposed as an alternative for the cementation of endodontic posts, because self-etching adhesives are generally used on dry dentin and do not require rinsing of the etchant. However, their efficiency at infiltrating thick smear layers like those produced during post space preparation remains controversial.^110,179 More recently, dual-curing adhesives have been developed to ensure a better polymerization of the resin deep inside the root canal. Dual-cured adhesives contain ternary catalysts to offset the acid-base reaction between the acidic monomers and the basic amines along the composite/adhesive interface.¹¹¹

Although both self-curing and light-curing luting cements can be used for cementation of prefabricated endodontic posts, most resin cements have a dual-curing process that requires light exposure to initiate the polymerization reaction. Dual-curing cements are preferred because there are concerns as to whether light-curing materials are properly cured, especially in areas of difficult light access such as the apical portion of the root canal. However, it has been reported that photocured composites generate more shrinkage stress and exhibit less flow than chemically cured composites.⁵⁴ Contraction stresses induced by polymerization also depend on the geometry of the post space and the thickness of the resin film. Previous research indicates that the restriction of flow of resin cements by the configuration of the root canal can significantly increase the contraction stress at the adhesive interface.^53,174

In recent years, a number of techniques have been used to measure the adhesion of resin-based luting cements to root canal dentin. These methods include the pull-out tests, the microtensile bond strength tests and the push-out tests.^49,65 Although laboratory tests confirmed that bond strengths ranging between 10 and 15 MPa can be obtained with modern resin-based luting cement, there is also evidence that frictional retention is a factor contributing to post retention.¹⁵ It is generally accepted that bonding to dentin of the pulp chamber is more reliable than to root canal dentin, especially at the apical level.¹³² The lowered bond strength values recorded at the apical third of the root canal are likely to be related to the reduced number of dentinal tubules available for dentin hybridization. Shorter posts may be used when successful bonding occurs between fiber-reinforced posts and root dentin, because current adhesive luting cements can assist in the retention of posts in the root canal space.¹³⁸

Another factor that may influence the performance of resin-based luting cements is the thickness of the cement layer. The cementation of endodontic fiber posts with thicker cement layers might be required when posts do not perfectly fit inside the root canal. Although a slight increase in cement thickness (up to 150 microns) does not significantly affect the performance of adhesive luting cements applied to root canal dentin, thicker layers may be detrimental to bond quality.^80,157

A recent study indicates that bond strength to radicular dentin might be maximized by adopting procedures that compensate for polymerization stresses.¹⁴ The bonding procedures are realized in two separate steps. The initial step allows optimal resin film formation and polymerization along the root canal walls, leading to more ideal resin-dentin hybridization without stresses imposed by the placement of the post. A second step bonds the post to the cured resin film. The polymerization shrinkage that occurs during the initial adhesive coating step will reduce the effects of stress imposed when the resin-coated post polymerizes, thereby preserving the bond integrity.

Although the bonding performance of resin-based luting cements is well documented, there are other reports indicating that resin-dentin bonds degrade over time.^23,60 The loss of bond strength and seal are attributed to the degradation of the hybrid layer created at the dentin-adhesive interface. This is particularly true for etch-and-rinse adhesives, because the gelatinization of collagen fibers caused by phosphoric acids may restrict the diffusion of the resin within the interfibrillar spaces and may leave unprotected fibers available for degradation. Removing organic components from the demineralized dentin prior to bonding procedures was recently suggested. The use of dilute NaOCl (0.5%) after acid etching or the conditioning of dentin smear layers with EDTA (0.1 M, pH 7.4) have been shown to produce more durable resin-dentin bonds made with single-step etch-and-rinse adhesives.¹⁵⁶

Other research indicates that the degradation of denuded collagen fibrils exposed in incompletely infiltrated hybrid layers is driven by an endogenous proteolytic mechanism involving the activity of matrix metalloproteinases (MMPs).^22,131 The release of MMPs such as collagenases has been evidenced in both coronal and root dentin of fully developed teeth of young patients.¹⁵⁴ Researchers suggest that conditioning root canal dentin with a broad-spectrum protease inhibitor such as chlorhexidine (2 wt% chlorhexidine digluconate solution) might be useful for the preservation of dentin bond strength over time.²¹

Interestingly, these dentin-conditioning procedures, which may improve the resistance of the resin-dentin bond to chemical degradation, also act as antibacterial agents; this might be of interest in the endodontic context.

Self-Adhesive Cements

More recently, self-adhesive resin cements have been introduced as an alternative to conventional resin-based luting cements. Self-adhesive luting cements contain multifunctional phosphoric acid methacrylates that react with hydroxyapatite and simultaneously demineralize and infiltrate dental hard tissue.¹¹⁴ They do not require any pretreatment of the tooth substrates, and their clinical application is accomplished in a single step. Therefore, the self-etching capability of these new cements reduces the risk for incomplete impregnation of the conditioned tissue by the resins and reduces technique sensitivity. The elastic moduli of chemically cured self-adhesive resin cements are relatively low (4 to 8 GPa) but generally increase when a dual-curing process is used. It is therefore recommended that all dual-cured resin cements receive maximal light to achieve superior material properties wherever clinically possible.¹⁴¹ Adhesion performance to dentin was found comparable to multistep luting cements, but bonding to enamel without prior phosphoric acid etching is not recommended.⁷⁶ However, their long-term clinical performances need to be assessed before making a general recommendation for their use.

Endodontic Evaluation

The prerestorative examination should include an inspection of the quality of existing endodontic treatment. New restorations, particularly complex restorations, should not be placed on abutment teeth with a questionable endodontic prognosis. Endodontic retreatment is indicated for teeth showing radiographic signs of apical periodontitis or clinical symptoms of inflammation. Restorations that require a post need a post space, which is prepared by removal of gutta-percha from the canal. Canals obturated with a silver cone or other inappropriate filling material should be endodontically retreated before starting any restorative therapy. Because the probability for periapical tissue to heal after endodontic retreatment is reasonably high, the chances to retain a well-restored tooth in asymptomatic function over time are excellent.¹²⁵

Periodontal Evaluation

Maintenance of periodontal health is also critical to the long-term success of endodontically treated teeth. The periodontal condition of the tooth must therefore be determined before the start of endodontic therapy and restorative phase. The following conditions are to be considered as critical for treatment success:

If one or more of the aforementioned conditions are not met owing to preexisting pathology or structural defects, treatment success or even feasibility can be compromised, sometimes suggesting extraction of weak teeth and replacement with dental implants rather than conventional therapy.

Biomechanical Evaluation

All previous events, from initial decay or trauma to final root canal therapy, influence the biomechanical status of the tooth and the selection of restorative materials and procedures. The biomechanical status can even justify the decision to extract extremely mutilated teeth that do not deserve extensive treatments that carry a limited probability of success. Important clinical factors include the following:

Teeth with minimal remaining tooth structure are at increased risk for the following clinical complications^120,173,178 (see Fig. 22-1; Fig. 22-4):

FIG. 22-4 Failure of prosthetic foundations can have dramatic consequences on both overlying restorations and surrounding tissues. A better understanding of compositional and structural changes that affect tooth resistance to repeated functional forces is mandatory to improve treatment success in endodontically treated teeth.

The amount and quality of remaining tooth substrate are far more important to the long-term prognosis of the restored tooth than any restorative material properties. One must consider that no restorative material can truly substitute for dentin or enamel and that a minimal amount of intact structure is mandatory to justify tooth maintenance and its strategic importance to the overall treatment plan. The presence of healthy adjacent teeth available as abutments or the option of dental implants are additional factors to be analyzed when justifying the restoration of endodontically treated teeth.

Tooth Position, Occlusal Forces, and Parafunctions

Teeth are subjected to cyclic axial and nonaxial forces. The teeth and associated restorations must resist these forces to limit potential damages such as wear or fracture. The degree and direction of forces depend on the location of the tooth in the arch, the occlusal scheme, and the patient’s functional status.

In most occlusal schemes, anterior teeth protect posterior teeth from lateral forces through anterolateral guidance. In the context of very steep anterior guidance and deep vertical overbite, maxillary anterior teeth are sustaining higher protrusive and lateral forces from the mandibular anterior teeth. Restorations of damaged anterior teeth with heavy function should therefore be designed to resist flexion. Restorative components should be stronger than would be required for teeth with an edge-to-edge relationship and therefore vertical forces.

Posterior teeth normally carry more vertical forces, especially when canine and anterior guidance are maintained; they also sustain greater occlusal loads than anterior teeth, and restorations must be planned to protect posterior teeth against fracture. In the case of parafunctions, protection by anterior contacts are likely to be reduced or lost and posterior teeth then submitted to more lateral stresses, generating higher demand for restorative materials.

The literature reports average biting forces varying between 25 and 75 N in the anterior region and between 40 and 125 N for the posterior region of the mouth, depending on food type, dental status (dentate or edentulous), and patient anatomy and functional habits.^59,75 Those forces can easily reach 1000 N or above in case of parafunctions, showing how potentially destructive they can be for intact teeth and even more so for nonvital, fragilized teeth. Parafunctional habits (clenching and bruxism) are major causes of fatigue or traumatic injury to teeth, including wear, cracks, and fractures. Teeth that show extensive wear or sequelae from parafunctions, especially heavy lateral function, require components with the highest physical properties to protect restored teeth against fracture.

In general, modern strategy focuses on tissue preservation and also on the use of adhesion to achieve stabilization of the restoration for improved short- and long-term service. However, in certain conditions like reduced tooth support, conventional materials are not obsolete.

Esthetic Evaluation and Requirements

Anterior teeth, premolars, and often the maxillary first molar, along with the surrounding gingiva, compose the esthetic zone of the mouth. Changes in the color or translucency of the visible tooth structure, along with thin soft tissues or biotype, diminish the chance for a successful esthetic treatment outcome.

Potential esthetic complications should be investigated before endodontic therapy is initiated. For instance, metal or dark carbon fiber posts or amalgam placed in the pulpal chamber can result in unacceptable esthetic results, such as a grayish appearance of the overlying prosthetic restoration (especially with modern, more translucent full-ceramic crowns) or gingival discoloration from the underlying cervical area or root (see Fig. 22-1). All teeth located in the esthetic zone also require critical control of endodontic filling materials in the coronal third of the canal and the pulp chamber to avoid or reduce the risk of discoloration. Careful selection of restorative materials, careful handling of tissues, and timely endodontic intervention are important to preserving the natural appearance of nonvital teeth and gingiva.

General Principles and Guidelines

The post, the core, and their luting or bonding agents form together the foundation restoration to support the coronal restoration of endodontically treated teeth. The recent evolution of foundation restorations has been to diminish invasiveness, to use adhesion rather than macromechanical anchorage, and to eliminate intraradicular components in selected cases. These changing clinical concepts derive from both an improved understanding of tooth biomechanics and advances in restorative materials.

The foundation and its different constituents are then aimed at providing the best protection against leakage-related caries, fracture, or restoration dislodgment. Therefore, all aforementioned local and general parameters are to be systematically analyzed in order to select the best treatment approach and restorative materials. Prosthetic requirements are also to be taken into consideration to complete each case analysis. In general, abutments for fixed or removable partial dentures clearly dictate more extensive protective and retentive features than do single crowns, owing to greater transverse and torquing forces. This modern biomechanical treatment strategy is summarized in Fig. 22-5.

FIG. 22-5 A, Current recommendations for the treatment of nonvital anterior teeth. *Normal function and anterior guidance; **moderate to severe parafunctions and abnormal occlusion/anterior guidance. B, Current recommendations for the treatment of nonvital posterior teeth. *Relatively flat anatomy and canine guidance, normal function; **group guidance, steep occlusal anatomy, parafunctions.

Structurally Sound Anterior Teeth

Anterior teeth can lose vitality as a result of a trauma with no or minimal structural damage. They generally do not require a crown, core, or post; restorative treatment is limited to sealing the access cavity and direct composite fillings. Discoloration, whenever present, is addressed by nonvital bleaching, or for untreatable or relapsing ones, with conservative restorative approaches such as direct or indirect veneers (Table 22-2).

TABLE 22-2 Clinical Protocols for Restoring Nonvital Teeth With Partial Restorations (Most Likely Procedures)

Nonvital Posterior Teeth With Minimal/Reduced Tissue Loss

The loss of vitality in posterior teeth resulting from trauma, decay, or restorative procedure does not necessarily lead to extreme biomechanical involvement and therefore allow in certain conditions for conservative restorations.

Occlusal cavities or mesio/disto-occlusal cavities can be restored with either direct- or indirect-adhesive intracoronal restorations, providing residual walls are thick enough (proximal ridges and buccolingual walls more then 1.5 mm thickness). The three additional clinical factors which must be analyzed to ensure optimal treatment success are the configuration factor (C-factor), cavity volume, and dentin quality. For instance, a large class I cavity with contaminated and sclerotic dentin would clearly be a contraindication to the direct approach, despite the fact it apparently falls within the indications of direct techniques. Conservative options must, however, always be analyzed under the light of functional and occlusal environment. They can only be considered in the absence of parafunctions and with anterior guidance, which limits overall functional loading and lateral or flexural forces. In less favorable biomechanical conditions (group guidance, steep occlusal anatomy, bruxism, clenching, etc.), a protective approach with full occlusal coverage (onlay or overlay) is mandated to minimize the risk of fatigue failures (see Table 22-2).

Structurally Compromised Teeth

The decision for placing a post as well as the selection of a post system (rigid or nonrigid) depends once again on the amount and quality of remaining tooth structure and the anticipated forces sustained by this tooth (Table 22-3).

TABLE 22-3 Clinical Protocols for Restoring Nonvital Teeth With Full Prosthetic Restorations (Most Likely Procedures)

In general, rigid posts made of stiff materials (metal and ceramics) are indicated for teeth with minimal tooth structure which rely on the post to hold the core and crown. Because rigid posts flex and bend less than other types of posts, they are supposed to limit movement of the core and possible disruption of the crown margins and cement seal. But one must remember that stiff posts transmit more stresses to the root, next to the post apex, when conventionally cemented. An attempt to strengthen a weak root by adding a stiff post can instead make the root weaker as a result of the force concentration behavior of a stiff rod in a more flexible material. Adhesion therefore plays a crucial role, since a well-bonded post can help absorb stresses more evenly throughout the remaining tooth structure. Benefit and increased risk of fissure and fracture must then be appropriately weighed against adhesion potential inside the root and post type, composition, and surface treatment.

In structurally sound teeth, nonrigid posts flex with the tooth under functional forces, reducing the transfer of force to the root and reducing the risk of root fracture. Flexion is of course related to post diameter. In structurally compromised teeth, which lack cervical stiffness from dentin and ferrule effect, excessive post flexion can be detrimental to the marginal seal and prosthesis longevity, so fiber posts are generally contraindicated.

White or translucent fiber posts are generally preferred underneath full-ceramic restorations, whereas stronger black carbon fiber posts, which can reflect through gingiva, tooth structure, or ceramic restorations, are usually used in teeth to be restored with gold or porcelain fused to metal crowns, as well as in zirconia-based restorations. The literature has largely overemphasized the impact of post color on restoration esthetics. Metal or carbon post color can be masked with resin opaquer and gold post and core ceramized to enhance esthetic integration. Such procedures can help to approach a more ideal restoration biomechanical behavior through the fabrication of rigid but more esthetic foundations. Upper lateral and lower incisors, together with extremely thin biotype, are probably the only real esthetic contraindication for metal or carbon fiber posts.

In cases of extreme tooth fragility secondary to caries, fracture, previous overenlargement of the root canal system, or immaturity, residual root structure can be unified and reinforced with adhesive bonding and composite before placing a normal-diameter post, forming altogether a cohesive unit, as previously described.

In conclusion, in a damaged tooth that is to be restored with a nonrigid post, 2 to 3 mm of cervical tooth structure must ideally remain to allow creation of a restoration as a whole that is resistant to flexion. Teeth with minimal tooth structure and limited ferrule effect need additional cervical stiffness from a more rigid post to resist distortion. In this situation, adhesive cementation is preferred to conventional cementation.

Structurally Compromised Anterior Teeth

Restoration of endodontically treated teeth becomes more complex as teeth or supporting structures become increasingly affected. A nonvital anterior tooth that has lost significant tooth structure requires restoration with a crown, supported and retained by a core and possibly a post as well.

When less than half the core height is present, or when remaining walls are extremely thin (less than 1 mm on more than three-fourths of the tooth circumference), a post is needed to increase retention and stabilize and reinforce the foundation. Many post options are available nowadays, including titanium, fiber-reinforced resin, and ceramics. Adhesion is now the preferred mode of post cementation unless a long-term contamination of root dentin is obvious (e.g., with eugenol), making adhesion highly questionable.

In the latter situation or in the presence of flared canals (possibly as well when a limited ferrule effect is present), cast gold post and core are still considered a feasible option. Actually, in this extremely unfavorable biomechanical environment, this traditional treatment approach provides a higher rigidity in the cervical area, which is mandatory for restoration stability. Here, a fiber-reinforced composite foundation having higher flexibility might present less favorable biomechanical behavior, such as suggested by FEM studies⁴³ (see Fig. 22-3 and Table 22-2). Tooth extraction and implant placement or bonded bridge (particularly for lateral incisors) are also to be considered in this situation.

In the esthetic zone, the post should not detract from the esthetics of the coronal tooth structure, ceramic crown, or gingiva. Current restorative procedures allow fabrication of highly esthetic ceramic coronal restorations that have no metal substructure. When such restorations with remarkable lifelike color and vitality are selected, it usually implies the use of nonmetal esthetic posts, either ceramic or resin fiber-reinforced ones.

Structurally Compromised Posterior Teeth

Slightly decayed posterior teeth in the context of parafunctions or significantly fragilized premolars and molars require cuspal protection afforded by onlay restoration, endocrown, or a full crown. The need for a post and core depends on the amount of remaining tooth structure. When remaining walls (buccal and lingual) provide more than 3 to 4 mm height (from pulpal chamber floor) and 1.5 to 2 mm thickness, core and restoration stability are granted through macromechanical retention and/or adhesion; then, posts are not needed (see Fig. 22-5). With current treatment strategy, the post has become the exception rather than the rule for the restoration of nonvital posterior teeth.

Additional Procedures

Periodontal crown lengthening surgery or orthodontic extrusion can expose additional root structure to allow restoration of a severely damaged tooth. In the smile frame, crown lengthening might however be limited by esthetic adverse consequences (proximal attachment reduction); basically, buccal crown lengthening only can be considered as a potential indication for this procedure. In the posterior region, crown lengthening is limited by tooth and furcation anatomy or by loss of bone structure, which complicates future implant placement. As regards to orthodontic extrusion, root length and anatomy are the limiting factors of this procedure; short roots or conical anatomy are contraindications to orthodontic extrusion. Once again, when a long-lasting, functional restoration cannot be predictably created, tooth extraction might be better than to pursue heroic efforts to restore extremely weak teeth, using complex, expensive, and unpredictable procedures.

Amalgam Core

A coronal-radicular amalgam foundation may be realized with or without post (Figs. 22-11 through 22-14). The amalgam coronal-radicular core without post is indicated for posterior teeth that have a large pulp chamber and lateral walls at least 4 mm high. This restoration extends slightly into the coronal portion of the canals (1 to 1.5 mm). The core is then retained by a combination of the divergence of the canals and natural undercuts in the pulp chamber. A single, homogenous material is used for the entire restoration, rather than the dual phases of a conventional preformed post and core.

FIG. 22-11 Amalgam core. Nonvital posterior teeth with reduced tooth structure and wall height. Amalgam core stability is granted by extension of the restoration into the coronal portion of the canal (curved roots) and post (only if straight root anatomy) and remaining pulpal chamber undercuts. Today, this option is considered obsolete or historical because of the intrinsic staining of amalgam, the nonadhesive technique, and the less-than-ideal biomechanical behavior of amalgam. A-B, Preparation. Existing restorative material and all remnants of endodontic sealer and gutta-percha are removed from the cavity to expose a clean dentin substrate. Thin and weak walls are removed until reaching sufficient thickness (1.5 to 2 mm); existing retentions are used to stabilize the amalgam core, together with a post. The metal post (with passive and anatomic design, preferably made of titanium) extends into the root for about the same length as its coronal extension. C-E, Restoration fabrication. After placing a matrix or cupper ring to envelop the preparation, amalgam is compacted into the cavity, superior root canal portion and around the post, until restoring the appropriate coronal volume. Preparation of cores is made to manage optimal space for PFM crowns. F, Posttreatment result. This treatment approach is feasible for teeth already restored with amalgam cores and when PFM restorations are mandated. Satisfactory esthetics can be attained with intrasulcular restoration margins.

FIG. 22-12 Composite core (without post). Nonvital tooth with more than half the coronal structure left and sufficient wall thickness. Core stability is granted by adhesion, remaining pulpal chamber, and residual wall height. A-B, Preparation. Existing restorative material and all remnants of endodontic sealer and gutta-percha are removed from the cavity to expose a clean dentin substrate. The tooth is prepared to evaluate precisely the height and thickness of residual tooth structure. Half of the abutment height and more than 1.5 mm wall thickness must be present to restore the tooth without post. If aforementioned conditions are met, a thin layer of hard cement (calcium hydroxide, zinc phosphate, or glass ionomer; not resin-modified glass ionomer) is applied at the entrance of the root canal to enable possible retreatment in the future. C, Adhesive procedures and core fabrication. All walls are covered with the adhesive system (etch-and-rinse or self-etch) before applying in layers a direct light-curing restorative composite or a self-curing material, in case of limited restoration volume. D, Postoperative view. The advantage of this approach is the ease of application and excellent esthetics. It is considered a good restorative solution for highly esthetic ceramic crowns placed on teeth with limited tissue loss and discoloration.

FIG. 22-13 Composite core with white post (resin reinforced fiber or zirconia post). Nonvital tooth with less than half the coronal structure left and/or insufficient wall thickness. Core stability is improved through adhesion and a tooth-colored post (resin reinforced fiber or ceramic posts). A, Preoperative view. B-C, Preparation. Existing restorative material and all remnants of endodontic sealer and gutta-percha are removed from the cavity to expose a clean dentin substrate. The tooth is prepared to evaluate precisely the height and thickness of residual tooth structure. If less than half of the abutment height or 1.5 mm wall thickness remain, the abutment requires a post to increase core retention and foundation stability. D-E, Post-and-core fabrication. Post placement: The post space is prepared with ad hoc instruments until reaching a length equivalent to core height or slightly less depending on remaining wall height. Increased length is not needed, since retention is predominantly achieved through adhesion. Any tooth-colored post can be used, but fiber posts are usually preferred to limit stress buildup into the root structure, even though they provide less rigidity to the foundation. Post cementation: A self-curing adhesive system is applied on all cavity walls and into post preparation before luting the post with a dual-cured composite cement or, as a simple alternative, a self-adhesive cement (then, no adhesive is used). Core fabrication: The remaining core volume is built up in layers, using a direct light-curing restorative composite or with a single increment of a self-curing material when only limited volume is to be completed. F, Postoperative view. The primary advantage of this approach is the excellent esthetic outcome. It is considered a good restorative solution for highly esthetic ceramic crowns placed on teeth with more tissue loss but moderate discoloration.

FIG. 22-14 Composite core with prefabricated metal post. Nonvital tooth with reduced coronal structure and ferrule effect and/or insufficient wall thickness. Core stability is improved through adhesion and a metal (opacified) post (titanium posts are usually preferred). A, Preoperative view. B, Preparation. Existing restorative material and all remnants of endodontic sealer and gutta-percha are removed from the cavity to expose remaining tooth structure and a clean dentin substrate. The absence of coronal tissue and ferrule effect mandates the use of a more rigid post (ceramic posts are considered too rigid and contraindicated in this situation). C-F, Post and core fabrication. Post space preparation: The post space is prepared with ad hoc instruments until reaching a length equivalent to about core height. More length is not needed, since retention is predominantly achieved through adhesion. Post preparation: The metal post (titanium here) is sandblasted (silicoating can be used to increase adhesion; i.e., Rocatec or CoJet systems, 3M), followed by silane application. An opaque resin or flowable composite can be used to mask metal dark color and improve esthetics. Post cementation: A self-curing adhesive system is applied on all cavity walls and into post preparation before luting the post with a dual-cured composite cement or, as a simple alternative, a self-adhesive cement (then, no adhesive is used). Core fabrication: The remaining core volume is built up in layers, using a direct light-curing restorative composite or a single increment of a self-curing material when only limited volume is to be completed.

When shorter lateral walls are present, a post is mandatory to improve core stability and retention. The preferred roots for post placement for molars are the palatinal root in the upper jaw and the distal root in the lower jaw. Other roots, thinner and presenting more pronounced curvatures, are normally not indicated for post placement. Coronal-radicular amalgam foundations are not indicated for premolars owing to their smaller dimensions; adhesive foundations or cast gold post and core are preferred.

Cast Gold Post and Core

Cast gold post and core restorations can be fabricated with either direct or indirect techniques.

Indirect Technique

With the indirect technique, a final impression of the prepared tooth and post space is made (Fig. 22-15). As with the direct technique, the path of withdrawal is made by undercut blockout, not by dentin removal. The castable final post and core pattern are fabricated on a die from this impression. The crown margins need not be accurately reproduced at this stage. Proprietary systems provide matched drills, impression posts, and laboratory casting patterns of various diameters. An impression post (preferably a repositionable one) is fitted to the post space, and a final impression is made that captures the form of the remaining coronal tooth structure and picks up the impression post. In the laboratory, a die reproduces the post space and the residual coronal tooth structure for fabrication of the post and core pattern.

FIG. 22-15 Nonvital teeth with limited coronal structure and/or numerous abutments. Indirect post and core fabrication helps to achieve proper parallelism and core anatomy. Core retention is assumed by a conventional or adhesive cement. Today, this solution is considered mostly for extended metal-ceramic restorations or sometimes for multiple full-ceramic restorations. A, Preoperative view. B, Preparation. Existing restorative material and all remnants of endodontic sealer and gutta-percha are removed from the cavity to expose remaining tooth structure and a clean dentin substrate. Despite the indirect approach, all healthy coronal structures must be maintained to improve foundation stability. Access cavity and coronal walls are prepared with minimal convergence (6 to 10 degrees) to allow for easy insertion. C-D, Cast gold post and core fabrication and luting. Post space preparation: The post space is prepared with ad hoc instruments until reaching a length equivalent to about core height or more. More length is needed here, since retention cannot always be achieved through adhesion (endodontic retreatment or highly sclerotic contaminated dentin). Then a conventional glass ionomer or zinc phosphate cement is used.

Post and core preparation: The gold post is sandblasted (or eventually sandblasted and coated if adhesive procedures are possible) using Rocatec or CoJet systems, 3M + silane. Post cementation: The post and core is luted with the aforementioned conventional cements or adhesive technique. Next, a self-curing adhesive system is applied on all cavity walls and into post preparation (before the insertion of the post and core) and covered with a dual-cured composite cement or, as a simple alternative, a self-adhesive cement (then, no adhesive is used). E-F, Restoration fabrication and placement. A second impression is needed for the final prosthetic work to optimize restoration precision and quality. Lab-made PFM or full ceramic crowns are made with optimal foundation design and alignment.

With both techniques, the fabrication of a temporary crown with intraradicular retention is needed. This provisional restoration must stay only for a limited time in the mouth to prevent decementation or canal reinfection. This is why this approach is less popular today and replaced by direct techniques in most cases.

The cast post and core is cemented at the second appointment. The cementation process must be passive in nature and an evacuation groove made on post side to facilitate cement expulsion and limit seating pressure. Rapid seating, excessive cement, and heavy seating pressure (e.g., occlusal force) can produce high hydraulic pressures inside the root that may be great enough to crack the root.