Restoration Of The Endodontically Treated Tooth PDF

CHAPTER 22 Restoration of the Endodontically Treated Tooth DIDIER DIETSCHI, SERGE BOUILLAGUET, and AVISHAI SADAN CHAPTER OUTLINE SPECIAL FEATURES OF ENDODONTICALLY Full Crowns TREATED TEETH...

CHAPTER 22 Restoration of the Endodontically Treated Tooth DIDIER DIETSCHI, SERGE BOUILLAGUET, CHAPTER OUTLINE SPECIAL FEATURES OF ENDODONTICALLY TREATED TEETH Compositional Changes in Nonvital Teeth and Influence of Endodontic Therapy Dentin Structure and Properties of Endodontically Treated Teeth Fracture Resistance and Tooth Stiffness of Endodontically Treated Teeth Esthetic Changes in Nonvital and Endodontically Treated Teeth RESTORATIVE MATERIALS AND OPTIONS Direct Composite Restorations Indirect Restorations: Composite or Ceramic Onlays and Overlays SPECIAL FEATURES OF ENDODONTICALLY TREATED TEETH Endodontically treated teeth are structurally different from vital teeth; major changes include altered tissue physical char- acteristics, loss of tooth structure, and possibly also discolor- ation. Tissue modifications must therefore be analyzed at different levels, including tooth composition, dentin micro- structure and tooth macrostructrure. It is fundamental to understand the implication of such features on tooth biome- chanics, since they will largely influence restorative approach and means (Table 22-1). Numerous in vitro studies dealing with the complexity of the nonvital tooth substrate are reported in the literature; ultimately, in vivo trials describe the global effect of tissue qualitative and quantitative changes on restora- tion, long-term behavior, and survival. Compositional Changes in Nonvital Teeth and Influence of Endodontic Therapy The loss of pulpal vitality is accompanied by a slight in tooth moisture content. This loss of moisture (9%) is attrib- uted to a change in free water but not in bonded water.69,73 This alteration was associated with a slight change in values for the Young modulus and proportional limit.79 However, no decrease in compressive and tensile strength values was associ- ated with this change in water content.79 Only one study 778 PART III RELATED CLINICAL TOPICS TABL E 2 2 - 1 Specific Tissue Modifications and Possible Clinical Implications Following Loss of Vitality or Endodontic Treatment Possible Clinical Alteration Level Specific Changes Implication Composition Collagen structure Increased Tooth moisture fragility Mineral composition Reduced adhesion and content to substrate Dentin structure Elasticity modulus Increased and behavior fragility Tensile and shear strength Microhardness Tooth Resistance to Increased macrostructure deformation fragility Resistance to Reduced retention/ fracture stability Resistance to fatigue prosthesis between peritubular and intertubular dentin and depends on tooth location. Peritubular dentin presents an elasticity modulus of 29.8 GPa, whereas intertubular dentin ranges from 17.7 GPa (close to pulp) to 21.1 GPa (close to root surface).74,91,107 Most if not all the decrease in hardness on approaching the pulp can be attributed to changes in hardness of the intertubular dentin.90,91 Overall dentin elasticity modulus is considered to be in the range of 16.5 to 18.5 GPa16,33,52 even though variations can be observed due to the measuring methods.92,127,145 The changes in mineral density due to the variation in the number and diameter of tubules within the tooth may also explain variations in the properties of dentin. It was shown that dentin hardness values are inversely related to dentin tubule density.130 Ultra microindentation measurements also demonstrated significantly higher values for hardness and elas- ticity modulus when forces were parallel to the tubules rather than perpendicular.139 Differences in maximum strength and compressive strength were also found to vary according to tubule orientation.127 The ultimate tensile strength (UTS) of human dentin is lowest when the tensile force is parallel to tubule orientation, showing the influence of dentin micro- structure and anisotropy of the tissue.99 No difference was found in the Young modulus of aged, transparent dentin (also called sclerotic) and normal dentin,20,93,175 but the mineral con- centration significantly increases and crystallite size is slightly smaller in transparent dentin, in relation with closure of the tubule lumens. Transparent dentin, unlike normal dentin, exhibits almost no yielding before failure. Its fracture tough- ness is also lowered by about 20% while the fatigue lifetime is deleteriously affected.93 However, no or only minor differences in microhardness values were found between vital and nonvital dentin of con- tralateral teeth after periods varying from 0.2 to 10 years.100,161 The literature does not support a widely held belief that attri- butes particular weakness or brittleness to nonvital dentin. It A C E G FIG. 22-1 The negative impact of poor initial biomechanical removal of the old metallic foundation. C, A new amalgam D, Prosthetic restorations on working model. E, Full periapical lesion. G, This untreatable tooth was finally a stable situation. Other teeth with less extensive 780 PART III RELATED CLINICAL TOPICS FIG. 22-2 Severe discoloration can significantly disturb even in the lateral area of the smile. When not treatable with bleaching agents or veneers, this condition might justify tooth preparation for a full crown. Esthetic Changes in Nonvital and Endodontically Treated Teeth Color change and darkening of nonvital teeth is a common clinical observation (Fig. 22-2), and improper endodontic techniques can contribute to discoloration. For instance, inad- equate cleaning and shaping can leave necrotic tissue in coronal pulp horns, resulting in tooth darkening. As well, root canal filling materials (gutta-percha and root canal cements) retained in the coronal aspect of anterior teeth can detract from the esthetic appearance. Opaque substances also adversely affect the color and translucency of most uncrowned teeth. Biochemically altered dentin modifies tooth color and appear- ance. It is generally accepted that organic substances present in dentin (e.g., hemoglobin) might play an important role in this color change and also food and drink pigment penetration triggered by the absence of pulpal pressure. However, the respective contribution of these two phenomena and precise physicochemical mechanisms leading to discoloration are poorly understood or described in the literature.35,71,137 Thin gingival tissue or in general, thin biotype is consid- ered a negative factor for esthetic outcome of restorative and prosthetic treatment of discolored teeth.116,117,124 Endodontic treatment and subsequent restoration of teeth in the esthetic zone require careful control of procedures and materials to retain a translucent, natural appearance. It is therefore strongly recommended to avoid the use of potentially staining endodontic cements and to clean all material residues in the pulpal chamber and access cavity. RESTORATIVE MATERIALS AND OPTIONS Endodontic treatment can result in significant loss and weak- ening of tooth structure. Tooth structure lost during endodon- tic treatment increases the risk of crown fracture, with fatigue mechanisms mediating the fracture of roots over time. Restora- tions of endodontically treated teeth are designed to (1) protect the remaining tooth from fracture, (2) prevent reinfection of the root canal system, and (3) replace the missing tooth structure. in vitro fibers to increase their mechanical resistance. Although most studies on the clinical performance of direct composite restorations were conducted on vital teeth, a recent clinical report indicates that direct in vitro fiber-reinforced composite restorations may represent a valuable alternative to conven- tional restorations of endodontically treated teeth.38 On the contrary, inserting an in vitro fiber post in the root canal of an endodontically treated tooth before bonding a direct MOD restoration significantly reduces its fracture resistance com- pared to the same composite restoration without a post.167 Indirect Restorations: Composite or Ceramic Onlays and Overlays Ceramic or resin composite onlays and endocrowns can also be used to restore endodontically treated teeth. Whereas over- lays incorporate a cusp or cusps by covering the missing tissue, endocrowns combine the post in the canal, the core, and the crown in one component.94 Both onlays and endocrowns allow for conservation of remaining tooth structure, whereas the alternative would be to completely eliminate cusps and perim- eter walls for restoration with a full crown.62 Onlays and over- lays are generally constructed in the laboratory from either hybrid resin composite or ceramics. Ceramics are a material of choice for long-term esthetic indirect restorations because their translucency and light transmission mimic enamel. Whereas traditional feldspathic porcelains were sintered from a slurry, new ceramic materials may be cast, machined, pressed, or slip-cast in addition to being sintered. New materials are either variations of feld- spathic porcelains (e.g., In-Ceram, Cerec, IPS Empress) or may be fabricated from other ceramic systems, including alumina, zirconia, or silica.3,39 Among these newer compositions is lithium disilicate, which offers high strength, high fracture toughness, and a high degree of translucency. Physical proper- ties of these materials have improved to the point where they can survive high stress-bearing situations such as posterior restorations in endodontically treated teeth.46,77 Researchers have examined 140 partial Cerec restorations (Vita MKII, feld- spathic porcelain) adhesively cemented to endodontically treated teeth and found this treatment approach satisfactory after an observation period of 55 months.10 Their results indi- cate that survival rates are higher for molars than for premolars. Onlays, overlays, and endocrowns can also be fabricated from resin composites processed in the laboratory. Using various combinations of light, pressure, and vacuum, these fabrication techniques are claimed to increase the conversion rate of the polymer and consequently the mechanical proper- ties of the restorative material. Other investigations have described the application of glass fiber–reinforced composite endocrowns on premolars and molars as single restorations or as abutment for fixed partial dentures.62,63 A recent in vitro study by another research team indicates that composite inlays can partially restore the resistance to fracture of endodontically treated molars and prevent catastrophic fractures after loading.29 Other investigators reported that composite resin MZ100 increased the fatigue resistance of overlay-type restora- tions in endodontically treated molars when compared to por- celain MKII.103 Another recent study used three-dimensional finite element analysis to estimate bone resorption around endocrowns made up of high- (alumina) or low-elastic modulus 782 PART III RELATED CLINICAL TOPICS 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 con- cavities 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 repeat- edly on the same root because of the nondestructive nature of the test. Study results indicate that root deformability is sig- nificantly 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 pre- paration of the post space. This is a reason why not every endodontically treated tooth needs a post and why more con- servative 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: ◆ Maximal protection of the root from fracture ◆ Maximal retention within the root and retrievability ◆ Maximal retention of the core and crown ◆ Maximal protection of the crown margin seal from coronal leakage ◆ Pleasing esthetics, when indicated ◆ High radiographic visibility ◆ Biocompatibility 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 would have an optimal combination of resilience, stiffness, flexibility, and strength. Resilience is the ability to deflect elasti- cally 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 resil- ient, 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 poten- tially devastating caries or endodontic leakage and apical rein- fection. Extensive caries extending into the root can be as irreparable as root fracture. Because rigid posts flex and bend A FIG. 22-3 A, Stress distribution within a metallic studies. The post is cemented and usually penetrates around the post and further inside the canal, around a fiber post (as shown in Fig. 22-3, B). This configuration untreatable root fractures. B, Stress distribution and FEM studies. The post is bonded to the canal walls the post in the cervical area. This configuration protects The presence of a ferrule is mandatory. (Adapted from endodontically treated teeth: a systematic review of 38:733–743, 2007.) 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 the post has a higher modulus than the dentin, the stress con- centration 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. 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 founda- tion 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 com- ponents used for fabricating a direct foundation restoration 784 PART III RELATED CLINICAL TOPICS active posts and passive posts. Active posts derive their primary retention directly from the root dentin by the use of threads. Most active posts are threaded and are intended to be screwed into the walls of the root canal. A major concern about threaded posts has been the potential for vertical root fracture during placement. As the post is screwed into place, it introduces great stresses within the root, causing a wedging effect.171 Therefore, it is generally accepted that the use of threaded posts should be avoided. Furthermore, the improved retention once offered by threaded posts can now be achieved with adhesive luting cements (see later).123 Passive posts are passively placed in close contact to the dentin walls, and their retention primarily relies on the luting cement used for cementation. The shape of a passive post may be either tapered or parallel.146 post is more retentive than a tapered post but also requires removal of more root dentin during the preparation of the post space. Parallel posts are reported to be less likely to cause root fractures than tapered posts, although they are less conforming to the original shape of the root.83,158,170 Unfortunately, modern techniques for root canal preparation use tapered nickel- titanium (NiTi) rotary shaping files, which results in a very wide tapered and unretentive canal exhibiting a significant divergence from apical to coronal.152 Longer posts are often necessary to accommodate this problem and offer adequate retention; adequate length in the root canal is considered to be greater than 6 mm. When teeth are protected by crowns with an adequate ferrule, longer posts do not further increase frac- ture resistance.84 Posts designed with mechanical locking fea- tures in the heads and roughened surface texture can show better retention of the core.30 Fiber Posts A fiber post consists of reinforcing fibers embed- ded in a resin polymerized matrix. Monomers used to form the resin matrix are typically bifunctional methacrylates (Bis- GMA, UDMA, TEGDMA), but epoxies have also been used. Common fibers in today’s fiber posts are made of carbon, glass, silica, or quartz but the type, volume content, and uniformity of the fibers and the matrix are proprietary and vary among fiber post systems. These differences in the manufacturing process may reflect the large variations observed among differ- ent types of fiber posts subjected to a fatigue resistance test.67 Fibers are commonly 7 to 20 µm in diameter and are used in a number of different configurations, including braided, woven, and longitudinal. The original fiber posts consisted carbon fibers embedded in epoxy resin, but quartz-fiber posts are currently preferred for their favorable mechanical proper- ties, esthetic qualities, and their ability to chemically bond to the polymer matrix.51 A recent study indicates that the flexural strength of glass-, silica-, or quartz-fiber posts appro- ximates 1000 MPa and that flexural modulus is about 23 GPa.36 Current fiber posts are radiopaque and may also conduct the light for polymerization of resin-based luting cements. A light- transmitting post results in better polymerization of resin com- posites in the apical area of simulated root canals, as measured by hardness values.148,182 To enhance bonding at the post/core/ cement interfaces, several physicochemical pretreatments such as silanization or sand blasting of the post surface have been described. Recent research indicates that silanization, hydro- fluoric etching, and sandblasting (with 30 to 50 µm Al2O3) do not modify the mechanical properties of different glass-, silica-, or quartz-fiber posts.5 cure adhesives.27 However, no adhesive has been shown to completely eliminate microleakage at the margins of the resto- ration.18 A degradation of the resin core or the marginal integ- rity of the crown can result in invasion of oral fluids. Therefore, as with all buildup materials for decimated teeth, more than 2 mm of sound tooth structure should remain at the margin for optimal composite resin core function. Composite core materials can be used in association with metallic, fiber, or zirconia posts. This is frequently observed in the presence of structurally compromised teeth. They may provide some protection from root fracture in teeth restored with metal posts compared with amalgam or gold cores. Loos- ening of the post, core, and crown with composite core can occur, but composite cores have been shown to fail more favor- ably than amalgam or gold.135 A recent retrospective study of the clinical performance of fiber posts indicates that fiber posts and cores have a failure rate ranging between 7% and 11% after a service period of 7 to 11 years and that post loosening may also occur.56 Composite core materials are typically two-paste, self-cured composites, but light-curing materials are also avail- able. The use of light-curing composite core materials generally eliminates the risk of chemical incompatibility between adhe- sives and self-curing resin core materials. Bonding resin composites to the irregular structure of the pulp chamber and canal orifices might eliminate the need for a post when sufficient tooth structure remains. Research indicates that bonding to the dentin walls of the pulp chamber is easier and superior to resin dentin bonds made on dentin canal walls.8 Amalgam Core Dental amalgam is a traditional core buildup material with a long history of clinical success. Although there are many variations in the alloy’s composition, more recent formulations have high compressive strength (400 MPa after 24 hours), high tensile strength, and a high modulus of elastic- ity. High-copper alloys tend to be stiffer (60 GPa) than low- copper alloys. Amalgam can be used with or without a post. Thirty years ago, investigators described the amalcore.119 With the amalcore technique, amalgam is compacted into the pulp chamber and 2 to 3 mm coronally of each canal. The following criteria were considered for the application of this technique: the remaining pulp chamber should be of sufficient width and depth to provide adequate bulk and retention of the amalgam restora- tion, and an adequate dentin thickness around the pulp chamber was required for the tooth-restoration continuum rigidity and strength. The fracture resistance of the amalgam coronal-radicular restoration with four or more millimeters of chamber wall was shown to be adequate, although the exten- sion into the root canal space had little influence.87 Amalgam can also be used in combination with a prefabri- cated metallic post when the retention offered by the remain- ing coronal tissue needs to be increased. Amalgam cores are highly retentive when used with a preformed metal post in posterior teeth; they require more force to dislodge than cast posts and cores.108 Others have suggested the use of adhesive resins to bond amalgam to coronal tissue.162 Significant disadvantages of amalgam cores are the “non- adhesive nature” of the material, the potential for corrosion, and subsequent discoloration of the gingiva or dentin. Amalgam use is declining worldwide because of legislative, safety, and environmental issues. 786 PART III RELATED CLINICAL TOPICS 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. Luting Cements A variety of cements have been used to cement endodontic posts and include traditional cements, glass ionomer cements, and resin-based luting cements. 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 phos- phate 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 mechan- ical 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 conven- tional 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 Con- 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 reten- tion 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 perfor- mance 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 com- pensate for polymerization stresses.14 The bonding procedures are realized in two separate steps. The initial step 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 particu- larly true for etch-and-rinse adhesives, because the tion of collagen fibers caused by phosphoric acids the diffusion of the resin within the interfibrillar may leave unprotected fibers available for degradation. Remov- ing 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 chemi- cal 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 intro- duced as an alternative to conventional resin-based luting 788 PART III RELATED CLINICAL TOPICS Maintenance of biologic width and ferrule effect before and after endodontic and restorative phases If one or more of the aforementioned conditions are not met owing to preexisting pathology or structural defects, treat- ment success or even feasibility can be compromised, some- times 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 treat- ments that carry a limited probability of success. Important clinical factors include the following: ◆ The amount and quality of remaining tooth structure ◆ The anatomic position of the tooth ◆ The occlusal forces on the tooth ◆ The restorative requirements of the tooth Teeth with minimal remaining tooth structure are at increased risk for the following clinical complications120,173,178 (see Fig. 22-1; Fig. 22-4): ◆ Root fracture ◆ Coronal-apical leakage ◆ Recurrent caries ◆ Dislodgment or loss of the core/prosthesis ◆ Periodontal injury from biologic width invasion 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 con- sider 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. FIG. 22-4 Failure of prosthetic foundations can have dramatic con- sequences 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. or reduce the risk of discoloration. Careful selection of restor- ative materials, careful handling of tissues, and timely end- odontic intervention are important to preserving the natural appearance of nonvital teeth and gingiva. Treatment Strategy General Principles and Guidelines The post, the core, and their luting or bonding agents form together the foundation restoration to support the coronal res- toration 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 restor- ative 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 system- atically 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 torqu- ing forces. This modern biomechanical treatment strategy is summarized in Fig. 22-5. 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. Discol- oration, 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). 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 (proxi- mal ridges and buccolingual walls more then 1.5 mm thick- ness). The three additional clinical factors which must be analyzed to ensure optimal treatment success are the configu- ration 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 indica- tions 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 favor- 790 PART III RELATED CLINICAL TOPICS Clinical conditions No discoloration responding !$% bleaching (internal ! Direct composite Conservative lingual access cavity !$% bleaching (internal ! Direct composites Class III cavities (! conservative lingual access cavity) !$% bleaching (internal ! Direct composites Class IV cavity (! conservative lingual access cavity) Limited over-bite and Adhesive core ! Full crown Large decay but "1/2 residual tooth structure and ferrule effect #1/2 residual tooth structure and/or limited ferrule effect A FIG. 22-5 A, Current recommendations for the treatment severe parafunctions and abnormal occlusion/anterior Limited Clinical conditions Small cavity size or conservative approach Class I Class I direct composite or inlay Class II MO/OD Class II direct composite or inlay Class II MOD Class II direct composite or inlay Conservative 1/2 residual tooth structure Endocrowns (ceramic or composite) 1/2 residual tooth structure B FIG. 22-5, cont’d B, Current recommendations for normal function; **group guidance, steep occlusal anatomy, 792 PART III RELATED CLINICAL TOPICS TABL E 2 2 - 2 Clinical Protocols for Restoring Nonvital Teeth With (Most Likely Procedures) Treatment Tooth Preparation Approach Indications (Critical Guidelines) Composite Minimal None restoration tissue loss Veneer Limited ≥1 mm Buccal tissue loss reduction, lingual enamel present, minimal to moderate discoloration only Overlay Thin Minimum 2 mm (composite/ remaining occlusal reduction ceramics) walls Endocrown Loss of Minimum 2 mm (composite/ occlusal occlusal reduction, ceramics) anatomy extension into pulpal chamber CAD-CAM, Computer-aided design/computer-aided machined; curing; PFM, porcelain fused to metal; SA, self-adhesive. 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 significan 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 founda- tion. 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 gold post and core are still considered a feasible option. Actu- ally, in this extremely unfavorable biomechanical environ- and AVISHAI SADAN Full Crowns Luting Cements PRETREATMENT EVALUATION AND TREATMENT STRATEGY Pretreatment Evaluation Treatment Strategy CLINICAL PROCEDURES Tooth Preparation Post Placement Adhesive Procedures Partial Restorations Foundation Restorations for Full Crowns Crown Preparation and Temporary Restoration showed no difference in moisture content between vital and nonvital teeth.129 No difference in collagen cross-linkage was found between vital and nonvital dentin.147 There is no other evidence of chemical alteration due to the removal of pulpal tissue. Sodium hypochlorite and chelators such as ethylenedi- amine tetra-acetic acid (EDTA), cyclohexane-1,2-diaminetetra- acetic acid (CDTA), ethylene glycol-bis-(β-amino-ethyl ether) N,N,N′,N′-tetra-acetic acid (EGTA), and calcium hydroxide (Ca[OH]2) commonly used for canal irrigation and disinfec- tion interact with root dentin, either with mineral content (chelators) or organic substrate (sodium hypochlorite).82,115,122 Chelators mainly deplete calcium by complex formation and also affect noncollagenous proteins (NCP), leading to dentin erosion and softening.82,88,151 Sodium hypochlorite supposedly demonstrates a proteolytic action by extensive fragmentation of long peptide chains such as collagen.72 These alterations are likely to fragilize dentin and root structure and reduce adhe- sion to this substrate. change Dentin Structure and Properties in Nonvital and Endodontically Treated Teeth It is important to know about the normal variations in dentin physical properties, which must be differentiated from altera- tions related to loss of vitality or endodontic treatment. For instance, dentin microhardness and elasticity actually varies 777 is also believed that the age-related reduction of pulp volume, progressively replaced by secondary or tertiary dentin, could account for a reduced fracture resistance of aged, nonvital teeth; this assumption is evaluated in the literature. The only impact of age-related tissue changes are the aforementioned reduction in fracture toughness and fatigue lifetime attributed to dentin sclerosis.93 The chemicals used for canal irrigation and disinfection, as tooth already mentioned, interact with mineral and organic contents and then reduce dentin elasticity and flexural strength to a significant extent,68,164 as well as microhardness.34,81,153 On the contrary, disinfectants like eugenol and formocresol increase tooth dentin tensile strength by way of protein coagulation and chelation with hydroxyapatite (eugenol). Dentin hardness, however, did not prove to be influenced by the latter products.118 tooth In conclusion, possible decrease in tooth strength can be attributed to dentin aging and to a smaller extent to dentin alteration by endodontic irrigants. of the Fracture Resistance and Tooth Stiffness of Nonvital and Endodontically Treated Teeth The major changes in tooth biomechanics are attributed to the loss of tissue following decay, fracture, or cavity preparation, including the access cavity prior to endodontic therapy. The loss of tooth structure following a conservative access cavity preparation affects tooth stiffness by only 5%.177 The influence of subsequent canal instrumentation and obturation lead only to a slight reduction in the resistance to fracture177 and ultimately have little effect on tooth biomechanics.144,177 Practically, one can expect alteration of tooth biomechanics only in cases of nonconservative canal preparation or through the chemical or structural alteration triggered by endodontic irrigants, as previously mentioned. In fact, the largest reduction in tooth stiffness results from additional preparation, especially the loss of marginal ridges. The literature reports 20% to 63% and 14% to 44% reduction in tooth stiffness following occlusal and MOD cavity prepara- tions, respectively.45,98,144 It was shown that an endodontic access cavity combined with an MOD preparation results in maximum tooth fragilization. The cavity depth, isthmus width, and configuration are therefore highly critical to the reduction in tooth stiffness and risk of fracture (Fig. 22-1).78,89,102,128 The presence of residual tissue in the cervical area (which comprises the ferrule for restorations) and a larger amount of residual tissue in general increase tooth resistance to frac- ture. It actually allows the axial walls of the crown to encircle the tooth, providing restoration retention and stabilization and reducing cervical tensile stresses.7,24,169 Crown prepara- tions with as little as 1 mm coronal extension of dentin above the margin doubles the fracture resistance of preparations, compared to those where the core terminates on a flat surface immediately above the margin109,169; therefore, a minimal 1-mm ferrule is considered necessary to stabilize the restoration.169 However, the widths of preparation shoulder and crown margin do not appear to influence fracture strength.2 It is critical to understand that the most important part of the restored tooth is the tooth itself, and no restorative material or combination of materials will perfectly substitute for tooth structure. CHAPTER 22 Restoration of the Endodontically Treated Tooth 779 B D F H status on restoration success. A-B, Preoperative radiographic views following the core, using post and self-anchorage into mesial root structure, was performed. arch view after 3 years. F, The tooth is symptomatic due to furcation involvement and extracted and replaced by an implant. H, Eight-year postoperative radiograph showing biomechanical damage overcame functional stresses. According to the amount of tissue to be replaced, restora- tions of endodontically treated teeth will rely on different materials and clinical procedures. As a general rule, most structurally damaged teeth should be restored with an artificial crown. Although the use of a crown built on post and core is a traditional approach, others have advocated the use of direct composite resins for restoring small defects in endodontically treated teeth. More recently, indirect restorations such as over- lays or endocrowns made of composite resins or ceramics have also been used. The selection of appropriate restorative materi- als and techniques is dictated by the amount of remaining tooth structure. This is far more relevant to the long-term prognosis of endodontically treated teeth than any properties of post, core, or crown materials. esthetics, Direct Composite Restorations When a minimal amount of coronal tooth structure has been lost after endodontic therapy, a direct resin composite restora- tion may be indicated. Composite resins are a mixture of a polymerized resin network reinforced by inorganic fillers. Contemporary composites have compressive strengths of about 280 MPa, and the Young modulus of composite resins is generally about 10 to 16 GPa, which is close to that of dentin.140 When properly cured, resin composites are highly esthetic, exhibit high mechanical properties, and can reinforce the remaining tooth structure through bonding mechanisms. Typi- cally, 500 to 800 mW/cm2 of blue light for 30 to 40 seconds is necessary to polymerize an increment of composite which must be 1 to 3 mm thick. Unfortunately, the shrinkage that accompanies polymerization of contemporary composite resins remains a significant problem to the long-term success of these restorations. The use of an incremental filling technique, which helps to reduce shrinkage stresses during polymeriza- tion, is highly recommended. The amount of shrinkage will also depend on the shape of the cavity preparation and the ratio of bonded to unbonded (or free) surfaces.37,55 This so- called C-factor is a clinically relevant predictor of the risk of debonding and leakage; restorations with high C-factors (>3.0) are at greatest risk for debonding.183 In other words, a direct composite restoration may be indicated when only one proxi- mal surface of the tooth has been lost; using an incremental filling technique is mandatory. Classically, direct composite restorations have been placed in anterior teeth that have not lost tooth structure beyond the endodontic access preparation. In such cases, the placement of a direct composite restoration offers an immediate sealing of the tooth, which prevents coronal leakage and recontamina- tion of the root canal system with bacteria. In vitro studies have demonstrated that the fracture resistance of small bonded res- torations is nearly as great as that of intact teeth.61,142 Although direct composite resins may also be used for small restorations in posterior teeth, they are contraindicated when more than a third of coronal tissue has been lost. In one study,142 it was reported that the resistance to fracture of end- odontically treated teeth is reduced by 69% in cases where MOD cavities are present.143 Under such conditions, a direct composite restoration may not be appropriate to prevent the tooth structure from fracture and reinfection. Furthermore, resin composite materials may require the use of reinforcing CHAPTER 22 Restoration of the Endodontically Treated Tooth 781 materials (resin composites). They concluded that the higher resilience of resin composite restorations acts positively against the risk of periodontal bone resorption by reducing the amount of force transferred to root dentin.4 Full Crowns When a significant amount of coronal tooth structure has been lost by caries, restorative procedures, and endodontics, a full crown may be the restoration of choice. In a few cases, the crown can be directly built on the remaining coronal structure which has been prepared accordingly (see Core Materials heading). More frequently, the cementation of a post inside the root canal is necessary to provide retention for the core mate- rial and the crown.172 The core is anchored to the tooth by extension into the root canal through the post and replaces missing coronal structure. The crown covers the core and restores the esthetics and function of the tooth. An additional role of the post and core is to protect the crown margins from deformation under function and thereby to prevent coronal leakage. Because most endodontic sealers do not completely seal the root canal space, the coronal seal provided by the placement of a post and core will positively influence the outcome of the endodontic treatment.155 The post’s ability to anchor the core is also an important factor for successful reconstruction, since the core and the post are usually fabricated of different materials. Finally, the luting material used to cement the post, the core, and the crown to the tooth will also influence the longevity of the restoration. The post, the core, and their luting or bonding agents together form a foundation restoration to support the future crown.112 The Foundation Restoration: General Considerations Although there are many materials and techniques to fabricate a foundation restoration, no combination of materials can sub- stitute 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 end- odontically 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: 1. The ferrule (dentin axial wall height) must be at least 2 to 3 mm. 2. The axial walls must be parallel. 3. The restoration must completely encircle the tooth. 4. The margin must be on solid tooth structure. 5. The crown and crown preparation must not invade the attachment apparatus. Root anatomy can also have significant influence over post placement and selection. Root curvature, furcations, develop- mental depressions, and root concavities observed at the exter- nal surface of the root are all likely to be reproduced inside the root canal. Within the same root, the shape of the canal 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 concentra- tion of a stiff rod in a more flexible material. Stress concentra- tion 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 signifi- cantly 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. ideal post 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 initia- tion and propagation of cracks; many failures of teeth or mate- rials 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. CHAPTER 22 Restoration of the Endodontically Treated Tooth 783 B post and core foundation and residual tooth structure, according to photoelastic and FEM the apical portion of the root. Functional stresses accumulate inside the foundation, slightly the apex of the post; there is less stress buildup in the cervical area compared to that with more ideally protects the coronocervical structures but when failing, results in severe within a fiber post/composite foundation and residual tooth structure, according to photoelastic and penetrates the canal less apically. Functional stresses mainly accumulate around the cervical area less efficiently but tends to prevent untreatable root fractures. Dietschi D, Duc O, Krejci I, Sadan A: Biomechanical considerations for the restoration of the literature—Part 1. Composition and micro- and macrostructure alterations. Quintessence Int (e.g., the endodontic post and core material) are described individually. dentin. If POSTS The large number of post designs and materials available on the market reflects the absence of consensus in that field. Based on what manufacturers or clinicians consider the most impor- tant properties, posts can be fabricated from metal (gold, tita- nium, stainless steel), ceramic, or fiber-reinforced resins. As a general rule, a post needs retention and resistance. Whereas post retention refers to the ability of a post to resist vertical forces, resistance refers to the ability of the tooth/post combina- tion to withstand lateral and rotational forces. Resistance is influenced by the presence of a ferrule, the post’s length and rigidity, and the presence of antirotational features. A restora- tion lacking resistance form is not likely to be a long-term success, regardless of the retentiveness of the post. Prefabricated Metallic Posts Prefabricated metallic posts are frequently used for the fabrication of a direct foundation res- toration. These posts are classified several ways, including by alloy composition, retention mode, and shape. Materials used to fabricate metallic posts include gold alloys, stainless steel, or titanium alloys. Metallic posts are very strong, and with the exception of the titanium alloys, very rigid.96 A recent study indicates that the flexural strength of stainless steel posts is about 1430 MPa and that flexural modulus approximates 110 GPa.136 On the other hand, titanium posts are less rigid (66 GPa) but exhibit a flexural strength (1280 MPa) similar to stainless steel. The retention of prefabricated posts inside the root canal is also essential for successful restorations. Two basic concepts have been used to promote the retention of endodontic posts: It is generally accepted that bonding fiber posts to root canal dentin can improve the distribution of forces applied along the root, thereby decreasing the risk of root fracture and contributing to the reinforcement of the remaining tooth struc- ture.13,19,58 A well-adapted adhesively cemented fiber post is considered the most retentive with the least stress generated on the canal walls. In a retrospective study which evaluated three types of bonded fiber posts, investigators reported 3.2% failure of 1306 fiber posts in recalls of 1 to 6 years.57 More recently, another study reported survival rates of 98.6% and 96.8% for parallel-sided and tapered fiber posts, respectively, placed in anterior teeth covered with full-ceramic crowns after a mean observation period of 5.3 years.163 A parallel Zirconia Posts Zirconia posts are composed of zirconium dioxide (ZrO2) partially stabilized with yttrium oxide and exhibit a high flexural strength. Zirconia posts are esthetic, partially adhesive, very rigid, but also brittle. Zirconia posts cannot be etched, and available literature suggests that bonding resins to these materials is less predictable and requires sub- stantially different bonding methods than conventional ceram- ics.12 When a composite core is built on a zirconia post, core retention may also be a problem. Controversies exist about the efficiency of airborne particle abrasion at establishing a durable resin bond to zirconia posts treated or not treated with a cou- pling agent.1,133 Overall, there are concerns about the rigidity of zirconia posts, which tends to make those posts too brittle. Other reports indicate that the rigidity of zirconia posts nega- tively affects the quality of the interface between the resin core material and dentin when subjected to fatigue testing.41,44 CORE MATERIALS The core replaces carious, fractured, or missing coronal struc- ture and helps to retain the final restoration. Desirable physical characteristics of a core include (1) high compressive and flexural strength, (2) dimensional stability, (3) ease of manipu- lation, (4) short setting time, and (5) the ability to bond to both tooth and post. Core materials include composite resin, cast metal or ceramic, amalgam, and sometimes glass ionomer materials. The core is anchored to the tooth by extension into the coronal aspect of the canal or through the endodontic post. The importance of retention between the post, the core, and the tooth increases as remaining tooth structure decreases. of Composite Resin Core Composite core materials take a number of strategies to enhance their strength and resistance; metal may be added, filler levels may be greater, or faster- setting ionomers may be used.140 Composite core materials have been shown to exhibit slightly better mechanical values than conventional materials, but improvements are negligi- ble.184 However, they appear to be superior to silver- glass-ionomer cement and amalgam.31 The advantages of composite core resins are adhesive bonding to tooth structure and many posts, ease of manipulation, rapid setting, and trans- lucent or highly opaque formulations. Composite cores have been shown to protect the strength of all-ceramic crowns equally to amalgam cores. Bond strength of composite cores to dentin depends on a complete curing of the resin materials, so dentin bonding agents must be chemically compatible with composite core materials. Self-cure composite resins require self-cure adhesives and are mostly incompatible with light- CHAPTER 22 Restoration of the Endodontically Treated Tooth 785 Glass Ionomer Core and Modified Glass Ionomer Core Glass ionomer and resin-modified glass ionomer cements are adhe- sive materials useful for small buildups or to fill undercuts in prepared teeth. The rationale for using glass ionomer materials is based on their cariostatic effect resulting from fluoride release. However, their low strength and fracture toughness result in brittleness, which contraindicates the use of glass ionomer buildups in thin anterior teeth or to replace unsup- ported cusps. They may be indicated in posterior teeth in which (1) a bulk of core material is possible, (2) significant sound dentin remains, and (3) caries control is indicated.180 Resin-modified glass ionomer materials are a combination of glass ionomer and composite resin technologies and have properties of both materials. Resin-modified glass ionomers have moderate strength, greater than glass ionomers but less than composite resins. As a core material, they are adequate for moderate-sized buildups, but hygroscopic expansion can cause fracture of ceramic crowns and fragilized roots.165 The bond to dentin is close to that of dentin-bonded composite resin and significantly higher than traditional glass ionomers. Today, resin composites have replaced glass ionomer materials for core fabrication. light-cured 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 resis- tance 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 signifi- cant. 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 com- pared to prefabricated parallel-sided posts. In a classic retro- spective 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 temporary 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 photopolymeriza- tion process, or a combination of both mechanisms. Photo- polymerization 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 oxi- dizing 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 mois- ture 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 gener- ally accepted that bonding to dentin of the pulp chamber is CHAPTER 22 Restoration of the Endodontically Treated Tooth 787 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 sensi- tivity. 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 recom- mended.76 However, their long-term clinical performances need to be assessed before making a general recommendation for their use. allows optimal PRETREATMENT EVALUATION AND TREATMENT STRATEGY Before any therapy is initiated, the tooth must be thoroughly evaluated to ensure treatment success. Each tooth must be examined individually and in the context of its contribution to the overall treatment plan and rehabilitation. This assess- ment includes endodontic, periodontal, biomechanical, and esthetic evaluations. Planning of the restoration for endodonti- cally treated teeth brings together all aforementioned biome- chanical and clinical factors, as well as the various materials and procedures designed to address them. gelatiniza- may restrict Pretreatment Evaluation spaces and Endodontic Evaluation The prerestorative examination should include an inspection of the quality of existing endodontic treatment. New restora- tions, particularly complex restorations, should not be placed on abutment teeth with a questionable endodontic prognosis. Endodontic retreatment is indicated for teeth showing radio- graphic 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 inappropri- ate filling material should be endodontically retreated before starting any restorative therapy. Because the probability for periapical tissue to heal after endodontic retreatment is reason- ably high, the chances to retain a well-restored tooth in asymp- tomatic 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 follow- ing conditions are to be considered as critical for treatment success: Healthy gingival tissue Normal bone architecture and attachment levels to favor periodontal health 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 over- bite, maxillary anterior teeth are sustaining higher protrusive and lateral forces from the mandibular anterior teeth. Restora- tions 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, espe- cially 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 CHAPTER 22 Restoration of the Endodontically Treated Tooth 789 able biomechanical conditions (group guidance, steep occlusal anatomy, bruxism, clenching, etc.), a protective approach with full occlusal coverage (onlay or overlay) is mandated to mini- mize 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). 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 sup- posed 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 appro- priately 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 esthet- ics. 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, frac- ture, previous overenlargement of the root canal system, or immaturity, residual root structure can be unified and rein- forced 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 Conservative approach or discoloration Discoloration resistant to bleaching to bleaching and/or external) Direct composite (pulpal chamber and access cavity) Veneer or Full crown and/or external) Direct composite (pulpal chamber and access cavity) Veneer or Full crown and/or external) Direct composite (pulpal chamber and access cavity) Veneer or Full crown Protective approach Deep over-bite and increased functional stresses* functional stresses** Fiber, ceramic, or metal post and core ! Full crown Fibre or Metal Post and core ! Full crown of nonvital anterior teeth. *Normal function and anterior guidance; **moderate to guidance. CHAPTER 22 Restoration of the Endodontically Treated Tooth 791 functional and lateral stresses* Increased functional and lateral stresses** Large cavity size or protective approach Overlay Overlay Overlay Conventional or Esthetic indication 4 mm 1/2 Composite core Full crown Post and core Full crown the treatment of nonvital posterior teeth. *Relatively flat anatomy and canine guidance, parafunctions. Partial Restorations Interface Treatment Tooth Restoration Restoration Fabrication DBA — Direct multilayer DBA 1. Sandblasting or CP direct multilayer or etching In laboratory: 2. Silane Etchable CER: fired, pressed, 3. Bonding resin or CAD-CAM DBA + 1. Sandblasting or In laboratory: CP: hand- composite etching shaped, light and/or heat lining 2. Silane cured, CAD-CAM 3. Bonding resin Etchable CER: fired, pressed or CAD-CAM DBA + 1. Sandblasting or In laboratory: CP: hand- composite etching shaped, light and/or heat lining 2. Silane cured, CAD-CAM 3. Bonding resin Etchable CER: fired, pressed, or CAD-CAM CER, ceramic; CP, composite; DBA, dentin bonding agent; Dual, dual curing; LC, light 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. t 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. present), cast Additional Procedures Periodontal crown lengthening surgery or orthodontic extru

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