Special Features of Endodontically Treated Teeth
Once endodontic therapy is completed, the tooth must be adequately restored. Indeed, given the large impact that poor or missing restorations have on the survival of endodontically treated teeth, one could make the argument that the restoration is actually the last step of endodontic therapy. However, it is important to realize that endodontically treated teeth are structurally different from nontreated vital teeth. Major changes following treatment include altered tissue physical characteristics, loss of tooth structure, and possibly discoloration. Research has analyzed these tissue modifications at different levels, including tooth composition, dentin microstructure, and tooth macrostructure. These studies indicate that it is critical to understand the implication of such features on tooth biomechanics, as they will largely influence the restorative approach and means ( Table 22-1 ). Additional in vitro studies dealing with the complexity of the nonvital tooth substrate are reported in the literature; ultimately, clinical studies have documented the global effect of these changes on the long-term survival of endodontically treated teeth.
|Alteration Level||Specific Changes||Possible Clinical Implication|
Compositional Changes in Nonvital Teeth and the Influence of Endodontic Therapy
The loss of pulpal vitality is accompanied by a slight change in tooth moisture content. This loss of moisture (9%) is attributed to a change in free water but not in water bonded to the organic and inorganic components. This alteration was associated with a slight change in values for the Young modulus and proportional limit. However, no decrease in compressive and tensile strength values was associated with this change in water content. Only one study showed no difference in moisture content between vital and nonvital teeth. No difference in collagen cross-linkage was found between vital and nonvital dentin. Thus, nonvital teeth undergo rather minor changes in physical characteristics.
Sodium hypochlorite and chelators such as ethylenediamine 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 disinfection interact with root dentin, with either mineral content (chelators) or organic substrate (sodium hypochlorite). Chelators mainly deplete calcium by complex formation and also affect noncollagenous proteins (NCPs), leading to dentin erosion and surface softening. Depending on concentration, duration of exposure, and other factors (see Chapter 6 ), sodium hypochlorite can demonstrate a proteolytic action by hydrolysis of long peptide chains such as collagen. These alterations are likely to impact dentin and root structure and alter bonding properties of this substrate.
Dentin Structure and Properties in Nonvital and Endodontically Treated Teeth
It is important to know that dentin displays a range of normal variations in its physical properties which must be distinguished from alterations related to loss of vitality or endodontic treatment. For instance, dentin microhardness and elasticity actually vary between peritubular and intertubular dentin and depend 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 the root surface). Most if not all the decreases in hardness on approaching the pulp can be attributed to changes in hardness of the intertubular dentin. Overall dentin elasticity modulus is considered to be in the range of 16.5 to 18.5 Gpa.
The changes in mineral density due to the variation in the number and diameter of tubules within the tooth may also contribute to variations in the properties of dentin. Not surprisingly, dentin hardness values are inversely related to dentin tubule density. Ultra microindentation measurements also demonstrated significantly higher values for hardness and elasticity modulus when forces were parallel to the tubules rather than perpendicular. Differences in maximum strength and compressive strength were also found to vary according to tubule orientation. The ultimate tensile strength (UTS) of human dentin is lowest when the tensile force is parallel to tubule orientation, showing the influence of dentin microstructure and anisotropy of the tissue. No difference was found in the Young modulus of aged, transparent dentin (also called sclerotic ) and normal dentin, but the mineral concentration 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 toughness is also lowered by about 20%, whereas the fatigue lifetime is deleteriously affected.
Interestingly, comparisons between vital and nonvital dentin of contralateral teeth demonstrate no or only minor differences in microhardness values after periods ranging up to 10 years after treatment. Thus, the literature does not support a widely held belief that attributes particular weakness or brittleness to nonvital dentin. Others have suggested that nonvital teeth in older patients may have greater risk for fracture because the age-related generation of secondary or tertiary dentin would be lost. However, this is not the case because the only impact of age-related tissue changes is the aforementioned reduction in fracture toughness and fatigue lifetime attributed to dentin sclerosis.
The chemicals used for canal irrigation and disinfection, as already mentioned, interact with mineral and organic contents and then reduce dentin elasticity and flexural strength to a significant extent, as well as microhardness. On the contrary, disinfectants like eugenol and formocresol increase 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.
In conclusion, a possible decrease in tooth strength can be attributed to dentin aging and to a smaller extent to dentin alteration by endodontic irrigants.
Fracture Resistance and Tooth Stiffness of Nonvital and Endodontically Treated Teeth
In contrast to the aforementioned factors, the major changes in tooth biomechanics appear to be due to the loss of hard tissue following decay, fracture, or cavity preparation (including the access cavity prior to endodontic therapy).
The loss of hard tooth structure following a conservative access cavity preparation affects tooth stiffness by only 5%. The influence of subsequent canal instrumentation and obturation lead only to a slight reduction in the resistance to fracture and ultimately have little effect on tooth biomechanics. From a clinical perspective, 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 excessive access preparation, especially the loss of marginal ridges. The literature reports a 20% to 63% and a 14% to 44% reduction in tooth stiffness following occlusal and MOD cavity preparations, respectively. 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 ). This important point has profound clinical implications.
The presence of residual hard tissue in the cervical area (which constitutes the ferrule for restorations) and a larger amount of residual mineralized tissue in general increase tooth resistance to fracture. It actually allows the axial walls of the crown to encircle the tooth, providing restoration retention and stabilization and reducing cervical tensile stresses. Crown preparations with as little as 1 mm coronal extension of dentin above the margin double the fracture resistance of preparations, compared to those where the core terminates on a flat surface immediately above the margin ; therefore, a minimal 1-mm ferrule (and ideally 2 to 3 mm) is considered necessary to stabilize the restoration. However, the widths of preparation shoulder and crown margin do not appear to influence fracture strength. Taken together, the height of the ferrule is one of the most important elements in the long-term survival of restorations. These considerations have led to the conclusion that the most important part of the restored tooth is the tooth itself, and no contemporary restorative material or combination of materials will perfectly substitute for lost tooth structure.
Aesthetic Changes in Nonvital and Endodontically Treated Teeth
Several aesthetic changes may also occur in nonvital or endodontically treated teeth. For example, color change or darkening of nonvital teeth is a common clinical observation ( Fig. 22-2 ). In addition, incomplete endodontic treatment can contribute to discoloration. For instance, inadequate cleaning and shaping can leave necrotic tissue in coronal pulp horns, resulting in tooth darkening. In addition, root canal filling materials (gutta-percha and root canal sealer cements, MTA-like materials) retained in the coronal aspect of anterior teeth can detract from the aesthetic appearance. Opaque substances also adversely affect the color and translucency of most uncrowned teeth. Biochemically altered dentin modifies tooth color and appearance. 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.
Thin gingival tissue or, in general, thin biotype is considered a negative factor for aesthetic outcome of restorative and prosthetic treatment of discolored teeth.
Endodontic treatment and subsequent restoration of teeth in the aesthetic zone require careful control of procedures and materials to retain a translucent, natural appearance. It is therefore strongly recommended that one avoid the use of potentially staining endodontic cements and clean all material residues left in the pulpal chamber and access cavity.
Restorative Materials and Options
As described previously, endodontic treatment, particularly excessive access preparations, can result in significant loss and weakening of tooth structure. Tooth structure lost during endodontic treatment increases the risk of crown fracture, with fatigue mechanisms mediating the fracture of roots over time. Restorations 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.
According to the amount of tissue to be replaced, restorations of endodontically treated teeth 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 overlays or endocrowns made of composite resins or ceramics have also been used. The selection of appropriate restorative materials 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.
Direct Composite Restorations
When a minimal amount of coronal tooth structure has been lost after endodontic therapy, a direct resin composite restoration 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.
When properly cured, resin composites are highly aesthetic, exhibit high mechanical properties, and can reinforce the remaining tooth structure through bonding mechanisms. Typically, 500 to 800 mW/cm 2 of blue light for 30 to 40 seconds is necessary to polymerize an increment of composite that 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 polymerization, 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. 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. In other words, a direct composite restoration may be indicated when only one proximal 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 recontamination of the root canal system with bacteria. In vitro studies have demonstrated that the fracture resistance of small bonded restorations is nearly as great as that of intact teeth.
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, it was reported that the resistance to fracture of endodontically treated teeth is reduced by 69% in cases where MOD cavities are present. 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 in vitro fibers to increase their mechanical resistance. Although most studies on the clinical performance of direct composite restorations were conducted on vital teeth, one clinical report indicates that direct in vitro fiber-reinforced composite restorations may represent a valuable alternative to conventional restorations of endodontically treated teeth. 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 compared to the same composite restoration without a post.
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 overlays 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. Both onlays and endocrowns allow for conservation of remaining tooth structure, whereas the alternative would be to completely eliminate cusps and perimeter walls for restoration with a full crown. Onlays and overlays are generally constructed in the laboratory from either hybrid resin composite or ceramics.
Ceramics are a material of choice for long-term aesthetic 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 either are variations of feldspathic porcelains (e.g., In-Ceram, Cerec, IPS Empress) or may be fabricated from other ceramic systems, including alumina, zirconia, or silica. Among these newer compositions is lithium disilicate, which offers high strength, high fracture toughness, and a high degree of translucency. Physical properties of these materials have improved to the point where they can survive high stress-bearing situations such as posterior restorations in endodontically treated teeth. Researchers have examined 140 partial Cerec restorations (Vita MKII, feldspathic porcelain) adhesively cemented to endodontically treated teeth and found this treatment approach satisfactory after an observation period of 55 months. Their results indicate 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 properties 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. One 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. Other investigators reported that composite resin MZ100 increased the fatigue resistance of overlay-type restorations in endodontically treated molars when compared to porcelain MKII. Another study used three-dimensional finite element analysis to estimate bone resorption around endocrowns made up of high-(alumina) or low-elastic modulus 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.
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 the Core Materials section). More frequently, the cementation of a post inside the root canal is necessary to retain the core material and the crown. 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 aesthetics 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. The post’s ability to anchor the core is also an important factor for successful reconstruction, because 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.
The Foundation Restoration: General Considerations
Although many materials and techniques can be used 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. 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. A longer ferrule increases fracture resistance significantly. 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:
The ferrule (dentin axial wall height) must be at least 2 to 3 mm.
The axial walls must be parallel.
The restoration must completely encircle the tooth.
The margin must be on solid tooth structure.
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, developmental depressions, and root concavities observed at the external surface of the root are all likely to be reproduced inside the root canal (see also Chapter 5 ). Within the same root, the shape of the canal will vary between the cervical level and the apical foramen. 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 that exhibit deep concavities on the furcal surface of their mesial root. 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. 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 increases significantly 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:
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 aesthetics, when indicated
High radiographic visibility
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 nonstiff 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 do high-modulus posts.
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 is likely to occur. 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. 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.
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 are 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. For clarity, the components used for fabricating a direct foundation restoration (e.g., the endodontic post and core material) are described individually.
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 important properties, posts can be fabricated from metal (gold, titanium, 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 combination 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 restoration 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 restoration. 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. One study indicates that the flexural strength of stainless steel posts is about 1430 MPa and that flexural modulus approximates 110 GPa. 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: 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. 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 (discussed later). 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. A parallel 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. Unfortunately, modern techniques for root canal preparation use tapered nickel-titanium (NiTi) rotary shaping files, which result in a very wide tapered and unretentive canal exhibiting a significant divergence from apical to coronal. 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 fracture resistance. Posts designed with mechanical locking features in the heads and roughened surface texture can show better retention of the core.
A fiber post consists of reinforcing fibers embedded 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 different types of fiber posts subjected to a fatigue-resistance test. Fibers are commonly 7 to 20 µm in diameter and are used in a number of configurations, including braided, woven, and longitudinal. The original fiber posts consisted of carbon fibers embedded in epoxy resin, but quartz-fiber posts are currently preferred for their favorable mechanical properties, aesthetic qualities, and ability to chemically bond to the polymer matrix. One study indicates that the flexural strength of glass-, silica-, or quartz-fiber posts approximates 1000 MPa and that flexural modulus is about 23 GPa. 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 composites in the apical area of simulated root canals, as measured by hardness values. 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. Research indicates that silanization, hydrofluoric etching, and sandblasting (with 30 to 50 µm Al 2 O 3 ) do not modify the mechanical properties of different glass-, silica-, or quartz-fiber 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 structure. 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 that evaluated three types of bonded fiber posts, investigators reported a 3.2% failure of 1306 fiber posts in recalls of 1 to 6 years. 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.
Zirconia posts are composed of zirconium dioxide (ZrO 2 ) partially stabilized with yttrium oxide and exhibit a high flexural strength. Zirconia posts are aesthetic, partially adhesive, rigid, but also brittle. Zirconia posts cannot be etched, and available literature suggests that bonding resins to these materials is less predictable and requires substantially different bonding methods than conventional ceramics. 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 coupling agent. 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 negatively affects the quality of the interface between the resin core material and dentin when subjected to fatigue testing.
The core replaces carious, fractured, or missing coronal structure 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 manipulation, (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.
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. Composite core materials have been shown to exhibit slightly better mechanical values than conventional materials, but improvements are negligible. However, they appear to be superior to silver-glass-ionomer cement and amalgam. The advantages of composite core resins are adhesive bonding to tooth structure and many posts, ease of manipulation, rapid setting, and translucent 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-cure adhesives. However, no adhesive has been shown to completely eliminate microleakage at the margins of the restoration. A degradation of the resin core or the marginal integrity 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. Loosening of the post, core, and crown with composite core can occur, but composite cores have been shown to fail more favorably than amalgam or gold. A 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. Composite core materials are typically two-paste, self-cured composites, but light-curing materials are also available. The use of light-curing composite core materials generally eliminates the risk of chemical incompatibility between adhesives and self-curing resin core materials. Bonding light-cured 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.
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 elasticity. High-copper alloys tend to be stiffer (60 GPa) than low-copper alloys.
Amalgam can be used with or without a post. In the 1980s, investigators described the amalcore. 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 restoration, 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 extension into the root canal space had little influence.
Amalgam can also be used in combination with a prefabricated metallic post when the retention offered by the remaining 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. Others have suggested the use of adhesive resins to bond amalgam to coronal tissue.
Significant disadvantages of amalgam cores are the “nonadhesive 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.
Glass Ionomer Core and Modified Glass Ionomer Core
Glass ionomer and resin-modified glass ionomer cements are adhesive 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 unsupported 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.
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. 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.
Indirect Foundation Restorations: Cast Post and Core
For many years, use of 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.
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.
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. 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.
One 6-year retrospective study reported a success rate higher than 90% using a cast post and core as a foundation restoration. 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.
A variety of cements have been used to cement endodontic posts and include traditional cements, glass ionomer cements, and resin-based luting 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 of 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. Contemporary resin-based luting cements have been shown to exhibit compressive strengths around 200 MPa and elastic moduli between 4 and 10 GPa. These materials may be polymerized through a chemical reaction, a photopolymerization 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. However, bonding to root canal dentin may be compromised by the use of endodontic irrigants such as sodium hypochlorite, hydrogen peroxide, or their combination. 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. 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, Plymouth Meeting, PA). Other reports indicate that the contamination of the dentin walls by eugenol diffusing from endodontic sealers may also affect the retention of bonded posts. 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. 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.
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. 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.
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. 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. It is generally accepted that bonding to dentin of the pulp chamber is more reliable than bonding to root canal dentin, especially at the apical level. 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.
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.
One study indicates that bond strength to radicular dentin might be maximized by adopting procedures that compensate for polymerization stresses. 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 reduces 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, other reports indicate that resin-dentin bonds degrade over time. 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 has been 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) has been shown to produce more durable resin-dentin bonds made with single-step etch-and-rinse adhesives.
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). The release of MMPs such as collagenases has been evidenced in both coronal and root dentin of fully developed teeth of young patients. 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.
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.
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. 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. Adhesion performance to dentin was found comparable to multistep luting cements, but bonding to enamel without prior phosphoric acid etching is not recommended. However, their long-term clinical performances need to be assessed before making a general recommendation for their use.