Post endodontic restorations

Special features of endodontically treated teeth

Alteration level	Specific changes	Possible clinical implication
Composition	No difference in collagen cross-linkage Loss of moisture (9%) attributed to change in free water not bonded water No decrease in compressive and tensile strength associated with this change in water content Dentin erosion and softening due to chelators and proteolytic action of sodium hypochlorite	Increased tooth fragility Reduced adhesion to substrate
Dentin structure	Elasticity modulus and behavior reduced dur to chemicals used for canal irrigation and disinfection No or minor differences in microhardness value Possible decrease in tooth strength can be attributed to dentin aging and to a smaller extent to dentin alteration by endodontic irrigants Reduction of tooth stiffness results from additional preparation especially loss of marginal ridges	Increased tooth fragility
Tooth macrostructure	Crown preparations 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 margin 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 fracture Resistance to fatigue	Increased tooth fragility Reduced retention / stability of the prosthesis
Esthetic changes	Improper endodontic techniques can contribute to discoloration Inadequate cleaning and shaping can leave necrotic tissue in coronal pulp horns, resulting in tooth darkening Root canal filling materials (gutta-percha and root canal cements) retained in the coronal aspect of anterior teeth can detract from the esthetic appearance Thin gingival tissue or in general, thin biotype is considered a negative factor for esthetic outcome of restorative and prosthetic treatment of discolored teeth	Avoid the use of potentially staining endodontic cements and to clean all material residues in the pulpal chamber and access cavity

Pretreatment evaluation

Endodontic evaluation	Inspection of the quality of existing endodontic treatment Good apical seal No sensitivity to pressure No exudate No fistula No apical sensitivity No active inflammation Endodontic retreatment is indicated for teeth showing radiographic signs of apical periodontitis or clinical symptoms of inflammation Canals obturated with a silver cone or other inappropriate filling material should be endodontically retreated before starting any restorative therapy
Periodontal evaluation	Critical for treatment success Healthy gingival tissue Normal bone architecture and attachment levels to favor periodontal health Maintenance of biologic width and ferrule effect before and after endodontic and restorative phases
Biomechanical evaluation	If coronal structures are largely intact and loading is favourable, simple filling can be placed in the access cavity If a substantial amount of coronal structure is missing, a cast post and core restoration is indicated Important clinical factors 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 Clinical complications for teeth with minimal remaining tooth structures Root fracture Coronal-apical leakage Recurrent caries Dislodgment or loss of the core/prosthesis Periodontal injury from biologic width invasion
Tooth position, occlusal forces, parafunctions	In most occlusal schemes, anterior teeth protect posterior teeth from lateral forces through anterolateral guidance Restorations of damaged anterior teeth with heavy function should therefore be designed to resist flexion Posterior teeth normally carry more vertical forces, especially when canine and anterior guidance are maintained; they also sustain greater occlusal loads than anterior teeth, and restorations must be planned to protect posterior teeth against fracture
Esthetic evaluation and requirements	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 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 All teeth located in the esthetic zone also require critical control of endodontic filling materials in the coronal third of the canal and the pulp chamber to avoid or reduce the risk of discoloration

Restorations of endodontically treated tooth are designed to

Protect the remaining tooth from fracture
Prevent reinfection of the root canal system
Replace the missing tooth structure

Considerations for endodontically treated anterior teeth

Do not always need a complete crown, except when the size of plastic restorative materials limits their prognosis (e.g., if the tooth has large proximal composite restorations and unsupported labial tooth structure)
Moisture content may be reduced but similar resistant to fracture compared to untreated teeth
Discoloration without significant tooth loss may be more effectively treated by bleaching than by the placement of a complete crown, although not all stained teeth can be bleached successfully. Resorption can be an unfortunate side effect of nonvital bleaching
When extensive loss of coronal tooth structure or the tooth will be serving as an abutment for an FDP or for a partial removable dental prosthesis, a complete crown is mandatory

Considerations for endodontically treated posterior teeth

Subject to greater loading than are anterior teeth because they are closer to the transverse horizontal axis + morphologic characteristics (having cusps that can be wedged apart) makes them more susceptible to fracture
Careful occlusal reshaping reduces potentially damaging lateral forces during excursive movements
Endodontically treated posterior teeth should receive cuspal coverage to prevent biting forces from causing fracture except mandibular premolars and first molars with intact marginal ridges and conservative access cavities not subjected to excessive occlusal forces (i.e., posterior disclusion in conjunction with normal muscle activity)
Complete coverage is recommended on teeth with a high risk of fracture especially maxillary premolars, which have been shown to have fairly high failure rates if two or three surfaces are restored with amalgam
When significant coronal tooth loss has occurred, a cast post and core restoration or an amalgam foundation restoration is needed

Requirements of successful crown restoration

Ferrule (dentin axial wall height) must be at least 2 to 3mm
Axial walls must be parallel
Restoration must completely encircle the tooth
Margin must be on solid tooth structure
Crown and crown preparation must not invade the attachment apparatus

Clinical features should be provided by posts

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

Mechanical features of endodontic post

Should not break
Should not break the root
Should not distort or allow movement of core and crown
Optimal combination of resilience, stiffness, flexibility and strength

Resilience : ability to deflect elastically under force without permanent damage

Stiffness : ability to resist deformation when stressed

Resilient enough to cushion an impact by stretching elastically, thereby reducing the resulting stress to the root
Stiff enough not to distort, permanently bend, or structurally fail under mastication forces

Features of successful post and core design

Adequate apical seal
Minimum canal enlargement
Adequate post length
Positive horizontal stop to minimize wedging
Vertical wall to prevent rotation
Extension of final restoration margin onto sound tooth structure

Disadvantages to the routine use of a cemented post

Placing the post requires an additional operative procedure
Preparing a tooth to accommodate the post entails removal of additional tooth structure
It may be difficult to restore the tooth later, when a complete crown is needed, because the cemented post may have failed to provide adequate retention for the core material
The post can complicate or preclude future endodontic re-treatment that may be necessary

For these reasons, a metal post is not recommended in anterior teeth that do not require complete crowns

Restorative materials and options

Direct composite restorations	When minimal amount of coronal tooth structure lost When properly cured, resin composites are highly esthetic, exhibit high mechanical properties, and can reinforce the remaining tooth structure through bonding mechanisms Shrinkage that accompanies polymerization of contemporary composite resins remains a significant problem to the long-term success of these restorations Use of an incremental filling technique helps to reduce shrinkage stresses during polymerization Restorations with high C-factors (ratio of bonded to unbounded surfaces) (>3.0) are at greatest risk for debonding Contraindicated when more than a third of coronal tissue has been lost
Indirect restorations : composite or ceramic onlays and overlays	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 Ceramics are a material of choice for long-term esthetic indirect restorations because their translucency and light transmission mimic enamel
Full crowns	When a significant amount of coronal tooth structure has been lost by caries, restorative procedures, and endodontics More frequently, the cementation of a post inside the root canal is necessary to provide retention for the core material and the crown An additional role of the post and core is to protect the crown margins from deformation under function and thereby to prevent coronal leakage

Principles of tooth preparation of post

Conservation of tooth structure	Preparation of the canal Remove only minimal tooth structure Excessive enlargement can perforate or weaken the root, which then may split during either cementation of the post or subsequent function Remaining dentin thickness is the prime variable in fracture resistance of the root Most roots are narrower mesiodistally than faciolingually and often have proximal concavities that cannot be seen on a periapical radiograph Root canal should be enlarged only enough to enable the post to fit accurately while strength and retention are ensured Along the length of a tapered post space, enlargement seldom needs to exceed what would have been accomplished with one or two additional file sizes beyond the largest size used for endodontic treatment Preparation of coronal tissue Save as much of the coronal tooth structure as possible because this helps reduce stress concentrations at the gingival margi When a cast post and core restoration is planned, he walls must be shortened to ensure strength. If more than 2 mm of coronal tooth structure remains, the post design probably has a limited role in the fracture resistance of the restored tooth Extension of the axial wall of the crown apical to the missing tooth structure provides ferrule effect as opposed to a crown that merely encircles core material. This is thought to help bind the remaining tooth structure together, while simultaneously preventing root fracture during function
Retention form	Anterior teeth Normal labiolingual convergence of anterior teeth, coupled with smaller tooth size, complicates achieving retention form Post retention is affected by the preparation geometry, post length, post diameter, post surface texture, and the luting agent
	Preparation geometry	Maxillary central incisors : nearly circular cross section. Can be prepared with twist drill or reamer to provide a cavity with parallel walls or minimal taper Canals with elliptical cross sections : prepared with a restricted amount of taper (usually 6 to 8 degrees) to ensure adequate retention while undesired undercuts are eliminated Retention : threaded > parallel-sided > tapered posts Threaded post is not recommended because of residual stress in the dentin Circular parallel-sided post systems are effective only in the most apical portion of the post space because the majority of prepared post spaces demonstrate considerable flare in the occlusal half When the root canal is elliptical, a parallel-sided post is not effective unless the canal is considerably enlarged, which would significantly weaken the root unnecessarily
	Post length	Retention increases with post length Maintain 5-mm apical seal If a post is shorter than the coronal height of the clinical crown of the tooth, the prognosis is considered unfavorable because stress is distributed over a smaller surface area, which increases the probability of root fracture A shortened apical seal of a minimum of 3 mm is considered acceptable for cases with a short root and a tall clinical crown
	Post diameter	Minimal retentive gain by increasing post diameter Overall prognosis is good when post diameter does not exceed one third of the cross-sectional root diameter
	Post surface texture	Serrated or roughened post is more retentive than a smooth one
	Luting agent	Choice of luting agent seems to have little effect on post retention or fracture resistance of dentin Adhesive resin luting agents have the potential to improve the performance of post and core restorations Resin cements are affected by eugenol-containing root canal sealers, which should be removed by irrigation with ethanol or by etching with 37% phosphoric acid if the adhesive is to be effective A post and core restoration should be remade if any rocking, rotation, or wobbling
	Posterior teeth Long posts should be avoided in posterior teeth, which often have curved roots and elliptical or ribbon-shaped canals If a reasonable amount of coronal tissue remains, use of a single metal post that is cemented in the largest canal can provide adequate retention for the core material When more than 3 to 4 mm of coronal tooth structure with reasonable wall thickness remains, use of a post in the root canals for retention is not necessary, and not having to prepare post space reduces the risk of perforation In mandibular premolars and molars with a reasonable amount of remaining coronal tooth structure, foundation restorations, coupled with a circumferential cervical band of tooth structure with restricted taper of about 2 mm, can typically be placed in composite resin or with amalgam directly condensed into the chamber
Resistance form	Stress distribution	Improve resistance to laterally directed forces by distributing them over as large an area as possible Glass fiber posts have an elastic modulus (flexibility) similar to that of dentin and therefore result in lower stress concentrations than do metal or ceramic posts; this concept is termed monoblock Greatest stress concentrations are found at the shoulder margin, particularly interproximally, and at the apex. Dentin should be conserved in these areas if possible Stresses are reduced as post length increases Parallel-sided posts may distribute stress more evenly than do tapered posts, which may have a wedging effect. However, parallel-sided posts heighten stresses at the apex Avoid sharp angles The cement layer results in a more even stress distribution to the root with lower stress concentrations
Resistance form	Rotational resistance	This usually does not present a problem when the remaining coronal tooth structure is sufficient because a vertical coronal wall prevents rotation Where coronal dentin has been completely lost, a small groove placed in the canal wall can serve as an antirotational element The groove is normally located where the root is bulkiest, usually on its lingual surface Alternatively, rotation can be prevented by an auxiliary pin in the root surface

Types of posts

Prefabricated metallic posts

Active posts

Derive their primary retention directly from root dentin by the use of threads
The use of threaded posts should be avoided due to potential of vertical root fracture

Passive

Passively placed in close contact to the dentin walls, and their retention primarily relies on the luting cement used for cementation
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, and less likely to cause root fracture

Fiber posts

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

Zirconia posts

Esthetic, partially adhesive, very rigid, but also brittle
Cannot be etched

Type of core materials

Luting cements

Traditional cements	Zinc phosphate cement is mostly used for cementing metal restorations and posts 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	Mechanically more resistant than zinc phosphate cements Major advantages of conventional glass ionomer cements are their ease of manipulation, chemical setting, and ability to bond to both tooth and post Not indicated for post cementation, because these cements exhibit hygroscopic expansion that can promote fracture of the root
Resin-based luting cements	Bonding posts to root canal dentin will reinforce the tooth and help retain the post and the restoration Most luting cements require a pretreatment of the root canal dentin with either etch-and-rinse or self-etching adhesives Bonding to root canal dentin may be compromised by the use of endodontic irrigants 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 Resin-dentin bonds degrade over time, 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
Self-adhesive cements	Do not require any pretreatment of the tooth substrates Clinical application is accomplished in a single step Reduces the risk for incomplete impregnation of the conditioned tissue by the resins and reduces technique sensitivity

Three stage operation

Removal of the root canal filling material to the appropriate depth
Enlargement of the canal
Preparation of the coronal tooth structure

Removal of the endodontic filling material	2 methods to remove gutta-percha Warmed endodontic plugger Rotary instrument sometimesin conjunction with chemical agents Warm plugger method Eliminates the possibility that the rotary instrument will inadvertently damage the dentin Does not disturb the apical seal Calculate the appropriate post length : post length equal to the height of the anatomic crown but leave 5mm of apical gutta-percha (minimum of 3 mm in short teeth) Determine the desired post space length Apply rubber dam to prevent aspiration of endodontic instrument Select an endodontic condenser large enough to hold heat well but not so large that it binds against the canal walls Mark it at the appropriate length Heat the condenser Place it in the canal to soften the gutta-percha Gates Glidden drill, Peeso Reamer drill Both are non end cutting Convex football shape of the cutting head of Gates Glidden drill often results in small divots in the wall of the post space Choose one that is slightly narrower than canal Ensure it follows the center of gutta percha and does not cut dentin Shape the canal as needed using endodontic file or low speed drill
Post system selection	Post should be no more than one third the root diameter with the root and walls at least 1 mm thick circumferentially Parallel-sided prefabricated posts are recommended for conservatively prepared root canals in teeth with roots of circular cross section Excessively flared canals (i.e., those found in young persons or in individuals after re-treatment of an endodontic failure) are most effectively managed with a custom post
Canal enlargement	Prefabricated posts Enlarge the canal one or two sizes with a drill, an endodontic file, or a reamer that matches the configuration of the post When using rotary instruments, alternate between the Peeso Reamer drills and twist drills that correspond in size Gain length with the Peeso Reamer drill, and then make the walls parallel with the twist drill A tapered post conforms better to the canal than does a parallel-sided post and requires less removal of dentin to achieve an adequate fit but slightly less retentive and results in greater stress concentrations Not to remove more dentin at the apical extent of the post space than is necessary Custom-made posts Often very little preparation is needed for a custom-made post Undercuts within the canal must be removed, and some additional shaping is usually necessary Be most careful on molars to avoid root perforation In mandibular molars, interradicular root concavities make the distal wall of the mesial root and the mesial wall of the distal root particularly susceptible In maxillary molars, the curvature of the mesiobuccal root increases the chance of mesial or distal perforation
Preparation of coronal tooth structure	Prepare the remaining tooth structure as if the coronal portion of the tooth is intact Be sure that the facial structure of the tooth is adequately reduced for good esthetics Remove all internal and external undercuts that will prevent withdrawal of the pattern Remove any unsupported tooth structure, but preserve as much of the crown as possible Be sure that at least part of the remaining coronal tissue is prepared perpendicular to the path of placement of the post This creates a horizontal flat surface that will serve as a positive stop to minimize wedging and potential splitting of the tooth Prevent rotation of the post by preparing a flat surface parallel to the post Eliminate sharp angles and establish a smooth finish line