Clinical Failure

Morphologic and functional differences between anterior teeth and posterior teeth necessitate that they be treated differently after endodontic therapy, mainly because different loading considerations apply.

In one retrospective analysis⁴ involving 638 patients, investigators evaluated 788 post and cores: 456 custom cast post and cores and 332 foundations with ParaPosts. Four to five years after cementation, reported failure rates in male patients were significantly higher than in female patients, and failure rates after age 60 were three times as high as failure rates for younger patients. Maxillary failure rates (15%) were three times as high as mandibular failure rates (5%) and more prevalent in lateral incisors, canines, and premolars than in central incisors and molars. Failure rate under FDPs was significantly lower than under single crowns. The latter finding may have been caused by load reduction resulting from bracing by the FDP. No correlation was apparent between failure and reduced marginal height of the encasing bone. Custom cast post and cores exhibited slightly higher failure rates than did amalgam foundations. This observation was also made by Sorensen and Martinoff.⁵ However, Torbjörner and colleagues⁴ suggested that custom cast post and cores tend to be used more often in teeth that already have considerably weakened root structure. Thus, regardless of the technique selected for subsequent restoration, the teeth themselves are already more prone to failure. Distal cantilevers appear to contribute to post and core failure in endodontically treated abutment teeth that support the cantilever.

Most of the failures just discussed are influenced by load. In general, as loading increases, failure rates appear to increase concomitantly. Failure has been shown to occur at lower loads as teeth are loaded less parallel to their long axes,⁶ which suggests that clinical failure occurs more readily under lateral loading.

In the planning of the restoration of endodontically treated teeth, the practitioner must account for the strength of the remaining tooth structure, weighed carefully against the load to which the restored tooth will be subjected.

Considerations for Anterior Teeth

Endodontically treated anterior teeth do not always need complete coverage by placement of a complete crown, except when plastic restorative materials have limited prognosis (e.g., if the tooth has large proximal composite restorations and unsupported tooth structure). Many otherwise intact teeth function satisfactorily with a composite resin restoration.

Although it is commonly believed, it has not been demonstrated experimentally that endodontically treated teeth are weaker or more brittle than vital teeth. Their moisture content, however, may be reduced.⁷ Laboratory testing⁸ has actually revealed a resistance to fracture similar between untreated and endodontically treated anterior teeth. Nevertheless, clinical fracture does occur, and attempts have been made to strengthen the tooth by removing part of the root canal filling and replacing it with a metal post. In reality, placement of a post requires the removal of additional tooth structure (Box 12-1), which is likely to weaken the tooth.

Cementing a post in an endodontically treated tooth is a fairly common clinical procedure, despite the paucity of data to support its success. In fact, a laboratory study⁹ and two stress analyses^10,11 have determined that no significant reinforcement results. This might be explained by the hypothesis that when the tooth is loaded, stresses are greatest at the facial and lingual surfaces of the root and an internal post, being only minimally stressed, does not help prevent fracture (Fig. 12-5). Results of other studies, however, contradict this assumption.^8,12 Cemented posts may further limit or complicate endodontic re-treatment options if these are necessary. In addition, if coronal destruction occurs, post removal may be necessary to provide adequate support for a future core.

Fig. 12-5 Experimental stress distributions in an endodontically treated tooth with a cemented post. When the tooth is loaded, the lingual surface (A) is in tension, and the facial surface (B) is in compression. The centrally located cemented post lies in the neutral axis (i.e., not in tension or compression).

(Redrawn from Guzy GE, Nicholls JI: In vitro comparison of intact endodontically treated teeth with and without endo-post reinforcement. J Prosthet Dent 42:39, 1979.)

For these reasons, a metal post is not recommended in anterior teeth that do not require complete coverage restorations. This view is supported by a retrospective study¹³ that did not show any improvement in prognosis for endodontically treated anterior teeth restored with a post. In another study, post placement did not influence the position or angle of radicular fracture.¹⁴ A conflicting report however, suggests that endodontically treated teeth not crowned after obturation were lost six times more frequently than teeth that were crowned after obturation.¹⁵

Discoloration in the absence of significant tooth loss may be more effectively treated by bleaching¹⁶ 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.¹⁷ However, when loss of coronal tooth structure is extensive or the tooth will be serving as an FDP or partial removable dental prosthetic abutment, a complete crown becomes mandatory. Retention and support then must be derived from within the canal, because a limited amount of coronal dentin remains once the reduction for complete coverage has been completed. Coupled with the loss of internal tooth structure necessary for endodontic treatment, the remaining walls become thin and fragile (Fig. 12-6), which often necessitates their reduction in height.

Fig. 12-6 Cross section through a central incisor. The dotted line indicates the original tooth contour before preparation for a metal-ceramic restoration. Even with minimum reduction for the extracoronal restoration, the facial wall is weakened and would not be able to support a prosthesis successfully. The sharp lingual wall complicates pattern fabrication.

Considerations for Posterior Teeth

Posterior teeth are subject to greater loading than are anterior teeth because of their closer proximity to the transverse horizontal axis. This, combined with their morphologic characteristics (having cusps that can be wedged apart), makes them more susceptible to fracture. Careful occlusal adjustment reduces potentially damaging lateral forces during excursive movements. Nevertheless, endodontically treated posterior tooth should receive cuspal coverage to prevent biting forces from causing fracture. Possible exceptions are 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. This is especially true for maxillary premolars, which have been shown to have fairly high failure rates if restored with two or three surface amalgam restorations.¹⁸ Complete coverage gives the best protection against fracture, because the tooth is completely encircled by therestoration. However, when a metal-ceramic crown is to be used, considerable tooth reduction is required, which results in further weakening of the remaining tooth structure. In general, when significant coronal tooth loss has occurred, a cast post and core (Fig. 12-7) or an amalgam foundation restoration is needed.

Fig. 12-7 A, Mandibular premolar and hemisected molar restored with cast post and cores. B, Waxed three-unit fixed dental prosthesis (FDP). C, The FDP cemented in place.

(Courtesy of Dr. F. Hsu.)

Conservation of Tooth Structure

Preparation of the canal

In creating post space, great care must be used to remove only minimal tooth structure from the canal (Fig. 12-8). Excessive enlargement can perforate or weaken the root, which then may split during post cementation or subsequent function. The thickness of the remaining dentin is the prime variable in fracture resistance of the root. Experimental impact testing of teeth with cemented posts of different diameters⁷ showed that teeth with a thicker (1.8 mm) post fractured more easily than those with a thinner (1.3 mm) one.

Fig. 12-8 Faciolingual cross-section through a maxillary central incisor prepared for a post and core. Six features of successful design are identified: 1, adequate apical seal; 2, minimum canal enlargement (no undercuts remaining); 3, adequate post length; 4, positive horizontal stop (to minimize wedging); 5, vertical wall to prevent rotation (similar to a box); and 6, extension of the final restoration margin onto sound tooth structure.

Photoelastic stress analysis also has shown that internal stresses are reduced with thinner posts. The root can be compared to a ring. The strength of a ring is proportional to the difference between the fourth powers of its internal and external radii. This implies that the strength of a prepared root comes from its periphery, not from its interior, and so a post of reasonable size should not weaken the root significantly.¹⁹ Nevertheless, it is difficult to enlarge a root canal uniformly and to judge with accuracy how much tooth structure has been removed and how thick the remaining dentin is. Most roots are narrower mesiodistally than faciolingually and often have proximal concavities that cannot be seen on a standard periapical radiograph. Experimentally, most root fractures originate from these concavities, because the remaining dentin thickness is minimal.²⁰ Therefore, the root canal should be enlarged only enough to enable the post to fit accurately and yet passively while ensuring strength and retention. 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. Because of the more coronal position of the post space, a much larger file must be used to accomplish this (Fig. 12-9).

Fig. 12-9 Use of a prefabricated post entails enlarging the canal one or two file sizes to obtain a good fit at a predetermined depth. A, Incorrect; the prefabricated post is too narrow. B, Incorrect; the prefabricated post does not extend to the apical seal. C, Correct; the prefabricated post is fitted by enlarging the canal slightly.

Preparation of coronal tissue

Endodontically treated teeth often have lost much coronal tooth structure as a result of caries, as a result of previously placed restorations, or in preparation of the endodontic access cavity. However, if a cast core is to be used, further reduction is needed to accommodate a complete crown and to remove undercuts from the chamber and internal walls. This may leave very little coronal dentin. Every effort should be made to save as much of the coronal tooth structure as possible, because this helps reduce stress concentrations at the gingival margin.²¹ The amount of remaining tooth structure is probably the most important predictor of clinical success. 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.^22,23 The once common clinical practice of routine coronal reduction to the gingival level before post and core fabrication is outmoded and should be avoided (Fig. 12-10). Extension of the axial wall of the crown apical to the missing tooth structure provides what is known as a restoration with a ferrule, which is defined as a metal band or ring used to fit the root or crown of a tooth, as opposed to a crown that merely encircles core material (Fig. 12-11). This is thought to help bind the remaining tooth structure together, simultaneously preventing root fracture during function.^24-26 Although there is evidence that preserving as much coronal tooth structure as possible enhances prognosis, it is less clear whether the prognosis is improved by creation of a ferrule in an extensively damaged tooth through a surgical crown-lengthening procedure. In this latter circumstance, although the crown lengthening allows fabrication of a crown with a ferrule, it also leads to a much less favorable crown/root ratio and therefore to increased leverage on the root during function (Fig. 12-12).

Fig. 12-10 A, It is preferable to maintain as much coronal tooth structure as possible, provided it is sound and of reasonable strength. B, Extensive caries has resulted in the loss of all coronal tooth structure. This is less desirable than the situation in A, because greater forces are transmitted to the root.

Fig. 12-11 Extending a preparation apically creates a ferrule and helps prevents fracture of an endodontically treated tooth during function. A, Preparation with a ferrule (arrows). B, Preparation without a ferrule.

Fig. 12-12 Effect of apical preparation on crown/root ratio. A, Schematic of extensively damaged premolar tooth. Apical extension of the gingival margin would encroach on the biologic width (Chapter 5). This preparation has no ferrule. C, crown length; R, root length. B, Creating a ferrule with orthodontic extrusion (see Fig. 6-21) reduces root length (R′), whereas crown length remains unchanged. C, Surgical crown lengthening also reduces root length (R′) but increases crown length (C′). This results in a much less favorable crown/root ratio, which may, in fact, weaken the restoration.

(Courtesy of Dr. A. G. Gegauff. From Gegauff AG: Effect of crown lengthening and ferrule placement on static load failure of cemented cast post-cores and crowns, J Prosthet Dent 84:169, 2000.)

One laboratory study showed that creating a ferrule through surgical crown lengthening resulted in a weaker, rather than a stronger, restored tooth.²⁷ In comparison, creating a ferrule with orthodontic extrusion may be preferred, because even though theroot is effectively shortened, the crown is not lengthened (see Fig. 12-12B).

Retention Form

Anterior teeth

Simultaneous dislodgment of an anterior crown with the post and core that retains it is frequently seen clinically and results from inadequate retention form of the prepared tooth.^13,28 The normal faciolingual convergence of anterior teeth, coupled with smaller tooth size, complicates achieving such retention form. Post retention is affected by the preparation geometry, post length, post diameter, post surface texture, and the luting agent.

Preparation geometry

Some canals, particularly in maxillary central incisors, have a nearly circular cross-section (see Table 12-3). These can be prepared with a twist drill or reamer to provide a cavity with parallel walls or minimal taper, allowing the use of a preformed post of corresponding size and configuration. Conversely, canals with elliptical cross-sections must be prepared with a restricted amount of taper (usually 6 to 8 degrees) to ensure adequate retention and eliminate undesired undercuts. This is analogous to an extracoronal preparation (see Chapter 7). With extracoronal preparations, retention increases rapidly as vertical wall taper is reduced (see Chapter 7). Although retention can be further increased by use of a threaded post, which screws into dentin, this procedure is not recommended because of residual stress in the dentin. If the procedure is used, however, threaded posts must be “backed off” to ensure passivity; otherwise, the root will fracture.

Table 12-3 ROOT CANAL CONFIGURATIONS

Circular	Elliptical
	Buccolingual	Mesiodistal
Maxillary central incisor	Maxillary lateral incisor Maxillary canine Mandibular incisors Mandibular canine
Maxillary first premolar (two roots)	Maxillary first premolar (single root) Mandibular first premolar
Mandibular second premolar	Maxillary second premolar
Maxillary molars (distobuccal roots)	Maxillary molars (mesiobuccal roots) Mandibular molars (mesial and distal roots)	Maxillary molars (palatal roots)

From Weine FS: Endodontic Therapy, 4th ed, pp 225–269. St. Louis, Mosby, 1989.

In accordance with this explanation, laboratory testing^29-31 has confirmed that parallel-sided posts are more retentive than tapered posts and that threaded posts are the most retentive (Fig. 12-13). However, these comparisons are relevant only if the post fits the root canal properly, because retention is proportional to the total surface area.

Fig. 12-13 Comparison of forces needed to remove different prefabricated post systems.

(Redrawn from Standlee JP, Caputo AA: The retentive and stress distributing properties of split threaded endodontic dowels. J Prosthet Dent 68:436, 1992.)

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. Similarly, 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 (Fig. 12-14).

Fig. 12-14 The use of a parallel-sided post in a tapered canal requires considerable enlargement of the post space, which can weaken the root significantly.

(Courtesy of Dr. R. Webber.)

Post length

Studies^29,31,32 have shown that as post length increases, so does retention. However, the relationship is not necessarily linear (Fig. 12-15). A post that is too short will fail (Fig. 12-16), whereas one that is too long may damage the seal of the root canal fill or risk root perforation if the apical third is curved or tapered (Fig. 12-17). Absolute guidelines for optimal post length are difficult to define. Ideally, the post should be as long as possible without jeopardizing the apical seal or the strength or integrity of the remaining root structure. Most endodontic texts advocate maintaining a 5-mm apical seal. However, 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, thereby increasing the probability of radicular fracture. A short root and a tall clinical crown present the clinician with the dilemma of having to compromise the mechanics, the apical seal, or both. Under such circumstances, an apical seal of 3 mm is considered acceptable.

Fig. 12-15 Effect of the depth of embedding a post on its retentive capacity.

(Data from Standlee JP, et al: Retention of endodontic dowels: effects of cement, dowel length, diameter, and design. J Prosthet Dent 39:401, 1978.)

Fig. 12-16 Faciolingual longitudinal sections through a maxillary central incisor. A, With a post of the correct length, a force (F) applied near the incisal edge of the crown generates a resultant couple (R). B, When the post is too short, this couple is greater (R′), which leads to the increased possibility of root fracture.

Fig. 12-17 A, Correct post length. B, The post is too short; the consequences are inadequate retention and increased risk of root fracture. C, These posts are too long, jeopardizing the apical seal.

Post diameter

Increasing the post diameter in an attempt to increase retention is not recommended because the results are minimal retentive gain and unnecessary weakening of the remaining root. Although one group of investigators³³ reported that increasing the post diameter increased retention, other reports do not confirm this.^29,30 Empirical evidence suggests that the overall prognosis is good when post diameter does not exceed one third of the cross-sectional root diameter.

Post surface texture

A serrated or roughened post is more retentive than a smooth one,³⁰ and controlled grooving of the post and root canal³⁴ (Fig. 12-18) considerably increases the retention of a tapered post.

Fig. 12-18 Effect of horizontal grooving on the retention of tapered posts. NS, Not significant.

(Modified from Wood WW: Retention of posts in teeth with nonvital pulps. J Prosthet Dent 49:504, 1983.)

Luting agent

In considering traditional cements, the choice of luting agent seems to have little effect on post retention^35,36 or the fracture resistance of dentin.³⁷ However, adhesive resin luting agents (see Chapter 31) have the potential to improve the performance of post and core restorations; laboratory studies have shown improved retention.^38,39 Resin cements may be indicated if a post becomes dislodged. Resin cements are affected by eugenol-containing root canal sealers, which should be removed by irrigation with ethanol or etching with 37% phosphoric acid if the adhesive is to be effective.⁴⁰ Zinc phosphate and glass ionomer have comparable retentive properties, whereas polycarboxylate and composite resin cements have slightly less.⁴¹ Some resin and glass ionomer cements have demonstrated significantly higher retention than resin-ionomer cements,⁴² although the choice of luting agent may become more important if the post has a poor fit within the canal.⁴³ A post and core should be remade if any rotation or wobble is present.

Posterior teeth

Relatively long posts with a circular cross-section provide good retention and support in anterior teeth but should be avoided in posterior teeth, which often have curved roots and elliptical or ribbon-shaped canals (Fig. 12-19). For these teeth, retention is better provided by two or more relatively short posts in the divergent canals.

Fig. 12-19 When preparing posterior teeth for intracoronal retention, the practitioner must be careful to avoid perforation, especially on the distal surface of mesial roots and the mesial surface of distal roots, where residual tooth structure is normally thinnest and where concavities are often present (arrows).

When amalgam is used as the core material, it can be condensed either around cemented metal posts or directly into short, prepared post spaces. 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 this reduces the risk of perforation.⁴⁴ When a post is not used, the chamber must provide adequate retention for the core material. It may then be advantageous to prepare several short divergent post spaces into which the core material extends. Use of the canals for retention can provide good results,⁴⁵ although once a complete crown has been provided, the strength of the tooth is not dramatically influenced by differences in technique.

Mandibular premolars and molars with a reasonable amount of remaining coronal tooth structure, when coupled with a circumferential cervical band of tooth structure with restricted taper of about 2 mm, can often be restored with amalgam directly condensed into the chamber. Core buildups in molars with one or more missing cusps benefit from one or more cemented posts around which the amalgam can be condensed. The posts provide the additional retention, which was compromised because of the missing tooth structure. In mandibular molars, the larger distal canal is recommended for post placement. In maxillary molars, the palatal canal is used (see Fig. 12-2C and D).

Although it is possible to restore a severely compromised molar with three or more missing cusps with multiple posts and amalgam, the tooth’s overall importance must be assessed, as the prognosis of such teeth is often guarded. If retaining the tooth is crucial and optimal strength is needed, a multipiece cast core can be used (made in sections that have different paths of withdrawal) (Fig. 12-20). An alternative preparation method for a posterior tooth is selecting the canals that are widest (normally the palatal of maxillary molars and the distal of mandibular molars) for the major post and then preparing short auxiliary post spaces in the other canals with the same path of withdrawal (Fig. 12-21).

Fig. 12-20 A to F, Cast cores for posterior teeth can be made in interlocking sections, with each section having its own path of withdrawal.

Fig. 12-21 Single-piece castings can be made by selecting the larger-diameter canal and extending a second post for a limited distance into the smaller canal. A, A maxillary first premolar. B, A maxillary first molar. C, A mandibular first molar. D to F, Post and core provided for a maxillary first premolar by the indirect technique.

Resistance Form

Rotational resistance

To minimize the risk of dislodgment, it is important that preparation geometry prevents a post with a circular cross-section from rotating during function (Fig. 12-22). This usually does not present a problem when sufficient coronal tooth structure remains, because rotation is prevented by a vertical coronal wall. 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 aspect. Alternatively, rotation can be prevented by an auxiliary pin in the root face. Rotation of a threaded post can also be prevented³¹ by preparing a small cavity (half in the post, half in the root) and condensing amalgam into it after the post is cemented.

Fig. 12-22 Rotational resistance in an extensively damaged tooth can be obtained by preparing a small groove in the root canal. This must be in the path of placement of the post and core.

Removal of the Endodontic Filling Material

The root canal system should first be completely filled; space should then be made for a post, thus ensuring that lateral canals are sealed. A post cannot be placed if the canal is filled with a full-length silver point, so these must be removed and the tooth retreated with gutta-percha. It is not advisable to shorten previously cemented silver points, because leakage will result even if only a short portion is removed.^53,54

There are two commonly used methods to remove gutta-percha (Fig. 12-23): (1) using a warmed endodontic plugger, and (2) using a rotary instrument, sometimes in conjunction with chemical agents. Although more time consuming, the warmed endodontic plugger is preferred because it eliminates the possibility that the rotary instrument will inadvertently damage the dentin. If it is more convenient, the gutta-percha can be removed with a warmed condenser immediately after obturation. This does not disturb the apical seal.^55,56 This method offers the additional advantage of allowing the operator to work in an area where the root canal anatomy is still familiar.

Fig. 12-23 Gutta-percha can be removed from the canal with a heated endodontic plugger. (A and B), a non–end-cutting bur (C) (e.g., a Gates Glidden drill). A ParaPost drill (D) can be used to parallel the post space wall (with a rubber stop to ensure accuracy of the preparation depth).

(A and B, Courtesy of Dr. D. A. Miller.)

6. If the gutta-percha is old and has lost much of its thermoplasticity, use a rotary instrument, making sure that it follows the gutta-percha and does not engage dentin (this causes a root perforation). For this reason, high-speed instruments and conventional burs are contraindicated. Special post preparation instruments are available (Fig. 12-24). Peeso-Reamers and Gates Glidden drills are often used for this purpose. The football shape of the cutting head of the Gates-Glidden drill often results in small concavities in the wall of the post space. These are avoided with the more cylindrically shaped Peeso-Reamer. Both are considered “safe-tip” instruments because they are not end-cutting burs. The friction generated between the fill and the tip of these burs softens the gutta-percha, allowing the rotary instrument to track the canal with reasonable predictability. In one comparison of rotary instruments,⁵⁷ investigators concluded that the Gates Glidden drill conformed to the original canal more consistently than did the ParaPost drill, which is an end-cutting instrument. The latter is a twist drill and should be used only to parallel the walls of the post space. Considerable heat can be generated by these rotary instruments, especially during the ParaPost preparation stage.⁵⁸

Table 12-1 AVERAGE CROWN AND ROOT LENGTHS (IN MILLIMETERS)

Fig. 12-24 Commonly used instruments for gutta-percha removal and canal enlargement. A, Endodontic pluggers, two sizes of Peeso-Reamers with corresponding twist drills, and endodontic file. Note attached floss as a safety precaution. B, The ParaPost twist drill corresponds in size to an aluminum post used to fabricate interim restorations, a plastic post for patterns, and a stainless-steel or titanium post.

(Courtesy of Dr. J. A. Nelson.)

Of importance: End-cutting instruments should never be used to gain length because root perforation will result!

The post should be no more than one-third the root diameter,^1,59 with the root and walls at least 1 mm thick. Obviously, for deciding on appropriate post diameters, a knowledge of average root dimensions is important. These have been calculated⁶⁰ and are presented in Table 12-2. Knowledge of root canal cross-section also is significant in post selection. Prefabricated posts are circular in cross-section, but many root canals are elliptical, which makes uniform reduction with a drill impossible. Canal shapes are summarized in Table 12-3.

Enlargement of the Canal

Before enlargement of the canal, the type of post system to be used for fabrication of the post and core must be chosen.

The advantages and disadvantages of different post types are summarized in Table 12-4. Because no system has universal application, being familiar with more than one technique is a significant advantage. A wide range of prefabricated posts are available. They come in many shapes and sizes and have varying radiopacity that may assist in their identification (Table 12-5 and Figs. 12-25 and 12-26).

Table 12-4 AVAILABLE POST AND CORE SYSTEMS

Fig. 12-25 Classification of prefabricated posts. A, Tapered smooth posts. B, Tapered serrated posts. C, Tapered threaded posts. D, Parallel-sided smooth posts. E, Parallel-sided serrated posts. F, Parallel-sided threaded posts.

(Redrawn from Shillingburg HT, Kessler JC: Restoration of the Endodontically Treated Tooth. Chicago, Quintessence Publishing, 1982.)

Fig. 12-26 The various endodontic posts encountered in clinical practice present with varying degrees of radiopacity. Dentists accustomed to seeing traditional stainless steel and titanium posts may be deceived by more recently introduced systems. A, Nine representative posts: (1) ParaPost stainless steel (Coltène/Whaledent); (2) ParaPost titanium (Coltène/Whaledent); (3) FRC Postec Plus (Ivoclar Vivadent); (4) Glass Fiber Post (Ellman International); (5) Glass Fiber C-I Post (Parkell); (6) D. T. Light-Post (Bisco); (7) Twin Luscent Anchors (Dentatus USA); (8) Unicore (Ultradent Products); (9) PeerlessPost (SybronEndo). The pure carbon fiber posts (not included in A) are completely radiolucent. The type of cement that is used plays a role in the radiopacity of the post (see Fig. 31-6). B to I, Radiographs of the six categories: B, Endowel (Star Dental), tapered and smooth sided. C, Unimetric (DENTSPLY), tapered and serrated. D, Surtex (Dentatus USA), tapered and threaded. E, CTH Beta Post (CTH), parallel-sided and smooth. F, ParaPost (Coltène/Whaledent) (two sizes), parallel-sided and serrated. G, Flexi-Post (Essential Dental Systems) (in the right maxillary first molar), parallel-sided and threaded (note the split shank). H, ParaPost Fiber Lux (Coltène/Whaledent) cemented with RelyX Luting (3M ESPE). Note the radiolucency of the post in comparison with the radiopacity of the gutta-percha endodontic fill.

(B, Courtesy of Dr. D. A. Miller and Dr. H. W. Zuckerman; C, courtesy of Dr. I. A. Roseman; D, courtesy of Dr. F. S. Weine and Dr. S. Strauss; E, courtesy of Dr. J. F. Tardera; F, courtesy of Dr. J. L. Wingo; G, courtesy of Dr. L. R. Farsakian; H, courtesy of Dr. D. A. Miller and Dr. G. Freebeck.)

The diameters of popular prefabricated posts are given in Table 12-6. Parallel-sided prefabricated posts are recommended for conservatively prepared root canals in teeth with roots of circular cross-section. Excessively flared canals (e.g., those found in young persons or in individuals after re-treatment of an endodontic failure) are most effectively managed with a custom post. However, situations should be evaluated on an individual basis.

Table 12-6 DIAMETERS OF EIGHT COMMONLY USED PREFABRICATED POSTS (IN MILLIMETERS)

Prefabricated posts

1. Enlarge the canal one or two sizes with a drill, endodontic file, or reamer that matches the configuration of the post (Fig. 12-27A and B). When using rotary instruments, alternate between the Peeso-Reamers and twist drills that correspond in size. In the case of a threaded post, the appropriate drill is followed by a tap that prethreads the internal wall of the post space. Parallel-sided posts are more retentive and distribute stresses better than do tapered posts, but they do not conform well to the shape of a canal that has been flared to facilitate condensation of gutta-percha. In this situation, it may not be possible to enlarge the canal sufficiently to provide adequate retention for the post; in that case, a tapered custom-made post is preferred.

2. Use a prefabricated post (Fig. 12-27C) that matches standard endodontic instruments. A tapered post conforms better to the canal than a parallel-sided post and requires less removal of dentin to achieve an adequate fit. However, it is slightly less retentive and causes greater stress concentrations, although retention may be improved by controlled grooving.³⁴

3. Be especially careful not to remove more dentin at the apical extent of the post space than is necessary (see Figs. 12-14 and 12-27).

Fig. 12-27 A to C, Enlargement of the root canal for a prefabricated post.

Of importance: If careful measurement techniques have been followed, radiographs are not normally necessary to verify the post space preparation.

Most of the time, a preformed parallel-sided post fits only in the most apical portion of the canal. Modified posts are available with tapered ends, and these conform better to the shape of the canal, although they have slightly less retention than parallel-sided posts do, particularly when restoring shorter roots.³² In the absence of a vertical stop on sound tooth structure, such posts can also create an undesirable wedging effect.

Custom-made posts

1. Use custom-made posts (Fig. 12-28) in canals that have a noncircular cross-section or extreme taper. Enlarging canals to conform to a preformed post may lead to perforation. Often very little preparation is needed for a custom-made post. However, undercuts within the canal must be removed, and some additional shaping usually is necessary.

2. 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 makes mesial or distal perforation more likely⁶¹ (Fig. 12-29). Therefore, neither post size nor length should be excessive.

Fig. 12-28 Custom-made posts are indicated for teeth with root canals whose cross-section is not circular or is extremely tapered. Further enlargement of the root canal is often not necessary on these teeth.

Fig. 12-29 A and B, Distal root curvature contributed to this mesial perforation (arrow) of a mandibular molar and necessitated removal of the distal root segment.

(Courtesy of Dr. J. Davila.)

Preparation of the Coronal Tooth Structure

After the post space has been prepared, the remaining coronal tooth structure is reduced for the extracoronal restoration. Specific reduction depends on the type of crown that is planned. When esthetic requirements apply, as for anterior teeth, metal-ceramic crowns or all-ceramic crowns are indicated. (see Chapters 9, 11, 24, and 25).

Post Fabrication

Prefabricated posts

Technique simplicity and treatment expediency are advantages of prefabricated posts. A post is selected to match the dimensions of the canal, and only minimum adjustment is needed to seat it to the full depth of the post space. The coronal part of the post may have an inadequate fit because the root canal has been flared. This can be corrected by adding material when the core is made.

Available materials (see Table 12-5)

Prefabricated parallel-sided posts are made of platinum-gold-palladium (Pt-Au-Pd), nickel-chromium (Ni-Cr), cobalt-chromium (Co-Cr), or stainless steel wire. Serrated posts come in stainless steel, titanium, or nonoxidizing noble alloy. Tapered posts are available in Au-Pt, Ni-Cr, and titanium alloys. All these posts have a high modulus of elasticity and an elongated grain structure, which contribute to their more suitable physical properties in comparison to cast posts. Essentially, they are more rigid.

Failure of posts cast in type III gold when loaded at a 45-degree angle has been attributed to bending.⁶² Although posts cast in stiffer (type IV) gold or Ni-Cr alloys can be expected to resist bending better, prefabricated posts should possess even more desirable physical properties, although their properties can deteriorate when a core is cast to a wrought post.⁶³

Fiber composite posts have increased in popularity. These posts consist of bundles of stretched aligned glass or carbon fibers^* embedded in a resin matrix. The resulting post is strong but has significantly less stiffness and strength than do ceramic and metal posts.⁶⁴ Preliminary retrospective study of the carbon fiber system appears promising⁶⁵ (Fig. 12-30). However, in a laboratory study in which teeth restored with carbon fiber posts and composite-resin foundations were compared with teeth restored with custom post and cores cast in type III alloy, there were significantly higher fracture thresholds for the cast post and cores.⁶⁶ One advantage of a fiber composite posts is their ease of its removal for re-treatment. The preferred technique involves drilling in an apical direction. The very strong carbon fibers prevent the drill from tracking laterally, avoiding penetration of the dentin and preventing the post from shattering easily into small fragments (Fig. 12-31).

Fig. 12-30 Fiber composite posts. A and B, The ParaPost Fiber Lux system is available in various sizes. C, Gutta-percha is removed with hot instruments or a Gates Glidden drill. The canal is prepared sequentially with the drills provided by the manufacturer. D, The post is seated in the canal. E, The canal is prepared by etching and priming according to the manufacturer’s recommendations. F, The luting resin is introduced into the canal with a paper point. G, The post is coated with resin luting agent, seated and the resin polymerized (H). The translucent post allows light transmission to the luting agent. I, The core is built up with the recommended core resin. J, The preparation is finalized.

(Courtesy of Coltène/Whaledent AG, Altstatten, Switzerland.)

Fig. 12-31 A, Maxillary canine requires fiber post removal for endodontic re-treatment B, Composite resin core is removed first. C, Gates Glidden drill used to remove the fiber post. D, Endodontically re-treated tooth before fabrication of new post and core and extracoronal restoration. If concern exists about the long-term prognosis of an endodontically treated tooth, a carbon fiber post should be considered. The chief disadvantage of a carbon fiber post is its black appearance, which presents an esthetic problem (as can metal posts).

(Courtesy Dr. D.A. Miller.)

Manufacturers have developed high-strength ceramic^67,68 (zirconia) posts^† (Fig. 12-32) and ceramic composite^‡ (Fig. 12-33) and woven fiber (e.g., polyethylene) posts,^§ all of which have excellent esthetic properties (see also Chapters 25 and 27). Ceramic is very strong and rigid; woven fiber is less strong and more flexible.⁶⁹ Because the systems are relatively new, judging how well the foundations will perform in clinical practice is difficult, but they should be considered when esthetic demands are high.

Fig. 12-32 Zirconia posts, such as the CosmoPost, shown with the corresponding rotary instruments, are esthetic and strong. Special pressable ceramics are available to form the core (composite resin can also be used).

(Courtesy of Ivoclar Vivadent, Amherst, New York.)

Fig. 12-33 Ceramic composite post. A, The D.T. LIGHT-POST system uses quartz fibers in an epoxy resin matrix. Cross-sectional (B) and longitudinal (C) sections of the fiber composite.

(Courtesy of Bisco, Inc., Schaumburg, Illinois.)

Corrosion resistance

Several reports^70-72 have linked root fracture to corrosion of base metal prefabricated post and core systems. In one study,⁶⁷ a report on 468 teeth with vertical or oblique root fracture, investigators attributed 72% of these failures to electrolytic action of dissimilar metals used for the post and the core (reaction occurring between tin in the amalgam core and stainless steel, German silver, or brass in the post). The authors suggested that volume changes produced by corrosion products split the root. Although possible fracture mechanisms have been suggested,^68,69 these studies are confusing cause with effect: The corrosion may have occurred after root fracture rather than causing it.

Further study is needed to answer the question conclusively. However, in the meantime, avoiding the use of potentially corrodible dissimilar metals for post, core, and crown is recommended.

Custom-made posts

A custom-made cast post and core can be cast from a direct pattern fabricated in the patient’s mouth, or an indirect pattern can be fabricated in the dental laboratory. A direct technique with autopolymerizing or light-polymerized resin is recommended for single canals with good clinical access, whereas an indirect procedure is more appropriate for multiple canals or when access is more problematic. As an alternative to autopolymerizing resin, thermoplastic resin can be used.

Direct procedure

1. Lightly lubricate the canal and notch a loose-fitting plastic dowel (Fig. 12-34A). It should extend to the full depth of the prepared canal.

2. Use the bead-brush technique (Fig. 12-34B) to add resin to the dowel (Fig. 12-34C) and seat it in the prepared canal. This should be done in two steps: Add resin only to the canal orifice first. An alternative is to mix some resin and roll it into a thin cylinder. This is introduced into the canal and pushed into place with the monomer-moistened plastic dowel.

3. Do not allow the resin to harden fully within the canal. Loosen and reseat it several times while it is still rubbery.

4. Once the resin has polymerized, remove the pattern (Fig. 12-34D).

5. Form the apical part of the post by adding additional resin and reseating and removing the post, taking care not to lock it in the canal.

6. Identify any undercuts that can be trimmed away carefully with a scalpel.

Fig. 12-34 A to D, Fabrication of an acrylic resin pattern for a custom-made post.

(Courtesy of Dr. R. Webber.)

The post pattern is complete when it can be inserted and removed easily without binding in the canal. Once the pattern has been made, additional resin or light-polymerized resin^* is added for the core.

Pattern fabrication with thermoplastic resin. (Fig. 12-35)

1. Fit the plastic rod to the prepared post space. Trim the rod until the bevel area is approximately 1.5 to 2 mm occlusal to the finish line for the core.

2. Lubricate the canal with a periodontal probe and petroleum jelly.

3. Heat the thermoplastic resin over a flame until the material turns clear, or heat the resin in a low-temperature glue gun.^*

4. Apply a small amount of the heated resin to the apical end of the rod to cover two thirds of the anticipated length of the post pattern.

5. Fully insert the rod into the prepared post space. Lift after 5 to 10 seconds and reseat. Inspect the post pattern for completeness and, with a scalpel blade, remove any projections that result from undercuts in the canal.

6. For the direct technique, fabricate the core with conventional autopolymerizing resin, using the brush-bead technique, or use a syringe to apply a light polymerized pattern resin (an easier technique).

7. If the indirect technique is preferred, pick up the pattern with an elastomeric impression material, which can be poured in the conventional manner. Soak the cast in warm water to help release the pattern. Reseat the post pattern, and wax the core.

8. Invest and cast the post and core. Phosphate-bonded investment is recommended because of its higher strength.

Fig. 12-35 The Merritt EZ Cast Post system. A, The canal is lubricated and excess lubricant removed with paper points. The post was previously trimmed until its beveled portion protruded about 1.5 to 2 mm above the tooth preparation. B, A stick of the thermoplastic material is heated. C, The plastic rod is covered for about two thirds of the anticipated post length. D, The coated post is inserted and can be removed in 5 to 10 seconds. E, After any protrusions have been removed, the core is built from autopolymerizing resin and trimmed to ideal tooth preparation form. F, The completed custom post and core.

(From Rosenstiel SF, et al: Custom-cast post fabrication with a thermoplastic material. J Prosthet Dent 77:209, 1997.)

Indirect procedure

Any elastomeric material will make an accurate impression of the root canal (Fig. 12-36A) if a wire reinforcement is placed to prevent distortion.

Fig. 12-36 A to E, Indirect procedure for post-and-cores.

1. Cut pieces of orthodontic wire to length and shape them like the letter J (Fig. 12-36B).

2. Verify the fit of the wire in each canal. It should fit loosely and extend to the full depth of the post space. If the fit is too tight, the impression material will strip away from the wire when the impression is removed.

3. Coat the wire with tray adhesive. If subgingival margins are present, tissue displacement may be helpful. Lubricate the canals to facilitate removal of the impression without distortion (die lubricant is suitable).

4. Using a lentulo spiral, fill the canals with elastomeric impression material. Before loading the impression syringe, verify that the lentulo will spiral material in an apical direction (clockwise). Pick up a small amount of material with the largest lentulo spiral that fits into the post space. Insert the lentulo with the handpiece set at low rotational speed to slowly carry material into the apical portion of the post space. Then increase handpiece speed and slowly withdraw the lentulo from the post space. This technique prevents the impression material from being dragged out. Repeat until the post space is filled.

5. Seat the wire reinforcement to the full depth of each post space, use a syringe to fill in more impression material around the prepared teeth, and insert the impression tray (see Fig. 12-36C).

6. Remove the impression (see Fig. 12-36D), evaluate it, and pour the definitive cast (see Fig. 12-36E) as usual (see Chapter 17).

Access for waxing is generally adequate without placement of dowel pins or sectioning of the cast.

7. Roughen a loose-fitting plastic post (a plastic toothpick is suitable) and, using the impression as a guide, make sure that it extends into the entire depth of the canal.

8. Apply a thin coat of sticky wax to the plastic post and, after lubricating the stone cast, add soft inlay wax in increments (Fig. 12-37). Start from the most apical and make sure that the post is correctly oriented as it is seated to adapt the wax. When this post pattern has been fabricated, the wax core can be added and shaped.

9. Use the impression to evaluate whether the wax pattern is completely adapted to the post space.

Fig. 12-37 A and B, Post and core patterns made by adding wax to prefabricated plastic posts.

Core Fabrication

The core of a post and core restoration replaces missing coronal tooth structure and, combined with the remaining coronal tissue, forms the shape of the optimal tooth preparation. It can be shaped in resin or wax and added to the post pattern before the assembly is cast in one piece. It is cast directly onto a prefabricated post. Some concern arises that the casting process may unfavorably affect the physical properties of wrought metal posts. A third alternative is to make the core from a plastic restorative material, such as amalgam, or from composite resin or glass ionomer.

Plastic filling materials

The advantages of amalgam, glass ionomer, or resin^62,73,74 include the following:

1. Maximum tooth structure can be conserved, because undercuts do not need to be removed.

2. Treatment requires one fewer patient visit.

3. There are fewer laboratory procedures.

4. Testing generally shows good resistance to fatigue testing⁷⁵ and good strength characteristics,⁷⁶ possibly because of the good adaptation to tooth structure. However, these plastic restorative materials, especially the glass ionomers, have lower tensile strength than do cast metals.

Disadvantages include the following:

1. Long-term success may be affected by corrosion of amalgam cores, the low strength of glass ionomer,⁷⁷ or the continued polymerization⁷⁸ and high thermal expansion coefficients of composite resin cores.

2. Microleakage with temperature fluctuations (thermocycling) is greater under composite resin and amalgam cores than under conventional crown preparations⁷⁹ (however, the extent of leakage under cast cores has yet to be determined).

3. Difficulty may be encountered with certain operative procedures such as rubber dam or matrix application (particularly on badly damaged teeth).

Amalgam cores are suitable for restoring posterior teeth, particularly when some coronal structure remains. The procedure described by Nayyar and associates,⁴⁵ with amalgam also used for the posts, is conservative of tooth structure. The cores are placed during the same appointment as the root canal obturation, because then the teeth are still isolated by the rubber dam, the practitioner is still familiar with the root canal structure, and the cores can serve as a support for the interim restoration (Fig. 12-38).

Fig. 12-38 A to E, Retention for an amalgam core can be obtained from the root canal system, preserving as much tooth structure as possible.

(B to D, Courtesy Dr. M. Padilla.)

Step-by-step procedure for amalgam (see also Chapter 6)

1. Apply the rubber dam and remove gutta-percha from the pulp chamber, as well as 2 to 4 mm into each root canal, if less than 4 mm of coronal height remains. Use a warmed endodontic instrument.

2. Remove any existing restoration, undermined enamel, or carious or weakened dentin. Establish the cavity form, using conventional principles of resistance and retention form. Even if cusps are missing, pins are not normally required, because adequate retention can be gained by extending the amalgam into the root canals.

3. If it is suspected that the floor of the pulp chamber is thin, protect it from condensing pressures with a cement base.

4. Fit a matrix band. Where lack of tooth structure makes the application of a conventional matrix system difficult, an orthodontic or annealed copper band may be used.

5. Condense the first increments of amalgam (select a material with high early strength) into the root canals with an endodontic plugger.

6. Fill the pulp chamber and coronal cavity in the conventional manner.

7. Carve the alloy to shape. The impression can be made immediately. Alternatively, the amalgam can be built up to anatomic contour and later prepared for a complete crown. Under these circumstances, the patient must be cautioned to avoid forces that would fracture the tooth or the newly placed restoration.

Cast metal

Cast metal cores have the following advantages:

1. They can be cast directly onto a prefabricated post, providing a restoration with good strength characteristics.

2. Conventional high noble-metal content alloys can be used.

3. An indirect procedure can be used, making restoration of posterior teeth easier.

Direct procedure for single-root teeth

Direct patterns can be formed by combining a prefabricated post with autopolymerizing resin. Alternatively, a thermoplastic material can be used to create a post pattern,⁸⁰ and the core portion can be developed in autopolymerizing resin, light polymerized resin, or wax.

Pattern fabrication with autopolymerizing resin

1. Use a prefabricated metal or custom acrylic resin post.

2. Add resin by the “bead” technique, dipping a small brush in monomer and then into polymer and applying it to the post. Alternatively, light-cured resin can be used to facilitate this step.⁸¹

3. Slightly overbuild the core and let it polymerize fully (Fig. 12-39A).

4. Shape the core with carbide finishing burs or diamonds (Fig. 12-39B). Use water spray to prevent overheating of the acrylic resin. Correct any small defects with wax.

5. Remove the pattern (Fig. 12-39C); sprue and invest it immediately.

Fig. 12-39 A to C, Direct pattern for a single-root tooth.

Direct pattern for multiroot teeth

A direct pattern (Fig. 12-40) can be used for multiroot posterior teeth, although limited access may make the indirect approach easier. A single-piece core with auxiliary posts is used, as opposed to the multisection core recommended for indirect posterior cast post and cores. The core is cast directly onto the post of one canal. (The other canals already have prefabricated posts that pass through holes in the core.)

Fig. 12-40 A direct post and core for posterior teeth can be made by cementing a prefabricated post through a casting. Here the two buccal canals (A and B) had a common path of withdrawal and could be incorporated into the core casting. More typically, only one canal has a fixed post, and the others are cemented through the core.

The procedure is simple, as long as smooth parallel-sided or tapered posts are used:

1. Fit prefabricated posts into the prepared canals. One post is roughened; the others are left smooth and lubricated. All posts should extend occlusally beyond the eventual core.

2. Build up the core with autopolymerizing resin, using the bead technique.

3. Shape the core to final form with carbide finishing burs.

4. Grip the smooth, lubricated posts with hemostatic forceps, and remove them.

5. Remove, invest, and cast the core with the roughened single post. When this has been done, the holes for the auxiliary posts can be refined with the appropriate twist drill.

6. After verifying the fit at evaluation, cement the core and auxiliary posts to place.

Indirect pattern for posterior teeth (Fig. 12-41)

1. Wax the custom-made posts as described previously.

2. Build part of the core around the first post.

3. Remove any undercuts adjacent to other post holes and cast the first section.

4. Wax additional sections and cast them.

Fig. 12-41 A to D, Multipiece post and cores can be made by the indirect technique, waxing each section to ensure that no undercuts are created. E to H, Alternatively, interlocking sections can be made, but this complicates the laboratory phase.

Using dovetails to interlock the sections makes the procedure more complicated and is probably of limited benefit, especially because the final buildup is held together by the fixed cast restoration.

Interim Restorations (see Chapter 15)

To reduce the need for endodontic re-treatment, endodontically treated teeth should be restored as soon as practical after completion of the endodontic procedure. Zinc oxide–eugenol (ZOE) luting materials have been used for many years to achieve a seal before initiation of prosthetic treatment, However, such ZOE materials have been shown to leak at the dentin-material interface.⁸³

Thus, if definitive restoration of the tooth is delayed, it is appropriate to etch and seal the access cavity with an adhesive resin to reduce the risk of microleakage. However, teeth in the esthetic zone often require a well-adapted interim restoration.

Such interim restorations prevent drifting of the tooth itself and of opposing or adjacent teeth after completion of endodontics (Fig. 12-42). Of particular importance are good proximal contacts to prevent tooth migration that leads to unwanted root proximity. If a cast post and core is made, the tooth will require an interim restoration while the post and core is being fabricated. This can be retained by fitting a wire (e.g., a paper clip or orthodontic wire) into the prepared canal. The restoration is then conveniently fabricated with autopolymerizing resin by the direct technique.

Fig. 12-42 A to C, Interim restorations made for endodontically treated teeth by lining a polycarbonate crown with autopolymerizing resin. The post is made of metal wire (orthodontic wire or a paper clip; see Chapter 15).

(A, From Taylor GN, Land MF: Restoring the endodontically treated tooth and the cast dowel. In Clark JW, ed: Clinical Dentistry, vol 4. New York, Harper & Row, 1985.)

Investing and Casting

A cast post and core should fit somewhat loosely in the canal. A tight fit may cause root fracture. The casting should be slightly undersized, which can be accomplished by restricting expansion of the investment (i.e., by omitting the usual ring liner or casting at a lower mold temperature [see Chapter 22]). An accelerated casting technique may facilitate the laboratory phase.⁸³ The casting alloy should have suitable physical properties. Extra-hard partial dental prosthetic gold (American Dental Association type IV) or Ni-Cr alloys have high moduli of elasticity and are suitable for cast posts (see Chapter 19). A sound casting technique is essential because any undetected porosity could lead to a weakened casting that might fail in function (Fig. 12-43).

Fig. 12-43 Fractured post.

(Courtesy of Dr. D. Francisco.)

Casting a core onto a prefabricated post avoids such problems of porosity, but the preheating temperature of the investment mold should be restricted if recrystallization of the wrought post⁸⁴ is to be avoided. The latter would adversely affect its physical properties.

Evaluation

The practitioner must be particularly careful that casting defects do not interfere with seating of thepost; otherwise, root fracture will result. Post and cores should be inserted with gentle pressure. However, the marginal fit of a cast foundation is not as crucial as that of other cast restorations, because the margins will be covered by the final casting. Air-abrading the surface to a matte finish may help detect interferences at try-in (Fig. 12-44).

Fig. 12-44 A, The fitting surface of the casting must be carefully evaluated. B, Nodules, as can be seen here, could easily lead to root fracture and tooth loss.

The shape of the foundation is evaluated and adjusted as necessary.

Cementation

The luting agent must fill all dead space within the root canal system (Fig. 12-45). Voids may be a cause of periodontal inflammation via the lateral canals.

Fig. 12-45 Residual voids after cementation can cause inflammation.

(Courtesy of Dr. D. Francisco.)

A rotary (lentulo) paste filler or cement tube (Fig. 12-46) is used to fill the canal with cement. The post and core is inserted gently to reduce hydrostatic pressure, which could cause root fracture. If a parallel-sided post is being used, a groove should be placed along the side of the post to allow for improved escape of excess cement. Use of venting procedures has also been shown to reduce the necessary seating force, although the latter is probably cement specific.⁸⁵

Fig. 12-46 A, Lentulo rotary paste fillers or a cement tube are used to fill the post space completely. B, The post is first coated with cement. C, The canal is filled with cement. D, To avoid the risk of fracture, the post and core is very gently seated. A small cement line is not usually significant, because dissolution is prevented by the presence of the definitive restoration.

(B to D, Courtesy of Dr. M. Padilla.)

Removal of Existing Posts

On occasion, an existing post and core must be removed (e.g., for re-treatment of a failed root canal filling). Patients must understand in advance that post removal is a risky process and occasionally results in radicular fracture. If sufficient length of post is exposed coronally, the post can be retrieved with thin-beaked forceps. Vibrating the post first with an ultrasonic scaler weakens brittle cement and facilitates removal. A thin scaler tip or special post removal tip is recommended (Fig. 12-47). Although histologic examination with animal models reveals no harmful effect in the periodontal tissues,⁸⁶ ultrasonic removal is slower than other methods and may result in an increased number of canal and intradentin cracks.⁸⁷ Alternatively, a post puller can be used.⁸⁸ This device consists of a vise to grip the post and legs that bear on the root face. A screw activates the vise and extracts the post.

Fig. 12-47 Post removal by ultrasonic device. A, Preoperative radiograph of the left maxillary first premolar with a parallel-sided threaded post that had to be removed for endodontic retreatment. B, After the coronal portion of the post has been well isolated, the tip of the ultrasonic device is placed against it, and energy is applied to disrupt the cement interface. Note the suction tip, which removes water spray used with the ultrasonic handpiece. C, After a time, the post becomes loose within the canal and can be retrieved by forceps. D, Radiograph of the premolar after post removal.

(Courtesy of Dr. L. L. Lazare.)

A post that has fractured within the root canal cannot be removed with a post puller or forceps. The post can be drilled out, but great care is needed to avoid perforation. The technique is best limited to relatively short fractured posts (Fig. 12-48).

Fig. 12-48 Post removal by high-speed bur. A, Preoperative radiograph of the right maxillary lateral incisor, in which both the crown and part of a post have been fractured off. A portion of the Kurer-type, parallel-sided, threaded post remains within the canal. B, Because of the large diameter of the post and its position within the canal, a high-speed handpiece was chosen to drill it out. C, Radiograph to verify the correct orientation of the bur’s progress inside the canal. With this method of post removal, the operator must be extremely careful not to let the high-speed bur contact the canal wall, which would seriously compromise tooth structure. D, Radiograph of the incisor after post removal and re-treatment.

(Courtesy of Dr. D. A. Miller.)

Another means of handling an embedded fractured post (described by Masserann⁸⁹ in 1966) is to use special hollow end-cutting tubes (or trephines) to prepare a thin trench around the post (Fig. 12-49). This technique has shown success.⁹⁰ Retrieval can be facilitated by using an adhesive to attach a hollow tube extractor⁹¹ or by using a threaded extractor⁹² (Fig. 12-50).

Fig. 12-49 Masserann technique for the removal of fractured posts. A and B, Maxillary incisor with a post that has fractured inside the canal. C, The diameter of the post is gauged with a sizing tool. D, The selected trephine is carefully rotated counterclockwise to create a narrow channel around the post. E, When the instrument has removed sufficient material, the post is recovered. F, The fractured crown and post after removal.

Fig. 12-50 Post removal by extractor. A, The Thomas (Gonon) post-removing system. It includes pliers, trephine burs, mandrels, and washers. B, Preoperative radiograph of the left maxillary lateral incisor with a post. C, Note the flared shape of the post in this preoperative view and the height of the surrounding tooth structure. D, A high-speed bur is used to free the post from coronal tooth structure and parallel its sides. (Note: An ultrasonic device may be used at this point to disturb the cement interface.) E, A trephine bur machines the post to the correct diameter and places threads for the mandrel. F, The mandrel is threaded onto the post with special washers, which distribute the forces from the extractor evenly over the tooth. G, The beaks of the pliers are fitted onto the mandrel; the knob of the pliers is then rotated, which separates the beaks, and the post is extruded from the tooth. H, The removed post, still attached to the mandrel and pliers. I, Radiograph of the lateral incisor after post removal.

(Courtesy of Dr. D. A. Miller.)