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Impact of Outcomes Data on Diagnosis and Treatment Planning

The purpose of diagnosis and treatment planning is to devise the longest-lasting and most cost-effective treatment that not only addresses patients’ chief complaints but meets or exceeds their expectations. Treatment recommendations are then based on practitioners’ individual knowledge, and their individual professional experiences serve as expanded frames of reference.

Such an approach to diagnosis and treatment planning often proves successful because data from clinical or laboratory research frequently can point to one approach as “the best treatment” or a “best practice.” That said, other aspects in the clinical decision-making process can be drawn from the dental literature to render “evidence-based” recommended treatment modalities. When available, published reports based on randomized controlled trials are considered to provide the highest level of evidential rigor,¹ allowing their results and conclusions to guide clinical treatment decision making. Unfortunately, studies with this level of science are not only complex but also costly, and they are not always available for many aspects of dentistry. Yet outcomes derived from longer-term clinical studies, such as those of 5 years’ or more duration and completed by different investigators, do provide another rich resource to guide clinical decision making. Even the results of peer-reviewed laboratory studies offer valuable information such as potential clinical trends when actual clinical findings are not otherwise available.

Therefore, it is recommended that outcomes data (the results of various scientific investigations) be used whenever possible so clinical diagnosis and treatment planning are evidence-based processes. This recommendation certainly holds true in the management of teeth that have received root canal treatment.

This chapter is intended to present reviews of published clinical and laboratory research, provide an interpretation of those research results, and then offer clinical guidelines to aid development of the best possible treatment plan for endodontically treated teeth (ETT). Varying levels of clinical and laboratory data are available in support of clinical guidelines. Therefore, a number of these key variables will be presented and discussed in this chapter to aid clinicians in arriving at the best treatment plan for each endodontic situation.

Tooth Fracture and Survival

For decades, practitioners have recognized a difference between vital teeth and ETT. In fact, more than half a century ago, Healey wrote that, “the remaining coronal portion of the treated pulpless tooth quite apparently is more brittle or fragile than when it contained a vital pulp.”² Healey’s perception was accompanied by another observation. ETT have a greater tendency to fracture during extraction and, therefore, are more likely to be removed in pieces than as intact teeth.²

Another clinical perception about ETT is that they do not have the same longevity as vital teeth. Case in point, fixed partial dentures are more likely to fail if the abutment teeth are nonvital.^3–7 Moreover, when posterior ETT are not restored with a crown, they have been found to fracture more often than vital teeth.⁸ These perceptions have emerged from reports based on both clinical and laboratory research and now are sufficiently well recognized to guide clinical decisions.

Tooth fracture

A 1999 study by Chan et al⁹ involving 315 teeth with vertical root fractures over a 13-year period determined that 60% of those fractures occurred in ETT while 40% occurred in vital teeth. Furthermore, the incidence of fracture in both nonvital teeth and vital teeth was 1.4 times higher in male than in female patients. Vertical fractures occurred more frequently in first molars for both vital and nonvital teeth.

Among ETT, the fracture rate of mandibular first molars was more than twice that of maxillary first molars, maxillary second molars, and maxillary first and second premolars. In molars that received endodontic treatment, the roots most likely to fracture were the mesiobuccal roots of maxillary molars and the mesial roots of mandibular molars.⁹ Interestingly, nonvital canines were the teeth least susceptible to fracture. A subsequent survey of cusp fractures in general dental practices found that teeth with a history of endodontic treatment were susceptible to subgingival fracture in unfavorable locations.¹⁰

Tooth survival

Clinical evidence also is available to support the perception that ETT have a lower survival rate than their vital counterparts. According to a 1992 Swedish study assessing the reasons for tooth extraction among 200 patients, ETT were lost more often than vital teeth.¹¹

A subsequent retrospective study of 202 nonvital teeth by Caplan et al¹² compared the survival of ETT with their contralateral counterparts after a median time of 6.7 years. Of the 202 matched pairs, 16% were anterior teeth, 41% were premolars, and 44% were molars. For all types of teeth, those that were endodontically treated were three times more likely to be extracted then their vital, contralateral counterparts. Of particular note, molars that received endodontic therapy were seven times more likely to be extracted than endodontically treated premolars and anterior teeth.¹²

Presence or absence of proximal contact

The survival of ETT has even been correlated to the presence or absence of proximal contact. Analysis of data from multiple studies determined that ETT with two proximal contacts had significantly longer survival rates than teeth with one or no proximal contacts.^{8, 13} In a review of charts, radiographs, and computer databases of 400 teeth from 280 patients, it was determined that ETT with two proximal contacts had substantially better survival than teeth with fewer than two proximal contacts.¹³

A meta-analysis of 14 clinical studies also identified ETT with both mesial and distal proximal contacts as having an increased rate of survival.¹⁴ In addition, a 4-year, cumulative tooth survival analysis of 759 teeth with primary root canal treatment and 858 teeth with secondary root canal treatment (retreatment) determined that teeth with two proximal contacts had a 50% lower risk of being lost than teeth with one or no proximal contact. Terminal teeth in the arch were associated with almost a 96% higher risk of loss than teeth that were not the distalmost tooth in the arch.¹⁵ Furthermore, second molars had an appreciably poorer 10-year survival rate than all other types of teeth.⁸

Fixed partial denture survival rates

As mentioned previously, multiple clinical studies, some dating back more than a quarter of a century, have shown that fixed partial dentures fail more often when supported by endodontically treated abutment teeth than when they are supported by vital abutment teeth.^3–7 One clinical study compared single crowns and fixed partial dentures over a 16- to 20-year period. It was reported that the long-term survival rates of three-unit fixed partial dentures on vital teeth were comparable to those of fixed partial dentures with at least one endodontically treated retainer. However, fixed partial dentures with more than three units and those with cantilevered units had significantly more failures.¹⁶

Restoration Selection for Pulpless Posterior Teeth

There is a substantial body of evidence to indicate that endodontically treated posterior teeth are likely to fracture unless they are restored with a complete-coverage crown. In some instances, the resulting fractures are so significant as to result in tooth loss. According to a 20-year, retrospective study of 1,639 posterior teeth restored with amalgam but without cuspal coverage, maxillary premolars (with mesio-occlusodistal [MOD] amalgam restorations) had the highest fracture rates. In fact, 28% of the maxillary premolars fractured within 3 years of endodontic therapy, 57% fractured after 10 years, and 73% fractured after 20 years, in the absence of some type of complete crown.¹⁷

Tooth fracture rates

Of all the teeth that fractured in the 20-year study cited above,¹⁷ 4% experienced catastrophic vertical fractures. Within that 4% of fractured teeth, the maxillary second molars accounted for 34.5% (10 of 29 fractures) of the teeth requiring extraction. In another study involving 220 molars followed for as few as 6 months and as long as 10 years, there were a total of 101 failures due to caries, cracks in the tooth or the restoration, loss of the restoration, or root fracture. Of these 101 teeth, 14 (13.9%) were judged to be nonrestorable. However, teeth with maximal tooth structure, mirroring that of a Class I restoration with at least 2.0 mm of surrounding axial wall thickness, had the highest 5-year survival rate (78%).¹⁸

An even larger study of 837 endodontically treated posterior teeth, with and without coronal-coverage restorations, reported a significant increase in clinical longevity when cuspal-coverage crowns were provided on maxillary and mandibular premolars and molars. The maxillary premolars experienced only a 6% failure rate when crowns were present, while the failure rate for premolars that did not receive a cuspal-coverage cast restoration was 44%. Similarly, maxillary molars restored with crowns had a 2% failure rate, while molars that were not restored with a crown had a 50% failure rate.

Similar outcomes were noted in mandibular premolars. One study reported a 6% failure rate when crowns were present but a 38% failure rate for ETT without crowns. For mandibular molars, the failure rate was only 3% for ETT with crowns but 42% for ETT without crowns.¹⁹

Large-scale data analysis

When examining the results of initial endodontic treatment in 1,462,936 teeth, researchers found that 97% of teeth were retained in the oral cavity 8 years after the initial nonsurgical endodontic treatment.²⁰ Although the percentage of tooth loss was small, 41,973 teeth were actually extracted during this observation period, of which 85%, or 35,697 teeth, did not have complete-coverage crowns. There was a statistically significant difference (P < .001) between teeth with crowns and those without crowns for all types of teeth. In fact, the number of nonvital premolars without crowns that required extraction was 5.8 times higher than that of premolars restored with crowns. In the case of nonvital molars, the number of teeth without crowns that required extraction was 6.2 times higher than the number extracted when molars were protected by complete crowns.²⁰

An analysis of data from multiple studies has demonstrated that ETT without crowns were lost at six times the rate of teeth with crowns.⁷ In a systematic review of single crowns on endodontically treated teeth, the 10-year survival of teeth with crowns was 81%, whereas the 10-year survival of ETT restored with a direct restoration (composite resin, amalgam, or cement) was 63%.²¹ A meta-analysis involving 14 clinical studies determined that crown placement on ETT increased tooth survival.⁹ The results of a recent 4-year cumulative tooth survival analysis, after primary and secondary root canal treatments, indicated a reduction in tooth loss by approximately 60% when ETT were restored with a cast restoration.¹⁰ Maxillary premolars and mandibular molars had the highest frequency of extraction due to tooth fracture.¹⁰

Time until failure

The time interval until failure and tooth loss also has been assessed for ETT with and without complete crowns. Those teeth without crowns failed after an average period of 50 months while pulpless teeth restored with a complete crown were lost after an average of 87 months following placement of a complete cast restoration.²²

In contrast, a shorter, 3-year investigation found comparable success rates between endodontically treated premolars restored with only a post and direct Class II composite resin restoration and premolars restored with complete-coverage crowns.²³ Similarly, a retrospective cohort study¹⁸ indicated that endodontically treated molars, intact except for the endodontic access opening, were successfully restored using composite resin restorations. Interestingly, composite resin restorations had better clinical performance than dental amalgam restorations. The 2-year probability of survival of molars restored with composite resin restorations was 90% versus 77% for amalgam restorations. At the 5-year point, the survival probability declined markedly for both restorative materials, to 38% for composite resin and 17% for dental amalgam restorations.¹⁸

Interestingly, clinical studies and other laboratory investigations mentioned previously reported positive results when composite resin restorations were used to restore ETT. In fact, these results support the use of composite resin rather than a cuspal-coverage crown when the tooth is intact except for the endodontic access opening or is minimally restored. Unfortunately, there are no long-term clinical data comparing the survival of pulpless posterior teeth with composite resin restorations to that of teeth restored with complete crowns that vary in terms of the amount of the remaining tooth structure. It would be helpful if there were studies comparing the survival rates of premolar and molar teeth restored without crowns with the following conditions: (1) intact except for a conservative endodontic access opening; (2) a Class I restoration and an access opening; (3) a small Class II restoration and an access opening; (4) a large Class II restoration; and (5) a large MOD restoration.

Additionally, normal occlusal forces place substantial stresses on teeth.²⁴ These same stresses can cause vertical fractures in both nonvital and vital teeth.³ It also has been reported that parafunctional habits produce higher failure rates for fiber posts restored with composite resin.²⁵ Therefore, the effects of heavier-than-normal occlusal forces, as well as parafunctional habits, on ETT restored without crowns should be examined more extensively to determine their impact on tooth survivability.

Complex amalgam restorations versus complete crowns

Aside from complete crowns, complex amalgam restorations also have been used to restore both vital and nonvital teeth. In these cases, an evaluation of the remaining tooth structure should be made to determine whether to replace or cover (“cap”) weakened cusps with dental amalgam restorative material.

In one randomized, controlled clinical trial, vital teeth were carefully evaluated for cusp strength. Weak cusps were reduced, and an 88% survival rate was reported for the 268 teeth receiving extensive amalgam restorations after a 100-month period. Forty of the 268 restorations (14.9%) required some form of clinical treatment during the study to secure or increase their clinical lifetime, resulting in a 72% survival rate.²⁶ A retrospective study of 128 vital teeth with complex amalgam restorations, described by the authors as posterior restorations replacing one or more cusps, included a time-life survival analysis. In that report, the percentage of restorations surviving for 10 years was 54%, but that amount declined to 36% after 15 years and only 19% after 20 years.²⁷ In another study of 124 cusp-covered Class II amalgam restorations, the cumulative survival rate was 72% after 15 years.²⁸

When amalgam restorations were placed in nonvital teeth, positive results also were reported in both laboratory and clinical studies, provided that a sufficient thickness of amalgam covered the cusps. In one laboratory study, 48 teeth were restored with mesio-occlusal (MO) restorations with a 4.0-mm thickness protecting the buccal cusps and 3.0 mm of amalgam over the lingual cusps.²⁹ When an angular load was applied to the restored cusps, the authors concluded that amalgam was a suitable material for cuspal restoration of pulpless teeth.²⁹ In another laboratory study of 36 extracted, intact mandibular molars, endodontic access openings were placed in the teeth, and the root canals were instrumented as they would be clinically to initiate endodontic therapy. The teeth were then prepared and restored with either MO or MOD amalgam restorations, with and without cuspal coverage. Specimens receiving amalgam coverage of the entire occlusal surface, with at least 2.0 mm of the cuspal coverage, retained their cuspal stiffness.³⁰ However, without cuspal coverage, teeth with conventional Class II MO and MOD amalgam restorations were not considered adequately protected.³⁰

One clinical study of 100 pulpless teeth, restored with amalgam overlying the cusps, found that the amalgam restorations were successful after 3 years of service.³¹ The results of another investigation included a recommendation that all cusps adjacent to teeth with missing marginal ridges be covered (“capped”) with a sufficient thickness of amalgam.³²

Restoration Selection for Pulpless Anterior Teeth

One of the most misunderstood and perhaps challenging clinical decisions has to do with how to manage anterior teeth following root canal therapy. There is a clinical perception that endodontically treated anterior teeth without crowns are less prone to fracture than posterior teeth.¹⁹ A study of 1,273 ETT conducted more than 25 years ago found that crowns significantly increased the survival rate for posterior teeth, but the same outcome was not valid for anterior teeth.¹⁹

In this study, the maxillary premolar success rate for ETT increased from 56.0% when no coronal coverage was provided to 93.9% with coronal coverage. Maxillary molars exhibited similar results with the success rate increasing from 50.0% (no crown) to 97.8% (with coronal coverage). For maxillary anterior teeth, the success rate was 85.4% with no crown and 87.5% with a crown. For mandibular anterior teeth, the success rate was 94.4% with no crown and 97.5% with a crown. From these results it was concluded that intact pulpless anterior teeth, except for a conservative endodontic access opening, do not require complete crowns. The authors of this chapter suggest that the results of this study make it reasonable to restore pulpless anterior teeth with composite resin when they are intact except for the access opening; when they are weakened by large and/or multiple restorations; and when they require significant color or form changes that cannot be managed by some type of more conservative treatment.

Despite the findings of Sorenson and Martinoff,¹⁹ anterior teeth without crowns can indeed fail and require extraction, as was reported in the previously cited study of more than 1.4 million teeth.²⁰ In that extensive investigation, 83% of the anterior teeth that were extracted had not received a crown, while 9.7% of the extracted teeth had crowns and posts and 7.3% of the extracted teeth had crowns but no posts.²⁰ Thus, stronger evidence confirms that the longevity of endodontically treated anterior teeth is increased when they are restored with crowns, in contrast to the findings reported by Sorensen and Martinoff in 1984.¹⁹

Clinicians must carefully evaluate the amount of remaining coronal tooth structure in anterior teeth before deciding not to recommend complete coronal coverage. Additionally, as with posterior teeth, there are no tooth survival data comparing intact pulpless anterior teeth with similar teeth having small restorations and those with large restorations where substantial amounts of tooth structure are missing. Likewise, data regarding the impact of heavy occlusal forces are not available to guide treatment planning. Therefore, tooth conditions and structural integrity remain key factors when clinicians assess anterior ETT and propose the most appropriate treatment regimen.

Physical Properties and Characteristics of Pulpless Teeth

As to the question of whether or not ETT are as brittle as many perceive, numerous studies have compared different physical properties and characteristics of both vital and nonvital teeth. While some definitive differences have been identified, there also are some conflicting findings. In fact, not all the data conclusively support the presence of substantial differences between vital and nonvital teeth. Additionally, some of the outcomes have not been replicated in multiple studies by different investigators. Nevertheless, a comprehensive review of available evidence provides insight into what happens or may happen to teeth following endodontic therapy. Therefore, this particular question is best addressed by assessing various physical properties and characteristics of pulpless teeth.

Moisture content

Conflicting information exists as to the moisture content in teeth before and after root canal therapy. One study of the dentin in dogs determined that pulpless teeth had 9% less moisture than comparable vital teeth.³³ Yet in another investigation of 23 matched pairs of human ETT and their vital contralateral teeth, the moisture levels (12.3% in vital teeth and 12.1% in nonvital teeth) were not statistically significantly different.³⁴ In some teeth that had undergone root canal therapy as many as 15 to 20 years earlier, the moisture content was not necessarily reduced, even after extended periods of time.³⁴ It has even been stated that dehydration alone does not account for changes in physical properties of dentin.³⁵

Flexibility

At least two studies have shown that ETT have less flexibility (ability to bend and then return to their original shape) than corresponding vital teeth.^36,37 Another study found a measurable decrease in tooth stiffness and proportional limit as a result of root canal treatment.³⁵ Stiffness also has been assessed as it relates to the type of restoration placed in vital teeth and the corresponding impact on root canal treatment. For example, a one-surface occlusal preparation was found to produce a 20% decrease in stiffness while an MO preparation caused a 46% reduction. Tooth stiffness was reduced even further (a 63% reduction) following placement of an MOD preparation. However, according to one investigation, endodontic treatment alone decreased stiffness by only an additional 5%.³⁸

Cuspal deflection

Aside from moisture level and stiffness changes, cuspal movement or, better yet, resistance to deflection is another important characteristic. The cuspal deflection (separation of the cusps) that occurs on maxillary first premolars has been measured by applying a load to a steel ball positioned in the occlusal fossa. One study found the separation of the facial and palatal cusps to be 1.0 μm for an intact, vital tooth. The actual amount of deflection increased dramatically in premolars when these teeth were prepared for restorations. In fact, cuspal deflection increased from 1.0 μm (baseline) to 16.0 μm when there was a Class I occlusal cavity preparation, to 20.0 μm for a minimal width MO cavity preparation, and to 24.0 μm for a minimal width MOD cavity preparation in teeth that had not undergone endodontic therapy. Following a pulpotomy, the amount of deflection rose to the highest level, 28.0 μm.³⁹ The authors concluded that breaking the continuity of the enamel layer reduces tooth rigidity, and teeth that have a wide isthmus, as in a Class II MOD cavity preparation, should have some form of cuspal protection.³⁹

Another study of cuspal deflection determined that intact mandibular molars had cuspal deflections of up to 1.0 μm.⁴⁰ While MO cavity preparations changed the deflection to less than 2.0 μm of movement, MOD cavity preparations produced 3.0 to 5.0 μm of movement. Endodontic access preparations produced 7.0 to 8.0 μm of movement in the MO group and 12.0 to 17.0 μm of movement in the MOD group (a twofold to threefold increase).⁴⁰

A third study of cuspal movement in 10 maxillary premolars reported a mean deflection ranging from 3.0 to 12.0 μm in intact teeth. The amount of the mean deflection actually increased from 14.0 to 26.0 μm following root canal treatment, removal of the marginal ridges, and restoration with composite resin.⁴¹

Proprioception

The ability of teeth to respond to stimuli, such as possessing a sense of being contacted by opposing teeth or other hard objects, is known as proprioception. One clinical study used a spring device to apply force to 155 normal teeth (incisors, canines, premolars, and molars) until patients indicated that they first felt the sensation of pressure.⁴² The proprioception threshold, or point at which a pressoreceptive response is initiated, was significantly higher (57%) in nonvital teeth than vital teeth.⁴² This threshold level also increased significantly from anterior to posterior teeth.⁴²

Another clinical investigation, with three patients, involved crowns with buccal bars placed on vital teeth and their adjacent or contralateral endodontically treated teeth.⁴³ Weights and their corresponding loads were applied at different positions on the bars until the subjects experienced pain. Nonvital teeth had pain threshold levels that were more than twice as high as those of their contralateral or adjacent vital tooth.⁴³

Conversely, a study of 29 patients compared the response of 59 vital teeth with that of 22 endodontically treated maxillary teeth when a pushing force was directed from the incisal edge parallel to the long axis of each tooth. The load was applied at an incremental speed of 1 N/s until the patient pushed a button to indicate touch was sensed. In this investigation, the authors did not find a significant difference between the tactile sensibilities of ETT and vital teeth.⁴⁴

Classic physical properties

Considerable variation exists among the classic physical property tests, such as hardness, load to fracture, toughness, and strength (compressive, shear, and tensile), used to compare outcomes for vital and nonvital teeth. This makes it challenging to draw definitive conclusions and comparisons for specific properties. For example, it has been reported that pulpless teeth have decreased dentin strength.^{45, 46} It also has been reported that dentin strength is not decreased following endodontic therapy.^{47, 48} When vital and nonvital dentin hardness were measured, one group of researchers found comparable hardness values⁴⁹ while another group noted a significant (3.5%) reduction in hardness.⁵⁰

A study of shear strength and toughness determined that ETT exhibited significantly lower values than corresponding vital teeth for both these tests.⁴⁶ A subsequent study by different investigators did not find differences in shear strength, toughness, or load to fracture between vital and nonvital dentin.⁵⁰ Comparable compressive and tensile strengths also were recorded for vital and nonvital dentin.³⁵

Guidelines for Restoration Selection

Recommendations for the type of restoration to be placed following endodontic treatment differ for posterior and anterior teeth.

Restoration of posterior teeth

Most endodontically treated posterior teeth should be restored with complete-coverage crowns to enhance their longevity, particularly teeth previously restored with large MOD, MO, or disto-occlusal intracoronal restorations. Such teeth benefit from crowns that encompass the cusps to prevent fracture from the occlusal forces responsible for cuspal separation. However, posterior teeth that are intact or minimally restored with a conservative endodontic access opening and are not subjected to heavier-than-normal occlusal forces can be restored with composite resin restorations. Dental amalgam restorations may be placed in situations where the restorative material covers the cusps at a thickness of at least 2.0 mm.

Restoration of anterior teeth

In many instances, the access opening in endodontically treated anterior teeth can be restored with a conservative composite resin restoration. According to Sorensen and Martinoff,¹⁹ complete-coverage crowns are not always necessary unless a tooth has been weakened or compromised by large or multiple restorations or its color or form cannot be effectively corrected with conservative treatment. As previously discussed, however, Salehrabi and Rotstein²⁰ found that failure rates were lower in all types of teeth that received crowns than in those that did not. Studies of the physical properties of ETT have produced some differing results, but there are documented changes in some of the physical properties and characteristics of nonvital teeth that may make them more susceptible to fracture.

Outcomes Data for Posts

Laboratory data

A popular misconception is that posts strengthen ETT. However, research indicates otherwise. In fact, numerous studies have shown that, rather than strengthen teeth, posts and cores actually weaken extracted teeth (decrease their fracture resistance) or fail to increase their fracture resistance.^51–56 Maxillary incisors without posts were found to resist higher loads than were other types of teeth with posts and crowns.⁵⁷ Likewise, mandibular incisors with intact natural crowns exhibited greater resistance to transverse loads than ETT with either prefabricated posts or cast posts and cores.⁵⁸ However, if a ferruled cast post and core was used, the likelihood of root fracture was reduced.⁵⁸

A condition where a post and core may have a positive impact on a tooth was identified in a photoelastic stress analysis study.⁵⁹ That research indicated that posts reduced dentin stress in situations where the root canal space was excessively enlarged and the dentin walls were thinned.⁵⁹

Clinical data

Clinical data also fail to support the perception that posts enhance the survival of teeth. While one investigation demonstrated that teeth with and without posts had the same longevity outcome,⁶⁰ another clinical study determined that teeth with posts exhibited significantly more apical periodontitis than teeth without posts.⁶¹ Results from the latter study indicated that the preparation and placement of a post can actually compromise the apical endodontic seal. The previously cited examination of 1.4 million teeth with initial endodontic treatment found no significant difference between the extracted teeth with posts and those without posts.²⁰

Guidelines for use of a post

While there is no evidence to support the contention that posts promote tooth survival or strengthen the root of a tooth, there is evidence that posts actually can decrease the fracture resistance of ETT.^51–56 Consequently, clinicians should recognize that the main purpose of a post is to retain a coronal core that cannot otherwise be placed in tooth structure by some other means.

Clinical Complications of Post and Core Restorations

A review of the literature for clinical post and core studies identified the two most common types of postoperative complications as post loosening and root fracture.⁶² Both of these negative outcomes were reported in more than 10 studies and were found to be the most common reasons for failure.

Root perforation is a serious complication but was not reported in this literature review.⁶² Therefore, a literature search was completed to identify clinical studies with data related to the incidence of clinical root perforation. In 1984, Sorensen and Martinoff¹⁹ reviewed the patient treatment records and radiographs of nine general dentists. Among 420 posts, only three perforations (0.7%) were noted.¹⁹ In another study, a review of radiographs for 327 ETT with posts, most of which were referred by general dentists for a variety of reasons, only 5 teeth (1.5%) with root perforations were identified.⁶³ A third investigation involving a radiographic analysis of 3,178 ETT determined that 1.1% of the teeth had perforations of either the root walls or the floor of the pulp chamber.⁶⁴

Additional studies published between 2005 and 2010 were identified in which the endodontic treatment was provided by predoctoral dental students.^65–68 A review of 388 teeth with 620 treated root canals in an undergraduate dental clinic found perforations in 17 root canals (2.7%).⁶⁵ A second study involved the assessment of endodontic treatment performed by predoctoral dental students in single-rooted teeth; examination of 100 radiographs of obturated root canals found no evidence of root perforations.⁶⁶ A third investigation involving 550 teeth treated by predoctoral students reported the incidence of root perforation to be 7.0%.⁶⁷ A 2010 retrospective study from a university clinic reported a total of 116 root perforations in 5,048 ETT in 2,002 patients for an incidence rate of 2.3%.⁶⁸ Over a 26-year span, the incidence of root perforation by general dentists and predoctoral dental students ranged from 0% to 7%.⁶⁸

The literature also has information about perforations as a reported complication in insurance claims, a reason for tooth extraction, and the basis for referral to an endodontist. In a review of 966 dental complications, claims submitted to the Swedish patient insurance program included 183 root perforations related to preparation for posts.⁶⁹ The teeth identified as having the greatest number of perforations were the mandibular first molar followed by the maxillary first premolar.⁶⁹ In another study of 119 teeth, iatrogenic perforations and thinning of the dentin to the location of the cementum (a process known as stripping) were responsible for 4.2% of the extractions.⁷⁰ Of 2,000 patients referred to an endodontist, 1,688 patients required some type of treatment in 2,221 teeth. Perforations were identified in 119 teeth for an incidence of 5.4%,⁷¹ well within the range of 0% to 7% reported in other studies.

The aforementioned data on complications reveal why it is important to understand the factors responsible for post loosening, root fracture, and root perforation. Once identified, appropriate steps can be taken to minimize, if not totally avoid, such complications. As already mentioned, the main purpose of a post is to provide a core for a tooth when no other means is available for a coronal buildup; therefore, the following content relates only to clinical situations where a post is needed.

Post loosening

Influence of post form

The geometry, or form, of a post also has been identified as a factor that can contribute to post loosening.^{72, 73} Threaded posts generally are recognized as the most retentive type of post.^{72, 73} At the same time, threaded posts also are responsible for producing high stess,⁷⁴ and tapered threaded posts are the worst stress producers.^{75, 76} In fact, comparative clinical studies have linked high stress from threaded posts to root fracture rates that are higher than those associated with cemented posts.^{77, 78} One meta-analysis of clinical studies reported survival rates of 81% for threaded posts and 91% for cemented posts.⁷⁹ Based on these findings, and to avoid potential root fractures, use of threaded posts is not recommended.

Influence of post length

Optimizing the post length is an important and safer method of preventing post loosening than the use of threaded posts to increase retention.⁸⁰ Posts that are three-quarters the root length were found to be 24% to 30% more retentive than those that were half the root length or equal in length to the crown height.⁸¹ However, with posts that are three-quarters the length of the root, there is an accompanying risk of compromise to the seal of the root canal filling material in teeth with average or shorter-than-average root lengths. What is more, extension of the post spaces to three-quarters the root length also reduces the amount of gutta-percha to less than that required to ensure the maintenance of an adequate seal. Therefore, post length should not be extended to the point that it requires the removal of so much apical gutta-percha that the apical seal is unknowingly compromised.

A number of laboratory studies have reviewed the amount of gutta-percha needed and its effects on the apical seal. It was determined that a large number of ETT specimens leak when there is only 2.0 mm of apical gutta-percha,⁸² and most specimens leaked when left with 3.0 mm of gutta-percha.⁸³ In addition, a clinical study found significantly more posttreatment periapical radiolucencies in teeth with less than 3.0 mm of apical gutta-percha.⁸⁴ However, in the presence of at least 4.0 mm of apical gutta-percha, studies show there is little leakage^{82, 85} or no leakage.^{86, 87}

Based on these data, it appears that 4.0 mm should be considered the minimum amount of gutta-percha required for an adequate apical seal. However, because length determinations are frequently based on radiographic images and the angulations of radiographs vary clinically, it is proposed that 5.0 mm of gutta-percha be retained apically as measured on a radiographic image. Therefore, the guideline for appropriate length is to retain not 3.0 or 4.0 mm but rather 5.0 mm of apical gutta-percha, as determined radiographically, and to extend the post fully to the point where the gutta-percha is located.

Influence of root selection

This 5.0-mm apical guideline for gutta-percha length works well for all teeth except molars. In 1982 Abou-Rass et al⁸⁸ prepared post spaces to depths of 4.0 mm and 7.0 mm into a canal space using No. 2 (diameter of 0.8 mm), No. 3 (diameter of 1.0 mm), and No. 4 (diameter of 1.2 mm) Peeso instruments.⁸⁸ The thickness of remaining dentin was then measured at the 4.0- and 7.0-mm depths along with the incidence of root perforation or thinning to the dentin-cementum border. The authors concluded that the distal roots of mandibular molars and the palatal roots of maxillary molars are the roots best suited for post preparation. They also contended that these roots can be safely prepared to a depth of 7.0 mm using the No. 2 and No. 3 Peeso instruments. In contrast, perforations or root thinning occurred in the mesial roots of mandibular molars as well as the facial roots of maxillary molars.⁸⁸ Of the 75 mandibular molars included in the study, perforations or instrumenting to only a thin layer of root structure occurred in 27.7% (20) of the prepared mesial roots.⁸⁸

Based on these results, it is proposed that posts with diameters between 0.8 and 1.0 mm extend no more than 7.0 mm into a molar canal space. This depth guideline should only be applied to the distal roots of mandibular molars and the palatal roots of maxillary molars. The mesial roots of mandibular molars and the facial roots of maxillary molars should not be used for post placement.

Guidelines for prevention of post loosening

By design, threaded posts provide greater resistance to post loosening than do cast root-form posts. As mentioned previously, however, posts with a thread design are not recommended because of their potential for generating high levels of internal root stresses and the associated higher incidence of root fracture. Prevention of post loosening is best accomplished by following the recommended guidelines for length and selecting the appropriate primary root. The clinician should leave 5.0 mm of intact apical gutta-percha and extend a post the length of the root space right to the level of the gutta-percha. The exception is in molar teeth, where the post length should be limited to 7.0 mm into the primary roots (palatal root of maxillary molars and distal root of mandibular molars). Care should be exercised to avoid preparing canal spaces in secondary roots (buccal roots of maxillary molars and mesial root of mandibular molars).

Root fracture and root perforation

Influence of threaded posts

If the use of threaded posts is avoided, the threat of high internal root stresses is reduced along with the accompanying risk of root fracture.^74–76 Multiple clinical studies comparing threaded and cemented posts have shown higher root fracture rates with threaded posts.^77–89 One meta-analysis of clinical studies determined that cemented posts had a 10% higher tooth survival rate than did threaded posts.⁷⁹ An evaluation of 95 ETT where vertical root fractures had occurred identified threaded posts as the most common type of post (64 of 95 teeth) present in teeth that fractured.⁸⁹

Influence of post length

Studies have determined that short posts increase the stress in teeth^{90, 91} while increases in post length enhance the resistance of a tooth to root fracture.⁹² Additionally, posts of optimal length offer the greatest rigidity and produce the least amount of root deflection.⁹³ In an examination of 154 teeth that had been extracted because of vertical root fractures, 95 teeth had posts, and 66 of the posts ended in the coronal third of the root.⁸⁹

Influence of post diameter

In addition to post length, post diameter affects the potential for root fracture by increasing stress^{91, 94} and decreasing resistance to fracture.⁹² With large-diameter posts (those 1.5 mm or greater), root fracture increased sixfold for every millimeter of decreased root diameter.⁷⁶ Unfortunately, there have been no clinical studies comparing fracture resistance of teeth restored with several different post diameters to determine the most appropriate proportional relationship between post diameter and root diameter. However, in 2011 Du et al⁹⁵ reported results, based on finite-element analysis of the mandibular first premolar, showing that posts with a diameter that was 50% of the root diameter exhibited the most favorable stress distribution.⁹⁵ Nevertheless, the clinical guideline that has been used successfully is not to prepare post spaces that exceed one-third the root diameter in its smallest cross-sectional dimension.

Influence of residual dentin thickness

The amount of dentin thickness remaining after root canal treatment is an important factor because overenlargement of the root canal space for placement of a post can compromise tooth strength.

It has been shown that only five teeth have more than 1.0 mm of dentin wall thickness remaining after conventional endodontic therapy.⁹⁶ Those five teeth are the (1) maxillary central incisors, (2) maxillary lateral incisors, (3) maxillary canines, (4) mandibular canines, and (5) maxillary first molars (palatal root only).⁹⁶ According to Ouzounian and Schilder,⁹⁶ all other teeth have less than 1.0 mm of dentin thickness after endodontic cleaning and shaping.

A study of maxillary first premolars with posts prepared to a depth equal to the clinical crown height determined that 1.0 mm of residual dentin thickness was only present when a 0.7-mm diameter rotary instrument was used to prepare the post space.⁹⁷ It has been shown that any amount of root canal preparation in mandibular first and second molars following endodontic treatment decreased the residual dentin thickness to less than 1.0 mm.⁹⁸ Additionally, a study of residual dentin thickness in the distal root of 26 mandibular molars after endodontic treatment alone determined that the canal wall on the furcal side of the root was less than 1.0 mm thick 82% of the time and less than 0.5 mm thick 17.5% of the time.⁹⁹

Influence of instrument diameter

Because of the potential for root perforation or root thinning that can lead to tooth fracture or perforation, appropriate post diameters have been identified in relation to average root diameters. Establishing appropriate post diameters helps to determine the maximum diameter of instruments that can be used safely in each type of tooth to prepare post spaces.

One study measured 50 teeth of each type and recommended selection of a post provided its diameter was (1) no greater than one-third the root diameter at the cementoenamel junction; (2) at least 2.0 mm less than the root at the midpoint of the post; and (3) 1.5 mm smaller than the root at the apical end of the post.¹⁰⁰ For mandibular incisors, it was proposed that posts be 0.7 mm in diameter. However, the authors proposed a diameter of 1.1 mm for the distal roots of mandibular molars, 1.3 mm for the palatal root of maxillary molars, and 0.9 mm for maxillary first premolars. The largest recommended diameter was 1.7 mm, for maxillary central incisors.¹⁰⁰

Another study measured average root dimensions of 125 teeth of each tooth type. The authors recommended that post diameters not exceed one-third of the average root width,¹⁰¹ using a 95% confidence level.

Based on these post diameters, certain instruments should not be used to prepare post spaces. For instance, No. 5 and No. 6 Peeso instruments, No. 6 Gates Glidden instruments, No. 4 and No. 6 round burs, and prefabricated post drills such as the No. 6 and No. 7 ParaPost drills (Coltène/Whaledent) are too large and should not be used. The No. 4 Peeso, No. 5 Gates Glidden, and No. 5 ParaPost drills have diameters that are slightly larger than 1.1 mm and, therefore, are only recommended for teeth with large-diameter roots. Even a No. 4 Gates Glidden drill has a diameter that is too large for the distal root of mandibular molars.⁹⁹

Influence of post depth and length

The previously cited study of root thinning and perforations in molars by Abou-Rass et al⁸⁸ determined that posts can safely be placed to a depth of 7.0 mm when instruments with diameters between 0.8 and 1.0 mm are used in the distal roots of mandibular molars and the palatal roots of maxillary molars.⁸⁸ In another investigation where No. 4 Gates Glidden instruments were used to prepare post spaces in the distal roots of 26 mandibular molars, perforations occurred 7.3% of the time and even more frequently when larger drills were used.⁹⁹ As a result, the authors recommended that Gates Glidden drills larger than No. 3 not be used in the distal roots of mandibular molars. The authors went on to state that “post spaces in such teeth should be limited to the endodontically prepared canal.”⁹⁹

Findings from other research also support the recommendation that the root canal space not be enlarged after endodontic treatment in the distal roots of mandibular molars.⁹⁸ Again, it was determined that the residual dentin thickness after root canal treatment was less than 1.0 mm,⁹⁸ making root fracture or perforation more likely even with self-limiting Gates Glidden mechanical instruments. Similar characteristics and guidelines exist for mandibular second molars as well.

To be optimally successful with posts, it seems reasonable to adopt the more conservative diameter recommendations reported in the studies cited. Therefore, the following maximal post diameters are recommended: 0.6 mm for mandibular incisors; 0.9 mm for most teeth (maxillary lateral incisors, maxillary and mandibular premolars, the distal root of mandibular molars, and the palatal root of maxillary molars); 1.0 mm for canines; and 1.1 mm for maxillary central incisors.

When molar canals are prepared, the previously recommended guideline of 7.0 mm for post length must be followed in the primary roots (palatal roots of maxillary molars and distal roots of mandibular molars). However, as already mentioned, posts should not be placed in the mesial roots of mandibular molars; as one study showed, 20 of 75 teeth with 7.0-mm-long posts had only a thin layer of remaining dentin or were perforated.⁸⁸

Influence of root morphology

Root morphology has been well described,¹⁰² and an understanding of external root morphology is helpful in identifying the teeth and roots that are best suited for posts and less likely to be subjected to root thinning or root fracture.

Maxillary root morphology. The nature of the root morphology of teeth in the maxillary arch has been characterized well in the literature. An understanding of this information, coupled with proper root selection for post placement, aids clinicians in negotiating these delicate spaces when placement of a radicular post is planned.

Incisors. Maxillary central incisors have a cross-sectional root anatomy that is triangular or ovoid with a lingual taper.¹⁰² Their form and dimensions usually allow placement of posts with the proposed optimal diameter of 1.1 mm.¹⁰¹ Maxillary lateral incisors also possess a single root with a circular, oval, or ovoid cross-sectional form. The root canal is triangular in cross section in the cervical area and round apically.¹⁰² However, the smaller root diameter makes it desirable to follow the recommended optimal post diameter of 0.8 mm.¹⁰¹

Canines. Maxillary canines have an oval cross-sectional shape with prominent developmental depressions,¹⁰² making their form less ideal than that of maxillary incisors for post placement. However, the root dimensions are usually sufficient to permit posts with the recommended optimal diameter of 1.0 mm.¹⁰¹

Premolars. In a 10-year retrospective study of both metal prefabricated posts and custom posts, posts in maxillary first premolars had the highest failure rate (30%).¹⁰³ The canals in these teeth are not well suited for enlargement beyond the root canal diameter present after endodontic treatment. In fact, one study of residual dentin thickness validated the negative effect of post preparation on dentin thickness. When the first premolars were prepared using rotary instruments with diameters of 0.9 and 1.0 mm, 61% of the lingual roots and 77% of the buccal roots had less than the desired 1.0 mm of dentin remaining.¹⁰⁴

Maxillary first premolars have prominent mesial and distal developmental depressions on the root trunk as well as a relatively narrow mesiodistal root dimension.¹⁰⁵ Additionally, in two-rooted first premolars, the developmental root depressions on both the mesial and distal aspects of the root trunk deepen progressively from the cervical line to the furcation.¹⁰⁶ When measured, the depth of the mesial furcal concavity increased to slightly more than 1.0 mm at a distance of 4.7 mm apical to the cementoenamel junction.¹⁰⁷

In two-rooted maxillary first premolars, the palatal root is the more desirable location for a post because it is usually straighter and does not have the distal root curvature frequently present (66%) in the facial root.¹⁰⁸ The palatal root of two-rooted premolars also has a surface form that is more conducive to placement of a post because the facial root frequently has a concavity on the furcal aspect of the root.¹⁰⁹ In one study of 100 maxillary first premolars, 37 of the teeth had bifurcated roots, and 62% of these teeth had a concavity (with a mean depth of 0.46 mm) on the furcal aspect of the buccal root.¹⁰⁶ Another study found that 35 (78%) of 45 maxillary first premolars with two roots had this groove on the palatal aspect of the buccal root.¹¹⁰

A third study of 97 bifurcated first premolars determined that the residual dentin thickness after root canal treatment, at 6.0 mm apical to the cementoenamel junction, was less than 1.0 mm in 53% of the buccal roots on the palatal surface. After post preparation, the dentin thickness in the area of the furcal groove was less than 1.0 mm in 77% of the teeth. The authors recommended that the lingual root be used instead of the facial root when posts are necessary.¹⁰⁴

When a single-rooted maxillary first premolar requires a post, the diameter of the canal should be 0.7 mm or less because the mesial and distal developmental root depressions restrict the amount of available tooth structure in the centrally located single root canal.⁹⁷

Maxillary second premolars are better suited for post placement then are maxillary first premolars because they usually have one root with a slightly larger mesiodistal dimension at the cervical line and slightly greater root length than the first premolar.¹⁰⁵ While mesial and distal developmental root depressions are typically present on the root of maxillary second premolars,¹⁰⁸ the mesial depression is shallower than the one present on the first premolar.¹¹¹

Molars. In maxillary first molars, only the palatal root is well suited for post placement. This canal is ovoid in cross-sectional shape¹⁰² and has greater cross-sectional dimensions than the facial roots.¹⁰⁵ Additionally, while developmental depressions can be present on the facial and palatal surfaces of the lingual root, they are generally shallow.¹⁰² One complicating factor for the palatal root is the frequent presence of a facial curvature in the apical third.¹¹² However, when the recommendation of a 7.0-mm palatal post length is followed, the presence of facial curvature does not create a problem because the root curvature occurs apical to the end of an ideal post. The mesiofacial root of the first molar is relatively thin mesiodistally and also has prominent depressions or flutings on both the mesial and distal surfaces,^{110, 111, 113–115} making it unsuitable for post placement.

The distofacial root is rounded or ovoid in cross section¹⁰² and usually does not have a distal developmental depression,¹⁰⁸ but it does contain a developmental depression on its mesial surface.¹⁰² It is also smaller faciolingually than the other roots and narrower mesiodistally at its attachment to the root trunk, making it a poor candidate for post placement.¹¹¹

Maxillary second molars are similar to first molars and, therefore, only the palatal root is suitable for post placement. Their facial roots are even less suitable than the facial roots of first molars because of their distal curvature.

Mandibular root morphology. The root morphology of teeth in the mandibular arch is important to understand so that appropriate treatment recommendations are made and post loosening and root fracture are averted.

Incisors. Mandibular central and lateral incisors have roots that are broad faciolingually but narrow mesiodistally with substantial longitudinal depressions on both the mesial and distal surfaces.¹⁰² These depressions are usually deeper at the junction of the middle and apical thirds of the root.¹¹¹ The roots’ cross-sectional form is ovoid to hourglass in shape.¹⁰² Because of the small mesiodistal root dimension, these teeth are not suitable for canal space enlargement beyond that resulting from endodontic treatment itself. Because prefabricated posts are larger than the maximum diameter recommended for mandibular incisors, these teeth are best treated with custom cast posts and cores made to fit the existing root canal morphology; the optimal post diameter is 0.6 mm.¹⁰¹

Canines. Mandibular canines have an oval cross-sectional shape and prominent developmental depressions.¹⁰² They also generally have root dimensions sufficient to permit safe post placement provided that the maximum diameter does not exceed 1.0 mm.

Premolars. Mandibular first premolar roots typically are larger faciolingually than they are mesiodistally. Developmental root depressions are frequently found on both the mesial and distal surfaces of the root.¹⁰² Nonetheless, the root dimensions generally are large enough to permit the safe placement of posts when they adhere to the recommended diameter of 0.9 mm.

Mandibular second premolars too have a root that is ovoid in cross section and have sufficient dimensions to permit placement of a post with the recommended diameter of 0.9 mm. However, mandibular premolars with oval or ribbon-shaped canals should not be subjected to any preparation of the root canal space beyond that produced during endodontic treatment because it is likely to leave less than the desired 1.0 mm of residual dentin thickness.¹¹⁶

Molars. Data on mandibular first molar root morphology indicate that both the mesial and distal surfaces of the mesial root have developmental root depressions.^{108, 111} The depression is deep on the mesial surface but even deeper on the distal surface.¹⁰⁸ Nevertheless, the distal root of the mandibular first molar is better suited for post placement because the distal root is straighter than its mesial counterpart and rounder in cross-sectional form. Also, the mesial developmental depression on the distal root often is not very deep, and the distal surface may not even have a depression or may possess only a slightly concave surface.¹⁰⁸ The distal roots of mandibular molars should not be enlarged beyond the preparation produced by endodontic treatment because the residual dentin thickness after root canal treatment typically is less than 1.0 mm.⁹⁸ Similar characteristics are present on mandibular second molars.

Guidelines for prevention of root fracture and root perforation

Fracture is more likely to occur when roots are weakened by posts that are short because short posts also increase root stress. Therefore, the same guidelines used to minimize post loosening should be followed to minimize, if not prevent, root fracture. As stated previously, the recommended post length for most teeth (except molars) is what remains after 5.0 mm of apical gutta-percha is left in place. For molars, post length should not exceed a maximum length of 7.0 mm in the primary roots (the palatal root of maxillary molar and distal root of mandibular molars).

Root fracture is more likely to occur when teeth are weakened by post spaces of an excessive diameter. A good guideline to follow clinically is to prepare post spaces to no greater than one-third the root’s original diameter. In some teeth, any root canal enlargement beyond that produced during root canal treatment will create excessive space because the amount of residual dentin remaining after endodontic treatment typically is 1.0 mm. As mentioned previously, only five maxillary teeth have more than 1.0 mm of root wall thickness remaining after root canal treatment: (1) maxillary central incisors, (2) maxillary lateral incisors, (3) maxillary canines, (4) mandibular canines, and (5) the palatal root of the maxillary first molar. Teeth with less than 1.0 mm of residual dentin thickness following endodontic treatment should not be subjected to further canal enlargement for placement of a post.

Overenlargement of a canal space and subsequent fracture can occur when instruments with diameters that exceed the recommended dimensions are used. Therefore, most teeth should be prepared with instruments that do not exceed 0.9 to 1.0 mm in diameter. The largest recommended instrument diameter is 1.1 mm, and that is reserved for use in maxillary central incisors. Teeth with small roots, such as mandibular incisors, should be prepared with instruments that do not exceed 0.6 mm in diameter. Instruments used to prepare the distal roots of mandibular molars and the palatal roots of maxillary first molars should be between 0.8 and 1.0 mm in diameter, provided that the post does not extend more than 7.0 mm into the root canal.

As stated previously, root perforation is more likely to occur for any one of the following three reasons: (1) instruments with diameters that cannot be accommodated by the morphology of the root are used; (2) the post is extended too far into a root with developmental root depressions and/or root curvature; and (3) a rotary instrument cuts on an incorrect angulation, and the instrument does not follow the root canal. Root perforations can be avoided if the same guidelines as those recommended for preventing root fracture are followed. That is, post spaces should be prepared so that they do not exceed one-third the root diameter, and instruments with diameters that can be accommodated by the dimensions of the roots being prepared should be used. One of the best means for clinicians to avoid root perforation is to have an understanding of root morphology.

Although posts technically can be placed in any root, the maxillary roots most suitable for posts include maxillary central incisors as long as the post has the recommended diameter of 1.1 mm; maxillary lateral incisors when the post diameter does not exceed the proposed diameter of 0.9 mm; maxillary canines as long as the post diameter does not exceed 1.0 mm; maxillary second premolars as long as the post diameter does not exceed 0.9 mm; and the palatal roots of the maxillary first and second molars provided that their diameters are 0.8 to 1.0 mm. Posts should be avoided in the facial roots of maxillary molars.

Maxillary first premolars have root morphology that has to be carefully respected if a post is required. There should be no enlargement of the root canal space beyond that produced by root canal treatment due to root dimensions and the depths of developmental root depression. With two-rooted first premolars, the lingual root is preferred because it is generally straighter and has a more suitable surface form. Two-rooted premolars frequently have a furcal depression in the facial root that compromises the facial root when the bifurcation occurs in the cervical one-third of the root because a post extended into the facial root would approximate this depression. When a single-rooted maxillary first premolar with one root canal requires a post, the post diameter should be 0.7 mm or less because the mesial and distal developmental root depression depths restrict the amount of available tooth structure peripheral to the centrally located single root canal.

The mandibular roots best suited for posts include mandibular canines, provided the post diameter does not exceed 1.0 mm, and mandibular first and second premolars when the post diameter is no greater than 0.9 mm. However, mandibular premolars with oval or ribbon-shaped canals should not be subjected to any preparation of the root canal beyond that produced during endodontic treatment because it will result in less than 1.0 mm of dentin. Posts can be placed in the distal roots of mandibular molars but there should be no enlargement beyond that produced by the endodontic treatment because the residual dentin thickness after root canal treatment is less than 1.0 mm. Posts should be avoided in the mesial roots of mandibular molars.

Posts also should be avoided in mandibular incisors, if possible. When posts are absolutely needed, there should be no enlargement beyond that produced during root canal treatment. Posts in mandibular incisors should not exceed 0.6 mm in diameter; this dimension precludes the use of prefabricated posts because most commercially available posts have a diameter that exceeds this dimension.

Requirements for Remaining Sound Tooth Structure

Sound dentin tooth structure should extend cervically beyond any type of core material. In fact, it is essential that sound tooth structure remain circumferentially to produce a cervical ferrule. The minimum amount of sound tooth structure grasped by a crown is important to optimize a tooth’s resistance to fracture. It is generally recognized that more than 1.0 mm of circumferential tooth structure should be encompassed by the overlying crown,^117–120 with some studies^121–123 indicating that the minimum should be 2.0 mm. Furthermore, maximum potential resistance can be achieved when that 2.0-mm ferrule encompasses all four of the axial surfaces.¹²³

Post Materials and Their Selection

Prefabricated posts have become quite popular, and a wide variety of commercial systems are available. More recently, in response to a need for tooth-colored posts, several nonmetallic posts made of zirconia, glass fiber–reinforced epoxy resin, and ultrahigh polyethylene fiber–reinforced posts are available.

Fiber posts require optimal length to avoid post loosening. In laboratory studies, they produced fewer root fractures or more favorable root fractures than metal posts, making them desirable for avoiding clinical root fractures.^124–134 Most clinical studies found no root fractures,^135–146 but three studies^147–149 reported a relatively small number of root fractures (2 of 173, 1 of 106, and 15 of 985, respectively) while one other investigation¹⁵⁰ reported a modest number of root fractures (14 of 99). Fiber posts are not as resistant to post fracture as metal posts, and a small number of post fractures have occurred in clinical studies (1 of 173 and 7 of 105).^{147, 148} Fiber posts have lower failure risk when there are three or four walls of coronal tooth structure,¹⁵¹ indicating that crown ferrules are important to the success of fiber posts. When parafunctional habits are present, fiber posts have been reported to have higher failure rates.²⁵

The first author of this chapter and the editor of this textbook both have placed a number of fiber posts and observed both success and failure. The post failures involved either post loosening or actual post fracture. These clinical observations, although not being part of a rigorous scientific investigation, suggest that there are some conditions where fiber posts are more likely to fail clinically. Therefore, until more definitive clinical data are available that identify all the factors responsible for fiber post failures, the following guidelines are proposed to help minimize, if not prevent, clinical failure of fiber posts:

• Appropriate post length is required because fiber posts are more prone to loosening if their length is not optimal.

• Fiber posts can fracture when little or no circumferential ferrule (sound tooth structure) is present.

• Fiber posts can fracture, loosen, or induce root fracture when they are subjected to heavy occlusal forces, so excessive tooth contact and all occlusal interferences should be eliminated.

Some clinicians consider a fiber post to be a good choice for ETT even when the root is short, there is no tooth structure below the core material (no ideal cervical crown ferrule), and heavy occlusal forces are present. The thought process expressed by these clinicians is that failure of a fiber post, including post fracture, still leaves the root in a restorable condition. Indeed, this situation may occur clinically, but the premise that the remaining root is restorable may not always be true.

The first author of this chapter has encountered both loosening and fracture of fiber posts. Most, but not all, of those failures resulted in a root that could be restored again. However, challenges can arise with removal of the remaining post segment because the process is not as easy as some would portray. These fractures of fiber posts often occurred within the first 1 to 3 years in service. The thought of charging the patient to remove the fractured post, place another fiber post, and make a replacement crown was not appealing. Moreover, the patient might reasonably have concluded that the same type of failure could occur again before an appropriate period of clinical service had transpired.

In light of such failures, a metal post was used to replace the failed fiber post to provide more sustained clinical service for patients. Therefore, while fiber posts are an option, they are not the most cost-effective treatment when the post is shorter than ideal, when there will be little or no ferrule, or when excessive occlusal forces are present.

Summary

The information presented in this chapter is intended to permit clinicians to make data-driven decisions in their clinical treatment of ETT based on known clinical complications. Guidelines have been provided for the decision of whether or not to place a post, the selection of an appropriate type of post material (if a post is used), and the indications and contraindications to placing crowns on ETT.

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