Criteria For The Predictably Restorable Endodontically Treated Tooth 

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The Ferrule Effect

With the placement of a crown on a tooth the “ferrule effect” must be considered. A ferrule is an encircling band of cast metal that acts as a reinforcing sleeve around the coronal portion of the tooth. Tjan and Whang,⁹ using a short 60 degree collar, found that this collar did not increase resistance to root fracture. Other studies, such as those of Barkhordar et al¹⁰ and Sorensen and Engelman,¹¹ have shown that ferrules confer significant increases in resistance to fracture. The differences stem from a range of interpretations about what constitutes a ferrule.

Sorensen and Engelman¹¹ define the ferrule effect as a “360 degree metal collar of the crown surrounding the parallel walls of the dentin extending coronal to the shoulder of the preparation. The result is an elevation in resistance form of the crown from the extension of dentinal tooth structure.” Barkhordar et al¹⁰ used a 2-mm long ferrule wall, and Sorensen and Engelman¹¹ a 1-mm long ferrule. Accepting that the ferrule is only effective when the walls are very close to parallel, and allowing for a somewhat rounded transition from ferrule wall to the prep margins, it seems prudent to accept 2 mm of ferrule length as a minimum clinically.

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Intact Anterior Teeth

There is a consensus that there is no need to place crowns or posts in anterior teeth (incisors and cuspids) with essentially intact crowns, because posts do not reinforce endodontically treated teeth.^12-15 In such teeth, closure of the access preparation with a bonded composite resin restoration is all that is required, provided the tooth meets the occlusal requirements and the patient is happy with its appearance.

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Posterior and Anterior Teeth Requiring Crowns

Almost without exception, endodontically treated posterior teeth (distal to the cuspid) benefit from cuspal coverage with some form of crown.¹² In addition, many anterior teeth will require crowns for esthetic, structural or occlusal reasons. For such teeth to be successfully restored, there must be a minimum of 4.5 mm of solid tooth structure above the bone crest; given that a minimum of 2.5 mm is required for biologic width and 2 mm for an effective ferrule.

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Thickness Of Coronal Tooth Structure

What constitutes adequate thickness of coronal tooth structure is also a subject of some discussion. Sorensen and Engelman,¹¹ and Tjan and Whang,⁹ found that increasing the thickness of axial dentin beyond 1 mm did not increase the strength of the restored tooth. Both showed, however, that with thicker axial dentin, failure was much more likely to be due to dislodgement rather than root fracture. Tjan and Whang measured the thickness of buccal dentin in their work, while Sorensen and Engelman measured the minimum dentin thickness and did not test teeth with a dentin thickness of less than 1 mm. On some teeth it is virtually impossible to obtain this 1 mm thickness of dentin in the interproximal areas, after both endodontic treatment and crown preparation. However, with careful tooth preparation it can be approached, and in the more important buccal and lingual walls it can usually be obtained.

The work of Hunter et al,¹⁶ using photoelastic models, supports the concept that preservation of coronal dentin strengthens the tooth. Apparently, thicker coronal dentin reduces the odds of failure by fracture of tooth structure and improves the odds that a failure will involve a simple dislodgement of the core. In such a case the tooth can be saved. The evidence indicates that more coronal dentin is better than less.

A 1 mm thickness of axial dentin seems to be the accepted minimum.¹⁷ However, this figure is empirical, based on opinion and clinical experience rather than research. This 1 mm refers to the thickness of dentin between the inside of the canal wall and the outside surface of the ferrule as shown in Fig. 2.

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The Need For a Post

Endodontically treated teeth all require build-ups of some sort, if only to close the access preparation and fill the pulp chamber. Where a crown is to be placed on the tooth as a final restoration, this build-up is often called upon to provide resistance and retention form to the preparation. Retention of the build-up is often accomplished by placement of a post which engages root structure.

Nayyar et al¹⁸ and Kane et al¹⁹ have both described the use of amalgam cores as build-ups for posterior teeth retained by the pulp chamber or by extensions of amalgam into the coronal 2 mm to 4 mm of canal space. Kane et al showed that where 4 mm or more of pulp chamber height remained, there was no advantage to placing amalgam into the root canal space. In addition, even in teeth with only 2 mm of pulp chamber height left, the fracture loads were comparable to those of teeth with 4 mm high pulp chambers.¹⁹ On posterior teeth in which the core build-up can be retained by remaining pulp chamber anatomy there is no need or advantage to placement of a post.

With both anterior teeth and posterior teeth the decision to place a post is based on the answer to the question; is a post required to retain the build-up? The decision to place a post should not be based on the desire to strengthen the tooth. Sorensen and Martinoff²⁰ found no evidence that placement of a post and/or crown on an anterior tooth has a significant effect on the prognosis of the tooth. Where a post is necessary to retain the build-up, the length of the post and the length of the remaining apical seal can greatly influence the probability of the success of the restoration.

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Click to see the larger image

Fig 2: Minimum dimensions for a predictably restorable tooth.

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Apical Seal

The length of apical seal remaining after post preparation and the degree to which it was disturbed during post-hole preparation can influence the long-term success of the restoration. The realities of post-hole preparation are such that, in most cases, some form of mechanical preparation of the post-hole is required.

There is support^21-23 for leaving 4 mm to 5 mm of undisturbed apical gutta percha after post preparation. Various researchers ^21,22,24 have reviewed different methods of post-hole preparation and none found an increase in apical leakage attributable to any particular procedure for removing gutta percha. However, Haddix et al²⁵ found significantly less leakage when gutta percha was removed using heated pluggers. The apical seal was much better when 5 mm of gutta percha remained following post preparation, as opposed to when 3 mm remained.

The work of Ouzounian and Schilder²⁶ shows that at a distance of 5 mm from the apex, there is a 1 mm thickness of dentin, before post preparation, only on cuspids, upper incisors, lower bicuspids and the palatal root of upper molars. There is consistently less than 1 mm of remaining dentin on all other roots. This underlines the need for careful assessment of each root prior to post preparation.

Caputo et al¹⁷ suggested that a minimum of 1 mm of sound dentin surround the apical end of the post. It is difficult, if not impossible, to measure this thickness clinically. If this recommendation was followed rigidly, the use of posts in many teeth would be ruled out. Given the high success rate shown with long posts in Sorensen and Martinoff’s retrospective study,²⁰ thinner cross-sections of dentin apparently suffice.

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Post Length

Standlee et al²⁷ showed retention of passive posts increased directly with length. Morgano²⁸ recommends that the post be as long as possible, while maintaining adequate apical seal. Sorensen and Martinoff²⁰ showed that where post length in the canal was equal to or greater than the length of the crown, the success rate was 97.5%, regardless of post design. The success rate was greater than 97.7% with para-post, even when shorter post dimensions were included. Ideally the length of the post extending into the root should equal the length of the crown.

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What Loads Will the Restored Tooth Experience

The load borne by the restored tooth is a major indicator of the success of a restoration. Sorensen and Martinoff²⁹ found in their retrospective study that while the success rate for single unit crowns was 94.8%, the rate was 89.2% for fixed partial denture (FPD) abutments and dropped to 77.4% for removable partial denture (RPD) abutments (both distal extension and tooth born designs). Nyman and Lindhe³⁰ found fractures in abutment teeth occurred more frequently in root treated teeth, and primarily in root treated teeth fitted with posts, serving as terminal abutments for free end segments. Hatzikyriakos et al³¹ found in a three-year retrospective study that the failure rate for endodontically treated teeth used as abutments for FPDs and RPDs was over double that for teeth not used as abutments.

Testori et al³² recommended against anchoring a cantilever bridge on an abutment that was endodontically treated and fitted with a post. Testori also stated that attaching removable partial dentures with extracoronal attachments to individual endodontically treated teeth placed these teeth at significant risk. Abutment teeth for distal extension RPDs are far more likely to experience high bending stresses in a mesio-distal direction than either tooth borne or non-cantilevered FPD abutments. The stresses generated in the tooth structure by such loads can be greater that those of comparable bucco-lingual loading, as most of these abutment teeth are much narrower mesio-distally than bucco-lingually (see Fig. 3).

Based on the available data, the use of an endodontically treated tooth to support a precision attachment RPD, a distal extension RPD or a cantilevered FPD cannot be considered highly predictable. In addition, the use of root treated teeth to support non-cantilevered FPDs must be assessed carefully on an individual basis. Root treated teeth may not be suitable as abutments in individuals with a history of bruxism, or those requiring long span fixed bridges.

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Click to see the larger image

Fig 3: Effect of bending forces from cantelever loading.

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Criteria For Successful Restoration

For the endodontically treated tooth requiring a post, the minimum length of remaining solid tooth would be the sum of biologic width (2.5 mm), ferrule length (2 mm), apical seal (4 mm) and a post length (equal to crown length), (i.e. 8.5 mm + post length beyond crown margin) (see Fig. 2).

For those teeth not requiring a post, the requirements are for biologic width + ferrule length (i.e. 4.5 mm of supra-bony solid tooth; this assumes adequate bone support to provide a clinically acceptable level of mobility). In both situations, “solid tooth” refers to dentin at least 1 mm thick after preparation.

In addition, consideration of the functional loads to be borne by the tooth is essential. Single abutments supporting precision attachment RPDs, distal extension RPDs or cantilever FPDs that are endodontically treated, or likely to be in future, should be avoided. Careful assessment of the occlusal demands and other loads, such as from fixed or removable partial dentures, must be made before restoration. Endodontically treated teeth that meet these criteria can be restored with a high degree of confidence.

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Discussion

There is a limited amount of research that correlates structural and endodontic failure of teeth to the types of load they experience. Studies involving a large number of teeth looking at the impact of such load factors could be an invaluable aid to assessing what will and won’t work in clinical situations. The decision to place a crown, or crown and post on anterior root treated teeth is a difficult one; current evidence is inconclusive on the value of crowns and posts in these situations. This is an area where a large well-designed study could help guide the clinician. Many teeth don’t fall within the guidelines outlined here, and yet need to be restored. There is a very real need for guidelines on restoration of these teeth and for ways to assess the level of predictability associated with such restorations.

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Acknowledgements

Dr. Alex McLean is in private general practice in Kamloops, British Columbia. Reprint requests to: Dr. Alex McLean, 201-418 St. Paul St., Kamloops, BC V2C 2J6

The author has no declared financial interest in any company manufacturing the types of products mentioned in this article.