Restoring Endodontic Teeth in the Adhesive Age: A Rationale for Posts and Ferrules

Although adhesive dentistry has improved the quality and longevity of our restorative treatments, tried-and-true principles are still valid today. When teeth are approached from an engineering-principles standpoint, long-term prognosis increases. However, if the treatment protocol is from the “bonding holds it all together” approach, success suffers.

These principles are especially true when restoring endodontically treated teeth. Frequently, teeth requiring endodontic treatment have either existing restorative materials (amalgam or composite) or recurrent decay, and often both are present in the same tooth. Endodontic access and excavation of the decay may further weaken the remaining tooth, so reconstruction will be required to reinforce the remaining tooth to function under normal loading.¹ Which restoration is indicated is determined by how much native tooth structure remains. One of the goals in minimally invasive dentistry is the preservation of native tooth structure.² These conservative treatment goals also extend into endodontics during access and instrumentation.³

The Endodontic Component of Treatment
When treating the tooth endodontically, an expected goal is the removal of all decay. We cannot leave decay, as it weakens the tooth and can allow the remaining decay to progress over time. However, conservative treatment begins with understanding the tooth’s anatomy and where the canal orifices lay within that anatomy, allowing us to preserve tooth structure when locating and accessing the canals.⁴ The removal of old restorative materials should be the first step so that only the removal of tooth structure needed to unroof the pulp chamber and locate the canal orifices is required. Teeth with an existing crown can be a challenge, as the practitioner loses anatomical reference since the tooth may have been rotated or tipped prior to placement of the crown. Thus, the pulp chamber and canal orifices will not necessarily be positioned based on the anatomy of the crown.

Figure 1. Stress concentration under loading, with higher levels as the lines get closer together. (Image courtesy of Dr. Gene McCoy.)	Figure 2. Comparison of the amount of cervical tooth structure that would be removed with files of increasing taper.

When loaded under function, some force is directed in the long-axis in posterior teeth; however, chewing has an “envelope of function” in which laterally directed forces occur. Anterior teeth have their loads off-axis due to how they contact opposing teeth. These forces concentrate at the cervical of the tooth (Figure 1). Thus, teeth with weakened cervical regions will manage these loads under function less ideally and have a higher failure rate than those teeth with intact cervical areas. The maintenance of this important tooth structure is therefore critical to the long-term survival of the tooth. Over-instrumentation of the canals with wider tapered files removes greater tooth structure in the cervical area, and selection of the ideal taper should be dependent upon which tooth is being treated, as well as the cervical size of the canal prior to treatment. Typically, a file with a 0.06 taper is sufficient in palatal canals of maxillary molars, distal canals of mandibular molars, maxillary central incisors, and all canines. All other teeth typically can be effectively instrumented to a 0.04 taper. There will be exceptions to this, but wider taper files should be reserved for those rare clinical circumstances where a very wide canal is noted radiographically during diagnosis (Figure 2).

Are Posts Still Necessary in Endodontically Treated Teeth?
The utilization of posts is dependent on how much remaining tooth structure is left following endodontic treatment and removal of existing restorative material and decay.⁵ The purpose of a post is to aid in retaining the core to the remaining tooth, but is a post needed in every endodontically treated tooth? How the tooth will be reconstructed will depend upon how much native tooth structure remains. With regard to posterior teeth, under function, there is a minute amount of cuspal spread. This increases when the center of the occlusal surface has been prepared for a direct restoration; the wider this removal of native tooth structure, the more the cusps may flex away from each other under loading (Figure 3).

Figure 3. (a) Cuspal flexure occurs when the tooth is loaded. (b) Greater flexure occurs when the tooth has been restored with an occlusal restoration but has not been endodontically treated. (c) When endodontically treated, even greater cuspal flexure is possible, (d) which may lead to cuspal fracture after being restored with a restoration with no cuspal shoeing. (e) Yet, with cuspal shoeing, flexure is prevented and fracture potential is diminished.

If the mesial and distal marginal ridges are intact, and not replaced by amalgam/composite or undermined by decay, this acts as a limiting factor to cuspal flexure and as a brace for the cusps. When one of the marginal ridges is missing and has been replaced by restorative material, cuspal flexure is allowed to increase. When both marginal ridges are missing, the potential of cuspal fracture increases under repetitive loading over time (Figure 4). This potentially increases further in endodontically treated teeth as the dentin found in the roof of the intact chamber is missing, permitting greater microscopic flexure of the cusps when loaded. Although not encountered often clinically, a posterior tooth that requires endodontic treatment that has both marginal ridges intact may be restored with an adhesive composite as the final restoration (Figures 5 and 6). In these cases, the removal of obturation in the canal orifice (in some cases, several millimeters apical to it) is recommended, followed by an adhesively bonded composite restoration. Due to the depth of cure, it is recommended that a dual-cure composite is used to ensure complete curing of the resin. This can be overlaid by a light-curable composite for better aesthetics if required/desired. It is also recommended that a self-cure promoter compatible with the dentin adhesive be used due to the depth of the preparation and the inaccessibility of the light to reach the lowest depth. The author recommends checking what is available from the manufacturer of the adhesive that you’re using as a self-cure promoter and to not mix brands in an attempt to make the adhesive self- or dual-cure.

Figure 4. An endodontically treated tooth that had been restored with composite that suffered a cuspal fracture under function.	Figure 5. A premolar, following endodontic treatment with both marginal ridges intact.
Figure 6. A molar, following endodontic treatment with both marginal ridges intact.	Figure 7. A posterior tooth, following endodontic treatment with a marginal ridge missing.

Figure 8. An endo-dontically treated premolar that has lost both marginal ridges, with greater than one third of the occlusal width missing. It will be subject to greater cuspal flexure under loading once restored with a direct restoration.

Figure 9. An endodontically treated tooth with a native tooth remaining coronally (left) that would yield minimal tooth structure to support a core buildup and crown once prepared for the crown (illustrated in yellow on the right).

Figure 10. Illustrating posts in an endodontically treated molar to pin the core to the remaining tooth structure.	Figure 11. A clinical case, showing posts placed into each canal in an endodontically treated molar to pin the core to the remaining tooth structure.

When one marginal ridge is intact, but the other is missing, a restoration that will limit cuspal flexure is indicated (Figure 7). If the missing tooth structure at the center of the occlusal surface is less than one third of the buccal-lingual width of the tooth, an onlay may be chosen for restoration of the tooth, as this provides cuspal shoeing that will limit flexure and potential cuspal fracture. The author’s opinion is that inlay restorations, like direct resin restorations, do not restrict cuspal flexure, so they should be avoided when restoring endodontically treated teeth.

Frequently, greater than one third of the occlusal width is missing, and a full-coverage crown is the best restoration for these teeth (Figure 8). These clinical situations typically are the times when a post is needed to retain the core, as minimal tooth structure will be present following preparation for a crown (Figure 9). The practitioner may view the tooth that’s now ready to be restored as having sufficient tooth structure and that a post is not required. However, the remaining native tooth structure lies on the buccal and lingual of the tooth, and the majority of this will be removed with the crown preparation. The placement of a post that equals the diameter of the instrumented canal in the coronal of the root does not remove additional tooth structure, providing a locking mechanism for the core to the remaining tooth structure. With this in mind, the author advocates that, in multi-canal teeth, the placement of a post into each canal better locks the core to the remaining tooth structure, decreasing the potential for core dislodgement over time under function (Figures 10 and 11).

Figure 12. The ferrule is the band of natural tooth structure between the restorative margin and the top of the dentin before any core material begins.

Figure 14. Crown failure, resulting from inadequate ferrule.

Anterior teeth, with their contact with opposing teeth and their narrower cervical cross sections, frequently require posts following endodontic treatment. Maxillary anteriors, especially the laterals and centrals, are under greater off-axis loading. The contact in the cervical half of the lingual surface, combined with the patient biting into food, places a buccal directed force on the tooth that may be more prone for failure of the crown by fracture at the crown margin. With adhesively bonded fiber posts, and the use of a tapered post that is the diameter of the instrumented canal, a better-reinforced cervical region on the tooth can be created, thus improving longevity and decreasing the potential for fracture in this region.

Do We Really Need to Consider a Restorative Ferrule Today?
A restorative ferrule is defined as the band of natural tooth structure between the crown margin and the occlusal aspect of the tooth before any core material begins (Figure 12). Although adhesive dentistry has greatly improved over the past 30 years, and higher bond strengths are reported to dentin, basic engineering concepts, such as ferrules, are as important today as they were before dentin adhesives were introduced.⁶ No matter how high the dentin bond strength, when a properly designed ferrule with adequate height is not provided, it can lead to crown failure (Figure 13). This may occur even when a post is present, leading to separation of the crown with the attached post/core from the root (Figure 14).

When the crown is off-axis loaded, tension occurs at the crown margin on the side where the load is directed from, and compression occurs on the opposing side. An inadequate ferrule may lead to the breakdown in the luting cement between the crown and the tooth, leading to an open margin on the tension side. This will, in turn, lead to microleakage and then recurrent decay. As the tooth is endodontically treated, and the patient has no perception of sensitivity in the tooth, this decay may progress until the crown dislodges from the tooth or decay becomes visually apparent. Fatigue loading of crowns leads to preliminary failure of leakage between the restoration and tooth that is clinically undetectable.⁷ Additionally, in those cases where minimal or no ferrule is present, stress concentrates under loading at the cervical, which may lead to failure of the core. Repetitive loading when a cast or prefabricated post is present allows these higher stresses to potentially lead to vertical root fracture and the loss of the tooth. When a metallic post is dislodged from a tooth with the crown, the practitioner should check for radial cracks (spoke like) from the post hole, which would indicate the tooth has failed.

But how much ferrule is needed? Studies have demonstrated that when ferrules of 0.5 mm or 1.0 mm were present, these teeth failed at a significantly lower number of cycles than the 1.5- and 2.0-mm ferrules and control teeth.8 So, it is recommended that a ferrule of 2.0 mm is adequate to resist the forces under load that may lead to failure of the restoration/tooth. A study confirmed that a ferrule increases the mechanical resistance of the restoration.9 Additionally, when an adequate ferrule was present, less stress (lower magnitude) was noted cervically, and the loading forces were not concentrated at the crown margins. Yet, those teeth with minimal ferrule saw a higher magnitude of stress cervically and at the crown margins, thus increasing the potential for failure.

CLOSING COMMENTS
When approaching the restoration of an endodontically treated tooth, conserving tooth structure is key to improving the longevity of the tooth and its subsequent restoration. The process begins with conservative instrumentation during the endodontic treatment to maintain cervical tooth structure. Determining what restorative treatment is required is based on how much native tooth structure remains. The use of a post should be considered when less-than-adequate tooth structure will remain following preparation for a crown. As discussed, despite the advent of adhesive dentistry, establishing a proper ferrule remains important to manage off-axis loading that may lead to restorative failure or, in the worst-case scenario, total failure of the tooth itself. Few teeth fail due to over-engineering, but many fail when they are under-engineered.

References

1. Morgano SM, Rodrigues AH, Sabrosa CE. Restoration of endodontically treated teeth. Dent Clin North Am. 2004;48:vi, 397-416.
2. Baba NZ, Goodacre CJ, Daher T. Restoration of endodontically treated teeth: the seven keys to success. Gen Dent. 2009;57:596-603.
3. Marchionatti AME, Wandscher VF, Rippe MP, et al. Clinical performance and failure modes of pulpless teeth restored with posts: a systematic review. Braz Oral Res. 2017;31:e64.
4. Tang W, Wu Y, Smales RJ. Identifying and reducing risks for potential fractures in endodontically treated teeth. J Endod. 2010;36:609-617.
5. Sarkis-Onofre R, Fergusson D, Cenci MS, et al. Performance of post-retained single crowns: a systematic review of related risk factors. J Endod. 2017;43:175-183.
6. Naumann M, Schmitter M, Frankenberger R, et al. “Ferrule comes first. Post is second!” Fake news and alternative facts? A systematic review. J Endod. 2018;44:212-219.
7. Freeman MA, Nicholls JI, Kydd WL, et al. Leakage associated with load fatigue-induced preliminary failure of full crowns placed over three different post and core systems. J Endod. 1998;24:26-32.
8. Libman WJ, Nicholls JI. Load fatigue of teeth restored with cast posts and cores and complete crowns. Int J Prosthodont. 1995;8:155-161.
9. Ichim I, Kuzmanovic DV, Love RM. A finite element analysis of ferrule design on restoration resistance and distribution of stress within a root. Int Endod J. 2006;39:443-452.

Dr. Kurtzman is in private general practice in Silver Spring, Md. A former assistant clinical professor at the University of Maryland, he has earned Fellowship in the AGD, the American Academy of Implant Prosthodontics, the American College of Dentists, the International Congress of Oral Implantologists (ICOI), the Pierre Fauchard Academy, and the Association of Dental Implantology; Masterships in the AGD and ICOI; and Diplomate status in the ICOI and the American Dental Implant Association. He has lectured internationally, and his articles have been published worldwide. He has been listed in Dentistry Today’s Leaders in Continuing Education since 2006. He can be reached at dr_kurtzman@maryland-implants.com.

Disclosure: Dr. Kurtzman reports no disclosures.

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