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Inside Dentistry
September 2018
Volume 14, Issue 9
Peer-Reviewed

Principles of Post and Core Dentistry

Considerations to optimize success in endodontic cases

Joseph Chikvashvili, DDS

The topic of "post and core" is pertinent to both endodontics and general dentistry. Each clinician has a unique way in which he or she performs successful post and core dentistry; the field is highly complex. However, within the complexity, it is possible to focus on developing expertise in specific areas that occur at a high frequency in post and core dentistry, such as the use of prefabricated posts. Clinicians are often curious about which post system is best, when in fact, no post is the "best" post. The more conservative a clinician's access, the greater the likelihood of obtaining longevity from a root canal or restoration. This article explores methods for obtaining conservative access and ensuring clinical success.

Root Canal Access

Ultimately, the aim in endodontics is to preserve a tooth for as long as possible for the patient. A clinician may want to do a root canal, but must face realistic concerns, such as not having enough tooth.1 Figure 1 shows a case in which the clinician was able to gain limited access to four canals through a zirconium crown. Generally, it is preferable to err on the side of larger, well cleaned out openings. If the access is too small, clinicians may fail to clean effectively, there is a greater chance of separation due to the lack of a straight path, and canals may be missed. Conversely, if the access is too large, clinicians can potentially destroy the tooth. A happy medium can be achieved in which the result is fine-tuned and perfectly engineered. Although the perfect result is not always possible or ideal, clinicians continue to strive for it. One of the best approaches is to find the canals and then open them up under a microscope to get the correct amount of access. This allows the clinician to use whatever shape is needed to remain conservative.

Types of Posts

Many teeth require restoration with posts, including prefabricated metal posts, cast posts, or fiber posts. Although fiber posts are currently the best option, the most popular of the posts has been the prefabricated metal post.2 It is easier to use, cheaper, and provides more variability-clinicians can acquire many different sizes.3 The various options offer greater flexibility, depending on the manner in which each case presents itself. In endodontic restoration, the purpose of a post is to retain the core. Posts can be used to retain amalgam, metal, and composite cores and help strengthen the connection between the coronal buildup material and the remaining tooth structure.

Principles of Post Placement

Certain factors must be considered in order to place a successful post and core.4 These factors, including the quantity of tooth structure, the quality of tooth structure, the presence of adjacent tooth structures, occlusal considerations, and the need for future restoration, can act as guidelines for the clinician. Understanding how these factors inform the principles of restoration will enable clinicians to make the right decision based on the case at hand.

Quantity of Tooth Structure

Regarding quantity, in general, when more than half of a tooth's structure is missing, the clinician should consider placing a post. The issue is whether or not there is enough structure present to retain the crown. It depends on the ferrule, which can be defined as a 360° collar of the crown that surrounds the parallel walls of the dentine and extends coronally to the shoulder of the preparation.5 Sometimes, the clinician has a situation in which there is no ferrule, and other times, the ferrule is almost ideal. Although there are exceptions, the ideal is when no core is required-just the natural tooth functioning as the core.

When restoring a tooth, it is optimal to have 3 mm of the wall on one side and 2 mm on the other, which indicates at least a 2-mm ferrule.6With these conditions, the clinician is going to have a higher chance of success. Restorations can occur in other situations, but the patient should be warned that failure could be possible in certain scenarios.

Quality of Tooth Structure

When assessing the quality of the tooth structure, the clinician is looking for indications of nonideal conditions, such as discoloration, cracks, an undercut, and concavities. The first premolar has a concavity, but sometimes there are concavities in molars as well, such as on the distal aspect of a mandibular molar or the palatal aspect of a maxillary molar (eg, tooth No. 3). These are teeth that should get posts. The clinician should aim to place the post in the canal with the straightest path. In tooth No. 5, for example, it is usually the palatal canal. There are times when another canal, such as the buccal canal, can offer a straighter option, but generally, the palatal canal is the best option.6

Adjacent Structures

If a tooth stands alone, greater forces will be placed on it because it has no adjacent teeth to provide support. This situation will affect the decision-making process for placing a post. Figure 2 shows a tooth on an "island" that would ultimately have three implants placed in front of it. Even though the tooth had a good root canal, it would take a much greater beating than one that had support proximally. Therefore, during treatment planning, the clinician needed to account for whether the tooth would last until the implants integrated. Figure 3 shows a case that would quickly proceed to implants. For more than a decade, the patient had a less-than-ideal bridge with decent endodontics and functionality. There was also an overhang on the premolar. Placement of a post was not ideal in this case because there was only one distal canal and the gutta-percha had not been completely removed from it. Achieving an intimate fit should be the goal with the post and the walls. Whether the post is threaded or not, there should not be excess gutta-percha because it is not as solid as the tooth and will lead to greater movement. That movement on the tooth and bridge could create too much force and result in demise.

Occlusal Considerations

Occlusion is one of the most overlooked areas in endodontics; however, beyond remaining structure, it is the most important factor.7 Patients who are heavy bruxers can create the need for a root canal from the lateral forces generated. Patients who clench pose a different challenge with apical forces, yet the damage from both behaviors can be similar. Figure 4 shows a patient with bruxism. The patient had pain in response to hot and cold stimuli, but the clinicians needed to take an additional bitewing radiograph to determine which tooth was causing the pain. From the periapical image, it was clear that tooth No. 2 had a problem, tooth No. 3 had a crown, and tooth No. 4 had a large restoration. Any of the three could have been the source of the problem.

With the bitewing shown in Figure 5, it became clear that there was gross decay in tooth No. 3 that could not be seen on the periapical image and that there was a periodontal defect (ie, a pulp stone) in tooth No. 2. There was also a class V lesion revealing the bruxism, flat occlusion, multiple restorations in tooth No. 30, a post in tooth No. 31, and decay on tooth No. 2. Due to its length, the post in tooth No. 31 was functionally useless. When a post is placed, it must be the right length. If the post is too short, there will be a greater fulcrum and the tooth could break under stress. If it is too long, the clinician can break the tooth by wedging it. When a post is too short, it will not directly hurt the tooth, but it will not provide any valuable function. Overall, it is preferable to place posts in teeth that only have single restorations rather than abutments for bridges because those teeth are already under greater force. If the technique is not performed correctly, iatrogenic issues may result.

Need for Future Restoration

Figure 6 depicts a situation involving tooth No. 12 that may have been presumed to be easily resolvable. However, the dentist experienced problems during treatment, including significant bleeding due either to irreversible pulpitis or a perforation. Judging by the periapical image, something detrimental was occurring at the distal aspect. One lesson that can be learned from this case is that a solution does not have to look pleasant for it to work. Functionally, the repair was successful because the endodontist was able to fix the whole wall with mineral trioxide aggregate (MTA) and was able to find the canal (Figure 7). The endodontist also put in the post for the general dentist and closed it. Although the general dentist later removed the post and replaced it with one that was much larger and shorter than would be recommended, Interestingly, the result lasted for 8.5 years (Figure 8).

The endodontist took the path described because the patient's general dentist had inadvertently destroyed the wall, which caused the initial sensitivity to quickly progress into the need for a root canal and the possibility of losing the tooth. Unfortunately, the story does not have a happy ending. After 9.25 years, the repairs finally fractured. Despite the ultimate failure, the case is important because it shows that even a resolution for a "terrible" case can last almost 10 years and be considered, to some extent, a success. When the right principles are followed, there can still be a successful outcome. The specific reason that this restoration remained viable for so long can be attributed to a factor that was barely visible: the existence of 2 mm of ferrule and, therefore, coronal tooth structure above the perforation repair. The perforation was not only below the gum, but also the bone, which is generally a more ideal location for a perforation.

Figure 9 shows an example of a restoration with a post that is too long. Although it was a threaded post, there was about 1 mm of gutta-percha where, ideally, 4 to 5 mm of gutta-percha should be used.8 This also created an iatrogenic problem because the referring dentist placed the post directly into the furcation. This type of problem can be fixed by post removal and repair with MTA. However, because there was not much tooth structure in this case, the tooth became far more compromised.

Figure 10 shows an example in which there was likely a good root canal that was subsequently absorbed. This type of problem can be avoided when the person who does the root canal also places the post. This is not always possible, and some referring general dentists prefer to do the restorative work themselves, but the advantage of having the endodontist do both is that he or she is familiar with the curve of the canal and knows the thickness of the walls and what the canal will allow. In this case, the post was placed under a rubber dam, which was beneficial. Oftentimes, clinicians will see teeth that have good endodontics, but multiple years later, a radiolucency will appear because saliva seeped in when the post was placed.

The rules for placement of a post are dependent on retention: the greater the length of the post, the greater the retention.3 In addition, although placement of a post that is two-thirds of the length of the root is ideal, maintaining 4 to 5 mm of apical gutta-percha is essential because anything less than that will disrupt the seal.8 Figure 11 shows a case in which a patient previously received a post, core, and crown. The threaded post was shorter than ideal because the tooth was longer. However, if the decision was made to remove the post, the clinician would need to use a trephine drill or an ultrasonic instrument around it, making the access larger than ideal and less conservative. Potentially, the crown could come off as well. The patient did not have the finances to re-treat the tooth and was not an appropriate candidate for an apicoectomy because the mental foramen was located directly under the tooth.

The remaining options included extraction and placement of a bridge (or an implant at a later date when funds were acquired) or, as was done in this case, the performance of an intentional replant. The clinician extracted the tooth, cut it, placed MTA, and then put it back under occlusion using sutures (normally, the clinician would put composite in each corner to maintain it and would not etch). After 2 weeks, there is usually stability. Figure 12 shows the postoperative radiograph, and Figure 13 shows a radiograph taken 4.5 years later. The patient's probing depth went from 3 to 4 mm, but overall, the result was considered a success because something is always lost coronally, regardless of whether the case is a replant or an implant.

Post Diameter, Texture, and Material

Post diameter and texture are other important concerns. Ideally, the diameter of the post should be less than one-third of the diameter of the root, and 1.5 mm of dentin should be present circumferentially (1 mm is the minimum). Otherwise, the clinician would be inviting the potential of fracture. With regard to texture, serrated and roughened posts provide greater retention because they possess more surface area. To achieve a similar effect for smooth posts, the clinician can sandblast. The cement that is used is also important. Luting agents are not all equal. For these procedures, resin and glass-ionomer cements are generally superior to others.9

Custom cast post and core is more prudent for use in large canals or in very oval canals, such as those found in some maxillary anterior teeth (ie, Nos. 7, 8, 9, and 10). Prefabricated posts are better for circular canals and are less expensive; however, there are definitely situations involving these teeth where a clinician would want to do the cast and core, especially if they do not have a lot of tooth structure, because it will be significantly better for the retention of the tooth.10 Although the overall root diameter must be considered and respected, the canal diameter ultimately dictates the size of the post used. In larger, oval canals, sometimes the post and core can spin around, especially if they are cast. Adding a keyway to the bulkiest part of the remaining tooth structure can help to resist rotational forces.

As to which post material should be used, the literature indicates that fiber is better because it has greater flexibility and is less likely to permit vertical fractures.2,3 It also has better esthetics, which is significant for both anterior and posterior teeth. The problem with fiber posts is that they can snap off or pull out. When they snap off, the retained portion can be difficult to remove. To achieve removal, a tapered diamond bur or ultrasonic instrument can be used, but it must be done very carefully. Depending on the circumstances, metal post fragments can also be difficult to remove, but metal is often serrated or threaded and thus can be unwound with an ultrasonic instrument. Unlike metal, fiber is significantly more flexible because it has a better modulus of elasticity, which should be as close to the value of dentin's elasticity as possible. This can make it difficult to visualize on a radiograph. Figure 14 shows a case with a fiber post in the central that should definitely be longer than it is and a good fiber post the lateral that could be a little longer. Because the clinician believed that the posts and crowns were well done with good seals, when the patient presented with pain and apical radiolucencies, they were treated with apicoectomies. This was easier and less invasive than dismantling the post and core structures.

Other Considerations

In deciding whether or not to use a post, the clinician also must consider any additional factors that are unique to each case. Figure 15 depicts a case in which the post was essentially useless and not congruent with the walls of the tooth. Figure 16 reveals what the tooth looked like after the post was removed. There was some decay, but the clinician was still able to save the tooth. Figure 17 shows the postoperative radiograph; Figure 18 was taken 6 months later. The clinician told the patient that healing would need to be checked 6 months postoperatively to determine whether she was a candidate for a crown. Two options for the tooth were presented to the patient: (1) building internally as well as from mid-root with composite, which was what the clinician did, or (2) placing a fiber post. Several fiber posts could have been placed in the tooth to provide a better option, but this patient did not want to spend much and was unsure about the crown.

Another time when a post cannot be placed is when there is internal or external resorption. In the case shown in Figure 19, internal resorption had occurred. Today, this would be much easier to ascertain with cone-beam computed tomography (CBCT). For this case, the clinician filled the resorption with gutta-percha and then filled the mid-root coronal section above the gutta-percha with composite. Because of how weakened the tooth had already become, the clinician wanted to do whatever was possible to prevent further weakening. Figure 20 shows the postoperative radiograph, and Figure 21 shows the 2-year follow-up.

When considering cores, the clinician has many exceptional options available. The best quality cores mimic the hardness of dentin. Clinicians need only ensure that whatever they use to etch and bond is compatible with the core.

Finally, it is important to evaluate from a 3-dimensional perspective. Ignoring the root-canal access, many observers may believe that the case in Figure 22 looks good. However, Figure 23 reveals that the situation is actually far from ideal-if about 20% is filled, then 100% is not clean. When viewing radiographs, it is common to assess them from the incomplete perspective shown in Figure 22. To properly consider the true dimensions of a tooth, it may be useful to view it in 3D with CBCT, especially if a patient is experiencing sensitivity seemingly inexplicably.

Conclusion

The decision of whether or not to place a post and what technique will be used to place it depend on the particular conditions of each case. When placing a post, the best option is to aim for the most conservative access possible while determining whether a prefabricated or cast post will provide the most ideal option. To optimize the retention of cores and the strength of connections between the coronal buildup material and the tooth structure, clinicians should adhere to the principles of post placement. Successful post and core dentistry considers quantity and quality of tooth structure as well as the presence of adjacent teeth, occlusion, parafunction, and the need for future restorations. It also takes the patient's goals into account and focuses on reasonable possibilities for their budgets. Ultimately, providing the most optimal patient care requires careful decision-making before, during, and after the performance of root canals and other endodontic treatments.

About the Author

Joseph Chikvashvili, DDS
Private Practice
West Orange, New Jersey

References

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