Post and Core Decisions Based on Fact, Not Fiction

Barry L. Musikant, D.M.D., F.A.C.D.
Post and Core Decisions Based on Fact, Not Fiction

Barry Musikant

IKE EVERY OTHER DENTIST, I read the latest articles in the dental trade journals and can certainly be influenced by them . . . with a few major exceptions. When I see products that are in direct competition with the things we have developed for Essential Dental Systems, such as the Flexi-Post and Flexi-Flange, SafeSiders, and the EZ-Fill obturation system, I think extra-critically to determine in my own mind whether what they claim is mostly hype or is based on solid data that defines their products as a step forward.
    One area of product development that has caught my attention is the introduction of new bonded fiber-reinforced composite posts. Claims have been made in one article or another that posts of this type actually strengthen teeth, that they seal the tooth better than root canal cement and gutta percha, and that they hold up at least as well as metal posts while dramatically reducing the incidence of root fracture associated with metal posts.
    If even one of these claims were true, these new products would represent major competition for the split-shanked metal post. Certainly, those claiming superiority for fiber-reinforced composite posts cannot do it on the basis of retention. The Flexi-Post and Flexi-Flange have retention values in the range of 300 pounds, while the maximum retention of a passive post—whether metal or fiber-reinforced composite—does not exceed 90 pounds, which is equal to the maximum cohesive strength of the strongest cements that exist today.
    A case could be made that fiber-reinforced posts will distribute less functional stress to the root than metal posts do because they are more flexible than metal posts of the same diameter. This is true, but in the literature it has been amply documented that the forces that the human musculature can generate are not sufficient to cause a passively seated metal post to fracture the tooth if there is a minimum of 1 mm of lateral tooth structure surrounding the post at its most apical placement. It has also been documented that the Flexi-Post and Flexi-Flange generate no more insertional stresses upon cementation despite their high retention than a passively placed post does. At the same time, the Flexi-Post and Flexi-Flange have the additional benefit of distributing functional stresses more evenly around the entire shaft of the post than is the case for a passive parallel metal post, where the functional stresses are concentrated apically.
    Recently one paper has definitively stated that a composite post in combination with a methacrylate resin cement will strengthen teeth if the cement is bonded to the dentinal walls via a primer. Yet the primer, which is drawn up into the dentinal tubules, has a hydrophilic nature and has been documented to continue to absorb water. This process leads to nano leakage and the weakening of the original bond. Although some authors have hyhpothesized that what is needed is a bond that is hyrodphilic prior to curing and hydrophobic after it cures, it also has been stated that these shifting qualities are beyond the present capabilities of the bonding agents and cements. As a result, any increase in strength is transient at best because the hydrophilic primer embedded into the dentinal tubules continues to attract the water molecules that accelerate the degradation of this bond. The results are a decrease in initial strength and the start of nanoleakage, which is a foot in the door in the process leading to microleakage. Furthermore, significant degradation of the bond of the post to the interface between cement and primer results from thermocycling, a process that is unavoidable in the mouth.
    In contrast, this degradation process does not occur when the threads of a metal post are embedded and cemented into the dentinal walls. At a minimum, independent research disputes the company-sponsored research claims of superior characteristics for the products they sell. However, once they have their research in print, companies will continue to make claims that are far from being universally accepted because they believe that those claims will sell the product.
    It is important for dentists to know what is real and to have the ability to differentiate it from what is claimed. Yes, one could read all the conflicting research and come to his or her own conclusions, but if we are honest we have to admit that this is not a likely scenario in most cases, where time is our most valuable asset.   A shortcut approach, even though it is self-serving, is to read the viewpoints of those who, as I do, have competitive products and want to make sure that the alternative interpretations gain public attention. At worst, a protracted dialogue goes on, with each side defending its own position. Ultimately the winners are those who follow the dialogue. Lincoln had it right: You can fool all the people some of the time and some of the people all the time, but you cannot fool all the people all the time.
    So what can be said about fiber-reinforced composite posts? There is no question that they work when sufficient dentin exists, but so does every post—or, for that matter, no post at all. The only purpose of a post is to create a more stable and substantial core upon which to seat, so the real test of a post is how well it will do when little or no coronal dentin exists. In these situations the core must support the crown without the added stability of extra dentin. Without coronal dentin, the post must have high retention, because its anchoring is limited to the internal retention of the shank of the post within the root canal. If the post is passively held in by the strongest cement, it will have a maximum retention of 90 pounds. If it is held into the dentin with threads, like those of a Flexi-Post or Flexi-Flange, the post will have over 300 pounds of retention, mimicking the retention that a natural crown has to a natural root with far greater accuracy than a passive post, no matter what material it is made of.
    Many dentists have chosen to use fiber-reinforced composite posts because these posts don’t discolor the core, which in turn may affect the shade of the overlying ceramic crown. If a post is necessary, then insufficient dentin is present, which implies that the crown should have a ferrule, which means that the final restoration should be porcelain fused to metal. If this is the case, a full porcelain jacket that is incapable of creating a fine chamfered finish line is the wrong restoration. A porcelain to metal restoration allows the placement of the stronger, more retentive, metal post while supporting the core with a ferrule, a design feature that has been shown to be the most important aspect of a final restoration where minimal dentin exists. And the esthetics of the post are not an issue because the porcelain fused to metal restoration is not translucent.
    The worst situation would be to place a shoulder preparation around a core that is entirely or nearly entirely composed of a flexible fiber-reinforced composite core surrounded by a composite core material. A restortion of this design is most likely to open gaps under repeated functional loads of compression and tension. Gaps lead to leakage, which leads to secondary decay.
    The most that can be said for fiber-reinforced composite posts is that in the event of a traumatic blow a flexible post will absorb more stress than the stiffer metal posts and in so doing will reduce the chances that the traumatic force will produce a vertical fracture. While reducing the incidence of root fracture is a plus, employing a flexible post to prevent these rare events while increasing the chances of gap formation and secondary decay under normal function does not make for long-term success.
    Rare events should never be the criteria that determine the mode of treatment. Ideal treatment should reflect the elimination or minimizing of those events that are most likely to occur. Unless we realize this, we may make treatment decisions based on an overemphasis of unlikely events while missing the common-sense approach that will most likely succeed for the largest number of patients.
    This approach—working to eliminate or minimize the most likely events—applies to many aspects of dentistry, including endodontics in particular, which we will go into in further detail in future issues of Endo-Mail.

Fall 2004

The real test of a post is how well it will do when little or no coronal dentin exists.

In canals where you need to place a curve or 45-degree bend at the end of the instrument to negotiate the apical dilacerations, line up the mark or notch on the rubber instrument stop with the bend of the instrument. This will ensure that the bend is facing in the right direction in relation to the apical curve every time the instrument is inserted into the canal.

Doug Kase

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