Despite the success of dental implants, a well-placed post and core build-up can still be an excellent treatment option for your patients.
The only reason to even consider this question is if posted teeth predictably fail far more than the placement of implants. We are by no means questioning the use of implants, which have given dentists and their patients options when edentulous areas exist. The specific question is this: Are the success rates higher with implants than a posted tooth?
When the same criteria for success are applied to both endodontically treated teeth and implants, a review of the literature says no. Furthermore, the interaction with the dentinal walls of differently designed posts has a great impact on the insertional and functional stresses that a tooth will absorb--and that, in turn, significantly affects long-term viability.
If one is going to replace a tooth with an implant, the burden of proof should clearly be placed on the implant's increased benefits of the implant. If one cannot demonstrate increased benefits, then a tooth is being removed, a surgical site significantly deeper than the depth of the roots is prepared and a titanium fixture that will require ongoing maintenance is placed. That is a lot of extra work and exposes the patient to potential problems that are not necessarily providing an increased benefit.
Perhaps, one way to look at the decision-making process is to examine weakenesses that have been linked to posted teeth. The obvious one is fracture. Used as an abutment, a posted tooth may absorb stresses that lead to fracture undermining the bridge and requiring an implant that with 20/20 hindsight should have been placed from the start. This type of thinking makes sense, but only to the extent that the mental linkage behind post placement and vertical root fracture are taken as givens. If a post can be placed with minimal insertional stresses, distribute functional stresses evenly, bend like the tooth that the post is inserted into and ask no more of it than would be asked from an implant, then post placement would not lead to the vertical fracture that some assume will occur.
Even those who advocate implants would not create a 5-tooth span in an edentulous area supported only by two implants--one a mesial anchor and the other the distal anchor. The cardinal rule of implants is to avoid excessive stress that can lead to dieback and continuous loss of horizontal bone. To avoid this, there is a good chance that 3, or possibly 4, implants would be placed to support a 5-unit bridge.
Using the same criteria, don’t ask a posted endodontically treated tooth to act as one of two abutments in a long span bridge. In fact, implants should be placed in the edentulous areas, leaving the natural teeth, posted or not, only having to support their own function. The benefits to the patient include a higher success rate for the remaining teeth, less surgery and lower costs.
What we have talked about up to this point is a common sense approach regarding the relationship between posted teeth and implants. To be more specific we have many options when it comes to choosing a post system for endodontically treated teeth that will have an impact on their long-term integrity. For example, we all know that short, widely tapered cast or prefabricated posts lead to a high concentration of functional stresses that can cause vertical fractures. The longer the post and the more parallel its walls, the more evenly the stresses will be distributed.
Figure 1 |
However a passive post that is completely parallel will only have direct contact at its base. As a result all functional stresses will be concentrated apically (Fig 1).
Figure 2 |
One way to avoid this apical concentration of stresses when placing a parallel post is to have threads. The threads rather than the apical end of the post will be the location for the distribution of functional stresses (Fig 2). Because the threads are present throughout the length of the post, the stress will be more or less evenly distributed along its length.
Figure 3 |
Of course, threaded posts parallel or otherwise have long been associated with insertional stresses that can lead to vertical fracture. So while the threads are good distributors of functional stresses, the creation of insertional stresses that in itself could lead to fracture even before the tooth is exposed to functional stresses sounds counterproductive. It would appear that the pros and cons of a threaded post at best cancel each other out.
This dichotomy can be resolved by incorporating a split shank along the length of a parallel threaded post. The split (Fig 3) turns the post into a graduated tap which provides for a series of threads that gradually deepen their engagement with the canal walls resulting in great retention, while minimizing insertional stresses.
The post now has high retention as a result of the threads engaging along the length of the canal. Additionally, the threads, no longer producing high insertional stresses, are excellent distributors of the functional stresses to follow. Retention means stability while an even distribution of stresses--both insertional and functional--means long-term success. Endodontically treated teeth, built up with split-shank designed posts and not subject to the stresses of long span bridges, will easily support their own occlusion and last a long time.
There has been ample discussion of metal versus fiber-reinforced composite posts. The latter has been associated with a reduction in vertical fractures when excessive force is applied to a tooth. This should come as no surprise given the far greater flexibility of the fiber-reinforced post. Able to bend far more when a force is applied, it makes sense that the flexible post will absorb more of the applied load than a post that is stiffer.
A couple of points should be made here. The first is one that has long been used in the advertising of fiber-reinforced posts. Namely, that the modulus of elasticity of fiber-reinforced posts and dentin are similar, implying that they will both bend similarly when subjected to an applied load. The picture that the advertisers want to convey is one where both the post and the tooth it is embedded into will bend the same, keeping the margins of the crown intact. Nothing could be further from the truth.
Figure 4 |
The facts are different. Materials that have the same modulus of elasticity bend the same only when they have the same cross-sectional areas. Any post placed into a tooth will be approximately one-tenth the diameter of the tooth. The post with the same modulus of elasticity and one-tenth the diameter will bend 10 times more, subjecting the marginal buildup of the bonded core materials to tensile stresses that quickly lead to gap formation and subsequent potential for secondary decay. Given this disparity in bending potentials, fiber-reinforced posts should only be used in cases where significant dentin exists. They do have the advantage of being tooth colored which can lead to superior esthetics when porcelain restorations are placed. However, the benefits of a ferrule, which can only be produced with a metal collar, eliminate any esthetic needs.
Practically all fiber-reinforced posts are passively placed depending upon the bonding agents and luting cements to create retention which, at best, cannot exceed 90 pounds. This is another reason to use fiber posts only where sufficient coronal dentin exists to support post retention and compensate for its greater flexibility.
One way to increase post retention while improving upon functional stress distribution is to use a threaded fiber post which has been designed to have its flutes engage pre-threaded grooves in such a way that insertional stresses are minimized (Fig 4).
Figure 5 |
This post is a spin-off of the split shank metal post referred to earlier in this paper. The post space has grooves created along its length via a split shank tap deepening the grooves as they go coronally (Fig 5). The fiber post has threads that sequentially engage these grooves more deeply as they travel coronally. The result is high retention, minimal insertional stresses and a superior distribution of functional stresses.
This article is attempting to emphasize the designs of posts--both metal and fiber-reinforced--that create the greatest stability and consequently, lead to long-term success. Furthermore, we wanted to show when and how they should be used. Superior design used in a common sense way bring the most success to both posted teeth and implants. To put either implants or posted teeth into untenable positions and then say one or the other is superior is a dead end approach. The judicious use of implants actually increases the success rate of posted teeth. To extract perfectly viable teeth to place an implant which can have its own well documented problems is not necessarily in the best interests of the patient.