Barry Musikant
 

Allan Deutsch

The Modulus of Elasticity Fallacy
In addition, a marketing innovation includes great emphasis that the post are made of materials that endow them with a modulus of elasticity equal to that of dentin. This implies that because of the similarity, the post will bend in unison with the tooth, reducing functional stresses to the root that would have been far greater if a metal post had been inserted into the canal.
    The claim of similar flexibility of the post and the tooth is very attractive for the long-term success of the post-supported restoration. The reality of the concept is false, however, and not supported by the logical analysis of its claims. First of all, when claims are made that the materials have the same modulus of elasticity, it means that the samples tested have the same cross-sectional area. This should immediately become clear if one realizes that a redwood tree and a redwood tooth pick both have the same modulus of elasticity. The tree is immovable, while the toothpick can be snapped in two between your fingers. Obviously, the cross-sectional area affects the resistance to bending. 
    A post going into the root of a tooth has by necessity a much smaller cross-sectional area than the root it is being placed into. The smaller the cross-sectional area, the greater the flexibility, just as in the case of the toothpick. Because all posts are thinner than the roots they are going into, a similar modulus of elasticity will produce a post that bends far more than the root it is in. The claim of similar moduli of elasticity is a weakness, not a strength, and a post that bends significantly more than its supporting root will not bring longevity to the overlying restoration.
    More specifically, the modulus of elasticity of dentin is 8. The diameter of the shank of the average post going into a root is generally between 20 and 25 times smaller than the host root. To compensate for this smaller diameter, the modulus of elasticity should be proportionally higher, producing a stiffer material.  A stiffer material that has a thinner cross-sectional area could bend in a fashion similar to the bending of the root, if it has the proper modulus of elasticity. The modulus of elasticity of stainless steel is 200, 25 times that of dentin. 
    Yet, if the cross sectional area of the stainless steel post is about 25 times thinner than the root, the bending of both the post and the root will be the same when subjected to an outside force (200 / 25 = 8).  A careful analysis of the original claims for non-metallic posts not only demonstrates that they bend far more than the roots that they are in, but also that stainless steel, rather than being too stiff, has in fact an excellent modulus of elasticity for compatibility between a post and supporting root.

The Composite Core Fallacy
While claiming the benefits of a lower modulus of elasticity, a further part of the technique includes the addition of a composite core.  the claim is made that the combination of the two creates a mono-block that bends like a tooth, something that the post was supposed to be able to do on its own.  The term mono-block implies a homogeneity of materials, suggesting that the post and core are so similar that they act as one unit. However, the post—though flexible—is highly resistant to fracture because of carbon fibers palced in parallel in the matrix. 
    The composite core is not endowed with such fracture-resistant components and is subject to degradation under function from a post that bends far more than the root.  In fact, the first area of degradation would be along the composite core dentin interface producing a gap formation susceptible to decay and further widening. For the concept to produce longevity, the core would also need the incorporation of parallel carbon fibers, a product that does not exist today.

The Superior Esthetics Fallacy
A newer generation of non-metallic posts which are composed of either a reinforced composite or ceramic are making claims of superior esthetic results. They are being advocated when the teeth are being restored with the new ceramic crowns. The solidity of the final restoration is based on the bonding ability of the new adhesives. As any endodontist will tell you, endodontically treated teeth do not make ideal abutments!
    If possible when they are used, additional teeth that have not had endodontics are incorporated into a restored span for greater stability. It is traditional, unchallenged knowledge that when endodontically treated teeth are crowned, the margin should include a long bevel onto the root surface to create a superior ferrule effect. The new ceramic crowns, like their predecessors, require a butt joint. These restorations should not be placed on endodntically treated teeth, especially those with a post-supported core. If the better restoration is a porcelain fused to metal crown with a long bevel, the need for an esthetic post does not exist, and the post that is placed can have higher retention and better stress distribution pattern than any of the non-metallic posts can deliver.
    The one area where a tooth-colored post might have some irreplaceable benefit is thin roots covered by a thin labial gingiva where the color of the metallic post might show through. The incidence of this particular situation is very small and even there can be modified by the use of opaquing cements over the shank of the metal post when it is placed within the root. 

Conclusions
In summary, the claims made for non-metallic posts are not supported by the logic of many laboratory studies.
    Further, the use of esthetic non-metallic posts with butt joint full coverage restorations is counter-productive and should be avoided. Well designed stainless steel prefabricated posts (Flexi-Post and Flexi-Flange) are more compatible with functioning roots, producing far higher retention, minimum stress upon insertion, and even distribution of stress under function. 
   It would appear that the even distribution of stress under function is enhanced by a post that bends very similarly to dentin, the result of a significantly higher modulus of elasticity, such as that of stainless steel.