Barry Musikant

1. They have a modulus of elasticity similar to that of dentin, implying that the post will bend similarly to the tooth in which it is embedded.
2. They have  high retention because they are bonded into the teeth.
3. They bend enough to absorb parafunctional forces without acting as a conduit for excessive stresses that lead to root fracture.
4. They eliminate the high insertional stresses associated with threaded metal posts because they are of passive design.
5. They impose no esthetic challenges  because they have the color of teeth.
6. They strengthen the teeth.

1.  They have a modulus of elasticity similar to that of dentin, implying that the post will bend similarly to the tooth in which it is embedded.
    This claim made its way into post advertisements early on.  The thrust of these advertisements is the illogical conclusion that posts made of materials with a modulus of elasticity similar to that of dentin will bend the same as dentin.  In fact, materials with a modulus of elasticity similar to that of dentin will only bend the same if (and it is a crucial if) they have the same or similar cross-sectional area.   When one realizes that a redwood tree and a redwood toothpick both have the same modulus of elasticity it becomes clear that the modulus of elasticity alone does not define deformation under function.
    In the case of teeth, a post with the same modulus of elasticity as the tooth is likely to have a cross-sectional area approximately 1/10 to 1/15 that of the tooth it is embedded into, making the post 10 to 15 times more deformable under function than the surrounding tooth.  The differences in cross-sectional area of the tooth and post define the degree of deformation, unavoidably leading to increased compressive and tensile stresses within the core material simply because the core material is supported by the more highly deformable post.
    The effects of a deformable post on the core material are another issue that has not been accurately addressed. Advertisements claim that the post and core bond to one another, creating a monobloc that is stronger than either component alone.  That might be true if the two materials were enmeshed in each other’s structures to such an intimate extent that a new composite material was created, such as occurs in airplane propellers.  However, in the case of posts and cores the fiber-reinforced post stands as a separate entity and is then grossly overlaid with a composite material in which either no fibers are included or the fibers are randomly aligned, giving it minimal resistance to functional forces.  The resistance to deformation and the resistance to cyclic fatigue degradation is defined by the strongest link in the chain, the fiber-reinforced post, which we have already demonstrated is 10 to 15 times more deformable than the root it is in.  Adding a weaker composite overlay does nothing to strengthen the post’s resistance to deformation.   The end result of the core buildup is stress to the core material as it undergoes repeated cycles of compression and tension because of the supporting post’s low resistance to deformation.  To reinforce the above point, consider a post that is as flexible as a human hair supporting a core against lateral movement without any coronal dentin existing.  The only resistance encountered is the minimal support of the post, a hair in this case, and the frictional resistance of the bonded composite to the relatively flat surface of dentin.  In this example, it should be clear that the composite adds nothing to the strength of the post.
    The saving grace in this dismal scenario comes from the outer margins of the final restoration.  As long as these circumferential margins stay intact, the weakness of the post-and-core construct will not be tested, allowing any type of underlying support or no underlying support at all to succeed.  If and when the outer margins of the restoration degrade, as they often do over time, then all the functional forces will be directed internally and the weaknesses of the construct will take their toll.

2. They have  high retention because they are bonded into the teeth.
    Advertisements make claims that bonded posts have unusually high retention, rendering the retention of threaded posts irrelevant.  The factor that made threaded posts irrelevant was not the still-present need for high retention, but rather the inability to disassociate high retention from high insertional stresses.  High retention is not acceptable even though it is needed if it must come with high insertional stresses because these stresses too often lead to root fracture.  The maximum retention that a bonded passive post can attain is 90 pounds, far less than the 340 pounds attained with a threaded Flexi-Post and Flexi-Flange.  Most importantly, both the Flexi-Post and Flexi-Flange attain their high retention values without introducing high insertional stresses, which are at a level comparable to those of passive posts.  The split-shank design of these posts creates high retention by making the grooves for the threads in a sequential manner as it is screwed into the root.  The posts themselves are, in effect,  graduated taps that allow the dentist to enjoy the benefits of high retention, low insertional stresses, and an even distribution of functional stresses.

3. They bend enough to absorb parafunctional forces without acting as a conduit for excessive stresses that lead to root fracture.
    The one marginal advantage a fiber-reinforced post would have over a Flexi-Post or Flexi-Flange is the greater deformation displayed by the former when a sharp blow would be applied to the post-restored tooth.  Under these unique circumstances, the increased bending would lead to a lower chance of root fracture than in a tooth with a metal post.  However, short of these circumstances, namely during all the normal functions that dentition undergo, the rigidity of a metal post bending in unison with a root is far more likely to keep margins intact while supporting the external restoration.
    To design for traumatic blows while not meeting the needs for routine function is a poor choice of options in our opinion.

4. They eliminate the high insertional stresses associated with threaded metal posts because they are of passive design.
    Over the years, the split-shank design of the Flexi-Post and Flexi-Flange has shown that a metal post can be threaded into a tooth producing high retention and minimal stress at the same time.  This fact alone makes the claimed advantages of a fiber-reinforced post non-existent.

5. They impose no esthetic challenges  because they have the color of teeth.
    The problem of discoloration is pretty much nullified with opaquing bonding agents.  I typically will opaque out the color of metal by coating the post, as it exits the root, with C & B Metabond opaquing agent.  This same cement can be used even within the root to minimize any color that might show through the length of the root and the overlying gingival.  Because this problem is eliminated so efficiently, esthetic considerations impose no limitation on the use of these split-shank metal posts.

6. They strengthen the teeth.
    This is a claim that is more apparent than real when first considered.  To clarify logical thinking, consider a metal pipe that is strong enough to resist 300 pounds of force before bending.  Now place a material within the pipe that alone resists 200 pounds of force before bending.  Does it now require 500 pounds of applied force to bend the pipe filled with the 200-pound resistant material?  On the contrary, the bending is still ultimately resisted by the pipe alone, which will bend after 300 pounds of force is applied even though the pipe is now filled with a 200 pound resistant core.
    In the same way, a post that bends far more easily than the tooth it is embedded into will not increase the resistance to bending of the tooth. The concept has a nice sound, but it is not backed up by fact.

OVER THE YEARS, from the use of reamers rather than files and now relieved reamers and their incorporation of a reciprocating handpiece rather than a rotary driving force, to the scientifically proven advantages of a split-shank design of threaded metal posts, we have attempted to demonstrate the connection between sound design and practical mechanics producing more successful techniques and restorative components.  We hope this discussion is timely and helps dentists to think more critically when exposed to advertising claims that, in our estimation, do not reflect clinical reality.