Two of the more popular stances of conventional wisdom regarding posts and cores include the following:
1. They should never have a threaded design because it will lead to root fracture.
2. They should be made of fiber reinforced composite because they bend far more like tooth structure.
The first stance is understandable. Most of us think of a screw being rotated into a either a solid material where the resistance is enormous or into a space already created for the width of the screw minus the width of the threads still producing substantial amounts of insertional stresses. While these procedures might be acceptable when placed into a large block of material, a tooth with its very limited dimensions logically appears vulnerable to vertical fracture from easily generated insertional stresses. These logical deductions have limited the use of posts to passive insertions that are held in by cement interfaces(1,2,4).
As for the second stance, marketing has sold us on the idea that materials possessing a similar modulus of elasticity will bend the same. If our reasoning is limited to this fact alone then one would easily conclude that fiber-reinforced composites with a modulus of elasticity similar to dentin would be much more compatible when placed in a tooth rather than a metal post with a modulus of elasticity about 10 times higher.
In both cases, we want to extend our thinking so we can appreciate the far wider and more effective choices that exist. In the case of threaded posts, the conclusions made are based on designs that are limited to those incorporating a solid shank and they indeed are accurate. However, if we incorporate a split along the length of the threaded shank we now have a new model that doesn’t obey the principles of a solid threaded shank. The split transforms a solid shank into one with two flexible legs that can collapse upon themselves, a feature that allows the dentist to now insert a threaded post with minimal insertional stresses producing retention that far exceeds one held in by cement alone (1,3,4). To illustrate this point clearly please look at the three schematics below of a Flexi-Post being inserted into a root.
The threads on the post stick out 0.2 mm from the shank. The post-hole created for the post creates a space that is 0.1 mm wider than the internal diameter of the post in its uncollapsed space. Consequently, the maximum embedment of the post into the dentin would be 0.1mm. If a thread on a solid shank post were to embed its thread 0.1 mm into dentin it would encounter a lot of resistance and produce significant insertional stresses. However, this is not the case when a split is incorporated into the shank (3). As soon as the threads on the split shank threaded post are rotated into the canal preparation the legs collapse upon themselves, reducing the depth of the threads into the dentin from 0.1 mm to 0.01 mm. That is a significant reduction that produces minimal insertional stresses. In like manner, the second thread on the post, is situated slightly more coronally. As we approach the origin of the split, the legs become less flexible. (Think of a clothespin). Consequently, the second thread will insert itself 0.03 mm. That does not mean this thread is doing 0.03 mm worth of work. Rather it is doing 0.02 mm worth of incremental work because the original thread had already deepened the groove into dentin to a depth of 0.01 mm. Again in like manner, the third thread, located on a less flexible portion of the post will insert itself 0.05 mm. That depth requires only an additional 0.02 mm of work over what was done with the second thread when it was rotated into place (3).
By incorporating a split, we accomplish two significant goals. One, in absolute terms we minimize the amount of insertional stresses generated of perhaps more significance, we are distributing the stresses along the entire length of the post in the root rather than concentrating the forces in the lead thread, something that would automatically occur when a solid-shanked parallel threaded post is placed. We certainly don’t want a post with threads incorporated on a solid shanked tapered post. Each new thread would add to the insertional stresses and easily overcome the tensile strength of the tooth.
The important concept to learn is that via proper design, what was once thought to be completely unacceptable not only becomes acceptable, but improves the success of the post, higher retention without the accompanying stresses associated with solid-shanked threaded posts. In addition, threads safely placed into the dentinal walls now provide the additional benefit of distributing functional stresses evenly in contrast to a solid shanked parallel passive post that will concentrate its stresses apically where it abuts against the apical preparation of the post-hole.
Let’s now address the issue of modulus of elasticity. Marketeers have convinced a good portion of the dental community that the modulus of elasticity defines flexibility. That is only half true. Flexibility is also defined by cross-sectional diameter. It is a product of the two. Once one realizes this fact, the shortcomings of defining flexibility by modulus of elasiticity alone become obvious. A post composed of a material with a modulus of elasticity of dentin would need the same cross-sectional diameter to bend like the tooth it is inserted into. That is a physical impossibility. In reality, the post is usually about 10 times thinner than the tooth it is inserted into. It follows that a post composed of a material with a similar modulus of elasticity, but 10 times thinner in diameter will bend 10 times more than the tooth it is inserted into under function, an undesirable outcome.
What is required for compatible bending of both the post and the root is a post that compensates for its much thinner cross-sectional area by having a modulus of elasticity that is 10 times higher, namely a metal post.
My goal is to illustrate by these two examples the acceptance of information that at first seems quite logical, but upon closer inspection may not only be inaccurate, but destructively inaccurate. When properly designed a threaded post produces superior results in retention without incurring excessive stresses and distributes those stresses more evenly because of the incorporation of a thread (5). In the past the thread has been criticized as an isolated bad feature not realizing it could be placed on a split-shanked design maximizing the benefits of the post. The emphasis on the modulus of elasticity as the sole feature to judge a post was purely a successful result of marketing and evaporates in the face of the full story. We should be aware of statements that are passed off as facts when indeed there is far less to them than appear at first.
1. CRA – 1990;14:2. Comparison of Strains Generated During Placement of Five Endodontic Posts.
2. Boyarsky H, Davis R. Root Fracture with Dentin Retained Post. American Journal of Dentistry 1992; 5:11-4.
3. Midwest Dental Evaluation Group. Interface: Flexi-Post Prefabricated Endodontic Post System. 1989;1:3.
4. Brown JD, Mitchem JC. Retentive Properties of Dowel Post Systems. Oper Dent 1987;12-15-19
5. Cohen BI, Pagnillo MK, Musikant BL, Deutsch AS. Split-Shank Threaded Posts and Threaded Posts: Tensile Properties and Stress Levels. Journal of Esthetic Dent 1995;174-8