Common sense as a guide to post design

We want minimal insertional stresses and we want whatever functional stresses that are generated to be evenly distributed along the length of the post that is embedded in the canal.

Without a stable form of high retention, the post may loosen in time. If insertional stresses are not kept to a minimum, they can combine with functional stresses to fracture the root, especially if and when the functional stresses are not evenly distributed over the length of the root.

Given these common sense criteria we now have a way to gauge the effectiveness of a post’s design and the material it is made from. A passive post, one without threads, will have retention that is limited to the cohesive strength of the cement holding in place, limiting retention to a maximum of 90 pounds. A threaded post, on the other hand, with flutes embedded into the dentin will have significantly higher retention values.

However, the advantages of higher retention are offset by much higher insertional stresses. In the past, the disadvantages of a thread outweighed the benefits of higher retention with companies seeking higher retention values via highly adhesive cements. Yet this form of circular thinking gets us back to the 90 pounds of retention that is often inadequate for predictable long-term retention.

By applying a common sense design, we can turn a threaded post into a graduated tap simply by placing a split in the shank. The split produces two flexible legs that conform to the canal walls as it is threaded into place.

The most flexible portion of the post is at the location of the most apical thread. Here the twin legs of the post easily collapse upon themselves as soon as the rotated threads touch the walls of the canal, producing minimal embedment and also producing minimal insertional stresses.

The threads along the length of a split shank post design are increasingly resistant to closure the more coronally situated they are. This simple mechanical fact turns the post into a graduated tap where the threads along length embed themselves sequentially deeper into the canal walls (Fig. 1).

At no point is the insertional stress concentrated at any single point. Rather, the stresses of insertion are first mitigated by the collapse of the twin legs and then distributed evenly along the entire length of the embedding post.¹ The result is a combination of high retention, more than 300 pounds when cemented with a flowable composite while having little more insertional stresses than the placement of a passive post. By taking the common sense approach of creating a split in the post, we have satisfied the need for a post with high retention, minimal insertional stresses and the even distribution of functional stresses.²

We will elaborate on this last point. As long as threads have been on solid shank posts the threads will consistently produce excessive insertional stresses. This reality led many dentists to the rational conclusion that threads in themselves cause stress and should be avoided. However, this is true only if the threads are on a solid shank. Once the threads are on a split shank, the threads embed into the canal walls, gradually no longer producing stresses. At this point, the threads function as distributors of functional stresses evenly distributing the stresses around each thread without concentrating functional stresses apically the way they would if the post were passively placed.³ The split shank design has now produced a post that reflects common sense and a rational design.

In light of all the passive “dowel” designed fiber reinforced composite posts, one should be aware that while the reinforced fibers have a modulus of elasticity comparable to dentin, these posts will bend far more than the roots within which they are embedded. The similar values of the modulus of elasticity for both teeth and fiber posts have been used to make the claim that a fiber post will bend similarly to the tooth. In fact, bending of any material is based on its modulus of elasticity and cross-sectional area.

While the post and the root have similar moduli of elasticity, the cross-sectional area of the tooth is about 10 times that of the post. Given the similar modulus of elasticity and a cross-sectional area 1/10 that of the root, the post will bend approximately 10 times more than the root it is embedded into. To compensate for the discrepancies in bending due to dissimilar cross-sectional areas, the post must be made of a material that has a modulus of elasticity approximately 10 times that of the root it is embedded into. The most resilient material is stainless steel. If a dentist desires to use a fiber-reinforced post, to minimize the discrepancy of bending, he must make the post as thick as possible. However, this will weaken the root and produce a poorer long-term prognosis. The use of properly designed metal posts is an attempt to optimize the long-term success of the overall restoration.

The Flexi-Post and Flexi-Flange (Fig. 2) are made from stainless steel, incorporate the split-shank design and have a series of threads along their shank.⁴ They are best able to give the dentist what he needs from a post when minimal coronal tooth structure is present. There is no doubt that fiber posts will function well when sufficient coronal dentin is present, but the truth is that posts are not necessary when sufficient coronal dentin is available.

When no coronal dentin or very little is available, the most common sense approach is to place a Flexi-Post/Flange cemented in with a flowable composite producing more than 300 pounds of retention, minimal insertional stresses and an even distribution of functional stresses. You cannot go wrong if you let common sense principles guide you.

For those interested in the techniques I advocate, you may wish to avail yourself for a free one-on-one, two-hour workshop in my office. Call me at (212) 582-8161 to set up a workshop date. For those interested in an intense, 17-credit, two-day, hands-on workshop that is tuition-based, call Essential Seminars at (888) 542-6376 or visit www.essentialseminars.org.

About the Author

Dr. Barry Lee Musikant is a member of the American Dental Association, American Association of Endodontists, Academy of General Dentistry, The Dental Society of NY, First District Dental Society, Academy of Oral Medicine, Alpha Omega Dental Fraternity and the American Society of Dental Aesthetics. He is also a fellow of the American College of Dentistry (FACD).

Musikant’s lecture schedule has taken him to more than 250 international and domestic locations. He has co-authored more than 300 articles in dentistry in various international dental journals from Argentina to Spain, including the major journals of the United States and Canada. As a partner in the largest endodontic practice in Manhattan, Musikant’s 35-plus years of practice experience have crafted him into one of the top authorities in endodontics.

References
1. Ross RS, Nicholls JI, Harrington GW. Comparison of Strains Generated During Placement Of Five Endodontic Posts. J Endodon 1991;17:450–456.
2. Greenfeld RS, Roydhouse RH, Marshall FJ, Schoner B. Comparison of Two-Post Systems Under Applied Compressive-Shear Loads. J Prosthet Dent 1989; 61:17–24.
3. Keyf, F, Sahin, E. Retentive properties of three post-core systems. Australian Dental Journal 1994;39(1):20–24.
4. Brett I. Cohen, BI, Pagnillo, M, Musikant, BL, Deutsch, AS. Comparison of the Retentive and Photoelastic Properties of Two Prefabricated Endodontic Post Systems. J Oral Rehabilit 1999; 26:488–494.