Optimizing the Placement of Posts and Cores

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Optimizing the Placement of Posts and Cores

Postby BarryMusikant on Sat Jun 06, 2015 3:15 pm

I am writing this post at the request of Jeffrey Hoos, a wonderful dentist who has made significant contributions to dentistry over the years. It is a discussion on posts, when to use them, how they should be designed and best utilized. I should state from the beginning that posts do not strengthen roots. They are placed solely to provide a greater coronal structure upon which to place a crown. They may be placed immediately or some time after root canal therapy, but prior to the placement of the permanent restoration. They may also be done if a tooth requires endodontic therapy after a crown has been placed and in the judgment of the dentist insufficient coronal tooth structure remains to predictably support the crown. If the margins of the restoration have been breached by decay, replacement of the old crown is recommended regardless of the amount of tooth structure remaining.

Once the conditions for post placement have been met, we must decide what is the best post to place. Our criteria include:
1. the high retention
2. minimal insertional stresses
3. an even distribution of functional stresses
4. resistance to cyclic fatigue
5. maintenance of as much tooth structure as possible consistent with withstanding functional stresses
6. compatibility in flexibility between the post, the core buildup and the root

Post retention and insertional stresses are often in conflict. For that reason passive posts held in by a bonded cement are often recommended. This approach limits the retention of the post to about 90 pounds, the strength of the cement interface. Often times, this degree of retention proves in adequate and leads to loosening of the post over time. While dentists are well aware of the limitations of passive posts, they are equally or more aware of the shortcomings of actively threaded posts. Here the concern is not retention which is now quite high, but the stresses imparted to the surrounding dentinal walls as the flutes engage along length.

We resolved this conflict by designing a threaded parallel post with a split in the apical 2/3’s of the post’s length. The split gives the legs the flexibility to collapse upon themselves when inserted into the canal after the recommended reamers have widened the canal appropriately. The post can now be accurately described as a graduated tap creating indentations along the length of the root that accurately fit the flute design of the post. Most importantly, the post, acting first as a graduated tap internally producing a space for the flutes while minimizing insertional stresses and distributing those minimal stresses over the length of the post now inserted within the root.

To better clarify the actions of the post a few illustrations should be beneficial.

Here we see the first thread embedding itself into the most apical portion of the canal. The size of the flute extends 0.2 mm from the shank. The appropriate sized reamer for the post creates a space that is 0.1 mm wider than the internal parallel walls of the post-hole. Given the 0.1 mm wider preparation of the canal space than the parallel walls of the post, the maximum embedment of the thread into the canal walls is 0.1 mm. This may not sound like a lot, but it is and if there was a full 0.1 mm embedment of the thread into the dentin along length from the apex up the placement of the post would encounter a lot of resistance when it would be rotated in. Rightfully, there would be concerns about the introduction of stresses that could lead to vertical fracture.



The incorporation of a split, however, changes the mechanics of insertion completely. As soon as the threads on the legs of the post touch the walls of the canal, the two legs collapse upon themselves. Rather than embedding 0.1 mm into dentin, the most apical thread is limited to embedding itself into dentin 0.01 mm. That is an infinitesimal amount and induces little stress within the root. The second most apical thread now follows the first thread. As we approach the origin of the split, the two independent legs become less flexible. As a result this thread embeds itself 0.03 mm into the canal walls, deepening the embedment by an additional 0.02 mm still a very small amount.




In like manner, the third thread from the apex is on a portion of the post that is stiffer yet with embedment gaining a depth of 0.05 mm. Please note that to gain a depth of 0.05 mm only an additional 0.02 mm of depth was needed.




As a result of the split design, the insertional stresses are significantly minimized. Whatever small amount of stresses generated are distributed over the length of the post within the root and not concentrated in the lead thread. It is the minimization and distribution of the stresses that allow the placement of a highly retentive post without the concerns of high insertional stresses. The split design allows for the safe placement of a thread along length. The thread, in turn, offers a design that now allows functional stresses to be evenly distributed over the length of the post rather than having them concentrated in the most apical portion of the post as they are with all parallel passive post designs.

I personally prefer metal posts with the split shank. Fiber reinforced composite posts became popular because of esthetic needs when placing porcelain restorations and their supposed compatibility in bending due to similar moduli of elasticity. Conventional wisdom states that fiber posts, having a modulus of elasticity similar to dentin, will bend the same under function having the tooth bend as a single unit rather than tend to separate into its component parts under function. There is a key fallacy in this reasoning. Flexibility is only partially defined by the modulus of elasticity. The other factor defining flexibility is the cross-sectional area. Once one realizes that flexibility is defined by both factors, it becomes readily apparent that a post made of a material with a similar modulus of elasticity to dentin will only bend the same as the tooth if it has the same cross-sectional area, a physical impossibility for a post. Generally, a post is on average 1/10 the diameter of the tooth within which it is being inserted. For a post to bend like the tooth it is in under function the modulus of elasticity should be approximately 10 times higher than the tooth to compensate for its significantly thinner cross-sectional area. Materials with a modulus of elasticity of dentin 10 times that of tooth are known as metals with stainless steel and titanium being examples of materials that are often used in the fabrication of posts.

From an esthetic point of view, if and when a porcelain restoration is going to be placed it takes no time to opaque out the metal color. The advantages of using a post system that is inherently strong, produces high retention while minimizing insertional stresses and then distributing functional stresses throughout the length of the post are unmatched by any other approach I am aware of.

I am happy to hear the thoughts and opinions of other. Jeffrey, please make any comments you feel are appropriate.

Regards, Barry
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Re: Optimizing the Placement of Posts and Cores

Postby ASHOK on Sun Jun 07, 2015 8:39 pm

Great lecture on posts Dr.Barry, thanks much. Its like a revision for me, lol, and i really appreciate your patience and interest in explaining its mechanism of action in so detail from so many years. I dont think i could able to do this at your age !!!. Keep going.

From past 7yrs i have eliminated rotary system from my practice, now its only your reciprocation.  Failure rate - less than 3%, and thats failure is not due to your system, its because of my error in treatment planning and poor handling of the instruments. I really appreciate your"s and your team innovation in this field.


Which cement you recommend to lute this metal posts, G.I.C or Resin cements.

From past few years iam doing good number of implant cases, one of the recent beautiful concepts in preserving labial bundle bone in aesthetic area while placing dental implants is Socket shield and Root Submergence Technique, few works have been published by Team Atlanta group ( salama brothers) and Denis Tarnow , i am sure you must of heard about them. In their technique they use long shank burs to split the root mesio distally from cervical third till apex, which is highly technique sensitive. Iam using endo ultrasonic tips for the same purpose which is more accurate and less damaging to the adjacent tooth and bone, but it takes lot of time. I would like to know about your EDS tips, its available length, its compatibility with which company scalers and its effectiveness in cutting dentin. Iam fusing endo work with implant dentistry, lol.


Now Who is your distributor in India, do they have all of your products, if iam not wrong they keep changing over here, please let me know about their contact information. Best regards, Ashok.
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Re: Optimizing the Placement of Posts and Cores

Postby BarryMusikant on Fri Jun 12, 2015 12:25 pm

ASHOK wrote:Great lecture on posts Dr.Barry, thanks much. Its like a revision for me, lol, and i really appreciate your patience and interest in explaining its mechanism of action in so detail from so many years. I dont think i could able to do this at your age !!!. Keep going.

From past 7yrs i have eliminated rotary system from my practice, now its only your reciprocation.  Failure rate - less than 3%, and thats failure is not due to your system, its because of my error in treatment planning and poor handling of the instruments. I really appreciate your"s and your team innovation in this field.


Which cement you recommend to lute this metal posts, G.I.C or Resin cements.

From past few years iam doing good number of implant cases, one of the recent beautiful concepts in preserving labial bundle bone in aesthetic area while placing dental implants is Socket shield and Root Submergence Technique, few works have been published by Team Atlanta group ( salama brothers) and Denis Tarnow , i am sure you must of heard about them. In their technique they use long shank burs to split the root mesio distally from cervical third till apex, which is highly technique sensitive. Iam using endo ultrasonic tips for the same purpose which is more accurate and less damaging to the adjacent tooth and bone, but it takes lot of time. I would like to know about your EDS tips, its available length, its compatibility with which company scalers and its effectiveness in cutting dentin. Iam fusing endo work with implant dentistry, lol.


Now Who is your distributor in India, do they have all of your products, if iam not wrong they keep changing over here, please let me know about their contact information. Best regards, Ashok.


Ashok,

I spoke with Gary at EDS and he tells me that the best way for you to get information is from Joel, our CEO who has all the details on our distributors and pricing. His email address is:  jburstein@edsdental.com

I read up on the new implant technique. Quite interesting, although it sounds like it is highly techniically sensitive.

As usual always good to hear from you.

Warm regards, Barry
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Re: Optimizing the Placement of Posts and Cores

Postby ASHOK on Wed Jun 17, 2015 9:07 pm

Thanks Dr.Barry, i will contact them to procure the ultrasonic tips, warm regards, Ashok.
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