Re: Fibre Posts

We are on different pages. We don't have to agree when we are done
discussing the topic. I don't mind. Kindly scroll.

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Stephen Mancuso, D.D.S.
Troy, Michigan, USA
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This posting is intended for informational or conversational purposes only.
Always seek the opinion of a licensed dental professional before acting on
the advice or opinion expressed here. Only a dentist who has examined you
in person can diagnose your problems and make decisions which will affect
your health.
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"NOYB" <noyb@xxxxxxxx> wrote in message
news:O8Z4e.1891$sp3.1292@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>
> "Dr Steve" <nospam@xxxxxxxx> wrote in message
> news:DhX4e.15656$ZB6.950@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>> My point (sorry to repeat for those who are tired of hearing me), is that
>> for the cases where a ferrule would help, you are cutting away too much
>> tooth structure and end up with a thin sliver of tooth holding everything
>> together. In the cases where a ferrule leaves a lot of thickness (of
>> tooth structure) between the pulp canal space and the outer preparation,
>> the tooth would have held up without the ferrule.
>>
>> The average RCT tooth has had a few fillings in it of moderate to big
>> size. Now we cut a big hole down the center to do the RCT. Ream it out a
>> bit bigger for the post. Usually, you will place some core material to
>> have smooth contours you can prep for your retention and resistance form.
>> Of course, now you cannot see the inner surface the residual dentin. A
>> lot of these teeth have some gingival recession and you want your margin
>> 0.5-1.0 mm subgingival. So you reduce the buccal surface just shy of 2.0
>> mm and the lingual surface a "tad" less.
>
> The amount of reduction on the facial at the margin is 1mm...and a "tad"
> less on the lingual.
> I'm doing a lot of Wol-Ceram crowns, and you don't need to do as much
> reduction at the margin
> as you do for Cerecs.

I don't reduce the cervical third on a CEREC-- unless I am trying to change
the color of the tooth.

>> But to get that 2.0 mm reduction at a height just below the gingival
>> crest, you are removing 3-4 mm (sometimes more) of tooth at the height of
>> buccal (and lingual) contour.
>
> More like 1.5mm-2.0mm total reduction buccal-lingually at the margin.

2.0 mm reduction at the CEJ requires 3-5 mm of reduction at the height of
contour.

>>You get your prep just like we were taught in school and send it off to
>>the lab anticipating your lab tech telling you how perfect that prep looks
>>on the model. If you were to extract that tooth at this stage and section
>>it about 0.5 mm above the margin, you would find that you now have paper
>>thin dentin walls holding the whole thing together. The least little bit
>>of parafunction, or trauma and "S-N-A-P". If you made a really good post,
>>the "snap" occurs at the tip of the post and breaks the root in 2-3
>>pieces.
>
> The shell is not really that thin on any tooth besides lower incisors or
> maxillary laterals. Below,
> you claim that without reduction, the tooth is 3-5 mm thick both buccal
> and lingual. If you reduce the
> buccal by 1 mm, and the lingual by 1/2-1mm, then the tooth is 2-4 mm thick
> on the buccal, and 2 1/2-4 1/2mm thick on
> the lingual. However, you have much more favorable force vectors.

I suggest you remove all the filling from the enter of a molar after you
have done a full crown prep with 2.0 mm reduction down to the CEJ. The
tooth is 3-5 mm thick buccal and lingual if the cervical third of the
toothis NOT preped. With the cervical third of the tooth "un-prepped", the
lever arm becomes much shorter. IF you prep to the CEJ, the lever arm (ie
crown) is longer.

>> My recommendation is (except for teeth which are badly discolored), to
>> reduce the vertical height of the tooth 3-4 mm, remove all the
>> filling/core material, make the inner walls about 5-7 degrees of taper so
>> that the occlusal line angles are divergent. Round these occlusal line
>> angles. Round over the transition from the proximal boxes to the buccal
>> and lingual walls, and STOP. Impression or scan time. Make the
>> porcelain and bond in place. The center part of the restoration will be
>> 6-10 mm thick. Horrendously strong. The cervical third of the tooth will
>> be 3-5 mm thick both buccal and lingual--wonderfully strong.
>
> The internal stresses are enormous without the ferrule effect. In the
> scenario that you just described, you've created
> a pry-bar with an extra long lever-arm. A ferrule creates a *compressive*
> force on the wall of tooth that is on the same
> side that the force comes from. You are creating compression on a
> cylinder. A tooth without a ferrule creates a compressive force on the
> wall of tooth that is on the side *opposite* the force. Thus, you are
> creating tension on a cylinder. A cylinder is much stronger when loaded
> in compression.

You are thinnking of prepping down to the CEJ. Forget that concept. The
ferrule does not create any compresion on the tooth. Especially after the
lab uses die-separator. What you are doing is moving the fulcrum to the
base of the ferrule. All of the crown above the margin is your lever arm.
If the tooth is strong enough that parafunctional forces do not create force
vectors which cause the post to break the root at the post tip, then there
was enough tooth structure present that the post was NEVER needed in the
first place. Remember that a post will never strengthen a tooth. It will
only weaken it.

>> The entire occlusal surface will be covered so that parafunctional
>> activity cannot force the cusps apart and cause (or perpetuate existing)
>> fractures.
>
> The long prybar is forcing the root apart.

Nothing in the root except gutta percha.

>> You don't need the post if you have 3 mm of tooth sticking up.
>
> Agreed.
>
>> If you don't have 3 mm of tooth sticking up, the tooth will probably snap
>> off in the next 5-10 years--regardless of any post.
>
> I don't agree. A ferrule dissipates the lateral forces in such a way that
> much of tooth is in compression.
> Brittle materials are very strong in compression.

If all your forces are vertical and there is never any parafunction, the
restoration you describe will hold up fine (many do in this scenario). IF
there is lateral parafunctional vectors of force, forget it. Joel liked to
point out that shovel makers all put a ferrule on the end of the shovel
blade. Where does a shovel handle break every time?

>> The presence of a post places the vector of forces at the tip of post
>> with lateral parafunctional movement.
>
> Some of the force, yes. But with a long enough post, you have a small
> force at the tip due to the long lever arm. With a short post,
> you have a high force at the tip.

The longer the post, the narrower the root is at the tip. The longer the
post, the greater force that can be exerted at the post tip when the ferrule
weakens. You only need a post to retain a filling on the tooth. Any more
post than needed to hld the filling is weakening the tooth.

> With a ferrule, the side of the tooth that the force comes from also
> shares some of the load with the other side of the tooth, and withthe
> portion of tooth in contact with the tip of the post.

And all that force is transmitted to the thinnest part of the tooth.

>> When (and if) it fails, the root will break. Without the post, should
>> the restoration fail, it will typically just chip (in response to
>> parafunctional activity) in the occlusal third. I know which one I would
>> rather re-treat. I often ask my patients if they would prefer a rigid
>> post going half way down their root which will transmit forces down its
>> entire length and if it fails will break the root, or would they rather
>> skip the post and leave the weakest point somewhere were we can repair it
>> 50 times with damaging the tooth. No one wants a post.
>
> I avoid them as much as possible, but sometimes they're necessary.

Only for heroic measures. They are great when you are trying to sneak some
extra life out of a failed restoration.

>> Posts are great when you have to play hero and put an existing PFM back
>> on which snapped off at the gingival crest (wonder why?).
>
> Overprepped tooth because that darn Cerec requires a 2 mm shoulder prep
> all the way around.

I have never had a CEREC crown break a tooth off yet. I see PFM's ehich
break the tooth off all the time. Remember, my point is that the cervical
third of the tooth is strongest part of the tooth, and should be maintained
at all cost.

>>You tell the patient this is only going to last 1-4 years, set the post,
>>do a reverse-core and cement the PFM back on.
>
> Pull the tooth and place an implant if it snaps to the gingival crest. I
> don't do reverse cores anymore because 1-4 years just isn't good enough.

I don't like 1-4 year restorations either, but a) it gets the crown into
service while the patient decides, and b) it is the patient's choice what
they ultimately get after proper informed consent.

>> Ferrules, are what we were taught in dental school. If you stop, ignore
>> what you were taught in DS, and pretend you were looking at the problem
>> the first time in your life. You are staring down at the tooth from the
>> occlusal view. You have all the filling out of the tooth, the RCT is
>> done and you have trimmed the gutta percha back into to the root. You
>> visualize how much tooth will be cut off for the perfect ferrule at the
>> gingival crest. You look at how much good healthy strong tooth you are
>> cutting away to create paper thin axial walls. You would never choose to
>> do ferrules.
>
> I graduated Mechanical Engineering and entered dental school. I looked at
> it from an engineering perspective and deduced that a ferrule helps
> prevent posts from fracturing roots.

The ferrule actuallly just keeps the post from loosening as quickly.

> The only time that I use a post is when one entire wall of a tooth is gone
> (buccal or lingual). If I have two walls (even if one is only 2 mm high),
> I do a bonded core. If I have one wall, and a half wall (one cusp) on
> the other side, I do a bonded core. If I have zero walls (snapped to the
> gumline), I send to the surgeon for EXT and implant or bridge.

I used to do the same. I then started looking at every core I did with the
intra-oral camera.


.

Relevant Pages

  • Re: Fibre Posts
    ... > cutting away all of that extra core material while prepping the crown. ... That's the point of the ferrule. ... >>> the post first contacts the tooth. ... >> If a weaker fulcrum is further out along the lever arm than the stronger ...
    (sci.med.dentistry)
  • Re: Fibre Posts
    ... > But crowns don't snap off the tooth at the height of contour. ... If your crown prep does not extend epically beyond the height of contour, ... >>> The internal stresses are enormous without the ferrule effect. ... >>> in compression. ...
    (sci.med.dentistry)
  • Re: Fibre Posts
    ... > tooth structure and end up with a thin sliver of tooth holding everything ... In the cases where a ferrule leaves a lot of thickness (of ... >dentin walls holding the whole thing together. ... The long prybar is forcing the root apart. ...
    (sci.med.dentistry)
  • Re: Fibre Posts
    ... tooth structure and end up with a thin sliver of tooth holding everything ... In the cases where a ferrule leaves a lot of thickness (of tooth ... the root will break. ... >>>But it prevents root fractures when there's a post. ...
    (sci.med.dentistry)
  • Re: Fibre Posts
    ... >>> that for the cases where a ferrule would help, ... >>> much tooth structure and end up with a thin sliver of tooth holding ... >> in compression. ... >> The long prybar is forcing the root apart. ...
    (sci.med.dentistry)
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