Re: Fibre Posts


>>>> 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.
>
> But crowns don't snap off the tooth at the height of contour.

Actually they do, because the height of contour for a PFM prep is at the
margin.

If your crown prep does not extend epically beyond the height of contour,
the tooth never snaps off.


>> 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.
>
> I do that every time. I got out of the habit of leaving amalgams that
> "looked OK" because I got burned by decay
> from microleakage hiding beneath them. I always remove the old
> buildups/fillings.

If you have the filling material totoally removed while you are doing your
crown prep, then I commend you for at least paying attention. All too often
(because of our good training), we will clean out the caries first, remove
all filling material, then core the tooth. After the core is set, we prep
for the crown. I want you to be looking at the tooth with the core removed.

>> The tooth is 3-5 mm thick buccal and lingual if the cervical third of the
>> tooth is 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.
>
> The coronal lever arm of the post is the length of the post from the top
> of the post and core to the point where the post first touches tooth.
> The radicular lever arm of the post is the length of the post from the tip
> of the post to the same exact point as mentioned above. Whether or not
> a tooth is prepped on the buccal at the gingiva or not will not affect the
> lever arm of the post.

Semantics we can agree to.

>>>> 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.
>
> Sure it does...and it occurs on the same side of the tooth from where the
> force originates. That's the point of the ferrule. Until you can
> visualize that very important compressive force, you'll have a difficult
> time conceptualizing why the ferrule is important.

That is an uneven compression with resultant tension on the opposite side.
That would onlly serve to crush dentin.

>> Especially after the lab uses die-separator.
>
> The space created by the die-separator is filled with a non-compressible
> resin or resin-modified glass ionomer cement with a very high compressive
> strength. Consequently, the space from the die-separator is irrelevent.

I thought you ere talking about vertical forces creating compression in a
uniform pressure circumpherentially. We were on different pages there.

>> What you are doing is moving the fulcrum to the base of the ferrule.
>
> For the crown, maybe. The fulcrum of the post is still the point where
> the post first contacts the tooth. With a ferrule, if you have a load from
> the lingual, you effectively have three fulcrums: 1)one for the post, 2)
> one for the crown's buccal contact point, and one for the crown's lingual
> contact point . Consequently, the stress is dissipated over three
> different areas. That third area is a compressive force on the
> tooth/cylinder. With your design, there is one fulcrum...and the stress
> at that point is enormous.

Having worked industrial and commercial construction for eight years, I will
tell you that it does really work that way in the world. The strongest of
the three points will bear all the stress. If a weaker fulcrum is further
out along the lever arm than the stronger one, the weaker one will break
before transfering stress to the stronger one. After the first fulcrum
breaks, the stress will transfer to the second fulcrum further from the end
of the lever arm. If this is not the stronger fulcrum, this too will give
way. Once one of the fulcrums you describe give way, movement begins--due
to either loosness or (more likelly) flexure. The movement will put greater
stress on the remaining fulcrums. If the tooth does not snap, the post will
come out and you will find that the post hole is now much wider than the
post is (frictional wear). We can certainly discuss cantilevering 300
pound light fixtures over 60 foot drop-offs and discuss where the fulcrum
is. The situation is not that different.

> > All of the crown above the margin is your lever arm.
>
> With your design too...but all of that force is concentrated at one
> fulcrum point. Bad news!

If I have a 100 micron gap across the entire inner surface of the crown/core
combination restoration, and have it bonded to place, then I simply have to
design in resistance form so as to avoid shear forces to the bonded surface.

>> 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.
>
> True. The only reason your design seems to be working OK so far is
> because of that really thick amount of tooth structurethat you're leaving
> at the gingival third. However, the lack of ferrule creates a much larger
> force on the single fulcrum point. The thickness is overcoming the
> additional load. If there's abfraction, and you put a post in that tooth
> without a ferrule, that tooth will split down the root...almost
> guaranteed.

No what I am doing is reporducing what nature created in the first place.
You are placing a shovel blade over a wooden handle. (That sounds
disrespectful when I re-read it, and please don't take it that way, this
debate should be taken for what it truly is, two friends discussing a
conflicting topic).

>>>> 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.
>
> I thought we were talking about posts?

Not for my technique. Why do we need a post if we have 1/3 to 1/2 the
vertical height of the tooth "un-touched".??


>>>> 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?
>>
>
> A ferrule provides "encirclement" on a tooth. I don't see the analogy.

What I am *mostly* trying to get you to see [just so that we both are taking
the same thing :-) ], is that cutting away the height of contour and
destroying all that good tooth structure to create a ferrule does not make
sense--to me. Things (including teeth) break at their weakest point. The
weakest point--in the absence of fractures--is the narrowest point. Cutting
a ferrule on an endo-tooth creates residual dentin walls which rarely are
more than 1.0 mm thick. When you are lucky you have a circle of 1.0 mm
thick dentin surrounding your post/core matrix. Along some surfaces you
will usually have less than that. 1.0 mm of dentin will not last long when
used as a fulcrum. Dentin has lots of flexure to it. Your crown has none.
The root which is in bone has little ability to flex. Parafunctional forces
will try to bend the tooth half-way between the crest of the bone and apical
margin of the crown. If this region is thick, the tooth stays in place, and
you may see abfraction and gingival recession in time. If this region is
thin,,,,,,,, snap!

Going back to our shovel, the handles break when try to do too much with our
shovels. On the job site, we would sometimes use tham as pry-bars (since
the contractor did not always provide metal pry-bars). We would use the
shovel to lift peices of concrete, the lever forms into place for pole-light
foundations, to pry a 200 pound piece of pipe over an inch or so, etc. When
we did this enough times, the handle would develope a crack. Every crack
was in the same place. It would start just inside the ferrule, and extend
just outside the ferrule in a oblique direction to the opposite side of the
handle. The broken handle is the root and the metal balde is the crown.
Some handle would be left in the blade with virtually all of the cut ferrule
remaining inside the blade.

Encircling the root with a ferrule is what restorative dentists have taught
for 50 years. It has been passed on one generation to the next. No one has
stopped to say, "hey, is this right or not?" Encircling the tooth with the
ferrule simply aplifies the forces so that when it fails, it wil snap off.

Again, every tooth will not snap off. Most will survive very well. But, I
insist that those which survive never needed the post since there was so
much tooth structure remaining.

>>>> The presence of a post places the vector of forces at the tip of post
>>>> with lateral parafunctional movement.
>
> And at the coronal contact point, but on the *opposite* side of the tooth
> in an *opposite* direction. That's really, really bad for teeth...and
> really, really good at splitting them.

I think we may be seeing this point in agreement, but am not sure.

>> 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.
>
> If you compare a post that is 9 mm long (from the gingival crest to the
> tip) to a post that is 3 mm long, the 3 mm post is tranmitting 3 times as
> much stress on the root of the tooth at the tip than the 9 mm post.
> That's the definition of a lever arm. Let's say that each post has the
> same height above the gingiva and that height is 9mm. If you place a 500
> N force on the top of each post, then the force on the root with the 9mm
> post is 500N...and the force on the root with the 3 mm post is a whopping
> 1500N! I don't care how much thicker the root is at the coronal third vs.
> the apical third, the huge force from a short post more than negates the
> thickness advantage.

I think this is a question of whether or not there is movement of crown
relative to the tooth. No movement, it does not matter. The entire tooth
absorbs all of the shock equally as one peice. If movement is present, then
your equation comes into play.

As long as your crown/core/post complex has a different modulus of
elasticity than the tooth, then parafunctional forces will loosen it (if it
does not snap off). Assuming for this part of the discussion that the tooth
holds up and does not break off. Also, I want to restrict the discussion to
parafunctional forces as I think vertical forces do not cause much harm.
Lateral parafunctional forces delivered along an oblique vector will try to
bend the tooth at teh center of the most flexible region. If the crown/core
is significantly more resilient than the tooth structure, then the crown
will come loose or break the tooth at the bse of the ferrule. ONce the
crown/core is loose the post is free to abrade the post space or break the
root.


>>> 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.
>
> But it's a much smaller force.

With a ferrule, the side of the tooth opposite the force application will be
forced into compression at the base of the crown margin/ferrule. All of the
force will accumulate at this point. This is one of "your" fulcom points.
This compression will be in a pin-point or alond a narrow line-angle
depending on the curve of the margin. This can easily exceed the
compressive strength of the dentin with the force concentrated to this small
area. There will be light tension force on the opposite side away from the
margin, and huge compressive forces right at the fulcrum of the margin.

>
> I was just joking about those Cerecs. ;-)

Thanks for teasing me, that implies that I am your friend!

>> Remember, my point is that the cervical third of the tooth is strongest
>> part of the tooth, and should be maintained at all cost.
>
> Not at the expense of creating a much larger force due to design.

My typical Endo-Crown is strange looking from a dental school point of view.
It is shaped much like a mushroom. George W_B sat in with me on a couple of
these. What I do is remove all the existing filling material, reduce the
occlusal 1/3 of the tooth *flat* (I use a large disk shaped diamond). I
then flatten the inner walls of the pulp chamber (after trimming the gutta
percha down to the orifices of the root spaces). I try to get as parallel
as possible here. I lightly round the line-angles from the axial wall of
the pulp chamber to the flat surface of the reduced occlusal height. I am
done and ready to scan and mill. The final product looks like a mushroom
with an extension wherever a proximal bax was pre-exisiting. The occlusal
porcelain is 4.0 mm thick. The center stem is 8-12 mm thick. These things
are unbelievably strong. The actual tooth keeps its height of contour
intact. When viewed from the occlusal surface, the tooth is 4-6 mm thick on
all sides (except for where a proximal box was).

If you wish to share an email address, I will forward images. If you choose
not to share one, I don't blame you and understand.

>> The ferrule actuallly just keeps the post from loosening as quickly.
>
> No. It drastically changes the force vectors on the tooth.

I was taught the same thing. I no longer beleive it.

>>> 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.
>
> What did the I/O camera show?


With the I/O camera, I stop and study the inner surface more. Instead of
taking out the old filling, checking for caries, and shoving a core material
in right away and running fast to the prep, It slowed me down and made me
look closer. Fist off, I found a lot more fractures than I expected to
find. I found vertical fractures as well as horizontal ones. But, what I
found myself looking at was (in most cases) lots of nice thick tooth
structure that would have to be whacked off to make a conventional crown.
If I was replacing an existing crown, my eyes opened wider as I saw haw
little tooth structure remained with obvious strength. The camera allowed
me to see more detail, and forced me to look harder.


.

Relevant Pages

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  • 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. ...
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