Whenever you are preparing a cuspid, it is easy to lose track of what you have reduced, and the most common error is underreduction.
The problems are not total reduction, but more in the 2nd and 3rd planes of the labial reduction.
First, I would HIGHLY recommend cutting pilot grooves on all anterior preparations, but, if you insist on freehand, just be aware that the DISTAL 1/2 of the labial is the most commonly under-reduced (simply because we can see the mesial 1/2 better.
fmn
Sunday, May 31, 2009
Saturday, May 30, 2009
BB hint #3 Pre-prep "preparation" part B
Part B includes the path of insertion and removal or PI & R of the final crown.
Often if the tooth you are preparing for a crown has either had decay that has cavitated or collapsed mesially and distally, the net mesial/distal dimension of the tooth has actually decreased and to a small degree, the roots of the subject tooth to the adjacent teeth are closer together.
This affects the final "P,I and R" in the following ways:
1. the adjacent teeth tend to tilt toward the narrowing tooth
(photo)
2.the contacts now become more pointed for the new crown than ideal, due to the tilt of the adjacent teeth
(photo)
3. you can have a prep that "draws" with itself, but not necessarily drawing with the required P,I & R that the adjacent teeth present you with.
So what is the solution? I have to tell a story on one of my best and admired professors at USC, Dr Richard Kahn. I had come back as a part time faculty to teach in the operative department and was in one of my first days just following him around helping the students.
We were trying to help a student with a troublesome seating of a crown and as they struggled, I opened my big mouth and blurted out what would have been blasphemy while I was in school:
"why don't you just adjust the adjacent tooth"
I had looked at the BWX and noticed that the adjacent tooth, made by a dental student a few years earlier was excessively bulky and making it very difficult to seat the new crown due to concept #3 above. The crown seated on the die with no undercuts, but when placed on the "untouched model" we always had to do, it was hitting heavily on the distal where the bulky crown was. I noticed that the distal preparation was sort of deep and "tucked under" the old crown, thus not allowing the full seating simply due to the fact that the dimension of the occlusal access (photo) was narrower than the total length m/d of the prepared tooths outer dimensions. Thus a simple recontouring of the very bulky crown, while marking the contact point would allow full seating.
Dr Kahn looked up at me, smiled and gracefully said something like, "Dr Nelson, I knew I trained you well! I admired him for acknowledging to me and all the students there watching that he had gotten so narrowed down to the details of seating, he hadn't even looked at the adjacent teeth.
Another factor affecting the "assumptions" was that he knew the adjacent crown was made at the school, so his operational situation was an assumption it's contours were correct, or it wouldn't have been passed off, right?
In other words, he was acknowledging how in a school environment, it is easy to miss the "forest for the trees"
take care
fmn
Often if the tooth you are preparing for a crown has either had decay that has cavitated or collapsed mesially and distally, the net mesial/distal dimension of the tooth has actually decreased and to a small degree, the roots of the subject tooth to the adjacent teeth are closer together.
This affects the final "P,I and R" in the following ways:
1. the adjacent teeth tend to tilt toward the narrowing tooth
(photo)
2.the contacts now become more pointed for the new crown than ideal, due to the tilt of the adjacent teeth
(photo)
3. you can have a prep that "draws" with itself, but not necessarily drawing with the required P,I & R that the adjacent teeth present you with.
So what is the solution? I have to tell a story on one of my best and admired professors at USC, Dr Richard Kahn. I had come back as a part time faculty to teach in the operative department and was in one of my first days just following him around helping the students.
We were trying to help a student with a troublesome seating of a crown and as they struggled, I opened my big mouth and blurted out what would have been blasphemy while I was in school:
"why don't you just adjust the adjacent tooth"
I had looked at the BWX and noticed that the adjacent tooth, made by a dental student a few years earlier was excessively bulky and making it very difficult to seat the new crown due to concept #3 above. The crown seated on the die with no undercuts, but when placed on the "untouched model" we always had to do, it was hitting heavily on the distal where the bulky crown was. I noticed that the distal preparation was sort of deep and "tucked under" the old crown, thus not allowing the full seating simply due to the fact that the dimension of the occlusal access (photo) was narrower than the total length m/d of the prepared tooths outer dimensions. Thus a simple recontouring of the very bulky crown, while marking the contact point would allow full seating.
Dr Kahn looked up at me, smiled and gracefully said something like, "Dr Nelson, I knew I trained you well! I admired him for acknowledging to me and all the students there watching that he had gotten so narrowed down to the details of seating, he hadn't even looked at the adjacent teeth.
Another factor affecting the "assumptions" was that he knew the adjacent crown was made at the school, so his operational situation was an assumption it's contours were correct, or it wouldn't have been passed off, right?
In other words, he was acknowledging how in a school environment, it is easy to miss the "forest for the trees"
take care
fmn
#2 "Pre-crown prep" preparation! part A
It is a habit that we tend to all have to sometimes narrow our focus so much on our task, we don't see the "forest for the trees" or however that goes.
I was blissfully going along with my 2 hour crown preps (after all, if you do it fast, it has to be sloppy, right?) and my search for perfection when I had the opportunity to share a common lab tech with Michael Schuster back in 1984 or so. There was a dedicated lab tech named Pat Choate who had attended the lab training at the Pankey Institute and since I had just completed a week long course, I had to find a Pankey trained lab, so I looked up the list and was delighted to find one right there in my home town of Prescott Arizona.
I was over at his lab one day (in the days we used to do that!) and noticed that a couple of the cases he had were from the now famous Michael Schuster of the Scottsdale based "Center For Professional Development". I was a little surprised/ impressed that Pat was doing this work. As it turned out, Schuster's regular lab man was ill or temporarily out and he had called the Pankey institute and gotten Pat Choat's name. since we lived in Prescott and it was less than 100 miles from Scottsdale, I guess it was the closest one.
How does this apply to this post? Well, we were discussing the cases that Schuster sent, and I was very impressed that he practiced what he preached. they were mounted on a Denar DII semiadjustible articulator with facebow transfer and CR bite records just like we had just learned.
I was looking at the opposing teeth on a 4 unit single crowns on the bottom and noticed that the upper teeth looked strangely flat and appeared to be heavily ground off. I asked Pat why and he matter of fact explained to me, "those teeth were supererupted and Dr Schuster didn't want to duplicate that error in the new restorations.
so, lesson learned, Duh, I had never thought of pre-adjusting the opposing teeth and then take an impression of that "pre-equilibration that he was doing, so the simple "pre-restorative, pre-crown prep" steps and areas to look for became todays topic, or part A:
look at the opposing tooth for supereruption, obviously sharp, thin cusps, "plunger" cusps that will go deep into the lower occlusal anatomy
There are several good reasons to do this:
1. If the opposing tooth has been opposite a space you are now bridging, if there has been any significant length of time, depending on the age of the patient, density of their bone, etc, it will usually be both sticking down too far AND rotated around the palatal cusp if it is an upper molar, thus, creating an opposing that will:
a. not usually allow any, if much anatomy of the new crown
b. require more than is really needed of central pit reduction (thus closer to the pulp)
c. create marginal ridge discrepancies, adding to food impaction liklihood
d. and overall just make it hard to do!
2. If the opposing has been wearing due to the current tooth's filling eroding, the end result is a hidden supereruption where the "plunger cusp" (usually lingual upper, buccal lower) continues to grow into the deminishing filling and tooth structure of the current tooth. If this occurs:
a. many of the same problems occur as in #1 except to a lesser degree
b. often as the wear occurs, the height of the non-working (buccal upper, lingual lower) cusps is artificially higher than normal and it is easy to get complacent and reduce what you think is a lot, and you are just taking off the supererupted portion (when you place a pilot groove for instance) illustration to follow soon
I was blissfully going along with my 2 hour crown preps (after all, if you do it fast, it has to be sloppy, right?) and my search for perfection when I had the opportunity to share a common lab tech with Michael Schuster back in 1984 or so. There was a dedicated lab tech named Pat Choate who had attended the lab training at the Pankey Institute and since I had just completed a week long course, I had to find a Pankey trained lab, so I looked up the list and was delighted to find one right there in my home town of Prescott Arizona.
I was over at his lab one day (in the days we used to do that!) and noticed that a couple of the cases he had were from the now famous Michael Schuster of the Scottsdale based "Center For Professional Development". I was a little surprised/ impressed that Pat was doing this work. As it turned out, Schuster's regular lab man was ill or temporarily out and he had called the Pankey institute and gotten Pat Choat's name. since we lived in Prescott and it was less than 100 miles from Scottsdale, I guess it was the closest one.
How does this apply to this post? Well, we were discussing the cases that Schuster sent, and I was very impressed that he practiced what he preached. they were mounted on a Denar DII semiadjustible articulator with facebow transfer and CR bite records just like we had just learned.
I was looking at the opposing teeth on a 4 unit single crowns on the bottom and noticed that the upper teeth looked strangely flat and appeared to be heavily ground off. I asked Pat why and he matter of fact explained to me, "those teeth were supererupted and Dr Schuster didn't want to duplicate that error in the new restorations.
so, lesson learned, Duh, I had never thought of pre-adjusting the opposing teeth and then take an impression of that "pre-equilibration that he was doing, so the simple "pre-restorative, pre-crown prep" steps and areas to look for became todays topic, or part A:
look at the opposing tooth for supereruption, obviously sharp, thin cusps, "plunger" cusps that will go deep into the lower occlusal anatomy
There are several good reasons to do this:
1. If the opposing tooth has been opposite a space you are now bridging, if there has been any significant length of time, depending on the age of the patient, density of their bone, etc, it will usually be both sticking down too far AND rotated around the palatal cusp if it is an upper molar, thus, creating an opposing that will:
a. not usually allow any, if much anatomy of the new crown
b. require more than is really needed of central pit reduction (thus closer to the pulp)
c. create marginal ridge discrepancies, adding to food impaction liklihood
d. and overall just make it hard to do!
2. If the opposing has been wearing due to the current tooth's filling eroding, the end result is a hidden supereruption where the "plunger cusp" (usually lingual upper, buccal lower) continues to grow into the deminishing filling and tooth structure of the current tooth. If this occurs:
a. many of the same problems occur as in #1 except to a lesser degree
b. often as the wear occurs, the height of the non-working (buccal upper, lingual lower) cusps is artificially higher than normal and it is easy to get complacent and reduce what you think is a lot, and you are just taking off the supererupted portion (when you place a pilot groove for instance) illustration to follow soon
BB#1 Seating a Crown
One of the most important but often ignored skill is proper seating of a crown. In dental school, we had this old lady, I cannot remember her name, but she was one of those loyal workers who showed up every day at the clinic because she had a mission. I think she had been doing it for 50 years or so, but her job was to help us seat a crown.
I think I can recite what she said almost verbatim. At the time we were using Zinc phosphate cement that requires dissipation of the heat of mixing vs in one big blob, so she would carefully get the cement out, on a "chilled glass slab" and carefully divide it up into "nine exact proportions" and then proceded to show us how, for each of the 30 crowns I did, how to properly mix the cement, never deviating the order or tone of the words. We did all we could to trip her up by asking questions during the "presentation" and she deftly diverted our questions, and always got back right on topic.
"now first, use this pledget of cotton with hydrogen peroxide to thoroughly clean the tooth preparation. one coat, here is another and finally a third time to assure the cleanliness of the preparation "(and stop the bleeding in the sulcus from our "gingival curettage preparation of the day)
"Now, we will CAREFULLY incorporate each segment of the cement into the full liquid content like this, being careful to dissipate the heat of mixing all over the chilled glass slab we have here"
(as we are getting nervous it is taking so long, and muttering things like, just get it over, ma'am, etc) she points out to us that by doing what she is doing, she can safely and confidently be mixing the 8th and 9th segment of the divided powder knowing it will not prematurely set, since she had carefully dissipated all heat each and every time she picked up a new segment of powder to "incorporate" into the "entire mixed mass", etc
so, I got into practice and the doc I worked with immediately, of course, told me that was all hogwash and I could just use "Duralon" and besides it stuck to gold better and soothed the tooth rather than creating heat on setting.
then the glass ionomers were the newest and greatest invention (does anyone remember protech cem? I think I ended up replacing about 30 of those crowns before I wised up). After just one more crown that I cut off and replaced because the patients post operative sensitivity was so hard to fix, I finally figured it out (we as a profession) that I was drying the teeth too much and there needed to be moisture during setting, or it would always be sensitive.
A
After I figured that out, I no longer had problems with this new "miracle" cement that "leached fluoride" and strengthened the tooth, then they came out with the resin ionomers.
Maybe I should have stuck to that memorized technique of the old lady. I NEVER had post op sensitivity when we did that....
Anyway, here we are where Resin Ionomer is the standard and, often with all ceramic crowns, all resin cement where we (gasp!) place acid RIGHT ON THE TOOTH! no dycal! anyway, the principles now are pretty consistent.
We have the new "self etching" cements that have that same feature that if you get the tooth too desicated, the patient will predictably have post op sensitivity (probably due to hydrolic pressure created at the dentin tubule interface where it is chronically dehydrated and creates a negative pressure)
All I am sure of is that I do not want to get any of my vital crowns too dry and make sure there is a mist of moisture at seating to allow enough water for the glass ionomer setting reaction. I also use a self etching bond and primer (currently all in one by Kerr) to increase the strength of the cement.
More later, just setting up the topics now.
I think I can recite what she said almost verbatim. At the time we were using Zinc phosphate cement that requires dissipation of the heat of mixing vs in one big blob, so she would carefully get the cement out, on a "chilled glass slab" and carefully divide it up into "nine exact proportions" and then proceded to show us how, for each of the 30 crowns I did, how to properly mix the cement, never deviating the order or tone of the words. We did all we could to trip her up by asking questions during the "presentation" and she deftly diverted our questions, and always got back right on topic.
"now first, use this pledget of cotton with hydrogen peroxide to thoroughly clean the tooth preparation. one coat, here is another and finally a third time to assure the cleanliness of the preparation "(and stop the bleeding in the sulcus from our "gingival curettage preparation of the day)
"Now, we will CAREFULLY incorporate each segment of the cement into the full liquid content like this, being careful to dissipate the heat of mixing all over the chilled glass slab we have here"
(as we are getting nervous it is taking so long, and muttering things like, just get it over, ma'am, etc) she points out to us that by doing what she is doing, she can safely and confidently be mixing the 8th and 9th segment of the divided powder knowing it will not prematurely set, since she had carefully dissipated all heat each and every time she picked up a new segment of powder to "incorporate" into the "entire mixed mass", etc
so, I got into practice and the doc I worked with immediately, of course, told me that was all hogwash and I could just use "Duralon" and besides it stuck to gold better and soothed the tooth rather than creating heat on setting.
then the glass ionomers were the newest and greatest invention (does anyone remember protech cem? I think I ended up replacing about 30 of those crowns before I wised up). After just one more crown that I cut off and replaced because the patients post operative sensitivity was so hard to fix, I finally figured it out (we as a profession) that I was drying the teeth too much and there needed to be moisture during setting, or it would always be sensitive.
A
After I figured that out, I no longer had problems with this new "miracle" cement that "leached fluoride" and strengthened the tooth, then they came out with the resin ionomers.
Maybe I should have stuck to that memorized technique of the old lady. I NEVER had post op sensitivity when we did that....
Anyway, here we are where Resin Ionomer is the standard and, often with all ceramic crowns, all resin cement where we (gasp!) place acid RIGHT ON THE TOOTH! no dycal! anyway, the principles now are pretty consistent.
We have the new "self etching" cements that have that same feature that if you get the tooth too desicated, the patient will predictably have post op sensitivity (probably due to hydrolic pressure created at the dentin tubule interface where it is chronically dehydrated and creates a negative pressure)
All I am sure of is that I do not want to get any of my vital crowns too dry and make sure there is a mist of moisture at seating to allow enough water for the glass ionomer setting reaction. I also use a self etching bond and primer (currently all in one by Kerr) to increase the strength of the cement.
More later, just setting up the topics now.
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