A student was asking about choices for bridge designs for replacing tooth #3. Of course, the best choice will depend on many things, and one can't make a blanket statement. But the retainer preparations can be chosen between PFM, 3/4 crown and full gold crown.
If the facial surfaces of the #2 and #4 retainers are both poor and should be covered, then we will certainly use a PFM on #4 and maybe a FGC on #2, if the esthetics are OK. Some people have very wide smiles and the facial surface of #2 can be seen conversationally.
If the facial surface of #4 is fine and the extension in the MB area can be minimal and the facial cusp ridge is sufficiently far from the opposing tooth, then a 3/4 crown would be a conservative choice for that - but it depends on the mesial and distal tooth structure remaining as well. If there is a prior restoration which is a MOD, whether amalgam or composite, if it is not too undercut faciolingually and if the lingual walls of the boxes will have sufficient tooth structure backing them up (lingually) so that the tooth itself is not likely to break in these areas, then a 3/4 crown with box retention can be done.
If the facial surface of #2 is nice, and the esthetics of the situation demand that the facial surface be natural color, then a 3/4 crown might be done, with the same provisos as for the previous paragraph.
Provided that the requirements for 3/4 crown restorations can be met for BOTH #4 and #2, then a FPD can be done with two 3/4 crown retainers.
Since many dentists don't feel confident about doing 3/4 crowns, and the oral situation may mitigate against them anyway, the PFM is a likely choice for #4 and, depending on the esthetics, a FGC or PFM is often done on #2. Nonetheless, for most of us dentists, we would prefer for ourselves, to have the 3/4 crown retainer preparations.
In my mouth I have a 3/4 crown on #14 that had to extend mesiobucally beyond what would be hidden behind the contour of #13, so my dentist backfilled to a more conservative margin placement with composite after the gold crown was cemented. If you know what you are doing there are many options.
Thursday, October 15, 2009
Friday, October 9, 2009
Gold Onlays - Posting #8
Color me old fashioned, but there are times where the choice between a PFM and an onlay preparation in a patient's mouth is difficult. Into this mix we have to add the Cerec/full ceramic crown also. Speaking as a dentist to a dentist, when you walk into your dentist's office with a broken cusp, are you thinking gold onlay, ceramic onlay or full ceramic crown? Naturally, the most conservative possible, with the best margins and that people don't see a lot of gold in your mouth?
I, myself, have lots of crowns - several full and partial gold veneers, several PFMs and a Cerec molar onlay. I frequently ask my students after they have studied with me for a week or two how many crowns I have and how many lower crowns do I have with occlusal gold. They never have any idea, because, conversationally, it really doesn't show for me. I guess I don't yawn in public or scream a lot either. But for many people it really doesn't make any difference whether they have gold on the occlusal or not, especially in the upper arch. But often, as dentists, we make patients think it will make a difference. Many dentists do this because they aren't confident in being able to do conservative gold preparations, like the onlay, or because they have a Cerec unit and the monthly "nut" must be covered.
But, for whatever the reason, the gold onlay can be a very useful preparation and most dentists these days don't really understand it. My dental practice was in an area where there were many older patients, so most of the crowns I did were gold, because, even if the facial surface were prepared, it didn't make much difference to them. I could always restore the tooth with an absolute minimum of removed tooth structure, generally using just what was already missing for retention of the occlusal restoration, replacing one or more cusps. The principles that we learn from 3/4 crowns, inlays and onlays will allow design of preparations that are very conservative, even though they may not have names! I used to have a die from a patient case that I passed around in my classes and defied students to figure out what kind of preparation it was. It is lost now, but as I recall it replaced only the ML cusp on a maxillary molar, had both a box and groove for retention and axial reduction only on part of the lingual surface with a chamfer margin. Now, realistically, one can do the same restoration with adhesive technology - using porcelain - but the reduction would have to be much more. Don't forget that adhesive technology can be used with GOLD as well, but it is of little advantage since we can get good retention generally for this material with less reduction.
The other misconception about gold onlays is that the preparations are more difficult than for the PFM. I have done very few gold onlays where I had to make the boxes for retention in areas where there were NOT previous restorations. In other words, almost always, there is a restoration with boxes already on the tooth and these areas can be readily used, in most circumstances, for retention of the gold restoration. You need but to make the boxes DRAW, put on some bevels and a couple other refinements, and you are ready to take the impression - AND the impression is much easier than for the PFM!
So, at the very least, we owe it to our patients to know this restoration - and occasionally it will make our lives much easier too. But, of course, to be fair, for a badly broken down tooth, we may not have enough tooth structure left for an onlay or even a traditional crown of any sort, without a major buildup first. These days, the buildup/crown combination is rarely used, since we can often prepare the remaining tooth structure for an adhesive porcelain restoration without removing much more structure. So, realistically, as usual, there are occasions where either type of restoration may be the most appropriate - don't be one sided!
By the way - another important issue that we should cover in more detail is porcelain abrasivity. I have studied this in the lab extensively, and don't miss a chance to study it in patient's mouth also. When a traditional PFM with occlusal porcelain is in the mouth for some time, a couple of years or even less, you can feel that the surface is quite rough. There are reasons for this which I won't go into here unless I get a specific question about it. I have a Cerec crown on #30 that was placed about 9 months ago, and I just checked it with an explorer and it is much smoother in the grooves than at the cusp tips, where it wears against the opposing teeth. It should be theoretically possible for the Cerec crown procelain to be less abrasive, but I'm pessimistic in practice. The abrasivity of porcelain crowns in certain patients can be a major factor in ruling them out.
For the MOD gold onlay, where there are boxes on both the mesial and distal sides, there are pretty well circumscribed preparations designs that work quite well. The most important aspect is the depth of the boxes gingivally, because this is where the retention comes from, and also, depending on the gingival recession, we might not be able to get a good impression of the gingival margin if there is not enough gingival clearance to allow passage of the set impression material. As indicated before, often the boxes are already there, but they need to be made to diverge occlusally in the faciolingual direction and converge axially, in order to have draw, AND we may need to deepen them gingivally.
When working with a tooth with a prior amalgam restoration, or even MOD composite, it is often found that this preparation was done with a great deal of faciolingual convergence. Maybe this was done to remove decalcification at the F and L proximal margins near the gingival floor, but more often it is done out of careless on the part of the operator, or because they were taught that way. It only takes a degree or two of convergence to retain amalgam, not the 30 degrees that I often see, when it is absolutely unnecessary. Also, with this much convergence, in order to convert the direct restoration prep into an indirect prep we have to make it DIVERGE, and this will increase the faciolingual size of the box, often too much to be practical. In this case we might have no choice but for full coverage or extracoronal retention.
Again - I encourage you to read through these postings and be stimulated by the topics I have presented and bring up questions that occur to you. In this blog I am only dealing with those issues that relate to preparations that may show up on advanced standing admissions practical exams - but there are many dental topics that are widely misunderstood and would be fun to discuss as well. If you are interested in the general subject of cast gold restorations, I will refer you to the best place for this - and it is not dental schools any more. Try the Richard V. Tucker Cast Gold Study clubs. They have a website now, and many study clubs throughout the world.
I, myself, have lots of crowns - several full and partial gold veneers, several PFMs and a Cerec molar onlay. I frequently ask my students after they have studied with me for a week or two how many crowns I have and how many lower crowns do I have with occlusal gold. They never have any idea, because, conversationally, it really doesn't show for me. I guess I don't yawn in public or scream a lot either. But for many people it really doesn't make any difference whether they have gold on the occlusal or not, especially in the upper arch. But often, as dentists, we make patients think it will make a difference. Many dentists do this because they aren't confident in being able to do conservative gold preparations, like the onlay, or because they have a Cerec unit and the monthly "nut" must be covered.
But, for whatever the reason, the gold onlay can be a very useful preparation and most dentists these days don't really understand it. My dental practice was in an area where there were many older patients, so most of the crowns I did were gold, because, even if the facial surface were prepared, it didn't make much difference to them. I could always restore the tooth with an absolute minimum of removed tooth structure, generally using just what was already missing for retention of the occlusal restoration, replacing one or more cusps. The principles that we learn from 3/4 crowns, inlays and onlays will allow design of preparations that are very conservative, even though they may not have names! I used to have a die from a patient case that I passed around in my classes and defied students to figure out what kind of preparation it was. It is lost now, but as I recall it replaced only the ML cusp on a maxillary molar, had both a box and groove for retention and axial reduction only on part of the lingual surface with a chamfer margin. Now, realistically, one can do the same restoration with adhesive technology - using porcelain - but the reduction would have to be much more. Don't forget that adhesive technology can be used with GOLD as well, but it is of little advantage since we can get good retention generally for this material with less reduction.
The other misconception about gold onlays is that the preparations are more difficult than for the PFM. I have done very few gold onlays where I had to make the boxes for retention in areas where there were NOT previous restorations. In other words, almost always, there is a restoration with boxes already on the tooth and these areas can be readily used, in most circumstances, for retention of the gold restoration. You need but to make the boxes DRAW, put on some bevels and a couple other refinements, and you are ready to take the impression - AND the impression is much easier than for the PFM!
So, at the very least, we owe it to our patients to know this restoration - and occasionally it will make our lives much easier too. But, of course, to be fair, for a badly broken down tooth, we may not have enough tooth structure left for an onlay or even a traditional crown of any sort, without a major buildup first. These days, the buildup/crown combination is rarely used, since we can often prepare the remaining tooth structure for an adhesive porcelain restoration without removing much more structure. So, realistically, as usual, there are occasions where either type of restoration may be the most appropriate - don't be one sided!
By the way - another important issue that we should cover in more detail is porcelain abrasivity. I have studied this in the lab extensively, and don't miss a chance to study it in patient's mouth also. When a traditional PFM with occlusal porcelain is in the mouth for some time, a couple of years or even less, you can feel that the surface is quite rough. There are reasons for this which I won't go into here unless I get a specific question about it. I have a Cerec crown on #30 that was placed about 9 months ago, and I just checked it with an explorer and it is much smoother in the grooves than at the cusp tips, where it wears against the opposing teeth. It should be theoretically possible for the Cerec crown procelain to be less abrasive, but I'm pessimistic in practice. The abrasivity of porcelain crowns in certain patients can be a major factor in ruling them out.
For the MOD gold onlay, where there are boxes on both the mesial and distal sides, there are pretty well circumscribed preparations designs that work quite well. The most important aspect is the depth of the boxes gingivally, because this is where the retention comes from, and also, depending on the gingival recession, we might not be able to get a good impression of the gingival margin if there is not enough gingival clearance to allow passage of the set impression material. As indicated before, often the boxes are already there, but they need to be made to diverge occlusally in the faciolingual direction and converge axially, in order to have draw, AND we may need to deepen them gingivally.
When working with a tooth with a prior amalgam restoration, or even MOD composite, it is often found that this preparation was done with a great deal of faciolingual convergence. Maybe this was done to remove decalcification at the F and L proximal margins near the gingival floor, but more often it is done out of careless on the part of the operator, or because they were taught that way. It only takes a degree or two of convergence to retain amalgam, not the 30 degrees that I often see, when it is absolutely unnecessary. Also, with this much convergence, in order to convert the direct restoration prep into an indirect prep we have to make it DIVERGE, and this will increase the faciolingual size of the box, often too much to be practical. In this case we might have no choice but for full coverage or extracoronal retention.
Again - I encourage you to read through these postings and be stimulated by the topics I have presented and bring up questions that occur to you. In this blog I am only dealing with those issues that relate to preparations that may show up on advanced standing admissions practical exams - but there are many dental topics that are widely misunderstood and would be fun to discuss as well. If you are interested in the general subject of cast gold restorations, I will refer you to the best place for this - and it is not dental schools any more. Try the Richard V. Tucker Cast Gold Study clubs. They have a website now, and many study clubs throughout the world.
Friday, October 2, 2009
Partial Veneer Crowns - Posting #7
One can expect partial veneer crown preparations on some school practical examinations - notably USC, for their case-based exam it is sometimes most appropriate to do this type of crown preparation.
There are many times in dental practice that, even if you start off thinking that you are going to do a full gold crown on a patient, you end up doing a partial veneer crown, for example a 3/4 crown, without even knowing it!
For example - and I use this example in my classes all the time - you are doing a FGC on #18 and it is the most distal tooth in the arch, and the gingival level is very high on the distal side. The patient may be in their 20s and they have had very little attachment loss and the tissue is high on that side. Now, you proceed as normal to do the occlusal reduction for the FGC and the occlusal surface of the preparation is, on the distal side, already at the gingival level, and you haven't done the axial reduction on that side of the tooth! So you have no retentive wall on the distal. Presuming that you have three retentive axial walls out of a possible four, you are doing a 3/4 crown, like it or not. Now, how do we get some resistance form for mesial tipping? Place grooves on the buccal and lingual. Then make sure that the margins are smooth and continuous gingival to the grooves, and up onto the distal margin which is just outside the marginal ridge, insure that the gold at the distal margin will not be too thin, and you have a 90 degree rotated 3/4 crown!
The main thing about partial veneer crowns, including the 3/4 and the 7/8 is that they allow, many times, a much more conservative preparation than would be necessary for the PFM. The PFM will allow retention, in principle, more easily, since we are preparing the entire facial surface and extending deep subgingivally - but often we should be thinking about the wisdom of this when a lot of the lingual part of the tooth is broken off - does it make sense to cut off most of the buccal part of the tooth?
In general, we need to keep in mind that a 3/4 or 7/8 crown can be done, and many times can be done more easily than a PFM or full ceramic crown, and certainly more conservatively, and with less hazard to the opposing teeth and with much better margins. Then, for an individual patient, we can decide what makes the most sense, esthetically and otherwise. If all you know how to do is a PFM your practice will be limited, to be sure. We have a variety of model typodonts that are available, and one does have a 7/8 crown on a maxillary premolar! Even this can be the most appropriate restoration in certain circumstances.
If you have any questions about how grooves and flares are made, or whether the standard textbook groove design is really necessary for retention, or if other, easier methods may work, just post a comment on this section and I'll be happy to discuss these things.
We are close to having our Fundamental Restorative Techniques course on-line! This will enable you to see exactly how many of the restorative preparations are done and hear a lot of discussion of the use of different designs.
There are many times in dental practice that, even if you start off thinking that you are going to do a full gold crown on a patient, you end up doing a partial veneer crown, for example a 3/4 crown, without even knowing it!
For example - and I use this example in my classes all the time - you are doing a FGC on #18 and it is the most distal tooth in the arch, and the gingival level is very high on the distal side. The patient may be in their 20s and they have had very little attachment loss and the tissue is high on that side. Now, you proceed as normal to do the occlusal reduction for the FGC and the occlusal surface of the preparation is, on the distal side, already at the gingival level, and you haven't done the axial reduction on that side of the tooth! So you have no retentive wall on the distal. Presuming that you have three retentive axial walls out of a possible four, you are doing a 3/4 crown, like it or not. Now, how do we get some resistance form for mesial tipping? Place grooves on the buccal and lingual. Then make sure that the margins are smooth and continuous gingival to the grooves, and up onto the distal margin which is just outside the marginal ridge, insure that the gold at the distal margin will not be too thin, and you have a 90 degree rotated 3/4 crown!
The main thing about partial veneer crowns, including the 3/4 and the 7/8 is that they allow, many times, a much more conservative preparation than would be necessary for the PFM. The PFM will allow retention, in principle, more easily, since we are preparing the entire facial surface and extending deep subgingivally - but often we should be thinking about the wisdom of this when a lot of the lingual part of the tooth is broken off - does it make sense to cut off most of the buccal part of the tooth?
In general, we need to keep in mind that a 3/4 or 7/8 crown can be done, and many times can be done more easily than a PFM or full ceramic crown, and certainly more conservatively, and with less hazard to the opposing teeth and with much better margins. Then, for an individual patient, we can decide what makes the most sense, esthetically and otherwise. If all you know how to do is a PFM your practice will be limited, to be sure. We have a variety of model typodonts that are available, and one does have a 7/8 crown on a maxillary premolar! Even this can be the most appropriate restoration in certain circumstances.
If you have any questions about how grooves and flares are made, or whether the standard textbook groove design is really necessary for retention, or if other, easier methods may work, just post a comment on this section and I'll be happy to discuss these things.
We are close to having our Fundamental Restorative Techniques course on-line! This will enable you to see exactly how many of the restorative preparations are done and hear a lot of discussion of the use of different designs.
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