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.
Wednesday, September 23, 2009
Porcelain-fused-to-metal Crowns - Posting #6
Ceramometal crowns require careful attention to the desired form of the final crown, especially for the anterior teeth, and the thickness of porcelain required for adequate esthetics, especially for older patients, where the enamel is more transparent.
The key to the preparation is the facial reduction, which is frequently done incorrectly, so that the lab tech must either overcontour the facial surface of the crown, or make the porcelain thin so that it has a poor appearance.
The evaluation of the facial reduction requires that you check the axial reduction at the gingival margin, AND check the taper in relation to the adjacent teeth whose contours the crown will be designed to match.
In order to evaluate the facial axial reduction at the gingival margin, you can do it without caring about the margin form. For example, if you use a horizontal shoulder, an angled shoulder, or a shoulder-bevel (no longer desirable for anterior teeth in the esthetic zone), you can see the axial reduction (horizontal distance from the gingival margin to the axial wall) by looking gingivally down the facial wall and measuring the apparent width of the shoulder from this viewing direction. This viewing direction cancels out the effect of angulation so you get the actual axial reduction automatically. We'd like to get approximately a 1.0 mm axial reduction uniformly from the mesial to distal facial embrasures.
From the gingival shoulder to the incisal edge, we need to make certain that the angulation is correct on the facial surface. You can guarantee that this will be appropriate by comparing the facial taper to that of the middle third of another adjacent tooth, that the crown will match. This should be the angle of the facial reduction for the gingival 2/3 of the surface to the shoulder. The incisal 1/3 will typically have another angle, matching that of the incisal 1/3 of the comparison tooth - referred to as a secondary plane, recognizing that the curved surface of the facial contour will require two planes for uniform axial reduction from gingival to incisal.
The most common error in PFM preparations is to allow the shoulder to remain horizontal into the proximal embrasures, mesially and distally. The problem is that this margin will sometimes be in enamel, and if it is the enamel will be undermined due to the natural incisal divergence of the enamel contours at the margin. Often I have seen preparations done by students in patients that, after removal of the provisional crown, before cementation of the permanent restoration, the gingival enamel was found to be chipped and broken, leaving deficient margins in these areas. It is always best to angle the shoulder gingivally with the end of a shoulder diamond so that the EXTERNAL angles of porcelain AND the enamel are closer to 90 degrees!
The esthetics of shoulder-bevel margins are not acceptable for anterior teeth these days, because of inadequate gingival placement or thin tissue biotype to which dentists don't pay enough attention. For posterior PFMs out of the esthetic zone, the shoulder-bevel can be a nice approach for the facial reduction.
We can discuss other types of porcelain crowns, including full ceramic crowns and CAD/CAM crowns. There are advantages and disadvantages to each, and they can be subtle. How one does a FCC prep on a typodont can be a challenge and I can go into this in detail.
For comments or questions about any of these issues, I will love to have you reply to this posting and I'll answer or discuss anything that concerns you. Be sure to check out our programs at www.dentistCEprogram.com
The key to the preparation is the facial reduction, which is frequently done incorrectly, so that the lab tech must either overcontour the facial surface of the crown, or make the porcelain thin so that it has a poor appearance.
The evaluation of the facial reduction requires that you check the axial reduction at the gingival margin, AND check the taper in relation to the adjacent teeth whose contours the crown will be designed to match.
In order to evaluate the facial axial reduction at the gingival margin, you can do it without caring about the margin form. For example, if you use a horizontal shoulder, an angled shoulder, or a shoulder-bevel (no longer desirable for anterior teeth in the esthetic zone), you can see the axial reduction (horizontal distance from the gingival margin to the axial wall) by looking gingivally down the facial wall and measuring the apparent width of the shoulder from this viewing direction. This viewing direction cancels out the effect of angulation so you get the actual axial reduction automatically. We'd like to get approximately a 1.0 mm axial reduction uniformly from the mesial to distal facial embrasures.
From the gingival shoulder to the incisal edge, we need to make certain that the angulation is correct on the facial surface. You can guarantee that this will be appropriate by comparing the facial taper to that of the middle third of another adjacent tooth, that the crown will match. This should be the angle of the facial reduction for the gingival 2/3 of the surface to the shoulder. The incisal 1/3 will typically have another angle, matching that of the incisal 1/3 of the comparison tooth - referred to as a secondary plane, recognizing that the curved surface of the facial contour will require two planes for uniform axial reduction from gingival to incisal.
The most common error in PFM preparations is to allow the shoulder to remain horizontal into the proximal embrasures, mesially and distally. The problem is that this margin will sometimes be in enamel, and if it is the enamel will be undermined due to the natural incisal divergence of the enamel contours at the margin. Often I have seen preparations done by students in patients that, after removal of the provisional crown, before cementation of the permanent restoration, the gingival enamel was found to be chipped and broken, leaving deficient margins in these areas. It is always best to angle the shoulder gingivally with the end of a shoulder diamond so that the EXTERNAL angles of porcelain AND the enamel are closer to 90 degrees!
The esthetics of shoulder-bevel margins are not acceptable for anterior teeth these days, because of inadequate gingival placement or thin tissue biotype to which dentists don't pay enough attention. For posterior PFMs out of the esthetic zone, the shoulder-bevel can be a nice approach for the facial reduction.
We can discuss other types of porcelain crowns, including full ceramic crowns and CAD/CAM crowns. There are advantages and disadvantages to each, and they can be subtle. How one does a FCC prep on a typodont can be a challenge and I can go into this in detail.
For comments or questions about any of these issues, I will love to have you reply to this posting and I'll answer or discuss anything that concerns you. Be sure to check out our programs at www.dentistCEprogram.com
Tuesday, September 15, 2009
Full Gold Crowns - Posting #5
Full gold crown preparations are done to conserve as much tooth structure as possible, provide the best adaptation at the margins (second only to what is possible with exposed margins on inlays, onlays, etc), antagonize the pulp chamber to the minimal extent, and be the most kind to the opposing teeth, particularly when there is a loss of anterior guidance and parafunctional habits create posterior attrition in group function.
While many dentists are quite aggressive about occlusal reduction, for younger patients this can be a really bad idea with the proximity of the pulp and the gingival limitations on crown height and thus retention and resistance form. So the circumstances need to be studied with some care to see how conservative you really need to be, but, in general, a more strategic approach to the crown preparation will require little if any additional time, and yet allow total control of the reduction and/or clearance of the occlusal reduction.
What is the difference between occlusal reduction and clearance? Most of the time there is no difference. If the particular part of the occlusal surface is originally in occlusion, probably it will be waxed into occlusion, and therefore the amount of reduction is geometrically identical to the amount of clearance between the preparation and the opposing surface. When the original tooth is NOT in occlusion and we want it to be, then we will reduce only to the extent that the clearance is adequate to build the occlusal surface of the gold crown in that area - this may be 1.0 mm or 1.5 mm. On the other hand, if the original tooth surface at some point is not in occlusion and we don't want it to be, we simply reduce it the 1.0 or 1.5 mm to get the thickness of the gold right, and we don't care how much clearance there is at the end. If we stop reducing, say, the ML cusp on a mandibular first molar when there is only 1 mm clearance, it will have little reduction and the cusp will be much higher than original and the patient will feel that it is a very sharp area on the crown.
As far as axial reduction is concerned - the major issues involve appropriate wall form, generally flat until sloping gingivally at the gingival .5 mm or so, axial reduction at the gingival margin approximating .5 mm and appropriate taper and reduction to construct the proximal contacts and F/L surfaces with natural contours. Generally, we can select a particular diamond bur that will accomplish the axial reduction with minimal problems if the tip of the bur is kept at the margin, if the gingival position of the margin is appropriate, and the taper is 10-12 degrees. All three of these criteria can be controlled as long as the operator is looking in the correct direction relative to the surface being prepared.
If you would like to ask some questions about the issues mentioned here, or would like to contribute, or are uncertain about anything, feel free to answer this post - we can get a lively discussion going.
While many dentists are quite aggressive about occlusal reduction, for younger patients this can be a really bad idea with the proximity of the pulp and the gingival limitations on crown height and thus retention and resistance form. So the circumstances need to be studied with some care to see how conservative you really need to be, but, in general, a more strategic approach to the crown preparation will require little if any additional time, and yet allow total control of the reduction and/or clearance of the occlusal reduction.
What is the difference between occlusal reduction and clearance? Most of the time there is no difference. If the particular part of the occlusal surface is originally in occlusion, probably it will be waxed into occlusion, and therefore the amount of reduction is geometrically identical to the amount of clearance between the preparation and the opposing surface. When the original tooth is NOT in occlusion and we want it to be, then we will reduce only to the extent that the clearance is adequate to build the occlusal surface of the gold crown in that area - this may be 1.0 mm or 1.5 mm. On the other hand, if the original tooth surface at some point is not in occlusion and we don't want it to be, we simply reduce it the 1.0 or 1.5 mm to get the thickness of the gold right, and we don't care how much clearance there is at the end. If we stop reducing, say, the ML cusp on a mandibular first molar when there is only 1 mm clearance, it will have little reduction and the cusp will be much higher than original and the patient will feel that it is a very sharp area on the crown.
As far as axial reduction is concerned - the major issues involve appropriate wall form, generally flat until sloping gingivally at the gingival .5 mm or so, axial reduction at the gingival margin approximating .5 mm and appropriate taper and reduction to construct the proximal contacts and F/L surfaces with natural contours. Generally, we can select a particular diamond bur that will accomplish the axial reduction with minimal problems if the tip of the bur is kept at the margin, if the gingival position of the margin is appropriate, and the taper is 10-12 degrees. All three of these criteria can be controlled as long as the operator is looking in the correct direction relative to the surface being prepared.
If you would like to ask some questions about the issues mentioned here, or would like to contribute, or are uncertain about anything, feel free to answer this post - we can get a lively discussion going.
Thursday, September 10, 2009
Class II Amalgam Preps - Posting #4
The class II amalgam preparation design is based on the need to remove caries and the need to take into account the physical characteristics of the amalgam material, which should not have an angular bulk at any margin less than 90 degrees. In some areas of certain teeth it may be difficult to avoid an acute angle of amalgam, for example the facial occlusal proximal margin when the contact area is shifted unusually much to the buccal. This also makes it difficult to CARVE the amalgam in that area because the buccal margin is so much higher than the lingual margin in the area of the marginal ridge.
But, generally, we can always get close to a 90 degree angle of amalgam. Remember the material is very hard, but brittle. I had a student in a clinical course back in 1993 who had taken the Cal Board and done an amalgam prep on her daughter - a lower second premolar. She had neglected to extend into the lingual groove for the occlusal outline and was failed because of that. When she brought her daughter into the course it was interesting to see that the amalgam that had been done during the exam had fractured completely through in the isthmus area where the wall was short due to the lack of extension into the lingual groove. We need to adhere pretty well to the principles design for this material in order to do a conservative preparation and restoration that will not compromise the strength of the filling itself.
Remember that the lifetime of an amalgam restoration with todays materials should be a minimum of 50 years, probably 60 or more when the preparaton is done with reasonable precision. There was a study done in Chicago 20 years ago where they recalled many patients who had been given amalgam restorations 50 years earlier and most of these restorations were still there and doing fine! And those were old type materials!
I'd like to hear some discussions under this posting about amalgam preparation design and carving, or anything else you may be confused about for class II restorations. For example, when and where do you think the "reverse curve" should be done?
DMD
But, generally, we can always get close to a 90 degree angle of amalgam. Remember the material is very hard, but brittle. I had a student in a clinical course back in 1993 who had taken the Cal Board and done an amalgam prep on her daughter - a lower second premolar. She had neglected to extend into the lingual groove for the occlusal outline and was failed because of that. When she brought her daughter into the course it was interesting to see that the amalgam that had been done during the exam had fractured completely through in the isthmus area where the wall was short due to the lack of extension into the lingual groove. We need to adhere pretty well to the principles design for this material in order to do a conservative preparation and restoration that will not compromise the strength of the filling itself.
Remember that the lifetime of an amalgam restoration with todays materials should be a minimum of 50 years, probably 60 or more when the preparaton is done with reasonable precision. There was a study done in Chicago 20 years ago where they recalled many patients who had been given amalgam restorations 50 years earlier and most of these restorations were still there and doing fine! And those were old type materials!
I'd like to hear some discussions under this posting about amalgam preparation design and carving, or anything else you may be confused about for class II restorations. For example, when and where do you think the "reverse curve" should be done?
DMD
Wednesday, September 9, 2009
Practical Exams - Post #3
At Duggan Dental Studies we are particularly interested in helping dentists gain the level of skill and understanding of dental preparations that is required to give a stand-out performance at any practical examination. Make no mistake about it, as opposed to board examinations, these dental school admissions exams are competitive, and hotly competitive. Within our field we are working toward the equivalent of a 400 meter race. How do you stack up against the competition? Are you doing what it takes to do better than most of the other applicants?
Who are the schools comparing you to? How will you look in comparison to the third year traditional students? What does the average applicant know and what can they do? What will the exam be like? What preps do I need to know? These are all questions that we hear all the time and we can discuss most of these issues in this posting, but also this is a great place to share information you have gathered about various schools and what you observed there.
Feel free to provide any information you have or ask any questions you want. If I don't have the answer readily at my fingertips, I can probably find out, or another viewer may have the answer. Let's share whatever we know and I will make sure that the information gets passed on to the benefit of all.
Having said that these examinations are competitive, I will also say that any dedicated group of people working together can elevate their level of work considerably above the crowd - sometimes my students comprise a large proportion of an entering class at various schools because they tend to all know what they are doing and do well. There are always benefits to working together and not trying to "fly alone" - if you went through the interview process last year and "fell out of the sky", now may be the time to try something different!
Who are the schools comparing you to? How will you look in comparison to the third year traditional students? What does the average applicant know and what can they do? What will the exam be like? What preps do I need to know? These are all questions that we hear all the time and we can discuss most of these issues in this posting, but also this is a great place to share information you have gathered about various schools and what you observed there.
Feel free to provide any information you have or ask any questions you want. If I don't have the answer readily at my fingertips, I can probably find out, or another viewer may have the answer. Let's share whatever we know and I will make sure that the information gets passed on to the benefit of all.
Having said that these examinations are competitive, I will also say that any dedicated group of people working together can elevate their level of work considerably above the crowd - sometimes my students comprise a large proportion of an entering class at various schools because they tend to all know what they are doing and do well. There are always benefits to working together and not trying to "fly alone" - if you went through the interview process last year and "fell out of the sky", now may be the time to try something different!
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