Typodont Equilibration

For those who are taking Advanced Standing Entrance (practical) exams, during the on campus interviews, it is going to be a really good idea for you to be aware of what it takes to get a typodont in shape. That means that the proximal contacts are there and passive and the occlusion is reasonable and stable. I discussed in another post the difference between Columbia and Kilgore tpodonts, but didn't really get into how to make each of them good enough for an exam.

While the practical exams are mostly over for this year, the first one for 2010 will probably be in May - at least the Colorado exam was in early May this year, and this comes pretty fast. Those who didn't apply during this season or who are planning to apply next year, should start figuring out some of the issues right away - and one is control of the typodont.

In the next few months I will make a new videotape of typodont equilibration and put it on our DDSeClass.com website, so you can sign up and see how it is done. But there is no definite rule, except the same one that works in the patient's mouth - figure out what the problems are and then invent something that will fix them!

There is, however, a reasonably systematic approach to adjusting a typodont. I will go through some of it here and leave the rest of the details for the eClass. First, I will point out that the other day I equilibrated a student's typodont for the USC exam, and it was really a mess. The typodont was Columbia and only about 2-3 years old, but the teeth I have, which are newly manufactured, did not fit well at all. The problem is that when you put the teeth into the socket and look through the hole in the bottom of the typodont base, the screw hole in the tooth doesn't line up. In this instance the hole in the base must be enlarged so the screw will enter the tooth passively and not break the tooth.

Provided the holes all line up, we need first to tighten all of the screws on the typodont, so that if they are tightened later during equilibration, nothing changes. Also it will prevent rotation, which is a big problem on the anterior teeth. Then I will take out all of the second premolars and the maxillary lateral incisors. At this point I check to see how passive the contact is between the molars in all four quadrants. I also check to see how the contacts are anterior to the removed teeth. Also we need to know if there is room to put the second premolar back in the socket. Sometimes there is more than enough room, and sometimes, like the newer Columbia typodonts, there is not enough room. Then we have to decide which way to move the teeth anterior and posterior to the missing tooth so that there is just the right amount of space to return the premolar, and so that there are good contacts in the anterior and posterior regions, and they are passive contacts.

Sometimes a lot of tilting of the teeth is involved. Sometimes all of the proximal contacts in the anterior and posterior segments are fine, but there is not enough room to return the second premolar. In this case, the second premolar is usually recontoured to fit the space that is available.

I will describe here some techniques for tilting teeth, but I will save the occlusal adjustment for another posting.

If you wish to tilt a premolar mesially or distally, it is usually possible to cut the bottom of the tooth at an angle, so when the screw is tightened, the tooth will tilt accordingly. Sometimes the tooth won't tile because the head of the screw sits on the base in a bad way, so we recut the surface of the base where the head tightens at an angle. For molars it is generally best to take a small piece of aluminum foil (I generally use a strip from the edge of a scalpel blade package) and place it under the tooth, on one side or another. You can also use foil to raise a tooth into occlusion, or to raise AND tilt a tooth at the same time, by using different thicknesses of foil in two different locations. The scalpel foil can be folded a number of times to get different thicknesses. Sometimes I've had to use 5 layers on one side of a tooth and 7 layers on another to get it raised into occlusion, but without the facial cusp (say) too close to the opposing tooth surface.

When we get back into prime practical exam preparation "season" we'll be going over more of these things more carefully and I'll be answering more questions.

It is also relevant that typodont equilibration is important in some Regional Clinical Licensure Examinations, like the CRDTS, but a lot of people don't realize it due to the way the exam is graded.

Kilgore Typodonts

Just a few notes here with respect to the Kilgore versus the Columbia typodonts. Depending on which school you apply to, the practical exam at the interview could involve one or the other, and either could be on a manikin or hand-held. The USC exam that was just held yesterday involved hand-held typodonts - which is actually more complicated for some procedures and a lot less realistic, of course. But if you can do the preparations well in the manikin, you know exactly what to look for as you do the work hand-held - just practice a little.

BUT, the Kilgore and Columbia typodonts are significantly different in several important ways. The nature of the ideal preparations, of course, does not change, but the sensitivity of each typodont to any overextension or carelessness in margin placement is very different.

Mostly, we are looking at Kilgore typodonts for the Northern California schools, but there are others as well. Be sure to ask what typodont they are expecting you to use, and if you supply your own or not. Remember that ALL typodonts have problems with positioning of teeth so that there is good proximal contact, or at least some, and it is not ACTIVE, and that there is good occlusion, with solid vertical stops for posterior preparations.

But you must be aware that for the Kilgore typodont the proximal contacts are positioned much farther toward the facial than for Columbia, and the occlusogingival width of the contact is very small, typically, for the Kilgore. This means that the box for a class II is shifted to the facial with respect to the isthmus, and that very little gingival extension is required to get gingival clearance. In fact, if you are not careful about your pulpal depth, you may have gingival clearance before even making the box!

For the Kilgore, it is generally easier to cut a crown preparation through the proximal contact area without coming too near the adjacent tooth, because the gingival embrasures are so large, but the preparations tend to be shorter than for Columbia, because of the smaller clinical crown. This makes over reduction of the marginal ridges a more serious problem, due to the short resultant walls and limited retention.

For the Columbia, active proximal contacts is a continuing problem - necessitating adjustment of the bottom of the tooth at an angle, recutting the seat of the screw on the back of the base, or placing aluminum foil under one side or the other of the bottom of the tooth, mostly molars in this case. If the adjacent tooth shifts while cutting a class II box, it can cause real problems. To say nothing of the very large contacts occlusogingivally for Columbia, often requiring very deep boxes to get gingival clearance. The newer Columbia typodonts use a different plastic for the base than they used to - exchanging Melamine for, I think, Nylon. This new base does not lend itself well to the adjustments that are necessary to get teeth to fit with only passive proximal contact.

A few days ago I equilibrated an older typodont - only 2 years old - and had to replace most of the teeth with new ones. Normally this would cause difficulties, having to adjust ALL proximal contacts in a sequential fashion, but in this case it was doubly complex, because almost all of the teeth did not fit the screw holes in the base. That is, when you look at the bottom of the tooth through the screw hole in the base, the hole in the tooth is not centered, often points so that the screw will hit the side of the hole in the base (hence tipping the tooth), or the screw will not even go into the tooth. In this case I had to recut the holes in the base in all of these cases - if that were not done and the screw could be tightened into the tooth, the tooth would almost always break. Some of you might have had this problem before - but it is becoming more frequent with Columbia typodonts recently!

If you read this post and find the subject of typodont equilibration important for you now or in the future, with a little encouragement I will make a videotape of the procedure for a couple of typodonts so that you can see some of the techniques that are necessary. Realistically, most students that see how complex this can be, elect to have me do it for them if they are to take their own typodont to an exam. If you use a typodont supplied to you by the school, know at least enough to check it and see if it will make your life more complicated because they did not equilibrate it well enough!

Class II and Class III Composite - Posting #9

Some comments should well be made about composite preparations as they may be found on entrance exams - so far I only know of one that gives the class III and that is U Mich - see another posting. For the USC exam it could well be that the case descriptions that they give you would call for either a composite or amalgam class II filling, and you should know what the criteria are for either. But you should also know how to do them. There are some significant issues that one comes across with these preparations, and, it must be known, that not all examination entities agree on the ideal preparation designs.

For example, the class III composite preparation. Know, first of all, that if you are preparing a typodont tooth, you must have a clear picture of where the decay WOULD be. It's pretty much the same as for a class II, the decay is at the gingival portion of the contact area, but here we enter from a completely different direction. Now there is some question about how much of the contact needs to be involved. In reality, typically, very little of the contact is involved. The incisal margin is generally in contact with the adjacent tooth, but it is not placed too far into the contact - if it is IN contact, that is far enough incisally.

The facial extension is another issue - both for clinical exams and manikin exams. For the patients it is easy to see how far the facial margin will have to be extended, because the surface decalcification is generally found into the facial embrasure, at least far enough so that the facial clearance will allow a bevel to remove the last of it. For a typodont, various approaches may be mandated as to how far the facial margin is extended - there could be the directions "don't break facial contact" - which tells you exactly nothing. Generally, I council my students to extend the facial wall, angled outward, until there is just enough clearance facially so that a facial bevel can be placed all the way to the incisal wall. This most closely duplicates what we commonly find in patients.

Should there be a bevel on the class III margins? All the textbooks say yes. Regional clinical board exams, even for patient-based procedures, generally state that bevels are optional! What does this mean? It really means that you need to decide whether to place a bevel or not. I can picture situations where, just for retention, a bevel would be an absolute clinical requirement. And in all other cases I would still place a bevel for good marginal bonding, good esthetic gradient, and to allow a more conservative preparation that still removes all of the surface decalcification. But be careful about local beliefs here.

For the class II composite preparation it is clear that there should be a gingival bevel - and yet it is rarely done in practice. I sometimes think that the lack of requirements for a bevel on clinical board exams is more a reflection of what the examiners do in their practice - as opposed to what the schools teach. I admit, I have not been in any regional board calibration meetings for years, and never got the chance to ask this question, but that could be it. What will you choose to do in your practice? Personally, I won't place a class II composite when I do not have gingival enamel to bond to, and I will always take that extra 30 seconds to place the bevel - why not?

The other interesting thing about the class II composite preparation is the proximal wall exit angles. If they are flared - i.e. the angle of tooth structure is obtuse, the end of the enamel rods is exposed by the wall itself and no proximal wall bevel is necessary. On the other hand, a more conservative preparation can be made by underextending facially and lingually and then removing the last of the decalcification with the bevel. On the other hand, in the dental community, the burs that are most commonly used for class II preparations are too large to permit a conservative class II preparation anyway - whether amalgam or composite.

So we basically do the class II composite preparation just as we do the amalgam - the same technique exactly, but we flare the proximal walls a little, and bevel the gingival margin. The isthmus width can be smaller, the "axial depth" can be smaller, but otherwise it is the same in most respects.

I'd like to hear about what experiences you might have observing dentistry in the U.S. in the area of composite preparations. Many of my students are dental assistants, and they always have something interesting to say about how dentistry is practiced in their office. Generally, not how it is taught!

Univ of Michigan Entrance Exam

I recently heard from a student who came back from the Univ Michigan practical exam. They were pretty upset with the way the exam was conducted. I'll repeat some of what she said - but this is second hand and should be taken lightly until confirmed. She said that the typodonts were supported by a vertical rod, not a horizontal rod and ball joint as we usually have in manikins, AND that they weren't supplied with mirrors. If this is accurate, then I don't think I could do a decent job on any preparations, nor could anyone else. All of the preparations were on maxillary teeth, including an MOD amalgam preparation. AND they were required to do amalgam fillings. I didn't think that any school was requiring the filling - I have said that I miss teaching that part for these exams (I do it all the time for our clinical courses), because it is something that few people do well at all. On the other hand, most people never had time to even do the filling. Apparently the hand instruments that they supplied were dull.

Now, how much of this is literally true I don't know, but it certainly wouldn't seem to test a candidate's skills in clinical dentistry as well as I think they could have done.

If you are going to apply in 2010 to this school - better call ahead and see if the same thing will be done then. There are other details about the exam that were told to me, and if you are interested just post a comment and I'll fill you in.

Question Came Up - Post #10

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.

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.

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.