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

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.

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

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!

Advanced-Standing Schools - Post #2

There are approximately 38 schools in the U.S. that are offering advanced-standing admissions for foreign-trained dentists who have attended or been graduated from a program not accredited by the ADA. Some of these schools admit only one or two to make up for extra space in the class. Some have large programs. Some have programs where the students are merged with the traditional students in lecture classes, preclinical labs and clinical instruction,, and some have independent programs with separate instructors and clinics. Some have practical admissions exams and some do not. Some have "review programs" for various periods of time before you are officially admitted, and some do not, or they are more unofficial.

We have assembled a spreadsheet for the schools offering advanced-standing admissions, and this includes all information we can get from the schools, from ADEA, from the ADA and from forums where discussions of these programs take place. There are currently 20 categories of information about each school. This can save you a tremendous amount of time as you plan your strategy for admission to these programs. Not all of the information can be current, because the schools change things frequently, and some of the information is just passed on by those who have been to the school. But it is a great starting point. If you would like a copy of this emailed to you, register with this blog.

Otherwise, if you have anything that you have heard or experienced that would be helpful to others reading this posting, please register and respond on this posting #2. If it is confirmed by enough people we will add it to the information in the master spreadsheet.

Monday, September 7, 2009

International Dentists in US - Orientation - Posting#1

International Dentists - Posting #1

This Blog Set is directed specifically to dentists who were graduated overseas and are looking for the opportunity to practice in the U.S. It is estimated that there are approximately 1000 dentists per year who fall into this category. Considering the range of experience and performance on standardized examinations, foreign graduates can hope to enter dental school in the U.S. during the third or junior year.

Admissions to U.S. dental schools with advanced-standing status is extremely competitive – there are approximately 400 places available each year. Performance on National Board Dental Examinations must for many schools be better than the national average of U.S. dental students, proficiency in English must be demonstrated, and for most of the positions a competitive practical examination is required.

At Duggan Dental Studies, our goal is to help in whatever way possible the foreign graduate to successfully navigate the challenges to dental school admissions. Our NBDE I program will be online in coming months – we already have 65 DVDs of recorded and edited material for this exam. Our Fundamentals of Restorative Techniques course has been widely popular to help with the practical examinations. We are about to expand our FRT program to another, 18-day offering, for those who can stay longer and further refine their skills and understanding. Also, international dentists who gain acceptance into an advanced-standing program must be aware that in most schools these are provisional acceptances, dependent upon performance in a “summer review” program. Once “accepted” it is very useful to work on advancing skill levels further, to guarantee excellent progress in these school reviews. If you have already taken one of our FRT programs, our 18-day clinical dentistry review is deal preparations for whatever your admitting school might throw at you – virtually guaranteeing the making of a very favorable impression during your first months.

Most of the foreign graduates that work with me at DDS have never done a crown on a patient before, or very few. Most have never been taught to do preparations with a strategy so that the least tooth structure is removed and the remaining structure is very refined in form. Most have never been taught the rationale for materials selection and for preparation decisions based on the nature of the problem and the characteristics of the available materials. Most have never been taught how to LOOK and what must be SEEN while doing a dental preparation. Most have never been taught how to use a handpiece and bur with the kind of precision that gives pleasure each and every day of your life in dental practice. We make up for all of this with our FRT program.

Posting #1 is for general questions about dentistry in the U.S., dental schools in the U.S., how dental faculty think, what the 4-year traditional students are like, what a teaching career is like, what dental practice is like, and anything else that comes to mind. Other posting threads will cover: #2. Schools with Advanced Standing Programs and their requirements; #3. Practical Examinations at various schools; #4. Detailed questions about class II amalgam preparations; #5. Detailed questions about full gold crown preparations; #6. Detailed questions about porcelain-fused-to-metal crown preparations; #7. Detailed questions about partial veneer crowns – including ¾ and 7/8 crowns; #8. Detailed questions about gold onlays; #9. Detailed questions about class II composite and class III composite restorations.

We will attempt to use these postings both as question-answer sessions with me providing the answers, and also where responders can contribute their own input – perhaps about what particular schools are looking for, what they gave on the last exam, etc. Postings #1, #2, and #3 will often be seen to work like forums.

All questions regarding preparations in the Postings #4 – 9 will be answered in words as completely and well as possible, but some mental image of the preparations will be very helpful. This can either be obtained from a standard textbook on the subject, or from our models with DVDs that are available. These are articulated models with a variety of preparations done in a manikin , using a mirror, and recorded with a loupe-mounted video camera so you can look in the mirror with me! We will also be posting pictures and short videos to further illustrate some of the points made.

We also have an online course which was recorded from a Fundamentals of Restorative Techniques program given a few months ago. All lectures and presentations are presented as well as demonstrations and evaluations and modifications of student work, done with the recording “point-of-view” camera, developed at DDS. We are planning that this online course will be ready for registration before the end of September.

At DDS and its precursor company, Duggan Study Institute, we have trained over 3500 international dentists for various examinations throughout the country. It is likely that no one in the world has worked on as many preparations with students as I have – and I would very much like to add you to the rolls of successful graduates from our programs. We proud of our record over the last 17 years and still get most of our students as referrals from those who have studied with us before.