How does one find the long axis of a tooth? (under a preparation for a crown)


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When doing preparation on typodont teeth for a ceramic crown, inlay, abutment preparation etc., how does on get the walls of the preparation to be parallell or convergent with the long axis of the tooth? It seems like I always have a problem with finding the long axis, whether it is in the mandible (where the curve of wilson is confusing me), or in the maxilla where my preparation walls (mesio-distal) tend to be leaning to towards one quadrant. I typically place my bur perpendicular at the midpoint of the occlusal surface and define this as my long axis of the tooth

If we for instance consider the incisors;
I always end up with either by buccal or lingual walls being convergent and the other being either straight (prone to undercuts), or divergent (this depends on the quadrant since I'm right-handed)
I think this mistake is due to me not angling the bur towards the long axis of the tooth. When I am working I feel like my bur is relatively straight, but I always end up with one of my walls being not to convergent.


My second question is regarding the inter-proximal reduction of tooth structure;
how does one clear tooth structure inter proximally without touching the adjacent teeth and at the same time leave sound tooth structure behind during a preparation?
I always end up with walls high in convergence angle (i.e. not convergent) and also a rough finish line.


I will be happy if anyone has tips on angling the bur/holding the dental handpiece towards the long axis of the tooth; finding the correct path of insertion when doing a prep for a crown/abutment/bridge
(Note that I'm assuming that the ideal path of insertion for a crown is almost parallel with the long axis of the teeth due to the forces applied to tooth structure - tell me if i'm wrong)
 
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honestdoc

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On a typodont, all the teeth are aligned ideally like a text book. In the mouth, very rarely you will find textbook anything. There will be tipped, rotated, misaligned teeth, broken down teeth, etc. Considering prepping teeth for a crown, there is no such thing as parallel walls, only tapered (convergent) walls ideally of 6 degrees. I prep down the occlusal ~1.5 to 2 mm first since it is the easiest. Then I prep the buccal about 6 degrees to 1.5 mm, then lingual or palatal. After most of the tooth is prepped, I reduce what is left of the interproximals (use a thinner diamond bur and leave a thin shell next to the adjacent teeth) and smooth all the margins (finish line) so the impression material will capture it crystal clear. When you smooth the finish line, the thin shell will be safely removed. I spend the most time achieving the smoothest possible finish line so the crown will predictably seat and seal.

Keep things simple. Worrying about long axis of the tooth is pointless when you need 6 degree taper on all walls. Keep in mind the typodont is 100% easier than worrying about your patient's wide tongue, gagging, water and fluid mess, lack of visual and access, patient's TMJ pain, finishing under any deep restorations, and difficult anesthesia. There are plenty of sources and illustrations on crown preps that may help you visualize.

You know you have a great crown prep by studying your temporary crown fabrication. Make sure there are no thin spots on the temp crown and the retention is excellent.
 
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On a typodont, all the teeth are aligned ideally like a text book. In the mouth, very rarely you will find textbook anything. There will be tipped, rotated, misaligned teeth, broken down teeth, etc. Considering prepping teeth for a crown, there is no such thing as parallel walls, only tapered (convergent) walls ideally of 6 degrees. I prep down the occlusal ~1.5 to 2 mm first since it is the easiest. Then I prep the buccal about 6 degrees to 1.5 mm, then lingual or palatal. After most of the tooth is prepped, I reduce what is left of the interproximals (use a thinner diamond bur and leave a thin shell next to the adjacent teeth) and smooth all the margins (finish line) so the impression material will capture it crystal clear. When you smooth the finish line, the thin shell will be safely removed. I spend the most time achieving the smoothest possible finish line so the crown will predictably seat and seal.

Keep things simple. Worrying about long axis of the tooth is pointless when you need 6 degree taper on all walls. Keep in mind the typodont is 100% easier than worrying about your patient's wide tongue, gagging, water and fluid mess, lack of visual and access, patient's TMJ pain, finishing under any deep restorations, and difficult anesthesia. There are plenty of sources and illustrations on crown preps that may help you visualize.

You know you have a great crown prep by studying your temporary crown fabrication. Make sure there are no thin spots on the temp crown and the retention is excellent.
Let me clarify; I meant convergent walls (not parallell walls).

When you need 6 degree taper walls, how does one align the bur to create such convergent walls? I find it hard creating the walls that will provide me with the greatest retention and resistance form. My professor said to me that I initially need to place my bur more parallell with the long axis of the tooth on typodont teeth, then proceed to tilt the bur a little bit buccal/palatal to make it a bit more convergent. In spite of this, I always end up with excessive taper when finished. Could this be because I always tend to look at my bur from a top down view? I do this because I want to make sure that my bur is somewhat perpendicular to the occlusal plane. I prep a little, then stop, check my prep, then continue and repeat this step when doing the axial walls; is this a bad strategy?

Furthermore, for instance, when doing a metal-ceramic crown prep with a chamfer margin on the buccal, I always tend to get undercut on bucco-palatal side. Do you have any pointes on how to avoid this whilst having a decent anatomical form? Also when doing a knife-edge margarin, I sometimes cannot physical see a continuous margin/tend to get undercuts on the palatal side, how does one avoid this? I have been taught to draw around the cervical margin of the abutment with a pencil and close one eye and look at it from the view of path of insertion. I'm guess my problem boils down to how does one find the ideal path of insertion on typodont teeth where all the teeth are ideally aligned?
 

honestdoc

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Your crown prep diamond burs should have taper. I don't think you need to tip the bur and just prep as parallel to the tooth as you can since you tend to over-taper. Are you able to purchase numerous typodont teeth? I know they can be expensive but practicing (read up on subject of deliberate practice) will make it easier. KEEP IT SIMPLE. Don't stop and check your prep until you reduce the axial walls. If you don't feel comfortable and must stop every moment...practice, practice, practice. When you stop all the time to check, you lose continuity and yes you can get undercuts and choppy preps. You don't need to analyse perpendicular, axial details...you won't be able to in the mouth. I do mostly zirconia crowns and some full gold crowns...metal-ceramic crowns look ugly and the ceramic fractures off. I do smooth full chamfers and no knife edge. If you follow the steps I mentioned, undercuts should not be a problem...remember, in the mouth you must deal with the tongue, gagging, limited mouth openings, difficult access, etc. Test your crown prep with your temp crown. Again, does it seat properly? Does it have good retention? Is it thin and close to perforation or fracture?

25 years ago, I had a classmate that couldn't get it. She was trying to analyse the proper angles, stopping all the time, and practicing all the time (not deliberate practice). Her preps look terrible despite all the over analysing. She got into the patient's mouth and turned off the water so she could somewhat see and they had to dismiss her because she is causing harm to the patients. If deliberate practice is not working for you, you may need to find another career.
 
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Your crown prep diamond burs should have taper. I don't think you need to tip the bur and just prep as parallel to the tooth as you can since you tend to over-taper. Are you able to purchase numerous typodont teeth? I know they can be expensive but practicing (read up on subject of deliberate practice) will make it easier. KEEP IT SIMPLE. Don't stop and check your prep until you reduce the axial walls. If you don't feel comfortable and must stop every moment...practice, practice, practice. When you stop all the time to check, you lose continuity and yes you can get undercuts and choppy preps. You don't need to analyse perpendicular, axial details...you won't be able to in the mouth. I do mostly zirconia crowns and some full gold crowns...metal-ceramic crowns look ugly and the ceramic fractures off. I do smooth full chamfers and no knife edge. If you follow the steps I mentioned, undercuts should not be a problem...remember, in the mouth you must deal with the tongue, gagging, limited mouth openings, difficult access, etc. Test your crown prep with your temp crown. Again, does it seat properly? Does it have good retention? Is it thin and close to perforation or fracture?

25 years ago, I had a classmate that couldn't get it. She was trying to analyse the proper angles, stopping all the time, and practicing all the time (not deliberate practice). Her preps look terrible despite all the over analysing. She got into the patient's mouth and turned off the water so she could somewhat see and they had to dismiss her because she is causing harm to the patients. If deliberate practice is not working for you, you may need to find another career.

Thanks, this encourages me to review my way of thinking when doing a crown prep. I often come across as a meticulous person, which often comes in the way of seeing the bigger picture. Yes, we have multiple typodont teeth. I think also one of the reason I tend to get undercuts is due to wrong bur angulation; I tend to fixate myself on the cervical margin and in the process lose sight on the angulation and hence my initial question regarding 5-6 degree taper.

One thing, however, that is still not clear to me. I know my diamond shaped crown burs have a intrinsic taper which will give me the ideal 5-6 degree taper. But how does one exactly (strictly speaking on typodont teeth because I know this depends in every clinical situation) place the bur on the sound tooth structure? (i.e.find the right the bur angulation that is not too parallell (crown will not seat) and at the same time not overtapered? This seems trivial but I'm a bit confused. Is this just a matter of endless hours of deliberative practice?)
 
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honestdoc

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I don't know the answer to your question. On a typodont, picture where you will prep the margin. I don't angle my bur too much because I will over taper. Remember your teeth morphology. The height of contour for buccals of lower molars are at the cervical third and they tend to tip lingually. Lower molar buccals will be too easy to over taper. Don't stop mid prep and look...just finish your reduction. Let's go back to my first reply. Reduce the occlusion first, then buccal, then lingual/palatal. Last you have the proximals with occlusal, buccal and linguals reduced so there won't be as much tooth structure. With deliberate practice, prepping all surfaces should take about 10 minutes. Afterwards, go back and smooth all the margins. Remember it is a typodont, not a real person's teeth. Get really good and fast with the typodont...you don't have to mess with the tongue, cheeck, water, anesth, etc.
 
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