HELP: TIPS NEEDED --- Class I / Class II preps and Axial Reduction for Crowns

[helpplease911]

Hi everyone,

My school isn't the greatest at providing tips/guidance for clinic.

I'm having a lot of trouble with Class I and Class II preps..

In addition, I am having trouble getting the chamfer margin and the axial reduction. The professors are saying to make sure the bur stays at the same angle as the line of draw, but when I do I over-reduce and do not produce a margin at all. Should the bur stay at the same angle the whole time during the reduction or do I need to angle it differently to produce the chamfer margin? Any help/articles/videos would be greatly appreciated.

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[Vicviper]

When I was starting, I kept having this problem with my crowns preps that I felt like I was drilling a ton on the tooth, but the margin I was expecting just wouldn't show up - it was incredibly frustrating. A few things I learned really helped me with that:

1) Use Sharp Burs - for your main reduction you want to use coarse or super-coarse burs.

2) I realized I was tipping the bur too much, so while it would reduce at the top of the tooth, only the very edge was hitting it at the margin, and it would make more of a feather than anything.

3) I realized I got lazy, and stopped using depth cuts - using depth cuts is incredibly helpful in establishing your margin - take your bur, run the side of the into the tooth and saw a little up and down to get it to the depth you want, have the base of the bur be slightly above where you want your margin to be. For pre-molars and anteriors, I'll do 3-4 depth cuts a side, for molars, usually 4-5. Remember to do Plane 1 and Plane 2 (higher up on the axial wall) depth cuts. After that, take your bur, and join two of the depth cuts together with a back and forth motion, like 5-6 passes in that small area - especially if you're using an air driven handpiece, this really helps establish the margin - you can't just get the margin you want in one pass. Then blend in the next depth cut and repeat. By this time, you've got a good portion of your margin established on that side.

4) Commit to your cuts - if you're not really pushing the bur where it needs to go, it won't do the cut you want it to, it'll just scratch over the surface. Practice the motion of the cut you're going to make once or twice, and then go for it.

5) Use a 330 to cut occlusal depth cuts, that is 1.5mm, and usually right around where you want for occlusal reduction

6) In general you want to keep the same angle of the bur for buccal and lingual margins, but you'll have to shift that as you're rotating around the tooth. The most important thing, like I mentioned above, is that you need the tip of the bur to directly engage the tooth, if you're angling it too much, higher up is hitting the tooth, but not the tip, and that is what establishes the margin. Be careful though, because if tip it too much the other direction, you'll end up with undercuts.

Good luck, I hope that helps!

What issues are you having with Class 1 and Class 2 preps? Also, are you working on teeth or plastic?

 

[helpplease911]

2) I realized I was tipping the bur too much, so while it would reduce at the top of the tooth, only the very edge was hitting it at the margin, and it would make more of a feather than anything.

3) I realized I got lazy, and stopped using depth cuts - using depth cuts is incredibly helpful in establishing your margin - take your bur, run the side of the into the tooth and saw a little up and down to get it to the depth you want, have the base of the bur be slightly above where you want your margin to be. For pre-molars and anteriors, I'll do 3-4 depth cuts a side, for molars, usually 4-5. Remember to do Plane 1 and Plane 2 (higher up on the axial wall) depth cuts. After that, take your bur, and join two of the depth cuts together with a back and forth motion, like 5-6 passes in that small area - especially if you're using an air driven handpiece, this really helps establish the margin - you can't just get the margin you want in one pass. Then blend in the next depth cut and repeat. By this time, you've got a good portion of your margin established on that side.


What issues are you having with Class 1 and Class 2 preps? Also, are you working on teeth or plastic?

Were you tipping the bur too far lingually when reducing the axially? It seems like the line of draw leaves you with your bur tilted more lingually then is needed to establish the chamfer margin.

Also, I was not taught to do depth cuts for the axial reduction... this is something you were taught? I was taught to do the depth grooves only for the occlusal surface. I also wasn't taught to do 2 planes for the axial reduction, where exactly should the planes be located?

I know for the occlusal surface you have the buccal and lingual reductions and then you must form the functional cusp bevel, but for the axial wall we were literally just told what bur to use and how deep the margin should be/how much the axial reduction should be. It's frustrating because if you don't go over the EXACT same line every time with your bur then you end up just getting a slope instead of an actual margin.

We are using plastic teeth.

Thank you for your advice!!

 

[Vicviper]

- I was tipping the bur too far into the tooth - lingually or buccally depending on the side of the tooth I was prepping. Remember that the Bur itself has a taper, so in general if you leave the bur just about straight up and down, it will still make a 4-6 degree taper you want.
- I was taught that axial depth cuts are part of the fundamentals of crown preps, it helps you give a precise reduction on the axial without relying on a reduction stent
- A second plane is very important - the first plane is the gingival 2/3 of the buccal and lingual, the second plane is the occlusal / incisal 1/3 of the tooth.
-I used to think the same thing about not being able to go over the same area again of the margin without messing it up - what you'll find is that you are supposed to let that area guide the bur - you can essentially let it rest on that area, and as long as you aren't putting any downward pressure, it won't make the margin deeper while you're sweeping back and forth. This takes a while to get used to.
-Check out this document for details and diagrams:
http://www.ddsltdlab.com/wp-content/uploads/2017/02/PFM-Prep-Crown-Page.pdf
-Lastly, prepping plastic teeth is much more difficult than actual teeth. Burs are made to cut actual tooth structure, and don't have the same amount of chittering and jumping that will happen on plastic teeth. Natural teeth also smooth much more easily than plastic.
-Don't stress, it just takes practice

What issues were you having with the direct restorative preps?

 

[helpplease911]

Thank you so much for the PDF it's incredibly helpful! I'm shocked my own instructors haven't suggested depth grooves on the buccal and lingual sides. Does the lingual side of the mandibular molar tend to be a little bit more tricky to get the chamfer margin?

What burs do you typically use for the reduction? By the way, these preparations are for FGCs.

For direct restorative preps, I'm having issues keeping the isthmus narrow enough and then making sure the pulpal floor and cavosurface margins are smooth.

Thank you for all the help!!

 

[schmoob]

Thank you so much for the PDF it's incredibly helpful! I'm shocked my own instructors haven't suggested depth grooves on the buccal and lingual sides. Does the lingual side of the mandibular molar tend to be a little bit more tricky to get the chamfer margin?

What burs do you typically use for the reduction? By the way, these preparations are for FGCs.

For direct restorative preps, I'm having issues keeping the isthmus narrow enough and then making sure the pulpal floor and cavosurface margins are smooth.

Thank you for all the help!!

When it comes to crown preps, you need to look at things three dimensionally. Make sure your bur is perpendicular with the occlusal table, and then use your mirror to make sure your bur is perpendicular to the occlusal from another view. For example, if you're reducing the buccal surface, use you mirror to make sure your bur is perpendicular from a proximal view.

For Class I & IIs, make sure you have a new bur and a very smooth/light touch. When I was a D1 I had a hard time getting my isthmus narrow enough. Then I got high-mag loupes and my isthmus became to narrow! I could barely pass a chisel through it. Honestly, you shouldn't even need a slow-speed.

 

[TanMan]

If this is for preclinicals, use a dull bur. Less likely to make an error and replicates how a real tooth feels when cutting.

 

[Vicviper]

For plastic teeth, you need to work completely differently than you do on natural teeth, as the plastic chips instead of cutting cleanly a lot of the time. To get a narrow isthmus, you need to limit yourself to very passes, and use a small bur. It will only get large if you put the bur through the area multiple times. I strongly recommend using the 330, and not a 56 or 557. Make your initial cuts with the high speed, and then finish and fine tune everything with a slow speed. See if you can find an End Cutting bur to use in the slow speed.

Tips for making controlled cuts - you need to have a great finger rest, if you don't, your bur will waver around and make jagged cuts. Practice the cut you are going to make before you do it. Go Pedal to the Metal - with the highspeed on plastic teeth, you really want to just go for it, don't feather your rheostat.

 

[deleted696582]

I did decently on class II prep and restoration practicals, so maybe I can give a few pointers. What I do is different from the next student though, but taking in advice from others and then forming your own strategy is a great idea. I didn't go through all the previous responses, so apologies for any redundancy. Also, we weren't allowed to use anything to protect adjacent teeth. Haven't done crown preps yet.

- I used a new bur (245 and 330) for each prep.
- I started with the occlusal class I portion, and then extended each side to the adjacent tooth leaving a thin enamel shell for protection.
- I grabbed a 245 and dropped the box down almost to the required depth, but didn't extend the box much facially or lingually.
- I switched back to the 330 and cleaned out the box that I dropped for retention. I used hand tools like the hatchets or proximal/distal carvers (I think that's what they were called...) to open the contact and smooth out the floor.
- Most importantly, and even more important than all of the previous tips, is that I did like 10 or 12 practice preps before each practical. It starts to become a lot easier the more you practice.