short question about tilt of bur for anterior crown prep

[childofthesacredheart]

I was just wondering if you guys can help me out? How do you tilt the long diamond bur when you are prepping the first plane of a crown prep on the facial of a maxillary central incisor?

I have the problem of tilting the bur either too lingually or too buccally, and therefore my mix-axial reduction is either too much or too little.

Are there any landmarks that you use in order to know that you are correctly tilting your bur when making the first plane?

Do you think that I should align my bur with the middle third of the facial of the central incisor? Is that the right tilt?

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[ramborhino]

I just try keep the draw of the bur generally the same as the gingival 3rd of the adjacent central. I like to slightly cheat towards making the bur a bit more buccal, rather than lingual..because you can't go back and fix things after you over-reduce but you can certainly go back and reduce a bit more if you need to correct the draw if you stay conservative. Also I don't go the width of the margin right away; I make depth cuts about 0.2-0.3mm smaller than my desired margin. Leaves room for making slight corrections, finishing the axial walls and it results in the margin blending together nicely. Good luck!

 

[childofthesacredheart]

I just try keep the draw of the bur generally the same as the gingival 3rd of the adjacent central. I like to slightly cheat towards making the bur a bit more buccal, rather than lingual..because you can't go back and fix things after you over-reduce but you can certainly go back and reduce a bit more if you need to correct the draw if you stay conservative. Also I don't go the width of the margin right away; I make depth cuts about 0.2-0.3mm smaller than my desired margin. Leaves room for making slight corrections, finishing the axial walls and it results in the margin blending together nicely. Good luck!

thank you ramborhino for your response 🙂 i'm sorry, but i still don't understand... could you perhaps include a drawing or an illustration?

If you "keep the draw of the bur generally the same as the gingival 3rd" then wouldn't the first plane of the prep be tilted way too buccally? The gingival third of the adjacent central is the emergence profile of the tooth, meaning, the contour of the tooth as it emerges out of the gingiva is clearly buccal. Following this would make the first plane tilted way too bucally.

 

[THS]

I generally prep the facial part of an anterior tooth in 1-2 planes.The first plane matches the central 1/3 of the tooth. You're correct with your previous comment, if you follow the gingival 1/3rd, you risk following the emergence profile and creating an undercut at the margin. However, if you do do this, your reduction on the cingulum will have to match so that your path of draw will be more labial, and that may not even work if the lingual side of the tooth isn't overreduced.

My 2nd plane is on canines, at the incisal 1/3rd. It is not a very prominent plane (nothing like prepping the buccal of a molar).
Do you use Genie Putty or anything similar to make a matrix to check your reduction? This is a great way to check your reduction while prepping, so you can get an idea of what angle works best for you.

While it is probably smart to make depth cuts, I don't. I just shave off a bit at a time until I have the desired reduction. I found this has saved me some time.

 

[free99]

While it is probably smart to make depth cuts, I don't. I just shave off a bit at a time until I have the desired reduction. I found this has saved me some time.

I haven't done a crown prep on a patient yet, but in the sim clinic, my crown preps improved measurably when I stopped doing depth grooves and just started making smooth reductions.

 

[arlmay]

1st plane orient bur perpendicular to the facial height of contour. 2nd plane anatomic reduction parallel to the adjacent central incisal 1/2-1/3.

Skip the depth cuts by the way, I've only found them somewhat useful when doing occlusal reductions, not axial.

 

[childofthesacredheart]

I generally prep the facial part of an anterior tooth in 1-2 planes.The first plane matches the central 1/3 of the tooth. You're correct with your previous comment, if you follow the gingival 1/3rd, you risk following the emergence profile and creating an undercut at the margin. However, if you do do this, your reduction on the cingulum will have to match so that your path of draw will be more labial, and that may not even work if the lingual side of the tooth isn't overreduced.

My 2nd plane is on canines, at the incisal 1/3rd. It is not a very prominent plane (nothing like prepping the buccal of a molar).
Do you use Genie Putty or anything similar to make a matrix to check your reduction? This is a great way to check your reduction while prepping, so you can get an idea of what angle works best for you.

While it is probably smart to make depth cuts, I don't. I just shave off a bit at a time until I have the desired reduction. I found this has saved me some time.

wow - thank you THS for this answer- it was the most clear and helpful 🙂 To answer your question - I do use a matrix putty to check for the mid axial reduction on the facial - and I notice that a huge amount of tooth structure is removed. The ideal amount of reduction (for a PFM crown on #8) on the mid axial facial is 1.5 mm - and that might sound like a little, but when you actually do the prep, it feels like you are removing so much tooth structure. I guess this is for the bulk of the material - a PFM crown is made of three things: the inner metal coping for strength, a layer of opaque porcelain to mask any chance of metal showing through, and the outermost layer of porcelain that can be shaded and colored for aesthetic reasons. Therefore, a huge amount of tooth structure does have to be removed in order to allow for the bulk of the material.

However, I do not always use a matrix putty to guide my reduction. The reason for this is because you can use the "shadow" or the silhouette of the uncut adjacent central incisor to see how much mid-axial reduction you have on the prepped tooth. The matrix putty can be very expensive to buy.

 

[childofthesacredheart]

1st plane orient bur perpendicular to the facial height of contour. 2nd plane anatomic reduction parallel to the adjacent central incisal 1/2-1/3.

Skip the depth cuts by the way, I've only found them somewhat useful when doing occlusal reductions, not axial.

thank you arlmay for your answer 🙂 but I don't understand what you mean when you say, "perpendicular to the facial height of contour. what do you mean? could you provide a drawing or picture?

 

[arlmay]

I worded the first plane poorly, it should read tangent to the facial height of contour which is also perpendicular but tangent might be more clear to you. Here is a diagram showing the principle not necessarily good orientation since the drawing isnt perfect anatomically.

 

[arlmay]

I worded the first plane poorly, it should read tangent to the facial height of contour which is also perpendicular but tangent might be more clear to you. Here is a diagram showing the principle not necessarily good orientation since the drawing isnt perfect anatomically.

The longer line is the second plane which is just a reduction parallel to the incisal 1/3 of the tooth, in other words an anatomic reduction

 

[ramborhino]

Sorry if I was confusing..like the posters above said, it should be a 2-plane reduction...then you can blend the 2 planes together a bit as it shouldn't be a sharp transition. As you can tell from the diagram above, it is hard to be know exactly what is perpendicular to a point on a contoured tooth surface. What I wanted to get across is that, generally, the plane that is more incisal will flow/draw with the incisal portion of the adjacent central, and the gingival plane will flow/draw with the gingival portion of the adjacent central. Don't get caught up on fractions and orientations, etc...try to visualize the draw of the prep and make slight adjustments as you go.