PFM Crown Prep

[daMan]

So I'm starting down in clinic and I have my first crown coming up. Its a PFM on #30 where my patient wants to see no metal margin. I've asked around and have been receiving different advice from different instructors on how to do the prep. I want to see what the strong consensus on this prep should be and why, so please give me your input.

Advice I've been told by instructors and students:

1. Chamfer all the way around.

2. 120 disappearing margin should with a chamfer on the lingual.

3. 120 disappearing margin all around.

If you have other options please add them. Thanks

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[capisce?]

We were taught for pfms to have a light chamfer on the lingual and a heavier one on the facial for the proper thickness of the porcelain.

 

[scalpel2008]

damn, its been 5 years since i've done one of those but i remember we used to do a 1.5mm butt joint margin on the buccal for the all-porcelain margin that would go past the interproximal contacts and "wings" as a transition for a lingual chamfer. Keep in mind this advice is coming from someone in OMS residency for 5 years. Maybe i'll try it with a #703 on a hall handpiece before i section my next tooth for old times sake.

 

[toofache32]

Jeez I remember my first crown prep. It took me 3 appointments to finish it. The first appointment I did the occlusal and buccal. The 2nd visit I did the lingual. The last appointment I did the interproximals and had to redo the occlusal because it had erupted more by then.

 

[scalpel2008]

Jeez I remember my first crown prep. It took me 3 appointments to finish it. The first appointment I did the occlusal and buccal. The 2nd visit I did the lingual. The last appointment I did the interproximals and had to redo the occlusal because it had erupted more by then.

I hear ya. I think it's easier to ORIF a mandible.

 

[toofache32]

I hear ya. I think it's easier to ORIF a mandible.

Now that I think about it.....my last crown prep wasn't much different.

 

[crazy_sherm]

So I'm starting down in clinic and I have my first crown coming up. Its a PFM on #30 where my patient wants to see no metal margin. I've asked around and have been receiving different advice from different instructors on how to do the prep. I want to see what the strong consensus on this prep should be and why, so please give me your input.

Advice I've been told by instructors and students:

1. Chamfer all the way around.

2. 120 disappearing margin should with a chamfer on the lingual.

3. 120 disappearing margin all around.

If you have other options please add them. Thanks

Chamfer all the way around will leave a metal margin. A 120 degree disappearing metal will leave a less visible metal margin, but it can still be seen. If the patient really wants to not see the metal margin, it'll have to be a 90 degree butt joint (1.2-1.3mm axial reduction). Since the tooth is not in an aesthetic area, I don't see why the patient is so concerned with the margin. His cheek covers one side and his tongue will cover the other. Keep in mind the less margin showing, the more tooth structure needs to be removed. Make sure the patient understands the drawbacks. Also, have you considered doing a Cerec or Lava restoration instead since aesthetics seem to be a big concern?

 

[Lesley]

So I'm starting down in clinic and I have my first crown coming up. Its a PFM on #30 where my patient wants to see no metal margin. I've asked around and have been receiving different advice from different instructors on how to do the prep. I want to see what the strong consensus on this prep should be and why, so please give me your input.

Advice I've been told by instructors and students:

1. Chamfer all the way around.

2. 120 disappearing margin should with a chamfer on the lingual.

3. 120 disappearing margin all around.

If you have other options please add them. Thanks

For PFM crowns on the posterior, if the patient is telling you he doesn't want to see any metal on the crown, I think he means the lingual too. This sounds like a patient who is going to be looking in the mirror and moving his tongue around to check.

I think a chamfer all around the prep, slightly below the gingiva, .5mm will work out nicely. It is an easy prep to do and you have control over it. For PFM crowns in the posterior the chamfer on the facial can be a little deeper for esthetics and lighter on the mesial/distal/lingual.

Bevel edges can be used on the mesial/distal/lingual. When kept slightly under the gingivia they can be invisible, too. Note that recession can cause any margin to become obvious. Packing too agressively with cord and cutting the tooth too deeply below the gingiva can cause recession.

Bevel margins are desirable when the patient has a large existing interproximal restoration already below the gingiva and you just want to get past the metal. Why bother trying to place a chamfer margin below the gingiva, it's already below the gum line. Use a nice bevel instead.

To keep things simple, do the chamfer prep, provided there is no deep exisiting restoration. It will be easy to keep track of how close to the gingiva you are. Should you need to bevel, be careful it's easy to go slightly too far below the gingiva and cause bleeding making it difficult to see, finish you prep and control bleeding during the impression. The patient will have more discomfort after the appointment too. Bleeding can also prevent your temporary for adhering well.

Chamfers are easier to control, and it's easy to pack cord around them. Chamfer margin impressions are easy to read and look nice and clean. Use loopes if you have them.

I do a lot of ceramic crowns, Procera and Cercon are my favorites. For these I do use a deep chamfer or shoulder prep. Sufficient occlusal clearance is absolutely necessary. I do some all ceramic crowns in the posterior too. I find the fracture rate is a slightly greater than with PFM crowns. Many of the posteriors I'm crowing have large existing amalgams slightly below the gingiva, so I like the flexibility of using a bevel edge when I need it. PFM's will always have their place in my book.

*When you are prepping and moving the bur around the tooth remember to keep the margin slightly below the gingiva as you approach the MB and DB areas, crowns slightly short in these areas look crappy, especially on premolar or anterior teeth!

Practice on a plastic tooth to feel more comfortable before the prep appointment. I know this will go well for you! Dentistry is fun! Good Luck!
Lesley

 

[HuyetKiem]

Now that I think about it.....my last crown prep wasn't much different.

My first crown prep was those darn root tips of #30. With surgical bur, I removed most of bone around them to make way for my elevator.

 

[12YearOldKid]

If it has to be PFM I would go for the heavy chamfer just slightly subgingival. At least that's what we do at my school.

 

[ItsGavinC]

Jeez I remember my first crown prep. It took me 3 appointments to finish it. The first appointment I did the occlusal and buccal. The 2nd visit I did the lingual. The last appointment I did the interproximals and had to redo the occlusal because it had erupted more by then.

And that doesn't count the four appointments that it took to impress and provisionalize.

 

[ItsGavinC]

I would place a shoulder all the way around, or a facial shoulder transitioning into a chamfer on the lingual.

 

[toofache32]

And that doesn't count the four appointments that it took to impress and provisionalize.

It was just 3 wise guy...

 

[Dr.Brutus]

Jeez I remember my first crown prep. It took me 3 appointments to finish it. The first appointment I did the occlusal and buccal. The 2nd visit I did the lingual. The last appointment I did the interproximals and had to redo the occlusal because it had erupted more by then.

I have literally been laughing for five minutes over this! That is one of the funniest things I've read on here for a long time.

 

[PERFECT3435]

I would place a shoulder all the way around, or a facial shoulder transitioning into a chamfer on the lingual.

i was going crazy till i saw your post. i thought thats what you had to do with most pfm crowns. here at iu, the only 2 pfm preps we ever did was on #21 and #8 both of which had a shoulder, chamfer, and wings.

 

[drhobie7]

LAVA baby, LAVA. On the posterior?! Yeah, it can take it....or so I've heard. 😀

 

[aphistis]

i was going crazy till i saw your post. i thought thats what you had to do with most pfm crowns. here at iu, the only 2 pfm preps we ever did was on #21 and #8 both of which had a shoulder, chamfer, and wings.

What? How did you prep #6?

(You could tell me they made you do a PFM-Maryland #6-8 with a sanitary full gold pontic, and I'd probably believe you.)

 

[psiyung]

Chamfer for Porcelain? What have you guys been smoking?

 

[CorneliusFudge]

Chamfer for Porcelain? What have you guys been smoking?

Obviously you don't prep a chamfer for a true porcelain margin. In that case it's a flat shoulder or butt joint. But at my school we are rarely allowed to do porcelain shoulders (I think maybe at the lab's request? ) Like others have said, we almost always cut a deep chamfer, put the margin subg and request a 0.25 -0.5 mm metal collar.

I think that will change when I get out of school in 8 months - 8 months baby!!! But for now I do as I'm told.

 

[psiyung]

Obviously you don't prep a chamfer for a true porcelain margin. In that case it's a flat shoulder or butt joint. But at my school we are rarely allowed to do porcelain shoulders (I think maybe at the lab's request? ) We almost always cut a deep chamfer, put the margin subg and request a 0.25 -0.5 mm metal collar.

I think that will change when I get out of school in 8 months - 8 months baby!!! But for now I do as I'm told.

Just another reason to hate dental school

 

[Dr.Brutus]

Obviously you don't prep a chamfer for a true porcelain margin. In that case it's a flat shoulder or butt joint. But at my school we are rarely allowed to do porcelain shoulders (I think maybe at the lab's request? ) Like others have said, we almost always cut a deep chamfer, put the margin subg and request a 0.25 -0.5 mm metal collar.

I think that will change when I get out of school in 8 months - 8 months baby!!! But for now I do as I'm told.

This is clearly the most esthetic option.

 

[Asprin84]

1.2mm shoulder facially merging into 0.5mm chamfer inteproximally without any wings. Lingally 0.5mm chamfer . smooth, continuous finish line is a must for best aesthetics 🙂

 

[Asprin84]

oh , it is an old post , do you still go subgingivally to place the margin?

 

[DrJeff]

oh , it is an old post , do you still go subgingivally to place the margin?

Depends.

If I'm cutting a crown prep on a molar, and there's no existing restorations/decay that go subgingival, I'm going to place my finishline about 1mm supragingival to facilitate the ease of cleaning of that margin by my patient. 2nd Premolars, most of the time i'm doing the same. 1st premolar to 1st premolar, especially nowadays with all the predictability of all ceramic crowns, if that tooth is restoration and decay free subgingivally AND i'm 100% sure of the shade selection, I'm going to put that finishline just slightly supragingival upfront too.

Obviously if there's an existing restoration and/or decay that goes subgingivally, the finish line goes subgingivally (atleast in that specific area). Or anteriorly, lets say that there's been some gingival recession and there's some dentin showing, then I'm likely going to put that finishline slightly subgingival for esthetics.

In general, wherever feasible, I'm going to put my finishlines SUPRAgingival

 

[Halcion]

In school:

PFM - facial/buccal shoulder to the interproximal then chamfer from interproximal to lingual

In the real world:

PFM, Zirconia, Emax - heavy chamfer all the way around, shoulder on buccal if in the esthetic zone with heavy chamfer everywhere else

You want more than a chamfer for traditional PFM because you want enough room to stack porcelain against the underlying metal to minimize opacity/monochromatic shade of the crown. If doing an all-ceramic (Zirconia, emax) you can get away with a heavy chamfer because there is no metal underneath that you have to stack porcelain against to get the shade you want. Personally, I rarely do traditional PFMs anymore, and I'm only 1.5 years out. I love the esthetics of all-ceramics, and there are even Zirconia bruxer crowns for patients who grind their teeth.

 

[ziptree]

Depends.
1st premolar to 1st premolar, especially nowadays with all the predictability of all ceramic crowns, if that tooth is restoration and decay free subgingivally AND i'm 100% sure of the shade selection, I'm going to put that finishline just slightly supragingival upfront too.

How often are you 100% certain of your shade selection? You must be using some magical shade guides!

On anteriors, I usually prep to the gingival crest, pack a light cord, #00, then refine the margins with a fine (red band) diamond. That will usually tuck the margin slighty subgingival for optimal esthetics. Then pack a second cord or remove the first, light laser/electrosurgery on the lingual and interproximal areas, then image and send the impressions off via CEREC Connect for my lava copings.

 

[DrJeff]

How often are you 100% certain of your shade selection? You must be using some magical shade guides!

On anteriors, I usually prep to the gingival crest, pack a light cord, #00, then refine the margins with a fine (red band) diamond. That will usually tuck the margin slighty subgingival for optimal esthetics. Then pack a second cord or remove the first, light laser/electrosurgery on the lingual and interproximal areas, then image and send the impressions off via CEREC Connect for my lava copings.

My digital shade analyzer helps. Even then it's a triple check with myself and my assistant. That combined with the fact that i'm 100% certain that my lab tech, who I've used for the last 10 yrs, will give me a TRUE say 2M1 instead of say a 2L1.5, combined with some good intraoral photos with the shade tab, and i'm quite confident. If the lab tech who handles all my cases has any hesitation, then he schedules one of his employees to come to my office for the delivery appointment so if some chairside custom staining is needed, it gets done right then (Helps to have the lab I use only be about an hour car ride away too! 😉😀 )

 

[HupHolland]

My digital shade analyzer helps. Even then it's a triple check with myself and my assistant. That combined with the fact that i'm 100% certain that my lab tech, who I've used for the last 10 yrs, will give me a TRUE say 2M1 instead of say a 2L1.5, combined with some good intraoral photos with the shade tab, and i'm quite confident. If the lab tech who handles all my cases has any hesitation, then he schedules one of his employees to come to my office for the delivery appointment so if some chairside custom staining is needed, it gets done right then (Helps to have the lab I use only be about an hour car ride away too! 😉😀 )

Precision craft?

 

[toothache1]

if sound structure, more space better esthetics from the lab.

if this is not your competency and your patient asked you for a disappearing margin on all surfaces, 1.5 - 2.0 mm all around.

I don't know if you have to do PFM but why not e-max. e-max has just as good prognosis as PFM from recent studies.

my two cents

 

[OceanDMD]

Very rarely do PFM, even on posteriors. Dental school may not allow this material, but BRUXZIR is an excellent choice to eliminate any metal but still retain the strength. Can also prep a chamfer which as you know is a much better choice with respect to marginal leakage versus a shoulder. You may find down the road that those PFMs you prep on 19 and 30 will show back up in your office with the porcelain fractured from the ML or DL cusps. An all ziconium restoration is a nice way to prevent this as well.