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PFM prep question

Discussion in 'Dental Residents and Practicing Dentists' started by polarnut, Mar 27, 2007.

  1. polarnut

    polarnut Senior Member

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    hello,

    i have a question regarding finish line design (chamfer and shoulder) for PFM crown preparation. i understand that for PFM preparation, the lingual should be chamfer because metal sits on it and buccal should be shoulder because porcelain sits on it. i understand that porcelain is likely to fracture if it sits on chamfer design, but why?

    also, i am very confused as to how to go about the transition area going from chamfer to shoulder in the proximal margin area. do i sort of blend in the two designs into a transition zone or do i make a uniform chamfer design on the lingual half and abruptly began making a shoulder design on the buccal half?
     
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  3. 12YearOldKid

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    Porcelain on a chamfer creates a thin wedge of porcelain at the margin. Porcelain needs significant bulk to provide strength; metal doesn't.

    Most people gradually blend the two margin designs, but ask your instructors what they want. There are a hundred different ways to prep a crown... and they are all correct.
     
  4. OMFSZADE

    OMFSZADE Health practitioner

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    Dear Polarnut,

    There are three general types of PFM crowns that are routinely used. Partly depending the area in the mouth you will be restoring:

    Lets first assume that we don;t really care about esthetics on the lingual, regardless of the tooth, ant that the margin will be ending on the metal on lingual. Then, the finish line that would be the best in this case is 1-1.5 mm chafer for the best seal.

    Now Buccal:

    1. In anterior teeth you can either have regular pfm margin, which is metal ending at the same level as porcelain, although the metal is very thin at the margin. In this case you would use a DEEP CHAMFER finish line to make sure that the metal is masked as much as possible. REduction here would be 1.5-2 mm.

    2. In anterior teeth you can also do a PFM crown, but buccally metal would end about 2-3 mm before the margin, so you would have an ALL-PORCELAIN buccal margin. In this case you would of course use BUTT JOINT margin...because otherwise porcelain would be too thin and would crack.

    3. On the posterior teeth, apart from "regular" pfm margin (see #1 above), a lot of people prefer using METAL COLLAR margin 360 degrees. This means that you would have all metal margins , with procelain starting about 1.5 mm coronal from the margin. In this case, you would still reduce throughout 1.5 - 2 mm...but your margins would NOT be a deep chamfer...but a regular...almost beveled chafer

    hope this helps
     
  5. polarnut

    polarnut Senior Member

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  6. docj1

    docj1 Walleye Whacker

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    Hi, I just wanted to add that in private practice, most patients will not go for any metal showing. Therefore, porc butt margin B+L or slightly subgingival margins with champfer. I prefer butt margins, I think they don't show the blue as fast. Procera for incisors. I had a patient who wanted me to send a crown on #18 back to the lab because of a 1mm lingual collar. She said that she could see the metal when she held her tongue to the side with her fingers. No S--t
     
  7. SquidsLife

    SquidsLife Navy to NiTi

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    Psalms 81:10:" . . . Open thy mouth wide, and I will fill it."

    :laugh: I had to laugh at this one when I read it...divine dentistry....love it! :laugh:
     
  8. Flapster

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    With the advent of Zirconia and Lava crowns with superior esthetics and durability (5 year warranty from the manufacturer) why place PFM crowns? I believe PFM's are going to be obsolete soon.
     
  9. aphistis

    Moderator Emeritus

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    Nice aesthetics (sidenote: am I the only person alive who learned to spell that word with the A in front?) and fracture resistance aren't the only considerations by a long shot. PFM restorations are still clearly indicated in certain instances, and can be made to produce highly satisfactory aesthetic outcomes. I'm in the middle of a 10-unit cosmetic fixed case as we speak, and 8 of the 10 units are PFM's.
     
  10. Flapster

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    You are right, for a bridge with a long span there is no replacement for PFM.

    For a single unit crown or a multiple of single unit crowns, if esthetics is not an issue then covering the occlusal with the metal of a PFM or a full gold crown makes sense. But if esthetics is an issue and you are going to cover the entire occlusal surface with porcelain, then why not place zirconia or lava all the time?

    For a series of crowns in the anterior segment I prefer Lava crowns as I can get the coping shade the same as the tooth and that is unbeatable for esthetics. I find that with PFM crowns unless you are going to use a high yellow gold like "captek" then the tooth appears to lack the opalescence of the natural tooth and does not mimic the vitality of healthy natural tooth color.

    You mention that 8 out of 10 units are PFM. May I ask what the other units are going to be? How is your ceramist making sure to match the shade of your PFM units to the other 2 units?

    I dropped the "A" in esthetic as I was addressing a mostly American audience and did not want to offend with British English. If you notice I also dropped the "U" from colour. God that sounds nerdy.

     
  11. aphistis

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    For single units in cosmetically sensitive areas, I agree that all-ceramic provides superior aesthetically superior results. From what I've seen, I agree that Captek PFM's are excellent. I didn't mean to infer that all-ceramics aren't the preferred treatment some of the time--I just don't believe they're always the answer, and that PFM crowns will be, as you stated before, that "PFM's are going to be obsolete soon."

    To answer your other question, the case in question is #4-#13, #4-#11 are the PFM's due to gingival recession and resultant cementum crown margins. #12 and #13 are planned for veneers pending any surprises when I prep them.
     
  12. Flapster

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    You have had me dust off my dictionary and check it twice today. With respect to PFM placement I must go with the dictionary definition of "no longer in general use; fallen into disuse". Within the next ten years PFM crowns will no longer be in general use and will be used in extenuating circumstances.

    I don't envy you for having to match veneer shade and characteristics to a PFM crown next to it. I hope your patient is not picky.

     
  13. polarnut

    polarnut Senior Member

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    can someone please explain what the "butt margin" or "butt joint margin" is? is that the shoulder finish line? :oops:
     
  14. aphistis

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    I guess we'll simply have to agree to disagree, then; and I expect she's much less picky than you.

    I think everything is going to turn out fine, but if everything crashes & burns I'll come back and own up to let you have your "I told you so." ;)
     
  15. Flapster

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    I have given you esthetic reasons behind the obsolescence of PFM crowns. Here is some information on the economic pinciples behind the theory that PFMs will become obsolete one day.

    "The world market for dental crowns and bridges for tooth restoration amounts to approximately 110 million teeth per year. Approximately 85% of the dental crowns and bridges are based on metal and some 15% on ceramic materials. Of the metal restorations, approximately 35% are used in bridges. The all-ceramic segment of the market is growing with an estimated 12-15% a year, while the metal segment is estimated to grow by 2-3% annually."

    The most important reason why PFM's will become obsolete is due to the fact that the price of gold has risen from $297.5 USD/ounce in 2002 to $668.75 USD/ounce today. That price will continue to rise to the point that the restoration containing precious metals will no longer be cost effective.

    I thought I'd throw those statistics your way just in case you wanted to invest in your children's college fund by buying gold or investing in all ceramic companies.













     
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  17. Flapster

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    I have given you esthetic reasons behind the obsolescence of PFM crowns. Here is some information on the economic pinciples behind the theory that PFMs will become obsolete one day.

    "The world market for dental crowns and bridges for tooth restoration amounts to approximately 110 million teeth per year. Approximately 85% of the dental crowns and bridges are based on metal and some 15% on ceramic materials. Of the metal restorations, approximately 35% are used in bridges. The all-ceramic segment of the market is growing with an estimated 12-15% a year, while the metal segment is estimated to grow by 2-3% annually."

    The most important reason why PFM's will become obsolete is due to the fact that the price of gold has risen from $297.5 USD/ounce in 2002 to $668.75 USD/ounce today. That price will continue to rise to the point that the restoration containing precious metals will no longer be cost effective.

    I thought I'd throw those statistics your way just in case you wanted to invest in your children's college fund by buying gold or investing in all ceramic companies.















     
  18. Flapster

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    I have given you esthetic reasons behind the obsolescence of PFM crowns. Here is some information on the economic pinciples behind the theory that PFMs will become obsolete one day.

    "The world market for dental crowns and bridges for tooth restoration amounts to approximately 110 million teeth per year. Approximately 85% of the dental crowns and bridges are based on metal and some 15% on ceramic materials. Of the metal restorations, approximately 35% are used in bridges. The all-ceramic segment of the market is growing with an estimated 12-15% a year, while the metal segment is estimated to grow by 2-3% annually."

    The most important reason why PFM's will become obsolete is due to the fact that the price of gold has risen from $297.5 USD/ounce in 2002 to $668.75 USD/ounce today. That price will continue to rise to the point that the restoration containing precious metals will no longer be cost effective.

    I thought I'd throw those statistics your way just in case you wanted to invest in your children's college fund by buying gold or investing in all ceramic companies.















     
  19. Flapster

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    I don't know why that post came up three times, I wasn't going for that effect. Sorry folks.
     

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