Dr. W. asks:
I want to minimize the amount of chipping of porcelain on my implant crown and bridge cases. What are the strongest alloy and porcelain combinations? Or does this not make any difference? Is the type of cement used a factor in porcelain fracture off implant supported crowns and bridges? I am worried that I will not be able to retrieve the crowns and bridges for repair. What is everybody else doing about this?
Strongest Alloy and Porcelain Combinations for Implants?
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The problem is not the porcelain or the alloy,but is the amount of occlusal charges on the porcelain: these should be out of contact, even in the strongest centric relation.
Just use “normal” combination of metal/porcelain. Remember to have the metal substructure thick enough to eliminate unsupported porcelain. If you are still chipping porcelain, consider a nightguard. For multiple units, I prefer the Leclerq-Martinez bridge, which my office and lab have perfected in the US. Check out prosthotech for pictures. This type of bridge allows repair of individual crowns. It can work for 3 unit to full mouth restorations.
I do not leave my implants out of occlusion, but I do make sure that they only occlude in the heaviest maximum intercuspal position.
I do not have any greater porcelain fracture with my implant crowns than I do with my regular crowns, thus I cement with glass ionomer, with no intention of removing the prosthesis. If it does fracture and cannot be simply repaired, I cut the crown completely off, or if the angle of insertion for the abutment is favorable, I cut an hole through the top to find the screw access hole and remove it with the crown. New fixture level impression and sent to lab for new crown and abutment. I warranty all my prosthetics for 3 years after delivery. It is rare to remake anything.
As Dr Kim suggested, it’s not necessary to make any special provisions for strength when it comes to restoring implant cases, with one caveat; In order to eliminate the chances of a galvanic cell forming between disimilar metals, never use NP metal. You probably already knew that, but it never hurts to say it again. Why does this matter?
Tests show the best bond to machined implant parts, such as UCLA abutments is had with alloys that are silver free, and contain at least 65% gold. It’s also the ideal alloy for non-cast to applications. Of course using higher gold alloys means you are using softer alloys, and in the case of long span bridges or bars, sagging can be a problem and contribute to porcelain failures.
There are a number of 65% gold, silver free alloys, such as Argen 65SF, that are reasonably priced and very high quality, exhibiting strength of 120,000 PSI or better.
Proper frame design and porcelain support should be considered, as well as flat occlusal surfaces. While we are on the subject, proper implant placement also affects procelain failures, because off axis loads on the porcelain are multiplied as the bending moment changes.
FYI.
I think we are using PD200 at the lab for the castings. Its silver feww and not subject to galvanization. I had been using aspen and did not regognize the significane of silver content until about six or seven years ago. No problems with it to date, but still wonder about those patients Ive restored who have moved on. Bill