July 1, 2006
by Dan Huber, RDT, DD
Heraeus Kulzer, manufacturer of porcelains for porcelain fused to metal, zirconia or titanium crowns, specify that proper substructure support must be achieved to create a crack-free restoration.
The frameworks must be designed to allow for no more than a 2.0mm thickness for porcelain and be designed with rounded corners (Fig. 8).
All porcelain manufacturers specify these requirements for frameworks, to eliminate stress, which may result in chips and cracks in fired porcelain.
The research department of Cendres & Metaux SA, the Swiss gold manufacturing company, notes that an irregular thickness layer of the porcelain can lead to cracks in the ceramics (Fig. 7).
The metal framework has to correspond, in a reduced shape, to the final contour of the ceramics. This axiom applies to all frameworks, regardless of the composition of the material used in creating the framework.
Copings can be grouped into two categories:
a) dipped copings
b) anatomical copings
Dental technicians create copings in different ways, depending on the philosophy of the laboratory owners.
Dipped copings are fabricated, as the name suggests, by simply dipping the die into the wax bath, trimming the excess to define the margin and creating a shoulder for the porcelain support (Figs. 1A-C). The method is commonly used in mass production laboratories.
Anatomical copings are created in two ways:
a) Started as a dipped coping, the technician adds wax to create a reduced shape of the final restoration (Fig. 2).
b) A full wax up of the restoration is created, then, with the aid of specific instruments, the wax is cut back to allow for proper porcelain build up.
These techniques are time consuming and must be performed by highly skilled dental technicians, resulting in higher unit cost for the restoration.
With introduction of CAD/ CAM technology to the dental laboratory, anatomical copings can now be created digitally, thereby dramatically reducing internal as well as external remakes.
Today our laboratory produces seventy percent of anatomically designed copings utilizing CAD-CAM technology.
The software can be programmed to create copings of any thickness, create a full wax up, then cut back by predetermined amount leaving space for porcelain (Figs. 4A & B). The preferences of individual dentists for contacts, occlusion etc. can be stored and automatically taken into account on any work the lab performs.
A virtual articulator, part of the software, indicates on the screen, the space or lack of it for porcelain (Figs. 3A-C).
The virtual articulator is a digital, three dimensional articulator appearing on the screen. It is fully adjustable and can be programmed to simulate readings from the original articulation such as condyle settings and incisal guidance.
Should there be a lack of space, the program will indicate this (Figs. 5A & B) showing an area in red that must have more space for porcelain (Fig. 6). The technician will then consult the practitioner to determine how best to resolve this issue.
The finalized information is then transmitted to an in-house wax printer, or to an off-site milling centre to create frames in Zircon or Titanium. The frame that the ceramist received will be as near to an ideal base for porcelain application as the intra-oral situation allows.
For the dental office, these technological advances mean less time spent adjusting occlusion, contacts and contour at chairside, as well as fewer remakes which add costly extra appointments to a patient’s treatment.
Dan Huber has been in the dental laboratory business for the past 50 years. He is currently the owner and chief executive officer of Lindberg Homburger Modent Dental Studios. He is the past chairman of the College of Dental Technologists of Ontario and a past member of the Advisory Committee of the Department of Dental Technology at George Brown College.
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