Biomedical Engineering Reference
In-Depth Information
one missing tooth. It has a strong glass-ceramic base, which is coated
with an apatite glass-ceramic. The amazing thing about these materials
is not just their mechanical properties. Their optical properties are such
that it is difficult to tell the difference between the glass-ceramics and
natural teeth - they even have the right amount of chroma (products:
IPS e.max
®
Press and IPS e.max
®
Ceram).
7.5 GLASS-CERAMICS THAT ARE MOLDABLE
AND MACHINABLE
Glass-ceramics containing leucite crystals (KAlSi
2
O
6
) are produced by
controlling the crystallization process at the surface of the glass. This
method was discovered after that of internal nucleation. Even though
this process was difficult to control at first, various methods have been
developed that have resolved the problem. Recently, materials of this
kind have become available that can be shaped by cutting or molding.
This characteristic is beneficial in the biocompatible materials used in
dentistry, since dental inlays, crowns, and veneers have to be produced
in different shapes and sizes, depending on the tooth that needs to
be repaired. In addition, these glass-ceramics must allow the dental
professional who makes the tooth replacement to adjust the color and
the light-transmitting ability of the materials to suit the individual
patient. Of course, these materials have to be chemically and biologically
compatible. The glass-ceramics mentioned combine all these character-
istics and they can be produced in large quantities. Nevertheless, they
cannot match the strength of lithium disilicate glass-ceramics. However,
because they combine such a large number of favorable characteristics,
these materials have a wide variety of uses (product: IPS Empress
®
).
Recent research results have shown that certain types of lithium
disilicate glass-ceramics can also be shaped by molding or cutting. If the
material is to be cut into the desired shape, it has to be done while it is
in a soft condition. Afterwards, the shaped object is fired to obtain the
strong glass-ceramic end product (IPS e.max
®
CAD).
7.6 OUTLOOK
Today, glass-ceramics with a large number of different characteristics
and uses are available. Their variety is achieved by taking advan-
tage of special mechanisms called controlled nucleation and controlled
