Porcelain
inlays and onlays
Porcelain has been used for many years to
simulate the appearance of tooth structure. Dental
labs painstakingly hand crafted porcelain over metal
structures and fired them in an oven much like porcelain
dishes are made. Most porcelain crowns had a metal
structure underneath to provide rigidity because porcelain
is strong but brittle. While it takes a lot of force
to “flex” porcelain, when it was flexed
as little as .001% it would shatter. The metal helped
to resist the flexing of the porcelain under load
but it also prevented a porcelain fused to metal crown
form looking totally natural. Like will reflect internally
in a tooth but it can only penetrate and reflect as
deep as the metal layer in a crown.
It was discovered that hand made porcelain always
contained voids on a microscopic basis and these voids
were fractures begin under stress. Experiments were
conducted to use very small particle size, mix the
porcelain under vacuum and producing a paste, which
could be extruded into a die. This material proved
to be much stronger. A technique was developed to
use a lost wax die process and produce a very accurate
and strong porcelain. When the firing process was
controlled by computers, the increase and decrease
of temperature could be controlled to minimize the
internal stress produced in porcelain during cooling.
Again an increase in strength was found.
When these procedures were all put together and special
equipment was produced to allow dental labs to fabricate
porcelain the process was called “pressed porcelain”
and the use of all porcelain restorations became practical.
This process became commercially available about 1988
and more than 15 million of these have been placed
in teeth. Further improvements came when a special
lithium disilicate framework was covered with fluorappetite
glass ceramic. The framework provides the rigidity
previously provided by metal and the fluorappetite
crystal closely mimics the physical and optical properties
of tooth enamel. The framework handles light much
like the inner dentin layer of tooth structure. The
combination of materials yields a product three times
stronger than the first pressed material and is in
fact stronger than natural tooth structure. It also
looks very natural.
It is possible to bond these restorations to teeth
with a force stronger than tooth structure. The result
is a tooth/restoration structure stronger than the
original tooth! Teeth that would have needed crowns
can now be restored with bonded high strength porcelain.
Because there is much less drilling required for the
inlay/onlay procedure versus a crown procedure, There
is less tooth sensitivity and fewer root canal treatments
required.
