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CERECDOCTORS.COM
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QUARTER 3
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2015
M AT E R I A L S
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B Y D E N N I S J . FA S B I N D E R , D . D . S . , A . B . G . D .
dentists, as well as dental manufacturers, have been on a long-term quest for the “holy grail”
of restorative
dentistry: a tooth-conserving, esthetic material with predictable clinical longevity for crowns.
For many years, cast gold restorations satisfied the predictable, tooth-conserving and longevity feature requirements —
that is, until esthetic concerns and cost influenced the move toward tooth-colored materials. As esthetic outcomes became a significant
feature, the quest has continued to evolve for a “white gold” restorative material.
Ensuring Outcomes With High-strength
Chairside CAD/CAM Restorations
The Search for the Perfect Restorative Material With Minimal Reduction
A key quality in this quest has been the strength of the restor-
ative material on the rationale that a stronger material will result
in a more “bullet-proof” restoration that will perform well over
time.
Laboratory testing can reveal important physical properties of
a material that may influence a logical consideration of treatment
options. However, the nature of clinical practice is to rarely use
materials in situations that are “ideal.” There are common clinical
limitations of short clinical crowns and limited interocclusal space
that can test the limits of what is needed for strength.
Strength can be measured in a number of ways. One popular
test is flexural strength. This is described as the breaking load
of a material as it is subjected to a bending force. The higher the
flexural strength, the stronger the material is considered. And it
is commonly perceived that this improved resistance to flexure
will increase the restoration fracture resistance. Although it
seems logical to expect that higher strength materials may have
better clinical performance, this does not take into consideration
a number of contributory factors that influence clinical success
such as occlusal load, restoration support, cementation material
and parafunctional stress. For this reason, flexural strength alone
is not predictive of clinical success.
A significant advantage of high-strength monolithic materials
is the ability to avoid surface chipping or fracture that may occur
with bi-layered, veneered crown substructures such as PFM or
zirconia crowns. The primary high-strength ceramic material
for chairside CAD/CAM restorations has been IPS e.max CAD
(Ivoclar). It is a 70 percent crystallized lithium disilicate ceramic
with a flexural strength of 360 MPa (Li, et al, 2014).
The complementary ingots of e.max that are used in the press-
fit process have a flexural strength of 400MPa. Although this may
be a statistically significant difference in flexural strength, there
is no clinical evidence that it translates into improved clinical
performance. Systematic reviews of ceramic materials will gener-
ally combine clinical studies on e.max CAD, and the press-fit
version of e.max as two different processes for lithium disilicate
restorations.
An important contributing factor to the strength of a restoration
is not only the restorative material but also how it is luted to the
tooth. The two most common techniques are: adhesive bonding
with some combination of tooth etching, bonding agent and resin
cement; or conventional cementation with either a glass ionomer
cement or a resin modified glass ionomer cement.
The difference is that resin cements integrate the ceramic resto-
ration to the tooth, providing support for the ceramic and limiting
internal crack propagation. Glass ionomer or resin modified glass
ionomer cements do not provide an adhesive bond to the ceramic,
and rely on the strength of the ceramic alone to provide the needed
strength to resist the stress of occlusal function.
Strength is also a function of the thickness of the material. Most
ceramic restorations are recommended by manufacturers to be
1.5 mm to 2.0 mm in thickness to ensure the maximum strength
potential of the ceramic. Reducing the ceramic thickness can
significantly reduce the strength, as strength is inversely related
to the square of the ceramic thickness. For example, reducing the
thickness from 2 mm to 1 mm essentially reduces the strength of
the ceramic by a factor of four (Seghi, et al, 1990).
One recent laboratory study measured the effect of reducing
the thickness of the material for both IPS e.max and IPS Empress
onlays (Bakeman, et al, 2015). Groups of extracted molar teeth
were prepared for full cuspal coverage onlays that were 1 mm
and 2 mm in occlusal thickness. A dental laboratory technician
fabricated onlays for each thickness from IPS Empress and IPS
e.max.
