CEREC doctors.com - Q3 2016 - page 55

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“... Unlike strength, fracture tough-
ness has been observed to generally
correlatewith clinical indications.” 1
CAN YOU DESCRIBE THE TEST
AND HOW IT IS MEASURED?
Several methods can be used to
obtain fracture toughness. In the
most common method (SEVNB),
specimens are generated in the form of bars. A V-shaped notch is
cut into each bar. The specimen then has force applied until the
crack propagates all the way through the bar (Fig. 1).
HOW IS FRACTURE TOUGHNESS CLINICALLY RELEVANT?
First, it’s inherent to a material and allows for meaningful compar-
isons of dental ceramics. In the material science world, this is a
strong statement and allows for an “apples to apples” comparison.
Second, the higher the fracture toughness, the lower the prob-
ability for failure at any given load.
IF THAT’S THE CASE, CAN THE VALUES BE USED TO DEFINE
MATERIAL INDICATION LIMITATIONS?
Yes. The ISOStandardusesmaterial toughness as a guide to indicate
which materials are recommended in specific indications (Fig. 2).
Fig. 2
RECOMMENDED FRACTURE TOUGHNESS VALUES
FOR CERAMICS BASED ON CLINICAL INDICATION
Here are recommended clinical indications with minimum
fracture toughness MPa · m1/2. This table is
derived from
the classification system and recommended methods for
measuring fracture toughness in Annex A of ISO 6872:2008.
CLASS 1: 0.7
• Monolithic ceramic for single-unit anterior prostheses,
veneers, inlays, or onlays adhesively cemented.
• Ceramic for coverage of a metal framework or a ceramic
substructure.
Commercial examples: Many, including Vita CEREC blocks
CLASS 2: 1.0
• Monolithic ceramic for single-unit anterior or posterior
prostheses adhesively cemented.
Commercial example: Leucite-reinforced
• Fully covered substructure ceramic for single-unit anterior
or posterior prostheses adhesively cemented.
Commercial example: In-Ceram Alumina
CLASS 3: 2.0
• Monolithic-ceramic for single-unit anterior or posterior
prostheses and for three-unit prostheses not involving molar
restoration, adhesively, or non-adhesively cemented.
• Fully covered substructure for single-unit anterior or
poster-ior prostheses and for three-unit prostheses not
involving molar restoration, adhesively, or non-adhesively
cemented.
Commercial examples: Lithium disilicate, In-Ceram Alumina
or Leucite-reinforced
CLASS 4: 3.5
• Monolithic-ceramic for three-unit prostheses involving molar
restoration.
Commercial example: Lithium disilicate
• Fully covered substructure for three-unit prostheses involving
molar restoration.
Commercial example: Zirconia
CLASS 5: 5.0
• Monolithic-ceramic for prostheses involving four or more
units or fully covered substructure for prostheses involving
four or more units.
Commercial example: Zirconia
derivative to fracture toughness and extremely sensitive to sur-
face condition, varying 1,000-fold depending on how specimens
are made. Strength is measured by simplified bars or discs and
does not include very influential variables in crown failure, such
as the effects of water, laboratory or clinical surface treatment
(e.g., sandblasting, etching, silanization), cement effects, and
the elastic modulus of the cement and build-up material.
So, fracture toughness is more of an inherent material property
than strength, and fracture toughness is predictive of the dura-
bility of single crowns. With resin-based composites, fracture
toughness correlates with marginal integrity and attritive wear.
We now have at least five to 10 years of clinical data on exist-
ing all-ceramic systems or closely related ones (Della Bona and
Kelly, JADA, 2008). The ISO working group has developed a novel
classification scheme for the 2008 revision of ISO 6872 based on
indications from clinical data. As part of this, fracture toughness
was also introduced; although still in an annex, fracture tough-
ness will eventually become a required measurement.
Fig. 1:
SEVNB SPECIMEN
Span = 4W
F/2 F/2
F
B
W = B or 2B
a