For more than a decade, clinical trials have recorded high
chipping rates in porcelain fused to zirconia (PFZ) dental restorations because
of the relatively weak bond between the porcelain veneer and strong zirconia
core. Porcelain fused to metal (PFM) dental restorations experience veneer
chipping as well, however, clinical evidence has shown they perform better than
PFZ restorations [1]. The major problem with PFM restorations is the metal
substructure does not provide the most appealing esthetics. Therefore,
researchers and clinicians developed monolithic all-ceramic dental restorations
in an attempt to resolve these issues. One such monolithic all-ceramic dental restoration
was developed by Ivoclar-Vivadent called IPS e.max® lithium disilicate.
When considering U.S. dental patient’s high demand for
esthetically driven restorations, a dental professional must make informed
decisions about the quality, longevity and esthetics of a dental restoration in
order to have a high acceptance rate among their patients. Thus, the dental
practice will be more successful by means of new and retained patients. This
article will briefly analyze the pros and cons of lithium disilicate
glass-ceramics.
Microstructure of
Lithium Disilicate (LS2)
Dental ceramics can be classified by their microstructure (i.e.,
composition of glass-to-crystalline ratio) [2]. Lithium Disilicate (LS2)
has a glass-based system (mainly silica) with crystalline fillers, formed by
the controlled crystallisation of the glass material [2]. The crystalline
filler particles are added to increase the strength of the glass-ceramic; cracks
caused by stress are “arrested by the lithium disilicate crystals, providing a
substantial increase in the flexural strength” [2].
Pros
One pro to lithium Disilicate glass-ceramic restorations is
an observed low chipping and fracture rate [1]. Lithium disilicate has an
unusual microstructure that contributes to its relatively high flexural
strength of 360-400MPa. The spread of cracks is deflected, branched and blunted
by lithium disilicate needle-like crystal structure – providing a substantial
increase in the flexural strength [2]. In a 2015 study, Valenti evaluated the
clinical performance of 110 lithium disilicate crowns over a 9-year period and
found 2 crowns had core fractures and 1 crown chipped. They found the “overall survival probability was 96.1% up to 9 years, with a
failure rate of 1.8%” [3].
Six other clinical studies (Richter et al. 2009; Nathanson
2008; Reich et al. 2010; Fasbinder et al. 2010; Bind 2011; Sorensen et al.
2009b) showed 97.9% of 237 IPS e.max ® lithium disilicate (CAD crowns)
restorations survived after a mean observation period of 4 years. The 2.1%
failure rate included 0.4% irreparable chipping, 0.4% secondary caries and
1.3% fractures. Glass-ceramics exhibit lower strength relative to Porcelain
fused to zirconia but higher chip fracture resistance. High chip fracture
resistance is due to an elongated grain structure that inhibits crack
propagation and contributes to its toughness, while it limits the strength by
“promoting crack initiation within a relatively coarse microstructure” [1].
Another pro is lithium disilicate has superior esthetics
compared to other ceramic materials. The monolithic structure and glass-ceramic
system of lithium disilicate allows the material to take on translucent and
light diffusing properties. Natural enamel has translucent qualities that
transmits light rather than reflect or scatter it. The optical compatibility
between the glassy matrix and the crystalline phase minimizes internal
scattering of the light as it passes through the material—high translucency
depends on the transmission of light through the core material [2]. Sravanthi
et al. study argues that the “core translucency therefore becomes one of the
primary factors in controlling aesthetics and is a critical consideration in
selection of materials.” The study used a reflectance spectrophotometer
(records the amount of light reflected) to compare the translucency of crowns
fabricated with three different commercially available all-ceramic materials
including alumina - CAD-CAM Procera, Lithium disilicate - Pressable IPS e.max
Press, Zirconia - CAD-CAM Lava [4]. Out of all three ceramic materials, lithium
disilicate was found to have the maximum translucency even when the coping was
at a 0.8mm thickness.
Lastly, glass-ceramics can be “readily etched and silanized
at the intaglio surfaces, with attendant superior bonding to a support
substrate” [1]. In other words, lithium disilicate has the ability to be easily
etched or milled out of a homogenous material by computer-aided-design/computer-aided-manufacturing
(CAD/CAM) digital systems. This technology assures acute accuracy of the
fabrication of the designed restoration. The silanization of the restoration’s
surface helps increase the hydrophobicity of the glass-ceramic.
Cons
Although lithium disilicate crown applications have a high survival
and success rate, they may be “less suitable for applications where stress
concentrations can be high, e.g. FDP [fixed dental prosthesis] connectors” [1].
The 360-400MPa flexural strength of lithium disilicate is relatively weak
compared to the 1,000MPa flexural strength of zirconia-based restorations. For
this reason, FDPs with frameworks made of glass-ceramics require “larger
connector areas to meet load-bearing requirements relative to their stronger
zirconia counterparts” [1]. Whereas, lithium disilicate single crowns are
supported by underlying dentin that exhibits greater strength and stability
then lithium disilicate multiple-unit restorations. However, a 10-year study evaluated
the long-term outcome of three-unit FDPs made from monolithic lithium
disilicate ceramic (IPS e.max Press, Ivoclar Vivadent) and found that the
survival and success rates were similar to those of conventional metal-ceramic
FDPs [7].
Next, one
study analyzed the friction and wear behavior of human enamel that opposed lithium
disilicate restorations and found evidence of abrasive wear behavior of the
opposing enamel [5]. Lee et al. found the “enamel that opposed
lithium disilicate glass ceramic exhibited cracks, plow furrows, and surface
loss, which indicated abrasive wear as the prominent wear mechanism.” It’s worth noting that adequate surface
finishing of dental ceramics influences wear on opposing enamel and requires
competent skills of the dental laboratory to achieve an acceptable polished surface. A rough surface will abrade
the opposing enamel of a restoration and “it is highly recommended that the
surface is finished and polished appropriately” [6].
To conclude, the flexural strength of previous glass-ceramic materials was relatively
weak until the development of highly esthetic and strong lithium disilicate glass-ceramics. The
highly specialized monolithic structure of IPS e.max ® lithium disilicate
by Ivoclar Vivadent can achieve
great structural integrity with superior translucent esthetics compared to
other all-ceramic materials. For this reason, Lithium disilicate is a viable
option for dental patients who are seeking longevity and beautiful esthetics
for their dental restoration.
About the Company:
Since 1991, Iverson Dental laboratory has provided dentists
with high quality dental lab products to ensure
their patients are completely satisfied with the restoration outcome. Iverson’s highly
knowledgeable team of certified dental technicians specialize in all-ceramic
restorations, dental implants, cosmetic and digital dentistry. Iverson dental
labs strongly believe in using high quality certified materials and authentic
manufacturing components to fabricate their dental restorations.
To inquire more about all-ceramic dental lab products, please visit http://www.iversondental-labs.com.
Reference:
1. Zhang Y, Chai H, Lee JJ,
Lawn BR. Chipping resistance of graded zirconia ceramics for dental crowns. J Dent Res. 2012;91:311–5. [PMC free article] [PubMed]
2. Shenoy A, Shenoy N. Dental ceramics: An update. J Conserv Dent. 2010;13:195–203. [PMC free article] [PubMed]
3. Valenti M, Valenti A. Retrospective survival analysis of
110 lithium disilicate crowns with feather-edge marginal preparation. Int J Esthet Dent. 2015
Summer;10(2):246-57.
4. Sravanthi, Y., Ramani,
Y. V., Rathod, A. M., Ram, S. M., & Turakhia, H. (2015). The Comparative
Evaluation of the Translucency of Crowns Fabricated with Three Different
All-Ceramic Materials: An in Vitro Study. Journal
of Clinical and Diagnostic Research : JCDR, 9(2), ZC30–ZC34.
doi:10.7860/JCDR/2015/12069.5559
5. Lee, A., Swain, M., He, L., & Lyons, K.
(n.d.). Wear behavior of human enamel against lithium disilicate glass ceramic
and type III gold. The Journal of
Prosthetic Dentistry, 1399-1405.
6. Rashid, H. (n.d.). The effect of surface
roughness on ceramics used in dentistry: A review of literature. European
Journal of Dentistry Eur J Dent, 571-571.
7. Kern, M., Sasse, M., & Wolfart, S.
(n.d.). Ten-year outcome of three-unit fixed dental prostheses made from monolithic
lithium disilicate ceramic. The Journal
of the American Dental Association, 234-240.
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