48
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CERECDOCTORS.COM
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QUARTER 2
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2016
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 .
chairside cad/camceramic restorations have relied on an adhesive bond
between the tooth and ceramic to ensure
a dependable, long-term clinical result. The adhesive bond is a trusted technique for retention of the restoration as well as
achieving maximum strength of the ceramic by creating a micromechanical bond to the tooth preparation and ceramic.
Is Adhesive Bonding an Option
for Zirconia Restorations?
What If the Retentive Feature of Preparation Design Is Not Optimal?
The recent innovation of CEREC Zirconia, a full-contour zirconia
material for restorations in a chairside CAD/CAMworkflow, encour-
ages the review of evidence on recommended materials and tech-
niques for delivery of the zirconia restorations. Zirconia has obvious
physical properties that make it desirable for high-stress clinical
applications. It has a high flexural strength of 900-1200 MPa, frac-
ture resistance of > 2000 N, and fracture toughness of 9-10 MPa that
is generally at least twice as great as other glass ceramicmaterials.
1
Etching the surface of a glass ceramic restorationwith hydroflu-
oric (HFl) acid selectively creates microporosities in the surface
of the ceramic. Coating the etched surface with a silane coupler
promotes adhesion between the inorganic ceramic surface and the
organic phase of the bonding adhesive through siloxane bonds.
2
The silane coupler also increases the surface energy of ceramic
surface, and improves cement wettability.
3
Zirconia cannot be
adhesively bonded similar to other glass ceramic materials.
Zirconia is a polycrystallinematerial with < 1 percent glass compo-
nent and very low silicon dioxide content. Due to the lack of a glass
component, zirconia is resistant to HFl acid etching. This negates
the usual etching process to create microporosities in the surface
of the restoration for adhesive bonding. Laboratory or bench-top
studies show that as-fabricated full-contour zirconia crowns have
little to no bond capability with resin cements.
The high strength properties of zirconia allow it to be cemented
rather than adhesively bonded, which is generally preferred by clini-
cians rather than adhesive bonding. However, this implies that the
main retentive feature for zirconia restorations is the preparation
design. Appreciating that theremaybe clinical situations inwhich the
retentive features of the preparationmay not be optimal, can zirconia
be predictably bonded to the tooth preparation to improve retention?
Several systematic reviews of adhesive bonding to zirconia have been
published that explore thevarious techniques inconsiderabledetail.
4,5
Surface roughening of the zirconia, combined with a surface
coating, have been considered as a means to create an adhesive
bond to zirconia. One consistently evaluated technique to prepare
the zirconia surface for bonding is to use airborne-particle abra-
sion (AA). Particle sizes of 30 mm-50 mm with lower pressures of
20-40 psi for 10-20 seconds are generally recommended.
This will increase the surface roughness of the zirconia mate-
rial. It also will increase the surface energy and reduce organic
contaminants (basically, it cleans the surface). The increase in
surface roughness and surface energy also improves the wetta-
bility of the zirconia surface.
There are two potential risks for airborne-particle abrasion of
zirconia. One is the creation of surface cracks. And the second is
to cause a phase transformation from tetragonal to monoclinic.
Both of these outcomes may reduce the physical properties of
the zirconia. These risks can be avoided primarily by using less
aggressive airborne-particle abrasion.
This includes using larger particle sizes and lower pressures for
a shorter time period. Think “cleaning” rather than “cutting” for
the application of airborne-particle abrasion. These risks have been
documented in laboratory studies; however, there are no long-term
clinical studies that have been published indicating that these risks
actually result in a lower success rate for zirconia restorations.
Although itmay be tempting to just use a diamondbur to roughen the
internal surfaceof a zirconia restoration, grinding the internal surface
does nothing but aggressively roughen itwhile thinning the axialwall
of the restoration, and significantly increasing the riskofmicrocracks.
One specific type of airborne-particle abrasion for zirconia surface
preparation involves the use of aluminumoxide particles coatedwith
silica (Fig. 1). This tribochemical silica coating technique increases
surface roughness aswell as embeds silicon on the zirconia surface. A
silane coupler can produce a chemical bond between the silica on the
modified zirconia surface and adhesive resin cement. Tribochemical
silica coating has beendocumented in laboratory studies to be signifi-
cantly better than just conventional airborne-particle abrasion, espe-
cially with the use of a silane coupler.
