Orthotown November 2017 - 35
Fig. 2
Fig. 3
Chemically etching artificial substrates
If clinicians decide not to use a sandblaster to prepare
ceramic surfaces, the bonding protocol is significantly
lengthened. First, the glaze must be mechanically removed
with a diamond bur to achieve initial rough-surface
retention. The rotational movement of a bur will only create
shelves and gouges through the crown (Fig. 2), so more
retention is needed. To further the mechanical retention of
a diamond-bur-roughened surface, hydrofluoric acid must
be placed on the tooth for four minutes to chemically etch
the glass fi ller in both porcelain and lithium disilicate.
Hydrofluoric acid is very caustic and the soft-tissue margin
must be protected with a barrier gel before any acid is
applied in the mouth. Once four minutes has expired, the
etch is carefully wiped off the tooth and rinsed into suction.
Utilizing this method generates shear values almost half
the strength of using a microetcher.
Clinicians may also be under the impression that
they're bonding to a porcelain surface, but the substrate
ends up being zirconia. When hydrofluoric acid is placed
on a zirconium substrate, the surface will not etch. (See
Figs. 3 and 4.) Thus, the clinician must forgo the typical
step of applying silane or wait long enough for hydrolyzed
silane to evaporate off the tooth-without interfering with
the fi nal primer step.
There are two scenarios where a clinician may encounter
a dual surface of artificial and enamel in the bonding area:
amalgam and composite restorations.
When bonding to a dual surface, the enamel must
be chemically etched and the artificial surface properly
roughened. However, there is a common misconception
that phosphoric acid will simply "clean" the tooth surface.
Th is often stems from the restorative protocol of using
phosphoric acid to clean the inside of a crown after
trying-in and removing from the patient's mouth. (Any
blood and salivary proteins that have adhered to the
crown are neutralized by phosphoric acid and bonding
can commence.) In orthodontics, however, this method
Fig. 4
of "cleaning" a non-enamel surface with phosphoric acid
can be disastrous on one surface. Phosphate ions have an
affinity for metal and will bond to zirconia-preventing the
following bonding agents from adhering to the zirconium
substrate. If there is no enamel present, do not etch.
orthotown.com \\ NOVEMBER 2017
35
http://www.orthotown.com
Table of Contents for the Digital Edition of Orthotown November 2017
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