CEREC doctors.com - Q1 2017 - page 44

42 | CERECDOCTORS.COM | QUARTER 1 | 2017
C A S E S T U D Y | | | B Y D E N N I S J . FA S B I N D E R , D . D . S .
the introduction of cerec zirconia for chairside restorations makes the management of the surface finish for
zirconia a much more important consideration for clinicians. There has been concern that full-contour zirconia restorations
are significantly abrasive to opposing functional surfaces due to the high hardness of zirconia. However, wear is actually a
direct result of surface smoothness, not surface hardness. A smooth surface is less abrasive to opposing surfaces compared to a rough
surface, regardless of the hardness of the surface. Therefore, creation of a very smooth surface is critical to prevent abrasive wear of
opposing functional surfaces.
Surface Finish of Full-contour
Zirconia Restorations
Preventing Abrasive Wear of Opposing Dentition
So, the obvious question is what is the best technique to create
the smoothest zirconia surface to prevent abrasive wear of the
opposing dentition? Glazed surfaces are often considered the
smoothest surfaces resulting in a high-gloss finish. However, the
glazing porcelain is a very weak porcelain material that is easily
worn away from the zirconia surface by either occlusal function
or occlusal adjustment by the clinician. This leaves a potentially
abrasive zirconia surface if it was not contoured or polished prior
to application and firing of the glaze.
The surface roughness of polished zirconia, glazed zirconia,
polished then reglazed zirconia, veneering porcelain and enamel was
assessedafterexposure to functionagainst enamel antagonists inan in
vitro wear simulator.1 Volumetric wear was measured at baseline and
after simulated wear testing. Polished zirconia surfaces had surface
roughness values thatwere smoothest compared to theother surfaces
that were evaluated. Polished zirconia surfaces caused the least
amount of opposing enamelwear; glazed zirconia surfaces caused the
most; andpolished thenreglazedzirconiawas intermediate inenamel
wear. Polished zirconia produced less wear than enamel against
enamel, and all the treatments produced less wear than veneering
porcelain. Quite a few other laboratory studies confirm that polished
zirconia surfaces caused significantly less abrasive wear of opposing
functional surfaces compared to glazed zirconia.2-6
There are two times in the clinical workflow where polishing
would be considered. The first is during processing of the crown,
either prior to or after sintering. Extraoral contouring and
polishing is the easiest technique to efficiently achieve a smooth
zirconia surface. The second time is after occlusal adjustment
following delivery of the crown. Intraoral adjustment leads to a
rough surface that should be repolished to ensure a compatible
surface with the opposing dentition.
Some concern has been expressed that the mechanical stress
imposed on the surface of zirconia by contouring and polishing
may lead to a phase shift in thematerial. Zirconia exists as a tetrag-
onal phase that contributes significantly to the superior physical
properties of the material.7 Stress and heat may induce the tetrag-
onal phase to shift to a weaker monoclinic phase. This phase shift
also induces a compressive layer that opposes the progression
of cracks through the body of the zirconia. However, this crack
initiation resistance may be decreased or lost with greater surface
cracks that may be caused by aggressive contouring or adjust-
ment.8 Several studies did not document a phase shift or change in
the zirconia following polishing, in spite of the concern.8
The milled restoration in the pre-sintered state is very easy to
adjust and contour before the restoration is sintered. This leads to
the consideration to do the contouring and finishing of the restora-
tion in the softer, pre-sintered phase, rather than the significantly
harder, sintered phase. The pre-sintered phase is soft enough
that additional occlusal anatomy may be carved into the restora-
tion using a hand instrument. However, this is not without risk of
damaging the restoration. Figure 1 shows the very smooth margin
integrity of a pre-sintered CEREC zirconia restoration. Figure 2
shows the potential margin damage created in the same restora-
tion by using a diamond-impregnated rubber wheel to contour
the margin area. This demonstrates the potential risk of margin
damage from contouring a pre-sintered restoration.
Realizing that adjustment ofmargins in the pre-sinteredphase is
a potentially risky endeavor, could the surface finish be improved
by polishing the bulk of the pre-sintered restoration while
avoiding the margin areas? One suggestion has been to do gentle
contouring with spiral polishers in the pre-sintered phase when it
is easier to do so compared to the final sintered restoration. A pilot