CEREC doctors.com - Q4 2016 - page 54

52 | CERECDOCTORS.COM | QUARTER 4 | 2016
Test samples were fabricated from type 3 cast gold, two CAD/CAM
high-strength ceramics and a CAD/CAM feldspathic porcelain.
Celtra Duo samples included both milled (not polished) and glaze-
fired surfaces. The wear resistance was analyzed by measuring the
vertical substance loss (mm) and the volume loss (mm3). Antagonist
wear (mm) was also measured. There was no significant difference
in the surface wear between milled and glazed Celtra Duo. Glazed
CeltraDuohadwear depth and volume similar to enamel. Therewas
no significant difference in wear depth or volume for the glaze-fired
CeltraDuo or other ceramicmaterials compared to enamel. The fine
microstructure of Celtra Duo allows for the creation of a smooth
surface tominimize wear of opposing teeth.
The option of glazing Celtra Duo reportedly increases the flexural
strength to 370 MPa. Glaze firing at 820 degrees Celcius causes the
glassy matrix to “heal” any surface microfractures that arise during
milling. An important consideration is support for the Celtra Duo
restoration during the glaze firing. The restoration should not be
filled with refractory putty, as it is already fully crystalized and no
significant volumetric changes occur during glaze firing. In fact, the
restoration can be placed directly on a firing padwithout the need for
any internal support. If a supportive material is required to prevent
the glaze surface from touching the firing pad, SuperPegII is the
preferredmaterial as it has a similar coefficient of thermal expansion
(CTE) as Celtra Duo. A small amount should be used to fix the resto-
ration to the firing pin, but it should not completely fill or encase the
entire restoration. Some degree of material movement will take place
during firing, and it should not be restricted. This is not a volumetric
change in the restoration, but more a thermo relaxation as the mate-
rial is heated and cooled. The restoration should not be restricted
frommovement during firing, or internal stress may be incorporated
into the restoration that may be a contributor to future fracture.
The increase in strength of the Celtra Duo as a result of glaze
firing is not a function of the glaze layer, but more related to heating
the material sufficiently to have the glass matrix heal the surface
microfractures. This suggests that a similar increase in strength
can be achieved without the glaze layer application, and place the
restoration directly in the porcelain oven in a process referred to as
“dry firing.” There is no significant structural or crystal change that
occurs to thematerial during firing as the block is already fully crys-
tallized. And dry firing would not significantly improve the surface
roughness of the material. So, the restoration should be polished
prior to dry firing to take advantage of the ability of polishing it
prior to the increase in strength. Although this potentially is an effi-
cient technique for creating a high-strength restoration, there is no
clinical evidence that this more efficient firing process would also
provide longevity expected of high-strength ceramic materials.
Another significant process that would impact on the strength
and possible longevity of the restoration is the cementationmaterial
and technique. Lower-strength ceramics, such as feldspathic and
leucite-reinforced, mandate adhesive cementation to improve the
fracture resistance of the glass ceramics. Most ceramic fractures are
initiated on the internal surface of the restoration and the adhesive
resin cement prevents the initiation of fractures internally, as well
as provides excellent retention for the restoration. As the inherent
strength of the ceramic increases, such as for zirconia, there is an
opportunity to conventionally cement the restoration.
One in vitro study investigated the influence of the type of cement
on the fracture resistance of zirconia reinforced lithium silicate
(Celtra Duo) crowns.3 Two conventional glass ionomer cements, a
self-etching, self-adhesive resincement andadual-cure resincement
were used to cement the oven-glazed Celtra Duo crowns. The frac-
ture resistance was compared to a control group of crystallized
e.maxCAD crowns cementedwith dual-cure resin cement. Thermal
cycling and mechanical loading against enamel antagonists was
performed in a chewing simulator for a calculated five years of clin-
ical serviceprior to loading the crowns to failure inauniversal testing
machine. The highest fracture load was reported for both Celtra
Duo and e.max CAD crowns cemented with dual cure resin cement.
However, there was no significant difference in fracture resistance
found between cement types. This study concluded that Celtra Duo
may be delivered with other cements than dual-cure resin cements
counter to the manufacturer’s recommendations. The careful clini-
cianwouldbe advised touse adhesive resincement until such time as
clinical evidence became available that non-adhesive cements were
equally successful in supporting Celtra Duo restorations.
A plan for longevity with Celtra Duo restorations would seem
dependent on understanding the unique material properties of a
zirconia-reinforced lithium silicate material. And careful atten-
tion to factors including tooth preparation, material processing
technique and adhesive delivery will provide clinicians the best
possible outcome.
For questions or more information, Dr. Fasbinder can be reached at
djfas@umich.edu.
REFERENCES
1 Krüger S., Deubener J., Ritzberger C., HölandW. Nucleation kinetics of lithium
metasilicate in ZrO2-bearing lithiumdisilicate glasses for dental application.
International Journal of Applied Glass Science. 2013;4(1):9–19. doi: 10.1111/ijag.12011
2 D’ArcangeloC,VaniniL,RondoniGD,andDeAngelisF.Wearpropertiesofdentalceramics
andporcelainscomparedwithhumanenamel.JProsthetDent2016;115:350-355
3 Preis V, Behr M, Hahnel S, Rosentritt M. Influence of cementation on in vitro
performance, marginal adaptation and fracture resistance of CAD/CAM-fabricated
ZLSmolar crowns. Dent Mater 2015 31:1363-1369.
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