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48 | CERECDOCTORS.COM | QUARTER 4 | 2014
disilicate and infiltration ceramic were
cemented to composite dies using zinc
phosphate cement andPanavia 21. Adhe-
sive cementation provided a significant
reinforcing effect on the lithium disili-
cate and infiltration ceramic crowns at
start of fracture. The use of the adhe-
sive cement also prevents penetration
of oral fluids to the internal surface of
the ceramic materials preventing water
corrosion of the ceramic.
Another in vitro study compared the
fracture load of lithium disilicate (e.max
CAD/Ivoclar) and Lucite-reinforced
ceramic (EmpressCAD/Ivoclar) based
on cementation technique and crown
thickness. Thirty crowns were fabri-
cated for each crown material at a thick-
ness of >1.5 mm and <1.0 mm.
Fifteen crowns for each material and
thickness were cemented on composite
dies using a conventional glass ionomer
cement (Vivaglass/Ivoclar) and an adhe-
sive resin cement (MultiLink/Ivoclar).
Analysis of the fracture load indicated
that the lithium disilicate crowns <1.0
mm and cemented with the GIC were
significantly weaker than all other
lithium disilicate crowns. There was
no significant difference between the
>1.5 mm cemented and <1.0 mm adhe-
sively bonded lithium disilicate crowns.
Further, the >1.5 mm adhesively bonded
crowns were significantly stronger than
all other lithium disilicate crowns.
The results indicate the significant
contribution adhesive cementationmakes
to the fracture load of lithium disilicate
crowns, as well as the importance of the
recommended thickness of at least 1.5mm
to ensure the increased flexural strength
possible with lithiumdisilicate.
These types of in vitro studies indicate
the potential importance of adequate
thickness of the lithium disilicate and
adhesive bonding to ensuring maximum
strengthandfractureresistancetolithium
disilicate crowns. However, as with all
in vitro studies, they are not necessarily
predictive of clinical performance.
One systematic review article has been
published on the clinical outcomes of
lithiumdisilicate crowns and bridges.The
review sorted 2,033 manuscripts to deter-
mine 12 acceptable clinical studies on
lithiumdisilicatecrownsandbridgesusing
both press-fit and CAD/CAM lithium
disilicate materials. A closer inspection of
the 12 clinical studies revealed that only
a few of the studies involved chairside
CAD/CAM lithiumdisilicate crowns.
One of the clinical studies evaluated
posterior lithium disilicate crowns after
four years of clinical service. All crowns
were fabricated chairside and adhesively
cemented with a self-etching, self-adhe-
sive resin cement (MultiLink Sprint/
Ivoclar). Twenty-nine of the initial 41
crowns were available for recall evalua-
tion after four years. One crown fractured
at 2.8 years; two crowns had recurrent
caries that was repairedwith a composite
restoration; and two crowns required
endodontic treatment. A Kaplan-Meier
survival rate of 96.3%was reported.
Another clinical study is an ongoing
longitudinal clinical study evaluating 100
e.max CAD crowns over five years. The
first 62 lithium disilicate crowns were
fabricated chairside with the CEREC
system. Then the crownswere adhesively
bondedwith a self-etching bonding agent
and dual-cure, adhesive resin cement
(MultiLink/Ivoclar), as well as an experi-
mental self-etching, self-adhesive resin
cement. Thirty-eight additional crowns
were delivered chairside with a commer-
cial self-etching, self-adhesive resin
cement (MultiLink Sprint/Ivoclar). No
post-operative sensitivity was reported
with any of the cements at any recall
interval. No crown fractures or crown
failures were reported after four years.
Anecdotal reports of clinicians success-
fully using non-adhesive cements — such
as resinmodified glass ionomer cements—
todeliver lithiumdisilicatecrowns are rela-
tively common. Howcan this be successful
in spite of the in vitro evidence to the
contrary? In the previously mentioned
study comparing crown thickness and
cement type, >1.5 mm-thick lithium disili-
cate was not significantly different in frac-
ture strength to <1.0 mm-thick adhesively
bonded lithium disilicate. Although both
groups were significantly weaker than >1.5
mm-thick lithiumdisilicate that was adhe-
sively bonded, they both are similar in frac-
tureloadto1.5-mmthickleucite-reinforced
ceramic (EmpressCAD/Ivoclar) that is
adhesively bonded. And leucite-reinforced
chairsideCAD/CAMrestorations havehad
very good clinical success in spite of the
fact they do not reach the level of fracture
resistance of lithium disilicate. Clinicians
should realize that reducing the thickness
of lithiumdisilicate andusing conventional
cements do not ensure the potentially high
flexural strength available with lithium
disilicate crowns.
The systematic review of lithium disil-
icate crowns and bridges reported that
the short-term evidence (less than five
years) indicated an excellent survival rate
with a two-year cumulative survival rate
of 100 percent, and a five-year cumula-
tive survival rate of 97.8 percent. Notably,
all of the published clinical studies used
an adhesive bonding technique to deliver
the lithium disilicate crowns.
The reviewed laboratory evidence
strongly supports the use of the recom-
mended thickness of 1.5 mm for lithium
disilicate restorations, and the maximum
flexural strength is ensured when deliv-
ered with an adhesive resin cement.
Published clinical studies provide very
good evidence of the success of lithium
disilicate crowns when delivered with
an adhesive resin cement. No clinical
studies have been published using a
conventional cementation technique for
lithium disilicate crowns.
For questions and more information,
Dr. Fasbinder can be reached at
djfas@umich.edu.
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