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thickness did not crack during artificial aging, whereas two of the
crowns with 0.5 mm wall thickness did fracture. Thermocycling
and chewing simulation only had a significant effect for crowns
with d= 0.5 mm.
Although the results of these two laboratory studies might be
encouraging support for reduced thickness e.max CAD restora-
tions, the results should be considered with a degree of caution.
Both studies were done on extracted teeth and did not account
for fatigue failure of the restorations. Intraoral restorations gener-
ally fracture due to fatigue failure rather than a catastrophic load
applied until the tooth or restoration fractures. Both studies also
adhesively bonded the restorations to the prepared teeth. Several
laboratory studies have shown the increased strength of ceramic
restorations that are adhesively bonded rather than cemented, and
it is doubtful that the results would be duplicated with cemented
rather than adhesively bonded restorations. A significant question
to consider is whether there is any clinical evidence to support
reduced thickness e.max restorations.
Systematic reviews provide very good summaries of clinical
evidenceover a specific timeperiod. Systematic reviews for lithium
disilicate are complicated by the inclusion of not just e.max CAD
and e.max press, but also Empress 2 (Ivoclar) studies. Empress 2
was an earlier version of a type of lithium disilicate; however it is
not the same as e.max. But since all of these are lithium disilicate
materials, they tend to be combined in systematic reviews.
One systematic review analyzed the short-term (up to five
years) and medium-term (up to 10 years) survival rates of lithium
disilicate single crowns on natural teeth (Pieger, et a, 2014). They
reviewed the literature between January 1998 and June 2013, and
discovered 136 abstracts from which 12 clinical studies qualified
for inclusion in the review. This included 696 lithium disilicate
crowns for 519 patients. There were nine total crown failures,
none of which occurred in the first three years of clinical service.
Four failures involved fracture of the veneering porcelain and five
failures involved facture of the lithium disilicate core. The five-
year cumulative survival rate was calculated to be 97.8 percent for
lithium disilicate single crowns.
One clinical study that focused specifically on e.max reported on
860 lithium disilicate restorations placed by six prosthodontists
(Fabbri et al 2014). The retrospective study assessed the clinical
performance of anterior and posterior lithium disilicate crowns,
onlays and veneers on both natural teeth and implants over six
years of clinical service (Table 1). The crown preparations were
0.3 to 1.0 mm axially and at least 1.5 mm occlusally.
IPS e.max press was used in both monolithic and layered
techniques. Of the 428 tooth-supported single crowns, 231 were
anterior and 197 were posterior crowns. Four-hundred-and-
seven of the restorations were pressed with 274 layered and 154
monolithic, and 21 crowns were CAD/CAM. Implant-supported
restorations accounted for 52 of the crowns. All the restorations
were adhesively cemented with several different resin cements
including RelyX Unicem (3M ESPE), Multilink Automix (Ivoclar)
and Variolink II (Ivoclar). There were five veneers that had small
chips that were smoothed and remained in clinical service. There
was one veneer that debonded. It was re-cemented and remained
functional. There were eight crowns with minor chipping, with
two of the crowns beingmonolithic e.max. Two of the crowns frac-
tured, requiring replacement, and two crowns debonded. Three
of the onlays had minor chipping, and one onlay debonded. Ante-
rior layered crowns had a 97.6 percent success rate, while ante-
rior monolithic crowns had a 95.5 percent success rate. Posterior
layered crowns had a 95.4 percent success rate, while posterior
monolithic crowns had a 96.2 percent success rate. Monolithic
restorations had the lowest number of technical complications,
with no failures of monolithic veneers and only three monolithic
crowns affected by chipping.
Clinical outcomes are very good for lithium disilicate restora-
tions. There are several clinical studies that have reported specifi-
cally on e.max CAD. One short-term clinical study evaluated
CAD/CAM-fabricated monolithic crowns made of lithium disili-
cate ceramic, as well as complete CAD/CAM-fabricated crowns
consisting of a framework made of zirconia and a milled, sintered
veneer made of lithium disilicate, as possible clinical solutions
to avoid surface chipping (this is problematic for conventional
veneered ceramic crowns) (Seydler & Schmitter, 2015). Thirty
monolithic lithium disilicate crowns were made using the CEREC
system (Bluecam) using e.max CAD blocks, and 30 bilayer crowns
were made from ZirCAD zirconia veneered with milled e.max
CAD with the CAD-on technique. All crowns were laboratory
fabricated requiring two appointments to prepare and deliver
the crowns. All crowns were adhesively cemented with Multilink
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FA S B I N D E R
TABLE 1:
LITHIUM DISILICATE RESTORATIONS PLACED
Crowns
Onlays
Veneers
Maxillary
329 21 203
Mandibular
151 41 115
Total
480 62 318
