CEREC doctors.com - Q1 2017 - page 10

8 | CERECDOCTORS.COM | QUARTER 1 | 2017
M AT E R I A L S | | | B Y A N D R E VA L C A N A I A , D . D . S . , A N D G I S E L E F. N E I VA , D . D . S . , M . S . , M . S .
teeth with substantial loss of structure often need to be treated endodontically before a final restoration is
placed. The longevity of root canal-treated teeth has been greatly extended due the continuous advances in endodontic therapy
as well as restorative procedures. Placement of a post and core is often required for replacement of the missing coronal aspect
of teeth, and are a routine procedure in clinical practice in order to increase length of the coronal portion of the preparation and reestab-
lish the functional integrity of the tooth to be restored.1,2
Which Block Should I Use?
Influence of Restoration Thickness on Metal Core
Show Through Under e.max CAD Anterior Crowns
The choice between pre-fabricated or cast post and core gener-
ally depends on the final shape of the canal as well as on the degree
of taper of the internal walls. The literature reports a success rate
that exceeds 90% percent after five years of clinical service.1
The presence of a metallic core that blocks out light transmission
gives the ceramic crown an unfavorable aspect, often interfering
with the final shade of the restoration.2,3,4 Given that the effect of
incident light largely determines the appearance of natural denti-
tion, all ceramic systems have been used to improve the optical limi-
tations of metal-ceramic or zirconia crowns. This is because they
transmit and refract light in the same manner as natural teeth. The
amount of light that is absorbed, reflected and transmitted depends
on the amount of crystals within the core matrix, the chemical
nature and the size of the particles compared to the incident light
wavelength. The final result is improved color and translucency of
the restoration.2,4,5,6 However, the optimized translucency of these
crowns may allow for show through of the underlying metal post
and core, especially for more conservative preparations.4
CAD/CAM materials that present low translucency (LT) and
high translucency (HT) options have different luminousness char-
acteristics due to variation in crystalline formation in each compo-
sition. Lithium disilicate ceramics, in a partially crystallized state,
can present two crystal nuclei: lithium metasilicate (Li2SiO3) and
lithium disilicate (Li2Si2O5). The translucency can be adjusted by
varying the crystallization heat treatment, which dictates crystalline
content and crystal size. Thus, HTceramic exhibits crystals of 0.8μm
- 1.5μm in a glassymatrix, whereas LT ceramic exhibits smaller crys-
tals of 0.2μm - 0.8μm interlocked in a high-density matrix.7
Recently, medium translucency e.max CAD MT blocks have
been introduced, with luminousness values that range between
the LT and HT blocks. The MT blocks are intended for restora-
tions that require a higher level of translucency than LT restora-
tions and a higher brightness level than HT restorations.8
Increasing the thickness of the ceramic material and utilizing a
more opaque luting agent have been suggested as ways of masking
the pre-existing metal or dark dentin substrate.4,9 This pilot study
was designed to address a couple of common clinical questions.
First, do e.max CAD crowns block out metal cores at different
translucency levels? Second, what is the necessary axial reduction
for the block out to be effective?
MATERIALS AND METHODS
In order to assess the influence of metal post and core on light
transmission through CAD/CAM e.max CAD anterior restorations
of various thicknesses and different translucency values, a typodont
tooth #9 was prepared with a well-defined chamfer finish line and
3 mm remaining tooth core. Post and core wax patterns were fabri-
cated and cast in gold color alloy (Fig. 1) and then cemented with
Panavia 21. The preparation was imaged three times with CEREC
Omnicam, software version 4.4.3. Each proposal was designed using
different thicknesses on the buccal surfaces, resulting in restora-
tions that were 0.9 mm, 1.2 mm and 1.5 mm in thickness (Fig. 2).
Three crowns were milled for each thickness using IPS e.max CAD
blocksHT,MTandLT in shadeA1, for a total of nine crowns (Fig. 3).
These were then divided into three experimental groups:
• Group I – 0.9 mm (HT, MT, LT)
• Group II – 1.2 mm (HT, MT, LT)
• Group III – 1.5 mm (HT, MT, LT)
The crowns were glazed and crystallized in a Programat P310
ceramic furnace for 24:30 minutes at 840°C. The crowns were
stabilized on the typodont preparation using Variolink Esthetic
Neutral try-in paste. Each group was photographed with a Nikon
D90 camera and Nikkor Medical 120 mm lens, using the same
distance, lighting and set-up with both black and gray back-
grounds. Two evaluators independently carried out visual analysis
among crowns under standardized lighting conditions.