Key words:
Computer-aided design - dental - denture - esthetics - fixed - partial
INTRODUCTION
In the search for esthetics materials that are mechanically similar to metal, the
high resistance ceramics are an alternative to the metal ceramics.[1]
[2] The yttrium oxide partially stabilized tetragonal zirconia (Y-TZP) stands out for
meeting such requirements[1]
[2]
[3] thanks to a computer-aided design/computer-aided manufacturing (CAD/CAM) system,
which enabled the milling of all-ceramic extensive dental prostheses.[1]
[4]
CASE REPORT
A 46-year-old female patient wished to replace her removable denture for a fixed one,
refusing dental implants and removable denture [[Figure 1]]. She displayed an open bite with the absence of both central and left lateral upper
incisors, including substantial loss of the corresponding soft tissue [[Figure 2]]. The oral environment was adjusted before the rehabilitation.
Figure 1: Initial photo with provisional prosthesis unsatisfactory
Figure 2: Loss of upper front dental elements
Thereafter, a six-element fixed partial denture (FPD) was proposed, whereby the remaining
upper front teeth would be used for abutment, with dental element 13 receiving an
intraradicular retainer [[Figure 3]]. By esthetic requirements, the planning was made digitally following the digital
smile design principles, and the chosen materials were to be all ceramic.
Figure 3: Abutments were prepared by high-speed diamond-coated (KG SorensenR, Sao Paulo, Brazil)
Occlusal stability led to the planning of an FDP keeping the central occlusion pattern.
Ceramic artificial gum was associated with the white esthetics concept in an attempt
to minimize open bite and tissue loss. Color A2 was chosen for the dental elements,
while photographs were used to select the ceramic gum.
The lateral incisor and right canine teeth presented pulp vitality and adequate bone
support, while the left canine, endodontically treated, requiring a cast metal post.
One week after postcementation, the three abutments were prepared by high-speed diamond-coated
milling with reductions of approximately 1.5 mm (axial), and 2.0 mm (occlusal), with
finish line in rounded shoulder with 1.0 mm wide. Sequentially, the impression was
made by the double-cord technique using putty and light body addition polyvinyl siloxane.
Adjustment and temporary cementation were done [[Figure 4]]. The cast model was scanned by the CAD/CAM system, followed by digital planning
and milling of the infrastructure from a single presintered block of high translucency
[[Figure 5]].
Figure 4: Right and left lateral view during the provisional prosthesis
phase
Figure 5: Zirconia infrastructure test (Y-TZP-ZirkonzahnR, Bruneck,
Italia)
The framework was veneered with leucite-reinforced feldspar ceramic in the dental
elements, and Ceramax in the artificial gum. FPD underwent blasting with aluminum
oxide and chemical treatment with a single layer of silane for 60”, followed by adhesive
and photoactivation for 40”. Dental prophylaxis with pumice stone preceded usage of
the self-adhesive resin cement. After removal of the cement excesses, each abutment
was photopolymerized for 60”/face [Figures 6]
[7]
[8]. Oral hygiene orientations were reinforced at the end of cementation.
Figure 6: Fixed partial denture finished. Stratification with feldspathic ceramics based on
leucite (IPS Empress E-Max®, Ivoclar Vivadent, São Paulo, Brazil) and Gingival ceramics (Ceramax, Talmax®, Curitiba, Brazil)
Figure 7: (a) Silane (Monobond S, Ivoclar Vivadent) chemical activation for 60 s, followed
by single layer of adhesive (Scotchbond™ Universal Adhesive, 3M). (b) Insertion of
dual auto adhesive resin cement (RelyX™ U200, 3M ESPE®). (c) dental positioning and fixed partial denture were photopolymerized (Optilight
LD MAX, Gnatus®, São Paulo, BR) for 60 s face in each abutment. (d) cemented prosthesis
Figure 8: Prosthesis installed. (a) by occlusal view; (b and c) by right and left lateral view.
(d-f) front view
DISCUSSION
Digital planning revealed the necessity of replacement of the lost soft tissue for
display of the gum through the high smile line.[5] Initially, several attempts of longer provisional crowns were made to compensate
soft tissue loss, but they turned out esthetically unsatisfying. Finally, fixed dentogingival
prosthesis with ceramic artificial gum was proven to be the best choice.[6]
The endodontic-treated abutment needed a cast metal post due to the limited coronal
remnants, beside the abutment function.[7] The structural opacity of the FPD concealed the metallic aspect, maintaining esthetics.[8]
The Y-TZP gained space in dentistry for combining resistance to esthetics,[8] unlike the metal.[1]
[3] The improvement of this ceramic guarantees flexural strength and high tenacity due
to its crystalline composition, increasing its indications,[1] although limiting their optical properties of translucency and hence being employed
as a core to be veneered with glass ceramics.[8] All ceramic FPDs are consensus in dental practice and seem to be a worldwide trend,
yet, they have to be well understood to optimize its characteristics and physical
properties.[1]
Although zirconia resistance is questioned in high masticatory forces areas, it can
be considered the most suitable substructure ceramic in rehabilitation for anterior-free
contact teeth.[9]
[10] Other two studies[2]
[11] that followed patients which use prostheses, totalizing 27 (from up to 6 units),
and thirty prostheses all exclusively ceramic, for 7 and 5 years, 0 and 2 structural
failures occurred, respectively . . The occurring failures were posterior, with the
first in the region of the connector, supposing that the location of the prostheses
is determinative to longevity, corroborating with the results found in studies, where
zirconia has its limited use on molar and posterior teeth due to loads of mastication
present in the region.[9]
[11] The connectors exhibit limited design and dimensions, increasing the susceptibility
to concentrate tension and risk of fractures.[1] A consensus about the ideal dimension for each material is undefined because it
involves variables such as available space, the posterior region being more critical
than the anterior hereby rehabilitated. In the present clinical case, there were two
abutments close to each other, making its design even more challenging. However, the
software used for virtual design ensured dimensions of all components with adequate
resistance,[1] only allowing milling with requirements within its own predefined patterns. This
data confer the prosthesis with favorable longevity characteristics.
The CAD/CAM system has expanded the indications of fixed ceramic prosthesis and improved
their adaptation, guaranteeing increased internal and marginal adaptation.[1]
[4] However, Y-TZP, commercialized in blocks, is machined in mono block, requiring adequate
preparation of the three abutments with reductions as referenced according to biomechanical
principles that guaranteed a coinciding insertion axis, retention, and structural
rigidity simultaneously.[12] The cervical preparation is responsible for the adaptation and marginal integrity
and consequently, for the longevity of the system. Therefore, the finishing in rounded
shoulder[1] propitiated the precise adaptation and constant ceramic thickness, favoring load
distribution and stress resistance while minimizing the tensions and consequently
preventing cohesive fractures.[12] Yet to be considered, even long-extension bridges are milled in one piece, which
might lead to strain development, especially in angled-type bridges. Hence, they must
present passive fit, since soldering, as is performed in metal ceramics, cannot be
done here.[13] Another concern is with regard to the veneering technique, which may also lead to
strain development. In spite of the precision of the framework, the application of
ceramic veneering results in an increase in strain development.[14] Chipping of the veneering ceramic is frequently associated with the failure of all-ceramic
prostheses. As evidenced in several clinical follow-up studies,[1]
[2]
[9]
[12] this failure generates uncertainties as to the longevity of these prostheses and
is associated with factors such as modulus of structural elasticity, different coefficients
of thermal expansion of the ceramics, application technique and firing, inappropriate
thickness, and occlusal overload.[2] In the present clinical case, the FPD followed the mechanical principles required
by the materials.
The low silica content of Y-TZP jeopardizes the conventional adhesive technique.[1] Blasting is recommended to increase surface roughness and assure micromechanical
retention, including the association of primers and/or adhesives.[1]
[8] After aluminum oxide blasting, FPD was subjected to a silane coupling agent.[2] Although silanization to zirconia is uncertain, it is done on the assumption of
increased surface energy and wettability, when made associated with sandblast,[15] favoring the action of cement.[1]
[8] The universal adhesive in use disposed the methacryloyloxydecyl dihydrogen phosphate
monomer[1] that structurally interacts with the surface oxides of zirconia[1]
[8] and with the methacrylate matrix of the resin cement seems uniting them chemically.[8] The self-etching cement guaranteed union to the dental structure even without previous
treatment, while its monomeric composition also seems to react to the available oxides,
promoting some chemical adhesion additional to the mechanical interlocks.[16] Despite aforementioned related, long-term stability of the adhesion is a concern
yet.[15]
One year after the cementation, FPD was checked, stable, with no modifications, and
the patient related to be pleased and comfortable with the rehabilitation.
CONCLUSION
Although all-ceramic FPDs are extensively used in clinical practice, dentists have
to consider several aspects of the material to overcome its limitations, as though
as to take benefit of its advantages. All ceramic rehabilitations are world widespread,
with improvements been launched frequently, demanding more studies to confirm them
evidence based to allow clinical use.
Financial support and sponsorship
Nil.
