On core and bi-layered all-ceramic fixed dental prostheses, design and mechanical properties : studies on stabilized zirconiumdioxide

Doctoral dissertation in odontology
Loss of teeth can affect a person’s self-esteem, social life, appearance
and oral function. Reconstruction of a missing tooth has scientifically
been shown to increase self-esteem and quality of life and to maintain
oral function. For many patients a fixed dental prostheses (FDP)
is preferred, either tooth- or implant-supported. Improvement and
development of all-ceramic materials have made them preferable to
other alternatives. However, despite properties of dental ceramics’
well known biocompatibility, good chemical and mechanical,
the materials have their weaknesses, such as brittleness and some
difficulties with the layering porcelain. Many all-ceramic materials
cannot withstand minor flexure; more than 0.1 - 0.3 %, will lead
to fracture. Oxide-ceramic, specifically yttria stabilized tetragonal
zirconia polycrystals (Y-TZP) has become the most commonly used
all-ceramic material. This material has the potential to be used for
larger restorations. In addition, one of many challenges is to ensure
durable zirconia-based restorations in the oral cavity.
In the clinical situation, crowns and bridges are supported by a
combination of different structures with differing properties, i.e.
bone, dentine and enamel. The complexity of the supporting tissues
in the oral cavity creates stress patterns in the prosthetic material,
which need to be considered when designing a dental restoration.
The durability of all-ceramic FDPs is dependent on knowledge of
the material and design of the FDPs. In particular the design, shape of
the connector and the radius of curvature at the gingival embrasure
play a significant role in the load-bearing capacity of FDPs.
The overall aim of this thesis is to evaluate design of zirconia-based
restorations in relation to achieving increased fracture resistance.
Another aim is related to how the choice of material used for
supporting tooth analogues in the test set-up and how this influences
test results relating to fracture strength of all-ceramic FDPs.
Study I evaluates different radii (0.60 and 0.90 mm) of curvature
in the embrasure of the connector area and different connector
dimensions (2 x 2, 3 x 2 and 3 x 3 mm) and their effects on the fracture resistance of 3-unit all-ceramic FPDs made of Y-TZP. The
results show that by increasing the radius of the gingival embrasure
from 0.6 to 0.9 mm, the fracture strength for a Y-TZP FPD with
connector dimension 3 x 3 mm will increase by 20%.
Study II investigated how the choice of material (aluminium,
polymer and DuraLay) used for supporting tooth analogues and
support complexity influence test results concerning the fracture
strength of FDPs made of a brittle material Y-TZP. The outcome
of the study demonstrated that Y-TZP FDPs cemented on tooth
analogues made of aluminium, with high E-modulus showed a
significantly higher load at fracture and a different fracture mode
than shown in clinical situations.
Study III evaluates how factors as different default settings for
connector design of two different CAD/CAM systems and different
radii of curvature in the embrasure area of the connector will affect
the fracture strength and the fracture mode of 3-unit, i.e. 4-unit allceramic
FDPs made from Y-TZP and further to investigate how the
number of pontics affect the fracture strength of Y-TZP. The results
showed that the most crucial factor for the load-bearing capacity
is the design of the radius of the gingival embrasures. Increasing
the number of pontics from three to four decreases the load-bearing
capacity nearly twice.
Study IV investigate and compare the fracture strength and fracture
mode in 11 groups of the currently most used multilayer all-ceramic
systems for Y-TZP FDPs, with respect to the choice of core material,
veneering material area, manufacturing technique (split-file, overpress,
built-up porcelains and glass-ceramics), design of connectors
and radius of curvature of FDP cores. The results show that the
design of a framework is a crucial factor for the load bearing capacity
of an all-ceramic FDP. The state-of-the-art designs are preferable,
since the split-file designed cores call for a cross-sectional connector
area, at least 42% larger, to have the same load bearing capacity as
the state-of-the-art designed cores. Analyses of the fracture patterns
demonstrated differences between the milled veneers and over-pressed
or built-up veneers, where the milled ones showed numerically more
veneer cracks whereas the other groups only showed complete
connector fractures. All veneering materials/techniques tested were
found, with great safety margin to be sufficient for clinical use both
anteriorly and posteriorly.